Rapid synthesis of scheelite SrWO4 particles using a natural SrSO4 ore under alkaline hydrothermal conditions

The conversion of three differently sized (< 38-165 μm) SrSO4 mineral powders into SrWO4 particles with scheelite structure was investigated in highly concentrated NaOH solutions saturated with WO42 − ions under hydrothermal conditions. The treatments were conducted over the range of 150-250 °C for several reaction intervals (1-48 h). The complete conversion of SrSO4 into SrWO4 under non-stirred conditions was found to proceed at 250 °C in 48 h in a 5 M NaOH solution. SrWO4 synthesis occurred faster using powders with fine particle sizes, < 38 and 69 μm; in comparison with the 165 μm powder. The SrWO4 particle morphology resembled a quasi-octahedral shaped and consisted of fine particles with bimodal particle size between 2.5 and 15 μm. The ultimate conversion of SrSO4 into SrWO4 is not affected by the autoclave filling ratio, the conversion proceeding more rapidly during early reaction intervals at a low filling ratio of 22.3% but more slowly at filling ratios above 45%, due to diffusion controlled interfacial molar ratio WO4/OH, this factor varying with the amount of solvent added in the reactor. The SrWO4 particles were produced by a coupled process involving both the bulk dissolution of the SrSO4 powder and the bulk precipitation of SrWO4 in an alkaline solution (5 M NaOH) saturated with WO42 − ions. Kinetic analysis shows that the activation energy required for the production of the SrWO4 quasi-octahedral shaped particles was low, in the range 13.13 to 20.0 kJ mol− 1 for those powders with fine particle sizes (< 38 and 69 μm). The activation energy required to transform the coarser (165 μm) SrSO4 particles was of 27.20 kJ mol− 1.