Experimental Measurement of the Solid–Liquid Equilibrium of the Systems MF₂ + H₂O (M = Mg, Ca, Zn) from 298.15 to 353.15 K

Abstract

The solubility of MgF 2 , CaF 2 , and ZnF 2 in aqueous solution is seldom studied. Their solubility data available in the literature conflict with each other, and their change tendency with temperature is hard to draw. In this work, we elaborately measured their solubility over the temperature range from 298.15 to 353.15 K. To guarantee the data reliability, various methods including ethylenediamine tetraacetic acid disodium salt dihydrate (EDTA) titration and fluoride ion selective electrode (FISE) have been used for ion content analysis in the saturated solutions and X-ray diffraction (XRD) for equilibrium solid phase identification. Besides, sufficient solid−liquid contact time has been taken to guarantee the equilibrium. The determined results were compared with those reported in the literature, and their differences have been discussed. It was found that the MgF 2 solubility decreases; however, the CaF 2 solubility increases with increasing temperature. In the ZnF 2 + H 2 O system, the equilibrium solid phase is ZnF 2 •4H 2 O (s) at temperatures below 333 K and ZnOHF (s) above 333 K. While the ZnF 2 •4H 2 O (s) solubility is sensitive to temperature, the ZnOHF (s) solubility is insensitive.

Solubility Phase Equilibrium of the Quaternary Reciprocal System Ca2+, Mn2+//F–, SO42– + H2O at 298.15 K

“…Also presented in Figure is the MnSO 4 ·H 2 O (s) solubility in pure water for comparison. In pure water, the CaF 2(s) solubility 0.000191 mol·kg –1 determined in this work agrees reasonably with the literature data of 0.00022 mol·kg –1 within the reported uncertainty. The solubility of CaF 2(s) increases monotonically with an increasing MnSO 4 concentration and reaches 0.00525 mol·kg –1 in the MnSO 4 -saturated solution, corresponding to an increase factor of 27 relative to its solubility in pure water.…”

“…This requirement necessitates the removal of fluoride and calcium ions from MnSO 4 aqueous solution. It is well-known, that the solubility of CaF 2(s) in pure water is quite low with a value of approximately 0.22 × 10 −3 mol•kg −1 at 298.15 K. 1 If the solubility of CaF 2(s) in MnSO 4 aqueous solution was the same as in pure water, calcium and fluoride ions could be easily removed simultaneously through CaF 2(s) precipitation. Unfortunately, the CaF 2(s) solubility in MnSO 4 aqueous solution has not been reported to date.…”

Solubility Measurement of the Systems MF2 (M = Ca, Mg, Zn) + ZnSO4 + H2O and MF2 (M = Ca, Mg) + ZnF2 + H2O at 298.15 K

“…In fact, the solubility of CaF 2(s) and MgF 2(s) in pure water is quite low. 7 In the case that their solubility behavior stays as low in the ZnSO 4 aqueous solution as in pure water, it is desirable to precipitate these impurities from the ZnSO 4 aqueous solution. However, the awareness of the solubility of these sparing salts is so scarce that the influence of temperature on the CaF 2(s) and MgF 2(s) in pure water was not determined until recently.…”

“…However, the awareness of the solubility of these sparing salts is so scarce that the influence of temperature on the CaF 2(s) and MgF 2(s) in pure water was not determined until recently. 7 The latter is the prerequisite and basis to develop a precipitation approach to remove these impurities. In our previous works, the CaF 2(s) solubility in CaSO 4 aqueous solution and MgF 2(s) solubility in MgSO 4 aqueous solution have been experimentally determined.…”

“…To remove these impurities from electrolyte solution, researchers have developed a lot of approaches, among which precipitating the impurities as hardly or slightly soluble salts is a prospective for its process simplicity and low cost. In fact, the solubility of CaF 2(s) and MgF 2(s) in pure water is quite low . In the case that their solubility behavior stays as low in the ZnSO 4 aqueous solution as in pure water, it is desirable to precipitate these impurities from the ZnSO 4 aqueous solution.…”