Statistical analysis of cobalt removal from zinc electrolyte using the arsenic-activated process

This paper presents the results from a statistical study of a continuous solution purification procedure. The purpose of the work was to study the role of various reagents acting simultaneously and their correlations with the calculated deposition efficiencies of impurities. Cobalt cementation is the second part of the electrolyte purification section at Outokumpu Kokkola Zinc, Finland. At Kokkola, zinc is produced by an aqueous electrowinning process. The zinc sulphate solution is processed in a three-part electrolyte purification process where impurities more noble than zinc are removed. In the second part of the purification, arsenic solution and zinc powder are added to create conditions where aqueous impurities are reduced to solid metals/arsenides. The deposition process is, in principle, simple, but purification results show that there are large variations in the calculated deposition efficiencies. The deposition efficiencies depend on several accelerating and decelerating components, of which copper is the best known and documented. The data is collected from a pilot scale continuous purification process. The significance of the composition of the incoming solution, the deposition components, together with zinc and arsenic feeds are evaluated by using a feed solution consisting of variable amounts of the impurities under study. Process data is evaluated by calculating correlations to the impurity deposition efficiencies. Results reveal that the success of purification is a combination of several components and that all of these components have a fairly low correlation to deposition efficiencies by themselves. In the purification process, deposition efficiencies tend to decline more slowly than components affecting them in the feed solution. Positively affecting components are a high copper content in the feed solution, combined with a high zinc powder feed and low arsenic feed. Weaker positive effects are formed by high aluminium with low calcium and silicon contents in the feed solution. (C) 2004 Elsevier B.V. All rights reserved.