Effect of hydroxyl alkyl amines on the solubility behavior of calcium sulphate dihydrate (gypsum) in the aqueous sodium chloride system at 35°C

Abstract

The solubility behavior of calcium sulphate dihydrate (gypsum, CaSO₄&middot;2H₂O) in aqueous NaCl solutions upon addition of hydroxyl alkyl amines, such as, ethanolamine (EA), diethanolamine (DEA), or triethanolamine (TEA) at 35 &deg;C is reported.&nbsp; The addition of hydroxyl alkyl amines alters the solubility of CaSO₄&middot;2H₂O in NaCl solutions significantly. At any equal concentration of hydroxyl alkyl amines in the solution, the order of CaSO₄&middot;2H₂O solubility enhancement is found to be: EA &lt; DEA &lt; TEA. A three-fold increase in CaSO₄&middot;2H₂O solubility maximum is observed in the solutions containing 15 wt% TEA. The addition of hydroxyl alkyl amines shifts the solubility maximum of CaSO₄&middot;2H₂O towards higher concentration of NaCl in solution. Solution properties such as density (<em>&rho;</em>) and speed of sound (<em>u</em>) data have been measured for the quaternary systems (CaSO₄&middot;2H₂O+NaCl+hydroxyl alkyl amines+H₂O). All the measurements have been carried out at 35 &deg;C under atmospheric pressure. Solution isentropic compressibility (<em>&kappa;</em>_S) values have been derived from <em>u</em> and <em>&rho;</em>&nbsp; data. Solubility, density, speed of sound, and isentropic compressibility data have been fitted with suitable polynomial equations in these systems.