Basic Research Reduced Cost of Uranium Production

Although little uranium production occurs in the U.S. today, U.S. uranium mills were able to reduce their costs in the 1980s as a result of fundamental understanding of molybdenum extraction gained in research at Oak Ridge National Laboratory. This understanding was important because molybdenum is often co-mineralized with uranium in ore and represents both a processing problem and potential by-product in uranium recovery. Common processing interferences in the solvent extraction of uranium from ore leach solutions arise from coextraction of molybdenum followed by troublesome precipitation of crystalline compounds in the solvent-extraction equipment in the mills. Fundamental research revealed the identity and structure of the extraction complexes formed in the extraction of molybdenum and the factors that controlled crystallization of these compounds. This information helped extractant manufacturers to improve their products and services by supplying the uranium mills with extractants tailored for higher molybdenum concentrations. In addition, U.S. uranium producers benefited directly by application of appropriate process controls to avoid costly sludge formation at several U.S. uranium mills. For example, controls demonstrated in 1986 at Chevron's Panna Maria facility produced annual savings of almost $300,000.

Sometimes studying a particular problem gives insights into general chemical behavior, and interestingly, the understanding of molybdenum extraction has led to an improved understanding of a broad class of separations. The extractant in this case belongs to a class of nitrogen compounds known as amines and contains three hydrocarbon chains. In its effective form, this amine has an additional hydrogen atom to give it an overall positive charge. The resulting ammonium ion must have a negative ion, called an anion, nearby. In extractions, usually this anion is exchanged for a larger anion. A structure for a common type of extraction complex formed in such extractions is shown in the figure. This complex consists of the long-chain ammonium ions and two anions X^- and Y^- of different sizes. The small anion X^- receives two hydrogen bonds from a pair of ammonium ions, and the larger anion Y^- receives none. In the molybdenum problem studied at ORNL, the small anion was chloride Cl^- and the large anion had the complicated formula PMo₁₂O₄₀^3-. The entire complex contained this large anion, three chloride ions, and six ammonium ions. Unfortunately, an X-ray structure of this unwieldy complex could not be obtained, but the general structure shown at right was in fact demonstrated for the first time on a related compound. The latter has two tributylammonium ions, one chloride, and one tetraphenylborate. Thus, an investigation related to a real-world problem explained a great deal about an important class of separations.