A nitro binary molten salt and a preparation method thereof

Abstract

The present application relates to the technical field of fused salt, in particular to a nitro binary fused salt and a preparation method thereof. The preparation method comprises the following steps: mixing sodium carbonate and potassium hydroxide solution, and then adding a starch-based grafting crystal inhibitor to inhibit crystallization; adding nitric acid aqueous solution in two stages, and introducing a co-seed crystal at the end of the first stage to guide nucleation; concentrating by falling film evaporation and setting partial reflux to optimize crystal distribution; after spray granulation, short-term melting reforming and controlled cooling are carried out in an inert atmosphere. The method significantly improves the composition uniformity and structural stability of the fused salt, so that the product has a melting point close to the theoretical eutectic point, low supercooling degree, high melting latent heat, excellent thermal cycle stability and corrosion resistance, and is suitable for long-term running industrial heat storage system.

Description

Technical Field

[0001] This invention relates to the field of molten salt technology, and in particular to a nitro binary molten salt and its preparation method. Background Technology

[0002] In the field of molten salt thermal energy storage technology, nitro binary molten salts have attracted widespread attention due to their suitable operating temperature range and high thermal stability. Traditional preparation processes typically involve mixing sodium carbonate and potassium hydroxide solutions, followed by a direct neutralization reaction with nitric acid. However, the solubility of sodium carbonate aqueous solutions varies significantly at different temperatures, especially during mixing and evaporation concentration, easily leading to crystallization due to localized supersaturation and resulting in uneven distribution of sodium and potassium ions. This component segregation not only causes the eutectic point of the final molten salt to deviate from the theoretical value but also triggers problems such as increased supercooling and decreased thermal energy storage capacity during subsequent phase transitions.

[0003] Furthermore, during the neutralization reaction stage, if nitric acid is added to the mixed alkaline solution all at once, the system will experience a drastic change in pH. Excessive local acidity will cause the carbonate to decompose rapidly, producing a large amount of carbon dioxide bubbles and forming coarse intermediate salt crystals. These crystals are difficult to redissolve during evaporation and concentration, becoming heterogeneous nucleation sites and exacerbating the microscopic inhomogeneity of composition and structure. In addition, uncontrolled crystal growth easily leads to scaling on the evaporator walls and inside the pipes, reducing heat transfer efficiency and increasing energy consumption and cleaning burden.

[0004] In the subsequent processing of the concentrate, conventional spray drying or direct crystallization processes are insufficient to effectively eliminate the compositional gradient introduced in the early stages. The resulting molten salt particles often contain micropores, component segregation, or metastable phases. During long-term thermal cycling, these defects gradually accumulate, manifesting as melting point drift, latent heat decay, and decreased thermal conductivity. Simultaneously, regions where the composition deviates from the eutectic point are more prone to corrosion of metallic materials at high temperatures, affecting the safety and service life of the thermal storage system. While existing methods attempt to optimize these processes by adjusting the raw material ratio or through post-treatment heat treatment, they fail to achieve precise control over crystal composition and morphology from the nucleation and growth stage. Insufficient synergy among the various process stages hinders further improvement in the overall performance of the molten salt. Summary of the Invention

[0005] In view of this, the purpose of this invention is to provide a nitro binary molten salt and its preparation method, so as to solve the problems of existing methods in which it is difficult to suppress component segregation and crystal coarsening during neutralization, concentration and crystallization, resulting in deviation of molten salt eutectic composition, large undercooling, poor thermal cycling stability and serious equipment scaling.

[0006] To achieve the above objectives, the present invention provides a method for preparing a nitro binary molten salt, comprising the following steps:

[0007] (1) Sodium carbonate and potassium hydroxide are dissolved in deionized water to prepare sodium carbonate aqueous solution and potassium hydroxide aqueous solution, respectively. The sodium carbonate aqueous solution and the potassium hydroxide aqueous solution are mixed to obtain a first mixture, and a starch-based grafting inhibitor is added to the first mixture.

[0008] Preferably, in step (1), the starch-based grafting inhibitor is prepared by the following steps: corn starch is added to deionized water, heated and stirred at about 80°C to gelatinize it, and then cooled to 75°C. Potassium persulfate and glycidyl methacrylate are added to continue the reaction. Then the temperature is lowered to 60°C, sodium p-aminobenzenesulfonate and sodium 3-mercapto-1-propanesulfonate are added in sequence, the pH is adjusted to alkaline and the reaction continues. The reaction solution is spray-dried to obtain the starch-based grafting inhibitor.

[0009] Preferably, the corn starch is food grade.

[0010] Preferably, the mass ratio of corn starch, glycidyl methacrylate, sodium p-aminobenzenesulfonate, and sodium 3-mercapto-1-propanesulfonate in the raw materials for preparing the starch-based grafting inhibitor is 100:0.5:1.5:1.5.

[0011] Preferably, in step (1), the sodium carbonate aqueous solution is prepared by 374 kg of anhydrous sodium carbonate and 748 kg of deionized water, the potassium hydroxide aqueous solution is prepared by 222 kg of potassium hydroxide and 430 kg of deionized water, and the mass of starch-based grafting inhibitor added is 6-15 g.

[0012] (2) Nitric acid and water are mixed to prepare a nitric acid aqueous solution. Under stirring conditions, the nitric acid aqueous solution is added to the first mixture in two stages. When the first stage of nitric acid aqueous solution is added, a suspension formed by sodium nitrate / potassium nitrate binary molten salt powder and deionized water is added to the system as a eutectic seed crystal. The second stage of nitric acid aqueous solution is added and the system temperature is kept below 70°C by cooling to complete the neutralization reaction and obtain the second mixture.

[0013] Preferably, in step (2), the nitric acid aqueous solution is obtained by mixing 1068 kg of concentrated nitric acid with a mass fraction of 65% with 474 kg of deionized water. The nitric acid aqueous solution is added to the first mixture in two stages, wherein the amount of the first stage nitric acid aqueous solution is 600 kg and the amount of the second stage nitric acid aqueous solution is 942 kg.

[0014] Preferably, in step (2), the eutectic seed crystal is formed by mixing a binary molten salt powder with a particle size of no more than 1 mm with deionized water after melting and solidifying sodium nitrate / potassium nitrate in a mass ratio of 60:40 and then cooling it; the amount of the binary molten salt powder is 0.8-1.5 kg and the amount of deionized water is 16-30 kg.

[0015] (3) The second mixture is fed into a falling film evaporator for evaporation and concentration, so that the total solute mass fraction of the effluent is 66.9%-68.7%, and during the evaporation process, 15%-25% of the mass fraction of the high-concentration effluent from the final effect is returned to the feed end of the second mixture and mixed with the unevaporated material to obtain the third mixture;

[0016] Preferably, in step (3), the falling film evaporator is a multi-effect falling film evaporator, the steam side temperature of the first effect is controlled at 118°C, the evaporation temperature of the last effect is controlled at 98°C, and the temperature difference between each effect does not exceed 20°C.

[0017] (4) The third mixture is sprayed and granulated to obtain wet molten salt particles; the wet molten salt particles are heated to 275-285℃ under an inert atmosphere and kept at the temperature in a fluidized state for 10-15 min, then cooled to 100℃ at a cooling rate of 8-12℃ / min, and then naturally cooled to room temperature to obtain a nitro binary molten salt composed of sodium nitrate and potassium nitrate.

[0018] Preferably, in step (4), the spray granulation is performed by heating the third mixture to 180°C in a high-level tank and then atomizing it through the nozzle at the top of the spray granulation tower, so that it comes into countercurrent contact with the 265°C hot air sent from the bottom of the tower, and controlling the temperature of the gas at the bottom of the tower to be 140°C.

[0019] Preferably, in step (4), the moisture content of the wet molten salt particles is 1.2%-1.4% by mass.

[0020] Preferably, in step (4), the particle size D50 of the nitro binary molten salt is 0.69-0.81 mm.

[0021] Furthermore, the present invention also provides a nitro binary molten salt, which is obtained by the above-described method for preparing nitro binary molten salt.

[0022] The beneficial effects of this invention are:

[0023] This invention significantly improves the compositional uniformity and structural stability of nitro binary molten salts by optimizing the preparation process. During the reaction, the acid-base environment is controlled in stages to ensure a stable neutralization reaction, effectively suppressing carbonate decomposition and coarse crystal precipitation caused by local over-acidity, thus laying the foundation for the formation of a uniform nitrate eutectic composition.

[0024] During the crystal nucleation stage, seed crystals with specific compositions are introduced to guide nucleation, allowing sodium nitrate and potassium nitrate molecules to arrange themselves in an orderly manner during precipitation. This reduces the risk of compositional fluctuations and metastable phase formation, thereby decreasing the undercooling of the molten salt and making the phase transition process more acute and reversible.

[0025] By setting up a reflux mixing mechanism in the high-concentration effluent, the crystal grains in the evaporation and concentration process undergo multiple dynamic equilibrium processes of dissolution and recrystallization, which further eliminates microscale component segregation and improves the stability of the eutectic point and the retention rate of latent heat of fusion.

[0026] The added polymeric crystal inhibitor adsorbs onto the crystal growth interface in the solution through steric hindrance and charge interaction, inhibiting abnormal crystal growth and aggregation, reducing the tendency of equipment to scale, and improving the flowability and dispersibility of subsequent particles.

[0027] In the post-granulation treatment, short-time high-temperature melting and controlled cooling cause local recrystallization inside the particles, eliminating micro-stress and structural defects, enhancing the phase transformation reversibility of molten salt in long-term thermal cycling, promoting the formation of a dense oxide film, and improving compatibility with metallic materials.

[0028] The resulting molten salt product has a consistent particle size distribution and a dense structure, and its thermal conductivity and chemical stability at high temperatures are significantly improved, making it suitable for industrial thermal storage systems that require long-term stable operation. Attached Figure Description

[0029] To more clearly illustrate the technical solutions in this invention or the prior art, the accompanying drawings used in the description of the embodiments or the prior art will be briefly introduced below.

[0030] Figure 1 The infrared spectra of the nitro binary molten salts prepared in Example 2 and Comparative Example 5 of this invention are shown. Detailed Implementation

[0031] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to specific embodiments.

[0032] Example 1: Step 1: Preparation of starch-based grafting inhibitor

[0033] 900g of deionized water and 100g of corn starch (food grade) were added to a glass-lined reactor equipped with a mechanical stirrer, thermometer, and reflux condenser. The mixture was stirred and heated at 80℃ for 60min, then cooled to 75℃, and 0.5g of potassium persulfate and 0.5g of glycidyl methacrylate were added. The mixture was stirred and reacted at 75℃ for 3h, then cooled to 60℃, and 1.5g of sodium p-aminobenzenesulfonate and 1.5g of sodium 3-mercapto-1-propanesulfonate were added sequentially. The pH was adjusted to 8.5 with sodium hydroxide solution, and the reaction was continued for 2h. After the reaction was completed, undissolved particles were removed by filtration, and the filtrate was sent to a spray drying tower for spray drying at an inlet air temperature of 160℃ and an outlet air temperature of 85℃ to collect the starch-based grafting inhibitor.

[0034] Step 2: Pre-preparing the solution

[0035] Weigh 374 kg of anhydrous sodium carbonate and add it to 748 kg of deionized water while stirring. Stir at 45°C until completely dissolved to obtain a sodium carbonate aqueous solution. Weigh 222 kg of potassium hydroxide and add it to 430 kg of deionized water. Stir at 25°C until completely dissolved to obtain a potassium hydroxide aqueous solution. Weigh 1068 kg of 65% concentrated nitric acid and add it to an acid-resistant stirred tank. Slowly add 474 kg of deionized water while maintaining the system temperature below 40°C under continuous stirring and cooling. Mix thoroughly to obtain a nitric acid aqueous solution.

[0036] Step 3: Prepare the first mixture and add a crystal inhibitor.

[0037] In an atmospheric pressure stainless steel mixing vessel equipped with a stirrer and jacket temperature control, 1122 kg of sodium carbonate aqueous solution was first added and stirred at 55°C for 30 min. Then, 652 kg of potassium hydroxide aqueous solution was added, with the addition time controlled at 30 min, so that the two alkaline solutions were mixed at 55°C to obtain the first mixture. Subsequently, 6 g of starch-based grafted crystal inhibitor was weighed and added to the first mixture, and the mixture was stirred at 55°C and 300 rpm for 20 min to obtain the first mixture containing the crystal inhibitor.

[0038] Step 4: Neutralize with nitric acid in stages and introduce eutectic seed crystals to prepare the second mixture.

[0039] The temperature of the first mixture was adjusted to 48°C. Under the condition of stirring speed of 300 rpm, 600 kg of nitric acid aqueous solution was added to the first mixture as the first stage of nitric acid at a constant flow rate over 60 min. At the end of this stage, 0.8 kg of binary molten salt powder with a particle size of no more than 1 mm, which was melted and solidified by cooling, was weighed and mixed with 16 kg of deionized water to form a suspension. This suspension was added to the mixing vessel over 10 min and stirred for another 20 min. Then the system temperature was raised to 65°C, and 942 kg of nitric acid aqueous solution was added as the second stage of nitric acid over 90 min. During the process, the temperature was maintained at no more than 70°C by cooling to obtain the second mixture.

[0040] Step 5: Falling film evaporation and partial reflux to prepare the third mixture.

[0041] The second mixture is pumped into a conventional four-effect falling film evaporator. The steam side temperature of the first effect is controlled at 118℃, and the evaporation temperature of the last effect is controlled at 98℃, ensuring that the temperature difference between each effect does not exceed 20℃. During the evaporation process, the concentration of the effluent is monitored by an online density meter, and the feed and steam flow rates are adjusted to keep the total solute mass fraction of the effluent from the last effect stable at 66%-69%. At the same time, 15% of the high-concentration effluent from the last effect is returned to the inlet of the feed buffer tank of the second mixture through the circulation pipeline. It is fully mixed with the new second mixture from step three before entering the evaporator, forming an online mixing mode of high-concentration reflux liquid and medium-concentration feed. Finally, after the evaporation system is in steady-state operation, a third mixture with a mass fraction of 66.9% is collected.

[0042] Step Six: Spray Granulation and Melt Reforming

[0043] The third mixture was heated to 180°C in a high-level tank and, under heat preservation conditions, was pumped into the top nozzle of a conventional spray granulation tower via a high-pressure pump. The third mixture was atomized into fine droplets by a pressure nozzle. Hot air at 265°C was introduced from the bottom of the tower and came into countercurrent contact with the droplets. The surface moisture evaporated rapidly and solidified sodium nitrate / potassium nitrate particles were formed. The temperature of the gas at the bottom outlet of the tower was controlled at 140°C, and wet molten salt particles with a moisture content of 1.4% were collected. The wet particles were then transported to a conventional fluidized bed dryer and heated to 275°C in a nitrogen atmosphere. The dryer was kept at this temperature for 10 minutes in a fluidized state to allow for localized micro-melting and recrystallization within the particles. After the heat preservation was completed, the temperature was reduced to 100°C at a controlled cooling rate of 8°C / min, and then allowed to cool naturally to room temperature. Excess agglomerates were removed by sieving, resulting in a nitro binary molten salt with a particle size D50 of 0.69 mm.

[0044] Example 2: Step 1: Preparation of starch-based grafting inhibitor

[0045] 900g of deionized water and 100g of corn starch (food grade) were added to a glass-lined reactor equipped with a mechanical stirrer, thermometer, and reflux condenser. The mixture was stirred and heated at 80℃ for 60min, then cooled to 75℃, and 0.5g of potassium persulfate and 0.5g of glycidyl methacrylate were added. The mixture was stirred and reacted at 75℃ for 3h, then cooled to 60℃, and 1.5g of sodium p-aminobenzenesulfonate and 1.5g of sodium 3-mercapto-1-propanesulfonate were added sequentially. The pH was adjusted to 8.5 with sodium hydroxide solution, and the reaction was continued for 2h. After the reaction was completed, undissolved particles were removed by filtration, and the filtrate was sent to a spray drying tower for spray drying at an inlet air temperature of 160℃ and an outlet air temperature of 85℃ to collect the starch-based grafting inhibitor.

[0046] Step 2: Pre-preparing the solution

[0047] Weigh 374 kg of anhydrous sodium carbonate and add it to 748 kg of deionized water while stirring. Stir at 45°C until completely dissolved to obtain a sodium carbonate aqueous solution. Weigh 222 kg of potassium hydroxide and add it to 430 kg of deionized water. Stir at 25°C until completely dissolved to obtain a potassium hydroxide aqueous solution. Weigh 1068 kg of 65% concentrated nitric acid and add it to an acid-resistant stirred tank. Slowly add 474 kg of deionized water while maintaining the system temperature below 40°C under continuous stirring and cooling. Mix thoroughly to obtain a nitric acid aqueous solution.

[0048] Step 3: Prepare the first mixture and add a crystal inhibitor.

[0049] In an atmospheric pressure stainless steel mixing vessel equipped with a stirrer and jacket temperature control, 1122 kg of sodium carbonate aqueous solution was first added and stirred at 55°C for 30 min. Then, 652 kg of potassium hydroxide aqueous solution was added, with the addition time controlled at 30 min, so that the two alkaline solutions were mixed at 55°C to obtain the first mixture. Subsequently, 10 g of starch-based grafting inhibitor was weighed and added to the first mixture, and the mixture was stirred at 55°C and 300 rpm for 20 min to obtain the first mixture containing the inhibitor.

[0050] Step 4: Neutralize with nitric acid in stages and introduce eutectic seed crystals to prepare the second mixture.

[0051] The temperature of the first mixture was adjusted to 48°C. Under the condition of stirring speed of 300 rpm, 600 kg of nitric acid aqueous solution was added to the first mixture as the first stage of nitric acid at a constant flow rate over 60 min. At the end of this stage, 1 kg of binary molten salt powder with a particle size of no more than 1 mm, which was melted and solidified by cooling, was weighed and mixed with 20 kg of deionized water to form a suspension. This suspension was added to the mixing vessel over 10 min and stirred for another 20 min. Then the system temperature was raised to 65°C, and 942 kg of nitric acid aqueous solution was added as the second stage of nitric acid over 90 min. During the process, the temperature was maintained at no more than 70°C by cooling to obtain the second mixture.

[0052] Step 5: Falling film evaporation and partial reflux to prepare the third mixture.

[0053] The second mixture is pumped into a conventional four-effect falling film evaporator. The steam side temperature of the first effect is controlled at 118℃, and the evaporation temperature of the last effect is controlled at 98℃, ensuring that the temperature difference between each effect does not exceed 20℃. During the evaporation process, the concentration of the effluent is monitored by an online density meter, and the feed and steam flow rates are adjusted to keep the total solute mass fraction of the effluent from the last effect stable at 66%-69%. At the same time, 20% of the high-concentration effluent from the last effect is returned to the inlet of the feed buffer tank of the second mixture through the circulation pipeline. It is fully mixed with the new second mixture from step three before entering the evaporator, forming an online mixing mode of high-concentration reflux liquid and medium-concentration feed. Finally, after the evaporation system is in steady-state operation, a third mixture with a mass fraction of 67.4% is collected.

[0054] Step Six: Spray Granulation and Melt Reforming

[0055] The third mixture was heated to 180°C in a high-level tank and, under heat preservation conditions, was pumped into the top nozzle of a conventional spray granulation tower via a high-pressure pump. The third mixture was atomized into fine droplets by a pressure nozzle. Hot air at 265°C was introduced from the bottom of the tower and contacted the droplets countercurrently. The surface moisture evaporated rapidly and solidified sodium nitrate / potassium nitrate particles were formed. The temperature of the gas at the bottom outlet of the tower was controlled at 140°C, and wet molten salt particles with a moisture content of 1.2% by mass were collected. The wet particles were then transported to a conventional fluidized bed dryer and heated to 280°C in a nitrogen atmosphere. The dryer was kept at this temperature for 12 minutes in a fluidized state to allow for localized micro-melting and recrystallization within the particles. After the heat preservation was completed, the temperature was reduced to 100°C at a controlled cooling rate of 10°C / min, and then allowed to cool naturally to room temperature. Excess agglomerates were removed by sieving, resulting in a nitro binary molten salt with a particle size D50 of 0.75 mm.

[0056] Example 3: Step 1: Preparation of starch-based grafting inhibitor

[0057] 900g of deionized water and 100g of corn starch (food grade) were added to a glass-lined reactor equipped with a mechanical stirrer, thermometer, and reflux condenser. The mixture was stirred and heated at 80℃ for 60min, then cooled to 75℃, and 0.5g of potassium persulfate and 0.5g of glycidyl methacrylate were added. The mixture was stirred and reacted at 75℃ for 3h, then cooled to 60℃, and 1.5g of sodium p-aminobenzenesulfonate and 1.5g of sodium 3-mercapto-1-propanesulfonate were added sequentially. The pH was adjusted to 8.5 with sodium hydroxide solution, and the reaction was continued for 2h. After the reaction was completed, undissolved particles were removed by filtration, and the filtrate was sent to a spray drying tower for spray drying at an inlet air temperature of 160℃ and an outlet air temperature of 85℃ to collect the starch-based grafting inhibitor.

[0058] Step 2: Pre-preparing the solution

[0059] Weigh 374 kg of anhydrous sodium carbonate and add it to 748 kg of deionized water while stirring. Stir at 45°C until completely dissolved to obtain a sodium carbonate aqueous solution. Weigh 222 kg of potassium hydroxide and add it to 430 kg of deionized water. Stir at 25°C until completely dissolved to obtain a potassium hydroxide aqueous solution. Weigh 1068 kg of 65% concentrated nitric acid and add it to an acid-resistant stirred tank. Slowly add 474 kg of deionized water while maintaining the system temperature below 40°C under continuous stirring and cooling. Mix thoroughly to obtain a nitric acid aqueous solution.

[0060] Step 3: Prepare the first mixture and add a crystal inhibitor.

[0061] In an atmospheric pressure stainless steel mixing vessel equipped with a stirrer and jacket temperature control, 1122 kg of sodium carbonate aqueous solution was first added and stirred at 55°C for 30 min. Then, 652 kg of potassium hydroxide aqueous solution was added, with the addition time controlled at 30 min, so that the two alkaline solutions were mixed at 55°C to obtain the first mixture. Subsequently, 15 g of starch-based grafting inhibitor was weighed and added to the first mixture, and the mixture was stirred at 55°C and 300 rpm for 20 min to obtain the first mixture containing the inhibitor.

[0062] Step 4: Neutralize with nitric acid in stages and introduce eutectic seed crystals to prepare the second mixture.

[0063] The temperature of the first mixture was adjusted to 48°C. Under the condition of stirring speed of 300 rpm, 600 kg of nitric acid aqueous solution was added to the first mixture as the first stage of nitric acid at a constant flow rate over 60 min. At the end of this stage, 1.5 kg of binary molten salt powder with a particle size of no more than 1 mm, which was melted and solidified by cooling, was weighed and mixed with 30 kg of deionized water to form a suspension. This suspension was added to the mixing vessel over 10 min and stirred for another 20 min. Then the system temperature was raised to 65°C, and 942 kg of nitric acid aqueous solution was added as the second stage of nitric acid over 90 min. During the process, the temperature was maintained at no more than 70°C by cooling to obtain the second mixture.

[0064] Step 5: Falling film evaporation and partial reflux to prepare the third mixture.

[0065] The second mixture is pumped into a conventional four-effect falling film evaporator. The steam side temperature of the first effect is controlled at 118℃, and the evaporation temperature of the last effect is controlled at 98℃, ensuring that the temperature difference between each effect does not exceed 20℃. During the evaporation process, the concentration of the effluent is monitored by an online density meter, and the feed and steam flow rates are adjusted to keep the total solute mass fraction of the effluent from the last effect stable at 66%-69%. At the same time, 25% of the high-concentration effluent from the last effect is returned to the inlet of the feed buffer tank of the second mixture through the circulation pipeline. It is fully mixed with the new second mixture from step three before entering the evaporator, forming an online mixing mode of high-concentration reflux liquid and medium-concentration feed. Finally, after the evaporation system is in steady-state operation, a third mixture with a mass fraction of 68.7% is collected.

[0066] Step Six: Spray Granulation and Melt Reforming

[0067] The third mixture was heated to 180°C in a high-level tank and, under heat preservation conditions, was pumped into the top nozzle of a conventional spray granulation tower via a high-pressure pump. The third mixture was atomized into fine droplets by a pressure nozzle. Hot air at 265°C was introduced from the bottom of the tower and came into countercurrent contact with the droplets. The surface moisture evaporated rapidly and solidified sodium nitrate / potassium nitrate particles were formed. The temperature of the gas at the bottom outlet of the tower was controlled at 140°C, and wet molten salt particles with a moisture content of 1.2% by mass were collected. The wet particles were then transported to a conventional fluidized bed dryer and heated to 285°C in a nitrogen atmosphere. The dryer was kept at this temperature for 15 minutes in a fluidized state to allow for localized micro-melting and recrystallization within the particles. After the heat preservation was completed, the temperature was reduced to 100°C at a controlled cooling rate of 12°C / min, and then allowed to cool naturally to room temperature. Excess agglomerates were removed by sieving, resulting in a nitro binary molten salt with a particle size D50 of 0.81 mm.

[0068] Comparative Example 1:

[0069] The difference between Comparative Example 1 and Example 2 is that in step four, all the nitric acid aqueous solution is continuously added to the first mixture at a constant flow rate within 10 minutes. The nitric acid aqueous solution is no longer divided into the first and second stages of addition, and the pH of the first mixture is not adjusted or controlled at any time point to be between the carbonate and bicarbonate buffer. However, when the cumulative amount of nitric acid aqueous solution added is equivalent to the amount of nitric acid added in the first stage in Example 2, the suspension formed by sodium nitrate / potassium nitrate binary molten salt powder and deionized water is added to the mixing vessel. The other conditions are the same as in Example 2.

[0070] Comparative Example 2:

[0071] The difference between Comparative Example 2 and Example 2 is that in step four, when the first stage of nitric acid aqueous solution is added and the system temperature rises to 65°C, the suspension formed by sodium nitrate / potassium nitrate binary molten salt powder and deionized water is no longer added, that is, the step of adding eutectic seed crystal is omitted, and the second stage of nitric acid aqueous solution is added directly to complete the neutralization. The remaining conditions are the same as in Example 2.

[0072] Comparative Example 3:

[0073] The difference between Comparative Example 3 and Example 2 is that in step three, after stirring the first mixture containing sodium carbonate aqueous solution and potassium hydroxide aqueous solution at the same temperature and stirring speed, starch-based grafting inhibitor is no longer added, that is, the step of adding starch-based grafting inhibitor is omitted, and the remaining conditions are the same as in Example 2.

[0074] Comparative Example 4:

[0075] The difference between Comparative Example 4 and Example 2 is that in step five, the second mixture is fed into a four-effect falling film evaporator and evaporated and concentrated to a total solute mass fraction of 67.4% under the same steam side temperature, final effect evaporation temperature and operating pressure conditions as in Example 2. However, instead of recirculating 20% ​​of the high-concentration final effect effluent to the inlet of the feed buffer tank, all the final effect effluent is directly collected as the third mixture. The remaining conditions are the same as in Example 2.

[0076] Comparative Example 5:

[0077] The difference between Comparative Example 5 and Example 2 is that in step six, after the third mixture is spray-dried in a spray granulation tower to obtain wet molten salt particles with a moisture content of 1.2%, nitrogen gas is introduced only into the fluidized bed dryer to raise the bed temperature to 220°C and keep it at that temperature for 5 minutes in a fluidized state to complete the drying. The temperature is not raised to 280°C for micro-melting and recrystallization, and the cooling rate is not controlled at 10°C / min. The other conditions are the same as in Example 2.

[0078] Comparative Example 6:

[0079] The difference between Comparative Example 6 and Example 2 is that in step one, 1.5g of sodium 3-mercapto-1-propanesulfonate was not added, but only 1.5g of sodium p-aminobenzenesulfonate was added for the reaction. That is, the starch-based grafting inhibitor does not contain sodium benzenesulfonate groups. The other conditions are the same as in Example 2.

[0080] Performance testing:

[0081] Infrared spectroscopy characterization: Qualitative analysis of inorganic salt samples was performed using a Fourier transform infrared spectrometer. 2 mg each of samples from Example 2 and Comparative Example 5 (sieved through a 100-mesh sieve) were mixed with 200 mg of potassium bromide that had been dried at high temperature, ground into a fine powder, and pressed into tablets to obtain sample tablets with acceptable transparency. Scanning was performed with air as a blank background. The results are shown below. Figure 1 As shown.

[0082] Differential scanning calorimetry (DSC) was used to determine the melting point, freezing point, and latent heat of fusion of nitro binary molten salts. A differential scanning calorimeter was used, and the heating and cooling programs were set according to GB / T19466.1-2004. 10 mg of sample was placed in a covered alumina crucible, with air as the protective gas. The heating program was as follows: from 50 °C to 350 °C at a heating rate of 10 °C / min, held at this temperature for 10 min, and then cooled to 50 °C at the same rate. The endothermic peak during heating and the exothermic peak during cooling were recorded. The melting point, freezing point, and supercooling of the nitro binary molten salts were recorded, and the latent heat of fusion was calculated. The results are shown in Table 1.

[0083] Thermal conductivity determination: In accordance with GB / T 10297-2015, the thermal conductivity was determined using a steady-state hot wire method thermal conductivity testing device. Each sample was melted and cast into a cylindrical specimen with a diameter of 50 mm and a thickness of 20 mm in a stainless steel mold. After cooling to room temperature, the upper and lower surfaces were polished flat. The specimen was clamped between constant temperature plates at 200℃ and 300℃, and a hot wire probe was arranged along the thickness direction of the specimen. The temperature difference and heat flux density were recorded under steady-state conditions, and the thermal conductivity was calculated. The results are shown in Table 1.

[0084] Thermal cycling stability evaluation: To simulate the long-term thermal cycling conditions of molten salt in the thermal storage system, a differential scanning calorimeter (DSC) was used to conduct repeated heating and cooling cycle tests. The heating and cooling program was set based on the thermal analysis repeatability requirements specified in GB / T 19466.1-2004: the sample was heated from 260℃ to 450℃ at a heating rate of 5℃ / min, held at the temperature for 30 min, and then cooled to 260℃ at the same rate, which constituted one cycle. Five hundred cycles were performed continuously. After five hundred cycles, complete DSC curves were collected to calculate the melting point and latent heat of fusion, and further calculated the melting point change and latent heat retention rate. The results are shown in Table 1.

[0085] High-temperature molten salt corrosion compatibility with metallic materials: To evaluate the compatibility of nitro binary molten salt with commonly used equipment materials, austenitic stainless steel test pieces were selected as the test metal, referring to GB / T 42912-2023. High-temperature corrosion tests were conducted in static molten salt. Stainless steel test pieces with dimensions of 20×10×2mm were polished to 1200# with sandpaper, ultrasonically cleaned with acetone and ethanol, dried, and weighed initially. They were then placed in nickel-based alloy crucibles containing the samples, ensuring the test pieces were completely immersed in the molten salt. The temperature was raised to 565℃ under nitrogen protection and maintained at a constant temperature for 500h. After the test, the test pieces were removed, surface corrosion products were removed, and the samples were cleaned, dried, and weighed again. The corrosion rate was calculated based on the mass loss and exposed area. The results are shown in Table 1.

[0086] Table 1 Performance Test Results

[0087]

[0088] Data Analysis:

[0089] As can be seen from the data of Examples 1-3 in Table 1, the nitro binary molten salt prepared by the present invention has a relatively narrow range in terms of melting point, freezing point and supercooling. The melting point is close to the phase transition temperature of the theoretical eutectic composition, the freezing point is uniformly distributed, and the supercooling is small. This indicates that by stepwise nitric acid neutralization, introducing eutectic seed crystals with the same ratio as the finished product in the carbonate / bicarbonate buffer zone, and partial high-concentration reflux in the falling film evaporation stage, the component segregation and early precipitation of coarse crystals during the neutralization and concentration process are effectively suppressed, making the phase boundary stable and the crystal nucleation more controllable during the heating and cooling process of the molten salt. From the overall level and fluctuation range of latent heat of fusion and thermal conductivity, Examples 1-3 all maintained high heat storage density and good thermal conductivity in the same system, indicating that the starch-based grafted crystal inhibitor controlled the nucleation and growth of carbonate crystals through multi-point adsorption and steric hindrance during the neutralization and evaporation stages of sodium carbonate / potassium hydroxide, reducing the effective heat exchange area loss caused by dead-angle scaling; at the same time, the trace carbonaceous residues formed by the moderate decomposition of the crystal inhibitor during the melting and reforming process near 280°C are uniformly dispersed in the nitrate matrix, slightly improving the apparent specific heat and thermal conductivity without destroying the eutectic point. Furthermore, after 500 thermal cycles, the melting point changes of Examples 1-3 were relatively small, the latent heat retention rate was generally maintained at a high level, and the metal corrosion rate was in a low range. This indicates that the process path of spray granulation combined with fluidized bed short-time melt reforming and controlled cooling, on the one hand, eliminates the microscopic composition inhomogeneity and metastable phase caused by the preparation process by completing a solid-state annealing inside the particles, and on the other hand, forms a stable and dense protective oxide film at the molten salt / stainless steel interface, which works synergistically with the dispersed carbon particles. Under long-term high-temperature cycling, it takes into account both the stability of heat storage performance and the compatibility with austenitic stainless steel equipment materials.

[0090] As can be seen from the data in Table 1 for Example 2 and Comparative Example 1, when the nitric acid aqueous solution is added all at once in a short period of time without segmented control of pH, the melting point shifts upward overall, the supercooling increases significantly, and the latent heat of fusion and thermal conductivity decrease. After thermal cycling, the melting point drift and latent heat decay are more pronounced, and the metal corrosion rate also increases. The main reason is that the one-time addition of acid leads to severe over-neutralization in local areas in a short period of time. Sodium carbonate easily forms coarse intermediate salt crystals and high-sodium phase enrichment areas near the acid impact front. Even with the subsequent addition of eutectic seed crystals and melt reforming, it is still difficult to completely eliminate the early-formed component segregation, thus causing the molten salt system to deviate locally from the theoretical eutectic ratio. Coarse grains and high-sodium phases are more likely to generate stress concentration and interface defects during thermal cycling, which amplifies the drift of phase transformation temperature and latent heat and provides easily corroded active sites for oxide film rupture. Therefore, it is evident that the phased addition of nitric acid and the control of the system within a buffer zone are key steps to ensure the uniformity of the eutectic composition and long-term thermal stability. Relying solely on subsequent seeding and heat treatment is insufficient to compensate for the adverse effects caused by a single addition of acid.

[0091] As can be seen from the data in Table 1 for Example 2 and Comparative Example 2, when the sodium nitrate / potassium nitrate eutectic seed crystal with the same ratio as the finished product is omitted, and only the segmented neutralization and crystal inhibitor control are retained, although the melting point only shifts slightly upward, the temperature gap between the melting point and the freezing point increases significantly, the supercooling is significantly amplified, the latent heat of fusion decreases slightly, and the amount of melting point change and latent heat retention rate after thermal cycling are worse than in Example 2. The main reason is that without the eutectic seed crystal template, the nitrate crystals generated subsequently can only rely on spontaneous nucleation. The fluctuation of the instantaneous sodium-potassium ratio in the local liquid phase is more difficult to lock near the target ratio, and it is easy to form small but metastable grains with compositional deviations in the solid phase. These grains melt and crystallize successively during heating and cooling, which will manifest as a widening of the phase transformation process, an increase in supercooling, and a decrease in the effective value of latent heat. At the same time, grains with slightly deviated composition from the eutectic are more prone to micro-phase separation at high temperatures, resulting in cumulative drift of melting point and latent heat during cycling. It is evident that the eutectic seed crystal used in conjunction with segmented neutralization has an unexpected amplification effect in guiding the nucleation components to converge to the target ratio. Its contribution is not simply additive, but rather amplifies the homogenization effect brought about by segmented neutralization and the inhibitor at the microstructural level.

[0092] As can be seen from the data in Table 1 for Example 2 and Comparative Example 3, when no starch-based grafted crystal inhibitor was added to the sodium carbonate / potassium hydroxide mixture, the melting point and supercooling both deteriorated significantly. The latent heat of fusion and thermal conductivity were lower than in Example 2, and the melting point drift and latent heat loss after thermal cycling were also more severe, with the corrosion rate being at a relatively high level among all samples. The main reason for this is the lack of multi-point adsorption of bifunctional groups of aromatic sulfonates and aliphatic sulfonates on the starch skeleton. During neutralization and evaporation concentration, carbonate crystals grow freely with their natural crystal habit, making it easier to form large, irregularly shaped crystal clusters. This not only exacerbates scaling in the evaporator and pipelines but also introduces a large number of heterogeneous interfaces during subsequent nitrate nucleation. These coarse crystal faces are difficult to completely rearrange during melt reforming, and the stress and porosity remaining inside the particles weaken the effective contact area for heat conduction and exchange. At the same time, they provide more anodic active sites for the molten salt / stainless steel interface, reducing the stability of the high-temperature oxide film. It is evident that although starch-based grafted crystal inhibitors are present in ppm-level dosages, they produce significant synergistic effects in controlling crystal size, inhibiting scaling, and improving interface uniformity. The performance degradation caused by their absence is far greater than the negative impact of additive residues on molten salt performance in the conventional sense.

[0093] As can be seen from the data in Table 1 for Example 2 and Comparative Example 4, when the partial reflux of the high-concentration effluent at the end of the falling film evaporation stage is eliminated, and all the effluent is directly collected as the third mixture, the melting point further shifts towards the high-temperature side, the supercooling remains at a large level, the dispersion of latent heat of fusion and thermal conductivity increases, the change in melting point after thermal cycling is larger among all samples, and the latent heat retention rate is also significantly lower than that of Example 2. The main reason is that the lack of online mixing of high-concentration reflux liquid and medium-concentration virgin material in the feed buffer section means that the coarse grains and local compositional fluctuations that are occasionally formed cannot undergo multiple dissolution-renucleation reshaping processes in the reflux dilution zone, thus retaining more crystal subgroups that deviate from the eutectic composition. These subgroups may still exist in the form of fine inclusions or local phase domains after spray granulation and melt reforming, participating in phase transformation successively during thermal cycling, causing the melting point to gradually drift and the latent heat to slowly decay. It can be seen that the partial recirculation of the high-concentration discharge at the end of the process does not only play a steady-state regulation role in the conventional sense, but also, in conjunction with the crystal inhibitor and eutectic seed crystal, achieves dynamic reshaping of the grain composition and size distribution at the microscale, demonstrating a synergistic effect of 1+1>2 under the series of multiple processes.

[0094] As can be seen from the data in Table 1 for Example 2 and Comparative Example 5, when the wet molten salt particles after spray granulation are dried only at a lower temperature for a short time, without being heated to 280°C for micro-melting and recrystallization, and without controlled cooling, the initial melting point and latent heat of fusion are close to those of Comparative Examples 1-4, but the supercooling remains at a relatively high level. After thermal cycling, the latent heat retention rate drops to one of the lowest values ​​among all samples. Although the change in melting point is not necessarily the largest, it exhibits a slow cumulative shift. The main reason for this is that without a short-term melting and reforming stage, the dendrites, fine voids, and metastable phases locally rich in a certain component formed during spray granulation cannot be annealed at the solid particle level. These implicit inhomogeneities are not obvious in a few cycles, but they gradually amplify after hundreds of heating and cooling cycles, manifesting as a gradual loss of effective latent heat and a slow drift in phase transformation temperature. In addition, the particles that have not undergone melting and reforming still retain many open pores, which easily accumulate decomposition products or residual moisture in the molten state, weakening the continuity of thermal conductivity and providing conditions for localized corrosion of the stainless steel surface. This shows that fluidized bed short-time melt reforming is not simply a drying extension, but rather a process of phase structure rebalancing within the particles, which has a significant impact on long-term cycle stability.

[0095] As can be seen from the data in Table 1 for Example 2 and Comparative Example 6, when sodium 3-mercapto-1-propanesulfonate is missing from the starch-based grafted inhibitor and only sodium p-aminobenzenesulfonate is retained for grafting, the melting point and supercooling are still significantly worse than in Example 2. There are also considerable differences in the melting point change and latent heat retention rate after thermal cycling. However, some indicators, such as latent heat of fusion and thermal conductivity, are slightly higher than in other comparative examples. This may be because the grafting density and spatial configuration of single aromatic sulfonate side groups on the starch skeleton differ from those of the disulfonate system, making it easier to form locally more rigid organic-inorganic composite phase domains. These phase domains initially improve crystal defect density, thus slightly increasing specific heat and thermal conductivity. However, they also introduce stronger anisotropic adsorption and local component enrichment during neutralization and evaporation, reducing the adaptability of the inhibitory effect. During long-term thermal cycling, this seemingly advantageous rigid phase domain is more likely to evolve into a source of stress concentration and a location for microcrack initiation, which reduces the stability of the phase transformation temperature and latent heat, and forms an uneven oxide film at the molten salt / metal interface, resulting in a corrosion rate higher than in Example 2.

[0096] from Figure 1 It can be seen that the sample of Example 2 and the sample of Comparative Example 5 are at 1380 cm. -1 The presence of bending vibration characteristic peaks on both sides indicates that the main structure of both samples is a binary molten salt of sodium nitrate / potassium nitrate; however, the nitrate characteristic peak of the sample in Example 2 is concentrated and sharp, with the corresponding peak at 3400 cm⁻¹. -1 and 1640cm -1 The absorption of OH and HOH is relatively weak at 1415 cm⁻¹. -1and 870cm -1 The trace carbonate peak was also significantly weakened, while that of the comparative example 5 sample was at 1380 cm⁻¹. -1 The region shows obvious shoulders and broadening, and the OH and carbonate-related absorption is enhanced, indicating that the nitrate lattice environment in Example 2 is more uniform and the residual moisture and carbonate content is lower.

[0097] Those skilled in the art should understand that the discussion of any of the above embodiments is merely exemplary and is not intended to imply that the scope of the invention is limited to these examples; within the framework of the invention, the technical features of the above embodiments or different embodiments can also be combined, the steps can be implemented in any order, and there are many other variations of the different aspects of the invention as described above, which are not provided in detail for the sake of brevity.

Claims

1. A method for preparing a nitro binary molten salt, characterized in that, Includes the following steps: (1) Sodium carbonate and potassium hydroxide are dissolved in deionized water to prepare sodium carbonate aqueous solution and potassium hydroxide aqueous solution, respectively. The sodium carbonate aqueous solution and the potassium hydroxide aqueous solution are mixed to obtain a first mixture, and a starch-based grafting inhibitor is added to the first mixture. (2) Nitric acid and water are mixed to prepare a nitric acid aqueous solution. Under stirring conditions, the nitric acid aqueous solution is added to the first mixture in two stages. When the first stage of nitric acid aqueous solution is added, a suspension formed by sodium nitrate / potassium nitrate binary molten salt powder and deionized water is added to the system as a eutectic seed crystal. The second stage of nitric acid aqueous solution is added and the system temperature is kept below 70°C by cooling to complete the neutralization reaction and obtain the second mixture. (3) The second mixture is fed into a falling film evaporator for evaporation and concentration, so that the total solute mass fraction of the effluent is 66.9%-68.7%, and during the evaporation process, 15%-25% of the mass fraction of the high-concentration effluent from the final effect is returned to the feed end of the second mixture and mixed with the unevaporated material to obtain the third mixture; (4) The third mixture is sprayed and granulated to obtain wet molten salt particles; the wet molten salt particles are heated to 275-285℃ under an inert atmosphere and kept at the temperature in a fluidized state for 10-15 min, then cooled to 100℃ at a cooling rate of 8-12℃ / min, and then naturally cooled to room temperature to obtain a nitro binary molten salt composed of sodium nitrate and potassium nitrate. In step (1), the starch-based grafting inhibitor is prepared by the following steps: corn starch is added to deionized water, heated and stirred at about 80°C to gelatinize it, and then cooled to 75°C. Potassium persulfate and glycidyl methacrylate are added to continue the reaction. Then the temperature is lowered to 60°C, sodium p-aminobenzenesulfonate and sodium 3-mercapto-1-propanesulfonate are added in sequence, the pH is adjusted to alkaline and the reaction continues. The reaction solution is spray-dried to obtain the starch-based grafting inhibitor. The mass ratio of corn starch, glycidyl methacrylate, sodium p-aminobenzenesulfonate, and sodium 3-mercapto-1-propanesulfonate in the raw materials for preparing the starch-based grafting inhibitor is 100:0.5:1.5:1.

5.

2. The method for preparing a nitro binary molten salt according to claim 1, characterized in that, The corn starch is food grade.

3. The method for preparing a nitro binary molten salt according to claim 1, characterized in that, In step (1), the sodium carbonate aqueous solution is prepared by 374 kg of anhydrous sodium carbonate and 748 kg of deionized water, the potassium hydroxide aqueous solution is prepared by 222 kg of potassium hydroxide and 430 kg of deionized water, and the mass of starch-based grafting inhibitor added is 6-15 g.

4. The method for preparing a nitro binary molten salt according to claim 1, characterized in that, In step (2), the nitric acid aqueous solution is obtained by mixing 1068 kg of concentrated nitric acid with a mass fraction of 65% with 474 kg of deionized water. The nitric acid aqueous solution is added to the first mixture in two stages, wherein the amount of the first stage nitric acid aqueous solution is 600 kg and the amount of the second stage nitric acid aqueous solution is 942 kg.

5. The method for preparing a nitro binary molten salt according to claim 1, characterized in that, In step (2), the eutectic seed crystal is formed by mixing a binary molten salt powder with a particle size of no more than 1 mm with deionized water after melting and solidifying sodium nitrate / potassium nitrate in a mass ratio of 60:40 and then cooling it. The amount of the binary molten salt powder used is 0.8-1.5 kg, and the amount of deionized water used is 16-30 kg.

6. The method for preparing a nitro binary molten salt according to claim 1, characterized in that, In step (3), the falling film evaporator is a multi-effect falling film evaporator, the steam side temperature of the first effect is controlled at 118℃, the evaporation temperature of the last effect is controlled at 98℃, and the temperature difference between each effect does not exceed 20℃.

7. The method for preparing a nitro binary molten salt according to claim 1, characterized in that, In step (4), spray granulation is performed by heating the third mixture to 180°C in a high-level tank and then atomizing it through the nozzle at the top of the spray granulation tower. The mixture is then in countercurrent contact with the 265°C hot air sent from the bottom of the tower, and the temperature of the gas at the bottom of the tower is controlled to be 140°C.

8. The method for preparing a nitro binary molten salt according to claim 1, characterized in that, In step (4), the moisture content of the wet molten salt particles is 1.2%-1.4% by mass.

9. The method for preparing a nitro binary molten salt according to claim 1, characterized in that, In step (4), the particle size D50 of the nitro binary molten salt is 0.69-0.81 mm.

10. A nitro binary molten salt, characterized in that, It is obtained by the preparation method of nitro binary molten salt according to any one of claims 1-9.

Images