It is necessary to clarify the influence of thermal history on the conversion of aluminate species in sodium aluminate solution in order to optimize Bayer alumina production. The interconversion of various solution species in the systems was investigated by measuring the infrared spectra of sodium aluminate solution with different compositions after separate heat treatment, dilution and concentration. The results show that increasing temperature or prolonging holding time favors the transformation of Al2O（OH）2- to Al—OH vibration（condensed Al O4 tetrahedral aluminate ion） at about 880 cm-1 and Al（OH）-4. A12O（OH）2-66 and Al—OH tetrahedral dimer ions convert rapidly to Al（OH）-4 during the dilution process; however, the back transformation of Al（OH）-4 to the Al—OH tetrahedral dimer ions can occur in diluted sodium aluminate solution. As for the concentration process, the transformation of Al（OH）-4 to A12O（OH）2-6 and Al—OH tetrahedral dimer ions can take place, while it is relatively difficult to transform to A12O（OH）2-6.
Surface tension of sodium aluminate solution and the contact angle between Al（OH）3 particles and aluminate solution were measured, then the dependence of Al（OH）3 solubility on its particle size was calculated and thus the variation of the critical nucleus sizes was determined based on the Ostwald ripening formula. The results show that the Al（OH）3 solubility in sodium aluminate solution decreases with the increment of particle size, and the critical nucleus sizes increase with the rise of alkali concentration, caustic ratio and precipitation temperature. The results also imply that the presence of small particles in seeded precipitation system is an important factor to limit the depth of precipitation.
