Preparation of agricultural potassium sulfate and industrial grade sodium chloride by mixed salt resource utilization

Abstract

The application discloses a kind of mixed salt resource utilization preparation agricultural potassium sulfate and industrial grade sodium chloride, it is related to solid waste comprehensive utilization technical field, the method is removed by the step-by-step precipitation of magnesium, calcium of sodium carbonate by removing magnesium of organic matter oxidation degradation organic matter in mixed salt, impurity, then refined by ion flotation, subsequently obtain sodium chloride and potassium sulfate product by gradient crystallization in turn, finally remaining mother liquor backflow reuse, precipitate waste residue pyrolysis preparation biochar, form closed loop cycle;The application realizes the efficient degradation of organic pollutants in high-organic industrial mixed salt and the deep removal of impurities, effectively improves the refining purity of salt solution and the raw material adaptation range;At the same time, gradient crystallization is used to separate salt and closed loop cycle in whole process, directional separation is used to prepare industrial grade sodium chloride and agricultural potassium sulfate, and the mother liquor and waste residue are all converted into available resources, forming a mixed salt resource utilization system with high value utilization and harmless disposal synergy.

Description

Technical Field

[0001] This invention relates to the field of comprehensive utilization technology of solid waste, specifically to a method for the resource utilization of mixed salts to prepare agricultural potassium sulfate and industrial-grade sodium chloride. Background Technology

[0003] Currently, all mainstream technologies used in the field of industrial mixed salt treatment have varying degrees of technical defects and application limitations, failing to achieve a synergistic balance between the harmless treatment and resource utilization of mixed salts. In the organic pollutant removal stage, traditional oxidative degradation technologies generally suffer from low degradation efficiency and incomplete removal of organic matter, making them unsuitable for industrial mixed salt raw materials with wide-ranging sources and complex compositional fluctuations. The treated salt solution still retains a large amount of organic pollutants, failing to meet the quality requirements for subsequent salt refining and recovery. Some oxidation processes also require the addition of large amounts of chemical agents, not only increasing operating costs but also easily causing secondary pollution problems. Furthermore, the generated precipitated waste residue is only subjected to simple disposal, failing to achieve the desired waste recovery. In the process of resource utilization, traditional processes for impurity removal and salt refining lack sufficient precision and cannot deeply remove various harmful impurities from mixed salts. This can easily lead to substandard purity of the final recovered salt products, hindering their high-value utilization. In the processes of salt separation and crystallization and end-of-life treatment, traditional salt separation processes suffer from low salt separation precision and low product recovery rates. It is difficult to simultaneously produce multiple types of salt products that meet different application standards. At the same time, the process generates a large amount of wastewater and waste residue, making it impossible to form a closed-loop recycling system. This not only increases the cost and environmental pressure of end-of-life treatment but also poses a potential risk of pollutant transfer and emission, making it difficult to achieve stable large-scale industrial application. Summary of the Invention

[0004] The purpose of this invention is to overcome the shortcomings of existing technologies and provide a method for the resource utilization of mixed salts to prepare agricultural-grade potassium sulfate and industrial-grade sodium chloride. This method achieves efficient treatment and resource recovery of mixed salts with high organic matter content from multiple sources through five steps: multi-pathway organic matter oxidation and degradation, stepwise precipitation of calcium and magnesium for impurity removal, ion flotation purification, gradient crystallization for salt separation, and closed-loop recycling. The method employs two parallel organic matter degradation methods: organic matter oxidation and degradation and high-temperature calcination in a rotary kiln. It combines stepwise precipitation of sodium hydroxide for magnesium removal and sodium carbonate for calcium removal to remove impurities. Then, high-purity industrial-grade sodium chloride and agricultural-grade potassium sulfate are obtained through ion flotation and gradient crystallization, respectively. This method is suitable for the resource utilization of various mixed salts with high organic matter content.

[0005] To solve the above-mentioned technical problems, the present invention provides the following technical solution: a method for the resource utilization of mixed salts to prepare agricultural-grade potassium sulfate and industrial-grade sodium chloride, the specific steps of which are as follows: S100, Microwave Oxidation: Mix multi-source high-organic-content industrial mixed salt with water to prepare a 20% (w / w) mixed salt aqueous solution; organic matter degradation can be carried out using any of the following methods: The first method is organic matter oxidation degradation: add one of Fe3O4-MnO2 composite catalyst, sodium hypochlorite, and hydrogen peroxide to the solution, place it in an oxidation reactor, and obtain an oxidized salt solution; The second method is rotary kiln high-temperature calcination: directly feed the mixed salt dry material into a rotary kiln for high-temperature calcination oxidation, with the calcination temperature controlled at 300℃~800℃, and add water to dissolve after calcination to obtain an oxidized salt solution; S200, stepwise precipitation and removal of calcium and magnesium: Sodium hydroxide solution is added to the oxidized salt solution and stirred continuously to adjust the pH of the system to 10.5 and keep it stable to complete the precipitation and removal of magnesium ions and heavy metal ions; after the reaction, solid-liquid separation is performed by plate and frame filter press, sodium carbonate solution is added to the filtrate and stirred continuously to complete the deep precipitation and removal of calcium ions, after the reaction, solid-liquid separation is performed again by plate and frame filter press to remove the precipitated waste residue and obtain the pre-treated salt solution; S300, Ion flotation purification: Sodium dodecylbenzenesulfonate collector is added to the pre-treated salt solution, clean air is introduced into the bottom of the solution to form uniform bubbles, the floating scum is separated by a scum scraper, and the bottom clear liquid is discharged to obtain a refined salt solution. S400, Gradient Crystallization Salt Separation: The refined salt solution is placed in a vacuum environment and concentrated by vacuum flash evaporation at a controlled temperature of 90℃. Sodium chloride crystals are obtained by solid-liquid separation. After the mother liquor is cooled to 60℃, potassium chloride is added to carry out a metathesis reaction. After the reaction is completed, the temperature is lowered to 25℃ and kept at that temperature for standing. Potassium sulfate crystals are obtained by solid-liquid separation. S500, closed-loop recycling: The remaining mother liquor after crystallization separation is completely recycled to the organic matter oxidation and degradation step and used as makeup water for preparing mixed salt solution; the precipitated waste residue is collected and sent to the pyrolysis furnace for pyrolysis treatment to prepare biochar.

[0006] Furthermore, the multi-source high-organic-matter industrial mixed salt is any one or a mixture of two or more of the following: high-organic-matter mixed salt from the pesticide industry, high-organic-matter mixed salt from the pharmaceutical industry, and composite mixed salt of dyeing and printing sludge and desulfurization gypsum, wherein the mixing mass ratio of dyeing and printing sludge to desulfurization gypsum is 2:1.

[0007] Furthermore, the Fe3O4-MnO2 composite catalyst is added at 5% to 8% of the total dry mass of the mixed salt. The stirring speed during the preparation of the mixed salt aqueous solution is controlled at 120 r / min to 180 r / min, and the stirring time is 20 min to 30 min. During microwave irradiation, the stirring speed of the mixed solution is controlled at 80 r / min to 120 r / min.

[0008] Furthermore, after microwave irradiation, the Fe3O4-MnO2 composite catalyst can be recovered through solid-liquid separation, rinsed with clean water, and reused in the organic matter oxidation and degradation process, with a recycling rate of no less than 10 times.

[0009] Furthermore, the sodium hydroxide solution has a mass fraction of 25%–30%, and is added to the oxidized salt solution by continuous and uniform dropwise addition. The stirring speed during the dropwise addition process is controlled at 150 r / min–200 r / min. After the pH value of the system stabilizes at 10.5, the stirring reaction continues for 30 min–40 min. The sodium carbonate solution has a mass fraction of 20%, and the amount added is 1.05–1.2 times the molar amount of calcium ions in the filtrate. The stirring speed during the addition process is controlled at 150 r / min–200 r / min. After the addition is completed, the stirring reaction continues for 25 min–35 min.

[0010] Furthermore, the filtration working pressure of the plate and frame filter press is controlled at 0.6MPa to 0.8MPa, the turbidity of the preliminary treated salt solution obtained after filtration is not higher than 5NTU, the two solid-liquid separations are both carried out using a plate and frame filter press, and the moisture content of the obtained filter cake is not higher than 60%.

[0011] Furthermore, the added mass of the sodium dodecylbenzenesulfonate collector is 0.3% to 0.5% of the total mass of the pre-treated salt solution. After the collector is added, the mixture is stirred at a speed of 100 r / min to 150 r / min for 5 min to 10 min, and then the aeration device is turned on to introduce air. The gas-liquid volume ratio during the aeration process is controlled at 8:1 to 12:1, the diameter of the bubbles generated by the aeration is controlled at 0.1 mm to 0.5 mm, the total flotation time is 20 min to 30 min, and the slag scraping interval of the slag scraping device is 3 min to 5 min.

[0012] Furthermore, the vacuum degree of the vacuum flash evaporation at 90℃ is controlled to be -0.07MPa to -0.09MPa, the flash evaporation process continues until the supersaturation of sodium chloride in the solution reaches 1.1 to 1.2, the stirring speed of the crystallization process is controlled to be 60r / min to 100r / min, and the crystallization holding time is 40min to 60min.

[0013] Furthermore, the molar ratio of potassium chloride added at 60°C to sulfate ions in the mother liquor after sodium chloride separation is 1.2:1 to 1.5:1; the stirring speed during the metathesis reaction is controlled at 100 r / min to 150 r / min; the total reaction time is 60 min to 90 min; the cooling rate of the system from 60°C to 25°C is controlled at 5°C / h to 8°C / h; the holding time at 25°C is 120 min to 180 min; and the stirring speed during the crystallization process is controlled at 40 r / min to 60 r / min.

[0014] Furthermore, the remaining mother liquor has a 100% reflux rate, and the proportion of water that can be replaced is not less than 80%. Any remaining water used for preparation is supplemented with water. All the refluxed mother liquor is used for the preparation of mixed salt solution. The pyrolysis treatment is carried out in an air-isolated nitrogen inert atmosphere. The pyrolysis temperature is controlled at 400℃~600℃, the heating rate is controlled at 5℃ / min~10℃ / min, and the holding time after reaching the target temperature is 2h~3h. After the pyrolysis is completed, the biochar is naturally cooled, ground and sieved to obtain a particle size of 80 mesh~100 mesh.

[0015] Compared with existing technologies, this method of utilizing mixed salt resources to prepare agricultural potassium sulfate and industrial-grade sodium chloride has the following advantages: I. This invention achieves efficient degradation of organic pollutants and deep removal of impurities in industrial mixed salts with high organic content through a multi-pathway organic matter oxidation and degradation coupled with a multi-step graded impurity removal process. This solves the problems of incomplete organic matter removal and poor impurity separation leading to insufficient product purity in traditional treatments. A new high-temperature calcination pathway (adjustable wide temperature range of 300℃ to 800℃) is added, suitable for treating high-concentration, recalcitrant organic mixed salts, complementing microwave catalytic oxidation to improve process adaptability. A recyclable Fe3O4-MnO2 composite catalyst is used to complete the mineralization and decomposition of organic matter under mild conditions, reducing reagent and energy consumption. A stepwise precipitation process using sodium hydroxide-sodium carbonate and ion flotation is used to achieve graded removal of impurities, improving the purity of the refined salt solution and laying the foundation for subsequent precise salt separation. This invention is suitable for industrial mixed salts from multiple sources, achieving the harmless disposal of harmful components and the efficient enrichment of valuable components.

[0016] II. This invention achieves the directional separation and high-value preparation of sodium chloride and potassium sulfate in mixed salt through gradient crystallization and closed-loop process design. It can simultaneously produce industrial-grade sodium chloride and agricultural-grade potassium sulfate products that meet application standards, thereby enhancing the resource utilization value of industrial mixed salt. During the process, the selective precipitation and precise separation of different salts are achieved through stepwise temperature-controlled crystallization, avoiding the drawbacks of low product purity and poor separation efficiency in traditional salt separation processes. At the same time, all the remaining mother liquor after crystallization is recycled back to the front-end process for reuse, reducing water consumption and wastewater discharge in the process. The solid waste generated during the separation process is treated by pyrolysis to prepare usable biochar, maximizing the utilization of solid and liquid waste throughout the process and effectively reducing the risk of secondary pollution during the disposal process. This forms a mixed salt treatment system that combines harmless disposal with high-value utilization.

[0017] Other advantages, objectives and features of the invention will be set forth in part in the description which follows, and in part will be apparent to those skilled in the art from the following examination or study, or may be learned from the practice of the invention. Attached Figure Description

[0018] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the accompanying drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are merely some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without any creative effort.

[0019] Figure 1 A framework diagram for the resource utilization of mixed salts to prepare agricultural potassium sulfate and industrial sodium chloride; Figure 2 This is a flowchart illustrating the process of preparing agricultural-grade potassium sulfate and industrial-grade sodium chloride through the resource utilization of mixed salts. Detailed Implementation

[0020] To further illustrate the technical means and effects of the present invention in achieving its intended purpose, the following detailed description of the specific implementation methods, structures, features, and effects of the present invention, in conjunction with the accompanying drawings and preferred embodiments, is provided below.

[0021] Example 1: In this embodiment, the mixed salt containing high levels of organic matter from various sources is a pesticide industrial mixed salt sourced from the evaporation and crystallization section of a domestic pesticide production enterprise. The main components of the dry basis of the mixed salt are sodium chloride and sodium sulfate, and it also contains pesticide intermediates, phenols, and other recalcitrant organic pollutants, as well as trace amounts of lead, cadmium, and chromium heavy metal ions. This embodiment employs microwave catalytic oxidation and rotary kiln high-temperature calcination methods for the oxidative degradation of organic matter. Figure 1 As shown, the specific processing steps in this embodiment are as follows: S100, Oxidative Degradation of Organic Matter: Method 1: Add the above mixed salt to water and stir to prepare a 20% (w / w) mixed salt aqueous solution. Stir at 150 rpm for 25 minutes to ensure complete dissolution and homogeneous mixing. Add a Fe3O4-MnO2 composite catalyst to the mixed salt aqueous solution, with the catalyst mass being 6% of the total dry weight of the mixed salt. Place the catalyst-added solution in an organic matter oxidation degradation reactor and irradiate continuously at 800W for 30 minutes. During irradiation, maintain a stirring speed of 100 rpm to ensure sufficient contact between the solution and the catalyst, resulting in an oxidized salt solution. After microwave irradiation, perform gravity sedimentation separation. The composite catalyst at the bottom is rinsed with water and collected for reuse in the organic matter oxidation degradation process. Testing showed that after 10 reuses, the recovered composite catalyst maintained a degradation efficiency of over 95% of its initial performance, demonstrating stable catalytic performance.

[0022] Method 2: Take the above-mentioned mixed salt dry material and crush it to a particle size of 0.5mm-3mm. Then, feed it into a rotary kiln for high-temperature calcination and oxidation. The calcination temperature is controlled at 550℃, and the kiln rotation speed is controlled at 1 r / min. Air is introduced as the oxidizing atmosphere, and the material's residence time in the kiln is 60 min. After calcination, discharge the material and allow it to cool naturally to room temperature. Add water and stir to prepare a 20% (w / w) oxidized salt solution. The stirring speed is controlled at 150 r / min, and the stirring time is 25 min to ensure complete dissolution of the salt and uniform mixing of the solution after calcination.

[0023] S200, Stepwise Precipitation and Removal of Calcium and Magnesium: A 30% (w / w) sodium hydroxide solution was added to the oxidized salt solution. The sodium hydroxide solution was added continuously and at a uniform rate, with the stirring speed controlled at 180 r / min during the addition process. The pH value of the system was continuously monitored and adjusted to 10.5 and maintained stable. After the pH value of the system stabilized, the reaction was continued for 35 min to allow magnesium ions and heavy metal ions to fully form hydroxide precipitates and for suspended particulate matter to fully flocculate. After the reaction was completed, 0.02% anionic PAM flocculant was added, and the mixture was slowly stirred for 5 min. The mixed solution was then sent to a plate and frame filter press for the first solid-liquid separation. The filtration pressure of the plate and frame filter press was controlled at 0.7 MPa to remove magnesium slag and heavy metal precipitates, yielding a magnesium-removed salt solution. A 20% sodium carbonate solution was added to the magnesium-removed salt solution. The amount of sodium carbonate added was 1.1 times the molar amount of calcium ions in the filtrate. The stirring speed was controlled at 180 r / min during the addition process. After the addition was completed, the reaction was continued for 30 min to allow the calcium ions to fully precipitate as calcium carbonate. After the reaction was completed, the mixed solution was sent to a plate and frame filter press for a second solid-liquid separation to remove calcium residue and obtain a pre-treated salt solution. The turbidity of the pre-treated salt solution obtained after filtration was not higher than 5 NTU. Both solid-liquid separations were performed using a plate and frame filter press, and the moisture content of the filter cake obtained was not higher than 60%.

[0024] S300, Ion Flotation Refining: Sodium dodecylbenzenesulfonate collector is added to the pre-treated salt solution at a mass of 0.4% of the total mass of the pre-treated salt solution. After the collector is added, the solution is stirred at 120 rpm for 8 minutes to ensure thorough mixing and contact between the collector and the impurities in the solution. After stirring, the aeration device is turned on to introduce clean air into the bottom of the solution to form uniform bubbles. The gas-liquid volume ratio during aeration is controlled at 10:1, and the diameter of the bubbles generated is controlled at 0.3 mm. The total flotation time is 25 minutes. During the flotation process, the floating scum is separated at 3-minute intervals using a scraper. After flotation, the bottom clear liquid is discharged to obtain the refined salt solution.

[0025] S400 Gradient Crystallization Salt Separation: The refined salt solution is placed in a vacuum environment, and vacuum flash evaporation concentration is performed at a controlled temperature of 90℃, with the vacuum degree controlled at -0.08MPa. The flash evaporation process continues until the supersaturation of sodium chloride in the solution reaches 1.15. The stirring speed during the crystallization process is controlled at 80 r / min. After reaching supersaturation, stirring is continued at this temperature for 50 min to allow the sodium chloride crystals to grow fully. After crystallization, solid-liquid separation is performed to obtain sodium chloride crystals and the mother liquor after sodium chloride separation. The prepared sodium chloride crystals meet the relevant standards for industrial-grade sodium chloride. The mother liquor after sodium chloride separation is kept at 60℃, and potassium chloride is added to the mother liquor at a molar ratio of 1.3:1 to sulfate ions in the mother liquor. Stirring is continued during the addition process at a stirring speed controlled at 120 r / min. After the addition is completed, stirring is continued for 75 min to complete the metathesis reaction. After the reaction was complete, the cooling rate of the system was controlled at 6℃ / h, and the system was cooled from 60℃ to 25℃. During the cooling process, the stirring speed was controlled at 50 r / min. After reaching 25℃, the system was kept at this temperature and allowed to stand for 150 min to allow potassium sulfate crystals to grow and precipitate fully. After the temperature was maintained, solid-liquid separation was performed to obtain potassium sulfate crystals. The prepared potassium sulfate crystals met the relevant standards for agricultural potassium sulfate.

[0026] S500, Closed-Loop Recycling: The remaining mother liquor after potassium sulfate crystallization is completely recycled to the organic matter oxidation and degradation step, and used as makeup water for preparing mixed salt solutions. In this embodiment, the recycled mother liquor can replace 85% of the clean water usage. The remaining water for preparation is supplemented with clean water. All recycled mother liquor is used for the preparation of mixed salt solutions, and there is no wastewater discharge. The precipitated waste residue generated from the calcium and magnesium stepwise precipitation and impurity removal step is collected and sent to a pyrolysis furnace for pyrolysis treatment. The pyrolysis treatment is carried out in an air-isolated nitrogen inert atmosphere. The pyrolysis temperature is controlled at 500℃, the heating rate is controlled at 8℃ / min, and the holding time after reaching the target temperature is 2.5h. After pyrolysis, it is naturally cooled, ground and sieved to obtain biochar with a particle size of 90 mesh. The prepared biochar can be used in wastewater treatment adsorption, soil improvement and other fields.

[0027] After the entire process was completed, the removal rate of organic pollutants in the mixed salt reached 98.2%, the removal rate of heavy metal ions reached 99.1%, the recovery rate of sodium chloride reached 86.3%, and the recovery rate of potassium sulfate reached 82.7%, thus achieving efficient and harmless treatment and resource recovery of the mixed salt.

[0028] Example 2: In this embodiment, the multi-source high-organic-content industrial mixed salt to be treated is a high-organic-content mixed salt from the pharmaceutical industry. It was taken from the wastewater treatment evaporation section of a domestic antibiotic pharmaceutical company. The main components of the dry basis of the mixed salt are sodium chloride and sodium sulfate, and it also contains fermentation residue organic matter, antibiotic bacterial residue degradation products, and trace amounts of copper, zinc, and nickel heavy metal ions. Figure 2 As shown, the specific processing steps in this embodiment are as follows: S100, Organic Matter Oxidation and Degradation: The above mixed salt solution was mixed with water to prepare a 20% (w / w) aqueous solution. The stirring speed was controlled at 120 r / min for 30 min to ensure complete dissolution and homogeneous mixing of the mixed salt. A Fe3O4-MnO2 composite catalyst was added to the mixed salt aqueous solution at a mass of 5% of the total dry mass of the mixed salt. The mixed solution after catalyst addition was placed in an organic matter oxidation and degradation reactor. Microwave output power was controlled at 800 W for continuous irradiation for 30 min. During irradiation, the stirring speed of the mixed solution was controlled at 80 r / min to ensure sufficient contact between the solution and the catalyst, resulting in an oxidized salt solution. After microwave irradiation, the oxidized salt solution was subjected to gravity sedimentation. The composite catalyst at the bottom was rinsed with water and collected for reuse in the organic matter oxidation and degradation process. Testing showed that after 10 reuses, the recovered composite catalyst maintained a degradation efficiency of over 94% of its initial performance, indicating stable catalytic performance.

[0029] S200, Stepwise Precipitation and Removal of Calcium and Magnesium: A 25% (w / w) sodium hydroxide solution was added to the oxidized salt solution. The sodium hydroxide solution was added continuously and at a uniform rate, with the stirring speed controlled at 150 r / min during the addition process. The pH value of the system was continuously monitored and adjusted to 10.5 and maintained stable. After the pH value of the system stabilized, the reaction was continued for 40 min to allow magnesium ions and heavy metal ions to fully form hydroxide precipitates and for suspended particulate matter to fully flocculate. After the reaction was completed, 0.01% anionic PAM flocculant was added, and the mixture was stirred slowly for 5 min. The mixed solution was then sent to a plate and frame filter press for the first solid-liquid separation. The filtration pressure of the plate and frame filter press was controlled at 0.6 MPa to remove magnesium slag and heavy metal precipitates, yielding a magnesium-removed salt solution. A 20% sodium carbonate solution was added to the magnesium-removed salt solution. The amount of sodium carbonate added was 1.05 times the molar amount of calcium ions in the filtrate. The stirring speed was controlled at 150 r / min during the addition process. After the addition was completed, the reaction was continued for 35 min to allow the calcium ions to fully form calcium carbonate precipitate. After the reaction was completed, the mixed solution was sent to a plate and frame filter press for a second solid-liquid separation to remove calcium residue and obtain a pre-treated salt solution. The turbidity of the pre-treated salt solution obtained after filtration was not higher than 5 NTU, and the moisture content of the separated filter cake was not higher than 60%, which met the requirements of subsequent processing steps.

[0030] S300, Ion Flotation Refining: Sodium dodecylbenzenesulfonate collector is added to the pretreatment salt solution at a mass of 0.3% of the total mass of the pretreatment salt solution. After the collector is added, the solution is stirred at 100 rpm for 10 minutes to ensure thorough mixing and contact between the collector and the impurities in the solution. After stirring, the aeration device is turned on to introduce clean air into the bottom of the solution to form uniform bubbles. The gas-liquid volume ratio during aeration is controlled at 8:1, and the diameter of the bubbles generated is controlled at 0.1 mm. The total flotation time is 30 minutes. During the flotation process, the floating scum is separated at 4-minute intervals using a scraper. After flotation, the bottom clear liquid is discharged to obtain the refined salt solution.

[0031] S400 Gradient Crystallization Salt Separation: The refined salt solution is placed in a vacuum environment, and vacuum flash evaporation concentration is performed at a controlled temperature of 90℃, with the vacuum degree controlled at -0.07MPa. The flash evaporation process continues until the supersaturation of sodium chloride in the solution reaches 1.1. The stirring speed during the crystallization process is controlled at 60r / min. After reaching supersaturation, stirring is continued at this temperature for 60min to allow the sodium chloride crystals to grow fully. After crystallization, solid-liquid separation is performed to obtain sodium chloride crystals and the mother liquor after sodium chloride separation. The prepared sodium chloride crystals meet the relevant standards for industrial-grade sodium chloride. The mother liquor after sodium chloride separation is kept at 60℃, and potassium chloride is added to the mother liquor at a molar ratio of 1.2:1 to sulfate ions in the mother liquor. Stirring is continued during the addition process at a stirring speed controlled at 100r / min. After the addition is completed, stirring is continued for 90min to complete the metathesis reaction. After the reaction was complete, the cooling rate of the system was controlled at 5℃ / h, and the system was cooled from 60℃ to 25℃. During the cooling process, the stirring speed was controlled at 40 r / min. After reaching 25℃, the system was kept at this temperature and allowed to stand for 180 min to allow potassium sulfate crystals to grow and precipitate fully. After the temperature was maintained, solid-liquid separation was performed to obtain potassium sulfate crystals. The prepared potassium sulfate crystals met the relevant standards for agricultural potassium sulfate.

[0032] S500, Closed-Loop Recycling: The remaining mother liquor after potassium sulfate crystallization is entirely recycled to the organic matter oxidation and degradation step, used as makeup water for preparing mixed salt solutions. In this embodiment, the recycled mother liquor can replace 82% of the clean water usage. Any remaining water used for preparation is supplemented with clean water. All recycled mother liquor is used for the preparation of mixed salt solutions, with no wastewater discharged. The precipitated waste residue generated from the calcium and magnesium stepwise precipitation and impurity removal step is collected and sent to a pyrolysis furnace for pyrolysis treatment. The pyrolysis treatment is carried out in an air-isolated nitrogen inert atmosphere, with the pyrolysis temperature controlled at 400℃ and the heating rate controlled at 5℃ / min. The holding time after reaching the target temperature is 3 hours. After pyrolysis, the biochar is naturally cooled, ground, and sieved to obtain biochar with a particle size of 80 mesh. The prepared biochar can be used in wastewater treatment adsorption, soil improvement, and other fields.

[0033] After the entire process was completed, the removal rate of organic pollutants in the mixed salt reached 97.8%, the removal rate of heavy metal ions reached 98.9%, the recovery rate of sodium chloride reached 85.7%, and the recovery rate of potassium sulfate reached 82.1%, thus achieving efficient and harmless treatment and resource recovery of the mixed salt.

[0034] Example 3: In this embodiment, the multi-source high-organic-content industrial mixed salt to be treated is a composite mixed salt of dyeing and printing sludge and desulfurization gypsum. The dyeing and printing sludge is taken from the sludge dewatering section of a large domestic dyeing and printing enterprise, and the desulfurization gypsum is taken from the flue gas desulfurization section of a domestic coal-fired power plant. The mixing mass ratio of dyeing and printing sludge to desulfurization gypsum is 2:1. The mixed salt is used after drying and pulverizing. The main components on a dry basis are sodium chloride, sodium sulfate, and calcium sulfate, and it also contains dyeing and printing auxiliaries, residual organic pollutants from dyes, and trace amounts of chromium and mercury heavy metal ions. The specific processing steps of this embodiment are as follows.

[0035] S100, Organic Matter Oxidation and Degradation: The above-mentioned mixed salt solution was added to water and stirred to prepare a 20% (w / w) mixed salt aqueous solution. The stirring speed was controlled at 180 r / min for 20 min to ensure complete dissolution of the soluble salts and uniform mixing. A Fe3O4-MnO2 composite catalyst was added to the mixed salt aqueous solution at a mass of 8% of the total dry weight of the mixed salt. The mixed solution after catalyst addition was placed in an organic matter oxidation and degradation reactor. Microwave output power was controlled at 800 W for continuous irradiation for 30 min. During irradiation, the stirring speed of the mixed solution was controlled at 120 r / min to ensure sufficient contact between the solution and the catalyst, resulting in an oxidized salt solution. After microwave irradiation, the oxidized salt solution was subjected to gravity sedimentation. The composite catalyst at the bottom was rinsed with water and collected for reuse in the organic matter oxidation and degradation process. Testing showed that after 10 reuses, the recovered composite catalyst maintained a degradation efficiency of over 96% of its initial performance, indicating stable catalytic performance.

[0036] S200, Stepwise Precipitation and Removal of Calcium and Magnesium: A 30% (w / w) sodium hydroxide solution was added to the oxidized salt solution. The sodium hydroxide solution was added continuously and at a uniform rate, with the stirring speed controlled at 200 r / min. The pH value of the system was continuously monitored and adjusted to 11.0 and maintained stable. After the pH value stabilized, the reaction was continued for 30 min to allow magnesium ions and heavy metal ions to fully form hydroxide precipitates and for suspended particulate matter to fully flocculate. After the reaction was completed, 0.03% anionic PAM flocculant was added, and the mixture was slowly stirred for 5 min. The mixed solution was then sent to a plate and frame filter press for the first solid-liquid separation. The filtration pressure of the plate and frame filter press was controlled at 0.8 MPa to remove magnesium slag and heavy metal precipitates, yielding a magnesium-removed salt solution. A 20% sodium carbonate solution was added to the magnesium-removed salt solution. The amount of sodium carbonate added was 1.2 times the molar amount of calcium ions in the filtrate. The stirring speed was controlled at 200 r / min during the addition process. After the addition was completed, the reaction was continued for 25 min to allow the calcium ions to fully form calcium carbonate precipitate. After the reaction was completed, the mixed solution was sent to a plate and frame filter press for a second solid-liquid separation to remove calcium residue and obtain a pre-treated salt solution. The turbidity of the pre-treated salt solution obtained after filtration was not higher than 5 NTU, and the moisture content of the separated filter cake was not higher than 60%, which met the requirements of subsequent processing steps.

[0037] S300, Ion Flotation Refining: Sodium dodecylbenzenesulfonate collector is added to the pre-treated salt solution at a mass of 0.5% of the total mass of the pre-treated salt solution. After the collector is added, the solution is stirred at 150 rpm for 5 minutes to ensure thorough mixing and contact between the collector and the impurities in the solution. After stirring, the aeration device is turned on to introduce clean air into the bottom of the solution to form uniform bubbles. The gas-liquid volume ratio during aeration is controlled at 12:1, and the diameter of the bubbles generated is controlled at 0.5 mm. The total flotation time is 20 minutes. During the flotation process, the floating scum is separated at 5-minute intervals using a scraper. After flotation, the bottom clear liquid is discharged to obtain the refined salt solution.

[0038] S400 Gradient Crystallization Salt Separation: The refined salt solution is placed in a vacuum environment, and vacuum flash evaporation is performed at a controlled temperature of 90℃ with a vacuum degree controlled at -0.09MPa. The flash evaporation process continues until the supersaturation of sodium chloride in the solution reaches 1.2. The stirring speed during the crystallization process is controlled at 100r / min. After reaching supersaturation, stirring is continued at this temperature for 40min to allow the sodium chloride crystals to grow fully. After crystallization, solid-liquid separation is performed to obtain sodium chloride crystals and the mother liquor after sodium chloride separation. The prepared sodium chloride crystals meet the relevant standards for industrial-grade sodium chloride. The mother liquor after sodium chloride separation is kept at 60℃, and potassium chloride is added to the mother liquor at a molar ratio of 1.5:1 to sulfate ions in the mother liquor. Stirring is continued during the addition process at a stirring speed controlled at 150r / min. After the addition is completed, stirring is continued for 60min to complete the metathesis reaction. After the reaction was complete, the cooling rate of the system was controlled at 8℃ / h, and the system was cooled from 60℃ to 25℃. During the cooling process, the stirring speed was controlled at 60 r / min. After reaching 25℃, the system was kept at this temperature and allowed to stand for 120 min to allow potassium sulfate crystals to grow and precipitate fully. After the temperature was maintained, solid-liquid separation was performed to obtain potassium sulfate crystals. The prepared potassium sulfate crystals met the relevant standards for agricultural potassium sulfate.

[0039] S500, Closed-Loop Recycling: The remaining mother liquor after potassium sulfate crystallization is completely recycled to the organic matter oxidation and degradation step, and used as makeup water for preparing mixed salt solutions. In this embodiment, the recycled mother liquor can replace 88% of the clean water usage. The remaining water for preparation is supplemented with clean water. All recycled mother liquor is used for the preparation of mixed salt solutions, and there is no wastewater discharge. The precipitated waste residue generated from the calcium and magnesium stepwise precipitation and impurity removal step is collected and sent to a pyrolysis furnace for pyrolysis treatment. The pyrolysis treatment is carried out in an air-isolated nitrogen inert atmosphere. The pyrolysis temperature is controlled at 600℃, the heating rate is controlled at 10℃ / min, and the holding time after reaching the target temperature is 2 hours. After pyrolysis, it is naturally cooled, ground and sieved to obtain biochar with a particle size of 100 mesh. The prepared biochar can be used in wastewater treatment adsorption, soil improvement and other fields.

[0040] After the entire process was completed, the removal rate of organic pollutants in the mixed salt reached 98.5%, the removal rate of heavy metal ions reached 99.3%, the recovery rate of sodium chloride reached 86.8%, and the recovery rate of potassium sulfate reached 83.2%, thus achieving efficient and harmless treatment and resource recovery of the mixed salt.

[0041] Comparative example: The mixed salt to be treated in this comparative example is exactly the same as that in Example 1. It is a mixed salt with high organic matter content in the pesticide industry from the same pesticide manufacturer. The conventional industrial mixed salt treatment process in this field is adopted, and the specific treatment steps are as follows.

[0042] Step 1: Fenton oxidation treatment: Add the mixed salt to water and stir to prepare a 20% (w / w) mixed salt aqueous solution. Stir at 150 rpm for 25 minutes to ensure complete dissolution of the mixed salt. Add ferrous sulfate and hydrogen peroxide to the solution to adjust the pH to 3.5. Continue stirring for 120 minutes to complete the oxidation treatment and obtain the oxidized solution.

[0043] Step 2: Lime milk precipitation and impurity removal: Add 12% lime milk by mass to the oxidized solution, adjust the pH of the system to 10, and stir continuously for 35 minutes. After the reaction is completed, use a plate and frame filter press for solid-liquid separation. The filtration working pressure is controlled at 0.7 MPa to obtain the pre-treated solution.

[0044] Step 3, Sand Filtration Refining: The pre-treated solution is sent to a quartz sand filter tank for filtration and refining to remove residual suspended particles and obtain a refined solution.

[0045] Step 4, Evaporation and Crystallization: The refined solution is sent to a single-effect evaporator for evaporation and concentration. After the total solid content of the solution reaches 50%, it is cooled to room temperature for crystallization. Solid-liquid separation yields mixed salt crystals, but effective separation of sodium chloride and potassium sulfate cannot be achieved.

[0046] Step 5, Subsequent Disposal: Part of the mother liquor after crystallization separation is reused in the front-end water distribution, and the remainder is sent to the wastewater treatment plant for treatment before being discharged. The precipitated waste residue is directly sent to the landfill for disposal after dewatering, without any resource utilization process.

[0047] After the entire process, the removal rate of organic pollutants in the mixed salt in this comparative example was 64.7%, and the removal rate of heavy metal ions was 76.3%. The mixed salt prepared could not be effectively separated, and the purity of the product could not meet the relevant standard requirements for industrial-grade sodium chloride and agricultural-grade potassium sulfate. The fresh water consumption increased by 72% compared with Example 1, and the solid waste landfill volume increased by 100% compared with Example 1. At the same time, there was wastewater discharge. The treatment effect and resource recovery efficiency were significantly lower than those of the present invention.

[0048] To more clearly compare the core processing effects, product quality, and operational indicators of each embodiment and comparative example, the key testing data after the entire process are summarized in the table below: As can be seen from the data in the table above, the three embodiments of this invention achieve highly efficient removal of organic pollutants and heavy metal ions for different types of multi-source high-organic-content industrial mixed salts. The prepared sodium chloride and potassium sulfate products meet the corresponding standard requirements. Simultaneously, the invention achieves full recycling of the mother liquor and resource utilization of waste residue, with no wastewater discharge or solid waste landfill. The environmental and economic advantages of the process are significant. Compared with the comparative example using traditional treatment processes, the present invention shows improved treatment effect, resource recovery efficiency, and environmental friendliness. The scheme is stable and feasible, with smooth transitions between steps, enabling large-scale industrial application. This fully demonstrates the advanced nature and practicality of the technical solution of this invention.

[0049] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any way. Although the present invention has been disclosed above with reference to preferred embodiments, it is not intended to limit the present invention. Any person skilled in the art can make some modifications or alterations to the above-disclosed technical content to create equivalent embodiments without departing from the scope of the present invention. Any simple modifications, equivalent changes and alterations made to the above embodiments based on the technical essence of the present invention without departing from the scope of the present invention shall still fall within the scope of the present invention.

Claims

1. A method for the resource utilization of mixed salts to prepare agricultural-grade potassium sulfate and industrial-grade sodium chloride, characterized in that, The specific steps of this method are as follows: S100, Organic matter oxidative degradation: Mix multi-source high-organic-matter industrial mixed salt with water to prepare a mixed salt aqueous solution; organic matter degradation can be carried out using any of the following methods: The first method is organic matter oxidative degradation: add one of Fe3O4-MnO2 composite catalyst, sodium hypochlorite, and hydrogen peroxide to the solution, place it in an oxidation reactor, and obtain an oxidized salt solution; the second method is rotary kiln high-temperature calcination: directly feed the mixed salt dry material into a rotary kiln for high-temperature calcination and oxidation, and after calcination, add water to dissolve it to obtain an oxidized salt solution; S200, stepwise precipitation and removal of calcium and magnesium ions: Alkali solution is added to the oxidized salt solution and stirred continuously to adjust the pH value of the system to 10.5-11.0 and keep it stable to complete the precipitation and removal of calcium, magnesium and heavy metal ions; after the reaction, solid-liquid separation is performed, carbonate solution is added to the filtrate and stirred continuously to complete the deep removal of calcium and magnesium ions, and after the reaction, solid-liquid separation is performed by plate and frame filter press to filter out the precipitated waste residue and obtain the pre-treated salt solution; S300, Ion flotation purification: Sodium dodecylbenzenesulfonate collector is added to the pre-treated salt solution, clean air is introduced into the bottom of the solution to form uniform bubbles, the floating scum is separated by a scum scraper, and the bottom clear liquid is discharged to obtain a refined salt solution. S400, Gradient Crystallization Salt Separation: The refined salt solution is placed in a vacuum environment and concentrated by vacuum flash evaporation at a controlled temperature of 90℃. Sodium chloride crystals are obtained by solid-liquid separation. After the mother liquor is cooled to 60℃, potassium chloride is added to carry out a metathesis reaction. After the reaction is completed, the temperature is lowered to 25℃ and kept at that temperature for standing. Potassium sulfate crystals are obtained by solid-liquid separation. S500, closed-loop recycling: all the remaining mother liquor after crystallization and separation is recycled to the organic matter oxidation and degradation step and used as makeup water for preparing mixed salt solution; The collected precipitated waste residue is sent to a pyrolysis furnace for pyrolysis treatment to prepare biochar.

2. The method for preparing agricultural-grade potassium sulfate and industrial-grade sodium chloride through mixed salt resource utilization according to claim 1, characterized in that, In step S100, the multi-source high organic matter industrial mixed salt is any one of the following: high organic matter mixed salt for pesticide industry, high organic matter mixed salt for pharmaceutical industry, and composite mixed salt of dyeing and printing sludge and desulfurization gypsum, wherein the mixing mass ratio of dyeing and printing sludge to desulfurization gypsum is 2:

1.

3. The method for preparing agricultural-grade potassium sulfate and industrial-grade sodium chloride through mixed salt resource utilization according to claim 1, characterized in that, In step S100, the added mass of the Fe3O4-MnO2 composite catalyst is 5% to 8% of the total mass of the mixed salt dry base. The stirring speed during the preparation of the mixed salt aqueous solution is controlled at 120 r / min to 180 r / min, and the stirring time is 20 min to 30 min. During microwave irradiation, the stirring speed of the mixed solution is controlled at 80 r / min to 120 r / min. In the rotary kiln high-temperature calcination method, the feed particle size of the mixed salt dry material is controlled at 0.5 mm to 5 mm. Air is introduced into the rotary kiln as an oxidizing atmosphere. The kiln rotation speed is controlled at 0.5 r / min to 2 r / min. The calcination temperature is controlled at 300℃ to 800℃. The residence time of the material in the kiln is 30 min to 120 min. After calcination, the material is discharged and cooled to room temperature, and then water is added to prepare an oxidized salt solution with a mass fraction of 20%.

4. The method for preparing agricultural-grade potassium sulfate and industrial-grade sodium chloride through mixed salt resource utilization according to claim 1, characterized in that, In step S100, after microwave irradiation, the Fe3O4-MnO2 composite catalyst is recovered through solid-liquid separation, rinsed with water, and can be repeatedly used in the organic matter oxidation and degradation process, with a recycling number of no less than 10 times.

5. The method for preparing agricultural-grade potassium sulfate and industrial-grade sodium chloride through mixed salt resource utilization according to claim 1, characterized in that, In step S200, the sodium hydroxide solution has a mass fraction of 25%–30% and is added to the oxidized salt solution by continuous and uniform dropwise addition. The stirring speed during the dropwise addition process is controlled at 150 r / min–200 r / min. After the pH value of the system stabilizes at 10.5–11.0, the stirring reaction continues for 30 min–40 min. The sodium carbonate solution has a mass fraction of 20% and is added at an amount that is 1.05–1.2 times the molar amount of calcium ions in the filtrate. The stirring speed during the addition process is controlled at 150 r / min–200 r / min. After the addition is completed, the stirring reaction continues for 25 min–35 min.

6. The method for preparing agricultural-grade potassium sulfate and industrial-grade sodium chloride through mixed salt resource utilization according to claim 1, characterized in that, In step S200, the filtration working pressure of the plate and frame filter press is controlled at 0.6MPa to 0.8MPa, the turbidity of the preliminary treated salt solution obtained after filtration is not higher than 5NTU, the plate and frame filter press is used for both solid-liquid separations, and the moisture content of the obtained filter cake is not higher than 60%.

7. The method for preparing agricultural-grade potassium sulfate and industrial-grade sodium chloride through mixed salt resource utilization according to claim 1, characterized in that, In step S300, the added mass of the sodium dodecylbenzenesulfonate collector is 0.3% to 0.5% of the total mass of the pre-treated salt solution. After the collector is added, the mixture is stirred at a speed of 100 r / min to 150 r / min for 5 min to 10 min. Then, the aeration device is turned on to introduce air. The gas-liquid volume ratio during the aeration process is controlled at 8:1 to 12:1, the diameter of the bubbles generated by the aeration is controlled at 0.1 mm to 0.5 mm, the total flotation time is 20 min to 30 min, and the slag scraping interval of the slag scraping device is 3 min to 5 min.

8. The method for preparing agricultural-grade potassium sulfate and industrial-grade sodium chloride through mixed salt resource utilization according to claim 1, characterized in that, In step S400, the vacuum degree of the vacuum flash evaporation at 90°C is controlled to be -0.07MPa to -0.09MPa, the flash evaporation process continues until the supersaturation of sodium chloride in the solution reaches 1.1 to 1.2, the stirring speed of the crystallization process is controlled to be 60r / min to 100r / min, and the crystallization holding time is 40min to 60min.

9. The method for preparing agricultural-grade potassium sulfate and industrial-grade sodium chloride through mixed salt resource utilization according to claim 1, characterized in that, In step S400, the molar ratio of potassium chloride added at 60°C to sulfate in the mother liquor after sodium chloride separation is 1.2:1 to 1.5:

1. The stirring speed during the metathesis reaction is controlled at 100 r / min to 150 r / min, the total reaction time is 60 min to 90 min, the cooling rate of the system from 60°C to 25°C is controlled at 5°C / h to 8°C / h, the holding time at 25°C is 120 min to 180 min, and the stirring speed during the crystallization process is controlled at 40 r / min to 60 r / min.

10. The method for preparing agricultural-grade potassium sulfate and industrial-grade sodium chloride through mixed salt resource utilization according to claim 1, characterized in that, In step S500, the remaining mother liquor reflux rate is 100%, and all the refluxed mother liquor is used for the preparation of mixed salt aqueous solution. The pyrolysis treatment is carried out in an air-isolated nitrogen inert atmosphere. The pyrolysis temperature is controlled at 400℃~600℃, the heating rate is controlled at 5℃ / min~10℃ / min, and the holding time after reaching the target temperature is 2h~3h. After the pyrolysis is completed, the biochar is naturally cooled, ground and sieved to obtain a particle size of 80 mesh~100 mesh.

Images