KINETICS OF PYRITE OXIDATION IN SODIUM-HYDROXIDE SOLUTIONS

The kinetics of pyrite oxidation in sodium hydroxide solution were inv estigated in a stirred reactor, under temperatures ranging from 50 deg rees C to 85 degrees C, oxygen partial pressures of up to 1 atm, parti cle size fractions from -150 + 106 to -38 + 10 mu m (-100 + 150 mesh t o -400 mesh + 10 mu m), and pH values of up to 12.5. The surface react ion is represented by the rate equation:-dN/dt = SbK '' pO(2)(0.5)[OH- ](0.25)/(1 + K'''pO(2)(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, pO(2) is the oxygen partial pressure, an d [OH-] is the hydroxyl ion concentration. The corresponding fractiona l conversion (X) vs time behavior follows the shrinking particle model for chemical reaction control: 1 - (1 - X)(1/3) = k(c)t The rate incr eases with the reciprocal of particle size and has an activation energ y of 55.6 kJ/mol (13.6 kcal/mol). The relationship between reaction ra te and oxygen partial pressure resembles a Langmuir-type equation and thus suggests that the reaction involves adsorption or desorption of o xygen at the interface. The square-root rate law may be due to the ads orption of a dissociated oxygen molecule. The observed apparent reacti on order with respect to the hydroxyl ion concentration is a result of a complex combination of processes involving the oxidation and hydrol ysis of iron, oxidation and hydrolysis of sulfur, and the oxygen reduc tion.