Gypsum solubility in seawater, and its application to bauxite residue amelioration
P. M. Kopittke A C , N. W. Menzies A and I. M. Fulton BA Centre for Mined Land Rehabilitation, University of Queensland, St Lucia, Qld 4072, Australia.
B EHS Department, Alcan Gove Pty Ltd, PO Box 21, Nhulunbuy, NT 0881, Australia.
C Corresponding author. Email: p.kopittke@uq.edu.au
Australian Journal of Soil Research 42(8) 953-960 https://doi.org/10.1071/SR04034
Submitted: 12 March 2004 Accepted: 23 July 2004 Published: 14 December 2004
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.
Additional keywords: bauxite residue sand, CaCO3 coating, gypsum dissolution rate, seawater, solubility, surface area.
Acknowledgments
The authors acknowledge the assistance of Graham Kerven and David Appleton with chemical analyses and Bernhard Wehr for his suggestions, ideas, and comments. The statistical assistance provided by Rosemary Kopittke is also gratefully acknowledged as is the assistance of Ron Rasch and Kim Sewell (Centre for Microscopy and Microanalysis, The University of Queensland) with the use of the SEM. The support of Ian Fulton is also gratefully received as is his assistance in placing the research in the context of the alumina industry.
This work was conducted as part of an environmental research program funded by Alcan Gove Pty Ltd.
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