KR20190027722A - Fluoride-based flux compositions for magnesium extraction through solid oxide membrane - Google Patents

Abstract

Provided is a molten salt composition for smelting magnesium using a solid oxide membrane (SOM) process. The present invention relates to a low-temperature molten salt composition that can be applied to an SOM process and contains 42-47wt % of MgF_2, 42-47 wt% of CaF_2, 6-16 wt% of at least one of LiF and NaF, and the remainder consisting of inevitable impurities.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention [0001] The present invention relates to a molten salt composition for a magnesium smelting process using a solid oxide membrane process (SOM)

The present invention relates to a composition of a molten salt necessary for smelting magnesium at a low temperature by using a solid oxide membrane (SOM). More specifically, the present invention relates to a composition of a molten salt for smelting magnesium at a low temperature using a solid oxide membrane To a low-temperature molten salt composition capable of maximizing energy efficiency for smelting magnesium.

The solid oxide membrane process is a process for reducing various metals (magnesium, aluminum, silicon, etc.) from metal oxides.

For example, U.S. Patent No. US20160362805A1 discloses a method for improving the current resistance of a membrane to increase corrosion resistance with respect to an energy efficiency optimization method for a metal smelting process using an oxygen permeable membrane.

Also, Canadian Patent No. 2363647 A1 proposes a process for smelting titanium from titanium slag and a molten salt for the smelting process in the eutectic composition of CaF ₂ -MgF ₂ , CaF ₂ -BaF ₂ -LiF and CaF ₂ -LiF.

In addition, U.S. Patent US20150047745 A1 proposes a process for smelting aluminum from an aluminum alloy and a molten salt for this process in a NaF-KF equimolar composition. In Korean Patent Laid-Open No. 10-2006-0061048, the composition of the molten salt for magnesium alloy refining is proposed as 35 to 55 wt% of LiCl and 45 to 65 wt% of KCl.

However, the inventions proposed in this prior art suggest a method of smelting a metal at a high temperature in the range of 1100-1300 ° C.

In the SOM process, the process temperature is set at a temperature at which the molten salt represents the liquid phase region. Thus, conventional solid oxide membrane processes typically involve smelting at a process temperature of 1100-1300 ° C using an eutectic composition of 45MgF ₂ -55CaF _{2, as} the liquid phase appears near 1000 ° C. In other words, the solid oxide membrane process utilizes a composition of 45MgF ₂ -55CaF ₂ to produce magnesium in a reducing atmosphere with a positive electrode of yttrium-stabilized zirconia (YSZ) at a process temperature (1150-1300 ° C) above eutectic temperature It is not preferable from the viewpoint of energy efficiency and the like.

Therefore, if the SOM process temperature is reduced, the energy efficiency can be improved with cost reduction.

Therefore, unlike the molten salt composition used in the conventional SOM process, the present invention has been made in order to overcome the limitations of the prior art described above, and it is an object of the present invention to provide a low- And to provide a molten salt composition.

 Further, the technical problems to be solved by the present invention are not limited to the technical problems mentioned above, and other technical problems which are not mentioned can be understood from the following description in order to clearly understand those skilled in the art to which the present invention belongs .

According to an aspect of the present invention,

The present invention relates to a low-temperature molten salt composition which can be applied to a SOM process including 42 to 47% of MgF ₂ , 42 to 47% of CaF ₂ , and 6 to 16% of at least one of LiF and NaF and residual unavoidable impurities .

The low-temperature molten salt composition preferably has a MgF ₂ -CaF ₂ (= 1: 1) -MF (M = Li, Na) composition.

The solubility of the low-temperature molten salt composition in MgO may be 1.5 wt% or more at 950 ° C.

The present invention having the above-described constitution can propose a low-melting-point molten salt composition in a magnesium smelting process using a solid oxide membrane, so that the SOM process can be performed at a lower temperature than the conventional process, The effect can be expected to improve overall process efficiency.

FIG. 1 is a graph showing the potential change of the metal anion with the temperature of the cation using the Nernst Equation (assuming the activity of the cation to be 0.1).
Fig. 2 is a diagram showing a ternary state diagram. Fig. 2 (a) shows a ternary system of MgF ₂ -CaF ₂ -LiF and Fig. 2 (b) shows a ternary system diagram of MgF ₂ -CaF _2- NaF.
3 is a graph showing the melting point of the MgF ₂ -CaF ₂ -LiF molten salt composition and the partial vapor pressure of LiF according to the present embodiment.
4 is a graph showing the melting point of the MgF ₂ -CaF ₂ -NaF molten salt composition and partial pressure of NaF according to the present embodiment.

Hereinafter, the present invention will be described.

In a magnesium smelting process using a solid oxide membrane process, the molten salt is used as an electrolyte to produce gaseous oxygen and magnesium in the anode and the cathode, respectively. In general, halide-based molten salts are suitable for low-melting-point molten salts used as electrolytes because they have relatively low melting points and large ionization tendencies. Halide-based molten salts can be largely divided into fluorides and chlorides. Chlorides have a great deal of deliquescence and corrosion problems in the reactor. Accordingly, the present invention proposes a fluoride-based molten salt composition as an appropriate molten salt.

Specifically, the low-temperature molten salt composition applicable to the SOM process of the present invention comprises 42 to 47% of MgF ₂ , 42 to 47% of CaF ₂ , and at least one of LiF and NaF of 6 to 16% %, Residual unavoidable impurities.

Since MgF ₂ exists in a state of Mg ²⁺ after being ionized, it can be used as a raw material of magnesium smelting. The CaF ₂ is used to lower the melting point of MgF _{2, and} the melting point can be lowered in an economical manner because the cost is lower than that of other fluoride materials. Therefore, MgF ₂ -CaF ₂ was set to the main molten salt composition.

In the present invention, since the process point of 45MgF ₂ -55CaF ₂ appears at about 976 ° C, trial and error have been made to find additional materials for forming a low-temperature molten salt below 950 ° C. As a result, It is confirmed that addition of NaF can effectively lower the melting point of the low-temperature molten salt composition, and the present invention is presented.

Specifically, in order to reduce the magnesium ion from the cathode to magnesium, the reduction potential of the magnesium ion in the cations in the molten salt must be the largest. The standard reduction potentials of the cations are shown in the following Table 1, and the reduction potentials according to the temperatures are shown in FIG.

Reduction Half Reaction E ^o (V) Ti ²⁺ (aq) + 2e ^- ? Ti (s) -0.34 Fe ²⁺ (aq) + 2e ^- ? Fe (s) -0.44 O ₂ (g) + e ^- ? O ^2- (aq) -0.56 Zn ²⁺ (aq) + 2e ^- ? Zn (s) -0.76 Mn ²⁺ (aq) + 2e ^- ? Mn (s) -1.18 Al ³⁺ (aq) + 3e ^- ? Al (s) -1.66 Mg ²⁺ (aq) + 2e ^- ? Mg (s) -2.36 Na ⁺ (aq) + e ^- ? Na (s) -2.71 Li ⁺ (aq) + e ^- > Li (s) -3.05

In Table 1, ions having a smaller value than the reduction potential of magnesium are Li ⁺ and Na ⁺ . In order to investigate the effect of temperature on the reduction potential, the reduction potential of metal cations was calculated using the Nernst Equation.

[Relation 1]

Me → Me ^{n +} + ne ^-

[Relation 2]

[Relation 3]

In the above equation 1-3, E is _N-reduction potential, E _N ⁰ is the standard reduction potential, R is the gas constant (8.314 J / mol), T is absolute temperature, F is the Faraday constant (9.6458x10 ^-4), aMe ^{n +} Is the activity of the metal cation, and aMe is the activity of the metal.

At this time, the activity of the cation was assumed to be 0.1, and the results are shown in FIG. As shown in FIG. 1, the reduction potential does not show a large difference depending on the temperature, and the reduction potential of Li ⁺ and Na ⁺ ions is lower than that of Mg ²⁺ ions. This means that Mg ²⁺ ions can be reduced at the cathode only when LiF or NaF is added to the molten salt of MgF ₂ -CaF ₂ so that the magnesium can be smelted. Therefore, in the present invention, MgF ₂ -CaF ₂ molten salt was selected as a material to which LiF and NaF can be added.

That is, based on the above points, in the present invention, it is preferable that in the present invention, at least one of 42 to 47% of MgF ₂ , 42 to 47% of CaF ₂ and 6 to 16% of LiF and NaF, Temperature molten salt composition.

On the other hand, the low-temperature molten salt composition component content of the present invention was determined in consideration of two factors. First, the melting point according to the composition of the molten salt was calculated using FactSage ^™ 7.0 (FTsalt database), a thermodynamic software, that can be used at temperatures below 950 ° C.

FIG. 2 (ab) is a diagram showing a state diagram of the molten salt (MgF ₂ -CaF ₂ -LiF, MgF ₂ -CaF ₂ -NaF) proposed by the present invention using FactSage ^™ 7.0 which is a thermodynamic program. As shown in FIG. 2, the blue shaded portion means a range of the composition that exhibits a liquid phase at 950 ° C. When MgF ₂ and CaF ₂ are present in a ratio of 1: 1 and LiF or NaF is added, the composition Lt; RTI ID = 0.0 > 950 C. < / RTI >

Second, the volatility of the fluoride-based molten salt was investigated by calculating the vapor pressure. The linear fitting of log P (atm) according to 1 / T (K) was calculated with reference to literature values (Luxel Vapor Pressure Table, Luxel Corporation, 2017). At this time, the relational expressions of the vapor pressures depending on the temperatures of MgF ₂ , CaF ₂ , LiF, and NaF are defined by the following Equation 4-7, respectively.

[Relation 4]

log P = -15981.85 / T + 6.43

[Equation 5]

log P = -16760.64 / T + 6.02

[Relation 6]

log P = -11903.78 / T + 8.27

[Relation 7]

log P = -12097.19 / T + 8.25

According to the above relation, the saturated vapor pressures of MgF ₂ , CaF ₂ , LiF, and NaF at 950 ° C. are 2.3 × 10 ^-7 atm, 2.07 × 10 ^-8 atm, 0.03 atm, and 0.02 atm, respectively. Since the saturated vapor pressures of MgF ₂ and CaF ₂ are much lower than those of LiF and NaF, the volatility of the molten salt increases as the activity of LiF or NaF in the molten salt increases. The activity of LiF or NaF in molten salts (MgF ₂ -CaF ₂ -LiF, MgF ₂ -CaF ₂ -NaF) can be calculated using FactSage ^™ 7.0 (FTsalt database).

The higher the partial vapor pressure of the calculated LiF or NaF, the more volatilization of the molten salt occurs. Therefore, in the present invention, it is required that the melting point of the molten salt is 950 占 폚 or less and that the partial pressure of LiF or NaF at each melting point is less than 2.0 占^10-3 atm.

In consideration of this, the molten salt composition of the present invention contains 42-47% of MgF ₂ , 42-47% of CaF ₂ , and 6-16% of at least one of LiF and NaF, .

Specifically, in the molten salt of MgF ₂ -CaF ₂ -LiF, when the amount of CaF ₂ is relatively larger than that of MgF ₂ , the melting point is higher than 950 ° C. and the activity of LiF is increased to 0.36, so that the partial vapor pressure of LiF To about 0.01 atm. On the contrary, when MgF ₂ is relatively larger than CaF ₂ , the melting point is lowered, but the activity of LiF is increased to 0.25, and the partial vapor pressure of LiF is also increased to about 0.007 atm. When MgF ₂ and CaF ₂ are added in an amount of less than 6 wt% when LiF is added in an amount of less than 6 wt%, the activity of LiF is lowered but the melting point is increased to more than 950 ° C. When the amount of LiF is more than 16 wt% . Therefore, the molten salt of MgF ₂ - (42 ~ 47%) CaF ₂ - (16 ~ 6%) LiF is considered appropriate as the weight percentage (42 ~ 47%).

In the molten salt of MgF ₂ -CaF ₂ -NaF, when the amount of CaF ₂ is relatively larger than that of MgF ₂ and the amount of MgF ₂ is relatively larger than that of CaF ₂ , the melting point is higher than 950 ° C. and the activity of NaF is increased to 0.25 The partial vapor pressure of NaF also increases to 0.005 atm. In the case of MgF ₂ and CaF ₂ at a ratio of 1: 1, if NaF is added below 6 wt%, the activity of NaF is low but the melting point is higher than 950 ° C. If more than 16 wt% is added, the activity of NaF is increased The partial vapor pressure of NaF exceeds 2.0 × 10 ^-3 atm. Therefore, it is suggested that the molten salt of MgF ₂ - (42 ~ 47%) CaF ₂ - (16 ~ 6%) NaF is suitable as the weight percentage (42 ~ 47%).

From the viewpoint of the melting temperature and the vapor pressure, a more preferable molten salt composition satisfies MgF ₂ -CaF ₂ (= 1: 1) -MF (M = Li, Na).

On the other hand, in the smelting of magnesium through the solid oxide membrane process, how much MgO, which is the main material, melts into the molten salt is an important factor. The solubility of the low-temperature molten salt composition of the present invention to MgO is not less than 1.5% by weight at 950 ° C.

Hereinafter, the present invention will be described in detail by way of examples.

(Example 1)

A molten salt composition for SOM having the composition components shown in Table 2 was prepared. Then, the melting point of each of these molten salts was calculated using FactSage ^™ 7.0 (FTsalt database), a thermodynamic software, and the results are shown in Table 2 below. For each of the molten salts, the activity and partial vapor pressure of LiF or NaF at the melting point was calculated, and the results are also shown in Table 2 below.

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

As it is shown in Table 2, and have a lower melting point of less than 950 ℃ than in the case of Comparative Sample Nos. 9-12 and 26-28, which meet the composition range of the present invention component not, for example, as well as, 2.0 × 10 ^{- It} has a low partial vapor pressure of ³ atm or less. Therefore, it can be confirmed that the molten salt composition satisfies the requirements required for the low-temperature operation in the SOM process.

On the other hand, in the conventional example in which neither LiF nor NaF is added to the molten salt, the melting point of the molten salt exceeds 950 ° C.

(Example 2)

How much MgO is dissolved in the molten salt is an important factor in the smelting of magnesium through the solid oxide membrane process. Therefore, the melt quenching experiment was conducted to derive MgO solubility in the molten salt.

That is, a fluoride-based molten salt having a composition as shown in Table 3 below was placed in a carbon crucible, and a cap was placed on the top of the crucible to prevent volatilization of the fluoride-based molten salt. Inside the crucible, molten salt and MgO bulk of a certain size were placed and quenching was performed at the temperature of 950, 1000, 1100, 1200 ℃ in the vertical resistance furnace.

The molten salt used in the experiment the composition are two compositions of 46.5MgF ₂ -46.5CaF ₂ included in the invention example - the composition of the three 7LiF and 45MgF ₂ -45CaF ₂ -10NaF was used. After the test, the oxygen concentration in the molten salt was analyzed using a combustion analyzer (NO, TC-300, LECO). The results are shown in Table 3.

Molten salt composition (% by weight) MgO solubility (wt%) MgF2 CaF2 LiF NaF 950 ℃ 1000 ℃ 1100 ℃ 1200 ℃ Honor 46.5 46.5 7 - 1.5 1.9 2.5 3.3 Honor 45 45 - 10 1.5 1.7 1.7 1.9

As can be seen from Table 3, the molten salt of 46.5MgF ₂ -46.5CaF ₂ -7LiF had MgO solubilities of 1.5 wt% and 2.3 wt% at 950 ° C and 1200 ° C, respectively, and 45MgF ₂ -45CaF ₂ -10NaF Of the molten salt has MgO solubility of 1.5 wt% and 1.9 wt% at 950 ° C and 1200 ° C, respectively. That is, it can be seen that the molten salt composition of the present invention has a MgO solubility of 1.5 wt% or more at a temperature of 950 ° C., and thus MgO can be effectively reduced even in a low-temperature operation at 950 ° C. or lower in the SOM process.

While the present invention has been particularly shown and described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments, Of course, this is possible. Therefore, the scope of the present invention should not be limited to the above-described embodiments but should be defined by the following claims as well as equivalents thereof.

Claims (4)

The present invention relates to a low-temperature molten salt composition capable of being applied to a SOM process including 42 to 47% of MgF ₂ , 42 to 47% of CaF ₂ , and 6 to 16% of at least one of LiF and NaF and residual unavoidable impurities .
The low-temperature molten salt composition according to claim 1, wherein the low-temperature molten salt composition has a composition of MgF ₂ -CaF ₂ (= 1: 1) -MF (M = Li, Na) Composition.
The low-temperature molten salt composition according to claim 1, wherein the solubility of the low-temperature molten salt composition in MgO is at least 1.5 wt% at 950 ° C.
The low-temperature molten salt composition according to claim 1, wherein the low-temperature molten salt has a melting point of 950 ° C or less and a partial pressure of LiF or NaF at each melting point is less than 2.0 × 10 ^-3 atm.

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