Thermochemical Properties Over LiCl-KCl-UCl₃ Ternary Salt System: Knudsen Effusion Mass Spectrometric Study

Abstract

Vaporisation studies were carried out over the solid region of LiCl(cr), KCl(cr) and liquid region of LiCl-KCl-UCl₃ ternary salt system by using Knudsen Effusion Mass Spectrometry (KEMS) in the temperature range of 715–913 K. Monomeric and dimeric species were observed in the vapour phase in equilibrium with their respective salts, LiCl(cr) and KCl(cr). LiCl(g), Li₂Cl₂(g), KCl(g), K₂Cl₂(g), and UCl₃(g) were the neutral species observed in the equilibrium vapour over ternary salt. Partial pressure–temperature relations for vapour species were derived using in-situ calibration from pressure dependent equilibrium constants as well as using pure silver as external calibrant. Using p-T relations, various heterogeneous reaction equilibria that exist between condensed phase-gas phase and the dissociation equilibra of following gas phase reactions: Li₂Cl₂(g) = 2LiCl(g); K₂Cl₂(g) = 2KCl(g) were evaluated by using 2nd and 3rd law methods. Also, the enthalpies of pressure-independent reactions: LiCl(cr) + LiCl(g) = Li₂Cl₂(g); KCl(cr) + KCl(g) = K₂Cl₂(g) were evaluated by using 3rd law method. Knudsen effusion mass spectrometric studies on LiCl-KCl-UCl₃ ternary salt system were carried out for the first time.

1 Introduction

Studies on pyro-metallurgical processes dealing with molten salt medium has been difficult because of corrosive nature of the medium which is highly aggressive to process equipment.^[1,2] The vapour pressures and the heat of vaporisation of common metallic chlorides at high temperature (up to 1523 K) were reported by Maier et al.,^[2] by using a static method. The group IA halides vaporise predominantly as monomers and dimers with minor concentrations of trimer and tetramer species.^[3] The presence of polymeric species in the equilibrium vapour over alkali halides was first experimentally shown in early fifties by means of the molecular-beam nuclear-magnetic-resonance technique^[4,5] and velocity distribution measurements.^[6] With the advent of the commercially available sector magnetic and quadrupole mass spectrometers, the presence of monomeric, dimeric, and to a lesser extent trimeric species were reported by various investigations.^{[7,8,9,10,11,12,13,14]} Berkowitz and Chupka^[7] reported the vaporisation behaviour of alkali halides species employing Knudsen Effusion Mass Spectrometry (KEMS). Milne and Klein^[8] reported the vaporisation behaviour of five alkali chlorides by using the Bendix time-of-flight mass spectrometer and deduced their heats of sublimation and dimerisation energies. Grimley and Joyce^[9] reported partial pressures of KCl(cr) using dual-cell high temperature mass spectrometry. The ionisation and dissociation processes resulting from the electron impact ionisation (EI) of atomic and molecular vapour species of LiCl(cr), NaCl(cr) and KCl(cr) were reported by Hobson^[10] by using 180° sector magnetic mass spectrometer. Milne et al.^[11] examined the vapours in equilibrium with the alkali metal chlorides by mass spectrometric analysis and reported various ion intensity ratios over LiCl(cr), KCl(cr), RbCl(cr) and CsCl(cr) salts. Berkowitz et al.^[15] reported fragmentation patterns, ionisation cross sections of dimers relative to those of monomers and dimerisation energies of lithium halides vapour by using double-oven apparatus coupled with a mass spectrometer. Niwa^[16] and Nesmeyanov and Sazonov^[17] reported partial pressure over LiCl(cr) using Knudsen effusion mass loss method. The vapour phase composition and partial pressures over KCl(cr) have been reported by Van Der Kemp et al.^[18] Rudnyi et al.^[19] had reviewed the literature work on potassium chloride. The vapour pressures over the crystalline salts (chloride and bromide salts of sodium, potassium, and rubidium) and their heat of vaporisation data were reported by Mayer et al.^[20] using Knudsen effusion mass loss method (KEML) in the range of 856-944 K. Deitz^[21] reported the vapour pressure of KCl(cr) and CsI(cr) by using KEML in the range of 873-973 K. The vapour pressure, heats, and entropies of vaporisation of crystalline KCl, KBr, KI and NaCl have been reported by Zimm and Mayer^[22] using surface ionisation method. Bloom et al.^[23] studied the ionisation and dissociation of the alkali halides (NaCl, KCl, RbCl, CsF, and CsCl) using a quadrupole mass spectrometer. The enthalpy of fusion and the vaporisation behaviour of UCl₃(cr) were reported by Kovacs et al.^[24] No literature report is available on vaporisation behaviour of LiCl-KCl-UCl₃ ternary salt.

Vaporisation studies over this ternary salt system are of fundamental importance to the pyro-metallurgical reprocessing of spent metallic nuclear fuel by employing molten salt electrorefining process, for the separation and recovery of precious actinides from the fission products. The molten salt electrorefining process was developed by Argonne National Laboratory for the reprocessing of irradiated U-Pu-Zr alloy fuels of the Integral Fast Reactors (IFR).^[25,26] This process is carried out at 773 K. The chopped spent nuclear fuel present in the anode basket is immersed in molten salt (LiCl-KCl eutectic salt mixture with UCl₃ concentration ranging from 4 to 6 wt.% as the medium). The fissile materials are selectively electrotransported and deposited on suitable cathodes of an electrorefining cell. This process exploits the difference in thermodynamic stabilities of the chlorides of fuel materials and fission products to separate them. Pyro-reprocessing enables the handling of short cooled and high burn-up fuels (highly radioactive). It also requires relatively less process volumes for reprocessing a given amount of fuel compared to aqueous reprocessing resulting in a compact plant. Other advantages include less criticality problems, actinide recycling potential and low waste generation. Complete details of the process can be found in the literature.^[25,26] Nature and composition of vapour phase and partial pressure data over LiCl-KCl-UCl₃ ternary salt system are relevant for the cathode processing step. Partial pressure data are required for the effective removal of electrolyte salt from uranium metal (deposited onto solid cathode as “dendrite-deposit”). Uranium obtained from electrorefining process is coated with chloride salts (typically 20 wt.% of the product).^[27] Hence, in the present work we have undertaken a detailed vaporisation study on ternary salt by using KEMS.

2 Experimental

2.1 Sample Preparation

LiCl(cr) (purity > 99%, M/s Fluka, Switzerland), analytical grade KCl(cr) (purity > 99.8%, M/s Glaxo Laboratories, India) and uranium metal of nuclear grade (BARC, Mumbai, India) were used for the preparation of LiCl-KCl-UCl₃. LiCl-KCl eutectic was purified by passing chlorine gas at 673 K and was melted subsequently under chlorine atmosphere. The purified salt was loaded with UCl₃ by equilibrating the salt containing CdCl₂ with uranium metal at 773 K in an argon atmosphere glove box. The amount of uranium in LiCl-KCl eutectic was determined by potentiometric technique using Davis and Gray method^[28] and the concentration of UCl₃ in the salt was calculated. The complete details of preparation of LiCl-KCl-UCl₃ with 4.46 wt.% UCl₃ (58.1 mol% LiCl − 41.2 mol% KCl − 0.7 mol% UCl₃) used for KEMS experiments are given in the reference.^[29]

2.2 Knudsen Effusion Mass Spectrometric Studies

The details of the instrumentation of KEMS used in the present study are described elsewhere.^[30] The schematic of the KEMS facility employed in the present study is depicted in Fig. 1. For each salt system (LiCl(cr), KCl(cr) and LiCl-KCl-UCl₃, about 200 mg of sample was taken in a zirconia Knudsen cell fitted with a zirconia lid having an orifice of diameter 0.5 mm at its centre and this was placed in an outer cup and lid made of tantalum having an orifice of 2 mm diameter collinear with the Knudsen cell orifice. A typical vacuum of ~ 3-4 × 10⁻⁵ Pa was maintained during the vaporisation experiments. Samples were heated by bombarding with the electrons emitted from the two tungsten filaments surrounding the Knudsen cell assembly. The temperatures of sample were measured by a Type K (Chromel-Alumel) thermocouple which is in contact with outer tantalum cup. Since these salts are hygroscopic in nature, it is essential to remove the moisture from them prior to KEMS studies. During the experiments, each salt system was slowly heated under a high vacuum, from room temperature to a temperature where I(H₂O⁺) was detected with shutterable intensity. Subsequently, the temperature of the sample was increased at 10 K per step and was maintained until the net intensity of H₂O⁺ became indistinguishable from the background. It took ~ 12 days for complete removal of moisture through this vacuum drying process. Hiden Analytical make Quadrupole Mass Spectrometer (Hiden HAL/3F RC 1001 PIC series, having mass range up to 1000 amu) was employed in the present study. The effusion flux was ionised by electrons of energy 13.2 eV (for LiCl and KCl vapour species over pure and ternary salt) and 28.2 eV (for UCl₃ vapour species over LiCl-KCl-UCl₃ system) at an emission current 100 µA. The ions were mass filtered by using a quadrupole triple filter geometry and subsequent detection by a secondary electron multiplier operating in pulse ion counting mode (PIC). For each system, one lot of samples (LiCl(cr), KCl(cr) and LiCl-KCl-UCl₃ was subjected to vaporisation experiments and two temperature dependence runs for all the vapour species were carried out over ternary salt. In each of these runs, sample was maintained at first temperature for about 1 h and subsequently sample was maintained at every temperature for ~ 30 min. The temperatures of sample were varied in cyclic manner. The mean value pressure calibration constant (k′) deduced from calibration experiments using pure silver was employed for converting the measured ion intensity data into partial pressure. k′ was also derived using in-situ calibration from pressure dependent equilibrium constant. The thermocouple was calibrated against the melting point of pure silver. The electron energy scale was calibrated using the first ionisation energies of In⁺, Hg⁺, Ag⁺, Ar⁺ and He⁺. The uncertainties associated with the measured temperatures and the electron energy scale were found to be ± 3 K and ± 0.5 eV, respectively.

Fig. 1

Schematic diagram of Knudsen effusion mass spectrometry facility used in the present study^[30]

3 Results and Discussion

⁷Li⁺, ⁴²LiCl⁺ and ⁴⁹Li₂Cl⁺ were the major ionic species observed in the mass spectrum of equilibrium vapour over pure LiCl(cr). ³⁹ K⁺, ⁷⁴KCl⁺ and ¹¹³K₂Cl⁺ were the major ionic species observed over pure KCl(cr). ⁷Li⁺, ⁴²LiCl⁺, ⁴⁹Li₂Cl⁺, ³⁹ K⁺, ⁷⁴KCl⁺, ¹¹³K₂Cl⁺, ²⁷³UCl⁺, ³⁰⁸UCl₂⁺ and ³⁴³UCl₃⁺ were the major ionic species observed over LiCl-KCl-UCl₃ ternary salt system. No shutterable peaks were observed for trimeric, polymeric species over LiCl(cr), KCl(cr) and UCl₃ dimeric species over ternary salt. The ionisation efficiency curves for the ions ³⁹K⁺, ⁷⁴KCl⁺ and ¹¹³K₂Cl⁺ were recorded over pure KCl(cr) as a function of electron energy and the appearance energies of these ions were obtained from these plots by linear extrapolation method. These plots are shown in Fig. 2(a), (b) and (c). The appearance energies deduced for the ionic species over KCl(cr) and the literature data for the ions Li⁺, LiCl⁺, Li₂Cl⁺, K⁺, KCl⁺ and K₂Cl⁺ reported^[14,22] are shown in Table 1.

Fig. 2

Ionisation efficiency curves of K⁺, KCl⁺ and K₂Cl⁺ recorded over KCl(cr)

Table 1. The appearance energy (AE) data for LiCl and KCl vapour species

Appearance energy values for KCl⁺ and K₂Cl⁺ are in good agreement with the data reported in the literature (Table 1). KCl⁺ results from the simple ionisation of KCl(g) whereas K₂Cl⁺ results from dissociative ionisation of K₂Cl₂(g). Appearance energy of K⁺ obtained in the present study (10.5 eV) is greater than the first ionisation energy (4.3 eV) of K(g) indicating that K⁺ is the fragment species and it originates from KCl(g) since the electron energy used (13.2 eV) is just below the fragmentation energy (13.3 eV) of dimer (Fig. 2a). From mass-to-charge ratio (m/z) and appearance energies, the neutral species were deduced as KCl(g) and K₂Cl₂(g) over pure KCl salt. It has been reported^[5,31] that electron bombardment ionisation of dimer of the alkali halides leads predominantly to fragmentation rather than simple ionisation; i.e., M₂Cl₂(g) + e⁻ → M₂Cl⁺ + Cl + 2e⁻ [M = Li, K] is two orders of magnitude more probable^[14] than M₂Cl₂(g) + e⁻ → M₂Cl₂⁺ + 2e⁻. Grimley et al.,^[32] from their angular distribution studies, showed that the curves for KCl⁺ and K⁺ are identical and are independent of the ionising electron energy in the range 12-25 eV. They concluded that the monomer KCl(g) undergoes both simple and dissociative ionisation and results in the production of KCl⁺ and K⁺ species, respectively and K₂Cl₂(g) undergoes preferentially dissociative ionisation to K₂Cl⁺. The present results are in accordance with that reported by Grimly et al.^[32] As has been shown by Grimley and Joyce^[9] and Hastie et al.,^[33] the simplified ionisation pattern of alkali-halide vapours, by electron bombardment ionisation, which contains MX(g) and M₂X₂(g) can be written as M₂X₂(g): M₂X⁺ and MX(g): MX⁺ and M⁺. In the case of pure LiCl(cr), based on the mass numbers of the major ionic species observed in the present study and appearance energies reported in the literature,^[14] LiCl(g) and Li₂Cl₂(g) were possible major vapour species. Based on neutral species over pure LiCl(cr) and KCl(cr), and from the mass numbers of ions, LiCl(g), Li₂Cl₂(g), KCl(g), K₂Cl₂(g) and UCl₃(g) were possible major vapour species over LiCl-KCl-UCl₃ ternary salt.

The vapour phase over metal halides generally contains monomers, gaseous homo-complexes such as (MX_i)_n and gaseous hetero-complexes MM$\mathrm{^{\prime}}$X_i+j.^[34] In these complexes, the metallic ions are bound by halogen bridges.^[34] It was reported in the literature that the vaporisation of KCl-LnCl₃(Ln = Nd and / Er) equimolar mixture contains complex molecule—KLnCl₄(g) in addition to monomer and dimer species in the vapour phase.^[35] Complex metal halide molecules over similar systems were also observed and their thermochemical data are summarized in the review article by McPhail et al.^[36] However, we could not observe the complex molecular species and the mixed species/reaction products in the present study. The reason for not observing complex molecules may be due to very small enthalpy change of the iso-molecular exchange of reaction type as indicated in the literature^[36]: MNCl₄(g) + MCl(g) = NCl₃(g) + M₂Cl₂(g) (where M = Li or K and N = U, Nd, Er etc)

Another reason for not observing species like LiUCl₄(g)/KUCl₄(g) in the present study may be attributed to (1) The mol% of UCl₃ in the condensed phase (very low concentration of UCl₃) (2) The temperature range (715-913 K) of present study is lower compared to literature works reported such species and the type of mass spectrometer employed. Also vapour species like LiKCl₂(g)/Li₂UCl₅(g)/K₂UCl₅(g) were not observed in the present study.

The ion intensities of ⁷Li⁺, ⁴²LiCl⁺, ⁴⁹Li₂Cl⁺, ³⁹K⁺, ⁷⁴KCl⁺, ¹¹³K₂Cl⁺, ²⁷³UCl⁺, ³⁰⁸UCl₂⁺ and ³⁴³UCl₃⁺ were recorded as a function of temperature in the range of 715-913 K. Two runs were conducted for LiCl species over pure salt and ternary salt system. For KCl species, one run over pure salt and two runs over ternary salt were conducted. Pure silver was used for deducing pressure calibration constant.

3.1 Determination of Partial Pressures

3.1.1 ${{\varvec{k}}}_{{\varvec{i}}}^{\boldsymbol{^{\prime}}}$ from Calibration Experiments Using Pure Silver as External Calibrant

Pressure calibration constant (k′_i) obtained from the experiment over pure silver was used for deducing partial pressures. The electron energy of $13.2\mathrm{ eV}$ was used during the temperature dependence of ion intensity measurements over pure LiCl, KCl and LiCl-KCl-UCl₃ ternary (for measuring LiCl and KCl monomeric and dimeric vapour species) to minimise contribution due to fragmentation of dimer. For ion intensity measurements of U bearing species over ternary salt, electron energy of 28.2 eV was used because of poor signal to noise ratios at lower energies.

The measured ion intensity of the species ‘i’ was converted to partial pressure using the following relation:

$${\text{p}}_{{\text{i}}} = {\text{ k}}_{{\text{i}}}^{\prime} { }.{\text{ I}}_{{{\text{i}}^{ + } }} { }.{\text{ T}}$$

(1)

$${\text{k}}_{{\text{i}}}^{\prime} = {\text{ k}}_{i} /\left\{ \sigma{_{i } . \gamma_{{i^{ + } }} . n_{{i^{ + } }} } \right\},$$

(2)

where i = ${\text{ Ag}}$

$${\text{k}}_{{\text{i}}}^{\prime} = {\text{ k}}_{{\text{j}}}^{\prime} \left\{ {\frac{{\sigma_{j} }}{{\sigma_{i} }}} \right\}\left\{ {\frac{{ \gamma_{{j^{ + } }} }}{{\gamma_{{i^{ + } }} }}} \right\}\left\{ {\frac{{n_{{j^{ + } }} }}{{n_{{i^{ + } }} }}} \right\},{ }$$

(3)

where j = Ag and i = LiCl, Li₂Cl₂, KCl, K₂Cl₂ and UCl₃

where $k_{i}^{\prime} = \frac{k}{{\left( {\sigma \gamma n} \right)_{i} }}$ is the pressure calibration constant; k is the instrument calibration constant; I⁺ is the ion intensity of LiCl⁺, Li₂Cl⁺, KCl⁺ , K₂Cl⁺, UCl₃⁺; T is the temperature in Kelvin; σ is the ionisation cross section; γ is the yield of secondary electron multiplier and n is the isotopic abundance. (n(³⁴³UCl₃⁺) = 0.432, n(³⁰⁸UCl₂⁺) = 0.570, n(²⁷³UCl₃⁺) = 0.752, n(LiCl⁺) = 0.701, n(Li₂Cl⁺) = 0.648, n(KCl⁺) = 0.707, n(K₂Cl⁺) = 0.660). The relative ionisation cross section data required were computed using the empirical relation σ(A_xB_y) = 0.75[xσ(A) + yσ(B)] used from Mann’s compilation.^[37]$\upsigma$(LiCl) = 2.45 × 10⁻¹⁶ cm², $\upsigma$(Li₂Cl₂) = 4.89 × 10⁻¹⁶ cm², $\upsigma$(KCl) = 4.07 × 10⁻¹⁶ cm², $\upsigma$(K₂Cl₂) = 8.14 × 10⁻¹⁶ cm², $\upsigma$(UCl₃) = 16.47 × 10⁻¹⁶ cm², $\upsigma$(UCl₂) = 14.94 × 10⁻¹⁶ cm², $\upsigma$(UCl) = 13.41 × 10⁻¹⁶ cm². Since ion intensity measurements were performed in pulse counting mode, γ is independent of mass and equals unity.^[38] The pressure calibration constant was obtained at two electron energies (13.2 and 28.2 eV). The required partial pressure of Ag(g) over pure silver was taken from reference.^[39] For a given temperature dependence run, the logarithmic values of partial pressures of species were least square fitted with the reciprocal of the temperature to get the partial pressure–temperature relation (p–T).

3.1.2 ${{\varvec{k}}}_{{\varvec{i}}}^{\boldsymbol{^{\prime}}}$ from In-Situ Determination from Gas Phase Equilibria

In-situ determination of the instrument calibration constant is possible if a gas phase equilibrium between monomer and dimer species exists over the sample. From the knowledge of equilibrium constant for the reaction

$${\text{dimer }}\left( {{\text{A}}_{{2}} \left( {\text{g}} \right)} \right) \, = {\text{ monomer }}\left( {{\text{2A}}\left( {\text{g}} \right)} \right)$$

(4)

Knowing the data of equilibrium constant $K_{p} =$ $\frac{{p}_{A}^{2}}{{p}_{{A}_{2}}}$, ionisation cross section, isotopic abundance, and multiplier gain and from the measured ion intensities of monomer and dimer, the instrument constant was deduced by following equation:

$$k = \frac{{\left( {\sigma_{A} \gamma_{A} n_{A} } \right)}}{{\left( {\sigma_{A2} \gamma_{A2} n_{A2} } \right)}}\frac{{I_{A2} }}{{I_{A}^{2} }}\frac{1}{T}K_{p}$$

(5)

By measuring the ion intensities of ${A}^{+}$ and ${A}_{2}^{+}$ and using the ionisation cross sections of A(g) and A₂(g) and detector responses for ${A}^{+}$ and ${A}_{2}^{+}$, the pressure calibration constant can be determined and the same was employed to calculate the partial pressures from the measured ion intensities over the pure and ternary system.

The pressure-temperature relation for a given run was obtained from the least-squares fitting of logarithm of partial pressures with reciprocal of the temperatures. Figure 3 shows the combined 2nd law plot of log(p_i/Pa) versus 1/T for all the species obtained over LiCl(cr), KCl(cr) and ternary salt using in-situ calibration method. The recommended p-T relations were obtained by plotting all the data points from the individual runs. The uncertainties in slopes and intercepts are the standard deviations. Tables 2and3 shows the p–T relations deduced for various species over LiCl(cr), KCl(cr) and LiCl-KCl-UCl₃ ternary salt from in-situ and external calibration methods using silver, from individual runs, recommended equation and pressure calculated at mean experimental temperature. Partial pressure data deduced from in-situ calibration method are higher by a factor 2.3 for LiCl species compared to those derived through external calibration method (both for pure salt and ternary salt). In the case of KCl species, partial pressure deduced using in-situ calibration is higher by 35.5 and 23.4 for monomer and dimer respectively compared to those obtained from silver calibration method. Ion intensity measurements were conducted at lower electron energy (13.2 eV) to minimise the contribution due to fragmentation. In the case of in-situ calibration, the mean value of instrument calibration constants (k) obtained from experiments carried out over pure salt (R-1 and R-2) and ternary salt (R-1 and R-2) is used to deduce the pressure calibration constant for respective species. Partial pressures deduced from in-situ calibration can be more reliable than those deduced from external calibration using pure silver, since the pressure calibration constant data also depends on the ionisation cross section of the reference species, reproducibility of Knudsen cell position and day to day variation of ion source emission characteristics and detector responses. The ascendancy of in-situ calibration method is evident from the data of instrument constant obtained over different samples [pure salt (R-1 and R-2) and ternary salt (R-1 and R-2)], at different temperatures and on different days were found to be reproducible. Hence, partial pressure data deduced from in-situ calibration method were used in the 2nd and 3rd law evaluation of reaction enthalpies for various reaction equilibria. There is a good agreement between ${\Delta }_{\mathrm{r}}{\mathrm{H}}_{298.15}^{^\circ }$ data for all the reactions deduced using p–T relations obtained from in-situ method and, those given in the compilation by Barin.^[40]

Fig. 3

Combined 2nd law plots of log(p_i) vs. 1/T for the all the species recorded over pure LiCl(cr), pure KCl(cr) and LiCl-KCl-UCl₃ ternary salt deduced using in-situ method.

Table 2. p-T relations for vapour species over LiCl(cr), KCl(cr) and LiCl-KCl-UCl₃ ternary salt deduced using in-situ calibration method

Table 3. p-T relations for vapour species over LiCl(cr), KCl(cr) and LiCl-KCl-UCl₃ ternary salt deduced from external calibration with silver

At 900 K, the partial pressure of KCl(g) is higher than that of K₂Cl₂(g) over pure KCl(cr) and ternary salt by a factor of 2 and 8, respectively. The partial pressures of KCl species over KCl(cr) and ternary salt at 900 K (monomer, dimer, and their ratios) obtained in the present study are presented in Table 4 along with those available in literature.^[18,19] In the case of KCl(cr), the trend in the partial pressures of monomer and dimer obtained in the present study, i.e., (P_m > P_D) is like the values reported in the literature.^[18,19]

Table 4. Comparison of partial pressures of KCl(g) and K₂Cl₂(g) deduced over pure KCl(cr) and ternary salt with those available in literature^[18,19]

At 900 K, partial pressure of Li₂Cl₂(g) over pure LiCl(cr) is higher than that of LiCl(g) by a factor of 3.2 whereas partial pressure of LiCl(g) monomer over ternary salt is higher that of dimer Li₂Cl₂(g) by a factor of 1.3.

3.2 Thermodynamic Quantities

3.2.1 Reaction Enthalpies

From the partial pressures, the equilibrium constant as well as enthalpy changes for the following heterogeneous reaction equilibria were evaluated by 2nd law methods:^[41]

$${\text{LiCl}}\left( {{\text{cr}}} \right) \, = {\text{ LiCl}}\left( {\text{g}} \right)$$

(6)

$${\text{2LiCl}}\left( {{\text{cr}}} \right) \, = {\text{ Li}}_{{2}} {\text{Cl}}_{{2}} \left( {\text{g}} \right)$$

(7)

$${\text{KCl}}\left( {{\text{cr}}} \right) \, = {\text{ KCl}}\left( {\text{g}} \right)$$

(8)

$${\text{2KCl}}\left( {{\text{cr}}} \right) \, = {\text{ K}}_{{2}} {\text{Cl}}_{{2}} \left( {\text{g}} \right)$$

(9)

$$log K_{p} = \frac{A}{T} + B$$

(10)

where $K_{p} = [\frac{{p\left( {products} \right)}}{{p\left( {reac\tan ts} \right)}}]^{v}$; ${\text{p}}^{^\circ } = 10^{5} {\text{ Pa}}$; A = $\frac{{ - \Delta_{{\text{r}}} {\text{H}}_{{{\text{Tm}}}}^{^\circ } }}{{2.303 \times {\text{R}}}}$; B = $\frac{{ - \Delta_{{\text{r}}} {\text{S}}_{{{\text{Tm}}}}^{^\circ } }}{{2.303 \times {\text{R}}}}$

Enthalpy changes of the reactions were also evaluated by using 3rd law method.^[41]

$${\Delta }_{{\text{r}}} {\text{H}}_{298.15}^{^\circ } = - T \left[ {R\ln K_{p} + \Delta \frac{{\left( {G_{T}^{^\circ } - H_{298.15}^{^\circ } } \right)}}{T}} \right]$$

(11)

where ${\Delta }_{{\text{r}}} \frac{{\left( {G_{T}^{^\circ } - H_{298.15}^{^\circ } } \right)}}{T}$ = ${\Delta }_{{\text{r}}} \frac{{\left( {H_{T}^{^\circ } - H_{298.15}^{^\circ } } \right)}}{T}$ + ${\Delta }_{{\text{r}}} {\text{S}}_{{\text{T}}}^{^\circ }$ is the change of the Gibbs energy function and can be obtained from Gibbs energy functions of the constituents involved in the reaction.

Required thermal functions were taken from the reference.^[40] The reaction enthalpies obtained from 2nd law and 3rd law and the recommended values are listed in Tables 5 and 6. The ${\Delta }_{{\text{r}}} {\text{H}}_{298.15}^{^\circ }$ data are compared with those calculated using literature^[40] and are also given in the Tables 5 and 6.

Table 5. 2nd and 3rd law evaluation of the reaction enthalpies, LiCl(cr) = LiCl(g) and KCl(cr) = KCl(g)

Table 6. 2nd and 3rd law evaluation of the reaction enthalpies, 2LiCl(cr) = Li₂Cl₂(g) and 2KCl(cr) = K₂Cl₂(g)

Table 7 shows the comparison of molar enthalpy of sublimation of monomer and dimer species over KCl(cr) and ternary salt deduced from 3rd law methods with the literature.^{[6,16,18,20,21,22,40]} As can be seen from the Table 7, Δ_subH deduced in the present study is in good agreement with the values reported in the literature.^{[6,16,18,20,21,22,40]}

Table 7. Third-law enthalpies of sublimation for potassium chloride

Dissociation energy of dimeric or polymeric species is of fundamental importance and thermochemical and spectroscopy methods are usually employed to deduce these data. In the present study, the dissociation equilibria of the following gas phase reactions were evaluated by using 2nd and 3rd law methods from the partial pressures of monomer and dimer species deduced over pure LiCl (Cr), KCl(cr) and ternary salt:

$${\text{Li}}_{{2}} {\text{Cl}}_{{2}} \left( {\text{g}} \right) \, = {\text{ 2LiCl}}\left( {\text{g}} \right);{\text{ K }} = \frac{{p_{{{\text{LiCl}}\left( {\text{g}} \right)}}^{2} }}{{{ }p_{{Li_{2} Cl_{2} \left( {\text{g}} \right)}} }}$$

(12)

$${\text{K}}_{{2}} {\text{Cl}}_{{2}} \left( {\text{g}} \right) \, = {\text{ 2KCl}}\left( {\text{g}} \right);{\text{K }} = \frac{{p_{{{\text{KCl}}\left( {\text{g}} \right)}}^{2} }}{{{ }p_{{K_{2} Cl_{2} \left( {\text{g}} \right)}} }}$$

(13)

Required thermal functions were taken from the reference.^[40] The 2nd law and 3rd law reaction enthalpies and the recommended values are listed in Table 8. These are compared with those computed using literature^[40] and presented in Table 8.

Table 8. 2nd and 3rd law evaluation of the reaction enthalpies, Li₂Cl₂(g) = 2LiCl(g) and K₂Cl₂(g) = 2KCl(g)

The enthalpies of following pressure-independent reactions were also evaluated by using 3rd law method. The equilibrium constants for these reactions are given below:

$${\text{LiCl}}\left( {{\text{cr}}} \right) \, + {\text{ LiCl}}\left( {\text{g}} \right) \, = {\text{ Li}}_{{2}} {\text{Cl}}_{{2}} \left( {\text{g}} \right);{\text{K }} = \frac{{{\text{p}}_{{Li_{2} Cl_{2} \left( {\text{g}} \right)}} }}{{{\text{a}}_{{{\text{LiCl}}\left( {{\text{cr}}} \right)}} { } \times {\text{ p}}_{{{\text{LiCl}}\left( {\text{g}} \right)}} }}$$

(14)

$${\text{KCl}}\left( {{\text{cr}}} \right) \, + {\text{ KCl}}\left( {\text{g}} \right) \, = {\text{ K}}_{{2}} {\text{Cl}}_{{2}} \left( {\text{g}} \right); {\text{K }} = \frac{{{\text{p}}_{{K_{2} Cl_{2} \left( {\text{g}} \right)}} }}{{{\text{a}}_{{{\text{KCl}}\left( {{\text{cr}}} \right)}} { } \times {\text{ p}}_{{{\text{KCl}}\left( {\text{g}} \right)}} }}$$

(15)

The auxiliary thermal functions required are taken from the literature.^[40] The results are given in Table 9.

Table 9. Third law evaluation of pressure-independent reactions, LiCl(cr) + LiCl(g) = Li₂Cl₂(g) and KCl(cr) + KCl(g) = K₂Cl₂(g)

Due to the corrosive nature of high temperature alkali metal halide vapours, limited number of vaporisation experiments could only be conducted.

4 Conclusions

Vaporisation studies on LiCl-KCl-UCl₃ ternary salt were carried out by using KEMS for the first time in the temperature range of 715-913 K. LiCl(g) and Li₂Cl₂(g) were possible major vapour species over LiCl(cr). KCl(g) and K₂Cl₂(g) were the major vapour species ascertained over pure KCl(cr). Based on neutral species over pure LiCl(cr) and KCl(cr) and mass numbers of ions, LiCl(g), Li₂Cl₂(g), KCl(g), K₂Cl₂(g) and UCl₃(g) were assumed to be possible major vapour species over LiCl-KCl-UCl₃ ternary salt. Temperature dependence measurements of intensities of ionic species over pure LiCl, pure KCl and LiCl-KCl-UCl₃ ternary salt system were carried out in the range 715-913 K. Partial pressure–temperature relations for vapour species were derived using (1) in-situ calibration from pressure dependent equilibrium as well as (2) using pure silver as external calibration. Using p-T relations various condensed phase-gas phase and gas phase reactions were evaluated by 2nd and 3rd law methods. The dissociation equilibria of following gas phase reactions: Li₂Cl₂(g) = 2LiCl(g) and K₂Cl₂(g) = 2KCl(g) were evaluated using the partial pressures of monomer and dimer over pure and ternary salts deduced from the present study. These data were compared with the results of previous literature. Knudsen effusion mass spectrometric studies on LiCl-KCl-UCl₃ ternary salt system were reported for the first time.