Method for preparing high-purity mullite by coupling coal gangue pretreatment and calcination

Abstract

The application 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, which takes coal gangue as raw material, and high-purity mullite is prepared through the steps of crushing, grinding, screening, adding soluble salt and phosphoric acid in alkaline hydrothermal reaction, roasting and impurity removal of ammonium salt, modification of composite mineralizer-silicon aluminum sol coating, three-stage calcination in a suspended state and the like. The method solves the technical problems of insufficient activation of coal gangue in the traditional process, difficulty in deep removal of impurities, low purity of mullite and low resource utilization efficiency of coal gangue, and realizes efficient resource conversion of coal gangue. Through the synergistic effect of each process link, 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 related fields of high-temperature structural materials and the like.

Description

TECHNICAL FIELD

[0001] The application relates to the technical field of coal gangue treatment, and particularly relates to a method for preparing high-purity mullite through coal gangue pretreatment-calcination coupling. BACKGROUND

[0002] When coal gangue is used to prepare high-purity mullite, the existing process faces key technical bottlenecks: on the one hand, the coal gangue contains harmful impurities such as Fe2O3 and TiO2, which can seriously affect the crystal phase purity and performance of mullite, and the existing single impurity removal process (such as simple acid leaching and alkali leaching) cannot achieve deep removal of impurities; on the other hand, the silicon-aluminum components in the coal gangue exist in the form of inert mineral phases such as kaolinite and illite, and the existing activation technology (such as conventional calcination) has low efficiency and cannot promote the full conversion of inert silicon-aluminum phases into active components, resulting in uneven development of mullite crystal phases and insufficient purity, which cannot meet the use requirements of high-end fields. At the same time, the traditional high-temperature solid-phase sintering process needs to be carried out at a temperature above 1600 DEG C, which not only has high energy consumption but also further increases the product cost pressure, seriously limiting the high-value utilization of coal gangue and the low-cost industrialization development of high-purity mullite. Therefore, it is urgent to develop a high-purity mullite preparation technology for deep removal of harmful impurities in coal gangue to get rid of the current industry development difficulties. SUMMARY

[0003] The application provides a method for preparing high-purity mullite through coal gangue pretreatment-calcination coupling, which realizes the efficient directional conversion of coal gangue into high-purity mullite through continuous suspension calcination process with staged temperature control in a suspension state calcination furnace after the pretreatment of coal gangue through crushing and grinding, and improves the product purity and crystallization quality, which has the advantages of resource recycling and energy saving and environmental protection.

[0004] In order to achieve the above purpose, one technical scheme adopted by the application is:

[0005] The application provides a method for preparing high-purity mullite through coal gangue pretreatment-calcination coupling, which realizes the efficient directional conversion of coal gangue into high-purity mullite through continuous suspension calcination process with staged temperature control in a suspension state calcination furnace after the pretreatment of coal gangue through crushing and grinding, and improves the product purity and crystallization quality, which has the advantages of resource recycling and energy saving and environmental protection.

[0006] Step 1. Taking coal gangue as a raw material, crushing, grinding and sieving to obtain coal gangue powder;

[0007] Step 2. Adding the coal gangue powder into a hydrothermal reactor, adding deionized water to completely dissolve after the first stirring, adding sodium hydroxide solution to adjust the pH value of the reaction system, adding soluble salt and phosphoric acid, and carrying out the third stirring reaction after the hydrothermal reactor is sealed and heated, then filtering, washing with deionized water until the filtrate is neutral, and then drying to obtain a dry precursor;

[0008] Step 3. The dry precursor is mixed with ammonium salt uniformly and placed in a calcination furnace to be calcined under air atmosphere to obtain a decontaminated precursor;

[0009] Step 4. The decontaminated precursor is added into deionized water to be dispersed by stirring to obtain a slurry, and then a composite mineralizer is added into the slurry to continue stirring to obtain a slurry containing the composite mineralizer;

[0010] Step 5. The slurry containing the composite mineralizer is slowly added into a silica-alumina sol to be reacted by stirring, and then aged by standing to obtain a coated precursor after filtration;

[0011] Step 6. The coated precursor is sent into a suspension calcination furnace, and a gas flow is introduced to make the coated precursor in a suspended state, and the coated precursor is sequentially subjected to a drying stage, a decomposition stage and a crystallization stage, and then cooled and collected after calcination to obtain high-purity mullite.

[0012] In the present application, in Step 2, the high temperature and high pressure in the hydrothermal reaction can destroy the stable Al-O-Si lattice bond in the highlandite and illite, weaken the structural stability of the mineral phase, and on the other hand, the molecular kinetic energy in the system is improved, and the OH⁻ in the alkaline environment precisely attacks the broken Al-O-Si bond to combine with the silicon aluminum component to form a soluble aluminosilicate, so that the silicon aluminum component is converted into a soluble active state, and at the same time, the lattice structure is destroyed to completely expose and release the impurity ions of Fe 3+ , Ti 4+ inside the silicon aluminum mineral to the system to create conditions for subsequent decontamination. At the same time, phosphoric acid preferentially forms a stable AlPO4 intermediate phase with Al 3+ through chemical bonding, avoiding the loss of effective components, and the heteroion (Y³⁺ / Ce³⁺ / Fe³⁺) introduced by the soluble salt is embedded into the lattice defects to form a catalytic site by forming a stable coordination bond with the unsaturated O²⁻ at the defect, and the catalytic site can adsorb the active SiO2 and Al2O3 components in the subsequent silicon aluminum recombination process to reduce the reaction energy barrier, thereby reducing the activation energy required for the directional recombination of silicon and aluminum.

[0013] In Step 3, during the ammonium salt calcination process, the acidic atmosphere produced by the decomposition of the ammonium salt can selectively dissolve impurities such as Fe2O3 and TiO2 to form soluble salts, while the AlPO4 intermediate phase remains stable, realizing efficient separation of impurities and effective components;

[0014] In the step 4, after the addition of the composite mineralizer, the low-melting liquid phase is generated to accelerate the diffusion of silicon and aluminum ions, and meanwhile, the stable impurity phase particles are formed with the residual trace impurities, which facilitates the subsequent separation; in the step 5, the silicon and aluminum sol coating constructs the core-shell structure, which not only completes the precise silicon and aluminum metering ratio required for the formation of mullite, but also forms a physical barrier to block the intrusion of external impurities; finally, in the step 6, in the suspension state three-stage calcination, the adsorbed water and the crystal water in the material are removed in the drying section, the volatile components are removed and the AlPO4 intermediate phase is converted into active Al2O3 in the decomposition section, and under the synergistic catalysis of the uniform temperature field and the mineralizer, the active SiO2 and Al2O3 are directionally generated into the mullite crystal phase in the crystallization section; the strong heat and mass transfer characteristics of the suspension state ensure the uniformity of the material temperature, avoiding the generation of corundum, cristobalite and other impurity phases, and finally generating high-purity mullite.

[0015] Preferably, in the step 1, the grinding time is 30-60 min; and the mesh number of the sieving is 200-400.

[0016] Preferably, in the 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 the step 2, the pressure of the hydrothermal reaction kettle is 3-8 MPa; the temperature of the temperature rise is 160-220℃; the rotating speed of the first stirring is 200-300 r / min, and the time is 15-30 min; the rotating speed of the second stirring is 300-500 r / min, and the time is 20-40 min; the rotating speed of the third stirring 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 value of the reaction system is adjusted to 10-12 by adding the sodium hydroxide solution.

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

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

[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); and the addition amount of the composite mineralizer is 3%-8% of the mass of the precursor after impurity removal.

[0021] In the application, AlF3 constructs ion transmission channels and solidifies impurities, B2O3 reduces the liquid phase generation temperature to promote sintering, and MgO directionally guides mullite crystallization and regulates the liquid phase viscosity, which can efficiently solve the problems of slow diffusion of silicon and aluminum ions and easy generation of impurities, 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 silicon-aluminum sol to the slurry is 1:(2-5), the mass ratio of SiO2 to Al2O3 in the silicon-aluminum sol is (2.5-3.5):2, the stirring temperature is 30-50 DEG C, the stirring time is 1-3 h, and the standing time is 4-6 h.

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

[0025] Compared with the prior art, the application has the following beneficial effects:

[0026] The application provides a method for preparing high-purity mullite by coupling coal gangue pretreatment and calcination. In the application, the specific surface area of the coal gangue is increased after crushing and grinding, thereby providing sufficient active sites for subsequent chemical modification; the crystal lattice structure of the high-activity coal gangue is destroyed in an alkaline high-temperature and high-pressure environment, so that the silicon and aluminum components are converted into a soluble active state, and impurity ions such as Fe 3+ , Ti 4+ and the like wrapped in the silicon and aluminum crystal lattices are released. In the hydrothermal reaction, the crystal lattice structure of inert silicon and aluminum mineral phases such as kaolinite and illite in the coal gangue is destroyed in an alkaline high-temperature and high-pressure environment, so that the silicon and aluminum components are converted into a soluble active state and the impurity ions such as Fe 3+ , Ti 4+ and the like wrapped therein are released, and at the same time, phosphoric acid and Al 3+The stable AlPO4 intermediate phase is formed to avoid the loss of effective components, the heterogeneous ions introduced by soluble salts are embedded into lattice defects to form catalytic sites, and the activation energy of silicon-aluminum recombination is reduced. During the calcination of ammonium salt, 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 intermediate phase remains stable, realizing efficient separation of impurities and effective components. After the impurities are removed, the precursor is generated by a composite mineralizer to form a low-melting-point liquid phase to accelerate the diffusion of silicon and aluminum ions, and at the same time, the stable heterogeneous phase particles are formed with the residual trace impurities to facilitate subsequent separation. Then, the silicon-aluminum sol is coated to complete the precise silicon-aluminum ratio required for the formation of mullite and form a physical barrier to block the intrusion of external impurities. Finally, in the three-stage calcination in suspension, the adsorbed water and crystallization water in the material are removed in the drying section, the volatile components are removed and the AlPO4 intermediate phase is converted into active Al2O3 in the decomposition section, and under the synergistic catalysis of the uniform temperature field and the mineralizer, active SiO2 and Al2O3 are directionally generated into mullite crystal phase. The strong heat and mass transfer characteristics of the suspension ensure uniform material temperature and avoid the generation of corundum and cristobalite, etc. Finally, high-purity mullite is generated. BRIEF DESCRIPTION OF DRAWINGS

[0027] Figure 1 A method for preparing high-purity mullite by coupling coal gangue pretreatment and calcination. DETAILED DESCRIPTION

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

[0029] In the present application, the terms used in the present application are only for the purpose of describing specific embodiments, and are not intended to limit the present application.

[0030] In the present application, the singular forms "is", "or", "a", "any" and "the" are intended to include the plural forms, unless the context clearly indicates otherwise.

[0031] In addition, in the present application, the terms "first", "second" appear only for the purpose of description, and cannot be understood as indicating or implying relative importance.

[0032] The following will specifically describe a method for preparing high-purity mullite by coupling coal gangue pretreatment and calcination provided by the present application in conjunction with different embodiments.

[0033] Embodiment 1

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

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

[0036] Step 2. Add the coal gangue powder into a hydrothermal reactor, add deionized water, and stir at a speed of 200 r / min for 15 min to completely dissolve, then add a 20wt% sodium hydroxide solution to make the pH value 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, put into a sealed hydrothermal reactor, stir at a speed of 300 r / min for 20 min under a pressure of 3 MPa, then heat to 160℃ and stir at a speed of 100 r / min for 4 h, after the reaction, filter, wash with deionized water until the filtrate is neutral with a pH of 6.5, then dry at 100℃ for 4 h to obtain a dry precursor;

[0037] Step 3. Mix the dry precursor with (NH4)2SO4 in a mass ratio of 1:0.1, and place in a calcination furnace, calcine under air atmosphere at a heating rate of 5℃ / min to 400℃ for 1 h to obtain a de-impurity precursor;

[0038] Step 4. Add the de-impurity precursor into deionized water, stir and disperse at a speed of 300 r / min for 20 min to obtain a slurry with a solid content of 20%, then add a composite mineralizer to the slurry, continue to stir at a speed of 300 r / min for 20 min to obtain a slurry containing a composite mineralizer; wherein the composite mineralizer is composed of AlF3, B2O3 and MgO in a mass ratio of 3:2:1, and the addition amount of the composite mineralizer is 3% of the mass of the de-impurity precursor;

[0039] Step 5. Slowly add silicon-aluminum sol into the slurry containing the composite mineralizer, stir and react at 30℃ for 1 h, then stand for aging for 4 h, filter to obtain a coated precursor; wherein the volume ratio of silicon-aluminum sol to slurry is 1:2, and the mass ratio of SiO2 to Al2O3 in the silicon-aluminum sol is 2.5:2;

[0040] Step 6. The coated precursor is sent into a suspension calcination furnace, and a high-temperature gas flow is introduced to make the coated precursor in a suspended state, and the coated precursor sequentially passes through a drying stage, a decomposition stage and a crystallization stage, wherein the high-temperature gas flow is air or oxygen, the gas flow speed is 1 m / s, the temperature of the drying stage is 200℃, the time of the drying stage is 10 min, the temperature of the decomposition stage is 600℃, the time of the decomposition stage is 10 min, the temperature of the crystallization stage is 1350℃, the time of the crystallization stage is 10 min, and after the calcination is completed, the coated precursor is cooled and collected to obtain high-purity mullite.

[0041] Example 2

[0042] As shown in the following figure, a method for preparing high-purity mullite by coal gangue pretreatment-calcination coupling includes the following steps: Figure 1

[0043] Step 1. Coal gangue is taken as a raw material, crushed and ground for 45 min, and then passed through a 300-mesh sieve to obtain coal gangue powder;

[0044] Step 2. The coal gangue powder is added into a hydrothermal reaction kettle, deionized water is added, and after complete dissolution by first stirring at a speed of 250 r / min for 22 min, a sodium hydroxide solution with a mass concentration of 25wt% is added to make the pH value of the reaction system be 11, Ce(NO3)3 and phosphoric acid are added, wherein the mass ratio of the coal gangue powder, the deionized water, the Ce(NO3)3 and the phosphoric acid is 1:8:0.03:0.01, the sealed hydrothermal reaction kettle is put into a high-pressure reactor, second stirring is carried out at a speed of 400 r / min for 30 min under a pressure of 5 MPa, third stirring is carried out at a speed of 150 r / min for 6 h after the temperature is raised to 180℃, and after the reaction is completed, the reaction product is filtered, washed with deionized water until the filtrate is neutral with a pH value of 7, and then dried at 105℃ for 5 h to obtain a dry precursor;

[0045] Step 3. The dry precursor is uniformly mixed with ammonium salt NH4Cl according to a mass ratio of 1:0.2, and placed in a calcination furnace for calcination, and the temperature is raised to 450℃ at a temperature raising rate of 8℃ / min under an air atmosphere, and the calcination is carried out for 2 h to obtain a decontaminated precursor;

[0046] Step 4. The decontaminated precursor is added into deionized water, and stirring and dispersion are carried out at a speed of 400 r / min for 30 min to obtain a slurry with a solid content of 30%, and then a composite mineralizer is added into the slurry, and stirring is continuously carried out at a speed of 400 r / min for 30 min to obtain a slurry containing the composite mineralizer; wherein the composite mineralizer is composed of AlF3, B2O3 and MgO according to a mass ratio of 4:2.5:1.5, and the addition amount of the composite mineralizer is 6% of the mass of the decontaminated precursor;

[0047] ​Step 5. The slurry containing the complexing agent is slowly added to the silica-alumina sol, and the reaction is stirred at 40℃ for 2h, and then aged for 5h. After filtration, the coated precursor is obtained; wherein the volume ratio of silica-alumina sol to slurry is 1:4, and the mass ratio of SiO2 to Al2O3 in the silica-alumina sol is 3:2;

[0048] Step 6. The coated precursor is fed into a suspension calcination furnace, and a gas stream is introduced to make the coated precursor in a suspended state, and sequentially passes through a drying stage, a decomposition stage and a crystallization stage. The high-temperature gas stream is air, the gas stream speed is 1-3m / s, the temperature of the drying stage is 250℃, the time is 13min, the temperature of the decomposition stage is 700℃, the time is 15min, the temperature of the crystallization stage is 1400℃, the time is 18min. After calcination, the high-purity mullite is obtained after cooling and collection.

[0049] Example 3

[0050] As shown in Figure 1 , a method for preparing high-purity mullite by coupling coal gangue pretreatment and calcination, comprising the following steps:

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

[0052] Step 2. The coal gangue powder is added to a hydrothermal reactor, deionized water is added, and after first stirring at a speed of 300r / min for 30min to completely dissolve, a sodium hydroxide solution with a mass concentration of 30wt% is added to make the pH value of the reaction system 12. Fe(NO3)3 and phosphoric acid are added, wherein the mass ratio of coal gangue powder, deionized water, Fe(NO3)3 and phosphoric acid is 1:10:0.05:0.02. Put into a sealed hydrothermal reactor, stir at a speed of 500r / min for 40min under a pressure of 8MPa, and then stir at a speed of 200r / min for 8h at a temperature of 220℃. After reaction, filter, wash with deionized water until the filtrate is neutral with a pH of 7.5, and then dry at 110℃ for 6h to obtain a dry precursor;

[0053] Step 3. The dry precursor is mixed with NH4NO3 at a mass ratio of 1:0.3, and then placed in a calcination furnace and calcined at a temperature increasing rate of 10℃ / min to 500℃ under an air atmosphere for 3h to obtain a decontaminated precursor;

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

[0055] Step 5. The slurry containing the composite mineralizer is slowly added to a silicon-aluminum sol, and stirred at 50℃ for 3 h, and then aged for 6 h, and then filtered to obtain a coated precursor; wherein the volume ratio of the silicon-aluminum sol to the slurry is 1:5, and the mass ratio of SiO2 to Al2O3 in the silicon-aluminum sol is 3.5:2.

[0056] Step 6. The coated precursor is fed into a suspension calcination furnace, and a high-temperature gas stream is introduced to make the coated precursor in a suspended state, and the coated precursor sequentially passes through a drying stage, a decomposition stage and a crystallization stage; wherein the high-temperature gas stream is oxygen, the gas stream speed is 3 m / s, the temperature of the drying stage is 300℃, the time of the drying stage is 15 min, the temperature of the decomposition stage is 800℃, the time of the decomposition stage is 20 min, the temperature of the crystallization stage is 1450℃, and the time of the crystallization stage is 25 min, and after calcination, the coated 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 is different from Example 3 in that step 5 of silicon-aluminum sol coating is omitted.

[0059] Comparative Example 2

[0060] A method for preparing high-purity mullite by coal gangue pretreatment-calcination coupling, which is different from Example 3 in that no soluble salt is added in step 2.

[0061] Comparative Example 3

[0062] A method for preparing high-purity mullite by coal gangue pretreatment-calcination coupling, which is different from Example 3 in that no phosphoric acid is added in step 2.

[0063] Comparative Example 4

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

[0065] Comparative Example 5

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

[0067] Performance tests are as follows:

[0068] 1. Purity detection: Acid dissolution weight loss method was used, specifically, the samples of Example 1-Example 3 and Comparative Example 1-Comparative Example 5 were accurately weighed, then placed in a 98% sulfuric acid solution at 100°C for 2h, after the reaction, filtered, washed, and dried to constant weight, and the weight loss rate was calculated by the mass difference before and after the reaction of each group of samples.

[0069] 2. Total amount of harmful impurities detection: Inductively coupled plasma optical emission spectrometry (ICP-OES) was used, specifically, the samples of Example 1-Example 3 and Comparative Example 1-Comparative Example 5 were treated by acid digestion, then introduced into the ICP-OES instrument, and the total content of Fe and Ti elements in the sample was quantitatively analyzed by comparison with the standard curve.

[0070] 3. Microhardness detection: Specifically, the samples of Example 1-Example 3 and Comparative Example 1-Comparative Example 5 were prepared into flat polished samples, under the test conditions of 100g loading force and 15s pressure retention, the indenter of the hardness tester formed an indentation on the sample surface, and the microhardness value was calculated according to the diagonal length of the indentation.

[0071] Table 1 Performance test data of high-purity mullite prepared by Example 1-Example 3 and Comparative Example 1-Comparative Example 5

[0072] Group Acid-soluble 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 shown in Table 1, the performance differences between Example 1-Example 3 and Comparative Example 1-Comparative Example 5 are mainly due to the synergistic effect of the five core technical links in the preparation process, namely, precise coating of silicon-aluminum sol, catalytic crystallization of soluble salt, directional stabilization of aluminum source by phosphoric acid, selective impurity removal by ammonium salt calcination, and densification by suspension calcination.

[0074] In terms of acid dissolution weight loss rate, the acid dissolution weight loss rate of Examples 1-3 is as low as 1.2%-1.8%, which is significantly better than 3.8%-10.2% of Comparative Examples 3, thanks to the coupling effect of the five links: the silicon-aluminum sol is precisely supplemented in the theoretical proportion of 2.5-3.5:2 of mullite, avoiding the generation of corundum, cristobalite and other easily acid-dissolved impurities due to unbalanced components; the heterogeneous ions introduced by soluble salt are embedded in the lattice defects to form catalytic sites, reducing the activation energy of silicon-aluminum recombination and promoting the directional growth of pure phase mullite; phosphoric acid preferentially combines with Al³⁺ to form stable AlPO4 intermediate phase, locking effective aluminum source to avoid loss and reducing the formation of impurities combined with aluminum source; ammonium salt roasting selectively dissolves Fe, Ti and other impurities through acidic atmosphere; the uniform temperature field and strong heat and mass transfer characteristics of suspension calcination ensure that the mullite is fully crystallized and the amount of impurities is minimized. Comparative Example 1 lacks silicon-aluminum sol coating, the silicon-aluminum ratio deviates from the theoretical value, and a large amount of impurities is generated, with an acid dissolution weight loss rate of 8.5%; Comparative Example 2 does not add soluble salt, and the lack of catalytic sites leads to incomplete crystallization, an increase in the proportion of impurities, and a weight loss rate of 3.8%; Comparative Example 3 does not add phosphoric acid, the effective aluminum components are lost, and the impurities cannot be effectively separated, resulting in the highest content of impurities, with a weight loss rate of 10.2%; Comparative Example 4 reverses the order of hydrothermal and ammonium salt roasting, and the impurities in the coal gangue lattice cannot be released by pre-roasting, and the impurities are not completely removed after subsequent hydrothermal activation, resulting in a large amount of residual impurities, with a weight loss rate of 7.8%; Comparative Example 5 uses traditional sintering process, and the uneven heat and mass transfer leads to incomplete crystallization in local areas, and the impurities are not completely converted, with a weight loss rate of 4.5%, which is significantly higher than that of Examples.

[0075] In terms of total amount of harmful impurities, the total amount of impurities in Examples 1-3 is controlled at 0.45%-0.75%, which is much lower than 0.90%-1.85% of Comparative Examples, which is due to the dual synergistic effect of "ammonium salt roasting and silicon-aluminum sol coating": the hydrothermal reaction destroys the coal gangue lattice and releases the wrapped Fe³⁺, Ti 4 ⁺ impurities, which are selectively dissolved and removed by the acidic atmosphere produced by subsequent ammonium salt roasting; the dense core-shell structure formed by silicon-aluminum sol coating effectively blocks the invasion of external impurities, while avoiding secondary pollution of the precursor after impurity removal. Comparative Example 3 does not add phosphoric acid, Al³⁺ is not directionally stabilized, and mixed with Fe, Ti impurities, resulting in a significant decrease in impurity removal efficiency, with a total amount of impurities of 1.85%; Comparative Example 4 reverses the process sequence, and the coal gangue lattice is not activated during pre-roasting, so the impurities cannot be released, and the impurities are not completely removed after subsequent hydrothermal reaction, with a total amount of impurities of 1.60%; Comparative Example 1 lacks silicon-aluminum sol coating, and the precursor is easily contaminated by external impurities after impurity removal, and the unbalanced silicon-aluminum ratio leads to the absorption of impurities by impurities, with a total amount of impurities of 1.20%; Comparative Example 5 uses traditional sintering process without the strong separation effect of suspension state, and the trace amount of residual impurities cannot be removed, with a total amount of impurities of 0.90%, which is higher than that of Examples.

[0076] In terms of microhardness, the microhardness of Examples 1-3 is 1520-1650 HV, which is significantly higher than 1050-1280 HV of Comparative Examples, which is due to the dual support of high-purity pure phase and densification structure: the synergistic effect of the five core links makes the purity of mullite crystal phase ≥98%, the impurity phase is less and uniformly distributed, and the structural defects are reduced; the suspension calcination promotes the close combination between particles, the low-melting-point liquid phase generated by the composite mineralizer accelerates ion diffusion, and the material density is improved, and the high-purity mullite itself has excellent mechanical properties, and the densification further strengthens the hardness. Comparative Example 3 has the highest impurity phase content, the lowest purity, and the loosest structure due to the lack of phosphoric acid, the loss of aluminum source, and the lack of phosphoric acid, the hardness is only 1050 HV; Comparative Example 1 lacks silicon-aluminum sol coating, and the imbalance of silicon-aluminum ratio leads to more impurity phases and high structure porosity, and the hardness is 1180 HV; Comparative Example 4 has more impurities and insufficient crystallization due to the reversed process sequence, and the structure has poor integrity, and the hardness is 1120 HV; Comparative Example 5 has uneven heat transfer during traditional sintering, and the density is insufficient, and there are more pores inside, and the hardness is 1230 HV; Comparative Example 2 has incomplete crystallization, small crystal grains and loose combination due to the lack of soluble salt, and the hardness is 1280 HV, which is significantly lower than Examples.

[0077] In summary, the excellent performance of Examples 1-3 is due to the synergistic effect of the whole process, and each link supports each other and is indispensable: the silicon-aluminum sol ratio lays the foundation for pure phase generation, phosphoric acid provides protection for aluminum source retention, soluble salt catalysis accelerates crystallization and reduces energy barrier, ammonium salt impurity removal removes impurity obstacles for pure phase generation, and suspension calcination provides conditions for densification and crystallization; and the comparative examples have component imbalance, more impurity phases, impurity residues, and insufficient densification due to the lack of any core link or the change of key process sequence, and the final performance is significantly worse than the examples, which fully proves the irreplaceability and synergistic effect of the core technical links of the scheme.

[0078] The above results show and describe the basic principles and main features of the present application and the advantages of the present application.

[0079] Those skilled in the art should understand that the present application is not limited to the above examples, and the above examples and descriptions in the specification are only to illustrate the principles of the present application. Without departing from the spirit and scope of the present application, various changes and improvements can be made to the present application, and these changes and improvements all fall within the scope of the claimed present application. The scope of protection claimed by the present application is defined by the equivalents of the appended claims.

Claims

1. A method for preparing high-purity mullite by coal gangue pretreatment-calcination coupling, characterized in that, The method comprises the following steps: Step 1. Taking coal gangue as raw material, after crushing, grinding and sieving, coal gangue powder is obtained; Step 2. The coal gangue powder is added into a hydrothermal reactor, deionized water is added and completely dissolved after first stirring, sodium hydroxide solution is added to adjust the pH value of the reaction system, soluble salt and phosphoric acid are added, the hydrothermal reactor is sealed after second stirring, and the reactor is heated for third stirring, after the reaction is completed, the mixture is filtered, washed with deionized water until the filtrate is neutral, and then dried to obtain a dry precursor; the soluble salt comprises any one of Y(NO3)3, Ce(NO3)3 and Fe(NO3)3; Step 3. The dry precursor is mixed with ammonium salt uniformly, and is placed in a calcination furnace for calcination, and is heated and calcined in an air atmosphere to obtain a decontaminated precursor; Step 4. The decontaminated precursor is added into deionized water and stirred and dispersed to obtain a slurry, and then a composite mineralizer is added into the slurry and continuously stirred to obtain a slurry containing the composite mineralizer; the composite mineralizer is composed of AlF3, B2O3 and MgO in a mass ratio of (3-5):(2-3):(1-2); Step 5. The slurry containing the composite mineralizer is added into a silica-alumina sol, and stirred and reacted, and then is aged, filtered and coated to obtain a coated precursor; Step 6. The coated precursor is sent into a suspension calcination furnace, a gas flow is introduced to make the coated precursor in a suspended state, and the coated precursor is sequentially subjected to a drying stage, a decomposition stage and a crystallization stage, and after calcination is completed, the coated precursor is cooled and collected to obtain high-purity mullite.

2. The method for preparing high-purity mullite by coupling coal gangue pretreatment and calcination according to claim 1, characterized in that, In step 1, the grinding time is 30-60 min, and the sieving mesh number is 200-400 meshes. 3.The method for preparing high-purity mullite by coupling coal gangue pretreatment and calcination according to claim 1, characterized in that, In step 2, 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 coupling coal gangue pretreatment and calcination according to claim 1, characterized in that, In step 2, the pressure of the hydrothermal reactor is 3-8 MPa, the temperature of the heating is 160-220℃, the rotating speed of the first stirring is 200-300 r / min and the time is 15-30 min, the rotating speed of the second stirring is 300-500 r / min and the time is 20-40 min, the rotating speed of the third stirring is 100-200 r / min and the time is 4-8 h, the neutral is pH 6.5-7.5, the drying temperature is 100-110℃ and the time is 4-6 h, and the pH value of the reaction system adjusted by the sodium hydroxide solution is 10-12.

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

6. The method for preparing high-purity mullite by coupling coal gangue pretreatment and calcination 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 1-3 h. 7.The method of claim 1, wherein the method is characterized by, In step 4, the solid content of the slurry is 20%-40%, and the addition amount of the composite mineralizer is 3%-8% of the mass of the decontaminated precursor. 8.The method of claim 1, wherein the method is characterized by, In step 4, the stirring speed is 300-500 r / min, and the stirring time is 20-40 min. 9.The method of claim 1, wherein the method is characterized by, 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℃, the stirring time is 1-3 h, and the standing time is 4-6 h. 10.The method of claim 1, wherein the method is characterized by, In step 6, the airflow is air or oxygen, the airflow speed is 1-3 m / s, the drying temperature is 200-300℃, the drying time is 10-15 min, the decomposition temperature is 600-800℃, the decomposition time is 10-20 min, the crystallization temperature is 1350-1450℃, and the crystallization time is 10-25 min.

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