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

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In order to assess the role of zinc sulfide complexes in ore-forming solutions, the solubility of sphalerite was measured in NaOH---H 2 S 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 2 S(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.

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