A method for separating ultrafine molecular sieve from alkaline ultrafine molecular sieve synthesis mother liquor

Abstract

The present invention provides a method for separating ultrafine molecular sieves from an alkaline ultrafine molecular sieve synthesis mother liquor, and relates to the field of chemical separation technology. The present invention uses an aqueous solution of ammonium chloride, urea, acrylonitrile and polyacrylamide as a first flocculant to perform a first flocculation treatment on the alkaline ultrafine molecular sieve synthesis mother liquor, which can eliminate the repulsive force between the molecular sieve grains, so that the grains interact with each other to form flocculated microclusters; the present invention uses an aqueous solution of polyacrylamide as a second flocculant to perform a second flocculation treatment on the mother liquor, which can cause the microclusters to quickly agglomerate and settle, and form a cross-linked structure between the clusters to prevent the clusters from being suspended again under stirring, shaking, etc. The method provided by the present invention not only avoids the step of adding acid and alkali to adjust the pH value, but also can directly obtain a sedimentation product after static sedimentation after the secondary flocculation, and the sedimentation product can directly achieve the separation of the aqueous phase and the solid phase without filtering, which greatly improves the separation efficiency of the ultrafine molecular sieve.

Description

Technical Field

[0001] The invention relates to the technical field of chemical separation, in particular to a method for separating ultrafine molecular sieves from alkaline ultrafine molecular sieve synthesis mother liquor. Background Art

[0002] Microporous zeolite molecular sieve is an inorganic silicon-containing crystal with a skeleton that has regular and orderly arranged pores close to the molecular size. It is widely used in catalysis, ion exchange, separation and other fields. For example, ZSM-5 zeolite has a pore system composed of 10-membered rings and has three-dimensional pores. It is widely used in reactions such as alkylation, isomerization, disproportionation, selective cracking and the synthesis of gasoline from methanol.

[0003] Most of the active sites of molecular sieves are located in the micropore structure (pore size is only about 0.55nm), and large molecules are difficult to diffuse and react in it, which limits its excellent catalytic performance. Therefore, people are prompted to develop and prepare ultrafine molecular sieves with smaller particles to shorten the transfer distance of reactant molecules in the pores. In addition, the smaller the molecular sieve grains, the more active centers required for the catalytic reaction exposed on the surface of the molecular sieve, and the higher the catalyst activity. Therefore, for most reactions, the smaller the molecular sieve grains, the higher the catalytic reaction efficiency. However, the smaller the molecular sieve particles, the higher their total surface energy. The more intense the solization of the molecular sieve particles in the molecular sieve alkaline suspension (i.e., the molecular sieve synthesis mother liquor) after crystallization, the more difficult it is to separate from the molecular sieve alkaline suspension after crystallization.

[0004] Molecular sieve crystals are separated from mother liquor by filtration, centrifugation or membrane separation. However, filtration is limited by the pore size of the filter cloth and is not suitable for the separation of ultrafine molecular sieve crystals (particle size below 200nm); centrifugal separation is only suitable for the preparation of small batches of molecular sieves in the laboratory; membrane separation requires a large amount of pure water, and the membrane pores are easily clogged, so it is only suitable for separating molecular sieve crystals with a particle size greater than 200nm.

[0005] Patent 201510216500.3 provides a method for separating ultrafine titanium silicon molecular sieve from an alkaline suspension of molecular sieve, comprising the following steps: (1) adding a dilute acidic solution to the alkaline suspension of molecular sieve after crystallization to adjust the pH value of the molecular sieve suspension to weak acidity; (2) then adding acidic silica sol and polyacrylamide solution in sequence, (3) then adding a dilute alkaline solution to adjust the pH value to weak alkalinity, (4) then adding a dilute acidic solution to reversely adjust the pH value to neutrality, static precipitation, filtration, separation and recovery to obtain ultrafine titanium silicon molecular sieve. Although this method achieves the separation of ultrafine molecular sieves, it requires the alternating addition of acid and alkali to adjust the pH value of the solution, and the steps are cumbersome. Patent CN108793183A discloses a method for separating titanium silicon molecular sieve mother liquor, comprising the following steps: (1) adding a water-soluble flocculant to the titanium silicon molecular sieve mother liquor after crystallization, specifically one or more of chitosan cross-linked cationic polymer flocculant, polydimethyldiallyl ammonium chloride, dimethyldiallyl ammonium chloride-acrylamide copolymer, sodium polystyrene sulfonate, etc.; (2) adding an organic acid or inorganic acid solution to adjust the pH value of the molecular sieve mother liquor to 3-9; (3) standing and settling for 0.5-24 hours, filtering, drying and roasting, and separating the titanium silicon molecular sieve raw powder. This method still requires the addition of an organic acid or an inorganic acid to adjust the pH value of the mother liquor, and the resulting precipitate still needs to be filtered and separated. Summary of the invention

[0006] In view of this, the object of the present invention is to provide a method for separating ultrafine molecular sieve from alkaline ultrafine molecular sieve synthesis mother liquor. The separation method provided by the present invention is simple in process, does not need to add acid and alkali reagents to adjust the pH value, and can directly obtain the sedimentation product after secondary flocculation without filtering. The present invention greatly improves the separation efficiency of the ultrafine molecular sieve.

[0007] In order to achieve the above-mentioned object of the invention, the present invention provides the following technical solutions:

[0008] The present invention provides a method for separating ultrafine molecular sieve from alkaline ultrafine molecular sieve synthesis mother liquor, comprising the following steps:

[0009] The alkaline ultrafine molecular sieve synthesis mother liquor after crystallization is mixed with a first flocculant to perform a first flocculation treatment to obtain a primary flocculation mother liquor; the first flocculant is an aqueous solution of ammonium chloride, urea, acrylonitrile and polyacrylamide;

[0010] The first flocculation mother liquid is mixed with a second flocculant for a second flocculation treatment to obtain a second flocculation mother liquid; the second flocculant is an aqueous solution of polyacrylamide;

[0011] The secondary flocculation mother liquor is allowed to settle, the supernatant is extracted, and the obtained precipitate is an ultrafine molecular sieve.

[0012] Preferably, the particle size of the ultrafine molecular sieve in the alkaline ultrafine molecular sieve synthesis mother liquor is 50-200 nm; the types of the ultrafine molecular sieve include ZSM-5 molecular sieve, BETA molecular sieve, SAPO-34 molecular sieve, Silicalite-1 molecular sieve or mordenite molecular sieve.

[0013] Preferably, the polyacrylamide in the first flocculant is anionic polyacrylamide, and the molecular weight of the anionic polyacrylamide is 6 to 10 million.

[0014] Preferably, the mass contents of ammonium chloride, urea, acrylonitrile and polyacrylamide in the first flocculant are 0.1-1.0%, 0.2-1.5%, 0.5-2.0% and 0.3-2.0% respectively; the volume of the first flocculant is 0.5-5% of the volume of the alkaline ultrafine molecular sieve synthesis mother liquor.

[0015] Preferably, the first flocculation treatment lasts for 3 to 5 minutes, and the first flocculation treatment is performed under stirring conditions.

[0016] Preferably, the polyacrylamide in the second flocculant is anionic polyacrylamide, and the molecular weight of the anionic polyacrylamide is 16 million to 25 million.

[0017] Preferably, the mass content of polyacrylamide in the second flocculant is 0.2-1%; the volume of the second flocculant is 0.5-8% of the volume of the primary flocculation mother liquid.

[0018] Preferably, the second flocculation treatment lasts for 2 to 8 minutes, and the second flocculation treatment is performed under stirring conditions.

[0019] Preferably, the static sedimentation time is 5 to 10 minutes.

[0020] Preferably, after extracting the supernatant, the obtained precipitate is sequentially washed with water, filtered, dried and roasted.

[0021] The present invention provides a method for separating an ultrafine molecular sieve from an alkaline ultrafine molecular sieve synthesis mother liquor, comprising the following steps: mixing the alkaline ultrafine molecular sieve synthesis mother liquor after crystallization with a first flocculant for a first flocculation treatment to obtain a primary flocculation mother liquor; the first flocculant is an aqueous solution of ammonium chloride, urea, acrylonitrile and polyacrylamide; mixing the primary flocculation mother liquor with a second flocculant for a second flocculation treatment to obtain a secondary flocculation mother liquor; the second flocculant is an aqueous solution of polyacrylamide; allowing the secondary flocculation mother liquor to settle, extracting the supernatant, and obtaining the precipitate as an ultrafine molecular sieve. The present invention uses an aqueous solution of ammonium chloride, urea, acrylonitrile and polyacrylamide as the first flocculant to perform a first flocculation treatment on the alkaline ultrafine molecular sieve synthesis mother liquor, which can eliminate the repulsive force between the molecular sieve grains, so that the grains interact with each other to generate flocculated microclusters, and form primary flocculation; the present invention uses an aqueous solution of polyacrylamide as the second flocculant to perform a second flocculation treatment on the mother liquor, which can cause the microclusters to quickly agglomerate and settle, and form a cross-linked structure between the clusters to prevent the clusters from being suspended again under stirring, shaking, etc. The method provided by the present invention not only avoids the step of adding acid and alkali to adjust the pH value, but also can directly obtain a sedimentation product after static sedimentation after the secondary flocculation, and the sedimentation product can directly achieve the separation of the aqueous phase and the solid phase without filtering. The method provided by the present invention greatly improves the efficiency of separating and recovering ultrafine molecular sieves from the alkaline suspension of the molecular sieve after crystallization. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 This is a physical picture of the ZSM-5 molecular sieve mother liquor in Example 1;

[0023] Figure 2 This is a physical picture of the mother liquor system obtained after the first flocculant is added to the ZSM-5 molecular sieve mother liquor in Example 1;

[0024] Figure 3 This is a physical picture of the mother liquor system obtained after adding the second flocculant in Example 1;

[0025] Figure 4 This is the SEM image of the 200 nm ultrafine ZSM-5 molecular sieve raw powder separated in Example 1. DETAILED DESCRIPTION

[0026] The present invention provides a method for separating ultrafine molecular sieve from alkaline ultrafine molecular sieve synthesis mother liquor, comprising the following steps:

[0027] The alkaline ultrafine molecular sieve synthesis mother liquor after crystallization is mixed with a first flocculant to perform a first flocculation treatment to obtain a primary flocculation mother liquor; the first flocculant is an aqueous solution of ammonium chloride, urea, acrylonitrile and polyacrylamide;

[0028] The first flocculation mother liquid is mixed with a second flocculant for a second flocculation treatment to obtain a second flocculation mother liquid; the second flocculant is an aqueous solution of polyacrylamide;

[0029] The secondary flocculation mother liquor is allowed to settle, the supernatant is extracted, and the obtained precipitate is an ultrafine molecular sieve.

[0030] In the present invention, unless otherwise specified, the raw materials involved are commercially available products well known to those skilled in the art.

[0031] The present invention mixes the alkaline ultrafine molecular sieve synthesis mother liquor after crystallization with the first flocculant to perform the first flocculation treatment to obtain a primary flocculation mother liquor. The present invention has no special requirements on the source of the alkaline ultrafine molecular sieve synthesis mother liquor (also called molecular sieve alkaline suspension), and the alkaline ultrafine molecular sieve synthesis mother liquor obtained after crystallization by methods familiar to those skilled in the art is suitable for the present invention; the pH value of the alkaline ultrafine molecular sieve synthesis mother liquor is generally 10 to 13, which is a strongly alkaline mother liquor. In the present invention, the particle size of the ultrafine molecular sieve in the alkaline ultrafine molecular sieve synthesis mother liquor is preferably 50 to 200 nm, more preferably 100 to 200 nm; the type of the ultrafine molecular sieve preferably includes ZSM-5 molecular sieve, BETA molecular sieve, SAPO-34 molecular sieve, Silicalite-1 molecular sieve or mordenite molecular sieve.

[0032] In the present invention, the first flocculant is an aqueous solution of ammonium chloride, urea, acrylonitrile and polyacrylamide; the polyacrylamide in the first flocculant is preferably anionic polyacrylamide, and the molecular weight of the anionic polyacrylamide is preferably 6 to 10 million, more preferably 6 to 8 million; the first flocculant is specifically obtained by dissolving ammonium chloride, urea, acrylonitrile and polyacrylamide in deionized water. In the present invention, the mass contents of ammonium chloride, urea, acrylonitrile and polyacrylamide in the first flocculant are preferably 0.1 to 1.0%, 0.2 to 1.5%, 0.5 to 2.0% and 0.3 to 2.0%, respectively, and more preferably 0.15 to 0.5%, 0.3 to 0.5%, 0.9 to 1.5% or 0.4 to 1.0%, respectively; the volume of the first flocculant is preferably 0.5 to 5% of the volume of the alkaline ultrafine molecular sieve synthesis mother liquor, more preferably 2.5 to 4%. The present invention preferably drops the first flocculant into the alkaline ultrafine molecular sieve synthesis mother liquor, and the present invention has no special requirements for the rate of the drop addition. In the present invention, the time of the first flocculation treatment is preferably 3 to 5 minutes, and the first flocculation treatment is preferably carried out under stirring conditions; after the treatment of the first flocculant, ultrafine molecular sieve flocculation microclusters are formed, and after 3 to 5 minutes of flocculation treatment, the mother liquor appears to be in a coagulated state, that is, the primary flocculation mother liquor.

[0033] After obtaining the primary flocculation mother liquor, the present invention mixes the primary flocculation mother liquor with the second flocculant to perform a second flocculation treatment to obtain a secondary flocculation mother liquor. In the present invention, the second flocculant is an aqueous solution of polyacrylamide, and the polyacrylamide in the second flocculant is preferably anionic polyacrylamide, and the molecular weight of the anionic polyacrylamide is preferably 16 to 25 million, and more preferably 18 to 20 million; the second flocculant is specifically obtained by dissolving polyacrylamide in deionized water. In the present invention, the mass content of polyacrylamide in the second flocculant is preferably 0.2 to 1%, and more preferably 0.4 to 0.5%; the volume of the second flocculant is preferably 0.5 to 8% of the volume of the primary flocculation mother liquor, and more preferably 0.5 to 3%. The present invention preferably drops the second flocculant into the primary flocculation mother liquor, and the present invention has no special requirements for the rate of the dropping. In the present invention, the time of the second flocculation treatment is preferably 2 to 8 minutes, more preferably 2 to 5 minutes, and the second flocculation treatment is preferably carried out under stirring conditions; after treatment with the second flocculant, the microclusters quickly agglomerate and settle, and a cross-linked structure is formed between the clusters. After 2 to 8 minutes of flocculation treatment, large particle aggregates appear in the mother liquor, which is the secondary flocculation mother liquor.

[0034] After obtaining the secondary flocculation mother liquor, the present invention allows the secondary flocculation mother liquor to settle and extract the supernatant, and the obtained precipitate is an ultrafine molecular sieve. In the present invention, the settling time is preferably 5 to 10 minutes, more preferably 8 to 10 minutes; the extraction of the supernatant is specifically to pour out the supernatant. The present invention can directly obtain a sedimentation product by settling after the secondary flocculation, and the sedimentation product can directly separate the water phase and the solid phase without filtering.

[0035] After extracting the supernatant, the present invention also preferably sequentially washes, filters, dries and roasts the obtained precipitate to obtain the ultrafine molecular sieve raw powder. In the present invention, the specific operation of the washing is: (a) adding deionized water to the precipitate, stirring and then settling, and pouring off the supernatant to obtain the washed sedimentation product; (b) repeating the operation (a) on the washed sedimentation product until the pH value of the supernatant is less than 8. In the present invention, the stirring time in (a) is preferably 2 minutes, and the sedimentation time is preferably 10 minutes. In the process of washing the precipitate with water, the present invention does not need to add additional flocculants, and sedimentation can still be well achieved. The present invention has no special requirements for the conditions of the drying and roasting, and the drying and roasting conditions familiar to those skilled in the art can be used.

[0036] The separation method provided by the present invention has a simple process and low cost, and does not require the addition of acid or alkali reagents to adjust the pH value. After secondary flocculation, a sedimentation product can be directly obtained without filtering, thereby greatly improving the separation efficiency of the ultrafine molecular sieve.

[0037] The method for separating the ultrafine molecule sieve from the alkaline ultrafine molecule sieve synthesis mother liquor provided by the present invention is described in detail below in conjunction with the embodiments, but they should not be construed as limiting the scope of protection of the present invention.

[0038] Comparative Example 1 (Synthesis and membrane separation of ultrafine ZSM-5 molecular sieve)

[0039] The silicon source, the aluminum source and the water are mixed evenly to obtain a colloidal mixture, the molar ratio of each substance in the mixture is SiO2:Al2O3:H2O=60:1:200, and the pH value is 12; the above mixture and the template agent tetrapropylammonium hydroxide are mixed, 5% by mass (relative to the total mass of the mixture and the template agent) of seed crystals are added, and the mixture is placed in a crystallization kettle and crystallized at 170°C for 50 hours to obtain an ultrafine grain ZSM-5 molecular sieve mother liquor with an average grain size of 200nm.

[0040] The ultrafine-grained ZSM-5 molecular sieve mother liquor is separated by a ceramic membrane to obtain an ultrafine-grained ZSM-5 molecular sieve. The above mother liquor separation needs to be washed with water 5 to 8 times (the amount of water is 20 to 30 times the volume of the mother liquor) to obtain a molecular sieve slurry with a pH value of 7 to 8, a solid content of less than 10%, and cannot be filtered, and drying will waste a lot of heat.

[0041] Example 1

[0042] According to the synthesis method of Comparative Example 1, 2 L of ZSM-5 molecular sieve mother liquor was obtained, the average crystal size was 200 nm, and the pH value was 12. Figure 1 This is a physical picture of ZSM-5 molecular sieve mother liquor.

[0043] 2g ammonium chloride, 6g urea, 10g acrylonitrile, 10g polyacrylamide (molecular weight 6-8 million, anionic type) are added to 1L deionized water and fully dissolved to obtain the first flocculant; 5g polyacrylamide (molecular weight 18-20 million, anionic type) is dissolved in 1L deionized water to obtain the second flocculant. 50mL of the first flocculant solution is added dropwise to the ZSM-5 molecular sieve mother liquor, stirred for 3-5min until the mother liquor begins to agglomerate; then 20mL of the second flocculant is added, stirred for 2min, and the stirring is stopped after large particle agglomerates appear; after precipitation for 10min, the supernatant is poured out, and 1L deionized water is poured into the precipitate, stirred for 2min, and settled for 10min, and this is repeated 4 times. After the pH value of the solution is less than 8, the precipitate is filtered, dried and roasted to obtain the ultrafine grain ZSM-5 molecular sieve raw powder.

[0044] Compared with membrane separation, the amount of washing water is 4 to 8 times the volume of the mother liquor, which saves a lot of water. At the same time, the solid content of the filter cake of the filter press is as high as 60 to 75%, which saves a lot of energy in the drying process.

[0045] Figure 2 This is a physical picture of the mother liquor system obtained after adding the first flocculant to the ZSM-5 molecular sieve mother liquor. Figure 3 This is a physical picture of the mother liquor system obtained after adding the second flocculant. Figure 2 and Figure 3 It can be seen that after adding the first flocculant, the molecular sieve mother liquor has shown obvious sedimentation. After further adding the second flocculant, larger flocculants appear in the molecular sieve mother liquor, and the molecular sieve slurry shows further sedimentation.

[0046] Figure 4 This is a SEM image of the 200nm ultrafine ZSM-5 molecular sieve powder separated in Example 1. Figure 4 It can be seen that the separated molecular sieve has a uniform particle size distribution of about 200 nm, and the addition of flocculant does not cause the agglomeration of the molecular sieve grains, indicating that the above separation method has no effect on the morphology of the molecular sieve.

[0047] Example 2

[0048] The silicon source, aluminum source, water and sodium hydroxide are uniformly mixed to obtain a colloidal mixture, wherein the molar ratio of each substance in the mixture is SiO2:Al2O3:H2O=20:1:50, and the pH value is 13; the above mixture is mixed with the template agent tetraethylammonium hydroxide, 8% by weight (relative to the total weight of the mixture and the template agent) of seed crystals are added, and the mixture is placed in a crystallization kettle at 150°C for crystallization for 40 hours to obtain an ultrafine BETA molecular sieve mother liquor with an average grain size of 200nm. Take 2L of the BETA molecular sieve mother liquor.

[0049] 2g ammonium chloride, 6g urea, 10g acrylonitrile, 5g polyacrylamide (molecular weight 6-7.5 million, anionic type) are added to 1L deionized water and fully dissolved to obtain the first flocculant; 5g polyacrylamide (molecular weight 16-18 million, anionic type) is dissolved in 1L deionized water to obtain the second flocculant. 80mL of the first flocculant is added dropwise to the BETA molecular sieve mother liquor, stirred for 3-5min until the mother liquor begins to agglomerate; then 30mL of the second flocculant is added, stirred for 2min, and the stirring is stopped after large particle agglomerates appear; after precipitation for 10min, the supernatant is poured out, and 1L deionized water is poured into the precipitate, stirred for 2min, and settled for 10min. This is repeated 5 times. After the pH value of the solution is less than 8, the precipitate is filtered, dried and roasted to obtain the ultrafine BETA molecular sieve raw powder.

[0050] Compared with membrane separation, the amount of washing water is 5 to 8 times the mass of the mother liquor, which saves a lot of water. At the same time, the solid content of the filter cake of the filter press is as high as 60 to 75%, which saves a lot of energy in the drying process.

[0051] Example 3

[0052] Ethyl orthosilicate, water and sodium hydroxide are uniformly mixed to obtain a colloidal mixture, wherein the molar ratio of each substance in the mixture is SiO2:H2O=20:50, and the pH value is 11; the above mixture is mixed with the template agent tetrapropylammonium hydroxide, 8% by mass (relative to the total mass of the mixture and the template agent) of seed crystals are added, and the mixture is placed in a crystallization kettle at 170°C for crystallization for 40 hours to obtain an ultrafine crystalline Silicalite-1 molecular sieve mother liquor, and the average crystal particle size is 100nm. Take 2L of the ultrafine crystalline Silicalite-1 molecular sieve mother liquor.

[0053] 5g of ammonium chloride, 4g of urea, 12g of acrylonitrile, and 5g of polyacrylamide (molecular weight 6-8 million, anionic type) were added to 1L of deionized water and fully dissolved to obtain the first flocculant; 5g of polyacrylamide (molecular weight 18-20 million, anionic type) was dissolved in 1L of deionized water to obtain the second flocculant. 50mL of the first flocculant was added dropwise to the Silicalite-1 molecular sieve mother liquor, and stirred for 5min until the mother liquor began to agglomerate; then 60mL of the second flocculant was added, stirred for 3-5min, and the stirring was stopped after large particle agglomerates appeared; after precipitation for 10min, the supernatant was poured out, and 1L of deionized water was poured into the precipitate, stirred for 2min, and settled for 10min, and this was repeated 3 times. After the pH value of the solution was less than 8, the precipitate was filtered, dried, and roasted to obtain the ultrafine Silicalite-1 molecular sieve raw powder.

[0054] Compared with membrane separation, the amount of washing water is 5 to 8 times the mass of the mother liquor, which saves a lot of water. At the same time, the solid content of the filter cake of the filter press is as high as 60 to 75%, which saves a lot of energy in the drying process.

[0055] It can be seen from the above embodiments that the method provided by the present invention not only avoids the step of adding acid and alkali to adjust the pH value, but also can directly obtain a sedimentation product after static sedimentation after the secondary flocculation. The sedimentation product can directly separate the water phase and the solid phase without filtering, which not only ensures the separation effect of the ultrafine molecular sieve, but also greatly improves the separation efficiency of the ultrafine molecular sieve, saving a lot of water resources and the energy required for drying.

[0056] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principle of the present invention. These improvements and modifications should also be regarded as the scope of protection of the present invention.

Claims

1. A method for separating ultrafine molecular sieve from alkaline ultrafine molecular sieve synthesis mother liquor, characterized in that: The following steps are involved: The alkaline superfine molecular sieve synthesis mother liquor after crystallization is mixed with the first flocculant for the first flocculation treatment to obtain a primary flocculation mother liquor; the first flocculant is an aqueous solution of ammonium chloride, urea, acrylonitrile and polyacrylamide; the pH value of the alkaline superfine molecular sieve synthesis mother liquor is 10-13; the particle size of the superfine molecular sieve in the alkaline superfine molecular sieve synthesis mother liquor is 50-200nm; the type of the superfine molecular sieve includes ZSM-5 molecular sieve, BETA molecular sieve or Silicalite-1 molecular sieve; the mass contents of ammonium chloride, urea, acrylonitrile and polyacrylamide in the first flocculant are 0.1-1.0%, 0.2-1.5%, 0.5-2.0% and 0.3-2.0% respectively; the polyacrylamide in the first flocculant is anionic polyacrylamide, and the molecular weight of the anionic polyacrylamide is 6 million to 10 million; the volume of the first flocculant is 0.5-5% of the volume of the alkaline superfine molecular sieve synthesis mother liquor; The primary flocculation mother liquor is mixed with a second flocculant for a second flocculation treatment to obtain a secondary flocculation mother liquor; the second flocculant is an aqueous solution of polyacrylamide; the polyacrylamide in the second flocculant is an anionic polyacrylamide, and the molecular weight of the anionic polyacrylamide is 16 million to 25 million; The secondary flocculation mother liquor is allowed to settle, the supernatant is extracted, and the obtained precipitate is an ultrafine molecular sieve.

2. The method according to claim 1, characterized in that The time of the first flocculation treatment is 3 to 5 minutes, and the first flocculation treatment is performed under stirring conditions.

3. The method according to claim 1, characterized in that The mass content of polyacrylamide in the second flocculant is 0.2-1%; the volume of the second flocculant is 0.5-8% of the volume of the primary flocculation mother liquid.

4. The method according to claim 1, characterized in that The second flocculation treatment lasts for 2 to 8 minutes, and the second flocculation treatment is performed under stirring conditions.

5. The method according to claim 1, characterized in that The static sedimentation time is 5 to 10 minutes.

6. The method according to claim 1, characterized in that After the supernatant is extracted, the obtained precipitate is sequentially washed, filtered, dried and roasted.

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