Cyanide oxidation by ozone in cyanidation tailings: Reaction kinetics

The treatment of cyanidation effluents is one of the main problems the gold mining industry faces. Different methods are available for the removal of cyanide but in most cases the consumption of reagents raises the cost to unaffordable levels, or there exists the formation of residual byproducts that are also toxic. Ozone gas is used in a new alternative approach that presents several advantages for oxidation of cyanide compounds. However, the lack of information on the reactions of cyanide compounds with ozone has limited ifs industrial application in cyanidation effluents. The oxidation of cyanide by ozone in aqueous solutions was studied using a bubble column reactor. The reactor was operated at different conditions of pH, ozone dosage rate, gas flow rate and cyanide concentration. Preliminary experiments were performed with synthetic cyanide solutions and the results showed that the rate of cyanide oxidation may be described by an equation which is first order with respect to ozone concentration in the oxidizing gas, and zero order with respect to cyanide concentration in the aqueous solution. The rate constant obtained for the reaction was about 10(-5) mol/L.s. The stoichiometry estimated for the reaction indicated one mole of cyanide reacts with one mole of ozone to form one mole of cyanate. These results were corroborated with industrial effluents from two mines in Mexico. (C) 2000 Elsevier Science Ltd. All rights reserved.