Kinetics of pyrite oxidation in sodium carbonate solutions

The kinetics of pyrite oxidation in sodium carbonate solutions were investigated in a stirred vessel, under temperatures ranging from 50 degrees C to 85 degrees C, oxygen partial pressures from 0 to 1 atm, particle size fractions from -150 + 106 to -38 + 10 mu m (-100 + 150 Mesh to -400 Mesh + 10 mu m) and pH values of up to 12.5. The rate of the oxidation reaction is described by the following expression: -dN/dt = SbkpO(2)(0.5) [OH-](0.1) where N represents moles of pyrite, S is the surface area of the solid particles, b is a stoichiometric factor, k is an apparent rate constant, pO(2) is the oxygen partial pressure, and [OH-] is the hydroxyl ion concentration. The experimental data were fitted by a stochastic model for chemically controlled reactions, represented by the following fractional conversion (X) vs time (t) equation: (1 - X)(-2/3) - 1 = k(ST)t The assumption behind this model, i.e., surface heterogeneity leading to preferential dissolution, is supported by the micrographs of reacted pyrite particles, showing pits created by localized dissolution beneath an oxide layer. In addition to the surface texture, the magnitude of the activation energy (60.9 kJ/mol or 14.6 +/- 2.7 kcal/mol), the independence of rate on the stirring speed, the inverse relationship between the rate constant and the initial particle diameter, and the fractional reaction orders are also in agreement with a mechanism controlled by chemical reaction.