Silicate melts thermodynamic models

The thermodynamics of corrosive alkali salt-oxide interaction is not well established. In an assessment of research needs for materials in coal conversion, the need for carbonate-silicate melt studies, including activity and phase equilibrium measurements, was stressed (31 ). The lack of thermodynamic data for fused salts, and their reactions with oxides and alloys leading to models of hot corrosion, was also indicated. 

Elemental and isotopic fractionations by evaporation of silicate liquids, in particular limiting circumstances, can be simulated by equilibrium calculations, provided that an adequate thermodynamic model of the melt is available. In this approach, a particular starting temperature, pressure, and initial composition of condensed material are chosen and the gas in equilibrium with the melt is calculated from thermodynamic data. The gas is then removed from the system and equilibrium is recalculated. Repeated small steps of this sort can simulate the kinetic behavior during vacuum evaporation (i.e., the limit of fast removal of the gas relative to the rate it is generated by evaporation). This approach has been taken by Grossman et al. (2000, 2002) and Alexander (2001, 2002). 

It can be assumed that the course of the above relationships will also be similar in the Me0-Si02 systems. On the basis of the above facts, the structure of the Me0-Si02 melts can be imagined as a lattice of Si04 tetrahedrons polymerized to a certain degree, where cations are situated in the free spaces between the tetrahedrons. This concept was used by Panek and Dandk (1977) to formulate the thermodynamic model of silicate melts. 

Application of the model to different systems. The thermodynamic model of silicate melts has been applied in the calculation of various types of binary, ternary, and pseudo-ternary phase diagrams. 

In the region of high content of SiOy, the calculation of the phase equilibrium fails as the formation of two liquids is not considered in the thermodynamic model of silicate melts. This is also the reason for the enlarged liquidus surface of CaTiSiOs up to the high content of silica. 

From the comparison of the calculated and experimental phase diagrams, it follows that the thermodynamic model of silicate melts is suitable for the description of the phase equilibrium also in titania-bearing silicate systems and provides deeper information on the behavior of Ti(IV) atoms. It was, however, shown that Ti(IV) atoms behave in silicate melts as network formers, except in the region of its high concentration, and in highly basic melts. 

From the comparison of the experimental and calculated liquidus curves, it follows that the thermodynamic model of silicate melts describes satisfactorily the courses of liquidus curves in these complex glass-forming systems. It is expected that this model can also be applied in other inorganic glass-forming systems, like germanates, phosphates, etc. 

In 1963 Dr. Danbk joined the Institute of Inorganic Chemistry of the Slovak Academy of Sciences in Bratislava, of which he was the director in the period 1991-1995. His main field of interest was the physical chemistry of molten salts systems in particular the study of the relations between the composition, properties, and structure of inorganic melts. He developed a method to measure the electrical conductivity of molten fluorides. He proposed the thermodynamic model of silicate melts and applied it to a number of two- and three-component silicate systems. He also developed the dissociation model of molten salts mixtures and applied it to different types of inorganic systems. More recently his work was in the field of chemical synthesis of double oxides from fused salts and the investigation of the physicochemical properties of molten systems of interest as electrolytes for the electrochemical deposition of metals from natural minerals, molybdenum, the synthesis of transition metal borides, and for aluminium production. 

Three different approaches have been used. Firstly, the distribution of the major elements between mineral phases and a coexisting silicate melt may be calculated from experimental phase equilibrium data using regression techniques. Secondly, mineral-melt equilibria can be determined from mineral-melt distribution coefficients. A third, less empirical and more complex, approach is to use equilibrium thermodynamic models for magmatic systems. These require a thermodynamically valid mixing model for the liquid and an internally consistent set of solid-liquid thermodiemical data. 

Bottinga Y., Weill D.F. and Richet P., 1981, Thermodynamic modelling of silicate melts. In Newton R.C., Navrotsky A, and Wood B.J. (eds.), Thermodynamics of minerals and melts. Advances in physical geochemistry series, Springer, Berlin, pp. 207-245. 

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