A mechanistic kinetic model for direct pressure leaching of iron containing sphalerite concentrate

In the current study, a numerical mechanistic kinetic model for direct leaching of zinc from iron containing sphalerite concentrate is presented. The model is based on the well-known shrinking-core model approach. However, in order to consider possible chemical changes that might take place in the liquid phase the model is solved numerically. This also makes it possible to separate and quantify different factors that might limit the leaching process. It is suggested that the most probable leaching mechanism is a combination of the acid attack and electrochemical reaction mechanisms. At first, zinc sulphide reacts with hydrogen ions at the surface of particles liberating zinc ions and forming hydrogen sulphide. Then in the next step, hydrogen sulphide is oxidized by ferric ions or molecular oxygen according to an electrochemical reaction mechanism. The estimated reaction orders for the oxidants are close to 0.5 which supports the electrochemical mechanism. The mechanism of sphalerite leaching also involves simultaneous surface reactions and internal diffusion of oxidants. The obtained results show that the rate of the leaching process is mainly limited by the surface reactions. However, internal diffusion might decrease the leaching rate in some cases up to 50% at higher sphalerite conversions. It is also demonstrated in the study that by using advanced statistical mathematical methods, such as the Markov chain Monte Carlo method, new information can be obtained about the kinetics of sphalerite leaching, the parameters affecting the leaching rate and the uncertainties involved in the model predictions.