Electrical Conductivity of Zirconium Tetrachloride Solutions in Molten Sodium, Potassium and Cesium Chlorides

1 Introduction

The chlorine method is one of the main methods used for zirconium (Zr) production with the extraction of zirconium tetrachloride (ZrCl₄) from ore [1], [2], [3]. Metallic Zr is produced by the electrolysis of ZrCl₄ containing molten salts. To perfect the technological processes of electrodeposition and electrorefining of Zr, the information on the electrical conductivity of the ZrCl₄ solutions in molten alkali metal chlorides is needed. ZrCl₄ is a highly volatile compound and thus molten mixtures, such as MCl-ZrCl₄ (M = alkali metals), generally have a high vapor pressure even at temperatures near the liquidus. However, in such systems, there are two concentration ranges in which the vapor pressure remains below the atmospheric one [1], [2], [3], [4], [5], [6], [7], [8], [9]. These are the eutectic ranges (0–33 mol.% ZrCl₄ and 55–75 mol.% ZrCl₄), which are separated by the congruently melting compound, see, for instance, Figure 1.

Figure 1:

The phase diagram of the NaCl-ZrCl₄ system [5]. The Na₂ZrCl₆ compound separates two eutectic ranges. The diagrams for the KCl-ZrCl₄ and CsCl-ZrCl₄ systems [1], [6], [7], [8] are similar to this diagram. Each diagram illustrates two eutectics.

The electrical conductivities of the MCl-ZrCl₄ melts near the second, lower-temperature eutectic region (55–75 mol.% ZrCl₄) have been studied in our previous works [10], [11] and in [12]. However, to date, there are only very old and fragmentary literature data on the conductivities of the MCl-ZrCl₄ systems near the first eutectic range ([ZrCl₄] <33 mol.%) [13]. Thus, the aim of the current work is to study the electrical conductivities of the molten NaCl-ZrCl₄, KCl-ZrCl₄ and CsCl-ZrCl₄ systems around the first eutectic range ([ZrCl₄] <33 mol.%).

2 Experimental

Commercial NaCl, KCl and CsCl (99.9+%) were vacuum dried and purified by 2–3-fold zone recrystallization. Commercial ZrCl₄ (98+%) was sublimated 2–3 times in a vacuum and once in the atmosphere of pure helium.

To prepare the solutions of the desired compositions, the weighed portions of NaCl, KCl and CsCl were placed together with freshly prepared ZrCl₄ in quartz ampoules, which were then evacuated and soldered. The ampoules were slowly heated for several hours under the temperatures of 820 °C–830 °C, that is, to the temperature above the melting point of the most high-melting component [5], [6], [7], [8]. The ampoules were kept at the maximum temperature for 1–2 h and then slowly cooled to room temperature. All weightings and loadings into the ampoules and measuring cells were performed in a dry box under nitrogen atmosphere with <3 ppm water content.

The electrical conductivity measurements were carried out in the quartz capillary cells with a special design [14]. In this work, we used platinum electrodes. The salt fusion cakes of the given compositions were loaded through a branch pipe (see Fig. 1 in [10]), which was then soldered up. The changes in the composition of molten mixtures caused by the evaporation of volatile ZrCl₄ during the experiments in the cells of this type were insignificant, because conductometric cells were sealed with porcelain rods that were tightly adjacent to the walls of the quartz tubes of the cell [14]. The maximum measurement temperatures were limited to the extent at which the saturated vapor pressure of ZrCl₄ over the melts did not exceed 0.1–0.25 atm [6], [7].

The conductometric cell was heated in an electric resistance furnace equipped with a massive metallic block in order to achieve a uniform temperature. The temperature was controlled by a high-precision temperature regulator VRT-3 with stability that is greater than 1 K. The temperature was measured with a Pt/Pt-Rh thermocouple with an accuracy of ±1 °C. The thermocouple hot junction was located near the measuring capillary of the cell. The melt resistance was measured using an R-5058 AC bridge at 10 kHz. It was established by preliminary measurements (see, for example, [11], [15]) that the melt electrical conductivity at the values exceeding 8 kHz is not frequency dependent. The Na, K, Cs and Zr concentrations in the solutions under investigation were defined before and after the experiments. The maximum discrepancy did not exceed 2 %. The analysis was performed using an atomic emission spectrometer OPTIMA 4300 DV (Perkin Elmer, USA) with inductively coupled plasma.

The conductometric cells were calibrated against the following salt-solvents: NaCl, KCl, or CsCl using available data [16], [17]. The values of our cell constant ranged from 94 to 108 cm⁻¹. The total error in the electrical conductivity measurements of the molten MCl-ZrCl₄ systems did not exceed 2 %. The design of the conductometric cell and measurement procedures are described in more detail in our previous papers [10], [14].

3 Results

The results of the electrical conductivity measurements of the molten NaCl-ZrCl₄, KCl-ZrCl₄ and CsCl-ZrCl₄ solutions are shown in Figures 2–4, respectively. In all cases, the electrical conductivity was found to increase as the temperature increased. The upper (higher-temperature) almost straight sections of the polytherms correspond to the electrical conductivity of the homogeneous melts. The temperature dependence of the specific conductivity of the homogeneous melts is approximated by the following equation:

Figure 2:

The specific conductivities of the molten and heterogeneous (melt + solid phase) NaCl-ZrCl₄ mixtures. The dotted lines are adopted from [13]. The concentration is expressed in mol.%.

Figure 3:

The specific conductivities of the molten and heterogeneous (melt + solid phase) KCl-ZrCl₄ mixtures.

Figure 4:

The specific conductivities of the molten and heterogeneous (melt + solid phase) CsCl-ZrCl₄ mixtures.

The A, B and C values and the root-mean-square scatter of the experimental points Δ are given in Table 1. As the temperature decreases, the inflection points appear on the polytherms of conductivity, after which the conductivity decreases faster. These inflection points correspond to the liquidus temperatures and their values are reported in Table 2. The second inflection point corresponds to the solidus temperature and the melting point of the eutectic. For the NaCl-ZrCl₄ system, we found that T_eut. = 818 K. The literature data report the following values for the same system: T_eut. = 812 K [1], [8] and 821 K [5], [6]. For the CsCl-ZrCl₄ system, we found that T_eut. = 858 K. In the published data for the same system, the following values are provided: T_eut. = 845 K [8] and 861 K [7].

The changes in the conductivities of the NaCl-ZrCl₄, KCl-ZrCl₄ and CsCl-ZrCl₄ solutions depending on the ZrCl₄ concentration are shown in Figures 5–7, respectively. In all cases, the conductivity decreases as the ZrCl₄ concentration increases.

4 Discussion

Molten alkali metal chlorides are almost entirely ionic liquids. In such melts, the volatile ZrCl₄ up to the concentration of about 33 mol.% of ZrCl₄ is considered part of the complex anionic group ZrCl62− [2], [18], [19]. In a series of salt-solvents from LiCl to CsCl, the strength of these complexes increases quickly as a result of a decrease in the counter-polarizing effect of alkali cations on chloride anions. As the complexes strengthen, the ZrCl₄ vapor pressure above the solutions decreases more and more [1], [2], [3], [4], [5], [6], [7].

Figure 5:

The specific conductivity isotherms of the ZrCl₄ solutions in molten NaCl. 933 K and 953 K represent our data. The dotted lines are adopted from [13].

Figure 6:

The specific conductivity isotherms of the ZrCl₄ solutions in molten KCl.

Figure 7:

The specific conductivity isotherms of the ZrCl₄ solutions in molten CsCl.

At the temperatures above the critical point of ZrCl₄ (T_cr = 778 K, P_cr = 58 atm [2]) we deal with the solutions of gaseous ZrCl₄ in the molten alkali metal chlorides and not with their mixtures with a hypothetically liquid ZrCl₄. When interacting with the NaCl, KCl or CsCl melts, the ZrCl₄ molecules ionize and form strong octahedral ZrCl62− anions [2], [18], [19]. Thus, as the concentration of ZrCl₄ in the molten alkali metal chlorides increases, the concentration of the less-mobile ZrCl62− complexes, which contain six strongly bound chloride anions, increases. This leads to a decrease in the concentration of electric charge carriers. The proportions of Na⁺, K⁺, Cs⁺ and, especially, the Cl⁻ ions decreases and, accordingly, the conductivity of the melts decreases. The experimental results (Figs. 5–7) are completely consistent with this model. The relative decrease in electrical conductivity caused by the ZrCl₄ concentration growth is more pronounced (Tab. 2) when the electrical conductivity of individual molten alkali chloride (κ_NaCl > κ_KCl > κ_CsCl [16]) is higher.

The thermodynamic studies of the MCl-ZrCl₄ systems and related solutions [4], [6] showed that the ZrCl₄ activity coefficient in the alkali metal chloride melts is much less than unity and that it decreases rapidly as the ZrCl₄ concentration decreases in the range of 33–5 mol.%. As the solutions are diluted, the ZrCl62− complexes become increasingly isolated from each other, it leads to the ever stronger hardening of the Zr⁴⁺-Cl⁻ bonds in these complexes. The enhancement of complexation is manifested in a pronounced negative curvature of the conductivity isotherms of quasi-binary MCl-ZrCl₄ (M = Na, K, Cs) molten mixtures, as illustrated in Figures 5–7, respectively.

As the temperature rises, the mobility of ions increases. This leads to the increase in the electrical conductivity, as shown in Figures 2–4. In Figures 2 and 5, the results of our electrical conductivity measurements are compared with the data for the NaCl-ZrCl₄ system obtained in [13]. In that work, a cylindrical (test tube-shaped) quartz cell with a measuring capillary and platinum electrodes was used. Under the conditions of their experiments (high temperatures and ZrCl₄ concentration), the ZrCl₄ vapor pressure above the solutions could reach several atmospheres [6], [7]. Therefore, it is very likely that, during the measurement of electrical conductivity, the melt composition changed continuously because of the rapid evaporation of ZrCl₄. The authors of a past work [13] have reported that they analyzed the electrolyte composition only after performing the experiments. The analysis of the initial composition was not performed, because the melt composition changed during the experiment. In addition, as the experiment was performed under the chlorine atmosphere, the measurements were carried out within the shortest possible time due to the possible corrosion of the platinum electrodes. Nevertheless, despite the quantitative differences, the tendencies, as identified by the authors of [13], agree qualitatively with ours (see Figs. 2, 5 and also Supplementary materials). At the moment, there is a lack of any other data on the electrical conductivity of MCl-ZrCl₄ (MCl-alkali metal chloride) high-temperature melts.

5 Conclusion

The electrical conductivities of the NaCl, KCl and CsCl melts containing volatile ZrCl₄ were measured at the ZrCl₄ concentration reaching 25–30 mol.%. In this work, the conductivities of the KCl-ZrCl₄ and CsCl-ZrCl₄ solutions were measured for the first time. It is found that the electrical conductivity increases as the temperature increases, and the electrical conductivity of the melt decreases as the concentration of ZrCl₄ increases and in the row of salt-solvents from NaCl to CsCl. Moreover, the values of the electrical conductivity of the melts studied in this work were significantly higher (0.6–3.5 S/cm) than those of the previously studied low-melting mixtures of ZrCl₄ with the same alkali metal chlorides (0.1–0.5 S/cm) with high ZrCl₄ content (55–75 mol.%) [10], [11], [12]. Such mixtures have great potential for industrial use [1], [2], [3], [9], [10].