Coal gasification slag-based glass ceramic based on microwave selective body heating as well as preparation method and application of coal gasification slag-based glass ceramic

Abstract

The invention provides coal gasification slag-based microcrystalline glass based on microwave selective body heating as well as a preparation method and application thereof, and belongs to the technical field of solid waste recycling and advanced ceramic preparation. The method comprises the following steps: mixing coal gasification slag powder and a microwave sensitizer in a dry method state to obtain composite powder; pressing the composite powder to obtain a green body; and carrying out microwave sintering on the green body, and naturally cooling to obtain the coal gasification slag-based microcrystalline glass. Compared with a traditional kiln, the microwave selective body heating comprehensive energy consumption can be reduced by 40-60%, the single-furnace sintering period is shortened to be within 2 h from tens of hours, and the production efficiency is improved by the order of magnitude; due to the uniform body heating characteristic, the homogeneous crack-free high-performance microcrystalline glass product with the thickness of 100-300 mm or even thicker thickness is possible to be prepared; rapid sintering inhibits overgrowth of grains, a fine grain structure is formed, and the mechanical property of the material is remarkably improved.

Description

Technical Field

[0001] This invention relates to the fields of solid waste resource utilization and advanced ceramic preparation technology, and in particular to a coal gasification slag-based microcrystalline glass based on microwave selective bulk heating, its preparation method and application. Background Technology

[0002] Tombstones, as important commemorative building materials, have long relied primarily on natural stone, especially black granite (such as China Black and Indian Black Gold Sand). However, natural stone faces two major challenges: First, the resources are non-renewable and increasingly depleted. High-quality black granite deposits have become increasingly scarce after long-term mining, leading to a continuous rise in the price of raw blocks, and the mining process severely damages the ecological environment. Second, the material itself has inherent defects. Natural stone is anisotropic, often containing hidden micro-cracks, color variations, and mineral inclusions, affecting the yield and appearance consistency of finished products; its main mineral component is silicate, which is prone to surface corrosion and decreased gloss when exposed to acid rain for a long time; its water absorption rate (usually >0.4%) makes it susceptible to freeze-thaw damage in cold regions and the growth of moss.

[0003] Utilizing industrial solid waste to produce artificial stone is an important direction for sustainable development. Coal gasification slag is a large-scale solid waste generated by the coal chemical industry, with a huge annual output. Its main components are SiO2, Al2O3, CaO, Fe2O3, and unburned residual carbon, which have the potential to form CaO-Al2O3-SiO2 system microcrystalline glass. Traditional methods of preparing microcrystalline glass by sintering waste slag in electric resistance furnaces or gas-fired kilns (such as patent CN201910123456.7A) have obvious bottlenecks: extremely high energy consumption, as the thermal efficiency is usually less than 50% due to reliance on external radiation / conduction heating; long sintering cycle, especially for blanks thicker than 50mm, where the heating rate needs to be reduced to 1~2℃ / min to avoid cracking due to excessive internal and external temperature differences, and the total sintering time often exceeds 24 hours; difficulty in producing ultra-thick products, as the inherent temperature gradient of traditional heating methods makes it difficult to densify the core of the blank, easily resulting in central porosity or cracks.

[0004] Microwave sintering, as a bulk heating technology, offers advantages such as rapid heating rate, high thermal efficiency, and selective heating. However, directly applying microwave sintering to coal gasification slag systems faces challenges: the main glassy phase components (SiO2, Al2O3) in the slag exhibit weak response to microwaves (especially 2.45GHz) at low temperatures, while residual carbon, although a good microwave absorber, is unevenly distributed, easily leading to uneven heating and localized overheating. Existing technologies (such as patent CN202010987654.3B) mention microwave treatment of waste slag, but these mostly focus on drying or activation, and a systematic technical solution has yet to be found that achieves rapid, uniform, and integrated sintering of coal gasification slag into large, thick-walled products by constructing a uniform microwave absorption network. Summary of the Invention

[0005] The purpose of this invention is to address the scarcity and performance defects of natural tombstone stone, as well as the technical difficulties in preparing microcrystalline glass using the existing coal gasification slag sintering method, which involves high energy consumption, long cycle time, and difficulty in producing thick, defect-free products. This invention provides a high-efficiency, energy-saving, and high-performance microcrystalline glass based on microwave selective bulk heating, which can be used to prepare ultra-thick tombstones, along with its preparation method and applications.

[0006] To achieve the above-mentioned objectives, the present invention provides the following technical solution: This invention provides a method for preparing coal gasification slag-based microcrystalline glass based on microwave selective bulk heating, comprising the following steps: 1) Coal gasification slag powder and microwave sensitizer are mixed in a dry process to obtain composite powder; 2) The composite powder is pressed to obtain a green body; 3) After microwave sintering, the green blank is naturally cooled to obtain coal gasification slag-based microcrystalline glass; The microwave sintering process includes a first heating stage, a second heating stage, and a holding stage; the atmosphere in the first heating stage is an air atmosphere or a weakly oxidizing atmosphere, and the microwave power density in the first heating stage is 0.5~1.5W / cm³. 3 The first heating stage lasts for 5 to 15 minutes, and the temperature rises to 550 to 650°C. The microwave power density during the second heating stage is 2~6 W / cm². 3 The heating rate in the second heating stage is 15~40℃ / min, and the temperature in the second heating stage is raised to 800~1000℃; The temperature during the heat preservation stage is 1050~1180℃, and the heat preservation time is 5~30min. The heat preservation stage adopts pulsed microwave mode or continuous wave, and the frequency of pulsed microwave mode is 900~930MHz.

[0007] Preferably, the microwave sensitizer in step 1) is one or more of silicon carbide, carbon fiber and graphene; the mass of the microwave sensitizer is 0.2 to 3.0% of the mass of the coal gasification slag powder.

[0008] Preferably, the particle size of the coal gasification slag powder is D50≤45μm, the average particle size of the microwave sensitizer is ≤1μm, and the dielectric loss tangent of the microwave sensitizer is ≥0.1 at room temperature and 2.45GHz.

[0009] Preferably, the mixing time in step 1) is 10-30 min; and the pressing pressure in step 2) is 100-300 MPa.

[0010] Preferably, in step 3), the oxygen content of the weakly oxidizing atmosphere in the first heating stage is ≤5%; and in the heat preservation stage, the duty cycle of the pulsed microwave mode is 30~70%, and the pulse frequency is 50~500Hz.

[0011] Preferably, the microwave sintering in step 3) is carried out in the heat preservation device of the microwave sintering furnace. The heat preservation device is made of porous alumina, mullite or quartz fiber felt. The dielectric constant ε' of the heat preservation device at room temperature is ≤5 and the dielectric loss tanδ is ≤0.001.

[0012] Preferably, the atmosphere during the heat preservation stage in step 3) is a protective atmosphere or a mixture of a protective atmosphere and air, wherein the protective atmosphere is nitrogen or argon.

[0013] The present invention also provides a method for preparing a coal gasification slag-based microcrystalline glass based on microwave selective bulk heating, wherein the coal gasification slag-based microcrystalline glass has a thickness of 100~300mm, no bubbles or cracks with a diameter greater than 50μm inside, a bulk density of ≥2.70g / cm³, and a water absorption rate of ≤0.05%.

[0014] As a preferred option, the main crystalline phases of the coal gasification slag-based microcrystalline glass are diopside, calcium iron pyroxene, and magnetite, and the average grain size of the coal gasification slag-based microcrystalline glass is ≤5μm, the compressive strength is ≥400MPa, and the Mohs hardness is ≥7.0.

[0015] The present invention also provides the application of the aforementioned coal gasification slag-based microcrystalline glass in tombstones.

[0016] The beneficial effects of this invention are: 1) Revolutionary energy saving and high efficiency: The thermal efficiency of the bulk heating mode exceeds 80%. Combined with the rapid sintering process, compared with traditional kilns, the overall energy consumption can be reduced by 40-60%, and the sintering cycle of a single furnace is shortened from tens of hours to less than 2 hours, resulting in an order-of-magnitude increase in production efficiency.

[0017] 2) Breakthrough solution to the problem of thick product preparation: The uniform volume heating characteristics make it possible to prepare homogeneous, crack-free, high-performance microcrystalline glass products with a thickness of 100~300mm or even thicker, solving the technical bottleneck that traditional processes cannot overcome.

[0018] 3) Excellent product performance: Rapid sintering inhibits excessive grain growth, forming a fine-grained structure (average grain size ≤5μm), significantly improving the mechanical properties of the material; Controllable atmosphere ensures that Fe element exists in the form of Fe3O4, giving the product a deep and stable black color; After 1000h xenon lamp accelerated aging test, the color difference ΔE of the microcrystalline glass tombstone is ≤2.0.

[0019] 4) Win-win for resources and environment: 100% utilization of solid waste provides a high-end outlet for coal gasification slag; short process and low energy consumption meet the requirements of green manufacturing. Detailed Implementation

[0020] This invention provides a method for preparing coal gasification slag-based microcrystalline glass based on microwave selective bulk heating, comprising the following steps: 1) Coal gasification slag powder and microwave sensitizer are mixed in a dry process to obtain composite powder; 2) The composite powder is pressed to obtain a green body; 3) After microwave sintering, the green blank is naturally cooled to obtain coal gasification slag-based microcrystalline glass; The microwave sintering process includes a first heating stage, a second heating stage, and a holding stage; the atmosphere in the first heating stage is an air atmosphere or a weakly oxidizing atmosphere, and the microwave power density in the first heating stage is 0.5~1.5W / cm³. 3 The first heating stage lasts for 5 to 15 minutes, and the temperature rises to 550 to 650°C. The microwave power density during the second heating stage is 2~6 W / cm². 3 The heating rate in the second heating stage is 15~40℃ / min, and the temperature in the second heating stage is raised to 800~1000℃; The temperature during the heat preservation stage is 1050~1180℃, and the heat preservation time is 5~30min. The heat preservation stage adopts pulsed microwave mode or continuous wave, and the frequency of pulsed microwave mode is 900~930MHz.

[0021] In this invention, the microwave sensitizer in step 1) is preferably one or more of silicon carbide, carbon fiber and graphene; the mass of the microwave sensitizer is preferably 0.2-3.0% of the mass of coal gasification slag powder, more preferably 0.5-2.5%, and even more preferably 1-2%.

[0022] In this invention, the particle size of the coal gasification slag powder is preferably D50≤45μm, more preferably D50≤40μm, and even more preferably D50≤35μm; the average particle size of the microwave sensitizer is preferably ≤1μm, more preferably ≤0.8μm, and the dielectric loss tangent (tanδ) of the microwave sensitizer is preferably ≥0.1 at room temperature and 2.45GHz.

[0023] In this invention, the coal gasification slag powder in step 1) is preferably obtained by pretreatment of coal gasification slag, wherein the pretreatment is carried out sequentially by drying, crushing and ball milling; the drying temperature is preferably 105~120℃, and more preferably 110~115℃.

[0024] In this invention, the mixing time in step 1) is preferably 10-30 min, more preferably 15-25 min, and even more preferably 20 min; the pressing pressure in step 2) is preferably 100-300 MPa, more preferably 150-250 MPa, and even more preferably 200 MPa.

[0025] In this invention, the oxygen content of the weakly oxidizing atmosphere in the first heating stage (step 3) is preferably ≤5%, more preferably ≤3%; the duty cycle of the pulsed microwave mode in the heat preservation stage is preferably 30~70%, more preferably 40~60%, more preferably 50%; the pulse frequency is preferably 50~500Hz, more preferably 100~400Hz, more preferably 200~300Hz; and the frequency of the continuous wave is preferably 2~3GHz, more preferably 2.45~2.5GHz.

[0026] In this invention, the microwave sintering in step 3) is carried out in the heat preservation device of the microwave sintering furnace. The heat preservation device is preferably made of porous alumina, mullite or quartz fiber felt. The dielectric constant ε' of the heat preservation device at room temperature is ≤5 and the dielectric loss tanδ is ≤0.001.

[0027] In this invention, the atmosphere of the heat preservation stage in step 3) is preferably a protective atmosphere or a mixture of protective atmosphere and air, and the protective atmosphere is preferably nitrogen or argon.

[0028] In this invention, the first heating stage completes the mild oxidation of residual carbon; the holding stage is used to regulate the valence state of iron elements inside the billet, so that the product exhibits a stable deep black color (Lab color space L). With a value ≤ 25, densification and crystal growth are completed; after the heat preservation stage, the microwave source is turned off, and the billet is allowed to cool naturally in the heat preservation device to obtain coal gasification slag-based microcrystalline glass.

[0029] The present invention also provides a method for preparing a coal gasification slag-based microcrystalline glass based on microwave selective bulk heating, wherein the coal gasification slag-based microcrystalline glass has a thickness of 100~300mm, no bubbles or cracks with a diameter greater than 50μm inside, a bulk density of coal gasification slag ≥2.70g / cm³, and a water absorption rate ≤0.05%.

[0030] In this invention, the main crystalline phases of the coal gasification slag-based microcrystalline glass are diopside, calcium iron pyroxene, and magnetite. The average grain size of the microcrystalline glass is preferably ≤5μm, more preferably ≤4μm, the compressive strength is preferably ≥400MPa, more preferably ≥450MPa, and the Mohs hardness is preferably ≥7.0, more preferably ≥7.5.

[0031] The present invention also provides the application of the aforementioned coal gasification slag-based microcrystalline glass in tombstones.

[0032] This invention utilizes the synergistic effect of residual carbon in coal gasification slag and an added trace amount of microwave sensitizer to construct a uniformly dispersed and highly efficient microwave-absorbing internal network. In a microwave field, this network is preferentially excited, instantly generating heat and conducting it outwards from the interior of the billet, achieving true "volume heating" and completely eliminating the internal and external temperature gradients inherent in traditional external heating methods. By precisely controlling microwave power, atmosphere, and time, the removal of organic matter / carbon, the regulation of the valence state of Fe2O3 (achieving self-coloring), and the densification and crystallization of the microcrystalline glass are simultaneously completed within an extremely short process window.

[0033] The technical solutions provided by the present invention will be described in detail below with reference to the embodiments, but they should not be construed as limiting the scope of protection of the present invention.

[0034] In the examples and comparative examples, the coal gasification slag was the coal gasification slag from a coal chemical enterprise, and its chemical composition was: SiO2 48.5wt%, Al2O3 22.1wt%, CaO 15.3wt%, Fe2O3 9.2wt%, and residual carbon 3.8wt%. The dielectric constant ε'≤5 and dielectric loss tanδ≤0.001 of the heat preservation device of the microwave sintering furnace at room temperature.

[0035] Example 1: Preparation of a 100mm thick black microcrystalline glass tombstone

[0036] The gasification slag was dried at 110℃ and then crushed by jaw crusher and ball milled by planetary mill to obtain gasification slag powder with D50=38μm. The gasification slag powder was mixed with silicon carbide powder (purity>99%) with an average particle size of 0.8μm in a three-dimensional mixer for 20min. The amount of silicon carbide added was 1.0% of the mass of the gasification slag powder to obtain composite powder.

[0037] The composite powder was loaded into a rubber mold and pressed under isostatic pressure of 200 MPa for 3 minutes to form a rectangular green body with dimensions of 210 mm × 105 mm × 100 mm.

[0038] The rectangular green blank is placed on a porous alumina fiber felt (density 0.25 g / cm³). 3 Inside the constructed insulation cavity, a 2.45GHz / 6kW multimode microwave sintering furnace was placed for three-stage microwave sintering; the first heating stage of microwave sintering involved introducing air at a flow rate of 2L / min into the furnace and applying a 1.0W / cm² temperature increase. 3 The microwave power density was used to uniformly heat the billet to 600℃ over 12 minutes. In the second heating stage, while maintaining an air atmosphere, the microwave power density was increased to 4.0 W / cm². 3The temperature was increased from 600℃ to 950℃ at a rate of 25℃ / min. During the holding phase, the atmosphere was switched to nitrogen gas with a flow rate of 3L / min, and the system was switched to 915MHz pulsed microwave mode with a duty cycle of 60% and a pulse frequency of 100Hz. The system was then held at 1120℃ for 20 minutes.

[0039] After the heat preservation stage is completed, all power and gas sources are turned off. The sintered product is then taken out after naturally cooling to below 80°C in a sealed furnace, resulting in a 100mm thick black microcrystalline glass tombstone.

[0040] Calculations show that the total power consumption for sintering from room temperature to 1120°C in this embodiment is 85 kWh; compared with the estimated energy consumption of the same output using a traditional gas-fired tunnel kiln, it saves 55% of energy.

[0041] The black microcrystalline glass tombstone prepared in this embodiment is a uniform deep black (L... =22.3, a =0.5, b =0.8), with a smooth surface; micro-CT scans showed no defects larger than 30μm in diameter throughout the entire 100mm thick section, and a porosity of <0.5%. The bulk density of the microcrystalline glass tombstone was 2.78g / cm³. 3 It has a water absorption rate of 0.02%, a Mohs hardness of 7.5, a compressive strength of 435 MPa, a flexural strength of 68 MPa, and an E value of 1.7 after 1000 hours of xenon lamp aging. After immersion in a 5% sulfuric acid solution for 30 days, the mass loss rate is 0.08%, and there is no change on the surface.

[0042] Example 2: Preparation of a 200mm thick black microcrystalline glass tombstone

[0043] The microwave sensitizer is a composite of carbon nanotubes and submicron silicon carbide, with each component accounting for 0.5% of the mass of the coal gasification slag powder. The composite powder is pressed into a 300mm×150mm×200mm rectangular green body under isostatic pressure of 250MPa for 3 minutes.

[0044] The microwave power density during the first heating stage is 0.8 W / cm². 3 The billet was uniformly heated to 600℃ over a period of 12 minutes; the power density was increased to 3.5W / cm³ in the second stage. 3 During the heat preservation stage, the temperature was maintained at 1100℃ for 30 minutes using a 2.45GHz continuous wave, with the atmosphere being a mixture of nitrogen and oxygen (the volume fraction of oxygen in the mixture was 2%, and the flow rate of the mixture was 3L / min).

[0045] Other process conditions are the same as in Example 1.

[0046] This embodiment successfully produced a crack-free black microcrystalline glass monument with a thickness of 200 mm, a core-to-edge density deviation of <1.5%, and a compressive strength of 418 MPa.

[0047] Comparative Example 1: Sintering in a conventional resistance furnace

[0048] The composition and preparation process of the cuboid green billet are the same as those in Example 1. The cuboid green billet is placed in a box-type resistance furnace and sintered according to the industry standard thick billet sintering curve: the temperature is increased to 600℃ at 1℃ / min and held for 2h, and then increased to 1120℃ at 2℃ / min and held for 4h, with a total heating time of more than 20h.

[0049] The product obtained in this comparative example has a dark brown surface, with obvious laminar fractures at a depth of 150 mm. Cross-sectional CT scans show numerous closed pores in the central region, and the overall density is only 2.62 g / cm³. 3 Its compressive strength is 285 MPa.

[0050] Comparative Example 2: Microwave sintering without microwave sensitizer

[0051] The silicon carbide powder of Example 1 is omitted, and the coal gasification slag powder is formed and microwave sintered using the same process as in Example 1.

[0052] In this comparative example, local "hot spots" appeared in the early stage of heating, and the blank cracked after about 3 minutes, making it impossible to obtain a complete sample.

[0053] The above embodiments fully demonstrate the outstanding effect, innovation, and industrial applicability of the method of the present invention in rapidly preparing thick, high-performance microcrystalline glass tombstones using coal gasification slag.

[0054] This invention utilizes pretreated coal gasification slag powder combined with a trace amount of microwave sensitizer, pressed into shape, and placed in a microwave sintering furnace. The microwave-absorbing network formed by residual carbon in the coal gasification slag and the sensitizer achieves rapid and uniform bulk heating of the material under a microwave field. By precisely controlling the multi-stage microwave power and atmosphere, gentle decarburization, rapid heating, and low-temperature short-time crystallization sintering are sequentially completed. This method can produce black microcrystalline glass tombstones with a thickness of 100-300 mm, free of macroscopic cracks, and with a uniform structure, within an extremely short period of 1050-1180℃ and a holding time of 5-30 minutes. Compared with traditional kiln sintering, this invention reduces energy consumption by 40-60%, shortens the sintering cycle by more than 80%, and produces a product with a compressive strength ≥400 MPa, a water absorption rate ≤0.05%, excellent weather resistance, and stable color, providing an innovative path for the high-value and large-scale utilization of coal gasification slag.

[0055] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for preparing coal gasification slag-based microcrystalline glass based on microwave selective bulk heating, characterized in that, It includes the following steps: 1) Coal gasification slag powder and microwave sensitizer are mixed in a dry process to obtain composite powder; 2) The composite powder is pressed to obtain a green body; 3) After microwave sintering, the green blank is naturally cooled to obtain coal gasification slag-based microcrystalline glass; The microwave sintering process includes a first heating stage, a second heating stage, and a holding stage; the atmosphere in the first heating stage is an air atmosphere or a weakly oxidizing atmosphere, and the microwave power density in the first heating stage is 0.5~1.5W / cm³. 3 The first heating stage lasts for 5 to 15 minutes, and the temperature rises to 550 to 650°C. The microwave power density during the second heating stage is 2~6 W / cm². 3 The heating rate in the second heating stage is 15~40℃ / min, and the temperature in the second heating stage is raised to 800~1000℃; The temperature during the heat preservation stage is 1050~1180℃, and the heat preservation time is 5~30min. The heat preservation stage adopts pulsed microwave mode or continuous wave, and the frequency of pulsed microwave mode is 900~930MHz.

2. The preparation method according to claim 1, characterized in that, Step 1) The microwave sensitizer is one or more of silicon carbide, carbon fiber and graphene; the mass of the microwave sensitizer is 0.2~3.0% of the mass of the coal gasification slag powder.

3. The preparation method according to claim 1 or 2, characterized in that, The particle size of the coal gasification slag powder is D50≤45μm, the average particle size of the microwave sensitizer is ≤1μm, and the dielectric loss tangent of the microwave sensitizer is ≥0.1 at room temperature and 2.45GHz.

4. The preparation method according to claim 3, characterized in that, Step 1) The mixing time is 10~30 min; Step 2) The pressing pressure is 100~300 MPa.

5. The preparation method according to claim 4, characterized in that, Step 3) The oxygen content of the weakly oxidizing atmosphere in the first heating stage is ≤5%; the duty cycle of the pulsed microwave mode in the heat preservation stage is 30~70%, and the pulse frequency is 50~500Hz.

6. The preparation method according to claim 4 or 5, characterized in that, Step 3) The microwave sintering is carried out in the heat preservation device of the microwave sintering furnace. The heat preservation device is made of porous alumina, mullite or quartz fiber felt. The dielectric constant ε'≤5 and the dielectric loss tanδ≤0.001 of the heat preservation device at room temperature are as follows.

7. The preparation method according to claim 6, characterized in that, Step 3) The atmosphere during the heat preservation stage is a protective atmosphere or a mixture of protective atmosphere and air, wherein the protective atmosphere is nitrogen or argon.

8. The coal gasification slag-based microcrystalline glass prepared by the method for preparing coal gasification slag-based microcrystalline glass based on microwave selective bulk heating according to any one of claims 1 to 7, characterized in that, The thickness of the coal gasification slag-based microcrystalline glass is 100~300mm, and there are no bubbles or cracks with a diameter greater than 50μm inside. The bulk density of the coal gasification slag-based microcrystalline glass is ≥2.70g / cm³, and the water absorption rate is ≤0.05%.

9. The coal gasification slag-based microcrystalline glass according to claim 8, characterized in that, The main crystalline phases of the coal gasification slag-based microcrystalline glass are diopside, calcium iron pyroxene and magnetite. The average grain size of the coal gasification slag-based microcrystalline glass is ≤5μm, the compressive strength is ≥400MPa, and the Mohs hardness is ≥7.

0.

10. The application of the coal gasification slag-based microcrystalline glass according to claim 9 in tombstones.