Crystallization kinetics of large-sized columnar α-hemihydrate gypsum by reaction of waste CaCl2 and Al2(SO4)3 without crystal modifiers

Abstract

Direct production of high-strength α-calcium sulfate hemihydrate (α-HH) with no additives was accomplished by the reaction of aluminum sulfate and waste CaCl2 discarded from Solvay soda plant. For scaling-up this new process, the crystallization kinetics of α-HH was experimentally investigated under varying conditions including temperature (323–363 K), concentration of CaCl2 (4–7 mol/kg), Al2(SO4)3 (0.6–3.0 mol/kg), and seed loading. The optimal operating conditions are 7 mol/kg CaCl2 solution, 0.8 mol/kg Al2(SO4)3 solution, 5 wt% seed loading, 2 mol/kg AlCl3 existing in the reactor, T = 353 K, and equilibrium time of 3 h. The α-HH with large columnar shape, as desired in industry, was successfully obtained at optimal conditions by use of a continuous stirred tank reactor (CSTR) without the addition of any additives. The stability test of various CaSO4 phases in aqueous AlCl3, CaCl2, and their solution mixtures elucidated that Al3+ ion and total concentration of Cl− ion play an important role in the growth and shape of α-HH crystal.