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Fundamental study of the deposition of cobalt from electrolytes containing zinc

University of British Columbia

The presence of cobalt in zinc electrowinning electrolytes acts to lower the current efficiency of zinc deposition. For this reason, industrially, cobalt is removed from zinc electrolytes by cementation with zinc dust. The process of cementation of cobalt with zinc dust is an inefficient use of zinc. Up to 200 times the stoichiometric amount of zinc dust is needed to effectively remove cobalt to levels sufficient to electrowin zinc at a high current efficiency. To increase the rate of cobalt removal, zinc dust activators, such as antimony, arsenic, and copper are added to the electrolyte to increase the removal rate of cobalt. The fundamental reasons for the inefficient reaction of zinc dust with cobalt were studied as they have not been determined previously. To study this system more effectively cobalt electrodepostion in the presence of zinc ions was studied. This method of deposition was shown to behave similarly to cementation where zinc metal rather than an electric current is used to reduce cobalt. The deposition of cobalt in the presence of zinc was studied by cyclic voltammety, potentiostatic deposition, AC impedance, and surface analysis. It was found that there were two regions of inhibition dependent of the overpotential of cobalt deposition. At low cobalt overpotentials it was proposed that zinc, in the form ZnOH⁺, adsorbs on the surface of the electrode blocking the deposition of cobalt. At higher cobalt overpotentials it was proposed that concurrent hydrogen generation increases the pH at the interface and Zn(OH)₂precipitates. A chemical model of inhibition as a function of cobalt overpotential is presented. Since the concentrations of the various species present in the cobalt-zinc-water system at the interface influences the concentration of ZnOH+ and the precipitation of Zn(OH)₂, the concentration of these species at the interface was calculated to test the proposed theories. This calculation showed that the onset of primary inhibition corresponds to a high concentration of ZnOH⁺, and secondary inhibition corresponds to conditions where the precipitation of Zn(OH)₂is favored.

Thesis/Dissertation

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2009-04-06

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http://hdl.handle.net/2429/6823

Materials Engineering

University of British Columbia

UBCV

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