EFFECT OF ADDITIVES ON THE DENSIFICATION AND MICROSTRUCTURAL DEVELOPMENT OF LOW-GRADE ALUMINA POWDERS

The densification and microstructural evolution of a metallurgical gra de alumina which is produced as an electrofilter residue during the Ba yer 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 w as applied during the processing for the homogeneous mixing of alumina powders and additives. The treatment of metallurgical low-grade alumi na powders through a series of calcination. hot washing and recalcinat ion is shown to improve the powder quality, resulting in microstructur es with densities of 90-96.7% of the theoretical density with the sint ering aids. It is found that the addition of 1% B2O3 improves the comp actibility 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 homog eneously-distributed finer grain size. Strength values in the 180-380 MPa range were obtained.