Ternary mixed eutectic molten salt and preparation method thereof

Abstract

The invention belongs to the technical field of molten salts, and provides a ternary mixed eutectic molten salt and a preparation method thereof, and the ternary mixed eutectic molten salt comprises the following components in percentage by mass: 50-56 wt% of potassium nitrate, 42-46 wt% of sodium nitrite and 1-4 wt% of sodium carbonate. The preparation method comprises the following steps: S1, uniformly mixing dried potassium nitrate, sodium nitrite and sodium carbonate according to a preset proportion; s2, putting the mixed salt into a high-temperature furnace, calcining until the mixed salt is completely molten, and then naturally cooling; and S3, grinding the cooled solid eutectic fused salt, and sealing and storing. The potassium nitrate-sodium nitrite-sodium carbonate eutectic molten salt is obtained by adjusting the formula of the molten salt, so that the molten salt is free of precipitation at the low-temperature end, the melting point is lower than 150 DEG C, the decomposition temperature is higher than 610 DEG C, and compared with an existing commercial salt working area, the working area is improved.

Description

Technical Field

[0001] The invention belongs to the technical field of molten salts, and in particular relates to a ternary mixed eutectic molten salt and a preparation method thereof. Background Art

[0002] As an excellent heat transfer and energy storage medium, molten salt has the advantages of high specific heat capacity, high stability, and a wide operating temperature window. It has become one of the most recognized heat transfer and storage media in the field of solar thermal power generation and thermal energy storage. Solar Salt (60wt% NaNO3-40 wt% KNO3) and Hitec salt (53wt% KNO3-40 wt% NaNO2-7 wt% NaNO3), which have been commercially applied, have been generally regarded as commercial salts for concentrated solar thermal power generation (CSP) systems due to their good thermophysical properties and low cost. However, in engineering applications, the high melting point of Solar Salt (223℃) can easily lead to pipeline freezing, and the decomposition temperature of Hitec Salt is low (500℃), resulting in low thermoelectric conversion efficiency. In order to further improve the power generation efficiency, the development of new molten salts with a wider operating temperature window and excellent thermophysical properties has important engineering value for solar thermal power generation, molten salt energy storage and other fields.

[0003] After preliminary research, it was found that the molten salt has a lower melting point than the commercial Solar Salt and a higher decomposition temperature than the Hitec Salt. However, in actual use, it was found that the molten salt of this component would have a single component solid precipitate during the cooling process, resulting in limited use. Summary of the invention

[0004] In response to the above technical problems, the present invention provides a ternary mixed eutectic molten salt and a preparation method thereof. By improving the formula of the molten salt, no precipitation is produced at the low-temperature end. At the same time, the melting point is less than 150°C and the decomposition temperature is greater than 610°C, which improves the working area compared to existing commercial salt.

[0005] To achieve the above object, the technical solution of the present invention is:

[0006] A ternary mixed eutectic molten salt is composed of 50-56wt% of potassium nitrate, 42-46wt% of sodium nitrite and 1-4wt% of sodium carbonate calculated by mass fraction.

[0007] Preferably, calculated by mass fraction, the ternary mixed eutectic molten salt consists of 51.5-55wt% potassium nitrate, 42.0-44.7wt% sodium nitrite and 1.5-3.8wt% sodium carbonate.

[0008] Preferably, calculated by mass fraction, the ternary mixed eutectic molten salt consists of 51.5wt% of potassium nitrate, 44.7wt% of sodium nitrite and 3.8wt% of sodium carbonate, or consists of 53wt% of potassium nitrate, 43.82wt% of sodium nitrite and 3.18wt% of sodium carbonate, or consists of 53.56wt% of potassium nitrate, 44.1wt% of sodium nitrite and 2.3wt% of sodium carbonate, or consists of 54.3wt% of potassium nitrate, 42.2wt% of sodium nitrite and 3.5wt% of sodium carbonate, or consists of 54.6wt% of potassium nitrate, 43.9wt% of sodium nitrite and 1.5wt% of sodium carbonate, or consists of 55wt% of potassium nitrate, 42.0wt% of sodium nitrite and 3.0wt% of sodium carbonate.

[0009] Based on the same inventive concept, the present invention also provides a method for preparing the above-mentioned ternary mixed eutectic molten salt, comprising the following steps:

[0010] S1: uniformly mixing the dried potassium nitrate, sodium nitrite and sodium carbonate according to a preset ratio;

[0011] S2: placing the mixed salt into a high temperature furnace for calcination until completely melted, and then cooling naturally;

[0012] S3: Grind the cooled eutectic molten salt and seal it for storage.

[0013] Preferably, in the drying step S1, potassium nitrate, sodium nitrite and sodium carbonate are dried in an oven at 105-120° C. for 24-48 hours.

[0014] Preferably, the calcination in step S2 is specifically as follows: heating to 120°C at a heating rate of 5-10°C / min, and then keeping warm for 30 minutes; then heating to 300-350°C at a heating rate of 5-10°C / min until the molten salt is completely melted, and then keeping warm for 1-12 hours and then cooling with the furnace.

[0015] The two-stage heating calcination is used to make the molten salt mix more evenly. First, the temperature is raised to 120°C, and then the mixed salt is dehydrated for a second time while keeping warm to increase the purity of the salt. The temperature is then raised to 300-350°C, which is higher than the melting point, to fully melt the molten salt into a uniform liquid. Heating at a heating rate of 5-10°C / min can affect the melting speed of the salt. The slower the melting, the more evenly the liquid components are mixed.

[0016] Due to the adoption of the above technical solution, the present invention has the following advantages and positive effects compared with the prior art:

[0017] The present invention improves the molten salt formula to provide a ternary molten salt composed of 50-56wt% potassium nitrate, 42-46wt% sodium nitrite and 1-4wt% sodium carbonate. The mixed molten salt in this range is a eutectic molten salt of potassium nitrate-sodium nitrite-sodium carbonate. The system is more stable and no precipitation will occur in the low temperature section. At the same time, the melting point is less than 150°C, which can be as low as 141.7°C, and the decomposition temperature is as high as 610.7°C, which is 81.3°C lower than the melting point of commercial salt, and the upper limit temperature is 45.7°C higher, which broadens the working area of ​​molten salt, can effectively reduce pipeline freezing and blockage, reduce heating costs, and improve thermoelectric conversion efficiency.

[0018] Moreover, this component molten salt is a new type of ternary molten salt composed of potassium nitrate, sodium nitrite and sodium carbonate. It does not contain precious metals, and sodium nitrite and sodium carbonate are cheaper than sodium nitrate, so the cost is lower than commercial nitric acid molten salt. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 The DSC curve of the ternary mixed eutectic molten salt provided in Example 1 of the present invention;

[0020] Figure 2 TG curve of the ternary mixed eutectic molten salt provided in Example 1 of the present invention;

[0021] Figure 3 The specific heat capacity curve of the ternary mixed eutectic molten salt provided in Example 1 of the present invention;

[0022] Figure 4 The DSC curve of the ternary mixed molten salt provided in Comparative Example 1 of the present invention;

[0023] Figure 5 TG curve of the ternary mixed molten salt provided in Comparative Example 1 of the present invention;

[0024] Figure 6 The specific heat capacity curve of the ternary mixed eutectic salt provided in Comparative Example 1 of the present invention;

[0025] Figure 7 The DSC curve of the ternary mixed eutectic molten salt provided in Example 2 of the present invention;

[0026] Figure 8 The DSC curve of the ternary mixed eutectic molten salt provided in Example 3 of the present invention;

[0027] Fig. 9 The DSC curve of the ternary mixed eutectic molten salt provided in Example 4 of the present invention;

[0028] Fig.10 The DSC curve of the ternary mixed eutectic molten salt provided in Example 5 of the present invention;

[0029] Fig.11The DSC curve of the ternary mixed eutectic molten salt provided in Example 6 of the present invention;

[0030] Fig.12 The DSC curve of the ternary mixed eutectic molten salt provided in Comparative Example 2 of the present invention;

[0031] Fig.13 This is the DSC curve of the ternary mixed eutectic molten salt provided in Comparative Example 3 of the present invention. DETAILED DESCRIPTION

[0032] The following is a further detailed description of a ternary mixed eutectic molten salt and a preparation method thereof proposed by the present invention in conjunction with the accompanying drawings and specific embodiments. The advantages and features of the present invention will become more apparent from the following description.

[0033] The invention provides a ternary mixed eutectic molten salt. Calculated by mass fraction, the ternary mixed eutectic molten salt consists of 50-56wt% of potassium nitrate, 42-46wt% of sodium nitrite and 1-4wt% of sodium carbonate.

[0034] The preparation method of the above-mentioned ternary mixed eutectic molten salt adopts the following steps:

[0035] S1: Dry the single components of potassium nitrate, sodium nitrite and sodium carbonate in an oven at 120° C. for 12 hours, mix them evenly according to the raw material ratio in Table 1, and place them in a clean and dry alumina crucible;

[0036] S2: The alumina crucible containing the salt mixture is transferred to a muffle furnace for heating. The temperature program is as follows: the temperature is increased from room temperature to 120°C at a heating rate of 10°C / min and kept at a constant temperature for 300 min; then, the temperature is increased to 300°C at a heating rate of 10°C / min to completely melt the molten salt, and then cooled to room temperature with the furnace after keeping the temperature for 6 hours;

[0037] S3: The cooled solid molten salt is transferred to an agate mortar for grinding and crushing, and then placed in a sealed bag for subsequent use.

[0038] The ternary molten salt prepared according to the above method, the required raw materials are weighed according to Table 1.

[0039] Table 1

[0040]

[0041] After the above comparative examples and embodiments are prepared, the melting point, initial crystallization point and specific heat capacity of the molten salt are tested using a calibrated differential scanning calorimeter (DSC). During the test, the protective gas and the purge gas are both high-purity nitrogen, the instrument heating rate is 10K / min, the test range is 30-500°C, and the DSC curve is obtained. The intersection of the tangent of the curve on the left side of the melting peak in the DSC curve and the baseline is used as the melting point of the molten salt, the area of ​​the melting peak is the melting enthalpy of the molten salt, and if the melting peak is a single peak, the molten salt is a eutectic salt. During the specific heat test, the test procedures of the baseline, standard sample and sample are consistent, and the specific heat capacity of the molten salt is calculated according to the test curve. The specific test results are shown in Table 2.

[0042] Then, the decomposition temperature of the molten salt was tested using a thermogravimetric analyzer (TG). During the test, the protective gas and the purge gas were both high-purity nitrogen, the instrument heating rate was 10K / min, the test range was 30-800℃, and the intersection of the tangent line of the weight loss curve and the baseline was the decomposition temperature. The specific test results are shown in Table 2.

[0043] Table 2

[0044] serial number Melting point(℃) Melting enthalpy (J / g) Specific heat capacity (J / g·K) Decomposition temperature(℃) Example 1 141.7 66.8 1.65 610.7 Example 2 146.5 85.6 1.59 623.0 Example 3 141.3 77.8 1.67 614.9 Example 4 143.2 74.9 1.65 608.3 Example 5 141.4 69.3 1.68 611.2 Example 6 143.0 81.4 1.65 617.5 Comparative Example 1 225.0 76.1 1.52 643.6 Comparative Example 2 166.5 83.8 1.62 566.0 Comparative Example 3 213.5 78.5 1.59 608.6

[0045] It can be seen from the experimental data in Table 2 that the molten salt prepared in this embodiment is in the range of 141.3-146.5°C, and the decomposition temperature is above 608.3, while the melting point of Solar Salt is 223°C, and the decomposition temperature of Hitec Salt is 500°C. Therefore, the molten salt prepared in this embodiment has a melting point 81.3°C lower than that of commercial salt, and a decomposition temperature 45.7°C higher, which broadens the working area of ​​the molten salt.

[0046] The embodiment 1 is specifically analyzed, and the test results are as follows Figure 1 , Figure 2 and Figure 3 As shown, the prepared molten salt has only a single peak in the melting curve of the DSC curve, and no other obvious secondary peaks appear, indicating that the molten salt prepared according to this ratio is a eutectic salt, and only solid in the eutectic state is precipitated during the cooling process of the molten salt (that is, the liquid is converted into a solid), and no single solid component will be precipitated during the cooling process or during the use in the low-temperature stage. Its melting point is 141.7°C, the latent heat of phase change is 66.8 J / g, the average specific heat capacity of the liquid stage is 1.65 J / g·K, and the decomposition temperature is 610.7°C.

[0047] Specific reference Figure 7-11 The molten salt prepared in Example 2-6 has only a single peak in the melting curve of the DSC curve, and no other obvious secondary peaks appear, indicating that the prepared molten salt is a eutectic salt.

[0048] See also Figure 4 , Figure 12-13However, the molten salt prepared in Comparative Examples 1-3 has a secondary peak in the melting curve of the DSC curve, indicating that the molten salt prepared according to this ratio is a non-eutectic salt, and a single solid cost is precipitated during the cooling process or at the low temperature stage.

[0049] By comparing Examples 1-6 with Comparative Examples 1-3, it can be concluded that the molten salt prepared in the Examples is a eutectic molten salt, and no single solid component will precipitate during the cooling process or during use in the low-temperature stage. The molten salt prepared in the Comparative Examples will precipitate a single solid component during the cooling process or in the low-temperature stage due to the appearance of a secondary peak in the melting curve of the DSC curve.

[0050] Therefore, the ternary molten salt provided by the present invention is a eutectic molten salt, and the molten salt system is more stable. No single component will be precipitated during the cooling process or when used in the low-temperature stage. In addition, the melting point is 81.3°C lower than that of commercial salt and the upper limit temperature is 45.7°C higher.

[0051] The embodiments of the present invention are described in detail above in conjunction with the accompanying drawings, but the present invention is not limited to the above embodiments. Even if various changes are made to the present invention, if these changes fall within the scope of the claims of the present invention and their equivalents, they still fall within the protection scope of the present invention.

Claims

1. A ternary mixed eutectic molten salt, characterized in that: Calculated by mass fraction, the ternary mixed eutectic molten salt consists of 50-56wt% of potassium nitrate, 42-46wt% of sodium nitrite and 1-4wt% of sodium carbonate.

2. The ternary mixed eutectic molten salt according to claim 1, characterized in that: Calculated by mass fraction, the ternary mixed eutectic molten salt consists of 51.5-55wt% of potassium nitrate, 42.0-44.7wt% of sodium nitrite and 1.5-3.8wt% of sodium carbonate.

3. The ternary mixed eutectic molten salt according to claim 1 or 2, characterized in that: Calculated by mass fraction, the ternary mixed eutectic molten salt consists of 51.5wt% of potassium nitrate, 44.7wt% of sodium nitrite and 3.8wt% of sodium carbonate, or consists of 53wt% of potassium nitrate, 43.82wt% of sodium nitrite and 3.18wt% of sodium carbonate, or consists of 53.56wt% of potassium nitrate, 44.1wt% of sodium nitrite and 2.3wt% of sodium carbonate, or consists of 54.3wt% of potassium nitrate, 42.2wt% of sodium nitrite and 3.5wt% of sodium carbonate, or consists of 54.6wt% of potassium nitrate, 43.9wt% of sodium nitrite and 1.5wt% of sodium carbonate, or consists of 55wt% of potassium nitrate, 42.0wt% of sodium nitrite and 3.0wt% of sodium carbonate.

4. A method for preparing a ternary mixed eutectic molten salt as claimed in any one of claims 1 to 3, characterized in that: The following steps are involved: S1: uniformly mixing the dried potassium nitrate, sodium nitrite and sodium carbonate according to a preset ratio; S2: placing the mixed salt into a high temperature furnace for calcination until completely melted, and then cooling naturally; S3: Grind the cooled solid eutectic molten salt and seal it for storage.

5. The method for preparing the ternary mixed eutectic molten salt according to claim 4, characterized in that: Drying in step S1: drying potassium nitrate, sodium nitrite and sodium carbonate in an oven at 105-120° C. for 24-48 hours.

6. The method for preparing the ternary mixed eutectic molten salt according to claim 4, characterized in that: The calcination in step S2 is specifically as follows: heating to 120°C at a heating rate of 5-10°C / min, and then keeping the temperature for 30 minutes; then heating to 300-350°C at a heating rate of 5-10°C / min until the molten salt is completely melted, and then keeping the temperature for 1-12 hours and then cooling with the furnace.

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