Semicontinuous Electrokinetic Dewatering of Clay Suspensions

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Title:
Semicontinuous Electrokinetic Dewatering of Clay Suspensions
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1 online resource (54 p.)
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english
Creator:
Kong,Rui
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University of Florida
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Gainesville, Fla.
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Degree:
Master's ( M.S.)
Degree Grantor:
University of Florida
Degree Disciplines:
Chemical Engineering
Committee Chair:
Orazem, Mark E
Committee Members:
Bloomquist, David G
Svoronos, Spyros

Subjects

Subjects / Keywords:
clarifier -- clays -- dewatering -- electrokinetics -- mining -- phosphate -- semicontinuous -- separation -- thickener
Chemical Engineering -- Dissertations, Academic -- UF
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Chemical Engineering thesis, M.S.
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theses   ( marcgt )
government publication (state, provincial, terriorial, dependent)   ( marcgt )
born-digital   ( sobekcm )
Electronic Thesis or Dissertation

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Abstract:
The processing and storage of the phosphatic clay suspensions which result as a waste product from phosphate mining is a longstanding problem. A very long time is required for the gravity-driven settling of clay suspensions with an initial 2-3 wt% solids content, and decades may be required to reach the desired target of 25-30 wt% solids content. In previous work, bench top experiments of electrokinetic dewatering were used to guide the development of a constitutive relationship describing the changes in solids content with time and electric field. In this thesis, a semi-continuous electrokinetic dewatering process was designed and tested. A unique feature of the system is that the electrodes were placed close to each other, leaving a large volume below the electrodes in which the clay could settle. The experimental results showed that electrokinetic dewatering occurs in semi-continuous operation and that the close electrode spacing, which greatly reduced the power requirement, did not impair separation. These results motivated a discussion of how electrokinetic effects could be incorporated into existing thickening equipment. Suggestions are made for future experiments.
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In the series University of Florida Digital Collections.
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Includes vita.
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Description based on online resource; title from PDF title page.
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This bibliographic record is available under the Creative Commons CC0 public domain dedication. The University of Florida Libraries, as creator of this bibliographic record, has waived all rights to it worldwide under copyright law, including all related and neighboring rights, to the extent allowed by law.
Statement of Responsibility:
by Rui Kong.
Thesis:
Thesis (M.S.)--University of Florida, 2011.
Local:
Adviser: Orazem, Mark E.

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UFE0043459:00001


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1 SEMICONTINUOUS ELECTROKINETIC DEWATERING OF CLAY SUSPENSIONS By RUI KONG A THESIS PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIEN CE UNIVERSITY OF FLORIDA 2011

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2 2011 Rui Kong

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3 To my parents and friends

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4 ACKNOWLEDGMENTS I would like to recognize all the faculty members in the Department of Chemical Engineering at University of Florida. I thank my advisor, Professor M ark Orazem, for his support and guidance. And I thank Professor s Spyros Svoronos and David Bloomquist for their kind support. I would l ike to thank Charlotte Brittain, Bryan Baylor and Pa ul Kucera of Mosaic Fertilizer LLC for their involvement in sponsoring this project. I would like to recognize my team members : Patric k Mckinney, Shaoling Wu, Bryan Hirschorn, Erin Patrick, Ya Chiao Chang Salim Erol Rodney Del Pei han Chiu, and Daniel Rood for their significant contribution toward the completion of the research work. Additionally, I would like to thank the staff membe r s in the Department of Chemical Engineering ; these thanks include Shirley Kelly, Deborah Sandoval, Denni s Vince and Jim Hinnant I also would like to thank the operators Andy Bristow and Michael Price, from the Water Reclamation Facility for their kind help with the turbidity measurement. Finally, I would like to thank my parents for their love, encouragement and guidance.

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5 TABLE OF CONTENTS page ACKNOWLEDGMENTS ................................ ................................ ................................ .. 4 LIST OF TABLES ................................ ................................ ................................ ............ 7 LIST OF FIGURES ................................ ................................ ................................ .......... 8 ABSTRACT ................................ ................................ ................................ ................... 10 CHAPTER 1 INTRODUCTION ................................ ................................ ................................ .... 11 2 LITERATURE REVIEW ................................ ................................ .......................... 12 Methods of Clay Dewatering ................................ ................................ ................... 12 Freeze Thaw ................................ ................................ ................................ .... 12 Moving Screen ................................ ................................ ................................ 12 Sand Clay Sandwich Process ................................ ................................ .......... 12 Flocculation Method ................................ ................................ ......................... 13 Electrokinetic Dewatering ................................ ................................ ................. 13 Principles of Electrokinetic Dewatering ................................ ................................ ... 14 Electrokinetic Dewatering Process ................................ ................................ ... 14 Sedimentation ................................ ................................ ............................ 14 Consolidation ................................ ................................ ............................. 15 Electrophoresis and Electro Osmosis ................................ .............................. 15 Chemical Reactions on the Electrodes ................................ ............................. 16 Fa ctors Influencing the Electrokinetic Process ................................ ................. 17 Grain size and mineral type ................................ ................................ ....... 17 pH value ................................ ................................ ................................ ..... 18 Salinity ................................ ................................ ................................ ....... 18 Current density ................................ ................................ ........................... 18 Electrode material ................................ ................................ ...................... 18 Electrode layout ................................ ................................ ......................... 18 Bench Top Experiment of Electrokinetic Dewatering ................................ .............. 19 Equipment Used in Clay Thickening ................................ ................................ ....... 21 Horizontal Flow Thickener ................................ ................................ ................ 21 Vertical Flow Thickener ................................ ................................ .................... 22 Inclined S urface Thickener ................................ ................................ ............... 23 Other Thickeners ................................ ................................ .............................. 24 Belt thickener ................................ ................................ ............................. 25 Screw thickener ................................ ................................ ......................... 25 Centrifuge thickener ................................ ................................ ................... 25

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6 3 EXPERIMENTAL EVALUATION OF SEMI CONTINUOUS FLOW OPERATION PROCEDURE ................................ ................................ ................................ ......... 29 Large Scale Semi Continuous Flow System ................................ ........................... 29 Cell Design ................................ ................................ ................................ ....... 29 Instruments ................................ ................................ ................................ ....... 29 Experiment Process ................................ ................................ ......................... 30 Separation Results ................................ ................................ ........................... 31 Deep Tank Semi Continuous Flo w System ................................ ............................ 32 Cell Design ................................ ................................ ................................ ....... 33 Instruments ................................ ................................ ................................ ....... 34 Experimental Process ................................ ................................ ...................... 34 Proof of Concept ................................ ................................ .............................. 34 Separation speed ................................ ................................ ....................... 35 Changes of solids cont ent ................................ ................................ .......... 36 Supernatant water ................................ ................................ ...................... 38 4 APPLICATION OF ELECTROKINETIC SEPARATION TO CONTINUOUS OPERATIONS ................................ ................................ ................................ ........ 48 Horizontal Flow Thickener ................................ ................................ ...................... 48 Vertical Flow Thickener ................................ ................................ ........................... 49 Inclined Surface Thickener ................................ ................................ ..................... 49 5 CONCLUSIONS AND FUTURE WORK ................................ ................................ 51 LIST OF REFERENCES ................................ ................................ ............................... 52 BIOGRAPHICAL SKE TCH ................................ ................................ ............................ 54

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7 LIST OF TABLES Table page 3 1 List of solids content in different height of clay suspensions. ............................. 39 3 2 List of turbidity of different supernatant water samples. ................................ ...... 39

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8 LIST OF FIGURES Figure page 2 1 Schematic of bench top cell with labeled locations of the electrodes and the temperature and voltage measurements. ................................ ........................... 26 2 2 Representation of a horizontal flow thickener. ................................ .................... 27 2 3 Representation of a vertical flow thickener. ................................ ........................ 27 2 4 Flow direction of clay suspensions in the inclined surface thickener. ................. 28 2 5 Equivalent floor space required by a horizontal flow thickener. .......................... 28 3 1 Large scale semi continuous flow system. ................................ ......................... 40 3 2 Top view of large scale semi continuous flow tank. ................................ ............ 40 3 3 Plastic cylinder used for sample collection. ................................ ........................ 41 3 4 Settl ing tank before and after separation with an applied potential. .................... 41 3 5 Side view of tank after electrokinetic separation (10 V, 18 hours). ..................... 41 3 6 Settling tank before and after separation without potential applied. .................... 42 3 7 Separation result after potential influenced on gravity thickened clay suspensions. ................................ ................................ ................................ ....... 42 3 8 Electrode configuration in deep tank semi continuous flow system. ................... 43 3 9 External appearance of whole system in operation. ................................ ........... 43 3 10 Initial clay suspensions at t=0. ................................ ................................ ............ 44 3 11 Separation after t=5 hours. ................................ ................................ ................. 44 3 12 Separation after t=11 hours. ................................ ................................ ............... 45 3 13 Separation after t=19 hours. ................................ ................................ ............... 46 3 14 Clay suspensions at the end of the experiment ................................ ................. 46 3 15 Calculation results of solids content after separation. ................................ ......... 47 3 16 Supernatant water collected from different experiments ................................ .... 47 4 1 Different arrangement of electrodes in horizontal flow thickener. ....................... 50

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9 4 2 Electric field between electrodes when plates are made from different materials. ................................ ................................ ................................ ............ 50

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10 Abstract of Thesis Presented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Master of Science SEMICON TINUOUS ELECTROKINETIC DEWATERING OF CLAY SUSPENSIONS By Rui Kong August 2011 Chair: Mark E. Orazem Major: Chemical Engineering The processing and storage of the phosphatic clay suspensions which result as a waste product from phosphate mining is a lon gstanding problem A very long time is required for t he gravity driven settling of clay suspensions with an initial 2 3 wt% solids content and decades may be required to reach the desired target of 25 30 wt% solids content. In previous work, b ench top exp eriments of electrokinetic dewatering were used to guide the development of a constitutive relationship describing the changes in solids content with time and electric field. In this thesis, a semi continuous electrokinetic dewa tering process was designed and tested A unique feature of the system is that the electrodes were placed close to each other, leaving a large volume below the electrodes in which the clay could settle. The experimental results showed that electrokinetic dewatering occurs in semi con tinuous operation and that the close electrode spacing, which greatly reduced the power requirement, did not impair separation. These results motivated a discussion of how electrokinetic effects could be incorporated into existing thickening equipment. Sug gestions are made for future experiments.

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11 CHAPTER 1 INTRODUCTION Phosphatic clay suspensions are a major waste product of the Florida phosphate mining industry which raise s environmental concerns for phosphate mining companies. The clay slurry which ha s an initial solids content of 2 3 wt % is pumped to large area disposal ponds for natural settling. This clay settling process takes decades to reach the demanded value of 20 25 wt % solids content. The se clay ponds currently cover an area of over 100,000 acres in Florida, which is approximately 30% of the mined land. Thus, there are benefits to develop ing a technique for phosphatic clay suspension dewatering, two of which are saving land currently used for disp osal ponds and recycling water. Previous benc h top experiments performed by Patrick McKinney demonstrated the effects of electrokinetic dewatering A constitutive relationship was established and p ower and energy consumption were calculated. The analy sis showed that energy consumption was in a reason able range but power consumption was too high. Thus, in this investigation a larger scale semi continuous flow system was designed and implemented to further evaluate the electrokinetic dewatering process. The separation speed, changes of solids content, and supernatant water turbidity were estimated in this process. The separation results were found to improve considerably with increasing applied potential. The idea of improving existing thickening instruments by introducing an electric field shal l be di scussed in C hapter 4 The discussion is focus ed on optimizing the electrode configuration for maxi mal efficiency Horizontal thickener s and inclined surface thickener s are believed to hold promis e for incorporating electrokinetic dewatering.

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12 CHAPTER 2 LIT ERATURE REVIEW Methods of Clay Dewatering Numerous methods have been attempted in the search for a better phosphate clay dewatering process. Some of them showed feasibility in laboratory scale experiments and were subsequently tested in large scale in sit u, environments. Freeze Thaw This method consist s of two stages: freeze and thaw. During freezing process clay particles and water are both frozen and separated from each other. Upon thawing, ice melt s in to water while clay remain s dehydrated allowing fo r clay particles to settl e Finally, supernatant w ater was removed from the clay suspension. Although it was reported that the solids content could be increased from 13.7 wt % to 42 wt % in this manner this method still failed because of the high energy cos t [ 1 ] Moving Screen This technique used the idea of passing a moving screen through the clay suspensions to destroy the structure of the gel like slurry. The supernatant water was removed periodically. It was found that the solids content increased upon t he decreasing speed of the moving screen. However, it is not practical to wait such a long time to reach the required level of dewatering [ 1 ]. Sand C lay Sandwich Proces s The sand tailings and clay slurry we re stacked layer by layer into a sandwich like structure. The sand layer simultaneously functioned as a drainage path and also put weight on the lower clay layer s to aid dewatering. In this procedure a sand layer is put on the bottom and a clay layer is p laced above it After the clay layer reache s a

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13 sufficiently high density and is stable enough to support another sand layer, the same operation is repeated. However, m any problems were seen in the field test. The waiting time between applying different lay ers was relatively long, thus more ponds were required for this process [ 1 ] D istribut ing each layer evenly is also a difficult problem when one is dealing with to a large area Flocculation Method This is curr ently the most widely used method in the mining industry for waste water treatment. The basic idea is to mix clay with flocculants which induce the small clay particles to coagulate thus form ing a larger cluster These much mo re rea dily than untreated suspensions This method has been used to achieve a 10 wt% solids content starting from 3 wt% Electrokinetic Dewatering The electrokinetic dewatering method use s a direct current applied across two electrode s which are placed on either side of the clay, causing electro osmosis of the water molecule s and electrophoresis of charged clay particles in the colloidal system. The study of this technique began in the when t he US Bureau of Mines began its pioneer ing research on the electrokinetic dewatering of tailings. Several successful field applications were reported, but there is still limited understanding of the electrokinetic principles which resulted in the following effects [ 2 ] : Large variance in the effectiveness of this t echnique with respect to the material to which it is applied High power consumption in some cases. Improperly designed operating systems.

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14 Principles of Electrokinetic Dewatering Electrokinetic Dewatering Process A typical electrokinetic clay dewatering p rocess can be divided into two stages: s edimentation and consolidation. Sedimentation In the process of sedimentation, suspended clay solids settl e under the combin ed influence of electrokinetic gravitational and electrochemical forces. The first of thes e concerns the motion of electrically charged par ticles under an applied field. Here we will be concerned primarily with the electrokinetic processes of e lectrophoresis and dielectrophore sis osmosis). T his e ffect is used in conjunction with the gravitational force which causes the clay particles to slowly drift to the bottom of the suspension. Finally, electrochemical forces are important since they determine the rate of interaction and aggregation amongst i ndividual clay particles, and this, in turn, determines how effective the previous two influences are. A theory of electrokinetic sedimentation has been proposed based on the combined action of electrophoretic, Stokesian, and interparticle forces on fine c lay particles suspended in water while under the influence of an external direct current electric field. The theory predict s that the sedimentation velocity of solid suspensions is proportional to the applied current density in the free settling stage and is dominated by the porosity of the suspensions in the subsequent hindered settling stage. A theoretical and experimental study have been conducted to verify this theory [ 3 ]

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15 Consolidation Once a porous soil is formed it is further consolidated ; primarily by electro osmosis. This is the consolidation stage. Mechanical dewatering methods based on gravitational settling, filtration, centrifugation, or hydraulic flow induced by applied pressure or vacuum techniques, all become ineffective in dewatering suspens ions of particles smaller th by mechanical methods, the particles move closer together, thus decreasing the size of pores through which the water must flow and drastically diminishing the rate of water removal Thus electro osmotic dewatering becomes the ideal method for the further removal of water trapped in the rather compacted fine clays because this mechanism is based on the electrostatic effects operating in the electrochemical double layers formed at the cla y particle/water interfaces in the wet clays [ 4 ] Electro osmotic consolidation has two functions: further reducing the soil water content and balancing electrochemical effects. Electrophoresis and Electro O smosis Electrophoresis and electro osmosis are amongst the most important principles in the electrokinetics of clay dewatering, and they have been discussed extensively. The former is the movement of colloidal particles in a direct current electric field while the latter is the flow of water in porous media due to a direct current electric field. During the process of electrophoresis and electro osmosis, the water velocity can be expressed as a function of the applied electric field (E) via : (2 1)

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16 w here is the permittivity of water, represents the zeta potential which is the potential at the inner limit of the diffuse layer, and is the viscosity of water [ 5 ] The effectiveness of electro osmotic de watering is governed by the electro osmotic permeability which can be defined by the empirical relation [ 6 ] : (2 2) w here is the flow rate of water in m 3 /s; is the sectional area normal to the direction of current density in m; is the current density in A/m; is the electrical conductivity of clay slurry in S/m. Chemical Reactions on the E lectrodes The issue of chemical reactions on the electrodes is widely discussed in connection with the electrokinetic process. This is because the quality of water collected is one of the primary concerns for investigators, and chemicals released by the electrodes may have an adverse effect. The electrode reactions can be summarized as follows [ 7 ] : At the anode: (2 3) (2 4) At the cathode: (2 5) (2 6) (2 7)

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17 w here represents the anode metal and is the dissolved cation species i, in solution. Equation 2 3 states that the anode hydrolysis generates oxygen and reduces the solutio n pH value. As a result, a metallic anode will corrode, as shown in Equation 2 4. The solution pH will increase at the cathode and hydrogen will be gener ated, as shown in Equation 2 5. Cations are driven to the cathode by the electric field where they may reduc e to element metals, as shown in Equation 2 6, or more likely form hydroxides, as shown in Equation 2 7. With only several exceptions, such as KOH and NaOH, most hydroxides are insoluble at pH > 5 [ 8 ] During the electrokinetic process, the movement of H + and OH will change the sludge pH drastically. A pH gradient will be generated across the soil as a result of the electrode reactions. The acid front at the anode will advance across the clay suspensions toward the cathode by advection and diffusion effects [ 9 ] The net effect is the decrease of cathode pH in the later stage of treatment. Factors Influencing the Electrokinetic Process Discussions about the influe ntial factors during the electrokinetic process include both internal and external factor s. The internal or intrinsic factors are the physical and chemical properties of materials, including [ 2 ] : Grain size and mineral type In materials which have a fine grain size, the surface properties of par ticles are dominant, and thus electrokinetics works more efficiently. It is particularly effective in brown clay, which is likely a more favorable cover material than what is currently used due to its special properties.

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18 pH value The electrokinetics is ver y effective in a high pH environment (pH > 9), but not effective in a low pH environment (pH < 6). Salinity Techniques based on e lectrokinetics are not effective in materials with high salt concentrations [ 10 ] The external factors governing electrokinetic processes are those which are controlled by the experimenter by means an external operation system including: Current density The effective current density in an electrokinetic dewatering process is dependent upon the material properties of the medium und er consideration The magnitude and spatial distribution of the current density are determined by the applied voltage and spacing of electrodes which are the primary consideration s when designing the operation system. Electrode material The efficiency, co rrosion rate, and lifespan of electrodes are influenced by the materials used. Excluding prohibitively expensive materials such as silver and platinum, iron and copper are amongst the best materials for field applications; being more effective than aluminu m, lead and carbon black [ 11 ] Iron or steel electrodes have the advantage of low cost whereas copper or brass electrodes have higher conductivities Electrode layout This includes the choice of horizontal or vertical configuration for the electrode s It i s preferred to use horizontal electrode configurations for new disposal ponds. For

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19 existing tailing ponds, installation of horizontal electrode arrays may not be techn ically or economically feasible. Bench T op Experiment of Electrokinetic Dewatering McKin ney and Orazem [ 12 ] perform ed a series of bench top experiment s to examine the effect of the electric field on the phosphatic clay dewatering process. The clay suspensions were sampled from a mine in central Florida. The e xperimental set up is shown in F igu re 2 1. A Plexiglas cylinder filled with clay suspensions having a solids content of around 10 wt% was used in this experiment An e lectric field was added by a pair of mesh electrode s with a top cathode and bottom anode configuration. Sensors at different height s were used to evaluate the potential within the cell The parameters affecting the dewatering process were investigated in both short term and long term experiments. A constitutive relationship describing the dewatering process was formulated as; (2 8) w here is the change in solids content referenced to the initial composition, t is the elapsed time in hours, E is the electric field in V/cm, and n is a dimensionless parameter that controls the tr ansition from short time to long time behavior. E quation 2 8 shows a good agreement with experimental data indicating the change in solids content can be predicted for a given electric field as a function of elapsed time. From E quation 2 8 an upper limi t on the change in solids content can be found for a given electric field. The upper limit fraction b is given by : (2 9)

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20 The energy requirement can be expressed by (2 10) w here is the energy required (in W h/kg water removed), is the cell potential, is the current, t is the operation time, and is mass of water removed. The energy requirement for remov ing water in the McKinney and Orazem experiments ranged from 1.25 to 175 Wh/kg. The calculation result from Equation 2 10 had a good agreement with the experiment values and validated the constitutive relationship. Th e constitutive relationship, coup led with a boundary element model for sol ving was then used to estimate the power requirements for appl ying electrokinetic dewatering to a large scale settling area. This simulation was conducted with the mathematical model program CP3D [ 13 ] Two types of electrode configuration were investigated, horizontal and vertical. The simulation modeled a clay settling area with the dimensions of 1 mile in width, 1 mile in length and 40 feet in depth. The simulation results indicated that there was a uniform electric field when utilizing a horizontal electrode configuration and a non uniform electric field with a vertical configuration. The time and energy require d to achieve a 15 wt% increase in solids content when using a non uniform electric f ield were both greater than with a unifor m electric field. The notable result of the simulation is the short time required for the separation of water and the huge power requirement s The power required for increasing the solids content of a one square mi le clay settling area from 10 wt% to 25 wt% was 44,000 MW

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21 over 19 hours. While the energy requirement of 8.4 10 8 kWh is moderate, the question was focused on the power requirement. The power is proportional to the square of cell potential and was thus sign ificantly reduced for a smaller electric field. Equipment Used in Clay Thickening Based on the success of bench top experiments, electrokinetic dewatering is considered to be a promising method for extension to large scale continuous dewatering operation s However, it is worthwhile to first investigate the existing dewatering techniques which are currently used in the manufacturing industry. Horizontal Flow Thickener One traditional way of designing equipment for clay dewatering is to simply us e gravity, w hich means to allow clay particles to settle down under the influence of their own weight In these kinds of designs, each thickener has an inlet zone, an exit zone, a collection device, and a sludge withdrawal area. Figure 2 2 shows one typical gravity th ickener with circular tank. Slurry with low solids content (usually 1 2 wt %) will be pumped through the influent pipe into the tank. During the process of separation, the high density clay particles settle down at the bottom of the thickener, get collected in the hopper, and then scrapped out of the tank through a sludge pipe set at the bottom. The low density water fl o w s up to the top layer and then leaves through the effluent pipe. This apparatus can be operated continuously for clay sedimentation Some s imilar thickener designs are in the form of rectangular tanks [ 14 ]. The rectangular tank avoid s the short circuiting problems which frequently occur in circular tanks. If multiple thickeners are used, a rectangular tank will help to save space and materials in construction.

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22 Horizontal flow thickener s are widely used in the separation industry. Its advantages include the l ow cost o f operation and maintenance easy operation and thus small operation staffs [ 15 ] However, this method suffer s from some shortcomi ngs which include the l ong waiting time s for settl ing and less clear s upernatant water compared with other methods Vertical Flow Thickener Another thickener has a shape which is similar to the horizontal flow thickener but with a different flow direction Clay treated by vertical flow thickener s usually flow upward in the tank, through a layer of floc. During this process, heavy clay particles settle down at the bottom and clear water effluent flow s out pipe at the top. Vertical flow thickeners are someti mes called solid contact thickener or sludge blanket thickener This class of thickener usually has a cone shaped unit in the center of thickening tank [ 16 ] as shown in F igure 2 3 This area is called mixing zone Slurry enter s this area to mix with the flocculants, and then the mixture fl ows through the suspended layer of sludge near the bottom of tank entering into the space outside the center cone. This sludge blanket act s as a filter, keeping the flocculate solids in the bulk clay while rel eas ing the water to top. Due to the inverted cone design, the velocity decrease s as the fluid fl ows toward the upper layer. A t a certain point, the upward velocity of the water is balanced the downward velocity of solids. The solids become suspended at the bottom and act as a blanket layer to b lock new flowing sludge. This accelerat es flocculation and reduce s the amount of coagulants used. The vertical flow thickener is very efficien t for dewatering clay suspensions combined with the chemical flocculants. F urthermore, its special structure prevent s the

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23 short circuiting problem often appearing in horizontal flow thickener s However, due to the unstable property of the sludge, the control of the thickener is very complex. The s ludge blanket level is sensitive to changes in coagulant concentration raw water chemistry and temperature. It needs a very accurate distribution of inlet slurry across the entire tank and precise control of the sludge. The limited compression zone volume is also a disadvantage built in t o this system. Inclined S urface T hickener The inclined surface thickener (also known as lamella clarifier ) usually consists of a rectangular or circular tank inserted with a bunch of parallel spaced plates or tubes inclined at some specified angle. Tak in g inclined plates as an example [ 17 ] raw slurry fl ows from bottom to top through these plates. Under the influence of gravity, heavy solids settled down on the lower surface s of the plates and then slip down along the inclined surface to the hopper of the tank. As show n in F igure 2 4 B, Clay flowing through the inclined surface thickener has a larger settling distance D, than in the horizontal flow tank. This distance is related to the space between the plates by the relation: (2 11) w here is the angle the plates measured from the horizontal; d is the distance between two plates; and D is the settling distance Thus settling distance is increased by the factor O bviously the la rger the angle is, the smaller the value of and th us the larger the settlement distance will be. However, the maximum allowed angle is 60 because the angle of inclination must not exceed the angle of repose of the

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24 separated soli ds. Consequently, the maximum settlement distance is twice the distance between the plates. This type of lamella design is up to 10 times as space efficient as conventional gravity thickener since one may increas e the effective area onto which settling may occur One may see from Figure 2 5 that a reduction of the required floor space is achieved by inclining the plates and stacking them. Using an angle of 45 for heavy particles and 60 for light particles reduces the required horizontal projected area by a factor of The surface area diagram ( F igure 2 5 ) graphically compares the floor space requirements of an inclined surface thickener with the equivalent horizontal projected settling area [ 18 ] The plate angle are commonly chosen from 45 to 60 The solids are unlikely to settle down on surface s with angle s larger than this. On the other hand, i f the angle is too small, solids will accumulate on the plates instead of sliding down. Sometimes people use inclined tubes instead of pla tes. The se are specially designed cellular structures consisting of hexagonal tubes and usually designed for use in oil separati on pools. The mechanism used is the same as with the lamella above and they both need laminar flow for operation. This requires a Reynolds number of less than 800 [ 19 ] Other T hickeners There are several thickeners which take advantages of mechanisms other than gravity. These thickeners include belt thickener s screw thickener s and centrifuge thickener s

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25 Belt thickener The belt thi ckener is usually composed of a moving belt with a conveying surface arranged horizontally. The initial slurry feeding from one side of the thickener is conveyed to the other side on the surface of the belt. A feeding reactor ensure s a uniform distribution over the full width of the continuously moving filter belt. The water filtered through the belt filter cloth is drained off into collection troughs. The volume of initial clay slurry can be reduced by approx 85%. Once the sludge cake ha s been discharged i nto the thick sludge collection trough a spray bar then clean s the filter belt [ 20 ] This method is used as a primary clarifier and the resultant thickened clay has a solids content of about 6%. Screw thickener The screw thickener, used for wastewater treat ment, look s like an inclined baby bottle. In this tank, a screw slowly rotate s and convey s the sludge upward through the inclined basket. The thickened clay is collected in another tank at the top of the basket. Water is drained through the basket and fl ow s out from the bottom effluent pipe. It is reported that the sludge volume reduction can reach to up to 90%. Power consumption is around 35 W/m 3 [ 20 ] Centrifuge thickener Centrifuges have been applied in wastewater treatment ever since the 1930s. A c entrifuge thickener consists of a cylindrical bowl wit h a scroll inside. S lurry initially fl ows into the tank and is then separated by the high speed of rotation. As a result, clear liquid fl ows out from one side and thickened clay is removed from the other end of cylindrical.

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26 This separation is driven by the force due to the rotation of the central scroll. The percent solid output can be varied by changing the operation parameter s The centrifuge thickener has a low space requireme nt, and thus it is economical for a small plant. The cleaning work for this instrument is easy, and the operation and maintenance cost is fairly low. However, this thickener still suffer s from the disadvantages of high power consumption and capital cost wh ich inhibit it s usage in many field applications [ 21 ] Figure 2 1. Schematic of bench top cell with labeled locations of the electrodes and the temperature and voltage measurements.

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27 Figure 2 2. Representation of a horizo ntal flow thickener. Figure 2 3 Representation of a vertical flow thickener. Effluent Influent Sludge b low off Agitator Mixing Zone

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28 A B Figure 2 4 Flow direction of clay suspensions in the inclined surface thickener. A) Slurry flow through the inclined plate surface. B) Relation between settling distance and inclined angle. Figure 2 5 Equivalent floor space required by a horizontal flow thickener. Inclined Plate s Equivalent Space

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29 CHAPTER 3 EXPERIMENTAL EVALUAT ION OF S EMI CONTINUOUS FLOW OPERATION PROCEDURE La r ge S cale S emi C ontinuous F low S ystem The success of bench top electrokinetic dewatering has been verified by a series of experiments. However, to adapt th is method to mining industry, larger scale equipment with semi continuous feature s would be necessary Thus, in this investigation, a cubic settling tank with large dimensions was designed and evaluated for separation. Cell D esign The basic separation process is similar to the normal gravity settling process The i nit ial clay suspensions were pumped into the settling tank and allowed to settle under the influence of an electric field. After separation, water fl owed out from the surface effluent pipe and thickened clay accumulated at the bottom of the tank. As shown in F igure 3 1, a plastic storage box with dimensions of 88.3 cm 41.9 cm 15.2 cm was used as a settling tank Two mesh plate electrodes made of titanium with a ruthenium oxide coating (Siemens, Inc.) were set horizontally in the tank with a top cathode and bot tom anode configuration. The d istance between the electrodes wa s 10 cm. Each mesh plate electrode was connected to a potentiostat instrument by a titanium wire. The wire was sealed in a silicon tube to prevent exposure to water. Two PVC tubes were used to con duct the influent and effluent ( Figure 3 2). The y were ke pt horizontal to ensur e an even distribution of liquid along the whole tube. Instrument s The influent clay suspensions were controlled by a Masterflex Model 77202 60 digital pump (Cole Parmer Inst rument Company).

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30 The results were obtained using an EG&G Princeton Applied Research (PAR) Model 273A Potentiostat/ Gavanostat, under potentiostat control using CorrWare software (Scribner Associates, Inc.). Operations were conducted during daytime and sto pped at night. The duty cycle for the experiments included 30 minutes with potential applied, followed by 2 minutes without potential applied. Experiment P rocess The clay samples were provided by Mosaic Fertilizer, LLC, pretreated by flocculent addition a nd allowed to reach an initial solid s content of around 11 wt%. The p otential was held at 10 V (resulting in an electric field of 1 V/cm) and the flow rate was 20 ml/min. The potentiostat and pump started at t=0. During the separation process, p otential an d current values were recorded and ph otos were taken every hour. To minimize the evaporation effect, the tank was covered at night when the operation was suspended After the operation supernatant water was removed thoroughly and clay samples were collec ted. In the sample collection process, a plastic cylinder of 28 cm in length and 2.6 cm in diameter was used. The upper opening was covered when t he cylinder was inserted straight into the clay. Due to the air pressure, the clay samples remain ed in the cy linder when it was pulled out of the tank. As electrodes covered nearly the entire surface area of tank, sampling spots could only be selected on one side of the tank. We chose several sampl ing spots in a straight line at a certain interval. The solids con tent of clay samples was obtained by the following method. A n empty 200 ml beaker was w eigh ed and the result was labeled as W 1 C lay samples were then transported in to the beaker, and the wet clay weight in the beaker was

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31 record ed as W 2 S amples were then moved to the oven to dry. After drying the dry clay weight (including the beaker) was measure d and record ed as W 3 The solids content was then calculated with E quation 3 1 : (3 1) Clay samples were taken before and after the ex periment and compared using the above formula. To evaluate the separation results, a parallel control experiment was taken under the same condition s The only difference was that no potential was applied during the process, and gravity was the only force i nducing the settling process Separation R esults The e xperiment was term inated after applying the potential for 18 hours using the previously described work cycle The significant change produced during the course of operation (F igure 3 4 ) indicate s the pow erful effect of electrokinetic separation on the larger scale basin. Solids content was increased from 11.85 wt% to 19.55 wt%. Clear water was separated from the clay suspensions and moved to the top layer of tank. The surface was divided into two distinct areas the left part contained clear water which was moving out, and the right part was the influent clay suspensions waiting to settle down. In principle, i f the separation speed were lower than the influent speed of the clay suspensions and the process were continuous, the boundary would slowly move from right to left until a steady state was achieved Because this is not a completely continuous process, this theoretic steady state could not be realized The side view of tank shown in F igure 3 5 g ives a clear picture of the result s

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32 A control experiment was taken under the same conditions and the result is shown in F igure 3 6. In this process, s olids content was increased from 11.72 wt% to 14.34 wt%. The o peration was terminated when the containers becam e overfill ed Finally the whole tank was filled with clay suspensions and no supernatant water was observed. To further verify the effect of an electric field, a controlled potential of 10 V was added to the separated clay suspensions from the blank expe riment. After 6 hours of operation, the solids content was increased from 14.34 wt% to 18.70 wt% and an obvious layer of supernatant water appeared as shown in F igure 3 7. Thus a change in solids of =7.7 wt% was achieved for an electric field of 1 V/cm. In the gravity settling process, th e value was =2.62 wt%, indicating a higher efficiency was achieved by applying an electric field. This difference could also be observed from the external appearance o f clay suspensions. Supernatant water occu pying about 1/3 the heig ht of the tank could be seen after electrokinetic separation. In contrast, in the gravity settling experiment, clay suspensions occupied the whole tank during the entire operation process D ue to the limited tank height the treatment capacity was too low to support a longer operation The basin would be filled with clay in 20 hours and the operation would be forced to terminate. Thus, to further estimate the semi continuous electrokinetics d ewatering process the settling tank was replaced by a deeper one Deep Tank Semi C ontinuous Flow System As has been explained, electrokinetic dewatering is more efficient than natural gravity thickening. However, simulations performed using the CP3D softw are show that applying this process to dewatering ponds with standard dimensions leads to

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33 unacceptably high power requirements. To overcome this problem, we studied the efficiency of deep tank semi continuous flow systems with a unique feature of suspended electrodes Cell D esign Since the shallow tank did not have enough capacity for semi continuous steady state operation a deep tank is needed instead However, if a deep tank with the same design as before were used the distance between two electrodes w ould be too large and the resultant power requirement would be too high T o avoid this problem, the bottom electrode was designed to be suspend ed in the clay suspensions to reduce the distance. The new electrode configuration is shown in F igure 3 8 A plas tic storage box with the dimensions of 88.9 cm 42.5 cm 32.7 cm was used as the settling tank. This tank had a larger surface area and a height compar able to the bench top cell. Two mesh plate electrodes made of titanium with a ruthenium oxide coating (Siem ens, Inc.) were suspended in the tank at an adjustable position. In the following experiment s the distance between the two electrodes was adjusted to 10 cm Thus, the influent clay suspensions experience d two stages in the separation process In the first step, the clay particles separate d from water and move d downward in the tank under the influence of the electric field When these particles passed the lower electrode, they settle d down and accumulate d at the bottom by gravity. The overall flow system is shown in F igure 3 9. I nitial clay su spensions in the right bucket were pumped in to the tank for settling, and supernatant water fl owed out via the left tube to the left bucket.

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34 Instrument s The results were obtained using an EG&G Princeton Applied Research (PAR) Model 273A Potentiostat/ Gavanostat, under potentiostat control by using CorrWare softwa re (Scribner Associates, Inc.). The influent clay suspensions were controlled by a Masterflex Model 77202 60 digital pump (Cole Parmer Instrument Company). The tu rbidity of supernatant water was measured using a HACH 2100N Laboratory Turbidimeter. Experiment al P rocess The tank was filled with clay suspensions provided by Mosaic Fertilizer, LLC, pretreated by flocculent addition and then allowed to reach an initial solid s content of around 10 wt% The o peration process was similar to that of the shallow tank. One bench mode experiment without continuous flow was conducted to evaluate the separation result s for different height level s Two flow by operations were car ried out to compare the result of electrokinetic dewatering and gravity settling After the experiment, supernatant water was removed thoroughly and clay samples were collected. To collect clay samples at different height s a device consisting of a glass tu be with the inner diameter of 6 mm connected to a syringe was constructed Th is tube could be moved to any layer of the clay to collect samples Proof of C oncept Two parallel experiments were conducted o ne with the constant potential applied between two e lectrodes and another with out potential applied. The control experiment produced results that were similar to what is seen in the settling process as it occurs naturally in a settling pond in mining industry. In this experiment, t he advantages of

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35 electroki netic dewatering were seen in the separation speed, change s of solid s content, and the turbidity of supernatant water. Separation s peed At the beginning of the experiment s both tanks were filled with well mixed clay suspensions T he same operation c onditi ons were used in both cases, with the exception of the applied electric field (Figure 3 10) After the 5 hours of operation clear difference s between the two suspensions can be observed ( Figure 3 11 ) For the clay suspensions with potential applied, an ob scure boundary between water and condensed clay appeared from the side view. In the top view, the clay suspensions became much more dilute ; b ubbles produced at the electrode surface during the process float ed up to the top and disturbed the supernatant wa ter. In the control experiment, few changes could be observed f rom the side view. Some evidence of sedimentation was seen in t op view by the clay sheets floating at the water surface. After 11 hours, significant effect s caused by electric field appeared b oth i n the side view and top view. Observing from the top of the system, the supernatant water became so transparent that the top electrode was visible through it From the side view, a distinct boundary between water and clay was formed as an uneven line. This is because of the accumulation of the influent on the right side of the tank. The control experiment did not show any changes from the side view. However, from the top view of the tank, the clay displayed an interesting structure containing many nar row cr acks.

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36 After 19 hours, in the system affected by electric field, supernatant water bec a me very clear. Condensed clay accumulated at the right side of the tank and forced water to move to the left. A clear boundary was formed in this process and slowl y extended to occupy approximately 1/4 the l ength of the tank. This system seem ed approach steady state. In the corresponding control experiment, because the influent flow speed was higher than the settling speed, some clay accumulated at the surface of th e effluent pipe and blocked at the edge which lead the effluent to become mixed with a lot of clay particle s After 32 hours, the surface boundary m oved to the middle of the tank. At this point t he operation was terminated and the electric field was remov ed. Although there was some supernatant water indicating the settling in the control experiment, the significant difference from both top view and side view between the two experiments proved the eff icacy of electrokinetics dewatering C hanges of solid s co ntent Bench mode experiment al results are listed in T able 3 1. Nine different locations were selected by dividing the horizontal and vertical direction s into three segments The distance between the clay surface and sampling point s were 4 cm, 10.2 cm, and 16.2 cm in order of decreasing height. The data indicate s that the surface (4 cm) solids content has the lowest value and that there is no significant difference between the middle (10.2 cm) and bottom (16.2 cm) layer s The low solids content in the upper layer might due to the incomplete decant of the supernatant water which may have remain ed in the clay structure. Thus, it could be s peculate d that the solids content is uniform with respect to height.

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37 In the semi continuous flow experiment, t he average i nitial solid s content s were 10.74 wt % and 10.56 wt % correspond ing to the experiment s with and without electric field. Final solid s content was evaluated by sampling different spots along the horizontal flow direction. The sample locations and resul ts are represented in F igure 3 15. The gray box represent s the experiment al tank and t he blue cylinders inserted in to the box denote the sampling locations and the shape of sampling area. They are marked with the solid s content of the final of measurements The average solid s content with an applied potential was 15.08 wt % which correspond s to a change in solids content of 4.34 wt%. The result from the constitutive relationship (Equation 2 8) suggest ed that (Changes of solids content) should be achieved when the system reache s stead y st ate Thus, the change of solids content from semi continuous flow system wa s lower than expected on the basis of the relationship developed from bench top experiment. The data shows that the solids conten t decreas es from 16.96 wt % to 13.70 wt % along the flow direction. This result can be explained by the shape of the thickened clay. Because of the inclined surface formed during the accumulation process, water tend ed to flow to lower level s Thus, the efflu ent part of the tank maintain ed more water both in the bulk and on the surface. As we did not wait a sufficiently long time to decant the surface water thoroughly, this upper portion appear ed to have a lower solid s content. However, another issue affecting the solid s content on the influent side is the evaporation effect from the expos ed part of clay. With the boundary line moving from right to left, more and more surface clay was exposed to evaporation. This affect ed the

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38 clay surface up to the mid point a s this is the final position of the boundary layer A future experiment that would avoid this evaporation effect was proposed The average final solid s content in the control experiment was 13.47 wt %, correspond ing to a change in solids content of 2.91 wt% The solids content displayed no obvious relationship with horizontal position s This observation was consistent with the result from the experiment with applied potential B ecause almost the entire clay surface was exposed to the air, surface water could be distribute d evenly everywhere. Thus, the separation result s showed that the change s of solids content achieved by the electrokinetic dewatering process was larger than the gravity only process but smaller tha n what is expected according to the constitu tive relationship. Supernatant w ater One of the purposes for develop ing the phosphat ic clay dewatering technique is to recycle water which makes it valuable to examine the quality of supernatant water. As shown in F igure 3 1 6 water in t he first beaker wa s collected at the very beginning of the separation process in the experiment with an applied potential It contain ed a lot of solids and appeared very turbid. The second beaker contained water collected from the first day. It wa s less turbid but still had some clay particles accumulated at the bottom of beaker. The third beaker contain ed the water collected after the first day and it wa s very clear and transparent. W ater in the fourth beaker was collected from the control experiment ; it was transparent but light brown in color. From the external appearance of these beakers of water, one may see that the supernatant water became clear er with increasing settling time Furthermore, the supernatant water from experiments both with and without an applied potenti al achieved transparen cy after

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39 a sufficient length of time. From the comparison, it is apparent that water removed by an electric field has a higher quality than water from normal gravity settling. The turbidity values of supernatant water samples were mea sured and listed in T able 3 2 Two measurements were taken f or each sample, the supernatant and the stirred value. The former represented the turbidity after a long settlement time and the latter represented the turbidity right after sampling. For the exp eriment with an applied potential the turbidity decreased with increas ing time, which is in consistent with the visual inspection Results from sample s C and D indicate the two methods achieved similar supernatant water turbidity, but the latter had a sli ghtly lower turbidity level. Table 3 1. List of solids content in different height of clay suspensions. Length measured from top surface /cm Solids content near effluent /wt% Solids content in middle /wt% Solids content near influent /wt% 4 14.07 13.69 12.29 10.2 15.72 14.98 14.80 16.2 15.26 14.85 15.72 Table 3 2 List of turbidity of different supernatant water samples. Sample Supernatant water turbidity/NTU Stirred water turbidity/NTU A 44 6635 B 12.2 5170 C 0.460 7.55 D 0.434 3.6 7

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40 Figure 3 1. Large scale semi continuous flow system. A B Figure 3 2. Top view of large scale semi continuous flow tank. A) Detail structure of effluent. B) Detai l structure of Influent.

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41 Figure 3 3. Plastic cylinder used for sample collection. A B Figure 3 4. Settling tank before and after separation with an applied potential. A) Initial clay suspensio ns with solids content of 11.85 wt%. B) After separation, with solids content of 19.55 wt%. Figure 3 5. Side view of tank after electrokinetic separation (10 V, 18 hours)

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42 A B Figure 3 6. Settling tank before and after separation without potential applied. A) Initial clay suspensions with solids content of 11.72 wt%. B) After separation, with solids content of 14.34 wt%. Figure 3 7. Separation result after potenti al influenced on gravity thickened clay suspensions.

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43 Figure 3 8. Electrode configuration in deep tank semi continuous flow system. Figure 3 9. External appearance of whole system in operation.

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44 A B C D Figure 3 10 Initial clay suspensions at t=0. A) Side view with potential. B) Side view without potential. C) Top view with potential. D) Top view without potential. A B C D Figure 3 11. Separation after t=5 hours. A) Side view with potential. B) Side view with out potential. C) Top view with potential. D) Top view without potential.

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45 A B C D Figure 3 12. Separation after t=11 hours. A) Side view with potential. B) Side view without potential. C) Top view with potential. D) Top view without potential.

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46 A B C D Figure 3 13. Separation after t=19 hours. A) Side view with potential. B) Side view without potential. C) Top view with potential. D) Top view without potential. A B C D Figure 3 14. Clay suspensions at the end of the experiment. A) Side view with potential. B) Side view without potential. C) Top view with potential. D) Top view without potential.

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47 A B Figure 3 15. Calculation results of solids content af ter separation A) With potential. B) W ithout potential A B C D Figure 3 1 6. Supernatant water collected from different experiments. A) Water from the beginning of separation, with electric field B) Water from the first day of separation, with electric field C) Water after the first day, with electric field D) Water from the blank experiment, without electric field

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48 CHAPTER 4 APPLICATION OF ELECT ROKINETIC SEPARATION TO CONTINUOUS OPERATIONS Now that we have demonstrated the feasibility of applying an electric field to improve the dewatering efficiency in a larger scale exper iment we shall now discuss the possibility of using electrokinetic separation in continuous flow operations. This discussion is base d on the existing instruments used in continuous dewatering ; namely, the horizontal flow thickener, vertical flow thickener and inclined surface thickener. Horizontal F low T hickener To apply electrokinetic dewatering in a horizontal flow thickener, designs similar to the bench top experiments were considered. Specifically two ways to add electrodes have been studied In the first one an anode is placed at the bottom of the tank and a cathode is positioned at the top surface ( F igure 4 1A ). In this design there is very limited space in which to set the surface electrode. It is necessary to be extremely careful to avoid the ex isting devices in the tank such as the center shaft and influent baffle. This structure is actually very similar to a n enlarged lab scale experiment. However, because of the scale of the thickener, the distance between two electrodes might be very large, a nd the electric field between them will be too great to meet power consumption specifications Thus, a modified design is considered. As shown in F igure 4 1B the position of the anode is raised to a certain height while the cathode remain s at the original position. In this manner the power cost is significantly reduced. The raw slurry influent will settle down under the effect of both gravity and electric field in the first stage, a nd accumulate only by gravity after it has passed anode With this method, we can take advantages of the electric field and avoid wasting energy as much as possible.

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49 If the efficiency of this process could be verified in future experiments, it is likely that this would become an attractive alternative to replace currently used d esigns Vertical F low T hickener Although the vertical flow thickener seems very similar to the horizontal flow thickener in shape, it is much harder to design electrode configurations on it. Because of the special mechanism s of vertical flow thickener s se paration efficiency is not determined by gravity alone Instead, multiple settling conditions should be considered. Obviously it is not proper to simply set the electrodes on vertical flow thickener s the same way as i n horizontal flow thickener s If electr odes are placed above the sludge blanket layer, this method itself be comes meaningless because most solids are locked in the previous process, and not too much effort need be expended at the upper layer. If the electrodes are moved to a slightly lower posi tion, with the cathode remaining above the sludge blanket layer and the anode below it, the existing subtle balance in this complex situation might be broken and a negative effect would appear. Thus, this kind of thickener is not easily modified with an el ectric field based on our current knowledge. Inclined S urface T hickener One reasonable method for introducing electrodes into an inclined surface thickener is to place the electrodes horizontally with the cathode at the top and the anode at the bottom. The selection of different plate materials is worth discussing here. If a conductive material is used ( F igure 4 2 A ), polarization of the plates will occur. This will result in a diminished potential along a segment of the inclined plates and create a free fal ling zone. T his effect will decrease the efficiency of the electrode plates. To avoid this negative effect, non conductive materials should be considered.

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50 If a non conductive material is used ( F igure 4 2 B ), the electric field lines will be oriented paralle l to the inclined plates. The component of the electric field that is parallel to gravity will act in the same direction as the natural deposition of the solids. The perpendicular component of the electric field will oppose the flow direction of the sludge This opposition to the flow will delay the settling of the solids allowing for more time before the sludge reaches the outlet area, which is the result we are seeking for. Thus, if one wishes to upgrade the inclined surface thickener by placing electrod es at the top and bottom, a non conductive plate material would be a better choice. A B Figure 4 1. Different arrangement of electrodes in horizontal flow thickener. A) Top cathode and bottom anode B) Top cathode and suspended anode A B Figure 4 2. Electric field between electrodes when plates are made from different materials A) Conductive plates B) Nonconductive plates

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51 CHAPTER 5 CONCLUSIONS AND FUTU RE WORK Motivated by the success of earlier bench top electrokinetic dewatering experiments and guided by the constitutive relationship developed therein, we have designed and performed a larger scale electrokinetic dewatering experiment thus establishing a proof of concept for this technique As c ompared to gravity settling the separation proc ess with a 10 V applied potential was much faster and more efficient. The final solids content with potential was higher than that of the blank experiment. The advantage of the electric field can also be demonstrated by the good clarity of the supernatant water. The water collected from the electrokinetic dewatering process was much clearer after a period of settlement Several designs for improving the existing continuous separation instruments with electrokinetic separation were considered These include several different electrode configuration s in the horizontal flow thickener, vertical flow thickener, and inclined surface thickener. The feasibility of these ideas needs to be verified by further effort s One of the future studies to be conducted is to ap ply the benc h top experiment constitutive relationship to the results of the larger scale semi continuous system The parameters affecting the dewatering behavior will be estimated by semi continuous flow experiment results. The residence time will be defi ned and the time required to reach steady state will be calculated using the constitutive relationship. A second future effort involves con struct ion of the equipment proposed for electrokinetic dewate ring in continuous flow system s

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52 LIST OF REFERENCES [1] W.E. Pittman, J.W. Sweeney A r eview of p hosphatic c lay d ewatering r esearch Final Report, Research Project FIPR Grant No 81 02 017, Florida I nstitute of Phosphate Research, 1983 [2] J.Q. Shang, K.Y. Lo, Electrokinetic dewatering of a phosphate clay J. Hazard Mater 55 (1997) 117 133. [3] J.Q. Shang, Electrokine tic sedimentation a theoretical and experimental study, Can. Geotech. J. 34 (1997) 305 314. [4] A.K. Vijh. Electro o smotic d ewatering of c lays, s oils, and s uspensions, Mod Aspects. Electroc 32 (2002) 301 332. [5 ] J. S. Newman, K E. Thomas Alyea Electroche mical S ystems, 2nd edition, Prentice Hall, Englewood Cliffs, New Jersey, 1991. [6] J.K. Mitchell. Fundamentals of S oil B ehavior, 2nd edition, John Wiley & Sons, New York, 1993. [7] J.Q. Shang, Electrokinetic dewatering of clay slurries as enginee red soil covers, Can. Geotech. J. 34 (1997) 78 86. [8] W.L. Lindsay. Chemical E quilibria in S oi ls John Wiley & Sons, New York 1979. [9] Y.B. Acar, J.T. Hamed, A.N. Alshawabkeh, R.J. Gale, Removal of cadmium (II) from saturated kaolinite by the application of electrical c urrent, Geotechnique. 44(2) (1994) 239 254. [10] N.C. Lockhart, Electroosmotic dewatering of clays, II. Influence of salt, acid and flocculants, Colloids Surf. 6(3) (1983) 239 251. [11] R.H. Sprute, D.J. Kelsh. Dewatering fine particle suspensions with direct current, In: Proc. Int. Symp. Fine Particle Processes, Las Vegas, Nevada. 2 (1980) 1828 1844. [12] J. P. McKinney M. E. Orazem, A c onstitutive r elationship for e lectrokinetic d ewatering of p hosphatic c lay s lurries, M iner Metall Proc 28 (2011) 49 54. [13] J.P. McKinney, M.E. Orazem, Electrokinetic d ewatering p hosphatic c lay s ettling a reas: n umerical s imulation and e conomic a ssessment, Miner Metal l Proc (2011) in press. [14] D. L. Russell, Practical W astewate r T reatment John Wiley & sons, Hoboken, New Jerse y 2006

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53 [15] S. R. Qasim, Wastewater t reatment p lants: p lanning, d esign, and o peration. Lancaster, Tec hnomic Publishing Company, Penn Sylvania 1994 [16] GC3, GC3 Technical m anual : c larificatio n, website. http://www.gc3.com/Default.aspx?tabid=89 accessed 18 October, 2010. [17] WaterWorld, Cooling t ower b lowdown t reatment u sing an i nclined p late c larifier, website. http://www.waterworld.com accessed 18 May, 2011. [1 8 ] Rexon, Clarifier, website. http://www.rexonassociates.com/WasteWaterTreatment_files/Clar ifier.htm, accessed 13 May, 2011. [1 9 ] K. M. Yao, Theoretical study of high rate sedimentation J. Water Poll. Control Fed. Water 42 (2) (1970) 218 228 [ 20 ] Huber Technology, Belt t hickener f or s ludge website. http://www.huber.de/ accessed 13 October, 2010. [2 1 ] United States E nvironmental P rotection A gency B iosolids t echnology f act s heet : c entrifuge t hickening and d ewatering EPA 832 F 00 053 2000

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54 BIOGRAPHICAL SKETCH Rui Kong graduated from China Unive rsity of Petroleum (East China) with a Bachelor of Science degree in materials chemistry in July of 2009. She entered graduate school in August of 2009 at the University of Florida into the Master of Engineering program in chemical engineering. Then s he joined Professor Mark E. in January of 2010 and transferred to the M aster of Science program for advanced study on the project of phosphate clay suspension dewatering spons ored by Mosaic Fertilizer, LLC. She received her M.S. from the University of Florida in the summer of 2011.