Experimental Determination of Ferberite Solubility in the KCl–HCl–H₂O System at 400–500 °C and 20–100 MPa

Abstract

The solubility of ferberite, FeWO₄ was studied at 400–500 °C, pressures of 20, 25, 40, 50 and 100 MPa, oxygen fugacity corresponding to the Ni–NiO, Fe₃O₄–Fe₂O₃ buffers, in 0.7 ÷ 8.9 mKCl solutions and acidity controlled by quartz-microcline-muscovite buffer assemblage. The parameters of the experiments cover both the field of homogeneous solutions and the region of immiscibility in the KCl–H₂O system. The total W concentration depends upon mCl, T and fO₂ in the system and range from 1 · 10⁻⁴–0.05 mol kg⁻¹ in 0.7 mKCl to 0.01–0.15 in 8.9 mKCl. The results suggest a large bulk solubility in the dense, salt-rich phase of the two-phase fluid. Ferberite dissolution in KCl solutions under pH and fO₂ buffered conditions at 400–500 °C proceeds congruently as well as incongruently with accompanying potassium tungsten bronzes formation, K_xWO₃, (x = 0.2–0.3). Thermodynamic calculations performed for a homogeneous solution at P = 100 MPa indicate that the predominant aqueous species of tungsten in KCl–HCl solutions at fO₂ = fO₂(Ni–NiO) may be W(V, VI) species: ${\text{WO}}_{3}^{ - }$, ${\text{HWO}}_{4}^{ - }$, ${\text{H}}_{2} {\text{W}}_{2} {\text{O}}_{7}^{ - }$ at 500 °C and ${\text{HWO}}_{4}^{ - }$, ${\text{W}}_{5} {\text{O}}_{16}^{3 - }$ at 400 °C. Application of the extended FeWO₄ solubility model to natural systems suggest that deposition of tungsten from ore-bearing solutions is due to interaction with wall rocks containing feldspars, and iron oxides together with decreasing temperatures. In the magnetite bearing system, the equilibrium tungsten concentration does not exceed 2 · 10⁻⁵ mol kg⁻¹ at temperatures of 400 °C.