Oxide complexes in alkali-alkaline-earth chloride melts

Abstract

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(l) +nMO(s)=Mm+nOnCl2m(l) In NaCl-MCl2 containing 25 mol % MCl2 the MO solubilities at 850 °C are xMgO= 6×l0-5 xCa0=10-3-10-4 and xSrO=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- = Mm+nOnCl2m 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(l) to from BaCl2(l) and MgO(s).