Cyanide dissolution of gold-bearing iron oxide minerals: kinetic studies and optimization using response surface methodology

This study investigated the cyanidation behavior of a low-grade gold-bearing ore (1.1 g/t Au) rich in iron oxides, using a combined approach involving mineralogical analysis, experimental leaching tests, statistical modeling, and kinetic evaluation. The final experimental design was conducted using the central composite design (CCD) method to assess the effects of pH and leaching time. A second-order response surface methodology (RSM) model with excellent goodness of fit (R² = 0.9962; predicted R² = 0.9640) was developed. The optimum conditions for maximum gold recovery were predicted at pH 10.25 and a leaching time of 26.2 h, showing strong agreement with experimental data. Kinetic analysis was performed using two approaches: nonlinear regression and linear shrinking core models (SCM). Among the nonlinear models, the power model showed the best fit (R² = 0.937), capturing rapid gold dissolution within the first 7 h (88.69% recovery), followed by a slower increase to a final recovery of 94.77%. In the linear approach, the rate constants for all models significantly decreased after 7 h, indicating a slower leaching process over time. These findings support the hypothesis that mixed control (R² = 0.996) governs the early stages of leaching, while diffusion control (R² = 0.986) becomes dominant in later stages.