Decomposition and oxidation of pyrite

An assessment of the mechanisms and kinetics of pyrite transformation in inert and oxygen-containing atmospheres at elevated temperatures has been performed based on a literature review. In inert atmospheres, the transformation of pyrite proceeds through a multi-step process in the sequence of pyrite --> pyrrhotite --> troilite --> iron. The whole process is determined by the temperature and the total sulfur gas pressure in the system. Under equilibrium conditions, the composition of the resulting pyrrhotite from pyrite decomposition is determined by the temperature. The decomposition of pyrite to pyrrhotite follows the unreacted core model and may be controlled by zero-order surface reaction, gas film/product layer diffusion or a combination of these mechanisms, depending on the reaction conditions. The transformation of pyrite in oxygen-containing atmospheres can take place in two different ways depending on the conditions, such as temperature, oxygen concentration, flow and particle size. At lower temperatures ( <about 800 K) and higher oxygen concentrations, pyrite will be directly oxidized. The direct oxidation process follows the unreacted core model and may be controlled by chemical reaction or the inward diffusion of oxygen due to the pore-blocking effect by the formation of ferric/ferrous sulfate. At higher temperatures (> about 800 K) or lower oxygen concentrations, pyrite will be transformed by a two-step process: the thermal decomposition of the pyrite to form porous pyrrhotite as the first step, and the successive oxidation of the formed pyrrhotite as the second step. The first step proceeds similarly to the thermal decomposition of pyrite in an inert atmosphere, but may proceed with a much higher rate due to the extra heating effect from the oxidation of the formed sulfur gas and the reduction of eventual diffusion resistance of the outgoing sulfur gas through the product layer and/or the gas film. The second step can proceed with pyrrhotite in a solid or a molten state depending on the particle temperature. Iron oxides (mainly hematite, Fe2O3 and magnetite, Fe3O4) are the main products of the oxidation of pyrite. Hematite usually forms at lower temperatures (< 1173-1273 K) and at high oxygen concentrations, whereas magnetite usually forms at higher temperatures and/or at low oxygen concentrations. Sulfates (mainly ferrous sulfate, FeSO4 and ferric sulfate, Fe-2(SO4)(3)) may be formed as minor products during the oxidation of pyrite at temperatures lower than 873-923 K. The formation of the sulfates is probably determined by the gas composition at the reaction front. Ferrous sulfate usually forms in a gas that is rich in SO2, whereas ferric sulfate usually forms in a gas that is rich in SO3. (C) 2006 Elsevier Ltd. All rights reserved.