Experimental Study of Stibnite Solubility in Aqueous Sulfide Solutions from 25 to 90 degC

Abstract

Modelling antimony transport in hydrothermal systems and developing mitigation procedures for stibnite (Sb2S3) scaling in geothermal power stations, such as those at the Ngawha and Rotokawa geothermal systems, requires precise thermodynamic data to determine stibnite solubility under different physicochemical conditions. Specifically, accurate modelling requires knowledge of the dependence of stibnite solubility and complexation on pH, sulfide concentration and temperature. However, there is uncertainty in the stability and stoichiometry of antimony(III) sulfide/ hydrosulfide complexes at 25 degC and higher temperatures that prevents detailed modelling of the transport and precipitation chemistry of antimony in aqueous sulphide media at elevated temperatures. We have conducted solubility experiments with natural stibnite in a flow-through apparatus to determine the solubility of stibnite in aqueous sulfide solutions from pH 6.0 to 12.5 and sulfide concentrations from 0.002 to 0.2 mStotal at 25 degC. The apparatus also allows measurement of stibnite solubility at elevated temperatures up to 90 degC. These experiments are underway. Our experimental results are similar to the solubilities found at 25 degC by Krupp (1988). He concluded that HxSb2S4x-2 species are dominant between pH 3 to 12, but other authors have suggested HxSb2S2O2x-2 and HxSbS3x-3 species (Wood, 1989; Tossell, 1994). We note that for arsenic, H3AsS3 is apparently the dominant thioarsenite stoichiometry (Bostick et al., 2005; Beak et al., 2008; Zakaznova-Herzog and Seward, 2012). At 90 degC, our solubilities are similar to solubilities extrapolated to higher pH from Krupp (1988) Our measurements will provide a complete set of stibnite solubility measurements between 25 and 90 degC in reduced, sulfide-containing fluids and permit a new evaluation of the stoichiometry and stability of thioantimonite species at these conditions.