Model for heap leaching of gold ores by cyanidation

Abstract

The objective of this study is to develop a model which describes the heap leaching process for gold extraction from non-refractory ores. The model considers the cyanidation reaction in a bed of uniformly sized spherical porous mineral particles, through which the leaching solution trickles. The leaching agents diffuse into the porous particles to react with the grains of metallic gold. The model considers the effects of transport mechanisms on the process and uses an electrochemical mechanism to describe gold cyanidation at the grain level. Mass transfer of the reactants and products in the heap are described using the concepts developed in research on trickle bed reactors. The effective wetting of the particles is estimated with correlations and employed to calculate an apparent effective diffusivity for the different species within the partially wetted particles. Numerical solution of the model equations allows the description of reactant and product concentration profiles within the particles and in the heap. From these profiles, mass balances are performed to show how the gold reacts and is transported until it finally leaves the heap. The effects of different parameters, such as mineral porosity, particle radius, leaching solution feed rate, heap height and concentration of reagents, on the gold extraction rate are determined. The results show that the rate-controlling step in this process, under normal operating conditions, is the diffusion of reactants within the porous particles. For this reason, particle radius, mineral porosity and solution flow rate greatly influence the overall leaching rate. One of the most important findings of this study is that the time required for the gold to react with cyanide and oxygen within the mineral is small compared to the time for the gold cyanide complex to diffuse out of the ore particle and leave the heap.