R. Suzuki, W. Li, M Schwartz and K. Nobe Department of Chemical Engineering University of California Los Angeles, CA

Transcription

1 Electrochemical Treatment of Metal Plating Wastes Using Flow-through Porous Carbon Electrodes R. Suzuki, W. Li, M Schwartz and K. Nobe Department of Chemical Engineering University of California Los Angeles, CA Abstract A three-dimensional, porous electrode composed of reticulated vitreous carbon (RVC) has been examined for the selective removal of metals by electrodeposition from single and multi-component solutions. A three-stage system was envisioned where each metal from a ternary mixture could be selectively removed in each stage. Single and multi-component solutions of copper, nickel, zinc and silver were tested. The metal ion concentration of each component in all the solutions was 500 ppm. The flow-through system was operated potentiostatically to selectively remove each metal. Copper, nickel and zinc were easily removed from single component solutions using the RVC electrode. Copper was selectively removed from binary mixtures with nickel and zinc without significant loss of the other metal. nickel could not be removed from a binary mixture with zinc due to co-deposition. However Copper was readily deposited from a ternary mixture of copper/nickel/zinc without loss of the other metals. Ternary mixtures of silver/copper/zinc and silver/coppednickel were tested using a three-stage system. Silver was selectively removed from each mixture without loss of other metals. Copper was also readily removed from both mixtures but with some loss of zinc in the silver/copper/zinc mixture

2 I. Introduction Sludge-free electrolytic removal and recovery of metal ions from solution is becoming an increasingly important industrial technique which not only alleviates environmental concerns but also recovers metals for re-use. Since electrochemical reactions are heterogeneous and metals in metal-bearing waste streams frequently occur in very dilute solutions, the use of high surface area electrodes capable of efficient mass transfer is essential. Fixed flow-through porous electrodes have been reported to be effective for the removal of copper [l-41, antimony 151, and silver [6] from dilute solutions. Recently, a three dimensional flow-through, carbon-fiber electrode was utilized for recovering copper and lead from printed circuit board waste streams [7]. For removal and recovery of metals from isolated, segregated streams, the system can be operated in either a potentiostatic or galvanostatic mode. Potentiostatic operation is usually required to accomplish simultaneous removal and recovery of several metal ions from mixed solutions or commingled streams. Porous electrode studies which involve multiple electrochemical reactions have been reported for separation of metal mixtures by deposition and stripping 181. Alkire and Could have performed theoretical analysis of multiple reaction sequences in flow-through porous electrodes 191. This paper reports on a feasibility study of an electrochemical process for selective removal and recovery of copper, nickel and zinc from blended metal streams using a segmented, porous flow-through electrode where each section is under separate potentiostatic control. 11. Experimental Description of ccll and flow system A stationary electrochemical system was employed in preliminary studies of the 694 2

3 polarization behavior of copper, nickel and zinc in dilute solutions. The cathode was a glassy carbon disk and the anode consisted of a coiled platinum wire. A saturated calomel electrode (SCE) was used as the reference electrode. A three-stage flow system for the removal and recovery process is envisioned; copper, nickel and zinc are removed sequentially in each stage. Samples of electrolyte were withdrawn from the reservoir at regular intervals and analyzed by atomic absorption spectrophotometry. An individual experimental flow cell is shown in Fig 1. The flow direction is upwad. The principle feature of the cell is the porous carbon electrode fabricated from reticulated vitreous carbon which has a specific surface area of cmz/cm3. The porous electrode was 1.59 cm in diameter and 2.54 cm in length, providing a surface area of cm'. It was contained in a plastic (polymethyl methacrylate) holder. Another plastic tube with a sawtooth pattern was placed on the top of the porous carbon electrode to provide uniform flow. A 1/8" graphite carbon rod inserted through the holder was used as the conductor to the porous carbon bed. The counter electrode consisted of a platinum wire. A reference saturated calomel electrode was placed in the reservoir. Two threaded plastic covers were provided at both ends. O-rings were used to insure sealing, and the cell was connected to the flow system through taper joints with Tygon tubing. The solutions used to study the cell performance were made with dilute copper sulfate, nickel sulfate and zinc sulfate at concentrations of 500ppm. Sodium sulfate (0.5 M) was the supporting electrolyte. The solution ph and temperature were monitored by a pwtemperature probe placed in the reservoir. The ph was maintained between 5 and 6 by the addition of either sulfuric acid or sodium hydroxide. Approximately 100 ml of solution were circulated through the system using a variable speed tubing pump. Flow rates were determined using graduated 3 695

4 cylinders to obtain a known volume over a specific time period. A flow rate of 54.8 ml/min was used. The system was operated at room temperature. A potentiostat was used to control the potential and measure the current during the experiments Results and Discussion Polarization Behavior A linear potential sweep was applied to the glassy carbon electrode from the open circuit potential to -1.5 V vs SCE. The ph of the solutions was 5. Figure 2 shows the current- potential behavior of single component solutions of nickel and zinc. Deposition of nickel commenced at V and dissolution occurred at -0.5 V vs SCE. Zinc deposition began at -1.1 V and dissolution at V vs SCE. However, in mixed solutions, nickel and zinc were found to co-deposit. Figure 3 shows the ternary system where nickel and zinc are seen to co-deposit at -1.1 V vs SCE. Copper deposition from the ternary mixture began at 0.05 V, and dissolution occurred at 0.15 V vs SCE Metal removal from single, binary and ternary mixtures The removal rate of metal ions from single component solutions using the experimental flow-through cell is shown in Figure 4. The metal ions were extracted by constant potential electrodeposition onto the reticulated vitreous carbon bed. The potentials were stepped from the rest potentials to the values determined by the polarization curves shown in Fig.2 and Fig.3. The applied potentials for the electrodeposition of copper, nickel and zinc were -0.3 V, -0.9 V and V vs SCE, respectively. For the electrodeposition of copper, the deposition current reached a minimum constant current, and the copper concentration dropped to levels below lppm after 30 minutes. Zinc 696 4

5 removal occurred at a slower rate. Almost two hours were required to lower the zinc concentration to under 1 ppm. Nickel proved to be the most difficult metal to remove from solution. Seven hours were required to lower the concentration of nickel in solution to under 20 ppm. Selective removal of one metal from a binary mixture was then examined. Copper removal from a mixture of copper and nickel at initial concentrations of 500ppm is shown in Figure 5. At a potential of -0.3 V vs SCE, copper readily deposited and was completely removed from the solution in under 30 minutes. Removal of copper from a copper and zinc binary mixture is shown in Figure 6. Initial concentrations were 500ppm and the potential was again set at -0.3 V vs SCE. Again copper readily deposited and was completely removed from the solution in under 30 minutes. Difficulties were encountered in the attempt to selectively remove nickel from a binary mixture of nickel and zinc. Although the deposition of nickel from a single component solution occurs at V vs SCE, in a binary mixture with zinc, co-deposition occurs. Figure 7 shows this co-deposition of nickel and zinc at a potential of -0.9 V vs SCE for 6 hours. Thus, nickel could not be selectively recovered from a binary mixture with zinc. The results of the electrodeposition of copper from a ternary Cu-Ni-Zn solution are shown in Fig. 8. At -0.3 V vs SCE, the copper ion concentration was reduced to less than 1 ppm in less than 30 minutes, while the concentrations of both nickel and zinc remained relatively unchanged. The selective removal of copper from a temary mixture containing nickel and zinc was readily accomplished. The difficulty in the selective removal of nickel from zinc lead to an investigation of other ternary metal mixtures, which could be more readily separated and used to test the feasibility of a three-stage flow-through system. Silver was found to selectively deposit from binary mixtures 5 697

6 with copper, nickel and zinc using the flow-through cell at a potential of +0.3 V vs SCE. A three-stage system was set up to separate each metal from two ternary mixtures. The first ternary mixture consisted of silver/copper/nickel with the metal ion concentration of each component at 500 ppm. The second temary mixture consisted of silver/copper/zinc with the metal ion concentration of each component also at 500 ppm. controlled due to precipitation of the silver hydroxide. The ph of the solution could not be Silver and copper were both selectively removed from the first ternary mixture without loss of nickel but in the second ternary mixture some loss of zinc occurred during the copper removal stage. Acknowledeement This work has been supported by an AESF Research Fellowship (Project 79). References 1. D.N. Bennion and J. Newman, J. Appl. Elecrrochem, 2, 113 (1972) 2. A.K.P. Chu, M. Fleischmann and G.J. Hills, J. Appl. Electrochem, 4, 323 (1974) 3. A.K.P. Chu and G.J. Hills,.I. Appl. Elecrrochem, 4, 331 (1972) 4. R.S. Wenger and D.N. Bennion, J. Appl. Electrochem, 6, 385 (1976) 5. A.T. Kuhn and R.W. Houghton, J. Appl. Elecrrochem, 4, 69 (1974) 6. J.V. Zee and J. Newman, J. Appl. Electrochem, (1977) 7. D. Bailey, M. Chan and D.Billings, flaring and Sur$ Finish., 26, April W.J. Blaedel and J.H. Strohl, Anal. Chem., 36, 1245 (1964) 9. R. Alkire and R. Gould, J. Appl. Elecrrochem, 123, 1842 (1976) 6 698

7 t OUTLET YIJ 3 I in f 114 in. INLET T i Fig. 1. Diagram of the flow-through porous electrode cell 7 699

8 + c N,'r ' I I I I \ "I I Ir iti 0 r- - I 1 I I I 700 8

9 I, I I 0.25 Sweep rate: 10 mv/sec W n w 0 v, rji > > V' -.- W r U Q) 4 0 a 0.0c oo cu zn Current (MA) Fig. 3. Cyclic voltammogram of the cathodic deposition and anodic dissolution of a ternary system with 500 ppm Cu++, 500 ppm Ni++, and 500 ppm Zn

10 I ' I i I Q CU, E=-0.3 V VS. SCE A Zn, E=-1.2 V vs. SCE n E n W.- C 0 4 e Y t d 0) 0 0 C 0 \ EBsBl \ e -m-n.@ 81 Ni, E=-0.9 V vs. SCE * I.... l... ; ". 1.. " 1 ". ' ~ '... I " Time (hr) Fig. 4. Metal ion concentrations versus time during potentiostatic removal from single component systems. Flow rate: 54.8 ml/min. Initial metal ion concentration: 500 ppm

11 LooLooLooLooLoo~ (udd) UO!~DJJU~~UO~

12 /A- A-A m cu A Zn 3P Time min) Fig. 6. Metal ion concentration versus time during potentiostatic removal of copper from a binary mixture containing Cu++ and Zn++. Flow rate: 54.8mllmin. E = -0.3 V VS. SCE 50 I 60 I ' : i ~

13 I i I I 500 W 450!\ W.- c,.c, c $ 200 c # A A Zn Ni loo# Time (hr) Fig. 7. Metal ion concentration versus time during potentiostatic removal of nickel from a binary mixture containing Ni++ and Zn++. Flow rate: 54.8mVmin. E = -0.9 V vs. SCE

14 ~~ E U.w +J t g 200 c cu A ~n - 0,\ 8 I * 0. ' ' I. 10 ' '. I ' (b: : : :( 50 t Time (min) Fig. 8. Metal ion concentration versus time during potentiostatic removal of copper from a ternary mixture containing Cu++, Ni++ and Zn++. Flow rate: 54.8ml/min. E = -0.3 V vs. SCE Ni I