We have investigated the solubility behavior of calcium sulfate dihydrate (gypsum, CaSO 4 ·2H 2 O) in aqueous NaCl solutions upon the addition of ammonium- or imidazolium-based ionic liquids (ILs), viz., ethylammonium lactate (EAL), 1-ethyl-3-methyl imidazolium hydrogen sulfate ([C 2 mim]HSO 4 ), and 1-butyl-3-methyl imidazolium hydrogen sulfate ([C 4 mim]HSO 4 ) at 30 °C. The addition of ammonium lactate ILs increased solubility significantly, ∼500%, whereas the addition of imidazolium hydrogen sulfate ILs decreased the solubility by ∼60%. The addition of ILs shifted the solubility maximum of CaSO 4 ·2H 2 O toward either higher or lower concentration of NaCl in solution depending on the nature of IL additive. Solution properties such as density (ρ) and speed of sound ( u ) have been measured for the quaternary systems (CaSO 4 ·2H 2 O + NaCl + ILs + H 2 O) at 30 °C. Solution isentropic compressibility (κ S ) has been derived from measurements of u and ρ. Physical and derived properties have been fitted with suitable polynomial equations. Crystal growth formation and surface morphology of CaSO 4 ·2H 2 O crystals recrystallized from different aqueous IL solutions have also been examined. IL additives have shown good potential for calcium sulfate scale removal and as calcium sulfate precipitation agent.
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Shukla et al. (2018) studied this question.
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