While some fractions of the uranium ores from Crownpoint, NM, are amenable to leaching with mild chemical reagents such as hydrogen peroxide or high-pressure oxygen in sodium bicarbonate solution, other ores are refractory to this treatment, possibly because the uranium mineral is embedded in a kerogenic organic matrix. Such refractory ore occurs only in selected areas of the general Crownpoint properties. Previous papers have described the use of chemically severe leaching systems (sodium hypochlorite, oxygen/sulfuric acid) to recover uranium from such refractory ore. In this paper we report results of an experimental study on the effects of heating refractory Crownpoint ore at moderate temperatures in the presence of a mild (oxygen/sodium bicarbonate [O2/NaHCO3] or hydrogen peroxide/sodium bicarbonate [H2O2/NaHCO3]) chemical leaching system. peroxide/sodium bicarbonate [H2O2/NaHCO3]) chemical leaching system. When a composite sample of refractory ore from the Crownpoint uranium properties was leached with high-pressure (600 to 800 psig [4140 to 5520 properties was leached with high-pressure (600 to 800 psig [4140 to 5520 kpa]) O2/NaHCO3 at ambient temperature in batch or pack tests, only 30 to 40% recovery and slow reaction rates were observed. However, this same leachate, when heated to 140F [60C], gave 60 to 65% recovery in batch, pack, and core studies at much more rapid rates. Several other pack, and core studies at much more rapid rates. Several other temperatures were studied, but 140F [60C] appears to be close to optimum for the group of ore samples tested. When we used rough kinetic approximations, relatively low apparent energies of activation were observed. The uranium recovered with heated leachate was accompanied by dissolved organic matter, which suggests oxidative attack on and breakdown of the highly aromatic organic carbonaceous matrix. Some trace quantities of organic molecules identified in the effluent were consistent with kerogen degradation products. Some dissolved silica also was produced. Reservoir heat balance estimations suggest that the concept of using heated leachate for in-situ leaching can be technically feasible in reservoirs such as Crownpoint, particularly if injection rates are high and ore bodies are reasonably thick. Thus, this technique represents a possible alternative for recovering uranium from refractory ore. possible alternative for recovering uranium from refractory ore. Introduction
Previous work has shown that uranium ore from the Crownpoint region of Previous work has shown that uranium ore from the Crownpoint region of northwest New Mexico has been amenable to leaching with a variety of chemical leachates. In these ore samples, both uraninite and coffinite have been identified, and Rhett has described a cryptocrystalline or possible organometallic form of residual uranium. Coffinite, when present, frequently is encysted or otherwise embedded in kerogenic organic material with crystallite size ranging from less than 1 to 30 microns [less than 1 to 30 mu m]. The uranium-bearing sandstones are classified as arkosic, and contain detrital quartz, feldspar, and other rock debris, with some clay and carbonate. Overall, we have observed considerable heterogeneity in ore characteristics, even within a relatively small area, and noted wide variation in leaching rates when a chemically mild leachate (O2 or H2O2 in NaHCO3 solution) was employed. Ore from a number of areas at Crownpoint did not leach efficiently or to high recoveries with such mild leaching systems and has been termed "refractory." In the ore samples that displayed this refractory or "intractable" nature, the coffinite was shown to be at least partially encapsulated in the carbonaceous organic matrix. We also observed that when chemically more powerful leachates such as sodium hypochlorite (NAOCl) or O2/sulfuric acid (H2SO4) were used, rapid leaching and high recoveries were obtained with Crownpoint refractory ore samples. Another way to introduce severity into a chemical reaction is, of course, to increase the temperature. That this is an effect of considerable magnitude at temperatures near 77F [25C] can be seen by simply proportioning temperature coefficients, for reactions at 77 and 95F proportioning temperature coefficients, for reactions at 77 and 95F [25 and 35C] (EA is the apparent energy of activation, kcal/mol [kJ/kmol], R equals 1.987 cal/K-mol, and T is temperature, K). For "typical chemical reactions" with EA = 15 and 20 kcal/mol [62 220 and 83 680 kJ/kmol], reaction rates are approximately doubled and tripled, respectively.