Rapid synthesis of scheelite SrWO 4 particles using a natural SrSO 4 ore under alkaline hydrothermal conditions

Abstract

Abstract The conversion of three differently sized ( 4 mineral powders into SrWO 4 particles with scheelite structure was investigated in highly concentrated NaOH solutions saturated with WO 4 2 − 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 SrSO 4 into SrWO 4 under non-stirred conditions was found to proceed at 250 °C in 48 h in a 5 M NaOH solution. SrWO 4 synthesis occurred faster using powders with fine particle sizes, 4 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 SrSO 4 into SrWO 4 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 WO 4 /OH, this factor varying with the amount of solvent added in the reactor. The SrWO 4 particles were produced by a coupled process involving both the bulk dissolution of the SrSO 4 powder and the bulk precipitation of SrWO 4 in an alkaline solution (5 M NaOH) saturated with WO 4 2 − ions. Kinetic analysis shows that the activation energy required for the production of the SrWO 4 quasi-octahedral shaped particles was low, in the range 13.13 to 20.0 kJ mol − 1 for those powders with fine particle sizes ( 4 particles was of 27.20 kJ mol − 1 .