Bipolar cells for electrowinning lead from molten lead chloride

Abstract

Alternative routes for extracting lead and zinc from complex ore bodies are being evaluated by industry as replacements for existing pyrometallurgical smelting processes. The most promising is a hydrometallurgical route in which the metal is converted to, for example, its chloride or sulphate. The successful development of such a process is dependant upon the efficient extraction of metal values from this salt. Part of the development of new technologies associated with metal extraction has been the successful implementation of multiple bipolar cells for electrowinning the metal from their molten salts. However specific details of much of this work is unavailable. Consequently the objective of the research in this presentation is to determine the design variables that influence the performance of these cells and to identify an optimum design strategy for the development of bipolar cells for the electrowinning of lead from lead chloride. Hydrodynamic modelling and aqueous bipolar cell trials were undertaken to determine the significance of electrode design and orientation on the cell performance. It was found that by orientating the electrodes at a slope of 15� the flow rate of the electrolyte could be maximised. This resulted in a reduction in the cell voltage and therefore minimised the energy demand of the cell. The poor Faradaic efficiencies of bipolar cells is understood to be the result of current bypass. Because of the nature of construction of these cells current can bypass intermediate electrodes reducing the efficiency. Methods for increasing the resistance of these bypass currents were investigated. These studies showed that while electrode end covers resulted in an improvement in current efficiency this was accompanied by a corresponding increase in cell voltage which negated the efficiency gains. Indeed it was concluded that insulating covers had no advantage for improving the overall cell performance. Quantitative sampling of the metal values along the cathode illustrated that the current density was not uniform and by careful electrode design the effect of bypass currents could be minimised. Mathematical equations were developed to describe the electrochemical dynamics and these were used in the construction of an elementary computer model to predict the performance of these cells. A number of molten salt bipolar cell designs were evaluated using PbCl2/NaCl as the electrolyte. The performance of these cells was characterised by a wide variation in current efficiencies; 40-81%, and the energy consumption was in the range 3.03-4.7kJ.kg_1 Pb. In general the optimum operating current density was about 0.63A.cm'2 and the cell performed better with electrode spacings less than 10mm. The influence of PbSO, and PbO as impurities was evaluated. Lead sulphate was found to be non-electroactive while lead oxide was rapidly removed from the electrolyte as a mixture of the carbon oxides, C02 and CO, and 02. It was concluded that bipolar cells are practical alternatives to existing pyrometallurgical process for refining lead ores, but the low energy efficiencies make their economics questionable.