Control of alkali-metal oxide activity in molten silicates

A new thermochemical reactor is designed to control alkali-metal oxide activity simultaneously in several molten silicates at temperature up to 1400 °C. The method consists in imposing an alkali metal vapor pressure in a closed system by Na(g) evaporation from Na2O–xSiO2 melt and equilibrating this vapor with the molten silicate samples. By comparison of experiments carried out in regular furnaces, the drastic reduction of the working volume ensures a better control of sodium metal vapor pressure for durations of the order of a hundred of hours. This device has been applied to measuring the sodium solubility and sodium-metal oxide activity at 1350 °C in the anorthite–diopside eutectic melt composition of the CaO–MgO–Al2O3–SiO2 system.