C.-D. Zhou, E. C. Stortz, E.J. Taylor, and R.P. Rem Faraday Technology, Inc Research Blvd. Dayton, OH 45420

Transcription

1 METAL RECOVERY WITH A NOVEL ELECTRODE C.-D. Zhou, E. C. Stortz, E.J. Taylor, and R.P. Rem Faraday Technology, Inc Research Blvd. Dayton, OH P ABSTRACT Metals are used in a broad range of industrial processes and products. Due to the toxicity of heavy metals, metal containing waste water must be adequately treated before being discharged. Electrowinning can recover metal in a metallic or concentrated solution form for recycle and purify water for reuse. However, due to the low reaction rate per unit electrode area and the mass transfer limitation, electrowinning is not efficient to treat dilute waste waters. In this study, we modified the graphite packed-bed electrode by coating the electrode with a cation exchange material. In this case, the reactions at the cathode include an ionexchange reaction on the coating and an electrochemical reaction on the graphite. Consequently, the overall metal reaction rate is enhanced. The ion-exchange material can also concentrate metal ions near the electrode prior to electrodeposition to enhance metal electrochemical reaction rate. Pulse current (PC) was used to increase mass transfer rate. The effects of the coating material and pulse current on the copper recovery process were studied. Our approach to metal recovery effectively integrates ion-exchange and electrochemical technologies into one unit. INTRODUCTION Metals are widely used in various industrial processes and products, including 1) metal plating, 2) paint additives, 3) ore processing and 4) the manufacture of photographic films. Due to the toxicity of heavy metals, metal containing waste water must be adequately treated before being discharged. The U. S. Environmental Protection Agency published its regulations for waste water discharge. Conventional methods for metal removal/recovery include precipitation, ionexchange, reverse-osmosis, evaporation and freeze crystallization. Precipitation often inadequately detoxifies waste water and generates undesirable sludge as a by-product, which creates new waste disposal problems. The ion-exchange, reverse osmosis, evaporation and freeze crystallization technologies can provide water with low metal concentration for discharge. However, the concentrated stream generated needs tbrther treatment. The electrochemical method for metal recovery has an advantage over the conventional methods. It can recover metal in metallic form for recycle and purify waste water for reuse as a rinse water. For dilute solutions, such as plating rinse water, electrolytic recovery of metal is not efficient due to the mass transfer limitation and low reaction rate per unit electrode area. A number of electrolytic cells have been designed to either enhance mass transfer rate or have a large surface area. Some of these designs are: porous electrode, concentric cylinder, l rotating cylinder,12) packed-bed,13-61 fluidized-bed,l6 *I and carbon fibre. A metal recovery approach coupled an electrochemical reactor for hgh concentration metal removal with an ionexchange unit for polishing of the low concentration metal. A drawback of this 1

2 approach is the high capital cost associated with the two metal recovery units. In this study, we modified the graphite packed-bed electrode by coating the electrode with a cation exchange material. In this case, the reactions at the cathode include an ionexchange reaction on the coating and an electrochemical reaction on the graphite. Consequently, the overall metal reaction rate is enhanced. The ion-exchange material can also concentrate metal ions near the electrode prior to electrodeposition to enhance metal electrochemical reaction rate. Pulse current (PC) was used to increase mass transfer rate. The effects of the coating material and pulse current on the copper recovery process were studied. Our approach to metal recovery effectively integrates ion-exchange and electrochemical technologies into one unit. Consequently, the capital investment is greatly reduced. Electrochemical Cell EXPERIMENTAL The structure of the electrochemical cell used in experiments is shown in Figure 1. The cell size is 0.12 m2. It includes three cell modules and each module has a cathode compartment and an anode compartment. A Nafionm membrane is used to separate the catholyte and the anolyte. The electrolyte volume in the cell is approximately 3 liters. Graphite particles coated with the novel material are used as a cathode. The equivalent diameter of the particles is 1 nun. The void fraction is about 0.5. The packing depth is 9 mm. The anode is DSA/02, ph below 2 electrode (Electrode Corporation, Chardon, OH). The distance between the anode surface and the membrane is approximately 2.5 mm. The packed-bed cathode almost reaches the membrane on the other side. There is a thin net between the cathode and the membrane. Experimental Setup Experiments were conducted to recover copper with a batch operation. The experimental setup is schematically shown in Figure 2. The system consisted of an electrochemical cell, a power supply, a catholyte holding tank, an anolyte holding tank, two metering pumps, two ph meters, two temperature meters and a voltammeter. Two metering pumps (Masterflex UP, Model 7529, Cole-Panner, Niles, IL) were used to feed catholyte and anolyte to the cathode compartment and anode compartment of the electrochemical cell, respectively, from the catholyte and anolyte holding tank. Two ph meters (Model , Cole-Parmer, Niles, IL) and two temperature meters (Model 2427, Westcon Corporation, Marion, OH) were used to monitor catholyte/anolyte ph and temperature during experiments. Two mixers (Model 6000, Arrow Engineering Co., Inc., Hillside, NJ) were used to stir catholyte and anolyte in the holding tanks. A pulse reverse power supply (Model DPR , Kraft Dynatronix Inc., Amery, WI) was used to supply either a constant current or a pulsed current to the electrochemical cell. A voltammeter (Model AP-101, Asahi Keiki Co., Ltd., Ohta-ku, Tokyo, Japan) was used to monitor the cathode-to-anode voltage during the experiment. ExDerimental Procedure Batch cell tests were carried out to recover copper from an acid copper sulfate solution with the novel material coated graphite electrode. Experiments were also conducted with the graphite electrode to serve as a baseline. DC and PC experiments were conducted at a solution recirculation rate of 0.5 L/min and 2.5 L/min. The catholyte was a 24 liter acid copper sulfate solution and the anolyte was 16 liters of 0.01 M H2S04. The graphite particles were dried and weighed. Afterwards, these particles were mixed with the novel material at a weight ratio of 1 part of the material to

3 parts of graphite particles and cured at 100 C for 1 hour. The treated graphite particles were put in the cell and used as a packed-bed cathode. Before the experiment, the acid copper sulfate solution and sulfuric acid solution were put in the catholyte and anolyte holding tanks, respectively. The mixers were turned on to stir the solution and a sample was taken from catholyte holding tank to analyze initial copper concentration. Two metering pumps were turned on to feed catholyte and anolyte to the electrochemical cell. Afterwards, a current from the power supply was applied to the cell. Samples were taken from the catholyte holding tank every 15 minutes to analyze copper concentration with a cyclic voltasnmetry method. During the experiment, the catholyte/anolyte temperatures were 70 to 80 F and the catholyte/anolyte ph was 2.2. RESULTS AND DISCUSSION DC and PC tests with average cell currents of 5 A, 10 A, 20 A and 40 A were conducted. The effects of coating material (with 1:lO weight ratio) and PC on copper recovery were studied. The Effect of Coating. Material at Low Cell Current The effect of coating material on copper recovery is studied at low average cell current of 5 A and 10 A with a solution recirculation rate of 0.5 Wmin and 2.5 L/min. Figure 3 shews t!e comparison of dimensionless copper concentration vs. dimensionless electrolysis time between the novel material coated graphite electrode and a graphite electrode for a PC run at an average cell current of 5A, 10% duty cycle, and 10 Hz frequency. The dimensionless copper concentration and dimensionless time are defined as: c* = CC,/C,,, (1) T = It/(nF)(VCc,,) (2) where, C,,, is the initial copper concentration (mom); and V is the solution volume (L). As shown in the Figure, it took much less time to decrease copper concentration for the coated graphite electrode compared to the graphte electrode when the average cell current was 5 A. Figures 4, 5, and 6 show copper concentration vs. electrolysis time at a solution flow rate of 0.5 L/min with PC of 10 Hz, 100 Hz, and 1000 Hz, respectively. All the PC runs had a 10% duty cycle and 10 A average current. As shown in these figures, copper concentration decreased much faster with the coated graphite electrode. Specifically, compared to the graphite electrode, it only took about 60% of the time for the coated electrode to treat the same amount of copper waste water. Figure 7 shows the comparison of instantaneous current efficiency for the PC 10 Hz run with the coated electrode and graphite electrode. Compared to the graphite electrode, the current efficiency improved considerably with the coated electrode. Figure 8 shows energy consumption per kilogram of copper recovered vs. copper concentration for these two runs. The cell-voltage was higher (3.2 V) with the coated graphite electrode than that (2.2 V) with the graphite electrode because of the lower conductivity of the coating. However, due to the higher operating efficiency with the coated graphite electrode, the power consumption per kilogram of copper recovered was comparable for these two runs. Table 1 summarizes the results of DC and PC at a solution flow rate of 0.5 Wmin with the coated electrode and the graphite electrode. the average cell current for all the PC runs was 10 A. As shown in the table, the average current efficiency is higher for all the runs with the coated electrode compared to the graphite electrode. To treat the same amount of waste water, electrolysis time can be reduced about 40% 3 479

4 with the coated electrode compared to the graphite electrode. Although the average cell voltage is higher with the coated electrode, the power consumption per kilogram of copper recovered is comparable due to the higher operating efficiency with the coated electrode. Consequently, compared to the graphite electrode, even though the operating cost is not reduced with the coated electrodes because they have comparable power consumption, the capital cost will be reduced with the coated electrodes since it takes less time to decrease the copper concentration. In other words, a smaller cell can be used to treat the same amount of waste water with the coated graphite electrode. Consequently, the cost for the treatment of the same amount of waste water with the coated electrode at average cell current of 10 A will be only 60% of the cost with graphite electrode. The Effect of Coating Material at Hi& Cell Current The effect of the coating material on the copper recovery process was studied at high cell currents of 20 A and 40 A with the weight ratio of coating material to graphite particles of 1 : 10. Figures 9 and 10 show the copper concentration vs. electrolysis time for a PC 20 A and a DC 40 A, respectively. The duty cycle was 10% and frequency was 10 Hz for the PC 20 A. As shown in Figure 9, there was no improvement with the coated graphite electrode (weight ratio of coating material to the graphite particle was 1 : 10) at a PC of 20 A compared to the graphite electrode. Figure i0 shows that the results of the coated electrode was comparable to graphite electrode at 40 A. cell currents of 5 A and 10 A; however, there was no improvement at high cell currents of 20 A and 40 A. The novel material coated on the graphite electrode has ion-exchange properties. Therefore, with the coated graphite electrode, the reactions at the cathode included an ionexchange reaction on the coating and an electroplating reaction on the graphite particles. The electroplating reaction rate is smaller at low cell currents of 5 A and 10 A compared to high cell currents of 20 A and 40 A. However, the ion-exchange reaction on the coating is comparable for both cases since the amount of the ion-exchange material coating is the same in both cases. Consequently, the effect of the ion-exchange reaction, i.e. the effect of the coating, on the overall copper recovery process was more apparent at low cell currents of 5 A and 10 A. Since we only coated a small amount of the material on the graphite particles, the effect of the ion-exchange reaction, i.e. the effect of coating, on the overall copper recovery rate may be negligible compared to the electroplating reaction at high cell currents of 20 A and 40 A. In the future, we will make electrodes with high coatinggraphite weight ratios to conduct experiments. It is expected that the metal recovery process will be improved at high cell current as well with the electrode having high coatinglgraphite weight ratio. The Effect of PC on Copper Recovery Process To examine the effect of PC on the copper recovery, Pe' with various frequencies and duty cycles were tested at a solution flow rate of 0.5 L/min. Table 2 summarizes the copper recovery Experiments were conducted at a DC of results for the coated electrode and the 20 A and a PC of average current of 10 A graphite electrode. As shown in Table 2, with with various frequencies at a 10% duty the coating to graphite particle weight ratio cycle. Figure 11 shows the copper of l:lo, copper recovery process was concentration vs. electrolysis time for a DC improved with the coated electrode at low of 20 A and several PC tests with average 480 4

5 cell current of 10 A at a solution flow rate of 0.5 L/min. As shown in the figure, it took less time for the PC 10 Hz run (the average current was 10 A) to decrease copper concentration compared to the DC 20 A run. The electrolysis time was comparable for the PC 100 Hz, 1000 Hz runs (10 A average current) and the DC 20 A. Due to the high cathode-to-anode voltage at DC 20 A, the energy consumption per kilogram of copper recovered was much higher for DC 20 A than that for these PC runs. Figures 12 and 13 show the instantaneous cathodic current efficiency and energy consumption per kilogram of copper recovered vs. copper concentration for these runs, respectively. As shown in the figures, current efficiency was much higher and energy consumption was much lower for PC electrolysis with a 10 A average current compared to DC 20 A. The results indicate that PC is superior to DC for copper recovery from an acid copper sulfate solution at a solution flow rate of 0.5 L/min. ACKNOWLEDGMENT This study was supported by the Advanced Research Project Agency (MA), Defense Sciences Oflice, under Contract MDA C REFERENCES [ 11 Keating, K.B., and Williams, J.M., 80th Annual AIChEi Meeting, Boston, MA, ) Holland, F.S., Chem. Ind., 7, 453 (1978). [3] Chu, A.K.P., Fleischmann, M., and Hills, G.J., J. Appl. Electrochem., 4, 323 (1974). 141 Chin, D.-T., and Eckert, B., Plat. and Surf. Fin., 63(10), 38 (1976). [5] Ho, S.P., Wang, Y.Y., and Wan, C.C., Water Res., 24, 1317 (1990). [6] Van der Heiden G., Raats, C.M.S., and Boon, H.F., Chem. Ing. tech., 51,651 (1979). 17) Barker, B.D., and Plunkett, B.A., Trans. Inst. Met. Fin., 54, 104 (1976). [S] Tyson, A.G., plat. and Surf. Fin., 71 (12), 44 (1984). 191 Vachon, D.T., et al., Plat. and Surf Fin,,73(4),68( 1986)

6 ~ Table 1: Results of copper recovery from an acid copper sulfate solution at a flow rate of 0.5 L/min. All PC runs have 10 A average current and 10% duty cycle. (1): graphite electrode; (2): the coated graphite electrode. Initial Cu (ppm) Final Cu (PPm) Time (min) Charge (kc) Ave. Cell Voltage Ave. Current Efficiency ( A) Ave. Energy Consumption (kwh/kg Cu) DC 2 : 20A PC : 10 Hz Pe2 : IO Hz 2.2 I 3.1 pc : loohz pc 2 : loohz PC : loo0hz pc 2. 1 OOOHZ Table 2: Comparison of Electrode withodwith Coating at a Solution Flow Rate of 2.5 L/min *. 5A 10 A 20 A 40A (without/with (without/with (withodwith coating) (withodwith coating) coating) coating) 10% IOHz, 10% 010% DC Time 195/ /150 I 150/135 I 120/ 135 *: initial copper concentration: 1000 ppm final copper concentration: < 5 ppm

7 1 cell unit 2 3 D 1 Anolyte 2 Catholyte ' T, '1 Figure 1 : The structure of electrochemical cell. electrolytic unit I 1 1 I-- & Catholyte metal containing holding tank water waste acid metering pumps 1 anolyte 1 holding tank 1 Figure 2: Experimental setun 7 483

8 PC: 10Hz,lO% duty cycle, lave=ba; 2.5 Umin c (II o z dimensionless time Figure 3 : The Comparison of Dimensionless Copper Concentration vs. Dimensionless Electrolysis Time for the Coated Graphite Electrode and a Graphite Electrode at a Solution Flow Rate of 2.5 L/min With a PC 5 A Average Current, 10 Hz, and 10% Duty Cycle. PC: lave=loa, 10 Hz, 10% duty cycle; 0.5 Umin I ia I U. I +without coating 200 +with coating t A I i I I I t Time (min) Figure 4: The comparison of copper concentration vs. electrolysis time for the coated electrode and graphite electrode at a solution flow rate of 0.5 L/min with a PC 10 Hz, 10% duty cycle, and 10 A average current

9 PC: lave=loa, 100 Hz, 10% duty cycle; 0.5 Umin I n g 20 a 0- I Time (min) Figure 5: The comparison of copper concentration vs. electrolysis time for the coated lectrode and graphite electrode at a solution flow rate of 0.5 L/min with a PC loohz, 10% duty cycle, and 10 A average current. PC: lave=loa, 1000 Hz, 10% duty cycle; 0.5 Umin I I I I I Time (min) Figure 6: The comparison of copper concentration vs. electrolysis time for the coated electrode and graphite electrode at a solution flow rste of0.5 Lhin with a PC :000iz, ig% duty cycie, and io A average current

10 PC: 10 Hz, 10% duty cycle, lave=loa; 0.5 Umin I i with coating I without coating Copper Conc. (ppm) I Figure 7: The comparison of instantaneous current efficiency for the coated electrode and graphite electrode at a solution flow rate of 0.5 L/min with a PC lohz, 10% duty cycle, and 10 A average current. PC: 10 Hz, 10% duty cycle, lave=loa; 0.5 Umin without coating Copper Conc. (ppm) Figure 8: The comparison of energy consumption per kilogram of copper recovered for the coated electrode and graphite electrode at a solution flow rate of 0.5 L/min with a PC lohz, 10% duty cycle, and 10 A average current

11 . PC: lave=poa, 10 Hz, 10% duty cycle; 2.5 Umin +with coating Time (min) Figure 9: The comparison of copper concentration vs. electrolysis time for the coated electrode and graphite electrode at a solution flow rate of 2.5 L/min with a PC IO&, 10% duty cycle, and 20 A average current. DC: 40 A; 2.5 Umin.- c m c r 0 z 0 u I I I I I electrolysis time (min) Figire IO: Tie Comparison of Copper Concentration vs. Electrolysis Time for the Coated Graphite Electrode and a Graphite Electrode at a Solution Flow Rate of 2.5 Lhin with a DC 40 A

12 ~~ Comparison of DC 20A and PC 1OA: 0.5 Umin --IcPC:100M,D=10% & PC:1 OM,Dl 0% -+-PC:1000M,D10% -rc- Dc:20A Time (min) Figure 11: Comparison of DC 20 A and several PC runs of average current of 10 A at a solution flow rate of 0.5 L/min with the coated graphite electrode. The Comparison of PC and DC 0.5 Umin PC:lOOHz,D=l O%,lave=l OA 45 4 I I PC:lOHz.D=lO%.lave=lOA PC:1000Hz,D=lO%,lave=1 OA 0 DC:20 A , A4 0 0 O0 1 I Copper Conc. (ppm) Figure 12: Instantaneous current efficiency comparison of DC 20 A and several PC runs of average current of 10 A at a solution flow rate of 0.5 L/min with the coated graphite electrode

13 1 ' I 120, The Comparison of PC and DC 0.5 Umin I 100 x x PC:lOOHz,D=l O%,lave=l OA PC:lOHz,D=l O%,lave=lOA A PC:1000Hz,D=l O%,lave=lO 1 A e M x M I Copper Conc. (ppm) Figure 13: Energy consumption comparison of DC 20 A and several PC runs of average current of 10 A at a solution flow rate of 0.5 L/min with the coated graphite electrode

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