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

## TL;DR

This paper studies how amorphous silicates in tricalcium aluminate hydrates affect acid neutralization, which is important for treating bauxite residue.

## Contribution

The study reveals new insights into how silicate phases alter the acid neutralization mechanism and capacity of tricalcium aluminate hydrates.

## Key findings

- Pseudo-KAT contains amorphous silicates and has a lower buffering range compared to TCA.
- The presence of silicates reduces acid consumption by about 17%.
- TCA forms Friedel’s salt during neutralization, while PKAT does not.

## Abstract

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)

x

(OH)12–4x
), 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 x ≈ 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.