Technology and theory of producing APT from tungsten concentrates by sulfuric acid conversion-ammonium salt leaching
Tungsten is considered as a strategic and critical raw material, due to its remarkable physical and chemical properties, wide industrial applications and non-substitutability. Ammonium paratungstate is the main intermediate product extracted from tungsten ores with metallurgical processes, intrinsically linking to the tungsten industry across all supply chain stages. An efficient and cleaner technology for producing ammonium paratungstate is crucial to the sustainable development of the tungsten industry. This work presents a novel process by the sulfuric acid conversion-ammoniacal ammonium carbonate leaching route. The complete conversion of tungsten concentrates in H2SO4 solutions can be achieved by controlling the sulfuric acid concentration and adding an oxidizing agent. The formation mechanism of the H2WO4 layer on unreacted tungsten particles is explained by isopolytungsten ions diffusion-tungsten acid deposition in the diffusion layer. The more difficulty of wolframite conversion than scheelite in H2SO4 solutions is attributed to the thermodynamics, especially the accumulation of Fe2+ and/or Mn2+ in the solutions. The kinetics of scheelite conversion in moderate H2SO4 solutions agrees with the shrinking core model under chemical surface reaction control. Subsequently, an ammonium tungstate solution is directly obtained by leaching converted products in ammoniacal (NH4)2CO3 solutions at 30 °C, with WO3 leaching yield of >99%. The transformation of calcium sulfates to carbonates influences the WO3 leaching yield by secondary reactions, which can be suppressed by an excess (NH4)2CO3 in solutions through the formation of more stable vaterite and calcite. The leaching agent can be recovered to restore the leaching system. Additionally, thermodynamic modelling of the CaSO4–H2O system is carried out to better understand the thermodynamic property of CaSO4 solution and facilitate the solutions cycle and scaling prevention, as well as the upcoming CaSO4–H2SO4–H2O system. The critically revaluated solubility data were assessed with the NPL Pitzer model by MTDATA software to optimize the Pitzer parameters. The obtained model predicts very well the CaSO4–H2O system up to 300 °C, and agrees with most published solubility data.This work makes it possible to produce ammonium paratungstate cleanly and economically, featuring circulation of the leaching reagents and bypassing the conversion of Na2WO4 to (NH4)2WO4, which will hopefully be adopted in industrial practices.
|Tila||Julkaistu - 2019|
|OKM-julkaisutyyppi||G5 Tohtorinväitöskirja (artikkeli)|