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

Abstract

Solubility of sparingly soluble salts, i.e., CaF2(s), MgF2(s), and ZnF2(s), in ZnSO4 aqueous solution is necessary for the process design to remove Ca2+, Mg2+, and F– ions from zinc hydrometallurgical systems. However, these data have been unavailable in any literature up to now. In this work, the solubilities of MF2(s) (M = Ca, Mg, Zn) in ZnSO4 and MF2(s) (M = Ca, Mg) in ZnF2 aqueous solutions have been elaborately measured at 298.15 K. The results showed that the CaF2(s) and MgF2(s) solubilities increase with the increasing ZnSO4 concentration; for example, their solubility in 2 mol·kg–1 ZnSO4 aqueous solution increases to about 6 times of that in pure water. The ZnSO4 has a slightly stronger salt-in effect on CaF2(s) than on MgF2(s). However, the ZnF2(s) solubility in ZnSO4 aqueous solution decreases at first and then increases with increasing ZnSO4 concentration. As expected, ZnF2 possesses strong salting-out effect on the solubility of CaF2(s) and MgF2(s). A remarkable character is that the CaF2(s) solubility goes through a minimum point and then increases again with the ZnF2 addition; inversely, the MgF2(s) solubility decreases with the ZnF2 addition with time. Their different solubility characters have been interpreted from the aspect of ion association interaction. An ion association model has been applied to predict the CaF2(s) and MgF2(s) solubility in ZnF2 aqueous solution. The predicted results agree with the experimental ones quite well, which indicates that the ion association dominates the excess properties of these concerned systems.

LaF3 Solubility in Pure Water and in MSO4 (M = Zn, Mn, Mg) Aqueous Solutions at T = 298.15 and 348.15 K

“…All of the factors result in the decrease of the F – ion activity and consequently the increase of the LaF 3(s) solubility. Similar phenomena have been observed for CaF 2 and MgF 2 in our previous work, − as shown in Figure .…”

“…After equilibrium was reached, the magnetic stirring was stopped and the solid–liquid mixture was kept static for 3 days. The sampling method at 298.15 K was the same as described in our previous work . The sampling process at a high temperature is depicted in Figure .…”

“…The sampling method at 298.15 K was the same as described in our previous work. 8 The sampling process at a high temperature is depicted in Figure 1. Concretely, first, the sample bottle was opened, and then, the bottle mouth was fixed by a plastic wrap with a rubber band.…”

“…Among all defluorination methods, precipitating F – ions as sparingly soluble salts is the most simple, effective, and economical method. Theoretically, CaF 2 and/or MgF 2 solubility is low enough to remove fluoride ions; however, their solubilities in the ZnSO 4 electrolyte solution are much higher than those in pure water because of the strong salting-in effect. − This makes the requirement (F – content: 2 × 10 –3 mol·kg –1 ) hard to achieve by precipitating CaF 2(s) or MgF 2(s) solely.…”

CaF2(s) Solubility Isotherm in the CaF2–CaSO4–MSO4–H2O (M = Zn, Mn, Mg) Quaternary Systems and Their Subsystems at Various Temperatures

“…2. CaF 2 (s) solubility in the ZnSO 4 aqueous solution at 348.15 K increases with increasing ZnSO 4 concentration, which is generally higher than that at 298.15 K. Specifically, at 348.15 K, CaF 2 (s) solubility increased from 4.79 × 10 −4 mol•kg −1 in pure water to 6.21 × 10 −3 mol•kg −1 in a 3 mol•kg −1 ZnSO 4 aqueous solution, which is 13 times of that in pure water and 3.1 times of that at the same ZnSO 4 concentration at 298.15 K. 6 These results suggest that removing fluoride as CaF 2 (s) precipitates becomes more difficult at higher temperatures compared to removing them at lower temperatures.…”