Effect of solution chemistry on particle characteristics during metal sulfide precipitation.

Metal sulfide precipitation forms an important component of acid mine drainage remediation systems based on bacterial sulfate reduction. The precipitation reaction is thermodynamically favorable, but a number of technical issues remain. In this study the effect of metal to sulfide molar ratio and operating pH on the nature and settling characteristics of copper and zinc sulfide precipitates was studied in a CSTR. A large number of small copper sulfide particles, with highly negatively charged surfaces and poor settling characteristics, were formed in the presence of a stoichiometric excess of sulfide at pH 6. The size and the settling characteristics of the particles were significantly improved, while the number of particles and magnitude of their zeta potential decreased when experiments were conducted at pH values <6. By comparison, for zinc sulfide, a small change in the number and size of the particles was observed for all metal to sulfide molar ratios and tested operating pH values. Precipitates generated at pH 6 had the most negative zeta potential, while operating at pH values <6 reduced the magnitude of the negative surface charge and improved the settling and dewatering characteristics of the precipitate. The data indicated that the amount of reactive sulfide species (HS(-) and S(2-) ions) available in solution during the precipitation process was important in determining the nature and surface characteristics of the particles produced and this was mainly dependent on pH.