Abstract: The activity of CaO and La_2O_3 in the liquid ternary slay CaO-SiO_2-La_2O_3 has been de- termined by equilibrating Sn with the slag phase in a graphite crucible under l arm of CO at 1600C.The experimental results ,showed that the activity of CaO and La_2O_3 changes slowly while the ratio of mole fraction.x_(CaO)/x_(SiO_2),is less than unit.hut varies sharply while the ra- rio increases from 1 to 1.2.In addition,by equilibrating Sn with the binary slag B_2O_3-La_2O_3 saturated witlt solid La_2O_3 at three different temperatures,the relation between Igγ_(La)~0 and I/T is determined as follows: Igγ_(La)~0=-20400/T+11.37(1500-1600C) A random network model of molten silicate structure was applied to CaO-SiO_2-La2O_3. It was found that the experimentally determined activity-composition curve of CaO are qualitatively reproduced by the curves calculated with the model,but the curves of La_2O_3 are considerahly different from the calculated ones.It implies that the model used is over-simpli- fied and that the difference in interaction energy between the metal cations of different valence and silicate anions should be considered.

Key words: CaO-SiO_2-La_2O_3 ternary system slag, slag structure, activity