Vogt, T.C., SPE, Mobil Research and Development Corp. Strom, E.T., Mobil Research and Development Corp. Dixon, S.A., Mobil Research and Development Corp. Johnson, W.F., SPE, Mobil Research and Development Corp. Venuto, P.B., SPE, Mobil Research and Development Corp.

Abstract

This paper describes laboratory leaching studies involving Crownpoint uranium ore samples and a mild leaching system. Batch leach tests with sodium bicarbonate solution and either high-pressure oxygen or low-pressure hydrogen peroxide gave qualitative data used to estimate leach rate and potential recovery. Using pseudo-firstorder rate constants derived from the batch test data, ore leachability was characterized as fast, intermediate, or slow. It was observed that leach rates varied by a factor of 50 for samples taken from different areas at Crownpoint; samples from the same ore trend often varied by a factor of 10. Packed-column and core-leach tests with oxygen at pressures up to 800 psig (5520 kPa) provided more quantitative estimates of leach rate and uranium recovery. Batch test results were correlatable with leach rates and uranium recoveries in packed-column or core tests. In ore samples where uraninite was the predominant uranium mineral, leach rates and recoveries were high. In samples containing coffinite, leach rates were generally lower than those with uraninite. Very low leach rates and recoveries were encountered where coffinite was intimately associated with carbonaceous material. However, the slow leaching rates are not caused by differences in reactivity of coffinite and uraninite. Mineralogical studies before and after leaching using electron microprobe analyses indicated that exposed coffinite crystals are dissolved easily, but finely disseminated coffinite crystallites persist after leaching if they are encapsulated in the carbonaceous matrix. Slow-leaching ores that did not respond to the mild oxidant system are called "refractory."

Introduction

Uranium-need projections of the late 1970's indicated annual requirements of 60,000 tons (54 X 10 kg) by 2050 to meet world energy demands. Recently uranium demand has dropped markedly, but increasing future energy demands dictate a revival of nuclear power. Typically, uranium has been produced by conventional mining and milling methods. In-situ leaching has emerged recently as an attractive alternative for uranium recovery from ore deposits beneath the water table and too deep for open-pit mining. In-situ leaching expands the potential uranium resource because it makes lower grade ore zones accessible. Hydrological disturbance is minimal because groundwater is recirculated. Ore handling is eliminated. and manpower requirements are lower.

In this paper we describe laboratory leaching studies that characterize leaching rate and ultimate uranium recovery for an areally broad sampling of Crownpoint uranium ore. The tests reported here involve a mildly alkaline leaching system.

The first essential step in leaching uranium from ore deposits is oxidation of uranium from the +4 state to the +6 state. This reaction has been the subject of many investigations. However, in the in-situ leaching process, metal sulfides such as pyrite and molybdenite also compete with the uranium for the oxidant in side reactions. Oxidation transforms the insoluble mineral form of uranium to the soluble uranyl ion, UO2++. This ion is mobilized in the form of a sulfate or a carbonate complex. In alkaline carbonate leaching, the soluble and stable uranyl tricarbonate ion, UO2(CO3)3, is formed. The formation constant for this complex is in the range of 10(18) to 10(23) as shown in recent compilations.

SPEJ

P. 1013^

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