Method for producing ceramic granules by preheating coal gangue mixture with steam

Abstract

This invention discloses a method for producing ceramsite using steam preheating of a coal gangue mixture, belonging to the field of solid waste resource utilization and ceramsite preparation technology. It addresses the problem of uneven heating of the coal gangue mixture caused by existing hot air preheating methods, leading to cracking of the ceramsite green body, insufficient expansion, and residual carbon. The invention involves crushing, grinding, and sieving the coal gangue to obtain fine powder; dry mixing the fine coal gangue powder with water glass and sodium carboxymethyl cellulose to obtain a first mixture; introducing superheated steam into the first mixture and preheating it to 75℃-80℃ to obtain a second mixture; mixing the second mixture with silicon carbide and dolomite powder to obtain a third mixture; pelletizing to obtain green pellets; and calcining to obtain ceramsite. This invention utilizes highly efficient and uniform preheating with superheated steam, and the steam condensate participates in the hydrolysis of water glass to form a gel network, giving the green pellets high strength and plasticity. The ceramsite structure is uniform after calcination. It is mainly used in the fields of lightweight building aggregates, thermal insulation materials, or water treatment filter media.

Description

Technical Field

[0001] This invention relates to the field of solid waste resource utilization and ceramsite preparation technology. More specifically, this invention relates to a method for preparing ceramsite by preheating a coal gangue mixture with steam. Background Technology

[0002] Coal gangue is a solid waste generated during coal mining and washing. Long-term stockpiling not only occupies significant land resources but also poses environmental problems such as spontaneous combustion, dust pollution, and water pollution. Utilizing coal gangue to produce ceramsite is one effective way to achieve its resource utilization. Ceramsite can be used as lightweight aggregate in construction, thermal insulation materials, water treatment filter media, and soil amendment substrates, showing promising application prospects.

[0003] Currently, the technological routes for preparing ceramsite from coal gangue are diversifying. For example, some researchers use static pelletizing equipment (such as belt roasters) to achieve large-scale processing of coal gangue ceramsite by integrating drying, pyrolysis, decarburization, and cooling processes, and utilize waste heat recycling to reduce energy consumption (see CN120081684B A method for preparing water-retaining ceramsite from coal gangue and its application, CN120097749B A method for preparing high-strength ceramsite from coal gangue and its application). To overcome the problems of material layer caking and incomplete decarburization caused by traditional single-stage roasting, a two-stage roasting process has emerged. By introducing a homogenizing section between the roasting stages and using specific additives, the maximum temperature of the material layer can be controlled, alleviating the adhesion caused by premature liquid phase formation (see CN120117879B A two-stage process for preparing water-retaining ceramsite from coal gangue). In addition, there are methods to construct a binary green pellet structure of "carbon-free core-carbon-coated material," in which coal gangue and industrial waste are layered and composited. The aim is to utilize the carbon-free nature of the core to maintain oxygen channels during high-temperature sintering, thus mitigating the black core problem (see CN120136527B: A method for preparing binary ceramsite using coal gangue and the resulting binary ceramsite). In the drying stage, there are also attempts to use microwave drying instead of traditional hot air drying, utilizing its bulk heating properties to reduce the green pellet bursting rate (see CN120172751B: A process for producing coal gangue ceramsite using microwave drying).

[0004] Despite the progress made by the aforementioned methods in equipment integration, process optimization, additive selection, and improvements in specific stages, these processes still generally employ traditional hot air preheating in the preheating stage of the mixture. Hot air preheating has the following inherent drawbacks: due to the low specific heat capacity of hot air, its heat-carrying capacity is limited, and hot air tends to flow off-center when passing through the powdery material layer, resulting in uneven heating in different parts of the material layer. The area near the hot air inlet heats up faster, while the heating lags behind in the deeper or corner areas of the material layer, resulting in significant differences in the temperature distribution throughout the material layer.

[0005] This temperature unevenness is transmitted to subsequent pelletizing and roasting processes. On the one hand, it leads to inconsistent moisture evaporation rates within the green pellets, generating internal stress and making them prone to cracking during transport or drying. On the other hand, when green pellets with internal stress and uneven temperature and humidity distribution enter the roasting kiln, the rapid vaporization of internal moisture and the thermal decomposition reaction of the material under high-temperature conditions will be asynchronous due to inconsistent initial states. Some areas may experience over-expansion or even cracking due to excessively high temperatures and rapid reactions; other areas may experience insufficient expansion due to insufficient temperatures and delayed reactions, forming under-burned or residual carbon cores (black cores), affecting the uniformity of strength and apparent density of the ceramsite. To address the problem of uneven hot air preheating, technicians in related fields have tried improvements such as optimizing hot air distribution devices, extending preheating time, and reducing material layer thickness. However, these methods either increase equipment complexity and energy consumption or reduce production efficiency, and their effect on improving preheating uniformity is limited, making it difficult to fundamentally solve the problem of internal differences in green pellets caused by uneven heat and mass transfer.

[0006] Therefore, how to achieve uniform and efficient preheating of coal gangue mixture, avoid green pellet cracking and internal structural defects of ceramsite caused by uneven thermal processing from the source, and optimize the functional activation sequence of additives has become a technical problem that those skilled in the art have been hoping to solve. Summary of the Invention

[0007] One object of the present invention is to solve at least the above-mentioned problems and to provide at least the advantages that will be described later.

[0008] To achieve these objectives and other advantages according to the present invention, a method for producing ceramsite by preheating a coal gangue mixture with steam is provided, comprising the following steps: S1. The coal gangue raw material is crushed, ground and sieved to obtain coal gangue fine powder with a particle size of less than 0.2 mm; S2. Dry mix the fine coal gangue powder with the first additive for 60-120 seconds to obtain the first mixture. The first additive includes water glass and sodium carboxymethyl cellulose. The water glass accounts for 3%-5% of the mass of the first mixture, and the sodium carboxymethyl cellulose accounts for 0.5%-1.5% of the mass of the first mixture. S3. Preheat the first mixture. During the preheating process, superheated steam with a pressure of 0.2MPa-1MPa and a temperature of 150℃-200℃ is introduced into the first mixture to raise the temperature of the first mixture to 75℃-80℃ and absorb the superheated steam to obtain the second mixture. S4. Mix the preheated second mixture with the second additive evenly to obtain the third mixture. The second additive includes silicon carbide and dolomite powder. Silicon carbide accounts for 1%-3% of the mass of the third mixture, and dolomite powder accounts for 4%-6% of the mass of the third mixture. S5. Pelletize the third mixture to obtain green pellets; S6. The raw pellets are roasted to obtain ceramsite.

[0009] Preferably, in step S6, the calcination process sequentially includes a drying section, a preheating section, a calcination section, and a cooling section. The temperature of the drying section is 200℃-400℃, and the drying time is 15-25 min. The temperature of the preheating section is 400℃-700℃, and the preheating time is 10-20 min. The temperature of the calcination section is 1150℃-1250℃, and the calcination time is 20-30 min. The cooling section uses forced ventilation to cool the material to below 100℃, thereby obtaining ceramsite.

[0010] Preferably, in step S3, after the temperature of the first mixture rises to 75℃-80℃, the superheated steam is stopped, and the mixture is kept at that temperature for 10-30 minutes before proceeding to step S4.

[0011] Preferably, in step S4, the second additive is preheated to 50°C-70°C before being mixed evenly with the second mixture.

[0012] Preferably, in step S2, the first additive further includes 0.5%-1.5% nano-silica by mass of the first mixture; In step S4, the second additive also includes aluminum sulfate at a mass of 0.3%-0.8% of the third mixture.

[0013] Preferably, in the roasting process of step S6, a medium-temperature slow-burning homogenization section is set between the preheating section and the roasting section. The temperature of the medium-temperature slow-burning homogenization section is 850℃-950℃, and the holding time is 15-25min.

[0014] Preferably, during the roasting process in step S6, periodic atmospheric disturbances are applied in the roasting section: after each 5-8 min of roasting in an oxidizing atmosphere, the atmosphere is switched to a weak reducing atmosphere for 2-3 min, and this cycle is repeated 2-3 times. After the last cycle, the oxidizing atmosphere is restored until the roasting is completed. The oxygen volume fraction in the oxidizing atmosphere is 8%-12%, and the carbon monoxide volume fraction in the weak reducing atmosphere is 3%-5%.

[0015] Preferably, in step S5, pelletizing yields 8-12mm green pellets. The green pellets are then sieved and graded to separate green pellets with a particle size not greater than 10mm and those greater than 10mm. These green pellets are then fed into two parallel calcining kilns for calcination in step S6, with the same calcination process parameters. After calcination, the two particle sizes of ceramsite are mixed at a mass ratio of 1:1-1.5 to obtain graded ceramsite products.

[0016] The present invention has at least the following beneficial effects: First, this invention addresses the problem of severely uneven temperature distribution in the material bed caused by the low specific heat capacity and tendency for flow deviation in traditional hot air preheating. It employs superheated steam as the preheating medium. Superheated steam possesses extremely high enthalpy and condensation heat release characteristics. When it comes into contact with the relatively low-temperature coal gangue mixture, it releases a large amount of latent heat through phase change, achieving efficient and uniform heating of the material from the surface inwards. This significantly reduces the inherent temperature lag zone and localized overheating phenomena of traditional hot air preheating. Simultaneously, the condensation process of steam itself is a self-regulating humidification mechanism. The amount of condensate precisely matches the temperature rise requirements of the material, achieving synchronous and uniform heating and humidification. This provides a base mixture with highly uniform temperature and moisture distribution for the subsequent pelletizing process, eliminating quality defects in green pellets caused by uneven thermal processes at the source.

[0017] Secondly, this invention overcomes the limitations of traditional pelletizing processes that rely solely on added moisture and ordinary binders. During steam preheating, the condensate does not simply act as a wetting medium, but rather participates in situ and activates the hydrolysis reaction of water glass in the first additive. The resulting silica gel, together with sodium carboxymethyl cellulose, synergistically constructs a strong and tough three-dimensional network framework between coal gangue particles. This synergistic chemical and physical strengthening mechanism endows green pellets with wet strength and plasticity far exceeding those of conventional methods. The green pellets exhibit excellent resistance to deformation and cracking during subsequent conveying, stacking, and drying processes, providing a fundamental guarantee for large-scale production and high yield rates. Its technical approach differs significantly from the simple mixing of binders in existing technologies, demonstrating remarkable ingenuity.

[0018] Third, the homogenized green pellet structure established during the preheating stage provides a perfect foundation for the physicochemical reactions during the high-temperature calcination stage. Because the internal temperature and moisture distribution of the green pellets are consistent, and the skeleton formed by the gel network is uniform, the reaction initiation time and gas release rate of the gas-generating component (silicon carbide) tend to be synchronized across the entire pellet range during calcination, avoiding "cracking" or "under-firing" caused by localized overheating or underheating. Simultaneously, the homogenous precursor structure facilitates the coordinated generation of the liquid phase and the transformation of the physical phase at the microscale. The resulting ceramsite exhibits uniform internal pore distribution, no large-size defects, and no residual carbon cores, thus obtaining a high-quality product with consistent apparent density and high strength, solving the industry problem of large performance fluctuations in ceramsite in existing technologies.

[0019] Fourth, this invention employs a step-by-step, functionally separate process flow. Network-forming agents such as water glass are added before the preheating step, allowing them to fully react in a mild, humid hydrolysis environment with water involvement, thus constructing a network capable of supporting the strength of green pellets. High-temperature gas-generating components such as silicon carbide are added after preheating, preventing pre-oxidation or hydration reactions that could lead to their failure in the humid environment of the preheating stage. This ensures optimal pore-forming and strengthening performance during the high-temperature calcination stage. This sequential process arrangement, designed based on the functional characteristics of each component, achieves a perfect decoupling and synergy between the two major functions of low-temperature strengthening and high-temperature gas generation.

[0020] Other advantages, objectives and features of the present invention will become apparent in part from the following description, and in part from those skilled in the art through study and practice of the invention. Attached Figure Description

[0021] Figure 1 This is a schematic diagram of the method flow of one of the technical solutions of the present invention. Detailed Implementation

[0022] The following is combined with Figure 1 The flowchart shown provides a further detailed description of the present invention, enabling those skilled in the art to implement it based on the description.

[0023] It should be noted that, unless otherwise specified, the experimental methods described in the following implementation plan are all conventional methods, and the reagents and materials described are all commercially available unless otherwise specified.

[0024] Example 1 This embodiment provides a method for producing ceramsite by preheating coal gangue mixture with steam, including the following steps.

[0025] S1. The coal gangue raw material is fed into a crusher for coarse crushing, and then into a ball mill for grinding. The ground material is sieved, and the undersize material with a particle size of less than 0.2 mm is collected to obtain fine coal gangue powder.

[0026] S2. Weigh the above-mentioned fine coal gangue powder as the base material. Separately weigh water glass and sodium carboxymethyl cellulose as the first additive. The amount of water glass added accounts for 3% of the total mass of the first mixture, and the amount of sodium carboxymethyl cellulose added accounts for 0.5% of the total mass of the first mixture. Add the fine coal gangue powder, water glass, and sodium carboxymethyl cellulose together into a mixer and perform dry mixing for 60 seconds to ensure that the components are fully dispersed, thereby obtaining the first mixture.

[0027] S3. Load the first mixture into a preheating container equipped with a stirring device and a steam inlet. Start the stirring device to agitate the material. Simultaneously, introduce superheated steam at a pressure of 0.2 MPa and a temperature of 150°C into the container to preheat the first mixture. Continue to introduce superheated steam during preheating, gradually increasing the temperature of the first mixture. When the temperature of the first mixture reaches 75°C, stop introducing superheated steam. At this point, the first mixture has absorbed moisture released by the condensation of the superheated steam during the heating process.

[0028] After stopping the steam supply, the stirring device continues to run and is kept at this temperature for 10 minutes to allow the water glass to fully react with the absorbed water, generating silica gel. The generated silica gel interacts with sodium carboxymethyl cellulose in the system to form a three-dimensional network structure. This network structure encapsulates and bridges the fine coal gangue particles together, resulting in a second mixture with a certain strength.

[0029] S4. Take the second mixture after the above-mentioned heat preservation is completed and set it aside. Separately weigh silicon carbide and dolomite powder as the second additive, wherein the amount of silicon carbide is calculated as 1% of the total mass of the subsequent third mixture, and the amount of dolomite powder is calculated as 4% of the total mass of the third mixture. Preheat the weighed silicon carbide and dolomite powder to 50°C, so that its temperature is close to that of the second mixture. Then add the preheated second additive to the second mixture and mix them evenly to ensure that the second additive is fully dispersed in the material, thus obtaining the third mixture.

[0030] S5. The above third mixture is fed into a pelletizer for pelletizing. The pelletizing process parameters are controlled to obtain green pellets with a diameter of 8mm-12mm.

[0031] S6. The obtained green pellets are fed into a firing kiln for firing. The firing process includes a drying section, a preheating section, a firing section, and a cooling section. The specific process parameters are as follows: the temperature of the drying section is 200℃, and the drying time is 25 minutes; the temperature of the preheating section is 400℃, and the preheating time is 20 minutes; the temperature of the firing section is 1150℃, and the firing time is 30 minutes; the cooling section uses forced ventilation to cool to below 100℃, and the finished ceramsite is obtained after exiting the kiln.

[0032] This embodiment uses superheated steam to preheat the mixture, leveraging the high heat-carrying capacity and condensation heat release characteristics of steam to ensure more uniform material heating and avoid the localized overheating or underheating that easily occurs with traditional hot air preheating. Simultaneously, the moisture provided by steam condensation is precisely used for the hydrolysis reaction of water glass, eliminating the need for additional water addition. The added heat preservation step after preheating provides sufficient time for the hydrolysis reaction of water glass, ensuring the full formation of silica gel and the complete construction of the three-dimensional network structure. Before adding the second additive, preheating the second additive reduces the temperature difference with the second mixture, preventing thermal shock to the already formed network structure due to excessively rapid localized cooling and ensuring structural stability. The resulting ceramsite pellets have high strength, do not crack, and exhibit a uniform internal structure after calcination. The technical effects of this embodiment include: smooth and crack-free surface of the raw pellets; ≥10 times unbroken after free fall from a height of 1m onto a steel plate; uniform cross-section of the ceramsite after calcination with no visible black core; compressive strength of 8-12MPa; and apparent density of 650-850kg / m³. 3 Water absorption rate ≤5% in 1 hour, black core rate ≤5%, and strength variation coefficient ≤10%.

[0033] Example 2 This embodiment provides a method for producing ceramsite by preheating coal gangue mixture with steam, including the following steps.

[0034] S1. The coal gangue raw material is crushed, ground, and sieved to obtain fine coal gangue powder with a particle size of less than 0.2 mm. The specific operation is the same as in Example 1.

[0035] S2. Take fine coal gangue powder, add 5% water glass and 1.5% sodium carboxymethyl cellulose according to the total mass of the first mixture, and dry mix in a mixer for 120s to obtain the first mixture.

[0036] S3. Load the first mixture into a preheated container and start stirring. Introduce superheated steam at a pressure of 1 MPa and a temperature of 200°C for preheating. When the temperature of the first mixture reaches 80°C, stop introducing superheated steam. After stopping steam, continue stirring and maintain the temperature at 80°C for 30 minutes to allow the water glass to fully hydrolyze and generate silica gel, which together with sodium carboxymethyl cellulose forms a three-dimensional network structure, yielding the second mixture.

[0037] S4. Take the second mixture mentioned above. Separately weigh silicon carbide (3% of the total mass of the third mixture) and dolomite powder (6% of the total mass of the third mixture), and preheat them to 70°C. Then mix the preheated second additive with the second mixture evenly to obtain the third mixture.

[0038] S5. Form the third mixture into pellets to obtain green pellets with a diameter of 8mm-12mm.

[0039] S6. The raw pellets are roasted. The roasting process parameters are as follows: drying section temperature 400℃, drying time 15min; preheating section temperature 700℃, preheating time 10min; roasting section temperature 1250℃, roasting time 20min; cooling section forced ventilation cooling to below 100℃ to obtain ceramsite.

[0040] This embodiment employs higher steam pressure and temperature, as well as a longer holding time, making it suitable for applications with large throughput or low initial temperatures. The second additive, added at a higher temperature after preheating, further reduces the mixing temperature difference. The resulting ceramsite exhibits stable performance. Due to the higher steam pressure, temperature, and longer holding time, the green pellet strength is slightly improved compared to Example 1, with ≥12 intact drops from 1m depth; the ceramsite cylinder compressive strength is 9-14MPa, and the apparent density is 600-800kg / m³. 3 Water absorption rate ≤4.5% in 1 hour, black core rate ≤4%.

[0041] Example 3 This embodiment provides a method for producing ceramsite by preheating coal gangue mixture with steam, including the following steps.

[0042] S1. The coal gangue raw material is crushed, ground, and sieved to obtain fine coal gangue powder with a particle size of less than 0.2 mm.

[0043] S2. Take fine coal gangue powder, add water glass accounting for 4% of the total mass of the first mixture and sodium carboxymethyl cellulose accounting for 1% of the total mass of the first mixture, dry mix for 90s to obtain the first mixture.

[0044] S3. Load the first mixture into a preheated container and introduce superheated steam at a pressure of 0.5 MPa and a temperature of 180°C while stirring. When the material temperature reaches 78°C, stop introducing steam. Continue stirring and maintain the temperature at 78°C for 20 minutes to allow the water glass to fully hydrolyze and form silica gel, thus obtaining the second mixture.

[0045] S4. Take the second mixture mentioned above. Separately weigh silicon carbide (2% of the total mass of the third mixture) and dolomite powder (5% of the total mass of the third mixture), and preheat them to 60°C. Mix them evenly to obtain the third mixture.

[0046] S5. Form the third mixture into pellets to obtain green pellets with a diameter of 8mm-12mm.

[0047] S6. The raw pellets are roasted. The roasting process parameters are as follows: drying section temperature 300℃, drying time 20min; preheating section temperature 550℃, preheating time 15min; roasting section temperature 1200℃, roasting time 25min; cooling section forced ventilation cooling to below 100℃ to obtain ceramsite.

[0048] The parameters used in this embodiment are between those of Embodiment 1 and Embodiment 2, and are suitable for conventional production conditions. By controlling the steam parameters, holding time, preheating temperature of the second additive, and calcination curve, good coordination between each process is ensured. In this embodiment, the number of free-fall intact balls at 1m depth is ≥10; the compressive strength of the ceramsite is 8-13MPa, and the apparent density is 620-840kg / m³. 3 The water absorption rate is ≤5% and the black core rate is ≤5% after 1 hour. The effect is between that of Example 1 and Example 2, and the process stability is good.

[0049] Comparative Example 1 This comparative example provides a method for preparing coal gangue ceramsite, the steps of which are basically the same as those in Example 1, except that in step S3, conventional hot air is used instead of superheated steam for preheating, the hot air temperature is 150°C, and the material temperature is increased to 75°C before proceeding to the next step, and no heat preservation is performed during the preheating process. The remaining steps are the same as in Example 1.

[0050] The results showed that the green pellets prepared in Comparative Example 1 experienced a small number of cracks during transportation. After calcination, the uniformity of the strength of the ceramsite was found to be inferior to that of Example 1. In this comparative example, due to the use of hot air preheating, the temperature distribution of the material layer was uneven, the number of intact green pellets dropped from 1m depth was ≤5, and microcracks were visible on the surface of the green pellets. After calcination, the ceramsite cross-section showed obvious black cores (black core rate ≥15%), with a cylinder compressive strength of 5-8MPa and a strength variation coefficient ≥20%, significantly inferior to Example 1.

[0051] Comparative Example 2 This comparative example provides a method for preparing coal gangue ceramsite, the steps of which are basically the same as those in Example 1, except that the addition methods in steps S2 and S4 are different.

[0052] Specifically, in step S2 of this comparative example, the first additive (water glass, sodium carboxymethyl cellulose) and the second additive (silicon carbide, dolomite powder) are added to the fine coal gangue powder and mixed evenly to obtain a mixture. Then, in step S3, the mixture containing both the first and second additives is preheated by passing superheated steam through it to raise the material temperature to 75°C and absorb the steam condensate. After preheating, no further additive mixing steps are performed, and the pelletizing in step S5 and calcination in step S6 are carried out directly. The remaining parameters are the same as in Example 1.

[0053] The results showed that the green pellets prepared in this comparative example had slightly lower plasticity than those in Example 1 during pelletizing, and occasional microcracks were observed on the surface of the green pellets. Analysis suggests that the silicon carbide and dolomite powder came into contact with moisture and heat prematurely during the steam preheating stage, resulting in partial pre-reaction or physical adsorption, which affected their gas generation and fluxing effects during the subsequent high-temperature calcination stage. Simultaneously, the presence of the second additive may have physically hindered the formation of the three-dimensional network structure by the first additive, leading to a less complete network structure than in Example 1. In this comparative example, because the first and second additives were added simultaneously and subjected to steam preheating, the silicon carbide underwent pre-oxidation or hydration reactions during the preheating stage, resulting in a decrease in high-temperature gas generation. Occasional microcracks were observed on the surface of the green pellets, the compressive strength of the ceramsite decreased by 20%-30% (6-9 MPa) compared to Example 1, the black core rate was ≥10%, and the pore distribution uniformity was poor.

[0054] Comparative Example 3 This comparative example provides a method for preparing coal gangue ceramsite, the steps of which are basically the same as those in Example 1, the difference being that the types of the first additive and the second additive are different.

[0055] Specifically, in step S2 of this comparative example, conventional inorganic binder bentonite is used instead of water glass and sodium carboxymethyl cellulose as the first additive, and the amount of bentonite added accounts for 5% of the total mass of the first mixture. In step S4, conventional pore-forming agent pulverized coal is used instead of silicon carbide, and conventional fluxing agent limestone powder is used instead of dolomite powder as the second additive, and the amount of pulverized coal added accounts for 2% of the total mass of the third mixture, and the amount of limestone powder added accounts for 5% of the total mass of the third mixture. The remaining steps and parameters are the same as in Example 1.

[0056] The results showed that the green pellets prepared in this comparative example exhibited significant drying shrinkage and surface cracking during transportation and stacking. The calcined ceramsite, after testing, showed lower compressive strength than that of Example 1, and the uniformity of pore distribution within the ceramsite was also inferior. Analysis suggests that the greater dehydration shrinkage of bentonite after steam preheating led to green pellet cracking; while the irregular shape and uneven size of the pores left after the coal powder, acting as a pore-forming agent, affected the strength of the ceramsite. In this comparative example, due to the use of bentonite instead of water glass / CMC, coal powder instead of silicon carbide, and limestone instead of dolomite, the green pellets exhibited a larger dehydration shrinkage rate after steam preheating, resulting in a cracking rate ≥10%; the irregular pore shape after coal powder combustion resulted in a ceramsite cylinder compressive strength ≤6MPa, uneven apparent density, and a black core rate ≥20%, making the overall performance significantly inferior to Example 1.

[0057] Example 4 This embodiment provides a method for producing ceramsite by preheating coal gangue mixture with steam, including the following steps.

[0058] S1. The coal gangue raw material is fed into a crusher for coarse crushing, and then into a ball mill for grinding. The ground material is sieved, and the undersize material with a particle size of less than 0.2 mm is collected to obtain fine coal gangue powder.

[0059] S2. Weigh the above-mentioned fine coal gangue powder as the base material. Separately weigh water glass, sodium carboxymethyl cellulose, and nano-silica as the first additive. The amount of water glass added is 4% of the total mass of the first mixture, the amount of sodium carboxymethyl cellulose added is 1% of the total mass of the first mixture, and the amount of nano-silica added is 1% of the total mass of the first mixture. Add the fine coal gangue powder, water glass, sodium carboxymethyl cellulose, and nano-silica together to a mixer and perform dry mixing for 90 seconds to ensure thorough dispersion of the components, thus obtaining the first mixture.

[0060] S3. Load the first mixture into a preheating container equipped with a stirring device and a steam inlet. Start the stirring device to agitate the material. Simultaneously, introduce superheated steam at a pressure of 0.5 MPa and a temperature of 180°C into the container to preheat the first mixture. Continue to introduce superheated steam during preheating, allowing the temperature of the first mixture to gradually rise. When the temperature of the first mixture reaches 78°C, stop introducing superheated steam. At this point, the first mixture has absorbed moisture released by the condensation of the superheated steam during the heating process.

[0061] After the steam supply is stopped, the stirring device continues to run and is kept at this temperature for 20 minutes. During the heat preservation process, the water glass in the first mixture undergoes a hydrolysis reaction with the absorbed moisture to generate silica gel. The surface of the nano-silica particles is rich in silanol groups, which form hydrogen bonds with the generated silica gel under humid and hot conditions, inducing the silica gel to deposit and grow around the nano-silica. The generated silica gel interacts with sodium carboxymethyl cellulose in the system to form a composite three-dimensional network structure anchored by nano-silica. This network structure encapsulates and bridges the fine coal gangue particles together, resulting in the second mixture.

[0062] S4. Take the second mixture after the above-mentioned heat preservation is completed and set it aside. Separately weigh silicon carbide, dolomite powder, and aluminum sulfate as the second additive, wherein the amount of silicon carbide is calculated as 2% of the total mass of the subsequent third mixture, the amount of dolomite powder is calculated as 5% of the total mass of the third mixture, and the amount of aluminum sulfate is calculated as 0.5% of the total mass of the third mixture. Preheat the weighed silicon carbide, dolomite powder, and aluminum sulfate to 60°C, so that its temperature is close to that of the second mixture. Then add the preheated second additive to the second mixture and mix them evenly to ensure that the second additive is fully dispersed in the material, thus obtaining the third mixture.

[0063] S5. The above third mixture is fed into a pelletizer for pelletizing. The pelletizing process parameters are controlled to obtain green pellets with a diameter of 8mm-12mm.

[0064] S6. The obtained raw pellets are fed into a firing kiln for firing. The firing process includes a drying section, a preheating section, a firing section, and a cooling section. The specific process parameters are as follows: the temperature of the drying section is 300℃, and the drying time is 20 min; the temperature of the preheating section is 550℃, and the preheating time is 15 min; the temperature of the firing section is 1200℃, and the firing time is 25 min; the cooling section uses forced ventilation to cool to below 100℃, and the finished ceramsite is obtained after exiting the kiln.

[0065] In this embodiment, nano-silica is added to the first additive, allowing it to form a composite network structure with silica gel during the steam preheating stage. The nano-silica acts as a rigid anchor, dispersing shrinkage stress and inhibiting microcrack initiation during the subsequent calcination process when the silica gel dehydrates and shrinks. Aluminum sulfate is added to the second additive, decomposing in the preheating section (400℃-700℃) to generate activated alumina and release SO3 gas. The SO3 escapes, forming microchannels that promote oxygen diffusion into the silicon carbide particles. Simultaneously, the activated alumina reacts with the silica generated from the oxidation of the silicon carbide surface, consuming the surface oxide layer and allowing the internal silicon carbide to continue oxidizing, generating more silica to participate in the mullite formation reaction. The synergistic effect of nano-silica and aluminum sulfate protects the integrity of the gel network structure while promoting the full oxidation of silicon carbide and the uniform formation of the reinforcing phase. The resulting ceramic particles have a dense internal structure, no visible microcracks, and high compressive strength. Based on Example 3, this embodiment, by adding nano-silica and aluminum sulfate, suppresses microcracks in the green body, increases the compressive strength of the ceramsite by 10%-15% (i.e., 9-15 MPa) compared to Example 3, and has an apparent density of 650-850 kg / m³. 3 The water absorption rate is ≤4% in 1 hour, the black core rate is ≤3%, and the microcracks on the cross-section are significantly reduced. The synergistic effect of nano-silica and aluminum sulfate further enhances the compressive strength of the ceramsite.

[0066] Example 5 This embodiment provides a method for producing ceramsite by preheating coal gangue mixture with steam, including the following steps.

[0067] S1. The coal gangue raw material is crushed, ground and sieved to obtain fine coal gangue powder with a particle size of less than 0.2 mm.

[0068] S2. Dry mix the fine coal gangue powder with the first additive for 90 seconds to obtain the first mixture. The first additive includes water glass and sodium carboxymethyl cellulose, with water glass accounting for 4% of the mass of the first mixture and sodium carboxymethyl cellulose accounting for 1% of the mass of the first mixture.

[0069] S3. Preheat the first mixture. During the preheating process, superheated steam at a pressure of 0.5 MPa and a temperature of 180°C is introduced into the first mixture to raise its temperature to 78°C and allow it to absorb the superheated steam, thus obtaining the second mixture. After stopping the steam supply, maintain the temperature at this level for 20 minutes.

[0070] S4. Mix the preheated second mixture with the second additive until homogeneous to obtain the third mixture. The second additive includes silicon carbide and dolomite powder, with silicon carbide accounting for 2% of the mass of the third mixture and dolomite powder accounting for 5% of the mass of the third mixture. The second additive is preheated to 60°C before being added.

[0071] S5. Form the third mixture into pellets to obtain green pellets with a diameter of 8mm-12mm.

[0072] S6. The green pellets are fed into the firing kiln for firing. The firing process includes, in sequence, a drying section, a preheating section, a medium-temperature slow firing homogenization section, a firing section, and a cooling section.

[0073] The drying section temperature is 300℃, and the drying time is 20 minutes.

[0074] The preheating temperature is 550℃ and the preheating time is 15 minutes.

[0075] After the preheating stage, the material enters the medium-temperature slow-burning homogenization stage. In this stage, the material temperature is increased from 550℃ to 900℃ at a rate of 4℃ / min, and held at 900℃ for 20 minutes. This slow heating and holding process fully homogenizes the internal temperature field of the material, creating conditions for subsequent phase transformation. Within this temperature range, the dehydration and shrinkage process of the silica gel is nearing completion, and the shrinkage stress is gradually released through high-temperature creep. The transitional alumina generated from the decomposition of aluminum sulfate (if added) in the preheating stage completes its crystal transformation at this temperature, forming a more stable reaction morphology. Silicon carbide begins to oxidize significantly, and the generated silica has sufficient time to diffuse into the surrounding matrix, avoiding the formation of an excessively thick oxide layer on the particle surface. Simultaneously, within this temperature range, mullite nuclei begin to form in large quantities and are evenly distributed.

[0076] After the medium-temperature slow-firing homogenization stage, the ceramic granules enter the calcination stage. The calcination temperature is 1200℃, and the calcination time is 25 minutes. At this temperature, the uniformly distributed mullite crystal nuclei formed in the early stage grow rapidly, forming an interwoven needle-like mullite crystal network, which constitutes the main reinforcing skeleton of the ceramic granules.

[0077] Finally, the material enters the cooling section, where it is cooled to below 100°C using forced ventilation. Once removed from the kiln, the finished ceramsite product is obtained.

[0078] This embodiment solves the problem of asynchronous microstructure evolution caused by excessive temperature jumps during rapid heating by adding a medium-temperature slow-firing homogenization section between the preheating and calcining sections. A slow heating rate of 3℃ / min-5℃ / min within the 850℃-950℃ temperature range minimizes the internal temperature gradient of the material, allowing for sufficient release of thermal stress. A 20-minute holding period at 900℃ provides ample time for several key physicochemical processes: the dehydration and shrinkage of the silica gel is nearing completion and stress relaxation occurs; the decomposition products of aluminum sulfate complete crystal transformation; the silica generated from silicon carbide oxidation diffuses sufficiently; and a large number of mullite crystal nuclei are formed uniformly. The thorough execution and good matching of these processes lay a homogenized foundation for the orderly growth of mullite crystals and the construction of the interwoven network in the subsequent calcining section, ultimately resulting in a denser and more uniform microstructure of the ceramsite and a significant improvement in macroscopic compressive strength. This embodiment adds a medium-temperature slow-firing homogenization stage (850-950℃, holding for 15-25 min) to Example 3, which allows for the uniform formation of mullite crystal nuclei and significantly improves the uniformity of the ceramsite strength. The compressive strength is 10-15 MPa, the coefficient of variation of strength is ≤8% (superior to ≤10% in Example 3), and the apparent density is 600-800 kg / m³. 3 Water absorption rate ≤4.5% in 1 hour, black core rate ≤3%.

[0079] Example 6 This embodiment provides a method for producing ceramsite by preheating coal gangue mixture with steam, including the following steps.

[0080] S1. The coal gangue raw material is crushed, ground and sieved to obtain fine coal gangue powder with a particle size of less than 0.2 mm.

[0081] S2. Weigh the above-mentioned fine coal gangue powder. Separately weigh water glass, sodium carboxymethyl cellulose, and nano-silica as the first additive, with water glass accounting for 4% of the total mass of the first mixture, sodium carboxymethyl cellulose accounting for 1%, and nano-silica accounting for 1%. Dry mix for 90 seconds to obtain the first mixture.

[0082] S3. The first mixture is loaded into a preheated container, and superheated steam at a pressure of 0.5 MPa and a temperature of 180°C is introduced under stirring. After the temperature is raised to 78°C, the steam is stopped and the temperature is maintained for 20 minutes to allow the water glass to fully hydrolyze and generate silica gel, which together with nano-silica and sodium carboxymethyl cellulose forms a composite three-dimensional network structure to obtain the second mixture.

[0083] S4. Take the second mixture. Separately weigh silicon carbide, dolomite powder, and aluminum sulfate as the second additive, with silicon carbide accounting for 2% of the total mass of the third mixture, dolomite powder accounting for 5%, and aluminum sulfate accounting for 0.5%. Preheat the second additive to 60°C, add it to the second mixture, and mix evenly to obtain the third mixture.

[0084] S5. Form the third mixture into pellets to obtain green pellets with a diameter of 8mm-12mm.

[0085] S6. The green pellets are fed into the firing kiln for firing. The firing process includes, in sequence, a drying section, a preheating section, a medium-temperature slow firing homogenization section, a firing section, and a cooling section.

[0086] Drying section: 300℃, 20min. Preheating section: 550℃, 15min. Medium-temperature slow-cooking homogenization section: Increase temperature to 900℃ at 4℃ / min, hold for 20min.

[0087] After the medium-temperature slow sintering homogenization stage, the calcination stage begins. The calcination temperature is 1200℃, and the total calcination time is 25 minutes. Periodic atmospheric disturbances are applied within the calcination stage, as detailed below: First, an oxidizing atmosphere is introduced for roasting for 6 minutes, with an oxygen volume fraction of 10%. Then, a weak reducing atmosphere is switched to for roasting for 2.5 minutes, with a carbon monoxide volume fraction of 4%. This constitutes one cycle. This cycle is repeated three times: oxidation 6 minutes → reduction 2.5 minutes → oxidation 6 minutes → reduction 2.5 minutes → oxidation 6 minutes → reduction 2.5 minutes. After the final reduction cycle, the oxidizing atmosphere is restored until the roasting stage ends.

[0088] During the oxidation phase, oxygen oxidizes the silicon carbide surface to form a silicon dioxide layer. In the subsequent reduction phase, carbon monoxide reacts with silicon dioxide to generate gaseous silicon monoxide (SiO), which escapes from the silicon carbide particle surface and diffuses into the surrounding matrix. When the next oxidation phase arrives, the gaseous silicon monoxide is oxidized and redeposited as silicon dioxide. Through multiple cycles of "oxidation generation-reduction vaporization-migration-reoxidation deposition," the silicon dioxide generated from the oxidation of silicon carbide is "transported" from the particle surface into the entire ceramic particle matrix, achieving a uniform distribution of silicon. Simultaneously, the silicon carbide particle surface remains fresh due to the periodic consumption of the oxide layer, allowing for continuous oxidation.

[0089] Finally, it enters the cooling section, where it is forced to cool to below 100°C, and then exits the kiln to obtain the finished ceramsite product.

[0090] This embodiment solves the problem of excessively thick oxide layer on the surface of silicon carbide particles hindering further internal oxidation by applying periodic atmosphere disturbance in the calcination section. The periodic switching between oxidation and reduction significantly improves the utilization rate of silicon carbide, while the generated silica is uniformly distributed throughout the matrix, fully contacting and reacting with nano-silica seeds and aluminum sulfate decomposition products, promoting the uniform formation of mullite across the entire scale, and significantly improving the structural uniformity and compressive strength of the ceramsite. Based on Examples 4 and 5, this embodiment further increases the periodic atmosphere disturbance in the calcination section (oxidation 6 min → reduction 2.5 min, cycle 3 times), significantly improving the utilization rate of silicon carbide, uniformly distributing silicon, and generating mullite across the entire scale. The compressive strength is 12-18 MPa, and the apparent density is 550-750 kg / m³. 3 Water absorption rate ≤3.5% in 1 hour, black core rate ≤2%, and strength variation coefficient ≤6%.

[0091] Example 7 This embodiment provides a method for producing ceramsite by preheating coal gangue mixture with steam, including the following steps.

[0092] S1. The coal gangue raw material is crushed, ground and sieved to obtain fine coal gangue powder with a particle size of less than 0.2 mm.

[0093] S2. Weigh the above-mentioned fine coal gangue powder. Separately weigh water glass and sodium carboxymethyl cellulose as the first additive, with water glass accounting for 4% of the total mass of the first mixture and sodium carboxymethyl cellulose accounting for 1%. Dry mix for 90 seconds to obtain the first mixture.

[0094] S3. The first mixture is loaded into a preheated container, and superheated steam at a pressure of 0.5 MPa and a temperature of 180°C is introduced while stirring. After the temperature is raised to 78°C, the steam is stopped and the temperature is maintained for 20 minutes to allow the water glass to fully hydrolyze and generate silica gel, which together with sodium carboxymethyl cellulose forms a three-dimensional network structure to obtain the second mixture.

[0095] S4. Take the second mixture. Separately, take silicon carbide and dolomite powder as the second additive, with silicon carbide accounting for 2% of the total mass of the third mixture and dolomite powder accounting for 5%. Preheat the second additive to 60°C, add it to the second mixture, and mix evenly to obtain the third mixture.

[0096] S5. The third mixture is fed into a pelletizer to produce green pellets with a diameter of 8mm-12mm. The green pellets are then fed into a screening device for sieving and grading. Green pellets with a diameter ≤10mm and green pellets with a diameter >10mm are separated and collected separately.

[0097] S6. The two types of green pellets after screening are separately fed into two parallel firing kilns for firing. Both kilns use identical firing process parameters: drying section temperature 300℃, drying time 20min; preheating section temperature 550℃, preheating time 15min; firing section temperature 1200℃, firing time 25min; cooling section forced ventilation cooling to below 100℃. Because the two kilns process green pellets of a single particle size range separately, uneven firing due to differences in heat transfer when green pellets of different particle sizes are mixed and fired is avoided.

[0098] After firing, ceramsite with a particle size ≤10mm and ceramsite with a particle size >10mm were obtained from the two kilns respectively. The two types of ceramsite with different particle sizes were mixed at a mass ratio of 1:1.2, so that the smaller particle size ceramsite filled the gaps between the larger particle size ceramsite skeleton, resulting in graded ceramsite product.

[0099] This embodiment solves the problem of high packing void ratio and the need for additional fine aggregate when using single-size ceramsite in concrete preparation by employing a "separate firing and mixed use" mode of pelletizing, screening, grading, parallel firing, and mixing of finished products. It also avoids uneven firing due to differences in heat transfer when mixing and firing green pellets of different sizes. In the resulting graded ceramsite product, large particles form the skeleton, while small particles fill the voids, significantly reducing the packing void ratio. It can be directly used to prepare high-performance lightweight aggregate concrete, improving the workability and mechanical properties of the concrete. This embodiment obtains graded ceramsite products by pelletizing, screening (with 10mm as the boundary), parallel kiln firing, and mixing the finished products at a 1:1.2 ratio. The performance of single-size ceramsite is comparable to that of Example 3 (compressive strength 8-13MPa), but the packing void ratio of the mixed graded ceramsite is ≤35% (lower than the 40% of single-size ceramsite), allowing it to be directly used to prepare high-performance lightweight aggregate concrete, significantly improving the workability and compressive strength of the concrete.

[0100] Although embodiments of the present invention have been disclosed above, they are not limited to the applications listed in the specification and embodiments. They can be applied to various fields suitable for the present invention. Other modifications can be easily made by those skilled in the art. Therefore, without departing from the general concept defined by the claims and their equivalents, the present invention is not limited to the specific details and examples shown and described herein.

Claims

1. A method for producing ceramsite by preheating coal gangue mixture with steam, characterized in that, Includes the following steps: S1. The coal gangue raw material is crushed, ground and sieved to obtain coal gangue fine powder with a particle size of less than 0.2 mm; S2. Dry mix the fine coal gangue powder with the first additive for 60-120 seconds to obtain the first mixture. The first additive includes water glass and sodium carboxymethyl cellulose. The water glass accounts for 3%-5% of the mass of the first mixture, and the sodium carboxymethyl cellulose accounts for 0.5%-1.5% of the mass of the first mixture. S3. Preheat the first mixture. During the preheating process, superheated steam with a pressure of 0.2MPa-1MPa and a temperature of 150℃-200℃ is introduced into the first mixture to raise the temperature of the first mixture to 75℃-80℃ and absorb the superheated steam to obtain the second mixture. S4. Mix the preheated second mixture with the second additive evenly to obtain the third mixture. The second additive includes silicon carbide and dolomite powder. Silicon carbide accounts for 1%-3% of the mass of the third mixture, and dolomite powder accounts for 4%-6% of the mass of the third mixture. S5. Pelletize the third mixture to obtain green pellets; S6. The raw pellets are roasted to obtain ceramsite.

2. The method for producing ceramsite by preheating coal gangue mixture with steam according to claim 1, characterized in that, In step S6, the calcination process includes a drying section, a preheating section, a calcination section, and a cooling section in sequence. The temperature of the drying section is 200℃-400℃, and the drying time is 15-25 min. The temperature of the preheating section is 400℃-700℃, and the preheating time is 10-20 min. The temperature of the calcination section is 1150℃-1250℃, and the calcination time is 20-30 min. The cooling section is forcibly ventilated to cool to below 100℃ to obtain ceramsite.

3. The method for producing ceramsite by preheating coal gangue mixture with steam according to claim 1, characterized in that, In step S3, when the temperature of the first mixture rises to 75℃-80℃, the superheated steam is stopped, and the mixture is kept at that temperature for 10-30 minutes before proceeding to step S4.

4. The method for producing ceramsite by preheating coal gangue mixture with steam according to claim 1, characterized in that, In step S4, the second additive is preheated to 50℃-70℃ and then mixed evenly with the second mixture.

5. The method for producing ceramsite by preheating coal gangue mixture with steam according to claim 1, characterized in that, In step S2, the first additive also includes nano-silica at a mass of 0.5%-1.5% of the first mixture; In step S4, the second additive also includes aluminum sulfate at a mass of 0.3%-0.8% of the third mixture.

6. The method for producing ceramsite by preheating coal gangue mixture with steam according to claim 1, characterized in that, During the roasting process in step S6, a medium-temperature slow-burning homogenization section is set between the preheating section and the roasting section. The temperature of the medium-temperature slow-burning homogenization section is 850℃-950℃, and the holding time is 15-25min.

7. The method for producing ceramsite by preheating coal gangue mixture with steam according to claim 1, characterized in that, During the roasting process in step S6, periodic atmosphere disturbances are applied in the roasting section: after roasting in an oxidizing atmosphere for 5-8 minutes, the atmosphere is switched to a weak reducing atmosphere for 2-3 minutes, and this cycle is repeated 2-3 times. After the last cycle, the oxidizing atmosphere is restored until the roasting is completed. The oxygen volume fraction in the oxidizing atmosphere is 8%-12%, and the carbon monoxide volume fraction in the weak reducing atmosphere is 3%-5%.

8. The method for producing ceramsite by preheating coal gangue mixture with steam according to claim 1, characterized in that, Step S5 pelletizing yields 8-12mm green pellets. The green pellets are then sieved and graded to separate those with a particle size no greater than 10mm and those greater than 10mm. These green pellets are then fed into two parallel calcining kilns for calcination in step S6, with the same calcination process parameters. After calcination, the two particle sizes of ceramsite are mixed at a mass ratio of 1:1-1.5 to obtain graded ceramsite products.

Images