Effect of Amorphous Silicates on the Neutralization of Tricalcium Aluminate Hexahydrate Relevant to Bauxite Residue Treatment

Acid neutralization is an efficient way to lower the solution pH of bauxite residue slurries (generated via the Bayer process) to a level that enables safe handling and further utilization. Tricalcium aluminate hexahydrate (TCA, Ca₃Al₂(OH)₁₂) and its silicate-substituted polymorph, katoite (KAT, Ca₃Al₂(SiO₄)<i>_<i>x</i></i>(OH)_12-4<i>x</i>), are one of the major sources that contribute to the high alkalinity of residues. Yet, the neutralization chemistry of KAT phases is poorly understood. To this end, we synthesized TCA and KAT and studied their acid-base reactions. We find the as-prepared pseudo-KAT (PKAT) phase to be a poorly substituted TCA with <i>x</i> ≈ 0.05 while containing amorphous calcium silicate hydrate as well as sodium aluminosilicate minor phases. Upon addition of HCl, TCA first transforms to Friedel's salt, a layered double hydroxide hosting Cl^- ions, which is absent for PKAT. Further, the lack of LDH precipitation, closely related to calcium silicate hydrate, gives rise to a lower buffering range (by ∼0.5-1.2 pH units) for PKAT. Another striking consequence of minor silicate phases in PKAT is the ∼17% smaller acid consumption as compared to TCA. In conclusion, amorphous silicates markedly affect both the neutralization mechanism and capacity of tricalcium aluminate hydrates.