The effect of sulphur on tungsten recovery from scheelite concentrates by alkali fusion

A low-grade scheelite concentrate, obtained from Uludag Tungsten Plant (Bursa, Turkey), was used in this study. It contained approximately 35% WO₃, 5.2% Sand 27% Fe. The optimum conditions of alkali fusion were: a temperature of 900°C, twice the stoichiometrically required Na₂CO₃, and 1 h treatment time. Tungsten was dissolved with a maximum efficiency at the end of 1 h of leaching with water at a solid/liquid weight ratio of 1/5 at 80aC under continuous stirring. A dissolution efficiency of 97% was obtained when the sulphur content of scheelite concentrate was decreased to 0.7% by roasting in a rotary furnace operating at 750°C and with 100% excess amount of air, while only 78% of tungsten dissolution efficiency could be reached when the original scheelite concentrate was not roasted prior to the alkali fusion. Phase analyses showed that the sulphur in concentrate has formed a matte phase as Na₃FeS₃ and confined some part of the tungsten as CaWO₄, during the alkali fusion, and decreased its dissolution efficiency.