An experimental study of Cobalt (II) complexation in Cl~- and H2S-bearing hydrothermal solutions

摘要

The speciation of cobalt (II) in Cl~- and H_2S~-bearing solutions was investigated spectrophotometrically at temperatures of 200, 250, and 300°C and a pressure of 100bars, and by measuring the solubility of cobaltpentlandite at temperatures of 120-300°C and variable pressures of H_2S. From the results of these experiments, it is evident that CoHS~+ and CoCl42- predominate in the solutions except at 150°C, for which the dominant chloride complex is CoCl_3~-. The logarithms of the stability constant for CoHS~+ show moderate variation with temperature, decreasing from 6.24 at 120°C to 5.84 at 200°C, and increasing to 6.52 at 300°C. Formation constants for chloride species increase smoothly with temperature and at 300°C their logarithms reach 8.33 for CoCl42-, 6.44 for CoCl_3~-, 4.94 to 5.36 for CoCl20, and 2.42 for CoCl~+. Calculations based on the composition of a model hydrothermal fluid (Ksp-Mu-Qz, KCl=0.25m, NaCl=0.75m, ΣS=0.3m) suggest that at temperatures ≤200°C, cobalt occurs dominantly as CoHS~+, whereas at higher temperatures the dominant species is CoCl_4~(2-).