Effect of Na 2 O additions on thermal conductivities of CaO–SiO 2 slags

Abstract

Thermal conductivities of Na₂O–CaO–SiO₂ slags have been determined using the non-stationary hot wire method over a wide temperature range between room temperature and 1773 K, to propose a prediction equation for the thermal conductivities of this slag system. Samples used were xNa₂O–yCaO–zSiO₂ (x+y+z=100 mol-%), where 5<x<40 mol-% and 10<y<40 mol-%. With increasing temperature up to glass transition temperatures, the thermal conductivities of the samples increased and then decreased even beyond liquidus temperatures. The thermal conductivities of slags having the same SiO₂ concentration, i.e. the same NBO/T (number of bridging oxygens per tetrahedrally coordinated atom), decreased by replacement of CaO with Na₂O but took almost the same value at their liquidus temperatures T _L, irrespective of the (%Na₂O)/(%CaO) ratio. Furthermore, plotting the thermal conductivities against T/T _L (temperature normalised by T _L) shows that the thermal conductivities in the temperature range T/T _L>0·9 are not dependent upon the (%Na₂O)/(%CaO) ratio as long as the SiO₂ concentration is kept constant. This finding indicates that in this system the relationship between thermal conductivities at T _L and NBO/T also applies at temperatures other than T _L using the temperature scale of T/T _L. On the basis of this relationship, the following prediction equation has been proposed for thermal conductivities λ of this slag system in liquid and supercooled liquid states: λ=a[1/(T/T _L)]+b, where a and b are constants specified by the concentration of SiO₂. This equation applies, for example, in the temperature range 0·9<T/T _L<1·3 for slags containing 70 mol-%SiO₂ and 0·9<T/T _L<1·1 for slags containing 40 mol-%SiO₂.