The direct conversion of celestite (SrSO4) to Strontium carbonate by mechanochemical processing was studied under wet milling conditions by varying the amount of sodium carbonate with respect to the stoichiometric amount required for the conversion reaction. The effect of solid to liquid ratio and change of particle size distribution during the milling were examined. The resulting conversion-time curve was fitted using the shrinking particle model. It was observed that celestite completely converted to strontium carbonate within 45 min, when the solid to liquid ratio was 0.1. With 50% excess sodium carbonate was used at a solid to liquid ratio of 0.2 the apparent rate constant was 0.0131 min(-1), at a solid to liquid ratio of 0.1 the rate was 0.0211 min(-1). The order of mechanochemical conversion was proportional to the 1.21 power of the CO32- concentration. (C) 2006 Elsevier B.V. All rights reserved.