Anodic oxidation of copper cyanide on graphite anodes in alkaline solution

Abstract

The anodic oxidation of copper cyanide has been studied using a graphite rotating disc with reference to cyanide concentration (0.05–4.00 M), CN:Cu mole ratio (3–12), temperature (25–60 °C) and hydroxide concentration (0.01–0.25 M). Copper had a significant catalytic effect on cyanide oxidation. In the low polarization region (about 0.4 V vs SCE or less), cuprous cyanide is oxidized to cupric cyanide complexes which further react to form cyanate. At a CN:Cu ratio of 3 and [OH⁻] = 0.25 M, the Tafel slope was about 0.12 V decade⁻¹. Cu(CN)₃ ²⁻ was discharged on the electrode and the reaction order with respect to the predicted concentration of Cu(CN)₃ ²⁻ is one. With increasing CN:Cu mole ratio and decreasing pH, the dominant discharged species shifted to Cu(CN)₄ ³⁻. Under these conditions, two Tafel slopes were observed with the first one being 0.060 V decade⁻¹ and the second one 0.17–0.20 V decade⁻¹. In the high polarization region (about 0.4 V vs SCE or more), cuprous cyanide complexes were oxidized to copper oxide and cyanate. Possible reaction mechanism was discussed.