Thermodynamic analysis of the decomposition of scheelite by oxalic acid

Abstract

The equilibrium diagram of the Ca–W–Oa–H₂O system for the H₂C₂O₄ leaching of scheelite at 298 K was constructed. This diagram clarifies the dominant regions of each component in the system as well as the variation rules of key components with pH value and total concentration of free H₂C₂O₄, thereby confirming the thermodynamic conditions for the H₂C₂O₄ leaching of scheelite. Theoretical analysis results indicate that H₂C₂O₄ can effectively leach scheelite, and the concentration of H₂C₂O₄ is a crucial factor influencing the leaching efficiency. When 1 < pH < 6.4, scheelite reacts with H₂C₂O₄ to form precipitates of H₂WO_4(s) and CaC₂O_4(s). Under the condition of maintaining a high concentration of free H₂C₂O_4(aq), H₂WO_4(s) further reacts with H₂C₂O_4(aq), and the formation of H₂[WO₃(C₂O₄·H₂O)] with high solubility is predicted, which promotes the leaching of scheelite. When 6.4 < pH < 14.6, the ionization of H₂C₂O₄ is enhanced with the increase of pH, leading to the rise in the concentrations of C₂O₄²⁻ and WO₄²⁻. When pH > 14.6, the binding effect between OH⁻ and Ca²⁺ in the system is sharply strengthened, and CaC₂O_4(s) is gradually converted into Ca(OH)_2(s). Based on the thermodynamic analysis results, experimental verification of H₂C₂O₄ leaching of scheelite was carried out. Under the optimal conditions of H₂C₂O₄ concentration of 2.2 mol L⁻¹, reaction temperature of 80 °C, reaction time of 5 h, liquid-to-solid ratio of 15 : 1, and stirring speed of 100 rpm, the leaching rate of WO₃ reached 99.54%, with only 0.53% of WO₃ remaining in the leaching residue. Comparison of the XRD, EDS, and FTIR characterization results of raw scheelite and oxalic acid-leached residue reveals that during the leaching process, most of the tungsten enters the leachate in a highly soluble form, while calcium combines with C₂O₄²⁻ to form CaC₂O₄·H₂O precipitate, which is retained in the leaching residue.