Restoration of Uranium In-Situ Leaching Sites

In recent years in-situ leach mining has emerged as a new technology for the recovery of uranium from strata that cannot be mined economically by other means. Because the ore bodies lie within groundwater aquifers, a significant determinant in the process' viability is the requirement that such aquifers be protected from contamination. Since ammonia is one of the constituents of the leach solutions now being field tested, one environmental problem to be resolved is the removal of ammonia at the end of mining. A second related question is the fate of the ammonia that is not removed by the restoration procedure. This paper considers the displacement and migration of ammonium cations in a flowing electrolyte with concomitant ion exchange. The ion exchange is an important feature since, during the solution mining phase, ammonium cations adsorb onto the mineral exchange sites and must be removed from these sites. A mathematical model is used to simulate this process, and the model is tested against the results of laboratory experiments. It is found that the simulations are adequate if an appropriate selection of parameters is made. The model then is used to simulate restoration procedures and to determine the rate of migration of unrecovered ammonium in the groundwater. It is concluded that ammonium removal can be accomplished best using high concentrations of a cation that is exchanged selectively relative to ammonium cation. Introduction In-situ solution mining is a process rapidly being developed for the recovery of uranium from sandstone ore bodies. This mining technique is applicable when the uranium ore is too deep, too small in extent, or of too low a grade to justify using conventional mining techniques. Such ore bodies are numerous in south Texas, occurring along a broad band of the U.S. gulf coastal plain. The solution mining process being used in Texas is primarily an alkaline leach. The sandstone ores that may be solution-mined occur in aquifers, and the uranium is in the insoluble +4 state of oxidation. To be mobilized, the uranium must be oxidized to the +6 state and then complexed with carbonate ions to form the highly soluble uranyl dicarbonate or uranyl tricarbonate ions. Thus, alkaline leach solutions contain an oxidant (usually hydrogen peroxide) and a mixture of carbonates and bicarbonates. To minimize formation damage, most solution mining now employs ammonium carbonate/bicarbonate as the carbonate source. These solutions have been found effective in dissolving the uranium minerals found in south Texas sandstone ores.1 However, the restoration of the mining site is also a primary consideration. Since the ore bodies that can be solution-mined occur in aquifers, government regulations require that water quality at the mining site not be degraded below the quality that existed at the inception of mining. Furthermore, the permitting procedures require that groundwater restoration be completed at one site before the next site on a particular lease may be mined.2 Obviously, environmental aspects will be an important consideration governing the success of in-situ solution mining.