An analysis of radial segregation for different sized spherical solids in rotary cylinders
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INTRODUCTION
ROTARY reactors are commonly used in the chemical and metallurgical industries to process granular and powdered solids. The many and varied applications of these reactors include ball mills for comminution, driers, mixers, blenders, and kilns. In each of these unit operations granular solids are set in motion radially in the reactor. The design and mode of operation of rotary reactors in the metallurgical industry vary depending on the application. In general, though, rotary kilns tend to have length to diameter ratios, (L/D), greater than 10/1. They are operated continuously with not more than 20 pct solids (by volume). The burden moves down the kiln by virtue of its rotation (up to Fr = 10 -3) and inclination (up to 5 pct). Driers, on the other hand, are operated with higher loadings (about 30 pct), higher Froude numbers (-10-2), and may not be inclined. They are operated continuously and have an L/D as low as 3/1. Ball mills generally have L/D's up to 2/1 and are operated at Fr of 0.5 to 0.75, with 50 pet loadings. They are not inclined but are operated continuously. Finally, mixers and blenders are commonly batch operated with Fr ranging between driers and ball mills. The type of bed motion used, slumping, rolling, cascading, cataracting, or centrifuging,~ varies with the application and the type of reactor. For example, in ball mills cataracting is preferred, while in the sintering of iron oxides both cascading and rolling actions are required. In the processing of oxide dispersion strengthened composites, cascading would be the preferred mode of bed behavior to completely mix metal and ceramic powders. In all of these systems, the feed material may consist of more than one component, each having a different density and its own size and shape distribution. As solids travel through the reactors, they may undergo physical and chemical changes. In the N. NITYANAND, formerly a Graduate Student, B. MANLEY, formerly an Undergraduate Student, and H. HENEIN, Associate Professor, are with the Department of Metallurgical Engineering and Materials Science, Carnegie-Mellon University, Pittsburgh, PA 15213. Manuscript submitted June 27, 1985. METALLURGICALTRANSACTIONS B
calcination of limestone, the size distribution of the feed may be broadened by degradation and fines generation. For other applications such as the direct reduction of iron, coal, iron ore, and flux with varying shape, size and density are charged to a rotary kiln. Due to these differences, segregation occurs often resulting in poor reactor performance, an unacceptable product, or poor energy efficiencies. The aim of this work is to quantify the effect of material, design, and operating variables on radial segregation. This work deals with segregation due to size differences of spherical solids applicable to the design and operating variables of kilns and driers. An experimental technique to be described in this paper was developed to quantify the kinetics of segregation and to identify the mechanism and scaleup of the radial segregation phe
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