Heap leaching: a gas diffusion rate-limited model.

The consequences are examined of assuming that the oxidation rate of pyrites in an overburden dump is determined by the rate at which the oxygen required can be supplied from the top surface of the heap by diffusion through the pore space of the heap. It is shown that the oxidation reaction proceeds at a reaction front that moves away from the top surface of the heap at a rate determined entirely by the concentration of pyrites in the heap, the diffusion coefficient of oxygen in the pore space, the concentration of oxygen in the air and the ratio of the mass of oxygen to the mass of pyrites consumed in the oxidation. Applied to White's overburden dump at Rum Jungle, the model indicates oxidation rates of the same order as those inferred from field measurement. It also predicts that at the present stage of that heap's development the reaction front lies between 3 and 5 m from the surface, depending on the porosity, which is assumed to be between 20 and 40 per cent, and that the front advances 7.5 to 13 cm per year. If the reaction is catalysed by iron-oxidising bacteria, then on the basis of the model the bacterial population will be greatest in this comparatively small region, zero in the anaerobic region below the front, and small in the upper levels of the heap. Estimates are also made of the temperature rise at the reaction front and the temperature distribution, assuming that the only heat loss is by conduction through the soil. However, heat loss to water passing through the heap could be significant and a more sophisticated treatment may be required