A process for removing anionic impurities from a caustic aluminate
solution having aqueous tetrahydroxy aluminate ions is described. A
caustic aluminate solution is obtained such that anionic impurities are
partially or substantially substituted into tetrahydroxy aluminate ions
to form substituted aluminate ions. A suitable calcium compound, such as
quicklime, is reacted in a conventional slaker (10) with a slaking
solution, for example water, and stored in an agitated storage tank (12).
The resulting slurry is pumped to a reaction vessel (14) where it is
reacted with a caustic aluminate solution under appropriate conditions
such that the calcium compound is converted into tricalcium aluminate
(TCA) precipitate. Upon reaction substituted aluminate ions are
substantially incorporated within the TCA precipitate to form a
substituted TCA precipitate enabling the anionic impurities to be removed
from the solution via the substituted TCA precipitate for disposal or
recovery. The caustic aluminate liquor may be optionally fortified with
an anionic impurity by mixing it in a mixing tank (16) with the anionic
impurity.