Method for self-propagating high-temperature synthesis of coal-based mullite composite ceramic from coal gangue

Abstract

The invention provides a method for self-propagating high-temperature synthesis of coal-based mullite multiphase ceramic from coal gangue, which comprises the following steps: proportioning coal gangue, carbon black and silicon powder according to a certain mass ratio, ball-milling and mixing by absolute ethyl alcohol, drying, placing in a high-pressure reaction device together with an ignition agent titanium powder, and electrifying to ignite the titanium powder to initiate a self-propagating reaction under the nitrogen atmosphere of 6-8MPa. The coal gangue is converted into a functional reaction raw material and a sintering aid from waste to be treated by utilizing the characteristic of complex components of the coal gangue, and impurity oxides such as K2O, Na2O, Fe2O3 and the like contained in the coal gangue can form a low-melting-point silicate liquid phase at a self-propagating high temperature, so that a self-catalysis sintering effect is realized. According to the method, only mechanical crushing and ball milling are needed, so that the energy consumption and the cost are reduced.

Description

Technical Field

[0001] This invention relates to the fields of industrial solid waste resource utilization and advanced ceramic material preparation technology, and in particular to a method for self-propagating high-temperature synthesis of coal-based mullite multiphase ceramics from coal gangue. Background Technology

[0002] Mullite multiphase ceramics are a high-quality refractory material with advantages such as high temperature resistance, high melting point, high strength, good creep resistance, low thermal conductivity, good thermal shock resistance, and significant energy-saving effect. Existing technologies for preparing mullite commonly utilize diatomaceous earth rich in silicon and aluminum as the main raw materials for synthesizing coal gangue ceramics (e.g., publication number CN110698188 A). The drawback is the high cost of raw materials and the complex preparation process. Many existing technologies also utilize pure raw materials containing Al and Si elements to prepare mullite (e.g., publication number CN113896514A), which similarly suffers from high raw material costs and complex preparation processes. Some technologies, while not using pure Al and Si elements, employ a calcination method (e.g., publication number CN113233879 A). However, the traditional calcination method for preparing mullite multiphase ceramics suffers from a long reaction cycle.

[0003] Therefore, how to synthesize mullite multiphase ceramic materials using inexpensive resources and simple processes has become one of the important research directions in the field of refractory materials. Summary of the Invention

[0004] The main components of coal gangue are alumina and silicon dioxide, which are commonly used raw materials for ceramic production. However, directly using coal gangue, which has a complex composition and uneven reactivity, to prepare high-performance multiphase ceramics still faces challenges such as difficulty in reaction control and poor performance of the prepared multiphase ceramic materials.

[0005] This invention utilizes the main components of solid waste material coal gangue, namely alumina and silicon dioxide, and specifically matches other raw materials as reactants, thus solving the technical problems of difficult reaction control and poor performance of the prepared multiphase ceramic materials.

[0006] Specifically, the method for self-propagating high-temperature synthesis of coal-based mullite multiphase ceramics from coal gangue of the present invention uses coal gangue, carbon black, silicon powder, and NH4Cl as reactants to participate in a combustion synthesis self-propagating reaction to obtain coal-based mullite multiphase ceramics. Carbon black, used as a raw material for carbothermal nitriding reduction, can synthesize β-SiAlON as a secondary phase material and further act as an exothermic reducing agent in self-propagating (SHS) combustion. The specific reaction is as follows: ;

[0007] NH4Cl decomposes endothermally during heating (sublimation temperature approximately 340°C) and produces gas. This process absorbs a significant amount of heat from the reaction system, effectively reducing the temperature and rate of combustion. In the SHS reaction of the Al-N2 system, adding NH4Cl is one of the key technologies for preparing high-quality AlN secondary phase powder. It significantly promotes the gas-solid reaction between Al and N2 by forming gaseous AlCl3, resulting in a more complete reaction, purer products, and more uniform grain size.

[0008] Furthermore, the mass percentages of coal gangue, carbon black, silica powder, and NH4Cl are as follows: coal gangue 55-65%, carbon black 10-15%, silica powder 15-25%, and NH4Cl 1%-1.5%.

[0009] Furthermore, the coal gangue contains the following components by mass percentage: 50-60% SiO2, 20-30% Al2O3, 2-6% iron oxides, 1.2% K2O, and 0.6% Na2O. The impurity oxides such as K2O, Na2O, and Fe2O3 in the coal gangue will form a low-melting-point silicate liquid phase under self-propagating high temperatures. This liquid phase acts as a classic liquid-phase sintering aid, promoting particle rearrangement through capillary force and accelerating the material diffusion process, thereby significantly improving the sintering rate of the product and achieving a "self-catalyzing" sintering effect. Furthermore, the "impurity" components in the coal gangue are retained and transformed into effective sintering aids, turning waste into treasure and fully demonstrating the advantages of this technology in resource utilization.

[0010] Furthermore, the igniter for the self-propagating reaction is titanium powder, which accounts for 4-6% of the total mass of the raw materials. The purity of the titanium powder is not less than 99%, and the particle size is 38-45 μm.

[0011] Furthermore, the carbon black has a fixed carbon content of not less than 98% and a particle size of 0.1~1μm.

[0012] Furthermore, the purity of the silicon powder is not less than 99%, and the particle size is 44~74μm.

[0013] Furthermore, the NH4Cl particle size is 75–100 μm.

[0014] This invention features a synergistic design of the particle size of all reaction raw materials, controlling the particle size to be between tens of micrometers and one hundred micrometers. This particle size range design has significant advantages: the similar particle size distribution avoids component segregation caused by differences in gravity during mechanical mixing, laying the foundation for achieving uniform mixing at the molecular level. This is a prerequisite for ensuring the stable propagation of the self-propagating reaction and the uniform distribution of the product phase.

[0015] Furthermore, the specific preparation steps are as follows: Step S1: Mix the reaction raw materials, ball mill them, and dry them to obtain a homogeneous mixture; In step S2, the mixed material and the igniter titanium powder are placed together in a high-pressure reactor. Under a nitrogen atmosphere of 6-8 MPa, the titanium powder is ignited by an electric current to initiate a self-propagating reaction of the mixed powder, and the reaction product is obtained.

[0016] Furthermore, in step S1, drying is carried out in a vacuum drying oven at a temperature of 110–130°C for 6–10 hours. During ball milling, anhydrous ethanol is used as the mixing medium, with a ball-to-material mass ratio of (5:1) to (10:1), and the milling time is 4–8 hours, resulting in a particle size of 250 mesh. This invention proposes that coal gangue requires no chemical pretreatment or high-temperature activation; it only needs mechanical crushing and ball milling to achieve a particle size of 250 mesh before direct use. This significantly simplifies the process and reduces energy consumption and costs.

[0017] Furthermore, in step S2, titanium powder is laid on one side of the homogenized material in the graphite crucible as an ignition source. The self-propagating reaction takes place in the graphite crucible under a nitrogen atmosphere, with the internal pressure controlled at 6-8 MPa. This invention achieves stable and continuous propagating combustion reaction of the combustion synthesis system by implementing combustion synthesis in high-pressure nitrogen atmosphere. It utilizes the carbonization and nitriding system provided by coal gangue powder, carbon black, and NH4Cl to release a large amount of heat energy, and optimizes the process parameters to rationally control the combustion reaction characteristics to synthesize multiphase ceramic materials.

[0018] This invention relates to a self-propagating reaction apparatus comprising a high-pressure reactor, a precision pneumatic control system, and an electric ignition system. Unlike traditional vertical self-propagating reactors, the powder receiving unit within the high-pressure reactor is designed as a horizontally placed graphite crucible boat structure, eliminating the influence of vertical pressure and powder gravity within the reactor vessel. The electric ignition system uses a high-melting-point tungsten filament as the ignition element, generating high temperature through instantaneous energization to reliably ignite the titanium powder igniter laid on the raw materials. The integrated design of this apparatus combines multiple steps, including loading, sealing, pressurization, ignition, and cooling, into a single closed system. Its advantages include: isolating oxygen to prevent raw material oxidation and product contamination; providing a stable and controllable external environment for the reaction; and achieving operational safety and process repeatability.

[0019] This invention sets the reaction environment to a nitrogen atmosphere of 6-8 MPa, and this parameter range is one of the key innovations of this invention. Its effect far exceeds that of a simple protective gas, mainly manifested in: First, it promotes nitrogen participation in the reaction: Under this medium-high pressure, the partial pressure of nitrogen is significantly increased, enhancing its chemical reactivity. At instantaneous high temperatures exceeding 2000℃ through self-propagation, nitrogen can undergo nitriding reactions with metallic silicon (such as unreacted Si) or generated silicides in the system, generating a certain amount of silicon nitride (Si3N4) or sialon phase in situ. The introduction of these high-hardness, high-toughness nitride phases, together with SiC and Al2O3, constitutes a multiphase composite ceramic, which can play a toughening role such as pinning and bridging, thereby synergistically improving the overall mechanical properties of the final product, especially fracture toughness.

[0020] Second, it inhibits the volatilization and excessive vaporization of elements: The high-pressure environment effectively inhibits the volatilization of low-boiling-point metals (such as alkali metals) and the excessive and violent generation of CO gas in the reaction system, making the reaction process more stable, the product morphology easier to control, and the pore structure finer and more uniform.

[0021] The method for self-propagating high-temperature synthesis of coal-based mullite multiphase ceramics from coal gangue of the present invention has the following significant advantages: 1. This invention forms a complete, efficient, and low-cost innovative technical solution for preparing high-performance multiphase ceramics from coal gangue through functional design of raw materials, synergistic optimization of particle size, integration of specialized equipment, and precise control of the reaction environment.

[0022] 2. This invention innovatively utilizes the complex composition of coal gangue, transforming it from "waste to be treated" into "functionalized reaction raw material and sintering aid." The abundant SiO2 and Al2O3 in coal gangue are the main sources for generating ceramic matrices (such as silicon carbide and mullite). In-situ generation of sintering aids: Impurities and oxides such as K2O, Na2O, and Fe2O3 contained in coal gangue form a low-melting-point silicate liquid phase under self-propagating high temperatures. This liquid phase acts as a classic liquid-phase sintering aid, promoting particle rearrangement through capillary forces and accelerating the material diffusion process, thereby significantly improving the sintering rate of the product and achieving a "self-catalyzed" sintering effect.

[0023] 3. Unlike traditional methods that require high-temperature activation and purification of coal gangue, this invention proposes a method where coal gangue requires no chemical pretreatment or high-temperature activation. It only needs mechanical crushing and ball milling to achieve a particle size of 250 mesh before direct use. This significantly simplifies the process, reduces energy consumption and costs. Furthermore, the "impurities" in the coal gangue are retained and transformed into effective sintering aids, turning waste into treasure and fully demonstrating the resource utilization advantages of this technology. Attached Figure Description

[0024] Figure 1 The image shows the XRD pattern of the coal-based multiphase ceramic material sample prepared in Example 1. Detailed Implementation

[0025] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0026] The technical solution of the present invention will be further explained below with reference to implementation examples.

[0027] Example 1 This embodiment provides a method for the self-propagating high-temperature synthesis of coal-based mullite multiphase ceramics from coal gangue, comprising the following steps: (1) Raw material preparation: Mix coal gangue (SiO2 to Al2O3 atomic ratio of 1.8, particle size 74μm), carbon black (fixed carbon >98%), silicon powder (purity >99%, particle size 50μm) and ammonium chloride in a mass ratio of 10:2:2:1; (2) Mix the above-mentioned raw materials with anhydrous ethanol, with the mass ratio of anhydrous ethanol to the raw materials to be ball-milled being 2:1. Use corundum balls as grinding balls, with a ball-to-material ratio of 8:1, and ball-mill for 6 hours to obtain ball-milled slurry. (3) The ball-milled slurry was dried in a vacuum drying oven at 110°C for 8 hours to obtain a uniformly mixed dry powder; (4) Load the dried powder into a graphite boat mold, and evenly cover the empty grooves on the side of the mixed raw material powder with titanium powder (accounting for 7% of the mass of the mixed powder). Place the mold in a high-pressure reactor, evacuate it, and then fill it with nitrogen to a pressure of 7 MPa. Turn on the power to instantly heat the tungsten wire and ignite the titanium powder, thereby initiating a self-propagating reaction of the system; (5) Once the temperature inside the instrument rises to 1000℃, the ignition button can be released. After that, the furnace is cooled to room temperature to obtain coal-based mullite multiphase ceramics.

[0028] Comparative Example 1 The method of this comparative example is the same as that of Example 1. The difference between this comparative example and Example 1 is that the raw materials of this comparative example do not include silicon powder.

[0029] After the comparative experiment, only the titanium powder igniter burned completely, while the raw materials were not ignited. Due to the lack of elemental silicon source and the absence of self-propagating heat, it was impossible to burn and synthesize porous or dense ceramic materials.

[0030] The XRD pattern of the coal-based multiphase ceramic material sample prepared in Example 1 is detailed in [reference needed]. Figure 1 The X-ray diffractometer used in this experiment was an MXP21VAHF, which analyzed the 10-80° range at a rate of 10° / minute. Figure 1XRD analysis of the self-propagating reaction products in Example 1 showed that, in addition to the expected silicon carbide and silicon oxide main phases, characteristic diffraction peaks of β-SiAlON, mullite, and a small amount of aluminates were clearly detected. This result directly confirms that the 6-8 MPa high-pressure nitrogen atmosphere successfully participated in the reaction, promoting the formation of high-performance nitride ceramic phases from silicon, aluminum, and other elements in the system through an in-situ nitriding mechanism. The formation of these nitrides, together with the matrix, constitutes a multiphase composite microstructure, strongly verifying the key role and innovation of the high-pressure nitrogen environment in this technical solution.

[0031] The embodiments described above are merely examples of several implementations of the present invention, and while the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the invention. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these modifications and improvements all fall within the scope of protection of the present invention.

Claims

1. A method for self-propagating high-temperature synthesis of coal-based mullite multiphase ceramics from coal gangue, characterized in that, Coal gangue, carbon black, silicon powder, and NH4Cl were used as reactants to participate in a combustion synthesis self-propagating reaction to prepare coal-based mullite multiphase ceramics.

2. The method according to claim 1, characterized in that, The mass percentages of coal gangue, carbon black, silica powder, and NH4Cl are as follows: coal gangue 55-65%, carbon black 10-15%, silica powder 15-25%, and NH4Cl 1%-1.5%.

3. The method according to claim 1, characterized in that, The coal gangue contains the following components in the indicated mass percentages: 50-60% SiO2, 20-30% Al2O3, 2-6% iron oxides, 1.2% K2O, and 0.6% Na2O.

4. The method according to claim 1, characterized in that, The igniter for the self-propagating reaction is titanium powder, which accounts for 4-6% of the total mass of the raw materials. The purity of the titanium powder is not less than 99%, and the particle size is 38-45 μm.

5. The method according to claim 1, characterized in that, The carbon black has a fixed carbon content of not less than 98% and a particle size of 0.1~1μm.

6. The method according to claim 1, characterized in that, The purity of the silicon powder is not less than 99%, and the particle size is 44~74μm.

7. The method according to claim 1, characterized in that, The NH4Cl particles have a size of 75–100 μm.

8. The method according to claim 1, characterized in that, The specific preparation steps are as follows: Step S1: Mix the reaction raw materials, ball mill them, and dry them to obtain a homogeneous mixture; In step S2, the mixed material and the igniter titanium powder are placed together in a high-pressure reactor. Under a nitrogen atmosphere of 6-8 MPa, the titanium powder is ignited by an electric current to initiate a self-propagating reaction of the mixed powder, and the reaction product is obtained.

9. The method according to claim 8, characterized in that, In step S1, the material is dried in a vacuum drying oven at a temperature of 110–130°C for 6–10 hours. During ball milling, anhydrous ethanol is used as the mixing medium, with a ball-to-material mass ratio of (5:1) to (10:1), and the milling time is 4–8 hours.

10. The method according to claim 8, characterized in that, In step S2, titanium powder is laid on one side of the homogenized material in the graphite crucible as an ignition source. The self-propagating reaction takes place in the graphite crucible under a nitrogen atmosphere, and the internal pressure of the crucible is controlled at 6~8 MPa.

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