Particle suspension in air-agitated pachuca tanks: Investigation of hysteresis and a novel split air injection technique
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I. INTRODUCTION
PACHUCA tanks are cylindrical, air-agitated slurry reactors with a conical bottom primarily used as leaching vessels in the extraction of nonferrous metals such as uranium, gold, zinc, and copper. Details about the classification, design, and operation of Pachuca tanks are available in the literature.[1,2] As these tanks are operated with slurries containing 50 to 60 wt pct solids, the principal objective in Pachuca leaching is to keep the solids in suspension.[1,2] With the help of extensive experiments and mathematical modeling. Roy et al.[1] have clarified the effect of the design and operating parameters of Pachuca tanks on the minimum superficial air velocity, defined as the volumetric air flow rate divided by the tank cross-sectional area and referred to as the critical air velocity required to keep the particles in suspension. However, several important questions still remain unanswered. Critical air velocity can be defined separately for maintaining and generating suspensions. For maintaining suspensions, critical air velocity is defined as the superficial air velocity below which particles start settling from a state of complete off-bottom suspension. Superficial air velocity, in turn, is defined as the volumetric air flow rate divided by the tank cross-sectional area. On the other hand, the critical air velocity for generating suspension is defined as the superficial air velocity above which particles are suspended from a completely settled state. There is a possibility of hysteresis, because the two critical air velocities may be different. Particle suspension has been characterized with respect to both maintaining[1] and generating[3,4] suspensions. Moreover, both definitions of critical air velocity have practical relevance. In practice, several Pachuca tanks are operated in series and slurry is continuously fed through each tank. Hence, the critical air velocity should be such that the solid S.P. MEHROTRA, Professor, and R. SHEKHAR, Associate Professor, are with the Department of Materials and Metallurgical Engineering, Indian Institute of Technology, Kanpur 208 016, India. Manuscript submitted March 16, 2000. METALLURGICAL AND MATERIALS TRANSACTIONS B
content of the slurry is maintained in suspension. Also, in the case of a power failure or maintenance-related work, air injection into the Pachuca tank will be disrupted, leading to a complete settling of particles from the slurry. Under these conditions, the air compressor should have the capability to resuspend the settled solids; its power rating, therefore, should conform with the critical air velocity required to generate suspension. However, hysteresis in particle suspension has not been adequately characterized.[5,6] Moreover, studies on hysteresis have been limited to bubble columns only, which substantially differ from Pachuca tanks both in terms of dimensions and mode of operation.[1] The mode of air injection is also an important aspect of Pachuca design. In most conventional Pachucas, air is injected from the bottom. There
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