Additives, zinc chloride electrowinning

The Performance of the laboratory scale cell for zinc chloride electrowinning at 400 A/m was very good. The TBACl and MTBCl are the best additives at a current density of 400 A/m and for a NaCl concentration of 2 M. Table VII summarises the results for these experiments and compares them with those obtained from zinc sulphate electrowinning at the laboratory scale and at the industrial scale. 

For the optimisation work, the deposition time was increased to 24 h, which would normally give a good indication of the performance at the industrial scale. The previous testing period was so short that the effect of additives could not be properly demonstrated. The level of additives was then included in the optimisation work. Current density was definitely to be tested on a 24-h time period, considering the major impact it had on all responses. A new factor, not tested before, was the catholyte recirculation rate. A new series of optimisation tests with a five level central composite design of experiments was initiated to obtain more precise results on the effects of four factors ([HCl], [TBACl], current density and catholyte recirculation flowrate) on zinc electrowinning. The temperature was fixed at 35°C, the concentration of zinc chloride in solution was maintained at 50 g/L and the concentration of sodium chloride was fixed at 0.25 M. 

MacKinnon and coworkers [391-393] have studied a number of organic compounds as additives and have shown that tetrabutylammonium chloride improves surface morphology and current efficiency during zinc electrowinning from acidic chloride solutions. A similar influence was observed using other tetraalkylammo-nium compounds as additives [371, 374, 375, 394],... 

Cadmium production is often connected with zinc production. The impure cadmium metal produced in the purification of zinc electrowinning electrolyte is dissolved in 150 g L-1 sulfuric acid solution and electrowon in a process similar to that for zinc. The impure metals that would disturb electrowinning, for example, Cu, Ni, As, are removed by cementation with impure cadmium. The solution is neutralized with lime. The electrolyte contains 140-180 gL-1 Cd, 30-40gL x Zn, some chlorides, and animal glue as additives. The electrolysis is done using... 

D.J. MacKinnon and J.M. Brannen, Evaluation of Organic Additives as Levelling Agents for Zinc Electrowinning from Chlorides Electrolytes , Journal of Applied Electrochemistry. Vol. 12,1982,21-31. 

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