Investigative study into the hydrodynamics of heap leaching processes
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I. INTRODUCTION
THERE have been several attempts during the last 25 years to model heap and dump leaching processes, by which metal values are extracted from ore particles and recovered from the leaching solution percolating through the ore bed. Early models involved two-dimensional governing equations to describe the diffusion and the convection of air into the pore spaces of the heap/dump.[1,2] It was widely held that the supply of oxygen to the acidophilic bacteria colonizing the ore surfaces was the rate-limiting process. During the last 10 years, attention has shifted toward the leaching solution and the ore particles, as the installation of lowpressure blowers has fast become standard practice in sulfide heap leaching, thus eliminating the oxygen limitation. The flow and transport equations defining the rate at which solutes travel through the bed interstices, exchange across phase boundaries, and diffuse through water-filled pores now constitute the backbone of any heap leaching model. Phenomena such as leaching reactions, bacterial attachment and growth, solute precipitation, gas/liquid mass transfer, heat generation, and ore decrepitation and compaction can be incorporated, as required, as source terms into the principal governing flow equations. The hydrodynamics of single and multicomponent solutes in porous media, as well as those of trickle-bed reactors in the chemical engineering industry, are very well documented in the literature. Although heaps, packed towers, and tricklebed reactors can all be portrayed as packed-bed reactors, the size of their packing material and their operating liquid and gas flow rates are radically different. Hence, this wealth SYLVIE C. BOUFFARD, Ph.D. Candidate, and DAVID G. DIXON, Associate Professor, are with the Department of Metals and Materials Engineering, The University of British Columbia, Vancouver, BC, Canada V6T 1Z4. Manuscript submitted December 18, 2000. METALLURGICAL AND MATERIALS TRANSACTIONS B
of information is of limited use to the heap modeler. Let us, thus, examine more closely the approaches taken by Dixon and Hendrix,[3] as well as Sa´nchez-Chaco´n and Lapidus,[4] to model heap hydrodynamics. One of the two critical facets considered was the distribution of the leaching solution throughout the heap and its relationship to the flow rate. A fraction of the leaching solution was assumed to be held up in the pores of the particles. In a dry ore bed, this solution would progressively push the air out of the smaller voids to form stagnant water pockets, thereby saturating the pores. Although Dixon and Hendrix[3] assumed that this liquid fraction existed only in the pores of the particles, Sa´nchez-Chaco´n and Lapidus[4] accounted also for the partial wetting of the external surface of the particles by a stagnant liquid film. Whether the water film is uniform or contiguous, or whether the stagnant pockets exist in the pores of the particles and/or at their interfaces, suffice it to say that the stagnant liquid fraction accounts typically for 17 to 32 pct of th
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