ELECTROCHEMICAL RECOVERY OF THE ZINC PRESENT IN THE SPENT PICKLING BATHS COMING FROM THE HOT DIP GALVANIZING INDUSTRY BY MEANS OF THE USE OF TWO DIFFERENT MEMBRANES

Spent pickling baths coming from the hot dip galvanizing process represents an environmental problem due to high ZnCl2, FeCl2 and HCl (among other substances) concentrations. This problem, together with the decrease of natural reserves of nonferrous metals, makes zinc electrowinning coming from spent pickling solutions an interesting alternative. Moreover, membranes, or other kind of diaphragms, must be placed between the anode and cathode of the electrochemical reactor to minimize the presence of chlorine gas generated in the anode, which attacks the zinc deposits causing their dissolution. Therefore, the purpose of this work is to compare the behaviour of an Anionic Exchange Membrane (IONICS AR-204) and a Cationic one (NAFION 117). The electrochemical reactor consisted of two connected pyrex glasses of 250 ml each, with the possibility of placing a membrane between them. Two graphite electrodes were used as working and counter electrodes and a standard Ag/AgCl electrode acted as reference one. Both anode and cathode were made of two cylindrical graphite bars with an effective area of 14.15 cm(2). All the experiences were performed using an Auto lab PGSTAT20 potentiostat/galvanostat. Samples were taken from the reactor every 30 min and potential, current, cell voltage and pH were recorded during the electrolysis. The determination of zinc and iron was performed by atomic absorption spectrophotometry (AAS) on a Perkin-Elmer AAnalyst 100 model. For the experiments in the presence of the anion-exchange membrane (AEM), the diluted spent pickling bath was charged in the cathodic chamber while a 0.1M HCl solution was placed in the anodic one. In contrast, in the case of the cation-exchange membrane (CEM) assays, the diluted spent bath was placed in the anodic chamber and an acidic solution was poured in the cathodic one. This acidic solution consisted of 0.1M HCl or 0.1M ZnCl2 in 0.1M HCl. The CEM allows the passage of zinc and iron cations. When an acidic solution in the absence of zinc is placed in the cathodic chamber, iron codeposits with zinc from the beginning of electrolysis. In contrast, when the acidic solution contains zinc, its presence inhibits iron deposition due to the anomalous codeposition phenomenon. Moreover, the figures of merit (zinc conversion and current efficiency) are higher when the acidic solution initially contains zinc since less amount of energy is employed in the iron deposition process. On the other hand, when the AEM is used, iron deposition starts if its concentration is at least two-fold the zinc concentration value due to the anomalous codeposition phenomenon. The iron codeposition must be taken into account if a separated zinc recovery is required.