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Analysis and Discussion on Optimizing Characteristics of Firebrick
Some high-temperature kilns in China use imported refractory materials for their masonry linings, which can operate at temperatures up to 1760°C and have shown excellent performance. However, the high cost of importing these refractory bricks has prompted the need for a domestic alternative. To address this challenge, we developed lightweight corundum-mullite firebricks with properties comparable to imported products, conducting extensive research on factors that influence their performance.
The composition of raw materials plays a crucial role in determining the high-temperature behavior of corundum-mullite lightweight bricks. High levels of Al₂O₃ and SiO₂ are essential, while impurities should be minimized to ensure optimal performance. The selection of raw materials is based on the ability to form mullite during firing. For example, the amount of C-alumina added must react with clay below 1600°C to produce mullite, while A-alumina must be added in an amount sufficient to react with SiO₂ to form mullite with approximately 72.8% Al₂O₃. The reaction should be complete at 1700°C. The quantity of corundum is adjusted based on the ultrafine powder fraction (10 μm) during the first firing, ensuring that any remaining corundum can continue to react with Al₂O₃ during long-term use at 1750°C, forming a mullite solid solution with about 78% Al₂O₃ and a slight excess.
Based on these considerations and the minimum clay content required for shaping, seven experimental formulations were created. In all formulations, the same amounts of four raw materials were used: 13% Suzhou soil, 4% montmorillonite clay, 15% sillimanite, and 13% C-alumina. The only difference was the proportion of A-alumina and fused corundum. Sample No. 1 contained 10% A-alumina and 45% fused corundum, while each subsequent sample increased A-alumina by 5% and decreased fused corundum by 5%. Sillimanite had a particle size of 74 μm, decomposing around 1600°C to generate mullite and offset firing shrinkage. C-alumina and A-alumina had a particle size of 10 μm, while fused corundum was 43 μm. After adding sawdust, the mix was extruded, dried, and fired at 1700°C for 20 hours. The final density was controlled to 1.42 g/cm³, and key properties such as linear shrinkage, load softening temperature, and re-burning line change were measured.
As the corundum content increased, both firing and re-burning shrinkage decreased. However, the load softening temperature did not follow a linear trend; it reached its peak when the ratio of A-alumina to corundum was 20:35. This is because corundum has low reactivity, and excessive amounts lead to more liquid phase and residual corundum, reducing the load softening temperature. Firing temperature and time significantly affect production costs. Under the condition of meeting performance requirements, the firing temperature should be minimized, and the holding time reduced.
To study the effect of firing temperature, Sample No. 3 was fired at various temperatures (1600–1780°C) for 20 hours, then re-fired at 1700°C for 12 hours. The results showed changes in weight loss, line change, and load softening temperature. Similarly, the effect of holding time was tested by sintering Sample No. 3 at 1700°C for 4 to 32 hours. As the holding time increased, the reburning shrinkage decreased rapidly, and the load softening temperature rose quickly. However, after 20 hours, both values stabilized, indicating an optimal point beyond which further increases in time had minimal impact.