(74b) Air Current Segregation of Alumina Powder

Although the production of aluminium is a well- established industrial process, problems frequently occur in the handling of the main feedstock, alumina. One cause of problems is segregation of fine particles during the storage of the alumina in silos. Identified mechanisms include air current segregation, which is object of this study. During the filling of a storage silo with alumina powder, air is simultaneously entrained and dusty air is often extracted from the silo. As a result, air flow circulation arises and separates fine particles (less than 42 microns) from the bulk stream which are then deposited to the outer areas of the silo. Upon discharge of the silo, the fraction of fines in the powder varies significantly and influences negatively the aluminium production process. It is the objective of this study to identify key parameters such as the feeding rate or the air extraction rate to develop a scaling model which scales experimental results from a small scale silo to industrial equipment. Preliminary experiments have included the investigation of the air extraction rate and air current segregation in a small cylindrical silo (40cm diameter x 1 m high) which was filled with alumina powder. The results showed the deposition of fine material on the outer areas of the silo. The silo allowed measurements of the particle velocities with Laser Doppler Anemometry (LDA). One result of the preliminary experiments revealed that air current segregation was greater when no air extraction was used. To develop a scaling model the dimensional analysis method has been applied to form dimensionless groups out of the significant parameters. Five dimensionless groups were obtained which is unwieldy. To reduce the number of dimensionless groups the physical properties were lumped into the terminal velocity. This simplified approach gives three dimensionless groups. Initial experiments justify further research to establish weather the simplified approach can scale the dynamic of the flow and the degree of segregation from a small scale silo to industrial equipment.