Effect of additives on the densification and microstructural development of low-grade alumina powders

The densification and microstructural evolution of a metallurgical grade alumina which is produced as an electrofilter residue during the Bayer process was investigated using a variety of additives. TiO2, MnO2, SiO2 and MgO were used as sintering aids to enhance densification and microstructural development during sintering. A colloidal technique was applied during the processing for the homogeneous mixing of alumina powders and additives. The treatment of metallurgical low-grade alumina powders through a series of calcination. hot washing and recalcination is shown to improve the powder quality, resulting in microstructures with densities of 90-96.7% of the theoretical density with the sintering aids. It is found that the addition of 1% B2O3 improves the compactibility of particles. to yield higher green densities. An additive with the composition 1%TiO2-1.5%MnO2-2%SiO2-0.5%MgO results in a homogeneously-distributed finer grain size. Strength values in the 180-380 MPa range were obtained.