Abstract
Electrolytic oxidation of cyanide is a potential alternative to chemical oxidation methods in the treatment of gold mill effluents. Previous experiments using 304 stainless steel electrodes showed that the addition of small amounts of copper provided a dramatic improvement in cyanide oxidation kinetics. However, visible corrosion of the 304 stainless steel electrodes was observed, which disqualified it from being a viable electrode material. As an alternative, 316 stainless steel was identified as candidate electrode material as it is already a common electrode material in industrial gold electrowinning cells. Cyclic voltammetry experiments were initially performed to characterize the redox behavior of 316 over a wide range of electrolyte conditions. Galvanostatic studies were then performed using electrolytes more typical of gold mill effluents, at a pH of ca. 11, containing 250 mg L-1 of cyanide with and without 50 mg L-1 of copper (as copper(I) cyanide). These experiments were conducted at current densities of both 20 A m-2 and 100 A m-2 over 48 hour periods, during which periodic sampling of the electrolyte was done. The residual cyanide concentrations were measured in the solution samples by potentiometric titration using a silver nitrate solution and fit to a first order rate model. The presence of copper was found to dramatically enhance the oxidation of cyanide, in a similar manner as shown for 304 stainless steel. The enhanced kinetics was attributed to the formation of loosely adherent copper(II) oxide particles at the anode. In this regard, SEM-EDS analysis of the anodes after being used in experiments with copper-containing electrolytes showed evidence of the presence of copper particles on the electrode surfaces. In comparison to 304 stainless steel, SEM-EDS analyses of the 316 stainless steel electrode surfaces and ICP-MS analyses of the solutions samples did not show significant evidence of corrosion after 48 hours, validating its potential use for industrial applications.