Experimental determination of the solubility of WO 3(s) and the thermodynamic properties of H 2WO 4(aq) in the range 300–600°C at 1 kbar: Calculation of scheelite solubility

Abstract

Solubility measurements were carried out to assess the importance of chloride to the hydrothermal transport of tungsten. The solubility of WO 3(s) was measured in 10 −3 to 0.49 molal HCl solutions at 1 kbar and temperatures from 300 to 600°C. These experiments were carried out using conventional cold-seal pressure vessels, with WO 3(s) powder contained within a tightly crimped inner Pd capsule surrounded by the fluid and then an outer Au capsule. This arrangement permitted contact between WO 3 and solution at high temperature and pressure, but effectively eliminated back reaction on quench and entrainment of fine particulate WO 3 upon fluid sampling. There is no significant dependence of measured W concentration on initial HCl concentrations between 5 x 10 −3 and 0.49 molal at all temperatures investigated. However, W concentrations were often slightly higher in 10 −3 molal HCl. These results indicate that significant chloride complex formation does not take place with W under supercritical conditions and the species responsible for W transport under the conditions of these experiments is H 2WO 4(aq). The increase in solubility of WO 3 at the lower HCl concentrations can be explained by ionization of H 2WO 4(aq) to HWO 4 − at higher pH values. The logarithms of the equilibrium constants for the reaction: WO 3( s) + H 2 O(1) = H 2 WO 4(aq) are calculated to be −4.0 ± 0.2 (300° C); −3.4 ± 0.4 (400° C); −3.1 ± 0.1 (500° C); −2.7 ± 0.3 (600° C). From these data the values ΔH r ≈ 41 kJ/mol and ΔC p, r ≈ 0 may be derived. Thermodynamic data for H 2WO 4(aq) were also calculated. Using the free energy data for aqueous tungstic acid determined in the solubility study, combined with that in the literature for scheelite, it is possible to show that the solubility of the mineral scheelite as a simple oxyacid species can attain values between 10 and 100 ppm under geologically reasonable conditions, and hence complexation by additional ligands is not required for W transport by hydrothermal solutions. However, W mineral solubility may be further increased by ion pairing of alkali metal cations with tungstate or chloride complexation of Fe, Mn (wolframite), or Ca (scheelite).