Waste salt resourceful treatment process

Abstract

The invention relates to a waste salt resourceful treatment process, which is characterized in that the content of divalent sulfate radicals is reduced by freezing; the frozen mixed waste brine is efficiently separated into two material flows, namely light-side produced water mainly containing sodium chloride and potassium chloride and concentrated-side concentrated water mainly containing sodium sulfate, sodium chloride and potassium chloride; then, the two material flows are independently fed into a subsequent deep purification process and an evaporative crystallization system in a coupling manner respectively, and finally high-purity sodium chloride, potassium chloride and sodium sulfate products are produced. According to the method, the potassium salt and the sodium salt are separated through freezing denitration, nanofiltration pre-separation and evaporation thermal crystallization, the problem that the mixed salt is difficult to separate is fundamentally solved, the technological process is simple, the product purity is high, and efficient and high-valued resource recovery of the waste salt is achieved.

Description

Technical Field

[0001] This invention belongs to the field of waste salt treatment technology, and specifically relates to a waste salt resource utilization process. Background Technology

[0002] Waste salt mainly originates from chemical reactions, neutralization crystallization, and mother liquor drying processes. Its composition is complex, containing high concentrations of inorganic salts (such as NaCl, KCl, Na₂SO₄, K₂SO₄, etc.) and toxic and harmful organic pollutants (such as benzene compounds, halogenated hydrocarbons, heterocyclic compounds, pesticide intermediates, etc.). It is listed in the "National Hazardous Waste List" (categories HW02, HW04, HW11, HW12, etc.) and is toxic and reactive. Direct landfilling or stockpiling will cause soil salinization, groundwater pollution, and long-term environmental risks from organic matter. Pyrolysis waste salt refers to the process of converting organic matter into small-molecule gases (pyrolysis gas), liquid oils, and solid carbon residues through chemical reactions such as bond breaking, decomposition, and condensation under anaerobic or hypoxic conditions by heating (usually 350~600℃). The hypoxic environment avoids the risk of dioxins from the complete combustion of organic matter. Relatively moderate temperatures (below the melting temperature) can effectively decompose organic matter while ensuring that the physical structure of inorganic salts remains essentially unchanged. Organic matter is converted into recyclable fuel gases (such as H2, CH4, CO), thus achieving energy recovery.

[0003] Currently, due to the complex composition of pyrolysis waste salt, which contains NaCl, KCl, Na2SO4, and K2SO4, even if organic matter is perfectly removed through pyrolysis, the product obtained is still a mixed salt. The disposal of such mixed salt is very narrow, and it is usually classified as a "byproduct that cannot be utilized at a high value". In many cases, it can only be landfilled or disposed of as general solid waste, which seriously affects the economics and feasibility of the entire resource utilization process. Summary of the Invention

[0004] Therefore, the technical problem solved by this invention is that the waste salt after traditional pyrolysis has a complex inorganic salt composition, so a waste salt resource utilization treatment process is proposed, which is a waste fly ash washing liquid wastewater treatment process with high treatment cost and high automation efficiency.

[0005] To solve the above-mentioned technical problems, the technical solution of the present invention is as follows: This invention provides a waste salt resource utilization process, the process comprising the following steps: S1. Dissolve the waste salt after pyrolysis in water; S2. Waste Salt Dissolution Solution Removal: After the waste salt is dissolved in water, the main water-soluble inorganic salts such as sodium chloride, potassium chloride, and sodium sulfate will dissolve in the water, while other water-insoluble inorganic salt impurities such as heavy metals and calcium sulfate will precipitate in the dissolution tank and need to be removed by chemical reaction. The first step uses a "chemical reaction plus precipitation" process, by adding lime, polyferric sulfate, and PAM to generate calcium fluoride and fluorapatite. After precipitation separation, the supernatant is cleaned of fluoride and phosphorus ions. The second step uses a "chemical reaction plus ultrafiltration membrane filtration" process, by adding soda ash and liquid alkali to generate calcium carbonate, magnesium ammonium phosphate, and heavy metal hydroxide flocs. After ultrafiltration membrane filtration, the turbidity is removed. S3, Primary nanofiltration desalination: The waste salt solution after impurity removal is separated by nanofiltration membrane to obtain divalent salts, resulting in fresh water with sodium chloride and potassium chloride as the main components, which is stored in nanofiltration fresh water storage tank, as well as concentrated water containing sodium chloride, potassium chloride and high concentration of sodium sulfate, which is stored in nanofiltration concentrated water storage tank. S4, the freeze crystallization and nitrification system: the concentrated water stored in the nanofiltration concentrate tank enters the freeze crystallizer, and the concentrated water is cooled by heat exchange with the cryo-liquid. Sodium sulfate is basically crystallized out in the form of sodium sulfate decahydrate (sodium sulfate). The remaining sodium chloride, potassium chloride and the freeze mother liquor containing a small amount of sodium sulfate are also transported to the mixed solution tank. S5. Sodium sulfate recrystallization: Sodium sulfate obtained from the freeze crystallization precipitation system is crystallized in the form of sodium sulfate. By heating the sodium sulfate, it is dissolved in its own water of crystallization. Sodium sulfate industrial salt is obtained by centrifugation. The mother liquor after centrifugation is then put into the evaporation crystallization system to evaporate the water in the mother liquor and the sodium sulfate is evaporated and crystallized.

[0006] S6. The evaporation crystallization unit coupled with the flash cooling crystallization unit separates salts. Fresh water stored in the nanofiltration fresh water storage tank enters the evaporation crystallization unit. Based on the sodium chloride and potassium chloride phase diagram data, sodium chloride is first evaporated and crystallized to obtain sodium chloride product. After potassium chloride is enriched in the concentrate to saturation, it is transported to the flash cooling crystallization unit. By cooling down, the solubility of potassium chloride decreases, and the saturated potassium chloride is cooled and crystallized to obtain potassium chloride product. S7. Secondary nanofiltration desalination: The potassium-precipitated mother liquor obtained from the evaporation crystallization unit coupled with the flash cooling crystallization unit, the frozen mother liquor obtained from the freeze crystallization of nitrate, and the potassium chloride and potassium chloride enrichment mother liquor obtained from the recrystallization of Glauber's salt are all fed into the secondary nanofiltration membrane. The divalent salts are then separated through the secondary nanofiltration membrane to obtain fresh water with sodium chloride and potassium chloride as the main components, which is stored in the nanofiltration fresh water storage tank. Concentrated water containing sodium chloride, potassium chloride, and high-concentration sodium sulfate is stored in the nanofiltration concentrated water storage tank. Sodium sulfate is enriched on the concentrated water side to a concentration of 7%, while the sodium sulfate concentration on the fresh water side is reduced to 0.2%, achieving the purpose of recycling.

[0007] Preferably, the waste salt resource utilization process includes: a salt dissolving system, an impurity removal unit, a primary nanofiltration salt separation unit, a forced circulation evaporative thermal crystallization plus cooling crystallization salt separation unit, a freeze crystallization nitrate precipitation unit, a mirabilite recrystallization unit, and a secondary nanofiltration salt separation unit. The salt dissolving system is connected to the impurity removal unit. The primary nanofiltration unit is connected to both the forced circulation evaporative thermal crystallization plus cooling crystallization salt separation unit and the freeze crystallization nitrate precipitation unit. The freeze crystallization nitrate precipitation unit is connected to the mirabilite recrystallization unit. The secondary nanofiltration salt separation unit is connected to the forced circulation evaporative thermal crystallization plus cooling crystallization salt separation unit, the freeze crystallization nitrate precipitation unit, and the mirabilite recrystallization unit.

[0008] Preferably, in the salt dissolving system, most of the water used to dissolve the waste salt is condensate from the downstream evaporation system, with a small amount of dissolved water added to redissolve the pyrolysis waste salt, thereby reducing the amount of water used.

[0009] Preferably, step S2 specifically involves the following steps: The waste salt solution is dissolved and transported to a primary dosing tank for impurity removal. Lime, polyferric sulfate, and PAM are added to generate insoluble substances such as calcium fluoride and fluorapatite. After sedimentation and separation, the sludge at the bottom of the primary dosing tank is transported to a primary filter press for filtration. The filtered sludge is transported off-site, and the top supernatant overflows to a secondary dosing tank for impurity removal, achieving the purpose of removing fluoride and phosphorus ions from the supernatant. In the secondary dosing tank, soda ash and liquid alkali are added to generate insoluble substances such as calcium carbonate, magnesium ammonium phosphate, and heavy metal hydroxide flocs. After sedimentation and separation, the sludge at the bottom of the secondary dosing tank is transported to a secondary filter press for filtration. The filtered sludge is transported off-site, and the top supernatant is transported to an ultrafiltration membrane for filtration, achieving the purpose of removing turbidity.

[0010] Preferably, in step S3, the nanofiltration membrane retains divalent sulfate ions, enriching sodium sulfate on the concentrate side to a concentration of 7%, while reducing the sodium sulfate concentration on the desalination side to 0.2%.

[0011] Preferably, in step S4, a heat exchange cooling process is used between the cryosol and the concentrated water to lower the temperature of the concentrated water to -5°C, at which point the solubility of sodium sulfate in the solution decreases to 0.7%.

[0012] Preferably, in step S5, since the mother liquor carried in the recrystallization of Glauber's salt contains sodium chloride and potassium chloride, and sodium chloride and potassium chloride are enriched in the centrifuged mother liquor, in order to ensure the quality of the recrystallized sodium sulfate, the mother liquor needs to be transported to the mixing solution tank after the total concentration of sodium chloride and potassium chloride is enriched to 20%.

[0013] Preferably, in step S6, the evaporation temperature for evaporating and crystallizing sodium chloride is 90~100℃, the concentration of potassium chloride when it reaches saturation is 21.7%, and then the flash evaporation and cooling crystallization temperature is 40~45℃. At this time, the sulfate concentration in the mother liquor after centrifugation increases due to evaporation and concentration, and the mother liquor after cooling and crystallizing potassium needs to be transported to the mixed solution tank to ensure the quality of potassium and sodium salts.

[0014] Preferably, in step S7, the nanofiltration membrane is made of an antifouling material. The nanofiltration membrane retains divalent sulfate ions, and sodium sulfate is enriched on the concentrate side, with a concentration of up to 7%, while the sodium sulfate concentration on the desalination side is reduced to 0.2%.

[0015] Compared with the prior art, the above-mentioned technical solution of the present invention has the following advantages: The core of the waste salt resource recovery process of the present invention lies in using freezing to reduce the content of divalent sulfate. Based on the selective separation characteristics of nanofiltration membranes for monovalent and divalent salts, the frozen mixed waste brine is efficiently separated into two material streams: a dilute product mainly composed of sodium chloride and potassium chloride, and a concentrated product mainly composed of sodium sulfate, sodium chloride, and potassium chloride. Subsequently, these two material streams are independently and coupledly sent to subsequent deep purification processes and evaporation crystallization systems, ultimately producing high-purity sodium chloride, potassium chloride, and sodium sulfate products. The present invention fundamentally solves the problem of difficult separation of mixed salts through freezing denitrification, nanofiltration pre-separation, and evaporative thermal crystallization separation of potassium and sodium salts. The process is simple, the product purity is high, and efficient and high-value resource recovery of waste salt is achieved. Attached Figure Description

[0016] To make the content of this invention easier to understand, the invention will be further described in detail below with reference to specific embodiments and accompanying drawings, wherein... Figure 1 This is a schematic diagram of a waste salt resource utilization process according to an embodiment of the present invention.

[0017] The reference numerals in the diagram are as follows: 1-Salt dissolving tank; 2-Primary dosing and impurity removal tank; 3-Primary filter press; 4-Secondary dosing and impurity removal tank; 5-Secondary filter press; 6-Ultrafiltration membrane filtration device; 7-Primary nanofiltration device; 8-Nanofiltration concentrate storage tank; 9-Freezing crystallizer; 10-Refrigeration unit; 11-Glauber's salt centrifuge; 12-Hot melting tank; 13-Sodium sulfate centrifuge; 14-Recrystallization evaporation device; 15-Mixed solution tank; 16-Nanofiltration desalination water storage tank; 17-Evaporation crystallization device; 18-Sodium salt centrifuge; 19-Flash cooling crystallization unit; 20-Potassium salt centrifuge; 20-Secondary nanofiltration device. Detailed Implementation Example 1

[0018] This embodiment provides a waste salt resource utilization process. The core of this process lies in utilizing the selective separation characteristics of nanofiltration membranes for monovalent and divalent salts. By freezing, the content of divalent sulfate is reduced, and the frozen mixed waste brine is then efficiently separated into two material streams: a dilute product mainly composed of sodium chloride and potassium chloride, and a concentrated product mainly composed of sodium sulfate, sodium chloride, and potassium chloride. Subsequently, these two material streams are independently and coupledly fed into subsequent deep purification processes and evaporation crystallization systems, ultimately producing high-purity sodium chloride, potassium chloride, and sodium sulfate products.

[0019] The waste salt resource utilization process includes the following steps: S1. The waste salt after pyrolysis is dissolved in water. In this embodiment, the waste salt treatment capacity is 4t / h, of which the sodium chloride content is 2.8t / h, the potassium chloride content is 0.6t / h, and the sodium sulfate content is 0.6t / h. First, the waste salt is sent to the salt dissolving tank 1. The condensate from the downstream evaporation system can be used to supplement a small amount of dissolving water. A total of 16t / h of dissolving water is added to completely dissolve the waste salt and form a solution. The dissolving solution volume is 20t / h.

[0020] S2. Waste salt dissolution solution removal: The 20t / h dissolution solution obtained in step S1, mainly composed of sodium chloride, potassium chloride, and sodium sulfate, is first fed into the primary dosing and impurity removal tank 2. Then, by adding lime, polyferric sulfate, and PAM, insoluble substances such as calcium fluoride and fluorapatite are generated. After sedimentation and separation, the sludge at the bottom of the primary dosing and impurity removal tank 2 is transported to the primary filter press 3 for filtration. The filtered sludge is transported off-site, and the top supernatant overflows into the secondary dosing and impurity removal tank 4, achieving the purpose of removing fluoride and phosphorus ions from the supernatant. In the secondary dosing and impurity removal tank 4, by adding soda ash and liquid alkali, insoluble substances such as calcium carbonate, magnesium ammonium phosphate, and heavy metal hydroxide flocs are generated. After sedimentation and separation, the sludge at the bottom of the secondary dosing and impurity removal tank 4 is transported to the secondary filter press 5 for filtration. The filtered sludge is transported off-site, and the top supernatant is transported to the ultrafiltration membrane filtration device 6, achieving the purpose of removing turbidity.

[0021] This invention utilizes the synergistic effect of lime, polyferric sulfate (PFSA), and polyammonium phosphate (PAM), achieving significantly better results than using any single or two agents in removing fluoride and phosphate ions. The process involves first adjusting the pH with lime to initiate precipitation, then adding PFSA for adsorption and co-precipitation, and finally adding PAM to aid coagulation. This produces the following synergistic effects: 1) Enhanced precipitation: Lime provides Ca²⁺ and an alkaline environment, generating "nuclei" for calcium fluoride and calcium phosphate; 2) Enhanced adsorption and co-precipitation: The Fe(OH)₃ colloid formed by PFSA under alkaline conditions not only adsorbs the remaining free fluoride ions but also... - and PO4 3-Furthermore, it can adsorb, encapsulate, and connect the fine CaF2 and other precipitates generated in the first step, forming a "calcium salt-iron floc" complex, which greatly increases the volume and density of the precipitate. 3) Qualitative change in separation efficiency: The addition of PAM transforms the above-mentioned composite flocs from a loose state into dense, large flocs, potentially increasing the settling speed by several times or even more than ten times. This not only makes the mud-water separation more thorough and the effluent clearer, but also makes the resulting sludge easier to concentrate and dewater; S3. First-stage nanofiltration desalination: The purified solution obtained in step S2 is passed through nanofiltration membrane device 6 to separate divalent salts, yielding fresh water with a flow rate of ~11.8 t / h, mainly composed of sodium chloride and potassium chloride, stored in nanofiltration fresh water storage tank 16, and concentrated water containing sodium chloride, potassium chloride, and high-concentration sodium sulfate, with a flow rate of ~8.2 t / h, stored in nanofiltration concentrated water storage tank 8. The nanofiltration membrane retains divalent sulfate ions, enriching sodium sulfate on the concentrated water side to a concentration of up to 7%, while reducing the sodium sulfate concentration on the fresh water side to 0.2%.

[0022] S4. The freeze crystallization and nitrification system stores the nanofiltration concentrate in the nanofiltration concentrate storage tank 8 from step S3. This concentrate is then sent to the freeze crystallizer 9. The -11℃ cryogenic liquid generated by the refrigeration unit 10 is used to exchange heat with the nanofiltration concentrate at room temperature (~25℃). According to the phase diagram data of sodium sulfate, sodium chloride, and potassium chloride solutions, the solubility of sodium sulfate in aqueous solution is only 0.7% at -5℃. Therefore, when the nanofiltration concentrate is frozen to -5℃, sodium sulfate is basically crystallized out in the form of sodium sulfate decahydrate (sodium sulfate). The solution containing sodium sulfate crystal slurry is sent to the sodium sulfate centrifuge 11 for centrifugation. The separated sodium sulfate is sent to the hot melt tank 12. The remaining sodium chloride, potassium chloride, and the frozen mother liquor containing a small amount of sodium sulfate are also sent to the mixed solution tank 15. Since the limiting factor for sodium is the sulfate content, if the sulfate content is high, the nanofiltration membrane cannot separate divalent salts. After removing and reducing the sulfate content in the mother liquor by freezing, it can then enter the secondary nanofiltration stage to separate the divalent salts.

[0023] S5. Recrystallization of Glauber's salt: The Glauber's salt obtained in step S4 is sent to the hot melt tank 1 for preheating and melting. The sodium sulfate obtained from the freeze crystallization precipitation system crystallizes out in the form of Glauber's salt. By heating the Glauber's salt, the residual heat of the tail gas or waste gas can be used to preheat the Glauber's salt to 90°C, so that the Glauber's salt dissolves in its own water of crystallization and forms a saturated solution. At the same time, some of the Glauber's salt loses its water of crystallization and precipitates out in the form of anhydrous sodium sulfate. This is separated by a sodium sulfate centrifuge 13 to obtain sodium sulfate industrial salt. The mother liquor after centrifugation is then sent to the recrystallization evaporation device 14 to evaporate the water in the mother liquor. The sodium sulfate continues to evaporate and crystallize, resulting in a solution containing sodium sulfate crystal slurry, which is then sent to the sodium sulfate centrifuge 13 to obtain sodium sulfate industrial salt. This cycle continues. Since the mother liquor entrained in the recrystallization of Glauber's salt contains sodium chloride and potassium chloride, which are enriched in the centrifuged mother liquor, in order to ensure the quality of the recrystallized sodium sulfate, the mother liquor needs to be transferred to the mixing solution tank 15 after the total concentration of sodium chloride and potassium chloride reaches 20%.

[0024] S6. The evaporation crystallization device is coupled with the flash cooling crystallization device to separate salts. The nanofiltration freshwater stored in the nanofiltration freshwater storage tank 16 in step S3 is sent to the evaporation crystallization device 17. By evaporating the water from the nanofiltration freshwater, according to the sodium chloride and potassium chloride phase diagram data, the sodium chloride in the nanofiltration freshwater first reaches saturation. The sodium chloride crystal slurry solution obtained by evaporation crystallization is sent to the sodium salt centrifuge 18 to obtain sodium chloride industrial salt product. As potassium chloride is enriched in the concentrate in the evaporation crystallization device 17 and reaches saturation, it is sent to the flash cooling crystallization unit 19. By cooling, the solubility of potassium chloride decreases, and the saturated potassium chloride is cooled and crystallized to obtain a potassium chloride crystal slurry solution, which is sent to the potassium salt centrifuge 20 to obtain potassium chloride product.

[0025] S7. Secondary nanofiltration desalination: The frozen mother liquor obtained from the freeze crystallization and nitrate precipitation in step S4, the potassium chloride and potassium chloride enrichment mother liquor obtained from the recrystallization of Glauber's salt in step S5, and the potassium precipitation mother liquor obtained from the desalination of the evaporation crystallization device coupled with the flash cooling crystallization device in step S6 are stored together in the mixed solution tank 15 and then transported together to the secondary nanofiltration membrane device. The divalent salts are separated through the secondary nanofiltration membrane to obtain fresh water with sodium chloride and potassium chloride as the main components, which is stored in the nanofiltration fresh water storage tank 16. The concentrated water containing sodium chloride, potassium chloride and high concentration of sodium sulfate is stored in the nanofiltration concentrated water storage tank 8. Sodium sulfate is enriched on the concentrated water side to a concentration of 7%, while the sodium sulfate concentration on the fresh water side is reduced to 0.2%, achieving the purpose of recycling.

[0026] The core of this invention's waste salt resource recovery process lies in utilizing freezing to reduce the content of divalent sulfate. Based on the selective separation characteristics of nanofiltration membranes for monovalent and divalent salts, the frozen mixed waste brine is efficiently separated into two streams: a desalinated product mainly composed of sodium chloride and potassium chloride, and a concentrated product mainly composed of sodium sulfate, sodium chloride, and potassium chloride. These two streams are then independently and coupledly fed into subsequent deep purification processes and an evaporation crystallization system, ultimately producing high-purity sodium chloride, potassium chloride, and sodium sulfate products. This invention fundamentally solves the problem of difficult mixed salt separation through freezing denitrification, nanofiltration pre-separation, and evaporative thermal crystallization to separate potassium and sodium salts. The process is simple, the product purity is high, and it achieves efficient and high-value resource recovery of waste salt.

[0027] Understandably, cryogenic denitrification involves cooling a sodium sulfate solution to below zero degrees Celsius. Utilizing the extremely low solubility of sodium sulfate at low temperatures, it precipitates out as sodium sulfate decahydrate crystals, achieving efficient and selective removal of sodium sulfate from the solution. Nanofiltration pre-separation, on the other hand, utilizes the properties of nanofiltration membranes as a crucial initial step in the pyrolysis waste salt resource recovery process. It prioritizes the separation of divalent salts (sodium sulfate) from monovalent salts (sodium chloride and potassium chloride), creating conditions for subsequent separation to obtain sodium sulfate, sodium chloride, and potassium chloride products.

[0028] Evaporative thermal crystallization separation of potassium and sodium chloride: Based on the difference in solubility-temperature characteristics of sodium chloride and potassium chloride, a physicochemical stepwise crystallization method of "first evaporating sodium chloride at high temperature and then cooling to precipitate potassium chloride" is adopted to produce two solid products of high purity, sodium chloride and potassium chloride, from a mixed solution.

[0029] Preferably, the waste salt resource utilization process includes: a salt dissolving system, an impurity removal unit, a primary nanofiltration salt separation unit, a forced circulation evaporative thermal crystallization plus cooling crystallization salt separation unit, a freeze crystallization nitrate precipitation unit, a mirabilite recrystallization unit, and a secondary nanofiltration salt separation unit. The salt dissolving system is connected to the impurity removal unit. The primary nanofiltration unit is connected to both the forced circulation evaporative thermal crystallization plus cooling crystallization salt separation unit and the freeze crystallization nitrate precipitation unit. The freeze crystallization nitrate precipitation unit is connected to the mirabilite recrystallization unit. The secondary nanofiltration salt separation unit is connected to the forced circulation evaporative thermal crystallization plus cooling crystallization salt separation unit, the freeze crystallization nitrate precipitation unit, and the mirabilite recrystallization unit.

[0030] Specifically, the salt dissolving system is connected to the impurity removal unit. After the pyrolysis of waste salt, impurities need to be removed before it enters the first-stage nanofiltration unit. The nanofiltration membrane has high requirements for pollutants in the influent and needs to be removed first to prevent membrane fouling and ensure the long-term stable operation of the nanofiltration system. The primary nanofiltration unit is connected to the forced circulation evaporative thermal crystallization plus cooling crystallization and salt separation unit and the freeze crystallization and nitrification unit. The primary nanofiltration unit produces nanofiltration concentrate and nanofiltration desalinated water. The nanofiltration desalinated water mainly contains monovalent salts: sodium chloride and potassium chloride, which can enter the forced circulation evaporative thermal crystallization plus cooling crystallization and salt separation unit to separate sodium chloride and potassium chloride products. The nanofiltration concentrate retains divalent ions and mainly contains divalent and monovalent salts: sodium sulfate, sodium chloride, and potassium chloride. It can enter the freeze crystallization and nitrification unit to freeze sodium sulfate in the form of sodium sulfate. The freeze crystallization and nitrification unit is connected to the sodium sulfate recrystallization unit to melt and evaporate the frozen sodium sulfate to obtain sodium sulfate product. The secondary nanofiltration salt separation unit is connected to the forced circulation evaporative thermal crystallization, cooling crystallization and salt separation unit, freeze crystallization and nitrification unit, and sodium sulfate recrystallization unit. The mother liquor from the three units is discharged into the secondary nanofiltration salt separation unit to separate the divalent salts. The secondary nanofiltration unit produces nanofiltration concentrate and nanofiltration desalinated water, and the entire system forms a closed loop.

[0031] Preferably, in the salt dissolving system, most of the water used to dissolve the waste salt is condensate from the downstream evaporation system, with a small amount of dissolved water added to redissolve the pyrolysis waste salt, thereby reducing the amount of water used.

[0032] Dissolving waste salts requires a large amount of process water. Using only fresh water (tap water, river water, etc.) would be costly and increase the overall water treatment load of the system. The condensate produced by the evaporation system is high-quality water that has undergone phase change separation. The closed-loop utilization of water resources within the system significantly reduces dependence on external fresh water (only a small amount is needed to replenish system losses). Wastewater generation is reduced at the source because the water circulates within the system, rather than being discharged after a single pass.

[0033] Preferably, step S2 specifically involves the following steps: The waste salt solution is dissolved and transported to a primary dosing tank for impurity removal. Lime, polyferric sulfate, and PAM are added to generate insoluble substances such as calcium fluoride and fluorapatite. After sedimentation and separation, the sludge at the bottom of the primary dosing tank is transported to a primary filter press for filtration. The filtered sludge is transported off-site, and the top supernatant overflows to a secondary dosing tank for impurity removal, achieving the purpose of removing fluoride and phosphorus ions from the supernatant. In the secondary dosing tank, soda ash and liquid alkali are added to generate insoluble substances such as calcium carbonate, magnesium ammonium phosphate, and heavy metal hydroxide flocs. After sedimentation and separation, the sludge at the bottom of the secondary dosing tank is transported to a secondary filter press for filtration. The filtered sludge is transported off-site, and the top supernatant is transported to an ultrafiltration membrane for filtration, achieving the purpose of removing turbidity.

[0034] This impurity removal stage represents a classic, complete, and highly efficient treatment chain for the deep purification of pyrolysis waste salt solutions (especially those containing complex impurities such as fluoride, phosphorus, and heavy metals). The multi-stage series design offers significant advantages, with its core principle being "staged treatment, step-by-step purification, and each stage performing its specific function," ultimately ensuring that the permeate meets the stringent requirements of subsequent nanofiltration membrane systems and evaporation crystallization feed water. Its greatest benefit is minimizing the operational risks, maintenance costs, and downtime probability of the entire system (especially expensive membrane systems), ensuring long-term, stable, and economical process operation.

[0035] Preferably, in step S3, the nanofiltration membrane retains divalent sulfate ions, enriching sodium sulfate on the concentrate side to a concentration of 7%, while reducing the sodium sulfate concentration on the desalination side to 0.2%.

[0036] It cleverly utilizes the high retention capacity of nanofiltration membranes for divalent sulfate ions to remove components (sulfate ions) that affect the separation of monovalent salts from the mixed salt wastewater after the dissolution of pyrolysis waste salts. This enables the separation of sodium chloride and potassium chloride, yielding the most valuable potassium chloride salt. This is a key turning point in realizing the technological and economic feasibility of pyrolysis waste salt resource utilization.

[0037] Preferably, in step S4, a heat exchange cooling process is used between the cryosol and the concentrated water to lower the temperature of the concentrated water to -5°C, at which point the solubility of sodium sulfate in the solution decreases to 0.7%.

[0038] The concentrated water enriched with divalent salts obtained through nanofiltration membranes is then frozen to extract sodium sulfate as Glauber's salt. The separated Glauber's salt crystals, after recrystallization, can be sold directly as an industrial product, thus achieving the separation of the third salt. This process also reduces the concentration of divalent sulfate in the mother liquor, allowing it to re-enter the secondary nanofiltration stage.

[0039] Preferably, in step S5, since the mother liquor carried in the recrystallization of Glauber's salt contains sodium chloride and potassium chloride, and sodium chloride and potassium chloride are enriched in the centrifuged mother liquor, in order to ensure the quality of the recrystallized sodium sulfate, the mother liquor needs to be transported to the mixing solution tank after the total concentration of sodium chloride and potassium chloride is enriched to 20%.

[0040] "Controlling the concentration of impurities (NaCl, KCl) in the mother liquor and periodically discharging them" is an optimized control strategy based on phase diagram theory and industrial practice. It is a key operation in the sodium sulfate recrystallization process for achieving "product purity control" and "system stable operation." When the total NaCl / KCl concentration in the mother liquor exceeds a certain limit, it significantly reduces the solubility of sodium sulfate and alters its crystallization behavior. More seriously, when the concentration reaches the co-saturation point, sodium chloride or potassium salts will co-crystallize and precipitate with sodium sulfate, affecting the purity of the sodium sulfate product.

[0041] Preferably, in step S6, the evaporation temperature for evaporating and crystallizing sodium chloride is 90~100℃, the concentration of potassium chloride when it reaches saturation is 21.7%, and then the flash evaporation and cooling crystallization temperature is 40~45℃. At this time, the sulfate concentration in the mother liquor after centrifugation increases due to evaporation and concentration, and the mother liquor after cooling and crystallizing potassium needs to be transported to the mixed solution tank to ensure the quality of potassium and sodium salts.

[0042] High-temperature evaporation and crystallization yields sodium chloride, while low-temperature cooling and crystallization yields potassium chloride, achieving separation of the two salts. This process is based on phase diagram theory and industrial practice. However, because nanofiltration desalinated water still contains a small amount of sulfate ions, these ions accumulate in the mother liquor during this unit, altering the solution phase diagram and causing the crystallization conditions of NaCl and KCl to deviate from their optimal range. This can lead to co-crystallization of sodium chloride with sodium sulfate, or potassium chloride with potassium sulfate, severely impairing the purity of the main product. Therefore, it is necessary to remove the mother liquor enriched with "sulfate impurities" to prevent the infinite circulation and accumulation of sulfate impurities in the main crystallization system, ensuring the quality of sodium chloride and potassium chloride salts.

[0043] Preferably, in step S7, the nanofiltration membrane is made of an antifouling material. The nanofiltration membrane retains divalent sulfate ions, and sodium sulfate is enriched on the concentrate side, with a concentration of up to 7%, while the sodium sulfate concentration on the desalination side is reduced to 0.2%.

[0044] The mother liquor discharged from the forced circulation evaporation hot crystallization, cooling crystallization salt separation unit, freezing crystallization nitrate separation unit, and Glauber's salt recrystallization unit is then removed from the mixed mother liquor by utilizing the high retention characteristics of divalent sulfate ions by nanofiltration membranes. This removes the components (sulfate ions) that affect the separation of monovalent salts from the mixed mother liquor, thus forming a closed loop for the entire system.

[0045] In this embodiment, the production and operating energy consumption of sodium chloride, potassium chloride and sodium sulfate in the waste salt resource utilization process are shown in Table 1.

[0046] Table 1 Serial Number project numerical values unit 1 Treatment of pyrolysis waste salt volume 4 t / h 2 Dissolved water volume 16 t / h 3 Sodium chloride production 2.8 t / h 4 Potassium chloride production 0.6 t / h 5 Sodium sulfate production 0.6 t / h 6 Power consumption 1400 kw.h 7 Steam consumption 1.8 t / h 8 Circulating water consumption 300 m³ / h 9 Low-temperature cold water consumption 50 m³ / h The above results show that the waste salt resource utilization process described in this embodiment has the advantages of low energy consumption, continuous automated production, and low operating cost. Example 2

[0047] This embodiment provides equipment for the waste salt resource utilization process described in Embodiment 1, such as... Figure 1As shown, the equipment includes a salt dissolving tank 1, a primary dosing and impurity removal tank 2, a primary filter press 3, a secondary dosing and impurity removal tank 4, a secondary filter press 5, an ultrafiltration membrane filtration device 6, and a primary nanofiltration device 7 connected in sequence. In the salt dissolving tank 1, most of the water used to dissolve the waste salt is condensate from the downstream evaporation system, with a small amount of dissolved water added to redissolve the pyrolysis waste salt and reduce the amount of water used. The primary nanofiltration unit 7 is connected to a nanofiltration concentrate storage tank 8 and a nanofiltration desalination tank 16. The nanofiltration concentrate storage tank 8 is sequentially connected to a freeze crystallizer 9, a Glauber's salt centrifuge 11, a hot melt tank 12, a sodium sulfate centrifuge 13, and a recrystallization evaporation unit 14. The cold source for the freeze crystallizer 9 comes from a refrigeration unit 10. The nanofiltration concentrate storage tank 16 is sequentially connected to an evaporation crystallization unit 17, a sodium salt centrifuge 18, a flash evaporation cooling crystallization unit 19, and a potassium salt centrifuge 20. The recrystallization evaporation unit 14, the potassium salt centrifuge 20, and the Glauber's salt centrifuge 11 are all connected to a mixed solution tank 15. The mixed solution tank 15 is connected to a secondary nanofiltration unit 20, which is also connected to the nanofiltration concentrate storage tank 8 and the nanofiltration desalination tank 16.

[0048] Obviously, the above embodiments are merely illustrative examples for clear explanation and are not intended to limit the implementation. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations here. However, obvious variations or modifications derived therefrom are still within the scope of protection of this invention.

Claims

1. A waste salt resource utilization treatment process, characterized in that, Includes the following steps: S1. Dissolve the waste salt after pyrolysis in water; S2. Waste salt dissolution solution impurity removal: After the waste salt is dissolved in water, the inorganic salts dissolve in the water, while the water-insoluble inorganic salt impurities precipitate in the dissolution tank. The first step adopts the "chemical reaction plus precipitation" process, by adding lime, polyferric sulfate and PAM to generate calcium fluoride and fluorapatite. After precipitation separation, the supernatant is used to remove fluoride ions and phosphorus ions. The second step adopts the "chemical reaction plus ultrafiltration membrane filtration" process, by adding soda ash and liquid alkali to generate calcium carbonate, magnesium ammonium phosphate and heavy metal hydroxide flocs. After ultrafiltration membrane filtration, the turbidity is removed. S3, Primary nanofiltration desalination: The waste salt solution after impurity removal is separated by nanofiltration membrane to obtain divalent salts, resulting in fresh water with sodium chloride and potassium chloride as the main components, which is stored in nanofiltration fresh water storage tank, as well as concentrated water containing sodium chloride, potassium chloride and high concentration of sodium sulfate, which is stored in nanofiltration concentrated water storage tank. S4. The cold crystallization and nitrification system: The concentrated water stored in the nanofiltration concentrate tank enters the cold crystallizer, where the concentrated water is cooled by heat exchange between the cold liquid and the cold liquid, so that sodium sulfate crystallizes in the form of sodium sulfate decahydrate. The cold mother liquor containing sodium chloride, potassium chloride and sodium sulfate is transported to the mixed solution tank. S5. Recrystallization of Glauber's salt: Sodium sulfate obtained from the freeze crystallization precipitation system is crystallized in the form of Glauber's salt. Glauber's salt is heated to dissolve in its own water of crystallization. Sodium sulfate industrial salt is obtained by centrifugation. The mother liquor after centrifugation is then put into the evaporation crystallization system to evaporate and crystallize the water and sodium sulfate in the mother liquor. S6. The evaporation crystallization unit coupled with the flash cooling crystallization unit separates salts. Fresh water stored in the nanofiltration fresh water storage tank enters the evaporation crystallization unit. Based on the sodium chloride and potassium chloride phase diagram data, sodium chloride is first evaporated and crystallized to obtain sodium chloride product. After potassium chloride is enriched in the concentrate to saturation, it is transported to the flash cooling crystallization unit. By cooling down, the solubility of potassium chloride decreases, and the saturated potassium chloride is cooled and crystallized to obtain potassium chloride product. S7. Secondary nanofiltration desalination: The potassium-precipitated mother liquor obtained from the evaporation crystallization unit coupled with the flash cooling crystallization unit, the frozen mother liquor obtained from the freeze crystallization of nitrate, and the potassium chloride and potassium chloride enrichment mother liquor obtained from the recrystallization of Glauber's salt are all fed into the secondary nanofiltration membrane. The divalent salts are then separated through the secondary nanofiltration membrane to obtain fresh water with sodium chloride and potassium chloride as the main components, which is stored in the nanofiltration fresh water storage tank. Concentrated water containing sodium chloride, potassium chloride, and high-concentration sodium sulfate is stored in the nanofiltration concentrated water storage tank. Sodium sulfate is enriched on the concentrated water side to a concentration of 7%, while the sodium sulfate concentration on the fresh water side is reduced to 0.2%, achieving the purpose of recycling.

2. The waste salt resource utilization process according to claim 1, characterized in that, The waste salt resource utilization process includes: a salt dissolving system, an impurity removal unit, a primary nanofiltration salt separation unit, a forced circulation evaporative thermal crystallization and cooling crystallization salt separation unit, a freeze crystallization and nitrate precipitation unit, a mirabilite recrystallization unit, and a secondary nanofiltration salt separation unit. The salt dissolving system is connected to the impurity removal unit. The primary nanofiltration unit is connected to both the forced circulation evaporative thermal crystallization and cooling crystallization salt separation unit and the freeze crystallization and nitrate precipitation unit. The freeze crystallization and nitrate precipitation unit is connected to the mirabilite recrystallization unit. The secondary nanofiltration salt separation unit is connected to the forced circulation evaporative thermal crystallization and cooling crystallization salt separation unit, the freeze crystallization and nitrate precipitation unit, and the mirabilite recrystallization unit.

3. The waste salt resource utilization process according to claim 2, characterized in that, In the salt dissolving system, most of the water used to dissolve the waste salt comes from the condensate of the downstream evaporation system, with a small amount of dissolved water added to redissolve the pyrolysis waste salt, thereby reducing the amount of water used.

4. The waste salt resource utilization process according to claim 2, characterized in that, The specific steps of step S2 are as follows: The waste salt dissolving solution is transported to the primary dosing and impurity removal tank. Insoluble matter is added, and after sedimentation and separation, the slurry at the bottom of the primary dosing and impurity removal tank is transported to the primary filter press for filtration. The filtered sludge is transported off-site, and the top supernatant overflows to the secondary dosing and impurity removal tank to remove fluoride and phosphorus ions from the supernatant. In the secondary dosing and impurity removal tank, soda ash and liquid alkali are added to generate insoluble matter containing calcium carbonate, magnesium ammonium phosphate, and heavy metal hydroxide flocs. After sedimentation and separation, the slurry at the bottom of the secondary dosing and impurity removal tank is transported to the secondary filter press for filtration. The filtered sludge is transported off-site, and the top supernatant is transported to the ultrafiltration membrane for filtration to remove turbidity.

5. The waste salt resource utilization process according to claim 2, characterized in that, In step S3, the nanofiltration membrane retains divalent sulfate ions, enriching sodium sulfate on the concentrate side to a concentration of 7%, while reducing the sodium sulfate concentration on the desalination side to 0.2%.

6. The waste salt resource utilization process according to claim 2, characterized in that, In step S4, the cooling liquid is exchanged with the concentrated water to reduce the temperature of the concentrated water to -5°C. At this point, the solubility of sodium sulfate in the solution is reduced to 0.7%.

7. The waste salt resource utilization process according to claim 2, characterized in that, In step S5, since the mother liquor carried in the recrystallization of Glauber's salt contains sodium chloride and potassium chloride, and sodium chloride and potassium chloride are enriched in the centrifuged mother liquor, in order to ensure the quality of the recrystallized sodium sulfate, the mother liquor needs to be transported to the mixing solution tank after the total concentration of sodium chloride and potassium chloride is enriched to 20%.

8. The waste salt resource utilization process according to claim 2, characterized in that, In step S6, the evaporation temperature for evaporating and crystallizing sodium chloride is 90~100℃, the concentration of potassium chloride when it reaches saturation is 21.7%, and then the flash evaporation and cooling crystallization temperature is 40~45℃. At this time, the sulfate concentration in the mother liquor after centrifugation increases due to evaporation and concentration. The mother liquor after cooling and crystallizing potassium needs to be transported to the mixed solution tank to ensure the quality of potassium and sodium salts.

9. The waste salt resource utilization process according to claim 2, characterized in that, In step S7, the nanofiltration membrane is made of an antifouling material. The nanofiltration membrane retains divalent sulfate ions, and sodium sulfate is enriched on the concentrate side, with a concentration of up to 7%, while the sodium sulfate concentration on the desalination side is reduced to 0.2%.

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