Ferric leaching of the sphalerite contained in a bulk concentrate: Kinetic study

A novel process for the integral treatment of polymetallic sulphide ores is proposed. The process consists of a global flotation, two stages of ferric leaching, the first stage dissolves the sphalerite and the rest of secondary sulphides, and the second dissolves the chalcopyrite with a silver catalyst, and finally a brine leaching to recover lead and silver (added as catalyst). The proposed process offers several advantages comparing to traditional pyrometallurgical techniques. This study is focused on the sphalerite dissolution with ferric sulphate, as the first stage of the treatment of Cu-Zn-Pb concentrates. The ferric ion concentration, the temperature and the particle size have an important role in the sphalerite oxidation. However, the initial sulphuric acid, ferrous ion and sulphate ion concentrations have no influence in the process rate. The formation of an elemental non-porous layer of sulphur along the reaction hinders the ferric ion transport to the sphalerite surface. Two kinetics regimes are proposed, in the beginning the chemical reaction is the rate controlling step, and, at 30% Zn extraction, the rate controlling step changes to diffusion through a non-porous film of elemental sulphur. The activation energy obtained for the chemical reaction has a value of 51.3 kJ/mol, and the apparent activation energy in the diffusional stage is the 47.7 kJ/mol. The reaction order with respect ferric ion is 0.26 and the reaction rate is proportional to the zinc sulphide amount in the first stage. The behaviour of sphalerite, contained in a bulk concentrate, is similar to the pure sphalerite or the sphalerite contained in a differential concentrate.