System and method for preparing kaolin by calcining and whitening coal gangue

Abstract

The invention discloses a system and a method for preparing kaolin by calcining and whitening coal gangue, the system comprises a preheater, a calcining furnace, a heat preservation bin and a cyclone cooling unit which are communicated in sequence, a blanking pipe of a penultimate cyclone separator of the preheater is connected with the calcining furnace, a combustor is arranged on the calcining furnace, and the cyclone cooling unit is connected with the calcining furnace. A discharging pipe of the cyclone separator at the bottom end of the preheater is connected with the calcining furnace and the heat preservation bin respectively; the cyclone cooling unit comprises a first cooling unit and a second cooling unit; a discharging port of the heat preservation bin is connected with a feeding port of the first cooling unit, and a discharging port of the first cooling unit is connected with a feeding port of the second cooling unit. The decarburization time is prolonged by adopting calcining furnace material circulation and a heat preservation bin, so that carbon in the coal gangue is completely removed, the whiteness of the kaolin product is further improved, the production line is energy-saving and emission-reducing, the product quality is stable, the process flow and equipment are simple, carbon emission reduction is realized, the production line is suitable for large-scale industrial production, the cost is reduced, and the cost performance is high.

Description

Technical Field

[0001] The invention relates to the technical field of coal gangue processing, and in particular to a system and method for preparing kaolin by calcining and whitening coal gangue. Background Art

[0002] Coal gangue is solid waste discharged during coal mining and coal washing. It is produced in large quantities and is generally stored in piles. It has a great impact on the environment and can cause spontaneous combustion, acid rain, underground infiltration, siltation of rivers, photochemical smog and mudslides. Therefore, the reuse and resource utilization of coal gangue is imperative.

[0003] Kaolin is one of the four major non-metallic minerals and a key industrial raw material, widely used in a variety of industries, including ceramics, papermaking, coatings, rubber, plastics, and building materials. However, with the continued mining of high-grade, conventional kaolin, the availability of raw kaolin for processing and producing refined kaolin that meets industrial requirements has become increasingly scarce.

[0004] The process flow of calcining kaolin with coal gangue mainly includes the following steps: raw material preparation: crushing and screening the coal gangue to obtain the appropriate particle size; calcination: placing the crushed coal gangue into a high-temperature kiln for calcination to volatilize the carbon in it and obtain kaolin material with kaolin structure; grinding: grinding the calcined kaolin to obtain finer particles and better application performance; grading: grading the ground kaolin according to the needs of different application fields to obtain products of different particle sizes; packaging: packaging the graded kaolin for easy transportation and application.

[0005] Whiteness is the most important indicator of calcined kaolin quality. Since gangue contains a large amount of inorganic impurities, as well as associated carbon and organic matter, high-temperature calcination can decompose the organic matter, converting carbon into carbon dioxide while also removing some inorganic impurities. Therefore, when preparing kaolin from gangue, the focus is on removing carbon and black to improve whiteness.

[0006] Chinese Patent Publication No. CN104150497A discloses a method for preparing ultra-high whiteness calcined kaolin from all-gangue. This method calcines raw materials composed entirely of gangue at a temperature between 945-955°C for more than 6 hours, and adjusts the calcination atmosphere so that the carbon in the gangue is oxidized to carbon monoxide. The calcination is a muffle calcination, which means that the calcined material does not directly contact the flame. This patent achieves whitening of the gangue, but the calcination time is too long, requiring more than 6 hours, and it is a muffle calcination, with low heat utilization efficiency and relatively high cost. Chinese patent publication number CN111847468A discloses a method for producing high-whiteness calcined kaolin by multi-stage suspension calcination of coal-based kaolin. The residence time of the powder in the suspension dehydration calciner is 2 to 10 minutes, the residence time of the dehydrated material in the suspension dehydroxylation calciner is 10 to 30 minutes, and the residence time of the dehydroxylation material in the suspension decarbonization calciner is 40 to 90 minutes. Coal gas needs to be introduced in all three stages. This patent performs suspension calcination of coal-based kaolin with high heat and mass transfer efficiency, and adopts "segmented precise control" of "dehydration-dehydroxylation-decarbonization", but each period is relatively long, which will lead to complex process flow and equipment, and poor operability and economy.

[0007] Therefore, based on market demand, circular economy and key scientific and technological challenges faced, a system and method for preparing kaolin by calcining and whitening coal gangue is provided to solve the problems of low whiteness value and poor economy of kaolin prepared by suspended calcination of coal gangue, so that the entire production system has high whiteness, low energy consumption, low cost, simple process flow, stable product quality, and realizes carbon emission reduction and large-scale industrial production, which has important practical significance. Summary of the Invention

[0008] In order to solve the problems of low whiteness value and poor economic efficiency of kaolin prepared by suspended calcination of coal gangue, the present invention provides a system and method for preparing kaolin by calcining and whitening coal gangue. The system adopts material circulation of the calcining furnace and the insulation bin to increase the decarbonization time, so that the carbon in the coal gangue is completely removed, and the whiteness of the kaolin products is further improved. In addition, the production line saves energy and reduces emissions, the product quality is stable, the process flow and equipment are simple, carbon emission reduction is achieved, it is suitable for large-scale industrial production, reduces costs, and has a high cost performance.

[0009] The present invention is achieved as follows: a system for preparing kaolin by calcining and whitening coal gangue, comprising a preheater, a calcining furnace, an insulation bin and a cyclone cooling unit connected in sequence, the discharge pipe of the penultimate cyclone separator of the preheater being connected to the calcining furnace, a burner being provided on the calcining furnace, the discharge pipe of the cyclone separator at the bottom end of the preheater being respectively connected to the calcining furnace and the insulation bin; the cyclone cooling unit comprising a first cooling unit and a second cooling unit; the discharge port of the insulation bin being connected to the feed port of the first cooling unit, and the discharge port of the first cooling unit being connected to the feed port of the second cooling unit.

[0010] In the above technical solution, preferably, multiple distribution valves are provided on the discharge pipe of the penultimate cyclone separator of the preheater, so that the material enters the calcining furnace at multiple points. The multiple distribution points control the temperature distribution in the calcining furnace, and the outlet temperature of the calcining furnace is 720-1020°C.

[0011] In the above technical solution, preferably, the high-temperature kaolin products produced in the calcining furnace are separated into gas and solid by the cyclone separator at the bottom end of the preheater, and a part of them is circulated into the calcining furnace and a part of them is entered into the insulation bin. The temperature of the insulation bin is controlled at 700-1000°C to remove the carbon in the coal gangue.

[0012] In the above technical solution, preferably, there are two insulation bins, so that the materials enter the two insulation bins alternately for insulation.

[0013] In the above technical solution, preferably, the discharge port of the heat preservation bin is connected to the feed port of the first cooling unit through a high-temperature reamer or a feed pipe.

[0014] In the above technical solution, preferably, cooling air is introduced into the air inlet of the second cooling unit.

[0015] In the above technical solution, preferably, a dust collector and a circulating fan are sequentially provided on the air outlet pipe of the second cooling unit, the outlet of the circulating fan is connected to the circulating air duct, the circulating air duct is connected to the air inlet of the first cooling unit, and the air outlet of the first cooling unit is connected to the air inlet of the calcining furnace; a valve is provided on the circulating air duct, and the circulating air volume is adjusted according to the amount of oxygen required for the fuel combustion design in the calcining furnace.

[0016] In the above technical solution, it is further preferred that the circulating fan outlet is also connected to a raw mill system, a drying system, other waste heat utilization systems or an exhaust gas treatment system.

[0017] In the above technical solution, preferably, the discharge pipe of the cyclone cooler at the bottom end of the second cooling unit is aligned with the finished zipper machine, and an emergency buffer bin is provided at the bottom of the air inlet pipe where the air inlet of the second cooling unit is located.

[0018] In the above technical solution, preferably, the number of stages of the preheater is 2 to 6.

[0019] A method for preparing kaolin by calcining and whitening coal gangue comprises the following steps:

[0020] The gangue raw material is fed into the preheater, where it undergoes heat exchange and gas-solid separation with the flue gas from the calcining furnace to remove moisture and some hydroxyl groups and carbon. The gangue raw material then enters the calcining furnace, where the heat released by the combustion of fuel is used to remove hydroxyl groups and decarbonize the material. The outlet temperature of the calcining furnace is 720-1020°C, and the material residence time in the calcining furnace is ≥20s. High-temperature kaolin products are obtained, and a large amount of flue gas is generated. The flue gas generated in the calcining furnace enters the preheater, where it exchanges heat with the raw material to become low-temperature flue gas, which is discharged through the air outlet of the cyclone separator at the top of the preheater.

[0021] The high-temperature kaolin products produced in the calcining furnace are separated into gas and solid by the cyclone separator at the bottom of the preheater. Part of the kaolin products are circulated into the calcining furnace, and part of them enter the insulation bin. The temperature of the insulation bin is controlled at 700-1000℃, and the insulation time is ≥20min to completely remove the carbon in the coal gangue. The hot materials leaving the insulation bin enter the first cooling unit.

[0022] The hot air after heat exchange in the second cooling unit is partially circulated to the first cooling unit according to the oxygen amount required by the calciner design, and enters the calciner after heat exchange with the hot material. The hot material separated by the first cooling unit enters the second cooling unit. The second cooling unit is introduced with the designed cooling air amount required for cooling the hot material, and after heat exchange with the hot material, it is further cooled to the temperature required by the product; part of the air out of the second cooling unit is circulated to the first cooling unit, and part enters the raw material grinding system or drying system or other waste heat utilization system or exhaust gas treatment system.

[0023] The advantages and positive effects of the present invention are:

[0024] (1) In order to fully decarbonize the material and increase the whiteness, the present invention sets a material circulation in the calcining furnace, and a part of the discharge of the cyclone separator at the bottom end of the preheater re-enters the calcining furnace, which increases the calcination time, improves the decarbonization efficiency, and further increases the whiteness.

[0025] (2) The present invention designs a heat preservation bin based on the decarbonization mechanism, and reasonably designs the heat preservation temperature and time, so as to promote the removal of carbon in the coal gangue and further improve the whiteness of the coal gangue after calcination; and the heat preservation bin is designed to be two, working alternately, to ensure the continuity and stability of the system.

[0026] (3) The present invention can adjust the temperature in the calcining furnace by feeding the material at three points to adapt to the decarbonization effect of different types of coal gangue, thereby ensuring the stability of the system and the whiteness of the product.

[0027] (4) The present invention sets a first cooling unit and a second cooling unit, and reasonably designs the cooling air volume, which can achieve full combustion of fuel, full decarbonization of coal gangue, and full cooling of finished products, and can effectively reduce system heat consumption and investment costs.

[0028] (5) The preheater outlet temperature of the entire system of the present invention is low, the air volume is small, and the system energy consumption is low. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 Schematic diagram of a system for preparing kaolin by calcining and whitening coal gangue provided by an embodiment of the present invention Figure 1 ;

[0030] Figure 2 Schematic diagram of a system for preparing kaolin by calcining and whitening coal gangue provided by an embodiment of the present invention Figure 2 .

[0031] In the picture:

[0032] 1-preheater, 101-top cyclone separator, 102-second-to-last cyclone separator, 103-bottom cyclone separator, 104-dispensing valve;

[0033] 2-Calcining furnace, 201-Burner, 3-Insulation chamber, 4-High-temperature reamer, 5-First cooling unit, 6-Second cooling unit, 7-Dust collector, 8-Circulating fan, 9-Circulating air duct, 10-Valve, 11-Finished zipper machine; 12-Emergency buffer chamber;

[0034] a-coal gangue, b-finished kaolin, A-cooling air;

[0035] The dotted line with an arrow is the direction of airflow, and the solid line with an arrow is the direction of material flow. DETAILED DESCRIPTION

[0036] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0037] In the description of the present invention, it should be understood that the terms "up", "down", "front", "back", "left", "right", "inside", "outside", etc., indicating directions or positional relationships, are based on the directions or positional relationships shown in the accompanying drawings. They are only for the convenience of describing the present invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific direction, be constructed and operated in a specific direction. Therefore, they should not be understood as limiting the present invention.

[0038] In the description of the present invention, it should be noted that, unless otherwise expressly specified or limited, the terms "mounted," "connected," and "connected" should be understood in a broad sense. For example, they may refer to fixed, detachable, or integral connections; mechanical or electrical connections; direct or indirect connections through an intermediate medium; and internal communication between two components. Those skilled in the art will understand the specific meanings of the above terms in the present invention based on the specific circumstances.

[0039] Example 1

[0040] See also Figure 1 and Figure 2 , an embodiment of the present invention provides a system for preparing kaolin by calcining and whitening coal gangue, comprising a preheater 1, a calcining furnace 2, a heat preservation bin 3 and a cyclone cooling unit connected in sequence, wherein the preheater 1 is provided with a raw material inlet, the raw material inlet of the cyclone separator 101 at the top of the preheater 1 is used for feeding raw material, the air outlet of the cyclone separator 101 at the top of the preheater 1 discharges low-temperature flue gas, the discharge pipe of the penultimate cyclone separator 102 of the preheater 1 is connected to the calcining furnace 2, a burner 201 is provided at the bottom of the calcining furnace 2 to control the outlet temperature of the calcining furnace to 720-1020°C, and the air inlet of the cyclone separator 103 at the bottom of the preheater 1 is connected to the air outlet pipe of the calcining furnace 2; The discharge pipe of the cyclone separator 103 at the bottom end of the preheater 1 is connected to the calcining furnace 2 and the insulation bin 3 respectively. By adjusting the material distribution of the cyclone separator 103 at the bottom end, part of the material is circulated into the calcining furnace 2, and part of the material is fed into the insulation bin 3. The insulation bin 3 uses a heat-insulating material with low thermal conductivity and good heat preservation performance to minimize heat dissipation. The temperature in the insulation bin 3 is 700-1000°C, and the insulation time is ≥20 minutes, so that the carbon in the coal gangue is completely removed. The cyclone cooling unit includes a first cooling unit 5 and a second cooling unit 6. The discharge port of the insulation bin 3 is connected to the feed port of the first cooling unit 5, and the discharge port of the first cooling unit 5 is connected to the feed port of the second cooling unit 6. It should be noted that the number of cyclone separators and cooling units is only for reference, and those skilled in the art can set it according to actual needs.

[0041] As a preferred embodiment, multiple distribution valves 104 are provided on the discharge pipe of the penultimate cyclone separator 102 of the preheater 1, allowing the material to enter the calcining furnace 2 at multiple points. These distribution points control the temperature within the calcining furnace 2. To regulate the temperature field distribution within the calcining furnace 2, two distribution valves 104 are provided on the discharge pipe of the penultimate cyclone separator 102 in this embodiment. By adjusting the distribution of the material at the upper, middle, and lower distribution points of the penultimate cyclone separator 102, different temperature zones are formed within the calcining furnace, and the calcining furnace outlet temperature is controlled to 720-1020°C.

[0042] As a preferred embodiment, there are two insulation bins 3, so that the material is alternately fed into the two insulation bins 3 for insulation. When one of the insulation bins is filled with hot material for insulation, the hot material is switched to the other insulation bin 3. The two insulation bins 3 are switched and used alternately to ensure the continuity and stability of the system. The hot material in the insulation bins 3 is discharged to the first cooling unit 5.

[0043] As a preferred embodiment, the discharge port of the heat preservation bin 3 is connected to the feed port of the first cooling unit 5 through a high-temperature reamer or a feed pipe. Figure 1 In the process, the material in the heat preservation bin 3 is discharged to the first cooling unit 5 through a high-temperature reamer. The material of the high-temperature reamer should be able to withstand a temperature of ≥950°C to prevent high-temperature deformation that is not conducive to stable discharge. Figure 2 In the process, the material in the heat preservation bin 3 is discharged to the first cooling unit 5 through the discharge pipe, and the discharge pipe is equipped with an air locking device to prevent the cooling gas of the first cooling unit 5 from flowing back into the heat preservation bin 3.

[0044] As a preferred embodiment, cooling air is introduced into the air inlet of the second cooling unit 6. The hot material separated by the first cooling unit 5 enters the second cooling unit 6, which is fed with the designed amount of cooling air required for cooling the hot material. After heat exchange with the hot material inside, the second cooling unit 6 is further cooled to the temperature required for the product, and a portion of the exhausted hot air is circulated to the first cooling unit 5.

[0045] As a preferred embodiment, the air outlet duct of the second cooling unit 6 is provided with a dust collector 7 and a circulating fan 8 in sequence. The outlet of the circulating fan 8 is connected to a circulating air duct 9, which is connected to the air inlet of the first cooling unit 5. The air outlet of the first cooling unit 5 is connected to the air inlet of the calcining furnace 2. The circulating air duct 9 is provided with a valve 10, and the circulating air volume is adjusted according to the amount of oxygen required for the fuel combustion design in the calcining furnace 2. The hot air after heat exchange in the second cooling unit 6 is partially circulated to the first cooling unit 5 according to the amount of oxygen required by the calcining furnace 2 after dust collection, and further increases the temperature after heat exchange with the hot material before entering the calcining furnace 2.

[0046] As a preferred embodiment, the outlet of the circulating fan 8 is also connected to the raw mill system, the drying system, other waste heat utilization system or the exhaust gas treatment system. Part of the hot air exhausted by the second cooling unit 6 enters the raw mill system, the drying system, other waste heat utilization system or the exhaust gas treatment system.

[0047] As a preferred embodiment, the discharge pipe of the cyclone cooler at the bottom end of the second cooling unit 6 is aligned with the finished zipper machine 11, and an emergency buffer bin 12 is provided at the bottom of the air inlet pipe where the air inlet of the second cooling unit 6 is located. This can be used to deal with emergencies such as when the circulating fan 8 stops and the material cannot be lifted and collapses. The discharge port of the emergency buffer bin 12 is aligned with the finished zipper machine 11.

[0048] As a preferred embodiment, the number of stages of the preheater 1 is 2 to 6. This embodiment is preferably a five-stage cyclone separation preheater, and air lock valves are provided on the discharge pipes of each stage of the cyclone separator.

[0049] Gangue is fed into the raw material grinding system to produce raw powder of the required particle size. The raw material is then fed into the preheater 1, undergoes gas-solid heat exchange in a multi-stage cyclone separator, and then enters the calciner 2 for decarbonization. The main fuel inlet is located at the bottom of the calciner 2. After the decarbonized material is separated by gas and solids in the cyclone separator, a portion is returned to the calciner 2 for further material circulation and decarbonization, while a portion enters the holding bin 3 for extended holding time, thereby removing all carbon from the gangue and further improving its whiteness after calcination. The flue gas discharged from the outlet of the preheater 1 enters the raw material mill or flue gas treatment system, and after flue gas treatment, it is discharged into the atmosphere through a chimney. The hot material leaving the holding bin 3 is sequentially cooled by the first cooling unit 5 and the second cooling unit 6. The hot air from the second cooling unit 6 is partially circulated to the first cooling unit 5 according to the oxygen content required by the calciner 2 design. After heat exchange with the hot material, the temperature is further increased before entering the calciner 2. The hot material separated by the first cooling unit 5 enters the second cooling unit 6 for further cooling to the required product temperature. The second cooling unit 6 is fed with the designed cooling air volume required for hot material cooling. After heat exchange with the hot material, part of the air is circulated to the first cooling unit 5, and part of the air enters the raw material grinding system or drying system or other waste heat utilization system or exhaust gas treatment system.

[0050] Example 2

[0051] See also Figure 1 and Figure 2 The present invention provides a method for preparing kaolin by calcining and whitening coal gangue, comprising the following steps:

[0052] The coal gangue a raw material that has been ground to an appropriate particle size is fed into the preheater 1. The raw material undergoes heat exchange and gas-solid separation with the flue gas exiting the calcining furnace 2 in the preheater 1 to remove moisture and some hydroxyl groups and carbon. The material after multi-stage gas-solid heat exchange enters the calcining furnace 2, and fuel is sprayed into the burner 201. The heat released by the combustion of the fuel in the calcining furnace 2 is used to remove hydroxyl groups and decarbonize the material. The outlet temperature of the calcining furnace is controlled at 720-1020°C, and the residence time of the material in the calcining furnace is ≥20s. High-temperature kaolin products are obtained, and a large amount of flue gas is generated. The flue gas generated in the calcining furnace 2 enters the preheater 1 to exchange heat with the raw material to become low-temperature flue gas, which is discharged through the air outlet of the cyclone separator 101 at the top of the preheater 1.

[0053] After the high-temperature kaolin products produced in the calcining furnace 2 are separated into gas and solid by the cyclone separator 103 at the bottom end of the preheater 1, a part of them circulates into the calcining furnace 2 and is further calcined in the calcining furnace 2 to form kaolin products with higher whiteness. A part of them enters the insulation bin 3 to extend the insulation time. The temperature of the insulation bin 3 is controlled at 700-1000℃, and the insulation time is ≥20min, so that the carbon in the coal gangue can be completely removed, further improving the whiteness of the coal gangue after calcination; the hot material leaving the insulation bin 3 enters the first cooling unit 5.

[0054] The hot air after heat exchange in the second cooling unit 6 is partially circulated to the first cooling unit 5 according to the oxygen amount required by the design of the calcining furnace 2, and the temperature is further increased after heat exchange with the hot material. The wind out of the first cooling unit 5 enters the calcining furnace 2, reducing the energy consumption of the system. The hot material separated by the first cooling unit 5 enters the second cooling unit 6, and the second cooling unit 6 is introduced with the designed cooling air A required for cooling the hot material. After heat exchange with the hot material, it is further cooled to the temperature required by the product. The finished kaolin b collected by the second cooling unit 6 enters the finished zipper machine 11; part of the wind out of the second cooling unit 6 is circulated to the first cooling unit 5, and part enters the raw material grinding system or drying system or other waste heat utilization system or exhaust gas treatment system.

[0055] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or make equivalent replacements for some or all of the technical features therein, and these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A system for preparing kaolin by calcining and whitening coal gangue, characterized by: It includes a preheater, a calcining furnace, an insulation bin and a cyclone cooling unit that are connected in sequence. The discharge pipe of the penultimate cyclone separator of the preheater is connected to the calcining furnace. A burner is provided on the calcining furnace. The discharge pipe of the cyclone separator at the bottom end of the preheater is respectively connected to the calcining furnace and the insulation bin; the cyclone cooling unit includes a first cooling unit and a second cooling unit; the discharge port of the insulation bin is connected to the feed port of the first cooling unit, and the discharge port of the first cooling unit is connected to the feed port of the second cooling unit.

2. The system for preparing kaolin by calcining and whitening coal gangue according to claim 1, characterized in that: A plurality of distribution valves are provided on the discharge pipe of the penultimate cyclone separator of the preheater, so that the material enters the calcining furnace at multiple points. The temperature distribution in the calcining furnace is controlled by the multiple distribution points, and the outlet temperature of the calcining furnace is 720-1020°C.

3. The system for preparing kaolin by calcining and whitening coal gangue according to claim 1, characterized in that: The high-temperature kaolin products produced in the calcining furnace are separated into gas and solid by the cyclone separator at the bottom of the preheater. Part of them circulates into the calcining furnace and part of them enters the insulation bin. The temperature of the insulation bin is controlled at 700-1000℃ to remove the carbon in the coal gangue.

4. The system for preparing kaolin by calcining and whitening coal gangue according to claim 1, characterized in that: There are two heat preservation bins, so that the materials are alternately put into the two heat preservation bins for heat preservation.

5. The system for preparing kaolin by calcining and whitening coal gangue according to claim 1, characterized in that: The discharge port of the heat preservation bin is connected to the feed port of the first cooling unit through a high-temperature reamer or a feed pipe.

6. The system for preparing kaolin by calcining and whitening coal gangue according to claim 1, characterized in that: Cooling air is introduced into the air inlet of the second cooling unit.

7. The system for preparing kaolin by calcining and whitening coal gangue according to claim 1, characterized in that: The air outlet pipe of the second cooling unit is provided with a dust collector and a circulating fan in sequence. The outlet of the circulating fan is connected to the circulating air duct, and the circulating air duct is connected to the air inlet of the first cooling unit. The air outlet of the first cooling unit is connected to the air inlet of the calcining furnace; a valve is provided on the circulating air duct, and the circulating air volume is adjusted according to the amount of oxygen required for the fuel combustion design in the calcining furnace.

8. The system for preparing kaolin by calcining and whitening coal gangue according to claim 7, characterized in that: The circulating fan outlet is also connected to a raw material grinding system, a drying system, other waste heat utilization systems or an exhaust gas treatment system.

9. The system for preparing kaolin by calcining and whitening coal gangue according to claim 1, characterized in that: The discharge pipe of the cyclone cooler at the bottom end of the second cooling unit is aligned with the finished zipper machine, and an emergency buffer bin is provided at the bottom of the air inlet pipe where the air inlet of the second cooling unit is located.

10. The system for preparing kaolin by calcining and whitening coal gangue according to claim 1, characterized in that: The number of stages of the preheater is 2 to 6.

11. A method for preparing kaolin by calcining and whitening coal gangue using the system according to any one of claims 1 to 10, characterized in that: The following steps are involved: The gangue raw material is fed into the preheater, where it undergoes heat exchange and gas-solid separation with the flue gas from the calcining furnace to remove moisture and some hydroxyl groups and carbon. The gangue raw material then enters the calcining furnace, where the heat released by the combustion of fuel is used to remove hydroxyl groups and decarbonize the material. The outlet temperature of the calcining furnace is 720-1020°C, and the material residence time in the calcining furnace is ≥20s. High-temperature kaolin products are obtained, and a large amount of flue gas is generated. The flue gas generated in the calcining furnace enters the preheater, where it exchanges heat with the raw material to become low-temperature flue gas, which is discharged through the air outlet of the cyclone separator at the top of the preheater. The high-temperature kaolin products produced in the calcining furnace are separated into gas and solid by the cyclone separator at the bottom of the preheater. Part of the kaolin products are recycled into the calcining furnace, and part of the products are sent to the insulation bin. The temperature of the insulation bin is controlled at 700-1000℃, and the insulation time is ≥20min, so that the carbon in the gangue can be completely removed. The hot material leaving the insulation bin enters the first cooling unit; The hot air after heat exchange in the second cooling unit is partially circulated to the first cooling unit according to the oxygen amount required by the calciner design, and enters the calciner after heat exchange with the hot material; the hot material separated by the first cooling unit enters the second cooling unit, and the second cooling unit is introduced with the designed cooling air amount required for cooling the hot material, and after heat exchange with the hot material, it is further cooled to the temperature required by the product; part of the air leaving the second cooling unit is circulated to the first cooling unit, and part enters the raw material grinding system or drying system or other waste heat utilization system or exhaust gas treatment system.

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