Factors affecting purity of ammonium para-tungstate (APT) prepared from wolframite ((Fe, Mn)WO4) concentrate by NaHSO4·H2O roasting, water/ammonia leaching and evaporation crystallization

Traditional alkaline decomposition process for tungsten ore is facing significant challenges, including release of large amounts of hazardous waste and lengthy procedures. A previously proposed novel process for scheelite using NaHSO₄·H₂O roasting yielded ammonium para-tungstate (APT) of high-purity. However, the applicability of this method to wolframite ((Fe, Mn)WO₄) concentrate remains uncertain. In this work, the process of wolframite decomposition by NaHSO₄·H₂O roasting for the preparation of APT was systematically investigated. It was found that 98.6 % of the tungsten in wolframite can be converted into WO₃ under the conditions of 3.0 times the stoichiometric amount of NaHSO₄·H₂O, a roasting temperature of 650 °C and a roasting time of 120 min. The main phases in the roasted product were WO₃, MnSO₄, and 3Na₂SO₄·Fe₂(SO₄)₃. Additionally, the water leaching conditions for WO₃ enrichment of the roasted product were investigated. At a liquid–solid ratio of 6:1 mL/g, a stirring rate of 300 rpm, a leaching time of 180 min and a leaching temperature of 25 °C, the removal efficiencies of Fe and Mn in the roasted product were 96.5 % and 99.3 %, respectively. Consequently, the concentration of WO₃ in the water-leaching product (residue) enriched from 25.2 % to 93.1 %. Furthermore, at an ammonia concentration of 4 mol/L, a liquid–solid ratio of 4:1 mL/g, a dissolution temperature of 80 °C, and a stirring rate of 350 rpm, the leaching efficiency of WO₃ in the WO₃ enrichment reached 98.5 % after 120 min. Subsequently, an APT product of high purity was obtained through impurity removal and evaporation crystallization. Overall, the results showed that NaHSO₄·H₂O roasting method has a broad range of applications in the decomposition of tungsten ore.