The leaching kinetics of a zinc sulphide concentrate in acid ferric sulphate

This work examines the dissolution kinetics of an iron–rich zinc sulphide concentrate in acid ferric sulphate medium. The effects of temperature, ferric ion and sulphuric acid concentrations, agitation speed and particle size on the leaching kinetics were investigated. The leaching process could be separated into two stages. Initially, the dissolution kinetics was controlled by the chemical reaction at the surface of the zinc sulphide particles followed by a second step where the reaction was controlled by diffusion of the reagents or products through the elemental sulphur (ash) layer. The activation energy of the chemical controlled step was 27.5 kJ/mol and the value determined for the diffusion controlled step was 19.6 kJ/mol. The reaction order with respect to ferric ion and sulphuric acid concentrations were approximately 0.50 and 1.00, respectively. Analysis of the unreacted and reacted sulphide particles by SEM-EDS showed a progressive increase of the thickness of the elemental sulphur layer on the solid surface. The development of this sulphur layer is further evidence of the change on the rate-controlling step as the reaction progress.