Thermodynamic And Kinetic Studies On The Phase Transformation Between CaSO₄·2H₂O And CaSO₄in MnSO₄Aqueous Solution

Abstract:It has been found that the thermodynamically metastable phase CaSO4-2H2O(DH) is formed preferably, which will harm the production process by scaling on the pipe wall and the surface of the filtration cloth. To convert the metastable CaSO4-2H2O to stable CaSO4(AH) thermodynamic and kinetic conversion mechanisms have been investigated in this paper and following conclusions have been drawn:1) A Pitzer Model was used to simulate the DH and AH solubility as function of temperature and MnSO4concentration. The obtained results demonstrated that AH is the stable phase at95℃and in any concentration of MnSO4aqueous.2) The influence of MnSO4concentration and the initial amount of solid phase on the transformation rate between AH and DH was researched at95℃. It was found that:(1) When the initial amounts of the solid phase DH and AH are20.0g·kg-1and2.0g·kg-1, respectively, and the MnSO4concentration is0.1mol·kg-1, it takes28days for DH to transform completely to AH in solid phase, and more than40days are needed for Ca2+in the aqueous solution to reach equilibrium with AH. What more, induction period didn’t appear in the transformation of solid phase. In2.4mol·kg-1MnSO4solution and the solid phase DH and AH are20.0g·kg-1and2.0g·kg-1, respectively, DH is completely converted to AH in18hours, Ca2+concentration reached equilibrium with DH in more than50hours.(2) When the initial amounts of the solid phase DH and AH are both10.0g·kg-1and the MnSO4concentration is0.1mol·kg-1, it takes4days for DH transforming completely to AH in solid phase and more than6days are needed for Ca2+to achieve equilibrium with AH. What more, in2.0mol·kg-1MnSO4solution DH is completely converted to AH in5hours, Ca2+concentration reached equilibrium with DH in more than16hours.Both theoretical calculation and experiments indicated that in the ternary system of CaSO4-MnSO4-H2O at95℃DH will transform to AH, and the transformation time is reduced with the increasing concentration of MnSO4aqueous, and when the added amount of CaSO4is more than2.0g·kg-1and the MnSO4concentration exceeds0.1mol·kg-1, induction period doesn’t exist in the transformation of solid phase. And also the results show the equilibration time between Ca2+and CaSO4in liquid is much longer than that of DH transforming to AH, which illustrates that the changing of the Ca2+concentration is controlled by the crystallization rate of CaSO4and the rate of DH transforming to AH is mainly controlled by the steps of calcium crystallizing.