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Effective inhibition of Cr(VI) in the Al2O3-CaO-Cr2O3 refractory castables system through silica gel assisted in-situ secondary phase tuning

Nath, Mithun, Song, Shengqiang, Xu, Tengteng, Wu, Yingjiang, Li, Yawei
Journal of cleaner production 2019 v.233 pp. 1038-1046
calcium, carcinogenicity, cement, chromium, corrosion, oxidation, silica gel, strength (mechanics), temperature, water solubility
Cr₂O₃ is the best-known corrosion resistant refractory materials under high temperature extremely harsh environment. However, the formation of toxic, carcinogenic & water-soluble Cr(VI) compounds during its application remained a major predicament. Here, we have introduced a facile route to fabricate the Al₂O₃-CaO-Cr₂O₃ refractory castables to mitigate Cr(VI) using basic silica sol (pH∼9) via in-situ secondary phase modification. Basic silica sols 0.25, 0.49 and 0.98 wt% (CaO/SiO₂ ratio of 5.8, 2.9 and 1.45 respectively) were doped by keeping the castables properties reasonable. Addition of silica drastically reduces the mid-temperature (500–1100 °C) formation of Cr(VI) phase (CaCrO₄), while relatively higher temperature Cr(VI) phase (hauyne, Ca₄Al₆CrO₁₆) totally disappeared from 900 to 1300 °C. However, at 1300–1500 °C, silica addition promote the solid solution phase formation like Ca(Al,Cr)₁₂O₁₉ and (Al,Cr)₂O₃ where chromium exists as +3 oxidation state. Also, silica addition reduces the dimensional tolerance of the castables at a higher temperature associated with the secondary phases (dehydration of cement phases and expansive formation of CaAl₁₂O₁₉). A comparable mechanical strength (up to 156 MPa) and a remarkable Cr(VI) reduction (up to 94%) could be achieved upon silica addition.