Kinetics of pyrite oxidation in sodium hydroxide solutions

The kinetics of pyrite oxidation in sodium hydroxide solution were investigated in a stirred reactor, under temperatures ranging from 50 °C to 85 °C, oxygen partial pressures of up to 1 atm, particle size fractions from -150 + 106 to -38 + 10μm (-100 + 150 mesh to -400 mesh + 10 μ), and pH values of up to 12.5. The surface reaction is represented by the rate equation:-dN/dt = Sbk″pO^0.5 ₂[oH^{- 0.25}/(1 +k‴ pO₂ ^0.5) where N represents moles of pyrite, S is the surface area of the solid particles, k″ and k″ are constants, b is a stoichiometric factor, pO₂ is the oxygen partial pressure, and [OH^-] is the hydroxyl ion concentration. The corresponding fractional conversion (X) vs time behavior follows the shrinking particle model for chemical reaction control: 1 - (1 -X)^1/3 =k _ct The rate increases with the reciprocal of particle size and has an activation energy of 55.6 kJ/mol (13.6 kcal/mol). The relationship between reaction rate and oxygen partial pressure resembles a Langmuir-type equation and thus suggests that the reaction involves adsorption or desorption of oxygen at the interface. The square-root rate law may be due to the adsorption of a dissociated oxygen molecule. The observed apparent reaction order with respect to the hydroxyl ion concentration is a result of a complex combination of processes involving the oxidation and nydrolysis of iron, oxidation and hydrolysis of sulfur, and the oxygen reduction.