Oxidation of cyanide in effluents by Caro’s Acid

Abstract

Caro’s Acid (peroxymonosulphuric acid: H2SO5) is a powerful liquid oxidant made from hydrogen peroxide that has been adopted for the detoxification of effluents containing cyanides in gold extraction plants in recent years.The present work reports the findings of a study on the kinetics of aqueous cyanide oxidation with Caro’s Acid. Experiments were conducted in batch mode using synthetic solutions of free cyanide. The experimental methodology employed involved a sequence of two 23 factorial designs using three factors: initial [CN−]: 100–400mg/L; H2SO5:CN− molar ratio: 1–1.5–3–4.5; pH: 9–11; each one conducted at one level of Caro’s Acid strength which is obtained with the H2SO4:H2O2 molar ratio used in Caro’s Acid preparation of 3:1 and 1:1. The objective was the evaluation of the effect of those factors on the reaction kinetics at room temperature. Statistical analysis showed that the three investigated variables were found to be significant, with the variables which affected the most being the initial [CN−] and the H2SO5:CN− molar ratio. The highest reaction rates were obtained for the following conditions: H2SO5:CN− molar ratio=4.5:1; pH=9; and Caro’s Acid strength produced from the mixture of 3mol of H2SO4 with 1mol of H2O2. These conditions led to a reduction of [CN−] from an initial value of 400mg/L to [CN−]=1.0mg/L after 10min of batch reaction time at room temperature. An empirical kinetic model incorporating the weight of the contributions and the interrelation of the relevant process variables has been derived as: −d[CN−]/dt=k [CN−]1.8 [H2SO5]1.1 [H+]0.06, with k=3.8 (±2.7)×10−6L/mgmin, at 25°C.