Ag₂S solubility in sulfide solutions up to 250°C

Abstract

In order to estimate possible silver sulfide complexes in ore solutions, the solubility of Ag₂S was measured between 25° and 250°C in NaOH-H₂S-H₂O solutions of 0.0 to 4.1 m NaHS concentration. Solubility changes as a function of temperature, total reduced sulfur concentration ΣS (m_H2S + m_HS-) and pH. A maximum solubility of 2140ppm Ag was obtained at 250°C in 4.1 m NaHS concentration under P_H2S of 29.1 atm. From these solubility data, reactions to form silver sulfide complexes are estimated as follows: Ag₂S + H₂S = Ag₂S(H₂S), Ag₂S + H₂S + HS^- = Ag₂S(H₂S) (HS)^-, Ag₂S + H₂S + 2HS^- = Ag₂S (H₂S) (HS)₂^2- and Ag₂S + 2HS^- = Ag₂S (HS)₂^2-. The equilibrium constants for these reactions are given in Table 2. Assuming that such silver sulfide complexes as above are in an ore solution, Ag₂S (argentite or acanthite) is precipitated in response to changes of temperature, pH and total sulfur concentration (ΣS). Decrease of pH and ΣS is more effective than that of temperature. Silver sulfide complexes are more important than silver chloride complexes in ore solution of high ΣS, neutral to slightly alkaline pH and geologically reasonable chloride concentrations.