Simultaneous removal of calcium and sulfate ions from flotation water of complex sulfides

This paper presents and discusses the results of an experimental study designed to investigate the feasibility of removing calcium and sulfate from saturated solutions by using calcium aluminate-based compounds. The results obtained show that the treatment of saturated solutions (0.016 mol/L CaSO4) alkalinized at pH above 12 by adding 0.75 g/L CaO and treated with a calcium aluminate-based compound (C70: 70% Al2O3, 30% CaO), promotes the precipitation of calcium sulfate as ettringite, which is a hydrated calcium sulfoaluminate, in equilibrium with residual calcium and sulfate concentrations below 200 mg/L. The active species of compound C70 is the monocalcium aluminate (CaO·Al2O3), which is the responsible of promoting ettringite precipitation. The use of a large excess of monocalcium aluminate significantly increases the removal kinetics but has the disadvantage that the excess is eventually hydrolyzed releasing calcium and aluminate ions to the solution. It was also observed that an excessively long contact time (e.g., 50 h), results in the acidification of the suspension due to absorption of atmospheric CO2, causing the dissolution of the ettringite, which is stable at pH above 11.5. The best experimental conditions observed are the use of a slight excess of reactants (with respect to the stoichiometrically required): 0.75 g/L CaO and 2 g/L of C70 and a reaction time of 6 h.