Reduction of ferric to ferrous with sphalerite concentrate, kinetic modelling

Reduction of dissolved ferric iron (Fe3+) to ferrous iron (Fe2+) with sphalerite (zinc sulphide) concentrate (Red Dog, Alaska) as the reducing agent was studied in an isothermal batch reactor under nitrogen flow. The influence of temperature (75–95 °C), initial concentration of ferric iron (0.1–0.3 mol/L), reducing agent-to-ferric iron molar ratio (0.5:1–2.1:1), as well as sulphuric acid concentration (0.20–1.02 mol/L) on the reduction kinetics was investigated. The sulphuric acid had no influence on the reduction kinetics. Analysis of unreacted and reacted sphalerite particles was performed by SEM-EDS and laser diffraction techniques to study the changes of the solid particles with time and to confirm whether a product layer was present. The result from SEM-EDS gave no evidence of product layer formation. The analysis with laser diffraction technique suggested that sulphur was formed, creating particles distinct from the unreacted material. The reaction kinetics was modelled by comparing the experimental data with several models proposed for solid–liquid reactions, and the parameters were estimated by nonlinear regression analysis. The models represented first- and second-order dependence of the reducing agent, surface reaction on spherical and cylindrical particles, as well as diffusion through a liquid film and a product layer. From the estimation results, it was concluded that a good description is obtained with a simple model, where the reaction rate is proportional to the concentration of reducing agent.