XU Lüe-wei, HE Bing-xuan, XU Guo-zuan, LEI Xin, HU Hao-dong, LIANG Yong, LIU De-gang. Study of equilibrium dissolution of WO3–CaSO4 in HCl–Na2SO4 solution[J]. Chinese Journal of Engineering, 2023, 45(6): 883-889. DOI: 10.13374/j.issn2095-9389.2022.03.25.001
Citation: XU Lüe-wei, HE Bing-xuan, XU Guo-zuan, LEI Xin, HU Hao-dong, LIANG Yong, LIU De-gang. Study of equilibrium dissolution of WO3–CaSO4 in HCl–Na2SO4 solution[J]. Chinese Journal of Engineering, 2023, 45(6): 883-889. DOI: 10.13374/j.issn2095-9389.2022.03.25.001

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.
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