Acidic rate- and flow-controlled dissolution of uraninite ores
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INTRODUCTION
I N SITU and dump leaching have become increasingly important methods for extracting uranium from low-grade ores. The basic technology of uranium leaching follows the sequence: (1) application of a leaching solution directly to the uranium ore, (2) dissolution and transport of the dissolved uranium through the ore, and (3) retrieval and recovery of the uranium from solution. Leaching solutions may be either acidic or basic, depending on the host rock mineralogy, but must contain an oxidant such as hydrogen peroxide, ferric ion, dissolved oxygen, or sodium hypochlorite for the efficient extraction of the uranium. ~Accordingly, the most important factors that control the rate of uranium leaching are the morphological properties of the ore, which control the amount of oxidant that contacts the uraniumbearing minerals, and the rates of dissolution of those uranium minerals by the leaching solutions. Numerous investigators, including Galichon et al.,Z Sundar, 3 Grant, 4 Tatorn, 5 and Goddard and Brosnahan, 6 have conducted column-leaching experiments with natural uranium ores to determine the rates of uranium extraction. However, in these studies unequivocal determinations of the relative importance of the chemical factors, which control the dissolution rates of the uranium minerals, compared to the morphological factors, which control the leachable uranium content in the ore, have been impeded by the complex nature of the reactions occurring between the natural ores and the leaching solutions, and by the lack of sufficient characterization of the ores. In many uranium deposits that have been selected for in situ leaching, uraninite (UO2-U308) is the most common uranium mineral and may be associated with iron sulfide minerals such as pyrite L. E. EARY, formerly a Graduate Student with the Department of Geochemistry anff Mineralogy, The Pennsylvania State University, is a Research Scientist with Battelle Pacific Northwest Laboratory, Richland, WA 99352. H. L. BARNES is a Professor of Geochemistry and Director, Ore Deposits Research Section, Department of Geosciences, The Pennsylvania State University, University Park, PA 16802. L. M. CATHLES is a Senior Research Geophysicist with Chevron Oil Field Research Company, La Habra, CA 90631. Manuscript submitted September 20, 1985. METALLURGICALTRANSACTIONS B
(FeS2). The chemical factors that control the rate of uraninite dissolution in acidic solutions have been determined in batch experiments, and rate expressions that describe the UO2 dissolution rate as a function of the oxidant concentration are given by Eary and Cathles 7 for hydrogen peroxide solutions and by Amell and Langmuir 8 for ferric ion solutions. This study was conducted to develop a generalized mathematical model that incorporates the rate expressions that are derived from batch experiments to describe the uranium leaching rates in columns as a function of solution chemistry and flow rate. The rate of aqueous oxidation of pyrite is also included in the model, because pyrite oxidation is a com
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