The effect of clay minerals on in-situ leaching of uranium

In recent years, with the development of techniques and methods for in-situ leaching (ISL), additional uranium extraction from previously worked-out blocks is becoming not only relevant, but also quite achievable. In this case, the extraction of residual uranium reserves from previously worked-out blocks does not require additional costs for the necessary infrastructure. One of the most important factors in the formation of residual uranium reserves in worked-out blocks is the presence of clay minerals in the ore horizon. In this regard, we conducted a number of studies on the adverse and positive effects of clay minerals on ISL process. Water permeability and relatively good filtration (not less than 0.5-1 m/day) of ores and rocks of a productive horizon (aquifer) is the most important hydrogeological factors affecting the performance of uranium ISL. The second most important hydrogeological factor is the lack of fluid communication between the productive aquifer and nonproductive aquifers, i.e., the obligatory presence of aquicludes. The role of clays in these hydrogeological factors is twofold. On the one hand, the presence of clays negatively affects both the solutions filtration rate and uranium extraction. On the other hand, the presence of clay minerals (forming an aquiclude) enhances the effect of ISL. The study findings allowed the role of clay minerals in uranium ISL to be assessed. The diffusion coefficients of nitrate ions in the clays were determined, and the protective effect of aquicludes was calculated. The effect of the clay content in the ore sand horizon on the solutions filtration coefficients was also established. The static uranium exchange capacity of clays was determined by studying the process of uranium sorption by clay samples from sulfate and bicarbonate solutions. The studies established the diffusion coefficients of nitrate ions in montmorillonite and kaolinite clays, which amounted to 3.34 · 10-6 and 2.14 · 10-6 cm2/s. Taking into account the calculated values of diffusion coefficients, the protective time of the clayey aquiclude for nitrate ions was 43 years. At 20 % clay minerals content, the solution filtration coefficient decreases to values where ISL conditions become unfavorable. It was found experimentally that the sorption of uranium by clay minerals depends on both the nature of the clays and the composition of the solution. Uranium sorption from sulfate solutions proceeds noticeably better than that from bicarbonate solutions. The highest values of the static uranium exchange capacity were obtained for bentonite (104 mg/g).