Does Alumina Particle Size Influence High-Alumina Wear-Resistant Plastic Refractory Properties? A Guide
Alumina micropowder is widely used in refractory materials due to its excellent dispersibility, sintering-promoting effect, and ability to enhance mechanical properties. Variations in the particle size of alumina micropowder can lead to significant changes in its specific surface area, thermodynamic properties, and other characteristics, which in turn affect the performance of high-alumina wear-resistant plastic refractory. Taking three types of alumina micropowder with different particle sizes as examples, this paper discusses the influence of alumina micropowder particle size on the properties of high-alumina wear-resistant plastic refractory.
Circulating Fluidized Bed (CFB) boilers, as an important type of clean combustion equipment, are widely used in the construction and renovation of thermal power plants. This has driven the continuous improvement and development of refractory materials suitable for CFB boilers. The lining structure of CFB boilers is complex, and a large number of coal particles and flue gas circulate inside the combustion chamber at high concentration and high velocity. Therefore, refractory materials used for CFB boiler linings must not only exhibit good construction performance, but also possess high strength and excellent wear resistance.
High-alumina wear-resistant plastic refractory is mainly used in CFB boiler lining areas with complex structures that cannot be formed by conventional casting with fixed molds, but can be installed by prefabrication, manual ramming, or troweling. Typical application areas include water-cooled walls, primary material return zones, and cyclone separators. High-alumina wear-resistant plastic refractory is generally a type of refractory material with a certain degree of viscosity and plasticity, prepared by mixing aggregates, fine powders, binders, and accelerators in specific proportions.
The selection of alumina micropowder is of great significance to both the construction performance of high-alumina wear-resistant plastic refractory and the service life of CFB boilers. Therefore, this paper analyzes the effects of three different alumina micropowders on the mechanical properties and wear resistance of the plastic refractory after firing.
01. Test Procedure
1.1 Raw Materials
The wear-resistant plastic refractory was prepared using premium bauxite, alumina micropowder, clay, and silica micropowder as the main raw materials, with aluminum dihydrogen phosphate serving as the binder and a hardening accelerator added as an auxiliary component. The chemical compositions of the main raw materials are shown in Table 1.
1.2 Preparation and Testing
Experiments were conducted using three types of alumina micropowder (A, B, and C) with different particle sizes, respectively.
The room-temperature modulus of rupture, room-temperature compressive strength, and wear resistance of the dried and heat-treated samples were tested in accordance with the relevant standards (GB/T 3001-2017, GB/T 5072-2008, and GB/T 18301-2012). The fracture morphologies of the samples heat-treated at 900 °C and 1100 °C were observed using a scanning electron microscope (SEM).
02.Results and Discussion
2.1 Effects of Alumina Micropowders with Different Particle Sizes on Strength at Different Heat-Treatment Temperatures
Figure 1 shows the effects of different heat-treatment temperatures on the modulus of rupture and compressive strength of the wear-resistant plastic refractories containing three types of alumina micropowder. As shown in the figure, the room-temperature modulus of rupture and compressive strength of all three samples increase with increasing heat-treatment temperature; however, the extent of these changes differs among the samples at different temperatures.
After drying at 110 °C, the modulus of rupture and compressive strength of the three samples are essentially the same. After heat treatment at 900 °C, Sample No. 2 exhibits the highest strength, followed by Sample No. 1 and Sample No. 3. However, after heat treatment at 1100 °C, Sample No. 1, which contains the alumina micropowder with the smallest particle size, shows higher strength than Sample No. 2 and Sample No. 3.
This behavior can be explained by the fact that when the particle size of the micropowder decreases to a certain extent, the filling effect is limited by interparticle agglomeration, and the strength does not increase significantly prior to sintering. When the temperature exceeds 1000 °C, sintering occurs; under these conditions, smaller micropowder particle sizes result in higher activity and consequently greater strength.
2.2 Effects of Different Alumina Micropowders on Wear Resistance at 900 ℃
As shown in Figure 2, after heat treatment at 900 °C, all three samples exhibit the same trend in both room-temperature and high-temperature wear resistance, following the order: Sample No. 2 > Sample No. 1 > Sample No. 3. Analysis indicates that at this heat-treatment temperature, the sample blocks remain unsintered, and the wear resistance at this stage mainly depends on the bonding strength between the aggregates and the matrix.
As shown in Figure 3, no liquid phase is formed at 900 °C. At this temperature, the bonding strength of the three samples mainly depends on the hydration of aluminate, the filling effect of alumina and silica micropowders, and the polymerization reaction of aluminum dihydrogen phosphate. The particle size of the alumina micropowder also affects the strength of the samples. As the particle size decreases, the filling effect becomes more pronounced. However, unlike castable refractories, excessively fine alumina micropowder tends to agglomerate, increasing interparticle resistance. This reduces the filling effect, increases porosity, and consequently decreases the strength of the samples.
Figure 4 shows the fracture morphology images of the three samples after heat treatment at 1100 °C.
As shown in the SEM images in Figure 4, at 1100 °C, the mullite phase has not yet formed from the alumina micropowder and liquid-phase SiO₂. In Sample No. 1, the matrix begins to melt, enveloping the bauxite and alumina micropowder particles. The formation of the liquid phase subsequently increases the porosity. Compared to Sample No. 1, Sample No. 2 has a slightly lower liquid-phase content, as indicated by the greater exposure of particles and the presence of smaller, fewer pores. The overall morphology of Sample No. 2 is very similar to that of Sample No. 1. The molten liquid phase from silica micropowder and clay forms wetting bonds with other components, resulting in a tight combination of aggregates and matrix, which enhances the sintering strength of the sample. In Sample No. 3, the liquid phase is at an initial stage of formation, with less liquid phase, densely packed materials, and low porosity. It can be concluded that for the three samples with identical other components, finer alumina micropowder promotes the generation of the liquid phase by silica micropowder at lower temperatures. For Sample No. 3, with a larger particle size, the temperature required for liquid-phase formation is correspondingly higher.
03 Conclusions
No liquid phase is formed in the high-alumina wear-resistant plastic refractory at 900 °C. The bonding strength at this temperature is mainly due to the cementation of silica micropowder and the hydration of the binder. As the particle size of the alumina micropowder decreases, agglomeration becomes more pronounced, and friction between components increases. This impairs the filling effect, leading to increased porosity and reduced strength.
The smaller the particle size of the alumina micropowder, the higher its activity. When the temperature exceeds 1000 °C, finer alumina micropowder lowers the temperature required for liquid-phase formation, promotes the sintering of the matrix, and improves the strength and wear resistance of the refractory.
In high-alumina wear-resistant plastic refractories, selecting alumina micropowder with an appropriate particle size ensures good construction performance, physical strength, and favorable cost performance.