Sustainable and efficient leaching of tungsten in ammoniacal ammonium carbonate solution from the sulfuric acid converted product of scheelite

To directly obtain solution of (NH 4 ) 2 WO 4 instead of Na 2 WO 4 is the key for developing a cleaner technology of ammonium paratungstate production. In this paper, ammoniacal ammonium carbonate solution was adopted for leaching tungsten from the converted product, a mixture of H 2 WO 4 and CaSO 4 obtained by treating scheelite with sulfuric acid . The research indicates that tungsten can be efficiently extracted in form of ammonium tungstate solution with WO 3 leaching yield of >99.5% under moderate leaching conditions. The WO 3 leaching yield is influenced by the transformation of calcium sulfate to calcium carbonate due to forming CaWO 4 through the secondary reaction between CaSO 4 and (NH 4 ) 2 WO 4 , whereas an excess (NH 4 ) 2 CO 3 (≥0.64 mol/L) can suppress the secondary reaction by facilitating the transformation. Additionally, the consumed ammonium carbonate can be recovered by treating the leaching residue with ammonium sulfate solution at above 70 °C. This work presents a cleaner and sustainable technique for producing ammonium paratungstate, with circulating the leaching reagents and bypassing the conversion of Na 2 WO 4 to (NH 4 ) 2 WO 4 .