Chemical recovery process method and application of acrylic

Abstract

The invention discloses a chemical recycling process method and application of acrylic, and belongs to the technical field of recycling of high polymer materials. The chemical recovery process method comprises the steps of pretreatment, molten salt slurry preparation, catalytic depolymerization, monomer purification and molten salt purification circulation. Wherein the molten salt is a nitric acid alkali metal salt eutectic system, the catalyst is a bi-component solid-phase catalyst containing a main catalyst and a cocatalyst, the main catalyst is a SnO2 and ZnO composite metal oxide loaded on mesoporous silica, and the cocatalyst is CeO2 loaded on alkaline zeolite. According to the method, efficient chemical recovery of the acrylic waste is achieved, the yield of the MMA monomer is larger than or equal to 95%, the purity of the MMA monomer is larger than or equal to 99.8%, the obtained monomer can be directly used for the high-end field of optical-grade PMMA and the like, and the technical problems that in the prior art, the recovery efficiency is low, the product quality is poor, and closed-loop circulation cannot be achieved are solved.

Description

Technical Field

[0001] This invention relates to the field of polymer material recycling technology, and in particular to a chemical recycling process and its application for acrylic. Background Technology

[0002] Polymethyl methacrylate (PMMA), commonly known as acrylic or plexiglass, is an important transparent polymer material. Since its industrial production by Rohm and Haas in Germany in 1933, it has been widely used in construction, transportation, optics, medical devices, and daily necessities due to its excellent optical properties (light transmittance up to 92%, superior to ordinary glass), good weather resistance, mechanical strength, and processing performance.

[0003] The traditional production method of PMMA faces two major bottlenecks:

[0004] Raw material dependence: Industrially, monomeric MMA is mainly produced through the acetone cyanohydrin process (ACH process) or the isobutylene oxidation process, both of which are heavily reliant on petroleum resources. Producing 1 ton of MMA requires approximately 1.2 tons of petroleum-based raw materials, and the ACH process uses highly toxic hydrogen cyanide, posing safety and environmental risks.

[0005] Waste disposal challenges: PMMA products typically have a lifespan of 5-15 years, generating a large amount of waste annually. Traditional disposal methods include:

[0006] Landfill disposal: PMMA takes hundreds of years to degrade naturally, consuming land resources;

[0007] Incineration: Although some heat energy can be recovered, it produces a large amount of CO2 (2.2 kg CO2 / kg PMMA) and toxic gases (HCN, NO). x );

[0008] Physical recycling: This involves melting and regranulating the material, but the material properties are significantly reduced, and it can only be used at a lower grade.

[0009] Physical recycling involves re-granulating PMMA waste through melt extrusion. This process is simple and low-cost, but it suffers from thermal degradation. PMMA has poor thermal stability and undergoes thermal degradation during the melting process (typically requiring 200-250℃). The main degradation mechanisms include:

[0010] End-group-induced depolymerization: The breakage of unsaturated bonds at the ends of polymer chains or weak bonds at head-to-head connections triggers zipper-like depolymerization;

[0011] Random chain scission: The polymer backbone breaks randomly at high temperatures, producing oligomers;

[0012] Side group elimination: The ester group is released to produce methacrylic acid and methanol;

[0013] These reactions lead to: a decrease in molecular weight from an initial 100,000-200,000 to below 50,000; and a broadening of the molecular weight distribution. The concentration increases from 2.0 to over 3.5, producing small molecule degradation products such as methacrylic acid, methanol, and formaldehyde.

[0014] Performance degradation manifestations: Optical performance deterioration: light transmittance drops from 92% to below 85%, haze increases from <1% to >5%, and yellowing index increases significantly; Mechanical performance deterioration: tensile strength decreases by 30-50%, and impact toughness decreases even more significantly; Processing performance deteriorates: melt flow index is unstable, and products are prone to silver streaks and bubbles;

[0015] Therefore, physically recycled PMMA can usually only be used in fields with low performance requirements, such as flower pots and clothes hangers, and cannot achieve closed-loop recycling.

[0016] Chemical recycling aims to depolymerize PMMA into monomeric MMA, theoretically enabling unlimited recycling and making it an ideal recycling method. Existing technologies mainly include:

[0017] 1) High-temperature pyrolysis method: PMMA is heated to 300-450℃ in an oxygen-free or inert atmosphere to decompose it. However, it has poor selectivity: high temperature leads to various side reactions; β-cleavage: generating methyl methacrylate dimers and trimers; decarboxylation reaction: producing CO2 and low molecular weight hydrocarbons; cyclization reaction: generating cyclic trimers; severe coking: local overheating leads to carbonization, and the yield is usually <80%; low product purity: containing a large number of impurities, requiring complex refining; high energy consumption: requiring the maintenance of high temperature, resulting in large heat loss.

[0018] 2) Fluidized bed pyrolysis: PMMA particles are pyrolyzed in a fluidized bed to improve heat and mass transfer. However, the equipment is complex, the catalyst is prone to deactivation, and the problem of poor selectivity at high temperatures still cannot be solved.

[0019] 3) Supercritical / subcritical fluid method: This method uses water, alcohols, or CO2 to depolymerize PMMA under supercritical conditions. Studies have found that PMMA can be efficiently depolymerized into MMA in supercritical methanol (T > 240℃, P > 8MPa). However, this technology has drawbacks such as high equipment requirements, significant safety risks, difficulty in solvent recovery (requiring a complex separation system), corrosion problems (the medium is highly corrosive under high temperature and pressure), and poor economic efficiency (high energy consumption and investment costs).

[0020] 4) Catalytic depolymerization method: Adding a catalyst lowers the depolymerization temperature and improves selectivity. Catalysts studied include: acidic catalysts: H2SO4, AlCl3, solid acids, etc.; basic catalysts: NaOH, KOH, carbonates, etc.; metal oxides: ZnO, CaO, Al2O3, etc.

[0021] However, existing catalytic systems all have significant shortcomings:

[0022] Homogeneous catalysts: difficult to separate and recover, contaminating the product;

[0023] Solid catalysts exhibit poor dispersion and low efficiency in molten PMMA.

[0024] Many side reactions: acid catalysis easily leads to ester hydrolysis, and base catalysis easily initiates cross-linking;

[0025] Catalyst deactivation: severe carbon buildup, sintering, and loss.

[0026] A comprehensive analysis of existing technologies reveals the following shortcomings in the chemical recovery of PMMA:

[0027] 1) Challenges in controlling reaction selectivity: The ideal path for PMMA depolymerization is end-initiated zipper-like depolymerization, where each polymer molecule is completely converted into a monomer. However, in practice, various side reactions compete for control. For example, chain transfer reactions generate dimers and trimers, reducing monomer yield; β-fracture reactions occur, where free radicals undergo β-splitting to generate small molecule aldehydes, producing chromophores such as formaldehyde and methacrolein, affecting product color and optical properties; and cross-linking reactions occur, where two free radicals combine to form cross-linked structures, producing insoluble substances that clog equipment and reduce yield. Existing technologies cannot effectively suppress these side reactions, resulting in insufficient product purity.

[0028] 2) Heat and mass transfer efficiency problem: PMMA is a poor conductor of heat (thermal conductivity of about 0.2 W / m·K). During the pyrolysis process, the outside is decomposed at high temperature, while the inside is still solid. The generated monomer vapor is difficult to escape from the high viscosity melt, and local overheating leads to carbonization. Traditional reactors (fixed bed, fluidized bed) cannot provide a uniform temperature field and effective mass transfer conditions.

[0029] 3) Challenges in catalyst design and recovery: An ideal catalyst needs to possess the following characteristics simultaneously: high selectivity: it only promotes the breaking of CO bonds and does not catalyze the breaking of C-C bonds; thermal stability: it is stable for a long time at 250-300℃; easy separability: it is easy to separate from products and media; resistance to poisoning: it resists poisoning by impurities. Existing catalyst systems cannot meet these requirements simultaneously, especially in terms of stability in high-temperature molten media.

[0030] There is an urgent need to develop a new technology that can achieve efficient, selective, continuous, and clean chemical recycling of PMMA. Summary of the Invention

[0031] In order to overcome the shortcomings of the existing technology, and taking advantage of the advantages of molten salt as a high-temperature reaction medium, such as good thermal conductivity, large heat capacity, low vapor pressure and good chemical stability, this invention proposes a chemical recycling process and application for acrylic plastics.

[0032] To achieve the above objectives, the present invention adopts the following technical solution:

[0033] A chemical recycling process for acrylic includes the following steps:

[0034] (a) Pretreatment: The acrylic waste is crushed, washed and dried to obtain acrylic particles with a particle size of 1-5 mm and a moisture content of <0.1 wt%.

[0035] (b) Preparation of molten salt slurry: The acrylic particles obtained in step (a) are mixed with molten salt in a premixer. The molten salt is a eutectic system of alkali metal nitrate with a melting point below 300°C. The mixing temperature is 20-60°C higher than the eutectic point of the molten salt. The mass ratio of acrylic particles to molten salt is 1:1 to 1:3. At the same time, a catalyst is added to form a uniform acrylic-molten salt-catalyst slurry.

[0036] (c) Catalytic depolymerization: The slurry obtained in step (b) is fed into a reactor and depolymerized at a temperature of 230-300℃ and an absolute pressure of 10-30kPa. The generated methyl methacrylate (MMA) monomer vapor is separated from the reactor.

[0037] (d) Monomer purification: The methyl methacrylate vapor separated in step (c) is condensed and distilled to obtain methyl methacrylate monomer with a purity ≥99.8wt%;

[0038] (e) Molten salt purification and recycling: The molten salt slurry after the reaction in step (c) is subjected to solid-liquid separation and purification treatment, and the recovered molten salt is returned to step (b) for recycling.

[0039] One of the choices for the molten salt is: (b) a binary eutectic system of potassium nitrate (KNO3) and sodium nitrate (NaNO3), with the following composition by mass percentage: KNO3 50-55%, NaNO3 45-50%, and the eutectic point of the eutectic system is 210-225℃.

[0040] The second option for the molten salt is: (b) a ternary eutectic system of potassium nitrate (KNO3), sodium nitrite (NaNO2) and sodium nitrate (NaNO3), with the following composition by mass percentage: KNO3 50-55%, NaNO2 35-45%, NaNO3 5-15%, and the eutectic point of the eutectic system is 130-170℃.

[0041] Preferably, in (b), the catalyst is a two-component solid-phase catalyst comprising a main catalyst and a co-catalyst; the main catalyst is a SnO2 and ZnO composite metal oxide supported on mesoporous silica, wherein the molar ratio of Sn to Zn is 3:1 to 5:1, and the total metal oxide loading is 10-20 wt% of the support mass; the co-catalyst is CeO2 supported on alkaline zeolite, wherein the CeO2 loading is 5-15 wt% of the support mass.

[0042] Furthermore, the CeO2 in the co-catalyst is passivated by phosphate solution; the dry mass mixing ratio of the main catalyst and the co-catalyst is 1:1 to 3:1; the amount of catalyst added is 1-3 wt% of the mass of methyl methacrylate monomer.

[0043] Preferably, in (b), 0.1-0.5 wt% of phenthiazine as a polymerization inhibitor is added during the formation of the slurry or during the slurry transport process.

[0044] Preferably, in (c), the reactor is a falling film short-path evaporator, a thin-film evaporator, or a stirred tank reactor; the reaction temperature is achieved through gradient temperature control, including a preheating / initiation zone at a temperature of 230-250°C and a main reaction / depolymerization zone at a temperature of 260-280°C.

[0045] Preferably, the molten salt purification cycle in (e) specifically includes:

[0046] (e1) High-temperature online filtration: The slurry after reaction is passed through a sintered metal filter with a filtration accuracy of 1-10μm at 230-270℃ to separate and recover the solid catalyst;

[0047] (e2) Adsorption purification: The molten salt filtered in step (e1) is passed through a fixed bed or moving bed adsorption tower containing a high-temperature adsorbent to remove dissolved organic impurities.

[0048] (e3) Composition adjustment and replenishment: The conductivity of the purified molten salt is monitored online and the composition is analyzed periodically. Fresh molten salt components are added according to the analysis results to maintain the stability of its eutectic composition and total amount.

[0049] Furthermore, in (e2), the high-temperature adsorbent is γ-alumina, activated carbon, or molecular sieve; in (e3), when the organic carbon content in the molten salt exceeds 0.5 wt% or the composition deviates from the eutectic point by more than 5%, part of the molten salt is removed from the system for offline deep regeneration or replacement.

[0050] Preferably, the methyl methacrylate (MMA) monomer prepared by the aforementioned chemical recycling process has a purity ≥99.8 wt%, a moisture content ≤0.02 wt%, and a color (APHA) ≤10.

[0051] The present invention also proposes the application of methyl methacrylate monomer obtained by the aforementioned chemical recycling process of acrylic, wherein the methyl methacrylate monomer is used as a monomer or mixed with other monomers and then polymerized to become a new material.

[0052] For example, an optical-grade cast polymethyl methacrylate (PMMA) sheet is made from previously recycled MMA monomers through bulk polymerization. The sheet has a light transmittance (3mm thickness) ≥92%, haze ≤1.0%, and a yellowing index change value ΔYI ≤2.0 after 1000 hours of xenon lamp aging.

[0053] The raw materials for its preparation include, by mass, 70-90 parts of recycled MMA monomer, 10-30 parts of virgin MMA monomer, 0.2-0.5 parts of initiator, 0.05-0.15 parts of chain transfer agent, and 0.3-0.8 parts of ultraviolet absorber.

[0054] Another example is a photosensitive resin used for photopolymerization 3D printing, which contains the aforementioned recycled MMA monomer, and its mass percentage in the resin formulation is 20-50%.

[0055] Its formulation, by mass percentage, includes: 25-45% of the 10MMA monomer as claimed in claim 1, 40-65% of the multifunctional acrylate monomer, 3-6% of the photoinitiator, and 0.5-2% of the additives; the viscosity of the photosensitive resin at 25°C is ≤500 mPa·s, and the tensile strength of the cured material is ≥40 MPa.

[0056] Another example is an acrylic emulsion, which is prepared by emulsion polymerization of a mixture of monomers including the aforementioned recovered MMA monomers, wherein the MMA monomers account for 30-60% of the total mass of the mixture of monomers; its solid content is 45-55%; the architectural coatings formulated from the emulsion have a scrub resistance of ≥10,000 cycles and a volatile organic compound (VOC) content of ≤50g / L.

[0057] like Figure 1 As shown, an online purification and circulation system for molten salt used in implementing the aforementioned chemical recovery process includes the following components connected in sequence:

[0058] Slurry buffer tank, used to receive molten salt slurry from the reactor;

[0059] A high-temperature online filter, connected to the outlet of a slurry buffer tank, is used to separate solid catalysts from the slurry;

[0060] The adsorption purification tower is connected to the outlet of the high-temperature online filter and is filled with high-temperature adsorbent to remove dissolved organic impurities from molten salt.

[0061] The molten salt circulation pipeline and replenishment device are connected to the outlet of the adsorption purification tower to transport the purified molten salt back to the reaction system. It is also equipped with a feed port for replenishing fresh molten salt components and an online analyzer for monitoring the salt composition.

[0062] Preferably, the high-temperature online filter is a backflush sintered metal tube filter with an operating temperature of 200-300℃ and a filtration accuracy of 5μm; the adsorption purification tower is a moving bed adsorption tower with two towers connected in parallel, and the adsorbent is spherical γ-alumina; the online analyzer includes a conductivity meter and / or an automatic sampling-ion chromatography analysis unit.

[0063] Compared with the prior art, the beneficial effects of the present invention are:

[0064] 1. This invention is the first to use an alkali metal nitrate eutectic system as the depolymerization reaction medium for PMMA, which has the following two characteristics:

[0065] 1) High-efficiency heat transfer medium: Low melting point design: The ternary eutectic system (KNO3 / NaNO2 / NaNO3) has a melting point of only 142℃, which is much lower than the traditional pyrolysis temperature (above 300℃); High heat capacity: The specific heat capacity reaches 1.5kJ / kg·K, which is twice that of PMMA, ensuring uniform temperature; Good fluidity: The viscosity at 180℃ is only 142mPa·s, which is much lower than that of molten PMMA (>10,000 mPa·s).

[0066] 2) Chemical reaction regulator: The weak oxidizing property of nitrate ions: NO3 - The release of reactive oxygen species at high temperatures can promptly scavenge free radicals; this reaction converts highly reactive carbon-centered free radicals into more stable oxygen-centered free radicals, effectively inhibiting β-cleavage side reactions; the reducing power of nitrite: NO2 - It can capture peroxides and prevent deep oxidation; ion exchange: Na in molten salt + K + It can undergo ion exchange with the carboxylic acid produced by the depolymerization of PMMA, preventing acid-catalyzed side reactions.

[0067] Compared with traditional pyrolysis, the molten salt system reduces the reaction temperature by 30-50℃ (from 300℃ to 270℃), reduces the amount of by-products generated by 70%, and increases the selectivity (MMA / total product) from 80% to over 95%.

[0068] 2. This invention utilizes a two-component catalyst for synergistic catalysis. The main catalyst is responsible for selective bond breaking, while the co-catalyst is responsible for free radical stabilization. Both achieve electron and mass transfer through ion conduction in the molten salt, forming a highly efficient catalytic cycle.

[0069] Main catalyst (SnO2-ZnO / mesoporous SiO2): Mesoporous support: pore size 8-10 nm, just large enough to accommodate PMMA chain segments (PMMA chain diameter approximately 2 nm); confinement effect: PMMA chains within the pores exhibit an extended conformation, with their ends preferentially contacting the catalyst active sites; bimetallic synergy: Sn 4+ (Strong Lewis acid): polarizes carbonyl oxygen, weakening the β-CO bond; Zn 2+ (Medium Lewis acid): Adjusts electron density to prevent excessive polarization from causing decarboxylation; when Sn:Zn=4:1, the optimal electronic structure is formed, and the activation energy of the CO bond is reduced from 210 to 185 kJ / mol;

[0070] Co-catalyst (CeO2 / basic zeolite): Oxygen storage capacity of CeO2: Ce 4+ / Ce 3+ Redox pairs can dynamically provide / accept oxygen atoms, a process that transforms alkyl radicals with a tendency to β-cleave into stable alkoxy radicals.

[0071] The key roles of phosphate passivation are: forming a 2-3 nm CePO4 thin layer on the CeO2 surface to inhibit sintering; increasing the specific surface area retention of CeO2 from 50% to 85% at high temperatures; preventing dissolution; reducing Ce dissolution from 15 ppm to <1 ppm; and regulating acidity and alkalinity by providing appropriate acid-base bifunctional sites.

[0072] 3. The gradient temperature control design of this invention generates an appropriate amount of free radicals (concentration approximately 10) in the preheating zone. -6 (mol / L), preheating for the next reaction; high-speed reaction in the reaction zone, shortening the free radical lifetime: from 15ms to 5ms, reducing the chance of side reactions, and timely removal of products: MMA vapor escapes rapidly, breaking the reaction equilibrium, and the monomer yield is ≥94%.

[0073] 4. This invention, through the innovative design of a synergistic system of molten salt eutectic medium, two-component catalyst, gradient temperature control, and online purification, successfully solves the defects of existing PMMA chemical recycling processes, such as low efficiency, high cost, and low product quality. The recycled MMA monomers achieve an optical grade purity of over 99.85%. Optical grade cast PMMA sheets prepared using recycled MMA fully meet the performance standards, proving that the recycled MMA has been restored to the level of virgin material at the molecular structure level, truly realizing a closed-loop cycle. Attached Figure Description

[0074] Figure 1 This is a flowchart of a chemical recycling process for acrylic proposed in this invention. Detailed Implementation

[0075] The technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with existing known technologies. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments.

[0076] Part 1. Overall Experimental Design and Specifications:

[0077] 1. Experimental Platform and General Conditions

[0078] 1.1 Raw materials:

[0079] Raw material source: All materials used were scraps from a certain PMMA sheet manufacturer, all grade CM-205; Raw material pretreatment: All experiments used the same batch of raw materials, which were mixed and homogenized; Initial analysis of raw materials: Moisture 0.15wt%, molecular weight Mw=120,000±5,000, glass transition temperature Tg=105℃.

[0080] 1.2 General Equipment Configuration:

[0081] Premixer: 100L capacity, jacketed heating, twin-screw agitator (speed adjustable from 0-100rpm); Reaction system: KDL-5 falling film short-path evaporator (evaporation area 5m²). 2 It can be switched to a tank reactor; purification system: packed distillation column (theoretical plates 30); analytical instruments: Agilent 7890B gas chromatograph (FID detector), Mettler Titrino Karl Fischer moisture analyzer, HACH colorimeter, TOC analyzer.

[0082] 1.3 General Operating Procedures:

[0083] Processing per batch of PMMA raw material: 10.00 kg (accurate to ±10 g); Experimental cycle: 3 complete cycles per group, record the data of the 3rd cycle; Sampling rule: 3 parallel samples are taken at each key node, and the average value of the results is taken; Operators: 3 technicians in the same group perform all experiments to reduce human error.

[0084] 1.4 Standards for Analytical and Testing Methods:

[0085] MMA purity: GB / T 17530.1-1998; Color (APHA): GB / T 3143-1982; Moisture content: GB / T 6283-2008; Molten salt composition: Ion chromatography (GB / T 30906-2014); Sheet transmittance / haze: GB / T 2410-2008.

[0086] Part Two. Experimental Design of Chemical Recycling Process for Acrylic:

[0087] Example 1. Optimization of a preferred ternary molten salt system + overall process optimization:

[0088] Phase 1: Preprocessing

[0089] 1. Crushing operation: Weigh 10.00 kg of PMMA scrap and put it into a twin-shaft shredder (power 5.5 kW). Set the speed to 25 rpm and crush it until it passes through a 5 mm screen. Screening and grading: 1-5 mm particles account for 94.5% (9.45 kg), <1 mm fine powder accounts for 5.0% (0.50 kg), and >5 mm rework 0.5% (0.05 kg).

[0090] 2. Cleaning procedure: Prepare cleaning solution: 30L of 60℃ deionized water, add 0.1% nonionic surfactant (TritonX-100); Ultrasonic cleaning parameters: frequency 40kHz, power 300W, time 15 minutes; Rinse twice: first with 50℃ deionized water, second with room temperature deionized water; Water quality test after cleaning: pH=6.8, conductivity 8.5μS / cm.

[0091] 3. Drying operation: Equipment: Fluidized bed dryer (hot air circulation type); Drying parameters: Inlet temperature 60℃, air velocity 1.2m / s, material layer thickness 50mm; Process monitoring: Moisture content is measured every hour; Drying endpoint: Moisture content is 0.075wt% (<0.1wt% standard) after 2.5 hours; Weight of dried material: 9.40kg (including 0.5kg fine powder).

[0092] Phase 2. Preparation of molten salt slurry:

[0093] 1. Molten Salt Preparation: Calculation of Ternary Eutectic Salt Ratio (Total 10.0kg): KNO3: 53.0% → 5.30kg (Use an electronic scale for precise weighing, error ±1g); NaNO2: 40.0% → 4.00kg; NaNO3: 7.0% → 0.70kg;

[0094] Melting process: The premixer is heated to 180℃; KNO3 and NaNO3 are added first, and the stirring speed is 20 rpm; after complete melting, NaNO2 is added (operation in the dark); melting time: 30 minutes; eutectic point test: DSC measurement shows the actual eutectic point to be 141.8℃; conductivity measurement: 445 mS / cm (standard value 450±15).

[0095] 2. Catalyst addition: Catalyst dosage calculation: Theoretical MMA yield: 10kg PMMA contains 9.30kg of MMA structural units; Total catalyst: 9.30kg × 2.0% = 186g; Main catalyst (SnO2-ZnO / SiO2): 186g × 2 / 3 = 124g; Co-catalyst (CeO2 / zeolite): 186g × 1 / 3 = 62g; Catalyst pretreatment: Preheat in an oven at 120℃ for 1 hour, then cool to 80℃ for later use (to prevent thermal shock);

[0096] 3. Slurry preparation: The molten salt temperature was stabilized at 180℃, and the catalyst was slowly added while the stirring speed was increased to 40 rpm; mixing time: 15 minutes; microscopic observation of samples: the catalyst was uniformly dispersed and there was no agglomeration; acrylic particles were added in 5 batches, each batch about 1.88 kg, with an interval of 3 minutes, and the stirring speed was maintained at 40 rpm during the addition. After the addition, mixing was continued for 30 minutes; addition of polymerization inhibitor: phenothiazine dosage: 9.30 kg × 0.3% = 27.9 g; preparation of 5% acetone solution: 27.9 g phenothiazine + 530 mL acetone; injection addition: injection pump rate 10 mL / min; acetone evaporation: stirring was continued for 15 minutes, and no acetone residue was detected in the tail gas; slurry condition check: temperature: 179.5℃; viscosity measurement (180℃): 142 mPa·s; density measurement: 1.85 g / cm³; final total slurry volume: about 20 kg.

[0097] Stage 3. Catalytic depolymerization:

[0098] 1. System preparation: Vacuum system leak test: Hold pressure at 15 kPa for 10 minutes, pressure drop < 0.1 kPa; Temperature setting: Preheating zone 240℃, reaction zone 270℃; Pressure setting: Absolute pressure 15 kPa; Condensation system: First stage 40℃, second stage -5℃; Nitrogen sealing of receiving tank: Oxygen content < 100 ppm;

[0099] 2. Feed reaction:

[0100] Slurry delivery: gear pump, flow rate 3.33 kg / h (total slurry 20 kg, reaction time 6 hours); reactor is a falling film short-path evaporator KDL-5.

[0101] Table 1. Monitoring Record of Catalytic Depolymerization Reaction Process

[0102] time Preheating zone temperature (°C) Reaction zone temperature (°C) System pressure (kPa) MMA steam temperature (°C) Remark 12:15 238.5 269.2 15.1 85.2 Start feeding 12:30 239.8 270.1 15.0 87.5 Stable operation 13:00 240.2 269.8 14.9 86.8 Peak reaction period 13:15 240.0 270.3 15.2 85.0 The reaction ended.

[0103] Reaction phenomenon record: 12:20: Small bubbles appear in the reaction zone; 12:35: A large number of bubbles are produced, and the liquid film fluctuates violently as observed through the sight glass; 13:00: The number of bubbles gradually decreases; 13:15: There are basically no bubbles, and the reaction ends.

[0104] 3. Product collection: Crude MMA collection amount: 8.85 kg (theoretical 9.30 kg); condensation efficiency calculation: first-stage condensation 85.2%, second-stage condensation 14.8%; molten salt collection after reaction: approximately 11 kg (including catalyst).

[0105] Phase 4. Monomer purification:

[0106] 1. Crude monomer pretreatment:

[0107] Sampling analysis (crude monomer): GC purity: 99.52%; moisture: 0.085%; main impurities: methanol 0.12%, methacrylic acid 0.02%; pretreatment agent added: add 0.1% anhydrous Na2SO4, stir for 15 minutes, and filter.

[0108] 2. Distillation operation:

[0109] Distillation parameter settings: Bottom temperature: 110℃; Top temperature: 100.5℃; Reflux ratio: 5:1; Feed rate: 1.5kg / h;

[0110] Table 2. MMA Monomer Distillation Process Record Sheet

[0111] ;

[0112] Material collection:

[0113] Light components: 0.18 kg (mainly methanol); MMA product: 8.45 kg; Heavy components: 0.22 kg.

[0114] 3. Post-processing of products:

[0115] Drying: Passed through a 4A molecular sieve column (50 mm in diameter, 500 mm in height);

[0116] Add polymerization inhibitor: MEHQ 5ppm;

[0117] Final product analysis: Appearance: colorless and transparent, free of suspended matter; Purity: 99.85%; Moisture: 0.018%; Color: 8 APHA; Acid value: 0.005 mg KOH / g.

[0118] Stage 5. Molten Salt Purification Cycle:

[0119] 1. High-temperature filtration:

[0120] Filtration conditions: Temperature: 250℃; Filtration accuracy: 5μm; Pressure difference: <0.08MPa;

[0121] Catalyst recovery: 179g of catalyst collected (initially 186g); recovery rate: 96.2%; catalyst activity test: conversion rate remained at 98.5% of the initial value.

[0122] 2. Adsorption and purification:

[0123] Adsorption tower operation: Adsorbent: spherical γ-alumina (particle size 3mm); Bed height: 1200mm; Empty tower linear velocity: 0.5m / h;

[0124] TOC removal efficiency: Imported TOC: 0.45%; Exported TOC: 0.32%; Removal rate: 28.9%;

[0125] 3. Composition adjustment:

[0126] Online Analysis: K + Concentration: decreased by 1.1%; NO2 - Concentration: decreased by 0.9%;

[0127] Supplementary calculations: Supplement KNO3: 5.30kg × 1.1% = 58g; Supplement NaNO2: 4.00kg × 0.9% = 36g;

[0128] Post-test results: Eutectic point: 141.6℃; Conductivity: 443mS / cm.

[0129] 4. Preparation for reuse:

[0130] Total molten salt volume after purification: approximately 10.0 kg (after replenishment);

[0131] Catalyst replenishment: 7g was added to compensate for the loss, restoring the total amount to 186g;

[0132] Standby status: Keep at 180℃ for standby use.

[0133] Example 2. Binary molten salt system:

[0134] The difference from Example 1 lies in the molten salt preparation stage:

[0135] Binary salt ratio (total 10.0 kg): KNO3: 52.0% → 5.20 kg; NaNO3: 48.0% → 4.80 kg;

[0136] Melting temperature adjustment: Eutectic point determination: 218.2℃; Mixing temperature: 250℃ (32℃ higher than the eutectic point); Melting time: 40 minutes (10 minutes longer than ternary salt);

[0137] Viscosity adjustment: Viscosity at 250℃: 185 mPa·s (30% higher than ternary salt); Stirring speed: Increase to 50 rpm to ensure uniform mixing during the depolymerization stage.

[0138] Temperature adjustment: Preheating zone: 255℃; Reaction zone: 280℃; Reason: Binary salts have high melting points and require higher temperatures to maintain fluidity;

[0139] Reaction time: extended to 6.5 hours (0.5 hours longer than in Example 1).

[0140] The other stages of the operation are the same as in Example 1.

[0141] Example 3. Isothermal reaction (without gradient temperature control):

[0142] The difference from Example 1 is the reactor replacement: the falling film evaporator is removed and a trough reactor with anchor stirring is installed; reactor specifications: volume 30L, material 316L, heating jacket, stirring speed 0-100rpm;

[0143] Correspondingly, the operation of the catalytic depolymerization reaction is changed:

[0144] Feeding method: All slurry (20kg) is added at once; stirring speed: 30rpm; constant temperature reaction: reaction temperature: 260℃ (constant temperature); reaction pressure: 15kPa; reaction time: 30 minutes, with samples taken every 5 minutes during the process;

[0145] Product collection: After the reaction is complete, the vacuum is turned on to extract the MMA vapor; Collection time: 40 minutes (10 minutes longer than gradient temperature control); Problem encountered: Local overheating: Thermocouple monitoring showed that the temperature near the stir bar reached 275℃, while the wall temperature was only 250℃;

[0146] Coking phenomenon: A thin brown layer (about 0.1 mm thick) appears on the reactor wall; Incomplete reaction: About 5% of solid particles are still not completely deagglomerated after the reaction.

[0147] Example 4. Co-catalyst without phosphate passivation:

[0148] The difference from Example 1 lies in the catalyst preparation: the phosphate solution impregnation treatment is omitted in the preparation of the co-catalyst, and it is directly calcined at 500°C for 4 hours. Other stages are the same as in Example 1.

[0149] Catalyst characterization comparison: BET specific surface area: 120m² before passivation 2 / g→65m after passivation 2 / g; Pore size distribution: The proportion of macropores increases after passivation; Surface acidity: The number of weak acid sites decreases after passivation.

[0150] Example 5. Molten salt purification eliminates online filtration and uses centrifugal separation:

[0151] The difference from Example 1 lies in the change of slurry filtration method:

[0152] The separation system was modified by adding a cooling system: the slurry after reaction is first cooled to 150℃ (to prevent MMA volatilization); cooling time: 30 minutes;

[0153] Centrifugal separation: Equipment: High-speed centrifuge (maximum speed 5000 rpm); Operating parameters: 3000 rpm, 10 minutes; Separation effect observation: Upper clear liquid: relatively turbid, light yellow; Lower solid: mixture of catalyst and part of molten salt;

[0154] Molten salt reheating: The clear liquid is transferred to a heating vessel; reheating to 250°C takes 40 minutes, increasing energy consumption;

[0155] Catalyst recovery issues: After centrifugation, the catalyst is in paste form and contains approximately 30% molten salt; it requires methanol washing for recovery, increasing solvent usage; the activity of the recovered catalyst decreases significantly.

[0156] Comparative Example 1. Traditional pyrolysis process without molten salt:

[0157] The process route is completely different from that in Example 1:

[0158] Pre-treatment stage: crush to 1-3mm (fineer than molten salt process); higher drying requirements: moisture content <0.05% (to prevent hydrolysis);

[0159] Catalyst mixing: The catalyst is directly dry-mixed with PMMA particles; the mixing uniformity is poor, and catalyst agglomeration is visible to the naked eye;

[0160] Pyrolysis reaction: Equipment: Fixed bed pyrolysis furnace (100mm in diameter, 500mm in height); Loading method: Layered filling, with each layer compacted;

[0161] Reaction conditions: Temperature: 280℃; Atmosphere: Nitrogen, flow rate 50 mL / min; Time: 60 minutes; No vacuum, operation at atmospheric pressure;

[0162] Serious problems occur: 12 minutes: Local temperature rises sharply to 320℃ (hot spot); 25 minutes: Large amount of smoke appears with an irritating odor; 45 minutes: Material clumps and heat transfer deteriorates; 60 minutes: Approximately 20% of the material carbonizes.

[0163] Product collection: Liquid product is collected by condensation system; Collection amount: 7.83 kg (yield 78.3%); Product characteristics: dark yellow with a burnt smell; Post-processing difficulties: Coking is easy to occur in the distillation column, requiring frequent equipment cleaning, and the product color can never meet the standard.

[0164] Comparative Example 2. No catalyst:

[0165] The operating steps differ from those in Example 1:

[0166] Slurry preparation: No catalyst was added at all, and other parameters were the same as in Example 1.

[0167] Abnormal reaction stage: slow reaction start-up: only a small number of bubbles appear after 30 minutes; the reaction temperature needs to be increased to 285℃ to maintain the reaction rate; the reaction time is extended to 50 minutes; incomplete reaction: obvious PMMA particles are still present in the slurry after the reaction.

[0168] Product characteristics: Low monomer yield: only 62.5%; many byproducts: more than 15 byproducts were detected by GC-MS; strong polymerization tendency: easy to polymerize during condensation process.

[0169] Comparative Example 3. Insufficient molten salt mixing temperature:

[0170] The operating steps differ from those in Example 1:

[0171] Mixing temperature setting: Set value: 147℃ (only 5℃ above the eutectic point); Actual temperature reached: fluctuated between 145-148℃;

[0172] Problems during mixing: High viscosity of molten salt: difficult to flow; After adding PMMA: forms a dough-like substance, causing the stirring motor to overload; forced to reduce the stirring speed to 15 rpm; Uneven mixing: undispersed PMMA lumps at the bottom;

[0173] Difficulties in conveying: frequent blockage of the slurry pump; manual unclogging required; unstable feeding, sometimes fast and sometimes slow;

[0174] Reaction process: Uneven temperature distribution: local high temperature zone reaches 310℃; severe coking: black coke blocks in the reactor; poor product color: always >20 APHA.

[0175] Comparative Example 4. Without polymerization inhibitor:

[0176] The operation steps differ from those in Example 1: no phenothiazine is added, but the rest of the operation is exactly the same as in Example 1.

[0177] Condensation stage: A misty appearance appears on the inner wall of the first-stage condenser; after 2 hours, a transparent film forms on the inner wall; condensation efficiency decreases from 85% to 70%.

[0178] Pipeline transportation: As pipeline pressure drop gradually increases, it is necessary to increase the transportation pressure;

[0179] Distillation stage: Polymer accumulation in the reboiler; shutdown and cleaning required after 3 batches of distillation; decreased tray efficiency: theoretical tray number drops from 30 to 25;

[0180] Economic losses: Equipment cleaning frequency: once every 3 batches (every 10 batches in Example 1); MMA loss: polymerization loss of about 5%; Product downgrade: color increased from 8 APHA to 25 APHA.

[0181] Comparative Example 5. Molten salt is used only once, without purification and recycling:

[0182] Molten salt was recycled without purification according to Example 1:

[0183] First cycle: Follow the steps in Example 1 exactly;

[0184] Second cycle simulation: using the molten salt from the first cycle (TOC=0.18%), without filtration and adsorption, and with a newly added catalyst;

[0185] Third loop (5th simulation):

[0186] Artificially accelerated aging of molten salt: addition of methacrylic acid to achieve an acid content of 0.5%; addition of carbon powder to simulate carbonization products; addition of an aldehyde mixture: formaldehyde + methacrolein total of 0.3%; addition of deactivated catalyst fine powder: below 5μm, concentration of 0.1%.

[0187] Molten salt condition: TOC: 2.1%; Color: Dark brown; Viscosity: 40% higher than fresh salt; Electrical conductivity: decreased by 15%;

[0188] Abnormal reaction process: severe foaming: feed rate needs to be reduced; dark product color: condensate is pale yellow; yield decreases significantly.

[0189] Part Three. Performance Analysis:

[0190] The process parameters of Examples 1-5 and Comparative Examples 1-5 are summarized in Table 3 below:

[0191] Table 3. Comparison of main process parameters between the examples and comparative examples

[0192] ;

[0193] The mass of MMA monomers in Examples 1-5 and Comparative Examples 1-5 is summarized in Table 4 below:

[0194] Table 4. Comparison of MMA monomer mass between the examples and comparative examples

[0195] ;

[0196] The molten salt cycle stability tests conducted in Examples 1-5 and Comparative Examples 1-5 are summarized in Table 5 below:

[0197] Table 5. Comparison of molten salt cycling stability between the examples and comparative examples (3rd cycle)

[0198] ;

[0199] An optical-grade cast polymethyl methacrylate (PMMA) sheet was prepared using the methyl methacrylate monomers from Examples 1-5 and Comparative Examples 1-5. This sheet was produced by bulk polymerization of previously recovered MMA monomers. The sheet exhibits a light transmittance (3mm thickness) ≥92%, haze ≤1.0%, and a yellowing index change ΔYI ≤2.0 after 1000 hours of xenon lamp aging. The raw materials, by weight, comprise: 80 parts recovered MMA monomer, 20 parts virgin MMA monomer, 0.35 parts initiator, 0.1 parts chain transfer agent, and 0.5 parts UV absorber. The performance of the sheet product is shown in Table 6 below.

[0200] Table 6. Performance Comparison of Optical PMMA Sheets Made from Products of Examples and Comparative Examples

[0201] ;

[0202] Table 7. Comparison of economic benefits and environmental indicators of some embodiments

[0203] ;

[0204] Summarize Tables 1-7 and perform data analysis:

[0205] 1. Analysis of the molten salt system:

[0206] Comparing Example 1 and Example 2, the essential difference between the ternary and binary systems was revealed by DSC and viscosity tests: Ternary salt (melting point 142℃): when operating at 180℃, with a superheat of 38℃, viscosity 142 mPa·s; Binary salt (melting point 218℃): when operating at 250℃, with a superheat of 32℃, viscosity 185 mPa·s.

[0207] The ternary salt has a 32% lower viscosity and a 25% higher heat transfer coefficient, which is the main reason for the 2.1% higher yield; the ternary salt contains NO2. - It has reducing properties and can capture free radicals; the ternary salt system has 4 ppm less aldehyde impurities and 1 APHA less color. The binary salt has a higher operating temperature of 70℃, increased heat loss, and consumes an additional 0.06 kWh of energy per kg of MMA, increasing costs by about 4%.

[0208] 2. The function of gradient temperature control:

[0209] Analyze the reaction process through online sampling:

[0210] In the preheating zone (240℃), chain-end initiation mainly occurred; the content of terminal double bonds was detected to decrease from 0.05% to 0.01%; a small amount of MMA was generated (approximately 15% of the total yield); preheating avoided excessively high free radical concentrations in the main reaction zone.

[0211] In the reaction zone (270℃), depolymerization is mainly zipper-like, and the free radical concentration remains stable at 10. -5 -10 -6 Within the mol / L range, the incidence of side reactions (β-fracture) is <0.5%;

[0212] If a constant temperature reaction is used in Comparative Example 3, at 260℃, chain initiation and chain propagation occur simultaneously, the peak free radical concentration is 3 times that of gradient temperature control, the β-fracture rate increases to 2.5%, and more small molecule aldehydes are produced (20 ppm more); a PMMA depolymerization kinetic model can be established subsequently.

[0213] 3. Catalyst action mechanism:

[0214] Synergistic effect of main catalyst (SnO2-ZnO):

[0215] Sn 4+ Functions: Strong Lewis acidity (L acid site); adsorption of PMMA carbonyl oxygen, electron cloud shift, and the bond energy of adjacent CO bonds decreases from 358 kJ / mol to about 320 kJ / mol;

[0216] Zn 2+ Regulatory role: Medium-strength L acid sites disperse SnO2 and prevent excessive aggregation;

[0217] Extensive experiments revealed that the acid site density was optimal when Sn:Zn = 4:1, with a synergistic mechanism: Sn 4+ Polarized C=O bonds, Zn 2+ The auxiliary stabilizing intermediate and recombination site reduce the depolymerization activation energy from 210 kJ / mol to 185 kJ / mol.

[0218] In contrast, Comparative Example 2 requires a reaction temperature of 285℃ without a catalyst, while with a catalyst, the temperature only needs to be 270℃.

[0219] Phosphate passivation mechanism of co-catalyst (CeO2):

[0220] Problem before passivation: CeO2 partially dissolves in molten salt (15ppm of Ce is leached out), the dissolved Ce... 3+ It becomes a homogeneous catalyst, but homogeneous catalysis is non-selective, leading to excessive pyrolysis;

[0221] The phosphate passivation layer, which may be about 2-3 nm thick, serves the following purposes: ① to prevent Ce dissolution; ② to inhibit sintering; ③ to adjust surface acidity.

[0222] Example 4, without passivation, resulted in Ce dissolution of 15 ppm, TOC of 0.52%, and a color intensity of 15 APHA; compared to Example 1, after passivation, Ce dissolution was <1 ppm, TOC was 0.32%, and the color intensity was 8 APHA; demonstrating that phosphate passivation is crucial for maintaining catalyst stability.

[0223] 4. The function of the molten salt purification system:

[0224] Establish a TOC cumulative model: TOC generated in each cycle: ΔTOC0 = 0.15%, purification system removal rate: η = 65%; after n cycles: TOC n =ΔTOC0×(1-η) n-1 / η;

[0225] Calculation results: Example 1 (η=65%): TOC=0.32% after 5 cycles; Comparative Example 5 (η=0%): TOC=0.75% after 5 cycles (actual measurement 2.1%, due to synergistic effect);

[0226] The effect of impurities on the reaction:

[0227] Organic acid-catalyzed side reactions: When the methacrylic acid content is >0.1%, it catalyzes transesterification, generating MMA dimers and trimers;

[0228] The photosensitivity effect of aldehydes: When the aldehyde content in the board is >100ppm, chromophores are easily formed under light. The linear relationship is that for every 10ppm increase in aldehyde content, ΔYI increases by about 0.15.

[0229] Nucleation effect of fine particles: catalyst fine powder <10μm becomes the core of impurity aggregation, accelerating TOC accumulation;

[0230] Comparing the centrifugal separation of Example 5 with the high-temperature online filtration of Example 1, online filtration (5μm) can remove 99% of particles >5μm; while centrifugal separation (3000rpm) can only remove 90% of particles >10μm.

[0231] Economic losses: 18% of the catalyst was lost during centrifugal separation (4% loss due to filtration). Based on a catalyst cost of 200 yuan / kg, the cost per kg of MMA increased by 2.8 yuan. Operational issues: Cooling-reheating energy consumption increased by 0.13 kWh / kg. Solvent washing losses: Methanol consumption increased by 0.2 L / kg.

[0232] 5. Correlation between product performance and impurities:

[0233] Product testing and analysis revealed that aldehydes are the impurities that have the greatest impact on optical properties, while metal ions (especially Fe and Cu) are catalysts for photo-oxidation, and moisture indirectly affects performance by influencing the polymerization process.

[0234] 6. Circular Economy Analysis:

[0235] The cost of the MMA monomer produced in Example 1 is calculated as follows:

[0236] Raw material cost (unit: RMB / kg MMA): 1.05 (PMMA waste purchase price); Catalyst: 0.56 (recycled, only replenishing losses); Molten salt: 0.25 (recycled, replenishing losses); Energy consumption: 1.05 (electricity + heat); Labor: 0.85; Equipment depreciation: 0.86; Environmental protection treatment: 0.23; Other: 0.90; Total: RMB 6.85 / kg;

[0237] Compared to the market price of 12-15 yuan / kg for petroleum-based MMA and 18-22 yuan / kg for bio-based MMA, the cost is reduced by 45-65%. Moreover, each kg of recycled MMA reduces CO2 emissions by 2.5 kg and saves 2.8 kg of petroleum feedstock per kg, reducing the need for PMMA landfill or incineration.

[0238] 7. Conclusion:

[0239] 1) Ternary eutectic salt (KNO3 / NaNO2 / NaNO3) is superior to binary salt in terms of low temperature, low viscosity and good chemical environment, which is one of the core innovations of this process.

[0240] 2) Catalytic system: The SnO2-ZnO and CeO2 bicomponent catalyst, after passivation with phosphate, exhibits excellent stability and selectivity in high-temperature molten salt, and the depolymerization efficiency is increased by 52% compared with no catalyst.

[0241] 3) Compared with the isothermal reaction, the product yield increased by 3.7% and the color decreased by 33% under gradient temperature control (240℃→270℃), which proves the key role of temperature field control in selectivity.

[0242] 4) The high-temperature online filtration and adsorption purification system keeps the TOC of the molten salt stable at 0.32% and the catalyst recovery rate is >95%, which is the technical guarantee for the long-term stable operation of the process.

[0243] 5) Product quality: The purity of recycled MMA is ≥99.8%, and the light transmittance of the optical PMMA sheets used for preparation is ≥92%, ΔYI≤2.0, which fully meets the requirements of high-end applications.

[0244] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A process for the chemical recycling of acrylonitrile, characterized in that, The method comprises the following steps: (a) Pretreatment: crushing, cleaning and drying the acrylic waste to obtain acrylic particles with a particle size of 1-5 mm and a water content of less than 0.1 wt%; (b) Preparation of molten salt slurry: mixing the acrylic particles obtained in (a) with a molten salt in a pre-mixer, the molten salt being a eutectic system of alkali metal nitrate with a melting point lower than 300℃, the mixing temperature being 20-60℃ higher than the eutectic point of the molten salt, the mass ratio of acrylic particles to molten salt being 1:1 to 1:3, and a catalyst being added at the same time to form a uniform acrylic-molten salt-catalyst slurry; (c) Catalytic depolymerization: feeding the slurry obtained in (b) into a reactor to carry out a depolymerization reaction at a temperature of 230-300℃ and an absolute pressure of 10-30 kPa, and the generated methyl methacrylate (MMA) monomer vapor being separated from the reactor; (d) Purification of monomer: condensing and rectifying the methyl methacrylate vapor separated in (c) to obtain methyl methacrylate monomer with a purity of ≥99.8 wt%; (e) Purification and recycling of molten salt: carrying out solid-liquid separation and purification treatment on the molten salt slurry after reaction in (c), and recycling the recovered molten salt for use in (b).

2. The process for chemical recycling of adhesives according to claim 1, characterized in that, In (b), the molten salt is a binary eutectic system of potassium nitrate and sodium nitrate, and the composition is 50-55 wt% of KNO3 and 45-50 wt% of NaNO3.

3. The process for chemical recycling of adhesives according to claim 1, characterized in that, In (b), the molten salt is a ternary eutectic system of potassium nitrate, sodium nitrite and sodium nitrate, and the composition is 50-55 wt% of KNO3, 35-45 wt% of NaNO2 and 5-15 wt% of NaNO3.

4. The process for chemical recycling of adhesives according to claim 1, characterized in that, In (b), the catalyst is a two-component solid-phase catalyst comprising a main catalyst and a co-catalyst; the main catalyst is a SnO2 and ZnO composite metal oxide supported on mesoporous silica, wherein the molar ratio of Sn to Zn is 3:1 to 5:1, and the total metal oxide loading is 10-20 wt% of the carrier mass; the co-catalyst is CeO2 supported on an alkaline zeolite, and the CeO2 loading is 5-15 wt% of the carrier mass.

5. The process for chemical recycling of acrylate according to claim 4, characterized in that, The CeO2 in the co-catalyst is surface passivated by a phosphate solution; the dry mass mixing ratio of the main catalyst to the co-catalyst is 1:1 to 3:1; the amount of catalyst added is 1-3 wt% of the mass of methyl methacrylate monomer.

6. The process for chemical recycling of adhesives according to claim 1, characterized in that, In (b), 0.1-0.5 wt% of phenothiazine is added as a polymerization inhibitor during the formation of the slurry or during the transportation of the slurry.

7. The process for chemical recycling of adhesives according to claim 1, characterized in that, In (c), the reactor is a falling film short path evaporator, a thin film evaporator or a tank reactor with stirring; the reaction temperature is achieved by gradient temperature control, including a preheating / initiation zone at a temperature of 230-250℃ and a main reaction / depolymerization zone at a temperature of 260-280℃.

8. The process for chemical recycling of adhesives according to claim 1, characterized in that, In (e), the purification and recycling of molten salt specifically comprises: (e1) high-temperature online filtration: the slurry after reaction is filtered through a metal sintered filter with a filtration accuracy of 1-10 μm at 230-270 °C to separate and recover the solid catalyst; (e2) adsorption purification: the molten salt after (e1) filtration is passed through a fixed bed or moving bed adsorption tower filled with high-temperature adsorbent to remove dissolved organic impurities; (e3) composition adjustment and replenishment: online conductivity monitoring and regular composition analysis are performed on the purified molten salt, and fresh molten salt components are added according to the analysis results to maintain the eutectic composition and total amount stable.

9. The process for chemical recycling of adhesives according to claim 8, characterized in that, The high-temperature adsorbent in (e2) is γ-alumina, activated carbon or molecular sieve; in (e3), when the organic carbon content in the molten salt exceeds 0.5 wt% or the composition deviates from the eutectic point by more than 5%, part of the molten salt is removed from the system for offline deep regeneration or replacement.

10. Use of methyl methacrylate monomer obtained by the process according to any one of claims 1 to 9, characterized in that, Methyl methacrylate monomer is polymerized as a new material, either as a monomer or mixed with other monomers.

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