Solubility of MgO in mixed chloride-fluoride melts containing MgCl2

The effect on MgO solubility of fluoride additions to MgCl2-containing melts has been studied. An almost linear increase in MgO solubility as function of MgF2 mole fraction was observed in MgCl2-MgF2 melts at 840 degrees C. In pure MgCl2 the MgO saturation mole fraction determined be x(sat)(MgO) = 0.0061+/-0.0002 at 840 degrees C in the MgCl2-MgF2 (x(MgF2)=0.40) mixture to X(MgO)(sat)=0.0109+/-0.0002 at 840 degrees C. The effect of temperature on the MgO solubility was determined for pure MgCl2 and for two MgCl2-MgF2 mixtures (x(MgF2)=0.1816 and x(MgF2)=0.2016): d(1n x(MgO)(sat))/d(1/T) was -5200 K for MgCl2 and -4200 K for the two mixtures, respectively. Three different oxide solubility experiments were performed for the MgCl2-NaCl-NaF ternary: 1. NaF and MgCl2 were added to a NaCl-MgCl2 mixture saturated with MgO at 850 degrees C such that x(MgCl2)=0.63. The MgO solubility increased with NaF content up to x(NaF)approximate to 0.08, whereafter it was constant to x(NaF)=0.20. 2. NaF was added to a NaCl-MgCl2 mixture saturated with MgO at 850 degrees C. x(MgCl2)(degrees)=0.63 was the initial MgCl2 concentration. An increase in MgO solubility up to about 8 mol% NaF was observed. On further NaF additions to x(NaF)=0.18 the MgO solubility decreased. 3. A melt closer in composition to the electrolyte used for electrowinning of magnesium was investigated. The MgCl2 mole fraction was kept constant at x(MgCl2)=0.10. The temperature was 900 degrees C. No effect of NaF content on the MgO solubility could be observed for x(NaF)less than or equal to 0.06.