A REACTION KINETIC-MODEL FOR THE LEACHING OF INDUSTRIAL ZINC FERRITE PARTICULATES IN SULFURIC-ACID MEDIA

Zinc ferrite of the general formula (Zn1-xFex2+)Fe23+O4 (x less-than-or-equal-to 0.4) is the principal constituent of the "neutral-leach residues" which are commonly treated by "hot-acid leaching" in electrolytic zinc plants. In this thorough investigation on the kinetics of zinc ferrite acid dissolution, well-characterized, porous zinc ferrite particulates coming from industrial neutral-leach residues are subjected to controlled aqueous leaching under hot-acid-leach conditions, i.e. temperature close to 100-degrees-C and sulphuric acid concentration higher than 0.25 mol l-1. The effect of key parameters such as temperature, particle surface area, [H2SO4], [ZnSO4], and [Fe2(SO4)3] is examined. The dissolution process is found to be described most adequately by the "grain model" with the surface reaction being the rate-controlling step. The estimated activation energy (64.8kJ mol-1), as well as the reaction order with respect to the activity of the hydrogen ion (0.54), indicate also a chemically controlled process. The retarding effect of high concentrations of metal (zinc and/or iron) sulphates on the dissolution kinetics is interpreted on the basis of the negative effect these sulphates have on the activity of H+. An electrochemical mechanism is adopted to explain the observed kinetic behaviour.