Degradable plastic particles, method for producing degradable plastic particles by replacing part of resin with aqueous salt mud or aqueous silt and application

Abstract

The application provides degradable plastic particles and a method and application for producing the degradable plastic particles by replacing part of resin with water-containing salt mud or water-containing silt, and belongs to the technical field of degradable plastic granulation.The method breaks through the limitation of traditional plastic granulation on absolutely dry raw materials, and can directly use silt with a water content of 40-80% or salt mud with a water content of 30-60% to replace part of degradable resin to produce plastic particles, realizes the melt blending granulation of the water-containing salt mud / water-containing silt and the degradable resin through a whole-process technical route of 'crushing treatment-heavy metal solidification-water content regulation-magnetic impurity removal-melt granulation', greatly simplifies the process, reduces the energy consumption and cost, simultaneously solves the environmental pollution problem, and realizes the large-scale utilization of the silt and the salt mud in the degradable plastic industry.

Description

Technical Field

[0001] This invention relates to the field of biodegradable plastic granulation technology, and more particularly to biodegradable plastic granules and methods and applications for producing them using hydrous salt mud or hydrous silt to replace part of the resin. Background Technology

[0002] Traditional plastic granulation industry requires the raw material moisture content to be below 0.1%~0.5% for the following reasons: (1) Moisture vaporization leads to bubble defects: The melting temperature of plastics usually exceeds 160℃, and residual moisture will vaporize instantly to form bubbles, resulting in pores inside the product and a sharp decline in mechanical properties; (2) Resin hydrolysis and degradation: Polar moisture at high temperatures will cause the molecular chains of biodegradable polyester resins (PBAT, PLA, etc.) to break, reducing molecular weight and product performance; (3) Poor process stability: Water-containing materials have poor flowability and uneven feeding, resulting in large fluctuations in extruder load and unstable product quality. Therefore, all inorganic fillers (including salt mud, silt, fly ash, white mud, etc.) must be strictly dried to a moisture content of <1% before being used in plastic granulation. This drying process is energy-intensive (it requires about 1.5~2 tons of steam to dry 1 ton of material), costly, and inefficient, which seriously restricts the large-scale application of industrial waste in the plastics industry.

[0003] Salt mud is a major waste product of the soda ash and chlor-alkali industries, with a significant output: approximately 50-60 kg of salt mud is generated for every ton of soda ash produced, and approximately 50-60 kg of salt mud is generated for every ton of caustic soda produced. The core challenge in the high-value utilization of salt mud is its high moisture content (typically 30%-60%), complex composition, and the presence of chloride and mercury ions. Meanwhile, silt (including sewage sludge and river / lake sediment), as an abundant industrial waste, faces the core challenge in its high-value utilization: extremely high moisture content (fresh silt typically has a moisture content of 60%-80%, and even after mechanical dewatering, it remains at 40%-60%), and it contains heavy metals and organic pollutants. Traditional processes require drying both salt mud and silt to <1% before granulation, with the energy cost of drying accounting for 30%-40% of the total cost. Furthermore, the drying process is prone to environmental pollution and safety hazards. Technical routes for using silt and salt mud as plastic fillers cannot bypass drying and purification processes, resulting in complex processes and high costs. Therefore, there is an urgent need to develop a technology that allows for the direct granulation of hydrated sludge or salt mud, eliminating drying energy consumption, reducing production costs, and solving environmental pollution problems, so as to realize the large-scale utilization of sludge and salt mud in the biodegradable plastics industry. Summary of the Invention

[0004] The purpose of this invention is to provide a biodegradable plastic granule and a method and application for producing biodegradable plastic granules by replacing part of the resin with hydrous salt mud or hydrous silt. The method provided by this invention can directly use silt with a water content of 40%~80% or salt mud with a water content of 30%~60% to replace part of the biodegradable resin to produce plastic granules. This realizes the melt blending and granulation of hydrous salt mud / hydrous silt with biodegradable resin, which greatly simplifies the process, reduces energy consumption and cost, and solves environmental pollution problems, thereby realizing the large-scale utilization of silt and salt mud in the biodegradable plastics industry.

[0005] To achieve the above-mentioned objectives, the present invention provides the following technical solution: This invention provides a method for producing biodegradable plastic granules by replacing part of the resin with hydrous salt mud or hydrous silt, characterized by comprising the following steps: (1) The water-containing salt mud or water-containing silt is subjected to crushing, heavy metal solidification, moisture content control and magnetic separation to obtain pretreated salt mud or pretreated silt. (2) After mixing the pretreated salt mud or pretreated sludge with biodegradable resin, chain extender and additives, the mixture is melt-granulated to obtain biodegradable plastic particles.

[0006] Preferably, according to the method of claim 1, the step (1) further includes: mixing the crushed salt mud with water, and then performing stirring, washing and solid-liquid separation in sequence to obtain dechlorinated salt mud.

[0007] Preferably, when the starting material water-containing salt mud in step (1) is low-mercury salt mud or mercury-free salt mud, heavy metal solidification is not required. When the water-containing salt mud is high-mercury salt mud, the heavy metal solidification includes: removing mercury by sodium sulfide precipitation or oxidative melting, or using equipment with pressure-controlled dehydration function to form a subcritical environment for solidification. When the starting material in step (1) is water-containing sludge, the heavy metal solidification includes: using microbial induced calcite precipitation technology (MICP), employing the high urease activity strain Sporosarcina ureilytica ML-2, and performing 5 rounds of MICP component irrigation for remediation.

[0008] Preferably, when the moisture content of the salt mud after heavy metal solidification in step (1) is ≤40%, no moisture content adjustment is required; When the moisture content of the salt mud after heavy metal solidification is greater than 40% and less than 50%, the moisture content control includes: adding water-absorbing material to the salt mud after heavy metal solidification to control the moisture content to 30%~40%; When the moisture content of the salt mud after heavy metal solidification is >50%, the moisture content control includes: mechanically dehydrating the salt mud after heavy metal solidification to make its moisture content greater than 40% and less than 50%, and then adding water-absorbing material to control the moisture content to 30%~40%; When the moisture content of the sludge after heavy metal solidification in step (1) is ≤50%, no moisture content adjustment is required; When the moisture content of the salt mud after heavy metal solidification is greater than 50% and less than 65%, the moisture content control includes: adding water-absorbing material to the salt mud after heavy metal solidification to control the moisture content to 40%~50%; When the moisture content of the salt mud after heavy metal solidification is >65%, the moisture content control includes: mechanically dehydrating the salt mud after heavy metal solidification to make its moisture content greater than 50% and less than 65%, and then adding water-absorbing material to control the moisture content to 40%~50%.

[0009] Preferably, the magnetic separation for impurity removal in step (1) includes: using a permanent magnet drum separator to perform magnetic separation on the salt mud or silt after the moisture content has been adjusted, and then measuring the amount of magnetic impurities removed by the magnetic separation; When the amount of magnetic impurities is ≤ 0.8% of the total mass of the salt mud or silt after moisture content adjustment, the stability of the material after magnetic separation is deemed qualified, and the material after magnetic separation is pretreated salt mud or pretreated silt; when the amount of magnetic impurities is > 0.8% of the total mass of the salt mud or silt after moisture content adjustment, the stability of the material after magnetic separation is deemed unqualified, the moisture content of the material after magnetic separation is measured, and the change in moisture content is calculated according to the formula: change in moisture content = (moisture content of the salt mud or silt after moisture content adjustment - change in moisture content of the material after magnetic separation) / moisture content of the total mass of all materials. When the change in moisture content is ≤3%, the change in moisture content is deemed qualified, and the material after magnetic separation is pretreated salt mud or pretreated sludge; when the change in moisture content is >3%, the viscosity of the material after magnetic separation is measured. When the viscosity of the magnetically separated material is ≤ 5000~8000 mPa·s (preset viscosity), the viscosity is deemed qualified, and the magnetically separated material is pretreated salt mud or pretreated sludge; when the viscosity of the magnetically separated material is > 5000~8000 mPa·s (preset viscosity), a moisture-regulating material is added to the magnetically separated material to adjust the viscosity to ≤ the preset viscosity, thus obtaining pretreated salt mud or pretreated sludge.

[0010] Preferably, in step (2), the mass ratio of pretreated salt mud or pretreated sludge to biodegradable resin is (0.8~2.5):1.

[0011] Preferably, in step (2), for pretreated salt mud, the additive is a lubricant and a dispersant; for pretreated sludge, the additive is an inorganic filler.

[0012] Preferably, the conditions for melt granulation in step (2) include: extrusion using a twin-screw extruder, an extrusion temperature of 150℃~200℃, a screw speed of 100~300rpm, and a melt pressure of 5~15MPa.

[0013] The present invention also provides a biodegradable plastic granule produced by the method described in the above technical solution, wherein the biodegradable plastic granule has a tensile strength of 18~28 MPa, an elongation at break of 80%~250%, a melt index of 2~10 g / 10min, a heat distortion temperature of 55℃~85℃, a chloride ion content ≤0.5wt%, a mercury content ≤20mg / kg, a biodegradability rate ≥90%, and a moisture content ≤0.5%.

[0014] The present invention also provides an application of the biodegradable plastic particles described above in packaging materials, agricultural mulch films, disposable tableware, injection molded products, fiber products, building materials, and automotive interiors.

[0015] This invention provides a method for producing biodegradable plastic granules by replacing part of the resin with hydrated salt mud or hydrated sludge. It breaks through the limitation of traditional plastic granulation on absolutely dry raw materials, and can directly use sludge with a moisture content of 40%~80% or salt mud with a moisture content of 30%~60% to replace part of the biodegradable resin in the production of plastic granules. Through the whole process technical route of "crushing treatment - heavy metal solidification - moisture content control - magnetic separation and impurity removal - melt granulation", it realizes the melt blending and granulation of hydrated salt mud / hydrated sludge and biodegradable resin, which greatly simplifies the process, reduces energy consumption and cost, and solves environmental pollution problems, realizing the large-scale utilization of sludge and salt mud in the biodegradable plastics industry.

[0016] Compared with existing technologies, this invention has the following significant advantages: (1) Breaking through traditional limitations: For the first time, it realizes direct granulation of silt with a water content of 40%~80% or salt mud with a water content of 30%~60%, without drying to <1%, reducing energy consumption by 60%~80%; (2) Significant environmental benefits: For water-containing salt mud, through water washing dechlorination and heavy metal solidification treatment, chloride ions are reduced by 60%-80%, and mercury removal rate is >95%. For water-containing silt, heavy metals are solidified through MIP technology, reducing the bioavailability of heavy metals by more than 30%, solving the environmental pollution problem; (3) Resource recycling: Industrial waste (salt mud or silt) is used to replace part of the biodegradable resin, reducing the use of petrochemical resources by more than 50%; (4) (5) Excellent product performance: The calcium carbonate, magnesium hydroxide, silicon dioxide and other components in the salt mud play a reinforcing and filling role, and the mechanical and thermal properties of the product meet the requirements of use. The biodegradable plastic particles prepared by the silt through chain extender and inorganic filler meet the requirements of use. (6) Simple and efficient process: The whole process is continuous production with a high degree of automation, which is suitable for large-scale industrial application. (7) Advantages of salt mud composition: Salt mud contains calcium carbonate (8.14%), magnesium hydroxide (4.34%), silicon dioxide (50.43%), etc., which are ideal plastic fillers and can reduce the dependence on mineral resources. Attached Figure Description

[0017] Figure 1 This is a photograph of the PBAT-based biodegradable plastic granules prepared in Example 1 of the present invention. Figure 2 This is a photograph of the PLA / PBAT blend biodegradable plastic granules prepared in Example 5 of the present invention. Figure 3 This is a photograph of the biodegradable plastic granules prepared in Example 6 of the present invention. Detailed Implementation

[0018] This invention provides a method for producing biodegradable plastic granules by replacing part of the resin with hydrous salt mud or hydrous silt, characterized by comprising the following steps: (1) The water-containing salt mud or water-containing silt is subjected to crushing, heavy metal solidification, moisture content control and magnetic separation to obtain pretreated salt mud or pretreated silt. (2) After mixing the pretreated salt mud or pretreated sludge with biodegradable resin, chain extender and additives, the mixture is melt-granulated to obtain biodegradable plastic particles.

[0019] Unless otherwise specified, all raw materials used in this invention are commercially available products in the art.

[0020] In this invention, the hydrous salt mud includes soda ash production salt mud (ammonia-soda process) and / or chlor-alkali industry salt mud. The main components of the soda ash production salt mud in this invention include silicon dioxide, calcium carbonate, magnesium hydroxide, and sodium chloride. The mercury content of the chlor-alkali industry salt mud in this invention must be controlled to ≤20 mg / kg. In this invention, the hydrous sludge includes sewage treatment sludge and river / lake bottom sediment, wherein the heavy metal content is controlled as follows: Cu≤500 mg / kg, Zn≤1000 mg / kg, Pb≤300 mg / kg, Cd≤5 mg / kg, Cr≤500 mg / kg, Ni≤100 mg / kg. Before use, this invention detects and controls the content of persistent organic pollutants such as polycyclic aromatic hydrocarbons and dioxins in the hydrous sludge, and employs high-temperature aerobic fermentation or lime stabilization treatment to kill pathogens in the hydrous sludge. In this invention, the particle size D90 of the crushed salt mud and sludge is preferably ≤0.15 mm. This invention controls the particle size of the crushed salt mud and silt within the above-mentioned range so that they can be easily and finely dispersed into the resin layer, fully preserving the natural and excellent properties of the solid waste itself, and giving the resulting particles better physical properties.

[0021] In this invention, after the water-containing salt mud is crushed, the process further includes: mixing the crushed salt mud with water, and then sequentially performing stirring, washing, and solid-liquid separation to obtain dechlorinated salt mud.

[0022] In this invention, the preferred mass ratio of the crushed salt mud to water is 1:(3~5). The preferred stirring and washing time is 15~30 min. This invention uses stirring and washing to dissolve sodium chloride in the crushed salt mud in the water. The preferred solid-liquid separation method is centrifugation or pressure filtration to dehydrate to a moisture content of 40%~50%, with the supernatant recovered for salt production or industrial water use. The preferred chloride ion content in the dechlorinated salt mud is ≤0.5 wt%. This invention, through the above stirring, washing, and solid-liquid separation, improves the dechlorination effect, reducing the chloride ion content of the salt mud by 60%~80%, from approximately 1.09 wt% to 0.2 wt%~0.4 wt%, thus preventing chloride ion corrosion of equipment and products.

[0023] In this invention, when the starting material, hydrous salt mud, is low-mercury or mercury-free, heavy metal solidification is unnecessary. When the hydrous salt mud is high-mercury, the heavy metal solidification preferably includes: removing mercury using sodium sulfide precipitation or oxidative leaching, or using equipment with pressure-controlled dehydration to create a subcritical environment for solidification. In this invention, the sodium sulfide precipitation method treats high-mercury salt mud by adding sodium sulfide to form mercury sulfide precipitate, achieving a mercury removal rate ≥95%. The oxidative leaching method involves introducing chlorine or calcium chloride into the high-mercury salt mud at high temperature, causing mercury to convert into mercuric chloride, which then volatilizes and is recovered through condensation, achieving a mercury recovery rate >80%. This invention, through heavy metal solidification of high-mercury salt mud, reduces the mercury content of the solidified salt mud to ≤20 mg / kg.

[0024] In this invention, when the starting material is water-containing sludge, the heavy metal solidification preferably includes: microbial induced calcite precipitation (MICP) technology, using the highly urease-active strain *Sporosarcina ureilytica* ML-2, and performing five rounds of MICP component irrigation for remediation. The five rounds of MICP component irrigation remediation in this invention rely on the highly active urease produced by the metabolism of the *Sporosarcina ureilytica* ML-2 strain itself. Its specific function is to hydrolyze urea into carbonate ions (CO3-). 2- ) and ammonium ions (NH4) + In a calcium-rich environment, these carbonate ions rapidly combine with calcium ions to form calcite (CaCO3) precipitate, which has a cementing effect. During this process, heavy metal ions are directly mineralized or adsorbed and encapsulated in the crystal structure of calcite, thus transforming from an exchangeable active state to a stable carbonate-bound or residual state, significantly reducing their migration ability and biotoxicity.

[0025] The five-round MICP component irrigation remediation technology of this invention adopts a batch-by-batch, multi-round irrigation method to ensure that the remediation components fully react and penetrate in the medium. In this invention, the basic operation process of the five-round MICP component irrigation remediation preferably includes: 1. Preparation of strains and materials, 2. One round of irrigation, 3. Reaction and settling, 4. Cyclic irrigation (repeated 5 times), 5. Completion of remediation.

[0026] The strain and material preparation described in this invention preferably include culturing the Sporosarcinaureilytica ML-2 strain with high urease activity and preparing a cementing solution composed of urea and calcium chloride. This is the starting point of the technology, utilizing the strain's highly efficient urease-producing ability to drive the entire mineralization process.

[0027] The preferred method of irrigation described in this invention includes: applying the prepared bacterial solution and cementing solution sequentially to the water-containing sludge to be treated in a certain proportion and manner. The bacterial solution provides a catalyst (urease), and the cementing solution provides a reaction substrate (urea) and mineral-forming raw materials (calcium ions), which react in situ in the sludge.

[0028] The preferred reaction and settling conditions described in this invention include providing sufficient reaction time for microbial metabolism and calcite crystal growth under natural or controlled environmental conditions. This is a crucial stage for the "locking in" of heavy metals, requiring ample time to ensure the reaction proceeds fully.

[0029] The cyclic irrigation (repeated 5 times) described in this invention preferably includes: repeating steps 2-3 above 5 times to form a multi-round cyclic treatment. This is the core of achieving efficient stabilization. Through multiple cycles, the reaction components can be continuously supplied, generating more calcite, thereby gradually improving the fixation rate and uniformity of heavy metals.

[0030] In this invention, the remediation process preferably includes: the reaction process ending, and the treated sludge being ready for subsequent resource utilization. The stability of heavy metals is significantly improved and ecotoxicity reduced after treatment, making safe land use possible.

[0031] In this invention, the parameters for the 5-round MICP component irrigation remediation preferably include: 1) Types and amounts of raw materials Functional strain: Sporosarcina ureilytica ML-2. This strain has strong ecological competitiveness, with a relative abundance of up to 53.18% in the treated sludge. Cementing solution: The core components are urea and calcium chloride (CaCl2). The molar ratio of urea to calcium source is usually 1:1, but some studies have shown that a ratio of 1:2 or 2:1 is more effective under different conditions. Key ratio (Ca) 2+ / Cd 2+ Molar ratio: In studies targeting cadmium (Cd) pollution, to ensure a removal rate exceeding 99.7%, the Ca in the cementitious solution was... 2+ with cd 2+ The molar ratio is controlled between 15:1 and 50:1; 2) Environmental parameter range Processing temperature: usually room temperature to 25~30℃, which is the common temperature range for urease-producing strains to maintain optimal activity; Treatment pH: Neutral to slightly alkaline (pH 7-9) environment is conducive to urease activity and calcite precipitation; Repair cycle: Five rounds of watering are used, and the reaction time of a single round depends on the actual situation, usually 24-48 hours.

[0032] This invention reduces the content of exchangeable heavy metals in water-containing sludge by more than 30% by solidifying the sludge.

[0033] In this invention, when the moisture content of the salt mud after heavy metal solidification is ≤40%, moisture content control is preferably not required; when the moisture content of the salt mud after heavy metal solidification is greater than 40% and less than 50%, the moisture content control preferably includes: adding water-absorbing material to the salt mud after heavy metal solidification to control the moisture content to 30%~40%; when the moisture content of the salt mud after heavy metal solidification is >50%, the moisture content control preferably includes: mechanically dehydrating the salt mud after heavy metal solidification to make its moisture content greater than 40% and less than 50%, and then adding water-absorbing material to control the moisture content to 30%~40%. This invention utilizes pretreated salt mud with a moisture content in the range of 30%~40%, which can replace part of the resin and directly enter the melt granulation process without complex drying treatment.

[0034] In this invention, when the moisture content of the sludge after heavy metal solidification is ≤50%, moisture content control is preferably not required. When the moisture content of the salt sludge after heavy metal solidification is greater than 50% and less than 65%, moisture content control preferably includes adding water-absorbing material to the salt sludge after heavy metal solidification to control the moisture content to 40%~50%. When the moisture content of the salt sludge after heavy metal solidification is >65%, moisture content control preferably includes mechanically dehydrating the salt sludge after heavy metal solidification to make its moisture content greater than 50% and less than 65%, and then adding water-absorbing material to control the moisture content to 40%~50%. In this invention, the water-absorbing material is preferably a mixture of coffee grounds, tofu residue, and rice bran in a mass ratio of 4:3:3. The amount of water-absorbing material added in this invention is determined according to the moisture content control requirements. This invention utilizes pretreated sludge with a moisture content in the range of 40%~50%, which can replace part of the resin and directly enter the melt granulation process without complex drying treatment.

[0035] This invention uses a drying method or an online microwave moisture meter to monitor the moisture content in real time; the moisture content fluctuation is controlled within ±2% to avoid affecting the stability of product quality; the moisture content is prone to increase in summer, so it is necessary to increase the amount of water-absorbing material or strengthen dehydration to achieve seasonal adjustment; the water-containing salt mud used should be stored in a rainproof shed to avoid rain causing the moisture content to increase.

[0036] In this invention, the magnetic separation for impurity removal preferably includes: using a permanent magnet drum separator to perform magnetic separation on salt mud or silt with controlled moisture content, and then measuring the amount of magnetic impurities removed by magnetic separation; When the amount of magnetic impurities is ≤ 0.8% of the total mass of the salt mud or silt after moisture content adjustment, the stability of the material after magnetic separation is deemed qualified, and the material after magnetic separation is pretreated salt mud or pretreated silt; when the amount of magnetic impurities is > 0.8% of the total mass of the salt mud or silt after moisture content adjustment, the stability of the material after magnetic separation is deemed unqualified, the moisture content of the material after magnetic separation is measured, and the change in moisture content is calculated according to the formula: change in moisture content = (moisture content of the salt mud or silt after moisture content adjustment - change in moisture content of the material after magnetic separation) / moisture content of the total mass of all materials. When the change in moisture content is ≤3%, the change in moisture content is deemed acceptable, and the material after magnetic separation is pretreated salt mud or pretreated sludge; when the change in moisture content is >3%, the viscosity of the material after magnetic separation is measured. When the viscosity of the magnetically separated material is ≤ 5000~8000 mPa·s (preset viscosity), the viscosity is deemed qualified, and the magnetically separated material is pretreated salt mud or pretreated sludge; when the viscosity of the magnetically separated material is > 5000~8000 mPa·s (preset viscosity), a moisture-regulating material is added to the magnetically separated material to adjust the viscosity to ≤ the preset viscosity, thus obtaining pretreated salt mud or pretreated sludge.

[0037] In this invention, the wet material is preferably anhydrous calcium sulfate or calcium oxide; the mass of the wet material is preferably 0.5% to 2% of the mass of the material after magnetic separation.

[0038] In this invention, the preferred conditions for magnetic separation are: a magnetic field strength of 8000~12000 Gauss and a drum rotation speed of 20~30 rpm for the permanent magnet drum separator. This invention removes magnetic metal impurities through magnetic separation and also provides preliminary drying, reducing the moisture content by 2%~5%. This invention improves the overall performance of the prepared biodegradable resin by measuring and controlling the viscosity of the material after magnetic separation.

[0039] In this invention, the biodegradable resin is preferably one or a blend of PBAT, PLA, PHA, and PPC. In this invention, the mass ratio of the pretreated salt mud or pretreated sludge to the biodegradable resin is preferably (0.8~2.5):1, more preferably (1~2):1. This invention improves the overall performance of the prepared biodegradable resin by controlling the mass ratio of the pretreated salt mud or pretreated sludge to the biodegradable resin.

[0040] In this invention, the chain extender is Joncryl ADR-4370S, and the mass of the chain extender is 0.5% to 2% of the mass of the biodegradable resin. In this invention, for pretreated salt mud, the additives are preferably lubricants and dispersants. In this invention, the lubricants and dispersants preferably include one or more of calcium stearate and polyethylene wax, and the total mass of the lubricants and dispersants is 1% to 3% of the total mass of all materials. This invention utilizes lubricants and dispersants to improve processing fluidity. The starting material, hydrous salt mud, used in this invention contains a large amount of inorganic components such as calcium carbonate, magnesium hydroxide, and silicon dioxide, making it an ideal plastic filler. In this invention, for pretreated sludge, the additives are preferably inorganic fillers. In this invention, the inorganic fillers include one or more of nano-calcium carbonate, talc, and silicon dioxide; the mass of the inorganic fillers is 5% to 15% of the total mass of all materials. The addition of inorganic fillers (such as calcium carbonate, talc, and silicon dioxide) in this invention can improve the crystallinity of the biodegradable resin system, thereby improving crystallization ability and thermal properties.

[0041] In this invention, the melt granulation includes: mixing the pretreated salt mud or pretreated sludge with biodegradable resin, chain extender, and additives, followed by sequential melt blending, cooling, pelletizing, sieving, and packaging to obtain biodegradable plastic granules. In this invention, the melt granulation conditions include: the melt blending method is extrusion using a twin-screw extruder, with an extrusion temperature of 150℃~200℃, a screw speed of 100~300rpm, and a melt pressure of 5~15MPa. In this invention, the uniformity coefficient of variation (CV value) of the mixture is preferably calculated; if the CV value ≤ 5%, the uniformity of the mixture is deemed acceptable, and melt granulation is directly performed; if the CV value > 5%, the mixing time is extended or the screw speed is adjusted until the CV value of the mixture is ≤ 5%. In this invention, the cooling method is preferably water cooling or air cooling to room temperature. In this invention, the length of the pellets obtained by pelletizing is preferably 3~5mm, and the diameter is preferably 2~4mm. In this invention, sieving removes excessively large or small particles to ensure particle uniformity. This invention achieves moisture-proof packaging by using packaging methods to prevent moisture absorption from affecting performance.

[0042] During the melt granulation process of this invention, the extrusion temperature must not exceed 200℃ to avoid resin degradation and chloride ion volatilization; the twin-screw extruder should be equipped with a vacuum exhaust device to promptly remove water vapor and volatile gases; a screw combination with moderate shear strength should be used to avoid excessive shearing leading to material overheating; during the die design process, a die with a large flow cross-section should be used to reduce melt pressure and reduce bubble generation; the cooling rate should be controlled to avoid excessively rapid cooling leading to internal stress concentration; since salt mud contains chloride ions, the equipment should be made of corrosion-resistant materials or undergo anti-corrosion treatment.

[0043] The waste gas generated during the twin-screw extrusion process in this invention should be collected and treated to meet emission standards (waste gas treatment); dust removal devices should be installed in the crushing and mixing processes to control dust concentration (dust control); the transportation, storage, and treatment of mercury-containing salt sludge should comply with hazardous waste management requirements (mercury-containing salt sludge management); operators should wear personal protective equipment such as dust masks and protective glasses (operator protection); emergency plans for mercury leaks and fires should be formulated, and emergency supplies should be provided (emergency plans); the saline wastewater generated during the process of preparing dechlorinated salt sludge through water washing and dechlorination should be recycled for salt production or treated to meet emission standards (wastewater treatment).

[0044] This invention also provides biodegradable plastic granules produced by the method described in the above technical solution. The biodegradable plastic granules have a tensile strength of 18-28 MPa, an elongation at break of 80%-250%, a melt index (MI) of 2-10 g / 10min (190℃, 2.16 kg), a heat distortion temperature of 55℃-85℃, a chloride ion content ≤0.5wt%, a mercury content ≤20 mg / kg, a biodegradability rate ≥90% (180 days under composting conditions), and a moisture content ≤0.5%. The heavy metal content of the biodegradable plastic granules prepared by this invention conforms to GB / T 20197-2006 "Definition, Classification, Labelling and Degradation Performance Requirements of Degradable Plastics".

[0045] The present invention also provides an application of the biodegradable plastic particles described above in packaging materials, agricultural mulch films, disposable tableware, injection molded products, fiber products, building materials, and automotive interiors.

[0046] In this invention, the packaging material preferably includes at least one of shopping bags, garbage bags, express delivery bags, and food packaging bags.

[0047] In this invention, the agricultural mulch film includes at least one of the following: covering film, greenhouse film, and drip irrigation tape. In this invention, the disposable tableware includes at least one of the following: lunch box, plate, cutlery, and straw. In this invention, the injection-molded product includes at least one of the following: daily necessities, toys, stationery, and electronic product casings. In this invention, the fiber product includes at least one of the following: non-woven fabric, textile fiber, and rope / net. In this invention, the building material includes at least one of the following: template, profile, pipe, and sheet material. In this invention, the automotive interior includes at least one of the following: dashboard, door panel, and seat. (Utilizing the flame-retardant properties of inorganic components in salt mud) The technical solutions of this invention will be clearly and completely described below with reference to the embodiments thereof. Obviously, the described embodiments are only a part of the embodiments of this invention, and not all of them. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without creative effort are within the scope of protection of this invention.

[0048] Unless otherwise specified, all experiments were repeated three times, and the results are expressed as averages.

[0049] The detection methods for each index or parameter in this embodiment of the invention are as follows: Tensile strength was measured using GB / T 1040.1-2018 (ISO 527); elongation at break was measured using GB / T 3923.1-2013 (ISO 13934-1); melt index (MI) g / 10min (190℃, 2.16kg) was measured using GB / T 3682.1-2018 (ISO 1133) or ASTM D1238; heat distortion temperature was measured using GB / T 1634.1-2004 (ISO 75) or ASTM D648; chloride ion content was measured using GB / T 3050-2000 potentiometric titration or GB / T 3051-2025 ion chromatography; mercury content was measured using GB / T 43865-2024 (direct injection), NY / T 4547-2025 (catalytic pyrolysis) or HJ. 694-2014 (Atomic Fluorescence Imaging) Method; Biodegradation rate: Primarily determined according to the GB / T 19277 series standards, which is also the domestic equivalent of the international standard ISO 14855. The determination is conducted under controlled composting conditions, and key parameters include: Temperature: Maintained at 58 ± 2℃ to simulate the high-temperature environment of industrial composting; Humidity: Relative humidity controlled at around 50-55% to maintain the activity of microorganisms; Oxygen: Requires continuous supply of CO2-free air or periodic ventilation to ensure sufficient oxygen supply. These conditions are measured after 180 days of composting. The moisture content is determined using the classic drying method.

[0050] The core of determining the coefficient of variation (CV) of moisture content uniformity is to measure the moisture content at different locations by taking multiple samples, and then calculate the ratio of the standard deviation to the mean of these data to quantify the uniformity of the material's moisture distribution. When all samples have the same moisture content, the coefficient of variation is 0, indicating absolute uniformity.

[0051] The CV value determination process includes the following steps: Step 1: Multi-point sampling. To accurately reflect the overall moisture distribution of the material, it is necessary to systematically obtain n representative samples. Step 2: Determine the moisture content of individual samples. The moisture content of all collected individual samples was determined. i (Moisture content of the i-th sample); Step 3: Calculate the coefficient of variation for uniformity; After obtaining the moisture content data for all samples, calculate the average value using the formula below. Calculate the standard deviation s (which measures the dispersion of moisture content data from the mean for each sample) and the coefficient of variation CV (%). ; ; ; Step 4: Interpretation and evaluation. The smaller the coefficient of variation, the higher the uniformity of the moisture content of the material.

[0052] Example 1 A method for producing PBAT-based biodegradable plastic granules using soda ash with a moisture content of 35% to produce salt mud as a substitute for part of the resin includes the following steps: (1) The starting material, water-containing salt mud, is soda ash production salt mud (35% water content, produced by ammonia-soda process), which is mercury-free salt mud; The water-containing salt mud is crushed to a particle size ≤0.15mm, and the crushed salt mud is mixed with water at a mass ratio of 1:4. The mixture is stirred and washed for 20 minutes to achieve water washing and dechlorination. After centrifugation and dehydration, dechlorinated salt mud with a moisture content of 38% is obtained. No heavy metal solidification and moisture content control are required. The impurities are removed directly by magnetic separation. The dechlorinated salt sludge is subjected to magnetic separation using a permanent magnet drum separator. The amount of magnetic impurities removed by the magnetic separation is then measured. If the amount of magnetic impurities is ≤ 0.8% of the total mass of the salt sludge or silt after moisture content adjustment, the stability of the material after magnetic separation is deemed qualified. The material after magnetic separation is then considered pretreated salt sludge. (2) According to the formula: the pretreated salt mud obtained in step (1) is PBAT=1.5:1 (mass ratio), the mass of ADR-4370S chain extender accounts for 1.5% of the total mass of all materials, the mass of calcium stearate accounts for 1% of the total mass of all materials, and the mass of polyethylene wax accounts for 1% of the total mass of all materials. Mix the components evenly to obtain a mixture. Calculate the coefficient of variation (CV) of uniformity ≤5%. Determine that the uniformity of the mixture is qualified. Directly melt-blend to obtain molten material. Then cool it to room temperature by air cooling. Cut it into pellets with a length of 2mm and a diameter of 1.5~2mm. After sieving and packaging with moisture-proof packaging, obtain PBAT-based biodegradable plastic pellets. The conditions for melt granulation are as follows: the melt blending method is extrusion using a twin-screw extruder, the extrusion temperature is 165℃, the screw speed is 180rpm, and the melt pressure is 12MPa; The performance of PBAT-based biodegradable plastic granules will be tested as follows: tensile strength 22 MPa, elongation at break 180%, melt index (MI) 4.5 g / 10min (190℃, 2.16kg), heat distortion temperature up to 90-100℃ (≥ 60℃ is acceptable), chloride ion content 0.3%, biodegradability ≥ 90%, and moisture content of biodegradable plastic granules ≤ 0.5%.

[0053] Example 2 A method for producing PLA / PBAT blend biodegradable plastic granules by replacing part of the resin with chlor-alkali industrial salt mud with a moisture content of 45%, comprising the following steps: (1) Starting material: water-containing salt mud: chlor-alkali industrial salt mud (moisture content 45%, mercury content about 100mg / kg), the water-containing salt mud is crushed to a particle size ≤0.15mm, the crushed salt mud is mixed with water at a mass ratio of 1:3, stirred and washed for 25 minutes to achieve water washing dechlorination, pressure filtration dewatering, and obtained dechlorinated salt mud with a moisture content of 42%; Mercury was removed by sodium sulfide precipitation (Na2S was added to the dechlorinated salt mud to form HgS precipitate), resulting in salt mud with a mercury content reduced to 15 mg / kg and a water content of 15%. After heavy metal solidification, the salt mud did not require water content control and could be directly removed by magnetic separation. The salt mud after heavy metal solidification is subjected to magnetic separation using a permanent magnet drum separator. The amount of magnetic impurities removed by magnetic separation is then measured. If the amount of magnetic impurities is less than or equal to 0.8% of the total mass of the salt mud or silt after moisture content adjustment, the stability of the material after magnetic separation is deemed to be qualified. The material after magnetic separation is the pretreated salt mud. (2) According to the formula: the pretreated salt mud obtained in step (1) is PLA:PBAT=1.2:0.6:0.6 (mass ratio), the mass of ADR-4370S chain extender accounts for 1.2% of the total mass of all materials, the mass of calcium stearate accounts for 1.5% of the total mass of all materials, and the mass of polyethylene wax accounts for 1% of the total mass of all materials. Mix the components evenly to obtain a mixture. Calculate the coefficient of variation (CV) of uniformity ≤5%. Determine that the uniformity of the mixture is qualified. Directly melt-blend to obtain molten material. Then cool it to room temperature by air cooling. Cut it into pellets with a length of 2mm and a diameter of 1.5~2mm. After sieving and packaging with moisture-proof packaging, obtain biodegradable plastic pellets. The conditions for melt granulation are as follows: the melt blending method is extrusion using a twin-screw extruder, the extrusion temperature is 170℃, the screw speed is 150rpm, and the melt pressure is 10MPa; The following properties of the biodegradable plastic granules will be tested: tensile strength 25 MPa, elongation at break 120%, melt index (MI) 2-10 g / 10 min (190℃, 2.16 kg), heat distortion temperature 72℃, chloride ion content 0.25%, mercury content 15 mg / kg, biodegradability ≥90%, and moisture content of the biodegradable plastic granules ≤0.5%.

[0054] Example 3 A method for producing biodegradable plastic pellets using fresh salt mud with a moisture content of 55% to replace part of the resin includes the following steps: (1) Starting material: water-containing salt mud: fresh salt mud with an original water content of 55%. It is used as the starting material after being dehydrated to a water content of 48% by plate and frame filter press in the production of soda ash. It is mercury-free salt mud. The water-containing salt mud is crushed to a particle size ≤0.15mm. The crushed salt mud and water are mixed at a mass ratio of 1:5 and stirred and washed for 30 minutes to achieve water washing and dechlorination. After centrifugation and dehydration, dechlorinated salt mud with a moisture content of 40% is obtained. Without the need for heavy metal solidification, a mixture of coffee grounds, tofu residue and rice bran with a mass ratio of 4:3:3 is directly added to adjust the moisture content to 35%. Then, a permanent magnet drum separator is used for magnetic separation. The amount of magnetic impurities removed by magnetic separation is measured. If the amount of magnetic impurities is ≤0.8% of the total mass of the salt mud or sludge after moisture content adjustment, the stability of the material after magnetic separation is determined to be qualified. The material after magnetic separation is the pretreated salt mud. (2) According to the formula: the pretreated salt mud obtained in step (1) is PBAT=1:1 (mass ratio), the mass of ADR-4370S chain extender accounts for 2% of the total mass of all materials, the mass of calcium stearate accounts for 2% of the total mass of all materials, and the mass of polyethylene wax accounts for 1.5% of the total mass of all materials. Mix the components evenly to obtain a mixture. Calculate the coefficient of variation (CV) of uniformity ≤5%. Determine that the uniformity of the mixture is qualified. Directly melt-blend to obtain molten material. Then cool it to room temperature by air cooling. Cut it into pellets with a length of 2mm and a diameter of 1.5~2mm. After sieving and packaging with moisture-proof packaging, obtain biodegradable plastic pellets. The conditions for melt granulation are as follows: the melt blending method is extrusion using a twin-screw extruder, the extrusion temperature is 165℃, the screw speed is 200rpm, and the melt pressure is 11MPa; The following properties of the biodegradable plastic granules will be tested: tensile strength 20 MPa, elongation at break 150%, melt index (MI) 3.8 g / 10min (190℃, 2.16kg), heat distortion temperature ≥ 60°℃, chloride ion content 0.35%, biodegradability ≥ 90%, and moisture content of the biodegradable plastic granules ≤ 0.5%.

[0055] Example 4 A method for producing PBAT-based biodegradable plastic pellets by replacing part of the resin with sewage treatment sludge with a moisture content of 45%, comprising the following steps: (1) Starting material: water-containing sludge: sewage treatment sludge with a moisture content of 45% after being filtered by plate and frame filter press; The water-containing sludge was crushed to a particle size ≤0.15mm, and 5 rounds of MIP component irrigation remediation (i.e. heavy metal solidification treatment) were carried out by adding the MIP strain Sporosarcina ureilyticaML-2. The heavy metal solidified sludge with a water content of less than 15% was obtained, and no water content control was required. It was then directly removed by magnetic separation. The key reference data for the five rounds of MIP component irrigation remediation are as follows: Type of bacterial agent: The strain with high urease activity, Sporosarcina ureilytica ML-2, was used in the study.

[0056] Bacterial concentration: The concentration of the bacterial solution used (OD600) was 2.02; Types of calcium sources: The most commonly used calcium source is calcium chloride (CaCl2). Cementing solution ratio: The molar ratio of urea to calcium source is usually 1:1; Cementing solution concentration: 0.5 mol / L; lower concentrations and multiple injections usually result in better uniformity. Temperature and Time: The suitable temperature range for the MICP reaction is 25~30 ℃. After the bacterial solution is injected, it is allowed to stand for 6 hours to allow the bacteria to adhere before injecting the cementing solution. The interval between each round is usually 16 hours. Table 1. Statistics of 5-round MICP component irrigation repair process and parameters

[0057] The “Porosity (PV)” in the table refers to the total volume of all pores in the sludge to be remediated. 1 PV is equal to the volume of the binder fluid equal to the total volume of these pores. For example, if the pore volume of 1 cubic meter of sludge is 0.4 cubic meters, then 1 PV is 0.4 cubic meters of binder fluid.

[0058] The sludge after the heavy metal solidification is subjected to magnetic separation using a permanent magnet drum separator. The amount of magnetic impurities removed by magnetic separation is then measured. If the amount of magnetic impurities is less than or equal to 0.8% of the total mass of the salt mud or sludge after moisture content adjustment, the stability of the material after magnetic separation is deemed to be qualified. The material after magnetic separation is the pretreated sludge. (2) According to the formula: the pretreated sludge obtained in step (1) is PBAT=1.2:1 (mass ratio), the mass of ADR-4370S chain extender accounts for 1% of the total mass of all materials, and the mass of nano calcium carbonate accounts for 10% of the total mass of all materials. Mix the components evenly to obtain a mixture. Calculate the coefficient of variation (CV) value of uniformity ≤5%. Determine that the uniformity of the mixture is qualified. Directly melt-blend to obtain molten material. Then cool it to room temperature by air cooling. Cut it into pellets with a length of 2mm and a diameter of 1.5~2mm. After sieving and packaging with moisture-proof packaging, obtain PBAT-based biodegradable plastic pellets. The conditions for melt granulation are as follows: the melt blending method is extrusion using a twin-screw extruder, the extrusion temperature is 180℃, the screw speed is 200rpm, and the melt pressure is 10MPa; The performance of the biodegradable plastic granules will be tested as follows: tensile strength 18 MPa, elongation at break 200%, melt index (MI) 5 g / 10min (190℃, 2.16kg), heat distortion temperature ≥ 60°℃, biodegradability ≥ 90%, and moisture content of PBAT-based biodegradable plastic granules ≤ 0.5%.

[0059] Example 5 A method for producing PLA / PBAT blend biodegradable plastic pellets by replacing part of the resin with river and lake sediment with a moisture content of 55%, comprising the following steps: (1) Starting material: water-containing silt: river and lake bottom sediment (water content 55%); The water-containing sludge was crushed to a particle size ≤0.15mm, and water-absorbing material (i.e., a mixture of coffee grounds, tofu residue and rice bran in a mass ratio of 4:3:3) was added to adjust the moisture content to 48%. The MICP strain Sporosarcinaureilytica ML-2 was added for 5 rounds of MICP component irrigation remediation (i.e. heavy metal solidification treatment) to obtain sludge with a heavy metal solidification moisture content of less than 15%. No moisture content adjustment was required, and it was directly removed by magnetic separation. The five rounds of MIP component irrigation repair were carried out according to the process and parameters of Example 4; The sludge after the heavy metal solidification is subjected to magnetic separation using a permanent magnet drum separator. The amount of magnetic impurities removed by magnetic separation is then measured. If the amount of magnetic impurities is less than or equal to 0.8% of the total mass of the salt mud or sludge after moisture content adjustment, the stability of the material after magnetic separation is deemed to be qualified. The material after magnetic separation is the pretreated sludge. (2) According to the formula: the pretreated sludge obtained in step (1) is PLA:PBAT=1.5:0.5:0.5 (mass ratio), the mass of ADR-4370S chain extender accounts for 1.5% of the total mass of all materials, and the mass of talc calcium accounts for 8% of the total mass of all materials. Mix the components evenly to obtain a mixture. Calculate the uniformity variation coefficient CV value ≤5%. Determine that the uniformity of the mixture is qualified. Directly perform melt blending to obtain melted material. Then cool it to room temperature by air cooling. Cut it into pellets with a length of 2mm and a diameter of 1.5~2mm. After sieving and packaging with moisture-proof packaging, obtain PLA / PBAT blended biodegradable plastic pellets. The conditions for melt granulation are as follows: the melt blending method is extrusion using a twin-screw extruder, the extrusion temperature is 180℃, the screw speed is 150rpm, and the melt pressure is 8MPa; The performance of the biodegradable plastic granules will be tested as follows: tensile strength 22 MPa, elongation at break 150%, melt index (MI) 2-10 g / 10 min (190℃, 2.16 kg), heat distortion temperature 65℃, biodegradability ≥90%, and moisture content of PBAT-based biodegradable plastic granules ≤0.5%.

[0060] Example 6 A method for producing biodegradable plastic pellets using fresh sludge with a moisture content of 70% to replace part of the resin includes the following steps: (1) Starting material: water-containing sludge: fresh sewage treatment sludge with an original moisture content of 70%, which is dehydrated to 60% by electro-osmosis and then mechanically filtered to 50% before being used as the starting material; The water-containing sludge was crushed to a particle size ≤0.15mm, and water-absorbing material (i.e., a mixture of coffee grounds, tofu residue and rice bran in a mass ratio of 4:3:3) was added to adjust the moisture content to 45%. The MICP strain Sporosarcinaureilytica ML-2 was added for 5 rounds of MICP component irrigation remediation (i.e. heavy metal solidification treatment) to obtain sludge with a heavy metal solidification moisture content of less than 15%. No moisture content adjustment was required, and it was directly removed by magnetic separation. The five rounds of MIP component irrigation repair were carried out according to the process and parameters of Example 4; The sludge after the heavy metal solidification is subjected to magnetic separation using a permanent magnet drum separator. The amount of magnetic impurities removed by magnetic separation is then measured. If the amount of magnetic impurities is less than or equal to 0.8% of the total mass of the salt mud or sludge after moisture content adjustment, the stability of the material after magnetic separation is deemed to be qualified. The material after magnetic separation is the pretreated sludge. (2) According to the formula: the pretreated sludge obtained in step (1) is PBAT=1:1 (mass ratio), the mass of ADR-4370S chain extender accounts for 2% of the total mass of all materials, and the mass of nano calcium carbonate accounts for 12% of the total mass of all materials. Mix the components evenly to obtain a mixture. Calculate the coefficient of variation (CV) value of uniformity ≤5%. Determine that the uniformity of the mixture is qualified. Directly perform melt blending to obtain melted material. Then cool it to room temperature by air cooling. Cut it into pellets with a length of 2mm and a diameter of 1.5~2mm. After sieving and packaging with moisture-proof packaging, obtain PLA / PBAT blended biodegradable plastic pellets. The conditions for melt granulation are as follows: the melt blending method is extrusion using a twin-screw extruder, the extrusion temperature is 180℃, the screw speed is 180rpm, and the melt pressure is 12MPa; The performance of the biodegradable plastic granules will be tested as follows: tensile strength 16 MPa, elongation at break 180%, melt index (MI) 4 g / 10min (190℃, 2.16kg), heat distortion temperature 65℃, biodegradability ≥90%, and moisture content of PBAT-based biodegradable plastic granules ≤0.5%.

[0061] Figure 1 This is a photograph of the PBAT-based biodegradable plastic granules prepared in Example 1. Figure 1 It is known that PBAT-based biodegradable plastic granules produced by replacing part of the resin with PBAT and salt mud have a full and smooth appearance, uniform granules, no cavitation (low water content), meet the design requirements, and can be used as raw materials for plastic products.

[0062] Figure 2 This is a photograph of the PLA / PBAT blend biodegradable plastic granules prepared in Example 5. Figure 2 It is known that the PLA / PBAT blend biodegradable plastic granules produced by replacing part of the resin with river and lake sediment have a full and smooth appearance, uniform particle size, no cavitation (low water content), meet the design requirements, and can be used as raw materials for plastic products.

[0063] Figure 3 This is a photograph of the biodegradable plastic granules prepared in Example 6. Figure 3 It is known that biodegradable plastic granules produced by replacing part of the resin with PBAT, fresh sludge, coffee grounds, tofu residue, and rice bran have a full and smooth appearance, uniform granules, no voids (low moisture content), meet design requirements, and can be used as raw materials for plastic products.

[0064] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for producing biodegradable plastic granules by replacing part of the resin with hydrous salt mud or hydrous silt, characterized in that, Includes the following steps: (1) The water-containing salt mud or water-containing silt is subjected to crushing, heavy metal solidification, moisture content control and magnetic separation to obtain pretreated salt mud or pretreated silt. (2) The pretreated salt mud or pretreated sludge is mixed with biodegradable resin, chain extender and additives, and then melt-granulated to obtain biodegradable plastic particles.

2. The method according to claim 1, characterized in that, The step (1) of crushing the water-containing salt mud further includes: mixing the crushed salt mud with water, and then performing stirring, washing and solid-liquid separation in sequence to obtain dechlorinated salt mud.

3. The method according to claim 1, characterized in that, When the starting material water-containing salt mud in step (1) is low-mercury salt mud or mercury-free salt mud, heavy metal solidification is not required. When the water-containing salt mud is high-mercury salt mud, the heavy metal solidification includes: removing mercury by sodium sulfide precipitation or oxidative melting, or using equipment with pressure-controlled dehydration function to form a subcritical environment for solidification. When the starting material in step (1) is water-containing sludge, the heavy metal solidification includes: using microbial induced calcite precipitation technology (MICP), employing the high urease activity strain Sporosarcina ureilytica ML-2, and performing 5 rounds of MICP component irrigation for remediation.

4. The method according to claim 1, characterized in that, When the moisture content of the salt mud after heavy metal solidification in step (1) is ≤40%, no moisture content adjustment is required; When the moisture content of the salt mud after heavy metal solidification is greater than 40% and less than 50%, the moisture content control includes: adding water-absorbing material to the salt mud after heavy metal solidification to control the moisture content to 30%~40%; When the moisture content of the salt mud after heavy metal solidification is >50%, the moisture content control includes: mechanically dehydrating the salt mud after heavy metal solidification to make its moisture content greater than 40% and less than 50%, and then adding water-absorbing material to control the moisture content to 30%~40%; When the moisture content of the sludge after heavy metal solidification in step (1) is ≤50%, no moisture content adjustment is required; When the moisture content of the salt mud after heavy metal solidification is greater than 50% and less than 65%, the moisture content control includes: adding water-absorbing material to the salt mud after heavy metal solidification to control the moisture content to 40%~50%; When the moisture content of the salt mud after heavy metal solidification is >65%, the moisture content control includes: mechanically dehydrating the salt mud after heavy metal solidification to make its moisture content greater than 50% and less than 65%, and then adding water-absorbing material to control the moisture content to 40%~50%.

5. The method according to claim 1, characterized in that, The magnetic separation for impurity removal in step (1) includes: using a permanent magnet drum separator to perform magnetic separation on the salt mud or silt after the moisture content has been adjusted, and then measuring the amount of magnetic impurities removed by the magnetic separation. When the amount of magnetic impurities is ≤ 0.8% of the total mass of the salt mud or silt after moisture content adjustment, the stability of the material after magnetic separation is deemed qualified, and the material after magnetic separation is pretreated salt mud or pretreated silt; when the amount of magnetic impurities is > 0.8% of the total mass of the salt mud or silt after moisture content adjustment, the stability of the material after magnetic separation is deemed unqualified, the moisture content of the material after magnetic separation is measured, and the change in moisture content is calculated according to the formula: change in moisture content = (moisture content of the salt mud or silt after moisture content adjustment - change in moisture content of the material after magnetic separation) / moisture content of the total mass of all materials. When the change in moisture content is ≤3%, the change in moisture content is deemed qualified, and the material after magnetic separation is pretreated salt mud or pretreated sludge; when the change in moisture content is >3%, the viscosity of the material after magnetic separation is measured. When the viscosity of the magnetically separated material is ≤ 5000~8000 mPa·s (preset viscosity), the viscosity is deemed qualified, and the magnetically separated material is pretreated salt mud or pretreated sludge; when the viscosity of the magnetically separated material is > 5000~8000 mPa·s (preset viscosity), a moisture-regulating material is added to the magnetically separated material to adjust the viscosity to ≤ the preset viscosity, thus obtaining pretreated salt mud or pretreated sludge.

6. The method according to claim 1, characterized in that, In step (2), the mass ratio of pretreated salt mud or pretreated sludge to biodegradable resin is (0.8~2.5):

1.

7. The method according to claim 1, characterized in that, In step (2), for pretreated salt mud, the additive is a lubricant and a dispersant; for pretreated sludge, the additive is an inorganic filler.

8. The method according to claim 1, characterized in that, The conditions for melt granulation in step (2) include: extrusion using a twin-screw extruder, extrusion temperature of 150℃~200℃, screw speed of 100~300rpm, and melt pressure of 5~15MPa.

9. A biodegradable plastic granule produced by the method according to any one of claims 1 to 8, characterized in that, The biodegradable plastic granules have a tensile strength of 18~28 MPa, an elongation at break of 80%~250%, a melt index of 2~10 g / 10min, a heat distortion temperature of 55℃~85℃, a chloride ion content of ≤0.5wt%, a mercury content of ≤20mg / kg, a biodegradability rate of ≥90%, and a moisture content of ≤0.5%.

10. The application of the biodegradable plastic granules of claim 9 in packaging materials, agricultural mulch films, disposable tableware, injection molded products, fiber products, building materials, and automotive interiors.

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