Method for preparing high-purity mullite through pretreatment-calcination coupling of coal gangue

Abstract

The invention relates to the technical field of coal gangue treatment, in particular to a method for preparing high-purity mullite through coal gangue pretreatment-calcination coupling. The high-purity mullite is prepared by the following steps: crushing, grinding and sieving, adding soluble salt, phosphoric acid and ammonium salt in an alkaline hydrothermal reaction, roasting and removing impurities, modifying by using a composite mineralizer, coating by using silicon-aluminum sol, carrying out suspended three-stage calcination and the like. The method solves the technical problems of insufficient activation of the coal gangue, difficulty in deep removal of impurities, low purity of mullite and low resource utilization efficiency of the coal gangue in the traditional process, and realizes efficient resource conversion of the coal gangue. Through the synergistic effect of all process links, the crystal phase purity and structural compactness of mullite are effectively improved, the obtained product has good application performance, a feasible path is provided for preparation of high-value mullite from coal gangue, and the method is suitable for the related fields of high-temperature structural materials and the like.

Description

Technical Field

[0001] This application relates to the field of coal gangue processing technology, specifically to a method for preparing high-purity mullite by coupled coal gangue pretreatment and calcination. Background Technology

[0002] my country's annual coal gangue emissions exceed 300 million tons. Long-term stockpiling not only occupies a large amount of land resources but also easily generates toxic and harmful gases due to spontaneous combustion and leachate water polluting groundwater, causing serious environmental problems. Its high-value utilization has become an urgent industry challenge. However, when using coal gangue to prepare high-purity mullite, existing processes face key technical bottlenecks: on the one hand, coal gangue contains harmful impurities such as Fe2O3 and TiO2, which seriously affect the crystal phase purity and properties of mullite, and existing single impurity removal processes (such as simple acid leaching and alkali leaching) are insufficient to achieve deep removal of impurities; on the other hand, the silica-alumina components in coal gangue mostly exist in the form of inert mineral phases such as kaolinite and illite. Existing activation technologies (such as conventional roasting) are inefficient and cannot fully convert the inert silica-alumina phase into active components, resulting in uneven mullite crystal phase development and insufficient purity, which is difficult to meet the requirements of high-end applications. Meanwhile, traditional high-temperature solid-state sintering processes require temperatures above 1600℃, resulting in high energy consumption and further exacerbating product cost pressures. This severely restricts the high-value utilization of coal gangue and the low-cost industrialization of high-purity mullite. Therefore, it is urgent to develop a high-purity mullite preparation technology that deeply removes harmful impurities from coal gangue to overcome the current industry development predicament. Summary of the Invention

[0003] This application provides a method for preparing high-purity mullite by coupled pretreatment and calcination of coal gangue. After pretreatment by crushing and grinding, the coal gangue undergoes a multi-stage hydrothermal activation reaction with stirring control, ammonium salt roasting for impurity removal, modification by composite mineralizer, and modification by silica-alumina sol coating. Finally, it is fed into a suspension calcination furnace and subjected to a continuous suspension calcination process with segmented temperature control. This process achieves efficient and directional conversion of coal gangue into high-purity mullite, improves product purity and crystallization quality, and has the advantages of resource recycling and energy conservation and environmental protection.

[0004] To achieve the above objectives, one technical solution adopted by the present invention is:

[0005] This application provides a method for preparing high-purity mullite by coupled pretreatment and calcination of coal gangue, comprising the following steps:

[0006] Step 1. Take coal gangue as raw material, crush it, grind it and then sieve it to obtain coal gangue powder;

[0007] Step 2. Add the coal gangue powder to a hydrothermal reactor, add deionized water and stir until completely dissolved, then add sodium hydroxide solution to adjust the pH of the reaction system, add soluble salt and phosphoric acid, stir a second time, seal the hydrothermal reactor and heat up for a third stirring reaction, filter after the reaction is completed, wash with deionized water until the filtrate is neutral, and then dry to obtain the dried precursor;

[0008] Step 3. Mix the dried precursor with the ammonium salt evenly, place it in a calcination furnace and calcinate it under air atmosphere to obtain the purified precursor;

[0009] Step 4. Add the purified precursor to deionized water, stir and disperse to obtain a slurry, then add the composite mineralizer to the slurry and continue stirring to obtain a slurry containing the composite mineralizer;

[0010] Step 5. Slowly add the slurry containing the composite mineralizer into the silica-alumina sol, stir and react, then let it stand and age, and filter to obtain the coated precursor;

[0011] Step 6. The coated precursor is fed into a suspension calcination furnace, and airflow is introduced to suspend the coated precursor. It then passes through a drying stage, a decomposition stage, and a crystallization stage in sequence. After calcination, it is cooled and collected to obtain high-purity mullite.

[0012] In this application, in step 2, the high temperature and pressure during the hydrothermal reaction disrupt the stable Al-O-Si lattice bonds in kaolinite and illite, weakening the structural stability of the mineral phases. Simultaneously, it increases the molecular kinetic energy within the system, and the OH groups in the alkaline environment... - It precisely attacks the broken Al-O-Si bonds, combining with the aluminosilicate to form soluble aluminosilicates, transforming the aluminosilicate into a soluble active state. Simultaneously, the disruption of the crystal structure allows Fe within the aluminosilicate mineral to... 3+ Ti 4+ The impurity ions are fully exposed and released into the system, creating conditions for subsequent deimpurification. Simultaneously, phosphoric acid preferentially reacts with Al. 3+ A stable AlPO4 intermediate phase is formed through chemical bonding, preventing the loss of effective components, while the heterogeneous ions (Y) introduced by the soluble salt... 3+ / Ce 3+ / Fe 3+ ) through unsaturated O at the defect 2- Stable coordination bonds are formed, which are then embedded in lattice defects to form catalytic sites. These catalytic sites can reduce the reaction energy barrier by adsorbing active SiO2 and Al2O3 components in the subsequent silicon-aluminum recombination process, thereby reducing the activation energy required for the directional recombination of silicon and aluminum.

[0013] In step 3, during the ammonium salt roasting process, the acidic atmosphere generated by the decomposition of ammonium salt can selectively dissolve impurities such as Fe2O3 and TiO2 to form soluble salts, while the AlPO4 mesophase remains stable, achieving efficient separation of impurities and effective components.

[0014] In step 4, the addition of the composite mineralizer accelerates the diffusion of silicon and aluminum ions by generating a low-melting-point liquid phase, while simultaneously forming stable impurity particles with residual trace impurities for subsequent separation. In step 5, the silicon-aluminum sol coating constructs a core-shell structure, which not only completes the precise silicon-aluminum stoichiometry required for mullite formation but also forms a physical barrier to prevent the intrusion of external impurities. Finally, in step 6, during the three-stage calcination in the suspension state, the drying section removes adsorbed water and crystal water from the material, the decomposition section removes volatiles and converts the AlPO4 intermediate phase into active Al2O3, and in the crystallization section, under the synergistic catalysis of the mineralizer and a uniform temperature field, active SiO2 and Al2O3 directionally generate the mullite crystal phase. The strong heat and mass transfer characteristics of the suspension state ensure uniform material temperature and avoid the formation of impurity phases such as corundum and cristobalite, ultimately producing high-purity mullite.

[0015] Preferably, in step 1, the grinding time is 30-60 minutes; and the sieve mesh size is 200-400 mesh.

[0016] Preferably, in step 2, the soluble salt includes any one of Y(NO3)3, Ce(NO3)3, and Fe(NO3)3; the mass ratio of the coal gangue powder, deionized water, soluble salt, and phosphoric acid is 1:(5-10):(0.01-0.05):(0.005-0.02); and the mass concentration of the sodium hydroxide solution is 20wt%-30wt%.

[0017] Preferably, in step 2, the pressure of the hydrothermal reactor is 3-8 MPa; the heating temperature is 160-220℃; the stirring speed for the first stirring is 200-300 r / min for 15-30 min; the stirring speed for the second stirring is 300-500 r / min for 20-40 min; the stirring speed for the third stirring is 100-200 r / min for 4-8 h; the neutral pH is 6.5-7.5; the drying temperature is 100-110℃ for 4-6 h; and the pH of the reaction system is adjusted to 10-12 by adding sodium hydroxide solution.

[0018] Preferably, in step 3, the ammonium salt includes any one of (NH4)2SO4, NH4Cl and NH4NO3; the mass ratio of the dried precursor to the ammonium salt is 1:(0.1-0.3).

[0019] Preferably, in step 3, the heating rate of the calcination is 5-10℃ / min, the calcination temperature is 400-500℃, and the calcination time is 1-3h.

[0020] Preferably, the solid content of the slurry in step 4 is 20%-40%; the composite mineralizer is composed of AlF3, B2O3 and MgO in a mass ratio of (3-5):(2-3):(1-2); the amount of the composite mineralizer added is 3%-8% of the mass of the precursor after impurity removal.

[0021] In this application, AlF3 constructs ion transport channels and solidifies impurities, B2O3 lowers the liquid phase formation temperature and promotes sintering, and MgO directionally guides mullite crystallization and regulates liquid phase viscosity. This can effectively solve the problems of slow diffusion of silicon and aluminum ions and easy formation of impurity phases, and improve the purity and structural compactness of mullite.

[0022] Preferably, in step 4, the stirring speed is 300-500 r / min and the stirring time is 20-40 min.

[0023] Preferably, in step 5, the volume ratio of the silica-alumina sol to the slurry is 1:(2-5); the mass ratio of SiO2 to Al2O3 in the silica-alumina sol is (2.5-3.5):2; the stirring temperature is 30-50℃ and the stirring time is 1-3h; the settling and aging time is 4-6h.

[0024] Preferably, in step 6, the airflow is air or oxygen, the airflow speed is 1-3 m / s, the temperature of the drying stage is 200-300℃ and the time is 10-15 min, the temperature of the decomposition stage is 600-800℃ and the time is 10-20 min, and the temperature of the crystallization stage is 1350-1450℃ and the time is 10-25 min.

[0025] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0026] This application provides a method for preparing high-purity mullite using a coal gangue pretreatment-calcination coupling process. In this method, the specific surface area of ​​the coal gangue increases after crushing and grinding, providing sufficient active sites for subsequent chemical modification. The highly active coal gangue undergoes lattice structure disruption in an alkaline, high-temperature, and high-pressure environment, converting the silicon-aluminum components into soluble active states. Fe is then encapsulated within the silicon-aluminum lattice. 3+ Ti 4+ Impurity ions are released. During the hydrothermal reaction, the alkaline, high-temperature, and high-pressure environment disrupts the crystal structure of inert silica-alumina minerals such as kaolinite and illite within the coal gangue, transforming the silica-alumina components into a soluble, active state and releasing the encapsulated Fe. 3+ Ti 4+ Impurity ions, and phosphoric acid and Al 3+A stable AlPO4 mesophase is formed, preventing the loss of effective components. The heterogeneous ions introduced by the soluble salt embed into lattice defects, forming catalytic sites and reducing the activation energy for silicon-aluminum recombination. During ammonium salt roasting, the acidic atmosphere generated by ammonium salt decomposition selectively dissolves impurities such as Fe2O3 and TiO2 to form soluble salts, while the AlPO4 mesophase remains stable, achieving efficient separation of impurities from effective components. After impurity removal, the precursor generates a low-melting-point liquid phase through a composite mineralizer, accelerating the diffusion of silicon-aluminum ions. Simultaneously, it forms stable impurity particles with residual trace impurities, facilitating subsequent separation. The silica-alumina sol coating not only completes the precise silica-alumina ratio required for mullite formation but also forms a physical barrier to prevent the intrusion of external impurities. In the final three-stage calcination in suspension, the drying stage removes adsorbed water and crystal water from the material, the decomposition stage removes volatiles and transforms the AlPO4 mesophase into active Al2O3, and the crystallization stage, under the synergistic catalysis of a uniform temperature field and mineralizer, directs the formation of mullite crystal phase from active SiO2 and Al2O3. The strong heat and mass transfer characteristics of the suspension state ensure uniform material temperature and avoid the formation of impurity phases such as corundum and cristobalite, ultimately producing high-purity mullite. Attached Figure Description

[0027] Figure 1 This is a schematic diagram of a process for preparing high-purity mullite by coupled pretreatment and calcination of coal gangue. Detailed Implementation

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

[0029] In this application, the terminology used is for the purpose of describing particular embodiments only and is not intended to be limiting of this application.

[0030] The singular forms “for,” “or,” “a,” “any,” and “the” used in this application are intended to include the plural forms unless the context clearly indicates otherwise.

[0031] Furthermore, the terms "first" and "second" appearing in this application are for descriptive purposes only and should not be construed as indicating or implying relative importance.

[0032] The following describes in detail, with reference to different embodiments, a method for preparing high-purity mullite by coupled pretreatment and calcination of coal gangue provided in this application.

[0033] Example 1

[0034] like Figure 1As shown, a method for preparing high-purity mullite by coupled pretreatment and calcination of coal gangue includes the following steps:

[0035] Step 1. Take coal gangue as raw material, crush and grind it for 30 minutes, and then pass it through a 200-mesh sieve to obtain coal gangue powder;

[0036] Step 2. Add the coal gangue powder to a hydrothermal reactor, add deionized water, and stir for 15 minutes at 200 r / min until completely dissolved. Then add a 20 wt% sodium hydroxide solution to make the pH of the reaction system 10. Add Y(NO3)3 and phosphoric acid, wherein the mass ratio of coal gangue powder, deionized water, Y(NO3)3 and phosphoric acid is 1:5:0.01:0.005. Place the mixture in a sealed hydrothermal reactor and stir for 20 minutes at 3 MPa pressure and 300 r / min. Then raise the temperature to 160℃ and stir for 4 hours at 100 r / min. After the reaction is completed, filter the mixture and wash it with deionized water until the filtrate is neutral at pH 6.5. Then dry it at 100℃ for 4 hours to obtain the dried precursor.

[0037] Step 3. Mix the dried precursor with (NH4)2SO4 at a mass ratio of 1:0.1, place the mixture in a calcining furnace and calcine it at 400°C for 1 hour in air atmosphere at a heating rate of 5°C / min to obtain the purified precursor.

[0038] Step 4. Add the purified precursor to deionized water and stir at 300 r / min for 20 min to obtain a slurry with a solid content of 20%. Then add a composite mineralizer to the slurry and continue stirring at 300 r / min for 20 min to obtain a slurry containing the composite mineralizer. The composite mineralizer is composed of AlF3, B2O3 and MgO in a mass ratio of 3:2:1, and the amount of composite mineralizer added is 3% of the mass of the purified precursor.

[0039] Step 5. Slowly add the slurry containing the composite mineralizer to the silica-alumina sol, stir and react at 30°C for 1 hour, then let it stand and age for 4 hours, and filter to obtain the coated precursor; wherein the volume ratio of silica-alumina sol to slurry is 1:2, and the mass ratio of SiO2 to A2O3 in silica-alumina sol is 2.5:2.

[0040] Step 6. The coated precursor is fed into a suspension calcination furnace, and a high-temperature gas flow is introduced to suspend the coated precursor. It then passes through a drying stage, a decomposition stage, and a crystallization stage in sequence. The high-temperature gas flow is air or oxygen, and the gas flow velocity is 1 m / s. The temperature of the drying stage is 200℃ and the time is 10 min. The temperature of the decomposition stage is 600℃ and the time is 10 min. The temperature of the crystallization stage is 1350℃ and the time is 10 min. After calcination, the precursor is cooled and collected to obtain high-purity mullite.

[0041] Example 2

[0042] like Figure 1 As shown, a method for preparing high-purity mullite by coupled pretreatment and calcination of coal gangue includes the following steps:

[0043] Step 1. Take coal gangue as raw material, crush and grind it for 45 minutes, and then pass it through a 300-mesh sieve to obtain coal gangue powder;

[0044] Step 2. Add the coal gangue powder to a hydrothermal reactor, add deionized water, and stir for 22 minutes at 250 r / min until completely dissolved. Then add a 25 wt% sodium hydroxide solution to make the pH of the reaction system 11. Add Ce(NO3)3 and phosphoric acid, wherein the mass ratio of coal gangue powder, deionized water, Ce(NO3)3 and phosphoric acid is 1:8:0.03:0.01. Place the mixture in a sealed hydrothermal reactor, and stir for 30 minutes at 5 MPa pressure and 400 r / min. Then raise the temperature to 180℃ and stir for 6 hours at 150 r / min. After the reaction is completed, filter the mixture and wash it with deionized water until the filtrate is neutral at pH 7. Then dry it at 105℃ for 5 hours to obtain the dried precursor.

[0045] Step 3. Mix the dried precursor with ammonium salt NH4Cl at a mass ratio of 1:0.2, place it in a calcining furnace and calcine it at 450°C for 2 hours under air atmosphere with a heating rate of 8°C / min to obtain the purified precursor.

[0046] Step 4. Add the purified precursor to deionized water and stir at 400 r / min for 30 min to obtain a slurry with a solid content of 30%. Then add a composite mineralizer to the slurry and continue stirring at 400 r / min for 30 min to obtain a slurry containing the composite mineralizer. The composite mineralizer is composed of AlF3, B2O3 and MgO in a mass ratio of 4:2.5:1.5, and the amount of composite mineralizer added is 6% of the mass of the purified precursor.

[0047] Step 5. The slurry containing the composite mineralizer is slowly added to the silica-alumina sol, stirred and reacted at 40°C for 2 hours, then allowed to stand and age for 5 hours, and filtered to obtain the coated precursor; wherein the volume ratio of silica-alumina sol to slurry is 1:4, and the mass ratio of SiO2 to Al2O3 in silica-alumina sol is 3:2.

[0048] Step 6. The coated precursor is fed into a suspension calcination furnace, and an airflow is introduced to suspend the coated precursor. It then passes through a drying stage, a decomposition stage, and a crystallization stage in sequence. The high-temperature airflow is air, and the airflow velocity is 1-3 m / s. The temperature of the drying stage is 250℃ and the time is 13 min. The temperature of the decomposition stage is 700℃ and the time is 15 min. The temperature of the crystallization stage is 1400℃ and the time is 18 min. After calcination, the precursor is cooled and collected to obtain high-purity mullite.

[0049] Example 3

[0050] like Figure 1 As shown, a method for preparing high-purity mullite by coupled pretreatment and calcination of coal gangue includes the following steps:

[0051] Step 1. Take coal gangue as raw material, crush and grind it for 60 minutes, and then pass it through a 400-mesh sieve to obtain coal gangue powder;

[0052] Step 2. Add the coal gangue powder to a hydrothermal reactor, add deionized water, and stir for 30 min at 300 r / min until completely dissolved. Then add a 30 wt% sodium hydroxide solution to make the pH of the reaction system 12. Add Fe(NO3)3 and phosphoric acid, wherein the mass ratio of coal gangue powder, deionized water, Fe(NO3)3 and phosphoric acid is 1:10:0.05:0.02. Place the mixture in a sealed hydrothermal reactor and stir for 40 min at 8 MPa pressure and 500 r / min. Then raise the temperature to 220℃ and stir for 8 h at 200 r / min. After the reaction is completed, filter the mixture and wash it with deionized water until the filtrate is neutral at pH 7.5. Then dry it at 110℃ for 6 h to obtain the dried precursor.

[0053] Step 3. Mix the dried precursor with NH4NO3 at a mass ratio of 1:0.3, place it in a calcining furnace and calcine it at 500°C for 3 hours in air atmosphere at a heating rate of 10°C / min to obtain the purified precursor.

[0054] Step 4. Add the purified precursor to deionized water and stir at 500 r / min for 40 min to obtain a slurry with a solid content of 40%. Then add a composite mineralizer to the slurry and continue stirring at 500 r / min for 40 min to obtain a slurry containing the composite mineralizer. The composite mineralizer is composed of AlF3, B2O3 and MgO in a mass ratio of 5:3:2, and the amount of composite mineralizer added is 8% of the mass of the purified precursor.

[0055] Step 5. The slurry containing the composite mineralizer is slowly added to the silica-alumina sol, stirred and reacted at 50°C for 3 hours, then allowed to stand and age for 6 hours, and filtered to obtain the coated precursor; wherein the volume ratio of silica-alumina sol to slurry is 1:5, and the mass ratio of SiO2 to Al2O3 in silica-alumina sol is 3.5:2.

[0056] Step 6. The coated precursor is fed into a suspension calcination furnace, and a high-temperature gas flow is introduced to suspend the coated precursor. It then passes through a drying stage, a decomposition stage, and a crystallization stage in sequence. The high-temperature gas flow is oxygen, and the gas flow rate is 3 m / s. The temperature of the drying stage is 300℃ and the time is 15 min. The temperature of the decomposition stage is 800℃ and the time is 20 min. The temperature of the crystallization stage is 1450℃ and the time is 25 min. After calcination, the precursor is cooled and collected to obtain high-purity mullite.

[0057] Comparative Example 1

[0058] A method for preparing high-purity mullite by coal gangue pretreatment-calcination coupling, which differs from Example 3 in that step 5, silica-alumina sol coating, is missing.

[0059] Comparative Example 2

[0060] A method for preparing high-purity mullite by coal gangue pretreatment-calcination coupling, which differs from Example 3 in that soluble salts are not added in step 2.

[0061] Comparative Example 3

[0062] A method for preparing high-purity mullite by coal gangue pretreatment-calcination coupling, which differs from Example 3 in that phosphoric acid is not added in step 2.

[0063] Comparative Example 4

[0064] A method for preparing high-purity mullite by coal gangue pretreatment-calcination coupling is different from Example 3 in that the order of steps 2 and 3 is changed: step 3 ammonium salt mixing and calcination is performed first, followed by the hydrothermal reaction in step 2.

[0065] Comparative Example 5

[0066] A method for preparing high-purity mullite by coal gangue pretreatment-calcination coupling, which differs from Example 3 in that step 6 uses a traditional sintering process instead of suspension calcination.

[0067] Performance tests are as follows:

[0068] 1. Purity detection: The acid dissolution weight loss method was used for determination. Specifically, the samples of Examples 1-3 and Comparative Examples 1-5 were accurately weighed and placed in 98% sulfuric acid solution at 100°C for 2 hours. After the reaction, the samples were filtered, washed, and dried to constant weight. The weight loss rate was obtained by calculating the mass difference of each group of samples before and after the reaction.

[0069] 2. Detection of total harmful impurities: Inductively coupled plasma optical emission spectrometry (ICP-OES) was used for determination. Specifically, the samples of Examples 1-3 and Comparative Examples 1-5 were acid digested and then introduced into the ICP-OES instrument. The total content of Fe and Ti elements in the samples was quantitatively analyzed by comparing with the standard curve.

[0070] 3. Microhardness test: Specifically, the samples of Examples 1-3 and Comparative Examples 1-5 were prepared into flat and polished specimens. Under the test conditions of 100g loading force and holding pressure for 15s, an indentation was formed on the surface of the specimen by the indenter of the hardness tester. The microhardness value was calculated based on the diagonal length of the indentation.

[0071] Table 1. Performance test data of high-purity mullite prepared in Examples 1-3 and Comparative Examples 1-5

[0072] Group Acid dissolution weight loss rate (%) Total amount of harmful impurities (%) Microhardness (HV) Example 1 1.8 0.75 1520 Example 2 1.5 0.60 1580 Example 3 1.2 0.45 1650 Comparative Example 1 8.5 1.20 1180 Comparative Example 2 3.8 0.95 1280 Comparative Example 3 10.2 1.85 1050 Comparative Example 4 7.8 1.60 1120 Comparative Example 5 4.5 0.90 1230

[0073] According to the data in Table 1, the performance differences between Examples 1-3 and Comparative Examples 1-5 are mainly due to the synergistic effect of five core technical steps in the preparation process: precise coating of silica-alumina sol, catalytic crystallization of soluble salts, directional stabilization of aluminum source with phosphate, selective deimpurification by ammonium salt roasting, and densification by suspension calcination.

[0074] Regarding the acid dissolution weight loss rate, the acid dissolution weight loss rate of Examples 1-3 was as low as 1.2%-1.8%, significantly better than the comparative example of 3.8%-10.2%, which is attributed to the coupling effect of five key processes: the silica-alumina sol was precisely balanced according to the theoretical mullite ratio of 2.5-3.5:2 to avoid the formation of easily acid-soluble impurity phases such as corundum and cristobalite due to component imbalance; the heterogeneous ions introduced by the soluble salt were embedded in the lattice defects to form catalytic sites, reducing the activation energy of silica-alumina recombination and promoting the directional growth of pure phase mullite; phosphoric acid preferentially combined with Al³⁺ to form a stable AlPO4 intermediate phase, locking in the effective aluminum source to prevent loss and reducing the combination of impurities with the aluminum source to form impurity phases; ammonium salt roasting selectively dissolved impurities such as Fe and Ti through an acidic atmosphere; and the uniform temperature field and strong heat and mass transfer characteristics of suspension calcination ensured sufficient mullite crystallization and minimized the amount of impurity phase formation. Comparative Example 1, lacking silica-alumina sol coating, had a silica-alumina ratio deviating from the theoretical value, resulting in a large amount of impurity phase formation and an acid dissolution weight loss rate of 8.5%. Comparative Example 2, without the addition of soluble salts, lacked catalytic sites, leading to insufficient crystallization, increased impurity phase content, and a weight loss rate of 3.8%. Comparative Example 3, without the addition of phosphoric acid, suffered from loss of effective aluminum components and ineffective separation of impurities, resulting in the highest impurity phase content and a weight loss rate of 10.2%. Comparative Example 4 reversed the order of hydrothermal and ammonium salt roasting; initial roasting failed to release impurities within the coal gangue lattice, and subsequent hydrothermal activation resulted in incomplete impurity removal, leaving a large amount of impurity phase residue and a weight loss rate of 7.8%. Comparative Example 5, using a traditional sintering process, suffered from uneven heat and mass transfer, leading to incomplete local crystallization and incomplete transformation of impurity phases, resulting in a weight loss rate of 4.5%, all significantly higher than the examples.

[0075] Regarding the total amount of harmful impurities, the total impurities in Examples 1-3 were controlled at 0.45%-0.75%, far lower than the comparative example of 0.90%-1.85%. The key lies in the dual synergistic effect of "ammonium salt roasting for impurity removal and silica-alumina sol coating for barrier": the hydrothermal reaction destroys the coal gangue lattice, releasing the encapsulated Fe. 3+ Ti 4+ Impurities are selectively dissolved and removed by the acidic atmosphere generated during subsequent ammonium salt roasting. The dense core-shell structure formed by the silica-alumina sol coating effectively blocks the intrusion of external impurities and avoids secondary contamination of the precursor after impurity removal. In Comparative Example 3, no phosphoric acid was added, and Al³⁺ was not oriented and stabilized. Mixing with Fe and Ti impurities led to a significant decrease in impurity removal efficiency, with the total impurity amount reaching 1.85%. In Comparative Example 4, the process order was reversed. During the initial roasting, the coal gangue lattice was not activated, and impurities could not be released. The subsequent hydrothermal reaction resulted in incomplete impurity removal, with a total impurity amount of 1.60%. In Comparative Example 1, the absence of silica-alumina sol coating made the precursor susceptible to contamination by external impurities after impurity removal. Furthermore, the imbalance in the silica-alumina ratio led to the adsorption of impurities by the impurity phase, resulting in a total impurity amount of 1.20%. In Comparative Example 5, the traditional sintering process lacked the strong separation effect of the suspended state, and trace impurities could not be removed. The total impurity amount was 0.90%, all of which were higher than those in the examples.

[0076] Regarding microhardness, the microhardness of Examples 1-3 reached 1520-1650 HV, significantly higher than that of the comparative example (1050-1280 HV). This is mainly due to the dual support of high-purity pure phase and densified structure: the synergistic effect of five core elements ensures that the purity of the mullite crystal phase is ≥98%, with few and uniformly distributed impurities, reducing structural defects; suspension calcination promotes tight bonding between particles, and the low-melting-point liquid phase generated by the composite mineralizer accelerates ion diffusion and improves the material density. Meanwhile, high-purity mullite itself has excellent mechanical properties, and densification further enhances hardness. Comparative Example 3, lacking phosphoric acid, suffered aluminum source loss and had the highest impurity phase content, resulting in the lowest purity, the most porous structure, and a hardness of only 1050 HV. Comparative Example 1, lacking silica-alumina sol coating, suffered from an imbalance in the silica-alumina ratio, leading to more impurities and higher structural porosity, resulting in a hardness of 1180 HV. Comparative Example 4, with its reversed process sequence, had more residual impurities and insufficient crystallization, resulting in poor structural integrity and a hardness of 1120 HV. Comparative Example 5, with its uneven heat transfer due to traditional sintering, resulted in insufficient density and numerous internal pores, resulting in a hardness of 1230 HV. Comparative Example 2, lacking soluble salt, suffered from insufficient catalysis, leading to incomplete crystallization, small grains, and loose bonding, resulting in a hardness of 1280 HV, all significantly lower than the examples.

[0077] In summary, the superior performance of Examples 1-3 stems from the synergistic effect of the entire process, with each step supporting and indispensable to the others: the silica-alumina sol ratio provides the component basis for pure phase generation, phosphoric acid stabilization ensures the retention of aluminum source, soluble salt catalysis accelerates crystallization and lowers the energy barrier, ammonium salt deimpurification removes impurities to clear obstacles for pure phase generation, and suspension calcination provides sufficient conditions for densification and crystallization. In contrast, the comparative examples, due to the absence of any core step or changes in the key process sequence, resulted in component imbalance, increased impurity phases, impurity residues, and insufficient densification, ultimately leading to significantly inferior performance compared to the examples. This fully demonstrates the irreplaceable nature and synergistic effect of the core technical links in this solution.

[0078] The above results demonstrate and describe the basic principles and main features of this application, as well as its advantages.

[0079] Those skilled in the art should understand that this application is not limited to the above embodiments. The embodiments and descriptions in the specification are merely illustrative of the principles of this application. Various changes and modifications can be made to this application without departing from the spirit and scope thereof, and all such changes and modifications fall within the scope of this application as claimed. The scope of protection of this application is defined by the equivalents of the appended claims.

Claims

1. A method for preparing high-purity mullite using a coal gangue pretreatment-calcination coupling process, characterized in that, Includes the following steps: Step 1. Take coal gangue as raw material, crush it, grind it and then sieve it to obtain coal gangue powder; Step 2. Add the coal gangue powder to a hydrothermal reactor, add deionized water and stir until completely dissolved. Then add sodium hydroxide solution to adjust the pH of the reaction system, add soluble salt and phosphoric acid, stir a second time, seal the hydrothermal reactor and heat up for a third stirring reaction. After the reaction is completed, filter and wash with deionized water until the filtrate is neutral. Then dry to obtain the dried precursor. Step 3. Mix the dried precursor with the ammonium salt evenly, place it in a roasting furnace and roast it under air atmosphere to obtain the purified precursor; Step 4. Add the purified precursor to deionized water, stir and disperse to obtain a slurry, then add the composite mineralizer to the slurry and continue stirring to obtain a slurry containing the composite mineralizer; Step 5. Add the slurry containing the composite mineralizer to the silica-alumina sol, stir and react, then let it stand and age, and filter to obtain the coated precursor; Step 6. The coated precursor is fed into a suspension calcination furnace, and airflow is introduced to suspend the coated precursor. It then passes through a drying stage, a decomposition stage, and a crystallization stage in sequence. After calcination, it is cooled and collected to obtain high-purity mullite.

2. The method for preparing high-purity mullite by coupled pretreatment and calcination of coal gangue according to claim 1, characterized in that, In step 1, the grinding time is 30-60 minutes; the sieve mesh size is 200-400 mesh.

3. The method for preparing high-purity mullite by coupled pretreatment and calcination of coal gangue according to claim 1, characterized in that, In step 2, the soluble salt includes any one of Y(NO3)3, Ce(NO3)3, and Fe(NO3)3; the mass ratio of the coal gangue powder, deionized water, soluble salt, and phosphoric acid is 1:(5-10):(0.01-0.05):(0.005-0.02); and the mass concentration of the sodium hydroxide solution is 20wt%-30wt%.

4. The method for preparing high-purity mullite by coupled pretreatment and calcination of coal gangue according to claim 1, characterized in that, In step 2, the pressure of the hydrothermal reactor is 3-8 MPa; the heating temperature is 160-220℃; the stirring speed for the first time is 200-300 r / min and the time is 15-30 min; the stirring speed for the second time is 300-500 r / min and the time is 20-40 min; the stirring speed for the third time is 100-200 r / min and the time is 4-8 h; the neutral pH is 6.5-7.5; the drying temperature is 100-110℃ and the time is 4-6 h; and the pH of the reaction system is adjusted to 10-12 by adding sodium hydroxide solution.

5. The method for preparing high-purity mullite by coupled pretreatment and calcination of coal gangue according to claim 1, characterized in that, In step 3, the ammonium salt includes any one of (NH4)2SO4, NH4Cl and NH4NO3; the mass ratio of the dried precursor to the ammonium salt is 1:(0.1-0.3).

6. The method for preparing high-purity mullite by coupled pretreatment and calcination of coal gangue according to claim 1, characterized in that, In step 3, the heating rate of the calcination is 5-10℃ / min, the calcination temperature is 400-500℃, and the calcination time is 1-3h.

7. The method for preparing high-purity mullite by coupled pretreatment and calcination of coal gangue according to claim 1, characterized in that, In step 4, the solid content of the slurry is 20%-40%; the composite mineralizer is composed of AlF3, B2O3 and MgO in a mass ratio of (3-5):(2-3):(1-2); the amount of the composite mineralizer added is 3%-8% of the mass of the precursor after impurity removal.

8. The method for preparing high-purity mullite by coupled pretreatment and calcination of coal gangue according to claim 1, characterized in that, In step 4, the stirring speed is 300-500 r / min and the stirring time is 20-40 min.

9. The method for preparing high-purity mullite by coupled pretreatment and calcination of coal gangue according to claim 1, characterized in that, In step 5, the volume ratio of the silica-alumina sol to the slurry is 1:(2-5); the mass ratio of SiO2 to Al2O3 in the silica-alumina sol is (2.5-3.5):2; the stirring temperature is 30-50℃ and the stirring time is 1-3h; the settling and aging time is 4-6h.

10. The method for preparing high-purity mullite by coupled pretreatment and calcination of coal gangue according to claim 1, characterized in that, In step 6, the airflow is air or oxygen, the airflow speed is 1-3 m / s, the temperature of the drying stage is 200-300℃ and the time is 10-15 min, the temperature of the decomposition stage is 600-800℃ and the time is 10-20 min, and the temperature of the crystallization stage is 1350-1450℃ and the time is 10-25 min.

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