Study of equilibrium dissolution of WO₃–CaSO₄ in HCl–Na₂SO₄ solution

There are numerous issues in the mainstream process of alkali decomposition of tungsten ores, such as large water consumption, large amounts of wastewater, and high processing costs, which add the dual pressure of economic and environmental protection on smelting enterprises and prevent them from meeting the industry’s development needs. As a result of a series of studies on scheelite roasting and decomposition processes, our team innovatively proposed the use of acid roasting to develop the process of sulfate decomposition of scheelite so that CaWO₄ in the scheelite could be transformed directly into WO₃. In addition to WO₃, the roasting products contained soluble Na₂SO₄ and insoluble CaSO₄. Because CaSO₄ can be dissolved in hydrochloric acid, it can be separated from WO₃ via hydrochloric acid leaching to further enrich WO₃, resulting in a higher-grade material for subsequent procedures. In the presence of Na₂SO₄, its effect on the dissolution of WO₃ or CaSO₄ in hydrochloric acid will directly determine the separation effect of calcium and tungsten in the roasting products. Thus, using pure substances such as WO₃, CaSO₄, and Na₂SO₄ as raw materials, the dissolution behaviors of WO₃, CaSO₄, and WO₃–CaSO₄ in HCl–Na₂SO₄ solution were investigated separately via isothermal equilibrium dissolution to investigate the effects of hydrochloric acid concentration, sodium sulfate concentration, dissolution time, and dissolution temperature on the solubility of WO₃, CaSO₄, and WO₃–CasO₄ in HCl–Na₂SO₄ solution. The analysis shows that WO₃ and CaSO₄ have very different solubilities in hydrochloric acid. The solubility of CaSO₄ in hydrochloric acid increases with temperature and hydrochloric acid concentration when the dissolution time is 0.5–2.5 h, the hydrochloric acid concentration is 1–5 mol·L⁻¹, the molar ratio of HCl and Na₂SO₄ is 1∶2–2∶1, and the dissolution temperature is 40–80 ℃. The solubility of calcium sulfate in hydrochloric acid increases with the increase in temperature and hydrochloric acid concentration. When the temperature is 80 ℃ and the concentration of hydrochloric acid is 3 mol·L⁻¹, the solubility of calcium sulfate in hydrochloric acid reaches a peak of 55 g·L⁻¹. Due to the same ion effect, Na₂SO₄ can significantly reduce the solubility of CaSO₄ and narrow the solubility difference between CaSO₄ and WO₃ in hydrochloric acid. CaSO₄ has the highest solubility in HCl–Na₂SO₄ solution at 17.04 g·L⁻¹. The dissolved WO₃, whose solubility is maintained at 0.3–3 g·L⁻¹, can be effectively recovered by using the current mature low-tungsten recovery process. Therefore, when CaSO₄ and WO₃ coexist in hydrochloric acid, increasing the concentration of hydrochloric acid and the dissolution temperature while decreasing the concentration of Na₂SO₄ can increase the solubility difference between them and achieve separation.