Effect of Organic Additives on the Solubility Behavior and Morphology of Calcium Sulfate Dihydrate (Gypsum) in the Aqueous Sodium Chloride System and Physicochemical Solution Properties at 35 °C

Abstract

The effect of the addition of organic solvents, such as ethyl acetate (EA), glycerol (GLY), isopropyl alcohol (IPA), dimethylformamide (DMF), oxalic acid (OA), and propylamine (PA), on the solubility behavior of calcium sulfate dihydrate (gypsum, CaSO4·2H2O) in aqueous NaCl solutions has been examined at 35 °C. Compared to organic solvent free system, the addition of EA slightly increased CaSO4·2H2O solubility, whereas only a little alteration in solubility was found with the addition of GLY. CaSO4·2H2O solubility decreased significantly and to nearly same extent with the addition of IPA or DMF. In the solutions containing PA or OA, CaSO4·2H2O dissolution was very limited probably due to precipitation of highly stable Ca salts of these organic solvents. Except OA, where no solubility maximum was observed, the addition of organic solvents did not shift the solubility maximum of CaSO4·2H2O as a function of NaCl concentration in solution, and the solubility pattern remained similar. Accurate data on density (ρ) and speed of sound (u) data have been recorded for the quaternary systems (CaSO4·2H2O + NaCl + H2O + organic solvent) at 35 °C. Measurements of u and ρ have been used to determine the solution isentropic compressibility (κS). Empirical equations describing the solubility, density, speed of sound, and isentropic compressibility in these systems are presented. The morphology of CaSO4·2H2O recrystallized from different aqueous–organic solvent solutions and aqueous-NaCl-organic solvent solutions has also been examined from scanning electron microscopy (SEM).