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A comprehensive physicochemical model is presented for nickel electrodeposition onto nickel and copper rotating disk electrodes in sulfate media buffered with boric acid at various NiSO 4 concentrations. The model accounts for three contributions comprising the total current: nickel deposition, H + reduction, and water reduction. Also considered are homogeneous reactions and transport of dissolved species to and from the electrode by diffusion and convection. The model is statistically fit to experimental linear sweep voltammetry data to obtain the various kinetic parameters. It is not possible to satisfactorily fit the curves obtained on a nickel substrate for the different NiSO 4 concentrations with a single set of parameters. Analysis of the experimental data and fitted model shows that the cathodic reactions reach completion in the following order as the potential becomes more negative during the scans: H + reduction, nickel deposition, and water reduction. The extent of water reduction during scans does not vary with the metal ion concentration in solutions containing 50 mol m −3 NiSO 4 or more, but increases when the NiSO 4 concentration is reduced to 10 mol m −3 . The model confirms that B͑OH͒ 3 hydrolysis mitigates the increase of the surface pH due to H + and water reduction.

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In the present research, the differences in the oxidation behavior of a high purity acanthite concentrate and silver sulfide ͑analytical reagent͒ in cyanide media were examined using two methods: electrochemical ͑Evans diagrams͒ and chemical ͑stirred tank reactor͒. On one hand, the kinetics of acanthite oxidation were determined to be mixed reaction-diffusion controlled, while, on the other, the analytical reagent oxidation was found to be solely diffusion controlled. The construction of the Evans diagrams was performed using the current vs. potential values of silver sulfide oxidation and oxygen reduction, obtained with direct pulse chronoamperometry on carbon paste electrodes. Analysis of these diagrams allowed calculation of the system's reaction velocity, which corresponded to 8.8 ϫ 10 Ϫ10 mol cm Ag 2 S Ϫ2 s Ϫ1 . When cyanidation was performed on a sulfide concentrate ͑1.4% acanthite͒, a normalized reaction velocity, several orders of magnitude lower, was obtained, even after compensating for the effects of diffusion. An analysis of the reactions involved, both in the electrochemical and the leaching systems, showed a notable difference between the two, reflected by dissimilar interfacial hydroxide ion concentrations. This difference in the chemical atmosphere questions the validity of employing Evans diagrams to describe the leaching kinetics in certain systems. Also, this study established that, for both systems, the aqueous phase lead ions accelerated silver sulfide oxidation because they acted as sequestering agents for hydroxide and/or sulfide ions.Cyanidation is still by far the method most employed in industry to extract gold and silver from their minerals, despite the environmental restrictions associated with it. Even though this method has been extensively studied and applied, doubts still exist regarding the reaction mechanism between silver sulfide and oxygen. 1-5 Two different techniques have been employed to study this process: in chemical and electrochemical systems. In the former, mineral concentrates are leached in stirred tank reactors, obtaining an estimate of the time that is required to extract the silver. The principal disadvantage of this type of research 4 is that it does not allow the direct elucidation of the chemical processes which take place during leaching. Consequently, it is difficult to identify the controlling phenomenon of the reaction, or the effect of solution parameters on the mechanism.In electrochemical systems, a major part of the work in cyanidation has been performed on gold and silver in their metallic forms. 1-3,5-6 However, it has been observed in practice that the behavior of metallic silver greatly differs from that of silver when it is present as a sulfide. In order to simulate the effect of those ions which are generally found associated with silver in its different mineralogical phases, studies have been performed with the addition of ions ͑sulfide and lead, among others͒ to the leaching solution. 2-3,6-7 However, the behavior of the silver ions in this situa...

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