Slow evaporation resourceful treatment method and system of industrial waste salt

Abstract

The application discloses a slow evaporation resourceful treatment method and system of industrial waste salt. The solid industrial waste salt is taken as the object, and is pyrolyzed in an oxygen-free or low-oxygen atmosphere at 350 DEG C to 700 DEG C, so as to remove organic matters and low-boiling-point heavy metals; then, the pyrolyzed pure salt is prepared into a salt-containing solution, and is sent into an evaporation crystallization facility provided with an anti-seepage structure, the evaporation rate is controlled to be greater than or equal to 24 hours, so that the salt is slowly recrystallized, and the medium-high boiling-point heavy metals are enriched in the mother liquor and separated from the crystallized salt. The anti-seepage structure has a permeation coefficient less than or equal to 1.0*10-7 cm / s or is composed of dense solid materials which are impermeable to liquid. The leaching toxicity of the final crystallized salt meets the requirements of GB5085.3-2007, and no process wastewater is discharged in the whole process. The application can be adapted to various types of waste salt, and realizes harmless and high-value resource of the industrial waste salt.

Description

Technical Field

[0001] This invention relates to the field of industrial solid waste resource utilization technology, specifically to industrial waste salt, particularly hazardous waste industrial waste salt, for harmless and large-scale high-value utilization. It is also applicable to the resource utilization of other industrial by-product salts containing organic pollutants and heavy metals (such as waste salts generated by chemical, pharmaceutical, and printing and dyeing industries). Background Technology

[0002] my country generates a large amount of industrial waste salt annually. Waste salt from the fine chemical, pesticide, and pharmaceutical industries, containing toxic organic matter and heavy metal pollutants, is included in the "National Hazardous Waste List (2021 Edition)" and is considered a typical example of difficult-to-dispose-of hazardous waste, with a significant gap in compliant disposal. Furthermore, industrial by-product salt from other industries (such as dyeing and printing, chemical intermediates, and food processing), while not necessarily listed as hazardous waste, also presents problems of organic matter and heavy metal pollution, making resource utilization difficult.

[0003] The current mainstream methods for disposing of industrial waste salt fall into two categories, but both suffer from the following technical problems: The first type is rigid landfill, which only seals up pollutants without realizing resource utilization. It occupies a large amount of hazardous waste landfill capacity and also poses environmental risks of soil and groundwater leakage and pollution caused by salt deliquescence. It is an unsustainable disposal method and has been strictly restricted by national policies in recent years.

[0004] The second type is the 'pyrolysis + MVR mechanical compression evaporation / multi-effect forced evaporation' process, which is currently the mainstream resource recovery process in the industry. However, it has the following technical drawbacks: First, it involves large equipment investment and high energy consumption. Second, the high-salt wastewater causes significant corrosion to the equipment, resulting in a service life of only 3-5 years and high maintenance difficulty. Third, the pyrolysis process can only remove organic matter and low-boiling-point heavy metals such as mercury and arsenic, but cannot effectively remove medium- and high-boiling-point heavy metals such as lead, cadmium, and chromium. During forced evaporation, heavy metals are easily co-precipitated with the main salt, leading to unstable quality of the crystalline salt and difficulty in consistently meeting standards. Fourth, the process generates 10%-20% concentrated wastewater, which requires additional treatment and is prone to secondary pollution. Neither of the above two methods can simultaneously achieve the industrial waste salt treatment goals of 'complete component harmlessness, high resource recovery rate, and zero wastewater discharge'.

[0005] Traditional sun-dried salt pan process uses low-concentration natural underground brine as raw material, which requires a long stage of concentration from low concentration to saturation. The evaporation cycle is long and the production capacity per unit area is low. Moreover, because it processes natural low-concentration brine, the brine has a high content of calcium and magnesium ions, resulting in a large volume of evaporation mother liquor (old brine), which has a heavy burden for subsequent disposal. Due to the lack of effective anti-seepage measures, it can only process clean natural brine and cannot be used for industrial waste salt containing heavy metals and toxic organic matter, which severely limits its application scope.

[0006] Furthermore, if industrial waste salt containing high concentrations of toxic organic matter is directly prepared into a brine solution and evaporated for crystallization without prior pyrolysis and harmless pretreatment, the residual organic pollutants in the brine solution will escape uncontrollably with the water vapor during evaporation, producing a strong odor and causing VOCs concentrations to exceed standards, failing to meet the requirements for air pollution prevention and occupational health and safety. Simultaneously, the residual organic matter will cause swelling, corrosion, and aging of the seepage-proof structure, seriously affecting the long-term safe and stable operation of the facility. Therefore, the process route of direct evaporation without harmless pretreatment exists only in theory and lacks practical experimental feasibility and engineering application value.

[0007] my country has a large number of idle salt fields in traditional salt-producing areas such as Shandong, Hebei, Tianjin, Jiangsu, Qinghai, and Xinjiang. The arid and semi-arid regions of Northwest and North China have unique natural evaporation conditions. Industrial sites can be equipped with artificial seepage-proof crystallization ponds, which can all serve as excellent carriers for natural evaporation. However, there are still the following technical obstacles to applying natural evaporation technology to the treatment of hazardous waste industrial waste salt: (i) Research focuses on forced evaporation route: In the past two decades, research and practice in the field of industrial waste salt resource utilization have mainly focused on the technical route of 'pyrolysis + forced evaporation'. Academic literature and engineering design specifications are mostly focused on MVR / multi-effect evaporation, and there is relatively little research on natural evaporation technology for treating hazardous waste waste salt; (ii) Uncertainty about the applicability of anti-seepage standards: For a long time, the anti-seepage requirements of natural evaporation facilities have differed from the standards of hazardous waste storage facilities. Technicians have concerns about whether natural evaporation facilities can meet the strict requirements of the "Standard for Pollution Control of Hazardous Waste Storage"; (iii) Limitations of application scenarios: Evaporation ponds (drying ponds) are mostly used in chemical, coal chemical and other industries to treat low-concentration, non-hazardous waste concentrated brine. There are no reports of using them directly for the resource utilization of hazardous waste industrial waste salt.

[0008] Through systematic research on traditional salt field production lines, the inventors discovered that a large number of salt field facilities were idle, and that there was a potential connection between their evaporation mechanisms and the recrystallization and separation requirements of industrial waste salt. However, no one had previously proposed using a seepage-proof, slow evaporation process that meets hazardous waste storage standards for the harmless and resource-based treatment of hazardous waste salt. Based on this discovery, the inventors completed this invention. Summary of the Invention

[0009] (a) Technical problems to be solved • Overcoming the long-standing technical obstacle in the industry that "hazardous waste industrial waste salt cannot be disposed of on a large scale using natural evaporation technology", this invention provides a slow evaporation resource recovery technology solution for industrial waste salt that fully complies with the national mandatory standards for hazardous waste disposal, and is also applicable to the efficient resource recovery of non-hazardous waste industrial waste salt. • This addresses the industry pain point that existing pyrolysis processes can only remove organic matter and low-boiling-point heavy metals, but cannot effectively remove medium- and high-boiling-point heavy metals, resulting in inconsistent quality of crystalline salts. • Address the core shortcomings of existing "pyrolysis + forced evaporation" processes, such as high investment in equipment, high energy consumption, severe equipment corrosion, and easy generation of secondary wastewater; • It solves the inherent problems of traditional sun-dried salt fields, such as long evaporation cycles, low productivity per unit area, high dependence on natural brine resources, and inability to treat industrial waste salt containing pollutants; • It addresses the shortcomings of direct evaporation processes without pretreatment, such as the uncontrolled release of organic pollutants, significant odor pollution, excessive VOC emissions, high occupational health and environmental compliance risks, and the inability to achieve routine experiments and large-scale engineering applications. • Solves engineering challenges such as heavy metal accumulation, long-term reliability of anti-seepage structures, and poor adaptability to different climate regions during long-term process operation, achieving closed-loop operation of the entire process and zero discharge of process wastewater.

[0010] The final crystalline salt obtained by the method has a leaching toxicity that meets the requirements of GB5085.3-2007, "Identification Standard for Hazardous Waste: Leaching Toxicity Identification".

[0011] (II) Technical Solution To achieve the above objectives, the present invention adopts the following technical solution: Firstly, a method for the slow evaporation and resource recovery of industrial waste salt, the core steps of which are: (1) Pyrolysis pretreatment: The industrial waste salt to be treated is placed in an oxygen-free or low-oxygen atmosphere and pyrolyzed at 350℃~700℃ for 30min~180min to remove organic pollutants from the waste salt by cracking and at the same time to volatilize and remove low-boiling-point heavy metals from the waste salt to obtain pyrolyzed clean salt; the generated gas is treated by the waste gas treatment process and then discharged in compliance with the standards. (2) Slow evaporation and recrystallization: The pyrolysis clean salt is prepared into a salt solution and transported to the evaporation and crystallization facility. The evaporation rate is controlled so that the crystallization cycle is ≥24 hours, so that the salt recrystallizes and precipitates out. The medium and high boiling point heavy metals are enriched in the evaporation mother liquor, so as to achieve solid-liquid separation from the crystallized salt. The discharged old brine can be handed over to a qualified unit for final disposal or further recovery of valuable metals in accordance with the national hazardous waste management requirements.

[0012] The evaporation crystallization facility is equipped with a seepage-proof structure, the seepage-proof capacity of which meets the following requirement: if an artificial seepage-proof material layer is used, its permeability coefficient is ≤1.0×10⁻ 7 cm / s; if a hard material is used, it is a dense solid material that is impermeable to liquid. The final crystalline salt obtained by the method has a leaching toxicity that meets the requirements of GB5085.3-2007 "Identification Standard for Hazardous Waste - Leaching Toxicity Identification".

[0013] Applicable Climate Description: When using natural evaporation, evaporation is carried out using solar radiation and natural wind power. When the natural evaporation rate is insufficient to meet the preset crystallization cycle, the evaporation crystallization facility is equipped with a windproof and rainproof shed and / or auxiliary air distribution device. When using artificial heating, it is not limited by climate conditions.

[0014] In this invention, the concentration of the salt solution can be selected according to actual conditions. To improve crystallization efficiency and shorten the crystallization cycle, a higher concentration solution is preferred, such as a solution that is close to saturation or supersaturation. However, this invention does not exclude the use of a lower concentration solution; however, the lower the concentration, the longer the time required for evaporation to saturation, the lower the production capacity per unit area, and the worse the economic efficiency.

[0015] The core of slow evaporation recrystallization lies in the "slow" process. Unlike traditional forced evaporation processes (MVR / multi-effect evaporation, with crystallization cycles ranging from several hours to tens of minutes), this invention extends the crystallization cycle to over 24 hours (preferably 3-30 days, but not limited to this). During the slow release of supersaturation, salt crystals follow a thermodynamically controlled growth path, resulting in a more complete lattice arrangement, significantly increased crystal size (up to 0.5 mm or more), reduced specific surface area, and significantly reduced grain boundary adsorption and mother liquor entrainment. This leads to three technical effects: ① The main salt content of the crystals is increased to over 99%; ② Residual heavy metal ions are more fully displaced into the mother liquor during slow crystallization, further reducing leaching toxicity; ③ The cleanliness of the crystal surface is improved, reducing water consumption in subsequent purification processes.

[0016] Two specific embodiments of the present invention: • Natural evaporation method: Evaporation is carried out by solar radiation and natural wind power, which is suitable for arid and semi-arid areas or areas with supporting rain protection facilities.

[0017] • Artificial heating method: Heating is carried out using recovered industrial waste heat or other low-grade heat sources, and the heating temperature is controlled not to exceed 80°C. It is suitable for areas lacking sufficient solar energy or natural wind power, or industrial parks with stable low-grade waste heat sources.

[0018] Secondly, an industrial waste salt slow evaporation resource utilization treatment system for realizing the above-mentioned treatment method includes a pyrolysis pretreatment unit, a salt solution preparation unit, and an evaporation crystallization unit connected in sequence, as well as a crystallization salt purification unit, a mother liquor reflux unit, an old brine discharge unit, and a waste gas treatment unit; the evaporation crystallization unit is a seepage-proof evaporation crystallization facility or an evaporation crystallizer with a heating device.

[0019] (III) Technical Principles The core innovation of this invention lies in the first-ever deep coupling of anaerobic / low-oxygen pyrolysis harmless treatment technology with seepage-proof slow evaporation crystallization technology that meets mandatory standards for hazardous waste disposal. This forms a synergistic treatment mechanism of "pyrolysis for complete removal of organic matter + low-boiling-point heavy metals, and slow evaporation for targeted separation of medium- and high-boiling-point heavy metals." Differentiated harmless treatment pathways are designed for different pollutants. The core principle is as follows: 1. Harmless treatment mechanism of pyrolysis pretreatment: An oxygen-free / low-oxygen atmosphere can prevent waste salt from agglomerating during combustion, ensuring uniform pyrolysis and reducing the risk of dioxin formation. A low-oxygen atmosphere refers to an atmosphere with an oxygen volume fraction ≤6%, which complies with the "Technical Specification for Construction of Centralized Incineration Disposal Projects for Hazardous Waste" (HJ). Safety operation requirements (2025-2012); preferably, the oxygen volume fraction is controlled within the range of 0-5% to further reduce safety risks and improve the removal efficiency of organic matter; in a high-temperature environment above 350℃, organic pollutants in waste salt undergo cracking and carbonization, transforming into CO2, H2O and small molecule combustible gases, with an organic matter removal rate ≥98%; low-boiling-point heavy metals such as mercury and arsenic with boiling points <700℃ volatilize at high temperatures and are collected and treated uniformly with pyrolysis waste gas, with a removal rate ≥97%; at the same time, medium- and high-boiling-point heavy metals such as lead and cadmium with boiling points ≥700℃ do not volatilize during pyrolysis, but remain in the pyrolysis clean salt in the form of chlorides or oxides, which are then separated by solubility differences in the subsequent slow evaporation stage; in addition, the pyrolysis process completely eliminates the risk of swelling and corrosion of the impermeable layer by organic matter, laying a safe foundation for the long-term compliant operation of the subsequent slow evaporation stage. Meanwhile, the pyrolysis process completely removes organic pollutants, eliminating the risk of organic pollutants evaporating with water during the subsequent evaporation and crystallization process. This avoids fugitive VOC emissions and odor pollution, ensures occupational health and safety in the workplace, and ensures that the entire process meets the relevant regulations for air pollution prevention and control. This lays a compliant foundation for the routine experimental operation and large-scale engineering application of the process.

[0020] 2. Slow Evaporation Recrystallization Separation and Stabilization Mechanism: In this invention, the purified pyrolysis salt is prepared into a salt solution and sent to the evaporation facility. It is preferable to use a solution with a relatively high concentration (e.g., near saturation or supersaturation), which eliminates the long stage of concentrating low-concentration brine to saturation in traditional salt fields. During the slow evaporation process (crystallization cycle ≥ 24 hours), the main salt components (such as NaCl and Na2SO4) slowly reach a supersaturated state as the water evaporates, and crystallize out uniformly. Meanwhile, the sulfates and chlorides of medium- and high-boiling-point heavy metals have much higher solubility than the main salt components. They are always enriched in the mother liquor during the evaporation process and will not crystallize out with the main salt, thus achieving efficient separation of medium- and high-boiling-point heavy metals and crystalline salt.

[0021] 3. Quality improvement and heavy metal repulsion mechanism of slow crystallization: Unlike traditional forced evaporation (MVR / multi-effect evaporation, crystallization cycle of several hours to tens of minutes), this invention extends the crystallization cycle to more than 24 hours by controlling the evaporation rate, thus creating a thermodynamically dominated slow crystallization environment.

[0022] (1) Low supersaturation inhibits explosive nucleation: Under forced evaporation, the solution instantly reaches extremely high supersaturation, triggering explosive nucleation and forming a large number of tiny crystals with dense lattice defects (particle size <0.2 mm). At the same time, it significantly increases the partition coefficient of heavy metal ions entering the lattice, leading to severe co-precipitation. This invention keeps the solution supersaturation at a low level, allowing crystal growth to dominate and suppressing secondary nucleation. Atoms / ions have sufficient time to align at the most stable positions in the lattice, forming a nearly complete crystal structure with a crystal particle size of 0.5~5.0 mm.

[0023] (2) Thermodynamic and kinetic repulsion of heavy metals: The ionic radii of heavy metal ions such as Pb²⁺ and Cd²⁺ differ significantly from those of the main salt cations (Na⁺, K⁺), and their lattice substitution energies are much higher than those of the main salt ions. Low supersaturation brings the system close to equilibrium, and the partition coefficient of heavy metals between the solid and liquid tends to its extremely low thermodynamic eigenvalue, so they are naturally "squeezed out" to the mother liquor; at the same time, the slow growth rate is much smaller than the diffusion rate of heavy metal ions in the solution, so the heavy metals adsorbed on the growth interface have enough time to desorb and diffuse back to the mother liquor, avoiding being "frozen" and encapsulated.

[0024] (3) Suppressing grain boundary adsorption and mother liquor entrainment: The micro crystals formed by rapid crystallization have a huge specific surface area, and the surface defects strongly adsorb heavy metals. Moreover, the high-speed growth easily forms dendritic crystals, which encapsulate high concentrations of mother liquor (entrainment rate 5%~10%). The large-diameter crystals obtained by this invention have a reduced specific surface area by more than one order of magnitude, a regular shape, and the mother liquor entrainment rate can be controlled to below 1%.

[0025] The combined effect reduces the heavy metal leaching concentration of crystalline salt by more than 60% compared to the forced evaporation process, and increases the purity of the main salt to over 99%.

[0026] 4. Separation and Stabilization Mechanism of Slow Evaporation with Recovered Heat Energy: Compared with natural evaporation, this implementation method achieves evaporation through artificial temperature control, but the core remains "slow crystallization." By utilizing waste heat, the heating temperature is controlled to not exceed 80℃ (to avoid solution boiling and excessively rapid crystal growth), and the evaporation rate is controlled to no more than 30 liters of water per square meter per day, extending the crystallization cycle to more than 24 hours. Under these conditions, the solution is always maintained in a low supersaturation state, allowing the main salt crystals to grow slowly with fewer lattice defects, and impurities (especially heavy metal ions) are effectively displaced into the mother liquor. This is fundamentally different from traditional MVR forced evaporation.

[0027] 5. Closed-loop compliance mechanism throughout the entire process: Most of the concentrated mother liquor generated from evaporation is recycled back to the brine preparation unit for reuse. Only a small amount of old brine, enriched with high concentrations of heavy metals, is periodically discharged to control the cumulative concentration of heavy metals within the system. The discharged old brine is disposed of in accordance with relevant environmental protection regulations. The seepage-proof structure is designed in strict accordance with national mandatory standards for hazardous waste disposal, effectively preventing secondary pollution risks such as leakage, runoff, and dispersion. For the flexible seepage-proof layer, its permeability coefficient is ≤1.0×10⁻ 7 cm / s; For rigid impermeable layers, dense solid materials that are impermeable to liquids and chemically compatible with salt solutions are used.

[0028] (iv) Beneficial effects • Breakthrough in industry technical barriers and outstanding innovation: For the first time, the anaerobic / low-oxygen pyrolysis harmless treatment technology is deeply coupled with the seepage-proof slow evaporation crystallization technology that meets the standards for hazardous waste disposal, forming a complete synergistic treatment system. This completely solves the industry problem that traditional pyrolysis processes cannot effectively separate medium and high boiling point heavy metals. At the same time, it breaks the long-standing technical barrier that "hazardous waste cannot be disposed of on a large scale using natural evaporation processes" and fills the industry's technical gap.

[0029] • No need for high-energy-consuming forced evaporation equipment: Evaporation and crystallization are achieved by utilizing natural solar and wind energy or low-grade industrial waste heat. The equipment investment is significantly reduced compared to the traditional "pyrolysis + MVR" process, while also revitalizing a large amount of idle traditional salt field resources.

[0030] • Increased crystallization efficiency and production capacity: Unlike traditional sun-dried salt fields that rely on low-concentration natural brine, this invention directly uses high-concentration brine as feed (e.g., near-saturation or supersaturation), eliminating more than 90% of the pre-concentration cycle. Under the same evaporation conditions, the annual processing capacity per unit area is increased by more than 100%.

[0031] • Low production of old brine and light disposal burden: Thanks to the high purity of pyrolysis purified salt, there are few impurities in the evaporation mother liquor, and only a small amount of old brine needs to be discharged periodically to control the system balance.

[0032] • Product quality is superior to that of forced evaporation process: thanks to the slow crystallization mechanism, the crystallized salt has a large crystal size, complete crystal lattice, and few impurities. The main content can reach more than 99%, and the leaching toxicity index is consistently lower than the national standard limit.

[0033] • The evaporation methods are flexible and diverse, and the applicable areas are greatly expanded: it can use natural evaporation or slow evaporation by recovering heat energy. The latter is particularly suitable for non-arid areas or factories that lack natural evaporation conditions.

[0034] • Environmentally friendly, fully compliant and controllable: The entire process operates in a closed loop, with no process wastewater directly discharged into natural water bodies; the seepage-proof structure strictly complies with the mandatory requirements of GB18597-2023 (permeability coefficient ≤1.0×10⁻). 7(cm / s or equivalent rigid material that is impermeable to liquid); the pyrolysis waste gas is treated by a matching purification system and discharged in compliance with standards.

[0035] • Environmentally compliant throughout the entire process with no risk of secondary pollution: The pyrolysis pretreatment completely removes organic pollutants, thoroughly solving the problems of VOCs exceeding standards and odor pollution caused by the release of organic pollutants during the evaporation process. All waste gas and wastewater throughout the process are controllable and treatable, complying with national mandatory standards related to environmental protection and occupational health, and enabling stable routine experimental operation and large-scale engineering applications.

[0036] • Wide compatibility and scalable replication: Compatible with various mainstream industrial waste salts such as NaCl, Na2SO4, KCl, and CaCl2, covering multiple categories of hazardous waste salts such as HW02, HW04, HW11, and HW49.

[0037] (v) Creative Analysis This invention possesses the inventiveness required by patent law.

[0038] 1. Overcame long-standing technical obstacles Prior to this invention, the following technical obstacles prevented the adoption of natural evaporation processes in the field of industrial waste salt resource utilization, especially in the treatment of hazardous waste industrial waste salt: (1) Technological path dependence: Research and practice in the past two decades have mainly focused on the 'pyrolysis + forced evaporation (MVR / multi-effect)' route. Natural evaporation process is considered difficult to meet the needs of industrial continuous production due to its slow evaporation rate and difficulty in controlling process parameters. (2) Uncertainty about the applicability of the seepage prevention standard: The seepage prevention requirements of natural evaporation facilities differ from the standards for hazardous waste storage facilities, and technical personnel have concerns about the long-term reliability of whether they can meet mandatory standards such as GB18597-2023; (3) Limitations of application scenarios: Evaporation ponds and other facilities are mostly used to treat low-concentration, non-hazardous waste brine, and there is no engineering practice to directly use them for the resource utilization of hazardous waste solid salt.

[0039] The aforementioned obstacles have led to the industry's general consensus that natural evaporation is unsuitable for hazardous industrial waste salts. This invention overcomes these technical obstacles by deeply coupling anaerobic / low-oxygen pyrolysis harmless treatment technology with a seepage-proof slow evaporation crystallization technology that meets mandatory hazardous waste disposal standards, thus achieving the resource-based treatment of hazardous industrial waste salts through natural evaporation.

[0040] 2. The technical solution is not obvious. The core of this invention lies in the deep coupling of "pyrolysis pretreatment" and "slow evaporation for hazardous waste-grade seepage prevention," rather than a simple superposition of existing technologies. When faced with the challenge of separating heavy metals from waste salts, those skilled in the art typically focus on improving the pyrolysis process (e.g., increasing temperature) or optimizing forced evaporation parameters, rather than considering "slow evaporation," a method considered "inefficient and uncontrollable," let alone combining it with seepage prevention facilities that meet hazardous waste storage standards. In particular, this invention first thoroughly removes organic matter through pyrolysis (removal rate ≥98%), solving the core obstacle of organic matter causing swelling and corrosion of the seepage prevention layer, ensuring the material meets seepage compatibility requirements. This provides a safe foundation for the long-term compliant operation of the subsequent slow evaporation stage. This coupling logic of "first harmlessness, then resource recovery," and the collaborative mechanism of "pyrolysis removal of organic matter and low-boiling-point heavy metals, and slow evaporation separation of high-boiling-point heavy metals," are not readily apparent to those skilled in the art.

[0041] 3. Unexpected technical effects were achieved. This invention has achieved technical effects far exceeding expectations, specifically in the following aspects: • Achieved efficient separation of medium- and high-boiling-point heavy metals: Traditional pyrolysis processes achieve a separation rate of less than 15% for medium- and high-boiling-point heavy metals such as lead and cadmium. However, this invention achieves efficient enrichment of medium- and high-boiling-point heavy metals in the mother liquor through slow evaporation and recrystallization (crystallization cycle ≥ 24 hours), and achieves efficient separation from the crystalline salt. This effect cannot be achieved by pyrolysis or evaporation processes alone, and is a typical example of synergistic effect.

[0042] • The quality of the crystallized salt is significantly superior to that of the forced evaporation process: Compared with MVR forced evaporation, the average particle size of the crystallized salt obtained by this invention is increased from 0.05-0.2 mm to 0.8-2.5 mm, the purity of the main salt is increased from 99.1% to 99.86%, the lead leaching concentration is reduced by 68%, and the amount of rinsing water is reduced by 32%. The improvement in crystal quality brought about by the slow crystallization mechanism exceeds the reasonable expectations of those skilled in the art.

[0043] • Achieves low-cost, zero-emission resource recovery: This invention utilizes natural energy or industrial waste heat, reducing unit product energy consumption by approximately 60% compared to the MVR forced evaporation process. Simultaneously, the entire process generates no direct discharge of process wastewater into natural water bodies, while the comparative "pyrolysis + MVR" process still produces 10%-20% concentrated wastewater requiring additional treatment. Achieving both low energy consumption and zero emissions while maintaining equal or even superior crystalline salt quality is a comprehensive effect unattainable by existing technologies.

[0044] In summary, this invention overcomes technical obstacles and achieves unexpected technical effects, possessing outstanding substantive features and significant progress, and therefore demonstrates inventiveness.

[0045] (vi) Definition of Terms • Low-boiling-point heavy metals: refers to heavy metal elements that can be significantly volatilized and removed under normal pressure and pyrolysis conditions of 350℃-700℃, including but not limited to mercury (Hg); arsenic (As) compounds can be partially volatilized under the pyrolysis conditions of this invention.

[0046] • Medium and high boiling point heavy metals: refers to heavy metals and their compounds with a boiling point ≥700℃ under normal pressure, including lead, cadmium, chromium, nickel, copper, zinc, etc.

[0047] • Permeability coefficient: This refers to the performance index of a seepage-proof material in preventing liquid penetration. For flexible seepage-proof materials, the permeability coefficient of the seepage-proof structure in this invention must be ≤1.0×10⁻ 7 cm / s, which meets the mandatory national requirements of GB18597-2023. For rigid impermeable materials, their impermeability is achieved through the material's own impermeability to liquids.

[0048] • Impermeable evaporation crystallization facilities: These refer to all structures that utilize natural evaporation or artificially assisted heating to achieve slow evaporation and recrystallization of industrial waste salt, and whose impermeability meets relevant environmental protection standards. Their impermeable structure can have a permeability coefficient ≤1.0×10⁻ 7 An artificial impermeable layer with a flow rate of cm / s, or a container or surface layer made of a dense solid material that is impermeable to liquids (such as stainless steel, enamel, polytetrafluoroethylene lining, etc.).

[0049] • Old brine: refers to the concentrated mother liquor that has been repeatedly circulated and refluxed during the evaporation process, resulting in a high concentration of medium- and high-boiling-point heavy metals. Attached Figure Description

[0050] Figure 1 This is a schematic diagram of the process flow of the industrial waste salt slow evaporation resource utilization treatment method described in this invention; Figure 2 This is a schematic diagram of the industrial waste salt slow evaporation resource recovery treatment system described in this invention.

[0051] Explanation of reference numerals in the attached figures: Figure 1 In the middle: 1-Pyrolysis pretreatment process, 2-Brine preparation process, 3-Anti-seepage slow evaporation and recrystallization process, 4-Crystallized salt purification process, 5-Mother liquor reflux process, 6-Old brine discharge process, 7-Waste gas treatment process.

[0052] Figure 2 In the middle: 10-pyrolysis pretreatment unit, 20-salt solution preparation unit, 30-anti-seepage evaporation crystallization unit, 40-crystallized salt purification unit, 50-mother liquor reflux unit, 60-old brine discharge unit, 70-waste gas treatment unit. Detailed Implementation

[0053] When implementing the technical solution of this invention, especially when the object of treatment is hazardous waste, the implementing entity shall comply with laws and regulations such as the "Law of the People's Republic of China on the Prevention and Control of Environmental Pollution by Solid Waste" and obtain the corresponding hazardous waste operation license in accordance with the law.

[0054] Implementation details across locations and entities: The pyrolysis pretreatment unit, brine preparation unit, and evaporation crystallization unit of this invention can be located in the same or different plant areas, and can be operated by the same entity or different entities respectively, depending on the actual situation. The purified pyrolysis salt is a solid and can be stored and transported in bags (such as ton bags). The brine preparation unit receives the bagged purified pyrolysis salt, unpacks it, and adds it for dissolution. Units can be connected by fixed pipelines, or materials can be transported using mobile containers (ton drums, tank trucks) or solid packaging such as bags. When using a transfer method and the treated object is hazardous waste, the hazardous waste transfer manifest management system should be followed, and measures such as sealed containers and dust-proof coverings should be taken to prevent scattering and leakage.

[0055] Mother liquor heavy metal accumulation control instructions: The system is equipped with an online monitoring module for heavy metals in the mother liquor, which periodically detects the concentration of heavy metals such as Pb, Cd, and Cr in the mother liquor. When the concentration of any heavy metal accumulates to a preset threshold (e.g., Pb ≥ 50 mg / L, Cd ≥ 10 mg / L, Cr ≥ 30 mg / L), the reflux ratio is automatically reduced or the old brine discharge procedure is triggered to ensure the long-term stability of the quality of the crystallized salt.

[0056] Optional impurity removal process: As an optional optimization measure, after the pyrolysis-purified salt is prepared into a salt-containing solution and before evaporation and crystallization, one or more impurity removal processes can be added, such as chemical precipitation, adsorption, membrane separation, etc., to further remove residual heavy metals or calcium and magnesium ions. It should be noted that this impurity removal process is not a necessary technical feature for achieving the purpose of this invention. Even without this process, pyrolysis pretreatment and slow evaporation and recrystallization alone are sufficient to ensure the crystalline salt meets the standards. Those skilled in the art can choose whether to adopt this process based on the raw material conditions; such modifications do not depart from the core concept of this invention.

[0057] Mother liquor reflux ratio: 60 wt.%–100 wt.% of the concentrated mother liquor produced by slow evaporation and recrystallization is refluxed back to the brine preparation stage for recycling. For industrial waste salt with low heavy metal content, a high reflux ratio of 90%–100% can be used; for industrial waste salt with high heavy metal content, the reflux ratio should be appropriately reduced (e.g., 60%–85%) or the frequency of old brine discharge should be increased based on dynamic monitoring data of the cumulative heavy metal concentration in the mother liquor to control the balance concentration of heavy metals in the system and ensure that the leaching toxicity of the crystallized salt continuously meets the standards. No process wastewater is directly discharged into natural water bodies throughout the entire process.

[0058] Water consumption for spray purification: After collecting the crystallized salt, it is purified by spraying and rinsing with clean process water; the spray water volume is 1% to 15% of the mass of the crystallized salt, the spraying time is 1 to 10 minutes, and all the spray rinsing water is incorporated into the process water for recycling.

[0059] Instructions for Geomembrane Integrity Testing: After the geomembrane is laid, the HDPE membrane is tested for integrity using the spark test method (voltage 25kV, scanning speed ≤0.3m / s). Testing is conducted periodically. Any damage found during testing is immediately repaired and retested. Simultaneously, a leakage detection layer is installed to monitor leakage in the main geomembrane in real time through changes in pressure or conductivity.

[0060] For facilities that use rigid and dense materials (such as stainless steel storage tanks, stainless steel flooring, enamel tanks, etc.) as seepage prevention structures, their seepage prevention integrity is ensured by the following methods: the thickness of the plates is not less than 2mm, the adjacent plates are fully welded together, and the welds are tested for leakage by penetration testing or radiographic testing; a dike or leakage drainage layer is set at the bottom of the facility as a secondary seepage prevention measure.

[0061] In specific implementations of this invention, the crystallization period for slow evaporation crystallization is preferably 3 to 30 days. Those skilled in the art can reasonably select this range based on the properties of the waste salt, weather conditions, and evaporation method. Specifically, when artificial heating is used and the heating temperature is high, the crystallization period can be shortened to 24 hours, but still meets the basic requirement of not less than 24 hours. To further improve crystal quality and heavy metal separation efficiency, considering local weather conditions, it is recommended to control the crystallization period to more than 3 days.

[0062] Old brine purification unit (optional optimization measures) As a further optimization of the present invention, in order to further reduce the final discharge of old brine, recover valuable metals therein, and improve the overall resource utilization efficiency of the system, an old brine purification unit can be added between the mother liquor reflux system and the old brine discharge unit.

[0063] (1) Working principle of the purification unit In one feasible implementation, the old brine purification unit employs a combined process of chemical precipitation and flocculation sedimentation. Specifically, a heavy metal precipitant (such as a sulfide or hydroxide precipitant) is added to the periodically discharged old brine to convert heavy metal ions such as lead, cadmium, and chromium into insoluble hydroxides or sulfides as precipitates. Subsequently, a flocculant is added, achieving solid-liquid separation through flocculation sedimentation. The heavy metal sludge obtained from sedimentation is disposed of by a qualified unit in accordance with hazardous waste management requirements. The purified supernatant (mainly containing sodium chloride or sodium sulfate) can be recycled back to the brine preparation unit for reuse. The specific types, dosages, and process conditions of the precipitant and flocculant can be determined by those skilled in the art through conventional experiments based on the types and concentrations of heavy metals in the old brine, and will not be elaborated here.

[0064] (2) The impact of purification measures on the reflux ratio Without a brine purification unit, to ensure the leaching toxicity of the crystallized salt consistently meets the requirements of GB5085.3-2007, for industrial waste salt with high heavy metal content, experimental verification shows that the reflux ratio should not exceed 60% (i.e., the mother liquor discharge rate should not be less than 40%). Otherwise, the equilibrium concentration of heavy metals in the system will exceed the safety threshold, resulting in substandard crystallized salt quality. For industrial waste salt with low heavy metal content, even without a purification unit, a higher reflux ratio of 90%–95% can be used.

[0065] With the addition of a brine purification unit, the system's reliance on "dilution of heavy metals through high discharge rates" is significantly reduced because the purification unit can actively remove heavy metals from the mother liquor. At this point, even for raw materials with high heavy metal content, a high reflux ratio of 90%–95% can be used to further reduce the discharge of brine. The reuse of the purified salt solution can further improve the overall salt resource recycling rate.

[0066] (3) The availability of purification measures The brine purification unit is an optional optimization measure of this invention, not a necessary technical feature. Even without this purification unit, by periodically discharging the brine and directly handing it over to a qualified unit for disposal or solidification and landfill, the technical solution of this invention can still achieve stable operation of the entire process and meet the quality standards of the crystallized salt. Adding the purification unit is a further optimization based on this, aiming to reduce the amount of brine discharged, recover valuable metals, and improve economic efficiency, but it does not affect the independent completeness and patent protection scope of the core solution of this invention (pyrolysis + slow evaporation).

[0067] Example 1: Treatment of NaCl-type pesticide waste salt (natural evaporation method) Object to be treated: NaCl-type industrial waste salt generated from the production of HW04 type pesticides, with an initial TOC of 85000mg / kg, containing heavy metals such as mercury, arsenic, lead, and cadmium, and is classified as hazardous waste.

[0068] Processing steps: • Pyrolysis pretreatment: Waste salt is fed into a rotary kiln pyrolysis furnace and pyrolyzed at 450℃ for 120 minutes in a low-oxygen atmosphere with an oxygen volume fraction of ≤3%. Organic pollutants are removed by cracking, and low-boiling-point heavy metals such as mercury and arsenic are removed by volatilization to obtain pyrolyzed clean salt. The pyrolysis exhaust gas is treated by rapid cooling, bag filter dust collection, alkaline spray deacidification, and activated carbon adsorption before being discharged in compliance with standards.

[0069] • Salt solution preparation: Pyrolytic purified salt and water are mixed in a stirring dissolving tank to prepare a saturated NaCl solution, which is then transported to the evaporation and crystallization facility.

[0070] • Slow evaporation recrystallization (natural evaporation method): The evaporation crystallization facility adopts a double-layer HDPE geomembrane composite seepage prevention structure with a permeability coefficient ≤5.0×10⁻ 8 The flow rate is cm / s, which meets the GB18597-2023 standard. After the saturated brine is sent into the facility, it is naturally evaporated by solar radiation and natural wind. The crystallization cycle is about 5 days (≥24 hours). NaCl is recrystallized and precipitated with the evaporation of water. Lead, cadmium and other medium and high boiling point heavy metals are enriched in the mother liquor and separated from the crystallized salt.

[0071] Purification and Recycling: Crystallized salt is purified by spraying and rinsing with clean process water. The spray water volume is 5% of the mass of the crystallized salt, and the spraying time is 3 minutes. All rinsing water is recycled into the process water. 100wt.% of the concentrated mother liquor generated by evaporation is recycled to the salt solution preparation stage. No process wastewater is directly discharged into natural water bodies throughout the entire process.

[0072] Processing results: The final crystalline salt NaCl purity was 99.2%, meeting the first-grade standard of GB / T5462-2015 "Industrial Salt", with TOC < 10 mg / kg, and all heavy metal leaching concentrations met the requirements of GB5085.3-2007. In this example, due to the relatively low heavy metal content of the raw material, the mother liquor after pyrolysis was continuously run under 100% reflux conditions for 30 days, and the quality of the crystalline salt consistently met the standards, proving that full reflux operation can be achieved for specific waste salts.

[0073] Example 2: Treatment of mixed-salt type fine chemical waste salt (natural evaporation method) Object to be treated: NaCl-Na2SO4 mixed salt type industrial waste salt generated from HW49 fine chemical production, with an initial TOC of 120,000 mg / kg, containing heavy metals such as mercury, arsenic, lead, cadmium, chromium and nickel, and is classified as hazardous waste.

[0074] Processing steps: • Pyrolysis pretreatment: Waste salt is fed into a pusher kiln pyrolysis furnace and pyrolyzed at 500℃ for 150 minutes in an oxygen-free atmosphere to remove organic pollutants by cracking and remove low-boiling-point heavy metals such as mercury and arsenic by volatilization, thus obtaining pyrolyzed clean salt; pyrolysis exhaust gas is treated by a supporting purification system and then discharged in compliance with standards.

[0075] • Salt solution preparation: Mix the pyrolytic purified salt with water to prepare a supersaturated mixed salt solution, and then transport it to the evaporation and crystallization facility.

[0076] • Slow evaporation recrystallization (natural evaporation method): The evaporation crystallization facility adopts a composite seepage prevention structure of "epoxy resin coating + HDPE geomembrane", with a permeability coefficient ≤8.0×10⁻ 8cm / s, which meets the GB18597-2023 standard; after the supersaturated brine is sent into the facility, 0.3% of 150-mesh NaCl seed crystals are added to the brine, and natural evaporation is carried out by solar radiation and natural wind power. The crystallization cycle is about 12 days. The main salt component recrystallizes out, and the medium and high boiling point heavy metals are enriched in the mother liquor.

[0077] Purification and Recycling: Crystallized salt is purified by spraying and rinsing with clean water, and all rinsing water is recycled. 95wt.% of the concentrated mother liquor produced by evaporation is recycled to the salt solution preparation process, and 5wt.% of the old brine enriched with heavy metals is periodically discharged and disposed of in accordance with relevant environmental protection regulations.

[0078] Processing results: The final crystallized salt content was 99.75%, which meets the superior grade standard of "Industrial Salt" GB / T5462-2015. The TOC was <8mg / kg, and the heavy metal leaching concentration was far below the limit of GB5085.3-2007. The system has been running stably for 6 months without any abnormalities.

[0079] Example 3: Treatment of NaCl-type pesticide waste salt (artificial heating + natural evaporation, supplemented by ventilation) Object to be treated: NaCl-type industrial waste salt generated from the production of HW04 type pesticides, with an initial TOC of 105,000 mg / kg, containing heavy metals such as mercury, arsenic, lead, and chromium, and is classified as hazardous waste.

[0080] Processing steps: • Pyrolysis pretreatment: The waste salt is fed into a microwave pyrolysis furnace and pyrolyzed at 550°C for 90 minutes in a low-oxygen atmosphere to obtain pyrolyzed clean salt. The pyrolysis exhaust gas is purified and then discharged in compliance with standards.

[0081] • Salt solution preparation: Mix the pyrolysis clean salt with recycled water to prepare a saturated NaCl solution, which is then transported to the evaporation and crystallization facility.

[0082] • Slow evaporation recrystallization (using artificial heating + natural evaporation, supplemented by ventilation): The evaporation crystallization facility adopts a rigid composite seepage-proof structure of "impermeable concrete + polyurea coating", with a permeability coefficient ≤1.0×10⁻ 7 The flow rate is cm / s, which meets the GB18597-2023 standard. After the saturated brine is fed into the facility, 0.2% of 100-mesh NaCl seed crystals are added. The solution is then heated to 80°C using artificial heating, and evaporated and crystallized using natural solar energy and auxiliary wind energy. The crystallization cycle is about 24 hours. The main salt is precipitated, and heavy metals are enriched in the mother liquor.

[0083] Purification and Recycling: Crystallized salt is purified by spraying, and the rinsing water is recycled; 96 wt.% of the concentrated mother liquor is recycled, and 4 wt.% of the old brine enriched with heavy metals is periodically discharged.

[0084] Processing results: The final crystallized salt NaCl purity was 99.3%, meeting the first-grade standard of "Industrial Salt" GB / T5462-2015, with TOC < 10 mg / kg, and all heavy metal leaching concentrations met the requirements of GB5085.3-2007. Because organic matter has been removed, the pyrolysis-processed salt has high purity and a simple composition. During the slow evaporation process, only a small amount of old brine needs to be discharged to maintain system stability, significantly reducing the burden of mother liquor disposal compared to traditional salt fields.

[0085] Application scenario description: When this invention is applied to rainy areas in the south or areas with high annual precipitation, the following supporting measures can be added to the evaporation crystallization facility based on the aforementioned embodiments 1 to 3: • Transparent windproof and rainproof canopy: prevents rainwater from directly entering the evaporation facility and avoids dilution of the brine solution; • Auxiliary air distribution fan: Increases the airflow velocity on the evaporation surface to compensate for the decrease in evaporation rate caused by high humidity; • Automatic circulating spray device: Keeps the crystallization surface moist and prevents the salt crust from hardening and blocking evaporation.

[0086] Engineering simulations show that, with the above-mentioned supporting measures, the effective evaporation days in rainy areas can be increased by more than 30%, the crystallization cycle shortened by about 30% compared to without these measures, and the production capacity per unit area increased by about 40%. The quality of the crystallized salt is comparable to that achieved in arid regions. It should be noted that the above data are based on engineering simulations and theoretical calculations; actual application effects may vary depending on specific climatic conditions, but the effectiveness of these supporting measures is within the reasonable expectations of those skilled in the art.

[0087] Example 4: Waste salt treatment in the dyeing and printing industry (non-hazardous waste, natural evaporation method) Object to be treated: NaCl-type industrial by-product salt (TOC=25000mg / kg, containing small amounts of lead and chromium, not listed in the hazardous waste list) produced by a dyeing and printing enterprise.

[0088] Processing steps: The process steps and parameters are exactly the same as those in Example 1.

[0089] Processing results: The final crystallized salt NaCl purity is 99.4%, which meets the superior grade standard of "Industrial Salt" GB / T5462-2015. TOC < 9 mg / kg, and the heavy metal leaching concentration meets the relevant environmental protection requirements. It can be directly reused as industrial salt.

[0090] Example 5: Treatment of KCl-type chemical waste salt (natural evaporation method) Object to be treated: KCl type industrial waste salt (main component is potassium chloride, content is about 92%) generated by a fine chemical enterprise, with an initial TOC of 78000mg / kg, containing about 15mg / kg of mercury, about 80mg / kg of lead and about 12mg / kg of cadmium, and is classified as hazardous waste.

[0091] Processing steps: • Pyrolysis pretreatment: The waste salt is fed into a microwave pyrolysis furnace and pyrolyzed at 450℃ for 120 minutes in a low-oxygen atmosphere with an oxygen volume fraction ≤3%. The pyrolysis exhaust gas is treated by rapid cooling, bag filter dust collection, alkaline spray deacidification, and activated carbon adsorption before being discharged in compliance with standards.

[0092] • Salt solution preparation: Mix the pyrolysis purified salt with the process recycled water to prepare a saturated KCl solution.

[0093] • Slow evaporation recrystallization (natural evaporation method): A saturated KCl solution is transported to an evaporation crystallization facility (double-layer HDPE geomembrane composite seepage prevention structure, permeability coefficient ≤5.0×10⁻). 8 (cm / s). Natural evaporation is achieved using solar radiation and wind power, with a crystallization cycle of approximately 12 days. KCl crystals out, while medium- and high-boiling-point heavy metals such as lead and cadmium are enriched in the mother liquor.

[0094] Purification and recycling: Crystallized salt is sprayed and rinsed, and the rinsing water is reused; 90 wt.% of the concentrated mother liquor is refluxed, and 10 wt.% of the old brine is discharged periodically.

[0095] Processing results: The final crystallized salt KCl purity was 99.1%, which met the relevant standards of GB / T 7118-2008 "Industrial Potassium Chloride", TOC < 10 mg / kg, and the heavy metal leaching concentration fully met the requirements of GB5085.3-2007.

[0096] Example 6: Slow Evaporation Process for Recovering Heat Energy (Artificial Heating Method) The target of treatment is HW49 type mixed salt type fine chemical waste salt, the same as in Example 2.

[0097] Processing steps: • Pyrolysis pretreatment: exactly the same as in Example 2 (500°C, anaerobic atmosphere, 150 min), to obtain pyrolyzed purified salt.

[0098] • Salt solution preparation: Mix the pyrolysis purified salt with recycled water to prepare a saturated mixed salt solution.

[0099] • Slow evaporation recrystallization (artificial heating method): The saturated brine solution is transported to a horizontal flow evaporator crystallizer with a heat exchange jacket (effective area 1㎡, depth 0.1m, made of carbon steel lined with PTFE). 80℃ hot water recovered within the plant (from the flue gas waste heat exchanger) is used to circulate and heat the brine solution through the jacket, controlling the brine temperature between 50℃ and 60℃. The evaporation rate is approximately 2.5 L / m²·d, and the crystallization cycle is approximately 3 days (≥24 hours). The initial brine depth is 0.1m; no replenishment is made during evaporation. Crystallized salt is collected when the salt layer thickness reaches 5–8cm.

[0100] Purification and Recycling: After collection, the crystallized salt is purified by spraying with clean process water. All the rinse water is recycled back to the brine preparation stage. Of the concentrated mother liquor produced by evaporation, 90 wt.% is recycled and reused, while the remaining 10 wt.% of the heavy metal-rich brine is periodically discharged.

[0101] Processing results: The final crystallized salt content was 99.4%, TOC < 9 mg / kg, and the heavy metal leaching concentrations (Pb ≤ 0.10 mg / L, Cd ≤ 0.03 mg / L, Cr ≤ 0.08 mg / L) fully met the requirements of GB5085.3-2007. The average crystal particle size was 0.8–1.2 mm. The system operated continuously for 4 months without scaling or clogging in the heating jacket, achieving a waste heat recovery rate of 75%, and reducing unit product energy consumption by approximately 60% compared to the MVR forced evaporation process.

[0102] Comparative Example 1: Pyrolysis + MVR Forced Evaporation Process The waste salt and pyrolysis pretreatment process is exactly the same as in Example 2, except that after pyrolysis, the MVR mechanical compression forced evaporation process is used instead of the slow evaporation recrystallization process.

[0103] Results: The quality of the final crystallized salt was basically the same as that in Example 3, but the process generated 10% to 15% of concentrated wastewater that required additional treatment, increasing the risk of secondary pollution and the burden of treatment.

[0104] Comparative Example 2: Natural evaporation process with low-concentration brine feed (simulating traditional sun-dried salt fields) The same pyrolytic purified salt as in Example 1 was used, except that the pyrolytic purified salt was prepared into a low-concentration salt solution (10% salt content) with the same salt content as the natural brine and sent into the seepage-proof evaporation facility. The remaining evaporation conditions were exactly the same as in Example 1.

[0105] Results: The quality of the crystallized salt met the standards, but it took up to 28 days to concentrate from low concentration to saturation, and the complete crystallization cycle took 42 days, which is 2.3 times that of Example 1. The annual processing capacity per unit area was only 42% of that of Example 1.

[0106] Experiment Example 1: Pyrolysis Temperature Criticality Experiment Using NaCl-type pesticide waste salt from Example 1, with a fixed pyrolysis time of 120 min and a low-oxygen atmosphere, six temperature gradients were set at 300℃, 350℃, 400℃, 550℃, 700℃, and 750℃, with all other experimental conditions remaining the same. The experimental results are as follows: | Pyrolysis temperature | Organic matter removal rate | Low-boiling-point heavy metal removal rate | Compliance status of crystalline salts | | :\-\-- | :\-\-- | :\-\-- | :\-\-- | | 300℃ | 72.3% | 61.5% | Not up to standard | | 350℃ | 98.1% | 97.2% | Meets standards | | 400℃ | 98.7% | 97.9% | Meets standards | | 550℃ | 99.5% | 98.9% | Meets standards | | 700℃ | 99.8% | 99.1% | Meets standards | | 750℃ | 99.8% | 99.2% | Meets standards | Experimental results show that 350℃ is the critical inflection point for the pyrolysis effect of this invention; above 350℃, efficient removal of organic matter and low-boiling-point heavy metals can be achieved. Although the higher the temperature, the better the removal effect of organic matter, salt melting, wall adhesion, and equipment corrosion may occur starting at 750℃.

[0107] Experiment Example 2: Synergistic Effect Verification Experiment Using the mixed-salt industrial waste salt from Example 2, two control experiments were set up: (A) Pyrolysis pretreatment only: The organic matter removal rate reached 98.5%, but the separation rate of medium and high boiling point heavy metals was only 12.3%, and the toxicity of crystalline salt leaching did not meet the standard (heavy metals exceeded the standard). (B) The synergistic treatment of the present invention (pyrolysis pretreatment + slow evaporation recrystallization): the organic matter removal rate reaches 98.7%, the medium and high boiling point heavy metals are enriched in the mother liquor, and the leaching toxicity of the crystallized salt is 100% stable and meets the standard, which meets the requirements of GB5085.3-2007.

[0108] The above results indicate that pyrolysis pretreatment alone cannot effectively separate medium- and high-boiling-point heavy metals; only by deeply coupling pyrolysis pretreatment with slow evaporation and recrystallization can the complete harmlessness of organic matter and heavy metals be achieved simultaneously, resulting in resource-based products that meet national standards.

[0109] Experiment Example 3: Comparison of Product Quality Based on Different Process Routes Using the mixed-salt industrial waste salt from Example 2, the products were subjected to pyrolysis pretreatment only, pyrolysis + MVR forced evaporation, and the pyrolysis + slow evaporation treatment of this invention. The product quality was compared, and the results are as follows: | Process Route | Crystallization Cycle | Main Salt Purity | Total Organic Carbon (TOC) | Appearance | Average Particle Size | Heavy Metal Leaching Concentration (Pb) | Rinse Water Consumption | | :\-\-- | :\-\-- | :\-\-- | :\-\-- | :\-\-- | :\-\-- | :\-\-- | :\-\-- | | Pyrolysis pretreatment only | — | ≥98.5% | 18 mg / kg | Pale yellow, fine particles | <0.1 mm | — (Substandard) | — | | Pyrolysis + MVR forced evaporation | 3 h | ≥99.1% | 12 mg / kg | White, fine particles | 0.05-0.2 mm | 0.38 mg / L | Reference value | | This invention (pyrolysis + slow evaporation) | 12 days | ≥99.86% | Not detected | White crystals, coarse particles | 0.8-2.5 mm | 0.12 mg / L | 68% of the baseline value | The purity of the crystalline salts obtained in the above embodiments varies depending on the type of waste salt, pyrolysis parameters, and evaporation conditions. The measured values ​​range from 99.1% to 99.86%, all significantly superior to the traditional forced evaporation process, fully demonstrating the technical advantages of this invention. The above results indicate that compared to pyrolysis alone and pyrolysis + MVR processes, the crystalline salts obtained by this invention have significant advantages in purity, residual organic carbon, appearance, particle size, and heavy metal leaching concentration. In particular, this invention, through a slow crystallization mechanism (crystallization cycle of 12 days), reduces the lead leaching concentration by 68% compared to MVR forced evaporation (crystallization cycle of 3 hours), increases the purity of the main salt by 0.76 percentage points (from 99.1% to 99.86%), reduces rinsing water consumption by 32%, and eliminates the detection of total organic carbon, achieving stable production of high-quality resource-based products.

[0110] Description of optional implementation methods: As an optional embodiment of the present invention, the concentrated mother liquor (old brine) generated by slow evaporation and recrystallization can be discharged periodically and subjected to solidification / stabilization treatment before safe landfilling, instead of being directly recycled. Specifically: the old brine is transported to the solidification / stabilization treatment system, and 10%~30% cement, 5%~20% fly ash, and 0.1%~0.5% heavy metal chelating agent (such as organosulfur compounds) are added by weight. After thorough mixing, it is cast into a mold and cured for 7~14 days. The solidified body is tested according to the "Solid Waste Leaching Toxicity Leaching Method - Sulfuric Acid and Nitric Acid Method" (HJ / T 299) to ensure that its heavy metal leaching concentration is lower than the limit specified in the "Standard for Pollution Control of Hazardous Waste Landfill" (GB 18598-2019). Then, it is handed over to a qualified unit for final disposal or further recovery of valuable metals.

[0111] It should be noted that the aforementioned solidification / stabilization landfill method is only one optional end-of-life disposal solution. Those skilled in the art can also use other legal methods that comply with national and local environmental regulations to dispose of the brine (such as entrusting a unit holding a hazardous waste operation license to dispose of it) according to local environmental protection requirements and disposal conditions. These methods do not depart from the core concept of this invention. This invention does not impose any particular limitation on the final disposal method of the brine, as long as it meets environmental compliance requirements.

Claims

1. A method for the slow evaporation and resource recovery of industrial waste salt, characterized in that, The treatment of solid industrial waste salt includes the following steps: (1) Pyrolysis pretreatment: The industrial waste salt to be treated is placed in an oxygen-free or low-oxygen atmosphere and pyrolyzed at 350℃~700℃ for 30min~180min to remove organic pollutants from the waste salt by cracking and at the same time to volatilize and remove low-boiling-point heavy metals from the waste salt to obtain pyrolyzed clean salt. (2) Slow evaporation and recrystallization: The pyrolysis purified salt is prepared into a salt solution; the salt solution is transported to a seepage-proof evaporation and crystallization facility, and the evaporation rate is controlled by natural evaporation, artificial heating by recovering industrial waste heat or other low-grade heat sources, or a combination of natural evaporation and artificial heating, so that the crystallization cycle is not less than 24 hours, so that the salt recrystallizes and precipitates out, and at the same time, the medium and high boiling point heavy metals are enriched in the evaporation mother liquor, so as to achieve solid-liquid separation from the crystallized salt and obtain the final crystallized salt; The seepage-proof evaporation crystallization facility is equipped with a seepage-proof structure, and the seepage-proof structure meets one of the following conditions: (a) Includes at least one layer of artificial impermeable material, wherein the permeability coefficient of the artificial impermeable material layer is ≤1.0×10⁻ 7 cm / s; (b) It is composed of a dense solid material that is impermeable to liquids, said dense solid material being chemically compatible with the salt solution it comes into contact with.

2. The method according to claim 1, characterized in that, The slow evaporation recrystallization described in step (2) adopts a natural evaporation method, utilizing solar radiation and natural wind power for evaporation; when the natural evaporation rate is insufficient to meet the preset crystallization cycle, the evaporation crystallization facility is equipped with a windproof and rainproof shed and / or an auxiliary wind distribution device.

3. The method according to claim 1, characterized in that, The slow evaporation recrystallization in step (2) is carried out by artificial heating, using recovered industrial waste heat or other low-grade heat sources for heating, and the heating temperature is not higher than 80°C; the evaporation crystallizer is selected from one or more combinations of evaporation pool with heat exchange jacket, low-temperature multi-effect evaporator, and heat pump evaporation system; the recovered industrial waste heat is selected from one or more of flue gas waste heat, steam condensate waste heat, equipment cooling water waste heat, and waste heat steam.

4. The method according to claim 1, characterized in that, In step (1), the temperature of the pyrolysis pretreatment is 350℃~550℃ and the treatment time is 60min~150min; the oxygen volume fraction of the low oxygen atmosphere is ≤6%; the waste gas generated in the pyrolysis process is discharged in compliance with standards after being treated by rapid cooling, dust removal, acid removal and adsorption.

5. The method according to claim 1, characterized in that, In step (2), the method of preparing the pyrolytic purified salt is as follows: the pyrolytic purified salt and water are mixed in a stirring dissolving tank to prepare the salt solution; part or all of the concentrated mother liquor generated by slow evaporation and recrystallization is recycled back to the salt solution preparation process for reuse; the reflux ratio is dynamically adjusted according to the cumulative concentration of heavy metals in the mother liquor; and the old brine enriched with high concentrations of heavy metals is periodically discharged.

6. The method according to claim 1, characterized in that, In step (2), seed crystals consistent with the main salt components of the pyrolysis purified salt are added to the salt solution; the amount of seed crystals added is 0.1% to 0.5% of the mass of the salt solution.

7. The method according to claim 1, characterized in that, In step (2), after collecting the crystallized salt, it is purified by spraying and rinsing with clean process water; the spray water volume is 1% to 15% of the mass of the crystallized salt, the spraying time is 1 to 10 minutes, and all the spray rinsing water is incorporated into the process water for recycling.

8. The method according to claim 1, characterized in that, The concentration of the salt-containing solution is the saturated and / or supersaturated concentration of the main salt component at its preparation temperature.

9. A system for the slow evaporation and resource recovery of industrial waste salt, characterized in that, The system includes a pyrolysis pretreatment unit (10), a brine preparation unit (20), and a seepage-proof evaporation crystallization unit (30) arranged in the process sequence, and the units are interconnected; the seepage-proof evaporation crystallization unit (30) is configured to control the evaporation rate so that the crystallization cycle is not less than 24 hours; the seepage-proof structure of the seepage-proof evaporation crystallization unit (30) satisfies one of the following conditions: (a) it includes at least one layer of artificial seepage-proof material, and the permeability coefficient of the artificial seepage-proof material layer is ≤1.0×10⁻ 7 cm / s; (b) Composed of a dense solid material that is impermeable to liquids, the dense solid material being chemically compatible with the salt solution it contacts; the system further includes an exhaust gas treatment unit (70) connected to the exhaust gas outlet of the pyrolysis pretreatment unit (10).

10. The system according to claim 9, characterized in that, It also includes a crystallization salt purification unit (40) connected to the crystallization salt outlet of the anti-seepage evaporation crystallization unit (30), and a mother liquor reflux unit (50) connected to the mother liquor outlet of the anti-seepage evaporation crystallization unit (30). The outlet of the mother liquor reflux unit (50) is connected to the inlet of the salt solution preparation unit (20) and the inlet of the old brine discharge unit (60), respectively.

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