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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Summary. One possible drawback to the use of in-situ leaching to recover uranium is the potential release of previously insoluble chemical species into the formation water. Before a pilot test of in-situ uranium leaching at Crownpoint, NM, was begun, extensive laboratory studies were undertaken to develop chemical methods for treating one possible contaminant, molybdenum (Mo). New Mexico regulations restrict the amount of Mo permissible in formation waters after leaching to less than 1 ppm. permissible in formation waters after leaching to less than 1 ppm. Two techniques to restore Mo after leaching were studied with core and pack tests. These studies suggest that if Mo restoration problems occur in the field, the use of precipitating agents such as Ca 2 + or reducing agents such as Fe 2 + may be helpful in ameliorating such problems. Introduction Typically, uranium has been produced by conventional underground mining and surface milling methods, but insitu leaching has recently emerged as an attractive alternative for uranium recovery. Before an in-situ leach operation to produce uranium in the Westwater Canyon member of the Morrison formation near Crownpoint, NM, was begun, comprehensive laboratory studies of ore mineralogy and leachate formulations were undertaken. The resultant pilot test was the deepest (2,000 ft [610 m]) successful in-situ uranium leach test carried out in the U.S. and the first operation of its type conducted in New Mexico. While in-situ leaching has significant advantages over conventional uranium recovery methods, one possible drawback is the release of previously insoluble chemical species into the formation water. Of course, conventional mining also has the potential to mobilize hazardous trace metals in the environment, as indicated by the elevated concentrations of Mo in waters downstream from the large Mo deposit at Climax, CO. Before the start of the Crownpoint pilot test, laboratory testing was undertaken to develop chemical methods for treating one possible contaminant, Mo. Ore analyses from the Westwater sands at the Crownpoint pilot site revealed Mo occurrences in the intended leach zone. In-situ production of uranium entails oxidizing uranium from the insoluble +4 oxidation state to the soluble, readily complexed +6 state, i.e., However, this process also transforms insoluble Mo 4+ minerals such as molybdenite or jordesite, both MoS2, into the soluble +6 form, molybdate, MoO4 2-, i.e., Current New Mexico environmental regulations restrict the amount of Mo permissible in formation waters after leaching to less than 1 ppm. Restoration of formation water to acceptable levels for each of the dissolved solids is the final phase of leaching operations. Three recent reports deal with restoration operations in in-situ uranium leaching in a most comprehensive manner. The main difficulty is that the original chemicalreducing environment underground has been changed to an oxidizing one during the leaching process. Undesirable species can continue to be slowly released from underground rock surfaces long after the primary leaching has ceased. Obviously, the contaminants need to be rendered insoluble. To reduce Mo levels in groundwater after leaching operations, there are at least two methods of restoration:the MoO4 2- may be precipitated by addition of a cation orthe oxidizing environment can be changed to a reducing one, converting the Mo back to the less soluble +4 oxidation state, perhaps to Mo3 O8. The latter technique has been characterized by Henry et al. as recreating the original mineralization process. Both techniques were studied in the experiments described in this report. Calcium ion (Ca2+ was used to precipitate MoO4 2 -. Ferrous ion (Fe2+ a reducing precipitate MoO4 2 -. Ferrous ion (Fe2+ a reducing agent, was studied to see whether it could render Mo insoluble. JPT P. 1301