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 H₂WO₄ and CaSO₄·nH₂O in H₂SO₄ solution was firstly studied in this work. The results show that H₂SO₄ concentration, temperature, time, stirring speed, and particle size affect the conversion efficiency. Moreover, this work reveals that solid H₂WO₄ layer on the surface of CaWO₄ particles forms and develops from thickness and looseness to thinness and denseness with increasing H₂SO₄ concentration, and thus hinders the conversion of CaWO₄ to H₂WO₄, whereas CaSO₄·nH₂O almost does not deposit in the layer. Meanwhile, H₂WO₄ layer on CaWO₄ surface is resulted by isopolytungstate ions diffusion-tungstic acid deposition in diffusion layer. Additionally, the performance of separating H₂WO₄ from CaSO₄·nH₂O 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 WO₃ 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 Na₂WO₄ to (NH₄)₂WO₄.