A MATHEMATICAL-MODEL OF THE LEACHING OF GOLD IN CYANIDE SOLUTIONS

The chemistry of the dissolution of gold in cyanide solutions has rece ived considerable attention. However, few rate expressions which descr ibe the leaching of gold ores have been developed. A model, based on t he shrinking-particle model, is presented here. In this work, it is pr oposed that a layer of passivating material forms on the surface of th e gold particle as it dissolves. As the total surface area diminishes in size, the passivating layer grows to increase its coverage of the s urface. The formation of the passive layer has little influence on the reaction in the initial stages of the batch experiment; however, in t he latter stages it has a dominant influence. In this work it is propo sed that the dissolution reaction is described by a mixed-potential mo del in which the reaction at the surface is rate controlling, Experime nts were conducted in which both the concentration of oxygen and cyani de were maintained at a fixed value throughout the batch experiment. T he concentration of cyanide was controlled by a computer operated syst em which uses an amperometric measurement of the concentration of cyan ide. It is shown that the mathematical model of the leaching reactions is an excellent description of the measured data reported here. The r ate constant for the dissolution reaction is shown to be dependent on the square root of the concentration of cyanide, which is in agreement with the proposed mixed-potential model. The results presented here c ontrast strongly with the previously accepted model of the dissolution reaction, which is based on the diffusion of either cyanide or oxygen . This previous model predicts a first-order dependence on the concent ration of cyanide in solution, which clearly contrasts with the result s presented here.