Zinc electrowinning under periodical reverse current (PRC): behavior of the anode and energy consumption

Laboratory-scale experiments of Zn electrowinning under periodical reverse current (PRC) were carried out in acidic sulfate solution. Direct current densities between 500-1500 A/m exp 2 with a direct duration pulse of 100 s and a reverse duration pulse of 0.1-1.0 s were investigated. The reverse current density was set at half the value of the direct current density. Comparisons were made with conventional dc electrolysis. The specific energy consumption under PRC electrowinning was found to be 2.49 kWh/kg at 500 A/m exp 2 , with a cell voltage of 3.00 V. Anode potential transients were studied to characterize the anodic reactions involved during current reversal. It was found that the reactions proceeding at the Pb--Ag electrode during current reversal are dependent upon the reverse charge density. For a reverse charge density beyond 125 C/m exp 2 , the beta -PbO sub 2 layer is reduced to PbSO sub 4 , which may be detrimental to the process. Indeed, in such a case, potential decay curves show that the surface of the anode becomes denser and, thus, the oxygen overpotential rises for a given current density. A lesser value of the reverse charge density was proven to be highly beneficial and was related to the desorption of shielding chemical species such as OH sub (ad) and SO sub 4 exp --2 . These results allow the use of PRC with a longer duration of the reverse current which, in the past, was the drawback of PRC electrolysis. Complementary results with longer reverse times and smaller reverse current densities have comfirmed the proposed mechanisms and showed a beneficial effect on the specific energy consumption of the process. A 24 h experiment was conducted with these parameters, showing the potential applicability of this technique. Nevertheless, more studies are needed on the effects of industrial additives such as permanganates, animal glue, etc., before the use of PRC can be implemented on a full-size scale. 53 ref.--AA