Scheelite conversion in sulfuric acid together with tungsten extraction by ammonium carbonate solution

Aiming to treat scheelite concentrate for producing ammonium paratungstate efficiently and cleanly, the conversion of scheelite to H 2 WO 4 and CaSO 4 ·nH 2 O in H 2 SO 4 solution was firstly studied in this work. The results show that H 2 SO 4 concentration, temperature, time, stirring speed, and particle size affect the conversion efficiency. Moreover, this work reveals that solid H 2 WO 4 layer on the surface of CaWO 4 particles forms and develops from thickness and looseness to thinness and denseness with increasing H 2 SO 4 concentration, and thus hinders the conversion of CaWO 4 to H 2 WO 4 , whereas CaSO 4 ·nH 2 O almost does not deposit in the layer. Meanwhile, H 2 WO 4 layer on CaWO 4 surface is resulted by isopolytungstate ions diffusion-tungstic acid deposition in diffusion layer. Additionally, the performance of separating H 2 WO 4 from CaSO 4 ·nH 2 O by leaching the converted product with aqueous ammonium carbonate solution was examined, showing that the converted product has an excellent leachability of tungsten in aqueous ammonium carbonate solution at ambient temperature with WO 3 leaching efficiency of about 99%. This work has the potential to develop a novel technique for tungsten extraction from scheelite concentrate under atmospheric pressure, bypassing the conversion of Na 2 WO 4 to (NH 4 ) 2 WO 4 .