Experimental and Numerical Investigation of Fluid Flow and Mixing in Pachuca Tanks
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s are hydrometallurgical reactors in which the suspension of mineral particles and the mass transfer processes controlling the kinetics of the reactions are intimately linked to the motion of the liquid that results from the injection of gas through the base and, in general, into a central draft tube. These tanks are large cylindrical vessels, commonly 4 to 10 m in diameter and 15 m in height, that normally include a tapered base with a 30- to 60-deg half angle and a draft tube typically 0.4 to 1.0 m in diameter.[1,2,3] These reactor vessels are widely used in the hydrometallurgical industry to carry out gold leaching, uranium leaching, the bacterial oxidation of pyrite, copper leaching, and celestite conversion, among other processes. Their relevance has been claimed to be due to their simple construction, to the absence of moving parts,[4] and additionally, according to Weiland,[5] to their low energy E. RODRI´GUEZ M., Postdoctoral Student, A.H. CASTILLEJOS E., Professor, and F.A. ACOSTA G., Associate Professor, are with the CINVESTAV–Unidad Saltillo, 25000 COAH, Saltillo, Mexico. Contact e-mail: [email protected] Manuscript submitted April 4, 2007. Article published online July 27, 2007. METALLURGICAL AND MATERIALS TRANSACTIONS B
consumption. However, this author[5] and others[1,6] indicate that if the ratio of the slurry flow rate to the air flow rate is taken as a measure of the energy performance of the Pachuca tank, this will be high at low gas flow rates, but will become poor after a critical air flow rate is exceeded. In fact, the energy performance will depend on the particular requirements of the process. Fraser[6] points out that a typical Pachuca tank, operating at an air rate of 0.02 Nm3/min/m3 of tank volume, may not be able to satisfy the necessary oxygen uptake rate for gold leaching, unless excess air is applied with the consequent waste of energy. For other processes, such as the acid leaching of uranium ores[4] and celestite conversion,[7] in which the purpose of the air is the agitation and suspension of particles and not aeration, the process goal may be achieved with a satisfactory energy performance. Lamont[4] indicated that operations requiring mainly particle suspension and scrubbing are best carried out in full-center column Pachuca tanks, while those needing aeration benefit from the use of free-airlift or stub column tanks. Therefore, based on the needs of a particular leaching process, the operating and design guidelines have to vary. In the Pachuca tank, the motion of a slurry of mineral particles is established when gas is centrally injected VOLUME 38B, AUGUST 2007—641
through its base into a draft tube, to produce a liquid upflow within this region and a downflow in the annular space formed between the draft tube and the tank wall. The driving force for this circulation is the imbalance in the hydrostatic heads between these two regions. Clark[1] and Zaisha et al.,[8] working with Pachuca tanks with a tapered base, found that, at low superficial gas velocities, the flow
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