OXIDE COMPLEXES IN ALKALI ALKALINE-EARTH CHLORIDE MELTS

The solubilities of alkaline-earth oxides in alkali-alkaline-earth chloride, NaCl-MCl2, melts are presented. The oxide solubility increases markedly with the MCl2 concentration and with increasing size of the alkaline-earth ion. The enhanced solubility of the oxide is explained through the reaction mMCl2(1) + nMO(s) = M(m+n)O(n)Cl2m(1) In NaCl-MCl2 containing 25 mol % MCl2 the MO solubilities at 850-degrees-C are x(MgO) = 6x10(-5), x(CaO) = 10(-3)-10(-4) and x(SrO) = 0.039, respectively. In the BaCl2 system the BaO/BaCl2 molar ratio at saturation is 1. The oxochlorocomplexes formed in these melts are further justified through cryoscopic measurements showing an increasing melting point of a binary NaCl-MCl2 melt when oxide is added. Model calculations in combination with the two sets of experimental data indicate that the M/O ratios for the different complexes are Mg2O, Ca3O, Ca4O, Sr3O, Sr4O, Ba3O and Ba4O2. The formation constants for neutral complexes based on the reactions (m + n)M2+ + nO2- + 2mCl- = M(m+n)O(n)Cl2m are very large. In technical melts for Mg electrolysis the MgO solubility is very small (in the ppm region). Even if BaO dissolves readily in NaCl-BaCl2 melts the solubility of BaO is very small in MgCl2-containing melts, since BaO(s) reacts with MgCl2(1) to form BaCl2(1) and MgO(s).