The precipitation of calcium sulphate dihydrate (CaSO4·2H2O, gypsum) was studied at 25 °C at constant calcium ion activity. The stability domain of the system and the kinetics of spontaneous precipitation of gypsum from unstable supersaturated solutions was investigated. The rates of precipitation showed parabolic dependence on the relative supersaturation, in agreement with literature reports for lower supersaturations and seeded growth. It was therefore suggested that the test phosphonates affected mainly the high apparent order nucleation process, allowing the crystal growth process of the supercritical nuclei. Moreover, the kinetics of precipitation of gypsum at sustained calcium ion activity at 25°C, pH 6.5 were measured in the presence of two phosphonates: benzo (1,3,5-triyltrisphosphonic acid (BTP) and 1,3,5-triazine triphosphonate (TATPA), over a range of concentrations between 4-16 ppm. The presence of TATPA resulted in significant reduction of the rate of gypsum precipitation, which reached total suppression of nucleation and growth at the highest test concentration. The presence of BTP, exhibited calcium sulfate dihydrate inhibition, which however was lower. The differences in inhibition efficiency were attributed to molecular characteristics, including flexibility and hydrogen bonding capacity. Both inhibitors acted as nucleation and as crystal growth inhibitors

Subject
Ions, Precipitation (solids), Phosphonates, Sulfates, Solids, Gypsum, Calcium, Supersaturation, Constants, Solutions, Crystal growth, Calcium sulfate, Corrosion inhibitors
Keywords:
calcium sulphate dihydrate, gypsum, precipitation, kinetics of, phosphonates inhibition
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2019
Association for Materials Protection and Performance (AMPP)
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