MOLTEN SALT COMPOSITION FOR SMELTING MAGNESIUM USING SOLID OXIDE MEMBRANE (SOM) PROCESS

Abstract

Provided is a molten salt composition for smelting magnesium using a solid oxide membrane (SOM) process. The low-temperature molten salt composition can be applied to a SOM process and contains, by wt %, 42% to 47% of MgF.sub.2, 42% to 47% of CaF.sub.2, 6% to 16% of one or more of LiF and NaF, and a remainder of inevitable impurities.

Claims

1. A low-temperature molten salt composition applicable to a solid oxide membrane (SOM) process, comprising, by weight%: 42% to 47% of MgF.sub.2, 42% to 47% of CaF.sub.2, 6% to 16% of one or more of LiF and NaF, and a remainder of inevitable impurities.

2. The low-temperature molten salt composition of claim 1, wherein the composition has a MgF.sub.2CaF.sub.2(=1:1)-MF(MLi, Na)-base composition.

3. The low-temperature molten salt composition of claim 1, wherein solubility of magnesium oxide (MgO) in the composition is 1.5 wt % or higher at 950 C.

4. The low-temperature molten salt composition of claim 1, wherein a melting point of the low-temperature molten salt is 950 C. or below, and a partial pressure of LiF or NaF is less than 2.010.sup.3 atm.

Description

BRIEF DESCRIPTIONS OF DRAWINGS

[0016] The patent or application file contains at least one drawing executed in color. Copies of this patent or patent application publication with color drawings will be provided by the Office upon request and payment of the necessary fee.

[0017] FIG. 1 is an image of potential changes of a metal anion according to a temperature of a cation using the Nernst Equation (on the assumption that activity of the cation is 0.1).

[0018] FIG. 2 is a ternary phase diagram; FIG. 2a illustrates a ternary phase diagram of MgF.sub.2CaF.sub.2LiF, and FIG. 2b illustrates a ternary phase diagram of MgF.sub.2CaF.sub.2NaF.

[0019] FIG. 3 is an image illustrating a melting point of a MgF.sub.2CaF.sub.2LiF molten salt composition and a partial pressure of LiF according to an exemplary embodiment.

[0020] FIG. 4 is an image illustrating a melting point of a MgF.sub.2CaF.sub.2LiF molten salt composition and a partial pressure of NaF according to an exemplary embodiment.

BEST MODE

[0021] Hereinafter, the present invention will be described.

[0022] Smelting of magnesium employing a solid oxide membrane process uses a molten salt as an electrolyte to produce gaseous oxygen and gaseous magnesium at an anode and a cathode. A halide molten salt generally has a comparatively low melting point and is likely to be ionized, and thus is appropriate for a low-temperature molten salt used as an electrolyte. Halide molten salts can be divided into fluoride-based and chloride-based molten salts, where the chloride-base has issues with great degradability and corrosion of a reactor. Accordingly, the present invention suggests a fluoride-based molten salt composition as the appropriate molten salt.

[0023] Specifically, a low-temperature molten salt composition of the present invention, applicable to the SOM process, contains, by weight %, 42% to 47% of MgF.sub.2, 42% to 47% of CaF.sub.2, 6% to 16% of one or more of LiF and NaF, and a remainder of inevitable impurities.

[0024] The MgF.sub.2 is present in the form of Mg.sup.2+ after being ionized, and thus can be used as a raw material for magnesium smelting. The CaF.sub.2 is used to lower the melting point of MgF.sub.2, and due to a comparatively lower cost thereof than other fluorides, a melting point can be lowered by an economically feasible method. Accordingly, MgF.sub.2CaF.sub.2 is determined as a main molten salt composition.

[0025] Meanwhile, as a eutectic point of 45MgF.sub.2-55CaF.sub.2 is observed at about 976 C. in the present invention, there have been undue trial and error exerted to find an additional substance to achieve a low-temperature molten salt composition of 950 C. or less. As a result, the addition of LiF or NaF to the low-temperature molten salt effectively lowers a melting point of the low-temperature molten salt composition, thereby suggesting the present invention.

[0026] Specifically, to reduce a magnesium ion to magnesium at a cathode, reduction potential of the magnesium ion needs to be the largest among cations in the molten salt. Table 1 shows standard reduction potential of the cations, and the reduction potential according to temperature is shown in FIG. 1.

TABLE-US-00001 TABLE 1 Reduction Half Reaction E (V) Ti.sup.2+ (aq) + 2e.sup. .fwdarw. Ti (s) 0.34 Fe.sup.2+ (aq) + 2e.sup. .fwdarw. Fe (s) 0.44 O.sub.2 (g) + e.sup. .fwdarw. O.sup.2 (aq) 0.56 Zn.sup.2+ (aq) + 2e.sup. .fwdarw. Zn (s) 0.76 Mn.sup.2+ (aq) + 2e.sup. .fwdarw. Mn (s) 1.18 Al.sup.3+ (aq) + 3e.sup. .fwdarw. Al (s) 1.66 Mg.sup.2+ (aq) + 2e.sup. .fwdarw. Mg (s) 2.36 Na.sup.+ (aq) + e.sup. .fwdarw. Na (s) 2.71 Li.sup.+ (aq) + e.sup. .fwdarw. Li (s) 3.05

[0027] As shown in Table 1, ions having a reduction potential lower than that of magnesium are Li.sup.+ and Na.sup.+. To review an effect of the temperature of the reduction potential, the Nernst Equation was used to calculate reduction potential according to the temperature of metal cations.

[00001] $\begin{matrix} Me -> {Me}^{n +} + {ne}^{-} & [Equation .Math. .Math. 1] \\ E_{N} = E_{N}^{0} - \frac{RT}{nF} .Math. \ln (\frac{{}^{a}{Me}^{n +}}{^{a} .Math. Me}) & [Equation .Math. .Math. 2] \\ E_{N} = E_{N}^{0} - \frac{1.98 10^{- 4}}{n} .Math. \log ({}^{a}{Me}^{n +}) .Math. T & [Equation .Math. .Math. 3] \end{matrix}$

[0028] In Equations 1 to 3, E.sub.N is reduction potential, E.sub.N.sup.0 is standard reduction potential, R is gas constant (8.314 J/mol), T is absolute temperature, F is Faraday constant (9.645810.sup.4), aMe.sup.n+ is activity of a metal cation, and aMe is activity of a metal.

[0029] Calculations were performed on the assumption that cation activity is 0.1, and a result thereof is shown in FIG. 1. As shown in FIG. 1, the reduction potential does not show a significant variation depending on the temperature, and the reduction potential of the Li.sup.+ and Na+ ions is lower than that of Mg.sup.2+ ions. This indicates that only in the case in which LiF and NaF is added to the MgF.sub.2CaF.sub.2 molten salt, the Mg.sup.2+ ions are reduced at the cathode, thereby smelting magnesium. Accordingly, it has been determined that LiF and NaF are substances which can be added to the MgF.sub.2CaF.sub.2 molten salt.

[0030] That is, based on the above, the molten salt composition containing, by weight %, 42% to 47% of MgF.sub.2, 42% to 47% of CaF.sub.2, 6% to 16% of one or more of LiF and NaF, and a remainder of inevitable impurities is suggested in the present invention.

[0031] Meanwhile, contents of ingredients of the low-temperature molten salt composition of the present invention are determined considering two factors. First, FactSage 7.0 (FTsalt database), thermodynamics software, was used to calculate melting points according to the molten salt composition so as to be used at a low temperature of 950 C. or below.

[0032] FIGS. 2a and 2b are images illustrating phase diagrams of the molten salts (MgF.sub.2CaF.sub.2LiF and MgF.sub.2CaF.sub.2NaF) suggested in the present invention using thermodynamics software FactSage 7.0 (FTsalt database). As illustrated in FIG. 2, a blue colored region indicates a range of a composition present in a liquid state at 950 C., in which MgF.sub.2 and CaF.sub.2 are present at a ratio of 1:1. It is also shown that when LiF or NaF is added, the composition of the molten salt can be 950 C. or less.

[0033] Second, partial pressure was calculated to observe volatility of the fluoride-based molten salts. As for the partial pressure, linear fitting of logP (atm) according to 1/T(K) was calculated based on reference values (Luxel Vapor Pressure Table, Luxel Corporation, 2017). Relations of vapor pressure with temperatures of MgF.sub.2, CaF.sub.2, LiF and NaF are defined as Equations 4 to 7 below.

logP=15981.85/T+6.43 [Equation 4]

logP=16760.64/T+6.02 [Equation 5]

logP=11903.78/T+8.27 [Equation 6]

logP=12097.19/T+8.25 [Equation 7]

[0034] According to the above Equations, saturated vapor pressures of MgF.sub.2, CaF.sub.2, LiF and NaF are 2.310.sup.7 atm, 2.0710.sup.8 atm, 0.03 atm and 0.02 atm, respectively. Since saturated vapor pressures of MgF.sub.2 and CaF.sub.2 are significantly lower than those of LiF and NaF, volatility of the molten salt increases as activity of LiF or NaF increases. The activity of LiF or NaF in the molten salts (MgF.sub.2CaF.sub.2LiF and MgF.sub.2CaF.sub.2NaF) can be calculated using FactSage 7.0 (FTsalt database).

[0035] It is understood that volatility of the molten salt increases as the calculated vapor pressure of LiF or NaF increases. Accordingly, it is required in the present invention that the melting points of the molten salts are 950 C. or less and the partial pressure of LiF or NaF is less than 2.010.sup.3 atm.

[0036] Considering the above, the molten salt composition of the present invention contains, by weight %, 42% to 47% of MgF.sub.2, 42% to 47% of CaF.sub.2, 6% to 16% of one or more of LiF and NaF, and a remainder of inevitable impurities.

[0037] Specifically, when a considerably larger amount of CaF.sub.2 is contained in the molten salt of MgF.sub.2CaF.sub.2LiF, compared to MgF.sub.2, the melting point was shown to be at 950 C. or higher. The activity of LiF was increased to 0.36, thus increasing the partial pressure thereof to be as high as about 0.01 atm. In contrast, when a considerably larger amount of MgF.sub.2 is contained, compared to CaF.sub.2, the melting point decreases, but the activity of LiF increases to 0.25, thereby increasing the partial pressure of LiF to about 0.007 atm. In the case in which MgF.sub.2 and CaF.sub.2 are contained at a ratio of 1:1, the activity of LiF is lowered but the melting point increases to 950 C. or higher when less than 6 wt % of LiF is added, whereas the activity of LiF increases although the melting point decreases when more than 16 wt % of LiF is added. Accordingly, it is determined that the molten salt of (42 wt % to 47 wt %) MgF.sub.2-(42 wt % to 47 wt %) CaF.sub.2-(16 wt % to 6 wt %) LiF is appropriate.

[0038] As for the molten salt of MgF.sub.2CaF.sub.2NaF, the melting point is higher than 950 C. and the NaF activity increases to 0.25 in both cases in which a larger amount of CaF.sub.2 is added than MgF.sub.2 and in which a larger amount of MgF.sub.2 is added than CaF.sub.2, such that the partial pressure of NaF increases to be as high as 0.005 atm. When MgF.sub.2 and CaF.sub.2 are contained at a ratio of 1:1, the NaF activity is reduced but the melting point is higher than 950 C. when less than 6 wt % of NaF is added, which may be problematic, whereas the NaF activity increases when more than 16 wt % of NaF is added, thereby leading to partial pressure of NaF greater than 2.010.sup.3 atm. Accordingly, it is suggested that molten salt of (42 wt % to 47 wt %) MgF.sub.2-(42 wt % to 47 wt %) CaF.sub.2-(16 wt % to 6 wt %) NaF is appropriate.

[0039] A more preferable composition of the molten salt with respect with melting point and partial pressure satisfies MgF.sub.2CaF.sub.2(=1:1)-MF(MLi, Na).

[0040] Meanwhile, when smelting magnesium through the SOM process, an amount of MgO, a main ingredient, dissolved in the molten salt is a critical factor. Solubility of MgO in the low-temperature molten salt composition of the present invention is at least 1.5 wt % at 950 C., indicating excellent solubility.

Mode for Invention

[0041] Hereinafter, the present invention will be described with reference to the following Examples.

Example 1

[0042] Molten salt compositions for SOM having the composition ingredients as shown in Table 2 below were prepared. A melting point was then calculated for each molten salt using FactSage 7.0 (FTsalt database), thermodynamic software, and a result thereof is shown in Table 2 below. Further, activity of LiF or NaF and partial pressure thereof at the melting point of each molten salt were calculated and results thereof are shown in Table 2.

TABLE-US-00002 TABLE 2 Partial Molten salt Activity pressure Sample composition(wt %) Melting of LiF or of LiF or No. Classification MgF.sub.2 CaF.sub.2 LiF NaF point( C.) NaF NaF(atm) 1 ***CoE 45 55 976.23 2 **CE 18 72 10 1114.23 0.17 0.00510 3 CE 16 64 20 1011.15 0.36 0.01068 4 CE 27 63 10 1052.27 0.15 0.00450 5 CE 25.5 59.5 15 1009.54 0.23 0.00683 6 CE 38 57 5 1007.80 0.05 0.00145 7 CE 36 54 10 983.16 0.11 0.00341 8 CE 48 48 4 957.47 0.03 0.00085 9 *IE 47 47 6 929.57 0.05 0.00138 10 IE 46 46 8 911.55 0.06 0.00182 11 IE 45 45 10 903.55 0.06 0.00179 12 IE 42 42 16 869.41 0.02 0.00048 13 CE 54 36 10 945.20 0.08 0.00236 14 CE 56 24 20 869.62 0.19 0.00579 15 CE 64 16 20 934.03 0.18 0.00533 16 CE 60 15 25 861.37 0.25 0.00750 17 CE 16 64 20 1085.72 0.19 0.00377 18 CE 46.5 62.4 22 1068.47 0.08 0.00170 19 CE 15 60 25 1040.98 0.25 0.00500 20 CE 27 63 10 1093.89 0.06 0.00112 21 CE 24 56 50 1042.41 0.41 0.00812 22 CE 36.8 55.2 8 1026.77 0.03 0.00054 23 CE 35.2 52.8 12 1021.66 0.05 0.00097 24 CE 32 48 20 992.82 0.11 0.000212 25 CE 48.5 48.5 3 978.66 0.01 0.00046 26 IE 47 47 6 943.72 0.01 0.00092 27 IE 45 45 10 945.75 0.02 0.00097 28 IE 42 42 16 942.48 0.05 0.00213 29 CE 54 36 10 967.67 0.05 0.00314 30 CE 44.4 29.6 26 921.81 0.11 0.00032 31 CE 42 28 30 935.59 0.16 0.00314 32 CE 63 27 10 1037.32 0.02 0.00032 33 CE 47.6 20.4 32 962.63 0.15 0.00309 34 CE 72 18 10 1100.83 0.01 0.00026 35 CE 64 16 20 978.18 0.04 0.00080 36 CE 52 13 35 988.34 0.16 0.00327 *IE: Inventive Example, **CE: Comparative Example, ***CoE: Conventional Example

[0043] As shown in Table 2, sample Nos. 9 to 12 and 26 to 28, satisfying the composition ingredient ranges of the present invention, have lower melting points of 950 C. or less and low partial pressure of 2.010.sup.3 atm or less, compared to the samples which do not satisfy the ranges. Accordingly, it is confirmed that the molten salt composition satisfies the requirements for the low temperature SOM process.

[0044] Meanwhile, in the case of a Conventional Example, in which neither LiF nor NaF is contained in the molten salt, the melting point of the molten salt was shown to be higher than 950 C.

Example 2

[0045] When smelting magnesium through the SOM process, an amount of MgO, a main ingredient, dissolved in the molten salt is a critical factor. In this regard, a melt quenching experiment was carried out to calculate MgO solubility in the molten salt.

[0046] That is, a fluoride-based molten salt having the compositions shown in Table 3 below was added to a carbon crucible, and a lid was attached to a top of the crucible to prevent volatilization of the fluoride molten salt. The molten salt and MgO bulk having a uniform size was added to the crucible and was allowed to react in a vertical resistance furnace at temperatures of 950 C., 1000 C., 1100 C. and 1200 C., followed by quenching.

[0047] The compositions of the molten salt used in the experiment were 46.5MgF.sub.2-46.5CaF.sub.2-7LiF and 45MgF.sub.2-45CaF.sub.2-10NaF. After the experiment was completed, oxygen concentrations in the molten salt were analyzed using a combustion analyzer (NO, TC-300, LECO), and a result thereof is shown in Table 3.

TABLE-US-00003 TABLE 3 Molten salt composition (wt %) MgO solubility (wt %) MgF2 CaF2 LiF NaF 950 C. 1000 C. 1100 C. 1200 C. IE* 46.5 46.5 7 1.5 1.9 2.5 3.3 IE 45 45 10 1.5 1.7 1.7 1.9 *IE: Inventive Example

[0048] As shown in Table 3 above, the molten salt of 46.5MgF.sub.2-46.5CaF.sub.2-7LiF has MgO solubility of 1.5 wt % and 2.3 wt % at 950 Cand 1200 C., respectively, and the molten salt of 45MgF.sub.2-45CaF.sub.2-10NaF has MgO solubility of 1.5 wt % and 1.9 wt % at 950 Cand 1200 C., respectively. That is, it is understood that the molten salt composition of the present invention has MgO solubility of 1.5 wt % or higher at 950 C., and accordingly, MgO can be effectively smelting reduced during a low-temperature SOM process of 950 C. or less.

[0049] While embodiments have been shown and described above, it will be apparent to those skilled in the art that modifications and variations could be made without departing from the scope of the present invention as defined by the appended claims.

MOLTEN SALT COMPOSITION FOR SMELTING MAGNESIUM USING SOLID OXIDE MEMBRANE (SOM) PROCESS

Inventors

Cpc classification

Classification Explorer

C22B9/10

CHEMISTRY; METALLURGY

Classification Explorer

C25C3/04

CHEMISTRY; METALLURGY

Classification Explorer

C25C7/005

CHEMISTRY; METALLURGY

Classification Explorer

C22B26/22

CHEMISTRY; METALLURGY

International classification

Classification Explorer

C22B26/22

CHEMISTRY; METALLURGY

Classification Explorer

C22B9/10

CHEMISTRY; METALLURGY

Abstract

Claims

Description