The use of aerodynamic turbo classifier in the production of calcined coke dust fraction

Abstract

Anode paste for aluminum reduction cells represents a composite material consisting of calcined coke and coal tar pitch. A coke aggregate mix includes 3 to 7 fractions with a particles size of 12 mm down to 63 μm, and even less. Grain fractions larger than 0.2 mm are prepared by crushing and screening. Grain fractions form a sort of skeleton, which makes the aggregate more stable. For filling the space between the grains, a dust fraction with particles less than 0.2 mm is used. The dust fraction comprises up to 50% of the aggregate and can contain up to 75% of particles with a size of <63 μm. The dust fraction is prepared in a fine grinding circuit of ball mills. The separation (or classification) of particles from a gas-dispersed flow, which is generated in the ball mill, takes place in classifiers of various designs. Modern air classification systems use aerodynamic turbo classifiers providing for a wide range of classification. This paper discusses the use of turbo classifiers in the process of preparing dust for the anode paste used in aluminum reduction cells. It contains the results of simulating flow separation boundaries based on different classification conditions: air flow, angle of rotation of guide vanes and turbine RPM. Particle size distributions (PSDs) were obtained for those products that result from separating the gas-dispersed flow. They were obtained empirically, under shop floor conditions, at turbine RPMs of 150 and 200 rpm. Based on the said data, separation curves were plotted and 50-percent separation boundaries were calculated: 55 μm at 150 rpm and 35 μm at 250 rpm.