Ultra-clean high-purity polyethylene purification process based on supercritical fluid extraction

Abstract

The invention relates to an ultra-clean high-purity polyethylene purification process based on supercritical fluid extraction, which is characterized in that supercritical CO2 containing an entrainer is used as a medium to wash polyethylene powder, and the entrainer comprises the following components: an acid solution, an alcohol solvent and a chelating agent. Compared with the prior art, the method has the advantages that the polyethylene powder is purified on the basis of the supercritical CO2 containing the entrainer, residual organic impurities in polyethylene can be deeply removed, meanwhile, the acid solution, the alcohol solvent, the chelating agent and the like are used as the entrainer, metal and other impurities which are difficult to dissolve in the supercritical CO2 are converted into soluble components, and the purity of the polyethylene powder is improved. And the polyethylene is separated from the polyethylene powder, so that deep purification of the polyethylene is realized.

Description

Technical Field

[0001] This invention relates to the field of ultra-high purity polymer technology, specifically to an ultra-clean high-purity polyethylene purification process based on supercritical fluid extraction. Background Technology

[0002] Polyethylene (PE) has become one of the most widely used general-purpose plastics due to its comprehensive advantages, including low relative density, chemical resistance, water resistance, mechanical strength, and electrical insulation. With the development of technology and industry, the application of high-end polyolefin materials, represented by ultra-high purity polyethylene (UHPE), in cutting-edge fields such as semiconductor manufacturing and biomedicine is becoming increasingly prominent. For example, in the semiconductor industry, UHPE can be used as a core membrane material in ultrapure water preparation systems, but it must meet the stringent requirement of ASTM D4326 that the total content of 12 metallic impurities, including sodium (Na), iron (Fe), and chromium (Cr), be less than 50 ppb. In the medical field, as an implant material, it must meet the requirements of ISO 5835. Furthermore, in semiconductor process auxiliary materials such as photoresist carriers, it must also meet the extreme control requirements for metal ion content in industry standards such as SEMIF57. Current polyethylene production processes still have significant limitations in preparing UHPE, making it difficult to achieve the ultra-low impurity levels required for high-end applications. Existing purification technologies, such as solvent washing and devolatilization, can reduce impurity content to some extent, but they still cannot achieve deep, stable, and cost-effective purification, thus limiting the production of ultra-high purity polyethylene. Therefore, developing a production process that also achieves deep purification is key to overcoming this barrier.

[0003] Existing technologies disclose methods for purifying polymers using supercritical fluids. For example, patent CN114014960B discloses a system and method for purifying polyolefins. This patent uses a supercritical fluid generator to convert hydrocarbons in polyolefins into supercritical fluids, washes the polyolefins with the supercritical fluid through a supercritical fluid washing device, and then separates the impurities in a depressurization device and an impurity separation device. Patent CN113500053A discloses a method for preparing high-purity polypropylene using supercritical CO2. This method involves mixing and stirring polypropylene with ethanol under supercritical CO2 conditions to prepare high-purity polypropylene. However, while CO2 is a non-polar solvent and can remove organic impurities, it is difficult to remove metallic impurities. Polyethylene powder agglomerates, reducing the contact area and making it difficult to thoroughly remove metallic impurities encapsulated in the polymer, thus failing to achieve deep purification of polyethylene. Removing metallic impurities from ultra-high molecular weight polyethylene is particularly difficult due to its large molecular weight, resulting in microscopic entanglement and close carbon chain distances, making traditional solvent cleaning ineffective. Therefore, it is still necessary to further develop a deep purification process that can efficiently produce ultra-high purity polyethylene. Summary of the Invention

[0004] The purpose of this invention is to provide an ultra-clean high-purity polyethylene purification process based on supercritical fluid extraction to solve the above-mentioned problems.

[0005] The objective of this invention is achieved through the following technical solution:

[0006] A purification process for ultra-clean high-purity polyethylene based on supercritical fluid extraction uses supercritical CO2 (scCO2) containing an entrainer as a medium to wash polyethylene powder, wherein the entrainer comprises the following components: acidic solution, alcohol solvent and chelating agent.

[0007] As a preferred embodiment of the present invention, the concentration of the polyethylene powder in supercritical CO2 is 50-300 g / L, more preferably, the concentration of the polyethylene powder in supercritical CO2 is 100-250 g / L, and even more preferably, the concentration of the polyethylene powder in supercritical CO2 is 150-200 g / L.

[0008] Further, based on the weight of the polyethylene powder, the amount of acidic solution added is 0.5-5 wt%, preferably 1-3 wt%, and even more preferably 1.5-2 wt%; the amount of alcohol solvent added is 5-15 wt%, preferably 8-12 wt%, and even more preferably 9-11 wt%; the amount of chelating agent added is 0.1-3 wt%, preferably 0.5-2 wt%, and even more preferably 1-2 wt%.

[0009] As a preferred embodiment of the present invention, the acidic solution is an inorganic acid or an organic acid, wherein the inorganic acid includes at least one of dilute nitric acid, hydrochloric acid, and dilute sulfuric acid, and the organic acid includes at least one of oxalic acid, citric acid, formic acid, and tartaric acid.

[0010] As a preferred embodiment of the present invention, the alcohol solvent includes at least one of isopropanol, ethanol, methanol, n-propanol, and n-butanol.

[0011] As a preferred technical solution of the present invention, the chelating agent is a chelating agent soluble in supercritical carbon dioxide, selected from at least one of hexafluoroacetylacetone, trifluoroacetylacetone, pentafluoropropionylacetone, heptafluorobutyrylacetone, and tributyl phosphate, preferably hexafluoroacetylacetone, trifluoroacetylacetone, and tributyl phosphate.

[0012] As a preferred embodiment of the present invention, the entrainer further includes a surfactant, wherein the amount of surfactant added is 0.1 to 3 wt% based on the weight of the polyethylene powder, preferably 0.1 to 2 wt%, and even more preferably 0.2 to 1 wt%.

[0013] As a preferred technical solution of the present invention, the surfactant is a CO2-affinity surfactant, selected from fluorocarbon chain surfactants (such as perfluorooctyl sulfonate), perfluorocarboxylic acids and their salt surfactants (such as ammonium perfluorooctanoate, sodium perfluorohexanoate), perfluorosulfonate surfactants (such as potassium perfluorobutyrate, sodium perfluorohexanesulfonate), perfluoroalkyl quaternary ammonium salt surfactants (such as perfluorohexyltrimethylammonium chloride), and perfluoropolyether surfactants (such as...). BH-50V).

[0014] As a preferred technical solution of the present invention, when washing polyethylene powder with supercritical CO2 containing an entrainer, the washing is carried out in a pressure-resistant and temperature-controllable mixing device under agitated conditions. The pressure and temperature in the mixing device are controlled above the supercritical state of CO2. Preferably, the washing time is 0.1 to 10 hours; more preferably, the washing time is 0.5 to 8 hours; even more preferably, the washing time is 1 to 4 hours. After the washing is completed, the purified polyethylene powder is obtained by liquid-solid and gas-solid separation.

[0015] As a preferred technical solution of the present invention, the entrainer is added at once in the form of a mixture along with CO2 or polyethylene powder, or added during the washing of polyethylene powder with supercritical CO2.

[0016] As a preferred embodiment of the present invention, the entrainer is added in steps according to the following procedures:

[0017] (1) First, add acidic solution and alcohol solvent to wash for a period of time. Supercritical CO2 containing acidic solution and alcohol solvent is used to acid wash polyethylene powder to dissolve alkaline impurities and metal oxides.

[0018] (2) Add a chelating agent to capture free metal ions and form a chelate;

[0019] Alternatively, a surfactant may be added after step (2).

[0020] Compared with the prior art, the present invention has the following beneficial effects:

[0021] This invention purifies polyethylene powder using supercritical CO2 containing an entrainer. Due to the unique properties of supercritical CO2, it can deeply remove residual organic impurities from polyethylene. Acidic solutions, alcohol solvents, and chelating agents are used as entrainers. The acidic solution reacts with residual alkaline impurities and metal oxides, decomposing metal compounds and releasing metal ions. These metal ions further react with the chelating agent to form chelates soluble in supercritical CO2. Supercritical CO2 has a swelling effect on polyethylene powder, allowing it to diffuse into the powder and fully release the impurities, thus facilitating deep purification. The alcohol solvent hydrolyzes metal residues (such as TiCl4 and AlRXCl3-x) into metal hydroxides and volatile components, causing the catalyst residue to expand and loosen, further enhancing the permeability of supercritical CO2 to the pores. In this process, impurities, including metals, that were originally difficult to dissolve in supercritical CO2 are converted into soluble components and separated from the polyethylene powder along with the supercritical CO2. In addition, CO2 does not damage the polyethylene powder itself. It is non-toxic, non-flammable, chemically stable, non-polluting, easy to recycle and reuse. It does not require drying after washing, and has great economic and environmental benefits. Detailed Implementation

[0022] The present invention will now be described in detail. Any technical solutions not described in detail herein are those already disclosed in the art.

[0023] This invention relates to a polyethylene purification process based on supercritical CO2 extraction. Supercritical CO2 exhibits similar kinetic properties to gaseous CO2, possessing gas-like diffusivity and a density close to that of a liquid, resulting in strong solubilizing capabilities. It is particularly effective at dissolving organic impurities (including solvents, residual monomers, and other non-volatile residues) and oligomers in polyethylene materials. Its viscosity is higher than that of a gas but much lower than that of a liquid, with a diffusion coefficient 10 to 100 times that of a liquid, demonstrating excellent mass transfer and good permeability. Supercritical CO2 can penetrate spaces larger than its molecules, effectively washing away metal components, such as active centers like Ti, that adhere to the surface via van der Waals forces. Due to its excellent swelling effect, a portion of the catalyst carrier can also be washed away, thus facilitating deep cleaning of polyethylene powder, especially removing impurities such as metals coated on the powder. Furthermore, its near-zero surface tension and low supercritical temperature prevent damage to the polyethylene powder itself. Because CO2 is non-toxic, non-flammable, chemically stable, pollution-free, easily recyclable, and reusable, drying is not required after cleaning.

[0024] However, while supercritical CO2, as a non-polar solvent, has good solubility for organic impurities, it has poor solubility for metallic impurities, making it difficult to effectively prepare ultra-clean, high-purity polyethylene with low metallic impurities. For example, high molecular weight polyethylene powder produced using a slurry process with a Ziegler-Natta catalyst mainly contains residual titanium chloride, magnesium chloride catalyst support, triethylaluminum co-catalyst, and other metallic impurities introduced into the production system. Some titanium chloride and magnesium chloride exist as hydrated magnesium chloride and hydroxyl-complexed titanium, and a significant portion of the metals are converted into metal oxides due to the interaction of water and oxygen. Therefore, the key to purifying polyethylene powder is removing these impurities. To fully utilize the advantages of supercritical CO2-based polyethylene purification, this invention has conducted in-depth research on this process. Through impurity analysis of polyethylene, the supercritical CO2 extraction and purification process has been improved by adding a specific entrainer, which washes the polyethylene powder along with supercritical CO2. During this process, impurities, including metals, that are not originally soluble in supercritical CO2 are converted into soluble components and separated with the supercritical CO2.

[0025] Based on the above-mentioned inventive concept, the present invention provides an ultra-clean high-purity polyethylene purification process based on supercritical fluid extraction, which uses supercritical CO2 containing an entrainer as a medium to wash polyethylene powder, wherein the entrainer includes the following components: acidic solution, alcohol solvent and chelating agent.

[0026] This process uses supercritical CO2 as a solvent to soak and wash polyethylene powder. The addition of an acidic solution reacts with impurities such as metal oxides in the polyethylene, decomposing the metal compounds and releasing metal ions. These ions then react with a chelating agent to form chelates soluble in supercritical CO2. Supercritical CO2 has a swelling effect on the polyethylene powder, allowing it to diffuse into the powder and fully release various impurities, thus facilitating deep purification. Alcohol solvents hydrolyze metal residues (such as TiCl4 and AlR3Cl) into metal hydroxides and volatile components, causing the catalyst residue to expand and loosen, further enhancing the permeability of supercritical CO2 to the pores. Impurities that were originally difficult to dissolve in supercritical CO2, including metals, are continuously converted into soluble components and separated from the polyethylene powder with the supercritical CO2, achieving deep purification of the polyethylene powder.

[0027] In a preferred embodiment, the polyethylene powder comes from a polyethylene production facility, for example, high molecular weight polyethylene powder produced by a slurry process using a Ziegler-Natta catalyst, and its ash content is typically between 80 and 250 ppm.

[0028] In a preferred embodiment, the concentration of the polyethylene powder in supercritical CO2 is 50-300 g / L. More preferably, the concentration of the polyethylene powder in supercritical CO2 is 100-250 g / L. Even more preferably, the concentration of the polyethylene powder in supercritical CO2 is 150-200 g / L. A suitable ratio of supercritical CO2 to polyethylene powder can achieve the purification of polyethylene powder in a more economical way.

[0029] In a preferred embodiment, based on the weight of the polyethylene powder, the amount of acidic solution added is 0.5-5 wt%, preferably 1-3 wt%, and even more preferably 1.5-2 wt%; the amount of alcohol solvent added is 5-15 wt%, preferably 8-12 wt%, and even more preferably 9-11 wt%; and the amount of chelating agent added is 0.1-3 wt%, preferably 0.5-2 wt%, and even more preferably 1-2 wt%.

[0030] In a preferred embodiment, the acidic solution is an inorganic acid or an organic acid. The inorganic acid includes at least one selected from dilute nitric acid, hydrochloric acid, and dilute sulfuric acid, and the organic acid includes at least one selected from oxalic acid, citric acid, formic acid, and tartaric acid. Preferably, an organic acid capable of dissolving basic metal oxides and salts is used, which will not damage polyethylene, avoid equipment corrosion, and is volatile and easily separated from polyethylene.

[0031] In a preferred embodiment, the alcohol solvent includes at least one of isopropanol, anhydrous ethanol, methanol, n-propanol, and n-butanol. Alcohol solvents help enhance the polarity of supercritical CO2 and can act as co-solvents for acids and chelating agents. Alcohols also help adjust the morphology and properties of supercritical CO2 microemulsions and break up the agglomeration of polyethylene powder.

[0032] In a preferred embodiment, the chelating agent is a chelating agent soluble in supercritical carbon dioxide (scCO2), selected from at least one of hexafluoroacetylacetone (HFAH), trifluoroacetylacetone (TFA), pentafluoropropionylacetone (PFPA), heptafluorobutyrylacetone (HFBA), and tributyl phosphate. The chelating agent has a metal-capturing effect, capable of chelating with free metal ions to form a chelate soluble in supercritical carbon dioxide, which is then discharged with the supercritical carbon dioxide, achieving the purpose of polyethylene purification.

[0033] In a preferred embodiment, the entrainer further comprises a surfactant, wherein the amount of surfactant added is 0.1 to 3 wt% based on the weight of the polyethylene powder, preferably 0.1 to 2 wt%, and even more preferably 0.2 to 1 wt%.

[0034] In a preferred embodiment, the surfactant is a CO2-affinity surfactant, selected from fluorocarbon chain surfactants (such as perfluorooctyl sulfonate), perfluorocarboxylic acids and their salts (such as ammonium perfluorooctanoate, sodium perfluorohexanoate), perfluorosulfonate surfactants (such as potassium perfluorobutyrate, sodium perfluorohexanesulfonate), perfluoroalkyl quaternary ammonium salt surfactants (such as perfluorohexyltrimethylammonium chloride), and perfluoropolyether surfactants (such as...). BH-50V), etc. Preferably, perfluoropolyether surfactants (such as...) are used. BH-50V, series, (Fluoropolyether products in the series). Surfactants can generate nanoscale polar regions in supercritical CO2, forming microemulsions. The presence of a polar microenvironment in these microemulsions enhances the solubility of supercritical CO2 for polar substances, further improving the solubility of metal hydroxides and polar byproducts, thereby effectively removing ionic metal residues.

[0035] As a preferred embodiment, washing is performed under disturbed conditions in a pressure-resistant and temperature-controllable mixing device, such as an extraction vessel, reaction vessel, or mixing tank. The mixing device is selected with a stirring and heat exchange jacket / heat exchange unit. The pressure and temperature within the mixing device are controlled above the supercritical state of CO2. Preferably, the washing time is 0.1–10 hours; more preferably, the washing time is 0.5–8 hours; even more preferably, the washing time is 1–4 hours. After the washing process is complete, purified polyethylene powder is obtained through liquid-solid and gas-solid separation.

[0036] In a preferred embodiment, when washing the polyethylene powder, the temperature and pressure are above the supercritical state of CO2. Preferably, the temperature is 40-90°C and the pressure is 7.4-20 MPa. More preferably, the temperature is 45-80°C and the pressure is 7.5-10 MPa. Even more preferably, the temperature is 50-70°C and the pressure is 7.5-8.5 MPa.

[0037] In a preferred embodiment, the entrainer can be added either in a mixed manner or in stages. For example, the entrainer can be added all at once with CO2 or polyethylene powder in the form of a mixture, or it can be added directly during the washing of polyethylene powder with supercritical CO2.

[0038] In a more preferred embodiment, the entrainer is added in steps according to the following steps:

[0039] (1) First, add acidic solution and alcohol solvent to wash for a period of time. Supercritical CO2 containing acidic solution and alcohol solvent is used to acid wash polyethylene powder to dissolve alkaline impurities and metal oxides.

[0040] (2) Add a chelating agent to capture free metal ions and form a chelate that can dissolve in supercritical CO2;

[0041] Alternatively, a surfactant may be added after step (2).

[0042] Stepwise addition allows for better utilization of each entrainer, achieving deeper and more efficient purification. The above steps achieve acid washing and dissociation, followed by chelation and re-permeation. In the acid washing stage, scCO2 containing acid and alcohol is added to dissolve alkaline impurities and oxides. In the chelation stage, scCO2 containing chelating agents and alcohol is added to capture free metal ions. Finally, deep washing with scCO2 containing surfactants is performed, meeting the needs of deep purification scenarios with extremely high purity requirements (such as electronic and medical grades).

[0043] Unless otherwise specified, all chemical reagents used in the embodiments of this invention were obtained through conventional commercial channels, and all reagents used were high-purity reagents, with ultrapure water used. For example, 20% dilute nitric acid, oxalic acid, citric acid, anhydrous ethanol, isopropanol, etc., can be purchased from Sinopharm Chemical Reagent Co., Ltd., Sigma-Aldrich, Aladdin, etc.; hexafluoroacetylacetone and trifluoroacetylacetone can be purchased from Bailingwei Technology Co., Ltd., Matrix Scientific, etc.; surfactants... BH-50V was purchased from Huntsman; the liquid carbon dioxide used is high-purity liquid carbon dioxide (≥99.9%), which can be purchased from gas suppliers such as Linde Group and Air Products.

[0044] The above process will be further explained below with reference to specific embodiments.

[0045] A small-scale experiment was conducted using a 1L supercritical fluid extraction apparatus equipped with mechanical stirring. The apparatus included a thermostatic jacketed extraction vessel, a CO2 high-pressure metering pump, a preheater, a back pressure valve, and a product collection system. The raw material to be purified was polyethylene powder (brand name LPF-8010U, viscosity-average molecular weight approximately 1.5 million) produced by Puxijing New Energy Materials (Shanghai) Co., Ltd. Before use, the raw material was dried in a vacuum oven at 60℃ for 12 hours.

[0046] Weigh approximately 100g of the above-mentioned polyethylene powder sample and place it in an extraction vessel. According to the formulation in Table 1, add the entrainer (e.g., 1.6g of 20% dilute nitric acid, 9g of anhydrous ethanol, and 1.2g of hexafluoroacetylacetone in Experiment 1) to the extraction vessel and mix it with the polyethylene powder. After sealing the extraction vessel, turn on the constant temperature system to raise the vessel temperature (50-60℃). Then, start the high-pressure metering pump to inject liquid CO2 into the extraction vessel until the system pressure reaches the supercritical state of CO2 (7.5-8MPa). Start stirring to wash and extract the polyethylene powder, continuing the treatment for a period of time (1-4 hours). After the treatment is completed, slowly open the back pressure valve to uniformly release the system pressure to atmospheric pressure within 30 minutes. During this process, filter the powder through the filter at the bottom of the device, and then wash it 1-2 times with liquid carbon dioxide to obtain purified polyethylene powder.

[0047] Experiments 1-3

[0048] Add approximately 100g of dry polyethylene powder experimental sample to the extraction vessel, add all the entrainer (acidic solution, alcohol solvent, chelating agent, or surfactant) at once, add liquid CO2, and then control the pressure and temperature inside the extraction vessel to be above the supercritical state of CO2 (approximately 55℃, 7.6MPa) to treat the polyethylene powder and obtain purified polyethylene powder.

[0049] Experiments 4-5

[0050] Add approximately 100g of dry polyethylene powder experimental sample to the extraction vessel, and add entrainer step by step according to the following steps: (1) Add acidic solution and alcohol solvent, add liquid CO2 accounting for about 1 / 3 of the total amount, control the pressure and temperature in the extraction vessel to be above the supercritical state of CO2 (about 55℃, 7.6MPa), treat the polyethylene powder for about 1 hour, and then release the pressure and filter; (2) Add chelating agent, add liquid CO2 accounting for about 1 / 3 of the total amount, control the pressure and temperature in the extraction vessel to be above the supercritical state of CO2 (about 55℃, 7.6MPa), treat the polyethylene powder for about 1 hour, and then release the pressure and filter; (3) Add surfactant, add liquid CO2 accounting for about 1 / 3 of the total amount, control the pressure and temperature in the extraction vessel to be above the supercritical state of CO2 (about 55℃, 7.6MPa), treat the polyethylene powder for about 1 hour, and then release the pressure and filter to obtain purified polyethylene powder.

[0051] Experiment 6

[0052] With reference to Experiment 3, except that no chelating agent was added, everything else was the same as in Example 3.

[0053] Experiment 7

[0054] With reference to Experiment 3, except that no alcohol solvent was added, everything else was the same as in Example 3.

[0055] Experiment 8

[0056] Ultrapure water was used as the solvent for washing. Following Experiment 3 as a reference, ultrapure water was used to replace supercritical liquid CO2. The specific steps were as follows: Approximately 100g of polyethylene powder was added to a 1L beaker, along with all the entraining agents (oxalic acid, isopropanol, and the water-soluble chelating agent EDTA), and approximately 500mL of ultrapure water. The temperature was raised to approximately 55℃, and the polyethylene powder was treated with stirring. After treatment, the powder was filtered, rinsed 1-2 times with ultrapure water, and dried to obtain the treated polyethylene powder.

[0057] Table 1. Specific amounts and composition of experimental materials

[0058]

[0059] The metal content of each polyethylene powder sample obtained in the above experiment was tested.

[0060] 1. The content of major metals was determined by inductively coupled plasma optical emission spectrometry (ICP-OES);

[0061] 2. The leaching experiment of the sample was carried out by ICP-MS to test the amount of main metal ions leached in the leaching solvent (electronic grade OK73, single metal <0.1ppb) of the purified sample (leaching time 7d). The amount of metal ions leached = (metal concentration of leaching solution - metal content concentration of blank solution) * volume of leaching solution / mass of PE material.

[0062] Table 2. ICP-OES Metal Content Test Results (mg / kg)

[0063] Metal content (ppm) Al Ti Mg Fe Cu K Na Zn Original sample 21 11.5 8.7 <2 <2 <2 <2 <2 Experiment 1 6.5 6.2 6.8 <2 <2 <2 <2 <2 Experiment 2 5.8 5.1 6.3 <2 <2 <2 <2 <2 Experiment 3 4.2 3 4.5 <2 <2 <2 <2 <2 Experiment 4 <2 <2 3.2 <2 <2 <2 <2 <2 Experiment 5 3 2.2 3.8 <2 <2 <2 <2 <2 Experiment 6 18.8 9.3 7.8 <2 <2 <2 <2 <2 Experiment 7 12.2 7.5 7.1 <2 <2 <2 <2 <2 Experiment 8 13.3 5.8 7.5 <2 <2 <2 <2 <2

[0064] Table 2 shows that in experiments 1 and 2, supercritical CO2 combined with acidic solution, alcohol solvents, and chelating agents can remove metal impurities from polyethylene powder. In experiments 3 to 5, the addition of surfactants resulted in the removal of even more metal impurities. Compared to adding the entrainer all at once, the staged extraction process in experiments 4 and 5 was more effective. Experiments 6 and 7 were less effective than experiments 1 to 5, indicating that chelating agents and alcohol solvents are key components for removing metal impurities; the lack of chelating agents or alcohol solvents makes it difficult to effectively remove metal ions. Experiment 8 used a water washing process, which also removed metal impurities, but the effect was not as good as the supercritical CO2 process. It required the use of ultrapure water, and an additional drying process was needed after washing.

[0065] Table 3. Metal ion precipitation amount (μg / kg)

[0066] Precipitation amount (ppb) Mg Al K Ti Cr Fe Co Ni Cu Zn total Experiment 1 3.35 1.88 3.99 0.15 0.79 2.87 1.20 0.35 0.42 5.12 20.13 Experiment 2 3.54 1.54 4.12 0.13 0.65 2.24 0.99 0.30 0.37 4.36 18.24 Experiment 3 2.15 0.99 2.87 <0.1 0.49 2.15 0.65 0.21 0.28 3.12 13.03 Experiment 4 0.85 0.45 1.25 <0.1 0.20 1.02 0.30 <0.1 0.12 1.55 5.91 Experiment 5 1.25 0.65 1.55 <0.1 0.28 1.32 0.40 0.12 0.15 1.99 7.79 Experiment 6 5.49 2.37 6.55 0.19 0.99 5.12 1.55 0.45 0.59 6.87 30.16 Experiment 7 5.12 2.15 6.32 0.18 0.93 3.99 1.48 0.42 0.55 6.55 27.70 Experiment 8 5.26 2.23 6.21 0.18 0.93 4.78 1.43 0.41 0.53 6.39 28.35

[0067] As shown in Table 3, compared with Experiments 6-8, the polyethylene samples obtained by the purification process in Experiments 1-5 have a lower total amount of metal ion precipitation in the leaching solvent (≤20ppb), with Experiment 4 having the lowest (<6ppb), reaching an ultra-low metal ion precipitation level, which can meet the high-end application requirements of core membrane materials and photoresist carriers in semiconductor ultrapure water preparation systems.

[0068] The above description of the embodiments is provided to enable those skilled in the art to understand and use the invention. It will be apparent to those skilled in the art that various modifications can be made to these embodiments, and the general principles described herein can be applied to other embodiments without inventive effort. Therefore, the present invention is not limited to the above embodiments, and any improvements and modifications made by those skilled in the art based on the disclosure of the present invention without departing from the scope of the invention should be within the protection scope of the present invention.

Claims

1. A purification process for ultra-clean high-purity polyethylene based on supercritical fluid extraction, characterized in that, Polyethylene powder is washed using supercritical CO2 containing an entrainer as a medium, wherein the entrainer comprises the following components: an acidic solution, an alcohol solvent, and a chelating agent.

2. The ultra-clean high-purity polyethylene purification process based on supercritical fluid extraction according to claim 1, characterized in that, The concentration of the polyethylene powder in supercritical CO2 is 50-300 g / L; The amount of acidic solution added is 0.5 to 5 wt% based on the weight of the polyethylene powder. The amount of alcohol solvent added is 5-15 wt%; The amount of the chelating agent added is 0.1–3 wt%.

3. The ultra-clean high-purity polyethylene purification process based on supercritical fluid extraction according to claim 1, characterized in that, The acidic solution is an inorganic acid or an organic acid, wherein the inorganic acid includes at least one of dilute nitric acid, hydrochloric acid, and dilute sulfuric acid, and the organic acid includes at least one of oxalic acid, citric acid, formic acid, and tartaric acid.

4. The ultra-clean high-purity polyethylene purification process based on supercritical fluid extraction according to claim 1, characterized in that, The alcohol solvent includes at least one of isopropanol, ethanol, methanol, n-propanol, and n-butanol.

5. The ultra-clean high-purity polyethylene purification process based on supercritical fluid extraction according to claim 1, characterized in that, The chelating agent is a chelating agent soluble in supercritical carbon dioxide, selected from at least one of hexafluoroacetylacetone, trifluoroacetylacetone, pentafluoropropionylacetone, heptafluorobutyrylacetone, and tributyl phosphate.

6. The ultra-clean high-purity polyethylene purification process based on supercritical fluid extraction according to claim 1, characterized in that, The entrainer further includes a surfactant, and the amount of the surfactant added is 0.1 to 3 wt% based on the weight of the polyethylene powder.

7. The ultra-clean high-purity polyethylene purification process based on supercritical fluid extraction according to claim 6, characterized in that, The surfactant is a CO2-affinity surfactant, selected from fluorocarbon chain surfactants, perfluorocarboxylic acid and its salt surfactants, perfluorosulfonate surfactants, perfluoroalkyl quaternary ammonium salt surfactants, and perfluoropolyether surfactants.

8. The ultra-clean high-purity polyethylene purification process based on supercritical fluid extraction according to claim 1, characterized in that, Washing is carried out in a pressure-resistant and temperature-controlled mixing device under agitation. The pressure and temperature inside the mixing device are controlled above the supercritical state of CO2. After the washing is completed, purified polyethylene powder is obtained through liquid-solid and gas-solid separation.

9. The ultra-clean high-purity polyethylene purification process based on supercritical fluid extraction according to claim 1, characterized in that, The entrainer is added in a single step along with CO2 or polyethylene powder in the form of a mixture, or added during the washing of polyethylene powder with supercritical CO2.

10. The ultra-clean high-purity polyethylene purification process based on supercritical fluid extraction according to claim 1, characterized in that, The entrainer is added step by step according to the following steps: (1) First, add acidic solution and alcohol solvent to wash for a period of time to dissolve alkaline impurities and metal oxides; (2) Add a chelating agent to capture free metal ions and form a chelate that can dissolve in supercritical CO2; Alternatively, a surfactant may be added after step (2).

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