Thermodynamic Properties of CaCl2-CaF2-CaO System: Phase Diagram Investigation

The CaCl2-CaF2-CaO phase diagram was investigated in the CaO low region (<40 mol.%). CaCl2-CaF2 and CaCl2-CaO binary diagrams, constituting the ternary system, were first studied by Differential Scanning Calorimetry (DSC) measurements and X-Ray Diffraction (XRD) characterization; a good agreement was obtained between the phase diagram models calculated with FactSage software (FTsalt database) and present experimental data. As the CaF2CaO liquidus could not be measured by DSC due to the high melting temperature, this diagram was calculated using FTsalt database combined with FToxid database of FactSage software. The ternary phase diagram was obtained by calculations and exhibits an eutectic at the composition CaCl2CaF2-CaO (78.2-15.7-6.1 mol.%) melting at 637 ̊C, and five peritectic points. Measurements of relevant vertical cross-sections for three CaCl2-CaF2 compositions (50-50, 40-60 and 30-70 mol.%) up to 18 mol.% CaO are in agreement with the ternary phase diagram model. For each section, the liquidus temperature is constant up to around 11 mol.% CaO and then sharply increases. Moreover, an increase of CaF2 content in CaCl2-CaF2 melt leads to a decrease of the CaO solubility in isothermal condition.


Introduction
and (64-23-13 mol.%) melting respectively at 660˚C and 670˚C. CaF 2 -rich part (>50 mol.% CaF 2 ) of the phase diagram is still unknown and needs to be investigated. To build it, binary phase diagrams constituting the ternary system, CaCl 2 -CaF 2 , CaCl 2 -CaO and CaF 2 -CaO, must be previously considered.
The CaCl 2 -CaF 2 system was investigated by several authors [1]- [6]: • in the CaCl 2 -rich region, they all suggested a single eutectics in the composition range 18.5 -25 mol.% CaF 2 at around 650˚C as well as a peritectic point at 735˚C around 41 mol.% CaF 2 , • the system exhibits CaFCl compound at 50 mol.% CaF 2 which is known to have an incongruent melting point [7]. In the composition range 41-50 mol.% CaF 2 , this phase changes to α-CaF 2 at around 735˚C, • the liquidus data are more scattered in the CaF 2 -rich part (>50 mol.% CaF 2 ).
Moreover, some of these results were obtained by visual-polythermal method which is not very accurate according to Chartrand et al. [8]. These authors proposed a calculated phase diagram using a modified quasi-chemical model, and confirmed it with data from several studies.
The CaCl 2 -CaO system was only investigated in the CaCl 2 -rich part and data were compiled by Shaw et al. [1] [9] [10] [11] [12]: • the system exhibits an eutectic point in the 5 -6.5 mol.% CaO composition range at around 750˚C. However, a different eutectic composition was found by Threadgill [13] (28.7 mol.% CaO and 593˚C), • a second eutectic was proposed by Neumann et al. [10] at 21 mol.% and 800˚C. These authors also reported the CaO(CaCl 2 ) 4 compound at 20 mol.% CaO, which melts congruently at 839˚C. Similar results were obtained by Perry et al. [11] specifying that the 20 -22 mol.% CaO region may exhibit a peritectic point instead of eutectic point. However, Wenz et al. [1] did not report it: they proposed a peritectic point at 18.5 mol.% melting at 835˚C and suggested the CaO(CaCl 2 ) 2 compound at 33 mol.% CaO which was considered in Shaw et al. study [12], • a sharp increase of the liquidus temperature for higher CaO  • no specific compound was identified in the diagram, but partial CaO solubility in solid CaF 2 phase was reported by Kim et al. [20] with a maximum around 5 mol.% CaO at the solidus temperature. This author also identified the CaF 2 allotropic transition α → β at 1146˚C without any effect on the CaO solubility, • Baak [26] proposed a two-liquid phase region in the 0.8 -10 mol.% CaO composition range, up to 1485˚C. According to the literature, discrepancies exist on the binary phase diagrams constituting the CaCl 2 -CaF 2 -CaO ternary system. In this study, both CaCl 2 -CaF 2 and CaCl 2 -CaO systems were investigated by combining experimental data and modeling. From these results, the ternary phase diagram was calculated and vertical cross-sections measurements were then performed to verify the model. Crystallographic phases were identified by X-Ray Diffraction analysis (XRD) with a Bruker D4-ENDEAVOR diffractometer. Patterns were recorded from 20˚ to 70˚ by 0.016˚ step (2θ) in the Bragg-Brentano geometry using the Kα radia-tion of the copper anode (40 kV, 40 mA). Samples were prepared in a glove box under inert argon atmosphere: the salt was protected by a Kapton film to prevent water absorption.

Experimental
Binary and ternary phase diagrams modeling was performed using Phase Diagram module from FactSage ® 6.3 software [27] [28]. In the Phase Diagram module, the Gibbs energy minimization is used for calculations where axis and constant parameters can be set, such as the volume, the temperature, the pres-sure… In this study, FTsalt and FToxid databases were used and the pressure was fixed to 1 atmosphere.

CaCl 2 -CaF 2 System
A classical DSC-signal from a binary sample of CaCl 2 -CaF 2 (70-30 mol.%) is shown in Figure 1 and exhibits a sharp endothermic peak at around 660˚C, solidus-followed by a second broad endothermic peak at around 710˚C, liquidus-typical of a binary mixture. The solidus temperature was measured as onset point whereas liquidus temperature at the peak maximum, as recommended by Höhne et al. [29]. The values were reported on the diagram in Figure 2 and are consistent with other authors [1]- [6] and FactSage ® model (FTsalt database).
Eutectic composition could not be accurately determined on DSC curves since the peaks are not well defined in the 15 -23 mol.% CaF 2 composition range. The determination of the eutectic composition was done by using of a Tamman diagram [30] [31] [32], representing the molar enthalpy associated to a first-order phase transition versus the molar fraction, based on the lever rule application. The resulting diagram has a triangle shape and the baseline endpoints indicate specific compositions (eutectics, peritectics, stoichiometric compound, solid so-lubility…).
In this case, the Tamman plot was established using enthalpy in the 5 -40 mol.% CaF 2 composition range. Both left and right endpoints from the baseline  To confirm the presence of the CaFCl stoichiometric compound, a melted mixture of CaCl 2 -CaF 2 (40-60 mol.%) annealed for 72 hours at 650˚C was prepared for phases identification by XRD analysis. The resulting pattern is shown in Figure 3 where CaFCl was identified, which is consistent with the model and the literature data [7]. As it was expected for this composition, α-CaF 2 was also detected without any segregation.
Unexpected signal occurred in the CaF 2 -rich part at around 647˚C and was not observed in the data available in the literature: it could be attributed to a segregation effect. One way to avoid it is to introduce an annealing step after the salt pre-melting: this procedure was successful as no more unexpected signal was observed.
Furthermore, the extrapolated curve reaches around 32 mol.% of CaO at 1400˚C and is in agreement with the CaO solubility value determined by Sano et al. [33].
To prove the existence of a stoichiometric compound in the binary system, a mixture of CaCl 2 containing 13 mol.% of CaO was melted and annealed for 48 hours at 650˚C. It was then characterized by XRD for phase identification. The pattern is shown in Figure 5 and reveals the presence of CaCl 2 and CaO(CaCl 2 ) 4 compound, confirming the endpoint composition of the Tamman triangle basis (20 mol.% CaO), as well as the presence of three peaks that could not be attributed.
A slight endothermic peak is observed at around 715˚C for every samples similarly to the thermal plateau observed by Neumann et al. [10] at 703˚C on the cooling curves; this signal can be attributed to the allotropic phase transition of CaO(CaCl 2 ) 4 compound.

CaF 2 -CaO System
Due to the high liquidus temperature (>1300˚C), the CaF 2 -CaO system was not investigated by DSC. However, the phase diagram was calculated using FTsalt database which offers satisfying models for both CaCl 2 -CaF 2 and CaCl 2 -CaO systems. The model was improved by including additional CaF 2 /CaO data from FToxid database and was plotted in Figure 6; it exhibits an eutectic point at 19.8 mol.% CaO melting at 1305˚C, and the CaO solubility domain into αand β-CaF 2 solids mentioned in Kim et al. study [20].

The Ternary Diagram
To build the CaCl 2 -CaF 2 -CaO diagram, the three binary diagrams constituting   The results are shown in Figure 8 and the following observations can be noticed: • the general shape of the liquidus temperature is in agreement with the model for each cross-section: a plateau from 0 to a threshold composition around 11 mol.% CaO and then a sharp increase,   • a slight difference of the threshold composition between measurements and the model, which could be attributed to the data selected to calculate the CaF 2 -CaO phase diagram and was not experimentally verified. At constant temperature, the CaO solubility decreases when the CaF 2 portion in the melt increases, as observed at 1400˚C by Sano et al. [33] in the investigated area. To compare his data with the present work, the logarithm of CaO solubility (x CaO ) above 11 mol.% was plotted as a function of the inverse of the absolute temperature in Figure 9. A linear relationship was obtained for all compositions and equations were extrapolated toward 1400˚C to determine the CaO solubility; data are reported in Table 2. The CaO solubility calculated at 1400˚C is slightly different than the ones determined by Sano et al. [33] with a maximum discrepancy reaching 4.2 mol.% in the equimolar CaCl 2 -CaF 2 medium. However, the CaO solubility evolution is similar with a decrease of the solubilized CaO amount while the CaCl 2 is replaced by CaF 2 at constant temperature (1400˚C).