Heat of Fusion of Na3AlF6 Eutectic Mixtures with CaF2 and Al2O3 104 D O I: 1 0. 15 82 6/ ch im te ch .2 01 9. 6. 3. 03 A. A. Redkina, S. V. Pershinaa, E. A. Il’inaa, A. A. Kataeva, Yu. P. Zaikova,b aInstitute of High-Temperature Electrochemistry of Ural Branch of RAS, 20 Akademicheskaya St., 620137, Ekaterinburg, Russian Federation bUral Federal University, 19 Mira st., Ekaterinburg, 620002, Russian Federation a.redkin@ihte.uran.ru 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. Keywords: heat of fusion; melting point; cryolite; heat balance; differential scanning calo- rimetry. Received: 01.08.2019. Accepted: 11.10.2019. Published: 15.10.2019. © Redkin A. A., Pershina S. V., Il’ina E. A., Kataev A. A., Zaikov Yu. P., 2019 Introduction Molten cryolites are applied as  electrolytes in  industrial production of  aluminum due to  high alumina solu- bility and high electrical conductivity [1]. However, it is difficult to  use them due to  their relatively high corrosion activ- ity. 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 knowl- edge of thermophysical properties of both liquid and frozen electrolyte is very im- portant, as  well as  freezing and melting processes themselves. The heat that is ab- sorbed 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 Na3AlF6) or 28.83 kJ·mol –1 (considering cryolite as 75 NaF — 25 AlF3 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 cry- olite 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 Redkin A. A., Pershina S. V., Il’ina E. A., Kataev A. A., Zaikov Yu. P. Chimica Techno Acta. 2019. Vol. 6, No. 3. P. 104–110. ISSN 2409–5613 105 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 measure- ment of the heat related to melting of such mixture. Heat of fusion measurements can be carried out by differential scanning calo- rimetry (DSC) and drop calorimetry. In drop calorimetry, the measured parameter is the enthalpy of the sample. The tempera- ture dependence of enthalpy is discontinu- ous at a melting point. The difference be- tween 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 fluo- ride. The measurements were carried out by DSC method, which can provide pre- cise results. Experimental The chemicals used for  the sample preparation are listed in  Table  1. Alu- minum fluoride was purified from oxy- gen containing admixtures by ammonium fluoride in a glassy carbon crucible. Part of NH4F (10 % of AlF3) was placed on the bottom of  the crucible, and the other part was mixed with aluminum fluoride in proportion as follows: 12 g of NH4F per 100 g of AlF3. 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: 6NH4F+Al2O3 = = 2AlF3+6NH3+3H2O (1) The analysis on oxygen after purifica- tion had been made using LECO element analyzer (USA). The mass content of oxy- gen 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 cal- cium fluoride and alumina, the aluminum fluoride was mixed with the other com- ponents of  eutectic mixture, placed into platinum crucible and heated up to 1323 K. To avoid the oxidation, a  small amount of NH4F was added to the mixture. Ammo- nium 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. The investigations were carried out us- ing a STA 449C Jupiter synchronous ther- mal analyzer (NETZSCH, Germany). The experimental setup ensures high accuracy of the measuring parameters: temperature (< 1 K); mass (± 1∙10–6 g); base line repro- ducibility (± 2.5 mW); enthalpy (± 3 %). The apparatus was calibrated using pure Table 1 Materials used in this work Compound Mass fraction purity, % Supplier Purification NaF 99.5 Vecton — AlF3 95 Vecton Treatment by NH4F Al2O3 99.5 Achinsk alumina plant — CaF2 99.5 Vecton — 106 salts supplied by NETZSCH (CsCl, AgSO4, BaCO3, RbNO3, KClO4). Monocrystalline sapphire was used to calibrate the sensi- tivity. The measurements were performed under following conditions: temperature interval  — 308–1300  K; heating rate  — 10  K min–1; atmosphere  — pure argon; crucibles with lids — Pt–Rh. All measure- ments were carried out under the same conditions. All calculations were per- formed with NETZSCH Proteus software. 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 [7–11]. The DSC curves are shown in Fig. 1. The weight loss was ob- served only after melting, and its value var- ied from 0.6 to 3 %. There are some solid- solid transitions on  the curves. The α–β cryolite solid transition is present in sam- ples 1 and 4 (Figs. 1, a, d). The temperature of this transition is in good agreement with literature [12]. All curves containing cal- cium 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 contain- ing 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  Na3AlF6–Al2O3, Na3AlF6–AlF3 and Na3AlF6–CaF2. The sample 1 is such quasi-binary Na3AlF6– Al2O3 and sample 4 is close to  quasi-bi- nary Na3AlF6–CaF2 due to  the low con- centration of alumina in this sample. The multi-component mixtures manifest other DSC peaks. Craig [9] investigated 8 eu- tectic mixtures of  Na3AlF6–AlF3–CaF2– Al2O3. 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 in- vestigation 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 litera- ture data on  heats of  fusion and melting points of cryolite salts are given in Table 4. Na3AlF6 is a  coordination compound. Coordination compounds are inorganic salts formed by  the combination of  two or  more simple compounds in  stoichio- metric ratio. In order to  compare molar properties of  coordination compounds and simple compounds, one must con- sider a  coordination compound (in  our case Na3AlF6) as a combination of simple Table 2 The composition of samples under investigation No Composition / mass% Composition / mol% NaF AlF3 CaF2 Al2O3 NaF AlF3 CaF2 Al2O3 1 56.4 37.6 0.0 11.7 70.5 23.5 0.0 6.0 2 50.1 33.4 14.8 3.1 66.1 18.5 10.2 1.7 3 50.5 33.6 10.0 5.9 67.2 22.4 7.2 3.2 4 46.2 30.8 19.9 3.1 62.8 21.0 14.6 1.7 107 compounds, i.e. 3 molecules of  NaF and 1 AlF3. In order to to equalize cryolite with simple compounds its molecular weight must be given for  1 molecule (0.75 mo- lecular weight of NaF and 0.25 molecular weight of AlF3). Fig. 1. DSC and thermogravimetric (TG) curves of cryolites with different composition: 1 (a), 2 (b), 3 (c), 4 (d) (see Table 2) Table 3 Heats of fusion and melting points of eutectics under investigation Comp. No Transition point / K ∆Htr / J g–1 ∆Htr / kJ mol–1 Melting point / K ∆Hm / J g–1 ∆Hm / kJ mol–1 1 831 43.3 2.4 1202 476.6 26.4 2 1080 95.5 5.1 1210 501.8 26.6 3 1075 71.7 4.0 1219 477.2 26.7 4 835 44.2 2.5 1204 455 26.0 1063 84.3 4.8 — — — Table 4 Heats of fusion and melting points of cryolites Compound Tmp / K Hfus / kJ mol –1 Compound Tmp / K Hfus / kJ mol –1 Li3AlF6 1058 [4] 21.0 [4] Na3AlF6 1284 [2] 28.3 [2] Li3AlF6 1058 [2] 22.0 [2] Na3AlF6 1284 [1] 28.9 [1] Na3AlF6 1284 [3] 28.8 [3] K3AlF6 1273 [2] 30.8 [2] 800 900 1000 1100 1200 1300 -0.4 -0.2 0.0 0.2 0.4 90 92 94 96 98 100 TG [% ] H ea t flo w [m W /m g] Temperature [K] Tons = 831 K exo Tons = 1202 K (a) 800 900 1000 1100 1200 -3 -2 -1 0 1 2 60 70 80 90 100 Tons = 1210 K Tons = 1080 K TG [% ] H ea t flo w [m W /m g] Temperature [K] exo (b) 800 900 1000 1100 1200 -4 -3 -2 -1 0 1 2 80 85 90 95 100 Tons = 1219 K Tons = 1075 K (c) TG [% ] H ea t flo w [m W /m g] Temperature [K] TG exo 800 900 1000 1100 1200 -1.0 -0.5 0.0 0.5 1.0 80 85 90 95 100 Tons = 1204 K Tons = 1063 K Tons = 835 K TG [% ] H ea t flo w [m W /m g] Temperature [K] exo (d) 108 Thus, the literature data were recalcu- lated using the molecular mass of mixture as a sum of 75 % of molecular mass of alkali halide and 25 % of molecular mass of alu- minium fluoride. It allows comparing re- sults with data on individual salts. All the results are presented in Fig. 2 in coordi- nates 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 thermo- dynamics, because Tm=ΔHm / ΔSm, (2) where Tm is the melting temperature, ΔHm is the enthalpy of  fusion, and ΔSm 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 ar- ticle [13]. The same correlation was shown for nitrates, carbonates and sulphates [14]. This trend is the part of more broad rela- tionship known as Trouton’s rule, which connect enthalpy of phase transition with its temperature. It is valid both for vapori- zation of pure elements [15] and for melt- ing [16, 17]. According to LSM (least-squares meth- od) estimations, the coefficients in  the equation H = A+B·T are as follows: BSQ = 0.034 kJ mol–1 K–1; ASQ = –14.25 kJ mol –1; T — temperature, K. The standard devia- tion σSQ is equal to 0.815 kJ mol –1, and the determination coefficient R2=0.94. Thus, the equation is: ΔHm / kJ mol –1 = =BSQ·Tm – ASQ = 0.034 Tm – 14.25 (3) The heat of  fusion temperature coef- ficient of cryolites is close to that of alkali halides. Thus, the temperature depend- ence of heat of fusion is the same for halide compounds and equal to the value for al- kali 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 pro- portional to the melting point for simple halide salts, salts compounds and eutec- tic 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 cry- olite with alumina and calcium cryolite. 2. The heat of fusion was found to be directly proportional to the melting point. Table 5 The values of coefficients in equations for heat of fusion dependence on the melting point Compounds Parameter ASQ / kJ mol –1 Parameter BSQ / kJ mol –1 K–1 Cryolites –14.25 0.034 Alkali halides [7] –12.12 0.036 Fig. 2. Heat of fusion dependence on the melting point for cryolites and alkali halides 700 800 900 1000 1100 1200 1300 12 16 20 24 28 32 36 Cryolites Alkali halides H ea t o f f us io n [k J/ m ol ] T melting [K] 109 Acknowledgements The work was supported by RFBR. Grants No 18-03-00785 А. 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