Leaching kinetics and stoichiometry of pyrite oxidation from a pyrite–marcasite concentrate in acid ferric sulfate media

A pyrite concentrate with minor marcasite was oxidized in an acidic ferric sulfate medium at temperatures from 45 to 75 °C and at constant potentials corresponding to Fe(III) to Fe(II) ratios from 10 to 300. Potassium permanganate (KMnO4) was found to be both a suitable oxidant for controlling the solution potential and a convenient and reliable indicator of leaching progress.The stoichiometry of pyrite oxidation was found to be essentially independent of temperature and only slightly dependent on solution potential over the range of conditions studied. Each unit of sulfide sulfur oxidized yielded 64 ± 2% sulfate, the rest elemental sulfur as discrete particles approximately 2 μm in diameter.The pyrite oxidation rate was very sensitive to the temperature, giving a large activation energy (83 kJ/mol), and was proportional to the Fe(III)/Fe(II) concentration ratio to the power of 0.57. The shrinking sphere model fitted very well the changing grain topology. A single mathematical expression combines the thermal, chemical, and topological functions to predict the pyrite conversion as a function of the known temperature, ferric concentration, ferrous concentration, particle size, and time. The model predictions are excellent over the range of conditions tested.