W. L. Bourcier and H. L. Barnes
Ore solution chemistry; VII, Stabilities of chloride and bisulfide complexes of zinc to 350 degrees C
Economic Geology and the Bulletin of the Society of Economic Geologists (November 1987), 82(7):1839-1863

Abstract:
The association constants for the dominant zinc chloride complexes were determined from solubility measurements of zincite (ZnO) and smithsonite (ZnCO ₃ ) in 0 to 5 m NaCl-CO ₂ -H ₂ O solutions from 100 degrees to 350 degrees C. For the important sphalerite solubility-controlling reactions: Zn (super +2) + Cl (super -) = ZnCl (super +) , Zn (super +2) + 2Cl (super -) = ZnCl ⁰ ₂ , Zn (super +2) + 3Cl (super -) = ZnCl (super -) ₃ , Zn (super +2) + 4Cl (super -) = ZnCl (super -2) ₄ , and ZnS (sub (s)) + 2H (super +) = Zn (super +2) + H ₂ S (sub (aq)) , the respective logs of the equilibrium constants for temperatures ( degrees C) equal to 100, 150, 200, 250, 300, and 350 are 1.2, 2.1, 3.1, 4.4, 5.7, 7.0; 1.9, 3.0, 4.3, 5.6, 7.2, 9.3; 2.3, 3.8, 5.2, 6.7, 8.1, 9.3; 1.4, 2.7, 4.4, 6.0, 7.4, 7.7; and -4.2, -3.8, -3.5, -3.1, -2.6, -1.7. The log K values for the last reaction were calculated here using free energy data for ZnS (sub (s)) , Zn (super +2) , and H ₂ S (sub (aq)) .The equilibrium constants for the dominant zinc bisulfide complexes were determined from solubility measurements of sphalerite (ZnS) in 0 to 4 m NaHS-H ₂ S-H ₂ O solutions from 100 degrees to 350 degrees C. The important solubility-controlling reactions are: ZnS (sub (s)) + H ₂ O (sub (l)) = Zn(OH)(HS) ⁰ , ZnS (sub (s)) + H ₂ S (sub (aq)) = Zn(HS) ⁰ ₂ , ZnS (sub (s)) + H ₂ S (sub (aq)) + HS (super -) = Zn(HS) (super -) ₃ , and ZnS (sub (s)) + H ₂ S (sub (aq)) + 2HS (super -) = Zn(HS) (super -2) ₄ , and the respective logs of their equilibrium constants for temperatures ( degrees C) equal to 100, 150, 200, 250, 300, and 350 are: -6.4, -6.1, -5.9, -5.6, -5.5, -5.3; -5.2, -5.0, -4.8, -4.7, -4.6, -4.5; -3.3, -3.1, -3.1, -3.2, -3.4, -3.6; and -4.6, -4.0, -3.7, -3.6, -3.6, -3.7.Enthalpies and entropies of complex formation derived from these data demonstrate the increasing importance of electrostatic effects in complex formation at higher temperatures. Good agreement was found between our measured and calculated solubilities of sphalerite from 100 degrees to 350 degrees C in an NaCl-H ₂ S-H ₂ O solution and in an NaCl-CaCl ₂ -CO ₂ -H ₂ S-H ₂ O solution in equilibrium with calcite. Therefore, the nine equilibrium constants provide confirmed resolution of sphalerite solubilities in ore solutions having chemistries quite different from those used in the solubility experiments. Sample calculations of sphalerite solubilities in Kuroko-type massive sulfide ore fluids are given which demonstrate the effectiveness of cooling in the temperature range of 250 degrees to 300 degrees C as a precipitation mechanism for sphalerite.