Recovery of Cu and EDTA from EDTA-Cu Solution by use of Electrodialysis Accompanied by Electrochemical Reaction

Transcription

1 Vol., No. () Article Recovery of Cu and from Solution by use of Electrodialysis Accompanied by Electrochemical Reaction Hiroshi TAKAHASHI *, Etsuko KASHIUCHI and Kenzo MUNAKATA Department of Engineering in Applied Chemistry for Environments, Graduate School of Engineering and Resources Science, Akita University, -, Tegatagakuen-cho, Akita city, Akita -, Japan. (Manuscript received May, ; Accepted August, ) Abstract Ethylenediaminetetraacetic acid () forms a stable complex with copper. Electrodialysis treatment accompanied by electrochemical reaction was studied for the regeneration of and Cu from a solution of the complex. The electrodialysis stack consisted of three units of two cells containing and compartments, and bipolar electrodes set between the units. The cells in a unit were separated by a cation-exchange membrane. Electrochemical reactions for the regeneration of and Cu were successfully achieved by electrodialysis with one unit in a stack. Both the rate of disappearance and the rate of formation were almost proportional to the current density. The Cu deposition on the also proceeded well. Experiments were also conducted using electrodialysis with bipolar electrodes in a stack, and the rates of formation and disappearance were faster than those for the single unit experiments, and increased with the current density. The current efficiency of the electrochemical reaction on the was close to %, and the current leakage from the bipolar electrode in the stack was negligibly small under the conditions employed. The results suggest that electrodialysis with bipolar electrodes enables the effective regeneration of Cu and from a solution of the complex. All the experimental results were analyzed using a mathematical model that considered electrochemical reaction on the, ionic flux in the cation-exchange membrane, and the material balances of,, and sodium. The simulated results were in good agreement with the experimental results. Keywords: Electrodialysis, Electrochemical reaction, Bipolar electrode, Ion-exchange membrane,. Introduction Ethylenediaminetetraacetic acid () is a strong chelating agent that is widely used in industrial applications such as metal finishing ), electric circuits ), and the remediation of soils ). forms strong complexes with metal ions and ligands; however, it does not biodegrade rapidly. Therefore, there has been much research to develop effective recovery processes such as adsorption by ion-exchange reactions, precipitation, electrochemical processes, and electrodialysis. Although adsorption by ion-exchange reactions is often used for the recovery of soluble metal ions from effluent, the presence of chelating agents reduces the effective recovery of metals and chelating agents. Precipitation is also used for the regeneration of from soil washing effluent ). While the chelating agent can be regenerated by metal substitution reaction, the metals are precipitated as phosphates during processing ). Therefore, another process to recover the metals should be developed. One promising technique to recover metals and is electrodialysis accompanied by electrochemical reaction. Applications of electrodialysis to metal recovery and recycling from simulated washing effluents of metal-contaminated soils have been reported,,7), where a metal solution including is electrically reduced to produce metallic copper on the and dissociated in solution. Although possibilities for the electrodialysis accompanied by electrochemical reaction have been discussed, *Corresponding author jr7mlf@ac.as.akita-u.ac.jp

2 J. ION EXCHANGE there is little quantitative data such as the effect of the operating conditions on the rate of regeneration and recovery rate of copper on the. In this study, we examine the recovery characteristics of Cu and in batch-recycle electrodialysis accompanied by electrochemical reaction under various experimental conditions. C BE C BE BE C Na+ Na + Na+. Theory When a voltage is applied to the electrodialyzer, the anolyte provides Na + to carry the current through the cationexchange membrane into the compartment. in the compartment is then electrically reduced and deposited on the. The experimental results were analyzed using a mathematical model ) that considered the electrochemical reaction at the, the ionic flux in the cation-exchange membrane, and the material balances of,, Cu, and sodium. The electrochemical reaction at the proceeds via ) the following equation : Cu + + Cu () For batch-recycle operation, the material balances of and in the tank are given as follows: = ( ) () = ( ) () where r and r represent the electrochemical reaction rates for disappearance and formation at the, respectively. These reaction rates are given by the sum of reaction rates at the s in the stack. ( ) = ( ) = = ( ) () The deposition rate of Cu on the is expressed by: = ( ) () In the compartment, hydrogen ions are produced by electrochemical reaction during electrodialysis, but disappear by neutralization. Therefore, only sodium ions permeate the cation-exchange membrane. The ionic flux of sodium through the cation-exchange membrane at current density I, is given by: = () The material balance of sodium in the compartment is expressed as:. Materials = (7). Experimental i= i= i=n C: cation-exchange membrane BE: bipolar electrode Fig. Schematic diagram of electrodialysis accompanied by electrochemical reaction. The catholyte was prepared by dissolving equimolar amounts of CuSO and -Na in distilled water. The initial concentration of ranged from to mol/m. The anolyte was prepared by dissolving Na CO in distilled water.. Apparatus Figure shows a schematic diagram of the experimental apparatus used for electrodialysis. The electrodialysis stack consisted of three units of two cells with and compartments. The cells in a unit were separated by cationexchange membrane (SELEMION CMV). Two stainless steel (SUS) plates were placed into the cells as and. The effective areas of the cation-exchange membrane and the electrode plates were.9 - and.7 - m, respectively. The gap between the electrodes was. - m and the volume of each reservoir was. - m. When electrodialysis was operated in one unit, the electrode was connected separately to a power source, and maintained the same electrical charge on the surface. In contrast, when two or three units were operated, the power source was connected to the two outer electrodes. The interior electrode plates between units act as bipolar electrodes; the two electrode surfaces on each interior electrode have opposite charges. The solution containing and the solution were circulated through the cells to the tanks at a constant flow rate. All electrodialysis experiments were conducted at a constant current with a direct-current power supply. The solution in each tank was sampled during the experiments. The copper and concentrations in the solution were measured using atomic absorption spectrometry, and by titration with CuSO, respectively. The sodium concentration in the was also measured using atomic absorption spectrometry. The amount of copper deposited on the was determined by weight difference.. Results and discussion. Electrodialysis with single unit Electrodialysis experiments were conducted using one

3 Vol., No. () typical experimental data for the change in the concentrations of,, sodium, and the weight of copper deposited under conditions of C = mol/m, I=A/m. When a voltage was supplied to the electrodialyzer, the concentration and weight of Cu deposited on the increased with electrodialysis time. The concentration decreased with a corresponding increase in concentration. The sodium ion in the compartment moved to the compartment. The ph in the compartment gradually decreased, which suggests that the hydrogen ions produced by electrochemical reaction neutralized the solution. The lines in the figures show the values calculated with the mathematical model, which well agree with the experimental data. The agreement indicates that regeneration proceeds as a single reaction according to the stoichiometric equation shown in Eq. (). Figure shows the effect of the liquid velocity in the electrodialyzer on the initial rates of disappearance r, and formation, r. The rates remained almost constant, irrespective of the increasing velocity and were almost in agreement with each other, which indicates that the liquid film resistance is negligibly small under conditions used. Figure shows the relationship between the initialreaction rates and current density. Both reaction rates are almost proportional to the current density. The line in the figure represents the values calculated using the mathematical model. The relationship between the rate of Cu deposition on the and the current density is shown in Fig.. The Cu deposition rate on the increased with current density, and agreed well with the calculated values. These results suggest that the electroreduction reaction shown in Eq. () proceeds rapidly, and that no side reactions occur at w [mol/m ] Cu ph [-] Fig. Time courses for the amount of copper on the, and the concentrations of,, and sodium during electrodialysis with one unit. Conditions: I= A/m, C, = mol/m, u=. m/s. I= A/m Na the under the experimental conditions. r [mol/m s] r [mol/m s] Fig. Relationship between initial-reaction rates and current density for and under the condition of u=. m/s. r Cu [mol/m s]..... u [m/s] Fig. Effect of liquid velocity on the initial rates of disappearance and formation. Conditions: I= A/m, C, = mol/m. I [A/m ] C [mol/m ] C = mol/m C = mol/m I [A/m ] Fig. Relationship between initial-deposition rate of Cu and current density under the condition of u=. m/s. -r [mol/m s] -r [mol/m s]

4 J. ION EXCHANGE. Electrodialysis with two units Electrodialysis experiments were also conducted using two units with a bipolar electrode within a stack. Thus, the voltage was only supplied to the outside electrodes. Figure shows time courses for the amount of copper on the, and the concentrations of,, and sodium during electrodialysis with two units in a stack. While the concentration decreased linearly with increasing electrodialysis time, the concentration was proportional to the electrodialysis time. The amount of regenerated at the was approximately times that with one unit (Fig. ). The amount of Cu deposited per effective area of the was the same as that for electrodialysis with only one unit, because the current densities used for both experiments were the same. In addition, the sodium concentration in the compartment gradually decreased. The lines in the figures show the values calculated with the theoretical model, and the results are consistent with the experimental data. Figure 7 shows the initial-reaction rates for the formation of and disappearance of in the compartment, and the current density during electrodialysis with two units in a stack. Both reaction rates increased in proportion to the current density and were almost in accordance with each other. In addition, the reaction rates were proportional to the number of units in the stack for the same current density. The current density dependency of the Cu deposition rate on the is shown in Figure. The deposition rate was increased with the current density and was in good agreement with the calculated values, which suggests that the current efficiency of the electrochemical reaction at the was close to %, and current leakage from the bipolar electrode r [mol/m s] I [A/m ] Fig. 7 Relationship between initial-reaction rates and current density for and. Conditions: C, = mol/m, u=.9 m/s. r Cu [mol/m s] i= i= -r [mol/m s] I [A/m ] Fig. Relationship between initial-deposition rate on the and current density during electrodialysis with two units. Conditions: C, = mol/m, u=.9 m/s. w [mol/m ] Cu I= A/m u=.9 m/s Fig. Time courses for the amount of copper on the, and the concentrations of,, and sodium during electrodialysis with two units. ph [-] Na w [mol/m ] Cu ph [-] Fig. 9 Time courses for the amount of copper on the, and the concentrations of,, and sodium during electrodialysis with three units in a stack. I= A/m u=. m/s Na

5 Vol., No. () r [mol/m s] bipolar electrode (n=) r r cal. n [-] bipolar electrode (n=) Fig. Photographs of s after electrodialysis with three units in a stack. Fig. Relationship between initial-reaction rates of and, and the number of units in a stack. Conditions: I= A/m, C, = mol/m. in the stack was negligible.. Electrodialysis with three units Figure 9 shows experimental results for electrodialysis with three units in a stack. The concentrations of and increased and decreased, respectively, with increasing electrodialysis time. The sodium concentration in the compartment gradually decreased as in the case for a single unit. The experimental values were in good agreement with the calculated values. Figure shows photographs of the s after the electrodialysis experiment with three units in a stack. The photographs are shown in the order of in the stack, surface on the bipolar electrode in the second unit, and that in the third unit. While the surface of deposited copper on the was course and bulky, the other surface on the bipolar electrode was almost homogeneous. Dissolution of the surface with sulfuric acid regenerated the copper as a CuSO solution. Figure shows the relation between the reaction rates and the number of units. Both reaction rates increased in proportion to the number of units, and were in good agreement with the calculated values. These results indicate that electrodialysis with the bipolar electrode design enables the -r [mol/m s] effective regeneration of Cu and from a solution of the complex.. Conclusions The recovery of and Cu from solution was examined using electrodialysis accompanied by electrochemical reaction. Electrodialysis with one unit in a stack successfully regenerated and Cu by electrochemical reaction. Both the rates of disappearance and formation were almost proportional to the current density. The deposition of Cu on the also proceeded well. Electrodialysis experiments were also conducted using two units in a stack. The rates of formation and - Cu disappearance were times faster than those in the single unit experiments, and increased with the current density. The current efficiency of electrochemical reaction at the was close to %, and current leakage from the bipolar electrode in the stack was negligible under the conditions examined. Electrodialysis with three units also resulted in the successful recovery of and Cu from their complex solution. The recovery rates for and Cu were proportional to the number of units. All the experimental results were analyzed using a mathematical model that considered the electrochemical reaction on the, the ionic flux in the cation-exchange membrane, and the material balances of,, Cu, and sodium. The calculated results were in good agreement with the experimental results. Acknowledgment This work was supported by a Kakenhi Grant-in-Aid for Scientific Research (C) (No. 9) from the Japan Society for the Promotion of Science. The authors are grateful to the Ministry of Education, Culture, Sports, Science and Technology of Japan for support of this research. Nomenclature C concentration [mol/m ] F Faraday s constant [C/mol] I current density [A/m ] J flux in cation-exchange membrane [mol/m s] n number of units [-] r reaction rate in the solution [mol/m s] r Cu deposition rate on the [mol/m s] S effective area of electrode [m ] S m effective area of cation exchange membrane [m ] t dialysis time [s] u liquid velocity in the cell [m/s] V volume of reservoir [m ] w weight of copper on the per area [mol/m ]

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