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-first-order 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."
