A low-melting-point molten salt, its preparation method and application

Abstract

This invention relates to the field of molten salt preparation technology, and in particular to a low-melting-point molten salt, its preparation method, and its applications. It comprises the following raw materials in the indicated mass percentages: sodium nitrate 10-30%, potassium nitrate 5-30%, sodium nitrite 5-50%, aluminum nitrate 1-5%, and calcium carbonate 1-10%. This invention uses five substances—sodium nitrate, potassium nitrate, sodium nitrite, aluminum nitrate, and calcium carbonate—as raw materials to prepare the molten salt. Each component has a low melting point, which lowers the crystallization temperature of the molten salt, resulting in a melting point range of 100-130℃. This helps to broaden the molten salt's heat storage temperature range and reduces the risk of crystallization. Simultaneously, since the presence of sodium nitrite in the molten salt may cause corrosion of metal materials, an appropriate amount of aluminum nitrate is added as a corrosion inhibitor, forming a passivation film to protect the metal surfaces in the system.

Description

Technical Field

[0001] This invention relates to the field of molten salt preparation technology, and in particular to a low-melting-point molten salt, its preparation method, and its application. Background Technology

[0002] Molten salt is a mixture of various chemical components in a certain proportion, with low-melting-point inorganic salts as the main component and high-melting-point inorganic salts as the minor component. When the main component, the low-melting-point inorganic salt, melts, the minor component, the high-melting-point inorganic salt, remains in a solid state. At this point, the solution is in an ionic state, hence the name molten liquid.

[0003] Typically, molten salts consist of two or more chemical substances, such as binary molten salts and ternary molten salts. Molten salts have a wide range of applications, including neutron moderators in nuclear reactors; catalyst carriers in the petroleum industry; chemical reaction media in the chemical industry; and conductive liquids in the power industry. Currently, molten salt thermal storage is widely used in solar thermal power generation, clean heating, and thermal power plant retrofitting, improving energy efficiency and system stability. However, due to the relatively high crystallization temperature of commonly used molten salts, there is a certain risk of pipeline crystallization, necessitating the development of low-melting-point molten salts. Therefore, this invention is proposed. Summary of the Invention

[0004] The first objective of this invention is to provide a low-melting-point molten salt with the advantages of a wide melting point range and a low crystallization temperature. The second objective of this invention is to provide a method for preparing and using the low-melting-point molten salt.

[0005] This invention provides a low-melting-point molten salt comprising the following raw materials by mass percentage: 10-30% sodium nitrate, 5-20% potassium nitrate, 5-50% sodium nitrite, 1-5% aluminum nitrate, and 1-10% calcium carbonate.

[0006] Preferably, the raw materials comprise the following percentages by weight: 25-30% sodium nitrate, 15-30% potassium nitrate, 25-40% sodium nitrite, 5% aluminum nitrate, and 5-10% calcium carbonate.

[0007] This invention provides a method for preparing the above-mentioned low-melting-point molten salt, comprising the following steps:

[0008] S1. Mix sodium nitrate, potassium nitrate, sodium nitrite, aluminum nitrate, and calcium carbonate to obtain a mixture;

[0009] S2. The mixture is heated under vacuum conditions. When the mixture is heated to 200°C, slag removal is performed. After slag removal, it is cooled to room temperature and then ground.

[0010] S3. The ground mixture is heated under vacuum to completely melt it, kept at that temperature, and then cooled to room temperature to obtain a low-melting-point molten salt.

[0011] Preferably, the crystal particles of the low-melting-point molten salt obtained in step S3 are 190-210 μm.

[0012] Preferably, in step S3, the temperature is raised to 200-300°C.

[0013] Preferably, the temperature is raised to 250°C in step S3.

[0014] Preferably, the heat preservation time in step S3 is 30-35 minutes.

[0015] This invention provides an application of the above-mentioned low-melting-point molten salt as a heat storage medium for molten salt energy storage.

[0016] Compared with the prior art, the present invention has the following advantages:

[0017] This invention uses five substances—sodium nitrate, potassium nitrate, sodium nitrite, aluminum nitrate, and calcium carbonate—as raw materials to prepare molten salt. Each component has a low melting point, which lowers the crystallization temperature of the molten salt, resulting in a melting point range of 100-130℃. This helps to broaden the molten salt's heat storage temperature range and reduces the risk of crystallization. Simultaneously, since the presence of sodium nitrite in the molten salt may cause corrosion of metal materials, an appropriate amount of aluminum nitrate is added as a corrosion inhibitor, forming a passivation film to protect the metal surfaces in the system. Detailed Implementation

[0018] It should be noted that the following detailed descriptions are illustrative and intended to provide further explanation of this application. Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application pertains.

[0019] It should be noted that the terminology used herein is for the purpose of describing particular implementations only and is not intended to limit the exemplary implementations according to this application. As used herein, the singular form includes the plural form unless the context clearly indicates otherwise. Furthermore, it should be understood that when the terms "comprising" and / or "including" are used in this description, they indicate the presence of features, steps, operations, devices, components, and / or combinations thereof.

[0020] The technical solution of the present invention will be clearly and completely described below with reference to the embodiments. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0021] Example 1

[0022] A low-melting-point molten salt is composed of the following raw materials in weight percentages: 30% sodium nitrate, 20% potassium nitrate, 35% sodium nitrite, 5% aluminum nitrate, and 10% calcium carbonate.

[0023] The preparation process of the above-mentioned low-melting-point molten salt is as follows:

[0024] S1. Mix sodium nitrate, potassium nitrate, sodium nitrite, aluminum nitrate, and calcium carbonate to obtain a mixture;

[0025] S2. Place the mixture into a pull-type vacuum melting furnace and heat it under vacuum conditions. When the mixture reaches 200°C, remove the slag. After removing the slag, cool it to room temperature and then grind it. The pull-type vacuum melting furnace is a pull-type vacuum melting furnace with added high-purity argon gas.

[0026] S3. Heat the ground mixture to 250°C under vacuum to completely melt it, keep it at that temperature for 30 minutes, and then cool it to room temperature to obtain a low-melting-point molten salt.

[0027] The low-melting-point molten salt prepared in this embodiment has a crystal particle size of 200 μm. DSC (Differential Scanning Calorimetry) analysis showed its melting point to be 101 °C; TG (Thermogravimetric Analysis) analysis showed its decomposition temperature to be 492 °C.

[0028] Example 2

[0029] A low-melting-point molten salt is composed of the following raw materials in weight percentages: sodium nitrate 25%, potassium nitrate 25%, sodium nitrite 40%, aluminum nitrate 1%, and calcium carbonate 9%.

[0030] The preparation process of the above-mentioned low-melting-point molten salt is as follows:

[0031] S1. Mix sodium nitrate, potassium nitrate, sodium nitrite, aluminum nitrate, and calcium carbonate to obtain a mixture;

[0032] S2. Place the mixture into a pull-out vacuum melting furnace and heat it under vacuum conditions. When the mixture reaches 200°C, remove the slag. After removing the slag, cool it to room temperature and then grind it.

[0033] S3. Heat the ground mixture to 300°C under vacuum to completely melt it, keep it at that temperature for 30 minutes, and then cool it to room temperature to obtain a low-melting-point molten salt.

[0034] The low-melting-point molten salt prepared in this embodiment has a crystal particle size of 200 μm. DSC analysis showed its melting point to be 123 °C; TG analysis showed its decomposition temperature to be 411 °C.

[0035] Example 3

[0036] A low-melting-point molten salt is composed of the following raw materials in weight percentages: sodium nitrate 28%, potassium nitrate 27%, sodium nitrite 35%, aluminum nitrate 3%, and calcium carbonate 7%.

[0037] The preparation process of the above-mentioned low-melting-point molten salt is as follows:

[0038] S1. Mix sodium nitrate, potassium nitrate, sodium nitrite, aluminum nitrate, and calcium carbonate to obtain a mixture;

[0039] S2. Place the mixture into a pull-out vacuum melting furnace and heat it under vacuum conditions. When the mixture reaches 200°C, remove the slag. After removing the slag, cool it to room temperature and then grind it.

[0040] S3. Heat the ground mixture to 250°C under vacuum to completely melt it, keep it at that temperature for 30 minutes, and then cool it to room temperature to obtain a low-melting-point molten salt.

[0041] The low-melting-point molten salt prepared in this embodiment has a crystal particle size of 200 μm. DSC analysis showed its melting point to be 114 °C; TG analysis showed its decomposition temperature to be 463 °C.

[0042] Example 4

[0043] A low-melting-point molten salt is composed of the following raw materials in weight percentages: 30% sodium nitrate, 30% potassium nitrate, 25% sodium nitrite, 5% aluminum nitrate, and 10% calcium carbonate.

[0044] The preparation process of the above-mentioned low-melting-point molten salt is as follows:

[0045] S1. Mix sodium nitrate, potassium nitrate, sodium nitrite, aluminum nitrate, and calcium carbonate to obtain a mixture;

[0046] S2. Place the mixture into a pull-out vacuum melting furnace and heat it under vacuum conditions. When the mixture reaches 200°C, remove the slag. After removing the slag, cool it to room temperature and then grind it.

[0047] S3. Heat the ground mixture to 300°C under vacuum to completely melt it, keep it at that temperature for 35 minutes, and then cool it to room temperature to obtain a low-melting-point molten salt.

[0048] The low-melting-point molten salt prepared in this embodiment has a crystal particle size of 200 μm. DSC analysis showed its melting point to be 130 °C; TG analysis showed its decomposition temperature to be 478 °C.

[0049] Comparative Example 1

[0050] A molten salt is composed of the following raw materials in weight percentages: 30% sodium nitrate, 20% potassium nitrate, 35% sodium nitrite, 5% lithium nitrate, and 10% calcium carbonate. Its preparation method is consistent with that in Example 1. DSC analysis shows its melting point to be 131°C; TG analysis shows its decomposition temperature to be 452°C.

[0051] Comparative Example 2

[0052] A molten salt is composed of the following raw materials in weight percentages: 30% sodium nitrate, 20% potassium nitrate, 35% sodium nitrite, 5% aluminum nitrate, and 10% calcium nitrate. DSC analysis shows its melting point to be 142°C; TG analysis shows its decomposition temperature to be 497°C.

[0053] Comparative Example 3

[0054] A molten salt is composed of the following raw materials in weight percentages: 30% sodium nitrate, 20% potassium nitrate, 35% sodium nitrite, and 15% calcium nitrate. DSC analysis shows its melting point to be 129°C; TG analysis shows its decomposition temperature to be 435°C.

[0055] The molten salts prepared in Examples 1-4 and Comparative Examples 1-3 of this invention were subjected to metal corrosion tests. The specific test procedures are as follows:

[0056] (1) The stainless steel plate commonly used in molten salt storage tanks was used as the sample. The size of each stainless steel plate was 20mm×20mm×3mm. The stainless steel plate was pretreated: the surface of the stainless steel plate was first polished with sandpaper until smooth, and then cleaned with acetone and anhydrous ethanol in turn to remove surface oil and impurities. Then it was dried with a hair dryer, the mass of the stainless steel plate was measured, and then it was placed in a dry container for later use.

[0057] (2) Put an appropriate amount of molten salt into a crucible, put the crucible into a high-temperature furnace, and heat it at a heating rate of 50℃ / min until the molten salt is completely melted and the temperature is stabilized at 400℃.

[0058] (3) Suspend the prepared sample with a thin iron wire or ceramic support, and slowly place it into the molten salt to ensure that the sample is completely submerged. Cover the crucible and soak it. The high-temperature furnace should be kept at a stable temperature of 400℃ for 10 days. Set up 3 parallel tests for each type of molten salt.

[0059] (4) After soaking, use tweezers to remove the sample from the molten salt, wash with deionized water to remove the molten salt adhering to the sample surface, then wash again with anhydrous ethanol, dry with a hair dryer, and measure the mass of the sample.

[0060] Record quality changes △ m = m0 - m1, where m0 is the mass of the sample before immersion and m1 is the mass of the sample after immersion; calculate the corrosion rate of the sample v =△ m / (st), where s is the surface area of ​​the sample and t is the soaking time. The test results are shown in Table 1.

[0061] Table 1 Corrosion rates of stainless steel plates in different molten salts

[0062] Group Corrosion rate (mm / d) Example 1 0.093 Example 2 0.15 Example 3 0.102 Example 4 0.076 Comparative Example 1 0.290 Comparative Example 2 0.265 Comparative Example 3 0.392

[0063] As can be seen from Table 1, the corrosion rate of the low melting point molten salt prepared in Examples 1-4 of the present invention on stainless steel plates is significantly lower than that of the molten salt prepared in Comparative Examples 1-3. The presence of sodium nitrite in the molten salt can cause corrosion of metal materials. The present invention adds an appropriate amount of aluminum nitrate as a corrosion inhibitor to protect the metal surface and delay metal corrosion by forming a passivation film.

[0064] The molten salt product prepared by this invention is pure and homogeneous, requires less equipment, and is simple and easy to operate. It uses five substances—sodium nitrate, potassium nitrate, sodium nitrite, aluminum nitrate, and calcium carbonate—as raw materials to prepare the molten salt. Each component in this five-element system has a low melting point, ranging from 100 to 130°C, which lowers the crystallization temperature of the molten salt. This helps to broaden the heat storage temperature range of the molten salt, reduce the risk of molten salt crystallization, and the added aluminum nitrate, in combination with the other components, has the effect of inhibiting metal corrosion.

[0065] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A low-melting-point molten salt, characterized in that, It is composed of the following raw materials by mass percentage: sodium nitrate 25-30%, potassium nitrate 15-30%, sodium nitrite 25-40%, aluminum nitrate 5%, and calcium carbonate 5-10%.

2. A method for preparing a low-melting-point molten salt according to claim 1, characterized in that, Includes the following steps: S1. Mix sodium nitrate, potassium nitrate, sodium nitrite, aluminum nitrate, and calcium carbonate to obtain a mixture; S2. The mixture is heated under vacuum conditions. When the mixture is heated to 200°C, slag removal is performed. After slag removal, it is cooled to room temperature and then ground. S3. The ground mixture is heated under vacuum to completely melt it, kept at that temperature, and then cooled to room temperature to obtain a low-melting-point molten salt.

3. The preparation method according to claim 2, characterized in that, The low-melting-point molten salt obtained in step S3 has crystal particles of 190-210µm.

4. The preparation method according to claim 2, characterized in that, In step S3, the temperature is raised to 200-300℃.

5. The preparation method according to claim 4, characterized in that, In step S3, the temperature is raised to 250°C.

6. The preparation method according to claim 4, characterized in that, The heat preservation time in step S3 is 30-35 minutes.

7. The application of the low-melting-point molten salt of claim 1 as a heat storage medium for molten salt energy storage.

8. A method for preparing a low-melting-point molten salt, characterized in that, It is composed of the following raw materials by weight percentage: sodium nitrate 25%, potassium nitrate 25%, sodium nitrite 40%, aluminum nitrate 1%, and calcium carbonate 9%. The steps are as follows: S1. Mix sodium nitrate, potassium nitrate, sodium nitrite, aluminum nitrate, and calcium carbonate to obtain a mixture; S2. Place the mixture into a pull-out vacuum melting furnace and heat it under vacuum conditions. When the mixture reaches 200°C, remove the slag. After removing the slag, cool it to room temperature and then grind it. S3. Heat the ground mixture to 300°C under vacuum to completely melt it, keep it at that temperature for 30 minutes, and then cool it to room temperature to obtain a low-melting-point molten salt.

9. An application of a low-melting-point molten salt obtained by the preparation method of claim 8 as a heat storage medium for molten salt energy storage.