Phase Transition And Crystallization Of Calcium Sulfite And Calcium Sulface In CaCl₂Solution

During the flue gas desulfurization (FGD) process, calcium based agent produces calcium sulfite (SH), which converts to calcium sulfate dihydrate (DH) after forced oxidation. The FGD byproduct mainly composed of SH and DH is termed as FGD gypsum. In the past15years, FGD gypsum has become a famous bulk industrial solid waste and the current discharge is about69,000,000ton. It demands urgent consumption as a useful resource in large quantities from the point of environmental protection and untilization. To make high value added use of FGD gypsum, this paper investigates the preparation of α-calcium sulfate hemihydrate (α-HH) from SH and DH and the corresponding phase transition and crystallization rules.In certain Ca-Mg-Mn chloride solution, SH oxidation and α-HH crystallization can be accomplished at the same time, the former of which turns to be the rate limiting step. Crystallization route of α-HH during the oxidation of SH depends heavily on the CaCl2concentration provided temperature and other factors are fixed. In2.50-3.50m CaCl2solutions, α-HH precipitated via intermediate phase DH, namely SHâ†'DHâ†'α-HH, and the existing time of DH shortened with the increase in CaCl2concentration. In a4.00m CaCl2solution, the transformation from SH to DH was not observed, namely, the existing time of DH was reduced to zero, presenting a direct SHâ†'α-HH transformation. Hence, it proposes a direct a-HH preparation method from SH in the salt medium under atmospheric pressure.To interpret the crystallization route evolution during SH-a-HH conversion in CaCl2solution, nucleation competition of calcium sulfate phases in homogenous solutions was investigated and the relative nucleation rate of a-HH (RHH) on the basis of classical nucleation theory (CNT) was simulated. Dominant calcium sulfate initially precipitated presentsâ‘ unstable DHâ†'metastable α-HHâ†'unstable DH andâ‘¡metastable DHâ†'unstable α-HHâ†'metastable DH evolution orders depending upon supersaturation in a-HH and DH metastable zones, respectively. The comprehensive effect of molecular volume (v0), interfacial energy (y) and supersaturation (S) of DH and a-HH leads to their competitive nucleation and hence selective crystallization. Concomitant nucleation occurs when the nucleation rates are comparable. The formation of DH at lower supersaturations in α-HH metastable zone and the formation of α-HH in DH metastable zone are attributed to their repsective heterogeneous nucleation. The occurrence of thermodynamically less stable phase at higher supersaturations conforms to the Ostwald’s rule of stages. The mole fraction of α-HH in the initial precipitate increases with the CaCl2concentration and temperature increasing. This is due to the larger supersaturation ratio of α-HH to DH (SHH/SSDH), which makes α-HH nucleation more competitive at higher CaCl2molality and temperature.The transformation accelerates with the temperature and CaCl2molality increment and DH particle size reduction. The transformation is a nucleation-growth limited process, which well fits to the dispersive kinetic model. Increment in temperature and CaCl2molality enlarges the solubility product ratio of DH to α-HH (KsP,DH/Ksp,HH) and lowers down the water activity, respectively, which both enlarges the supersaturation and activationentropy change, expediting the α-HH nucleation and growth. Reduction in DH particle size increases the specific surface area and the lattice deformity number, which lowers down the activation enthalpy and enlarges the activation entropy, and boosts α-HH surface nucleation, but has no evident effect on the growth rate.The study demonstrates SH can be transformed into α-HH in CaCl2solution, and the nucleation competition between DH and α-HH accounts for the transition route variation. Nucleation and growth of α-HH is the key step affecting the DH transformation to α-HH. The results provide theoretical guidance, processing method and important factors for the high value added utilization of FGD gypsum. Also, it deepens the understanding of calcium sulfate multi-phase crystallization and offers a method to investigate the phase-transition thermodynamics and kinetics control of inorganic minerals in aqueous solutions.