Particle suspension in (air-agitated) pachuca tanks
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INTRODUCTION
PACHUCA tanks are air-agitated slurry reactors used as leaching vessels for the extraction of nonferrous metals like uranium, gold, zinc, and copper. These tanks are operated with slurries containing 50 to 60 wt pct solids. Depending on whether the tank is equipped with a draft tube or not, Pachuca tanks are classified as (Figure 1) (1) free-air-lift (FAL), where the draft tube is absent; (2) full-center-column (FCC), which have a full-length draft tube; and (3) stub-column (SC), which are equipped with a foreshortened draft tube. Further details of Pachuca tanks can be obtained elsewhere.[1,2] The principal objectives in the operation of Pachuca leaching are suspension of particles, dissolution of solid ore particles (solid-liquid mass transfer), and mixing. In some cases, oxygen mass transfer from air bubble to slurry is also an important parameter. Oxygen mass transfer is the ratelimiting factor in the cyanide leaching of gold and the carbonate leaching of uranium. However, an earlier study[3,4] has shown that the design of the Pachuca tank should have no bearing on the efficacy of oxygen mass transfer. Moreover, investigations such as those of Harriot[5] have shown that beyond the point of complete off-bottom suspension, an increase in solid-liquid mass transfer is not as significant compared to the initial stages of suspension. Consequently, particle suspension may be the most important parameter in the design of energy-efficient Pachuca tanks. A review of the literature shows that experimental studies on particle suspension in Pachuca tanks have been carried out only by Hallett et al.[6] Their experiments, which were restricted only to FAL tanks, indicated that superficial air velocity, defined as volumetric flow rate (corrected to atG.G. ROY, formerly Graduate Student, is Visiting Lecturer, Department of Metallurgical Engineering, Indian Institute of Technology Kharagpur 721 302, India. R. SHEKHAR, Associate Professor, and S.P. MEHROTRA, Professor, are with the Department of Materials and Metallurgical Engineering, Indian Institute of Technology, Kanpur 208 016, India. Manuscript submitted April 1, 1996. METALLURGICAL AND MATERIALS TRANSACTIONS B
mospheric pressure) divided by tank cross section area, may not be used as a scale-up criterion for the design of Pachuca tanks. It was noted that an increase in tank diameter had the effect of decreasing the air velocity for off-bottom suspension. An interesting observation reported by Hallett et al.[6] is that industrial tanks operate at superficial air velocities considerably lower than those required for off-bottom suspension in their experiments. Several investigations on particle suspension have been carried out in bubble columns both in the presence[7] and absence[8–14] of a draft tube. Many of these investigations have been summarized by Abraham et al.[15] Because bubble columns are similar in configuration to Pachuca tanks, some important conclusions emerging from bubble column studies are presented subsequently. Perhaps the most rigorous inves
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