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1 SETTLING OF SUPERNATANT FROM SEMIC ONTINUOUS ELECTROKINETIC PROCESS ING OF PHOSPHATIC CLAY SUSPENSION S By PEI HAN CHIU A THESIS PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DE GREE OF MASTER OF SCIENCE UNIVERSITY OF FLORIDA 2012
2 2012 P e i H an C hiu
3 To my parents, sister, and friends
4 ACKNOWLEDGMENTS I thank my advisor, Professor Mark Orazem, for his support and guidance He has led me into the area of engineering research and the thinking of an engineer. I would like to thank Professor David Bloomquist for his concept of civil engineer ing with respect to clay dewatering. I would also like to thank Paul Kucera of Mosaic Fertilizer, LLC for his involvement in sponsoring this project and his advisement on the project from the aspect of the phosphate mining industry. I thank all of the students in Professor Orazem s research group for support ing the research work, including Rui Kong, Ya Chiao Chang, Salim Erol Christopher Cleveland, Chao L i u, Yan Yu, Alok Shankar Darshit Shah Vishnu vardhan Pinjala and Rodney Del Ri o I would like to thank the staff members in the Department of Chemical Engineering. This includes Shirley Kelly, Carolyn Miller, Deborah Sandoval, Dennis Vince, and Jim Hinnant. Finally, I would like to thank my parents, my sister, my brother, and my gra ndparents for their love, enlight enment, and support throughout my life.
5 TABLE OF CONTENTS page ACKNOWLEDGMENTS ................................ ................................ ................................ .. 4 LIST OF TABLES ................................ ................................ ................................ ............ 7 LIST OF FIGURES ................................ ................................ ................................ .......... 8 LIST OF SYMBOLS ................................ ................................ ................................ ...... 10 ABSTRACT ................................ ................................ ................................ ................... 12 CHAPTER 1 INTRODUCTION ................................ ................................ ................................ .... 13 2 LITERATURE REVIEW ................................ ................................ .......................... 15 The Origin of Phosphatic Clay Suspension s ................................ ........................... 15 Characteristics of Clay Suspensions ................................ ................................ ....... 16 Electrokinetic Dewatering Theory ................................ ................................ ........... 19 Electrokinetic Parameters ................................ ................................ ....................... 25 Electrode Fabrication and Design ................................ ................................ .... 25 Material Properties ................................ ................................ ........................... 26 Paramete rs Related to Operation Conditions ................................ ................... 27 Effluent Analysis ................................ ................................ ............................... 28 Other Dewatering Methods ................................ ................................ ..................... 30 3 EXPERIMENTAL ................................ ................................ ................................ .... 35 Source of Sludge ................................ ................................ ................................ .... 35 Equipment an d Instruments ................................ ................................ .................... 35 Apparatus ................................ ................................ ................................ ............... 35 Methods and Testing Procedure ................................ ................................ ............. 36 4 EXPERIMENTAL RESULTS AND DISCUSSION ................................ ................... 40 Proof of Concept ................................ ................................ ................................ ..... 40 Settling of Supernatant Water ................................ ................................ ................. 40 Monitored Supernatant Turbidity ................................ ................................ ............. 41 Effect of pH on Supernatant Settling ................................ ................................ ....... 43 Effect of Flow Rate on Supernata nt Settling ................................ ........................... 44 Effect of Electrical Potential Gradient on Supernatant Settling ............................... 47 5 CONCLUSIONS AND FUTURE WORK ................................ ................................ 58
6 LIST OF REFERENCES ................................ ................................ ............................... 61 BIOGRAPHICAL SKETCH ................................ ................................ ............................ 64
7 LIST OF TABLES Table page 4 1 The operating conditions of electrokinetic dewatering experiment s with a fixed applied electric field ................................ ................................ .................. 51 4 2 The operating conditions of electrokinetic dewatering experiments with a fixed flow rate. ................................ ................................ ................................ .... 51
8 LIST OF FIGURES Figure page 2 1 Regions of phosphate ore deposits in Florida. The inset is illustrated the genera l make up of phosphate ore deposits in depth. ................................ ........ 32 2 2 Schematic of m ajor destabilization mechanisms of liquid dispersions. ............... 32 2 3 Schem atic representation of mechanical dewatering and electro dewatering phenomena for negative charged particles with an applied electric field. ........... 33 2 4 Schematic representation of a bench top cell. The darker shaded area within the cell represents where the clay slurry is loaded for experiments ................... 33 2 5 Schematic of the idealized settling of a uniform, monodispersed suspension and t he normalized turbidity signal that will be expected as a function of time .. 34 3 1 Schematic representation of the semi continuous electrochemical cell ............. 38 3 2 Photograph of the empty experimental setup for semi continuous electrokinetic dewat ering ................................ ................................ ................... 38 3 3 Photograph of the semi continuous experimental setup. ................................ .... 39 4 1 Photographs of the electrokinetic cell before and after the semi continuous operation with a flow rate of 20 mL/min. ................................ ............................. 51 4 2 Photographs of the supernatant water collected during semi continuous operation before and after s ettling ................................ ................................ ..... 52 4 3 Turbidity of the supernatant liquid before and after settling with the applied electric field of 3 V/cm and flow r ate of 40 mL/min ................................ ............. 53 4 4 The samples of supernatant water from the electrokinetic cell after settling in the sample cell for three days. ................................ ................................ ............ 53 4 5 pH of the supernatant liquid before and after settling with the applied electric field of 3 V/cm and flow rate of 40 mL/min ................................ ......................... 54 4 6 Test of the influence of pH on clay se dimentation ................................ ............. 54 4 7 The turbidity of the supernatant water during free settling is as a function of settling time with different flow rates ................................ ................................ .. 55 4 8 The turbidity of the supernatant water during free settling is as a function of settling time with different flow rates (log log plot). ................................ ............. 55
9 4 9 The turbidity of the supernatant water during free settling is as a function of settling time with different applied electric fields ................................ ................ 56 4 10 The turbidity of the supernatant water during free settling varied with time at different applied electric field s ................................ ................................ ........... 56 4 11 The turbidity of the supernatant water during free settling varied with time at different applied electric fields (log lo g plot). ................................ ...................... 57
10 LIST OF SYMBOL S R OMAN a magnitude of initial turbidity A sectional area normal to the direction of current density, m 2 b length of time required to reach low level of turbidity D p particle diameter, m E applied electric field, V/cm H height of the optical window, h 2 h 1 cm h 1 height from the top of turbidimeter holder to the top of the aperture cm h 2 height from the top of turbidimeter holder to the bottom of the aperture cm I o intensity of incident light I t intensity of transmitted light j current density, A /m 2 k e electrokinetic permeability k h hydraulic permeability K (D p ) extinction coefficient of a sphere of diameter D p L optical path length, cm M a anode metals any kind of cationic species that can be reduced N p number concentration of particles m mol/ mL q charge, C/cm 2 q eo the flow rate of water, m 3 /s t settling time, minute or hour v water velocity cm/ s
11 V s settling velocity, cm/s y turbidity of supernatant water, NTU Greek absorption coefficient permittivity, F/cm zeta potential, mV Debye length, nm water viscosity g/cm s electrical conductivity of clay suspensions, S/m turbidity coefficient
12 Abstract of Thesis Presented to the Graduate School of the University of Florida in Partial Fulfillment of the Requi rements for the Degree of Master of Science SETTLING OF SUPERNATANT FROM SEMIC ONTINUOUS ELECTROKINETIC PROCESS ING OF PHOSPHATIC CLA Y SUSPENSION S By Pei Han Chiu August 2012 Chair: Mark E. Orazem Major: Chemical Engineering Dilute suspen sions of phosphatic clay, a waste product of the phosphate ore beneficiation, have been de pos it ed in clay settling areas (CSA) in Cen tral Florida for decades. The fine particle size and high surface charge density causes slow sedimentation of the suspensions, which may take decades to reach solids contents greater than 25 wt%. In this study, semicontinuou s benchtop electrokinetic dewaterin g of phosphatic clay suspensions was performed to accelerate the dewatering process. Turbidity measurements were used to monitor the effectiveness of the electrokinetic separation. While electrokinetic dewatering reduced the turbidity of the supernatant liquid subsequent settling greatly reduced t he t urbidity of the supernatant before The settling process of the supernatant was monitored for different applied electric field s or flow rate s A relations hip was established that related the decrease in turbidity to settling time The supernatant pH was found to have little effect on free settling. Thus, the decrease in turbidity was attributed to the electrokinetic process The settling model developed with different ope ratin g conditions can be used to guide design of electrokinetic dewatering.
13 CHAPTER 1 INTRODUCTION A dilute suspension of p hosphatic clay (containing clay, silica, and residual phosphate ore) is a waste product of phosphate mining. C lay suspension s which contain 3 5 wt% initial solid s content (Carrier 2001) are pumped to clay settling area s (CSAs) for storage and gravitational se dimentation The process of solid liquid separation is first sedimentation followed by thick en ing. Hindered settling and self consolidation require s 2 5 years to reach 40% solid s content The inefficiency of this process results in large numbers of clay settling impoundment s that occup y about 100 square m iles in Central Florida or 37 % of the land mined (Energy and the Environment 2006) Accelerating the dewatering of clay suspension would reduce clay settling area s and lead to more available land s for develop ment Previously, Patri ck McKinney ( 2010) used electrokinetic dewatering to enhance the dewatering process of the phosphatic clay suspension in a bench top exper iment. This benchtop electrokinetic cell significantly improved dewatering of clay suspension s with a cell potential of 4 V/cm i n 9 hours achieving 35% solid s content from approximately initial solids content of 10 wt%. Moreov er, McKinney established a constitut ive relationship between solids content variations, time and electric al potential gradient s, in different time period frame s Rui Kong ( 2011) also documented that in a deep tank semi continuous flow system the average solids content approached 15 wt% and the turbidity value of supernatant water decreased with elapsed time with the effect of the electrokinetic dewatering process In this study the settling supernata nt water collected during the semi continuous electrokinetic process was monitored A s imple analytic model was used to interpret the correlation of the supernatant turbidity with time.
14 A chemical property of the supernatant water, t he pH value of supernatant was examined for its effect on supernatant settling. The e lectrokinetic dewatering process was found to have an influence on free settling of supernatant liquid. The supernatant settling behavior can be a signal of the effectiveness of the e lectrokinetic process
15 CHAPTER 2 LITERATURE REVIEW The Origin of Phosphatic Clay Suspension s Florida has been the mother lode of p hosph ate mining. There are more than 270 square mile s of phosphate min ed lands in Florida. Figure 2 1 illustrate s the location and lay out of the phosphate min es in Central Florida (Bloomquist 1982) The phosphate mining industry wa s the third largest industry behind tourism and agriculture in the 20 th century (Barnett 2008) The industry produces 75% of the phosphate tha t is supplied to the United States and 25% of the worldwide needs (IMC Phosphates 2002) A major percenta ge as much as 90% of the mined phosphate is used for fertilizer production (McKinney 2010) Phosphate ore deposits lie under the top layer of sand, called overburden. In Figure 2 1 the inset presents the stratum of the Central Florida deposits (Bloomquist 1982) Under the vegeta tion layer, the overburden extends to a depth of approximately 25 feet, and the 25 50 feet phosphate matrix lies directly beneath it. Overburden is made of primarily sand and cl ay. Approximately one third s ilica one third clay and one third phosphate ores form the matrix, also known as phosphate ore deposits (Bloomquist 1982) In phosphate mining, a large dragline excavation system is used to remove the overburden that lies above the phosphate matrix. The exposed phosphate matrix is continuously excavated by the dragline to a more shallow area, termed as a pit. High pressure water is shot onto the pi t and turn s the matrix into a slurry, which is pumped through a pipeline from the mine to the phosphate beneficiation plant. The phosphate, sand and clay of slurry are separated from each other in the beneficiation plant. The
16 leftover clays are sent to lar ge impoundment areas, terme d clay settling areas (CSAs) (Barnett 2008) The dilute cl ay slurry contains initially 3 5 wt% solids (Carrier 2001) with particles finer than 150 mi crons in diameter (Bloomquist 1982) Disposal of the phosphatic clay suspensions poses a major environment impact on land utilization. In Central Florida, undeveloped clay settling areas occupy approximately 100 square miles which comprise 37 % of the mined land s (Energy and the Environment 2006, McKinney 2010) Furthermore, the poor settling characteristics of the clay cause that hinder ed settling and self consolidation tak es 25 years or longer to reach the solid s content of 40 wt% (McKinney 2010) The i ncreasing numbers of clay settling area s leads to an intense environmental awareness The requi rement to r educe the amount of land dedicated to clay settling areas imposes increasing demands upon conventional sludge dewatering technology for acceleration of clay dewatering process Therefore, the goal of this study is to reduce clay settling area s for more mined lands reclamation. Characteristics of Clay Suspensions In general, c lay is a class of coarse dispersion s known as a suspension A coarse dispersion is a heterogeneous mixture, in which solid particles suspend in liquid. The size of solid pa rticles, that are larger than 1 micrometer in diameter is suff iciently large to settle down eventually in the liquid Another type of dispersions called a colloid that contains particles with 1 1000 nanometer diameter s, is distinguished from a suspension The fine particles disperse in liquid and barely settl e Dispersion stability, which means the capability of the system to resist change in its properties over time is defined as the interaction forces between suspended particles at equilibrium (McClements 2005) Excluded volume repulsion, elec trostatic
17 interaction, van der W aals forces, entropic forces, and steric forces are involve d in the particle interaction s The unbalanced forces of the interaction caus e destabilization. Destabilizations are classified into two major processes: migration phenomena (sedimentation) and particle size increase phenomena ( flocculation, aggregation) (Figure 2 2). Sedimentation causes local changes in concentration, and floccul ation or aggregation causes global changes in size. A d ispersion is a system that consists of two separate phases: a dispersed phase (or internal phase) and a continuous phase (or dispersion medium). Gravitational p hase separation which causes migration p henomena, is a result of dispersion destabilization when the dispersed phase is denser than the continuous phase D ewatering process es accelerated by artificial forces are similar in concept t o natural destabilization that disturbs dispersion stability and leads to solid liquid separation. Multiple light scattering coupled with vertical scanning is widely used to monitor the dispersion state of a product, hence identifying and quantifying dest abilization phenomena (Roland et al. 2003, Lemarchand et al. 2003) Monitoring sedimentation ( a destabilization phen o men on ) of suspensions is of practical significance to evaluate the efficiency of a wide variety of techniques in diverse areas such as marine geology, coastal and ocean science, hydraulic engineering, and solid liquid separation technology (INTERCOH 2000 2002, Buah Bassuah et al. 1998, Gibbs 1985, Hill et al. 1994) Sedimentation has been studied by using very simple vessels and either measuring the extinction of radiation or visible light by scanning the length of a glass c ell containing the settling suspension to give the particle size or by determining the turbidity at fixed sedimentation depth.
18 Phosphatic clay s are composed of approximately one third phosphate ores o ne third silica, and one third clay. The size of clay particles is less tha n 100 microns in diameter, and this characteristic of clay results in poor settling suspension s However, the fin e particle size is convers ely a key element for electrokinetic dewatering process Another factor of poor settling is that the large internal surface area s and a very small density of particles can cause some clay minerals to scatter at the surface of the suspension s The average specific gravity of the dry particles of the phosphatic clays is 2.7 with a range between 2.6 to 2.9 The general shape of most clay particles is a plate or flat like shape leading to a large ratio of surface area to mass (Craig 1997) This high surface area causes hindered settling of clays. Other than the effe ct of particle size the surface propertie s of the clays also dominate to enhanced sedimentation when an electric field is applied Typically, clay particles immersed in water have a negative surface charge. The magnitude of charge is usually expressed instead in terms of the zeta potential. The charge is directly proportional to the zeta potential as (2 1) where is the permittivity and is the Debye length which represents the thickness of the diffuse part of the double la yer. There are short range attractions between particles due to van der Waals forces which decrease with increasing distance between particles (Craig 1997) When no electric field is applied, the repulsion due to the like char ges between the particles as well as their large specific surface area s dominate in keeping the clays suspended in water and hindering settling. T he charge s on the clay particles
19 aid sedimentation when an electric field is applied, in which there is a counter force as the interaction of like charges between cl ay particles enhanc ing settling processes Phosphatic clays consist of clay minerals and non clays. The composition of clay minerals are smectite (or montmorillonite), palygorskite and kaolini te, while carbonate fluorapatite, quartz, wavelite and dolomite represent non clay ingredients. For clay minerals, kaolinite particles have a plate like shape with a negative surface charge on their faces and a positive surface charge on their edges (Ma 1999) On the other hand, the charge s on palygorskite are much smaller than those on other clays and it has a higher surface area to mass ratio. Its minimal charge causes it not to respond favorably to an electric field and in addition, its high surface area to mass ratio does not allow it to settle well naturally Electrokinetic Dewatering Theory Electrokinetic dewatering process es provide an attractive method to increase the final dry solids content and to accelerate the d ewatering process of phosphatic clay suspension s with low energy consumption. The electrokinetic process is applied via an external electric field that induce s the relative movem ent of solid particles and liquid in a suspension. The induced movement of so l id particles and liquid accelerate s the dewatering pr ocess and increases the solids content of suspensions. Electric field assisted dewatering process is a technology that may be used in conjunction with a conventional dewatering mechanism, such as filter presses to improve solid liquid separation (Hwang, Min 2003) The mechanism is based on the interaction between the applied electric field and the diffuse double layer formed at the solid liquid interface. The water flow induced by an electrical potential difference leads to the electrokinetic phenomena, which include
20 electromigration, electrophoresis and electro osmosis (Newman, Thomas Alyea 2004) Electrop horesis is the movement of charge d particles under an applied electric field towards the el ectrode, which enhanc es sedimentation; the fluid driven towards the electrode, known as electro osmosis, enhances consolidation The el ectrokinetic process is applied an elect ric field to phosphatic clay suspensions with electrodes, which is the equivalent of an electrochemical cell. Therefore, electrochemical reactions are associated with an electrokinetic process. A n electrical double layer exists at the phase boundary when a solution is in contact with a solid. Fine particles in suspension s usually have negative surface charge s wh ich attract cations to the surrounding solution. In this condition, suspended solid particles natura lly settle under the combined gravitational and viscous forces. Moreover, as the repulsion force between same charged particles is l arger than the attractive force of opposite charged particles the clay suspension s will take a long period of time to settle completely The principle electro kinetic phenomena induced by an applied electric field are present ed in Fig ure 2 3 (Mahmoud et al. 2010) When an electric field is applied, the negative charged particles move relative to a stationar y liquid towards the posi tively charge d electrode This phenomenon is described as electrophoresis (Mahmoud et al. 2010) A condensed cake, which consists of a porous solid skeleton filled with mobile water, is formed du e to the electrokinetic force. T he liquid with cations migrate s through the porous media towards the negative ly charged electrode This process is called electro osmosis (Mahmoud et al. 2010, Shang, Lo 1997) The former phenomenon enhances the movement of negative charged particles towards the positively charge d
21 electrode, while the latter enhances the movement of liquid with cations towards the negatively charged electrode. These electrokinetic phenomena resul t in the acceleration of the dewatering process. Meanwhile, an electrochemical reaction electrolysis, occur s at the site of electrodes which cause s an effect on the performance of the electro dewatering process. The environment that the electrokinetic pr ocess creates is essentially an electrochemical cell. One electrode functions as the positively charged anode where oxidation reactions occur and the other functions as the negatively charged cathode where reduction reactions occur. At the anode, oxygen and hydro ge n ion s (H + ) are produced and then the surrounding s become acidic. Another possible reaction that is involved i s the corrosion of the electrode. (2 2) (2 3) where represents the anode metals. At the cathode, t he reaction invo lves hydrogen gas released and hydroxide ions (OH ) generated. T he local basic environment is developed (2 4) (2 5) where represents any kind of catio nic species that can be reduced (Mahmoud et al. 2010, Shang 1997) T he need for electrochemical reactions represents a disadvantage of the electrokinetic process. Hydrogen ion s produce d at anode and hydroxide ions create d at cathode generate a pH gradient across the clay suspension s (McKinney 2010) The pH
22 gradient changes the zeta potential and affects the dewatering process. For kaolinite suspensions, the zeta potential inc reases from 90 to 20 mV when the suspension pH decreases from 10 to 3 during the electrokinetic process. The dispersed suspensions co agulate due to the pH decrease (Mahmoud et al. 2010) Furthermore, oxygen gener ated at anode and hydrogen produced at cathode leads to voids in the solids and increases the electrical resistance of the system. The increasing resistance of the system results in a requirement of a higher energy consumption to maintain an electrokinetic process. Additionally, the corrosion of anode reduces the effectiveness of the electrokinetic process and increases the energy cost. This may also cause the clay contamination (Mahmoud et al. 2010) There are two anot her imp ortant electrokinetic phenomena occur during the electrokinetic process. One is r elated to the motion of ions called electromigration (Figure 2 3) Electromigration refers to the transport of ionic species in the pore fluid, and this is the main mechanism by which the electrical current flows through the sediment. This phenomenon is considered to be a speci al case of electrophoresis when the particle size is close to zero (Mahmoud et al. 2010, Reddy, Urbanek & Khodadoust 2006) The other is related to the movement of charged particles under a non uniform applied electric field termed as dielectrophoresis This phenomenon happens when non uniform electric fields impose on charged particle suspension s during electrokinetic dewatering processes Particles with surro unding diffuse double layers may be considered as equivalent dipoles controlled by charges in th e double layer. In a non uniform external electric field, the charges in the double layer located in the half of the
23 particle closest to the more intensive field will be acted upon by a stronger force, which generates a net force, dielectrophoretic force. This force is also a driving force that leads to the motion of part icles with respect to the fluid ; however, the magnitude and the influenced range of the force is smaller and shorter than that of electrophoresis. The total electrokinetic force on a partic le in an external electric field is the vectorial summation of electr ophoresis and dielectrophoresis (Shang, Lo 1997, Shang 1997, Pohl 1978, Shang, Inculet & L o 1994) During an electrokinetic process, the velocity of water outside the diffuse double layer can be expressed as a function of the applied electric field (2 6) where is the permittivity is the zeta potential, and is the viscosity of water The water velocity can also be defined in terms of the charge s in the diffuse layer or the charge s on the particle s (2 7) where is the Debye length which represents the thickness of the diffuse pa rt of the double layer and is the viscosity of water (McKinney 2010, Newman, Thomas Alyea 2004) The effectiveness of electrophoretic dewatering can be evaluated by the relative magnitude of gravitational and electrokinetic sedimentation (Shang 1997) The effectiveness of electro osmosis dewatering is governed by the electrokinetic permeability which can be calculated from an empirical relation (Mitchell 1993) (2 8)
24 where is the flow rate of water in m 3 /s is the sectional area normal to the direction of current density in m 2 is the current density in A/m 2 and is the electrical conductivity of clay s uspensions in S/m. The technique of e lectrokinetic dewatering has been studied in clays and other sludges, including oily sludges (Yang, Nakhla & Bassi 2005) harbor dredges (Reddy, Urbanek & Khodadoust 2006) contaminated river sediment (B uckland, Shang & Mohammedelhassan 2000) waste sludge (Raats et al. 2002) and activated sludge (Saveyn et al. 2005) on laboratory, pilot, and full scale. However, technical problems have hampered its widespread application to date. These problems include the requirement for corrosion resistant electrode materials and high electrical energy consumption (Raats et al. 2002) Therefore, s everal research studies are aim ed to optimize equipment designs, to evaluate the effect of various parameters o n electrokinetic dewatering process es and to establish theoretical equations to fit the process (Mahmoud et a l. 2010) McKinney and Orazem ( 2010) set up a cylindrical Plexiglas cell with a horizontal electrode configuration (Figure 2 4) to perform the electrokinetic dewatering process on phosphatic clay suspension s from Central Florida. The electrokinetic process with an applied electric field of 4 V/cm in the bench top cell for 9 hours achieved a solids content of 35 wt% from about initial solids content of 10 wt%. Additionally, a constitutive relationship was esta blished that relates the increase in solids content to operating time and to the applied electric field W hile the electric field applied for less than 30 hours, the time required to achieve a given solids content is inversely related to the applied
25 electr ic field, whereas for longer periods the value of the maximum solids content achievable was found to be a function of the applied electric field. Kong and Orazem ( 2011) conduc ted the electro kinetic dewatering process to phosphatic clay suspensions o n a larger scale ; a plastic storage box was used to operate a semi continuous dewatering process with a closer horizontal electro de configuration. The solids content of suspensions reached 15 wt% under the semi continuous opera tion, and the energy consumption was lower than that of the bench top experiments due to the closer electrode arrangement. The turbidity of supernatant water was found to be low under the operation. The supernatant liquid collected during the operation set tled to a low turbidity level after several days. The supernatant turbidity decreased with increasing time with the effect of the semi continuous operation Electrokinetic Parameters The eff ectiveness of the electro kinetic dewatering process is determined by the performance of electrok inetic phenomena induc ed by an applied electric field. The factors that influence the performance of electrokinetic dewatering are discussed in this section Electrode Fabrication and Design On e major issue f or the electro dewatering system is relat ed to the electrode materials and relative positioning In order to maintain the performance of the process, use of corrosion resistant electrode materials for the anode is essential to design conside rations (Mahmoud et al. 2010, Shang, Lo 1997, Raats et al. 2002) According to the previous study, steel electrodes have the advanta ge of low cost whereas brass electrodes ar e more efficient on current conduction (Shang, Lo 1997) Mesh or porous
26 electrodes are preferred in the process for removing water through the electrodes and releasing gas generated in the reaction during the process (McKinney 2010) Electrode s are confi gur ed in two layouts : vertical and horizontal. Conventional electro dewatering te chniques operated with a vertical electric field are usually performed in existing tailing ponds (Shang, Lo 1997) In recent literature, research es conducted with a horizontal electrode arrangement have be en applied to the dewater ing process. H orizontal electric field demonstrat ed in new disposal ponds is superio r to enhance the dissipation of gases produced at the electrodes and to keep the anode immersed in liquid during the dewatering process. The advantages of a horizontal electrode arrangement compared to those of the vertical elect rode arrangement are as fol lows : simple set up, high effectiveness and easy operation. H orizontal electrode configuration on a large scale is promising (Zhou et al. 2001) Material Properties The physical and chemical properties of materials including particle size, surface particle charge (zeta potential), pH value, salinity, conductivity and hydraulic permeability determine the magnitude of the interaction with the applied electric field (Shang, Lo 1997, Fourie, Johns & Jones 2007) The surface properties of particles are dominant in fine grained materials with large surfac e area. The critical surface propert y is surface particle charge express ed in term s of zeta potential, which is a useful indicator for the effectiveness of electro kinetic dewatering process Chen, Mujumdar & Raghavan ( 1996) found that the percentage of water removed during dewatering of fine gold tailings was directly proporti onal to the zeta potential. The electrical charges on particles vary with pH value and salin ity of materials which change the effectiveness of dewatering process and the power consumption.
27 Electrokinetics was not effective in a low pH environment (pH < ~ 6) but was very effective in a high pH environment (pH > 9) at even high salinity (Shang, Lo 1997) Electrochemical reactions associated with the dewatering process may result in a pH gradient across the filter cake, which may cause a deterioration of the product quality (Mahmoud et al. 2010) An environment with m oderate salt c oncentration s result s in a better dewateri ng effect, while the electrokinetic process is not effective in the environment with high salt concentrations which are associated with a low er zeta potential (Shang, Lo 1997) The c onductivity of an electrolyte solution is the ability to conduct electricity which represent s the ionic content of a solution O ver a material conductivity of 2.5 mS/cm, electrokinetic dewatering would not be feasible (Fourie, Johns & Jones 2007) The hydraulic permeability also plays a critical role in electro osmotic consolidation. The flow in electrophoresis sedimentation and electro osmosis consolidation depends on the relative magnitude of the hydraulic permeability k h and electrokinetic permeability k e of the material, respectively (Mitchell 1993) Parameters Related to Operation Conditions There are two main types of factors involved in operating conditions of the electrokinetic dewatering system: process parameters and operation mode. Proce ss parameter s, such as total energy input and energy output distribution in time influences the dewatering effect and overall energy efficiency. As an example of o peration mode one can hold voltage, current or electric field constant or change it with a certain pattern, such as a sine or block wave. The notable application is short to intermediate interruptions to the electric field, which apparently improves the process efficienc y and reduces power consumption (Gopalakrishnan, Mujumdar & Weber 1996, Yoshida 2000)
28 Other crucial operating factors have been reported in the literature. The effective current density is determined by the applied voltage and spacing of electrodes. The increase in the volume o f water collected accompanied high electric field intensity on electro osmosis dewatering of kaolinite clay (Buckland, Shang & Mohammedelhassan 2000) The voltage loss at electrodes during an electr o kinetic process is governed by the electrode materials (Buckland, Shang & Mohammedelhassan 2000) Effluent Analysis Concentration changes and the settling rate are two main indicat ors to understand the eff ectiven ess of the electrokinetic process. Solid and liquid phase of suspension s are involved in these two investigations. A l ight scattering technique is most widely applied to detect the concentration of residues in liquid phase of suspensions by backscattering intensity Optical turbidity is used as a su rrogate for sampling the residue concentration of supernatant water at a fixe d height during the electro kinetic dewatering process. Turbidity measurement s do not disturb the equilibrium of the dewatering system leave the sample int act for other studies and allow for rapid analysis of samples (Caron et al. 1996) According to the Lambert Beer law, the well established theory of photo sedimentation, the attenuation ( ) in the intensity of light after transmission through a suspension of monodisperse d sph erical particles is given by (2 9 ) where is the diameter of the particle, is the extinction coefficient for parti cle size of is the optical path length, and is the number of particles per unit volume. is a measure of the attenuation due to absorption and scattering. That is,
29 where is the scattering coefficient or turbidity coefficient and is the absorption coefficient. The e xtinction coefficient is a function of particle diameter, which is dominant for fine particles. Fine particles have a large ratio of surface area to mass, wh ich inclines the surface properties dominant in the optical technique. The scattering behavior of the particl es is favorable in the phenomen on of attenuation. For non absorbing particles that do not vary in size, the turbidity coefficient plays a prim ary role in the turbidity signal. The t urbidity coefficient determines the fractional decrease in intensity of light and is proportional to the number concentration (2 10 ) This relationship indicates that a normalized turbidity signal can directly relate to the concentration changes of particles during the settling process. For a uniform, dilute suspension of monodisperse d spherical particles that settle with a single settl ing the turbidity signal remains constant when the settling path of the particles is above the optical aperture. The turbidity decreases linearly while particles pass the aperture to zero when particles have traversed the aperture (Figure 2 5). The no rmalized turbidity differences are divided by positions (Coutinho, Harrinauth & Gupta 2008) (2 11 )
30 Other Dewatering Methods De watering techniques can be characterized as being either physical or chemical Many examples of physical dewatering have been documented in the literature. Most of the physical dewatering methods are based on mechanical process es which involve filtration, centrifugal applications, high temperature drying, magnetic mechanical dewatering, and acoustic mechanical dewatering (Mahmoud et al. 2010) For chemical dewatering floccula nt is a common a nd effective addi tive for the flocculation of clay suspension s The Florida Institute of Phosphate Research (FIPR) reported numerous dewatering methods. Flocculation is a behavior of colloidal particles that form s flocs or flakes suspended in the liquid by the addition of a clarifying agent (Figure 2 2) Inorganic salts and organic flocculants have been used to flocculate the clays. The former were inorganic electrolytes, including lime, calcium chloride, magnesium chloride, and alum, while the latter were either natural polymers or synthetic organic polymers. Starches, gum, tannins, and sod ium aliginate are natural polymers. Synthetic organ ic polymers are charged as anionic or cationic and the most applied synthetic organic flocculants are polyacrylamides ( PAM) and polyethylene oxide. P olyacrylamides were the most efficient flocculants (Bratby 1980, Rahman 2000) Both active and passive dosing of polyacrylamides reduced the turbi dity of construction site water, which is a kaolinite suspension, by up to 88%, with turbidity levels <50 NTU in discharges (Bhardwaj 2008) Anionic polyacrylamides substantially reduced the turbidity of subsoils w ith little smectite or vermiculite from across North Carolina by > 90% By contrast, the subsoils that had the higher content of smectite or vermiculite were in little response to polyacrylamide treatments (McLaughl in, Bartholomew 2007)
31 For phosphatic clay suspensions, flocculants accelerate the settling rate and release process water recycling to the phosphate beneficiation plant (Rahman 2000) The dilute suspension d irectly from the beneficiation plant is pretreated with flocculants that inc reases from 2 to 10 wt% solid s content
32 Figure 2 1. Regions of phosphate ore deposits in Flo rida (Bloomquist 1982) The inset is illustrated the general make up of phosphate ore deposits in depth. Figure 2 2. Schematic of m ajor destabilization mechanisms of liquid dispersions.
33 Figu re 2 3. Schem atic representation of mechan ical dewatering and electro dewatering phenomena for neg ative charged particles with an applied electric field (Mahmoud et al. 2010) Figure 2 4. Schematic representation of a bench top cell. The darker shaded area within the cell represents where the clay slurry is loaded for experiments (McKinney 2010)
34 Figure 2 5 Schematic of the idealized settling of a uniform, monodisperse d suspension and t he normalized turbidity signal that will be expected as a function of time (Coutinho, Harrinauth & Gupta 2008)
35 CHAPTER 3 EXPERIMENTAL Source of Sludge Phosphatic clay suspension s were provid ed by Mosaic Fertilizer, LLC The suspensions that are pretreated with flocculant have approximate ly 10 wt% of initial solid s content. Equipment and Instruments A Mastech P ower S upply HY10010EX provide d specific voltages for different operating conditions, and a Masterflex Model 77202 60 digital pump (Cole Parmer Inc. ) control led the influent flow rate of phosphatic clay suspensions. A HACH 2100Q Portable Turbidimeter wa s used to measure the turbidity value of supernatant water, and a n Orion 4 Star Plus pH /ISE Benchtop Multiparameter M e ter (Thermo Scientific Inc.) wa s used to measure the pH value of supernatant water. Apparatus The cell design for semi continuous operati on is shown in Figure 3 1 The electrode arrangement was, a s is shown in F igure 3 2 a plastic storage box (88.9 cm42.5 cm32.7 cm) with tw o horizontally suspended metallic mesh electrodes which are made of titanium coated with iridium oxide on the surface (Water Star Inc.) (Kong 2011) The distance within the horizontal electrode configuration i s adjustable, and gaps of 5 cm and 10 cm were set up in different operating conditions A DC power supply provided adjustable voltage Each mesh plate electrode was connec ted to a power supply by a titanium wire. The wire was sealed in a sili con tube to prevent exposure to water A digital pump sent initial clay suspension from the right bucket through a PVC tube into the box to maintain a semi continuous dewatering environment; meanwhile,
36 real time efflue nt flowed out at the left top through the PVC tube to the left bucket The solids accumulated in the bottom of the electrochemical cell. Figure 3 3 presents the semi continuous experimental setup (Kong 2011) Methods and Testing Procedure A pair of experiments was conducted to prove the concept of the semi continuous system design. One experiment was applied with an electric field of 1 V/cm and a flow rate of 20 mL /min o f clay suspensions This experiment was intended to examine the effect of electrokinetic dewatering between the electrodes and gravity sedimentation under the electrod es A second experiment with the same flow rate but no applied electric field was intended to explore a case where the c lay suspe nsion was affected only by gravity sedimentation. Supernatant water was collected at 1 hour, 8 hours, and 16 hours during operation in R ound M edia S torage B ottles (PYR EX Inc.), respectively. The settling of supernatant water was monitored. Experiments wit h applied electrical poten tial gradient s of 2 V/cm, 3 V/cm and 3.33 V/cm with a fixed flow rate and flow rate s of 30 mL /min, 40 mL /min, and 60 mL /min wit h a fixed electric field were perform ed for 8 hours during daytime and rest for 16 hours during night until the feed suspension filled the tank. During the dewatering process, effluent samples of supernatant were collected every 30 minutes in Fisherbrand Class B C lear G lass T hreaded V ials (Fisher Scientific Inc.) To monitor the changes of supernatant turbidity, turbidity was measured immediately after collected and after settling for 24 hours. To monitor the settling of supernatant water, effluent samples were taken during the pseudo steady state of the experiments at the beginning of the second day of operation after supernatant water became turbid. The supernatant turbidity was measured every 10 mi nutes in the first 8 hours and a t a longer period of time when the
37 value held steady The pH value of supernatant wate r was measured along with the turbidity measurements A pair of experiments was conducted to examine the influence of pH on clay sedimentation One cylinder contained the clay with the pH value adjusted by a ddition of KOH to 11.7, which is the same pH valu e of the supernatant liquid achieved under semi continuous operation while the other had the clay that re t ained the initial pH value of 7.1. The sedimentation phenomenon was observed before and after 10 day s of gravity settling.
38 Figure 3 1. Schemat ic representation of the semi continuous electrochemical cell Figure 3 2 Photograp h of the empty experimental setup for semi continuous electrokinetic dewatering (photograph by Rui Kong) (Kong 2011)
39 Figure 3 3 Photograph of the semi continuous experimental setup (photograph by Rui Kong) (Kong 2011)
40 CHAPTER 4 EXPERIMENTAL RESULTS AND DISCUSSION Proof of Concept A pair of experiments was performed to prove the concept of the new semi continuous electrokinetic dewatering system One of the experiment s was operated with a n applied elect ric field of 1 V/cm and the other was under the same conditions except that no el ectric field was applied The tank loaded with the clay suspension s before the experiments started is presented in Figure 4 1A. The side view of the tank shows that after 15 hours of operation, a layer of clear water approximately 1/7 height of the tank formed at the top of the tank in the test with applied potential (Figure 4 1 B ) By contrast, there is only small amount of clear water appearing after 15 hours of operation in the control test (Figure 4 1 C ) The semi continuous operation yielded a substantial solid liquid separation for clay slurries. Settl ing of Supernatant Water Rui Kong found that samples of supernatant water that collected during electrokinetic dewatering process after a few hours of operation were clearer than those collected a t the beginning of the operation Additionally, the turbidity of s upernatant water from the operation decreased wi th increasing time, which was consistent with the observation of supernatant samples (Kong 2011) These phenomena were observed in a new semi continuous electrokinetic e xperiment designed to verify the results S upernatant liquid was collected after 1 hour, 8 hour, and 16 hour of operation with an applied electric field of 1 V/cm and a flow rate of 20 mL /min respectively The clarity of supernatant water from the operation i s evident in Figure 4 2 A.B .C. A fter
41 settling overnight the supernatant water collected after 8 hours and 16 hours of the process became clear with deposits evident at the bottom of the bottles whereas the sample collected after 1 hour of operation settle d only slightly ( see Figure 4 2 D. E.F). The reproduc ible phenomena show ed that t he supernatant water collected during the electrokineti c process was clearer than that from the be ginning of the operation This suggested that the semi continuous operation enhanced the solid liquid separation and led to the clearer supernatant water. The supernatant liquid was collected and stored for 24 hours, and it wa s observed that t he supernatan t turbidity significant ly decreased after overnight settling The residues of clay suspensions under electrokinetic process presented a quick settling behavior. Monitor ed Supernatant Turbidity Turbidity measurements were conducted in order to evaluate the settling behavior of the supernatant liquid The approach is characterized by a n experiment with an applied el ectric field of 3 V/cm and an input flow ra te of 40 mL /min Effluent samples of supernatant were collected every 30 minutes in cells during the pr ocess of electrokinetic dewatering. Turbidity was measured before and after free settling of supernatant water: one was measured immediately after the effluent was collected (marked as black circles), and the other was measured after 24 hours settling in t he sample cell s (marked as red circles) (Figure 4 3). The initial turbidity of sample supernatant was 68 000 NTU. The turbidity decreased sharply at the beginning of experiment and reached the steady state condition after about 4 hours. After achieving the steady state condition, the value of turbidity was stable at the low level until the flow in clay suspensions filled with the tank,
42 which means the accumulating rate of the clay particles is larger than the effluent rate of the particl es resulted in the increasing turbidity of effluent. The supernatant turbidity before and after settling was in a similar bathtub pattern. The turbidity of supernatan t water after free settling decreased in the first 4 hours, and dropped to a value under 30 NTU during the steady state condition, and then increased in the last few hours (Figure 4 4) The supernatant water collected during the pseudo steady state operation was lower than 30 NTU after 24 hours settling, which satisfies the requirement for pr ocess water. S upernatant turbidity before and after free settling reve aled the influence of applied electric field on clay suspensions. An applied electrical potential promotes the movement of charged particles towards the bottom electrode. The concentration of charged particles in effl uent samples of supernatant decreased under sem i continuous operation Supernatant turbidity at the steady state of the operation was stable at the low level, which indicated that the electric field effectively sep arated charged and uncharged particles and the supernatant turbidity showed the concentration of particle residues at the low level The supernatant water from the steady state of the operation had low turbidity after 24 hours settling. This indicated th at the residues of supernatant demonstrated a quick settling behavior and uncharged particles were the main particles in supernatant water due to free settling These results suggested that the electrokinetic dewatering process performed effective solid l iquid separation, and enhance s the free settling of supernatant water.
43 Effect of pH on Supernatant Settling The pH of the supernatant was measured and plotted before and after free settling of supernatant liqu id in a similar mode (Figure 4 5 ) The pH of the initial cl ay suspensions was neutral, i.e., was eq ual to 7. As shown in Figure 4 5 the pH of the supernatant increased rapidly to 11.7 and was sta ble around this value until the solids content of the supernatant effluent increased due to a saturation of the solids holding capacity of the tank There was no significant change of pH before and after the free settling of supernatant water. The increasing pH of the supernatant is a result of the electrochemical reaction at the cathode which is o ccurring hydrolysis to generate hydrogen gas and hydroxide ions, OH To examine whether the alkaline character of supernatant water could be a factor of the settling process (Figure 4 7 4 9 ), experiments were performed in which sedimentation of clay susp ensions with different pH values were observed in two cylinders as shown in Figure 4 6 A. The alkalinity of o ne sample was adjusted by addition of KOH to a pH value of 11.7, which is the same as the pH of the supernatant water under semi continuous operatio n, and the other ha d a pH unchanged at the original value of 7.1. After 10 days, a layer of water was observed to form at the top of both clay cylinders, but the smaller and more turbid layer was observed in the cyl inder with higher pH (Figure 4 6 B). In c ontrast, the supernatant water from the electrokinetic cell showed a high degree of clarity after free settling (Figure 4 4). Supernatant water with a pH value of 11.7 demonstrated a free settling behavior, whereas clay suspensions with a pH value of 11.7 had a poorer sedimentation than those with pH value of 7.1. These two results show that the basic pH value of supernatant water does not improve the dewater i ng
44 process of clay suspensions The small value of supernatant turbidity after settling is attributed to the el e ctrokinetic process. This deduction suggest s that semi continuous operation effectively separated charged and uncharged particles. T he r esidues of clay suspensions under electrokinet ic process are a majority of uncharged particles in supernatant water. Thus, alkaline character of supernatant had a little effect on uncharged particles in supernatant liquid. Uncharged particles are dominant in free settling of supernatant water. Hindered settling, which is caused by the repulsion of cha rged particles, interferes the settling of clay suspensions The surface charges of particles are affected by increasing pH value during the operation, which enhances the repulsion effect of negative charged particles and hindered settling Before the elec trokinetic process, both charged a nd uncharged particles are in suspensions. Clay suspensions w ith the pH value of 11.7 show ed less sedimentation than those with the pH value of 7.1. This result indicated that t he pH value of 11.7 resulted in a stronger re pulsion of negative charged particles in clay suspensions and the particle dispersion Effect o f Flow Rate on Supernatant Settling Table 1 lists the operating conditions of the fixed electric field experiment s. E xperiment s w ere operated for 8 hours during daytime and rest ed for 16 hours during nig ht until suspensions filled the basin at an applied cell potential of 2 V /cm and flow rates of 30, 40, and 60 mL /min Supernatant samples taken during the steady state of electrokinetic process, indicate that an applied electric field can provide steady enhancement to the solid liquid separation At steady state, t he turbidity of the samples was at average 350 NTU under effective semi continuous operation Supernatant water grad ually settled over time. The turbi di ty data measured for free settling of supernatant
45 and presented as the change of turbidity as a function of settling time, are given in Fig ure 4 7 As shown in F igure 4 7 the initial turbidity of supernatant water collected during the steady state of the operation was 230 420 NTU The turbidity dropped quickly in the first 24 hours to a value less than 10 NTU for experiments conducted at flow rates of 30 and 40 mL /min and applied electric fields of 2 V/cm. A turbidity of 30 NTU could be achieved wit hin the first 12 hours of free settling. For a higher flow rate of 60 mL /min the turbidity of the initial sample had a higher turbidity of 1500 NTU and ha d a supernatant turbidity value greater than 50 NTU after settling over the first 24 hours. Supernatant turbidity was at average 350 NTU from t he steady state of the experiments with flow rates of 30 and 40 mL /min and applied electric fields of 2 V/cm which reduced from the initial turbidity of 68,000 NTU. This indicate d that experiments at flow rate s of 30 and 40 mL /min and applied electric field s of 2 V/cm performed effectively electrokinetic dewatering process of clay suspensions in bench top semi contin uous model. The electrokinetic process separated the charged particles from uncharged clay p articles and led to a low turbidity level of residues Contrarily, t he higher turbidity of 1500 NTU from the experiment of a flow rate of 60 mL /min showed that, some charged particles were in the residues with uncharged particles. T he flow rate of input suspensions was greater than the rate of motion of the charg ed particle that was generated by electrokinetic process. Some c harged particles flushed out with uncharged particles in effluents of supernatant water. An average supernatant turbidity o f 350 NTU at the steady state of experiments with flow rates of 30 and 40 mL /min and applied electric fields of 2 V/cm, dropped to a
46 value less th an 10 NTU in the first 24 hours, which indicated that the residues in supernatant water settle d in a quick process and supernatant water contained mostly uncharged particles. However, the supernatant sample collected from the experiment with the flow rate of 60 mL /min had a turbidity value more than 50 NTU after free settling. The high settled turbidity sugges ts that some charged particles were in the supernatant water and dispersed in it due to the repulsion of charged particles. The relationship i s evident in Fig ure 4 7 The dec r e ase in turbidity was dependent on the settling time. Th e turbidi ty of supernatant liquid was found to be a function of the elapsed time, as shown in Fig ure 4 7 A relationship between the turbidity and the settling time was found to be (4 1) w here is the elapsed time in hour s and is the supernatant turbidity in NTU. To interpret the physical meaning of parameters for the relationship, a log log plot was developed as shown in Figure 4 8 The experimental data agrees well with the linear decline in the physical picture shown in Figure 2 5. The parameter in the equation is the y intercept which indicates the magnitude of initial turbidity, and for the parameter of the equation, represents the slope of the linear correlation, which indicates the length of time required to reach low leve l of turbidity. The results suggest that the s emi continuous operati on with an applied electric field of 2 V/cm and flow rates of 30, 40, and 60 mL /min, effective ly separated charged particles from uncharged particles during the steady state, which had supernatant water contain most of uncharged particles. Uncharged particles demonstrated a similar settling behavior to the ideal settling model of a uniform, monodisperse d suspension.
47 The free settling of supernatant wat er shows qualitative agreement with a settling model of a uniform dilute suspension of monodisperse d spherical particles. The established relationship is valid only when the electrokinetic dewat ering process performs effectively on phosphatic clay suspensi ons which includes the operating conditions of an applied electric field of 2 V/cm and flow rates of 30, 40, and 60 mL /min. The agreement of the settling model of supernatant with experimental data showed in the experiments of an applied electric field of 2 V/cm and flow rates of 30, 40, and 60 mL /min. This sugges ts that the supernatant turbidity during the free settling can be predicted for a given flow rate as a function of settling time. Equation ( 4 1) is presented in Fig ure 4 7 with and as parameter s Furthermore, the relationship matches the changing turbidity of supernatant liquid with ti me during the electrokinetic process. It supports the experimental method and the analysis of the data a s a valid app roach to study the influence of electrokinetic process on supernatant water Effect of Electrical Potentia l Gradient on Supernatant Settling The operating conditions of the fixed flo w rate experiment s show in Table 2 Similar experiments were performed with different applied potentials Supernatant water was collected during the steady state of experiments with a flow rate of 40 mL /min and applied electric fields of 2, 3, and 3.33 V/cm Turbidity measurements of supernatant liquid were conducted continuously over settling time (Figure 4 9 ) In the experiment s at a flow rate of 40 mL /min and applied electric fields of 2 and 3 V/cm the turbidity of supernatant water was also related to settling time. Figure 4 9 shows that the s upernatant turbidity was found as a function of time as well. A s imilar relationship of supernatant turbidity and settling time was established (4 1)
48 w here is the settling time in hour s and is the supernatant t urbidity in NTU. Parameter proves to be the magnitude of initial turbidity, and parameter indicates the length of time required to achieve the low level of turbidity. This relationship suggests that the settling behavior of supernatant liquid with different applied potential s (experiments with a flow rate of 40 mL /min and applied electric fields of 2 and 3 V/cm) was similar to that with different flow rates (experiments with an applied electric field of 2 V/cm and flow rates of 30, 40, and 60 mL /min) Semi continuous operation led to a similar effect on phosphatic clay dewatering process. E xperiments at the flow rate of 40 mL /min and applied electric fields of 2 and 3 V/cm performed effectively solid liquid separation on phosphat ic clay suspensions under the ele ctrokinetic proce ss The results have a good agreement with the developed relationship (Figure 4 9 ) which means the turbidity v alue of supernatant during the free settling can also be calculated for a given time point in the semi continuous operations of a flow rate of 4 0 mL /min and applied electric fields of 2 and 3 V/cm T he relationship with parameters and (Equation 4 1) can be used to predict the supernatant turbidity during free settling. T h is model that monitor s the free settling of supernatant water fit s the settling process of supernatant accelerating by the semi continuous electrokinetic dewatering process. One of the great improvements in an electrokinetic dewatering technique is to reduce energy consumption A common method for cutting energy cost s is to reduce the distance of electrodes. Figure 4 10 demonstrated the settling process of supernatant water from the steady state of the experiments with the flow rate of 40 mL /min and applied electric
49 fields of 2, 3 and 3.3 3 V/cm. The turbidity of supe rnatant water from the experiment at an applied electrical potential of 3.33 V/cm dropped rapidly in the first 8 hours and reached a plateau at a value of around 0.4 NTU The experiment with higher applied electrical potential of 3.3 3 V/cm which was gener ated by the shorter distance of electrodes resulted in a faster settling process of supernatant liquid. This se ttling phenomenon suggested that a rise of electric field of only 0.3 3 V/cm could cause this drama tic change to settling behavior due to the reduction of electrode distance s The semi continuous operation with a higher applied electric field of 3.33 V/cm performed a better particle separation in the shorter electrode distance. The concentration of charged particles was lower in supernatant water from the operation with an applied electric field of 3.33 V/cm than that from the operation with lower electric fields. Higher concentration of uncharged particles could lead to a faster settling process. The settling of cha rged particles that was driven by an applied electric field might not be disturbed by the bubbles of g ases generated at the electrodes, especially with closer electrode spacing. The closer electrode spacing reduced the energy consumption but did not lessen the performance of semi continuous operation, and even could enhance the dewatering effect. To interpret the decay of supernatant water, a log log plot illustrate d the settling process of supernatant in a large time scale. Figure 4 1 1 indicate d the settling process of supernatant liquid from different experiments with the flow rate of 40 mL /min and applied electric fields of 2, 3 and 3.3 3 V/cm in a long time frame. The settling of supernatant water tended to a settling process of three phases with the experiment at
50 an applied electri c field of 3.3 3 V/cm. This result also show ed a faster settling process of supernat ant water from the experiment with a n applied electric field of 3.33 V/cm. In the log log plot, the settling process of the supernat ant water showed a normal decay of three phases under the experiment with the flow rate of 40 mL /min and the app lied electric field of 3.33 V/cm This result suggested that an electrokinetic dew atering process with a higher applied electric field that was operated in a shorter distance of elect rodes performed an effective particle separatio n The primary residues of semi continuous operation in supernatant water were uncharged particles. They could settle in a short period of time and demonstrate a whole settling decay. The relationship that supernatant turbidity as a function of time which i s established from the experimental data of the flow rate of 40 mL /min and a pplied electric field s of 2 and 3 V/cm could be one of t hese phases of the settling process (Figure 4 11) This indicated that t he separation of charged and uncharged particles wa s less effective in experiments with applied electric fields of 2 and 3 V/cm than that in the experiment with an applied electric field of 3.33 V/cm. The residues of the operation contained a little concentration of charged particles with uncharged particl es which might require a longer time to settle completely in supernatant water. A settling process of s upernatant water could reach other phases when supernatant water settled over a longer time. An entire settling process of supernatant water could be reproducible under experiments with a higher applied electric field in closer electrode distance in semi continuous or a larger scale.
51 Table 4 1. The operating conditions of electrokinetic dewatering experiment s with a fixed applied electric field Test N umber Electrical potential gradient (V/cm) Flow rate ( mL /min) 1 2 30 2 2 40 3 2 60 Table 4 2 The operating conditions of electrokinetic dewatering experiment s with a fixed flow rate Test Number Electrical potential gradient (V/cm) Flow rate ( mL /min) 1 2 4 0 2 3 40 3 3.33 4 0 A B C Figure 4 1 Photographs of the e lectrokinetic cell before and after the semi continuous operation with a flow rate of 20 mL /min A) side view before the operation B ) side view after 15 hours with an applied electric field of 1 V/cm C ) sid e view after 15 hours with no applied electric field (photograph s by Rui Kong and Pei Han Chiu )
52 A B C D E F Figure 4 2. Photographs of the supernatant water collected during semi continuous operation before and after settling A) supernatant water collected after 1 hour of operation, B) supernatant water collected after 8 hours of operation, C) supernatant water collected after 16 hours of oper ation, D) s upernatant water at 1 hour of operation after overnight settling E) s upe rnatant water at 8 hours of operation after overnight settling and F) s upernatant water at 16 hours of operation after overnight settling (photograph s by Rui Kong and Pei Han Chiu )
53 Figure 4 3 Tur bidity of the supernatant liquid before and after settling with the applied electric field of 3 V/cm and flow rate of 40 mL/min The black circles (upper line) represent the turbidity measured immediately after the sample is collected, and the red circles (lower line) represent the turbidity measured afte r 24 hours settling in the sample cell. Day 1 Day 2 Day 3 Figure 4 4 The samples of supernatant water from the electrokinetic cell after settling in the sample cell for three days
54 Figure 4 5 pH of the supernatant liquid before and after settling with the applied electric field of 3 V/cm and flow rate of 40 mL/min The black circles represent the pH value measured immediately after the sample is collected, and the red circles represent the pH val ue measured after 24 hours settling in the sample cell. A B Figure 4 6 Test of the influence of pH on clay se dimentation A) sample s of clay suspensions before sedimentation with pH of 7.1 (left) and pH of 11.7 (right), and B) sampl es of clay suspen sions after 10 days of gravity sedimentation.
55 Figure 4 7 The turbid it y of the supernatant water during free settling is as a function of settling time with different flow rates Figure 4 8 The turbid it y of the supernatant water during free settling is as a function of settling time with different flow rates (log log plot)
56 Figure 4 9 The turbid it y of the supernatant water during free settling is as a func tion of settling time with different applied electric fields Figure 4 10 The turbidity of the super natant water during free settling varied with time at different applied electric field s
57 Figure 4 11 The turbidity of the supernatant water during free settling varied with time at different applied electric field s (log log plot)
58 CHAPTER 5 CONCLUSIONS AND FUTU RE WORK An electrokinetic dewatering process of phosphatic clay suspensions was investigated in semi continuous model. The system of semi continuous operation accelerated the dewatering process o f phosphatic clay s uspensions. T he experiment with an applied electric field of 1 V/cm show ed that a layer of clear water was approximately 1/7 height of the tank was produced after 15 hours of operation ; whereas, there was no clear water layer observed in the expe riment with no applied electric field Electrokinetic param eters w ill be adjusted to optimize the operating conditions of semi continuous operation and those in large scale The semi continuous operation effectively performed a solid liquid separation to p hosphatic clay suspensions The turbidity of supernatant water during the steady state of the electrokinetic process was stable at average 350 NTU which decreased from the initial turbidity of 68,000 NTU Moreover, the semi continuous electrokinetic proce ss enhanced the subsequent settling of supernatant water. I n the experiment with an applied electric field of 2 V/cm and an input flow rate of 30 and 40 mL /min t he average 350 NTU of supernatant turbidity at the steady state of the operation dropped dramatically to a value less than 10 NTU in first 24 hour s A turbidity of 30 NTU could be achieved within first 12 hour s of free settling. Supernatant pH could not account for an improvement in the dewatering process of clay suspensions. There was no sig nificant change of pH before and after the free settling of supernatant water. However, clay sedimentation experiments after 10 days, a layer of water was smaller and more turbid observed for the basic sample cylinder than that for the original neutral sam ple. An increase in the pH value of clay suspensions did
59 not enhance solid liquid separation. Hence, the settling process of the supernatant water and the low supernatant turbidity achievable after settling were attributed to the electrokinetic dewatering process. A settling model of supernatant water was developed for the supernatant collected during semi continuous operation : the turbidity of supernatant water during free settling was found as a function of time. The e xperiment al parameters included different flow rates (30, 40 and 60 mL /min ) and the applied electric field s ( 2 3, and 3.33 V/cm) This mathematical relationship show ed a good agree ment with the experimental results even agreed with the settling process of clay suspensions enhanced by the semi continuous operation Thus, t his model can predict a time period that t he supernatant water reaches specific requirement s f or recycl ing process water. The model did not work as well for the settling behavior of supernatant from semi continuous operation with a higher applied electric field of 3.33 V/cm creat ed by reducing the electrode spacing Under these conditions, the supernatant settled faster than would have been predicted by the model. It is possible that h igher concentrat ion s of uncharged particles in the residues enhanced the fre e settling of supernatant water Future work should be conducted to determine the explanation for this improved performance. E xperiments should be performed with a higher electric field and shorte r electrode spacing in both semi continuous and continuous operation s It is assumed that the semi continuous operation accelerates the dewatering process of phosphatic clay suspensions due to the separation of charged and uncharged particles. An applied e lectric field induces the movement of charged particles which congregates into solids at the electrode while uncharged particles that remain in
60 the residues flush out with the supernatant water An effective electrokinetic dewatering process leads to a well performed solid liquid separation and a settling process of the supernatant liquid. A settling process of the supernatant water could be an indic ator to examine the effectiveness of the electrokinetic process on clay sus pensions with different operating conditions. The settling process of supernatant water would be modified to fit electrokinetic dewatering processes with different operating conditions in large scale. The composition of the solids precipitated from superna tant sh ould be analyzed to verify the separation theory. The work presented in this thesis may be used to guide the design of large scale dewatering equipment
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64 BIOGRAPHICAL SKETCH Pei Han Chiu graduated from National Taiwan University, with a Bachelor of Science degree in animal science and technology in June of 2006, and with a Master of Science degree in microbiology and immunology in June of 200 8 She entered the Master of Engineering program in chemical engineering in August of 2010 at the University of Florida In January of 2011 s he joined Professor Mark E. Orazem s research group which specializes i n electrochemical engineering Then, she transferred to the Master of Science program for advanced study on the project of phosphate clay suspension dewatering, sponsored by Mo saic Fertilizer, LLC. Pei Han complet ed a Master of Science degree in the summer of 2012.