Abstract
The solubility of covellite (CuS) was measured in aqueous sulfide solutions under varying conditions: up to 4.95 molal HS (super -) ; pH 3.5 to 13.5; 20 degrees to 200 degrees C; and 2 to 83 atm total pressure. The solubility of covellite is a function of the concentration of HS (super -) , H 2 S pressure, and temperature. Three important copper-bisulfide complexes are formed, the stability of which depends on pH and temperature. The following reactions represent the formation of the complexes predominating in the pH ranges given, at the temperatures, pH's, and ionic strength (mu ) indicated:At 25 degrees C: pH < 7.3 CuS(s) + H 2 S(g) + HS (super -) = Cu(HS) 3 (super -) K = 10 (super -4.40+ or -0.20) (mu = 0)K' = 10 (super -4.25+ or -0.20) (mu = 1.0)pH > 7.3 CuS(s) + 3HS (super -) = CuS(HS) 3 (super 3-) K = 10 (super -3.67+ or -0.20) (mu = 0)K' = 10 (super -3.67+ or -0.20) (mu = 1.0)At 200 degrees C: pH < 6.6 CuS(s) + H 2 S(g) + 2HS (super -) = Cu(HS) 4 (super 2-) K = 10 (super -2.30+ or -0.3) (mu = 0)K' = 10 (super -3.60+ or -0.3) (mu = 1.0)pH > 6.6 CuS(s) + 3HS (super -) = CuS(HS) 3 (super 3-) K = 10 (super -2.13+ or -0.20) (mu = 0)K' = 10 (super -2.12+ or -0.20) (mu = 0)The boundary between the complexes Cu(HS) 3 (super -) and CuS(HS) 3 (super 3-) is dependent on the total sulfur concentration (Sigma S = HS (super -) + H 2 S) of the solution and ionic strength. The pH limits given here are for mu = 1.00. The pH boundary at 200 degrees C is relative to a neutral pH of 5.6. There is no evidence for the existence of CuS-H 2 S complexes in the very low pH region ( approximately 3), or CuS-S (super 2-) complexes in the high pH region (10-13.5). Solubilities calculated using the above data compare well with those observed in these experiments.If it is assumed that a solubility of at least 10 ppm Cu is necessary for significant ore transport, bisulfide complexes are potential ore carriers at 200 degrees C and a total sulfur concentration of 0.25 molal, or greater, at neutral to weakly alkaline pH's. However, complexes between Cu and either NH 3 or Cl (super -) in bisulfide solutions are not geologically important, at least at low temperatures.Covellite transported as bisulfide complexes would be deposited by any mechanism that lowers the stability of these complexes, such as a decrease in temperature or pH, oxidation, or dilution. The conditions of covellite deposition obtained in the laboratory experiments agree well with those deduced from natural occurrences, although additional solubility data are needed for the more common copper sulfides digenite and chalcocite.
