Chen Wang,Feng Wu,Yanfeng Sui,Guoxiong Wang,Tielin Wang,Jiayu Ma
The solubility of K2SiF6 in MgCl2 and FeCl3 solutions within a temperature range between 20 and 70 °C was experimentally determined by the static method under atmospheric pressure. The equilibrium solids obtained under various conditions were analyzed by XRD, SEM, and FTIR to confirm the stability of corresponding solid phases. The solubility of K2SiF6 in MgCl2 and FeCl3 solutions increased with increasing temperature and concentration, and K2SiF6 remained stable in MgCl2 and FeCl3 solutions. Moreover, the experimental data were successfully correlated with the Apelblat empirical equation. The thermodynamic properties of the solution process, including the Gibbs energy, enthalpy, and entropy, were calculated by the van’t Hoff analysis and the Gibbs equation. The dissolution process of K2SiF6 in the measured solutions was found to be endothermic and nonspontaneous. The findings above can provide fundamental data and theoretical support for optimizing the crystallization of K2SiF6 in acid leaching solution from phosphorus–potassium associated ore and preventing scale formation of K2SiF6 in wet-process phosphoric acid (WPA) vacuum filtration.
An industrially feasible process was proposed to produce α-calcium sulfate hemihydrate (α-HH) from FGD gypsum without any additives. In this process, an AlCl3 solution was used to promote the conversion of calcium sulfate dihydrate (DH) to hexagonal prismatic α-HH with high strength at atmospheric pressure. The phase transition kinetics of FGD gypsum was experimentally investigated under conditions: AlCl3 (2–2.5 m), MgCl2 (0–0.4 m) existing in raw FGD gypsum, and the solid–liquid ratio (or solid content %) range of 0.1–2 at 363 K. It was found that increasing the concentrations of MgCl2 in AlCl3 solution accelerated the conversion. The uniform large hexagonal prism HH product was obtained at the folowing optimal conditions: 2.5 mol/kg AlCl3, 0.4 mol/kg MgCl2, solid–liquid ratio of 2, and equilibration time of 2 h at 363 K. The phase diagram of the AlCl3-MgCl2-H2O system was determined by measuring solubility of solids. The E-NRTL activity model was used to construct the phase transition diagram of CaSO4 in AlCl3–MgCl2 media and to judge the stable range of DH and HH.
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