Article pubs.acs.org/jced
Cite This: J. Chem. Eng. Data XXXX, XXX, XXX−XXX
Liquidus Temperature and Electrical Conductivity of Molten Eutectic CsCl-NaCl-KCl Containing ReCl4 Alexey Rudenko,† Andrey Isakov,*,† Alexey Apisarov,† Alexander Chernyshev,‡ Olga Tkacheva,‡ and Yurii Zaikov‡ †
Institute of High Temperature Electrochemistry, Academicheskaya St. 20, Ekaterinburg 620990, Russia Ural Federal University, Ekaterinburg 620002, Russia
J. Chem. Eng. Data Downloaded from pubs.acs.org by UNIV DE BARCELONA on 02/05/19. For personal use only.
‡
ABSTRACT: The liquidus temperatures and electrical conductivity of the (CsCl-NaCl-KCl)eut-ReCl4 melts, feasible for Re production, were studied by thermal analysis and impedance spectroscopy. A part of the quasi-binary phase diagram for [CsCl-KCl-NaCl]eut-ReCl4 in the ReCl4 concertation range of 0−7.61 mol % was compiled on the basis of obtained thermal analysis data. ReCl4 is most likely to dissolve in molten eutectic CsCl-KCl-NaCl with the formation of the ReCl62− ions. Compound Cs2ReCl6 was identified in the frozen samples by XRD analysis. The electrical conductivity of molten mixture (CsCl-KCl-NaCl)eut.-ReCl4, measured in the cell with the coaxial glassy carbon electrodes, was found to decrease with increasing ReCl4 content.
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INTRODUCTION
A promising low-temperature molten medium for producing the rhenium coatings is an electrolyte based on the CsCl-KClNaCl eutectic. This composition has a relatively low melting point (753 K),5 and it is less aggressive than the fluoride melts. Besides, the eutectic does not contain hygroscopic components and therefore has a low sensitivity to humidity.6 The principal options of obtaining the rhenium coatings are described in the literature.7 The process proceeds in the eutectic CsCl-KCl-NaCl containing rhenium(IV) in the form of an alkali hexachlororhenate. It was reported that a continuous deposit was obtained on a graphite substrate at 1123 K. The CsCl-KCl-NaCl system is quite in demand, and there is a significant number of studies related to the electroreduction of metals from molten electrolytes based on the CsCl-KClNaCl system.8−14 However, data related to the physicochemical properties of these melts are almost absent. Specifically, information regarding the electrical conductivity, in particular, the electrical conductivity of the CsCl-KCl-NaCl-ReCl4 melts, has not been found in the literature. Only the equations describing the temperature dependence of the electrical conductivity in CsCl-KCl (in the temperature range of 1030−1150 K) and CsCl-NaCl (1050−1150 K) are available.15 To improve the process of producing the continuous rhenium coatings from the CsCl-KCl-NaCl melts containing rhenium compounds, it is necessary to determine the region of homogeneity of the melts because the liquidus temperature of
The electroreduction of metals in molten salt media is a modern energy-efficient process for manufacturing materials and products. Moreover, it is possible to obtain continuous metal coatings with a density close to the physical density of the material. At the same time, the formation of the coating (or article) takes place at a temperature much lower than the melting point of the material. The techniques of electroforming a number of refractory and noble metals from the molten salts have been developed.1 The electrodeposition typically is carried out in the medium of the molten alkali halides containing potential-determining ions, participating in the electrode process. Recently, the method of high-temperature electroforming in molten salts has become topical in connection with the development of new materials, based on iridium and rhenium, which are used in aerospace engineering.2−4 Such materials can be operated in oxidizing environments at temperatures of up to 2200 °C, which opens the possibility of using new types of ecologically friendly rocket fuels. These developments are highly in demand and in the future can be introduced into the production of the combustion chambers for thruster engines. In general, the fundamentals of obtaining the Ir−Re coatings are fairly well developed. One of the main and most laborious stages of producing the Ir−Re composites in the alkali chlorides melts containing the rhenium compounds is the formation of continuous rhenium coatings.4 The process of obtaining the rhenium coating can proceed for up to several days, depending on the desired thickness. It is believed that changes in the electrolyte properties and concentration do not occur. © XXXX American Chemical Society
Received: August 27, 2018 Accepted: January 21, 2019
A
DOI: 10.1021/acs.jced.8b00758 J. Chem. Eng. Data XXXX, XXX, XXX−XXX
Journal of Chemical & Engineering Data
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Table 1. Specifications of Chemicals chemical name
source
initial mass fraction purity
purification method
cesium chloride potassium chloride sodium chloride rhenium composition -1b composition -2b composition -3b composition -4b composition -5b
Vekton, CJSC Vekton, CJSC Uralkalii, OJSC Pobedit, OJSC synthesis synthesisc synthesisc synthesisc synthesis
0.9999 0.9990 0.999 0.9999
none none none none none none none none none
a
final mass fraction purity
analysis method
0.999
ICPa ICPa ICPa ICPa ICPa
0.999
ICPa
b
c
Inductively coupled plasma atomic emission spectrometry. Composition of electrolytes is given in Table 2 Obtained by diluting composition 5.
Table 2. Compositions of Electrolytes CsCl-KCl-NaCl-ReCl4 CsCl
KCl
NaCl
ReCl4
no.
mol %
wt %
mol %
wt %
mol %
wt %
mol %
wt %
1 2 3 4 5
45.50 44.79 44.04 43.08 42.04
68.15 65.15 62.15 58.55 54.94
24.50 24.12 23.71 23.19 22.63
16.25 15.53 14.82 13.96 13.10
30.00 29.54 29.04 28.41 27.72
15.60 14.91 14.23 13.40 12.58
0.00 1.55 3.21 5.32 7.61
0.00 4.41 8.80 14.09 19.38
in a glassy carbon container. The container was covered with a lid having a hole for a glassy carbon tube, through which a gas could flow. The assembly was heated to 1000 K, which is 250 K higher than the eutectic melting point, in order to ensure the melt homogeneity because the solubility of the rhenium chloride in this melt is unknown. The chlorine gaseous was obtained by electrolysis of the molten PbCl2. The chlorine was blown through the melt at a rate of 1200 cm3/h (n.c.) for 24 h. Under conditions of the chlorine excess, the following reaction is likely to occur
the CsCl-KCl-NaCl-ReCl4 system may vary depending on the ReCl4 concentration. Until now, the mechanism of the rhenium chloride interaction with the multicomponent alkali chloride mixtures has not been given significant attention. According to Druce,16 during the crystallization of the CsCl-KCl-NaCl-ReCl4 melt the appearance of the Me2ReCl6 phases (where Me is Cs, K, or Na) can be expected. The composition of the crystallizing phase will determine the processes occurring during melting and, in particular, the value of the liquidus temperature, which is important for the practical application of these melts. However, in the works7,17 devoted to the Re electrodeposition in the multicomponent chloride melts containing rhenium chloride, the question of which MeCl (Me is Cs, K, and Na) interacts with the ReCl4 is not considered. For example, the electrodeposition of Re from the NaCl-KCl-ReCl4 melts within the temperature range of 680−970 °C was studied in ref 17. The rhenium ions were set to the NaCl-KCl melt by the chlorination of the metallic rhenium. It was assumed that the Me2ReCl6 component can form in the NaCl-KCl-ReCl4 melt. In ref 7, K2ReCl6 was added to ternary molten system NaClKCl-CsCl in order to carry out the Re electrodeposition. Thus, the purpose of this work was to determine the effect of ReCl4 on the liquidus temperature in the (CsCl-KCl-NaCl)eutReCl4 system, to define which compound is formed during crystallization, and to reveal the electrical conductivity in the molten multicomponent chloride mixtures with different contents of ReCl4.
2MeCl + 2Re + 5Cl 2 = 2MeReCl 6
(1)
where Me is an alkali metal. Then, the container with the salt mixture was cooled to room temperature. A minor amount of sublimates on the container walls was collected and returned to the mixture, which was melted again in the atmosphere of an inert gas (Ar) that blew the melt for 6 h. In this case, the following equilibrium was established in the melt: 2MeReCl 6 + 2MeCl = 2Me2ReCl 6 + Cl 2
(2)
The assumption that rhenium has a stable degree of oxidation of +4 in the chloride melts and most likely exists in the form of complex ion ReCl62− was made on the basis of the literature data.16,17 When the composition of the electrolytes was calculated, the concentration of the dissolved rhenium in the form of the ReCl4 was taken into account. The Re content in the melt was determined by the ICP analysis using the optical emission spectrometer with inductively coupled plasma (Thermo Scientific iCAP 6300 Duo). The obtained composition (CsCl-NaCl-KCl)eut-ReCl4 contained 11.0 wt % of the rhenium element. The original composition was diluted with the CsCl-KCl-NaCl eutectic in order to attain melts with lower contents of rhenium (2.5, 5.0, and 8.0 wt %). The compositions of electrolytes for studying the liquidus temperature and the specific electrical conductivity are given in Table 2.
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EXPERIMENTAL SECTION Materials. CsCl (99.99%) and NaCl (99.9%) (both supplied by Vekton) and KCl (99.9%) (Uralkalii) of high purity were used for the electrolyte preparation. The individual salts were mixed in a certain ratio in order to obtain a eutectic composition and melted in a glassy carbon container in an Ar atmosphere. The specifications of the chemicals are given in Table 1. The prepared eutectic (about 350 g) together with a metal rhenium powder of 99.99 wt % purity (about 45 g) was placed B
DOI: 10.1021/acs.jced.8b00758 J. Chem. Eng. Data XXXX, XXX, XXX−XXX
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X-ray diffraction (Rigaku D/MAX-2200VL/PC X-ray diffractometer) was used to study the composition and structure of the samples. Experimental Cell. Currently, two types of the electrochemical cells are widely used to measure the electrical conductivity of molten salts: the capillary type18 and the cell with parallel electrodes (Eger type).19 The design of both types of cells assumes the employment of metal electrodes. That is not acceptable for our study because a metallothermic reduction of the rhenium from its salts can occur. The presence of the Re metal in the melt does not make it possible to use Pt because of its ability to form Pt−Re alloys. The Pt− Re alloy on the surface of the Pt electrode will change the resistance of the electrode. Moreover, the process of alloy formation will continue for the duration of the experiment. The use of other non-noble metals can likely lead to a decrease in the Re concentration in the melt due to the metallothermic reduction and, consequently, to an error in the measurement of the electrical conductivity. In this work, a coaxial cell with the glassy carbon electrodes was developed. The glassy carbon is an indifferent material with respect to the complex metal ions in molten salts: it has relatively good conductivity, a small thermal expansion coefficient, and is not subject to a change in geometry as a result of the thermal fields. The electrochemical cells made of graphite materials have been successfully used to measure the electrical conductivity in alkali and alkali earth chlorides.20 A schematic diagram of the cell with the coaxially positioned electrodes for measuring the electrical conductivity of molten salts is shown in Figure 1. The same cell was used to determine the liqudus temperature of the tested electrolyte. The measuring device was a quartz container tightly covered with a fluoroplastic cap, protected from thermal radiation by the graphite shields. A glassy carbon crucible with the tested melt (140−150 g) was mounted on a graphite ring inside the quartz container. A unit of two glassy carbon electrodes (18 mm o.d., 14 mm i.d, 4 mm diameter of the central electrode, and 5 mm the distance between the inner wall of the outer electrode and the central wall), insulated by the BN separator in the lower part and a fluoroplastic lid at the top, and the Pt/Pt−Rh thermocouple in an alumina case was fixed in the fluoroplastic cap in such a way that they could be moved up and down while maintaining the inert atmosphere in the cell. The tightness of the measuring unit was ensured by means of the seals made of vacuum rubber. The assembled device was placed in a furnace and heated to the required temperature under an argon atmosphere. After that, the thermocouple and the electrode unit were immersed in the melt and the electrical conductivity and the cooling curves were determined in a predetermined temperature range. When the electrical conductivity was measured, the electrodes were always immersed in the melt at a constant depth of 15 mm. (The melt height in the crucible was 40 mm.) The depth of immersion was counted from the point of a contact of the electrodes with the melt, which was determined as follows. The electrodes, while above the melt were energized using a ZahnerElectric instrument, slowly descended until the sinusoidal “AC current - time” curve appears on the monitor. As soon as this curve acquired the regular shape, the device reported that the electrical circuit was closed (resistance from kΩ decreases to Ω).
Figure 1. Diagram of the cell for measuring the electrical conductivity and liquidus temperature: (1) quartz container, (2) graphite ring, (3) glassy carbon crucible, (4) thermocouple (Pt/Pt-Rh), (5) seals made of vacuum rubber, (6) alumina case, (7) fluoroplastic, (8) current lead (Mo), (9) graphite shields, (10) glassy carbon tube, (11) separator (BN), (12) glassy carbon rod, and (13) tested melt.
Measurement Procedure. The crystallization temperature was determined by the thermal analysis (TA) based on recording the thermal effects. The measurements were carried out during either the heating or cooling of the salt. As a rule, the difference in the values of the registered temperature at the points of the phase transitions obtained during both temperature cycles did not exceed 5°. The cooling and heating curves were registered in the coordinates of the “thermoEMF-time” using an APPA 109N multimeter. The temperature was automatically recorded at a frequency of 1 measurement per second. The average values of the cooling and heating rates were 2.8 and 7.1 K per minute, respectively. The magnitude of the total error in measuring the temperature by the TA method, estimated in accordance with the All-Union state standard, did not exceed ±5°. The electrical conductivity was measured by impedance spectroscopy using the ZAHNER-Elektrik IM6E impedance measurement unit in the frequency range of the alternating current from 100 Hz to 105 Hz with an amplitude of 5 mV. The resistance of the melt was determined in the impedance diagram from the impedance active-part at the point of intersection of the curve with the abscissa axis. The electrical conductivity was calculated according to κ = K /R
(3) −1
where κ is the electrical conductivity (Sm·cm ), K is the cell constant (cm−1), and R is the resistance of the melt (Ω). The C
DOI: 10.1021/acs.jced.8b00758 J. Chem. Eng. Data XXXX, XXX, XXX−XXX
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maximum relative error in determining the conductivity, calculated according to the All-Union state standard, is 5.5%. The electrochemical cell was calibrated by molten CsCl during heating and cooling in the temperature range of 933− 1145 K (Figure 2). The specific conductivity data for molten CsCl were taken from ref 15.
Table 3. Experimental Values of the Electrical Conductivity of (CsCl-KCl)eut at an Ar Pressure of 0.106 MPaa electrical conductivity, S·cm−1
u(k), S·cm−1
947
2.25
0.11
976 997 1016 1038 1062 1080 1105 1126 1136
2.31 2.37 2.43 2.48 2.58 2.67 2.71 2.75 2.85
0.12 0.13 0.12 0.12 0.14 0.13 0.14 0.14 0.14
no.
composition, mol % temp, K
1
CsCl(37.5)-KCl (62.5)
2 3 4 5 6 7 8 9 10
a Standard uncertainties are u(T) = 0.5 K and u(p) = 10 kPa. The standard uncertainties for electrical conductivity u(k) are given.
Figure 2. Cell constant obtained by calibration with molten CsCl.
The cell constant slightly increased with temperature: its change was 0.15 cm−1 in the temperature range of 170°. The constant of the coaxial cell is determined not only by the distance between the electrodes, the cross-sectional area of the electrolyte column, but also by the immersion depth of the electrodes. If the operating temperature increases by 100− 200°, then the coefficient of thermal expansion of a material such as glassy carbon remains almost unchanged but the volume or level of molten electrolyte slightly increases, which affects the cell constant. The cell temperature dependence (Figure 2) can be described by the equation K = 0.13 + 7.53 × 10−4T
Figure 4. Heating curves in the [CsCl-KCl-NaCl]eut-ReCl4 system with different contents of ReCl4 (mol %): (1) 1.55, (2) 3.21, (3) 5.32, and (4) 7.61 for (a) the liquidus area (liquidus temperature is 873 K (curve 1); 896 K (curve 2); 938 K (curve 3); and 988 K(curve 4)) and (b) the solidus area
(4)
the X axis has no the numerical values. The inflection points on the curves were assigned to the liquidus and solidus temperatures. To make the points of inflection more visible, the temperature regions corresponding to the liquidus and solidus are presented by the different charts (Figure 4A,B). According to these results, a part of the quasi-binary phase diagram [CsCl-KCl-NaCl]eut-ReCl4 was composed in the ReCl4 concentration range of up to 7.61 mol %. It is represented in Figure 5 and listed in Table 4. The melting
2
where T is the temperature (K) and R = 0.97. Equation 4 was taken into account in calculating the electrical conductivity of the tested electrolytes at different temperatures. Preliminarily, the cell operation was tested on the molten eutectic CsCl-KCl with known values of the electrical conductivity.15 The electrical conductivity of the molten (CsCl-KCl)eut, measured and calculated by the equation given in ref 15, is plotted in Figure 3 and listed in Table 3. The data matches within ±2.5%.
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RESULT AND DISCUSSION Liquidus Temperature. The heating curves obtained in the [CsCl-KCl-NaCl]eut-ReCl4 system with different contents of ReCl4 are collected in one chart (Figure 4). For this reason,
Figure 5. Quasi-binary phase diagram [CsCl-KCl-NaCl]eut-ReCl4.
point of eutectic CsCl(45.5)-KCl(24.5)-NaCl(30.0) was found to be 753 K, which coincides with the available data.5 The liquidus temperature in the [CsCl-KCl-NaCl]eut-ReCl4 system increases with increasing ReCl4 concentration. Apparently, the anticipated eutectic point in the system is omitted, which is reflected by the dashed line in Figure 5.
Figure 3. Electrical conductivity of the molten (CsCl-KCl)eut: points, experimentally measured; straight line, calculation with the equation in ref 15. D
DOI: 10.1021/acs.jced.8b00758 J. Chem. Eng. Data XXXX, XXX, XXX−XXX
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Table 4. Temperatures of Liquidus and Solidus in [CsClKCl-NaCl]eut-ReCl4 (0−7.61 mol %) at an Ar Pressure of 0.106 MPaa no. 1 2 3 4 5
composition [CsCl-KClNaCl]eutReCl4
Table 5. Equations for (CsCl-KCl-NaCl)eut-ReCl4 coefficients in κ = aT − b
u(L), K
solidus temp, K
u(S), K
no.
ReCl4 content, mol %
755
5.00
714
4.00
875 901 940 989
5.04 5.09 5.08 5.00
720 727 725 714
4.03 4.07 4.06 4.00
1 2 3 4 5
0.00 1.55 3.21 5.32 7.61
content of ReCl4, mol %
liquidus temp, K
0.00 1.55 3.21 5.32 7.61
a × 10−3 b × 10−3 2.20 2.15 2.10 2.07 2.15
508.70 703.74 711.18 777.43 960.45
R2
temperature range of applicability, K
0.9989 0.9969 0.9949 0.9948 0.9956
804−1146 873−1142 896−1155 938−1153 988−1158
a
The standard uncertainties for liquidus temperatures u(L), with solidus temperatures u(S) given.
To determine the mechanism of the ReCl4 interaction with the molten ternary system during the crystallization of CsClKCl-NaCl-ReCl4, a thermodynamic analysis was performed. It was established, that the following reactions are possible: 2NaCl + ReCl4 = Na 2ReCl 6, ΔG1000K = −43.5 kJ ·mol−1 (5) −1
2KCl + ReCl4 = K 2ReCl 6, ΔG1000K = − 82.8 kJ·mol
(6)
2CsCl + ReCl4 = Cs2ReCl 6, ΔG1000K = − 102.9 kJ·mol−1 (7)
Figure 7. Temperature dependence of the specific electrical conductivity for the following electrolytes (mol %): (1) CsCl(45.5)-KCl(24.5)-NaCl(30.0), (2) CsCl(50.0)-NaCl(50.0),15 (3) CsCl(50.0)-KCl(50.0), 15 (4) CsCl(44.79)-KCl(24.12)NaCl(29.54)-ReCl 4 (1.55); (5) CsCl(44.04)-KCl(23.71)NaCl(29.04)-ReCl 4 (3.21); (6) CsCl(43.08)-KCl(23.19)NaCl(28.41)-ReCl 4 (5.32); (7) CsCl(42.04)-KCl(22.63)NaCl(27.72)-ReCl4(7.61).
The ΔG of reactions 5−7 is negative, and its values decrease with an increase in the radius of the alkali ion in the series Na, K, Cs. Thus, reaction 7 predominantly proceeds to form Cs2ReCl6. This is confirmed by the XRD analysis of frozen samples CsCl-KCl-NaCl-ReCl4. The diffraction peaks were identified for the alkali chlorides and Cs2ReCl6. The XRD diagram is presented in Figure 6.
the fact that the electrical conductivity of compositions CsClKCl, CsCl- NaCl, and CsCl-KCl-NaCl (Figure 7, curves 1−3) have comparable values. However, the electrical conductivity of the ternary system cannot be additively calculated from the electrical conductivities of the binaries. The cumulative interaction, especially in the ternary system, is rather difficult. This is confirmed by the fact that the melts have different melting points. The comparable melts contain up to 50.0 mol % CsCl; however, they have a significant difference in the liquidus temperatures: 853 and 903 K for compositions (mol %) CsCl (50.0)-NaCl(50.0) and CsCl (50.0)-KCl(50.0), respectively. For original melt CsCl (45.5)-KCl (24.5)-NaCl (30.0), the melting point was found to be 753 K. The obtained temperature dependences of the electrical conductivity in the CsCl-KCl-NaCl-ReCl4 melts are approximated by the linear regression
Figure 6. XRD pattern of frozen system CsCl-KCl-NaCl-ReCl4.
Therefore, the increase in the ReCl4 content from 0 to 7.61 mol % in molten eutectic CsCl-KCl-NaCl resulted in the increase in the liquidus temperature from 753 to 988 K, which is likely due to the formation of the compound. Electrical Conductivity. The obtained results regarding the solubility of ReCl4 in the CsCl-KCl-NaCl melt allowed us to determine the homogeneity ranges in which no first-order phase transitions occur. The electrical conductivity was measured within the temperature ranges of applicability, as indicated in Table 5. The temperature dependence of the specific electrical conductivity for the CsCl-KCl-NaCl-ReCl4 electrolytes with different ReCl4 contents is presented in Figure 7 and given in Table 6. There are also results, borrowed from the literature,16 for equimolar mixtures CsCl-KCl and CsCl-NaCl. It is known21 that the charge transfer predominantly occurs through the cation sublattice of the melt. This correlates with
κ = (aT − b)
(8) −1
where κ is the specific electrical conductivity (S·cm ), a and b are the empirical coefficients, and T is temperature (K). The values of coefficients a and b are presented in Table 2. The temperature dependences have almost the same slope (i.e., the average temperature coefficient a for all compositions is (2.134 ± 0.004) × 10−3). The electrical conductivity of the CsCl-KCl-NaCl-ReCl4 melts with different ReCl4 contents at temperatures 988, 1038, and 1088 K is plotted in Figure 8. E
DOI: 10.1021/acs.jced.8b00758 J. Chem. Eng. Data XXXX, XXX, XXX−XXX
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Table 6. Experimental Values of the Electrical Conductivity of Various Electrolytes at an Ar Pressure of 0.106 MPaa composition, mol %
temp, K
electrical conductivity, S·cm−1
CsC(45.5)-KCl(24.5)-NaCl(30.0)
803 834 870 907 941 976 990 998 1022 1031 1073 1114 1124 1141 1146 892
1.27 1.35 1.43 1.50 1.60 1.64 1.69 1.72 1.73 1.79 1.92 2.01 1.98 2.01 2.02 1.21
0.08 0.07 0.07 0.08 0.08 0.08 0.10 0.09 0.09 0.09 0.10 0.10 0.10 0.11 0.10 0.06
918 948 985 1016 1048 1087 1115 1142 899
1.27 1.32 1.42 1.49 1.56 1.63 1.68 1.75 1.16
0.06 0.07 0.07 0.07 0.09 0.08 0.09 0.09 0.06
926 963 993 1033 1065 1103 1129 1155 938
1.22 1.30 1.39 1.47 1.54 1.61 1.65 1.69 1.16
0.06 0.07 0.07 0.07 0.08 0.08 0.09 0.08 0.06
975 1005 1033 1068 1100 1130 1153 995
1.23 1.31 1.38 1.44 1.51 1.55 1.60 1.18
0.06 0.07 0.07 0.07 0.08 0.08 0.08 0.06
1032 1063 1105 1130 1158
1.24 1.33 1.42 1.47 1.52
0.06 0.10 0.07 0.07 0.08
CsCl(44.79)-KCl(24.12)NaCl(29.54)-ReCl4(1.55)
CsCl(44.04)-KCl(23.71)NaCl(29.04)-ReCl4(3.21)
CsCl(43.08)-KCl(23.19)NaCl(28.41)-ReCl4(5.32)
CsCl(42.04)- KCl(22.63)NaCl(27.72)- ReCl4(7.61)
u(k), S·cm−1
Figure 8. Electrical conductivity of the CsCl-KCl-NaCl-ReCl4 melts versus the ReCl4 content at the following temperatures: (⧫) 1088 K, (■) 1038 K, and (▲) 988 K.
ions appear in the melt, the charge of which must be compensated for by the alkali ions.
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CONCLUSIONS Thus, the concentration and temperature ranges for the optimization of the electrochemical process of producing rhenium in molten eutectic CsCl-KCl-NaCl were determined. The ReCl4 additions to the CsCl-KCl-NaCl melts were found to increase the liquids temperature and decrease the electrical conductivity. The liquidus temperatures of the (CsCl-KClNaCl)eut‑(0−7.61 mol %) ReCl4 system are in the range of 753−988 K. The electrical conductivity of the (CsCl-KClNaCl)eut-ReCl4 melts was determined in the temperature range of homogeneous solutions. The electrical conductivity temperature regression obtained in the [CsCl-KCl-NaCl]eut-ReCl4 melts can be described by the linear equations. ReCl4 is most likely to exist in the CsCl-KCl-NaCl melt in the form of ReCl62− ions and crystallizes in the solidified melt as a Cs2ReCl6 phase.
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AUTHOR INFORMATION
Corresponding Author
*E- mail:
[email protected]. ORCID
Andrey Isakov: 0000-0002-0192-3048 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This study was carried out with the financial support of the Ministry of Education and Science of the Russian Federation within the framework of the Federal target program (agreement number 14.578.21.0238, identification number RFMEFI57817X0238). This study was performed using the equipment of the Shared Access Center “Composition of substance” no. ROSS RU.0001.515512 at IHTE UB RAS.
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REFERENCES
(1) Baraboshkin, A. N. Electrocrystallization of Metals from Molten Salts; Nauka: Moscow, 1976; p280. (2) Toenshoff, D.; Lanam, R.; Ragaini, J.; Shchetkovskiy, A.; Smirnov, A. Iridium Coated Rhenium Rocket Chambers Produced by Electroforming. 36th AIAA/ASME/SAE/ASEE Joint Propulsion Conference and Exhibit, Joint Propulsion Conferences, 17−19 July 2000, Huntsville, AL, DOI: 10.2514/6.2000-3166. (3) Etenko, A.; McKechnie, T.; Shchetkovskiy, A.; Smirnov, A. Oxidation-protective iridium and iridium-rhodium coatings produced by electrodeposition from molten salts. ECS Trans 2006, 3, 151−157.
a Standard uncertainties are u(T) = 0.5 K and u(P) = 10 kPa. The standard uncertainties for the electrical conductivity u(k) are given.
The ReCl4 additions to molten mixture CsCl-KCl-NaCl result in a decrease in the electrical conductivity, which is connected to the formation of the complex groupings containing rhenium. According to reaction 3, the ReCl62− F
DOI: 10.1021/acs.jced.8b00758 J. Chem. Eng. Data XXXX, XXX, XXX−XXX
Journal of Chemical & Engineering Data
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DOI: 10.1021/acs.jced.8b00758 J. Chem. Eng. Data XXXX, XXX, XXX−XXX
