The conversion reaction of three feedstock particle sizes (<38-165 mu m) of celestine (SrSO4) powders into SrWO4 particles with scheelite structure was investigated in highly concentrated NaOH solutions saturated with the chemical reagent Na2WO4. The hydrothermal treatments were carried out over the range of 150-225 degrees C for several reaction intervals (1-6 h). The complete conversion of SrSO4 into SrWO4 under non-stirred conditions was found to proceed rapidly on fine SrSO4 particles (<38 pm) by increasing the concentration of the alkaline fluid, and the conversion to SrWO4 was completed at 200 degrees C for 6 h in a 5 M NaOH solution. The crystallization of the SrWO4 particles occurred rapidly by stirring the autoclave at a speed of 20 rpm, even when larger SrSO4 particles (165 pm) were hydrothermally converted at a low temperature of 150 degrees C for 4 h. The conversion was further accelerated by increasing the reaction temperature and employing small feedstock particle sizes, <38 and 69 mu m. The SrWO4 particles morphology resembled a quasi-octahedral shaped and consisted of fine particles with bimodal sizes between 0.9 and 2.5 mu m. A typical tendency toward particle agglomeration was observed during the crystallization of the SrWO4 particles, which after the longest reaction intervals produced agglomerates with a size of 50 mu m. Bulk dissolution of the mineral SrSO4 powder coupled with a massive precipitation produced the formation of SrWO4 particles. A kinetic analysis was carried out for the conversion of large SrSO4 particles (165 mu m) into SrWO4 using the diffusion controlled kinetic model proposed by Dander. The activation energy determined for the global conversion process of the quasi-octahedral shaped SrWO4 particles was 18.30 kJ mol(-1). (C) 2016 Elsevier B.V. All rights reserved.