The influence of silica on the properties of the specimens has been a topic of interest in previous studies, particularly focusing on the hydroxylation and hydroxymethylation of lignin. However, the process of selecting raw materials is often complicated, especially when dealing with high concentrations. Removing impurities like silicon dioxide (SiO2) becomes challenging, and the presence of SiO2 can significantly enhance the high-temperature stability of refractory materials, which has drawn our attention.
By comparing the infrared spectra of Al-C brick specimens, we observed significant differences between those without and with silica separation. These differences become more pronounced as temperature increases. As the temperature rises from 900°C to 1300°C, an absorption peak corresponding to the Si-C bond (797–830 cmâ»Â¹) appears in the specimen without silica removal. The intensity of this peak increases with temperature, indicating the formation of more Si-C bonds. At 900°C, no SiC absorption peak is visible, but it becomes clear at 1100°C. This suggests that the reaction between SiO2 and organic carbon begins around 1100°C, which is slightly lower than the temperature reported by KLINGERN et al. This difference may be due to the gel state of SiO2 and the large contact area between the organic carbon and the binder.
Additionally, there is a strong Si-O bond absorption peak at 1093–1097 cmâ»Â¹ in the internal spectrum. However, this peak decreases rapidly as the temperature increases. In contrast, the surface spectrum shows only a strong Si-O absorption peak, with no evidence of Si-C bonds. This discrepancy may be attributed to the oxidation of organic carbon on the surface under non-protective conditions. As the temperature rises, the organic carbon on the surface starts to burn, creating a negative temperature gradient initially. Once internal combustion begins, the internal temperature exceeds the external temperature, leading to a positive gradient. However, the combustion remains incomplete, with oxygen coming from both residual air and the lignin’s phenolic bonds. Studies by HIGUCHI et al. and ADLERE suggest that lignin contains a high proportion of oxygen, contributing to its thermal stability.
At lower temperatures (below 400°C), the addition of excess binder leads to the formation of numerous black, porous particles within the specimen, reducing material strength and increasing brittleness. Despite this, lignin-based phenolic adhesives exhibit better high-temperature resistance compared to traditional phenolic or aldehyde-based adhesives, as noted by DIRES and BABONNEAUF. This heat resistance is believed to be related to the presence of phenyl groups, making lignin-based adhesives a superior choice for cold-solid pellet refractories.
At higher temperatures, the addition of more binder enhances the formation of SiC within the specimen. The introduction of silica occurs during the modification of papermaking black liquor. After phenolic hydroxylation, sodium silicate is added under acidic conditions, leading to rapid formation of silica gel. In methylolation processes, sodium silicate is used to increase alkalinity, acting as a catalyst. This method introduces a significant amount of silica into the Al₂O₃ powder, ensuring sufficient contact between the adhesive and silica. This improves reaction kinetics, allowing for more SiC to form at lower temperatures and shorter durations.
As SiC content increases, the resulting specimens show improved oxidation resistance, corrosion resistance, thermal shock resistance, and mechanical strength. Overall, removing impurities like SiOâ‚‚ from the papermaking black liquor before concentration allows direct use of the material. Residual fibers, organic components, and inorganic materials such as heterocells serve as precursors for in-situ SiC synthesis, simplifying the adhesive preparation process and promoting the comprehensive utilization of lignin.
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