GONTIJO, Glayson Stopa.
Resumen:
The alumina produced through the extraction of the gibbsite from the bauxite
ore by caustic liquor is constrained by the soluble sodium aluminate instability in
the supersaturated liquor processed in alumina refineries. The aluminum hydrate
precipitation, when takes place inside equipments not designed to this purpose,
like press filters, washers or thickeners, will generate significant losses to the
refinery. Therefore, the stabilization of the soluble sodium aluminate will reduce the
liquor supersaturation and, as consequence, the driving force to the aluminum
hydrate precipitation will be reduced as well. The supersaturation decrease was
obtained from the usage of stabilizers, named nucleation inhibitors. The
precipitation performance will not suffer negative impacts from the stabilizers
usage due to the extremely elevated driving force caused by the temperature drop
and aluminum hydrate seed addition. Stability trials were carried out dosing
nucleation inhibitors under two distinct forms: a polymer which is soluble in water
and a metallic cation solution. It was demonstrated that the elevated
supersaturation of the liquors in the filtration feed and first mud washer of the mud
washers train was reduced. The evidence for that was the significant ratio drop
decrease. The influence of the stabilizer amount dosed and the liquor solids
concentration were evaluated. The solids concentration presented a large potential
to reduce the liquor instability. The effects generated by the solids were the
precipitation rate increase and bigger ratio drop. The amount of stabilizer dosed
presented a small impact over the resultant stability. The stabilization effect
obtained from the metallic cation usage can be obtained only if it remains soluble.
After the addition of the metallic cation dosage into the liquor, the concentration of
this specie raised. However, this concentration started to drop tending to stabilize
in an equilibrium concentration. The results obtained from the plant trial using the
polymer as stabilizer were similar to the ones obtained in laboratory. This study
concluded that nucleation inhibitors present a large potential to increase sodium
aluminate concentration in Bayer liquor and, as consequence, the refinery
production. Besides this, the residual life of some equipments can be increased as
well.