A low melting point, low cost ZnCl2-based eutectic molten salt and its preparation method and application

Abstract

The present invention discloses a low-melting-point, low-cost ZnCl2-based eutectic molten salt and its preparation method and application. The formula of the quaternary chloride eutectic molten salt system is: 8.0-20.0wt.% NaCl, 18.0-35.0wt.% KCl, 1.0-17.0wt.% CaCl2, 41.0-58.0wt.% ZnCl2. The lowest eutectic point of the quaternary chloride molten salt system is 180.4°C, the specific heat capacity is 1.15±0.15J / g°C, and the thermal decomposition temperature is 620.7°C. The new ZnCl2-based eutectic molten salt developed by the present invention overcomes the disadvantages of the high melting point of Solar salt molten salt and the high cost, and provides a candidate heat transfer and heat storage medium for salt cavern energy storage, solar thermal power generation, industrial waste heat recovery and utilization and other fields.

Description

Technical Field

[0001] The present invention relates to the field of energy, and more specifically to a low melting point quaternary chloride molten salt and a preparation method thereof. Background Art

[0002] Low-melting-point molten salt has the advantages of low melting point, excellent heat transfer and storage characteristics, and wide sources. Therefore, it has great application potential in salt cavern energy storage, heating, and molten salt batteries. Nitrates are currently the most widely used in CSP systems. They have good thermal properties and low corrosiveness. The disadvantage is that they have a high freezing point, which can easily cause pipelines to freeze and block, greatly increasing the heating costs of pipelines. Chloride molten salt is recognized as a candidate material for the next generation of concentrated solar thermal power generation systems, but chloride molten salts generally have a high melting point, which greatly increases the energy consumption, cost, and safety risks required for their preparation. A large number of literature surveys have shown that many chloride salts are abundant in reserves and much cheaper than nitrates, so it is necessary to design and study low-melting-point chloride molten salt systems. Secondly, it is necessary to solve the problem of pipeline blockage, reduce power consumption, reduce costs, increase safety, and apply them to the civilian field during the preparation of molten salts.

[0003] To solve the above problems, Li Peiwen and others from the University of Arizona studied the NaCl-KCl-ZnCl2 system and found that the lowest eutectic point was 198.7°C, and the specific heat capacity was as low as 0.90 J / g·°C. Chinese patent application CN202110743467.5 discloses a NaCl-KCl-CaCl2 molten salt with high latent heat and high stability, but a melting point of up to 495°C. ZnCl2 is a covalent compound, and combining ionic compounds in a specific ratio will have a more obvious change in the thermal properties of the system. Because the melting point of CaCl2 is very high, it is difficult to form a low-melting-point chloride molten salt when it is combined with other chloride salts to form a molten salt system.

[0004] In summary, developing a low-melting-point, low-cost eutectic salt of a chloride molten salt system and its preparation method can improve stability, reduce the difficulty of preventing solidification, reduce power consumption, reduce costs, and increase safety, and apply it to the field of thermal energy storage and conversion, such as industrial waste heat recovery, regional centralized heating, solar thermal power generation, salt cavern energy storage, etc., which is of great significance in terms of social and economic benefits and energy conservation and emission reduction effects. Summary of the invention

[0005] In order to overcome the problems of high melting point and high cost of the commonly used molten salt system, the purpose of the present invention is to develop a ZnCl2-based eutectic molten salt system with low melting point, low cost and high thermal stability and a preparation method thereof. In order to solve the above problems, the technical solution adopted by the present invention is:

[0006] The quaternary ZnCl2-based chloride eutectic molten salt system of the present invention comprises the following components in terms of mass percentage: 8.0-20.0wt.% NaCl, 18.0-35.0wt.% KCl, 41.0-58.0wt.% ZnCl2, and 1.0-17.0wt.% CaCl2.

[0007] Another object of the present invention is to provide a method for preparing the above-mentioned quaternary chloride eutectic molten salt.

[0008] Another object of the present invention is to provide the use of the above-mentioned quaternary chloride eutectic molten salt.

[0009] Preferably, the preparation method of the quaternary chloride molten salt is:

[0010] NaCl, KCl, ZnCl2 and CaCl2 are mixed in a certain ratio and stirred evenly, placed in a pretreated crucible, dehydrated and kept warm for a certain period of time at a suitable temperature, then heated until all the solid molten salt is melted, then kept at a constant temperature for 0.5-12 hours, and then naturally cooled to room temperature to obtain the chloride eutectic molten salt system.

[0011] Preferably, the step of preparing the eutectic molten salt is carried out in a glove box with a high-purity inert atmosphere to avoid the influence of water and oxygen on the preparation of the molten salt.

[0012] Preferably, the prepared crucible is an alumina crucible or a quartz crucible.

[0013] Preferably, the step of pre-treating the crucible before the experiment is specifically as follows: washing with deionized water, placing the cleaned crucible in an oven to dry to remove its free water, then taking out the dried crucible, and placing it in a high-temperature resistance furnace covered with an inert atmosphere and calcining it at a high temperature of ≥700°C to remove the bound water therein. Preferably, the dehydration and insulation temperature for the preparation of NaCl-KCl-ZnCl2-CaCl2 eutectic molten salt is 100-150°C. Preferably, the dehydration and insulation time for the preparation of NaCl-KCl-ZnCl2-CaCl2 eutectic molten salt is 0.5-5 hours. Preferably, the melting temperature for the preparation of NaCl-KCl-ZnCl2-CaCl2 eutectic molten salt is 200-250°C. Preferably, the heating temperature rise rate for the preparation of NaCl-KCl-ZnCl2-CaCl2 eutectic molten salt is 2-10°C / min.

[0014] Preferably, the melting holding time of the NaCl-KCl-ZnCl2-CaCl2 eutectic molten salt is 2-12 hours. The CaCl2 content of the quaternary chloride is 1.0-17.0wt.%; if the CaCl2 content in the quaternary chloride is lower than 1.0wt.%, the CaCl2 content in the system is very small, and at this time, CaCl2 exists in the form of impurities, which has little effect on the thermophysical properties of the system and increases the cost of impurity removal and purification; if the CaCl2 content is higher than 17.0wt.%, the increase in calcium chloride content will lead to an increase in melting point, an increase in viscosity, and even the inability to form a eutectic system.

[0015] Further preferred: the raw material composition of the ZnCl2-based eutectic molten salt, by mass percentage, is: 9% NaCl, 33% KCl, 43% ZnCl2, 15% CaCl2; the melting point of the ZnCl2-based eutectic molten salt is 182.0±2.0℃; the specific heat capacity is 1.15±0.15J / g·K, and the thermal decomposition temperature is 620.7℃.

[0016] The NaCl-KCl-ZnCl2-CaCl2 molten salt of the present invention is a eutectic salt, and the components will melt or solidify at the eutectic point temperature at the same time, so it has good stability during long-term use. The long-term thermophysical properties of the quaternary eutectic salt of the present invention are better than those of non-eutectic molten salts. In the art, the melting point of CaCl2 is very high, and it is generally difficult to obtain a low-melting-point molten salt, and it is even more difficult to prepare a quaternary eutectic salt containing CaCl2 with only one melting peak. However, the present invention, through the preferred synergy of the amount of each component, enables the addition of CaCl2 to form a eutectic system, and ensures the low melting point of the CaCl2 eutectic system, producing unexpected effects.

[0017] The eutectic molten salt system prepared by the present invention overcomes the disadvantage of high melting point of solar salt molten salt, reduces the insulation energy consumption in the heat transfer and storage system, makes the molten salt less likely to solidify in the system pipeline, and overcomes the disadvantage of high cost of solar salt molten salt; the molten salt preparation process is simple and safe, easy to operate and control, has a short preparation cycle, is environmentally friendly, and is particularly suitable for large-scale molten salt preparation and production; the quaternary chloride eutectic molten salt system has low cost and low melting point, and has great application potential in the fields of solar thermal power generation, salt cavern energy storage, industrial waste heat recovery, etc. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 is the DSC variation curve of the quaternary chloride eutectic molten salt system of Example 1 of the present invention;

[0019] Figure 2 is a curve showing a change in specific heat capacity of the quaternary chloride eutectic molten salt system as a function of temperature in Example 1 of the present invention;

[0020] Figure 3is the DSC variation curve of the quaternary chloride eutectic molten salt system of Example 2 of the present invention;

[0021] Figure 4 is a curve showing a change in specific heat capacity of the quaternary chloride eutectic molten salt system as a function of temperature in Example 2 of the present invention;

[0022] Figure 5 is the DSC variation curve of the quaternary chloride eutectic molten salt system of Example 3 of the present invention;

[0023] Figure 6 is the DSC change curve of the quaternary chloride eutectic molten salt system of Comparative Example 1 of the present invention;

[0024] Figure 7 is the DSC change curve of the quaternary chloride eutectic molten salt system of Comparative Example 2 of the present invention;

[0025] Figure 8 is the TG variation curve of the quaternary chloride eutectic molten salt system of Example 1 of the present invention;

[0026] Fig. 9 This is the TG variation curve of the quaternary chloride eutectic molten salt system of Example 2 of the present invention. DETAILED DESCRIPTION

[0027] The specific implementation methods of the present invention are described below to explain the technical solutions of the present invention in detail. However, the following embodiments are only used to understand the present invention and cannot limit the present invention. The embodiments and features in the embodiments of the present invention can be combined with each other. The present invention can be implemented in a variety of different ways as defined and covered by the claims.

[0028] Store NaCl, KCl, ZnCl2, and CaCl2 salts in a glove box with a high-purity inert atmosphere in advance to eliminate the influence of water and oxygen for use in the next experiment.

[0029] Adjust the glove box and its high-temperature resistance furnace to the best condition and prepare them for use.

[0030] The experimental crucible was cleaned with deionized water, placed in an oven for drying to remove free water, and then the dried crucible was taken out and transferred to a high-temperature resistance furnace covered with an inert atmosphere, and the temperature was set to ≥700°C for high-temperature calcination to remove bound water, thereby obtaining a treated crucible.

[0031] Ensure that other molten salt accessories in the glove box, such as electronic balance (accuracy 0.001g), tweezers, salt container, crucible tongs, etc., are clean in preparation for molten salt preparation.

[0032] In a glove box, weigh NaCl, KCl, ZnCl2, and CaCl2 separately according to the corresponding mass fraction, mix and stir evenly. Place in a pretreated crucible, cover the crucible lid, transfer to a high-temperature resistance furnace covered with an inert atmosphere, set the temperature program, heat to 100-150°C at 2-10°C / min, and keep warm for 0.5-5 hours. After the insulation is completed, heat to 200-250°C at 2-10°C / min, keep warm for 0.5-12 hours, and cool naturally to room temperature to obtain the target product.

[0033] The prepared chloride molten salt system was tested for melting point using a differential scanning calorimeter (DSC). A 0.01 mg electronic balance was used to weigh about 10 mg of the sample, which was placed in an alumina crucible and heated to 50-100 ° C above the melting point. The heating rate was 10.0 ° C / min, and the Ar flow rate was 30.0 mL / min. The test process was repeated three times to ensure the accuracy of the results. The change in melting point after the test is used to determine whether a NaCl-KCl-ZnCl2-CaCl2 eutectic molten salt system is formed. The molten salt eutectic can be judged based on whether the endothermic peak and exothermic peak obtained by its DSC test are single peaks. If it is a single peak, it is a eutectic salt. If a secondary peak appears, it is a non-eutectic salt. The thermophysical properties of the eutectic molten salt are better than those of the non-eutectic molten salt.

[0034] The specific heat capacity of the prepared chloride molten salt system was tested using a differential scanning calorimeter (DSC), and the test conditions were the same as the melting point test. First, a baseline measurement was performed, and a pair of high-purity empty graphite crucibles with platinum crucible covers were heated from room temperature to 450°C as a baseline. Secondly, a standard sample test was performed, and about 20.0 mg of the standard sample sapphire was accurately weighed, placed in a graphite crucible, and heated to 450°C. Finally, the sample measurement was performed, and about 20.0 mg of the NaCl-KCl-ZnCl2-CaCl2 eutectic molten salt was accurately weighed and placed in the same graphite crucible as the standard sample, and the measurement was performed under the same experimental conditions.

[0035] The thermal weight loss of NaCl-KCl-ZnCl2-CaCl2 eutectic molten salt was measured using a synchronous thermal analyzer (STA, Setaram Labsys Evo). The Al2O3 crucible was selected as the container because it is inert to molten chloride salts and there is no chemical reaction between the two. First, the empty Al2O3 crucible was heated to 700°C at a heating rate of 2-10°C / min, and the high-purity argon gas flow rate was 30.0mL / min during the experiment. Then, about 30mg of NaCl-KCl-ZnCl2-CaCl2 eutectic molten salt was added to the Al2O3 crucible and tested under the same experimental conditions.

[0036] Example 1

[0037] After calibrating the electronic balance, weigh 1.8g, 6.6g, 8.6g, and 3.0g of NaCl, KCl, ZnCl2, and CaCl2 in the glove box, respectively. Their mass proportions are 9% NaCl, 33% KCl, 43% ZnCl2, and 15% CaCl2, respectively.

[0038] After mixing evenly, put it into a clean and dry alumina crucible and cover it with a lid. Place the alumina crucible in a high-temperature resistance furnace and heat it to 120°C at a rate of 10°C / min and keep it warm for 3 hours to remove free water in the molten salt.

[0039] After the water removal was completed, the temperature was continued to be raised to 250°C at a rate of 10°C / min, and after being kept at this temperature for 5 hours, the temperature was lowered to room temperature in the glove box along with the furnace to obtain a quaternary chloride molten salt system.

[0040] The melting point of the prepared quaternary chloride NaCl-KCl-ZnCl2-CaCl2 eutectic molten salt system was tested by differential scanning calorimetry (DSC). The test temperature range was 100-270°C, and the heating rate was 5°C / min. The melting peak of the molten salt system after the test was a single peak, such as Figure 1 The figure shows that the NaCl-KCl-ZnCl2-CaCl2 molten salt is a eutectic salt, and the eutectic temperature is 180.4℃.

[0041] The specific heat capacity of the prepared quaternary chloride NaCl-KCl-ZnCl2-CaCl2 eutectic molten salt system was tested by differential scanning calorimetry (DSC). The test temperature range was 200-450°C and the heating rate was 5°C / min. Figure 2 As shown, the specific heat capacity of NaCl-KCl-ZnCl2-CaCl2 eutectic molten salt is 1.15±0.15 J / g·K.

[0042] The thermogravimetric analysis of NaCl-KCl-ZnCl2-CaCl2 eutectic molten salt was performed using a synchronous thermal analyzer (STA, Setaram Labsys Evo). The test temperature range was 100-700°C, and the heating rate was 10°C / min. Figure 8 As shown, the thermal decomposition temperature of NaCl-KCl-ZnCl2-CaCl2 eutectic molten salt is 620.7℃.

[0043] Example 2

[0044] After calibrating the electronic balance, 3.8 g, 4.8 g, 11.2 g, and 0.2 g of NaCl, KCl, ZnCl2, and CaCl2 were weighed in the glove box, respectively. Their mass proportions were 19.0% NaCl, 24.0% KCl, 56.0% ZnCl2, and 1.0% CaCl2, respectively.

[0045] After mixing evenly, put it into a clean and dry alumina crucible and cover it with a lid. Place the alumina crucible in a high-temperature resistance furnace and heat it to 120°C at a rate of 10°C / min and keep it warm for 3 hours to remove free water in the molten salt.

[0046] After the water removal was completed, the temperature was continued to be raised to 250°C at a rate of 10°C / min, and after being kept at this temperature for 5 hours, the temperature was lowered to room temperature in the glove box along with the furnace to obtain a quaternary chloride molten salt system.

[0047] The prepared quaternary chloride NaCl-KCl-ZnCl2-CaCl2 eutectic molten salt system was tested by differential scanning calorimetry (DSC). The test temperature range was 100-270°C, and the heating rate was 5°C / min. The melting peak of the molten salt system after the test was a single peak, such as Figure 3 The figure shows that the NaCl-KCl-ZnCl2-CaCl2 molten salt is a eutectic salt with a eutectic temperature of 183.5°C.

[0048] The specific heat capacity of the prepared quaternary chloride NaCl-KCl-ZnCl2-CaCl2 eutectic molten salt system was tested by differential scanning calorimetry (DSC). The test temperature range was 200-450°C and the heating rate was 5°C / min. Figure 4 It is shown that the specific heat capacity of NaCl-KCl-ZnCl2-CaCl2 eutectic molten salt is 1.31±0.17J / g·K.

[0049] The thermogravimetric analysis of NaCl-KCl-ZnCl2-CaCl2 eutectic molten salt was performed using a synchronous thermal analyzer (STA, Setaram Labsys Evo). The test temperature range was 100-700°C, and the heating rate was 10°C / min. Fig. 9 As shown, the thermal decomposition temperature of NaCl-KCl-ZnCl2-CaCl2 eutectic molten salt is 603.5℃.

[0050] Example 3

[0051] After calibrating the electronic balance, weigh 2.4g, 5.6g, 10.4g, and 1.6g of NaCl, KCl, ZnCl2, and CaCl2 in the glove box, respectively. Their mass proportions are 12.0% NaCl, 28.0% KCl, 52.0% ZnCl2, and 8.0% CaCl2, respectively.

[0052] After mixing evenly, put it into a clean and dry alumina crucible and cover it with a lid. Place the alumina crucible in a high-temperature resistance furnace and heat it to 120°C at a rate of 10°C / min and keep it warm for 3 hours to remove free water in the molten salt.

[0053] After the water removal was completed, the temperature was continued to be raised to 250°C at a rate of 10°C / min, and after being kept at this temperature for 5 hours, the temperature was lowered to room temperature in the glove box along with the furnace to obtain a quaternary chloride molten salt system.

[0054] The prepared quaternary chloride NaCl-KCl-ZnCl2-CaCl2 eutectic molten salt system was tested by differential scanning calorimetry (DSC). The test temperature range was 100-270°C, and the heating rate was 5°C / min. The melting peak of the molten salt system after the test was a single peak, such as Figure 5 The figure shows that the NaCl-KCl-ZnCl2-CaCl2 molten salt is a eutectic salt, and the eutectic temperature is 182.8℃.

[0055] Comparative Example 1

[0056] After calibrating the electronic balance, weigh 2.4g, 4.2g, 8.4g, and 5.0g of NaCl, KCl, ZnCl2, and CaCl2 in the glove box, respectively. Their mass proportions are 12.0% NaCl, 21.0% KCl, 42.0% ZnCl2, and 25.0% CaCl2, respectively.

[0057] After mixing evenly, put it into a clean and dry alumina crucible and cover it with a lid. Place the alumina crucible in a high-temperature resistance furnace and heat it to 120°C at a rate of 10°C / min and keep it warm for 3 hours to remove free water in the molten salt.

[0058] After the water removal was completed, the temperature was continued to be raised to 250°C at a rate of 10°C / min, and after being kept at this temperature for 5 hours, the temperature was lowered to room temperature in the glove box along with the furnace to obtain a quaternary chloride molten salt system.

[0059] The prepared quaternary chloride NaCl-KCl-ZnCl2-CaCl2 eutectic molten salt system was tested by differential scanning calorimetry (DSC). The test temperature range was 100-270°C, and the heating rate was 5°C / min. The melting peak of the molten salt system obtained by the test was not a single peak, and secondary peaks appeared near 165°C and 215°C, such as Figure 6 The area marked by the red dotted line shows that the NaCl-KCl-ZnCl2-CaCl2 molten salt is a non-eutectic salt.

[0060] Comparative Example 2

[0061] After calibrating the electronic balance, weigh 2.0g, 4.6g, 9.2g, and 4.2g of NaCl, KCl, ZnCl2, and CaCl2 in the glove box, respectively. Their mass proportions are 10.0% NaCl, 23.0% KCl, 46.0% ZnCl2, and 21.0% CaCl2, respectively.

[0062] After mixing evenly, put it into a clean and dry alumina crucible and cover it with a lid. Place the alumina crucible in a high-temperature resistance furnace and heat it to 120°C at a rate of 10°C / min and keep it warm for 3 hours to remove free water in the molten salt.

[0063] After the water removal was completed, the temperature was continued to be raised to 250°C at a rate of 10°C / min, and after being kept at this temperature for 5 hours, the temperature was lowered to room temperature in the glove box along with the furnace to obtain a quaternary chloride molten salt system.

[0064] The prepared quaternary chloride NaCl-KCl-ZnCl2-CaCl2 eutectic molten salt system was tested by differential scanning calorimetry (DSC). The test temperature range was 100-270°C, and the heating rate was 5°C / min. The melting peak of the molten salt system obtained after the test was not a single peak, and a secondary peak appeared near 200°C, such as Figure 7 The area marked with a red dotted line indicates that the NaCl-KCl-ZnCl2-CaCl2 molten salt is a non-eutectic salt. The above is only a preferred embodiment of the present invention and is not intended to limit the scope of the present invention. The above embodiments of the present invention can also be changed in various ways. That is, all simple, equivalent changes and modifications made according to the claims and description of the present invention fall within the scope of protection of the patent of the present invention. The contents not described in detail in the present invention are all conventional technical contents.

Claims

1. A low melting point, low cost ZnCl2-based eutectic molten salt, characterized in that: The ZnCl2-based eutectic molten salt is a quaternary chloride eutectic molten salt prepared from NaCl, KCl, ZnCl2 and CaCl2, and its raw material composition is, by mass percentage, 9.0-19.0 wt.% of NaCl, 24.0-33.0 wt.% of KCl, 1.0-15.0 wt.% of CaCl2, and 43.0-56.0 wt.% of ZnCl2; the eutectic temperature of the ZnCl2-based eutectic molten salt is 180.4 ~183.5 ° C, and the thermal decomposition temperature is 603.5~620.7 ° C.

2. The method for preparing the low melting point, low cost ZnCl2-based eutectic molten salt according to claim 1, characterized in that: The preparation steps are: S1. Weigh NaCl, KCl, ZnCl2, and CaCl2, fully physically blend them to obtain a mixed salt, and then pour them into a pre-treated crucible; the mixed salt has the following composition by mass percentage: 9.0-19.0 wt.% NaCl, 24.0-33.0 wt.% KCl, 1.0-15.0 wt.% CaCl2, and 43.0-56.0 wt.% ZnCl2; S2. Place the crucible containing the mixed salt in a high-temperature resistance furnace and keep it at 120°C for 3 hours; S3. After keeping the temperature, heat the mixture to 250 °C at a heating rate of 10 °C / min until all the solid salt mixture is melted, then keep the temperature for 5 hours, cool naturally to room temperature, and crush to obtain ZnCl2-based eutectic molten salt.

3. The preparation method according to claim 2, characterized in that: The quaternary chloride eutectic molten salt is prepared in a glove box protected by a high-purity inert atmosphere.

4. The preparation method according to claim 2, characterized in that: The raw material composition of the ZnCl2-based eutectic molten salt is, by mass percentage, 9 wt.% of NaCl, 33 wt.% of KCl, 43 wt.% of ZnCl2, and 15 wt.% of CaCl2.

5. The low melting point, low cost ZnCl2-based eutectic molten salt prepared by the method according to claim 4, characterized in that: The melting point of ZnCl2-based eutectic molten salt is 182.0±2.0 ℃; the specific heat capacity is 1.15±0.15 J / g·K, and the thermal decomposition temperature is 620.7 ℃.

6. The ZnCl2-based eutectic molten salt prepared according to the method according to any one of claims 2 to 4 is used as a heat transfer and storage medium in salt cavern energy storage, solar thermal power generation, and industrial waste heat recovery.

Images