Key Process Parameters Analysis of Rotary Tube Furnace in Refractory Material Preparation

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Key Process Parameters Analysis of Rotary Tube Furnace in Refractory Material Preparation

1. Introduction
As core basic materials in high-temperature industrial fields, refractory materials are widely used in key parts such as furnace linings and high-temperature equipment components in metallurgy, building materials, chemical industry, energy and other industries. Their performance directly affects the stability, safety and economy of industrial production. Equipped with a unique structural design (e.g., continuous feeding/discharging, uniform mixing and heating of materials by constant rotation of the furnace tube), precise temperature control capability and flexible atmosphere adjustment function, the rotary tube furnace has become an indispensable core equipment in refractory material preparation, especially suitable for key process links such as powder sintering, particle modification and composite powder synthesis.

Based on industrial production practice and experimental data, this paper systematically analyzes the core process parameters (temperature, atmosphere, rotation speed, material residence time, etc.) of rotary tube furnace in refractory material preparation, deeply discusses the influence mechanism of each parameter on key indicators of refractory materials such as density, strength, high-temperature resistance and chemical stability, so as to provide theoretical support and practical reference for the precise regulation of process parameters in industrial production.

2. Core Process Parameters and Influence Mechanism of Rotary Tube Furnace

2.1 Temperature Parameters: Core Driving Factor for Refractory Sintering
Temperature is the most critical parameter affecting the sintering process of refractory materials, directly determining the phase transformation, grain growth and densification degree of materials. Its regulation needs to balance "sufficient sintering" and "avoiding over-sintering".
Sintering Temperature: There are significant differences in sintering temperatures of different types of refractory materials (e.g., the sintering temperature of alumina-based refractories is usually 1600-1800℃, silicon carbide-based is 1800-2000℃, and mullite-based is 1500-1700℃). When the temperature is lower than the critical temperature, only surface diffusion occurs between material particles, resulting in low density and insufficient strength of the sintered body; after reaching the sintering temperature, lattice diffusion and volume diffusion are intensified, particles fuse and pores are discharged, forming a dense structure. However, excessively high temperature will lead to abnormal grain growth and grain boundary cracking, which will reduce the high-temperature strength and thermal shock resistance of refractory materials.
Temperature Rise Rate: The temperature rise rate needs to match the heat conduction characteristics and thermal decomposition behavior of materials. Too fast temperature rise will cause huge temperature gradient inside the material, leading to thermal stress cracking; at the same time, the adsorbed water, crystal water and organic additives (such as binders) in the material volatilize/decompose rapidly, which is likely to cause defects such as pores and cracks in the sintered body. Usually, the temperature rise rate in refractory material preparation is controlled at 5-15℃/min, and for materials containing volatile components or large-sized green bodies, it needs to be reduced to 3-8℃/min.
Holding Time: The core function of the holding stage is to promote phase homogenization and densification process. Insufficient holding time will result in incomplete phase transformation, difficulty in completely discharging pores, and large fluctuations in the performance of the sintered body; excessively long holding time will lead to abnormal grain growth and increase energy consumption and production costs. Generally, the holding time is 2-6 hours, which needs to be adjusted according to material characteristics, sintering temperature and green body thickness (e.g., the holding time of thick-walled products can be extended to 8-12 hours).

2.2 Atmosphere Parameters: Regulating Reaction Path and Product Performance
The rotary tube furnace can construct oxidizing, reducing, inert or neutral atmospheres by introducing different gases (such as air, nitrogen, argon, hydrogen, carbon monoxide, etc.), thereby regulating the chemical reaction path in the preparation process of refractory materials, avoiding the oxidation or reduction of target components, and affecting the microstructure and performance of the sintered body.

Oxidizing Atmosphere: Usually achieved by introducing air or oxygen, it is suitable for the sintering of oxide refractories (such as alumina, zirconia, mullite, etc.). The oxidizing atmosphere can promote the oxidation and removal of impurities (such as carbon and sulfur) in the material, and at the same time help the uniform growth of oxide grains, improving the density and purity of the sintered body. However, it should be noted that for refractories containing low-valent oxides (such as FeO, MnO), the oxidizing atmosphere may cause them to be oxidized to high-valent oxides, leading to volume changes and affecting the dimensional stability of products.

Reducing Atmosphere: Constructed by introducing hydrogen, carbon monoxide or ammonia decomposition gas (N₂+H₂), it is mainly used for the preparation or sintering of carbide and nitride refractories (such as silicon carbide, silicon nitride, aluminum nitride, etc.). The reducing atmosphere can inhibit the oxidation of target components (e.g., SiC is easily oxidized to SiO₂ at high temperature, and the introduction of CO can inhibit this reaction), and promote the reduction reaction and sintering densification of materials. For example, in the preparation of silicon nitride refractories, the introduction of a mixed gas of nitrogen and hydrogen can promote the nitridation reaction of silicon powder to generate high-purity Si₃N₄.

Inert Atmosphere: Nitrogen and argon are commonly used as inert gases, which are suitable for refractories sensitive to oxygen or prone to phase transformation (such as zirconia-toughened refractories, aluminum titanate refractories, etc.). The inert atmosphere can isolate air, avoid material oxidation, and maintain the original phase structure of the material to ensure stable product performance. For example, in the sintering of zirconia refractories, the introduction of argon can prevent ZrO₂ from being reduced, and at the same time inhibit its crystal transformation, improving the thermal shock resistance of the material.

Atmosphere Flow Rate and Pressure: The atmosphere flow rate should ensure uniform and stable atmosphere in the furnace to avoid local atmosphere imbalance affecting product quality, usually controlled at 0.5-2L/min; the pressure in the furnace is generally maintained at a slight positive pressure (0.01-0.05MPa), which can prevent the infiltration of external air and ensure atmosphere purity.

2.3 Furnace Tube Rotation Speed and Material Residence Time: Ensuring Uniform Mixing and Heating
The rotation speed of the rotary tube furnace directly determines the mixing degree and residence time of materials in the furnace, thereby affecting the heating uniformity and reaction sufficiency of materials, and is a key regulation parameter in continuous production.

Furnace Tube Rotation Speed: When the rotation speed is too low, materials are prone to accumulate in the furnace tube, resulting in uneven local heating and incomplete sintering; excessively high rotation speed will shorten the material residence time, leading to insufficient reaction, and increase material wear and energy consumption. Generally, the rotation speed is controlled at 5-20r/min. For powdery materials with high viscosity, the rotation speed can be appropriately reduced (3-8r/min); for granular materials or systems requiring sufficient mixing, the rotation speed can be increased to 10-25r/min. In addition, the inclination angle of the furnace tube (usually 3-5°) will also affect the moving speed of materials, which needs to be adjusted in coordination with the rotation speed.

Material Residence Time: The residence time refers to the total time from when the material enters the furnace tube from the feed port to when it is discharged from the discharge port. Its length directly determines the reaction degree of the material under the target temperature and atmosphere. Too short residence time results in insufficient sintering and substandard product performance; excessively long residence time is likely to cause problems such as over-sintering and grain growth. In industrial production, the residence time can be achieved by adjusting the furnace tube rotation speed, inclination angle and furnace tube length, usually controlled at 30-120 minutes. For example, for refractories requiring deep sintering, the rotation speed can be reduced, the furnace tube length can be increased, and the residence time can be extended to 60-120 minutes; for rapid modification processes, the rotation speed can be increased, and the residence time can be shortened to 30-60 minutes.

2.4 Material Characteristics and Loading Capacity: Basic Factors Affecting Sintering Effect
Material Particle Size and Particle Size Distribution: The smaller the material particle size, the larger the specific surface area, the higher the surface activity, the stronger the sintering driving force, and the lower the required sintering temperature and shorter holding time. However, excessively fine particle size is prone to powder agglomeration, which affects mixing uniformity and sintering densification. Therefore, it is necessary to control the material particle size within a reasonable range (usually 1-50μm) and ensure uniform particle size distribution.

Material Moisture Content and Purity: Excessively high moisture content in the material will cause rapid evaporation of water during the sintering process, generating pores and cracks. Therefore, the material needs to be dried in advance (moisture content controlled below 0.5%); material purity directly affects product performance. Impurities (such as Fe₂O₃, CaO, MgO, etc.) may form low-melting phases, reducing the refractoriness and high-temperature strength of refractory materials. Therefore, the raw material purity should be guaranteed to be ≥99% (except for special requirements).
Material Loading Capacity: The loading capacity needs to match the furnace tube volume and rotation speed. Excessive loading will result in too thick material layer, hindering heat transfer and incomplete sintering of internal materials; insufficient loading will reduce production efficiency and increase energy consumption per unit product. Usually, the material loading capacity is 30%-50% of the effective volume of the furnace tube to ensure that the material can be fully stirred and uniformly heated during the rotation of the furnace tube.

3. Conclusion and Outlook
The regulation of process parameters of rotary tube furnace in refractory material preparation directly determines the microstructure and core performance of products. Among them, temperature, atmosphere, furnace tube rotation speed, material residence time and material characteristics are key regulation dimensions. The efficient and high-quality preparation of refractory materials can be achieved by scientifically setting the sintering temperature and heating/holding system, reasonably selecting the atmosphere type and parameters, collaboratively optimizing the furnace tube rotation speed and material residence time, and strictly controlling the material characteristics and loading capacity.

In the future, with the continuous improvement of performance requirements for refractory materials in high-temperature industries (such as higher refractoriness, better thermal shock resistance and corrosion resistance), the regulation of process parameters of rotary tube furnaces will develop in the direction of "precision and intelligence". By introducing automatic control systems (such as PID temperature control, atmosphere flow closed-loop control), online monitoring technologies (such as real-time monitoring of material density and temperature during sintering) and numerical simulation methods (such as CFD fluid mechanics simulation, sintering process numerical simulation), the dynamic optimization and precise control of process parameters can be realized, further improving the product quality and production efficiency of refractory materials, and providing strong support for the green and efficient development of high-temperature industries.

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Key Process Parameters Analysis of Rotary Tube Furnace in Refractory Material Preparation

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