OXIDATION OF PYRITE IN ALKALINE SOLUTIONS AND HETEROGENEOUS EQUILIBRIA OF SULFUR- AND ARSENIC-CONTAINING MINERALS IN CYANIDE SOLUTIONS.

摘要

The kinetics of pyrite oxidation in Na(,2)CO(,3) and NaOH solutions was investigated in a stirred vessel, temperatures of 50-85(DEGREES)C, oxygen partial pressures of 0-1 atm, size fractions from -100 + 150 Mesh to -400 Mesh + 10 micron and pH up to 12.5. In Na(,2)CO(,3) solutions, the reaction is 0.5 and 0.1 order with respect to pO(,2) and OH('-) , respectively, and the activation energy is 61 kJ/mol. The experimental data are fitted by a stochastic model for chemically controlled reactions. The reaction in sodium hydroxide solutions is 0.25 order with respect to OH('-) and the effect of oxygen on the reaction rate is described by a Langmuir-type equation where pO(,2) appears with a 0.5 exponent; the activation energy is 55 kJ/mol. The experimental data are fitted by the unreacted core model for chemical control.;The effects of decomposition of sulfur- and arsenic-containing minerals on gold ores cyanidation was analyzed with the aid of Eh-pH diagrams. The decomposition reactions produce metal cations, and a series of sulfur compounds that may react with cyanide and oxygen, increasing the consumption of reagents and consequently decreasing the efficiency of the extraction process. The thermodynamic stability of Fe(III) and Fe(II)-cyanide complexes suggests that the observed low solubility of pyrite and arsenopyrite in cyanide solutions is a result of a slow rate phenomenon.;The rate of pyrite oxidation in alkaline solutions increases in the sequence CaO < NaOH < Na(,2)CO(,3). The slow reaction in CaO solutions is attributable to the precipitation of calcium carbonate, identified by infrared analysis. The faster rate in Na(,2)CO(,3) solutions is explained by the buffering effect of carbonate ions, which minimizes the pH-drop at the reaction interface. At longer reaction times the conversion in Na(,2)CO(,3) systems approximates to that in NaOH solutions, due to the blockage of active sites by the iron oxide layer (amorphous hydrated-iron hydroxide). In NaOH solutions, the precipitation seems to occur mainly in the bulk solution, resulting in stable colloidal suspensions and pyrite particles covered by a thin oxide layer, which breaks down after some time and does not inhibit the reaction.