Solubility of sphalerite in aqueous sulfide solutions at temperatures between 25 and 240°c

Abstract

In order to assess the role of zinc sulfide complexes in ore-forming solutions, the solubility of sphalerite was measured in NaOHH 2S aqueous solutions of 0.0 to 3.0 m NaHS concentration at temperatures of 25 to 240°C. Solubilities vary with temperature, activities of H 2S(aq) and HS −, total reduced sulfur concentration (∑S), and pH. From the solubility data, the main reactions that form zinc sulfide complexes were determined as follows: ZnS( s) + H 2 S( aq) = Zn( HS) 2 0, ZnS( s) + H 2 S( aq) + HS − = Zn( HS) − 3, ZnS( s) + H 2 S( aq) + 2 HS − = Zn( HS) 4 2−, ZnS( s) + H 2 O(1) + HS − = Zn( OH)( HS) − 2, and ZnS( s) + H 2 O(1) + 2 HS − = Zn( OH)( HS) 2− 3. Their equilibrium constants (log K) are 25°C: −5.3, −3.3, −3.4, −4.4, −4.9; 100°C: −5.2, −3.5, −3.2, −4.1, −5.0; 150°C: −4.7, −3.8, −3.1, −4.7, −5.2; 200°C: −5.1, −3.4, −3.1, −4.6; 240°C: −4.9, −3.3, −3.1, −4.9, respectively. Zn(OHXHS) 2− 3 is not stable at temperatures higher than 200°C. Zinc sulfide complexes predominate over chloride complexes in relatively low temperature hydrothermal solutions which have high ∑S, low ∑C1 −, and high pH values. In these solutions, ZnS is precipitated in response to changes of temperature, pH, and ∑S. Among them, decrease of ∑S is more effective than that of temperature and pH. Zinc sulfide complexes do not transport significant zinc in those ore-forming solutions responsible for economic zinc sulfide deposits. However, they become predominant zinc species in certain geothermal solutions and ore-forming solutions responsible for some epithermal precious metal deposits.