Article pubs.acs.org/jced
Phase Equilibrium and Density Investigation of the Molten LiCl + NaCl + ZnCl2 System Blanka Kubíková,*,† Vladimír Danielik,‡ Eric Robert,§ Jarmila Mlynáriková,† and Miroslav Boča†,∥ †
Institute of Inorganic Chemistry, Slovak Academy of Sciences, Dúbravská cesta 9, 845 36 Bratislava, Slovakia Institute of Chemistry, Technology and Materials, Faculty of Chemical and Food Engineering, Radlinského 9, 812 37 Bratislava, Slovakia § Umicore Group R&D, Kasteelstraat 7, B-2250 Olen, Belgium ∥ Department of Chemistry, Faculty of Natural Sciences, Constantine The Philosopher University, Trieda Andreja Hlinku 1, 949 74 Nitra, Slovakia ‡
ABSTRACT: The investigation of phase equilibrium and density in the (LiCl + NaCl + ZnCl2) molten system was performed over a limited concentration range. Thermal analysis and the Archimedean method were used for the determination of these properties. Concentration dependences of the density at T = (923, 973, and 1023) K were calculated.
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INTRODUCTION Zinc coatings are mostly used on iron or steel substrates to protect the metals against corrosion. It is well-known that zinc resistant properties rise from the formation of the passive layers on the ferrous substrates as zinc is a less noble metal than iron and more reactive than iron or steel. The electrodeposition of zinc and its alloys is only one of the several techniques applied in the production of protective coatings.1 The quality of metal coatings is influenced by several factors, e.g., composition of the bath, pH, working temperature, substrate, current density, etc. In general, the metal electrodeposition can be provided from aqueous and nonaqueous solutions. Main disadvantages of the conventional aqueous solution are mostly hydrogen liberation, evaporation, narrow electrochemical window, low current efficiency, and low thermal stability. Therefore, many works were focused on the nonaqueous media for metal deposition. Simka et al.2 in their review summarized the possibility of metal deposition from nonaqueous solutions (electrodeposition from conventional organic solvents, electrodeposition from ionic liquids, and electrodeposition from molten salts). Many publications were devoted to the improvement of zinc electrodeposition from different nonaqueous media.3−17 The present work deals with a detailed investigation of the phase equilibria and density in the (LiCl (1) + NaCl (2) + ZnCl2 (3)) molten system up to x3 = 0.20 for the optimization process of the zinc deposition. The knowledge of basic physicochemical properties is crucial for these applications. © 2014 American Chemical Society
EXPERIMENTAL SECTION The experiments have been performed with the following chemicals: LiCl (Fluka, anhydride p.a.), NaCl (Merck p.a.), and ZnCl2 (Umicore, p.a.). Each of them was dried for 2 h: LiCl at 873 K, NaCl at 673 K, and ZnCl2 at 473 K. The chemicals have been stored in a glovebox with an inert nitrogen atmosphere. Phase Equilibria. The temperatures of individual phase transitions (primary and secondary crystallization) were determined by means of the thermal analysis method, recording the cooling and heating curves of the investigated mixtures. The temperatures of primary (Tpc/K) and secondary (T2/K) crystallization were obtained from the cooling curves. A cooling rate of (0.5 to 1) K·min−1 was used. Two platinum crucibles containing 10 g of the samples (of the same composition) were placed into the resistance furnace provided by a N2 atmosphere and adjustable cooling rate. One thermocouple was immersed into the each crucible, and two sets of data were obtained for one composition. The temperature control and the data processing were controlled programmatically via digital outputs of the Keithley Multimeter 2700. The temperature was measured using a Pt−PtRh10 thermocouple calibrated to the melting points of NaCl and LiCl, providing an uncertainty in the temperature measurement of u(T) = 2 K. The detailed description of the experimental device as well as the measuring Received: January 13, 2014 Accepted: July 8, 2014 Published: July 17, 2014 2408
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procedure can be found in a previous publication.18 The obtained temperatures of primary and secondary crystallization for both samples with the same composition were averaged. Density. The density of the investigated melts was measured using the Archimedean method. A platinum vessel hanged on a platinum wire of diameter d/mm = 0.3 (u(d) = 0.05 mm) attached below an electronic balance unit was used as the measuring body. The temperature dependence of the vessel volume was determined by calibration using molten NaCl and KF, all of analytic grade purity. The temperature was measured using a Pt−Pt10Rh thermocouple calibrated at the melting points of NaCl and KCl. The measurements were carried out in a temperature interval of approximately T = (100 to 150) K above the temperature of primary crystallization. An uncertainty in the temperature measurement was u(T) = 2 K. For control of the measuring device and for the evaluation of the experimental data we used an online PC XT computer. The necessary amount of the mixture (m = 45 g with the uncertainty u(m) = 0.0005 g) was weighted, homogenized, and placed in the Pt crucible in a glovebox and quickly transferred into the furnace that was preheated at T = 373 K. The Pt crucible was located just below the measuring body in an atmosphere of dried nitrogen. The samples were heated to T = (100 to 150) K above the temperature of primary crystallization; then the measuring body was immersed into the melt, and we started measurements in the cooling direction. Each experiment was performed down to the lowest temperature of the mixture, i.e., T = (20−30) K above the temperature of primary crystallization of the mixture. The density results were automatically recorded by the measuring device every t = 3 s (u(t) = 0.05 s) for each melt. The reproducibility of the density measurement was ± 0.03 g·cm−3. Further information about the measuring procedure and the device can be found in another publication.19
The graphical representation of the phase diagram of the (LiCl (1) + NaCl (2) + ZnCl2 (3)) system is in Figure 1. Figure 1 displays 15 compositions investigated by the thermal analysis along with the corresponding measured temperatures of first crystallization.
Figure 1. Figurative points, ●, investigated by thermal analysis with the corresponding measured temperatures of the first crystallization expressed in kelvin, of the (LiCl (1) + NaCl (2) + ZnCl2 (3)) molten system.
Density. The temperature dependence of the density was expressed in the form of the linear equation ρ = a − bT
where ρ/g·cm is the density and T/K is the temperature. The uncertainty in temperature measurement u(T) was u(T) = 2 K. The values of the constants a/g·cm−3 and b/g·cm−3·K−1 together with the standard deviations of approximations, sdρ/g·cm−3, obtained by the linear regression analysis of the experimentally determined data are given in Table 2. Table 2. Regression Coefficients a/g·cm−3, b/g·cm−3·K−1, and the Standard Deviations of Approximation, sdρ/g·cm−3, and Temperature Range, T/K, of Investigated Melts in the System (LiCl (1) + NaCl (2) + ZnCl2 (3))a
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RESULTS AND DISCUSSION Phase Equilibria. The average temperatures of primary (Tpc/K) and secondary crystallization (T2/K) of the investigated mixtures are summarized in Table 1. Table 1. Average Experimental Temperatures of Primary (Tpc/K) and Secondary (T2/K) Crystallization of the (LiCl (1) + NaCl (2) + ZnCl2 (3)) Molten Systema x1
x2
Tpc/K
T2/K
0.65 0.55 0.45 0.40 0.30 0.65 0.60 0.50 0.40 0.30 0.55 0.45 0.30 0.40 0.25
0.30 0.40 0.50 0.55 0.65 0.25 0.30 0.40 0.50 0.60 0.30 0.40 0.55 0.40 0.55
808 828 873 907 943 790 790 804 853 901 764 775 852 735 819
621 615 634 604 612 620 633 623 618 624 623 622 622 622 626
(1)
−3
a
b·104
sdρ·104
sample no.
x1
x2
g·cm−3
g·cm−3·K−1
g·cm−3
T/K
1 1 2 2 3 3 4 4 5 5 6 6 7 7 8 8 9 9 10 10
0.15 0.15 0.30 0.30 0.45 0.45 0.60 0.60 0.20 0.20 0.35 0.35 0.50 0.50 0.10 0.10 0.20 0.20 0.35 0.35
0.80 0.80 0.65 0.65 0.50 0.50 0.35 0.35 0.70 0.70 0.55 0.55 0.40 0.40 0.70 0.70 0.60 0.60 0.45 0.45
2.184 2.203 2.193 2.198 2.216 2.227 1.129 1.115 2.245 2.254 2.246 2.271 2.182 2.177 2.436 2.458 2.487 2.378 2.333 2.424
5.433 5.616 5.572 5.611 5.605 5.704 4.991 4.847 5.478 5.551 5.658 5.885 5.166 5.131 6.338 6.453 6.849 5.601 5.364 6.321
2.123 3.630 2.168 2.213 4.005 2.054 2.278 2.145 4.132 7.768 2.359 3.779 5.574 4.543 2.731 1.960 2.313 8.668 4.241 2.954
1023−1103 1018−1108 963−1043 963−1043 975−1025 953−1053 913−1023 913−1023 973−1073 968−1073 923−1023 923−1023 923−1023 922−1033 933−1023 933−1023 883−1023 879−1023 872−989 879−986
a
Standard uncertainties u are u(x1) = 0.01, u(x2) = 0.01, and u(T) = 2 K.
Two curves (one cooling and one heating) were recorded for each measurement. For each curve coefficients a and b were obtained. A graphical representation of the obtained results of the calculated linear regression equations is shown in Figure 2.
a
Standard uncertainties u are u (x1) = 0.01, u (x2) = 0.01, u(Tpc) = 2 K, and u(T2) = 2 K. 2409
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Figure 2. Experimental dependence of the density, ρ, on temperature, T, for 10 samples of the (LiCl (1) + NaCl (2) + ZnCl2 (3)) molten system; full line (), cooling curve; dotted line (····), heating curve.
As can be seen, the linear curves can be divided into three groups. The curves for the samples containing 5 mol % ZnCl2 (3) (sample nos. 1−4) are close to each other as well as the curves for the samples containing 10 mol % ZnCl2 (3) (sample nos. 5−7) and 20 mol % of ZnCl2 (3) (sample nos. 8−10). Densities at (923, 973, and 1023) K are calculated for each sample according to eq 2, and these values are presented in Table 3.
Figure 3. Composition dependence of the density, ρ, in the investigated part of the (LiCl (1) + NaCl (2) + ZnCl2 (3)) molten system at 973 K.
Table 3. Experimental Data for the Density ρ/g·cm−3, in the Molten System (LiCl (1) + NaCl (2) + ZnCl2 (3)) at Selected Temperaturesa ρ923K sample no.
x1
x2
1 1 2 2 3 3 4 4 5 5 6 6 7 7 8 8 9 9 10 10
0.15 0.15 0.30 0.30 0.45 0.45 0.60 0.60 0.20 0.20 0.35 0.35 0.50 0.50 0.10 0.10 0.20 0.20 0.35 0.35
0.80 0.80 0.65 0.65 0.50 0.50 0.35 0.35 0.70 0.70 0.55 0.55 0.40 0.40 0.70 0.70 0.60 0.60 0.45 0.45
−3
ρ973K −3
g·cm
g·cm
1.682 1.685 1.679 1.680 1.699 1.700 1.669 1.668 1.739 1.742 1.723 1.728 1.706 1.703 1.851 1.863 1.854 1.861 1.838 1.841
1.655 1.657 1.651 1.652 1.671 1.672 1.644 1.643 1.712 1.714 1.695 1.698 1.680 1.678 1.819 1.830 1.820 1.833 1.811 1.809
The composition dependence of the density expressed in mole fractions at for three given temperatures (923, 973, and 1023) K was formally described by the following simple additive equation
ρ1023K g·cm−3 1.628 1.629 1.623 1.624 1.643 1.643 1.619 1.619 1.684 1.686 1.667 1.669 1.654 1.652 1.787 1.798 1.786 1.805 1.784 1.778
ρ /g·cm−3 = Ax1 + Bx 2 + Cx1x3 + Dx1x33 + Ex 2x3 + Fx 2x33
(2)
The coefficients A to F were calculated using the program REGANAL (developed at IIC SAS) excluding all statistically nonimportant terms at a confidence level of 0.95. The obtained coefficients for eq 2 together with the standard deviations of approximations at the temperatures (923, 973, and 1023) K are given in Table 4. A comparison between the calculated and the experimental density values is given in Table 5. It is necessary to stress that eq 2 is a formal equation describing the acquired density values and valid only over the studied composition and temperature range. The difference between the experimental and the calculated density values was calculated according to eq 3 and is expressed in a percentage. ⎛ρ − ρ ⎞ ρ ·100 calc exp ⎟ = calc dev = 100·⎜⎜ − 100 ⎟ ρexp ρexp ⎝ ⎠
a
Standard uncertainties u are u(x1) = 0.01, u(x2) = 0.01, and u(T) = 2 K, and the combined expanded uncertainty Uc is Uc(ρ) = 0.003 g·cm−3.
(3)
A graphical representation of composition dependence of the density at the temperature T = 973 K is shown in Figure 3. One 2410
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Table 4. Values of the Coefficients, (A-F)/g·cm−3 and Their Standard Deviations of Approximation, sdi/g·cm−3 for eq 2 for Three Given Temperaturesa coefficient ± sdi/g·cm−3
923 K
A ± sdA B ± sdB C ± sdC D ± sdD E ± sdE F ± sdF a
1.506 1.636 3.634 6.388 3.159 6.939
± ± ± ± ± ±
973 K
0.019 0.017 0.18 1.2 0.16 0.9
1.488 1.608 3.520 6.642 3.111 6.835
± ± ± ± ± ±
1023 K
0.018 0.017 0.17 1.20 0.16 0.9
1.470 1.580 3.404 6.906 3.063 6.728
± ± ± ± ± ±
0.019 0.018 0.18 1.20 0.16 0.9
Standard uncertainties u are u(T) = 2 K.
Table 5. Comparison between the Experimental Density Values, ρex/g·cm−3, and Calculated Density Values Calculated from eq 3, ρcal/g·cm−3, in the (LiCl (1) + NaCl (2) + ZnCl2 (3)) Molten System at the Temperatures (923, 973, and 1023) Ka T = 923 K
a
T = 973 K
T = 1023 K
ρex.
ρcal.
dev.
ρex.
ρcal.
dev.
ρex.
ρcal.
x1
x2
g·cm−3
g·cm−3
%
g·cm−3
g·cm−3
%
g·cm−3
g·cm−3
%
0.15 0.15 0.30 0.30 0.45 0.45 0.60 0.60 0.20 0.20 0.35 0.35 0.50 0.50 0.10 0.10 0.20 0.20 0.35 0.35
0.80 0.80 0.65 0.65 0.50 0.50 0.35 0.35 0.70 0.70 0.55 0.55 0.40 0.40 0.70 0.70 0.60 0.60 0.45 0.45
1.682 1.685 1.679 1.680 1.699 1.700 1.669 1.668 1.739 1.742 1.723 1.728 1.706 1.703 1.851 1.863 1.854 1.861 1.838 1.841
1.689 1.689 1.673 1.673 1.657 1.657 1.641 1.641 1.746 1.746 1.734 1.734 1.721 1.721 1.854 1.854 1.851 1.851 1.845 1.845
0.399 0.223 −0.345 −0.425 −2.507 −2.618 −1.682 −1.610 0.392 0.236 0.594 0.332 0.912 1.050 0.195 −0.437 −0.214 −0.551 0.381 0.195
1.655 1.657 1.651 1.652 1.671 1.672 1.644 1.643 1.712 1.714 1.695 1.698 1.680 1.678 1.819 1.830 1.820 1.833 1.811 1.809
1.661 1.661 1.646 1.646 1.631 1.631 1.616 1.616 1.717 1.717 1.705 1.705 1.694 1.694 1.824 1.824 1.820 1.820 1.814 1.814
0.376 0.253 −0.280 −0.350 −2.409 −2.491 −1.700 −1.672 0.323 0.186 0.609 0.410 0.816 0.947 0.256 −0.355 −0.023 −0.709 0.167 0.241
1.627 1.629 1.623 1.624 1.643 1.643 1.619 1.619 1.684 1.686 1.667 1.669 1.654 1.652 1.787 1.798 1.786 1.805 1.784 1.778
1.634 1.634 1.619 1.619 1.605 1.605 1.591 1.591 1.689 1.689 1.677 1.677 1.666 1.666 1.793 1.793 1.789 1.789 1.783 1.783
0.354 0.284 −0.214 −0.273 −2.308 −2.361 −1.720 −1.737 0.252 0.134 0.624 0.489 0.715 0.837 0.321 −0.269 0.175 −0.872 −0.056 0.287
dev.
Standard uncertainties u are u(x1) = 0.01, u(x2) = 0.01, and u(T) = 2 K, and the combined expanded uncertainty Uc is Uc(ρ) = 0.003 g·cm−3.
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can see that the density continuously rises with the increasing amount of ZnCl2 (3) in the mixture.
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AUTHOR INFORMATION
Corresponding Author
*Tel.: +421 2 59410 414. Fax: +421 2 59410 444. E-mail:
[email protected]. Funding
This work was supported by the Slovak Research and Development Agency under the contract APVV-0460-10. The present work was financially supported by the Scientific Grant Agency of the Ministry of Education of the Slovak Republic and the Slovak Academy of Sciences under no. 2/0095/12 and 2/ 0116/14 and Umicore Group R&D, Olen, Belgium. This work is the result of the project Competence center for new materials, advanced technologies and energy ITMS 26240220073, supported by the Research and Development Operational Program funded by the European Regional Development Fund. Notes
The authors declare no competing financial interest. 2411
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