The influence of silica on the properties of the specimens has been a subject of interest in previous studies, particularly focusing on the modification of lignin through hydroxylation and hydroxymethylation. However, the selection of raw materials often involves complex procedures, especially when dealing with high concentrations. The removal of impurities such as silicon dioxide (SiO2) is challenging, and when SiO2 is not separated during the preparation of refractory adhesives, it significantly enhances the high-temperature stability of the resulting material, which has drawn our attention.
By comparing the infrared spectra of Al-C brick specimens, a noticeable difference was observed between those without silica separation and those with it. This difference becomes 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 separation. The intensity of this peak increases with temperature, indicating the formation of more Si-C bonds within the sample. At 900°C, no SiC absorption peak is visible, but by 1100°C, it becomes clearly detectable. This suggests that the reaction between SiOâ‚‚ and organic carbon begins at around 1100°C, which is slightly lower than the temperature reported by KLINGERN et al. This discrepancy may be due to the gel state of SiOâ‚‚ and the large contact area between the organic binder and silica.
Additionally, a strong Si-O bond absorption peak (1093–1097 cmâ»Â¹) is observed in the internal spectrum, but this peak decreases rapidly with increasing temperature. In contrast, the surface spectrum only shows a strong Si-O peak, with no evidence of Si-C bonding. This could be attributed to the incomplete combustion of organic carbon inside the specimen, where oxygen comes both from residual air and from the phenolic bonds in the lignin-based adhesive. Studies by HIGUCHI et al. and ADLERE have shown that lignin contains a relatively high proportion of oxygen atoms.
At lower temperatures (below 400°C), the specimen exhibits numerous black, porous particles distributed uniformly, leading to reduced mechanical strength and increased brittleness. Despite this, lignin-based phenolic adhesives demonstrate better high-temperature resistance compared to traditional phenolic and aldehyde-based adhesives. Researchers like DIRES and BABONNEAUF suggest that this property is related to the presence of phenyl groups. Therefore, lignin-based adhesives are considered superior for use in cold-solid pellet refractories.
At higher temperatures, the addition of excess binder can enhance the formation of SiC within the specimen. The introduction of silica occurs during the modification of papermaking black liquor. After the phenolic hydroxylation process, sodium silicate is added under acidic conditions, leading to the rapid formation of silica gel. In the case of methylolation modification, sodium silicate is used to adjust the alkalinity of the system, acting as a catalyst. This process naturally incorporates a significant amount of silica into the Al₂O₃ powder. During high-temperature reactions, the presence of silica in a gel or free state ensures maximum contact with the binder, improving reaction kinetics and enabling the production of more SiC at lower temperatures. As a result, the specimens exhibit enhanced oxidation resistance, corrosion resistance, thermal shock resistance, and mechanical strength as the SiC content increases.
In conclusion, during the synthesis of refractory adhesives from lignin in papermaking black liquor, simply removing the sediment before concentration allows direct use. The remaining fibers, along with organic and inorganic components such as heterocells, serve as raw materials for in-situ SiC synthesis. This approach greatly simplifies the adhesive preparation process and supports the comprehensive utilization of lignin.
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