| Autoclave oxidation of arsenical gold and silver ores in alkaline medium as a precyanidation step has been studied in terms of investigating the dissolution behavior of arsenopyrite in oxidizing hydrothermal alkaline systems and the effects of reaction parameters such as oxygen, partial pressure, temperature, alkaline concentration, particle size, agitation speed, slurry density, initial concentration of arsenic and ferric ions, and the presence of stibnite.; Presence of stibnite, at low and intermediate oxygen partial pressures, greatly retards the oxidation of arsenopyrite. Very little effect, however, exists at high oxygen pressures.; The reaction is characterized by (1) a half-order and a first-order with respect to oxygen partial pressure and alkaline concentration, respectively; (2) inverse first-order relationship between initial rate and particle size; (3) iron oxide shell formation around a shrinking arsenopyrite core, (4) an average reaction velocity of the order 10{dollar}sp{lcub}-4{rcub}{dollar} cm/sec, compared to theoretically estimated minimum mass transfer coefficient of the order 10{dollar}sp{lcub}-1{rcub}{dollar} cm/sec; and (5) an apparent activation energy of 5.7 Kcal/mole (23.9 KJ/mole). Under the specified standard conditions the reaction is controlled by mixed kinetics of chemical reaction and diffusion through iron oxide product layer. Rate controlling step is, however, dependent on reaction conditions. An attempt has been made to establish this relationship.; An electrochemical mechanism which involves a cathodic dissolution of arsenopyrite has been proposed for the chemical reaction. Using the Butler-Volmer equation, a theoretical analysis of the proposed electro-chemical reactions yielded the rate expression:(UNFORMATTED TABLE OR EQUATION FOLLOWS){dollar}{dollar}{lcub}rm {lcub}sp{lcub}dn{rcub}(FeAsS)over dt{rcub} = Kspstar Asb{lcub}c{rcub}Psb{lcub}Osb2{rcub}sp{lcub}1/2{rcub}lbrack OHsp{lcub}-{rcub}rbrack{rcub}{dollar}{dollar}(TABLE/EQUATION ENDS)which is consistent with experimental observations. |
