Low-energy-consumption crystallization and purification method of high-purity molten salt-grade potassium nitrate

Abstract

The invention discloses a low-energy-consumption crystallization purification method of high-purity molten salt-grade potassium nitrate, and belongs to the technical field of purification of inorganic chemical materials. The method comprises the steps of raw material pretreatment directional impurity removal, multi-stage gradient crystallization, mother liquor graded circulation and low-temperature drying, in the raw material pretreatment stage, impurity concentration online detection and dynamic pH regulation are combined, a chelating agent, an oxidizing agent and an adsorption flocculant are added step by step, precise directional removal of metal impurities and suspended solids is achieved, and the product purity is improved; a multi-stage gradient crystallization process combining gradient temperature control and sectional controllable cooling is adopted, and the crystallization efficiency is improved in cooperation with a crystal form regulating agent; a mother liquor graded circulation and drying waste heat recovery system is matched, so that the mother liquor utilization rate is increased, and the production energy consumption is reduced.

Description

Technical Field

[0001] This invention relates to the field of inorganic chemical material purification technology, specifically to a low-energy crystallization purification method for high-purity molten salt grade potassium nitrate. Background Technology

[0002] Potassium nitrate, as an important inorganic chemical raw material, is widely used in energy storage and other fields. Among them, molten salt-grade potassium nitrate is the core high-temperature heat transfer and heat storage medium in solar thermal power generation and other scenarios due to its excellent thermal stability and thermal conductivity.

[0003] Molten salt grade potassium nitrate requires high purity, especially for... Fe3+ , Al3+ , Ca2+ , Mg2+ The control of metallic impurities and suspended solids is quite strict. Excessive impurity content can lead to the decomposition of molten salt at high temperatures, corrosion of equipment, and a reduction in the heat transfer and storage performance of the molten salt, shortening its service life. Existing potassium nitrate purification methods mainly include recrystallization, ion exchange, and membrane separation. Recrystallization is widely used due to its simplicity and low cost, but traditional methods suffer from low crystallization efficiency, high energy consumption, and significant waste of mother liquor. Furthermore, it is difficult to achieve targeted and efficient removal of multiple impurities, resulting in limited purification effects. While ion exchange can effectively remove metallic impurities, it suffers from frequent resin regeneration, high reagent consumption, and complex post-processing, hindering large-scale industrial production. Membrane separation requires sophisticated equipment, suffers from membrane fouling issues, and has limited throughput, restricting its application in the preparation of high-purity potassium nitrate. Therefore, there is an urgent need to develop a low-energy-consumption crystallization purification method for high-purity molten salt-grade potassium nitrate. Summary of the Invention

[0004] This invention provides a low-energy crystallization purification method for high-purity molten salt grade potassium nitrate. Through the synergistic design of raw material pretreatment for directional impurity removal, multi-stage gradient crystallization, mother liquor graded circulation, and waste heat recovery, it effectively removes metal impurities and suspended solids from industrial-grade potassium nitrate, improves crystallization efficiency and product purity, reduces production energy consumption, and increases mother liquor utilization.

[0005] This invention is implemented through the following technical solution: a low-energy crystallization purification method for high-purity molten salt grade potassium nitrate, comprising the following steps: S1. Raw material pretreatment and targeted impurity removal: Industrial-grade potassium nitrate is dissolved in mother liquor circulating water and / or fresh water, and heated to a temperature that allows the industrial-grade potassium nitrate to completely dissolve without boiling, forming a raw material solution; the pH value of the solution is dynamically adjusted by online detection of impurity concentration, and chelating agents, oxidizing impurity removers and adsorption flocculants are added step by step. After each component is added, it is stirred until the corresponding impurities are removed to the standard. The complexed precipitates, oxidation products and adsorbed flocs are removed by a multi-stage solid-liquid separation process to obtain a purified solution; S2, multi-level gradient crystallization S21, Primary cooling crystallization: The purified liquid is cooled and crystallized using a gradient temperature control method, and solid-liquid separation is performed to obtain primary crystals and mother liquor I; S22, Secondary isothermal crystallization: After the mother liquor I is heated and concentrated to the point where the potassium nitrate solubility is saturated, it is naturally cooled and crystallized, and the solid and liquid are separated to obtain secondary crystals and mother liquor II; S23, Three-stage mild recrystallization: The primary crystal and the secondary crystal are mixed, dissolved in a solvent and heated until the crystal is completely dissolved. A crystal form regulator is added, and the crystals are cooled and crystallized at a segmented controllable cooling rate using a programmable temperature control device. The solid and liquid are separated to obtain high-purity potassium nitrate wet product. S3, Mother liquor staged circulation S31. Recycling Mother Liquor I: The mother liquor I is returned to step S1 for dissolving industrial-grade potassium nitrate raw material; S32, Mother Liquor II Purification and Classification Circulation: After cooling, Mother Liquor II is pretreated by ceramic membrane filtration. After filtration, the mother liquor is allowed to stand until some impurities precipitate. After impurity removal, part of the mother liquor is returned to step S1, and part is used for crystal dissolution in step S23. The precipitate generated by membrane filtration and standing is washed, concentrated, and crystallized to obtain by-product salt. The condensate generated in the process is reused in step S1. After the ceramic membrane runs continuously until the filtration efficiency decreases, an acid-base alternating cleaning process is used to restore the filtration performance. S4. Low-temperature drying: The high-purity potassium nitrate wet product is dried by hot air circulation until the moisture content meets the requirements for use in molten salt grade, thus obtaining the high-purity molten salt grade potassium nitrate.

[0006] Further, in step S1, the mother liquor circulating water includes mother liquor I from step S31 and mother liquor after impurity removal from step S32; the multi-stage solid-liquid separation specifically involves: firstly removing precipitates using plate and frame filtration, and then subjecting them to secondary interception via precision filtration.

[0007] Furthermore, in step S1, the pH value is monitored and adjusted in real time using an online pH meter; the online detection method for impurity concentration is an ion sensor; and the corresponding impurity removal meeting the standard is achieved when the ion sensor detects the impurity. Fe3+ , Al3+ , Ca2+ , Mg2+ The concentration no longer decreases; the stepwise addition order is as follows: first add the chelating agent, then stir the reaction until... Fe3+ , Al3+ Once the concentration reaches the target, add an oxidizing impurity remover and continue stirring the reaction until... Fe2+ Once the concentration reaches the target, add the adsorption-type flocculant and stir until the turbidity of the solution no longer decreases; maintain a stirring rate that ensures uniform dispersion of the reagent throughout the process; the chelating agent is diethylenetriaminepentamethylenephosphonic acid; the oxidizing impurity remover is hydrogen peroxide; and the adsorption-type flocculant is modified diatomaceous earth.

[0008] Furthermore, in step S21, the cooling rate of the gradient temperature control method is 0.8~1.2℃ / min, the cooling endpoint temperature is 32~48℃, and the temperature is maintained for 2.5~3.5 hours after crystallization.

[0009] Further, in step S23, the solvent is a mixture of fresh water and the mother liquor after impurity removal in step S32; the crystal form regulator is sodium citrate; the segmented controllable cooling rate is specifically: cooling at a rate of 0.6~1.0℃ / h in the early stage, cooling at a rate of 0.4~0.6℃ / h in the middle stage, and cooling to 8~12℃ at a rate of 0.2~0.4℃ / h in the later stage.

[0010] Further, in step S32, the ceramic film is α-Al2O3 The ceramic membrane is filtered using a cross-flow filtration method. The acid-base alternating cleaning process is as follows: first, cross-flow cleaning is performed with hydrochloric acid solution, then cross-flow cleaning is performed with sodium hydroxide solution, and finally, fresh water is used to rinse until the pH of the filtrate is 6.5~7.5. The precipitate is washed with hot water at 50~60℃, which is the condensate from the drying process in step S4.

[0011] Furthermore, in step S4, the hot air circulating dryer is equipped with a waste heat recovery device, and the recovered waste heat is used to preheat the raw material dissolved water in step S1.

[0012] Furthermore, a high-purity molten salt grade potassium nitrate, wherein the potassium nitrate is prepared by the method described in any of the preceding claims.

[0013] Beneficial effects Compared with the prior art, the present invention has the following beneficial effects: 1. This invention utilizes a targeted impurity removal design for raw material pretreatment, combined with online detection of impurity concentration and dynamic pH control. Chelating agents, oxidizing impurity removers, and adsorption-type flocculants are added stepwise, along with a multi-stage solid-liquid separation process, to effectively remove metallic impurities and suspended solids from industrial-grade potassium nitrate, laying the foundation for the preparation of high-purity molten salt-grade potassium nitrate.

[0014] 2. This invention employs a multi-stage gradient crystallization process, which improves crystallization efficiency through a combination of gradient temperature control, natural cooling, and segmented controllable cooling. At the same time, it optimizes crystal performance with crystal form regulators, thereby reducing production energy consumption compared to traditional recrystallization methods.

[0015] 3. This invention uses a mother liquor staged recycling design to return the mother liquor to the raw material dissolving process or use it as a tertiary crystallization solvent, while simultaneously reusing condensate and washing water, thus improving the utilization rate of the mother liquor. The ceramic membrane adopts cross-flow filtration and alternating acid-alkali cleaning processes to ensure the efficiency of mother liquor impurity removal, further improving the resource recycling rate and contributing to low-energy production.

[0016] 4. The present invention is equipped with a waste heat recovery device in the drying process to further reduce production energy consumption. At the same time, the multi-stage crystallization process adopts a mild temperature control method to reduce energy consumption in the high-temperature concentration stage and enhance the low-energy production effect. Attached Figure Description

[0017] Figure 1 This is a process flow diagram of an embodiment of the present invention; Figure 2 This is a temperature-time curve of multi-level gradient crystallization in one embodiment of the present invention; Figure 3 This is a flowchart of a ceramic membrane cleaning process in one embodiment of the present invention. Detailed Implementation

[0018] The following is in conjunction with the appendix Figures 1 to 3 The present invention will be further described in detail with reference to specific embodiments. The embodiments are only used to explain the present invention and are not intended to limit the scope of protection of the present invention.

[0019] Example 1: S1. Raw material pretreatment and targeted impurity removal Industrial-grade potassium nitrate is dissolved in mother liquor circulating water and / or fresh water. Mother liquor I is mixed with impurity-removed mother liquor at a volume ratio of 1:1. The mixture is heated to a temperature of 65-75°C, ensuring complete dissolution of the industrial-grade potassium nitrate without boiling, to form a raw material solution with a mass concentration of 32-38%. Online monitoring is performed using an ion sensor. Fe3+ , Al3+ , Ca2+ , Mg2+ The concentration was adjusted dynamically using an online pH meter to stabilize the solution pH at 6.2–6.8. First, the chelating agent diethylenetriaminepentamethylenephosphonic acid was added, with the amount dynamically adjusted based on the ion sensor detection results: when the iron ion concentration was >0.005%, the amount added was 0.05%–0.08% of the solution mass; when the iron ion concentration was ≤0.003%, the amount added was 0.02%–0.04% of the solution mass. The mixture was stirred for 30 minutes until the ion sensor detected the ion concentration. Fe3+ , Al3+ The concentration no longer decreases; then add hydrogen peroxide, an oxidizing agent for removing impurities, with a mass concentration of 30%~50%. The amount added is dynamically adjusted based on the detection results of the ion sensor. Fe2+ When the concentration is >0.005%, the amount added is 0.08%~0.1% of the solution mass; Fe2+ When the concentration is ≤0.003%, the amount added is 0.05%~0.07% of the solution mass; continue stirring for 20 minutes until... Fe2+The concentration no longer decreased; finally, adsorption-type flocculant modified diatomaceous earth was added, with an addition amount of 0.1%~0.2% of the solution mass; the mixture was stirred for 15 minutes until the turbidity of the solution no longer decreased; the precipitate was first removed by plate and frame filtration, and then subjected to a second interception by precision filtration with a filtration accuracy of 0.1µm to obtain the purified solution.

[0020] S2, multi-level gradient crystallization S21, Primary Cooling Crystallization: The purified liquid is cooled and crystallized using a gradient temperature control method. The cooling rate is controlled at 0.8~1.2℃ / min, and the final cooling temperature is 32~48℃. After crystallization, the liquid is kept at this temperature for 2.5~3.5 hours. Solid-liquid separation is performed using a conventional centrifuge or plate and frame filter press to obtain primary crystals and mother liquor I. S22, Secondary isothermal crystallization: The mother liquor I is heated to 55~65℃ until the potassium nitrate solubility is saturated, and then concentrated to a mass concentration of 48~55%. Then it is naturally cooled to 15~25℃ for crystallization reaction; plate and frame filter press is used for solid-liquid separation to obtain secondary crystals and mother liquor II. S23. Three-stage mild recrystallization: Primary and secondary crystals are mixed at a mass ratio of 2:1, and solvent (fresh water and mother liquor after impurity removal in step S32 are mixed at a volume ratio of 1:3) is added to dissolve them and form a solution with a mass concentration of 38-42%. The solution is heated to 70-75°C to completely dissolve the crystals. Sodium citrate, a crystal form regulator, is added at a mass ratio of 0.01-0.02% of the solution. The cooling rate is controlled by a programmed temperature control device: the initial cooling rate is 0.6-1.0°C / h, the middle cooling rate is 0.4-0.6°C / h, and the final cooling rate is 0.2-0.4°C / h to 8-12°C. The cooling process uses a combination of natural cooling and water bath cooling. Vacuum filtration is used to separate the solid and liquid to obtain high-purity potassium nitrate wet product.

[0021] S3, Mother liquor staged circulation S31. Mother liquor I recycling: Mother liquor I is directly returned to step S1 for dissolving industrial-grade potassium nitrate raw material; S32, Mother Liquor II Purification and Staged Circulation: After cooling Mother Liquor II to 5~8℃, the following steps are taken: α-Al2O3The ceramic membrane undergoes cross-flow filtration pretreatment. After filtration, the mother liquor is allowed to stand for 2-3 hours to precipitate some impurities. After filtration to remove impurities, 30% of the mother liquor is returned to step S1, and 70% of the mother liquor is used for crystal dissolution in step S23. The precipitate generated from membrane filtration and standing is washed three times with hot water at 50-60℃ (condensate from the drying process in step S4), and then concentrated by evaporation at 120℃ and vacuum crystallized at 40℃ to obtain by-product salt (mainly sodium chloride). The condensate generated during the process is reused in step S1. The ceramic membrane operates continuously for 72 hours. After a few hours, the filtration efficiency dropped to 70% of the initial efficiency. After treatment with an alternating acid-base cleaning process, the filtration efficiency was restored to 98% of the initial efficiency. A single cleaning can restore the ceramic membrane to stable filtration performance, and the cumulative number of cleaning times can reach more than 500 times. The alternating acid-base cleaning process is as follows: first, use a 5% hydrochloric acid solution for cross-flow cleaning for 30 minutes, then use a 5% sodium hydroxide solution for cross-flow cleaning for 30 minutes, and finally rinse with fresh water until the pH of the filtrate is 6.5~7.5 to restore the filtration performance.

[0022] S4. Low-temperature drying: Place the high-purity potassium nitrate wet product into a hot air circulating dryer equipped with a waste heat recovery device. The drying temperature is controlled at 75~80℃. Dry until the moisture content is ≤0.1%. The recovered waste heat is used to preheat the raw material dissolving water in step S1 to obtain high-purity molten salt grade potassium nitrate.

[0023] Implementation results: Testing showed the product purity to be ≥99.95%. Fe3+ , Al3+ , Ca2+ , Mg2+ The total impurity content is ≤50ppm and the moisture content is ≤0.1%, meeting the requirements for use in molten salt grade.

[0024] Example 2: S1. Raw material pretreatment and targeted impurity removal: exactly the same as in Example 1, to obtain a purified liquid.

[0025] S2, multi-level gradient crystallization S21, Primary Cooling Crystallization: The purified liquid is cooled and crystallized using a gradient temperature control method. The cooling rate is strictly controlled at 0.8℃ / min, and the final cooling temperature is 32℃. After crystallization, the liquid is kept at this temperature for 3.5 hours. Solid-liquid separation is performed using a conventional centrifuge to obtain primary crystals and mother liquor I. S22, Secondary isothermal crystallization: exactly the same as in Example 1, to obtain secondary crystals and mother liquor II; S23, Third-stage mild recrystallization: exactly the same as in Example 1, to obtain high-purity wet potassium nitrate.

[0026] S3, Mother liquor staged circulation S31. Mother liquor I recycling: exactly the same as in Example 1.

[0027] S32, Mother liquor II purification and graded circulation: exactly the same as in Example 1.

[0028] S4. Low-temperature drying: exactly the same as in Example 1.

[0029] Implementation results: Testing showed that the final product purity was ≥99.95%. Fe3+ , Al3+ , Ca2+ , Mg2+ The total impurity content is ≤48ppm and the moisture content is ≤0.1%, which meets the requirements for use in molten salt grade.

[0030] Example 3: S1. Raw material pretreatment and targeted impurity removal: exactly the same as in Example 1, to obtain a purified liquid.

[0031] S2, multi-level gradient crystallization S21, Primary Cooling Crystallization: The purified liquid is cooled and crystallized using a gradient temperature control method. The cooling rate is strictly controlled at 1.2℃ / min, and the final cooling temperature is 48℃. After crystallization, the liquid is kept at this temperature for 2.5 hours. Solid-liquid separation is performed using plate and frame filter press to obtain primary crystals and mother liquor I. S22, Secondary isothermal crystallization: exactly the same as in Example 1, to obtain secondary crystals and mother liquor II; S23, Third-stage mild recrystallization: exactly the same as in Example 1, to obtain high-purity wet potassium nitrate.

[0032] S3, Mother liquor staged circulation S31. Mother liquor I recycling: exactly the same as in Example 1.

[0033] S32, Mother liquor II purification and graded circulation: exactly the same as in Example 1.

[0034] S4. Low-temperature drying: exactly the same as in Example 1.

[0035] Implementation results: Testing showed that the final product purity was ≥99.95%. Fe3+ , Al3+ , Ca2+ , Mg2+ The total impurity content is ≤49ppm and the moisture content is ≤0.1%, which meets the requirements for use in molten salt grade.

[0036] Example 4 S1. Raw material pretreatment and targeted impurity removal: exactly the same as in Example 1, to obtain a purified liquid.

[0037] S2, multi-level gradient crystallization S21, Primary cooling crystallization: exactly the same as in Example 1, to obtain primary crystals and mother liquor I; S22, Secondary isothermal crystallization: exactly the same as in Example 1, to obtain secondary crystals and mother liquor II; S23, Three-stage mild recrystallization: The primary crystal and the secondary crystal are mixed at a mass ratio of 1:1 (different from the 2:1 in Example 1), and the remaining parameters are exactly the same as in Example 1.

[0038] S3, Mother liquor staged circulation S31. Mother liquor I recycling: exactly the same as in Example 1.

[0039] S32. Mother liquor II purification and classification circulation: After filtration and impurity removal, 50% of the volume of mother liquor is returned to step S1, and 50% of the volume of mother liquor is used for crystal dissolution in step S23 (different from 30%:70% in Example 1). The remaining parameters are exactly the same as in Example 1.

[0040] S4. Low-temperature drying: exactly the same as in Example 1.

[0041] Implementation results: Testing showed the final product purity was ≥99.94%. Fe3+ , Al3+ , Ca2+ , Mg2+ The total impurity content is ≤50ppm and the moisture content is ≤0.1%, meeting the requirements for use in molten salt grade.

[0042] Example 5 S1. Raw material pretreatment and targeted impurity removal: exactly the same as in Example 1, to obtain a purified liquid.

[0043] S2, multi-level gradient crystallization S21, Primary cooling crystallization: exactly the same as in Example 1, to obtain primary crystals and mother liquor I; S22, Secondary isothermal crystallization: exactly the same as in Example 1, to obtain secondary crystals and mother liquor II; S23, Three-stage mild recrystallization: The primary crystal and the secondary crystal are mixed at a mass ratio of 3:1 (different from the 2:1 in Example 1), and the remaining parameters are exactly the same as in Example 1.

[0044] S3, Mother liquor staged circulation S31. Mother liquor I recycling: exactly the same as in Example 1.

[0045] S32. Mother liquor II purification and classification circulation: After filtration and impurity removal, 70% of the volume of mother liquor is returned to step S1, and 30% of the volume of mother liquor is used for crystal dissolution in step S23 (different from 30%:70% in Example 1). The remaining parameters are exactly the same as in Example 1.

[0046] S4. Low-temperature drying: exactly the same as in Example 1.

[0047] Implementation results: Testing showed the final product purity was ≥99.94%. Fe3+ , Al3+ , Ca2+ , Mg2+ The total impurity content is ≤50ppm and the moisture content is ≤0.1%, meeting the requirements for use in molten salt grade.

[0048] Example 6 S1. Raw material pretreatment and targeted impurity removal: exactly the same as in Example 1, to obtain a purified liquid.

[0049] S2, multi-level gradient crystallization S21, Primary cooling crystallization: exactly the same as in Example 1, to obtain primary crystals and mother liquor I; S22, Secondary isothermal crystallization: exactly the same as in Example 1, to obtain secondary crystals and mother liquor II; S23, Three-stage mild recrystallization: After adding the crystal form regulator, the cooling rate is controlled by a programmable temperature control device: the initial cooling rate is 0.6℃ / h, the middle cooling rate is 0.4℃ / h, and the final cooling rate is 0.2℃ / h to 8℃. The remaining parameters are exactly the same as in Example 1.

[0050] S3, Mother liquor staged circulation S31. Mother liquor I recycling: exactly the same as in Example 1.

[0051] S32, Mother liquor II purification and graded circulation: exactly the same as in Example 1.

[0052] S4. Low-temperature drying: exactly the same as in Example 1.

[0053] Implementation results: Testing showed that the final product purity was ≥99.95%. Fe3+ , Al3+ , Ca2+ , Mg2+ The total impurity content is ≤47ppm and the moisture content is ≤0.1%, which meets the requirements for use in molten salt grade.

[0054] Example 7 S1. Raw material pretreatment and targeted impurity removal: exactly the same as in Example 1, to obtain a purified liquid.

[0055] S2, multi-level gradient crystallization S21, Primary cooling crystallization: exactly the same as in Example 1, to obtain primary crystals and mother liquor I; S22, Secondary isothermal crystallization: exactly the same as in Example 1, to obtain secondary crystals and mother liquor II; S23, Three-stage mild recrystallization: After adding the crystal form regulator, the cooling rate is controlled by a programmable temperature control device: the initial cooling rate is 1.0℃ / h, the middle cooling rate is 0.6℃ / h, and the final cooling rate is 0.4℃ / h to 12℃. The remaining parameters are exactly the same as in Example 1.

[0056] S3, Mother liquor staged circulation S31. Mother liquor I recycling: exactly the same as in Example 1.

[0057] S32, Mother liquor II purification and graded circulation: exactly the same as in Example 1.

[0058] S4. Low-temperature drying: exactly the same as in Example 1.

[0059] Implementation results: Testing showed that the final product purity was ≥99.95%. Fe3+ , Al3+ , Ca2+ , Mg2+ The total impurity content is ≤48ppm and the moisture content is ≤0.1%, which meets the requirements for use in molten salt grade.

[0060] The scope of protection of this invention is defined by the appended claims. Any equivalent substitutions or improvements made based on the technical solution and inventive concept of this invention shall fall within the scope of protection of this invention.

Claims

1. A low-energy crystallization purification method for high-purity molten salt grade potassium nitrate, characterized in that, Includes the following steps: S1. Raw material pretreatment and targeted impurity removal: Industrial-grade potassium nitrate is dissolved in mother liquor circulating water and / or fresh water, and heated to a temperature that allows the industrial-grade potassium nitrate to completely dissolve without boiling, forming a raw material solution; the pH value of the solution is dynamically adjusted by online detection of impurity concentration, and chelating agents, oxidizing impurity removers and adsorption flocculants are added step by step. After each component is added, it is stirred until the corresponding impurities are removed to the standard. The complexed precipitates, oxidation products and adsorbed flocs are removed by a multi-stage solid-liquid separation process to obtain a purified solution; S2, multi-level gradient crystallization S21, Primary cooling crystallization: The purified liquid is cooled and crystallized using a gradient temperature control method, and solid-liquid separation is performed to obtain primary crystals and mother liquor I; S22, Secondary isothermal crystallization: The mother liquor I is heated and concentrated until the potassium nitrate solubility is saturated, and then naturally cooled and crystallized to obtain secondary crystals and mother liquor II through solid-liquid separation; S23, Three-stage mild recrystallization: The primary crystal and the secondary crystal are mixed, dissolved in a solvent and heated until the crystal is completely dissolved. A crystal form regulator is added, and the crystals are cooled and crystallized at a segmented controllable cooling rate using a programmable temperature control device. The solid and liquid are separated to obtain high-purity potassium nitrate wet product. S3, Mother liquor staged circulation S31. Recycling Mother Liquor I: The mother liquor I is returned to step S1 for dissolving industrial-grade potassium nitrate raw material; S32, Mother Liquor II Purification and Classification Circulation: After cooling, Mother Liquor II is pretreated by ceramic membrane filtration. After filtration, the mother liquor is allowed to stand until some impurities precipitate. After impurity removal, part of the mother liquor is returned to step S1, and part is used for crystal dissolution in step S23. The precipitate generated by membrane filtration and standing is washed, concentrated, and crystallized to obtain by-product salt. The condensate generated in the process is reused in step S1. After the ceramic membrane runs continuously until the filtration efficiency decreases, an acid-base alternating cleaning process is used to restore the filtration performance. S4. Low-temperature drying: The high-purity potassium nitrate wet product is dried by hot air circulation until the moisture content meets the requirements for use in molten salt grade, thus obtaining the high-purity molten salt grade potassium nitrate.

2. The method according to claim 1, characterized in that, In step S1, the mother liquor circulating water includes mother liquor I from step S31 and mother liquor after impurity removal from step S32; the multi-stage solid-liquid separation specifically involves: firstly removing precipitates using plate and frame filtration, and then subjecting them to secondary interception via precision filtration.

3. The method according to claim 1, characterized in that, In step S1, the pH value is monitored and adjusted in real time using an online pH meter; the online detection method for impurity concentration is an ion sensor; and the corresponding impurity removal meeting the standard is determined by the ion sensor detecting the impurity. Fe3+ , Al3+ , Ca2+ , Mg2+ The concentration no longer decreases; the stepwise addition order is as follows: first add the chelating agent, then stir the reaction until... Fe3+ , Al3+ Once the concentration reaches the target, add an oxidizing impurity remover and continue stirring the reaction until... Fe2+ Once the concentration reaches the target, add the adsorption-type flocculant and stir until the turbidity of the solution no longer decreases; maintain a stirring rate that ensures uniform dispersion of the reagent throughout the process; the chelating agent is diethylenetriaminepentamethylenephosphonic acid; the oxidizing impurity remover is hydrogen peroxide; and the adsorption-type flocculant is modified diatomaceous earth.

4. The method according to claim 1, characterized in that, In step S21, the cooling rate of the gradient temperature control method is 0.8~1.2℃ / min, the cooling endpoint temperature is 32~48℃, and the temperature is maintained for 2.5~3.5 hours after crystallization.

5. The method according to claim 1, characterized in that, In step S23, the solvent is a mixture of fresh water and the mother liquor after impurity removal in step S32; the crystal form regulator is sodium citrate; the segmented controllable cooling rate is specifically as follows: in the early stage, the cooling rate is 0.6~1.0℃ / h, in the middle stage, the cooling rate is 0.4~0.6℃ / h, and in the later stage, the cooling rate is 0.2~0.4℃ / h to 8~12℃.

6. The method according to claim 1, characterized in that, In step S32, the ceramic film is α-Al2O3 The ceramic membrane is filtered using a cross-flow filtration method. The acid-base alternating cleaning process is as follows: first, cross-flow cleaning is performed with hydrochloric acid solution, then cross-flow cleaning is performed with sodium hydroxide solution, and finally, fresh water is used to rinse until the pH of the filtrate is 6.5~7.

5. The precipitate is washed with hot water at 50~60℃, which is the condensate from the drying process in step S4.

7. The method according to claim 1, characterized in that, In step S4, the hot air circulating dryer is equipped with a waste heat recovery device, and the recovered waste heat is used to preheat the raw material dissolved water in step S1.

8. A high-purity molten salt grade potassium nitrate, characterized in that, The potassium nitrate is prepared by the method according to any one of claims 1 to 7.

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