UBC Theses and Dissertations

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UBC Theses and Dissertations

The removal of cobalt from zinc sulphate electrolytes using the copper-antimoney process Lew, Richard W.


At Cominco's zinc refinery located at Trail, British Columbia, impurities are removed from zinc sulphate electrolyte by a two stage purification process using zinc dust. Copper, cadmium and thallium are removed in the first stage whereas cobalt is removed in the second stage. Cobalt removal kinetics are affected by soluble antimony and copper additions to the electrolyte. This study was focused on the kinetics of cobalt removal by zinc powder cementation. Experimental variables included copper and antimony concentrations, pH and temperature. Common organic additives employed in zinc plants were studied to determine potential interactions with the cobalt removal process. Cementation products obtained from laboratory experiments and plant practice were studied using the SEM. A number of significant findings were observed. 1) Cobalt removal was extremely slow in the absence of any activators in solution. Batch testing have shown that antimony activation performed better than copper activation, but the addition of both copper and antimony together increased the rate of cobalt removal significantly. Copper and antimony ions were essentially removed from solution during the early stages of the cementation reaction (within the first 15 minutes), presumably, establishing the cathode surface for cobalt removal. 2) The cobalt removal rate constant increased with increasing temperature, and the highest rate was obtained at 90°C (the highest temperature tested). An apparent energy of activation of 86.6 kJ/mol was calculated for this system, and this value was consistent with activation controlled processes. 3) pH significantly affected the cobalt cementation rate constant. Too low a pH (4.0) can result in passivation of the residue's surface. In both these extreme ranges, cobalt removal was detrimentally affected. 4) A drop in the cobalt removal rates can be correlated with a rise in the E^ of the slurry. Lignin sulphonate, Percol 351 and animal glue at levels of 2 mg/L significantly impede cobalt cementation. 5) SEM analyses of the cementation residues have shown that the activation agents and the cobalt impurity was collected onto an impurity shell surrounding the dissolving zinc particle. This shell consisted mainly of Zn and minor constituents of Co, Cu and Sb.

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