Superfine whitening kaolin and its preparation method and application

Abstract

The application relates to the field of kaolin preparation, and discloses superfined and whitened high-viscosity-concentration kaolin as well as a preparation method and application thereof, the preparation method comprises the following steps: preparing a pulp of raw kaolin, dispersing, removing sand and impurities, removing iron by magnetic separation, bleaching treatment, calcining treatment to obtain pretreated kaolin; preparing a pulp of the pretreated kaolin; performing spray drying and sintering on the kaolin suspension slurry; uniformly beating and stirring kaolin microsphere particles, adding sodium hexametaphosphate, adjusting the pH value of the system to 9-10, heating to 70-85 DEG C, dropwise adding sodium metaaluminate solution and dilute sulfuric acid solution in parallel flow, maintaining the pH value of the system, aging after stirring reaction, and obtaining alumina-coated kaolin; placing the alumina-coated kaolin in a tetraethyl orthosilicate impregnation liquid, uniformly impregnating, drying and calcining to obtain the product, and the product is added to a ceramic coating to give the coating high hardness, high bonding strength, excellent wear resistance, corrosion resistance and fire resistance.

Description

Technical Field

[0001] This invention belongs to the field of kaolin preparation technology, specifically relating to an ultrafine whitened kaolin, its preparation method, and its application. Background Technology

[0002] Kaolin has a relatively soft and fine physical property, appearing as a white powder. However, powdered kaolin is a product of purification; kaolin mined in actual production often has low purity and contains natural impurities, appearing gray or brown. It not only lacks the whiteness of pure kaolin in appearance but also exhibits poor chemical properties, affecting its performance and application. The excellent refractoriness of pure kaolin determines its excellent chemical properties; therefore, kaolin used in chemical production processes often requires purification and whitening treatment.

[0003] Thermal spraying of ceramic materials is a novel surface treatment and strengthening technology with wide applicability. It possesses superior properties unmatched by many metallic materials, leading to its rapid development and increasingly important role in numerous fields. Plasma spraying, as a type of thermal spraying, has become a major surface technology for preparing ceramic coatings due to its advantages of low cost, high deposition efficiency, ease of large-area preparation, and strong adaptability. However, kaolin, as a crucial raw material for ceramic materials, faces limitations in its application. The limited interfacial bonding and intrinsic brittleness of kaolin used in plasma spraying of ceramic coatings restrict significant improvements in corrosion resistance, mechanical properties, bonding strength, and other properties, thus hindering the large-scale industrial application of plasma-sprayed ceramic coatings. Summary of the Invention

[0004] To address the shortcomings mentioned in the background art, the present invention aims to provide an ultrafine whitening kaolin, its preparation method, and its application, which results in a ceramic coating with high hardness, good bonding strength, wear resistance, corrosion resistance, and fire resistance.

[0005] The objective of this invention can be achieved through the following technical solutions:

[0006] A method for preparing ultrafine whitening kaolin includes the following steps:

[0007] S1. The raw kaolin ore is pulped and dispersed, sand and impurities are removed, iron is removed by magnetic separation, bleaching is performed, and calcination is performed to prepare pretreated kaolin.

[0008] S2. Take the pretreated kaolin and prepare kaolin slurry in deionized water. Add sodium hexametaphosphate to the kaolin slurry, ultrasonically vibrate until uniform, then let stand and filter to prepare kaolin suspension slurry.

[0009] S3. The kaolin suspension slurry is spray-dried and sintered to prepare kaolin microspheres.

[0010] S4. Take kaolin microspheres in deionized water, slurry and stir evenly, add sodium hexametaphosphate, adjust the pH of the system to 9-10 with sodium hydroxide, heat to 70-85℃, and add sodium aluminate solution and dilute sulfuric acid solution dropwise to maintain the pH of the system. Stir the reaction for 0.5-1h, and after the reaction is completed, age for 24-28h. After filtration, washing and drying, alumina-coated kaolin is prepared.

[0011] S5. Prepare an impregnation solution of tetraethyl orthosilicate using ethanol as solvent, with a mass fraction of 5% based on silicon dioxide. Place the alumina-coated kaolin in the impregnation solution, and after uniform impregnation, dry it at 100~120℃ for 20~24h. Then, calcine it at 550~600℃ for 2~4h to obtain ultrafine whitened kaolin.

[0012] Preferably, the method for preparing pretreated kaolin in step S1 specifically includes the following steps:

[0013] (1) After the raw kaolin ore is coarsely crushed by a grinding mill, it is transported to a slurry machine. Deionized water and sodium hexametaphosphate are added to the slurry machine to make a slurry with a concentration of 20-40%.

[0014] (2) Remove coarse impurities from the initial slurry by desanding to obtain the slurry;

[0015] (3) The slurry is transported to a high gradient magnetic separator for iron removal to obtain iron-removed slurry;

[0016] (4) Add sulfuric acid with a concentration of 2~4 mol / L to the iron removal slurry to adjust the pH value of the iron removal slurry to 2~3. After adjustment, add 0.1~0.2 wt% sodium hydrosulfite and stir evenly. Then add 0.1~0.15 wt% oxalic acid to complex it. After filtration and washing, bleaching slurry is obtained.

[0017] (5) After the bleached slurry is filtered by a filter press, it is dried and ground. The resulting ground powder is mixed evenly with sodium chloride and calcined at 1000~1200℃ for 1~1.5h to prepare pretreated kaolin.

[0018] Preferably, the pretreated kaolin has a particle size of 2~5µm.

[0019] Preferably, the solid content of the kaolin slurry is 25-30%; the mass of the sodium hexametaphosphate is 0.1-0.3% of the mass of the pretreated kaolin.

[0020] Preferably, the process parameters for spray drying are: spray pressure of 1.2 MPa and inlet temperature of 140°C.

[0021] Preferably, the sintering process parameters are as follows: first, heat to 850~900℃, cool and then heat to 1100~1200℃ and hold for 1~1.5h, cool again and then heat to 1100~1200℃ and hold for 3.5~4h.

[0022] Preferably, the concentration of the kaolin microspheres is 175~200 g / L; the mass of the sodium hexametaphosphate is 0.1~0.3% of the mass of the kaolin microspheres.

[0023] Preferably, the concentration of the sodium aluminate solution is 1~2 mol / L; the concentration of the dilute sulfuric acid solution is 0.5~1 mol / L; and the coating amount in the alumina-coated kaolin is 3~4%.

[0024] An ultrafine whitening kaolin is prepared by the preparation method described above.

[0025] An application of the ultrafine whitening kaolin as described in claim 1, wherein the ultrafine whitening kaolin is applied to a spray-formed ceramic coating.

[0026] The beneficial effects of this invention are:

[0027] This invention purifies kaolin ore through pulping and dispersion, sand and impurity removal, magnetic separation for iron removal, bleaching, and calcination. The resulting pretreated kaolin exhibits improved whiteness and overall performance, making it highly valuable. The invention further involves spray-drying the pretreated kaolin to prepare micron-sized ceramic microspheres, and then using a secondary sintering method to prepare kaolin microsphere particles with excellent strength, hardness, and dispersibility. Using these microsphere particles as the core and sodium aluminate as the coating agent, a core-shell structure of alumina-coated kaolin is prepared. Subsequently, after modification with tetraethyl orthosilicate impregnation solution and calcination, ultrafine whitened kaolin is obtained. The alumina, acting as an intermediate layer, exhibits good chemical compatibility with both the kaolin microsphere particles and the silica generated by the subsequent tetraethyl orthosilicate, reducing interfacial defects and enhancing the bonding strength between particles within the coating. Simultaneously, the silanol groups formed by the tetraethyl orthosilicate modification replace those on the alumina surface. Aluminum hydroxyl groups form Al-O-Si bonds, improving interfacial bonding strength and mechanical properties. Furthermore, alumina coating enhances the flowability and dispersibility of spherical kaolin microspheres, resulting in more uniform particle distribution and higher coating density during spray molding. Additionally, the nano-scale silica coating formed on the alumina coating surface after the hydrolysis of tetraethyl orthosilicate further densifies the coating structure, effectively preventing the penetration of corrosive media. Moreover, the ultrafine whitened kaolin prepared in this invention, due to its spherical shape, can overcome the anisotropy of irregularly shaped reinforcing materials such as needle-like, fibrous, spindle-shaped, and sheet-like materials when used as a coating reinforcement material, reducing stress concentration at sharp corners and improving matrix strength and plasticity. The ultrafine whitened kaolin prepared in this invention can be applied to spray-molded ceramic coatings, imparting excellent wear resistance, bonding strength, refractoriness, corrosion resistance, and high hardness to the coating. Detailed Implementation

[0028] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0029] Example 1: A method for preparing pretreated kaolin includes the following steps:

[0030] (1) After coarse crushing of the raw kaolin ore by a grinding mill, it is transported to a slurry machine. Deionized water and sodium hexametaphosphate at 0.1% of the mass of the raw kaolin ore are added to the slurry machine to form a 30% concentration slurry initial material.

[0031] (2) Remove coarse impurities from the initial slurry by desanding to obtain the slurry;

[0032] (3) The slurry is transported to a high gradient magnetic separator for two iron removal processes to obtain iron-removed slurry;

[0033] (4) Add sulfuric acid with a concentration of 3 mol / L to the iron removal slurry to adjust the pH value of the iron removal slurry to 3. After adjustment, add 0.2 wt% sodium hydrosulfite and stir evenly. Then add 0.15 wt% oxalic acid to complex and filter and wash with water to obtain bleaching slurry.

[0034] (5) After the bleached slurry is filtered by a filter press, it is dried and ground. The resulting ground powder is mixed evenly with sodium chloride and calcined at 1200℃ for 1.5h. The amount of sodium chloride added is 1.5wt% of the ground powder to prepare pretreated kaolin.

[0035] Example 2: A method for preparing ultrafine whitened kaolin, comprising the following steps:

[0036] S1. Take the pretreated kaolin prepared in Example 1 and prepare a kaolin slurry with a solid content of 25% in deionized water. Add 0.1% sodium hexametaphosphate by weight of the pretreated kaolin to the kaolin slurry, stir and disperse evenly, then pour the obtained suspension into a ball mill for ball milling, followed by ultrasonic oscillation for 0.5 h, and finally let it stand for 1 h and filter to prepare a kaolin suspension slurry.

[0037] S2. The kaolin suspension slurry was spray-dried and sintered. The spray pressure was 1.2 MPa and the inlet temperature was 140℃. The sintering process parameters were set as follows: first heat up to 850℃, cool and then heat up to 1120℃ and hold for 1.5h, cool again and then heat up to 1120℃ and hold for 4h. The heating rate was 5℃ / min. Kaolin microspheres were prepared.

[0038] S3. Take kaolin microspheres in deionized water, slurry and stir evenly to obtain a slurry with a concentration of 180 g / L, add 0.1% sodium hexametaphosphate by mass of kaolin microspheres, adjust the pH of the system to 10 with sodium hydroxide, heat to 75℃, and add dropwise 1 mol / L sodium aluminate solution and 0.5 mol / L dilute sulfuric acid solution, with a coating amount of 3%, maintain the pH of the system, stir the reaction for 0.5 h, after the reaction is completed, age for 24 h, filter, wash and dry to prepare alumina-coated kaolin;

[0039] S4. Prepare an impregnation solution of tetraethyl orthosilicate using ethanol as solvent, with a mass fraction of 5% based on silicon dioxide. Place the alumina-coated kaolin in the impregnation solution, and after uniform impregnation, dry it at 120°C for 24 hours. Then, calcine it at 550°C for 4 hours to obtain ultrafine whitened kaolin.

[0040] Example 3 A method for preparing ultrafine whitening kaolin, comprising the following steps:

[0041] S1. Take the pretreated kaolin prepared in Example 1 and prepare a kaolin slurry with a solid content of 30% in deionized water. Add 0.2% sodium hexametaphosphate by weight of the pretreated kaolin to the kaolin slurry, stir and disperse evenly, then pour the obtained suspension into a ball mill for ball milling, followed by ultrasonic oscillation for 0.5 h, and finally let it stand for 1 h and filter to prepare a kaolin suspension slurry.

[0042] S2. The kaolin suspension slurry was spray-dried and sintered. The spray pressure was 1.2 MPa and the inlet temperature was 140℃. The sintering process parameters were set as follows: first heat up to 900℃, cool and then heat up to 1150℃ and hold for 1.5h, cool again and then heat up to 1150℃ and hold for 4h. The heating rate was 5℃ / min. Kaolin microspheres were prepared.

[0043] S3. Take kaolin microspheres in deionized water, slurry and stir evenly to obtain a slurry with a concentration of 200 g / L, add 0.2% sodium hexametaphosphate by mass of kaolin microspheres, adjust the pH of the system to 10 using sodium hydroxide, heat to 80℃, and add dropwise 1.5 mol / L sodium aluminate solution and 0.5 mol / L dilute sulfuric acid solution, with a coating amount of 4%, maintain the pH of the system, stir the reaction for 0.5 h, after the reaction is completed, age for 25 h, filter, wash and dry to prepare alumina-coated kaolin;

[0044] S4. Prepare an impregnation solution of tetraethyl orthosilicate using ethanol as solvent, with a mass fraction of 5% based on silicon dioxide. Place the alumina-coated kaolin in the impregnation solution, and after uniform impregnation, dry it at 120°C for 24 hours. Then, calcine it at 580°C for 3 hours to obtain ultrafine whitened kaolin.

[0045] Example 4 A method for preparing ultrafine whitening kaolin, comprising the following steps:

[0046] S1. Take the pretreated kaolin prepared in Example 1 and prepare a kaolin slurry with a solid content of 30% in deionized water. Add 0.3% sodium hexametaphosphate by weight of the pretreated kaolin to the kaolin slurry, stir and disperse evenly, then pour the obtained suspension into a ball mill for ball milling, followed by ultrasonic oscillation for 0.5 h, and finally let it stand for 1 h and filter to prepare a kaolin suspension slurry.

[0047] S2. The kaolin suspension slurry was spray-dried and sintered. The spray pressure was 1.2 MPa and the inlet temperature was 140℃. The sintering process parameters were set as follows: first heat up to 900℃, cool and then heat up to 1200℃ and hold for 1 h, cool again and then heat up to 1200℃ and hold for 3.5 h. The heating rate was 5℃ / min. Kaolin microspheres were prepared.

[0048] S3. Take kaolin microspheres in deionized water, slurry and stir evenly to obtain a slurry with a concentration of 200 g / L, add 0.3% sodium hexametaphosphate by mass of kaolin microspheres, adjust the pH of the system to 10 with sodium hydroxide, heat to 85℃, and add dropwise 2 mol / L sodium aluminate solution and 1 mol / L dilute sulfuric acid solution, with a coating amount of 4%, maintain the pH of the system, stir the reaction for 1 h, after the reaction is completed, age for 27 h, filter, wash and dry to prepare alumina-coated kaolin;

[0049] S4. Prepare an impregnation solution of tetraethyl orthosilicate using ethanol as solvent, with a mass fraction of 5% based on silicon dioxide. Place the alumina-coated kaolin in the impregnation solution, and after uniform impregnation, dry it at 120°C for 24 hours. Then, calcine it at 550°C for 4 hours to obtain ultrafine whitened kaolin.

[0050] Comparative Example 1: A method for preparing ultrafine whitened kaolin, comprising the following steps:

[0051] S1. Take the pretreated kaolin prepared in Example 1 and prepare a kaolin slurry with a solid content of 30% in deionized water. Add 0.3% sodium hexametaphosphate by weight of the pretreated kaolin to the kaolin slurry, stir and disperse evenly, then pour the obtained suspension into a ball mill for ball milling, followed by ultrasonic oscillation for 0.5 h, and finally let it stand for 1 h and filter to prepare a kaolin suspension slurry.

[0052] S2. The kaolin suspension slurry was spray-dried and sintered. The spray pressure was 1.2 MPa and the inlet temperature was 140℃. The sintering process parameters were set as follows: directly heated to 1200℃ and held for 4 hours, with a heating rate of 5℃ / min, to prepare kaolin microspheres.

[0053] S3. Take kaolin microspheres in deionized water, slurry and stir evenly to obtain a slurry with a concentration of 200 g / L, add 0.3% sodium hexametaphosphate by mass of kaolin microspheres, adjust the pH of the system to 10 with sodium hydroxide, heat to 85℃, and add dropwise 2 mol / L sodium aluminate solution and 1 mol / L dilute sulfuric acid solution, with a coating amount of 4%, maintain the pH of the system, stir the reaction for 1 h, after the reaction is completed, age for 27 h, filter, wash and dry to prepare alumina-coated kaolin;

[0054] S4. Prepare an impregnation solution of tetraethyl orthosilicate using ethanol as solvent, with a mass fraction of 5% based on silicon dioxide. Place the alumina-coated kaolin in the impregnation solution, and after uniform impregnation, dry it at 120°C for 24 hours. Then, calcine it at 550°C for 4 hours to obtain ultrafine whitened kaolin.

[0055] Comparative Example 2: A method for preparing ultrafine whitened kaolin, comprising the following steps:

[0056] S1. Take the pretreated kaolin prepared in Example 1 and prepare a kaolin slurry with a solid content of 30% in deionized water. Add 0.3% sodium hexametaphosphate by weight of the pretreated kaolin to the kaolin slurry, stir and disperse evenly, then pour the obtained suspension into a ball mill for ball milling, followed by ultrasonic oscillation for 0.5 h, and finally let it stand for 1 h and filter to prepare a kaolin suspension slurry.

[0057] S2. The kaolin suspension slurry was spray-dried and sintered. The spray pressure was 1.2 MPa and the inlet temperature was 140℃. The sintering process parameters were set as follows: first heat up to 900℃, cool and then heat up to 1200℃ and hold for 1 h, cool again and then heat up to 1200℃ and hold for 3.5 h. The heating rate was 5℃ / min. Kaolin microspheres were prepared.

[0058] S3. Take kaolin microspheres in deionized water, slurry and stir evenly to obtain a slurry with a concentration of 200 g / L. Add 0.3% sodium hexametaphosphate by mass of kaolin microspheres. Adjust the pH of the system to 10 using sodium hydroxide. Heat to 85℃ and add dropwise 2 mol / L sodium aluminate solution and 1 mol / L dilute sulfuric acid solution. The coating amount is 4%. Maintain the pH of the system and stir the reaction for 1 hour. After the reaction is completed, age for 27 hours. After filtration, washing and drying, ultrafine whitened kaolin is prepared.

[0059] Comparative Example 3: A method for preparing ultrafine whitening kaolin, comprising the following steps:

[0060] S1. Take the pretreated kaolin prepared in Example 1 and prepare a kaolin slurry with a solid content of 30% in deionized water. Add 0.3% sodium hexametaphosphate by weight of the pretreated kaolin to the kaolin slurry, stir and disperse evenly, then pour the obtained suspension into a ball mill for ball milling, followed by ultrasonic oscillation for 0.5 h, and finally let it stand for 1 h and filter to prepare a kaolin suspension slurry.

[0061] S2. The kaolin suspension slurry was spray-dried and sintered. The spray pressure was 1.2 MPa and the inlet temperature was 140℃. The sintering process parameters were set as follows: first, the temperature was raised to 900℃, then cooled and rapidly heated to 1200℃ and held for 1 hour, then cooled again and raised to 1200℃ and held for 3.5 hours. The heating rate was 5℃ / min. Ultrafine whitened kaolin was prepared.

[0062] Comparative Example 4 uses the pretreated kaolin prepared in Example 1.

[0063] Performance testing

[0064] Kaolin prepared in Examples 2-4 and Comparative Examples 1-4 was used as the raw material for spraying. Stainless steel 304, cleaned with acetone using ultrasonic cleaning, was used as the substrate. A ceramic coating was prepared using atmospheric plasma spraying technology, and its performance was tested: the hardness of the coating cross-section was tested using a Vickers hardness tester with a load of 200g (1.96N) and a holding time of 15s. Fifteen areas of each coating were randomly selected for testing, and the average value was taken. The bonding strength between the coating and the substrate was tested according to GB / T 8642-2002 using a universal testing machine. Under ambient temperature and relative humidity of 45%, a Si3N4 grinding ball (6.35mm in diameter) was used as the grinding pair. Wear tests were conducted on the coating in the ball-disc reciprocating wear mode of a pin-disc friction and wear testing machine. Before the wear test, the surface of the coating sample was first ground and polished to a roughness of 0.02µm. The wear test parameters were: 10N vertical load, rotation speed 400r / min, and test time 30min. The test was conducted according to GB / T... 9274-1988 tested the acid and alkali corrosion resistance. The acid resistance was tested with 10% sulfuric acid solution, and the alkali resistance was tested with 10% sodium hydroxide solution. Oxybutane (1200℃) was used as an instantaneous high-temperature heat source. The coating changes were observed at 2 min, 4 min, and 10 min during the burning process. The fire resistance performance was tested, and the data results are shown in Table 1.

[0065] Table 1 Sample performance test results

[0066]

[0067] As can be seen from the data in Table 1, the ceramic coatings prepared in Examples 2-4 of this invention have high hardness, good bonding strength, wear resistance, corrosion resistance, and fire resistance. In Comparative Example 1, a single sintering process was used, and its measured hardness was significantly lower than that of Examples 2-4. This is because the kaolin microspheres prepared using a two-stage sintering process have better strength and hardness. Comparative Example 2 did not undergo tetraethyl orthosilicate impregnation, and Comparative Example 3 did not undergo alumina coating and was impregnated with tetraethyl orthosilicate. The measured hardness, wear resistance, acid resistance, and alkali resistance of Comparative Examples 2-3 were lower than those of Examples 2-4, indicating that alumina coating and tetraethyl orthosilicate modification can synergistically improve mechanical properties and corrosion resistance. Furthermore, the bonding strength in Comparative Example 3 was lower than that in Examples 2-4. Comparative Example 4, which directly used pretreated kaolin, showed the most significant decrease in mechanical properties, bonding strength, and corrosion resistance compared to Examples 2-4.

[0068] In the description of this specification, references to terms such as "an embodiment," "example," "specific example," etc., indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of the invention. In this specification, illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples.

[0069] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The embodiments and descriptions in the specification are merely illustrative of the principles of the invention. Various changes and modifications can be made to the invention without departing from its spirit and scope, and all such changes and modifications fall within the scope of the claimed invention.

Claims

1. A process for the preparation of an ultrafine whitening kaolin, characterized in that, The method comprises the following steps: S1, preparing pretreated kaolin by dispersing, desanding, removing impurities, magnetic separation, bleaching treatment and calcination treatment of raw kaolin ore; S2, preparing kaolin slurry by adding sodium hexametaphosphate to the pretreated kaolin in deionized water, ultrasonic oscillation, and then standing and filtering; S3, preparing kaolin microsphere particles by spray drying and sintering the kaolin slurry; S4, preparing alumina-coated kaolin by adding sodium hexametaphosphate to the kaolin microsphere particles in deionized water, stirring, adjusting the pH value of the system to 9-10 with sodium hydroxide, heating to 70-85℃, and then adding sodium metaaluminate solution and dilute sulfuric acid solution drop by drop, stirring for 0.5-1h, and then aging for 24-28h, and then filtering, washing and drying; S5, preparing superfine whitening kaolin by preparing a tetraethyl orthosilicate impregnating solution with ethanol as the solvent, the mass fraction of silicon oxide being 5%, and then immersing the alumina-coated kaolin in the impregnating solution, drying at 100-120℃ for 20-24h, and then calcining at 550-600℃ for 2-4h.

2. The method of claim 1, wherein the ultrafine whitening kaolin is prepared by the steps of: The preparation method of the pretreated kaolin in step S1 comprises the following steps: (1) feeding the raw kaolin ore into a crusher after coarse crushing, and then feeding the crushed ore into a beater mill, adding deionized water and sodium hexametaphosphate to the beater mill to prepare a slurry, and then adding 0.1-0.3wt% of sodium chloride to the slurry to form a 20-40% concentration of the slurry; (2) removing coarse impurities from the slurry to obtain a slurry; (3) feeding the slurry into a high-gradient magnetic separator to remove iron to obtain a deironing slurry; (4) adding sulfuric acid with a concentration of 2-4mol / L to the deironing slurry to adjust the pH value of the deironing slurry to 2-3, adding 0.1-0.2wt% of sodium hydrosulfite after the adjustment, and then adding 0.1-0.15wt% of oxalic acid to form a bleaching slurry; (5) filtering the bleaching slurry by a filter press, drying and grinding the obtained powder, and then mixing the powder with sodium chloride, and then sintering the mixture at 1000-1200℃ for 1-1.5h to obtain the pretreated kaolin.

3. The method of claim 2, wherein the ultrafine whitening kaolin is prepared by the steps of: The particle size of the pretreated kaolin is 2-5µm.

4. The method of claim 1 wherein the ultrafine whitening kaolin is prepared by the steps of: The solid content of the kaolin slurry in step S2 is 25-30%, and the mass of sodium hexametaphosphate is 0.1-0.3% of the mass of the pretreated kaolin.

5. The method of claim 1 wherein the ultrafine whitening kaolin is prepared by the steps of: The process parameters of the spray drying are: the spray pressure is 1.2MPa, and the inlet temperature is 140℃.

6. The method of claim 1 wherein the ultrafine whitening kaolin is prepared by the steps of: The process parameters of the sintering are: first heating to 850-900℃, cooling, then heating to 1100-1200℃ and keeping for 1-1.5h, and then cooling, heating to 1100-1200℃ and keeping for 3.5-4h.

7. The method of claim 1 wherein the ultrafine whitening kaolin is prepared by the steps of: The concentration of the kaolin microsphere particles in step S4 is 175-200g / L, and the mass of sodium hexametaphosphate is 0.1-0.3% of the mass of the kaolin microsphere particles.

8. The method of claim 1 wherein the ultrafine whitening kaolin is prepared by the steps of: The concentration of the sodium metaaluminate solution is 1-2 mol / L; the concentration of the dilute sulfuric acid solution is 0.5-1 mol / L; and the coating amount in the alumina-coated kaolin is 3-4%.

9. An ultrafine whitening kaolin, characterized in that, The preparation method according to any one of claims 1-8.

10. Use of the ultrafine whitening kaolin according to claim 9, characterized in that, The superfined whitening kaolin is applied to the spray forming ceramic coating.

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