The ultrasonic enhancement of the reduction of dissolved ferric iron (Fe3+) to ferrous iron (Fe2+) with sphalerite (zinc sulfide) concentrate in a sulfuric acid solution, which is an industrially important reaction in zinc production, was studied. The temperature of the experiments varied between 75 degrees C and 95 degrees C. The initial concentrations of ferric iron and sphalerite were 0.179 mol/L and 0.107 mol/L, i.e. 1.2:2 reducing agent-to-ferric iron molar ratio was used in a 0.40 M sulfuric acid solution. The effect of ultrasonic power, duration of ultrasound cycles and the time between the cycles were studied. The concentration of ferric iron was determined by sequential injection analysis (SIA). The apparent effect of ultrasound varied depending on the conditions used, but it clearly enhanced the rate of the reaction especially when the impact of mass transfer was essential. Eleven different models were tested for silent systems. These models are utilized in the present kinetic study for explaining the kinetics of the reduction reaction of spherical particles when ultrasound is employed. (C) 2007 Elsevier B.V. All rights reserved.