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 MgF2, 42% to 47% of CaF2, 6% to 16% of one or more of LiF and NaF, and a remainder of inevitable impurities.

Description

TECHNICAL FIELD

The present invention relates to a molten salt composition required for smelting magnesium employing a solid oxide membrane (SOM) process, more specifically, to a low-temperature molten salt composition for smelting magnesium at a working temperature lower than a working temperature of a conventional high temperature process, which is reduced by about 200° C., employing a SOM process, thereby maximizing energy efficiency.

BACKGROUND ART

A solid oxide membrane (SOM) process is a method for reducing various metals (magnesium, aluminum, silicon, or the like) from metal oxides.

For example, U.S. Patent Publication Application No. US2016-0362805A1 discloses a method for increasing corrosion resistance by improving current resistance of a membrane with respect to a method for optimizing energy efficiency of a metal smelting process using an oxygen permeable membrane.

In addition, Canadian Patent Publication Application No. CA2363647A1 discloses a process of smelting titanium from a titanium slag, and discloses that molten salts required for such a smelting process have a eutectic composition of CaF₂—MgF₂, CaF₂—BaF₂—LiF and CaF₂—LiF.

U.S. Patent Publication Application No. US2015-0047745A1 also discloses a process of smelting aluminum from an aluminum alloy, and discloses that molten salts required for such a smelting process have an NaF—KF equimolar composition. And Korean Patent Publication Application No. KR10-2006-0061048A discloses that the composition of a molten salt for smelting a magnesium alloy includes 35 wt % to 55 wt % of LiCl and 45 wt % to 65 wt % of KCl.

The inventions proposed in the above prior art discloses a method for smelting a metal at a high temperature of 1100° C. to 1300° C.

A working temperature of the SOM process is determined in a temperature range in which a molten salt is liquid. In this regard, as a liquid phase is observed at around 1000° C. during the conventional SOM process, it was general that the smelting was performed at a working temperature of 1100° C. to 1300° C. using a eutectic composition of 45MgF₂-55CaF₂. In other words, the SOM process was not preferable with respect to energy efficiency, or the like, as magnesium was reduction smelting together with an anode of yttrium-stabilized zirconia (YSZ) at a temperature (1150° C. to 1300° C.) higher than the eutectic temperature (about 1000° C.) using the 45MgF₂-55CaF₂ composition.

Accordingly, there have been efforts to reduce working temperature of the described SOM process for reduced costs and higher energy efficiency.

DISCLOSURE

Technical Problem

Accordingly, in order to resolve the previously described limitations of the prior art, the object of the present invention is to provide a low-temperature molten salt composition for smelting reducing magnesium, or the like, using a SOM process at a low temperature of 1000° C. or less, contrary to a molten salt composition used in an existing SOM process.

Technical problems of the present invention are not limited to the above. Technical problems, which are not described, will be clearly understood by those skilled in the art through the following description.

Technical Solution

To achieve the above technical problem, the present invention relates to a low-temperature molten salt composition applicable to a solid oxide membrane (SOM) process, comprising, by weight%: 42% to 47% of MgF₂, 42% to 47% of CaF₂, 6% to 16% of one or more of LiF and NaF, and a remainder of inevitable impurities.

The low-temperature molten salt composition has a MgF₂—CaF₂(=1:1)-MF(M═Li, Na)-base composition.

Solubility of magnesium oxide (MgO) in the low-temperature molten salt composition may be 1.5 wt % or higher at 950° C.

Advantageous Effects

The present invention having the previously described constitution suggests a low-temperature molten composition for magnesium smelting, employing a solid oxide membrane (SOM) process, thereby enabling the SOM process to be exerted at a lower temperature than the existing process. Accordingly, overall working efficiency can be expected to be higher through energy efficiency improvements and cost reduction effects.

BRIEF DESCRIPTIONS OF DRAWINGS

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.

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).

FIG. 2 is a ternary phase diagram; FIG. 2a illustrates a ternary phase diagram of MgF₂—CaF₂—LiF, and FIG. 2b illustrates a ternary phase diagram of MgF₂—CaF₂₋—NaF.

FIG. 3 is an image illustrating a melting point of a MgF₂—CaF₂—LiF molten salt composition and a partial pressure of LiF according to an exemplary embodiment.

FIG. 4 is an image illustrating a melting point of a MgF₂—CaF₂—LiF molten salt composition and a partial pressure of NaF according to an exemplary embodiment.

BEST MODE

Hereinafter, the present invention will be described.

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.

Specifically, a low-temperature molten salt composition of the present invention, applicable to the SOM process, contains, by weight %, 42% to 47% of MgF₂, 42% to 47% of CaF₂, 6% to 16% of one or more of LiF and NaF, and a remainder of inevitable impurities.

The MgF₂ is present in the form of Mg²⁺ after being ionized, and thus can be used as a raw material for magnesium smelting. The CaF₂ is used to lower the melting point of MgF₂, and due to a comparatively lower cost thereof than other fluorides, a melting point can be lowered by an economically feasible method. Accordingly, MgF₂—CaF₂ is determined as a main molten salt composition.

Meanwhile, as a eutectic point of 45MgF₂-55CaF₂ 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.

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 1
Reduction Half Reaction  E ° (V)
Ti2+ (aq) + 2e− → Ti (s) −0.34
Fe2+ (aq) + 2e− → Fe (s) −0.44
O2 (g) + e− → O2− (aq)   −0.56
Zn2+ (aq) + 2e− → Zn (s) −0.76
Mn2+ (aq) + 2e− → Mn (s) −1.18
Al3+ (aq) + 3e− → Al (s) −1.66
Mg2+ (aq) + 2e− → Mg (s) −2.36
Na+ (aq) + e− → Na (s)   −2.71
Li+ (aq) + e− → Li (s)   −3.05

As shown in Table 1, ions having a reduction potential lower than that of magnesium are Li⁺ and Na⁺. 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.

$\begin{array}{cc}\mathrm{Me}->{\mathrm{Me}}^{n+}+{\mathrm{ne}}^{-}& \left[\mathrm{Equation}1\right]\\ E_N=E_N^0-\frac{\mathrm{RT}}{\mathrm{nF}}\ln \left(\frac{\mathrm{Me}}{{\hspace{0.17em}}^a\mathrm{Me}}\right)& \left[\mathrm{Equation}2\right]\\ E_N=E_N^0-\frac{1.98\times {10}^{-4}}{n}\log \left(\mathrm{Me}\right)T& \left[\mathrm{Equation}3\right]\end{array}$

In Equations 1 to 3, E_N is reduction potential, E_N⁰ is standard reduction potential, R is gas constant (8.314 J/mol), T is absolute temperature, F is Faraday constant (9.6458×10⁻⁴), aMeⁿ⁺ is activity of a metal cation, and aMe is activity of a metal.

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⁺ and Na+ ions is lower than that of Mg²⁺ ions. This indicates that only in the case in which LiF and NaF is added to the MgF₂—CaF₂ molten salt, the Mg²⁺ 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 MgF₂—CaF₂ molten salt.

That is, based on the above, the molten salt composition containing, by weight %, 42% to 47% of MgF₂, 42% to 47% of CaF₂, 6% to 16% of one or more of LiF and NaF, and a remainder of inevitable impurities is suggested in the present invention.

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.

FIGS. 2a and 2b are images illustrating phase diagrams of the molten salts (MgF₂—CaF₂—LiF and MgF₂—CaF₂—NaF) 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 MgF₂ and CaF₂ 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.

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 MgF₂, CaF₂, 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]

According to the above Equations, saturated vapor pressures of MgF₂, CaF₂, LiF and NaF are 2.3×10⁻⁷ atm, 2.07×10⁻⁸ atm, 0.03 atm and 0.02 atm, respectively. Since saturated vapor pressures of MgF₂ and CaF₂ 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 (MgF₂—CaF₂—LiF and MgF₂—CaF₂—NaF) can be calculated using FactSage™ 7.0 (FTsalt database).

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 2.0×10⁻³ atm.

Considering the above, the molten salt composition of the present invention contains, by weight %, 42% to 47% of MgF₂, 42% to 47% of CaF₂, 6% to 16% of one or more of LiF and NaF, and a remainder of inevitable impurities.

Specifically, when a considerably larger amount of CaF₂ is contained in the molten salt of MgF₂—CaF₂—LiF, compared to MgF₂, 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 MgF₂ is contained, compared to CaF₂, 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 MgF₂ and CaF₂ 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 %) MgF₂-(42 wt % to 47 wt %) CaF₂-(16 wt % to 6 wt %) LiF is appropriate.

As for the molten salt of MgF₂—CaF₂—NaF, 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 CaF₂ is added than MgF₂ and in which a larger amount of MgF₂ is added than CaF₂, such that the partial pressure of NaF increases to be as high as 0.005 atm. When MgF₂ and CaF₂ 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 2.0×10⁻³ atm. Accordingly, it is suggested that molten salt of (42 wt % to 47 wt %) MgF₂-(42 wt % to 47 wt %) CaF₂-(16 wt % to 6 wt %) NaF is appropriate.

A more preferable composition of the molten salt with respect with melting point and partial pressure satisfies MgF₂—CaF₂(=1:1)-MF(M═Li, Na).

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

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

Example 1

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 2
					Partial
		Molten salt		Activity	pressure
Sample		composition(wt %)	Melting	of LiF or	of LiF or
No.	Classification	MgF2	CaF2	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

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 2.0×10⁻³ 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.

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

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.

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.

The compositions of the molten salt used in the experiment were 46.5MgF₂-46.5CaF₂-7LiF and 45MgF₂-45CaF₂-10NaF. 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 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

As shown in Table 3 above, the molten salt of 46.5MgF₂-46.5CaF₂-7LiF has MgO solubility of 1.5 wt % and 2.3 wt % at 950° Cand 1200° C., respectively, and the molten salt of 45MgF₂-45CaF₂-10NaF 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.

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.

Claims

1. A low-temperature molten salt composition applicable to a solid oxide membrane (SOM) process, comprising, by weight%: 42% to 47% of MgF2, 42% to 47% of CaF2, 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 MgF2—CaF2(=1:1)-MF(M═Li, 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 2.0×10−3 atm.