Heat of Fusion of Na3AlF6 Eutectic Mixtures with CaF2 and Al2O3

The heat of fusion of eutectic mixtures of sodium cryolite with alumina and calcium fluoride was measured using differential scanning calorimetry. Melting temperatures were found to be in good agreement with literature data. The molar heat of fusion of cryolite salts and eutectic mixtures was found to be directly dependent on melting temperature. The temperature dependence coefficient is the same as that of alkali halides.


Introduction
Molten cryolites are applied as electrolytes in industrial production of aluminum due to high alumina solubility and high electrical conductivity [1]. However, it is difficult to use them due to their relatively high corrosion activity. One means of avoiding this issue is to create protective layer of frozen salts on the walls of electrolytic cell, but this layer, or ledge, is unstable due to high heat flows in the salt bath. In order to control the thickness of a side ledge, the knowledge of thermophysical properties of both liquid and frozen electrolyte is very important, as well as freezing and melting processes themselves. The heat that is absorbed or realized at melting or freezing is determined by the enthalpy of fusion. The main component of an aluminium bath is sodium cryolite, and its enthalpy of fu-sion was investigated by many researches. The very first results were very different from later data. Malinovsky [2] analyzed all results available in mid-eighties. His own results on heat of fusion were given as 115.4 kJ·mol -1 (considering cryolite as Na 3 AlF 6 ) or 28.83 kJ·mol -1 (considering cryolite as 75 NaF -25 AlF 3 in mol %.The data obtained by Malinovsky do not differ essentially from the previous ones [3,4]. Latest results are also close to these data [5]. Along with sodium cryolite, the other cryolites were studied in works of Holm and Bjorge [3,6]. The real bath consists not only of cryolite but also of the other components such as alumina and calcium fluoride. The composition of electrolyte is usually close to some eutectic mixture of sodium cryolite with alumina, calcium fluoride and aluminium fluoride plus some small quantities of initial components. Eutectic mixture behaviour at melting (freezing) is similar to that of individual substance, and it is possible to perform a precise measurement of the heat related to melting of such mixture. Heat of fusion measurements can be carried out by differential scanning calorimetry (DSC) and drop calorimetry. In drop calorimetry, the measured parameter is the enthalpy of the sample. The temperature dependence of enthalpy is discontinu-ous at a melting point. The difference between values for solid and liquid states is the heat of fusion. With the DSC method, the heat of fusion is calculated from the melting area.
The aim of the work was to measure the melting points and heats of fusion of some eutectic mixtures of cryolite with aluminium oxide and calcium fluoride. The measurements were carried out by DSC method, which can provide precise results.

Experimental
The chemicals used for the sample preparation are listed in Table 1. Aluminum fluoride was purified from oxygen containing admixtures by ammonium fluoride in a glassy carbon crucible. Part of NH 4 F (10 % of AlF 3 ) was placed on the bottom of the crucible, and the other part was mixed with aluminum fluoride in proportion as follows: 12 g of NH 4 F per 100 g of AlF 3 . The mixture was heated up to 723-773 K and kept for about 6 hours at that temperature. The reaction between aluminum oxide and ammonium fluoride is given below: (1) The analysis on oxygen after purification had been made using LECO element analyzer (USA). The mass content of oxygen was less than 0.1 %. The purity of other reagents was higher than 99.5 % content of main component (Table 1); therefore, their purification was not required.
For preparation of cryolites with calcium fluoride and alumina, the aluminum fluoride was mixed with the other components of eutectic mixture, placed into platinum crucible and heated up to 1323 K. To avoid the oxidation, a small amount of NH 4 F was added to the mixture. Ammonium fluoride was decomposed at 513 K and did not influence the composition of the mixture. After melting, the sample was poured into a graphite mould.

Results and discussion
Some eutectic compositions of cryolite with calcium fluoride and alumina were investigated; their compositions are given in Table 2.
The phase diagrams of these systems were widely studied and can be found in works [7][8][9][10][11]. The DSC curves are shown in Fig. 1. The weight loss was observed only after melting, and its value varied from 0.6 to 3 %. There are some solidsolid transitions on the curves. The α-β cryolite solid transition is present in samples 1 and 4 (Figs. 1, a, d). The temperature of this transition is in good agreement with literature [12]. All curves containing calcium fluoride have endothermic peaks in the interval of 1060-1080 K.
Fedotieff and Iljinsky found two temperature halts in the cooling curves in this region for calcium fluoride containing compositions [7]. There are no α / β transitions of cryolite in samples 2 and 3. The transitions occur in mixtures which are quasi-binary such as Na 3 AlF 6 -Al 2 O 3 , Na 3 AlF 6 -AlF 3 and Na 3 AlF 6 -CaF 2 . The sample 1 is such quasi-binary Na 3 AlF 6 -Al 2 O 3 and sample 4 is close to quasi-bi-nary Na 3 AlF 6 -CaF 2 due to the low concentration of alumina in this sample. The multi-component mixtures manifest other DSC peaks. Craig [9] investigated 8 eutectic mixtures of Na 3 AlF 6 -AlF 3 -CaF 2 -Al 2 O 3 . The lowest DSC peak temperature for these mixtures was found to be equal to 948 K. Melting peaks are very broad, but the same lines were observed by other scientists [8,10].
Melting points of mixtures under investigation are in the interval of 1200-1220 K. It is in good agreement with the results presented in the article [11]. The values of melting points and heats of fusion are given in Table 3. The literature data on heats of fusion and melting points of cryolite salts are given in Table 4. Na 3 AlF 6 is a coordination compound. Coordination compounds are inorganic salts formed by the combination of two or more simple compounds in stoichiometric ratio. In order to compare molar properties of coordination compounds and simple compounds, one must consider a coordination compound (in our case Na 3 AlF 6 ) as a combination of simple   Table 2)  Thus, the literature data were recalculated using the molecular mass of mixture as a sum of 75 % of molecular mass of alkali halide and 25 % of molecular mass of aluminium fluoride. It allows comparing results with data on individual salts. All the results are presented in Fig. 2 in coordinates as follows: heat of fusion -melting point.
There is a clear correlation between the enthalpy of melting and the melting point. The nature of this correlation is in thermodynamics, because where T m is the melting temperature, ΔH m is the enthalpy of fusion, and ΔS m is the entropy of fusion.
The correlation between the enthalpy of melting and the melting point for alkali halide salts was found in our previous article [13]. The same correlation was shown for nitrates, carbonates and sulphates [14]. This trend is the part of more broad relationship known as Trouton's rule, which connect enthalpy of phase transition with its temperature. It is valid both for vaporization of pure elements [15] and for melting [16,17].
According to LSM (least-squares method) estimations, the coefficients in the equation H = A+B·T are as follows: B SQ = 0.034 kJ mol -1 K -1 ; A SQ = -14.25 kJ mol -1 ; T -temperature, K. The standard deviation σ SQ is equal to 0.815 kJ mol -1 , and the determination coefficient R 2 =0.94.
Thus, the equation is: ΔH m / kJ mol -1 = =B SQ ·T m -A SQ = 0.034 T m -14.25 (3) The heat of fusion temperature coefficient of cryolites is close to that of alkali halides. Thus, the temperature dependence of heat of fusion is the same for halide compounds and equal to the value for alkali halide salts. The values of temperature coefficients are close to 4R. It is the same number as heat capacity of halide salts per atom [18].
Thus, the heat of fusion is directly proportional to the melting point for simple halide salts, salts compounds and eutectic mixtures. The fusion properties of all these substances are possibly connected due to the fact that the main components of these compounds are halide salts.

Conclusions
1. Heats of fusion were measured for some eutectic mixtures of sodium cryolite with alumina and calcium cryolite.
2. The heat of fusion was found to be directly proportional to the melting point.