An experimental study of segregation in rotary kilns
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INTRODUCTION
SEGREGATION is a naturally occurring phenomenon encountered when processing granular materials. In a rotary kiln, segregation in the solids bed frequently occurs radially and longitudinally because the feed consists of solids having size, shape, and density differences. Moreover, within the kiln the solids undergo chemical and physical changes which contribute further to the distributions of size, shape, and density. Segregation in the bed of a rotary kiln is important because it often results in poor kiln operation or in an unacceptable product. For example, if fine and coarse material is fed to a lime kiln, the fines segregate in the center of the bed where they receive less heat and calcine more slowly than the coarse particles. The lime product thus is calcined unevenly and is not marketable for most uses. The mechanism of segregation, radially or longitudinally, and its effect on the flow of the granular solids in a rotary kiln are not well understood. Hence, the objectives of the present study have been to examine the influence of segregation on the bed behavior and to determine the shape and composition of the segregated core. The operative mechanism for the segregation has been determined from the results.
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are observed to form a horizontal core in the bed, ~-11whereas in banding segregation alternate bands of coarse and fine or light and heavy particles form down the length of the rotary cylinder. 7"8'12'~8The end-longitudinal segregation is such that the finer or denser particles form two end-bands at the cylinder walls with the coarse or lighter particles in the center of the cylinder axis. 2'7'8'12"14 Segregation in rotary cylinders is enhanced by large differences in particle size, shape, and density. 1"4'6'9-11"12'1~-16 The most comprehensive analysis of segregation has been made by Donald and Roseman. 8~s 17Their findings showed that radial segregation always occurs while the type of longitudinal or axial segregation, if it occurs, may be predicted based on the static angles of repose of the components of the mixture. When the smaller sized particles have a static angle of repose greater than that of the coarse fraction, the banding
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(a)
Radial
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Bonding longitudinal
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End
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PREVIOUS WORK
Segregation studies conducted on horizontal rotary cylinders have identified three types of segregation patterns in terms of radial and longitudinal segregation as shown in Figure 1. In radial segregation, smaller or denser particles H HENEIN, formerly Graduate Student, Department of Metallurgical Engineenng, The Umversity of British Columbia, is now Assistant Professor with the Department of Metallurgical Engineering and Materials Science, Carnegie-Mellon University, Pittsburgh, PA 15213. J. K. BRIMACOMBE, Stelco/NSERC Professor and Director, The Centre for Metallurgical Process Engineering, Department of Metallurgical Engineering, and A. P WATKINSON, Professor, Department of Chemtcal Engineering, are with The University of British C
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