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Elutriation of particles from rotary kilns Tackie, Emmanuel Nii


The elutriation of fine materials from the solids bed in rotary kilns was studied in a laboratory scale steel cylinder of 0.2m in diameter and 2.4m in length. The cylinder was charged with a batch of fine alumina particles having a mean size of 64jum and the average elutriation rate measured at different air flow rates, rotational speeds and percentage of solids fill. All measurements were done at room temperature. To show the effect of fines concentration and segregation in the solids bed, a binary mixture of fine alumina and coarse Ottawa sand was used. Local dust concentration profiles were measured in the freeboard through a probe equipped with a filter. Design factors such as the geometry of the kiln exit dams, were found to influence dust carryover into the cleaning equipment by accelerating the flowing gas and or obstructing the flow of solids in the gas phase. Wall roughness and imperfections also affected elutriation especially at higher rotational speeds by exposing trapped fines directly into the flowing gas. Dust concentration measurements revealed that most of the solids in the gas phase travelled in saltation within about 2 cm above the bed surface. With the wall effect eliminated by an insert, increasing the rotational speed was found to exhibit a negative effect on the elutriation rate. Dust concentrations were higher in the gas phase above the lower edge of the rotating bed than at the upper edge or midpoint. However, while the concentration above the rest of the bed remained fairly constant with increased rotational speeds, at the lower edge of the bed it decreased. Banding segregation occurred in the beds composed of fine and coarse particles. Elutriation increased with the number of fine bands formed which was proportional to the concentration of fines. The location of the bands from the exit also influenced elutriation. Saltating particles returning to the bed close to the exit had a better chance of ejecting other particles if they landed on fine bands than they would if they landed on coarse bands. Gas velocity exhibited the strongest influence on elutriation rate. A correlation of experimental results showed a velocity dependence of U⁶ regardless of initial fines concentration in the bed. An entrainment mechanism has been formulated based on the collision of saltating particles on the solids bed. Subsequently, a simple mathematical model was developed to describe the influence of the operating variables on elutriation. The model predictions were verified with the experimental data and the scanty data in the literature. The model requires knowledge of the saltation height and the threshold shear stress for particle movement. Model predictions for typical industrial kilns are presented. The predictions are in fairly good agreement with values reported in a survey of industrial kiln operations made prior to the experimental program, given that the effect of kiln internals was not accounted for in the model.

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