Hydrosulfide/sulfide complexes of zinc to 250°C and the thermodynamic properties of sphalerite

Abstract

The solubility of synthetic ZnS (cr) was measured at 25–250 °C and P = 150 bars as a function of pH in aqueous sulfide solutions (~ 0.015–0.15 m of total reduced sulfur). The solubility determinations were performed using a Ti flow-through hydrothermal reactor. The solubility of ZnS (cr) was found to increase slowly with temperature over the whole pH range from 2 to ~ 10. The values of the Zn–S–HS complex stability constant, β, were determined for Zn(HS) 2 0 (aq) , Zn(HS) 3 − , Zn(HS) 4 2− , and ZnS(HS) − . Based on the experimental values the Ryzhenko–Bryzgalin electrostatic model parameters for these stability constants were calculated, and the ZnS (cr) solubility and the speciation of Zn in sulfide-containing hydrothermal solutions were evaluated. The most pronounced solubility increase, about 3 log units at m ( S total ) = 0.1 for the temperatures from 25 to 250 °C, was found in acidic solutions (pH ~ 3 to 4) in the Zn(HS) 2 0 (aq) predominance field. In weakly alkaline solutions, where Zn(HS) 3 − and Zn(HS) 4 2− are the dominant Zn–S–HS complexes, the ZnS (cr) solubility increases by 1 log unit at the same conditions. It was found that ZnS(HS) − and especially Zn(HS) 4 2− become less important in high temperature solutions. At 25 °C and m ( S total ) = 0.1, these species dominate Zn speciation at pH > 7. At 100 °C and m ( S total ) = 0.1, the maximum fraction of Zn(HS) 4 2− is only 20% of the total Zn concentration (i.e. at pH t ~ 7.5), whereas at 350 °C and 3 <pH t <10, the fraction of Zn(HS) 4 2− and ZnS(HS) − is less than 0.05% and 2.5% respectively, of the total Zn concentration and Zn(HS) 2 0 and Zn(HS) 3 − predominate. The measured equilibrium formation constants were combined with the literature data on the stability of Zn–Cl complexes in order to evaluate the concentration and speciation of Zn in chloride solutions. It was found that at acidic pH, and in more saline fluids having total chloride > 0.05 m, Zn–Cl complexes are responsible for hydrothermal Zn transport with no significant contribution of Zn–S–HS complexes. The hydrosulfide/sulfide complexes will play a more important role in lower salinity (< 0.05 m chloride) hydrothermal solutions which are characteristic of many epithermal ore depositing environments. The value of Δ f G ° (β-ZnS (cr) ) = − 198.6 ± 0.2 kJ/mol at 25 °C was determined via solubility measurements of natural low-iron Santander (Spain) sphalerite.