This item is only available as the following downloads:
1 DEVELOPMENT AND ASSESSMENT OF A NOVEL AMORPHOUS SILICA ENCAPSULATION TECHNOLOGY FOR MITIGATING BIOTOXICITY OF WELDING FUME PARTICLES By JUN WANG A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY UNIVERSITY OF FLORIDA 2013
2 2013 Jun Wang
3 To my parents, Dr. Xiaode Wang and Mrs. Xuhui Hou, my wife, Mrs. Yingjia Zhu, a nd lovely daughter, Ms. Ceci l ia Wang They always support and encourage me, give me the strength, and inspire me to achieve the milestone here.
4 ACKNOWLEDGMENTS First, I would like to thank my supervisor Dr. Chang Yu Wu, for his continuous patience an d guidance throughout my doctoral study. Dr. Wu is the role model for me in the research and academic world. He dedicated all his resources to light up my path I am sincerely gratef ul to my family Without their endless support and cheering up I would not be able to push the boundaries. My daughter, Cecilia Wang who was born at the same time with the completion of this dissertation, is the great est gift in my life. I w ant to acknow ledge the funding agencies supporting my doctoral study, particularly Mrs. Kathleen Paulson at Naval Facilities Engineering Command The funding agencies are the reason my studies can be completed without financial disruption I would like to thank my whol e Ph.D. committee namely Dr. Myoseon Jang, Dr. Wesley Bolch, Dr. Vito Ilacqua, and Dr. Jean Claude Bonzongo, for their time, attention, criticism, and encourage ment I would like to mention all the undergraduate student assistants, including Mr. Mark Kali voda, Ms. Jianying Guan, Ms. Jessica Sharby, and others who worked with me The undergraduate students enthusiasm help ed me achieve the research goal, while working with them honed my mentorship skill. Finally, I want to express my deepest gratitude to th ose who helped cared, and inspired me, in the past, present, and future.
5 TABLE OF CONTENTS page ACKNOWLEDGMENTS ................................ ................................ ................................ .. 4 LIST OF TABLES ................................ ................................ ................................ ............ 7 LIST OF FIGURES ................................ ................................ ................................ .......... 9 LIST OF ABBREVIATIONS ................................ ................................ ........................... 11 ABSTRACT ................................ ................................ ................................ ................... 13 CHAPTER 1 INT RODUCTION ................................ ................................ ................................ .... 15 Background ................................ ................................ ................................ ............. 15 Distribution of Welding Fume Particles in t he Human Body ................................ .... 17 Current Control Technologies ................................ ................................ ................. 19 Amorphous Silica Encapsulation Technology ................................ ......................... 21 Analysis of Silica Encapsulation ................................ ................................ ............. 23 Research Goals ................................ ................................ ................................ ...... 24 2 DETERMINATION OF SILICA COATING EFFICIENCY ON METAL PARTICLES USING MULTIPLE DIGESTION METHODS ................................ ...... 25 Objective ................................ ................................ ................................ ................. 25 Experimental Methods ................................ ................................ ............................ 29 Sample Preparation ................................ ................................ .......................... 29 Instrumentation and Chemical Reagen ts ................................ ......................... 30 Digestion Process ................................ ................................ ............................ 33 Method Detection Limit and Recovery Efficiency ................................ ............. 34 Results and Discussion ................................ ................................ ........................... 35 Method Detection Limit ................................ ................................ ..................... 35 Metal Recovery Efficiency ................................ ................................ ................ 36 Silica Coating Efficiency ................................ ................................ ................... 38 Summary ................................ ................................ ................................ ................ 42 3 DOUBLE SHROUD DELIVERY OF SILICA PRECURSOR TO REDUCE HEXAVALENT CHROMIUM IN WELDING FUME ................................ .................. 44 Objective ................................ ................................ ................................ ................. 44 Experimental Methods ................................ ................................ ............................ 46 Injection of Silica Precursor ................................ ................................ .............. 46 Welding Fume Sampling ................................ ................................ .................. 48 Analysis of Cr 6+ ................................ ................................ ................................ 51
6 Chemical Reagents and Labware ................................ ................................ .... 52 Hazard and Cost Estimation ................................ ................................ ............. 53 Statistics ................................ ................................ ................................ ........... 54 Results and Discussion ................................ ................................ ........................... 54 Chamber Sampling Results ................................ ................................ .............. 54 NO and CO Results ................................ ................................ .......................... 56 XPS Results ................................ ................................ ................................ ..... 58 Field Sampling Results ................................ ................................ ..................... 59 Effects on Welding Quality ................................ ................................ ............... 60 Hazard and Cost Estimation ................................ ................................ ............. 61 Summary ................................ ................................ ................................ ................ 62 4 ENCAPSULATION EFFECTIVENESS OF WELDING FUME P ARTICLES AND ITS IMPACT ON MECHANICAL PROPERTIES OF WELDS ................................ 64 Objective ................................ ................................ ................................ ................. 64 Experimental Methods ................................ ................................ ............................ 66 Welding Fume Generation ................................ ................................ ................ 66 Analysis of Silica Encapsulation ................................ ................................ ....... 69 Mechanical Property Test ................................ ................................ ................. 71 Quality Control and Statistics ................................ ................................ ........... 72 Results and Discussion ................................ ................................ ........................... 73 Silica Encapsulation ................................ ................................ ......................... 73 TEM Imagery ................................ ................................ ................................ .... 75 Mechanical Properties ................................ ................................ ...................... 78 Summary ................................ ................................ ................................ ................ 81 5 CONCLUSIONS AND RECOMMENDATIONS ................................ ....................... 83 APPENDIX A DETAIL RESULTS OF THE HAZARD AND COST ESTIMATION .......................... 87 B DETAIL RESULTS OF THE MECHANICAL STRUCTURE TEST .......................... 95 C CHARA CTERIZATION OF MERCURY IN CEMENT KILN BAGHOUSE FILTERED DUST (BFD) AND THE RELEASE OF VAPOR PHASE MERCURY FROM CONCRETE PROCESSING WITH BFD ADDED CEMENT ....................... 99 LIST OF REFERENCES ................................ ................................ ............................. 120 BIOGRAPHICAL SKETCH ................................ ................................ .......................... 133
7 LIST OF TABLES Table page 1 1 Exposure limits for airborne Cr 6+ Ni, Mn, and welding fume .............................. 17 2 1 Welding parameters and silica coating efficiencies under different welding conditions ................................ ................................ ................................ ........... 30 2 2 Operation conditions of Plasma 3200 ICP AES ................................ ................. 31 2 3 Wavelength of spectral lines (nm) and method detection limit (mg L 1 ) for each metal in this study ................................ ................................ ...................... 32 2 4 Operating programs of CEM MDS 81D microwave digesting system ................ 33 2 5 Measured concentrations (mg L 1 ) of metal elements digest ed by different acid mixtures ................................ ................................ ................................ ...... 40 3 1 Testing conditions of TMS and shielding gas flows in the chamber sampling .... 50 3 2 Testing cond itions of NO/Ar and CO/Ar in the chamber sampling ...................... 50 4 1 Flow rates of primary shielding gas and TMS carrier gas, and the corresponding mass of collected welding fume particles ................................ .... 69 4 2 Operating conditions of the ICP AES and the spectral wavelengths used for the analysis ................................ ................................ ................................ ......... 71 4 3 Chemical composition (wt%) of weld metals and standard requirements ........... 78 A 1 Unit cost used in the cost model ................................ ................................ ......... 90 A 2 Welded joint costs of traditional welding and ASE technology ........................... 91 A 3 Typical ventilation system cost ................................ ................................ ........... 93 A 4 Summary of the cost comparison of the ASE technology in different scenarios and tra ditional welding ................................ ................................ ........ 94 B 1 Composition profile of test welds and standard materials (%) ............................ 95 B 2 Tensile values of baseline and TMS tes t welds test specimen ................................ ................................ ................................ ..... 98 C 1 Hg classification in this study ................................ ................................ ............ 104 C 2 The recipe of the Portland cement concrete (kg) used in this study ................. 110
8 C 3 Hg concentration and speciation in the concrete constituents .......................... 111 C 4 Total Hg (g) releas ed from the BFD under different experimental conditions, in the OH trap ................................ ................................ ................................ ... 115
9 LIST OF FIGURES Figure page 1 1 Mechanistic illustration of the ASE tec hnology ................................ ................... 22 2 1 Schematic diagram for the sampling system ................................ ...................... 30 2 2 Recovery efficiencies for Fe, Cr, Ni, Mn, Cu by different digestio n acid ............. 37 2 3 TEM imaginary of welding fume particles with silica coating .............................. 38 3 1 Mechanistic illustration of silica precursor for reducing the formation of Cr 6+ ..... 45 3 2 Cross sectional sketch of the insulated double shroud torch (IDST) .................. 47 3 3 Schematic diagram ................................ ................................ ............................. 47 3 4 Cr 6+ concentration as a function of TMS carrier gas/primary shielding gas percentage ................................ ................................ ................................ .......... 55 3 5 XPS spectra ................................ ................................ ................................ ........ 59 3 6 Photographs of weld surface ................................ ................................ .............. 61 4 1 M echanistic illustration of the ASE technology in welding application ................ 65 4 2 Schematic diagram of the TMS feeding apparatus and the welding fume chamber ................................ ................................ ................................ ............. 67 4 3 SCE as a function of TMS carrier gas flow rate ................................ .................. 73 4 4 T EM images ................................ ................................ ................................ ....... 77 4 5 Macrostructure of welds. ................................ ................................ .................... 79 4 6 Microstructure of welds ................................ ................................ ....................... 79 4 7 Tensile test results ................................ ................................ .............................. 81 B 1 Microstructure of the weld zone in one plate. ................................ ..................... 96 B 2 Microstructure of the weld zone in sample F541. ................................ ............... 97 C 1 Simplified material flow of a typical cement kiln. ................................ ............... 101 C 2 Breakdown schematic diagram of the bench system. ................................ ....... 106
10 C 3 Cross sectional and lateral view of t he cylindrical tube loaded with 500 g and 100 g BFD ................................ ................................ ................................ ........ 109 C 4 Total Hg, SI Hg, and NSI Hg concentration in the 7 day time series study. ..... 113 C 5 Hg released from the BFD as a function of time ................................ ............... 115
11 LIST OF ABBREVIATIONS ACGIH A MERICAN C ONFERENCE OF G OVERNMENTAL I NDUSTRIAL H YGIENISTS A NOVA AN ALYSIS O F VA RIANCE ASE A MORPHOUS S ILICA E NCAPSULATION ASTM A MERI CAN S OCIETY FOR T ESTING AND M ATERIALS AWS A MERICAN W ELDING S OCIETY BP B ASE P LATE CO C ARBON M ONOXIDE ELPI E LECTRICAL L OW P RESSURE I MPACTOR FL F USION LINE GMAW G AS M ETAL A RC W ELDING GTAW G AS T UNGSTEN A RC W ELDING HAP H AZARDOUS A IR P OLLUTANT HAZ H EAT A F FECTED Z ONE IC I ON C HROMATOGRAPHY ICP AES I NDUCTIVELY C OUPLED P LASMA A TOMIC E MISSION S PECTROSCOPY IDST I NSULATED D OUBLE S HROUD T ORCH LEV L OCAL E XHAUST V ENTILATION LPM L ITER P ER M INUTE NIOSH N ATIONAL I NSTITUTE FOR O CCUPATIONAL S AFETY AND H EALTH NO N ITR IC O XIDE OSHA O CCUPATIONAL S AFETY AND H EALTH A DMINISTRATION PEL P ERMISSIBLE E XPOSURE L IMIT PPE P ERSONAL P ROTECTION E QUIPMENT REL R ECOMMENDED E X POSURE L IMIT
12 SCE S ILICA C OATING E FFICIENCY SMAW S HIELDED M ETAL A RC W ELDING SMPS S CANNING M OBILITY P ARTICL E S IZER TEM T RANSMISSION E LECTRON M ICROSCOPY TEOS T ETRAETHYLOXYSILANE TMS T ETRAMETHYLSILANE TWA T IME WEIGHTED AVERAGE WM W ELD METAL XPS X RAY PHOTOELECTRON SP ECTROSCOPY
13 Abstract of Dissertation Presented to the Graduate School of the University of F lorida in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy DEVELOPMENT AND ASSESSMENT OF A NOVEL AMORPHOUS SILICA ENCAPSULATION TECHNOLOGY FOR MITIGATING BIOTOXICITY OF WELDING FUME PARTICLES By Jun Wang Augu st 2013 Chair: Chang Yu Wu Major: Environmental Engineering Sciences Welding generates a large number of metallic nano particles and poses serious health risks to welders. The tightening occupational standard requires more reduction in exposure, which is not satisfied by current control technologies. The objective of this study wa s to develop an amorphous silica encapsulation (ASE) technology for limit ing chromium oxidation and encapsulat ing fume particles with an amorphous silica layer and to assess its e ffectiveness A quantitative method to determine silica coating efficiency (SCE) on metal particles was first developed. Different acid mixtures were tested as digestion method s targeting at metals and silica coating. SCE s were calculated based on the meas ured concentrations following digestion by HNO 3 /HF and aqua regia. T he results showed that 14 39% of fume particles were encapsulated The low SCEs were due to premature decomposition of silica precursor A n insulated double shroud torch ( IDST ) was de signe d to overcome the premature decomposition by separating the flows of the shielding gases The r esults from the laboratory and field demonstrated over 90% reduction of Cr 6+ The SCE s us ing the IDST were enhanced to around 48~64% at the
14 low and medium primar y shielding gas flow rates. The highest SCE of 76% occurred at the high shielding gas flow rate (30 Lpm) with a TMS carrier gas flow of 0.64 Lpm Transmission electron microscopy (TEM) images confirmed the amorphous silica layer on the fume particles The concentration of TMS was below safety threshold (1%) in the worst case scenario Mechanical structure test s showed that the quality of welds from the baseline and from the ASE technology were similar. With only a 3.8% additional cost, this novel ASE techno logy has the potential to cost effectively address the welding fume exposure issue which has been listed as the National Occupational Research Agenda. Upon successful demonstration, this novel technology can help the industries meet the occupational standard, and protect over 0.5 million welders in the US. Th e newly developed method to quantify SCEs can be used in other silica coated nanoparticles application s The study helps further understand how silica and metals interact in a high temperature aerosol system
15 CHAPTER 1 INTRODUCTION Background Stainless steel contains alloying metals such as chromium (Cr), nickel (Ni), and manganese (Mn) ( Peckner and Bernstein 1977 ; Marshall 1984 ) Stainless steel welding the join ing of pieces of metal with stai nless steel filler material is a common industrial practice. These metals in welding filler material s such as wires and electrodes can vaporize during the welding process due to the high temperature of the welding arc. The metals are oxidized and subseque ntly form nano sized particles as the temperature drops ( Hewett 1995 ; Zimmer and Biswas 2001 ; Moroni and Viti 2009 ) which may stay ( Schoonover et al. 2011 ) The welding fumes pose potential adverse health risks to welders who may inhale toxic metals such as hex a val e nt chromium (Cr 6+ ), Ni, Mn, and other toxic components. Because of their nanometer to sub micrometer size, the inhaled particles can pen etrate deeply into the human respiratory tract ( Yu et al. 2000 ; Biswas and Wu 2005 ; Geiser and Kreyling 2010 ) Upon contact with human organ s the se metals can be released from the particles, absorbed, distributed, and metabolized ( Li et al. 2004 ) Chromium because of its resistan ce to corrosion and discoloration ( Marshall 1984 ; Kropschot and Doebri ch 2010 ) is one of the ma jor components of stainless steel Cr 6+ is formed as a result of Cr oxidation with reactive species such as oxygen and ozone in the welding arc zone ( Gray et al. 1983 ; Dennis et al. 1997 ) Cr 6+ exposure continues to be a troublesome issue in industrial occupation environment s ( Korczynski 2000 ; Pellerin and Booker 2000 ) Among all the metals present in welding fume s Cr 6+ draws the most concern due to its carcinogenic effect ( IARC 1990 ) which has been
16 confirmed by extensive human and animal data ( Langrd 1988 ; USEPA 1998 ) By contrast, trivalent chromium (Cr 3+ ) is relative ly less toxic and actually one essential nutrient for human metabolism activity ( Anderson 1997 ) Among all the other metals in welding fumes, nickel is also a known human carcinogen ( IARC 1990 ) while manganese is a neurotoxin that can induce neurological symptoms such as cognition dysfu nction ( Harris et al. 2011 ; Laohaudomchok et al. 2011 ; Summers et al. 2011 ) Numerous toxicological studies have shown that welders inhaling fume particles are exposed to these toxic metals and bear the risk of respiratory diseases, neurological symptoms, and cancer ( Keskinen et al. 1980 ; Antonini et al. 1998 ; Antonini et al. 2003 ; Antonini et al. 2007 ; Flynn and Susi 2009 ; Harris et al. 2011 ; Rice et al. 2011 ; Summers et al. 2011 ; Zeidler Erdely et al. 2011 ; Thaon et al. 2012 ) The exposure limits set by regulatory agencies and professional organization s are listed in Table 1 1. T he Occupational Safety and Health Administration (OSHA) does not currently regulate total welding fumes ; t he National Institute for Occupationa l Safety and Health (NIOSH) conside rs welding fumes to be potential occupational carcinogens and has set the recommended exposure limit (REL) at the lowest feasible concentration ( NIOSH 2007 ) T he American Conference of Governmental Industrial Hygienists (ACGIH) has also assigned welding fumes a n 8 hour time weighted average (TWA) threshold limit value (TLV) of 5 mg/ m 3 ( ACGIH 2012 ) OSHA lowered the 8 hour time weighted average (8 hr TWA) permissible exposure limit (PEL) for Cr 6+ from 52 3 3 in 2006 ( OSHA 2006a ) NIOSH just lowered 10 hr TWA REL for Cr 6+ of 1 g/m 3 ( NIOSH 2008 ) to 8 hr TWA R EL of 0.2 g/m 3 in 2013 Because of the residual risk of lung cancer even at REL ( NIOSH 2008 ) NIOSH also recommended adoption of
17 reasonable control technology to further reduce Cr 6+ Although ACGIH recommend ed a TLV of 50 g/m 3 the value has not been updated since 2006. The tightenin g standards revealed the fact that the previous ly set high value was not enough for protecting the workers. All the relevant agencies regulate or recommend exposure limits for Ni and Mn from 15 ng/m 3 to 5 mg/m 3 level (Table 1 1), which are more tolerated t han for Cr 6+ It should be noted that NIOSH RELs are always order of magnitude lower than OSHA PEL. OSHA regulates the permissible exposure standard to an economically feasible level for the industries, and offers the appropriate protection for the workers NIOSH recommends the conservative values based on the newest research. Table 1 1. Exposure limits for airborne Cr 6+ Ni, Mn, and welding fume Airborne OSHA ( 2006a ) NIOSH ( 2008 ) ACGIH ( 2012 ) Cr 6+ 5 g/m 3 0.2 g/m 3 ( 2013 ) 50 g/m 3 Ni 1 mg/m 3 0.015 mg/m 3 0.1~1.5 mg/m 3 Mn 5 mg/m 3 1 mg/m 3 0.2 mg/m 3 Welding fume Not regulated Lowest feasible 5 mg/m 3 Di stribution of Welding Fume Particles in the Human Body The deposition pattern of welding fume particles in the human respiratory system are determined by various parameters, such as p article size distribution (PSD) ( ICRP 1994 ) and flow rate ( Geiser and Kreyling 2010 ) Some large size particles will be removed directly by mechanical exhal ation through mouth or nose. The median diameter of the welding fume particles is around 20 nm ( Zimmer and Biswas 2001 ) Particles in this nanometer range have a higher probability of reaching the conducting airways or the alveoli ( Oberdorster et al. 2005 ) from where they are not easily removed by mechanical exhalation A fter deposition the release of the metals from the
18 nanoparticle complex depends on the particle size and solubility of the metals. The metals dissolv e d in the biological fluid will then enter the systemic blood stream. The undissolved particles will be carried by the cilia towards the pharynx, where they were either swallowed or expelled via coughing. Inhalation exposure to Cr 6+ and Ni, which are carcinogens, can lead to the development of neoplasm s following initiation, promotion, and progression stag es of carcinogenesis ( ATSDR 2008a ) Cr 6+ is known to coordinate with DNA bases and disrupt DNA structure ( Hansen and Stern 1985 ) The absorption of Cr through i nhalation to the lungs depends on the oxidation state of the Cr, i.e., Cr 3+ or Cr 6+ The kinetic process for Cr 6+ follows a linear first order kinetics, with about 80% of the Cr 6+ compounds removed from the lung s in a week ( ATSDR 2008a ) Absorbed Cr is carried by blood to various systemic tissues. The targets of Cr distribution include the lymph no de s spleen, liver, kidney, and heart wall ( Zeidler Erdely et al. 2008 ) Cr 3+ is an essential nutrient to glucose, protein, and fat metabolism. T he higher redox potential of Cr 6+ and its ability to penetrate the cell membranes gives it a greater toxic potency ( Zatka 1985 ) Consequently, t he reduction of Cr 6+ to Cr 3+ or even Cr 0 could limit the toxicity of welding fume particles. Inhalation carcinogenesis of Ni has not been well studied, mainly due to the poor sensiti vity of current analytical methods for Ni and its compounds ( ATSDR 2005 ) In contrast to Cr 6+ stud ies found Ni exposure was most likely associated with non respiratory tract cancers, such as stomach and bone cancer ( Mayer et al. 1998 ; Kasprzak et al. 2003 ; Oller et al. 2008 ) IARC classified metallic Ni as a possible human carcinogen, while other Ni compounds were classified as human carcinogens ( IARC
19 1990 ) In this case, insulating Ni to prevent the formation of Ni compounds will reduce the carcinogenicity of welding fume particles. The site of action of Mn is in the brain due to its neurological toxi n nature ( Keane et al. 2010 ) Consequently, the distribution of Mn to the brain is crit ical for its toxicity ( ATSDR 2008b ) The absorption of Mn mainly depends on the welding fume particle size ( Antonini et al. 2011 ) Small size particle s deposit in the deeper airways, get absorbed in the blood, and are then transported to system ic organs v ia the bloodstream ( Richman et al. 2008 ) While the blood brain barrier is a mechanism to limit toxics reaching the central nervous system and the brain ( Sriram et al. 2010 ) particles deposited in the n asal mucosa c an be directly transported to the brain ( Richman et al. 2008 ) Current Control Technologies These occupational standards pushed the develop ment of Cr 6+ free welding technology to en ( CDC 2008 ) There are various ways to mitigate the emission of welding fume particles and reduce fume exposure ( Hewitt and Hirst 1993 ) The most effective methods are still personal protective equipment (PPE) such as respirators ( Cho et al. 2011 ) However, the majority of welders prefer not to wear a respirator as it is bulky and heavy. Although respirators are distributed in industrial facilities, it is not mandatory to do the fitting test and workers are not required to wear the respirators during welding operations M etal inert gas (MIG) welding uses shielding gas t o protect we lds from surrounding atmospheric components such as nitrogen and water The shielding gas also helps limit the oxidation of metals by lowering penetration of reactive oxygen species ( Dennis et al. 1997 ) and reducing fume generation rate s ( Ebrahimnia et al.
20 2009 ) However, there are numerous types of shielding gases ( Antonini et al. 1998 ) and some combinations of these gases actually could increase ultraviolet (UV) intensity and ozone concentration ( Dennis et al. 1997 ) S hielded metal arc welding (SMAW) uses vapor generated from flux coating to prevent oxidation and to ensure the purity of the weld. I t should be emphasized that the purposes of these shielding technologies aimed at expelling oxygen species are to ensure and promote the weld quality, rather than to reduce the toxicity of welding fume. L ocal exhaust ventilation (LEV) technology is commercially available to remove welding fumes from breathing zone s ( Lee et al. 2007 ; Meeker et al. 2010 ) However, LEV is inconvenient in outdoor welding where welders move frequent ly ( Hewitt and Hirst 1993 ; Flynn and Susi 2012 ) On gun extraction was developed to collect welding fume near the welding arc ; nevertheless, the current type of extraction gun is bulky and heavy from an operational perspective and less effective in vertical and overhead we lding ( Wallace et al. 2001 ; Flynn and Susi 2012 ) There are also concerns that the suction force of the LEV or on gun extraction w ould disturb the flow of the shielding gas and thus affect the weld quality Control technologies targeting Cr 6+ have also been developed. For example t he addition of reactive metals, such as zinc (Zn) and aluminum (Al) to welding wir es can reduce the formation of Cr 6+ ( Dennis et al. 1996 ) R educing agents used as shielding gas additives such as methane (C 2 H 4 ) or nitrogen oxide (NO) can reduce ozone formation in the welding fume s and hence limit the formation of Cr 6+ Dennis et al. ( 2002 ) test ed NO and C 2 H 4 as shielding gas additives, and found about 50~70% remo val efficiencies. However the flammabi lity of met hane and the formation of toxic NO 2 are
21 potential risks when using th e two reducing agents Carbon monoxide (CO) is also used in th e industry as a reduc ing gas ( Jie et al. 2008 ) to reduce metal oxidation, yet is itself a hazardous air pollutant (HAP) Replacing the Cr content in the stainless steel wire wit h other materials with identical functions such as ruthenium (Ru) can reduce the amount of Cr in welding fume ( Paulson et al. 2011 ) but this option is limited by the lack of availability of these rare metals. In summary, there is currently no widely adopted t echnology conforming to the new occupational standards to mitigate the formation o f airborne Cr 6+ emissions from welding and which is well received by welders. A technology that can be broadly applied to most welding conditions, well balanced in efficiency and feasibility, is critically needed for the better protection of welders heal th and safety, satisfaction of the occupational standards, as well as reduction of medical cost for the society. Amorphous Silica Encapsulation Technology C oating an amorphous silica layer on particle surfaces to insulate engineered nanoparticles from degr adation from exposure to the surrounding environment has been reported in various studies ( Liz Marzn et al. 1996 ; Biswas et al. 1997 ; Correa Duarte et al. 1998 ; Yi et al. 2005 ; Teleki et al. 2009 ) Amorpho us silica encapsulation (ASE) technology has been demonstrated to be an effective measure to control nano sized metal particles and to reduce the environmental pollution from combustion system and incinerators ( Biswas et al. 1995 ; McMillin et al. 1996 ; Owens and Biswas 1996 ; Biswas et al. 1997 ; Biswas and Wu 1998 ) The process is conceptually illustrated in Figure 1. Silica precursor injected into the high temperature zone reacts with reactive oxygen species, henc e limiting the oxidation of metals. Silica coating is achieved through the condensation of in situ generated amorphous silica onto metal particles. The
22 amorphous silica layer insulates the metal species from human organisms when inhaled. Silica formed from reaction also effectively increases the size of particles in the system The two fold approach of limiting oxidation potential and coating metal particles in an amorphous silica layer goes beyond previous control technologies by addressing all the toxic m etals regardless of their oxidation state. Figure 1 1. Mechanistic illustration of the ASE technology Initial proof of concept demonstration of the amorphous silica encapsulation technology applied to welding has be en successfully achieved in the prel iminary experiments ( Topham et al. 2010 ; Topham et al. 2011 ; Yu et al. 2011 ) Vapor phase s ilica precursor tetramethylsi lane (TMS) was added to shielding gas and decomposed at the welding arc zone. TMS reacted with the oxygen species to form silica, hence limiting the oxidation of Cr. In situ generated silica then co ndensed on metal particles to form an amorphous silica coating, which was observed in the TEM images However, the initial success of the ASE technology is also constrained to specific operating
23 condition s Expanding this technology to a wide range of weld ing operations is critical to its practical application. More importantly, the assessment of the change of welding physical, chemical and toxicological properties due to silica precursor is necessary to demonstrate the effectiveness of thi s technology. Analysis of Silica Encapsulation Bioavailable fraction of the welding fume particle is the toxicants dissolved and absorbed, thus enters the systematic circulation. The silica layer on the welding fume particles can effectively prevent the to xic metals from being bioavailable. Quantifying the coating efficiency of silica layer on the metal particles, i.e. the fraction of metals encapsulated that is not bioavailable is essential for assessing the ASE technology, as well as other silica metal c ore shell structure particle synthesis or control technology. The traditional method s to examine the silica encapsulation on particles were to visualize the particles using either Scanning Electron Microscopy (SEM) or Transmission Electron Microscopy ( TEM ) However, due to the two dimensional nature of microscopic image, metal appears to be encapsulated in the images could be non partially or fully encapsulated or even enriched on silica surface ( Maynard et al. 2004 ) in a three dimensional space. In addition, the microscopic images can only portray a portion of the particles on the microscope grid. Whether the images can represent the real conditions of the silica encapsulation have alwa ys been questioned. Silica is immune to most acid attack s except hydrofluoric acid ( HF ) ( Xu et al. 2005 ) Nitric acid or aqua regia is a common acid to dis solve metals in the environmental samples ( Dulski 1996 ; Myhnen et al. 2002 ) By using different digestion acids with different l evel s of abilities to dissolve silica, the amount of metals encapsulated inside the silica shell can be calculated.
24 Research Goals The hypotheses of this doctoral research are : (1) the proportion of the metals encapsulated inside the silica layer can be qu antified with different digestion methods ; (2) insulating the flow of the silica precursor from the other shielding gas flow should be able to eliminate premature decomposition of the silica precursor, thus further reducing Cr 6+ and improving the SCEs comp ared to the pre mixed mode; (3) the ASE technology will not noticeably change the cost and the mechanical qualities of the product compared to the conventional welding process. To prove the hypothesis a quantitative method to determine the proportion of m etals encapsulated inside the amorphous silica shell was first developed using HNO 3 /HF mixture and aqua regia To overcome the premature decomposition of silica precursor, an insulated double shroud torch (IDST) was designed and test ed for Cr 6+ reduction, as well as silica encapsulation efficienc y (SCE) using the newly developed quantitative method. During the study, the impact s of this technology on industrial c ost, hazard, and weld quality were examined and compare d to the baseline condition. Success of the demonstration of the ASE technology in welding will bring great health and safety benefits to welders, currently estimated to be 466,400 in United States ( BLS 2008 ) as well as reduc e tremendous medical cost associated with the toxic welding fume exposure
25 CHAPTER 2 DETERMINATION OF SILICA COATING EFFICIENCY ON METAL PARTICLES USING MULTIPLE DIGESTION METHODS Objective Metal p articles, including both elemental and oxidized metals, are commonly found in industrial systems such as coal combustor ( Biswas et al. 1995 ) incinerator ( Chen et al. 1999 ) furnace ( Gullett and Raghunathan 2002 ) boiler ( Frederick et al. 2004 ) engine ( Kittelson et al. 2006 ) and other high temperature processes involving metals including arc welding ( Zimmer and Biswas 2001 ) These metals vaporize at the flame/arc zone, and then quickly nucleate and condense to form nano sized particles as temperature drops, due to the sharp decrease of their saturation vapor pressure ( Sethi and Biswas 1990 ) Those nano sized metal particles are respirable and able to travel deep within the lung ( Oberdrster et al. 2007 ) Compared to their l arger size counterparts, nano sized particles are more likely to penetrate tissues and cannot be easily removed from human body ( Biswas and Wu 2005 ) The presence of toxic metal particles in the ambient and occupational environment has led to a rapid development of various control techniques. Howeve r, traditional emission control technologies exhibit less effectiveness for nano sized metal particles ( Kim et al. 2007 ) The tightening of emission standards and occupational exposure limits requires development of next generation control technologies for nano sized metal particles. The toxicity of metal particles is mainly decided by their surface composition that is i n contact with tissues rather than their bulk composition. A novel amorphous silica encapsulation (ASE) technology was recently developed to control nano sized metal particles and to reduce their toxicity. Silica coating was achieved through the condensati on of in situ generated amorphous silica onto metal particles ( McMillin et al.
26 1996 ; Biswas et al. 1997 ; Biswas and Zachariah 1997 ) The ASE technology had been demonstrated in several studies of controlling metal particles in flue gas of combustion system ( Biswas et al. 1995 ; McMillin et al. 1996 ; Owens and Biswas 1996 ; Biswas et al. 1997 ; Biswas and Zachariah 1997 ; Biswas and Wu 1998 ; 2005 ) Amorphous silica has a low s olubility in water ( Iler 1979 ) and is relatively non toxic compared to metals such as chromium (Cr), manganese (Mn) and nickel (Ni). Hence, amorphous silica coating outside the metal cores has the potential advantage of changing the biotoxicity of metal particles ( Yu et al. 2011 ) Recent studies applied this technique to gas t ungsten arc welding (GTAW) ( Topham et al. 2010 ) and gas metal arc welding (GMAW) ( Topham et al. 2011 ) from which the formed w elding fume mainly consisted of nano sized metal particles. The transmission electron microscopy (TEM) image of fume particles formed with the addition of silica precursor showed some silica shell/metal core particles. However, the silica coating was not p resent on all the metal particles due to the heterogeneous reaction existing in the welding fume formation. Moreover, different levels of silica coating were found under different shielding gas flow rates. Silica particles alone and uncoated metal particle s were also observed in the low shielding gas flow condition. Visual examination by TEM only focused on a small fraction of the fume particles and cannot be used to quantitatively determine the silica coating efficiency in bulk scale. Since silica coating efficiency (SCE) is critical in the silica encapsulation technique, it is necessary to quantify the coating efficiency of silica on metal particles, i.e. the fraction of metals encapsulated inside silica coating that is not bioavailable. Besides metal emis sion control, it should be noted that silica encapsulation has wide
27 applications in nanotechnology, e.g. to modify the surface function of metal particles ( McMill in et al. 1996 ; Biswas et al. 1997 ; Hall et al. 1999 ; Fu et al. 2006 ; Guo et al. 2010 ; Niitsoo and Couzis 2011 ) Hence, determination of silica coating efficiencies is important to evaluate the functionality of silica coating in those cases. Among the analytical systems which can determine the elemental composition of particles, Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP AES) is a recognized, versatile analytical instrument for its low detection limit, large dynamic range, and capability of si multaneous analysis of multiple elementals ( Wu et al. 1996 ) ICP AES operates on the principle of atomi c emission by atoms ionized in argon plasma flame. Light of specific wavelengths is emitted as electrons return to the ground state of the ionized elements and makes it possible to quantitatively identify the species present in the solution ( Atanassova et al. 1998 ) Comparing to Graphite Furnace Atomic Absorption Spectrometry (GF AAS) and Proton Induced X ray Emission (PIXE), ICP AES has the equivalent detection limit but less matrix effect ( Rubio et al. 1984 ; Menzel et al. 2002 ) It costs much less than Inductively Coupled Plasma Mass Spectroscopy (ICP MS ) ( Webb et al. 2005 ) All the above make ICP AES an inexpensive and relatively accurate way to determine the total metals ( Boevski et al. 2 000 ) However, metal particle samples need to be treated in order to transfer metal analyte from solid phase into liquid solutions. The common practice is to mix particle phase metals with strong acids and dissolve metals in a temperature controlled envi ronment. The digests are diluted to an acceptable total dissolved solids (TDS) concentration for ICP AES analysis.
28 Several digestion methods have been reported effective to dissolve metal particles ( Lamble and Hill 1998 ; Sandroni et al. 2003 ; Okorie et al. 2010 ) Different digestion methods have varied metal rec overy efficiencies. Nitric digestion alone is generally effective for most metals. For certain metals like copper (Cu), recovery efficiency can be greatly promoted by adding hydrofluoric acid (HF) ( Ikvalko et al. 1999 ) HF is also commonly employed in digestion of solid samples containing both or ganic and inorganic silicon ( Xu et al. 2005 ) The main concerns for the use of HF are corrosion to silica parts in the instrument and hazard s of exposure to the operators. This risk can be minimized by adding a stoichiometric amount of boric acid (H 3 BO 3 ) into the complex aft er digestion. Digestion with aqua regia is also a common method for the determination of trace elements in soils and sediments ( Myhnen et al. 2002 ) Aqua regia is a mixture of nitric acid (HNO 3 ) and hydrochloric acid (HCl) in a volume ratio of 1:3. There is almost no common metal that can survive the acid attack of aqua regia ( Dulski 1996 ) Other acids such as sulfuric acid and phosphoric acid were also mentioned in some studies ( Wu et al. 1996 ; Xu et al. 2005 ) Heating block, ultra sound assisted and microwave digestions were used to provide a high energetic environm ent to assist strong acid attack ( Kuss 1992 ; Filgueiras et al. 2000 ) Among those methods, microwave digestion is highly recommended in det ermination of trace metals with the advantages of fast decomposition rate, less volatile loss, less contamination, precise temperature and pressure control. It requires less acid and therefore can lower the method detection limit ( Lamble and Hill 1998 ) The objective of th is chapter was to develop a method for determining silica coati ng efficiency on metal particles generated from the ASE technology. Silica coated
29 metal particles were generated using gas metal arc welding (GMAW) with a silica precursor. The fume particles from the process mainly contained Fe, Cr, Ni, Mn, and Cu. Metal recovery efficiencies of three digestion acid mixtures were compared. Silica coating efficiencies were calculated by the apparent concentration differences between digestion methods which can effectively dissolve silica coated metal particles and which can only dissolve metals. Experimental Methods Sample Preparation A GMAW welding machine (Lincoln 140C) was used for generating welding fume particles using ER 308L stainle ss steel welding wires. Figure 2 1 shows the schema of the fume production and sampling system. The GMAW welding fume particles primarily come from the welding wire ( Castner and Null 1998 ) The 308L weldi ng wire nominally contains 20% Cr, 10% Ni, 1.7% Mn, 0.03% Cu, and balanced with Fe ( Vitek and David 1987 ) Silica coated particles were generated by adding tetram ethylsilane (TMS) as the silica precursor. Argon and carbon dioxide mixture (3:1, v/v), which is commonly used for stainless steel GMAW, was employed as the shielding gas in this study. A fraction of the shielding gas was used as the carrier gas for the TM S vapor. Injected TMS decomposed at the high temperature welding arc zone to form an amorphous silica coating on welding fume particles. The particles thus generated were then collected onto glass fiber filter (Whatman 90 mm), weighed and digested. Previou s study showed the higher shielding gas flow rate, the more silica coating observed ( Topham et al. 2011 ) Hence, three wel ding conditions were selected to represent the low, medium, and high shielding gas flow conditions studied previously. The operating parameters of
30 different welding conditions are listed in Table 2 1 Sampling under each condition was carried out at least four times. Figure 2 1 Schematic diagram for the sampling system Table 2 1 Welding parameters and silica coating efficiencies under different welding conditions Welding condition Primary shielding gas flow rate (Lpm) TMS carrier gas flow rate (Lpm) Silica coating efficiency SD (n) 1 30 2.8 38.67% 6.76% (5) 2 25 2.1 25.76% 4.21% (4) 3 20 2.1 13.82% 3.79% (5) Instrumentation and Chemical Reagents Analysis of total metals was carried out by an ICP AES system (Perkin Elmer Plasma 3200). The system is equipped with a plasma torch with an alumina injector which enables measurement of the samples with HF. Two monochromators covering the spectral range of 165 785 nm with a grated ruling of 3600 lines/mm are included in the system. The system also contains a crossflow nebulizer, a spray chamber, and a Gilson four channel peristaltic pump. The system is capable of analyzing metals in
31 digest solution with a detection limit range of less than part per million (ppm). The operating conditions for the IC P AES are listed in Table 2 2. Table 2 2. Operation conditions of Plasma 3200 ICP AES Incident Power (W) 1300 Plasma gas flow rate (Lpm) 13 Auxiliary gas flow rate (Lpm) 0.5 Nebulizer gas flow rate (Lpm) 0.8 Peristaltic pump flow rate ( mL min 1 ) 1 R eading delay time (s) 40 Reading per sample 5 replicate s The metals analyzed by ICP AES included Fe, Cu, Cr, Ni, and Mn. Two atomic emission spectral lines for each element were used simultaneously to reduce spectral interference introduced by other co existing elements in the matrix. The wavelengths of their spectral lines are listed in Table 2 3. The concentration of each metal in a sample was determined by averaging the results of five reading replicates. A single calibration curve was constructed for each metal with six aqueous standards. The concentration ranges of calibration solutions were 10 100 mg L 1 for Fe and 1 60 mg L 1 for Cr, Ni, Mn and Cu. All the calibration plots were linear in the investigated concentration ranges with the correlation c oefficients greater than 0.999 Blank samples were used to determine the method detection limits of different digestion methods. Standard samples were randomly inserted into sample queue to test the bias after a relatively long period of running ICP AES. The mass of collected samples were measured by an analytical scale (Sartorius MC210S) with a readability of 10 g. Each batch of samples was weighed three times and the mean value was calculated. Transmission electron microscopy (TEM, JEOL 2010F) was used to acquire visual evidence of silica coating on welding fume particles.
32 The fume particles were loaded onto a specialty TEM grids (Pelco Lacey Carbon Type A 300 mesh) inserted into the welding chamber. Table 2 3. Wavelength of spectral lines (nm) and met hod detection limit (mg L 1 ) for each metal in this study Metal Wavelengths ( interference metals ( McLaren and Berman 1985 ) ) Method detection limit by d igestion acid mixture HNO 3 alone HNO 3 /HF mixtu re Aqua regia Fe 238.204, 259.940, 0.49 0.32 0.36 Cr 205.552 (Fe, Mo), 267.716 (Mn, V) 0.28 0.33 0.29 Ni 231.604, 221.647 1.66 0.57 0.48 Mn 257.610 (Fe, Cr), 293.306 (Fe, Al) 0.66 0.47 0.85 Cu 324.754, 327.396 0.78 0.36 0.54 All chemicals used were analytical grade or higher purity. Calibration curve was obtained from external standard. Standard solutions were prepared by diluting high purity stock solutions with deionized (DI) water: 10 g L 1 Fe and 100 mg L 1 Cu (Fisher Scientific), 1000 mg L 1 Ni and 1000 mg L 1 Mn (Spex Certiprep). Cr standard was in the form of 1000 mg L 1 chromate (CrO 4 2 ) (Acros Organics) and thus conversion was necessary to get the Cr concentration. 2 1 All acid solutions (Fisher Scientific), HNO 3 HF, and HCl were at their original concentration and not diluted. Boric acid (H 3 BO 3 ) (Acros Organics) was obtained in solid phase. Water used for dilution and cleaning was deionized by a water purification cm. Labwares were cleaned before and after analysis to prevent exposure to potential residual contamination during the whole period of experiments. Polytetr afluoroethylene (PTFE) tubes with screw caps (VWR) were used as the
33 digestion vessel to eliminate the risk of HF to glassware. Screw caps can prevent the vaporization of analyte and environmental contamination. All glassware and PTFE tubes were rinsed by D I water, immersed in nitric acid bath (3M HNO 3 ) for at least 72 h, cleaned in an ultrasonic cleaner (FS220) for 4 h, and dried in an oven (230G Isotemp) in a laminar flow hood. To further minimize the memory effect of any acid employed in this study, each PTFE tube was labeled and fixed to a specific digestion method, e.g., the tube used for HNO 3 alone and aqua regia would not contact with HF under any circumstances. Digestion Process Digestion was assisted by using a microwave digesting system (CEM MDS 81D ). An 8 step microwave heating procedure was designed according to the review of experiments done on metal digestion ( Lamble and Hill 1998 ) and is listed in Table 2 4. The sample digests were cooled and filtered by ashless filter (Whatman 32 mm) to remove the solid residues. The filtered digests were diluted to 50 mL with 2% HNO 3 adde d. The diluted solutions were analyzed by ICP AES immediately after dilution. Table 2 4 Operating programs of CEM MDS 81D microwave digesting system Increment Power (W) Duration (s) 1 250 120 2 400 120 3 500 600 4 250 480 5 400 240 6 600 360 7 0 120 8 300 180 The basic principle of the digestion process is to utilize HNO 3 HCl, and HF acid to solubilize the metal particles. HNO 3 alone method and aqua regia method were
34 adopted to determine the most aggressive acid mixture without breaking the si lica coating on metal particles. The particle loaded filters were cut to quarters after weighing. Each quarter was digested with different acid mixtures. The homogenous mass distribution of metal particles on the filter was examined to avoid interference f rom mass difference among filter quarters. HNO 3 alone : Filter samples were placed in the PTFE vessels with 10 mL of HNO 3 added to each vessel. The screw caps on the vessels were tightened to prevent loss during digestion. HNO 3 /HF mixture : HF involved dig estion method was based on EPA method 3052 ( E.P.A. 1996 ) to co mpletely dissolve the metals and silica coating. Filter samples were placed in the PTFE vessels with 9 mL of HNO 3 and 1 mL of HF added to each vessel. After the microwave digestion, a stoichiometric amount of H 3 BO 3 was added to eliminate the free fluoride ion in order to prevent damage to the ICP AES sample loops. The amount of H 3 BO 3 was calculated based on the following chemical reaction. 2 2 Aqua Regia : Aqua regia (HNO 3 :HCl, 1:3, v/v) digestion method was based on ISO sta ndard 11466 ( ISO 1995 ) Filter samples were placed in the PTFE vessels with 7.5 mL of HCl and 2.5 mL of HNO 3 added to each vessel. All the acid mixtures we re placed at room temperature for 10 h before sent to the microwave digesting system. Method Detection Limit and Recovery Efficiency Blank filters were digested and used as lab blanks to determine the background level of metals. Method detection limit is d efined as 3 times standard deviations of metal
35 concentration in lab blank ( IUPAC 1997 ) Relative standard deviation (RSD) was calcul ated among metals and digestion methods. digestion method. Pure metal powders (Acros Organics) were weighed and spiked onto blank glass fiber filters. Measured and known mass of spike samples were compared to calcula te the metal recovery effici ency by the following equation. 2 3 Metal ratios in spiked samples were kept nearly constant and similar to the metal particles generated from the welding process. The spiked samples were dige sted and analyzed using the same methods as the generated particles. t test was used in comparison of metal recovery efficiency for different digestion methods. Results and Discussion Method Detection Limit Method detection limits determined from lab orator y blanks are shown in Table 2 3. Detection limits of five metals in this study were generally around 1 mg L 1 level. Based on the results, the choice among different digestion methods had little influence on the method detection limit since the metal conce ntration in this study is much higher than 1 mg L 1 The HNO 3 alone digestion method shows a slightly higher detection limit for Ni and Cu than the other two methods. However, the method detection limits hereby were limited by the instrument (ICP AES) and trace metal impurity levels of the glass fiber filter. The detection limit could be significantly decreased if analyzed by ICP MS though it costs more and the procedures are more complicated. The silica shell/metal core structure particles are always synth esized in bulk volume and likely tests will not be
36 carried out for a minute amount of particles. Hence, high resolution (lower than 1 ng L 1 ) is not necessary in most cases. Glass fiber filter was used in this study because it has good resistance to high t emperature generated by welding and it can handle the high flow rate generated by the high volume sampling pump. Since the trace metal level in glass fibers was much less than the collected metal mass, its impact on method detection limit is not critical t o the concentration measurement. In applications where high temperature and high flow rate are not encountered, alternative filters such as cellulose and membrane filters could be used to reduce the trace metal impurities ( Dams et al. 1972 ; Scott et al. 1976 ; Sievering et al. 1978 ) Filters are not necessary if the particles can be transferred to digestion vessel without being collected on filters. In the experiment, metal matrix effect and HF memory effect could also interfere with the resolution and affect the detection limit ( Tan and Horlick 1987 ; Todoli et al. 2002 ; Xu et al. 2005 ) Carefully acid washing the vessels af ter the experiment could also lower the detection limit by removing possible residual contaminations. Metal Recovery Efficiency Figure 2 2 shows the recovery efficiencies for different digestion methods and different metals. For all the elements and method s, the recovery efficiencies ranged from 90 110%. Recovery efficiencies of HNO 3 /HF mixture for all metals except Cu are close to 100% and statistically they are significantly different from the other two methods ( p <0.05). Cu has the highest average recover y efficiency of 103.7%. Metal recovery efficiencies over 100% can be attributed to the filter impurity level, e.g., glass fiber filter generally has 3 ng/cm 2 of Cu ( Scott et al. 1976 ) The selection of wavelengths of Cr and Mn cannot avoid the spectral inferences from Fe as shown in Table 3. Overestimation of Cr and Mn could happen due to spectral effect induced by high
37 conce ntration of Fe in the welding fume particles. In addition, non spectral matrix effect of ICP AES could either enhance or depress the signals for those metals ( Todoli et al. 2002 ) Figure 2 2. Recovery efficiencies for Fe, Cr, Ni, Mn, Cu by different digestio n acid The RSD for each metal in different methods was less than 3.5%. Except for Cr, HNO 3 /HF mixture showed the highest mean recovery efficiencies compared to the other two acid mixtures ( p <0.05). Aqua regia was higher than HNO 3 alone for Mn ( p <0.05), bu t no significant difference was found on other metals. HNO 3 alone showed a low recovery efficiency of 88.3% for Mn and a relatively high deviation. This represents the relatively low acid attack ability by HNO 3 alone. Hence, aqua regia was adopted instead
38 of HNO 3 acid alone in the following silica coating efficiency test to have a comparison with HNO 3 /HF mixture. However, mixture of HCl and HNO 3 requires a well sealed vessel such as PTFE tube due to the formation of highly evaporative gas phase NOCl and Cl 2 HNO 3 alone could be an alternative if the sealant cannot be perfectly achieved. In this study, the recovery efficiency test was done using elemental metals, while in real applications such as flame synthesis or welding fume, metal oxides could dominate t he particle compositions. Differences in their solubilities should be checked. However, the differences can be neglected in high energy environment provided by the microwave digestion system ( Nadkarni 1984 ) Silica Coating Efficiency Figure 2 3 shows the TEM imagery of fume part icles collected at welding condition 1. Metal particles are darker while silica is in light color, due to the penetration ability of electron when interacting with the particles. Figure 2 3. TEM imaginary of welding fume particles with silica coating
39 In the TEM image, some particles (e.g., Particle 1) have a high electron density (darker) region surrounded by a low electron density region (lighter). This is likely the result of metal encapsulated by silica coating layer. Nonetheless, due to the two dim ensional nature of TEM image, metal in particle 1 could be non partially or fully encapsulated or even enriched on silica surface ( Maynard et al. 2004 ) in a three dimensional space. To truly determine the level of encapsulation, the silica coating efficiency would be critically important. HNO 3 /HF mixture breaks the silica coating on the metal particles, and according to the metal recovery efficiencies results it had the most aggressive solubilization ability. Aqua regia showed the similar digestion ability on metal only particles but did not have any effect on particles with silica coating. The mass difference between the results obtained from these two digestion methods was therefore us ed to calculate the silica coating efficiency using the following equation. 2 4 where N: Number of metals involved C NF,i : Measured concentration of the i th metal digested by HNO 3 /HF mixture C AR,i : Measured concentration o f the i th metal digested by aqua regia which could not be dissolved by aqua regia. It should be noted that the silica coating efficiency for individual metal can be calc ulated through this equa tion without summation. However, due to the heterogeneous interaction between silica and metal in this study, the coating process is not targeting at any specific metal. Hence, the silica coating efficiency for individual metal was not calculated. C AR,i can be replaced with metal concentration measured by other acid digestion methods when appropriate, e.g.,
40 HNO 3 acid alone without Mn in the system. For specific applications, some rare metals such as Ru, Ta, Os, and Rh can withstand common acid attack such as aqua regia ( Craig and Anderso n 1995 ) Under such a circumstance, the equation is not capable of handling those rare metals. Silica coating efficiencies were acquired from three sets of samples generated under different welding conditions. Mean value and standard deviation of metal c oncentration were obtained for all the samples and are shown in Table 2 5. This shows a relatively consistent welding fume particle composition during the experiment. Cu was not detected at most conditions because of the relatively small proportion of Cu i n welding wire. The presence of Cu in some of the samples was likely the ablation of welding nozzle tip and shell, rather than the wire. In the samples where Cu was detected, concentration of Cu was close to the method detection limit. Except Cu, other met als showed a good agreement among replicas (RSD <14%). Table 2 5. Measured concentrations (mg L 1 ) of metal elements digested by different acid mixtures Metals Welding Condition 1 (n=5) Welding Condition 2 (n=4) Welding Condition 3 (n=5) C NF, Fe 56.34 (1.13) 78.79 (8.67) 63.51 (1.91) C AR, Fe 33.28 (3.00) 51.79 (2.07) 59.38 (3.56) C NF, Cr 31.02 (1.55) 45.45 (1.36) 37.19 (2.98) C AR, Ct 28.13 (2.79) 31.33 (2.19) 28.56 (1.71) C NF, Ni 12.82 (0.38) 20.15 (0.81) 13.41 (0.54) C AR, Ni n.d. ** 17.43 (1.57) 1 4.82 (1.78) C NF, Mn 19.73 (0.99) 20.92 (0.84) 17.65 (1.24) C AR, Mn 14.05 (1.83) 22.17 (3.10) 12.33 (1.11) C NF, Cu 3.12 (1.03) n.d. 1.78 (0.33) C AR, Cu n.d. n.d. n.d. Number in the parentheses indicated the standard deviation among sample replicates ** n.d. Not detected in the samples or lower than method detection limit.
41 Results of Mn in welding condition 2 and Ni in welding condition 3 suggest more metals have been digested by aqua regia than HNO 3 /HF mixture. These two anomaly values could be attr ibuted by uneven distribution of particles on different quarters of one glass fiber filter. Based on the metal concentrations acquired and the equation, silica coating efficiencies were calculated and are listed in Table 2 1. As shown, the silica coating e fficiencies were generally low (<40%), due to the heterogeneous nature of gaseous reaction between silica precursor and metals. The highest silica coating efficiency occurred under welding condition 1 which had a higher shielding gas flow rate. The higher shielding gas dispersed more heat generated from the welding arc. Examination of the inner shell of welding nozzle showed a large amount of white silica powder deposited under the low shielding gas flow condition. This confirmed that TMS in the low shieldi ng gas flow condition already decomposed before reaching the effective coating position, because heat was quickly transferred to the welding nozzle area resulting information of silica earlier than desired. As shown, the knowledge of silica coating efficie ncy is very important in understanding the effect of processing conditions (welding parameters) on the process performance (silica coating distribution). By understanding the defect of welding nozzle under low shielding gas flow condition, a new welding to rch should be developed to allow insulation from the heat transfer. The equation based on multiple digestion methods can be used in examining silica coverage of various silica shell/metal core structure particles applications ( Ohmori and Matijevic 1992 ; Liz Marzn et al. 1996 ; Correa Duarte et al. 1998 ; Lu et al. 2002 ; Graf et al. 2003 ; Yi et al. 2005 ; Teleki e t al. 2009 ) The silica coating efficiencies in
42 those cases could be an indicator for the effectiveness of the silica shell, e.g., change of functionality, biocompatibility, or colloidal stability by silica shell. Summary ICP AES and microwave digestion were capable for simultaneous analysis of multiple metals down to ~1 mg L 1 in this study. The accuracy of results was confirmed by metal recovery efficiencies by different digestion methods. HNO 3 /HF acid mixture and aqua regia digestion method were both very effective for treatment of ICP AES analyte. Major metals (Fe, Mn) and minor metals (Cr, Ni, Cu) in welding particles showed good recovery under those two methods. The calculation of silica coating efficiencies based on the measured mass difference bet ween two digestion methods quantified the metals sealed inside the silica shell. Five metals were tested in this study based on the composition of stainless steel wires used in metal particles generation. More metal speciation should be examined in the fut analytical method to applications of silica shell/metal core particles, where high silica coating efficiencies were expected from synthesis based on homogenous reaction but n o quantitative confirmation yet. Metals encapsulated by amorphous silica cannot be further extracted by general acid such as HNO 3 or aqua regia. Hence, silica encapsulation is a promising technique to reduce the biotoxicity of metal compounds in nanoparti cles. The experimental result showed silica coating efficiency increased with increasing shielding gas flow rate. The low silica coating efficiency under low shielding gas flow rate was due to the premature decomposition of silica precursors. Modification of welding gun structure to overcome
43 this shortage likely will improve the silica coating efficiencies under a wide range of flow rates.
44 CHAPTER 3 DOUBLE SHROUD DELIVERY OF SILICA PRECURSOR TO REDUCE HEXAVALENT CHROMIUM IN WELDING FUME Objective The amor phous silica encapsulation (ASE) technology has been shown to be an effective measure for controlling metal nano particles emissions from pyroprocesses such as combustors and incinerators ( Biswas et al. 1995 ; Owens and Biswas 1996 ; Biswas and Zachariah 1997 ; Biswas and Wu 1998 ) The use of shielding gas in welding process provides a medium for the introduction of a vapor phase silica precursor Silica precursor injected through the shielding gas decomposes in the high temperature of the welding arc zone and reacts wit h reactive oxygen species, hence providing a reducing environment (as shown in Figure 3 1) The in situ generated silica condenses on the metal particles, and further insulate s Cr f rom oxygen species by forming an amorphous silica layer An E. c oli growth e xperiment demonstrated that in situ generated amorphous silica has much less biotoxicity than the particles of Cr and other metals ( Yu et al. 2011 ) The byproducts from the decomposition of silica precursor are carbon dioxide and water. The ASE technology ap plied to welding was described in previous studies ( Topham et al. 2010 ; Topham et al. 2011 ; Wang et al. 2011 ; Yu et al. 2011 ) A v apor phase silica precursor tetramethylsilane (TMS) added to the shielding gas reduc ed Cr 6+ concentration over 90% under the high prima ry shielding gas flow rate of 30 liters per minute (L pm). The inhibition of Cr oxidation was not considerable effective under medium (25 Lpm) and low (20 Lpm) shielding gas flow rates This ineffectiveness constrain s the application to very specific operat ing condition s H igh shielding gas flow
45 c an significantly increase wel ding operation costs, which would impede the use of the ASE tec hnology in industrial practice. Figure 3 1. Mechanistic illustration of silica precursor for reducing the formation of Cr 6+ Medium and low shielding gas flows could not effectively disperse the heat generated from welding process TMS premix upstream with the shielding gas, being sensitive to high temperatures, decomposed before reaching the welding arc zone. Furtherm ore, a large quantity of silica powder was found deposited inside the welding gun under those conditions. A new welding gun design was needed to overcome these problems Another question to be addressed was that the analytical method (NIOSH 7604) ( NIOSH 1994 ) used in the previous study could not verify the oxidation state of Cr sealed inside the amorphous silica shell formed during the process.
46 The ob jecti ve of this chapter was to develop an alternative way to deliver silica precursor effectively into the welding arc zone. A prototype insulated double shroud torch ( IDST ) was designed to eliminate the premature heat transfer to the TMS. Laboratory experiment s were conducted to assess its ability to reduce the Cr 6+ concentration to below the limit of detection in all shielding gas flow rates. A preliminary field test was carried out in an industrial site to evaluate the ease of implementing this technology. Th e hazard and cost associated with the technology were also assessed. Success of the reduction of Cr 6+ generated by stainless steel welding would reduce the carcinogenic risk associated with this industrial process. Experimental Methods Injection of Silica Precursor To avoid the excess thermal energy transferred to the silica precursor, the silica precursor needs to flow separately from the primary shielding gas which is a carrier of heat. Dennis et al. ( Dennis et al. 2002 ) first designed a double shroud torch using two nozzles to introduce the primary shielding gas and secondary shielding gas. However, their t orch was designed only for conveniently mixing and switching different gas components. It did not address heat insulation and worked no differently with the silica precursor from the premixing it. In this study, a new double shroud torch was designed that incorporated a ceramic material to insulate against heat transfer between the primary and secondary shielding gases. Figure 3 2 shows the cross sectional sketch of the new IDST. The IDST wa s hypothesized to be able to minimize the premature decomposition o f si lica precursor by deliver ing the silica precursor directly to an effective position.
47 Figure 3 2. Cross sectional sketch of the insulated double shroud torch ( IDST ) Figure 3 3. Schematic diagram. A ) the a pparatus for feeding TMS B ) the weldin g fume chamber with a zoom view at the end of the torch C ) the a p paratus fo r feeding NO or CO D ) the fixed location sampling system in the field with a picture of the welder performing welding Traditional welding equipment was modified to introduce the silica precursor TMS into welding shielding gas (Figure 3 3 A ) TMS carrier gas a rgon (Ar) flowed through a
48 Teflon impinger (Apex Instruments T507G) with TMS liquid covering the bottom The impinger wa s immersed in an ice bath to lower the TMS vapor pressur e which prevented excess TMS vapor from getting in the system. The carrier gas bec a me saturated with TMS vapor which was delivered to the welding arc zone using the outer shroud of the IDST Welding Fume Sampling Laboratory sampling of welding fumes was p erformed in an enclosed stainless steel welding chamber shown in Figure 3 3 B The design of the chamber followed the American Welding Society (AWS) F1.2:2006 design. ( AWS 2006 ) The conical chamber measured 36 in ches in diameter at the b ottom 8 inches in diameter at the top and 36 inches in height. A high volume pump ( General Metal Works GL 2000 H) with a sampling rate of 50 Lpm was put on top of the chamber. Welding fume particles generated in the chamber were collected on a glass fiber filter (Whatman 90 mm) mounted on the pump A welding machine ( Lincoln Power MIG 140C ) was used in the l aboratory study, with a constant voltage of 19.5 V and a wire speed of 100 inches per minute (Ipm) was maintained throughout the study. The study used 0.035 inch diameter 308L stainless steel wire, which has about 20% (wt) Cr. The IDST was kept in a fixed position by a metal stand, and the round shape mild steel base metal was positioned on a constantly rotating turn table (MK Products Aircrafter T 25). The laboratory study simulated beading on the base metal at 1.5 min per sample. The chamber study us ed 75 % Ar/25% CO 2 as the primary shielding gas the primary shielding gas were selected based on previous studies to represent the low,
49 medium, and high shielding gas flow c onditions. Different injection rates of TMS carrier gas were tested, the testing conditions are listed in Table 3 1 The injection rate of TMS carrier gas per each flow rate of the primary shielding gas started at 0.16 Lpm and gradually increased to a perc entage of the primary shielding gas that could eliminate the airborne Cr 6+ concentration to the limit of detection. The flow rates of the primary shielding gas and the TMS carrier gas were respectively controlled by a rotameter ( Radnor HRF 1425 580 ) and a mass flow controller (Omega FMA5500) In addition to the silica precursor, nitric oxide (NO) and carbon monoxide (CO) were also tested during the study as alternative reducing reagents for comparison purposes. A commercial available NO/Ar shielding gas, MI SON, consists of 300 ppm (0.03%) NO in Ar. Theoretically, CO can absorb the ultraviolet (UV) light and react with oxygen species to provide a similar reduction. Different percentages of NO or CO in the final shielding gas (Ar) were tested. 1% (v/v) NO or 1 % (v/v) CO in Ar were injected as the secondary shielding gas through the IDST (Figure 3c), while pure Ar was used as the primary shielding gas. The testing conditions are listed in Table 3 2. A carbon monoxide monitor (Nighthawks KN COPP 3) was set near t he chamber to determine the CO concentration in the air. The field sampling was performed in an industrial facility that regularly conducts welding activities. A fixed location area sampling setup was used in the field as shown in Figure 3d. A certified we lder performed the welding task in front of the high volume pump with the glass fiber filter installed. The sampling period per each sample was extended to 5 minutes arc time to collect enough welding fume for analysis.
50 Table 3 1. Testing conditions of T MS and shielding gas flows in the chamber sampling Primary Shielding gas flow rate (Lpm) TMS Carrier gas flow rate (Lpm) Percentage of TMS Carrier gas to primary shielding gas flow rate (%) Estimated TMS concentration in the shielding gas ( v/v ) 20 0.16 0. 8 2.810 3 0.32 1.6 5 6 10 3 0.64 3.2 1. 1 10 2 25 0.16 0.6 4 2 3 10 3 0.32 1. 3 4 5 10 3 0.64 2. 6 8 9 10 3 1. 3 5.1 1 .810 2 30 0.16 0.53 1 9 10 3 0.32 1. 1 3 8 10 3 0.64 2.1 7 4 10 3 1. 3 4. 3 1. 510 2 Table 3 2. Testing conditions of NO/Ar and CO/Ar in the chamber sampling Primary shielding gas Injection rate (Lpm) Secondary shielding gas Injection rate (Lpm) Percentage of NO/Ar or CO/Ar Ar 30 1% (v/v) NO in Ar 0.3 0.01 0.9 0.03 3 0.1 1% (v/v) CO in Ar 0.3 0.01 0.9 0.03 3 0.1 A welding machine ( Miller Invision 456MP ) and a wire feeder ( Miller 70 series 24 V wire feeder ) were employed in the field sampling. The voltage was controlled by a pulse program with an average of 26 V. 310 stainless steel wire with a diameter of 0.045 inch and 310 stainless steel base metal with a thickness of 3/16 inch were used. The 310 stainless steel typically contains about 25% (wt) Cr. The wire speed was fixed at 192 Ipm. The primary shielding gas used was a mixture of 69% Ar, 31% He, and 1% CO 2 The flow rates of the primary shielding gas and the TMS carrier gas flow were kept constant at 15 Lpm and 0.24 Lpm, respectively
51 Analysis of Cr 6+ An Ion Chromatography (IC) system ( Dionex ICS 1500) was used to analyze Cr 6+ The IC was equipped with a cation analytical column (Dionex CS5A) for separating different ions, and a conductivity detector (Dionex DS6 ) to measure the Cr 6+ concentration. Sample extraction followe d NIOSH Method 7604 ( NIOSH 1994 ) P er CS5A column 5 mM sodium bicarbonate and 1 mM sodium carbonate were chosen as the eluent The Cr 6+ content on the glass fiber filter was extracted using 20 mL e luent heated in a water bath to 100 C for one hour. The extracted solution was filtered to remove the solid residue in order to protect the ICS 1500 injection loop. Standard solutions were prepared by diluting 1000 mg /L chromate solution (Acros Organics) with deionized (DI) water to a range of 1 60 mg /L of Cr 6+ T he calibration plot w as linear in the investigated concentration range with the correlation coefficient larger than 0.9999 The estimated Cr 6+ concentration (g/m 3 ) in the sampled air ( ) was calculated using the following equation. 3 1 where [Cr 6+ ] is the concentration (g/mL) of Cr 6+ in the 20 mL eluent measured by ICS 1500; Q is the flow r ate of the sampling pump, 0.05 m 3 /min; t is the sampling time, 1.5 min for the laboratory sampling and 5 min for the field sampling. If [Cr 6+ ] was less than the ICS 6+ concentration was conservativ ely assigned the corresponding values of the limit of detection (2.8 g/m 3 in the lab, 1.1 g/m 3 in the field).
52 In previous studies ( Topham et al. 2010 ; Topham et al. 2011 ; Wang et al. 2011 ; Yu et al. 2011 ) some fume particles were encapsulated in the amorphous sili ca layer generated by the condensation of silica vapor. However, the eluent used in the analytical protocol (5 mM NaHCO 3 and 1 mM Na 2 CO 3 ) was not able to dissolve the amorphous silica shell. The use of a strong acid such as hydrofluoric acid (HF) could dis solve the silica shell; however, it could also change the valence state of the original Cr 6+ as well as damage the instrument. In other words, ion chromatography cannot determine the Cr 6+ encapsulated in the silica shell. X ray photoelectron spectroscopy ( XPS) is a non destructive analytical process that could examine the valance state of Cr in the range of the penetration depth of an X ray. A thin layer of fume particles was scratched down from the glass fiber filter to a silicon substrate. Only samples fr om one baseline and one optimal injection rate were analyzed. The XPS ( Perkin Elmer 5100 ) gives the intensity as an arbitrary unit (a.u.) for each peak; therefore, the relative ratio of Cr 6+ /Cr 3+ was calculated. Chemical Reagents and Labware All chemicals used were analytical grade or higher in purity. DI water used for dilution and cleaning was deionized and purified by a Nanopure system (Barnstead cm. All the shielding gases used were ultra high purity and certified by the manufacturers (Airgas, Praxair, and Air Liquide). All the glassware used in the study was cleaned in an ultrasonic cleaner (FS220) for 4 hrs, and dried in an oven (FS230G Isotemp) in a laminar flow hood. The tubings in the sampling system were Polyt etrafluoroethylene (PTFE) or Tygon, and air leaking tests were performed regularly.
53 Hazard and Cost Estimation TMS is a flammable and volatile liquid ( Aston et al. 1941 ) High concentration s of TMS vapor may cause flash fire or explosion s in oxidizing environment s T he toxicological properties of TMS to humans have not been fully inves tigated. However, high temperatures in the welding arc zone are expected to decompose the trace amount of TMS. In this study, the TMS concentration was estimated for a room based on test conditions in the field sampling and a worst case scenario, i.e., all TMS vapor escaped without decomposition and ventilation. The v apor pressure of TMS was cal culated using the Antoine equation, ( Aston et al. 1941 ) 3 2 where A, B, C are 3.97703, 1047.242, 36.057 (unitless), respectively, and T is the t emperature of the ice bath ( 273 K ). The i d eal gas law was then used to calculate the concentration of TMS in the saturated carrier gas. The increase in operating costs when using the ASE technology can be a major barrier for industrial implementation. A comprehensive cost assessment of the silica precursor w as carried out using a model developed for estimating the welding cost. ( Lippold and Frankel 2009 ) Different scenarios of weld (pipe, butt, and fillet) production were utilized to compare the cost using the general welding technology and the AS E technology. The cost to acquire TMS is $65 per 100 mL and its consumption rate is about 0.02 mL per minute.
54 Statistics primary shielding gas flow rate and the TMS carrier gas f low rate in laboratory. The t test was used to examine the mean concentration of Cr 6+ under different circumstances. Statistical significance was evaluated using a significan ce level of p =0.05 All s tatistical analyses w ere performed using the statistical software (SAS 9.3). Results and Discussion Chamber Sampling Results Figures 3 4 A ~4 C show the airborne Cr 6+ concentrations under different combinations of the primary shielding gas and the TMS carrier gas flow rates. In addition to the current results from using the IDST and the limit of detection, the results from a previous study ( Topham et al. 2011 ) in which the TMS carri er gas and the primary shielding gas were premixed via a Y fitting at the welding gun hose, are illustrated in the figure as well. The Cr 6+ concentrations in baseline samples were 25(5.2) g/m 3 35(4.1) g/m 3 and 56(8.9) g/m 3 for the low, medium, and high primary shielding gas flow rates, respectively. It should be noted that the chamber sampling was a simulation of a continuous welding practice for 1.5 min, and it collected the full stream of welding fumes in an enclosed space. The c oncentrations meas ured in the chamber do not directly reflec t breathing zone exposure concentrations and cannot be directly compared to the OSHA 8 hr PEL of 5 g/m 3 The limit of detection of 2.8 g/m 3 was used as lower reference for the Cr 6+ reduction. The lower limit of t he reduced airborne Cr 6+
55 concentration was assumed to be equal to 2.8 g/m 3 the previously cited limit of detection in the laboratory study Figure 3 4. Cr 6+ concentration as a function of TMS carrier gas/primary shielding gas percentage A ) low B ) medium C ) high primary shielding gas fl ow condition D ) Cr 6+ concentration as a function of NO/Ar percentage and CO/Ar percentage Under the low shielding gas flow rate condition (Figure 4a), premixed TMS was unable to reduce the Cr 6+ ( p >0.1) reg ardless the increase of the percentage of TMS carrier gas to the primary shielding gas over the range studied (up to 6.3% v/v). In contrast, TMS injected by the IDST was able to reduce Cr 6+ to below the limit of detection (2.8 g/m 3 ) at 3.2% TMS gas/primar y shielding gas. The percentage was also
56 less than the needed percentage for the IDST to reduce Cr 6+ below the limit of detection under the medium and high shielding gas flow rates. In the case of separate flowing of TMS carrier gas and the primary shieldi ng gas, the low flow will create a relatively longer reaction time for the TMS in the welding arc zone, which reduces the ineffective reaction outside the microenvironment. The medium and high shielding gas flow rates (Figures 3 4 B & 3 4 C ) show similar red uction trends: As more TMS carrier gas was injected, the Cr 6+ concentration gradually decreased to the limit of detection of 2.8 g/m 3 ( p <0.05 for each conditions). It should be noted that the efficiency of the IDST was always higher than that of premix m ode at the same percentage of TMS carrier gas/primary shielding gas. This supports the hypothesis that the IDST could deliver more TMS without premature decomposition than premix mode. Other benefits are that the costs associated with TMS consumption were also reduced and the nozzle showed less silica powder after the IDST experiment. This is direct evidence that the premature reaction of the TMS inside the nozzle that occurred in premix mode was minimized with the IDST. In sum mary the chamber sampling res ults demonstrated that combining the IDST and the ASE technology could effectively reduce the airborne Cr 6+ emission over 90%. The positive results prove that the IDST led to a greater Cr 6+ reduction compared to the original premix mode. It should be noted that the lowest concentrations reported here all corresponded to the limit of detection (2.8 g/m 3 ) Hence, the true reduction efficiency might be beyond this conservative estimate. NO and CO Results As previously mentioned, NO and CO were intentionally a dded to the shielding gas to test their reduction of the oxygen species, thus controlling the formation of Cr 6+
57 Figure 4d shows the results under different percentages of NO and CO in Ar. The mean Cr 6+ concentration was 24 (5.8) g/m 3 when NO was added t o the Ar. Compared to the airborne Cr 6+ concentration of 56 (8.9) g/m 3 in the baseline, the difference indicated a mean reduction efficiency of 57%. The maximum reduction occurred at 0.1% of NO, which was much higher than that of the only commercially av ailable NO/Ar shielding gas MISON (0.03% v/v or 300 ppm). A downside of NO is that excess NO may influence the welding arc and lead to mechanical failure of weld ( Menzel 2003 ) Furthermore, the control of NO has to be precise due to a NIOSH REL of 25 ppm for NO ( NIOSH 2007 ) and OSHA PEL of 5 ppm for NO 2 ( OSHA 2006c ) There was no statistical difference among Cr 6+ concentrations with differe nt injection rates of CO ( p >0.1). The reduction of the UV intensity and the oxygen species due to CO could only lead to about 35% reduction of Cr 6+ The CO monitor also recorded a peak concentration of 100 ppm CO in the air while the percentage of CO in Ar was increased to over 0.1% (v/v), which could be hazardous with prolonged exposure. Even without the injection of NO, nitrogen oxides (NO x ) can appear in the welding arc zone due to the reaction of ambient oxygen and nitrogen at high temperature. CO 2 the product of reaction between CO and oxygen, can also dissociate to CO at high temperature (>1000 K). ( Lavoisier 1984 ) The thermodynamic equilibrium here inhibit ed the reaction of NO and CO with the oxygen species. Another reason for the low reduction efficiencies of these gases was that these gases only protected the metal vapor from oxidation near the welding arc zone, while the amorphous silica layer
58 formed on the metal particles by the ASE technology could insulate the metals from subsequent oxidation occurring outside the welding arc zone. As seen in Figure 3 4 D the injection of N O or C O into the primary shielding gas as a reducing agent was much less effecti ve at reducing the Cr 6+ emission than the IDST and ASE technology. In addition, the emission of unreacted NO or C O as well as the generation of NO 2 potentially pose additional health hazard s Hence, the ASE technology is better suited for reducing Cr 6+ in welding fumes. XPS Results Figure 3 5 shows the XPS spectra of selected samples. Significant bands were found at binding energies from 577.0~578.0 eV for Cr 2p3/2 and 587.0~588.0 eV for Cr 2p1/2. The Cr 3+ peak was assigned by Cr 3+ (577.2 eV) and Cr 2 O 3 (57 6.3 eV), while the Cr 6+ peak was assigned by CrO 3 (578.1 eV) and Cr2O 7 2 (579.2 eV). The ratio of the Cr 6+ to Cr 3+ was calculated by curve fitting. In this particular baseline sample (Figure 5a), the ratio of Cr 6+ to Cr 3+ was about 45:55. In the optimal TMS injected samples (Figure 5b), Cr 6+ was absent. The XPS result provides semi quantitative evidence of the absence of Cr 6+ inside the silica shell. This supports the hypothesis that elimination of the Cr 6+ occurred before the formation of the amorphous sili ca layer.
59 Figure 3 5. XPS spectra A ) baseline fume particle with peak assigned B ) XPS spectra of fume particle generated using 30 Lpm primary shielding gas and 4.2% TMS carrier gas Field Sampling Results Due to limited study resources, the only c omparison made was between the airborne Cr 6+ concentrations using no TMS gas versus using 1.6% TMS carrier gas /primary shielding gas T he Cr 6+ concentration without using TMS was 9.8 (3.4) g/m 3 whereas the Cr 6+ concentration was less than the limit of d etection ( 1.1 g/m 3 ) with 1.6% TMS carrier gas /primary shielding gas The baseline airborne Cr 6+ emission of 9.8 g/m 3 was less than the baseline Cr 6+ concentrations in the chamber samples (range: 25~56 g/m 3 ) due to the greater dilution ventilation in the field setting.
60 The percentage of TMS carrier gas/primary shielding gas needed to achieve maximum reduction in the field (1.6%) was also lower than the laboratory (3.2%). The possible reason was that the pulse welding process employed in the field sampling could decrease the heat input and increase the efficiency. This also indicates different welding machines and processes might have different optimal injection rates for silica precursor. Effects on Welding Quality Figure 3 6 shows phot ographs of two welds made using baseline ( no TMS gas) and 4.2% TMS carrier gas added to 30 Lpm primary shielding gas (75% Ar + 25%CO 2 ), respectively The welds were examined by a certified professional welder who reported no visual difference on buildup of the weld. The react ion between the TMS and oxygen species introduced extra heat input to the weld, which led to a longer cooling time for the weld puddle. This is similar to the role of CO 2 in the shielding gas. CO 2 is commonly added to the shielding gas to help weld penetra tion in cases where the welding machine does not have sufficient capacity. If extra heat is not desired, one can always use other components of the shielding gas to compensate for the heat, such as increasing the Ar content or increasing the primary shield ing gas flow rate. The TMS vapor also increased the globular size in the globular transfer mode. The globular tended to bounce at the edge of the weld and create more spots. This problem did not occur in the short circuit and the spray modes.
61 Figure 3 6. Photographs of weld surface A ) baseline B ) 30 Lpm primary shielding gas and 4.2% TMS carrier gas Apart from these two effects, there was no other evidence that TMS altered the welding process, and no mechanical structure change was expected. How ever, further in depth mechanical test such as transverse tensile and bend tests should be performed to confirm the effects of introducing TMS on the generated weld. Hazard and Cost Estimation Equation 3 3 was used to calculate the worst case volume concen tration of unreacted TMS vapor in the air in a 5m5m3m room with a 5 min duration of welding T h e scenario represents the conditions of the field sampling in this study ( d etail s of the calculation are given in Appendix A) This equation is based on diluti on of the saturated carrier gas (TMS vapor in Ar) entering the TMS free room air without ventilation. An assumption wa s made that the saturated carrier gas was at atmospheric pressure (760 mmHg). The saturation vapor pressure of TMS is 271 mmHg at 0 C (ic e bath). The corresponding maximum concentration was calculated to be 5.7 ppm when injecting
62 2.410 4 m 3 /min TMS carrier gas which was the flow rate (0.24 Lpm) used in the field sampling. This maximum concentration was three orders of magnitude below the T lower flammable limit/lower explosion limit (LFL/LEL) of 1%. This result indicates the use of TMS will not cause explosion risks for welding in a typically sized room, even without removal by ventilation. 3 3 where C TMS is the concentration of TMS (vppm), Q is the flow rate of TMS (2.410 4 m 3 /min), t is the duration of welding (5 min), and V is the space of the room (75 m 3 ). Using the model from SERDP PP 1415 ( Lippold and Frankel 2009 ) to assess t he additional costs of the TMS in various welding processes, the general material cost was calculated to increase by 3.8% ( see Appendix A) The cost of the shielding gas containing TMS was $0.6 per ft of weld. This cost does not include the initial capital c ost of the IDST, which might vary significantly from laboratory phase to industrial bulk production. The details of these calculation are available in another report ( Paulson and Wu 2012 ) The costs of implementing the ASE technology are comparable to those of other control technologies such as LEV and on gun extraction ( WTIA 1999 ; Pocock et al. 2009 ) Summary Overall, the use of the IDST with the ASE technology was shown to reduce the airborne Cr 6+ concentration to below the limit of detec tion (2.8 g/m 3 ) in the laboratory, under all primary shielding gas flow rates. The premature decomposition of silica precursor that occurred in the premix mode was minimized by injecting the primary shielding gas and TMS carrier gas separately. TMS was sh own to be more effective and
63 less hazardous than NO and CO as a reducing reagent in welding applications, with an acceptable cost. As observed in the previous studies, the in situ generated silica condenses on the fume particles that can insulate the metal s from the analytical eluent of IC. XPS result confirms that at the optimal ratio, TMS prevented the formation of all the Cr 6+ compounds. In addition to the laboratory chamber sampling, a fixed location area sampling study was carried out in an industrial welding facility to examine the practicality of this technology. By adding 1.6% TMS carrier gas to the primary shielding gas, the Cr 6+ concentration was reduced to below the limit of detection of 1.1 g/m 3 Furthermore, visual inspection of welds generated using the ASE technology showed no surface deterioration in weld quality. Nevertheless, a mechanical structure test will be helpful in determining if extra heat and increased globular size introduced by the TMS have any impact on weld quality Besides red ucing the Cr 6+ concentration in welding fumes, ASE technology also encapsulates other toxic metals such as Ni and Mn which will be discussed in the next chapter.
64 CHAPTER 4 ENCAPSULATION EFFECTIVENESS OF WELDING FUME PARTICLES AND ITS IMPACT ON MECHANICAL PROPERTIES OF WELDS Objective C oating an amorphous silica layer on particle surfaces to insulate engineered nanoparticles from degradation from exposure to the surrounding environment has been reported in various studies ( Liz Marzn et al. 1996 ; Biswas et al. 1997 ; Correa Duarte et al. 1998 ; Yi et al. 2005 ; Teleki et al. 2009 ) It has also been demonstrated to be an effective measure for controlling nano sized metal particle emissions from combustors and in cinerators ( Biswas et al. 1995 ; Owens and Biswas 1996 ; Biswas e t al. 1997 ; Biswas and Wu 1998 ) T his concept when implemented in welding (as shown in Figure 4 1), has been labeled amorphous silica encapsulation (ASE) and presents a potential solution that can reduc e the toxicity of welding fume particles provided the silica coating layer is in the amorphous phase Indeed, X ray diffractogram s (XRD) of the coated fume particles confirmed that the in situ generated silica w as all in the amorphous phase ( Topham et al. 2010 ) hence eliminating the potential hazard of crystalline silica The amorphous silica layer on metal particles can insulate the metal species from absorption when inhaled. Additionally, silica thus formed yield s a web like network structure that effectiv el y increases the size of the particles which shifts the particle deposition upward in the respiratory tract ( ICRP 1994 ) Furthermore, the decomposition of the silica precursor scavenges oxygen species, t hus suppressing the oxidation of Cr to Cr 6+ ( Topham et al. 2010 ; Topham et al. 2011 ; Wang et al. 2012 )
65 Figure 4 1. Mechanistic illustration of the ASE technology in welding application The results in Chapter 2 using a premixed shielding gas containing a silica precursor showed the SCE to be about 14~38%, dep ending on the flow rate used. The relatively low SCE resulted from the premature decomposition of the silica precursor, i.e., the spatial and temporal condensation At low shielding gas flow, when the gas cou ld not effectively disperse the heat thermal energy induced the decomposition of the silica precursor and the formation of silica particles inside the nozzle and outside the welding arc zone, before welding fume particles h ad even formed In Chapter 3, an i nsulated double shroud torch
66 (IDST) was developed to address this premature decomposition issue ( Wang et al. 2012 ) The IDST design involves a ceramic wall in the torch to insulate the heat between the primary shielding gas and the sili ca precursor carrier gas (as shown in Figure 4 1) thus preventing the premature decomposition of the silica precursor that occurs when the gases are premixed While the testing showed reduction of airborne Cr 6+ concentration to below the detection limit the im pact on SCE and the mechanical properties of the weld remained unknown. Knowledge of SCE is imperative due to the fact that uncoated fume particles are still available for worker inhalation and subsequent systemic biodistribution. Verification that t he mechanical properties of the weld ha d not been altered by the addition of t he silica precursor is critical, if the technology is to be a ccepted and adopted by the industry. The objectives of this Chapter were to assess the effectiveness of the ASE techn ology with IDST feeding to encapsulate the welding fume particles and to characterize the mechanical properties of welds. Both quantitative analysis of SCEs and qualitative TEM images were acquired for evaluating the conditions of encapsulation. The weld g enerated from the ASE technology underwent a series of mechanical property tests to validate the applicability of the ASE technology to welding practices. Experimental Methods Welding Fume Generation Sampling of welding fumes (Figure 4 2) followed the Amer ican Welding Society ( AWS ) fume hood design recommended in Method F1.2 2006 ( AWS 2006 ) Welding fumes were generated in an enclosed conical chamber of 36 inches in diameter at the base, 8 inches in diameter at the top, and 36 inches in height. A high volume flow pump ( General Metal Works GL 2000 H, Cleves, OH) was mounted on top of the cha mber.
67 The welding fume particles generated were collected onto a glass fiber filter (Whatman 90 mm pore size 1 m, Maidstone, Kent, UK ) Figure 4 2. Schematic diagram of the TMS feeding apparatus and the welding fume chamber A welding machine ( Lincol n Power MIG 140C Cleveland, OH) was used in the study. The voltage and wire speed were kept at 19.5 V and 100 inches per minute (ipm) throughout the study. The welding wires used were ER 308L stainless steel of 0.035 inch diameter, with a nominal composit ion of 19.5 ~ 22.0% Cr, 9.0 ~ 11.0% Ni, and 1.0 ~ 2.5% Mn A m ild steel base metal was used to minimize costs as well as to avoid interference from stainless steel components in the base metal. The base metal plates were placed on a rotating turntable (MK Produc ts Aircrafter T 25 Irvine, CA ) at the bottom of the chamber (as seen in Figure 4 2). A stand was used to hold the welding gun and to keep the torch at a constant height relative to the base metals. The trigger of the welding gun was modified to allow remo te control from outside the welding
68 chamber. The chamber study simulated welding on the rotating plates for 1.5 minutes per sample. The length of the sampling period was based on those used in previous studies to ensure sufficient fume particles were colle cted for analysis. An IDST replaced the conventional welding torch to allow separate flows of the primary shielding gas and the TMS carrier gas. The TMS carrier gas (argon) flowed through a Teflon impinger (Apex Instruments T507G, Fuquay Varina NC) filled with TMS liquid at the bottom. The impinger was immersed in an ice bath at 0 C to lower the vapor pressure of TMS for controlling the amount of vapor entering the carrier gas. The TMS saturated carrier gas was delivered to the welding arc zone through th e outer shroud of the IDST. A mixture of 75% argon and 25% carbon dioxide was chosen as the primary shielding gas, based on the low power capacity of the welding machine. The flow rates of the primary shielding gas and the TMS carrier gas (listed in Table 4 1) were respectively controlled by a rotameter (Radnor HRF 1425 580 Radnor, PA ) and a mass flow controller (Omega FMA5500 Stamford, CT) The welding fume particles collected on the filter were gravimetrically measured before and after sampling, using an analytical scale (Sartorius MC210S) with a readability of 10 g. Each sample was weighed three times and the mean value was calculated.
69 Table 4 1. Flow rates of primary shielding gas and TMS carrier gas, and the corresponding mass of collected welding fume particles Primary s hielding gas (Lpm) TMS c arrier gas (Lpm) Mass of collected fume particles (mg) 20 (low) 0.16 12.3 1.1 0.32 13.1 0.9 0.64 11.7 1.3 0.96 12.4 0.6 25 (medium) 0.16 13.6 0.9 0.32 12.8 1.7 0.64 14.2 0.8 0.96 13.0 1.2 30 ( high) 0.16 17.6 1.5 0.32 16.5 0.8 0.64 19.2 1.1 0.96 21.4 2.3 Analysis of Silica Encapsulation Determination of SCE followed the methodology developed in a Chapter 3 which used the acid resistance ability of the silica shell to measure SCE. Glass fiber filters loaded with welding fume particles were cut into two halves One half was digested using 9 mL nitric acid (HNO 3 68 % ) and 1 mL hydrofluoric acid (HF, 48~51%), which were aggressive enough to dissolve all the metals regardless of their coating conditions. The other half was digested using 10 mL aqua regia, a mixture of HNO 3 and hydrochloric acid (HCl, 38%) (1:3 v/v), which only dissolved metals not encapsulated in the silica shell. The SCE was thus calculated by using the differences in measure d metal mass in the following equation: 4 2 where SCE is in percentage (%), N is the n umber of metals involved (three in this study), C NF,i is the m easured concentration of the i th metal diges ted by HNO 3 /HF mixture C AR, i is the m easured concentration of the i th metal digested by aqua regia In other words, SCE represent s the ratio of encapsulated metals to total metals measured
70 The digestion was done using a microwave digestion system ( CEM MD S 81D Matthews, NC ) following the eight step protocol described in the previous study ( Wang et al. 2011 ) The digests were cooled, filtered, and diluted. Measurement of the mass of each metal in diluted solutions was conducted with inductively coupled plasma atomic emission spectroscopy (ICP AES, Perkin Elmer Plasma 3200 Norwalk, CT ). Two atomic emission spectral lines for each metal (Cr, Ni, and Mn) were used simultaneously to reduce spectral interference introduced by other co existing elements in the welding fume The operating conditions and spectra l lines selected are listed in Table 4 2. The concentration of each metal in a sample was determined by averaging the resu lts of five reading replicates. Transmission electron microscopy ( TEM, JEOL 2010F, Peabody, MA ) was used to observe the morphology and silica encapsulation conditions of the welding fume particles. The JEOL 2010F has an ultrahigh spatial resolution below 1 nm. A s pecialty grid (Pelco 300 mesh, Ted Pella, Redding, CA ) was inserted into the welding chamber, as shown in Figure 4 2. The grid was held for 30 s in the welding fume stream drawn upwards by the pump. Fume particles thus attached to the ultrathin lacy carbon film on the grid. Images of both the baseline and the ASE samples were captured. The microscopic image is a supplemental tool for determining the effectiveness of the ASE technology. However, TEM only focused on a fraction of the welding fume particles and was limited to a two dimensional plane.
71 Table 4 2. Operati ng condition s of the ICP AES and the spectral wavelengths used for the analysis Incident Power (W) 1300 Plasma gas flow rate (Lpm) 13 Auxiliary gas flow rate (Lpm) 0.5 Nebulizer gas flow rate (Lpm) 0.8 Peristaltic pump flow rate ( mL min 1 ) 1 Reading delay time (s) 40 Reading per sample 5 replicate s Cr spectral wavelengths (nm) 205.552, 267.716 Ni spectral wavelengths (nm) 231.604, 221.647 Mn spectral wavelengths (nm) 257.610, 293.306 Mechanical Property Test It is difficult to generate welds of high quality using t he chamber s ystem due to problems of the wel property tests were generated at an industrial facility that regularly conducts welding activities. The modification to the conventional welding system in the facility for the IDST was simi lar to the laboratory chamber setup, except with a different welding machine ( Miller Invision 456MP A ppleton WA ). The voltage was controlled by a pulse program with an average of 26 V. ER 310 stainless steel base plate of 0.18 inch thickness was welded w ith 0.045 inch diameter ER 310 stainless steel welding wire. The nominal ER 310 stainless steel plate composition is 24 26% Cr, 19 22% Ni, and 2% Mn, while the welding wire is 25 28% Cr, 20 22.5% Ni, and 1 2.5% Mn. The wire speed was fixed at 192 ipm. The primary shielding gas used was a mixture of 69% Ar, 30 % He, and 1% CO 2 The flow rates of the primary shielding gas and the TMS carrier gas flow were kept constant at 15 Lpm and 0.24 Lpm, respectively A total of six welds were produced, three for the base line and three for the ASE technology. Two of the six joints were analyzed for chemical composition ( ASTM 2011b ) prior to machining. Standard transverse tension test specimens were machined from
72 each weld ( ASTM 2011a ) Metallographic analysis w as performed on additional weld sections at both macro and microstructure scales for each weld Examinations highlighted fusion, penetration, microstructure macrostructure, and weld discontinuities. The transverse tensile tests ( AWS 1998 ; ASTM 2011a ) were designed to compare the performance of welds generated from the baseline and ASE technology. Quality Control and Statistics Al l chemicals used were analytical grade or higher in purity DI w ater used for dilution and cleaning was deionized and purified by a Nanopure system (Barnstead Nanopure D11901, Thermo Fisher Scientific Waltham, MA ) to a c onductivity of 18.2 cm. All aci d solutions (Acros Organics, Morris Plains, NJ ) were at their original concentration and not diluted. All the shielding gases used were ultra high purity and certified by the manufacturers (Airgas and Air Liquide). The glassware used in the study was cleane d in an ultrasonic cleaner (FS220 Thermo Fisher Scientific Waltham, MA ) for 4 h ours and dried in an Isotemp oven ( FS 230G Thermo Fisher Scientific Waltham, MA ) in a laminar flow hood. The tubing in the sampling system was Polytetrafluoroethylene (PTFE) or Tygon, and air leaking tests were performed regularly. The c alibration curve for chemical analysis was obtained from external standard s Standard solutions were prepared by diluting high purity stock solutions with DI water: 1000 mg / L Ni and 1000 mg / L Mn (Spex Certiprep Metuchen, NJ ) and 1000 mg / L chromate (CrO 4 2 ) ( Acros Organics, Morris Plains, NJ ) All the samples were pentaplicate per each combination of the primary shielding gas flow rate and the TMS carrier gas flow rate. t test was used to exami ne the results
73 from different combinations using a significan ce level of p = 0.05 S tatistical analyses w ere performed using the statistical software SAS 9.3. Results and Discussion Silica Encapsulation Figures 4 3 A ~ C show SCEs under different combinations of primary shielding gas and TMS carrier gas flow rates. Overall, the SCE ranged from 31~76%, which was a significant improvement from the SCE of 14~38% with premixed gases in the previous study ( Wang et al. 2011 ) Furthermore, the TMS carrier gas feed rate was below 1 Lpm (compared to over 2 Lpm when t he gases were premixed.) Inspection of the welding gun revealed few deposits of white silica powder verifying a lower degree of premature decomposition of the TMS. These pieces of evidence prove that the IDST design can deliver silica precursor to the wel ding arc zone more effectively and efficiently than the premixed gas method while decreasing the costs associated with feeding excessive TMS. Figure 4 3. SCE as a function of TMS carrier gas flow rate A ) 20 Lpm B ) 25 Lpm C ) 30 Lpm primary shie lding gas flows
74 Under the low and medium shielding gas flow rates, the highest SCE were similar at 64%. The highest SCE for 20 Lpm (low) primary shielding gas flow was 64 6.1%, feeding with only 0.16 Lpm TMS carrier gas and there was no statistical diffe rence in SCE among 0.32~0.96 Lpm TMS carrier gas flow rates ( p >0.1). The SCE of the 25 Lpm (medium) primary shielding gas flow reached a maximum of 64 9.4% at 0.32 Lpm TMS carrier gas flow rates, and no statistical difference for the rest of TMS carrier ga s flow rates. The 30 Lpm (high) shielding gas flow showed a different trend. The SCEs were low at 31 4.9% and 38.3 2.8% at 0.16 Lpm and 0.32 Lpm TMS carrier gas flow rates, respectively. Although a maximum SCE of 76 7.9% occurred at 0.64 Lpm TMS carrier ga s flow, it decreased to 43 9.0% after further increase of the TMS carrier gas flow rate. Table 4 1 lists the mass of collected fume particles under the different conditions tested. The gravimetric measurements of collected fume particles under low and medi um primary shielding gas flows were significantly lower than those of high shielding gas flow ( p <0.01). Hence, the ASE technology was able to deliver enough in situ generated silica to coat the relatively low amount of welding fume particles under low and medium primary shielding gas. However, coating under high primary shielding gas flow (30 Lpm) was less effective with low TMS feed rates, at least partially due to the high mass of welding fume generated. At very high TMS feed (0.96 Lpm), both the primary and the TMS carrier flows might disperse the silica vapor and metal particles to a larger mixing zone and therefore reduce the time of interaction. The chaotic condition at high gas flow was confirmed by the TEM images later, with a large quantity of unco ated metal particles and the formation of stand alone silica particles.
75 By delivering TMS into the effective mixing area using the IDST technology, it was hoped that the SCE would be near 100% and all metal particles would be fully encapsulated. However, t he welding process naturally contains a high intensity of ultraviolet (UV) light emitted from the welding arc. TMS inevitably undergo photolysis under UV ( Gonzlez et al. 1992 ) in undesirable locations such as inside the welding gun. This was confirmed by the trace amount of silica powder deposits in the torch. To further improve SCE, aerosol dynamic modeling o f the welding fume system will be needed to provide helpful insights into the effects of the UV and other influen t i al factors. These insights can be used to optimize the silica precursor delivery setup. The calculation of SCE was done by summing up the dif ferences in mass of individual metals. The mass of iron (Fe) was excluded from the calculation due to its insignificant toxicity, even though Fe is a major component among the metals in welding fume particles. Other trace metals present only in extremely l ow amounts in the welding fume particles were also disregarded. Hence, SCE as calculated was toxicity weighted with Cr, Ni, and Mn. In addition, SCE measurements only counted particles fully enclosed in a hermetical silica shell (i.e., partially encapsulat ed particles were not counted as encapsulated). In short, SCE measurements conservatively quantify the reduction in the bioavailable mass of selected toxic metals (Cr, Ni, and Mn) caused by encapsulation of these toxic particles in amorphous silica shells. TEM Imagery Figures 4 4 A ~ D display the TEM imagery of welding fume particles under various conditions. Because of the penetration ability of electron s when interacting with the particles metals with a high electron density are typically darker on a brigh t field TEM image while silica is lighter with its low electron density ( Williams and Carter 2009 )
76 Figure 4 4 A depicts the welding fume particles generated from 30 Lpm primary shielding gas without the introduc tion of TMS. The diameter of the welding fume particles ranged fro m 10~100 nm. From the 2D image, it was difficult to determine if the particles were chain like agglomerates or overlapped at different planes. Figures 4 4 B ~ C show welding fume particles generated from 30 Lpm primary shielding gas and 0.64 Lpm TMS carrier g as flow with SCE of 767.9%. The images showed a silica en c apsulated metal agglomerate, with a clear boundary between the amorphous silica layer and its metal components. By comparing with Figure 4 4 A it was observed that the primary particle size was sma ller than that in the baseline case. The result suggests that the silica coating prevented sintering of metal particles. Figure 4 4 C also showed that encapsulated metal particles have different shapes, from spherical to polygonal. The nano sized primary m etal particles were bound to an agglomerate particle with a larger equivalent diameter through the inter coagulation mechanism ( Lee and Wu 2005 ) This resultant increase in particle size can effectively reduce respiratory tract deposition, due to the low deposition of particles around 200~300 nm ( ICRP 1994 ) Figure 4 4 D shows the welding fume particles generated from 30 Lpm primary shielding gas and 0.96 Lpm TMS carrier gas flows with SCE of 439.0%. The particles are more randomly arranged, due to the high amount of welding fume particles generated and possible poor mixing interac tion between silica vapor and metal particles. In the situation of excessive TMS feed, the silica vapor was more likely to form stand alone silica particles than to condense on the surface of metal particles The same phen o m e non was also observed at the 0. 96 Lpm TMS carrier gas flow with 20 and 25 Lpm primary shielding gas. Although some metal particles were trapped in the
77 silica matrix, a high level of excess silica particles formed from feeding a high amount of TMS warrants investigation of the health eff ects. Some studies showed that amorphous silica particles may cause phagocytosis but the mechanism by which this is caused is not clear ( Lundborg et al. 2006 ; Yu et al. 2009 ; Costantini et al. 2011 ) TEM imagery supports the idea that there is an optimal TMS carrier gas flow rate for applying the ASE technolog y, i.e., a rate to allow sufficient silica vapor for the amount of metal particles, without causing an overflow of excess silica particles. Figure 4 4. TEM images A ) baseline welding fume p articles without any coating B ) welding fume particles a gglomerate with a distinct silica layer C ) 2 magnif ication view of the fume particles and silica layer in image B D ) welding fume particles with excessive silica particles formed
78 Mechanical Properties The chemical compositions of the welds in weight perce ntage (wt%) from the baseline and the ASE technology are listed in Table 4 3. Cr, Ni, Mn, and other elements in both conditions were almost identical to the standard ER 310 stainless steel ( AWS 2012 ) It was also noted that the ASE technology did not introduce extra Si content into the welds, while extra Si could adversely affect weld quality ( Lampman 1997 ) Table 4 3. Chemical composition (wt%) of weld metals and standard requirements Standard material/Welds Cr Ni Mn Si Mo Cu P S ER 31 0 stainless steel welding wire specification ( AWS 2012 ) 25.0~28.0 20.0~2 2.5 1.0 2.5 0.30 0.65 N/A* N/A* 0.05 0.03 Baseline 26.5 20. 0 1.19 0.47 0.14 0.10 0.02 0.01 ASE technology 26.0 19.7 1.21 0.47 0.14 0.10 0.02 0.01 No standard values for the elements Mo and Cu in the specification ( AWS 2012 ) The macrostructures of the welds are shown in Figure 4 5. The weld metal and base plate are labeled as WM and BP, respectively. The welds of the baseline technology revealed typical columnar grains adjacent to fine gra ins of the base plates. The welds from the ASE technology were identical to the baselines in macrostructure with no weld defect. Figure 4 6 A displays the microstructure of one particular baseline weld. The baseline weld was comprised of grains within the h undreds of micron range, adjacent to the fine grains in the heat affected zone (HAZ) and the fusion line (FL). The microstructures of the baseline weld showed the presence of intermetallic particles onto which the grains nucleated. The microstructures of o ne particular weld generated by the ASE tec hnology are shown in Figure 4 6 B The weld was generally similar to that of the
79 baseline, except for a tiny crack at the interface of the HAZ and weld metal (WM). It is possible that the addition of gas (TMS/Ar) c reated a gas pocket that coalesced into a crack. However, no crack was found in other welds from the ASE technology. Figure 4 5. Macrostructure of welds A ) B) C ) welds gen erated with baseline technology D ) E ) F ) welds generated with ASE technology. Fi gure 4 6. Microstructure of welds A ) baseline technology B ) ASE technology
80 The results of the tensile tests of the welds are shown in Figure 4 7 A and the detailed data are in Appendix B The yield strength (YS) of the welds from the baseline and the ASE technology were identical, 44 1 kilo pounds per square inch (ksi). Meanwhile, the ultimate tensile strength (UTS) of the welds from the baseline and the ASE technology were 83 4.3 ksi and 77 8.1 ksi, respectively with no statistical difference ( p >0.1). Figure 4 7 B shows the comparison of average elongation of welds from the baseline and the ASE technology. Again, the elongation values showed no statistical difference ( p >0.1). The AWS requirement for ER 310 stainless steel ( AWS 2012 ) is also displayed. It should be noted that the AWS minimums for UTS and elongation are established for standard materials with unif orm composition, not for welds involving a combination of welded metal, HAZ, and base metal. Hence these AWS minimums are included for reference only. As both the baseline and the ASE samples were lower than the AWS minimum, welder inexperience working wi th a new welding shielding gas additive likely is a major factor contributing to the imperfect welds. If not from the statistical aspect, the result indicated that the ASE technology reduced tensile strength in some samples. W hile the ASE technology did no t s tatistical ly deteriorate the mechanical qu ality of the welds, optimization of different welding parameters to achieve better tensile property certainly should be considered
81 Figure 4 7. Tensile test results A ) Yield strength (YS) and ultimate tens ile strength (UTS) B ) elongation of welds generated with baseline and ASE technology Summary The ASE technology applied to the stainless steel welding process is an emerging method for effective minimization of welder exposure to toxic nano sized we lding fume particles. Overall, 31 76% of Cr, Ni, and Mn in the welding fume particles were completely encapsulated in a layer of amorphous silica by feeding the TMS carrier gas into the welding arc zone through the IDST. At low and medium primary shielding gas flow rates, the SCE was similar and reached a maximum of 64% at a moderate TMS carrier gas flow rate. At the high primary shielding gas flow rate, the SCE was 76% at 0.64 Lpm TMS carrier gas flow rate, and much lower at other TMS carrier gas flow rate s. The high amount of welding fume generated at high primary shielding gas flow rate likely contributed to the low SCE. The high gas flows possibly caused a poor mixing, reduced the exposure time, and resulted in a low SCE and the formation of more stand a lone silica particles which was confirmed by the TEM images On the
82 other hand, the TEM images also showed a distinct silica layer on the primary metal particles and agglomerates at the optimal gas flow rate. The result also showed that introducing TMS as an additive to the shielding gas did not change the metal and Si contents in the welds. The metallography of welds generated from the baseline and the ASE technology were similar. A tiny crack was found in the microstructure of one particular weld from th e ASE technology. The yield strength, ultimate tensile strength, and elongation of welds from the baseline and the ASE technology showed no statistical difference. The ASE technology may be further optimized to achieve both a higher SCE and mechanical prop erties through tools such as computational fluid dynamic simulations combined with aerosol dynamics. Toxicological studies are also essential to help fully realize the potential health benefits brought by the ASE technology.
83 CHAPTER 5 CONCLUSIONS AND REC OMMENDATIONS In this doctoral study, an analytical method to determine the amount of metals encapsulated in the amorphous silica shell was developed. HNO 3 /HF acid mixture and aqua regia digestion method were used to determine silica coating efficiencies ba sed on the measured mass difference This method can be also applied to applications of silica shell/metal core particles Metals encapsulated by amorphous silica cannot be further extracted by general acid such as HNO 3 or aqua regia or weak acid environm ent inside human body Hence, this proportion of metal should not be bio accessible to human body. The experimental result for the pre mixed TMS feed showed silica coating efficiency increased with increasing shielding gas flow rate. The low silica coating efficiency under low shielding gas flow rate was due to the premature decomposition of silica precursors. Insulated double shroud torch ( IDST ) was designed and shown to reduce the airborne Cr 6+ concentration to below the limit of detection (2.8 g/m 3 ) in the laboratory, under all primary shielding gas flow rates. The premature decomposition was minimized by injecting the primary shielding gas and TMS carrier gas separately. TMS was shown to be more effective and less hazardous than NO and CO as a reducing reagent in welding applications, with an acceptable cost. XPS result confirmed that at the optimal ratio, TMS prevented the formation of all the Cr 6+ compounds regardless encapsulation status. In addition to the laboratory chamber sampling, field study wa s carried out to examine the practicality of this technology. By adding 1.6% TMS carrier gas to the
84 primary shielding gas, the Cr 6+ concentration was reduced to below the limit of detection of 1.1 g/m 3 Besides reducing the Cr 6+ concentration in welding f umes, the ASE technology also encapsulates other toxic metals such as Ni and Mn. Using IDST, 31 76% of Cr, Ni, and Mn in the welding fume particles were completely encapsulated in a layer of amorphous silica. At low and medium primary shielding gas flow ra tes, the SCE was similar and reached a maximum of 64% at a moderate TMS carrier gas flow rate. At the high primary shielding gas flow rate, the SCE was 76% at 0.64 Lpm TMS carrier gas flow rate, and lower at other TMS carrier gas flow rates. The high amoun t of welding fume generated at high primary shielding gas flow rate likely contributed to the low SCE. The high gas flows possibly caused a poor mixing, reduced the mixing time, and resulted in a low SCE and the formation of more stand alone silica particl es which was confirmed by the TEM images. The ASE technology did not change the metal and Si contents in the welds. The metallography of welds generated from the baseline and the ASE technology were similar. The yield strength, ultimate tensile strength, and elongation of welds from the baseline and the ASE technology showed no statistical difference. All the study results showed the potential of the ASE technology in addressing the welding fume exposure issues, by retrofitting the welding equipment with t he IDST, without adversely impacting the cost and quality of welding process. The ASE technology answered the call from the National Occupational Research Agenda, which listed developing effective welding fume control technology as priority research area. The ASE technology upon implementation can possibly help the welding industries
85 comply the newest tightened occupational standard for Cr 6+ and other metals. The technology may potentially protect over 500,000 welders in the US and more around the world, as welding activities continue to play an important role in the growing construction activities and economic revival. Th e newly developed method to quantify SCEs can be used in assessing other silica metal core shell structure nanotechnology applications Th e study on SCEs under different flow rates of gases allows a better understanding of how silica and metals dynamically interact in such an aerosol system, and this knowledge can be extended to help understand other applications utilizing similar in situ ge nerated silica such as nanoparticle flame synthesis, thermal plasma reaction, and chemical vapor deposition. There are several aspects that the ASE technology may be further optimized and assessed in the future: (1) to achieve both a higher SCE and mechani cal properties through tools such as computational fluid dynamic simulations combined with aerosol dynamics modeling. The welding experiments are expensive with complicated factorial designs. Thus, modeling can be a good supplement and validation; (2) to i nvestigate the cytotoxicity change caused by the amorphous silica shell. The human lung alveolar cells such as A549 cell can be cultured and exposed to the welding fume particles, to represent the inhalation toxicity. The cell exposure unit can be retrofit ted to connect to a welding chamber directly to achieve an in flight exposure of freshly generated welding fume particles; (3) to determine the bioavailability of welding fume particles before and after silica encapsulation using simulated lung fluid test. The biotoxicity can be greatly reduced if the adsorption of metals to human body is masked by an amorphous silica shell; (4) to develop a comprehensive systemic biokinetic model for welding fume,
86 coupling with respiratory deposition model, bioavailability and cytotoxicity test results. This model can be used to predict the health effects and to trace back the welding exposure. The tool will also be useful in assessing the effectiveness of the ASE technology.
87 APPENDIX A DET A IL RESULTS OF THE HAZARD AND COS T ESTIMATION Estimation of TMS in the worst case scenario One potential concern in implementation is the safe handling of TMS. Tetramethylsilane (TMS) is a flammable and volatile liquid. High concentration of TMS vapor may cause flash fire s or explosion s i n oxidizing environment s Exposure to TMS may cause skin, eye, and respiratory tract irritation, although the toxicological properties of TMS have not been fully investigated. The MSDS suggests that workers should avoid inhalation of TMS. There is no stand ard or published document for handling welding with TMS shielding gas since it is still under development. Nevertheless, there has been no incident of fire or explosion involved in welding with TMS shielding gas to date. We carried out a calculation of TMS concentration in a typical room in a scenario in which all TMS leaked into a room without decomposition. Vapor pressure (V p ) of TMS was calculated using the Antoine equation, where A, B, C were 3.97703, 1047.242, 36.057 ( Aston et al. 1941 ) re spectively. Temperature was set to 273 K because the TMS was in an ice bath. Accordingly, V p was 271 mmHg. The i deal gas law was used to calculate the concentration of TMS in saturated carrier gas, where pressure (P) is equal to 271/760=0.356 atm; gas constant (R) is equal to 82.0575 atm cm 3 /(Kmol); temperature (T) is equal to 273 K. Accordingly,
88 Consider the worst case scenario, in which all the TMS has leaked into the air without forming silica : Welding to join two test plates takes a maximum of about 3 min. According to the standard operation procedure (Appendix B) in field demon stration, the TMS flow would be stopped between two welding events while the welder replace d the plates and supplies. For a room of 74 m 3 and a pre s et carrier gas flow rate of 0.24 Lpm, the TMS concentration in the tent/room would be The value is orders of magnitude lower than the TMS lower flammable limit/lower explosion limit (LFL/LEL) of 1%. It should be noted this calculation is based on the worst case scenario which is very unlikely to occur. The ventilation system in the field will lower the accumulated TMS concentration. Result of the Cost Estimation This cost estimation intended to compare th re e baseline welding process es utilizi ng the TMS technology. The estimation incorporated (1) t he approximate cost differences between MIG welding using standard shielding gas and the TMS additive ; (2) t he approximate cost differences between standard ventilation systems and ventilation systems designed to meet the new occupational standards; (3) t he approximate costs associated with the welding process requalification if a change is
89 made to the ASE technology (4) assuming using the ASE technology without retrofitting conventional ventilation s ystems will fit the occupational standards. A detailed cost analysis for the substitution of Cr free welding consumables for standard Type 308 filler metals for the welding of stainless steel was developed in 2006 under SERDP Project PP 1415 of Chromium Free Welding Consumables for Stainless Steels Some aspects of this approach were utilized to assess and compare the costs associated with the ASE technology and traditional welding Ten different combinations of joint type and industry secto r were evaluated. These same combinations were evaluated below to assess the effect on costs as a result of using the ASE technology. The industry sectors from which the applications were selected included shipbuilding, transportation and storage tanks, an d general fabrication. The joint designs included V groove butt welds between both pipe and plate configurations, as well as T joints with fillet welds. The criteria developed in the PP 1415 were utilized for the various GMAW joints with the additional cos ts associated with the three TMS approaches included. The unit costs of each scenario are listed in Table A 1 These costs were then compared to costs of produc ing the same joints traditional welding Scenario 1 represented the mixing of TMS at the torch. Scenario 2 utilized customized TMS cylinder gas w here the gas wa s supplied from the customized gas department of Airgas a gas supply company. Scenario 3 wa s similar to S cenario 2, but with the estimated reduced cost of TMS cylinder gas once it is commerc ially available and in high volume production a net profit rate of 50% according to Airgas. G as flow costs were based on the
90 estimated total arc times associated with eac h joint type, and did not include mass flow control ler and nozzle equipment costs. Table A 1 Unit cost used in the cost model Unit p rice Consumption rate Cost per minute Scenario 1 Primary shielding gas (Ar/CO 2 ) $27/300 ft 3 29 Lpm 0.092226148 Carrie r gas (Ar) $25/300 ft 3 1 Lpm 0.002944641 TMS $65.16/100 m L 0.02 m L /min 0.013032 Scenario 2 Primary shielding gas (Ar/CO 2 ) $27/300 ft 3 29 Lpm 0.092226148 TMS premix cylinder gas $1264/44 L 1 Lpm 28.72727273 Scenario 3 (Commercial product of cylinder ga s) Primary shielding gas (Ar/CO 2 ) $27/300 ft 3 29 Lpm 0.092226148 TMS premix cylinder gas $205/300 ft 3 1 Lpm 0.024146054 The comparisons of c osts are summarized in Tables A 2 T he results found that using TMS mixing at the torch would slightly increase cost s while using customized TMS cylinder gas would dramatically increase cost s The estimated commercial TMS cylinder gas can reduce the cost s to the same level as TMS mixing at the torch. TMS in the cylinder would be more convenient to end users than th e other means. While customized TMS cylinder gas is already available, currently it is only available through custom order. The result suggest ed th at cost s w ould be greatly reduced once the ASE technology becomes widely adopted in the welding community, an d TMS gas in cylinder s becomes a commonly available commodity. When OSHA established the new ventilation requirements for reducing exposure to hexavalent chromium, it stated that the primary methods for reducing such an exposure would be local exhaust vent ilation and improvement of general dilution ventilation. In addition, it wa s anticipated that in many cases a welder w ould utilize personal protective equipment with a respirator when welding stainless steels.
91 Therefore, this cost assessment wa s based on the assumption that a typical fabrication facility w ould incur additional costs for improved general and local ventilation, as well as personal protective equipment, as a result of the n e w OSHA regulation. Table A 2 Welded joint costs of traditional wel ding and ASE technology Traditional ASE Scenario 1 ( mixed at torch ) Industry Joint description Process Cost/ft or cost/joint* ($) gas cost ($)** Cost/ft or cost/joint* ($) gas cost ($)** Ship building/pressure vessels 6" diameter pipe GMAW 24.5 1.2 25 .2 1.9 12" diameter pipe GMAW 56.2 2.3 57.8 3.9 3/16" fillet weld GMAW 7.4 0.5 7.7 0.8 Tanks 3/16" butt weld GMAW 5.4 0.5 5.7 0.8 3/8" butt weld GMAW 8.8 0.5 9.1 0.8 General fabrication 3/16" fillet weld GMAW 2.2 0.3 2.4 0.5 1/4" fillet weld GMAW 4 0.4 4.3 0.7 Scenario 2 (customized TMS shielding gas in cylinder) Ship building/pressure vessels 6" diameter pipe GMAW 24.5 1.2 542 509 12" diameter pipe GMAW 56.2 2.3 1092 1038 3/16" fillet weld GMAW 7.4 0.5 214 207 Tanks 3/16" butt weld GMAW 5. 4 0.5 223 218 3/8" butt weld GMAW 8.8 0.5 226 218 General fabrication 3/16" fillet weld GMAW 2.2 0.3 122 120 1/4" fillet weld GMAW 4 0.4 185 181 Scenario 3 ((commercial TMS shielding gas in cylinder) Ship building/pressure vessels 6" diameter pipe G MAW 24.5 1.2 25.4 2.1 12" diameter pipe GMAW 56.2 2.3 58.1 4.2 3/16" fillet weld GMAW 7.4 0.5 7.7 0.8 Tanks 3/16" butt weld GMAW 5.4 0.5 5.8 0.9 3/8" butt weld GMAW 8.8 0.5 9.2 0.9 General fabrication 3/16" fillet weld GMAW 2.2 0.3 2.4 0.5 1/4" f illet weld GMAW 4 0.4 4.3 0.7 There are numerous general considerations associated with ventilation decisions regarding the new OSHA ventilation requirements, including issues such as the size of the fabrication facility and whether or not welding is bei n g conducted in confined space. Every case will be different T his analysis was based on two typical cases: a relatively
92 large fabrication space and a relative ly small fabrication space. I t is important to point out that this comparison represents very gen eric cases, and should only be used as a guideline. In addition to the overall size of the facility, there are many specific factors that must be considered that will affect ventilation requirements for each location. Examples of other factors to be consi dered included the location and number of roof and wall ventilators, overhead doors and obstructions, make up air exchange systems, welding parameters, working hours, annual consumable usage, type of welding processes used, etc. For the purposes of this g eneric comparison, the two different weld shop sizes considered were a 60 by 30 ft shop with 12 welders, and a 200 by 100 ft shop with 36 welders. Assumptions in each case included a single shift, welding parameters rang ing from 90 to 150 amps, overhead o bstructions (cranes) and no wall ventilators, and an heating ventilation air conditioning ( HVAC ) system present as an air exchange system. In the case of the larger shop, it was assumed there were five roof ventilators at 1000 CFM each, 4 overhead doors, and the annual consumable usage was estimated at 60,000 lb/year. For the smaller shop, it was assumed there were 2 roof ventilators at 1000 CFM each, 2 overhead doors, and the annual consumable usage was estimated at 20,000 lb/year. In each case, it was assumed that SMAW, GMAW, and GTAW processes were used. The extent to which the SMAW process is used will play a significant role in filter replacement frequency (higher usages of SMAW will require more frequent filter replacements), but there was no attem pt to quantify this detail. One more major assumption was that 100% of the welding in these two shops was stainless steel, which in many cases would not be accurate.
93 Lincoln Electric provided quotes for ventilation systems that were used for comparison. T he system costs included both a general ventilation system a nd a source extraction system. The general system was a U shaped push pull type. This would provide a continuous positive and negativ e air flow over the weld area. The source ventilation system in cluded pivoting and telescopic extractio n arms for each welding booth. Other costs considered included the costs of personal protection ventilation suits and air monitoring. T he summary below incorporates the aforementioned assumptions and information and compares the typical ventilation system purchase cost differences between a shop that welds stainless steel ( and therefore is subject to the new OSHA requirements ) to those of a shop not subject to such requirements. These results are also summarized on T able A 3 Table A 3 Typical v entilation system cost Weld shop size Number of welders meet OSHA standard Initial purchase expense ($) Recurring expenses ($) 36 Yes 700 000 50 000 No 410000 20000 12 Yes 162000 20000 No 100 000 1 0000 For the purposes of better understanding the financial impact of the OSHA h exavalent c hro mium lower exposure requirement, and the additional cost s associated with the ASE technology, six scenarios involving the two welding shop sizes were comp ared. The results are summarized in Table A 4
94 Table A 4 Summary of the cost comparison of the ASE technology in different scenarios and traditional welding Room size Scenario Initial cost ($) Annual recurring cost ($) Ventilation cost ($) 200 ft by 100 ft T raditional welding with new ventilation system 700 000 50 000 N/A Mix at the torch 109000 59000 290000 Customized gas cylinder 44000000 Insignificant Insignificant Commercial gas cylinder 126 000 76 000 290 000 60 ft by 30 ft Traditional welding with new ventilation system 162000 20000 N/A Mix at the torch 37 000 20000 62000 Customized gas cylinder 15000000 Insignificant Insignificant Commercial gas cylinder 42000 25000 62000
95 APPENDIX B DETAIL RESULTS OF THE MECHANICAL STRUCTURE TEST The chemical composition analys e s of the b aseline weld and ASE weld are shown in Table B 1 The wei ght percentages of the solutes: chromium (Cr), nickel (Ni), moly b d en um (Mo), manganes e (Mn), and other metals were within the standard limits for the 310 stainless steel plate and wire chemical composition. Table B 1 Composition profile of test welds and standard materials (%) Weld ID C Cr Ni Mo Mn Si P S Cu Baseline weld 0.073 26.5 20.0 0.14 1.19 0.47 0.016 0.0006 0.097 ASE weld 0.081 26.0 19.7 0.14 1.21 0.47 0.020 0.0007 0.10 ER310 consumables 0.25 24.0 26.0 19.0 22.0 N/A 2.00 1.50 0.045 0.030 N/ Standard 310 Plate 0.08 0.15 25.0 28.0 20.0 22.5 0.75 1.0 2.5 0.30 0.65 0.03 0.03 0.75 The macrostructures of the three baseline plates are shown in Figur e 4 5 A ~ C The microstructures of the weld zone in one baseline plate are shown in Figure B 1 No analyses were performed to determine the phases of the microstructure (Ferrite, Austenite and Bainite). The macrostructures reveal ed a typical welded me tal fe ature of columnar grains which was controlled by gro wth due to the high temperature, adjacent to fine grains of the parent metal and buffered by the heat affected zone (HAZ) The macrostructures of the three baseline plates were similar and did not appear to contain any weld defects.
96 Figure B 1 Microstructure of the weld zone in one plate (a) 5, (b) 10, (c) 20. Figure B 1 reveals a microstructure of welded metal with grains of hundreds of micrometers length adjacent to the HAZ and the fusion line (FL). The HAZ and FL are of finer grain size. The microstructures also indicate d the presence of inter metallic particles where the grains probably nucleated on them. This is a typical feature of particle s affected nucleation of grains. No defects were ev ident in the microstructures shown. The macrostructures of the ASE plates are shown in Figure 4 5 D ~ F The general features were similar to those of the baseline plates. However, a crack at the interface between the WM and the HAZ appeared and is shown in F igure 4 5 E The microstructure of this plate in Figure B 2 reveal ed the length of the crack to be several hundred microns. The TMS addition or problems with shielding gas or improper welding technique might have caused gas pockets which could have coalesce d into a crack.
97 Figure B 2 Microstructure of the weld zone in sample F541 (a) 5, (b) 10, (c) 20. The tensile data are shown in Table B 2 The ultimate strength values of 5 out of 6 baseline specimens were above 80ksi, the minimum value for the E310 (AWS specification A5.4). The tensile strength of the sample that did not reach the minimum tensile strength was 75.5 ksi (94.4% of the minimum value). It should be noted that when comparing the properties of all weld metal (AWM) tests with those of trans verse tests, the structures were different. In the AWM, there was a uniform material, while in the transverse samples were composites of base metal, HAZ and the weld zone. These regions would stretch unequally in a tensile test which was problematic for determining whether the mechanical tensile properties we re acceptable or not. Th is is why yield strength and percent elongation properties are not usually reported in a transverse tensile test. The ultimate tensile strength is usually reported for comparis on methods. The results indicated that significant defects existed in all the six plates and it was equally significant that most of the specimens (5 out of 6) in the baseline and 1/3 of the ASE specimens (2 out of 6) had their tensile strength values exce ed the 80 ksi minimum value. In summary, while the ASE technology does not significantly
98 deteriorate the mechanical quality of the welds, it probably can use some optimization of the different parameters to achieve the expected mechanical tensile parameter s. Table B 2 Tensile values of baseline and TMS test welds test specimen Standard material/Welds YS, ksi* UTS, ksi El, %* ER 310 stainless steel welding wire specification N/A 80 30 Baseline 44.8 86.5 37 44.8 81.0 22 46 .2 85.5 28 44.4 86.5 29 44.9 75.5 15 43.2 84.5 32 ASE technology 43.5 76.5 17 43.7 76.5 18 45.3 64.0 9.0 45.1 87.5 37 46.0 83.0 24 44.4 73.5 16 American Welding Society A5.9: Specification for Bare Stainless Steel Welding Electrodes and Rods. ** All the fractures in the tensile tests were ductile in the welded area. Ductile fractures are required to ensure that catastrophic brittle failures are avoided in the welded area.
99 APPENDIX C CHARACTERIZATION OF MERCURY IN CEMENT KILN BAGHOUSE F ILTERED DUST (BFD) AND THE RELEASE OF VAPOR PHASE MERCURY FROM CONCRETE PROCESSING WITH BFD ADDED CEMENT Abstract The fate of mercury (Hg) in cement processing and products has drawn intense attention due to its contribution to the ambient emission invento ry. Feeding Hg loaded and the practice of replacing 5% of cement with the Hg loaded BFD by cement plants has raised environmental and occupational health concerns. The objective of this study was to determin e Hg concentration and speciation in BFD, as well as to investigate the release of vapor phase Hg from storing and processing BFD added cement. The results showed that Hg in the BFD from different seasons ranged 0 .91~1.44 mg/kg ( ppm ), with 62~73% as soluble inorganic Hg while Hg in the other concrete constituents were 1~3 orders of magnitude low er than the BFD. The time series study showed up to 21% of Hg loss while storing the BFD in the open environment by the e nd of 7th day. Real time monitoring in the bench system indicated that high temperature and moisture can facilitate Hg release at the early stage. Ontario Hydro (OH) traps showed that Hg emission from BFD is related to the air exchange surface area. In the bench simulation of concrete processing, 0.4~0.5% of Hg escaped from mixing and curing BFD added cement. Follow up head space study did not detect Hg releas e in the following 7 days. In summary, replacing 5% of cement with the BFD investigated in this stu dy has minimal occupational health concern for concrete workers, and proper storing and mixing of BFD with cement can minimize Hg emission burden for the cement plant.
100 Introduction Fly ash from coal fired power plants is one of the main raw materials in th e pyroprocess of the Portland cement kiln, ( ) and it provides the desired plasticity, permeability, sulphate resistance, and durability for cement and concrete. ( Pistilli and Majko 1984 ; Kula et al. 2001 ) However, as a result of utilizing activated carbon injection in the power plant flue gas treatment system ( Hassett and Eylands 1999 ) coal fly ash now contains mercury (Hg) loaded carbon material. The addition of coal fly ash to the raw mill therefore leads to an undesired Hg emission from the cement kiln stack. ( Mlakar et al. 2010 ) The Environmental Protection Agency (E PA) established the National Emission Standards for Hazardous Air Pollutants ( NESHAP ) and the New Source Performance Standards (NSPS) for Portland cement kilns, with a limit of 55 or 21 lb Hg emission per million tons of clinker production for the existing or new cement kiln, respectively. ( USEPA 2010 ) To address the Hg emission issue, a filtration baghouse is typically used downstream in the cement kiln to remove the Hg loaded dust from the flue gas. ( USEPA 2011 ) A material flow chart in a typical cement plant is illustrated in Figure C 1. baghouse filtered dust (BFD) i s either landfilled or recycled back into the cement production loop. ( Siddique 2006 ; Linero 2011 ) Although no adverse effect to the cement kiln and the p roduct the accumulated Hg in the kiln may eventually increase the Hg emission from the stack to the ambient air. ( Senior et al. 2009 ) As the cement demand and production continue to grow ( Edwards 2012 ) in the United States with potential economic revival and increasing construction activities, the excessive contribution to the Hg emission inventory from ceme nt plants becomes an important issue to the environment and ecosystem.
101 Figure C 1. Simplified material flow of a typical cement kiln. The BFD from the baghouse filter was either recycled back to the raw mill or added to the final cement product. In lieu of recycling BFD to the kiln, some cement plants in the State of Florida have been practicing to directly add BFD to the final cement product in the ball mill. The practice is based on the hypothesis that the concrete made from the BFD added cement can im mobilize the Hg. BFD has similar property to lime, and research showed adding BFD up to 5% mass of the cement has no significant effect on the mechanical properties of concrete. ( Maslehuddin et al. 2009 ) While this practice theoretically may reduce the Hg emission to the ambien t air, few studies have investigated the environmental consequences of storing the BFD added cement and the exposure of the workers processing the BFD added cement. In addition, the ball mill mixing the BFD and cement may not have a dust filtration device as the rotary kiln has. This may consequently increase the Hg emission from the cement plant, due to the unfiltered flue gas. It was reported that the total Hg concentration in the BFD is similar to the coal fly ash added to the cement kiln. ( Siddique 2006 ) However, the recent popularity of
102 selective non catalytic reduction (SNCR) devic es employed in the coal fired power plants has changed the composition of the fly ash, with more ammonia and possibly Hg. Information about Hg concentration for the other concrete constituents is also scarce. These concrete constituents may have trace leve l Hg due to the possibility for Hg entering these materials either in the kiln (cement) or by natural occurrence (aggregates). In addition, it is difficult to determine the speciation of Hg due to the fact that the amount of total Hg presented in these mat erials is at trace level. However, the environmental mobility, bio accumulation, and toxicity of Hg all depend on the speciation. Alkyl Hg and soluble inorganic Hg (SI Hg) are much more mobile and toxic than the non soluble inorganic Hg (NSI Hg) that is pr edominantly elemental Hg. ( Langford and Ferner 1999 ) Among th e studies with the Hg contained cement, most focused on the leaching characteristics of curing the concrete while very few focused on the vapor phase Hg release. A head space study ( Hamilton and Bowers 1997 ) showed that vapor phase Hg was released from the Hg doped (0.2% wt / wt ) solidified cement monolith. The release rate of Hg was a function of time and temperature. The study also prompted that the vapor phase Hg release from cement monolith was possibly due to moisture and temperature increase during mixing and curing. However, the study was car ried out with a very high concentration of doped Hg (0.2% wt / wt ) in cement which is not likely to happen in the industrial setting. Another study ( Golightly et al. 2009 ) demonstrated that about 0.31% of total Hg was released from curing concrete, with 55% of the cement replaced by coal fly ash. A study ( Goodrow et al. 2005 ) in New Jersey revealed that the
103 total Hg contribution to the atmosphere from cement stabilized waste was negligible (<4%), although the release rate (130 kg/y) was on par with other industrial sources. The objective of this study was to characterize the Hg concentration and speciation in the BFD as well as other concrete constituents, by using different extraction methods and high sensitivity analytical equipment A bench s ystem was built to simulate the storage, mixing, curing of the BFD added cement, under different environmental conditions. Both real time monitoring and Ontario Hydro (OH) method were used to examine the amount of vapor phase Hg released. The environmental impact and occupational hazard of adding BFD to cement were assessed. Experimental Material Characterization Fresh BFD was sampled from different running seasons (Oct, 2011; Dec, 2011; Feb, 2012, Aug 2012.) of one cement kiln in the State of Florida. Com mercially available Portland cement, coarse aggregate (rocks), and fine aggregate (sands) were acquired from local retail stores. All the materials were stored in either desiccators or desiccated buckets. The materials were homogenized by a rotatory drum p rior to the experiment. The digestion procedure for materials were derived based on the sequential extraction procedure described in EPA Method 3200, ( USEPA 2005 ) while the analysis of the materials were bas ed on EPA Method 7474. ( USEPA 2007 ) For each type of constituent, fifteen of 0.5 g sample were weighed using an analytical scale (Sartorius MC210S, Goettingen, German) with a readability of 0.01 mg. The samples were then put into threaded cap Polytetrafluoroethylene (PTFE) tubes 10 mL of the extraction
104 solvent and acids for different Hg species (listed in Table C 1) were added to the PTFE tube, with five replicates for each extraction method. The PTFE tubes were heated up to about 100 C for 30 minutes in a microwave digest ion system ( CEM MDS 81D Matthews, NC ) The digests were diluted with deionized (DI) water to a pre determined level, to accommodate the upper/lower detection limits of the analytical instruments. The diluted solution was transferred into a 50 mL centrifug e tube and analyzed by a h ydri de g eneration atomic fluorescence s pectrome ter (HG AFS) (Aurora Biomed 3300, Vancouver, BC, Canada). The detection limit of the HG AFS was around 1 ng /mL. The Hg concentration in each type of samples was averaged from the fi ve replicates. Table C 1. Hg classification in this study Hg Species Environmental mobility Toxicity Extraction Soluble inorganic Hg (SI Hg) Hg 2+ (HgCl 2 HgSO 4 HgO Hg(NO 3 ) 2 Hg(OH) 2 ) Mobile Toxic 10% HCl Alkyl Hg Methyl Hg, Ethyl Hg Mobile High ly to xic Toluene Non soluble inorganic Hg (NSI Hg) Hg 0 Semi mobile Less toxic Calculated from mass balance HgS Hg 2 Cl 2 Non mobile Less toxic Total Hg All of the above Aqua regia (HNO 3 : HCl, 1:3 v/v) In addition to the characterization, a 7 day time seri es study was carried out on two batches of the BFD (Oct, 2011; Dec, 2011) to evaluate the Hg loss from the BFD by exposing them to natural weather ( ~ 30 C 60~70% relative humidity). The BFD was laid down on a flat surface as a thin layer and left outdoor s. A small portion of the sample was fetched on a daily basis during the 7 day period The retrieved sample was dried in the desiccator to remove excessive water absorbed on the BFD digested and
105 analyzed by HG AFS. The condition was used to simulate stori ng the BFD in an open area with indefinite head space. Bench System Real time measurement of vapor phase Hg release from the materials and concrete processing were performed separately in a small enclosed cylindrical tube and air tight glove box (Plas Lab 818 GB, Lansing, MI) with an interior volume of 489 L. The conceptual illustration of the bench system is shown in Figure C 2. In Figure C 2 A gas flows from two air cylinders were mixed at a mixing chamber to create a total flow of 1~2 liter per minute ( L pm ). The ratio of flow rates between water saturated air and dry air was used to control the relative humidity (RH), which was monitored by a hygrometer (Omega HX94C, Stamford, CT) inside the mixing chamber. The gas stream then went through either apparatu s in Figure C 2 B or C 2 C depending on whether measuring vapor phase Hg release from the materials or concrete processing. The small enclosed cylindrical tube with heating tape wrapped around (Figure C 2 B ) was used to test vapor phase Hg release from the B FD, cement, and BFD/Cement mixture under given flow rate, RH, and temperature. This limited head space study was to simulate storing the BFD in an enclosed environment. Simulation of concrete processing was carried out in the glove box shown in Figure C 2 C Thermometers mounted in the cylindrical tube and inside the concrete were used to monitor the temperature during the earlier stage of mixing.
106 Figure C 2. Breakdown schematic diagram of the bench system A ) controlled humidified air generator B ) te mperature controlled tube C ) glove box wit h concrete proce ssing inside D ) Hg transformation unit, real time Hg analyzer, and OH trap The gas stream carrying any released vapor phase Hg was sent through a Hg transformation unit (Figure C 2 D ) developed in a previous study. ( Li and Wu 2006 ) By
107 switching to 10% potassium chloride (KCl) solution, the soluble inorganic Hg (SI Hg) was absorbed in the impinger while on ly non soluble inorganic Hg (NSI Hg) passed the unit On the other channel, SI Hg was reduced to Hg 0 by 10% t in(II ) chloride (SnCl 2 ) solution. In this channel, all the vapor phase Hg passed through the unit. The measur ed difference of these two channels wa s the amount of SI Hg. 10% s odium hydroxide (NaOH) solution in the unit removed any acidic gas, and a condenser reduced the moisture content in the gas stream. Both were to prevent potential damage to the Hg analyzer. A real time Hg analyzer (Ohio Lumex RA 915 +, T winsburg, OH ) monitored the Hg concentration in the gas stream, with a resolution of 2 ng /m 3 s. The Hg analyzer was calibrated using a Hg permeation device (VICI Metrics Dynacal Poulsbo, WA) which can release Hg 0 vapor at a constant rate. The anal yzer was zeroed using ultrahigh purity nitrogen cylinder gas prior to each test. The vapor phase Hg passed the Zeema n atomic absorption spectrometer in the real time Hg analyzer and then entered the Ontario Hydro (OH) trap. Since SI Hg was either removed or reduced by the Hg transformation unit, the original OH sampling train was modified to skip the 10% KCl solution impingers. 100 mL of h ydrogen peroxide (H 2 O 2 ) solution and 100 mL potassium permanganate (KMnO 4 ) solution with acids were used to collect Hg 0 10% KCl solution was added to the KMnO 4 trap solution after the sampling, to remove the SO 2 produced from the reaction The OH trap was diluted and immediately analyzed by a c old vapor atomic fluorescence spectrometer (CV AFS) ( Tekran 2600, Toronto, ON Canada). The CV AFS with a detection limit of 1 pg /mL was capable of detect ing trace amount of Hg in the OH traps.
108 The bench system was used for the first 24 hours due to two reasons: the data record length limit of the real time Hg analyzer, and instabi lity of Hg in the OH trap while feeding continuous air. ( Laudal and Heidt 1997 ) A long term head space experiment was therefore performed using the glove box as a supplement to the 24 hour study. The solidified concrete was put into the box without air exc hange. After a pre set period, vacuum was used to pull 20 L air from the glove box through the OH trap. The concentration in the OH trap was used to calculate the Hg vapor released from the solidified concrete. Test Conditions 500 g each of BFD, cement, an d 5% BFD/95% cement was weighed and laid on the bottom surface of the cylindrical tube in Figure C 2 B In addition to 500 g of BFD, 100 g of BFD was also tested, which h ad a similar surface area exposed to the air flow in the cylindrical tube (Figure C 3). Two flow rates (1 and 2 Lpm) and four relative humitidties (0%, 25%, 75%, 100%) were employed. The temperature in the tube was either room temperature (~23 C ) or elevated by the heating tape (~80 C ). This experiment was designed to identify the potentia l effect of environmental parameters on storing BFD and cement prior to handling.
109 Figure C 3. Cross sectional and lateral view of the cylindrical tube loaded with 500 g and 100 g BFD Simulation of concrete processing was conducted in the glove box in F igure C 2 C using Feb, 2012 BFD sample. The mixing ratio of different constituents followed the original recipe for Portland cement concrete ( Kosmatka et al. 2002 ) The recipe of concrete was scaled down to fit the size of the glov e box and is listed in Table C 2. was fixed at 50%. During concrete processing, large amount of water was supplied to the mixture and the relative humidity was already at 100%. Air exchange rate of the glove box was set to 1.5 Lpm The stirring of BFD, cement, water, and aggregates using a hand mixer lasted about 3 minutes while the concrete remained undisturbed and became completely solidified in 8 hours. The concrete processing was repeated fi ve times using 5 % BFD/95% cement mixture and 10% BFD/90% cement mixture, respectively. After 24 hours of curing, one
110 of the concrete s from each mixing ratio was subjected to a 7 day head space study with OH trap. Table C 2. The recipe of the Portland ceme nt concrete (kg) used in this study Constituent 5% BFD/95% Cement 10% BFD/90% Cement Water 0.54 0.54 BFD 0.05 0.10 Cement 0.95 0.90 Coarse aggregates 1.82 1.82 Fine aggregates 1.45 1.45 Total 4.81 4.81 Quality Control and Statistics All the solutio ns and dilution were made with DI water from a Nanopure system (Barnstead Nanopure D11901, Waltham, MA) with a conductivity of 18.2 cm. The chemicals used were analytical grade or higher in purity. The reservoir, bench system components, and tubing used in the s tudy were ultrasonically cleaned to remove any residues. Pressurized air leak test was performed before conducting each experiment The calibration curve for the Hg analysis was prepared by diluting high purity 1000 mg/L solutions (Ricca Chemical, Arlington, TX) with an R 2 equal to or larger than 0.9999. Lab blank (10% HCl solution) was randomly inserted to sample batch of the HG AFS and the CV AFS to examine signal drift residues, and background Hg level All the samples were at least tri plicates and the results were averaged t test was used to examine the results from different samples using a significance level of p = 0.05. Statistical analyses were performed using the statistical software package (SAS 9.3, Cary, NC).
111 Results and Discussion Hg Concentration and Speciation in the Materials Table C 3 lists the Hg concentration and speciation in the BFD, cement, and other constituents in the concrete processing. Alkyl Hg was below the detection limit in all the samples, and therefore was eliminated from following studies. The high temperature and combustion condition in the cement kiln may decompose any organic phase compounds. Table C 3. Hg concentration and speciation in the concrete constituents Material Total Hg (g/kg) SI Hg (mg/kg) Percenta ge of SI Hg (%) BFD Oct 2011 910 60 560 20 61.5 Dec 2011 1430 70 1050 40 73.4 Feb 2012 1520 90 990 100 65.4 Aug 2012 1440 230 1030 100 71.5 Cement 74.51 6.24 47.02 8.94 63.1 Coarse aggregates (rocks) 4.32 1.52 2.63 1.16 60. 8 Fine aggregates (sand) 0.44 0.06 0.33 0.04 73.2 The total Hg concentration in the BFD ranged from 0.91~1.52 mg/kg (ppm) and was consistent in each season. More mobile and toxic SI Hg counted about 61.54~73.43% of total Hg in the samples while the rest was in NSI Hg phase. The total Hg concentration in the BFD was higher than previous ly measured value of 0.66 mg/kg (mean) in the cement kiln dust by Portland Cement Association (PCA). ( PCA 1992 ) It should be noted that Hg concentration in the BFD measured by PCA has a large deviation with some samples having up to 25.50 mg/kg total Hg The BFD from different cement kiln plants does not have the same characteristics. The variance of Hg in fed coal fly ash, raw materials, wastes, fuels could contribute to the difference among different seasons of the same cement plant.
112 The total Hg conce ntration was 74.51 g/kg ( ppb ) in the commercial cement. It was about one order of magnitude lower than the BFD. When mixing 5% of BFD with 95% of cement, the significance of Hg contributed from BFD diminished due to the small proportion. The concentration in the cement was also higher than the available studies ( Pistilli and Majko 1984 ; PCA 1992 ) in past decades; the latter showed Hg in the cement was averaging below 14 g/kg with the highest to be 39 g/kg. The aggregates in the co ncrete processing showed much less Hg presence compared to the BFD and the cement The coarse aggregates (rocks) and the fine aggregates (sands) had 4.32 g/kg and 0.44 g/kg of total Hg. Hg in the sands was just slightly above the detection limit of CV AF S. Those constituents were natural ly occurring materials and the characteristic can substantial ly vary from different geological locations. All the constituents had a relative constant SI Hg /Total Hg ratio of 0.6~0.7. Vapor Phase Hg Released from the BFD a nd Cement (Bench System) Figure C 4 shows the change of Hg concentration in the 7 day time series study in the open area using the BFD. The vaporization of NSI Hg loss was significant on the first day, from 0.36 mg/kg to 0.27 mg/kg for Oct, 2011 sample, an d from 0.25 mg/kg to 0.15 mg/kg for Dec, 2011 sample. The concentration of NSI Hg stayed relatively unchanged afterward. The SI Hg remained constant during the 7 day period. The total Hg loss was 13.83~17.52% on the first day, while 20.21~21.16% during the 7 day period, mostly contributed from NSI Hg vaporization. This corresponded to 30.77~57.14% on the first day and 43.59~65.71% during the 7 day of the total NSI Hg in the samples.
113 Figure C 4. Total Hg, SI Hg, and NSI Hg concentration in the 7 day time series study Figure C 5 shows the total Hg concentration in the gas stream as a function of time in 24 hours in the bench system for the BFD samples. Generally, significant Hg release was detectable only in the first 2 hours after experiment started. The concentration was at the background level (2 ng /m 3 s) afterward. High RH and temperature facilitated the Hg release in the earlier stage while the gas flow rates did not affect the release pattern. High temperature increased the diffusion activity while l owering the adsorption affinity between Hg and the dust. Water in high moist air may compete with Hg on adsorption surface. ( Li et al. 2011 ) Hg speciation results are not presented in Figure 5, due to the fact that early detectable release was mostly NSI Hg i.e. switching the channel in the Hg transformation unit gave an identical result. The Hg
114 analyzer could not detect any significant release from the cement and the 5% BFD/95% cement mixture under all experimental conditions. This was due to the lower total Hg co ncentration in these materials. Averagely, 1.6 0.4% of Hg in the 500 g BFD was released as vapor phase, while almost all released Hg was in NSI Hg phase ( p <0.05). The amount of total Hg released from the BFD and consequently captured b y OH trap in different experimental condition is listed in Table C 4. There was no statistical correlation found between total Hg released with any environmental parameter. This indicated the total Hg released in 24 hours were independent of gas flow rate, temperature, and RH. Furthermore, the amount of Hg released from 500 g and 100 g BFD were identical, regardless the mass difference in total Hg. It was possibly due to their similar area of air exchange (Figure C 3). This suggests the release was limited to the top layer and the corresponding effective thickness was calculated to be 0.12 cm. This implies that the release of Hg from BFD can be correlated to the degree of contact between air and dust. The amount of Hg released from the cement was below the d etection limit at most environmental conditions, due to the low Hg concentration in the cement. Similarly, the 5% BFD/95% cement mixture did not show any detectable vapor phase Hg released. The higher concentration of Hg and porous structure of the BFD lik ely contributed to the higher vapor phase Hg released and captured by the OH traps.
115 Figure C 5. Hg released from the BFD as a function of time A ) 1 Lpm gas flow rate and room temperature B ) 1 Lpm gas flo w rate and elevated temperature C ) 2 Lpm gas flow rate and room temperature D ) 2 Lpm gas flow rate and elevated temperature Table C 4. Total Hg (g) released from the BFD under different experimental conditions, in the OH trap Amount of BFD Air flow rate Temperature Dry air 25% RH 75% RH 100% RH 500 g 1 Lpm 23 C 18 (2.4%)* 9 (1.2%) 10 (1.3%) 15 (2.0%) 80 C 11 (1.5%) 5 (1.3%) 13 (1.7%) 7 (0.9%) 2 Lpm 23 C 13 (1.7%) 18 (2.4%) 14 (1.8%) 13 (1.7%) 80 C 11 (1.5%) 11(1.5%) 11 (1.5%) 13 (1.7%) 100 g 2 Lpm 80 C 13 (8.7%) The percentage in the parenthesis denotes total Hg released versus total Hg in the 500 g or 100 g Feb, 2012 BFD.
116 Vapor Phase Hg Release during Concrete Curing The release of Hg from mixing and curing cement was more complicated than the BFD and the cement The mixing process involved several minutes of high speed blending and numerous bubbles were created in the mixtures. Considerable amount of water was added into the mixture during this process. However, the temperature was raised only about 11 C during the mixing, which could be considered to be negligible In this study, the BFD and the cement contribute d 1% and 20% mass balance to the concrete mixture, respectively. The rest of the concrete mixture was coarse and fine aggregates, which contained much less Hg. The total mass of concrete constituents was 4.8 kg for each batch of experiment and the material contained about 0.15~0.22 mg of total Hg, from the calculation using mass fraction in Table C 2 and Hg concentration in Table C 3. During the mixing and curing of the 5% BFD/95% cement and the 10 % BFD/90% cement, there was no detectable Hg release activity from the real time monitoring. It may be due to the lower total Hg content in the concrete mixture (3.4 times less) than the 100% BFD. Total Hg released from the 1 0% BFD/90% cement mixture captured by the OH trap was 0.8 0.09 g slightly higher than 0.7 0.07 g from the 5% BFD/95% cement mixture ( p <0.05). Again, all total Hg released was in NSI Hg phase ( p <0.05). The total Hg released from the concrete mixing was a bout 0.4~0.5% of the total Hg in the concrete constituents. This was higher than the study ( Golightly et al. 2005 ) using coal fly ash added cement while the latter had 0.1% of Hg escaped. Powdered activated carbon in coal fly ash was usually not saturated with Hg due to low Hg carbon ratio, ( Galbreath and Zygarlicke 2000 ) and it increased the ability of retaining Hg in the
117 dust, thus possibly led to the difference between study results. There was no detectable Hg in the OH trap for the 7 day head space study on solidified concrete. It ind icated that the majority of Hg release occurred in the first 24 hour. Impact on the Ambient and Occupational Environment From the experimental results, storing BFD in an open area contributed the highest Hg loss up to 21%. The Hg loss is related to the deg ree of contact between air and BFD, i.e., area of air exchange surface. Hence it is recommended that cement kiln plant store unused BFD in a closed area to avoid additional Hg emission from the plant The ball mill where clinkers are grinded and BFD is a dded to the final product is constantly rotating. Thus, it creates good mixing between air and BFD. All the volatile Hg (NSI Hg) can be possibly released from rotating and feeding air. However, there will be likely minimum oxidized Hg (SI Hg) loss due to t he low temperature profile (~240 F/116 C) of the ball mill. Based on mass balance, there would be 30 lb of additional Hg emitted from the ball mill per million ton of cement produced, by adding 5% of BFD containing 1 mg/kg of Hg content to the ball mill, assuming conservatively all the 30% NSI Hg is released. This is above the EPA limit of 21 lb per million tons of clinker produced. ( USEPA 2010 ) Therefore, it is recommended that the plant should direct the flue gas from the ball mill to a Hg removal device to prevent undesired emission of Hg to the ambient air. The worst case scenario is that if the 73 million tons of cement US produced ( USGS 2013 ) in 2012 all contained 5% BFD, and without Hg removal device attached. This would create 2190 lb additional Hg emission. This is significant compared to the total Hg emission of 965 8.2 lb in 2008 ( EPA 2012 ) from the cement industry as estimated by EPA.
118 During concrete curing and after being solidified, less than 0.5% of Hg was released in the vapor phase. Assuming 5% of the cement is replaced with the BFD containing 1 mg/kg of Hg, about 0.05 mg Hg per ton of concrete could be released. In a typical scenario of workers placing and finishing the concrete floor (about 16.5 ton concrete) in an enclosed room (6.72 m in length, 5 m in width, 3 m in height), the maximum ceiling concentration of Hg is less than 0.01 mg/m 3 if no ventilation at a ll. This is much lower than the permissible exposure limit (PEL) of 0. 1 mg Hg/m 3 ceiling concentration set by the Occupational Safety and Health Administration (OSHA). ( OSHA 2006b ) For an ou tdoor concrete curing scenario, t he Hg will be instantly diluted by ambient air and should cause no violation to the PEL. In summary, caution should be exercised how the BFD should be stored and mixed to minimize Hg emission to the environment. Meanwhile, the occupational exposure is minimal while replacing 5% of the cement with the BFD c ontaining Hg concentration around 1 ppm. It should be noted that the total Hg concentration in the concrete constituents may vary from geological location, hence affecting the released amount. Understanding the mechanism how Hg is trapped in the concrete w ill provide knowledge how to avoid potential release from concrete in the long term. Field study is also essential to validate the laboratory study of vapor phase Hg release in the cement plant and concrete processing site. Acknowledgements T he study was funded by the Florida Department of Transportation (FDOT) through contract BDK75 977 50 The authors would like to thank Mr. Oliver H. Sohn and
119 Mr. Eduardo R Ferrer for assisting the sample and data collection. The authors are also grateful to Mr. Al Line ro of Florida Department of Environmental Protection for his valuable input.
120 LIST OF REFERENCES ACGIH (2012). Documentation of the Threshold Limit Values and Biological Exposure Indices Cincinnati, OH. Anderson, R. A. (1997). Chromium as an essential nutrient for humans. Regul. Toxicol. Pharm. 26:S35 S41. Antonini, J., Taylor, M., Zimmer, A. and Roberts, J. (2003). Pulmonary Responses to Welding Fumes: Role of Metal Constituents. J. Toxicol. Environ. Health Part A 67:233 249. Antonini, J. M., Clarke, R. W., Murthy, G. G. K., Sreekanthan, P., Jenkins, N., Eagar, T. W. and Brain, J. D. (1998). Freshly Generated Stainless Steel Welding Fume Induces Greater Lung Inflammation in Rats as Compared to Aged Fume. Toxicol. Lett. 98:77 86. Antonin i, J. M., Keane, M., Chen, B. T., Stone, S., Roberts, J. R., Schwegler Berry, D., Andrews, R. N., Frazer, D. G. and Sriram, K. (2011). Alterations in welding process voltage affect the generation of ultrafine particles, fume composition, and pulmonary toxi city. Nanotoxicology 5:700 710. Antonini, J. M., Stone, S., Roberts, J. R., Chen, B., Schwegler Berry, D., Afshari, A. A. and Frazer, D. G. (2007). Effect of Short term Stainless Steel Welding Fume Inhalation Exposure on Lung Inflammation, Injury, and Defe nse Responses in Rats. Toxicol. Appl. Pharmacol. 223:234 245. ASTM (2011a). E8/E8M 11: Standard Test Methods for Tension Testing of Metallic Materials. ASTM (2011b). E1019: Standard Test Methods for Determination of Carbon, Sulfur, Nitrogen, and Oxygen in Steel, Iron, Nickel, and Cobalt Alloys by Various Combustion and Fusion Techniques. Aston, J. G., Kennedy, R. M. and Messerly, G. H. (1941). the heat capacity and entropy, heats of fusion and vaporization and the vapor pressure of silicon tetramethyl. J. A m. Chem. Soc. 63:2343 2348. Atanassova, D., Stefanova, V. and Russeva, E. (1998). Co precipitative pre concentration with sodium diethyldithiocarbamate and ICP AES determination of Se, Cu, Pb, Zn, Fe, Co, Ni, Mn, Cr and Cd in water. Talanta 47:1237 1243. A TSDR (2008a). Draft toxicological profile for chromium, HHS, ed., Atlanta, GA. ATSDR (2008b). Draft toxicological profile for manganese, HHS, ed., Atlanta, GA. ATSDR (2005). Toxicological profile for nickel, HHS, ed., Atlanta, GA.
121 AWS (2012). A5.9 Specific ation for Bare Stainless Steel Welding Electrodes and Rods, American Welding Society. AWS (1998). B4.0 Standard Methods for Mechanical Testing of Welds, American Welding Society. AWS (2006). F1.2:2006 Laboratory Method for Measuring Fume Generation Rates a nd Total Fume Emission of Welding and Allied Processes, American Welding Society. Biswas, P., Owens, T. M. and Wu, C. Y. (1995). Control of toxic metal emissions from combustors using vapor phase sorbent materials. J. Aerosol Sci. 26:S217 S218. Biswas, P. and Wu, C. Y. (1998). Control of toxic metal emissions from combustors using sorbents: A review. J. Air Waste Manage. Assoc. 48:113. Biswas, P. and Wu, C. Y. (2005). Nanoparticles and the Environment. J. Air Waste Manage. Assoc. 55:708 746. Biswas, P., Wu, C. Y., Zachariah, M. R. and McMillin, B. (1997). Characterization of Iron Oxide silica Nanocomposites in Flames: Part II. Comparison of Discrete sectional Model Predictions to Experimental Data. J. Mater. Res. 12:714 772. Biswas, P. and Zachariah, M. R. ( 1997). In situ immobilization of lead species in combustion environments by injection of gas phase silica sorbent precursors. Environ. Sci. Technol. 31:2455 2463. BLS (2008). National employment matrix, US Department of Labor, Washington, DC. Boevski, I., Daskalova, N. and Havezov, I. (2000). Determination of barium, chromium, cadmium, manganese, lead and zinc in atmospheric particulate matter by inductively coupled plasma atomic emission spectrometry (ICP AES). Spectrochim. Acta, Part B 55:1643 1657. Castn er, H. R. and Null, C. L. (1998). Chromium, Nickel and Manganese in Shipyard Welding Fumes. Weld. J. 77:223 231. CDC (2008). National occupational research agenda: Construction. Chen, J. C., Wey, M. Y. and Ou, W. Y. (1999). Capture of heavy metals by sorbe nts in incineration flue gas. Sci. Total Environ. 228:67 77. Cho, H. W., Yoon, C. S., Lee, J. H., Lee, S. J., Viner, A. and Johnson, E. W. (2011). Comparison of Pressure Drop and Filtration Efficiency of Particulate Respirators using Welding Fumes and Sodi um Chloride. Ann. Occup. Hyg. 55:666 680.
122 Correa Duarte, M. A., Giersig, M. and Liz Marzn, L. M. (1998). Stabilization of CdS Semiconductor Nanoparticles Against Photodegradation by a Silica Coating Procedure. Chem. Phys. Lett. 286:497 501. Costantini, L. M., Gilberti, R. M. and Knecht, D. A. (2011). The Phagocytosis and Toxicity of Amorphous Silica. PLoS ONE 6:e14647. Craig, B. D. and Anderson, D. S. (1995). Handbook of corrosion data ASM International, Materials Park, OH. Dams, R., Rahn, K. A. and Winch ester, J. W. (1972). Evaluation of filter materials and impaction surfaces for nondestructive neutron activation analysis of aerosols. Environ. Sci. Technol. 6:441 448. additives in concrete mixtures. Cem. Concr. Res. 26:1737 1744. Dennis, J. H., French, M. J., Hewitt, P. J., Mortazavi, S. B. and Redding, A. J. (1996). Reduction of hexaval ent chromium concentration in fumes from mteal cored arc welding by addition of reactive metals. Ann. Occup. Hyg. 40:339 344. Dennis, J. H., French, M. J., Hewitt, P. J., Mortazavi, S. B. and Redding, C. A. J. (2002). Control of Exposure to Hexavalent Chro mium and Ozone in Gas Metal Arc Welding of Stainless Steels by Use of A Secondary Shield Gas. Ann. Occup. Hyg. 46:43 48. Dennis, J. H., Mortazavi, S. B., French, M. J., Hewitt, P. J. and Redding, C. R. (1997). The effects of welding parameters on ultraviol et light emissions, ozone and CrVI formation in MIG welding. Ann. Occup. Hyg. 41:95 104. Dulski, T. R. (1996). A manual for the chemical analysis of metals ASTM, West Conshohocken, PA. E.P.A., U. S. (1996). EPA Method 3052, Microwave Assisted Digestion of Siliceous and Organically based Matrices, U.S. Government Printing Office, Washington, DC. Ebrahimnia, M., Goodarzi, M., Nouri, M. and Sheikhi, M. (2009). Study of the effect of shielding gas composition on the mechanical weld properties of steel ST 37 2 in gas metal arc welding. Mater. Des. 30:3891 3895. Edwards, P. (2012). Cement in the USA, in Global Cement Magazine EPA (2012). 2008 National emissions inventory data & documentation. Filgueiras, A. V., Capelo, J. L., Lavilla, I. and Bendicho, C. (2000). Comparison of ultrasound assisted extraction and microwave assisted digestion for determination of magnesium, manganese and zinc in plant samples by flame atomic absorption spectrometry. Talanta 53:433 441.
123 Flynn, M. R. and Susi, P. (2012). Local Exhaust Ventilation for the Control of Welding Fumes in the Construction Industry A Literature Review. Ann. Occup. Hyg. 56:764 776. Flynn, M. R. and Susi, P. (2009). Neurological Risks Associated with Manganese Exposure from Welding Operations A Literature Rev iew. Int. J. Hyg. Environ. Health 212:459 469. Frederick, W. J., Ling, A., Tran, H. N. and Lien, S. J. (2004). Mechanisms of sintering of alkali metal salt aerosol deposits in recovery boilers. Fuel 83:1659 1664. Fu, W., Yang, H., Hari, B., Liu, S., Li, M. and Zou, G. (2006). Preparation and characteristics of core shell structure cobalt/silica nanoparticles. Mater. Chem. Phys. 100:246 250. Galbreath, K. C. and Zygarlicke, C. J. (2000). Mercury transformations in coal combustion flue gas. Fuel Process. Tech nol. 65 66:289 310. Geiser, M. and Kreyling, W. (2010). Deposition and Biokinetics of Inhaled Nanoparticles. Part. Fibre Toxicol. 7:2. Golightly, D. W., Cheng, C. M., Weavers, L. K., Walker, H. W. and Wolfe, W. E. (2009). Fly ash properties and mercury sor bent affect mercury release from curing concrete. Energy Fuels 23:2035 2040. Golightly, D. W., Sun, P., Cheng, C. M., Taerakul, P., Walker, H. W., Weavers, L. K. and Golden, D. M. (2005). Gaseous mercury from curing concretes that contain ory measurements. Environ. Sci. Technol. 39:5689 5693. Gonzlez, P., Fernndez, D., Pou, J., Garca, E., Serra, J., Len, B. and Prez Amor, M. (1992). Photo induced Chemical Vapour Deposition of Silicon Oxide Thin Films. Thin Solid Films 218:170 181. Good row, S. M., Miskewitz, R., Hires, R. I., Eisenreich, S. J., Douglas, W. S. and Reinfelder, J. R. (2005). Mercury emissions from cement stabilized dredged material. Environ. Sci. Technol. 39:8185 8190. Graf, C., Vossen, D. L. J., Imhof, A. and van Blaaderen A. (2003). A General Method To Coat Colloidal Particles with Silica. Langmuir 19:6693 6700. Gray, C. N., Goldstone, A., Dare, P. R. M. and Hewitt, P. J. (1983). The evolution of hexavalent chromium in metallic aerosols. Am. Ind. Hyg. Assoc. J. 44:384 3 88. Gullett, B. K. and Raghunathan, K. (2002). Reduction of Coal Based Metal Emissions by Furnace Sorbent Injection. Energy Fuels 8:1068 1076.
124 Guo, B., Yim, H., Khasanov, A. and Stevens, J. (2010). Formation of Magnetic FexOy/Silica Core Shell Particles in a One Step Flame Aerosol Process. Aerosol Sci. Technol. 44:281 291. Hall, S. R., Davis, S. A. and Mann, S. (1999). Co condensation of organosilica hybrid shells on nanoparticle templates: a direct synthetic route to functionalized core shell colloids. L angmuir 16:1454 1456. Hamilton, W. P. and Bowers, A. R. (1997). Determination of acute Hg emissions from solidified/stabilized cement waste forms. Waste Manage. 17:25 32. Hansen, K. and Stern, R. M. (1985). Welding fumes and chromium compounds in cell tran sformation assays. J. Appl. Toxicol. 5:306 314. Harris, R. C., Lundin, J. I., Criswell, S. R., Hobson, A., Swisher, L. M., Evanoff, B. A., Checkoway, H. and Racette, B. A. (2011). Effects of Parkinsonism on Health Status in Welding Exposed Workers. Parkins onism Relat. Disord. 17:672 676. Hassett, D. J. and Eylands, K. E. (1999). Mercury capture on coal combustion fly ash. Fuel 78:243 248. Hewett, P. (1995). The Particle Size Distribution, Density and Specific Surface Area of Welding Fumes from SMAW And GMAW Mild steel and Stainless steel Consumables. Am. Ind. Hyg. Assoc. J. 56:128 135. Hewitt, P. J. and Hirst, A. A. (1993). A Systems Approach to the Control of Welding Fumes at Source. Ann. Occup. Hyg. 37:297 306. IARC (1990). Monographs on the Evaluation of Carcinogenic Risks to Humans: Chromium, Nickel and Welding ICRP (1994). Human Respiratory Tract Model for Radiological Protection. Anals of the ICRP, Publication 66 Ikvalko, E., Laitinen, T. and Revitzer, H. (1999). Optimised method of coal digestion fo r trace metal determination by atomic absorption spectroscopy. Fresenius J. Anal. Chem. 363:314 316. Iler, R. K. (1979). The Chemistry of Silica: Solubility, Polymerization, Colloid and Surface Properties and Biochemistry of Silica Wiley Interscience. ISO (1995). Extraction of Trace Elements Soluble in Aqua Regia, ISO 11466. IUPAC (1997). Detection Limit, in Compendium of Chemical Terminology Jie, S., De ren, W., Ye dong, H., Hui bin, Q. and Gao, W. (2008). Reduction of oxide scale on hot rolled strip ste els by carbon monoxide. Mater. Lett. 62:3500 3502.
125 Kasprzak, K. S., Sunderman Jr, F. W. and Salnikow, K. (2003). Nickel carcinogenesis. Mutation Research/Fundamental and Molecular Mechanisms of Mutagenesis 533:67 97. Keane, M., Stone, S. and Chen, B. (2010 ). Welding fumes from stainless steel gas metal arc processes contain multiple manganese chemical species. J Environ Monitor 12:1133 1140. Keskinen, H., Kalliomki, P. L. and Alanko, K. (1980). Occupational Asthma Due to Stainless Steel Welding Fumes. Clin Exp. Allergy 10:151 159. Kim, S., Harrington, M. and Pui, D. (2007). Experimental study of nanoparticles penetration through commercial filter media, in Nanotechnology and Occupational Health A. D. Maynard and D. Y. H. Pui, eds., Springer Netherlands, 1 17 125. Kittelson, D. B., Watts, W. F. and Johnson, J. P. (2006). On road and laboratory evaluation of combustion aerosols -Part1: Summary of diesel engine results. J. Aerosol Sci. 37:913 930. Korczynski, R. E. (2000). Occupational health concerns in the w elding industry. Appl. Occup. Environ. Hyg. 15:936 945. Kosmatka, S. H., Kerkhoff, B. and Panarese, W. C. (2002). Design and control of concrete mixtures Portland Cement Association, Skokie, IL. Kropschot, S. J. and Doebrich, J. (2010). Chromium Makes s tainless steel stainless USGS Mineral Resources Program. Kula, I., Olgun, A., Erdogan, Y. and Sevinc, V. (2001). Effects of colemanite waste, cool bottom ash, and fly ash on the properties of cement. Cem. Concr. Res. 31:491 494. Kuss, H. M. (1992). Applica tions of microwave digestion technique for elemental analyses. Fresenius J. Anal. Chem. 343:788 793. Lamble, K. and Hill, S. (1998). Microwave digestion procedures for environmental matrices Critical Review. Analyst 123:103R 133R. Lampman, S., ed. (1997) Weld Integrity and Performance ASM International Materials Park, OH. Langrd, S. (1988). Chromium carcinogenicity: A review of experimental animal data. Sci. Total Environ. 71:341 350. Langford, N. and Ferner, R. (1999). Toxicity of mercury. J. Hum. Hyp ertens. 13:651 656.
126 Laohaudomchok, W., Lin, X., Herrick, R. F., Fang, S. C., Cavallari, J. M., Shrairman, R., Landau, A., Christiani, D. C. and Weisskopf, M. G. (2011). Neuropsychological Effects of Low level Manganese Exposure in Welders. NeuroToxicology 32:171 179. Laudal, D. L. and Heidt, M. K. (1997). Evaluation of flue gas mercury speciation method, Energy and Environmental Research Center, Grand Forks, ND. Lavoisier, A. (1984). Elements of Chemistry Dover Publications Lee, M. H., McClellan, W., Cand ela, J., Andrews, D. and Biswas, P. (2007). Reduction of Nanoparticle Exposure to Welding Aerosols by Modification of the Ventilation System in a Workplace. J. Nanopart. Res. 9:127 136. Lee, S. R. and Wu, C. Y. (2005). Size Distribution Evolution of Fine A erosols Due to Intercoagulation with Coarse Aerosols. Aerosol Sci. Technol. 39:358 370. Li, G. J., Zhang, L. L., Lu, L., Wu, P. and Zheng, W. (2004). Occupational Exposure to Welding Fume among Welders: Alterations of Manganese, Iron, Zinc, Copper, and Lea d in Body Fluids and the Oxidative Stress Status. J. Occup. Environ. Med. 46:241 248. Li, H., Li, Y., Wu, C. Y. and Zhang, J. (2011). Oxidation and capture of elemental mercury over SiO 2 TiO 2 V 2 O 5 catalysts in simulated low rank coal combustion flue gas. C hem. Eng. J. 169:186 193. Li, Y. and Wu, C. Y. (2006). Role of moisture in adsorption, photocatalytic oxidation, and reemission of elemental mercury on a SiO2 TiO2 nanocomposite. Environ. Sci. Technol. 40:6444 6448. Linero, A. (2011). Synopsis of mercury ( Hg) controls at Florida cement plants, in 104th Air and Waste Management Association Annual Conference and Exhibition Orlando, FL, 782. Lippold, J. C. and Frankel, G. S. (2009). SERDP Project Report PP 1415: Development of chromium free welding consumable s for stainless steels, Ohio State University, 2009, Columbus, OH. Liz Marzn, L. M., Giersig, M. and Mulvaney, P. (1996). Synthesis of Nanosized Langmuir 12:4329 4335. Lu, Y., Yin, Y., Li, Z. Y. and Xia, Y. (2002). Synthe sis and Self Assembly of Au SiO2 Nano Lett. 2:785 788. Lundborg, M., Dahln, S. E., Johard, U., Gerde, P., Jarstrand, C., Camner, P. and Lstbom, L. (2006). Aggregates of Ultrafine Particles Impair Phagocytosis of Microorganisms by Hum an Alveolar Macrophages. Environ. Res. 100:197 204.
127 Marshall, P. (1984). Austenitic Stainless Steels: Microstructure and Mechanical Properties Elsevier Applied Science, London, UK. Maslehuddin, M., Al Amoudi, O. S. B., Rahman, M. K., Ali, M. R. and Barry, M. S. (2009). Properties of cement kiln dust concrete. Constr. Build. Mater. 23:2357 2361. Mayer, C., Klein, R. G., Wesch, H. and Schmezer, P. (1998). Nickel subsulfide is genotoxic in vitro but shows no mutagenic potential in respiratory tract tissues of Mutation Research/Genetic Toxicology and Environmental Mutagenesis 420:85 98. Maynard, A. D., Ito, Y., Arslan, I., Zimmer, A. T., Browning, N. and Nicholls, A. (2004). Examining Elemental Surface Enrichment in Ultrafine Aerosol Particles Using Analytical Scanning Transmission Electron Microscopy. Aerosol Sci. Technol. 38:365 381. McLaren, J. W. and Berman, S. S. (1985). Wavelength selection for trace analysis by ICP AES. Spectrochim. Acta, Part B 40:217 225. McMillin, B. K., Biswas, P. and Zachariah, M. R. (1996). In situ characterization of vapor phase growth of iron oxide silica nanocomposites: Part I. 2 D planar laser induced fluorescence and Mie imaging. J. Mater. Res. 11:1552 1561. Meeker, J. D., Susi, P. and Flynn, M. R. (2010). Hexavalent chromium exposure and control in welding tasks. J. Occup. Environ. Hyg. 7:607 615. Menzel, M. (2003). The influence of individual components of an industrial gas mixture on the welding process and the prop erties of welded joints. Weld. Int. 17:262 264. Menzel, N., Schramel, P. and Wittmaack, K. (2002). Elemental composition of aerosol particulate matter collected on membrane filters: A comparison of results by PIXE and ICP AES. Nucl. Instrum. Methods Phys. Res., Sect. B 189:94 99. (2010). Mercury species, mass flows and processes in a cement plant. Fuel 89:1936 1945. Moroni, B. and Viti, C. (2009). Grain Size, Chemistry and Structure of Fine and Ultrafine Particles in Stainless Steel Welding Fumes. J. Aerosol Sci. 40:938 949. Myhnen, T., Mntylahti, V., Koivunen, K. and Matilainen, R. (2002). Simultaneous determination of As, Cd, Cr and Pb in aqua regia digests of soi ls and sediments using electrothermal atomic absorption spectrometry and fast furnace programs. Spectrochim. Acta, Part B 57:1681 1688.
128 Nadkarni, R. A. (1984). Applications of microwave oven sample dissolution in analysis. Anal. Chem. 56:2233 2237. Niitsoo O. and Couzis, A. (2011). Facile Synthesis of Silver Core Silica Shell Composite Nanoparticles. J. Colloid Interface Sci. 354:887 890. NIOSH (2008). Criteria document update: Occupational exposure to hexavalent chromium, Cincinnati, OH. NIOSH (1994). M ethod 7604: chromium, hexavalent, Cincinnati, OH:. NIOSH (2013). NIOSH Publication No. 2013 128: Criteria for a Recommended Standard: Occupational Exposure to Hexavalent Chromium, Cincinnati, OH. NIOSH (2007). Pocket Guide to Chemical Hazards, Cincinnati, OH. Oberdorster, G., Maynard, A., Donaldson, K., Castranova, V., Fitzpatrick, J., Ausman, K., Carter, J., Karn, B., Kreyling, W., Lai, D., Olin, S., Monteiro Riviere, N., Warheit, D. and Yang, H. (2005). Principles for characterizing the potential human he alth effects from exposure to nanomaterials: elements of a screening strategy. Particle and Fibre Toxicology 2:8. Oberdrster, G., Stone, V. and Donaldson, K. (2007). Toxicology of nanoparticles: A historical perspective. Nanotoxicology 1:2 25. Ohmori, M. and Matijevic, E. (1992). Preparation and properties of uniform coated colloidal particles. VII. Silica on hematite. J. Colloid Interface Sci. 150:594 598. Okorie, A., Entwistle, J. and Dean, J. R. (2010). The optimization of microwave digestion procedures and application to an evaluation of potentially toxic element contamination on a former industrial site. Talanta 82:1421 1425. Oller, A. R., Kirkpatrick, D. T., Radovsky, A. and Bates, H. K. (2008). Inhalation carcinogenicity study with nickel metal powde r in Wistar rats. Toxicology and Applied Pharmacology 233:262 275. OSHA (2006a). Chromium (VI), in Toxic and Hazardous Substances, Occupational Safety and Health Standards OSHA (2006b). Mercury, in Toxic and Hazardous Substances, Occupational Safety and H ealth Standards OSHA (2006c). Nitrogen dioxide, in Toxic and Hazardous Substances, Occupational Safety and Health Standards Owens, T. M. and Biswas, P. (1996). Vapor phase sorbent precursors for toxic metal emissions control from combustors. Industrial & Engineering Chemistry Research 35:792 798.
129 Paulson, K. and Wu, C. Y. (2012). ESTCP Project Report WP 0903: Innovative welding technologies to control HAP emissions using silicon additives, University of Florida, Gainesville, FL. Paulson, K. M., Wang, J., Topham, N., Wu, C. Y., Alexandrov, B. T., Lippold, J. C. and Es Said, O. S. (2011). Alternatives for Joining Stainless Steel to Reduce Cr(VI) Emissions and Occupational Exposures. J. Ship Prod. Des. 27:91 97. PCA (1992). An analysis of selected trace metal s in cement and kiln dust, Portland Cement Association, Skokie, IL, 4. Peckner, D. and Bernstein, I. M. (1977). Handbook of Stainless Steel McGraw Hill. Pellerin, C. and Booker, S. M. (2000). Reflections on hexavalent chromium: Health hazards of an indust rial heavyweight. Environ. Health Perspect. 108. Pistilli, M. and Majko, R. (1984). Optimizing the amount of Class C fly ash in concrete mixtures 6. Pocock, D., Saunders, C. J. and Carter, G. (2009). Effective control of gas shielded arc welding fume, H. a S. Executive, ed. Rice, M. B., Cavallari, J., Fang, S. and Christiani, D. (2011). Acute Decrease in HDL Cholesterol Associated With Exposure to Welding Fumes. J. Occup. Environ. Med. 53:17 21. Richman, J. D., Livi, K. J. T., Spannhake, E. W., Macri, K. K ., Torrey, C. M. and Geyh, A. S. (2008). Assessment of Pulmonary Exposure to Manganese: Uptake of Metals from Inhaled Welding Fumes into the Circulatory System. Epidemiology 19:S262 S262. Rubio, R., Huguet, J. and Rauret, G. (1984). Comparative study of th e Cd. Cu and Pb determination by AAS and by ICP AES in river water : Application to a mediterranean river (Congost river, Catalonia, Spain). Water Res. 18:423 428. Sandroni, V., Smith, C. M. M. and Donovan, A. (2003). Microwave digestion of sediment, soils and urban particulate matter for trace metal analysis. Talanta 60:715 723. Schoonover, T., Conroy, L., Lacey, S. and Plavka, J. (2011). Personal Exposure to Metal Fume, NO 2 and O 3 among Production Welders and Non welders. Ind. Health 49:63 72. Scott, D. R., Loseke, W. A., Holboke, L. E. and Thompson, R. J. (1976). Analysis of Atmospheric Particulates for Trace Elements by Optical Emission Spectrometry. Appl. Spectrosc. 30:392 405.
130 Senior, C., Montgomery, C. J. and Sarofim, A. (2009). Transient m odel for behavior of mercury in Portland cement kilns. Ind. Eng. Chem. Res. 49:1436 1443. Sethi, V. and Biswas, P. (1990). Modeling of Particle Formation and Dynamics in a Flame Incinerator. J. Air Waste Manage. Assoc. 40:42 46. Siddique, R. (2006). Utiliz ation of cement kiln dust (CKD) cement mortar and concrete an overview. Resour. Conserv. Recycl. 48:315 338. Sievering, H., Dave, M. J., McCoy, P. G. and Walther, K. (1978). Cellulose filter high volume cascade impactor aerosol collection efficiency. A tec hnical note. Environ. Sci. Technol. 12:1435 1437. Sriram, K., Lin, G. X., Jefferson, A. M., Roberts, J. R., Chapman, R. S., Chen, B. T., Soukup, J. M., Ghio, A. J. and Antonini, J. M. (2010). Dopaminergic neurotoxicity following pulmonary exposure to manga nese containing welding fumes. Arch. Toxicol. 84:521 540. Summers, M. J., Summers, J. J., White, T. F. and Hannan, G. J. (2011). The Effect of Occupational Exposure to Manganese Dust and Fume on Neuropsychological Functioning in Australian Smelter Workers. J. Clin. Exp. Neuropsychol. 33:692 703. Tan, S. H. and Horlick, G. (1987). Matrix effect observations in inductively coupled plasma mass spectrometry. J. Anal. At. Spectrom. 2:745 763. Teleki, A., Suter, M., Kidambi, P. R., Ergeneman, O., Krumeich, F., Ne lson, B. J. and Pratsinis, S. E. (2009). Hermetically Coated Superparamagnetic Fe 2 O 3 Particles with SiO2 Nanofilms. Chem. Mater. 21:2094 2100. Thaon, I., Demange, V., Herin, F., Touranchet, A. and Paris, C. (2012). Increased Lung Function Decline in Blue c ollar Workers Exposed to Welding Fumes. Chest Todoli, J. L., Gras, L., Hernandis, V. and Mora, J. (2002). Elemental matrix effects in ICP AES. J. Anal. At. Spectrom. 17:142 169. Topham, N., Kalivoda, M., Hsu, Y. M., Wu, C. Y., Oh, S. and Cho, K. (2010). R educing Cr 6+ emissions from gas tungsten arc welding using a silica precursor. J. Aerosol Sci. 41:326 330. Topham, N., Wang, J., Kalivoda, M., Huang, J., Yu, K. M., Hsu, Y. M., Wu, C. Y., Oh, S., Cho, K. and Paulson, K. (2011). Control of Cr 6+ Emissions fr om Gas Metal Arc Welding Using a Silica Precursor as a Shielding Gas Additive. Ann. Occup. Hyg. 56:242 252. USEPA (2010). 75 FR 54970: National emission standards for hazardous air pollutants from the portland cement manufacturing industry and standards of performance for portland cement plants; Final rule, Washington, DC.
131 USEPA (2011). Materials characterization paper in support of the final rulemaking: identification of nonhazardous secondary materials that are solid waste cement kiln dust (CKD), Washin gton, DC. USEPA (2005). Method 3200: Mercury species fractionation and quantification by microwave assisted extraction, selective solvent extraction and/or solid phase extraction, Washington, DC. USEPA (2007). Method 7474: Mercury in sediment and tissue sa mples by atomic fluorescence spectrometry, Washington, DC. USEPA (1998). Toxicological review of hexavalent chromium, Washington, DC. USGS (2013). Cement statistics and information. Vitek, J. M. and David, S. A. (1987). The aging behavior of homogenized ty pe 308 and 308CRE stainless steel. Metall. Trans. A 18A:7:1195 1202. Wallace, M., Shulman, S. and Sheehy, J. (2001). Comparing exposure levels by type of welding operation and evaluating the effectiveness of fume extraction guns. Appl. Occup. Environ. Hyg. 16:771 779. Wang, J., Kalivoda, M., Guan, J., Theodore, A., Sharby, J., Wu, C. Y., Paulson, K. and Es Said, O. (2012). Double Shroud Delivery of Silica Precursor for Reducing Hexavalent Chromium in Welding Fume. J. Occupy. Environ. Hyg. 9:733 742. Wang, J ., Topham, N. and Wu, C. Y. (2011). Determination of silica coating efficiency on metal particles using multiple digestion methods. Talanta 85:2655 2661. Webb, E., Amarasiriwardena, D., Tauch, S., Green, E. F., Jones, J. and Goodman, A. H. (2005). Inductiv ely coupled plasma mass (ICP MS) and atomic emission spectrometry (ICP AES): Versatile analytical techniques to identify the archived elemental information in human teeth. Microchem. J. 81:201 208. Williams, D. B. and Carter, C. B. (2009). Transmission Ele ctron Microscopy: A Textbook for Materials Science Springer, New York, NY. WTIA (1999). Fume minimisation guidelines: welding, cutting, brazing and soldering. Wu, S., Zhao, Y. H., Feng, X. and Wittmeier, A. (1996). Application of inductively coupled plasm a mass spectrometry for total metal determination in silicon containing solid samples using the microwave assisted nitric acid hydrofluoric acid hydrogen peroxide boric acid digestion system. J. Anal. At. Spectrom. 11:287 296. Xu, Y. H., Iwashita, A., Naka jima, T., Yamashita, H., Takanashi, H. and Ohki, A. (2005). Effect of HF addition on the microwave assisted acid digestion for the
132 determination of metals in coal by inductively coupled plasma atomic emission spectrometry. Talanta 66:58 64. Yi, D. K., Selv an, S. T., Lee, S. S., Papaefthymiou, G. C., Kundaliya, D. and Ying, J. Y. (2005). Silica Coated Nanocomposites of Magnetic Nanoparticles and Quantum Dots. J. Am. Chem. Soc. 127:4990 4991. Yu, I. J., Kim, K. J., Chang, H. K., Song, K. S., Han, K. T., Han, J. H., Maeng, S. H., Chung, Y. H., Park, S. H., Chung, K. H., Han, J. S. and Chung, H. K. (2000). Pattern of Deposition of Stainless Steel Welding Fume Particles Inhaled into the Respiratory Systems of Sprague Dawley Rats Exposed to a Novel Welding Fume Ge nerating System. Toxicol. Lett. 116:103 111. Yu, K. M., Topham, N., Wang, J., Kalivoda, M., Tseng, Y., Wu, C. Y., Lee, W. J. and Cho, K. (2011). Decreasing biotoxicity of fume particles produced in welding process. J. Hazard. Mater. 185:1587 1591. Yu, K., Grabinski, C., Schrand, A., Murdock, R., Wang, W., Gu, B., Schlager, J. and Hussain, S. (2009). Toxicity of Amorphous Silica Nanoparticles in Mouse Keratinocytes. J. Nanopart. Res. 11:15 24. Zatka, V. J. (1985). Speciation of hexavalent chromium in welding fumes interference by air oxidation of chromium. Am. Ind. Hyg. Assoc. J. 46:327 331. Zeidler Erdely, P. C., Battelli, L. A., Stone, S., Chen, B. T., Frazer, D. G., Young, S. H., Erdely, A., Kashon, M. L., Andrews, R. and Antonini, J. M. (2011). Short term Inhalation of Stainless Steel Welding Fume Causes Sustained Lung Toxicity But No Tumorigenesis in Lung Tumor Susceptible A/J Mice. Inhalation Toxicol. 23:112 120. Zeidler Erdely, P. C., Kashon, M. L., Battelli, L. A., Young, S. H., Erdely, A., Roberts, J. R., Reynolds, S. H. and Antonini, J. M. (2008). Pulmonary inflammation and tumor induction in lung tumor susceptible A/J and resistant C57BL/6J mice exposed to welding fume. Part. Fibre Toxicol. 5. Zimmer, A. T. and Biswas, P. (2001). Characterization of the Aerosols Resulting from Arc Welding Processes. J. Aerosol Sci. 32:993 1008.
133 BIOGRAPHICAL SKETCH Jun Wang was born in Shanxi Province, China in J une 1984. He graduated from Nankai University twice, with a Bachelor of Engineering in environmental engi neering (2006), and a Master of Science in environmental management and economics (2009). From 2005 to 2009, he was actively involved in various studies at China State Environmental Protection Key Laboratory of Particulate Matters Pollution and Control. Th e research resulted in one first authored journal paper, one co authored journal paper, and two co authored books. He also worked as an instructor for environmental management courses at Tianjin Bohai Vocational College from 2006 to 2007. Jun joined Dr. Ch ang summer of 2009. His primary research was about welding fume control technologies, while also participated in several other projects. Du ring the four year doctoral study, h e published several fi rst autho red journal paper s He received various prestigious academic scholarships, travel grants and poster competition awards from different professional organizations. He served in the professional community, in the form of conference committee member, conference abstract reviewer, peer reviewed journal reviewer, student chapter president, and conference student volunteer. He graduated in Aug 2013 with a Doctor of Philosophy (Ph.D.) in environmental engineering sciences from the University of Florida, a nd immediately join ed the University of Oklahoma Health Science Center as tenure track assistant professor.