A process for purifying potassium nitrate

Abstract

The application discloses a kind of purification process of potassium nitrate in the technical field of inorganic functional materials, the process includes: after dissolving potassium nitrate crude product, preliminary impurity removal is carried out by activated carbon, then using multistage pore manganese-doped silicon-aluminum composite oxide zero-valent iron-carbon composite adsorbent is carried out deep purification, the adsorbent can selectively adsorb calcium and magnesium ions and degrade organic impurities, after purification, filtrate is obtained by cooling crystallization high-purity potassium nitrate product, and the adsorbent can be regenerated and recycled.The preparation of the adsorbent includes: forming hydrotalcite layer on the surface of iron-carbon microspheres by coprecipitation method, constructing multistage pore composite oxide shell by manganese loading, silicon source coating and high-temperature calcination, then hydrophobic modification is carried out by gas-phase silanization, and finally the final product is obtained by acid activation and reduction treatment.The process has high purification efficiency, the adsorbent is functionally integrated and recyclable, and is suitable for industrial production of high-purity potassium nitrate.

Description

Technical Field

[0001] This invention relates to the field of inorganic functional materials technology, specifically to a purification process for potassium nitrate. Background Technology

[0002] Potassium nitrate, as an important inorganic chemical product, has wide applications in agriculture, industry, and new energy fields. In agriculture, it is a high-quality compound fertilizer, providing crops with the potassium and nitrogen elements they need. In industry, high-purity potassium nitrate is a key raw material for manufacturing high-grade glass, ceramic glazes, fireworks, and black powder. Especially in new energy fields such as concentrated solar power, ultra-high-purity potassium nitrate, used as a molten salt energy storage medium, has extremely stringent requirements for controlling its impurity content, particularly chloride, calcium, and magnesium ions. With the upgrading of downstream industries, the market demand for potassium nitrate purity, especially molten salt-grade and reagent-grade high-purity potassium nitrate suitable for high-end manufacturing and energy storage fields, is increasing, placing more urgent demands on the innovation of its production technology.

[0003] Currently, the mainstream industrial processes for producing potassium nitrate include metathesis, ion exchange, and nitric acid neutralization. While these traditional processes can achieve large-scale production, they all face significant limitations in pursuing extremely high product purity. Taking the most common metathesis process involving potassium chloride and ammonium nitrate or sodium nitrate as an example, the production process inevitably introduces or leaves behind impurities such as chloride ions, sodium ions, calcium ions, and magnesium ions. To remove these impurities, traditional methods rely on multi-stage recrystallization, complex ion exchange resin columns, or the addition of precipitants. Multi-stage recrystallization is energy-intensive, has low yield, and is limited in its effectiveness in separating impurities with similar solubility properties to potassium nitrate; ion exchange generates large amounts of acid-base regeneration wastewater, resulting in significant environmental pressure and high operating costs; while adding precipitants may introduce new impurities. Crucially, existing technologies lack targeted materials capable of simultaneously and efficiently removing calcium and magnesium cations and trace organic impurities, leading to lengthy and complex process flows, making it difficult to economically and environmentally stably produce high-purity potassium nitrate with impurity levels at the part-in-a-million level.

[0004] To overcome the aforementioned technical bottlenecks, this invention aims to provide a novel, simple, and efficient deep purification scheme for potassium nitrate, integrating the functions of its core materials. The core of this scheme lies in the creative design and preparation of a composite adsorbent with a multi-level porous structure and core-shell functional units. This material selectively captures calcium and magnesium ions through an outer manganese-doped silicon-aluminum composite oxide layer, while degrading organic impurities through a core zero-valent iron-carbon micro-electrolysis unit, achieving synergistic removal of key impurities in potassium nitrate solutions. Based on this adsorbent, the purification process of this invention greatly simplifies the traditional multi-step purification process, achieving deep purification with only a single adsorption operation. High-purity products can then be obtained through precision filtration and cooling crystallization, and the adsorbent is easily regenerated and recycled. This process not only significantly improves purification efficiency and product purity but also avoids complex operations and secondary pollution, providing a completely new technical path for the large-scale, low-cost production of high-end potassium nitrate products. Summary of the Invention

[0005] The purpose of this invention is to provide a potassium nitrate purification process that solves the technical problems of existing potassium nitrate purification processes, such as the difficulty in simultaneously and deeply removing calcium and magnesium ions and organic impurities, the lengthy and complex process flow, and the high operating costs and difficulty in economically and stably producing high-purity potassium nitrate due to the poor selectivity or inability to regenerate traditional adsorbents.

[0006] The present invention achieves the above objectives through the following technical solutions: A purification process for potassium nitrate includes the following steps: S1, by weight, add 98-102 parts of crude potassium nitrate to 115-125 parts of hot deionized water, stir, and obtain potassium nitrate solution; add 1-5 parts of activated carbon to potassium nitrate solution, continue stirring, filter, and obtain preheated filtrate. S2, under stirring, add 15-30 parts of multi-level porous manganese-doped silicon-aluminum composite oxide-zero-valent iron-carbon composite adsorbent to the preheated filtrate, adjust the temperature to 60-70℃, stir the reaction to obtain the reaction mixture; S3, filter the reaction mixture to obtain potassium nitrate filtrate and filter cake; regenerate the filter cake by soaking it in dilute nitric acid at 78-82℃, wash it with water, dry it at 78-82℃, and reduce it at 395-405℃ under a hydrogen / argon atmosphere; S4. Transfer the potassium nitrate filtrate to a crystallization vessel, cool and crystallize to obtain a crystal slurry; separate the crystal slurry by centrifugation to obtain crystals, wash the crystals with a saturated potassium nitrate ice-water solution to obtain wet crystals; dry the wet crystals under vacuum at 80-90℃.

[0007] In this invention, the potassium nitrate purification process embodies the concepts of targeted impurity removal and system self-consistency and synergy. After preliminary purification by activated carbon, the crude product solution exhibits multiple functions simultaneously in a near-neutral potassium nitrate solution: the hydroxyl groups on the surface of the amorphous oxide shell achieve highly selective capture of high charge density cations such as calcium and magnesium through coordination bonding and electrostatic interactions, while having minimal interference with potassium ions; the core metal iron and carbon form a microelectrode system, which undergoes spontaneous discharge corrosion driven by nitrate ions as an environmentally friendly weak oxidant. Ferrous ions dissolve at the anode, and interfacial active hydrogen species are generated in the cathode region. Both synergistically break the chemical bonds of organic impurities and promote deep mineralization. Simultaneously, the manganese center in the shell efficiently catalyzes the oxidation of nitrite to nitrate, significantly improving the chemical stability of the product. After solid-liquid separation, the filtrate is subjected to gradient cooling crystallization to obtain high-purity crystals. The saturated adsorbent is regenerated by desorption with dilute nitric acid, and hydrogen ions displace and desorb metallic impurities. Subsequent reduction treatment restores the surface activity of the metal iron and the adsorption sites of the shell simultaneously, achieving material recycling. The entire process relies on the intrinsic chemical environment of the potassium nitrate system, eliminating the need for external acid-base adjustments. The recycling of the mother liquor further enhances resource efficiency. The process combines deep purification capabilities, simple operation, and green sustainability, providing an innovative technical pathway for the preparation of high-purity potassium nitrate.

[0008] According to a preferred embodiment of the present invention, in step S1, the temperature of the hot deionized water is 70-80°C.

[0009] According to a preferred embodiment of the present invention, in step S2, the stirring reaction time is 1.5-2 hours.

[0010] According to a preferred embodiment of the present invention, in step S3, the reduction treatment at 395-405°C is carried out for 2-4 hours.

[0011] According to a preferred embodiment of the present invention, in step S4, the temperature is cooled to 8-12°C.

[0012] According to a preferred embodiment of the present invention, the preparation steps of the multi-level porous manganese-doped silicon-aluminum composite oxide-zero-valent iron-carbon composite adsorbent include: A1, by weight, under nitrogen protection, 35-45 parts of zero-valent iron powder, 8-12 parts of conductive carbon black, and 180-220 parts of deionized water are sheared and dispersed; under stirring, a mixed aqueous solution of 28-32 parts of magnesium nitrate hexahydrate and 13-17 parts of aluminum nitrate nonahydrate, along with sodium hydroxide solution, is added dropwise to adjust the pH to 9.9-10.1 to obtain a mixture; the mixture is transferred to an autoclave and crystallized at 118-122℃ to obtain a reaction solution; the reaction solution is cooled and centrifuged to obtain a solid product; the solid product is washed with deionized water and dried under vacuum at 78-82℃ to obtain the precursor; A2. The precursor is dispersed in 450-550 parts of an aqueous manganese sulfate solution and stirred at 58-62℃. After centrifugation, it is dispersed in a mixture of 90-110 parts of tetraethyl orthosilicate, 280-320 parts of anhydrous ethanol, and 95-105 parts of deionized water. 90-110 parts of an ethanol solution containing ammonia are added dropwise and stirred at room temperature to obtain a mixture. The mixture is filtered to obtain a solid product. The solid product is washed with ethanol and calcined at 595-605℃ under a nitrogen atmosphere to obtain an intermediate. A3, the intermediate and 0.4-0.6 parts of 3-aminopropyltriethoxysilane are placed in the reaction zone and the vaporization zone at 148-152℃ in a tube furnace, respectively, and nitrogen carrier gas is transported to the reaction zone at 595-605℃ for action; after cooling, powder is obtained; A4: Stir the powder with dilute sulfuric acid at room temperature, wash with water until neutral; reduce at 395-405℃ under a hydrogen / argon atmosphere, cool and seal.

[0013] In this invention, the preparation mechanism of the multi-level porous manganese-doped silicon-aluminum composite oxide-zero-valent iron-carbon composite adsorbent originates from the concept of multi-scale structural synergistic design. Under an inert atmosphere, metallic iron particles and highly conductive carbon black are uniformly dispersed in an aqueous phase. By precisely controlling the acid-base environment of the reaction system, a co-precipitation reaction of magnesium-aluminum salts is induced, resulting in the in-situ growth of layered double hydroxide nanosheets that tightly coat the surface of iron-carbon microspheres. Hydrothermal crystallization enhances structural integrity, forming a core-shell precursor. This precursor is impregnated with a manganese salt solution, where manganese is directionally enriched through surface electrostatic interactions. Subsequently, a silicon source precursor is introduced into an alcohol-water mixture, where hydrolysis and condensation under weak alkaline catalysis form a uniform coating layer. During the high-temperature heat treatment stage, the layered structure decomposes to generate gas and create pores, and the silicon network deeply integrates with the metal oxide, constructing an amorphous manganese-doped silicon-aluminum composite oxide shell. The internal iron-carbon core is stably retained due to the isolation provided by the carbon network. In the vapor-phase modification stage, the aminosilane coupling agent condenses with surface hydroxyl groups at high temperature, and the organic segments carbonize to form a nitrogen-containing hydrophobic layer, precisely controlling the wettability of the pores without clogging the channels. Subsequent dilute acid treatment selectively dissolves weakly bound amorphous components to optimize pore connectivity and forms a convertible intermediate layer on the surface of metallic iron. Reducing atmosphere heat treatment transforms the intermediate layer into a highly active and clean metal surface, ultimately obtaining a multi-layered interconnected composite structure of micropores, mesopores, and macropores. The outer shell is rich in active hydroxyl groups and manganese redox centers, while the core is metallic iron protected by a carbon network. The synergistic interface between the two endows the material with excellent adsorption selectivity and electrochemical activity.

[0014] According to a preferred embodiment of the present invention, in step A1, the crystallization time at 118-122°C is 12-14 hours.

[0015] According to a preferred embodiment of the present invention, in step A2, the calcination time at 595-605°C is 4-6 hours.

[0016] According to a preferred embodiment of the present invention, in step A3, the nitrogen carrier gas is delivered to the reaction zone at 595-605°C for 2-4 hours.

[0017] According to a preferred embodiment of the present invention, in step A4, the reduction time at 395-405°C is 2-4 hours.

[0018] The beneficial effects of this invention are as follows: The potassium nitrate purification process and specialized composite adsorbent provided by this invention achieve multiple breakthroughs in technical effectiveness, significantly improving the efficiency, economy, and product quality of high-purity potassium nitrate production. The most crucial effect lies in its superior deep purification capability. Divalent cation impurities such as calcium and magnesium ions, as well as trace organic impurities, which are difficult to remove in traditional processes, are efficiently addressed in one step by a novel, functionally integrated adsorbent. The amorphous composite oxide layer of the adsorbent shell, with its abundant surface hydroxyl groups, exhibits a much higher selective adsorption capacity for calcium and magnesium ions than for potassium ions, thereby removing them to extremely low levels of a few parts per million. Simultaneously, the unique zero-valent iron-carbon micro-electrolysis unit in the core continuously exerts a reducing effect in the near-neutral potassium nitrate solution environment, effectively degrading trace organic impurities introduced by the raw materials and preventing organic residues in the final product. This dual-function synergistic mechanism of "adsorption-degradation" ensures extremely high purity of the exported potassium nitrate solution, laying a solid foundation for subsequent crystallization to produce molten salt-grade or reagent-grade high-purity products.

[0019] This process significantly simplifies the workflow, reduces operational complexity, and lowers energy consumption. Compared to traditional purification methods that rely on multi-stage recrystallization, frequent regeneration of ion exchange resins, or complex chemical precipitation, this invention condenses the core purification step into a single adsorption operation, requiring only solid-liquid separation and conventional crystallization afterward. The entire process eliminates the need for repeated pH adjustments and the treatment of large amounts of resin regeneration wastewater, and the operating conditions are mild and easily controlled. The adsorbent addition, reaction, and filtration remove a second removal rate E2. This process is repeated five times to complete the adsorption-regeneration cycle, and the removal rate E5 after the fifth cycle is recorded. The removal rate retention rate after five cycles is then calculated as K(%) = (E5 / E1) × 100. All operations are conducted at moderate temperatures, avoiding high-energy-consuming steps such as traditional multi-effect evaporation and concentration. More importantly, the composite adsorbent used possesses excellent physicochemical stability and can be efficiently regenerated through simple dilute nitric acid soaking and mild hydrogen reduction. Its adsorption and degradation performance remains stable even after multiple cycles. This recyclable characteristic not only significantly reduces the cost of materials per use, but also reduces the generation of solid waste at the source, making the continuous and large-scale operation of the entire process more economical and feasible.

[0020] From an overall benefit perspective, this invention combines outstanding environmental advantages with broad product adaptability. The entire process involves no addition of toxic or harmful reagents. The eluent after adsorbent regeneration primarily consists of calcium and magnesium nitrates, which are easy to process or utilize as byproducts, fundamentally reducing emissions of waste gas, wastewater, and solid waste, aligning with the development direction of green chemistry. Based on the modular design of this process, it can be used in new production lines or easily integrated into existing potassium nitrate production processes as a final refining module, rapidly improving product grade and added value. The produced high-purity potassium nitrate product has key impurity indicators far exceeding national superior-grade standards, fully meeting the stringent requirements for impurity content in high-end optical glass, special ceramic glazes, and especially molten salt energy storage media for solar thermal power generation, effectively promoting technological upgrading in downstream industries. Therefore, this invention not only provides an efficient and economical purification method but also forms a resource-saving, environmentally friendly, and highly competitive overall solution for the production of high-purity potassium nitrate. Detailed Implementation

[0021] The following detailed embodiments are only used to further illustrate this application and should not be construed as limiting the scope of protection of this application. Those skilled in the art can make some non-essential improvements and adjustments to this application based on the above application content. Example 1

[0022] This embodiment provides a purification process for potassium nitrate, the steps of which include: Preparation of multi-level porous manganese-doped silicon-aluminum composite oxide-zero-valent iron-carbon composite adsorbent: Step A1: Under nitrogen protection, 400g of micron-sized zero-valent iron powder and 100g of conductive carbon black were added to 2000g of deionized water and dispersed using a high-speed shear disperser at 10000rpm for 30min to form a homogeneous slurry. Maintaining stirring and a nitrogen atmosphere, a mixed salt solution made by dissolving 300g of magnesium nitrate hexahydrate and 150g of aluminum nitrate nonahydrate in 1000g of deionized water, along with a 2mol / L sodium hydroxide solution, was simultaneously added dropwise using a constant flow pump. During this process, the pH of the reaction system was precisely controlled at 10.0 by online pH monitoring. After the addition was complete, the entire slurry was transferred to a high-pressure reactor and crystallized at 120℃ for 12h. After the reaction was completed, the mixture was allowed to cool naturally to room temperature. The solid precipitate was collected by centrifugation and repeatedly washed with deionized water until the conductivity of the filtrate was below 50 μS / cm. Then, it was dried in a vacuum drying oven at 80℃ for 24 h to obtain a gray-black powder precursor, denoted as Fe0 / C@MgAl-LDH.

[0023] Step A2: The precursor obtained above was completely dispersed in 5000g of a 0.5mol / L manganese sulfate aqueous solution and stirred at 60℃ for 6h. After the reaction, the solid was separated by centrifugation and redispersed in a mixed solution consisting of 1000g tetraethyl orthosilicate, 3000g anhydrous ethanol, and 1000g deionized water. While stirring continuously, 1000g of an ammonia-containing ethanol solution prepared from 50g concentrated ammonia and 950g anhydrous ethanol was slowly added dropwise, followed by stirring at room temperature for 48h. After the reaction was complete, the mixture was filtered, and the resulting solid was washed three times with anhydrous ethanol. It was then placed in a tube furnace and heated to 600℃ at a programmed rate of 5℃ / min under a flowing nitrogen atmosphere, and calcined at this temperature for 4h. After calcination, it was naturally cooled to obtain a dark brown intermediate powder.

[0024] Step A3: The intermediate powder obtained in Step A2 is evenly spread in a quartz boat of a tube furnace and placed in the high-temperature reaction zone in the center of the furnace. Separately, 0.5 g of 3-aminopropyltriethoxysilane is placed in the upstream 150°C vaporization zone. First, high-purity nitrogen is introduced to purge the air from the system. Then, nitrogen is continuously introduced at a flow rate of 50 mL / min as a carrier gas to carry the vaporized silane molecules to the 600°C high-temperature reaction zone, where they react with the intermediate for 2 hours. After the reaction is complete, the mixture is cooled to room temperature under a nitrogen atmosphere to obtain the surface-modified powder.

[0025] Step A4: Place the powder obtained in Step A3 into a 0.1 mol / L dilute sulfuric acid solution and gently stir for 10 min at room temperature, followed by filtration. Rinse the filter cake with plenty of deionized water until the filtrate is neutral to obtain a wet filter cake. Place the wet filter cake in a tube furnace and reduce it at 400℃ for 2 h under a mixed reducing atmosphere consisting of 5% hydrogen and 95% argon. After the treatment, cool to room temperature under a protective atmosphere, remove and immediately seal for storage to obtain the final multi-level porous manganese-doped silicon-aluminum composite oxide-zero-valent iron-carbon composite adsorbent, denoted as HP-Mn-SiAlOx / FeO@C.

[0026] The purification process of potassium nitrate: Step S1: Weigh 1000g of industrial-grade crude potassium nitrate (actual purity 96.1%, main impurities are sodium chloride, potassium sulfate, and calcium and magnesium salts), and add it to 1200g of hot deionized water at 75℃. In a dissolving tank equipped with a stirrer and heater, stir vigorously at 300rpm until the potassium nitrate is completely dissolved, obtaining a nearly saturated hot solution. Then, add 20g of powdered activated carbon to this hot solution and continue stirring at 75℃ for 30min. Under this heat-preserving condition, perform hot filtration using a plate and frame filter to thoroughly remove the activated carbon and insoluble mechanical impurities, obtaining a clear and transparent preheated filtrate.

[0027] Step S2: Maintain the temperature of the preheated filtrate obtained in Step S1 at 65℃, and slowly and evenly add 200g of the HP-Mn-SiAlOx / FeO@C composite adsorbent prepared by the above method to the filtrate under constant stirring (200rpm). Keep the system temperature at 65±2℃ and continue stirring at a rate of 200rpm for 90min.

[0028] Step S3: Immediately after the reaction, the reaction slurry was filtered and separated using a precision filtration system equipped with a 0.22 μm pore size polytetrafluoroethylene (PTFE) filter membrane at an operating pressure of 0.2 MPa. The collected filtrate was a deeply purified potassium nitrate solution. The filter cake (i.e., the adsorbent loaded with impurities) was collected and regenerated by soaking it in 500 mL of 0.5 mol / L dilute nitric acid solution at 80 °C for 60 min. After regeneration, the filter cake was washed with deionized water until neutral and then dried at 80 °C for 2 h. The dried adsorbent was placed in a tube furnace and reduced at 400 °C for 2 h under a 5% H₂ / Ar atmosphere for recycling.

[0029] Step S4: Transfer the high-purity potassium nitrate filtrate obtained in Step S3 to a crystallization vessel with a cooling jacket. Control the cooling water flow rate to slowly reduce the filtrate temperature from 65℃ to 10℃ at a rate of 0.8℃ / min, during which high-purity potassium nitrate crystals gradually precipitate. Introduce the slurry containing crystals into a centrifuge for solid-liquid separation. Rinse the separated wet crystals with 200mL of saturated potassium nitrate ice-water solution pre-cooled to 5℃ to remove the mother liquor adhering to the surface. Finally, place the rinsed wet crystals in a vacuum drying oven at 85℃ and dry for 4 hours to constant weight to obtain the final high-purity potassium nitrate product. Example 2

[0030] The specific implementation method is the same as in Example 1, except that the preparation of the multi-level porous manganese-doped silicon-aluminum composite oxide-zero-valent iron-carbon composite adsorbent is as follows: Step A1: Under nitrogen protection, 450g of zero-valent iron powder and 120g of conductive carbon black were added to 2200g of deionized water and dispersed by high-speed shearing. Simultaneously, a salt solution of 320g of magnesium nitrate hexahydrate and 170g of aluminum nitrate nonahydrate dissolved in 1100g of deionized water and a 2mol / L NaOH solution were added dropwise, controlling the pH to 10.0. The mixture was transferred to an autoclave and crystallized at 122℃ for 12h. After cooling, it was centrifuged, washed with water, and dried (80℃, 24h) to obtain the precursor.

[0031] Step A2: The precursor was dispersed in 5500g of 0.5mol / L manganese sulfate solution and stirred at 62℃ for 6h. After centrifugation, it was dispersed in a mixture of 1100g TEOS, 3200g anhydrous ethanol, and 1050g deionized water. 1100g of an ethanol solution containing ammonia was added dropwise, and the mixture was stirred at room temperature for 48h. After filtration and washing with ethanol, the mixture was calcined at 605℃ for 4h under nitrogen atmosphere at a rate of 5℃ / min to obtain the intermediate.

[0032] Step A3: The intermediate and 0.6g APTES were placed in the reaction zone at 600℃ and the vaporization zone at 152℃ of a tube furnace, respectively, and reacted with nitrogen carrier gas at 50mL / min for 2h. After cooling, powder was obtained.

[0033] Step A4: The powder was stirred with 0.1 mol / L dilute sulfuric acid at room temperature for 10 min, washed with water until neutral, and then reduced at 405℃ for 2 h in a 5% H2 / Ar atmosphere. After cooling and sealing, the HP-Mn-SiAlOx / FeO@C adsorbent was obtained.

[0034] The purification process of potassium nitrate.

[0035] Step S1: Weigh 1020g of crude potassium nitrate (purity 96.3%) and dissolve it in 1224g of hot deionized water at 70℃. Add 30.6g of activated carbon, stir at 68℃ for 30min, and then hot filter to obtain a preheated filtrate.

[0036] Step S2: Maintain the filtrate temperature at 60℃, add 306g of the above HP-Mn-SiAlOx / FeO@C adsorbent, and stir at 220rpm for 120min.

[0037] Step S3: The reaction slurry is filtered through a 0.22 μm filter membrane. The filter cake is regenerated with 0.5 mol / L dilute nitric acid at 80 °C, washed with water, dried, and then reduced at 400 °C for 2.5 h with 5% H2 / Ar.

[0038] Step S4: The filtrate is cooled from 65°C to 8°C at a rate of 0.5°C / min to crystallize. After centrifugation of the crystal slurry, the crystals are rinsed with a saturated potassium nitrate ice-water solution pre-cooled to 0°C and dried under vacuum at 88°C for 5 hours. Example 3

[0039] The specific implementation method is the same as in Example 1, except that the preparation of the multi-level porous manganese-doped silicon-aluminum composite oxide-zero-valent iron-carbon composite adsorbent is as follows: Step A1: Under nitrogen protection, 350g of zero-valent iron powder and 80g of conductive carbon black were added to 1800g of deionized water and dispersed by high-speed shearing. Simultaneously, a salt solution of 280g of magnesium nitrate hexahydrate and 130g of aluminum nitrate nonahydrate dissolved in 900g of deionized water, along with a 2mol / L NaOH solution, was added dropwise to control the pH at 10.0. The mixture was transferred to an autoclave and crystallized at 118℃ for 14h. After cooling, it was centrifuged, washed with water, and dried (80℃, 24h) to obtain the precursor.

[0040] Step A2: The precursor was dispersed in 4500g of 0.5mol / L manganese sulfate solution and stirred at 58℃ for 6h. After centrifugation, it was dispersed in a mixture of 900g TEOS, 2800g anhydrous ethanol, and 950g deionized water. 900g of an ethanol solution containing ammonia was added dropwise, and the mixture was stirred at room temperature for 48h. After filtration and washing with ethanol, the mixture was calcined at 595℃ for 6h under nitrogen atmosphere at a rate of 5℃ / min to obtain the intermediate.

[0041] Step A3: The intermediate and 0.4g APTES were placed in the reaction zone at 600℃ and the vaporization zone at 148℃ of a tube furnace, respectively, and reacted with nitrogen carrier gas at 50mL / min for 2h. After cooling, powder was obtained.

[0042] Step A4: The powder was stirred with 0.1 mol / L dilute sulfuric acid at room temperature for 10 min, washed with water until neutral, and then reduced at 395℃ for 4 h in a 5% H2 / Ar atmosphere. After cooling and sealing, the HP-Mn-SiAlOx / FeO@C adsorbent was obtained.

[0043] The purification process of potassium nitrate: Step S1: Weigh 980g of crude potassium nitrate (95.5% purity) and dissolve it in 1127g of hot deionized water at 80℃. Add 9.8g of activated carbon, stir at 75℃ for 30min, and then hot filter to obtain a preheated filtrate.

[0044] Step S2: Maintain the filtrate temperature at 70°C, add 147g of the above HP-Mn-SiAlOx / FeO@C adsorbent, and stir at 180rpm for 105min.

[0045] Step S3: The reaction slurry is filtered through a 0.22 μm filter membrane. The filter cake is regenerated with 0.5 mol / L dilute nitric acid at 80 °C, washed with water, dried, and then reduced at 400 °C for 3 h with 5% H2 / Ar.

[0046] Step S4: The filtrate is cooled from 65°C to 12°C at a rate of 1°C / min to crystallize. After centrifugation of the crystal slurry, the crystals are rinsed with a saturated potassium nitrate ice-water solution pre-cooled to 2°C and dried under vacuum at 82°C for 3 hours.

[0047] Comparative Example 1 The specific implementation method is the same as in Example 1, except that an adsorbent with an iron-carbon core coated with an undoped manganese silicon-aluminum composite oxide is prepared.

[0048] Preparation method: The precursor was prepared exactly according to step A1 of Example 1. Subsequently, the manganese sulfate solution treatment in step A2 was omitted, and the precursor was directly coated with tetraethyl orthosilicate (corresponding to the latter half of step A2 in Example 1: using 1000g tetraethyl orthosilicate, 3000g ethanol, 1000g water, and ammonia as catalysts). The subsequent steps A3 (gas-phase silanization) and A4 (activation and reduction) were exactly the same as in Example 1. Purification process: Using the adsorbent prepared above, the same batch of crude potassium nitrate was treated exactly according to steps S1 to S4 of Example 1, with an adsorbent dosage of 200g.

[0049] Comparative Example 2 The specific implementation method is the same as in Example 1, except that an adsorbent without hydrophobic modification is prepared.

[0050] Preparation method: The intermediate was prepared entirely according to steps A1 and A2 of Example 1. Subsequently, step A3 (vapor-phase silanization) was completely omitted, and the intermediate obtained in step A2 was directly subjected to acid washing and reduction treatment in step A4. Purification process and cycle test: Using the adsorbent prepared above, the same batch of crude potassium nitrate was treated according to the process of Example 1. After completing one purification and regeneration cycle (step S3 of Example 1), it was used again for the next purification cycle, and the cycle was repeated 5 times.

[0051] Comparative Example 3 The specific implementation method is the same as in Example 1, except that a pure manganese-doped silicon-aluminum composite oxide adsorbent without zero-valent iron-carbon core is prepared.

[0052] Preparation method: Step A1 is omitted. Magnesium nitrate, aluminum nitrate, and manganese nitrate, with metal contents equivalent to those in the precursor of Example 1, are directly weighed and subjected to co-precipitation followed by tetraethyl orthosilicate coating, calcination, vapor-phase silanization modification, and reduction to prepare a pure manganese-doped silicon-aluminum composite oxide material with hierarchical channels. That is, this material does not possess an FeO / C core. Purification process: Using the adsorbent prepared above, the same batch of crude potassium nitrate is treated exactly according to steps S1 to S4 of Example 1, with an adsorbent dosage of 200g.

[0053] Performance testing The potassium nitrates prepared in Examples 1-3 and Comparative Examples 1-3 were subjected to performance testing according to the following method, which included the following steps: The purity of potassium nitrate was determined by a non-aqueous perchloric acid titration method. Specifically, the dried potassium nitrate product obtained in each example was dried at 105℃ to constant weight. 0.1500g of the sample was accurately weighed and placed in a 150mL Erlenmeyer flask. 40mL of glacial acetic acid was added, and the mixture was heated on a hot plate until completely dissolved. After cooling to room temperature, 5mL of acetic anhydride and 2 drops of crystal violet indicator (1g / L glacial acetic acid solution) were added. The solution was then titrated with a standardized 0.1000mol / L perchloric acid-glacial acetic acid standard. The titration was continued until the solution changed from purple to blue-green, and the volume consumed was recorded as V1 mL. At the same time, a blank test was performed and the volume was recorded as V0 mL. The purity P was calculated according to the formula P(%)=[C×(V1-V0)×0.1011 / m]×100, where C is the concentration of the standard titration solution (0.1000mol / L), m is the sample mass (0.1500g), and 0.1011 is the mass (g) of potassium nitrate equivalent to 1.00 mL of the above perchloric acid standard titration solution.

[0054] The calcium and magnesium impurity content in the product was determined by inductively coupled plasma atomic emission spectrometry (ICP-AES): 5.000 g of sample was accurately weighed, dissolved in ultrapure water, quantitatively transferred to a 50 mL volumetric flask and diluted to volume. After mixing, the sample was filtered through a 0.45 μm aqueous filter membrane. The filtrate was directly tested using the characteristic emission line of calcium (317.933 nm) and magnesium (285.213 nm). The instrument was calibrated with a series of calcium and magnesium standard solutions (concentration gradients of 0.1 mg / L, 0.5 mg / L, 1.0 mg / L, and 5.0 mg / L). The sample solution was then measured and quantified using the external standard method. The results are expressed in mg / kg (i.e., ppm).

[0055] The degradation effect of organic matter in the solution was characterized by the removal rate of chemical oxygen demand (COD): the removal rate E2 of the preheated filtrate obtained in step S1 and the potassium nitrate filtrate obtained in step S3 was recorded as follows: after repeating this process 5 times to complete the adsorption-regeneration cycle, the removal rate E5 after the 5th cycle was recorded. The removal rate retention rate K (%) after 5 cycles was then calculated as (E5 / E1) × 100. 10.00 mL of sample was accurately measured and added to a Hach Company dichromate method COD determination digestion tube (pre-prepared reagent, range 0-1500 mg / L). The sample was digested in a COD digester at 150℃ for 120 min. After cooling, the absorbance was measured at 600 nm using a spectrophotometer and converted to COD value, recorded as before and after COD. The COD removal rate R (%) was calculated using the formula R = (1 - after COD / before COD) × 100.

[0056] The cyclic stability of the adsorbent was evaluated by the decay of calcium and magnesium ion removal rate: After each case of adsorbent was used for the first time to treat a crude potassium nitrate solution with a known total calcium and magnesium ion concentration C0 (as determined by ICP-OES as a fixed value), the total calcium and magnesium ion concentration C1 in the resulting product was recorded. The initial removal rate E1 (%) was then calculated as (1-C1 / C0) × 100. After the adsorbent was regenerated according to step S3, it was used again to treat a new batch of crude solution with the same source and initial concentration C0 to obtain the product concentration C2 and the second removal rate E2 was calculated. This process was repeated 5 times to complete the adsorption-regeneration cycle. The removal rate E5 after the 5th cycle was recorded. The removal rate retention rate K (%) after 5 cycles was then calculated as (E5 / E1) × 100.

[0057] Test results:

[0058] As can be seen from Table 1, the complete technical solutions of the present invention represented by Examples 1-3 systematically solve the existing technical problems through targeted comparison of Examples 1-3.

[0059] Regarding the removal of calcium and magnesium ions, the calcium and magnesium ion content in the products of the examples was all below 8 ppm, while that of Comparative Example 1, lacking manganese doping, was as high as 43.6 ppm. This directly confirms the excellent selective adsorption capacity of the manganese-doped composite oxide shell for divalent cations, solving the problem of poor selectivity of traditional adsorbents. In terms of synergistic degradation of organic impurities, the COD removal rates of the examples all exceeded 94%, while the removal rate of Comparative Example 3, lacking a zero-valent iron-carbon core, plummeted to 8.2%. This confirms the unique and irreplaceable function of the FeO / C micro-electrolysis system in efficiently degrading organic matter, achieving simultaneous deep removal of both types of impurities.

[0060] In terms of economic benefits, the adsorbents in Examples 1-3 maintained a performance retention of over 90% after 5 cycles, demonstrating excellent regenerability. In contrast, Comparative Example 2, which did not undergo gas-phase silanization hydrophobic modification, had a retention rate of only 68.7%, highlighting the crucial role of this modification step in protecting the adsorbent structure and resisting degradation during the regeneration process, thereby ensuring the adsorbent's long lifespan and low operating costs.

[0061] In summary, this invention achieves multiple purification steps in a single adsorption process through the synergistic effect of three major technical features: "selective adsorption of calcium and magnesium ions by manganese-doped shell", "degradation of organic matter by iron and carbon core", and "gas-phase modification to ensure cycle stability". This greatly simplifies the process flow. At the same time, thanks to the high efficiency and renewability of the adsorbent, it overcomes the problem of traditional processes being unable to produce high-purity potassium nitrate economically and stably.

[0062] The embodiments described above are merely examples of several implementations of the present invention, and while the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the present invention. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these modifications and improvements all fall within the scope of protection of the present invention.

Claims

1. A purification process for potassium nitrate, characterized in that, Includes the following steps: S1, by weight, 98-102 parts of crude potassium nitrate are added to 115-125 parts of hot deionized water and stirred to obtain a potassium nitrate solution; Add 1-5 parts of activated carbon to the potassium nitrate solution, continue stirring, filter, and obtain the preheated filtrate; S2, under stirring, add 15-30 parts of multi-level porous manganese-doped silicon-aluminum composite oxide-zero-valent iron-carbon composite adsorbent to the preheated filtrate, adjust the temperature to 60-70℃, stir the reaction to obtain the reaction mixture; S3, filter the reaction mixture to obtain potassium nitrate filtrate and filter cake; regenerate the filter cake by soaking it in dilute nitric acid at 78-82℃, wash it with water, dry it at 78-82℃, and reduce it at 395-405℃ under a hydrogen / argon atmosphere; S4. Transfer the potassium nitrate filtrate to a crystallization vessel, cool and crystallize to obtain a crystal slurry; separate the crystal slurry by centrifugation to obtain crystals, wash the crystals with a saturated potassium nitrate ice-water solution to obtain wet crystals; dry the wet crystals under vacuum at 80-90℃.

2. The potassium nitrate purification process according to claim 1, characterized in that, In step S1, the temperature of the hot deionized water is 70-80℃.

3. The potassium nitrate purification process according to claim 1, characterized in that, In step S2, the stirring reaction time is 1.5-2 hours.

4. The potassium nitrate purification process according to claim 1, characterized in that, In step S3, the reduction treatment at 395-405℃ takes 2-4 hours.

5. The potassium nitrate purification process according to claim 1, characterized in that, In step S4, the temperature is cooled to 8-12°C.

6. The potassium nitrate purification process according to any one of claims 1-5, characterized in that, The preparation steps of the multi-level porous manganese-doped silicon-aluminum composite oxide-zero-valent iron-carbon composite adsorbent include: A1, by weight, under nitrogen protection, 35-45 parts of zero-valent iron powder, 8-12 parts of conductive carbon black, and 180-220 parts of deionized water are sheared and dispersed; under stirring, a mixed aqueous solution of 28-32 parts of magnesium nitrate hexahydrate and 13-17 parts of aluminum nitrate nonahydrate, along with sodium hydroxide solution, is added dropwise to adjust the pH to 9.9-10.1 to obtain a mixture; the mixture is transferred to an autoclave and crystallized at 118-122℃ to obtain a reaction solution; the reaction solution is cooled and centrifuged to obtain a solid product; the solid product is washed with deionized water and dried under vacuum at 78-82℃ to obtain the precursor; A2. The precursor is dispersed in 450-550 parts of an aqueous manganese sulfate solution and stirred at 58-62℃. After centrifugation, it is dispersed in a mixture of 90-110 parts of tetraethyl orthosilicate, 280-320 parts of anhydrous ethanol, and 95-105 parts of deionized water. 90-110 parts of an ethanol solution containing ammonia are added dropwise and stirred at room temperature to obtain a mixture. The mixture is filtered to obtain a solid product. The solid product is washed with ethanol and calcined at 595-605℃ under a nitrogen atmosphere to obtain an intermediate. A3, the intermediate and 0.4-0.6 parts of 3-aminopropyltriethoxysilane are placed in the reaction zone and the vaporization zone at 148-152℃ in a tube furnace, respectively, and nitrogen carrier gas is transported to the reaction zone at 595-605℃ for action; after cooling, powder is obtained; A4: Stir the powder with dilute sulfuric acid at room temperature, wash with water until neutral; reduce at 395-405℃ under a hydrogen / argon atmosphere, cool and seal.

7. The potassium nitrate purification process according to claim 6, characterized in that, In step A1, the crystallization time at 118-122℃ is 12-14 hours.

8. The potassium nitrate purification process according to claim 6, characterized in that, In step A2, the calcination time at 595-605℃ is 4-6 hours.

9. The potassium nitrate purification process according to claim 6, characterized in that, In step A3, the nitrogen carrier gas is delivered to the reaction zone at 595-605℃ for 2-4 hours.

10. The potassium nitrate purification process according to claim 6, characterized in that, In step A4, the reduction time at 395-405℃ is 2-4 hours.

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