The stable cross section of LiF - NaF - KCl quadruple mutual system Li, Na, K // F, Cl was studied with the differential thermal (DTA) and X-ray fluorescence (XRF) methods. It was established that in a system the eutectic composition crystallizing at 591 ° C is realized. Temperatures of starting solid-phase reactions were revealed in the ternary systems mutual Na, K // F, Cl and Li, Na // F, Cl, (715 and 650 °C, respectively) corresponding to the conversion of reactants of metastable diagonals into products of stable diagonals.
The liquidus surface of the system (LiF)2-Li2SO4-Li2CO3 was studied by the calculated and differential thermal method of physicochemical analysis. An analysis of the systems of lower dimensionality of the facets of the investigated object showed that the most informative, for the experimental study, is the sections located in the crystallization field of lithium fluoride. A study of the DTA of a number of compositions located at the initially chosen polythermal section in the lithium fluoride crystallization field, the ratios of lithium sulfate and carbonate in the eutectic are determined. The composition of the triple eutectic was revealed by studying a non-invariant cut from the vertex of the triangle (LiF), through a point showing a constant ratio of sulfate and lithium carbonate in the eutectic, to the fusion of the thermal effects of the primary and tertiary crystallizations. The complexity of the study was that lithium carbonate is the most fusible component in the system, and according to the literature, after the melting of lithium carbonate, decomposition begins, which greatly complicates the interpretation of research results. In order to avoid the decomposition of lithium carbonate, each experimentally studied composition was heated to the melting temperature of lithium carbonate and kept in isothermal mode, at a temperature below its melting. Thus, the theoretical melting calculations and the region of location of the non-invariant composition have been extrapolated, allowing to limit the number of experimentally studied samples, and the subsequent experimental investigation of DTA of two polythermal sections revealed a eutectic composition crystallizing at 476 ° C and containing LiF-20 eq.%, Li2SO4 - 51 eq.%, Li2CO3 – 29 eq.%. The discrepancies between theoretical calculations and experimental studies are 8.3% in temperature and 5.05% in composition.
The liquidus surface of the quasi-triple system LiF–NaCl–Na3FSO4 was studied by a differential-thermal method of physicochemical analysis. As a result of the studies, the crystallization temperature (554 °C) and the composition of the three-component eutectic, which can be used as a heat accumulator in thermal energy storage devices, are determined. When designing plants based on renewable energy sources, it is necessary to provide storage tanks for the concentration of thermal energy, so that the stored heat energy can be used even in the period of absence of solar radiation. The most suitable for thermal accumulation are salt eutectic mixtures. Priority in this respect is research devoted to the development of compositions as possible with large values of the latent heat of the solid-liquid phase transition. The experiment was carried out on the synchronous thermal analysis unit STA 449 F3 Phoenix, the company Netzsch, designed to operate in the temperature range from room temperature to 1500 ° C in an atmosphere of inert gases (argon). All facet triangle (LiF)2 – (NaCl)2 – Na3FSO4: stable diagonal (LiF)2 – (NaCl)2 of the triple mutual system Li, Na // F, Cl and quasibinary systems: LiF–Na3FSO4; NaCl – Na3FSO4 is of the eutectic type, therefore it can be assumed that a triple eutectic is formed in the system. To determine the thermo physical characteristics of the eutectic composition, the experiment is planned in accordance with the general rules of the projection-thermographic method. The one-dimensional polythermal section AB located in the crystallization field of lithium fluoride, where A is 50% (LiF)2+ 50% Na3FSO4, B is 50% (LiF)2+ 50% (NaCl)2 was experientally studied. The study of the AB section reveals the direction to the triple eutectic, from the poles of lithium fluoride crystallization, i.e. the study of this section revealed a constant ratio of sodium chloride and sulfate-sodium fluoride in the triple eutectic. At the point showing a constant ratio of the two initial components in the eutectic, the thermal effects of the secondary and tertiary crystallizations are combined, and the primary crystallization is fixed at 657 °C. This composition is the starting point for the investigation of the next section. The content of lithium fluoride in the eutectic is determined by studying the polythermal section of lithium fluoride from the crystallization pole and passing through the projection Ē to the side of the triangle (NaCl)2 – Na3FSO4. As a result of the studies, the crystallization temperature and the concentration of the initial salts in the triple eutectic have been established. The detected eutectic composition (EΔ) crystallizes at 554 °C and contains eq. %: (LiF)2 – 26; (NaCl)2 – 23; Na3FSO4 – 51.Forcitation:Omarova S.M., Verdieva Z.N., Alkhasov A.B., Magomedbekov U.G., Arbukhanova P.A., Verdiev N.N. Phase equilibria in system (LiF)2 – (NaCl)2 – Na3FSO4. Izv. Vyssh. Uchebn. Zaved. Khim. Khim. Tekhnol. 2017. V. 60. N 10. P. 4-8
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