A method for extracting proteins from ion exchange slurry based on pre-coated filtration

Abstract

This invention discloses a method for extracting protein from ion exchange slurry based on pre-coated filtration, comprising the following steps: mixing activated carbon, diatomaceous earth, chelating resin, and water to prepare a pre-coating solution; pumping the pre-coating solution into a pre-coated filter for pre-coating treatment; vacuum filtering the ion exchange slurry using the pre-coated filter after coating, collecting the filtrate and filter residue; dispersing the collected filter residue with water, centrifuging the dispersion, and spray-drying the supernatant to obtain compound fertilizer; and further filtering the filtrate and spray-drying the filtrate to obtain protein. This method effectively removes impurities and metal ions from the ion exchange slurry, producing a high-content protein as the main product and a compound fertilizer as the byproduct, thus improving the economic value of the ion exchange slurry. Furthermore, the waste liquid and residue generated during the process are fully utilized, resulting in energy conservation and environmental protection.

Description

Technical Field

[0001] This invention relates to the field of protein preparation technology, and more specifically to a method for extracting proteins from ionized slurry based on pre-coated filtration. Background Technology

[0002] Corn soaking water is organic wastewater generated during the wet starch production process from corn. Currently, the main method for developing and utilizing corn soaking water is to recover phytic acid through resin adsorption, followed by further treatment to prepare a series of products. However, this process generates a large amount of ion exchange slurry. Failure to treat this wastewater will not only cause secondary pollution to the environment but also result in resource waste.

[0003] The slurry from corn soaking contains a large amount of protein. The protein in this water has physiological activities such as enhancing human immunity and anti-oxidation, making it a protein resource that has not yet been fully developed and utilized. If the protein could be extracted from the corn soaking water, it would not only improve the factory's economic efficiency and solve environmental pollution problems, but also allow for the development of new protein resources.

[0004] Currently, the main methods for preparing proteins from ion exchange slurries are the removal of impurities and metal ions. However, filtration is difficult, and conventional clarification methods cannot be used. While membrane clarification methods offer good clarification results, they are prone to clogging pipelines, incurring high maintenance costs and overall operating expenses. Therefore, how to effectively recover proteins from ion exchange slurries has become a problem that needs to be solved by those skilled in the art. Summary of the Invention

[0005] The technical problem to be solved by the present invention is to provide a method for extracting protein from ion exchange slurry based on pre-coating filtration, which can effectively remove impurities and metal ions from the ion exchange slurry, and produce high-protein main product and compound fertilizer by-product, thereby improving the economic value of ion exchange slurry. In addition, the waste liquid and waste residue in the process are fully utilized, which is energy-saving and environmentally friendly.

[0006] To solve the above-mentioned technical problems, the technical solution of the present invention is as follows:

[0007] A method for extracting proteins from a cross-linked slurry based on pre-coated filtration includes the following steps:

[0008] (1) Preparation of pre-coating solution: Activated carbon, diatomaceous earth, chelating resin and water are mixed and stirred to prepare pre-coating solution;

[0009] (2) Pre-coating treatment: The above pre-coating liquid is pumped into the pre-coating filter for pre-coating treatment;

[0010] (3) Filtration: Vacuum filter the slurry after the coating in step (2) using the pre-coated filter, and collect the filtrate and filter residue;

[0011] (4) Recovering by-products: The filter residue collected in step (3) is added to water and dispersed. The dispersion is centrifuged and the supernatant is spray-dried to obtain compound fertilizer.

[0012] (5) Protein recovery: The filtrate from step (3) is filtered a second time and spray-dried to obtain protein.

[0013] Preferably, in step (1), the chelating resin is a mixture of D401 resin and D001×7 resin, and the mass ratio of D401 resin to D001×7 resin is 3:(1-5).

[0014] Preferably, in step (1), the activated carbon and diatomaceous earth are both 60-150 mesh.

[0015] Preferably, in step (1), the mass ratio of activated carbon, diatomaceous earth and chelating resin is 1:(0.9-1.2):(0.1-0.2).

[0016] Preferably, in step (1), the total volume ratio of the activated carbon, diatomaceous earth and chelating resin to the water is 1:(1-5).

[0017] Preferably, in step (2), during the pre-coating process, the filter cloth of the pre-coated filter has a mesh size of 60-100 mesh, and the pressure during pre-coating is -0.04 to -0.1 MPa.

[0018] Preferably, in step (2), the thickness of the pre-coating layer after the pre-coating treatment is 50-100 mm.

[0019] Preferably, in step (3), the vacuum filtration conditions are: the flow rate of the discharged slurry is 100-150 L / h·m. 2 The pressure is -0.04 to -0.1 MPa.

[0020] Preferably, in step (4), the volume ratio of filter residue to water during dispersion is 1:(1-3).

[0021] Preferably, in step (4), the centrifugation speed is 1000-3000 r / min and the centrifugation time is 3-8 min.

[0022] Preferably, in step (5), a plate and frame filter is used for secondary filtration. The filter cloth of the plate and frame filter has a mesh size of 60-100 mesh, and the pressure during secondary filtration is -0.04 to -0.1 MPa.

[0023] Preferably, in steps (4) and (5), the precipitate after centrifugation and the filter residue after secondary filtration are used as pre-coating agents in the preparation of the pre-coating liquid in step (1).

[0024] Due to the adoption of the above technical solution, the beneficial effects of the present invention are:

[0025] 1. This invention provides a method for extracting protein from ion exchange slurry based on pre-coated filtration. The method uses ion exchange slurry excluding phytic acid as raw material. First, it is filtered through a pre-coated filter containing activated carbon, diatomaceous earth and chelating resin as pre-coating. The filtrate is then filtered a second time through a plate and frame filter press to obtain a high protein content. This method has high production efficiency, low cost and is suitable for large-scale production.

[0026] 2. This invention uses activated carbon, diatomaceous earth, and chelating resin in a specific ratio to form a pre-coating. Activated carbon has a high specific surface area and excellent adsorption performance, effectively removing impurities from the ion exchange slurry and adsorbing metal ions through its surface active functional groups. Diatomaceous earth, as a natural mineral material, has a fine pore structure that effectively intercepts suspended solids and insoluble impurities. Its porous particle shape provides a larger filtration area and higher filtration efficiency, thereby increasing liquid throughput, reducing membrane clogging, and ensuring efficient filtration. The fine particles of diatomaceous earth combine with activated carbon to form a composite structure, further improving the removal of fine impurities. The chelating resin specifically complexes with metal ions in the ion exchange slurry, effectively removing metal ions while protecting water-soluble proteins, thus improving protein purity. In summary, based on the combined effects of activated carbon, diatomaceous earth, and chelating resin, the pre-coating optimizes the overall filtration process while removing small particulate impurities, improving protein purity and recovery rate.

[0027] 3. The present invention uses a chelating resin composed of D401 and D001×7 resins. D001×7 resin is a strongly acidic cation exchange resin with sulfonic acid groups on a styrene-divinylbenzene copolymer. D401 resin is a macroporous styrene ion exchange resin containing iminodiacetic acid chelating groups. The active groups on the surface of D401 resin can complex with metal ions such as calcium and magnesium ions in the ion exchange slurry, thereby achieving metal ion adsorption. Meanwhile, the hydroxyl groups in the sulfonic acid groups on the surface of D001×7 resin can bind with metal ions, thereby adsorbing metal ions in the ion exchange slurry. In summary, the chelating resin composed of D401 and D001×7 resins has a better surface area and more effective active sites, enhancing its ability to capture different metal ions. The ion exchange characteristics of D001×7 resin can further optimize the adsorption behavior of the resin, synergistically enhancing the removal of multiple metal ions, thereby producing proteins with higher purity.

[0028] 4. In this invention, the filter residue after filtration by a pre-coated filter is added to water for pulping and dispersion, and then further centrifuged. The supernatant from the centrifugation contains effective components such as lactic acid, sugars, magnesium salts, and calcium salts, which can be sold as compound fertilizer, thus improving the economic benefits for enterprises.

[0029] 5. This invention reuses the centrifugal sediment and filter residue from secondary filtration in the preparation of the pre-coating solution, further reducing energy consumption. Detailed Implementation

[0030] To better understand the above-mentioned objectives, features, and advantages of the present invention, the solutions of the present invention will be further described below. It should be noted that, unless otherwise specified, the embodiments of the present invention and the features thereof can be combined with each other.

[0031] To further understand the present invention, preferred embodiments of the present invention are described below in conjunction with examples. However, it should be understood that these descriptions are only for further illustrating the features and advantages of the present invention, and not for limiting the scope of the claims of the present invention.

[0032] In the following examples and comparative examples, the D001×7 resin was sourced from Zhejiang Zhengguang Industrial Co., Ltd., and the D401 resin from Tianjin Yunkai Resin Technology Co., Ltd. Unless otherwise specified, all other raw materials are commercially available products, and the conditions described are standard conditions in the art.

[0033] Protein yield (%) = [(protein yield × protein content) / (content of dry matter in slurry after delivery × / protein content in dry matter in slurry after delivery)] × 100%.

[0034] Example 1

[0035] A method for extracting proteins from a cross-linked slurry based on pre-coated filtration includes the following steps:

[0036] (1) Preparation of pre-coating agent:

[0037] Activated carbon (60 mesh), diatomaceous earth (60 mesh), and chelating resin (D401 resin and D001×7 resin are compounded in a mass ratio of 3:2) are mixed in a mass ratio of 1:1:0.15 to obtain a pre-coating agent. The pre-coating agent and water (the volume ratio of pre-coating agent and water is 1:3) are mixed and stirred to obtain a pre-coating liquid.

[0038] (2) Pre-coating treatment:

[0039] The above pre-coating liquid is pumped into a pre-coated filter (100 mesh filter cloth) for pre-coating treatment, so that a pre-coating layer with a thickness of 50 mm is deposited on the surface of the filter cloth of the pre-coated filter; the pressure during the pre-coating treatment is -0.075 MPa.

[0040] (3) Filtering:

[0041] 100L of slurry (12% dry matter and 45% protein) was vacuum filtered using a pre-coated filter after the coating in step (2), and the filtrate and filter residue were collected. The vacuum filtration conditions were: a flow rate of 120L / h·m for the slurry. 2 The pressure is -0.075 MPa;

[0042] (4) Recycling by-products:

[0043] The filter residue collected in step (3) was added to water (the volume ratio of filter residue to water was 1:2) and dispersed. The dispersion was centrifuged at 1500 r / min for 5 min. The supernatant was spray-dried (inlet temperature 180℃, outlet temperature 85℃) to obtain compound fertilizer. The centrifuged sediment was reused in the preparation of the pre-coating solution in step (1).

[0044] (5) Protein recovery:

[0045] The filtrate from step (3) was filtered a second time using a plate and frame filter press (80 mesh filter cloth). The pressure during the second filtration was -0.075 MPa. The filtrate was then spray-dried (inlet temperature 180℃, outlet temperature 85℃) to obtain 8.47 kg of protein with a protein content of 58 wt% and a protein yield of 91%. The filter residue from the second filtration was reused in the preparation of the pre-coating solution in step (1).

[0046] Example 2

[0047] A method for extracting proteins from a cross-linked slurry based on pre-coated filtration includes the following steps:

[0048] (1) Preparation of pre-coating agent:

[0049] Activated carbon (60 mesh), diatomaceous earth (60 mesh), and chelating resin (D401 resin and D001×7 resin are compounded in a mass ratio of 3:5) are mixed in a mass ratio of 1:1:0.1 to obtain a pre-coating agent. The pre-coating agent and water (the volume ratio of pre-coating agent and water is 1:3) are mixed and stirred to obtain a pre-coating liquid.

[0050] (2) Pre-coating treatment:

[0051] The above pre-coating liquid is pumped into a pre-coated filter (100 mesh filter cloth) for pre-coating treatment, so that a pre-coating layer with a thickness of 100 mm is deposited on the surface of the filter cloth of the pre-coated filter; the pressure during the pre-coating treatment is -0.1 MPa;

[0052] (3) Filtering:

[0053] 100L of slurry (12% dry matter and 45% protein) was vacuum filtered using a pre-coated filter after the coating in step (2), and the filtrate and filter residue were collected. The vacuum filtration conditions were: a flow rate of 150L / h·m for the slurry. 2 The pressure is -0.1 MPa;

[0054] (4) Recycling by-products:

[0055] The filter residue collected in step (3) was added to water (the volume ratio of filter residue to water was 1:3) and dispersed. The dispersion was centrifuged at 2000 r / min for 6 min. The supernatant was spray-dried (inlet temperature 180℃, outlet temperature 85℃) to obtain compound fertilizer. The centrifuged sediment was reused for the preparation of the pre-coating solution in step (1).

[0056] (5) Protein recovery:

[0057] The filtrate from step (3) was filtered a second time using a plate and frame filter press (80 mesh filter cloth). The pressure during the second filtration was -0.075 MPa. The filtrate was then spray-dried (inlet temperature 180℃, outlet temperature 85℃) to obtain 8.58 kg of protein with a protein content of 57 wt%. The protein yield was 90.6%. The filter residue from the second filtration was reused in the preparation of the pre-coating solution in step (1).

[0058] Example 3

[0059] A method for extracting proteins from a cross-linked slurry based on pre-coated filtration includes the following steps:

[0060] (1) Preparation of pre-coating agent:

[0061] Activated carbon (60 mesh), diatomaceous earth (60 mesh), and chelating resin (D401 resin and D001×7 resin are compounded in a mass ratio of 3:3) are mixed in a mass ratio of 1:1:0.15 to obtain a pre-coating agent. The pre-coating agent and water (the volume ratio of pre-coating agent and water is 1:3) are mixed and stirred to obtain a pre-coating liquid.

[0062] (2) Pre-coating treatment:

[0063] The above pre-coating liquid is pumped into a pre-coated filter (100 mesh) for pre-coating treatment, so that a pre-coating layer with a thickness of 90 mm is deposited on the surface of the filter cloth of the pre-coated filter; the pressure during the pre-coating treatment is -0.8 MPa;

[0064] (3) Filtering:

[0065] 100L of slurry (12% dry matter and 45% protein) was vacuum filtered using a pre-coated filter after the coating in step (2), and the filtrate and filter residue were collected. The vacuum filtration conditions were: a flow rate of 125L / h·m for the slurry. 2 The pressure is -0.085 MPa;

[0066] (4) Recycling by-products:

[0067] The filter residue collected in step (3) was added to water (the volume ratio of filter residue to water was 1:2) and dispersed. The dispersion was centrifuged at 2000 r / min for 5 min. The supernatant was spray-dried (inlet temperature 180℃, outlet temperature 85℃) to obtain compound fertilizer. The centrifuged sediment was reused in the preparation of the pre-coating solution in step (1).

[0068] (5) Protein recovery:

[0069] The filtrate from step (3) was filtered a second time using a plate and frame filter press (80 mesh filter cloth). The pressure during the second filtration was -0.075 MPa. The filtrate was then spray-dried (inlet temperature 180℃, outlet temperature 85℃) to obtain 8.65 kg of protein with a protein content of 59 wt%. The protein yield was 94.5%. The filter residue from the second filtration was reused in the preparation of the pre-coating solution in step (1).

[0070] Example 4

[0071] A method for extracting proteins from a cross-linked slurry based on pre-coated filtration includes the following steps:

[0072] (1) Preparation of pre-coating agent:

[0073] Activated carbon (60 mesh), diatomaceous earth (60 mesh), and chelating resin (D401 resin and D001×7 resin compounded in a mass ratio of 3:2) are mixed in a mass ratio of 1:0.95:0.15 to obtain a pre-coating agent. The pre-coating agent and water (the volume ratio of pre-coating agent to water is 1:3) are mixed and stirred to obtain a pre-coating liquid.

[0074] (2) Pre-coating treatment:

[0075] The above pre-coating liquid is pumped into a pre-coated filter (100 mesh filter cloth) for pre-coating treatment, so that a pre-coating layer with a thickness of 100 mm is deposited on the surface of the filter cloth of the pre-coated filter; the pressure during the pre-coating treatment is -0.085 MPa.

[0076] (3) Filtering:

[0077] 100L of slurry (12% dry matter and 45% protein) was vacuum filtered using a pre-coated filter after the coating in step (2), and the filtrate and filter residue were collected. The vacuum filtration conditions were: a flow rate of 140L / h·m for the slurry. 2 The pressure is -0.09 MPa;

[0078] (4) Recycling by-products:

[0079] The filter residue collected in step (3) was added to water (the volume ratio of filter residue to water was 1:1) and dispersed. The dispersion was centrifuged at 3000 r / min for 8 min. The supernatant was spray-dried (inlet temperature 180℃, outlet temperature 85℃) to obtain compound fertilizer. The centrifuged sediment was reused for the preparation of the pre-coating solution in step (1).

[0080] (5) Protein recovery:

[0081] The filtrate from step (3) was filtered a second time using a plate and frame filter press (80 mesh filter cloth). The pressure during the second filtration was -0.075 MPa. The filtrate was then spray-dried (inlet temperature 180℃, outlet temperature 85℃) to obtain 8.45 kg of protein with a protein content of 58 wt% and a protein yield of 90.8%. The filter residue from the second filtration was reused in the preparation of the pre-coating solution in step (1).

[0082] Example 5

[0083] A method for extracting proteins from a cross-linked slurry based on pre-coated filtration includes the following steps:

[0084] (1) Preparation of pre-coating agent:

[0085] Activated carbon (60 mesh), diatomaceous earth (60 mesh), and chelating resin (D401 resin and D001×7 resin compounded in a mass ratio of 3:1) are mixed in a mass ratio of 1:0.95:0.2 to obtain a pre-coating agent. The pre-coating agent and water (the volume ratio of pre-coating agent to water is 1:5) are mixed and stirred to obtain a pre-coating liquid.

[0086] (2) Pre-coating treatment:

[0087] The above pre-coating liquid is pumped into a pre-coated filter (60 mesh) for pre-coating treatment, so that a pre-coating layer with a thickness of 50 mm is deposited on the surface of the filter cloth of the pre-coated filter; the pressure during the pre-coating treatment is -0.05 MPa;

[0088] (3) Filtering:

[0089] 100L of slurry (12% dry matter and 45% protein) was vacuum filtered using a pre-coated filter after the coating in step (2), and the filtrate and filter residue were collected. The vacuum filtration conditions were: a flow rate of 100L / h·m for the slurry. 2 The pressure is -0.06 MPa;

[0090] (4) Recycling by-products:

[0091] The filter residue collected in step (3) was added to water (the volume ratio of filter residue to water was 1:1.5) and dispersed. The dispersion was centrifuged at 1000 r / min for 5 min. The supernatant was spray-dried (inlet temperature 180℃, outlet temperature 85℃) to obtain compound fertilizer. The centrifuged sediment was reused in the preparation of the pre-coating solution in step (1).

[0092] (5) Protein recovery:

[0093] The filtrate from step (3) was filtered a second time using a plate and frame filter press (80 mesh filter cloth). The pressure during the second filtration was -0.075 MPa. The filtrate was then spray-dried (inlet temperature 180℃, outlet temperature 85℃) to obtain 8.35 kg of protein with a protein content of 59 wt% and a protein yield of 91.2%. The filter residue from the second filtration was reused in the preparation of the pre-coating solution in step (1).

[0094] To better verify the effectiveness of the present invention, the following description takes Example 1 as a reference and combines several comparative examples for further explanation.

[0095] Comparative Example 1

[0096] Compared with Example 1, the difference is that in step (1), only diatomaceous earth was used as the pre-coating agent, and the other conditions were the same as in Example 1, resulting in 8.7 kg of protein with a protein content of 52% and a protein yield of 83.8%.

[0097] Comparative Example 2

[0098] Compared with Example 1, the difference is that in step (1), only activated carbon was used as the pre-coating agent, and the other conditions were the same as in Example 1, resulting in 8.78 kg of protein with a protein content of 50% and a protein yield of 81.3%.

[0099] Comparative Example 3

[0100] Compared with Example 1, the difference is that in step (1), only chelating resin was used as the pre-coating agent, and the other conditions were the same as in Example 1, resulting in 6445g of protein with a protein content of 62% and a protein yield of 74%.

[0101] Comparative Example 4

[0102] Compared with Example 1, the difference is that in step (1), only D001×7 resin was used as the chelating resin, and the other conditions were the same as in Example 1, resulting in 5981g of protein with a protein content of 65% and a protein yield of 72%.

[0103] Comparative Example 5

[0104] Compared with Example 1, the difference is that in step (1), only D401 resin was used as the chelating resin, and the other conditions were the same as in Example 1, resulting in 6750g of protein with a protein content of 60% and a protein yield of 75%.

[0105] Comparative Example 6

[0106] Compared with Example 1, the difference is that in step (3), the filtration pressure is -0.2MPa, and the other conditions are the same as in Example 1, resulting in 9200g of protein with a protein content of 50% and a protein yield of 85.2%.

[0107] As can be seen from the above embodiments and comparative examples, the present invention uses activated carbon, diatomaceous earth and chelating resin as a pre-coating layer, and effectively controls the pre-coating and filtration conditions to obtain a high protein content, with a protein yield of over 90%.

[0108] In Comparative Examples 1-3, only one of diatomaceous earth, activated carbon, and chelating resin was used as a pre-coating agent, resulting in poor filtration effects and impacting both protein purity and yield. In Comparative Examples 4-5, however, the present invention selected a combination of D001×7 and D401 resins for the chelating resin, which demonstrated better performance in protein recovery. This is because the chelating resin composed of D401 and D001×7 resins has a larger surface area and more effective active sites, enhancing its ability to capture different metal ions.

[0109] This document uses specific examples to illustrate the principles and implementation methods of the present invention. The descriptions of these embodiments are merely to aid in understanding the method and core ideas of the present invention, including the best mode, and to enable any person skilled in the art to practice the present invention, including manufacturing and using any device or system, and implementing any combined method. It should be noted that those skilled in the art can make various improvements and modifications to the present invention without departing from its principles, and these improvements and modifications also fall within the scope of protection of the claims. The scope of protection of this patent is defined by the claims and may include other embodiments that can be conceived by those skilled in the art. If these other embodiments have structural elements similar to those expressed in the claims, or if they include equivalent structural elements that are not substantially different from those expressed in the claims, then these other embodiments should also be included within the scope of the claims.

Claims

1. A method for extracting proteins from a slurry based on pre-coated filtration, characterized in that, Includes the following steps: (1) Preparation of pre-coating solution: Activated carbon, diatomaceous earth, chelating resin and water are mixed and stirred to prepare pre-coating solution; the mass ratio of activated carbon, diatomaceous earth and chelating resin is 1:(0.9-1.2):(0.1-0.2); the chelating resin is a mixture of D401 resin and D001×7 resin, and the mass ratio of D401 resin and D001×7 resin is 3:(1-5); (2) Pre-coating treatment: The above pre-coating liquid is pumped into the pre-coating filter for pre-coating treatment; (3) Filtration: Vacuum filter the slurry after the coating in step (2) using a pre-coated filter, and collect the filtrate and filter residue; (4) Recovering by-products: The filter residue collected in step (3) is added to water and dispersed. The dispersion is centrifuged and the supernatant is spray-dried to obtain compound fertilizer. (5) Protein recovery: The filtrate from step (3) is filtered a second time and spray-dried to obtain protein.

2. The method for extracting proteins from ionized slurry based on pre-coated filtration according to claim 1, characterized in that, In step (1), the activated carbon and diatomaceous earth are both 60-150 mesh.

3. The method for extracting proteins from ionized slurry based on pre-coated filtration according to claim 1, characterized in that, In step (1), the total volume ratio of activated carbon, diatomaceous earth and chelating resin to water is 1:(1-5).

4. The method for extracting proteins from ionized slurry based on pre-coated filtration according to claim 1, characterized in that, In step (2), during the pre-coating process, the filter cloth of the pre-coated filter has a mesh size of 60-100 mesh, and the pressure during pre-coating is -0.04 to -0.1 MPa; and / or after the pre-coating process, the thickness of the pre-coating layer is 50-100 mm.

5. The method for extracting proteins from ionized slurry based on pre-coated filtration according to claim 1, characterized in that, In step (3), the vacuum filtration conditions are: the flow rate of the slurry being discharged is 100-150 L / h·m. 2 The pressure is -0.04 to -0.1 MPa.

6. The method for extracting proteins from ionized slurry based on pre-coated filtration according to claim 1, characterized in that, In step (4), the volume ratio of filter residue to water during dispersion is 1:(1-3); and / or the centrifugation speed is 1000-3000 r / min, and the centrifugation time is 3-8 min.

7. The method for extracting proteins from ionized slurry based on pre-coated filtration according to claim 1, characterized in that, In step (5), a plate and frame filter is used for secondary filtration. The filter cloth of the plate and frame filter is 60-100 mesh, and the pressure during secondary filtration is -0.04 to -0.1 MPa.

8. The method for extracting proteins from ionized slurry based on pre-coated filtration according to claim 1, characterized in that, In steps (4) and (5), the precipitate after centrifugation and the filter residue after secondary filtration are used as pre-coating agents in the preparation of the pre-coating liquid in step (1).