Reaction mechanism for the ferric chloride leaching of sphalerite

Reaction mechanisms for the ferric chloride leaching of sphalerite are proposed based on data obtained in leaching and dual cell experiments presented in this work and in a previous study. The results from the leaching experiments show that at low concentrations the rate is proportional to [Fe³⁺]_T ^0.5 and [Cl^-]_T ^0.43 but at higher concentrations the reaction order with respect to both [Fe³⁺]_T and [Cl^-]_T decreases. Using dual cell experiments which allow the half cell reactions to be separated, increased rates are observed when NaCl is added to the anolyte and to the catholyte. The increase in rate is attributed to a direct, anodic electrochemical reaction of Cl^- with the mineral. When NaCl is added only to the catholyte, a decrease in the rate is observed due to a decrease in theE ⁰ of the cathode which is attributed to the formation of ferric-chloro complexes. Several possible electrochemical mechanisms and mathematical models based on the Butler-Volmer relation are delineated, and of these, one model is selected which accounts for the experimentally observed changes in reaction order for both Fe³⁺ and Cl^-. This analysis incorporates a charge transfer process for each ion and an adsorption step for ferric and chloride ions. The inhibiting effect of Fe²⁺ noted by previous investigators is also accounted for through a similar model which includes back reaction kinetics for Fe²⁺. The proposed models successfully provide a theoretical basis for describing the role of Cl^-, Fe³⁺, and Fe²⁺ as well as their interrelationship in zinc sulfide leaching reactions. Possible applications of these results to chloride leaching systems involving other sulfides or complex sulfides are considered.