A quinary fluoride molten salt and a method for preparing the same

Abstract

This invention discloses a five-component fluoride molten salt. The composition of the five-component fluoride molten salt is: 57.1–57.6 wt.% RbF, 8.2–8.6 wt.% LiF, 3.1–3.5 wt.% NaF, 16.6–17.0 wt.% KF, and 14.0–14.4 wt.% ZnF₂. This invention also discloses a method for preparing the five-component fluoride molten salt. This invention innovatively uses RbF as the main component, combined with appropriate amounts of Li, Na, K, and Zn fluorides, and for the first time proposes and prepares a five-component fluoride molten salt. This molten salt has a melting point of 410.0 ± 2.0 °C, a solid-liquid enthalpy change of 107.0 ± 6.0 J / g, a specific heat of 1.55 ± 0.55 J / g °C, and a decomposition temperature of 755 ± 2 °C. This molten salt has a melting point about 50°C lower than existing fluoride salt systems, exhibits excellent heat transfer and storage performance, can effectively improve the safety of heat transfer and storage systems, and has a wider range of application prospects.

Description

Technical Field

[0001] This invention relates to a fluoride molten salt, and more particularly to a five-component fluoride molten salt formulation and its preparation method. Background Technology

[0002] Molten salt is a liquid salt composed of cations and anions in a molten state. It has advantages such as a wide operating range, large heat capacity, and relatively high high-temperature stability. It is recognized as a heat transfer and storage medium in fields such as nuclear fission, spent fuel reprocessing, solar thermal power generation, and high-temperature hydrogen production.

[0003] Commonly used molten salts as heat transfer and storage materials are mainly classified into four categories: nitrates, carbonates, chlorides, and fluorides composed of alkali metals or alkaline earth metals. Nitrates have excellent performance and low corrosivity, and have been successfully applied in solar thermal power generation. However, their poor high-temperature thermal stability limits the potential for improving thermal efficiency, thus restricting their development in high-temperature applications. Carbonates and their mixtures have high heat of fusion and good heat storage capacity, but their high viscosity and the tendency of some carbonates to decompose at high temperatures limit their practical applications. Chlorides are diverse, have high latent heat of phase change, and good high-temperature stability, but their strong high-temperature corrosivity greatly limits their large-scale application. Fluorides, on the other hand, have advantages such as large volumetric heat of fusion, high thermal conductivity, low viscosity, and a wide operating temperature range. Due to their radiation resistance and excellent neutron properties, they are now widely recognized as heat transfer and storage media in nuclear reactors, spent fuel reprocessing, and high-temperature hydrogen production.

[0004] Among existing fluoride molten salts, LiF-NaF-KF (FLiNaK, 46.5:11.5:42.0 mol%) is a recognized fluoride heat transfer and storage medium with great application potential. However, its freezing point is relatively high (approximately 458℃), making it prone to freezing and clogging pipelines during application. This not only reduces system safety but also significantly increases the overall system cost due to the added costs of heat tracing and anti-freezing. This cannot fully meet the needs of comprehensive heat transfer and storage utilization. Therefore, it is necessary to develop fluoride molten salts with lower melting points. Summary of the Invention

[0005] The technical problem to be solved by the present invention is to overcome the shortcomings of the high melting point of FLiNaK molten salt and provide a five-element fluoride molten salt, which has the common advantages of fluoride molten salts, while significantly reducing the melting point compared with existing fluoride molten salts, and has better application prospects.

[0006] The present invention specifically adopts the following technical solutions to solve the above-mentioned technical problems:

[0007] A pentagonal fluoride molten salt has the following composition: 8.2–8.6 wt.% LiF, 3.1–3.5 wt.% NaF, 16.6–17.0 wt.% KF, 57.1–57.6 wt.% RbF, and 14.0–14.4 wt.% ZnF2.

[0008] One preferred formulation has the following composition: 8.3 wt.% LiF, 3.3 wt.% NaF, 16.9 wt.% KF, 57.1 wt.% RbF, and 14.4 wt.% ZnF2.

[0009] The second preferred embodiment has the following composition: 8.4 wt.% LiF, 3.3 wt.% NaF, 16.9 wt.% KF, 57.3 wt.% RbF, and 14.1 wt.% ZnF2.

[0010] The third preferred option has the following composition: 8.6 wt.% LiF, 3.5 wt.% NaF, 16.6 wt.% KF, 57.3 wt.% RbF, and 14.0 wt.% ZnF2.

[0011] The fourth preferred embodiment has the following composition: 8.4 wt.% LiF, 3.4 wt.% NaF, 16.7 wt.% KF, 57.2 wt.% RbF, and 14.3 wt.% ZnF2.

[0012] The fifth preferred embodiment has the following composition: 8.2 wt.% LiF, 3.1 wt.% NaF, 17.0 wt.% KF, 57.6 wt.% RbF, and 14.1 wt.% ZnF2.

[0013] The preparation method of the five-component fluoride molten salt as described in any of the above technical solutions includes the following steps: after mixing each component evenly according to the ratio, heating to 100-200℃ at a heating rate of 2-10℃ / min under an inert gas protection environment, and holding at the temperature for 0.5-5h; then heating at a rate of 2-15℃ / min until the molten salt is completely melted, holding at the temperature for 0.5-12 hours and then naturally cooling to room temperature.

[0014] Compared with the prior art, the technical solution of the present invention has the following beneficial effects:

[0015] This invention proposes a five-component fluoride molten salt system for the first time. It innovatively uses RbF as the main component, combined with appropriate amounts of fluorides of Li, Na, K, and Zn, to obtain a five-component fluoride molten salt system with extremely excellent performance, including a melting point of 410.0±2.0℃, a solid-liquid enthalpy change of 107.0±6.0 J / g, a specific heat of 1.55±0.55 J / g℃, and a decomposition temperature of 755℃±2. The melting point of this system is about 50℃ lower than that of existing fluoride salt systems, which can effectively improve the safety of heat transfer and storage systems and has a wider range of application prospects. Attached Figure Description

[0016] Figure 1 This is the DSC variation curve of Example 1;

[0017] Figure 2 This is the DSC variation curve of Example 2;

[0018] Figure 3 This is the DSC variation curve of Example 3;

[0019] Figure 4 This is the DSC variation curve of Example 4;

[0020] Figure 5 This is the DSC variation curve of Example 5;

[0021] Figure 6 This is the DSC change curve of Comparative Example 1;

[0022] Figure 7 The DSC variation curve is shown in Comparative Example 2;

[0023] Figure 8 The curve showing the specific heat of the pentagonal fluoride molten salt of Example 1 of the present invention as a function of temperature;

[0024] Figure 9 The curve showing the change in mass fraction of the pentagonal fluoride molten salt as a function of temperature in Example 1 of the present invention is shown. Detailed Implementation

[0025] While existing FLiNaK molten salts possess excellent overall performance, their melting point of approximately 458℃ easily leads to pipeline freezing and blockage during application. This not only reduces system safety but also drastically increases the overall system cost due to the added expenses for heat tracing and anti-freezing. To address this issue, this invention innovatively uses RbF as the main component, combined with appropriate amounts of Li, Na, K, and Zn fluorides, to obtain for the first time a five-component fluoride molten salt system. This system has a melting point of 410.0±2.0℃, a solid-liquid enthalpy change of 107.0±6.0 J / g, a specific heat of 1.55±0.55 J / g℃, and a decomposition temperature of 755℃±2. Not only does it exhibit superior overall performance, but more importantly, its melting point is reduced by up to 50℃ compared to existing fluoride salt systems, completely overcoming the problem of pipeline freezing and blockage caused by existing fluoride molten salts.

[0026] Specifically, the five-component fluoride molten salt proposed in this invention has the following composition: 8.2-8.6 wt.% LiF, 3.1-3.5 wt.% NaF, 16.6-17.0 wt.% KF, 57.1-57.6 wt.% RbF, and 14.0-14.4 wt.% ZnF2.

[0027] The present invention also proposes a method for preparing the five-component fluoride molten salt, comprising the following steps: after mixing the components evenly according to the ratio, heating to 100-200℃ at a heating rate of 2-10℃ / min under an inert gas protection environment, and holding at the temperature for 0.5-5h; then heating at a rate of 2-15℃ / min until the molten salt is completely melted, holding at the temperature for 0.5-12 hours and then naturally cooling to room temperature.

[0028] To facilitate public understanding of the technical solution and effects of this invention, the technical solution of this invention will be described in detail below through several specific embodiments and in conjunction with the accompanying drawings:

[0029] Example 1

[0030] After calibrating the electronic balance, weigh 8.3 wt.% LiF, 3.3 wt.% NaF, 16.9 wt.% KF, 57.1 wt.% RbF, and 14.4 wt.% ZnF2 in the glove box.

[0031] After mixing evenly, place the mixture into a clean and dry quartz crucible. Place the covered quartz crucible in a high-temperature resistance furnace covered with a high-purity inert atmosphere, and heat it to 100-200℃ at a rate of 2-10℃ / min. Hold the temperature for 0.5-5 hours to remove free water from the molten salt.

[0032] Then, the temperature was increased to 420-700℃ at a heating rate of 2-10℃ / min, and held at this temperature for 0.5-12h before being naturally cooled to room temperature for characterization.

[0033] The prepared pentagonal fluoride molten salt was tested using a pre-calibrated differential scanning calorimeter (DSC). The peak shape and melting point after testing were used to determine whether a eutectic molten salt had formed. Figure 1 As shown, the melting peak of the molten salt is regular and is a single peak, indicating that the molten salt system with this ratio forms a eutectic molten salt with a melting point of 410.8℃ and an enthalpy change of 109.3J / g.

[0034] Example 2

[0035] After calibrating the electronic balance, weigh 8.4 wt.% LiF, 3.3 wt.% NaF, 16.9 wt.% KF, 57.3 wt.% RbF, and 14.1 wt.% ZnF2 in the glove box.

[0036] After mixing evenly, place the mixture into a clean and dry quartz crucible. Place the covered quartz crucible in a high-temperature resistance furnace covered with a high-purity inert atmosphere, and heat it to 100-200℃ at a rate of 2-10℃ / min. Hold the temperature for 0.5-5 hours to remove free water from the molten salt.

[0037] Then, the temperature was increased to 420-700℃ at a heating rate of 2-10℃ / min, and held at this temperature for 0.5-12h before being naturally cooled to room temperature for characterization.

[0038] The prepared pentagonal fluoride molten salt was tested using a pre-calibrated differential scanning calorimeter (DSC). The peak shape and melting point after testing were used to determine whether a eutectic molten salt had formed. Figure 2 As shown, the melting peak of the molten salt is regular and is a single peak, indicating that the molten salt system with this ratio forms a eutectic molten salt with a melting point of 410.5℃ and an enthalpy change of 107.6J / g.

[0039] Example 3

[0040] After calibrating the electronic balance, weigh 8.6 wt.% LiF, 3.5 wt.% NaF, 16.6 wt.% KF, 57.3 wt.% RbF, and 14.0 wt.% ZnF2 in the glove box.

[0041] After mixing evenly, place the mixture into a clean and dry quartz crucible. Place the covered quartz crucible in a high-temperature resistance furnace covered with a high-purity inert atmosphere, and heat it to 100-200℃ at a rate of 2-10℃ / min. Hold the temperature for 0.5-5 hours to remove free water from the molten salt.

[0042] Then, the temperature was increased to 420-700℃ at a heating rate of 2-10℃ / min, and held at this temperature for 0.5-12h before being naturally cooled to room temperature for characterization.

[0043] The prepared pentagonal fluoride molten salt was tested using a pre-calibrated differential scanning calorimeter (DSC). The peak shape and melting point after testing were used to determine whether a eutectic molten salt had formed. Figure 3 As shown, the melting peak of the molten salt is regular and is a single peak, indicating that the molten salt system with this ratio forms a eutectic molten salt with a melting point of 409.2℃ and an enthalpy change of 110.3J / g.

[0044] Example 4

[0045] After calibrating the electronic balance, weigh 8.4 wt.% LiF, 3.4 wt.% NaF, 16.7 wt.% KF, 57.2 wt.% RbF, and 14.3 wt.% ZnF2 in the glove box.

[0046] After mixing evenly, place the mixture into a clean and dry quartz crucible. Place the covered quartz crucible in a high-temperature resistance furnace covered with a high-purity inert atmosphere, and heat it to 100-200℃ at a rate of 2-10℃ / min. Hold the temperature for 0.5-5 hours to remove free water from the molten salt.

[0047] Then, the temperature was increased to 420-700℃ at a heating rate of 2-10℃ / min, and held at this temperature for 0.5-12h before being naturally cooled to room temperature for characterization.

[0048] The prepared pentagonal fluoride molten salt was tested using a pre-calibrated differential scanning calorimeter (DSC). The peak shape and melting point after testing were used to determine whether a eutectic molten salt had formed. Figure 4 As shown, the melting peak of the molten salt is regular and is a single peak, indicating that the molten salt system with this ratio forms a eutectic molten salt with a melting point of 411.2℃ and an enthalpy change of 105.6J / g.

[0049] Example 5

[0050] After calibrating the electronic balance, weigh 8.2 wt.% LiF, 3.1 wt.% NaF, 17.0 wt.% KF, 57.6 wt.% RbF, and 14.1 wt.% ZnF2 in the glove box.

[0051] After mixing evenly, place the mixture into a clean and dry quartz crucible. Place the covered quartz crucible in a high-temperature resistance furnace covered with a high-purity inert atmosphere, and heat it to 100-200℃ at a rate of 2-10℃ / min. Hold the temperature for 0.5-5 hours to remove free water from the molten salt.

[0052] Then, the temperature was increased to 420-700℃ at a heating rate of 2-10℃ / min, and held at this temperature for 0.5-12h before being naturally cooled to room temperature for characterization.

[0053] The prepared pentagonal fluoride molten salt was tested using a pre-calibrated differential scanning calorimeter (DSC). The peak shape and melting point after testing were used to determine whether a eutectic molten salt had formed. Figure 5 As shown, the melting peak of the molten salt is regular and is a single peak, indicating that the molten salt system with this ratio forms a eutectic molten salt with a melting point of 408.5℃ and an enthalpy change of 107.3J / g.

[0054] The melting point and enthalpy change data of the five fluoride molten salts obtained in the above five examples are shown in Table 1. It can be seen that their melting points are all around 410℃.

[0055] Table 1 Melting point and enthalpy change of Examples 1-5

[0056] Example 1 Example 2 Example 3 Example 4 Example 5 Melting point (°C) 410.8 410.5 409.2 411.2 408.5 Enthalpy change (J / g) 109.3 107.6 110.3 105.6 107.3

[0057] The specific heat of the LiF-NaF-KF-RbF-ZnF2 molten salt in Example 1 was measured using a pre-calibrated differential scanning calorimeter (DSC). The experimental results were then analyzed using a specific heat comparison method. The specific heat was 1.55 ± 0.55 J / g℃. The curve showing the change in specific heat with temperature is shown below. Figure 8 As shown.

[0058] The weight loss of the LiF-NaF-KF-RbF-ZnF2 molten salt in Example 1 was measured using a simultaneous thermal analyzer (STA), and the experimental results were analyzed. The decomposition temperature was found to be 755℃, and the mass fraction change curve with temperature is shown below. Figure 9 As shown.

[0059] To verify whether other ratios could produce the same effect, the following comparative experiments were also conducted.

[0060] Comparative Example 1

[0061] After calibrating the electronic balance, weigh 8.1 wt.% LiF, 3.3 wt.% NaF, 16.4 wt.% KF, 57.9 wt.% RbF, and 14.3 wt.% ZnF2 in the glove box.

[0062] After mixing evenly, place the mixture into a clean and dry quartz crucible. Place the covered quartz crucible in a high-temperature resistance furnace covered with a high-purity inert atmosphere, and heat it to 100-200℃ at a rate of 2-10℃ / min. Hold the temperature for 0.5-5 hours to remove free water from the molten salt.

[0063] Then, the temperature was increased to 420-700℃ at a heating rate of 2-10℃ / min, and held at this temperature for 0.5-12h before being naturally cooled to room temperature for characterization.

[0064] The prepared pentagonal fluoride molten salt was tested using a pre-calibrated differential scanning calorimeter (DSC). The peak shape and melting point after testing were used to determine whether a eutectic molten salt had formed. Figure 6 As shown, the melting peak of the molten salt is irregular, indicating that the molten salt system with this ratio is not a stable single phase, and the melting point increases to 417.4℃.

[0065] Comparative Example 2

[0066] After calibrating the electronic balance, weigh 11.8 wt.% LiF, 4.7 wt.% NaF, 23.8 wt.% KF, 47.9 wt.% RbF, and 11.8 wt.% ZnF2 in the glove box.

[0067] After mixing evenly, place the mixture into a clean and dry quartz crucible. Place the covered quartz crucible in a high-temperature resistance furnace covered with a high-purity inert atmosphere, and heat it to 100-200℃ at a rate of 2-10℃ / min. Hold the temperature for 0.5-5 hours to remove free water from the molten salt.

[0068] Then, the temperature was increased to 420-700℃ at a heating rate of 2-10℃ / min, and held at this temperature for 0.5-12h before being naturally cooled to room temperature for characterization.

[0069] The prepared pentagonal fluoride molten salt was tested using a pre-calibrated differential scanning calorimeter (DSC). The peak shape and melting point after testing were used to determine whether a eutectic molten salt had formed. Figure 7 As shown, the melting peak of this molten salt is not a regular single peak, indicating that the molten salt system with this ratio has not formed a stable single phase, and the melting point has increased to 417.8℃.

[0070] In summary, the five-element fluoride molten salt system proposed in this invention effectively overcomes the shortcomings of existing fluoride molten salts with high melting points. It not only enriches the database of molten salt heat transfer and storage performance, but also provides an excellent choice for heat transfer and storage working fluids in fields such as molten salt reactor coolant, molten salt energy storage, pyrolysis biomass, and industrial waste heat recovery.

Claims

1. A pentagonal fluoride molten salt, characterized in that, Its composition is: 8.2-8.6 wt.% LiF, 3.1-3.5 wt.% NaF, 16.6-17.0 wt.% KF, 57.1-57.6 wt.% RbF, and 14.0-14.4 wt.% ZnF2.

2. The pentagonal fluoride molten salt as described in claim 1, characterized in that, Its composition is: 8.3 wt.% LiF, 3.3 wt.% NaF, 16.9 wt.% KF, 57.1 wt.% RbF, and 14.4 wt.% ZnF2.

3. The pentagonal fluoride molten salt as described in claim 1, characterized in that, Its composition is: 8.4 wt.% LiF, 3.3 wt.% NaF, 16.9 wt.% KF, 57.3 wt.% RbF, and 14.1 wt.% ZnF2.

4. The pentagonal fluoride molten salt as described in claim 1, characterized in that, Its composition is: 8.6 wt.% LiF, 3.5 wt.% NaF, 16.6 wt.% KF, 57.3 wt.% RbF, and 14.0 wt.% ZnF2.

5. The pentagonal fluoride molten salt as described in claim 1, characterized in that, Its composition is: 8.4 wt.% LiF, 3.4 wt.% NaF, 16.7 wt.% KF, 57.2 wt.% RbF, and 14.3 wt.% ZnF2.

6. The pentagonal fluoride molten salt as described in claim 1, characterized in that, Its composition is: 8.2 wt.% LiF, 3.1 wt.% NaF, 17.0 wt.% KF, 57.6 wt.% RbF, and 14.1 wt.% ZnF2.

7. The method for preparing the pentagonal fluoride molten salt according to any one of claims 1 to 6, characterized in that, Includes the following steps: After mixing all components evenly according to the formula, heat to 100-200℃ at a heating rate of 2-10℃ / min under an inert gas protection environment, and hold at that temperature for 0.5-5h; then heat at a rate of 2-15℃ / min until the molten salt is completely melted, hold at that temperature for 0.5-12 hours, and then cool naturally to room temperature.

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