Effect of flocculating agents on solubility behavior of calcium sulfate dihydrate (gypsum) in aqueous sodium chloride system and solution properties at 35 °C

Abstract

The effect of addition of flocculating agents, viz., alum (KAl(SO₄)₂·12H₂O) (up to 10 wt%) and polyaluminum chloride ([Al(OH)_aCl_b]_n, a >1.05; n~15) (up to 6 wt%) on the solubility behavior of calcium sulfate dihydrate (gypsum, CaSO₄·2H₂O) in aqueous NaCl solutions has been examined at 35 °C. The solubility of CaSO₄·2H₂O decreases with the addition of alum while it increases with the addition of polyaluminum chloride without any significant shift in solubility maximum. Density (ρ) and speed of sound (u) have also been determined for the quaternary systems (CaSO₄·2H₂O+NaCl+alum/PAC+H₂O) at 35 °C and used to estimate solution isentropic compressibility (κ_s) and ion-hydration (n_h) characteristics. Empirical equations describing the s, ρ, u and κ_s as a function of solution composition are presented.