Miscibility Gaps in Fused Salts. Note X. The Reciprocal Ternary System K, Li/Br, F

Abstract

Solid-liquid and liquid-liquid equilibria in the reciprocal ternary system K, Li/Br, F were fully measured.The projection of the miscibility gap occupies 20.8% of the composition square; the upper critical point of the gap is along the stable diagonal at 953 °C and £LiF = 0.70.Attempts to predict the liquid-liquid equilibria by means of the conformal ionic solution theory using temperature independent interaction parameters lead to an incorrect symmetry of the gap. The symmetry can be improved if the temperature dependence of the interaction parameters of the binary mixtures is taken into account.Detailed measurements of the liquid-liquid (LL) equilibria in reciprocal ternary systems are highly desirable both to complete basic data and to be used as particularly sensitive tests of current theories for ionic systems.The solid-liquid (SL) equilibria of the reciprocal ternary system K, Li/Br, F were reported by Volkov and Dubinskava [1] in 1953, whereas the LL equilibria were studied only along the stable diagonal (LiF + KBr) and reported in Note VII of this series [2].The present paper gives a complete experimental analysis of the SL and LL equilibria taking into account the full range of compositions and temperatures. Moreover, the results of the conformal ionic solution (CIS) theory [3] when applied to this system along with a discussion of the applicability limits of the theory are reported.The apparatus, the experimental techniques and the characteristics of the employed salts were described in previous papers [4], Figure 1 shows the SL equilibria, along with the coordinates of the eutectics, of the four binaries. Figure 2 reports the SL and LL equilibria along the unstable diagonal (KF + LiBr) and the stable one (LiF + KBr): the area in which demixing occurs is shaded. As regards the unstable diagonal, the primary crystallization temperature (PCT) varies from 801 °C (x LiB r = 0.41) to 798 °C (x Li Br = 0.59) going through a maximum at 822 °C (^LiBr = 0.50); the point of maximum of the miscibility gap is at 930 °C (#LiBr = 0.50). ResultsThe stable diagonal was fully described inThe general topology of the system K, Li/Br, F is shown in Figure 3. Figure 3a reports the projection of the 12 studied cuts along with various significant temperatures. The stratification lens (dashed) impinges on the LiF crystallization field and occupies 20.8% of the square. The main axis of the lens coincides with the principal diagonal, and along this axis the PCT is constant at 822 °C. The system presents two ternary eutectics at 320 °C OzLiF = 0.03 5 ; *LiBr = 0.54 5 ; ^KBr = 0.42) and at 465 °C (£LiF = 0.07s; ^KBr = 0.60s; £kf = 0.32), respectively.The comparison of the present results with the ones in Ref. [1] shows only a qualitative topological agreement, the numerical data being in some cases quite different. The invariant point E-i. for instance, is reported in Ref.[1] as a peritectic point at 558 °C (x LiF = 0.05; x KB r = 0.545; X K F = 0.395). DiscussionAmong the recent applications of the CIS theory the a priori predic...