A lumped kinetic model for gold ore cyanidation

The effect of particle size on the kinetics of gold cyanidation was investigated for a gold ore from the Abitibi region (Canada). Six size fractions representative of the plant operation were used to carry out the gold leaching and cyanide consumption kinetic tests. A two-level factorial experimental design was used for the gold leaching tests of the six size fractions. The free cyanide concentrations were selected as 260 and 650 mg/L, the dissolved oxygen concentrations as 8 and 40 mg/L, and the pH was maintained constant at 12. The gold concentration as a function of time was fitted to a lumped pseudo-homogeneous kinetic model, using a least-squares method. The results show that the gold dissolution kinetic reaction orders are approximately two for gold, one for free cyanide and a quarter for dissolved oxygen, and that the kinetic rate constant is a decreasing cubic function of the ore particle average diameter. In the cyanide consumption tests, conducted also for the six different size fractions, the pH was maintained at 12, the dissolved oxygen concentration was chosen as 8 mg/L, and the initial free cyanide concentration as 260 mg/L. The cyanide consumption lumped kinetic model shows that the reaction order is approximately three for free cyanide, and that the rate constant is a decreasing reciprocal square-root function of the ore particle size. Although finer grind of the ore enhances gold release, it considerably increases the cyanide consumption due to the liberation of the copper and iron bearing minerals, thus leading to a possible optimization of the ore size fraction that can simultaneously promote high gold recovery and acceptable cyanide consumption.