A method for removing impurities from a semi-finished product potassium nitrate

Abstract

This invention discloses a method for removing impurities from semi-finished potassium nitrate in the field of inorganic functional materials. The method uses mesoporous γ-alumina spherical particles co-modified with cerium and molybdenum oxides as the adsorbent. The method includes: filling the adsorbent to form a fixed bed; preheating and rinsing; injecting a saturated potassium nitrate hot solution into the bed for dynamic adsorption to remove ammonium, calcium, magnesium, and chloride ion impurities; collecting the purified liquid; after adsorption saturation, regenerating with a dilute nitric acid hot solution; washing and drying; and reusing the purified liquid. The purified liquid is then cooled, crystallized, filtered, washed, and dried to obtain high-purity potassium nitrate. The adsorbent is prepared by mixing boehmite with a colloidal solvent, molding it into a spherical carrier, calcining it at high temperature, and then sequentially loading cerium and molybdenum oxides and calcining the carrier. This method is simple, has good adsorbent selectivity, and is recyclable, effectively removing various impurity ions from potassium nitrate, and is suitable for industrial production.

Description

Technical Field

[0001] This invention relates to the field of inorganic functional materials technology, specifically to a method for removing impurities from semi-finished potassium nitrate. Background Technology

[0002] Potassium nitrate, as an important inorganic chemical product, has wide applications in agricultural fertilizers, industrial catalysts, glass manufacturing, and fireworks powder. Its purity directly affects the performance and quality of downstream products. Mainstream industrial methods for producing potassium nitrate, such as the metathesis process, often introduce or leave various ionic impurities in the product, including ammonium ions, calcium ions, magnesium ions, sodium ions, and chloride ions. The presence of these impurities not only reduces the grade and value of potassium nitrate products but is also unacceptable in certain high-end applications, such as as an auxiliary material for optoelectronic materials or as a clarifying agent for special glasses. Therefore, developing efficient and economical deep purification technologies to remove these coexisting impurities is a key step in enhancing the added value of potassium nitrate products and expanding their high-end applications, and it remains a focus of ongoing research in this technological field.

[0003] Currently, various technical approaches have been explored and applied in the purification of potassium nitrate solutions, but each has significant limitations. Crystallization is the most traditional method, gradually increasing purity through multiple recrystallizations. However, this method is inefficient, resulting in significant potassium nitrate yield loss, and its separation effect on impurity ions with similar physicochemical properties (such as sodium ions) is limited, leading to high operating costs. Adsorption, as a promising alternative, relies heavily on the selectivity of the adsorbent material. Common adsorbents such as activated alumina or activated carbon, while possessing some adsorption capacity for certain heavy metal ions, generally lack sufficient synergistic selective adsorption capacity for common mixed impurities in potassium nitrate systems, such as ammonium, calcium, magnesium, and chloride. Furthermore, their limited adsorption capacity makes deep purification difficult. Ion exchange resins have strong ion removal capabilities, but they face challenges such as frequent resin regeneration, high consumption of regeneration reagents, potential introduction of new organic impurities, and resin aging after treating high-concentration brine, resulting in high operating and maintenance costs. More importantly, existing technologies mostly focus on removing single or specific types of impurities, lacking an integrated solution capable of synergistically, efficiently, and selectively adsorbing multiple typical coexisting cationic and anionic impurities in potassium nitrate solutions. In particular, how to simultaneously and efficiently remove ammonium, alkaline earth metal ions, and chloride ions under the same process conditions, while ensuring the efficient passage of the target product, potassium ions, is a common challenge faced by existing separation technologies. Samples were taken and ion chromatography was used to determine the initial concentrations of chloride, ammonium, calcium, and magnesium ions in the feed solution.

[0004] In summary, the long-standing bottleneck in this technical field lies in the lack of a purification method that is simple in process, possesses high selectivity and is easily regenerable, and can deeply remove multiple typical impurity ions from potassium nitrate in a single step. Existing technologies either suffer from low separation efficiency, unsatisfactory selectivity, or high operating costs and complex processes, failing to meet the urgent needs of modern chemical production for efficient, economical, and green purification technologies. This technological gap restricts the upgrading and transformation of industrial-grade potassium nitrate to higher purity products. Therefore, there is an urgent need to innovate purification processes and adsorption material design, and to develop a novel potassium nitrate impurity removal technology that can achieve the synergistic removal of multiple impurity ions and is easy to implement continuously in industrial applications. Summary of the Invention

[0005] The purpose of this invention is to provide a method for removing impurities from semi-finished potassium nitrate, which solves the technical problems of existing potassium nitrate purification technologies, which are difficult to simultaneously and deeply remove multiple coexisting impurities such as ammonium, calcium, magnesium, and chlorine, and have complex processes, low efficiency, or high costs.

[0006] The present invention achieves the above objectives through the following technical solutions: A method for removing impurities from semi-finished potassium nitrate includes the following steps: S1. The adsorption column is fixed on the support, the bottom of the column is lined with quartz wool, and the column is filled with mesoporous γ-alumina spherical particles co-modified with cerium and molybdenum oxide to form a fixed bed. Preheated deionized water is pumped in from the top of the column for rinsing, and the fixed bed is blown with hot air. S2, after crushing the semi-finished potassium nitrate, dissolve it in hot deionized water and heat it to obtain a saturated potassium nitrate hot aqueous solution; inject the saturated potassium nitrate hot aqueous solution from the top of the adsorption column and let it flow through a fixed bed of mesoporous γ-alumina spherical particles co-modified with cerium and molybdenum oxides, and collect the solution containing purified potassium nitrate from the bottom of the column. S3. When the chloride ion concentration in the solution containing purified potassium nitrate increases, stop feeding; backwash the fixed bed with hot deionized water, pump dilute nitric acid solution into the adsorption column at 58-62℃ for regeneration, wash the adsorption column with deionized water until neutral, and dry it with hot air for reuse. S4. The solution containing purified potassium nitrate is cooled, allowed to stand, filtered, and washed with ice water to obtain wet crystals, which are then dried at 100-110℃.

[0007] In this invention, during the impurity removal application of semi-finished potassium nitrate, the mesoporous γ-alumina spherical particles co-modified with cerium and molybdenum oxides achieve efficient and targeted impurity removal through a multi-mechanism synergy. When the hot solution flows through the fixed bed, ammonium ions are electrostatically attracted and coordinated with hydroxyl groups on the material surface, resulting in their retention. The process temperature promotes their conversion to gaseous ammonia, and the microenvironment on the material surface facilitates ammonia molecule desorption. Combined with the timely exhaust structure of the system, this continuously drives the removal equilibrium forward. High-valence cations such as calcium and magnesium ions, due to their high charge density, preferentially and selectively capture hydroxyl groups on the alumina carrier surface through complexation and ion exchange. The cerium oxide component further enhances the adsorption capacity and selectivity for these ions. Chloride ions, relying on the positively charged region formed by protonation on the surface of cerium oxide under near-neutral conditions, achieve auxiliary adsorption through electrostatic attraction. After co-modification with cerium and molybdenum oxides, the surface charge distribution and pore microenvironment of the material are systematically optimized, exhibiting a significant selective adsorption advantage for impurity ions. Potassium ions, due to their hydration characteristics and charge compatibility, achieve efficient permeation, ensuring a high recovery rate of the main component. The regeneration stage employs a warm, dilute acid solution treatment, where hydrogen ions efficiently replace and adsorb impurities, restoring the active sites of the material. After washing with neutral water and drying, the material is recycled. The entire process relies on the intrinsic properties of the material and precise coordination of process parameters, with no external chemical reagents added and no secondary pollution, achieving green and deep purification of potassium nitrate solution. The final crystallization yields a high-purity product that meets the application requirements of high-end fields.

[0008] According to a preferred embodiment of the present invention, in step S1, deionized water is preheated to 78-82°C.

[0009] According to a preferred embodiment of the present invention, in step S2, the temperature is raised to 85-90°C.

[0010] According to a preferred embodiment of the present invention, in step S3, the temperature of the hot deionized water is 70-80°C.

[0011] According to a preferred embodiment of the present invention, in step S4, the temperature is cooled to 10-15°C.

[0012] According to a preferred embodiment of the present invention, the preparation steps of the mesoporous γ-alumina spherical particles co-modified with cerium and molybdenum oxide include: A1, by weight, 80-120 parts of pseudoboehmite powder are placed in a mixer, and a gelling agent composed of 3-8 parts of citric acid and deionized water is added. The mixture is then mixed to obtain a paste. The paste is extruded into strips and rolled into wet balls. The wet balls are dried at 118-122℃ and then calcined at 780-820℃ in air atmosphere and allowed to cool naturally to obtain a mesoporous γ-alumina spherical carrier. A2, immerse the mesoporous γ-alumina spherical support in an aqueous solution of cerium nitrate, let it stand at room temperature for impregnation, and stir; after impregnation, filter, dry at 108-112℃, and then calcine at 445-455℃ in air atmosphere to obtain the intermediate; A3. The intermediate is placed in a spin coater, and an aqueous solution of ammonium molybdate is sprayed onto the surface of the intermediate under rotation to obtain the coated material. The coated material is aged at room temperature and dried at 78-82℃. Then, it is calcined at 495-505℃ in air to obtain the product. A4. After cooling the product to room temperature in air, sieve it.

[0013] In this invention, during the preparation of mesoporous γ-alumina spherical particles co-modified with cerium and molybdenum oxides, boehmite powder is thoroughly mixed with citric acid aqueous solution to form a homogeneous paste. Citric acid, acting as a green complexing agent, is used throughout the process, effectively regulating the dispersion of metal ions and inhibiting high-temperature agglomeration. After extrusion, spheroidization, and moderate drying, the material undergoes deep thermal transformation in air. Boehmite is dehydroxylated and reconstructed into an alumina carrier with a high specific surface area and a three-dimensional interconnected mesoporous network. Its surface is enriched with active hydroxyl groups, laying a structural foundation for subsequent functional component anchoring. The cerium component penetrates deep into the pores of the carrier through an impregnation process. Cerium ions and hydroxyl groups form a stable coordination structure, which is then transformed into highly dispersed cerium oxide nanocrystals after heat treatment. These nanocrystals are firmly embedded in the inner wall of the pores, significantly enhancing the carrier's affinity for multivalent cations and its thermal stability. The molybdenum component is precisely modified using spin-coating technology. The solution uniformly covers the outer layer of the particles, and after aging and thermal transformation, an active molybdenum oxide phase is formed, mainly concentrated in the surface area of ​​the particles, forming a gradient functional distribution with the inner cerium oxide layer. This stepwise loading strategy avoids interference between components, ensuring that the cerium component enhances the bulk adsorption capacity, while the molybdenum component optimizes the surface microenvironment. The final programmed temperature-controlled cooling process effectively alleviates thermal stress, ensuring the integrity of the spherical particle structure and mechanical strength, resulting in a high-performance composite oxide material with a rational spatial distribution of components, well-developed pores, and synergistically optimized surface properties.

[0014] According to a preferred embodiment of the present invention, in step A1, the calcination time at 780-820°C is 4-6 hours.

[0015] According to a preferred embodiment of the present invention, in step A2, the calcination time at 445-455°C is 3-5 hours.

[0016] According to a preferred embodiment of the present invention, in step A3, the calcination time at 495-505°C is 2-4 hours.

[0017] According to a preferred embodiment of the present invention, in step A4, the cooling rate is 1-3°C / min.

[0018] The beneficial effects of this invention are as follows: The technical solution provided by this invention combines innovative adsorbent material design with optimized dynamic column adsorption purification process to achieve efficient, synergistic, and deep removal of multiple typical coexisting impurity ions in semi-finished potassium nitrate, while completely preserving the target product potassium nitrate. This achieves a comprehensive effect significantly superior to existing single purification technologies. The core lies in the use of a specially designed mesoporous spherical alumina adsorbent material co-modified with cerium and molybdenum oxides. This material is not a simple stacking of multiple functions, but rather, through careful structural design and surface modification, it simultaneously possesses specific adsorption sites for different impurities and a microenvironment that promotes the shift of adsorption equilibrium. The abundant hydroxyl groups on the alumina support surface provide ion exchange-dominated, high-capacity capture capacity for ammonium and alkaline earth metal cations such as calcium and magnesium; the introduction of cerium oxide optimizes the charge distribution on the support surface in near-neutral solutions, generating electrostatically assisted adsorption of anionic impurities such as chloride ions; and the modification with molybdenum oxide further regulates the surface chemical microenvironment. This integrated design with multiple active sites allows a single type of adsorbent to simultaneously address complex mixed ion systems, solving the problem of limited selectivity in traditional adsorbents. More importantly, the adsorption operation carried out at the optimal temperature not only significantly improves the kinetic rate of various ion exchanges and diffusions, but also provides a weak thermal decomposition driving force for the adsorbed and fixed ammonium ions. Combined with the system's exhaust design, it can promote the escape of trace amounts of ammonia gas generated, thereby continuously driving the adsorption equilibrium towards deep deammoniation and achieving an extremely low residual level that is difficult to achieve with conventional ion exchange.

[0019] From the perspective of the adsorbent material itself, its preparation method is scientifically sound, and the resulting structural characteristics bring many key performance advantages. Using citric acid as a colloidal solvent in a specific ratio, combined with boehmite molding and calcination at a suitable high temperature, a mesoporous spherical alumina support with good mechanical strength was successfully prepared. The regular shape and uniform packing density of the spherical particles effectively prevent channeling in the fixed bed, reduce column pressure drop, and ensure good hydrodynamic performance during solution flow. The well-developed mesoporous structure provides a large specific surface area and abundant surface hydroxyl groups, which is the material basis for achieving high adsorption capacity. Through stepwise loading and calcination, cerium and molybdenum oxides are highly dispersed and stably bound on the surface and within the pores of the support, rather than being simply physically mixed. This structure ensures the effective utilization of the modifying components, avoids the aggregation and failure of active sites, and allows the material to maintain structural stability and long-term performance during repeated adsorption-regeneration cycles. The stable composite structure formed between the support and the modifying components allows it to withstand the chemical erosion of hot potassium nitrate solution during adsorption and dilute nitric acid during regeneration, exhibiting excellent structural stability and service life, which is an important guarantee for achieving continuous industrial operation.

[0020] The entire impurity removal process demonstrates a high degree of efficiency, economy, and ease of operation. The process achieves simultaneous, deep purification of multiple key impurities such as ammonium, calcium, magnesium, sodium, and chlorine in a single, continuous flow, simplifying the cumbersome multi-step separation process required in traditional methods and significantly improving production efficiency. The highly selective adsorption of impurity ions by the adsorbent ensures an extremely high recovery rate of the main potassium nitrate component with virtually no loss, significantly improving raw material utilization and economy. Using dilute nitric acid at a specific concentration and temperature as the regenerator provides mild and efficient regeneration conditions, thoroughly eluting all adsorbed cations and anions, fully restoring the adsorbent's performance, and enabling the recycling and reuse of the adsorbent material, greatly reducing long-term material consumption costs. Simultaneously, the regenerated waste liquid has a relatively simple composition and is easy to treat, demonstrating good environmental friendliness. The entire method has clearly defined process parameters, mild operating conditions, and uses conventional equipment, making it easy to automate and scale up, and has broad prospects for industrial application. In summary, this invention provides a potassium nitrate deep purification solution that is highly efficient, low in operating cost, easy to operate, and environmentally friendly through synergistic innovation in materials and processes, effectively overcoming many shortcomings of existing technologies. 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

[0022] This embodiment provides a method for removing impurities from semi-finished potassium nitrate, the steps of which include: Preparation of mesoporous γ-alumina spherical particles co-modified with cerium and molybdenum oxides: Step A1: Weigh 100.0 g of boehmite powder and place it in a laboratory mixer. Weigh 5.0 g of citric acid and dissolve it in 65.0 g of deionized water to prepare a gelling agent. Slowly add the gelling agent to the mixer and mix it with the powder for 30 min until a uniform and plastic paste is formed. Transfer the paste to a screw extruder and extrude it into strips through a 3.0 mm diameter perforated plate. Place the strips in a ball rolling pan and roll them into spherical wet particles with a diameter of approximately 3.0 mm. Spread the wet spheres evenly in a tray and place them in a forced-air drying oven to dry at 120.0 °C for 10.0 h. Transfer the dried spheres to a box-type muffle furnace and heat them from room temperature to 800.0 °C at a heating rate of 2.0 °C / min under static air atmosphere, and calcine them at 800.0 °C for 5.0 h. After calcination, the heating power is turned off, and the furnace body is allowed to cool naturally to room temperature (approximately 25.0°C) to obtain mesoporous γ-alumina spherical carriers.

[0023] Step A2: Weigh 50.0 g of the above spherical support and place it in a 500 mL beaker. Prepare the impregnation solution: Weigh 1.6 g of cerium nitrate (hexahydrate) and dissolve it in 100.0 mL of deionized water. Pour the impregnation solution into the beaker containing the support, ensuring that the liquid completely submerges the support. Let it stand at room temperature (25.0 °C) for 6.0 h, stirring manually for 1.0 min every 1.0 h. After impregnation, separate the solid material by suction filtration using a Buchner funnel. Transfer the filtered solid material to a watch glass and dry it in a forced-air drying oven at 110.0 °C for 6.0 h. Place the dried material in a muffle furnace and calcine it at 450.0 °C at a rate of 2.5 °C / min under air atmosphere for 4.0 h. After calcination, allow it to cool naturally to room temperature to obtain the cerium-modified intermediate.

[0024] Step A3: Weigh 50.0g of the intermediate obtained in Step A2 and place it in the tray of a spin coater (speed set to 30rpm). Prepare the spraying solution: Weigh 3.0g of ammonium molybdate and dissolve it in 50.0mL of deionized water. Use a spray gun to evenly spray the spraying solution onto the continuously rotating surface of the intermediate within 10 minutes. After spraying, age (stand still) the material at room temperature (25.0℃) in a dust-free environment for 12.0h. Then, place the material in a forced-air drying oven and dry at 80.0℃ for 4.0h. Transfer the dried material to a muffle furnace and calcine it at 500.0℃ at a rate of 2.5℃ / min in an air atmosphere for 3.0h.

[0025] Step A4: After calcination in Step A3, immediately set the temperature control program of the muffle furnace to cool to room temperature (25.0℃) at a rate of 2.0℃ / min. After cooling, pass the product through a 30-mesh standard sieve and take particles with a particle size of approximately 0.8-1.2mm to obtain the final adsorbent, labeled CM-Al.

[0026] Method for removing impurities from semi-finished potassium nitrate: Step S1: Take a glass adsorption column with an inner diameter of 30.0 mm and a length of 500.0 mm and fix it vertically on an iron stand. Place a layer of quartz wool about 5.0 mm thick at the bottom of the column. Pack 150.0 g of CM-Al adsorbent granules into the column using a wet method (accompanied by the inflow of deionized water) to form a fixed bed with a height of about 300.0 mm. Preheat the deionized water to 80.0℃ and pump it into the column from the top at a flow rate of 150.0 mL / h using a constant flow pump to rinse the bed for 30.0 min. After rinsing, turn off the pump and drain the free water from the column. Connect the outlet of an air heater to the bottom inlet of the adsorption column and introduce hot air at 80.0℃ to purge the bed from bottom to top for 60.0 min until no obvious water vapor escapes from the top of the column.

[0027] Step S2: Weigh 200.0g of semi-finished potassium nitrate (industrial grade, containing impurities) and crush it using a mortar and pestle. Add the crushed material to a three-necked flask containing 500.0mL of deionized water. Set the water bath temperature to 88.0℃ and heat to dissolve while stirring until a saturated solution is formed, maintaining the temperature at 88.0±1.0℃. Inject this saturated potassium nitrate hot solution from the top of the adsorption column using a constant flow pump, controlling the flow rate at 300.0mL / h, allowing it to flow through the adsorbent fixed bed. Collect the purified solution flowing out through the bottom outlet pipe.

[0028] Step S3: Monitor the chloride ion concentration in the column effluent online using an ion chromatograph. Stop feeding when the chloride ion concentration rises from a stable background value (less than 2.0 mg / L) to 8.0 mg / L. Switch the tubing and pump preheated deionized water (80.0℃) from the bottom of the adsorption column to backwash the bed for 40.0 min at a flow rate of 200.0 mL / h. After backwashing, switch the tubing to the regeneration solution. Heat a 1.0 mol / L dilute nitric acid solution to 60.0℃ in a constant temperature water bath, then pump it into the adsorption column from the top at a flow rate of 150.0 mL / h for regeneration, consuming approximately 400.0 mL of regeneration solution. After regeneration, switch to rinsing with deionized water from the top of the column until the pH of the effluent stabilizes at 7.0. Finally, reconnect the air heater and circulate hot air at 110.0℃ into the column to dry the bed for 120.0 min. The dried adsorption column can be reused.

[0029] Step S4: Transfer all the purified solution collected in Step S2 to a crystallizing dish and place it on a magnetic stirrer. While continuously stirring, slowly cool the solution to 12.0°C at a rate of 0.5°C / min, and allow it to crystallize at this temperature for 12.0 h. After crystallization, filter the solution, and wash the resulting wet crystals three times with a small amount (approximately 50.0 mL) of ice water at 0°C. Spread the washed wet crystals evenly in a watch glass and dry them in an oven at 105.0°C for 6.0 h to obtain high-purity potassium nitrate crystals. Example

[0030] The specific implementation method is the same as in Example 1, except that the mesoporous γ-alumina spherical particles co-modified with cerium and molybdenum oxide are prepared as follows: Step A1: Weigh 90.0g of pseudoboehmite powder and place it in a mixer. Weigh 3.5g of citric acid and dissolve it in 60.0g of deionized water to prepare a gelling agent. After adding the gelling agent, mix and grind for 28 minutes to form a paste. Extrude into strips and roll into wet balls with a diameter of approximately 2.8mm. Dry the wet balls at 119.0℃ for 11.0h. Then, calcine them in a muffle furnace in an air atmosphere at a rate of 1.8℃ / min to 790.0℃, maintain the temperature for 4.5h, and allow to cool naturally to obtain the carrier.

[0031] Step A2: Weigh 50.0 g of the above carrier and immerse it in a solution prepared by dissolving 1.0 g of cerium nitrate (hexahydrate) in 100.0 mL of deionized water. Let it stand at room temperature for 5.5 h, stirring periodically during the process. After filtration, dry the solid at 109.0 °C for 5.5 h. Then, calcine it in air at a rate of 2.3 °C / min to 448.0 °C for 3.5 h to obtain the intermediate.

[0032] Step A3: Weigh 50.0g of the intermediate and place it in a spin coater (35rpm). Spray a 45.0mL aqueous solution containing 2.2g of ammonium molybdate evenly onto its surface. After spraying, age the material at room temperature for 10.0h, then dry it at 79.0℃ for 4.5h. Subsequently, calcine it in air at a rate of 2.6℃ / min to 498.0℃ for 2.5h.

[0033] Step A4: After calcination, cool to room temperature at a rate of 1.5℃ / min, sieve to obtain particles of the desired size, and obtain the adsorbent.

[0034] Method for removing impurities from semi-finished potassium nitrate: Step S1: Pack 150.0g of the above adsorbent into the column, with a bed height of approximately 290.0mm. Rinse with deionized water at 79.0℃ at a flow rate of 140.0mL / h for 35.0min. Then purge the column from the bottom with hot air at 79.0℃ for 65.0min.

[0035] Step S2: Dissolve 200.0 g of semi-finished potassium nitrate in 500.0 mL of deionized water to prepare a saturated solution at 86.0 °C. Pump this solution into the column top at a flow rate of 280.0 mL / h for adsorption.

[0036] Step S3: Stop when the Cl⁻ concentration in the effluent reaches 7.0 mg / L. Backwash with 79.0℃ hot water at a flow rate of 190.0 mL / h for 35.0 min. Regenerate with 0.9 mol / L dilute nitric acid at 59.0℃ at a flow rate of 140.0 mL / h. After washing with water until pH=7, dry with hot air at 108.0℃ for 110.0 min.

[0037] Step S4: The purified solution was cooled to 11.0℃ at 0.4℃ / min and allowed to stand for 10.0h to crystallize. After filtration, the wet crystals were washed with ice water and dried at 102.0℃ for 5.5h to obtain a high-purity product. Example

[0038] The specific implementation method is the same as in Example 1, except that the mesoporous γ-alumina spherical particles co-modified with cerium and molybdenum oxide are prepared as follows: Step A1: Weigh 110.0g of pseudoboehmite powder and place it in a mixer. Weigh 7.0g of citric acid and dissolve it in 70.0g of deionized water to prepare a gelling agent. After adding the gelling agent, mix and grind for 32 minutes to form a paste. Extrude into strips and roll into wet balls with a diameter of approximately 3.2mm. Dry the wet balls at 121.0℃ for 9.0h. Then, calcine them in a muffle furnace in air atmosphere at a rate of 2.2℃ / min to 810.0℃, maintain the temperature for 5.5h, and allow to cool naturally to obtain the carrier.

[0039] Step A2: Weigh 50.0 g of the above-mentioned carrier and immerse it in a solution prepared by dissolving 2.2 g of cerium nitrate (hexahydrate) in 100.0 mL of deionized water. Let it stand at room temperature for 6.5 h, stirring periodically during the process. After filtration, dry the solid at 111.0 °C for 6.5 h. Then, calcine it at 452.0 °C for 4.5 h in air at a rate of 2.7 °C / min to obtain the intermediate.

[0040] Step A3: Weigh 50.0 g of the intermediate and place it in a spin coater (25 rpm). Spray a 55.0 mL aqueous solution containing 4.0 g of ammonium molybdate evenly onto its surface. After spraying, age the material at room temperature for 14.0 h, then dry it at 81.0 °C for 3.5 h. Subsequently, calcine it in air at a rate of 2.4 °C / min to 502.0 °C for 3.5 h.

[0041] Step A4: After calcination, cool to room temperature at a rate of 2.5℃ / min, sieve to obtain particles of the desired size, and obtain the adsorbent.

[0042] Method for removing impurities from semi-finished potassium nitrate: Step S1: Pack 150.0g of the above adsorbent into the column, with a bed height of approximately 310.0mm. Rinse with deionized water at 81.0℃ at a flow rate of 160.0mL / h for 25.0min. Then purge the column from the bottom with hot air at 81.0℃ for 55.0min.

[0043] Step S2: Dissolve 200.0 g of semi-finished potassium nitrate in 500.0 mL of deionized water to prepare a saturated solution at 89.0 °C. Pump this solution into the column top at a flow rate of 320.0 mL / h for adsorption.

[0044] Step S3: Stop when the Cl⁻ concentration in the effluent reaches 9.0 mg / L. Backwash with 81.0℃ hot water at a flow rate of 210.0 mL / h for 45.0 min. Regenerate with 1.1 mol / L dilute nitric acid at 61.0℃ at a flow rate of 160.0 mL / h. After washing with water until pH=7, dry with hot air at 112.0℃ for 130.0 min.

[0045] Step S4: The purified solution was cooled to 14.0℃ at 0.6℃ / min and allowed to stand for 14.0h to crystallize. After filtration, the wet crystals were washed with ice water and dried at 108.0℃ for 6.5h to obtain a high-purity product.

[0046] Comparative Example 1 The specific implementation method is the same as in Example 1, except that this comparative example uses pure mesoporous γ-alumina spherical carrier (unmodified) as adsorbent, and the rest is the same as in Example 1.

[0047] Comparative Example 2 The specific implementation method is the same as in Example 1, except that this comparative example uses spherical alumina (C-Al) loaded only with cerium oxide (CeO2) as the adsorbent. Its preparation includes steps A1 and A2 (with the same parameters) as in Example 1, but step A3 (without loading molybdenum) is completely skipped. The cooling method for step A4 is the same as in Example 1, and the rest is the same as in Example 1.

[0048] Comparative Example 3 The specific implementation method is the same as in Example 1, except that this comparative example uses spherical alumina (M-Al) loaded only with molybdenum oxide (MoO3) as the adsorbent. Its preparation includes step A1 (with the same parameters) as in Example 1, but skips step A2 (without loading cerium), and directly performs the same molybdenum loading and subsequent step A4 treatment on the pure support as in step A3 of Example 1, with the rest being the same as in Example 1.

[0049] Performance testing The purification methods for the semi-finished potassium nitrate products prepared in Examples 1-3 and Comparative Examples 1-3 were tested for performance according to the following steps: The performance test was performed according to the following steps: First, 200.0g of semi-finished potassium nitrate raw material was accurately weighed, and a saturated solution was prepared according to the method described in the example. A sample was then taken and the initial concentrations of chloride ions, ammonium ions, calcium ions, and magnesium ions in the raw material solution were determined using an ion chromatograph and recorded as follows: At the same time, atomic absorption spectroscopy was used to determine the initial concentration of sodium ions. Dynamic adsorption experiments were then conducted. A prepared saturated hot potassium nitrate solution was passed through a fixed-bed column containing 150.0 g of adsorbent at a constant flow rate of 300.0 mL / h. Eluent was collected every 30.0 min from the bottom of the column outlet, and the collection time and effluent volume were recorded. The real-time concentrations of chloride and ammonium ions in each sample were immediately determined using ion chromatography. Plot the outflow volume on the x-axis. Plot a penetration curve on the ordinate, and The point is defined as the penetration point of the impurity ion. The point is defined as the adsorption saturation point.

[0050] The method for calculating the chloride ion breakthrough adsorption capacity is as follows: the total mass of chloride ions adsorbed by the adsorbent from the start of adsorption to the breakthrough point, divided by the mass of the adsorbent. ,in To achieve the cumulative outflow volume at the penetration point, This represents the mass of the adsorbent. The ammonium ion breakthrough adsorption capacity is calculated using the same formula.

[0051] The method for calculating the impurity ion removal rate is as follows: ,in This represents the concentration of the ion in the last effluent sample collected after adsorption saturation and before regeneration. The removal rates of calcium, magnesium, and sodium ions are calculated by sampling and measuring at the adsorption saturation point.

[0052] The method for testing product purity is as follows: Dissolve and dilute the high-purity potassium nitrate product obtained by final crystallization and drying, and determine the content of all impurity anions and cations using ion chromatography. The difference between the total impurity content and 100% is the main content of potassium nitrate.

[0053] The product yield is calculated as follows: (mass of dried high-purity potassium nitrate crystals obtained / theoretical mass of potassium nitrate in the raw material semi-finished product potassium nitrate) × 100%.

[0054] The method for testing the cyclic stability of the adsorbent is as follows: Using the same batch of adsorbent on the same adsorption column, five complete "adsorption-regeneration-drying" cycles are performed consecutively, with the raw materials and operating conditions being exactly the same as the first cycle. The removal rates of chloride and ammonium ions after the fifth cycle are recorded and compared with the removal rates of the first cycle to calculate the retention rate: Retention rate = (Removal rate of the fifth cycle / Removal rate of the first cycle) × 100%.

[0055] Test results:

[0056] As can be seen from Table 1, the technical solutions provided in Examples 1-3 of this invention, by using mesoporous γ-alumina spherical particles co-modified with cerium and molybdenum oxides as the core adsorbent, successfully solve the problems of existing potassium nitrate purification technologies, such as difficulty in simultaneously and deeply removing multiple coexisting impurities and high efficiency and cost. Performance test data clearly show that, compared to comparative materials modified or unmodified with a single component, the materials in these examples exhibit comprehensive and balanced superior performance.

[0057] Specifically, Comparative Example 1 used pure alumina as a carrier, and its chloride ion removal rate was only 45.3%, as shown in Table 1. This demonstrates that the single carrier has limited functionality and cannot cope with complex impurity systems. Comparative Example 2 only modified cerium oxide, which increased the chloride ion removal rate to 97.5%, but its removal capacity for ammonium ions (90.2%) was significantly weaker than that of the examples (over 99%), showing a clear shortcoming. Comparative Example 3 only modified molybdenum oxide, and its removal effect on ammonium ions (98.9%) was acceptable, but it had almost no selective adsorption capacity for chloride ions, with a removal rate as low as 46.1%. This set of comparisons strongly confirms the irreplaceable synergistic value of cerium and molybdenum oxide modification: the cerium component mainly enables the material to specifically adsorb chloride ions, while the molybdenum component crucially enhances the deep removal capacity of ammonium ions. Both are indispensable and together constitute a complete adsorption site design that can simultaneously and efficiently capture cations and anions.

[0058] Under this synergistic effect, Examples 1-3 also achieved removal rates of over 99.5% for calcium and magnesium ions, realizing "one-column" deep purification of multiple key impurities. The purity of the final product remained stable at over 99.9%, far superior to the comparative examples. Furthermore, Examples 1-3 also demonstrated significant advantages in key performance indicators. Their penetration adsorption capacities for chloride and ammonium ions reached approximately 18 mg / g and 15.5 mg / g, respectively, significantly higher than the comparative examples. This means a larger single adsorption capacity, higher purification efficiency, reduced equipment size, or extended operating cycles, directly improving the process economy.

[0059] More importantly, in five adsorption-regeneration cycle tests, Examples 1-3 maintained a removal rate of approximately 98% for both core impurities, demonstrating excellent regeneration stability and structural durability, while the comparative materials showed significant performance degradation. This characteristic allows the adsorbent of this invention to be used repeatedly for a long period, greatly reducing material consumption and operating costs in continuous production, and fundamentally overcoming the shortcomings of traditional methods, such as frequent resin regeneration, large amounts of waste liquid, or high costs associated with single-use of adsorbents.

[0060] In summary, the technical solutions of Examples 1-3, through collaborative innovation in material design, achieve a balance between deep removal, efficient operation, and low-cost maintenance, effectively solving the core problems existing in the background technology.

[0061] 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 method for removing impurities from semi-finished potassium nitrate, characterized in that, Includes the following steps: S1. The adsorption column is fixed on the support, the bottom of the column is lined with quartz wool, and the column is filled with mesoporous γ-alumina spherical particles co-modified with cerium and molybdenum oxide to form a fixed bed. Preheated deionized water is pumped in from the top of the column for rinsing, and the fixed bed is blown with hot air. S2, after crushing the semi-finished potassium nitrate, dissolve it in hot deionized water and heat it to obtain a saturated potassium nitrate hot aqueous solution; inject the saturated potassium nitrate hot aqueous solution from the top of the adsorption column and let it flow through a fixed bed of mesoporous γ-alumina spherical particles co-modified with cerium and molybdenum oxides, and collect the solution containing purified potassium nitrate from the bottom of the column. S3. When the chloride ion concentration in the solution containing purified potassium nitrate increases, stop feeding; backwash the fixed bed with hot deionized water, pump dilute nitric acid solution into the adsorption column at 58-62℃ for regeneration, wash the adsorption column with deionized water until neutral, and dry it with hot air for reuse. S4. The solution containing purified potassium nitrate is cooled, allowed to stand, filtered, and washed with ice water to obtain wet crystals, which are then dried at 100-110℃.

2. The method for removing impurities from semi-finished potassium nitrate according to claim 1, characterized in that, In step S1, deionized water is preheated to 78-82°C.

3. The method for removing impurities from semi-finished potassium nitrate according to claim 1, characterized in that, In step S2, the temperature is raised to 85-90℃.

4. The method for removing impurities from semi-finished potassium nitrate according to claim 1, characterized in that, In step S3, the temperature of the hot deionized water is 70-80℃.

5. The method for removing impurities from semi-finished potassium nitrate according to claim 1, characterized in that, In step S4, the temperature is cooled to 10-15°C.

6. The method for removing impurities from semi-finished potassium nitrate according to any one of claims 1-5, characterized in that, The preparation steps of the mesoporous γ-alumina spherical particles co-modified with cerium and molybdenum oxide include: A1, by weight, 80-120 parts of pseudoboehmite powder are placed in a mixer, and a gelling agent composed of 3-8 parts of citric acid and deionized water is added. The mixture is then mixed to obtain a paste. The paste is extruded into strips and rolled into wet balls. The wet balls are dried at 118-122℃ and then calcined at 780-820℃ in air atmosphere and allowed to cool naturally to obtain a mesoporous γ-alumina spherical carrier. A2, immerse the mesoporous γ-alumina spherical support in an aqueous solution of cerium nitrate, let it stand at room temperature for impregnation, and stir; after impregnation, filter, dry at 108-112℃, and then calcine at 445-455℃ in air atmosphere to obtain the intermediate; A3. The intermediate is placed in a spin coater, and an aqueous solution of ammonium molybdate is sprayed onto the surface of the intermediate under rotation to obtain the coated material. The coated material is aged at room temperature and dried at 78-82℃. Then, it is calcined at 495-505℃ in air to obtain the product. A4. After cooling the product to room temperature in air, sieve it.

7. The method for removing impurities from semi-finished potassium nitrate according to claim 6, characterized in that, In step A1, the calcination time at 780-820℃ is 4-6 hours.

8. The method for removing impurities from semi-finished potassium nitrate according to claim 6, characterized in that, In step A2, the calcination time at 445-455℃ is 3-5 hours.

9. The method for removing impurities from semi-finished potassium nitrate according to claim 6, characterized in that, In step A3, the calcination time at 495-505℃ is 2-4 hours.

10. The method for removing impurities from semi-finished potassium nitrate according to claim 6, characterized in that, In step A4, the cooling rate is 1-3℃ / min.

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