Gypsum solubility in seawater, and its application to bauxite residue amelioration

Abstract

The solubilities and dissolution rates of 3 gypsum sources [analytical grade reagent (AR), phosphogypsum (PG), mined gypsum (MG)] with 6 MG size fractions (>2.0, 1.0–2.0, 0.5–1.0, 0.25–0.5, 0.125–0.25, <0.125 mm) were investigated in triple-deionised water (TDI) and seawater to examine their suitability for bauxite residue amelioration. Gypsum solubility was greater in seawater (3.8 g/L) than TDI (2.9 g/L) due to the ionic strength effect, with dissolution in both TDI and seawater following first-order kinetics. Dissolution rate constants varied with gypsum source (AR > PG > MG) due to reactivity and surface area differences, with 1 : 20 gypsum : solution suspensions reaching saturation within 15 s (AR) to 30 min (MG >2.0 mm). The ability of bauxite residue to adsorb Ca from solution was also examined. The quantity of the total solution Ca adsorbed was found to be small (5%). These low rates of solution Ca adsorption, combined with the comparatively rapid dissolution rates, preclude the application of gypsum to the residue sand/seawater slurry as a method for residue amelioration. Instead, direct field application to the residue would ensure more efficient gypsum use. In addition, the formation of a sparingly soluble CaCO3 coating around the gypsum particles after mixing in a highly alkaline seawater/supernatant liquor solution greatly reduced the rate of gypsum dissolution.