CYANIDE OXIDATION AT NICKEL ANODES .2. VOLTAMMETRY AND COULOMETRY OF NI/CN-H2O SYSTEMS

Coulometric experiments with a Ni plate electrode in 1 kmol NaOH m-3 containing 10(-4)-1 kmol NaCN m-3 showed that cyanide could be oxidized at high current efficiency [greater-than-or-equal-to 2 mol e-(mol CN-)-1] with negligible nickel dissolution. However, current densities were low for potentials below those at which Ni(OH2) was oxidized to Ni(OH)3, when oxygen evolution occurred, causing a sharp decrease in current efficiency. The electrochemical behavior of nickel in alkaline cyanide solutions was studied by cyclic voltammetry using a rotating (Pt) ring-disk (Ni) electrode system. No cyanide oxidation products were detected at the platinum ring electrode. However, by plating the Pt with Pb from alkaline solution, and using potentials < -1.4V vs. HgO/Hg with cyanide concentrations > 10 mol m-3, species other than oxygen (reduction) and hydrogen (oxidation) generated by the nickel disk electrode could be detected transiently, prior to passivation. From thermodynamic calculations for Ni-CN-H2O systems in the preceding paper, the most likely species involved were predicted to be Ni(CN)4(2-) ions, formed by reduction of Ni(OH)3 and the simultaneous oxidative dissolution of the underlying metal. The reaction(s) was observed only on the negative-going potential sweep of the nickel disk; no nickel dissolution was found to have occurred when the electrode was anodically polarized at constant potentials due to the presence of Ni(CN)2/Ni(OH)2 adsorbed on the electrode, as predicted from the thermodynamic calculations.