Mathematical Modeling of In-Situ Uranium Leaching

This paper presents the development and results of a computer model of in-situ uranium leaching. This model uses a streamline-concentration balance approach and is useful with a wide range of reservoirs. It can be used with any type of well system, in a reservoir with or without boundaries, and with any form of discriptive kinetics. The model also includes the effects of dispersion and consumption of oxidant by minerals other than uranium. The effects of well pattern, variable uranium concentrations, and the pattern, variable uranium concentrations, and the presence of oxidant consumers on uranium presence of oxidant consumers on uranium production are discussed. production are discussed. Introduction The sandstone uranium deposits of south Texas represent a possible major energy source. These deposits consist mainly of widely scattered roll fronts (pods) of unoxidized uranium minerals in loosely packed sands. It is thought that these deposits were packed sands. It is thought that these deposits were formed by the downdip migration of groundwater carrying oxidized uranium leached from the host rock, Catahoula Tuff. When the uranium-bearing waters reached a reducing zone, the uranium was precipitated, forming mainly the mineral uraninite, precipitated, forming mainly the mineral uraninite, UO2(). Much of the uranium ore in the area is low grade (less than 0.05% U3 O8) and is at depths of 100 to 1,500 ft.Since 1960, various companies have been mining some of the higher-grade deposits to depths of up to 200 ft, using conventional strip-mining techniques. The concomitant surface disruption is extensive, and the costs of mining and transporting, to a mill such large amounts of material prohibit the utilization of low-grade ore.A mining technique that may overcome these difficulties to some extent and ultimately make more of the south Texas uranium deposits amenable to recovery is in-situ solution mining. This technique consists of pumping through the ore body a chemical solution that will dissolve the uranium minerals so that they may be leached from the ore and recovered from the solution. For this process to be economically feasible, a low-cost solution must be available that will dissolve a large portion of the uranium present, the uranium must be easily recoverable from the leach solution, the physical attributes of the ore body must be such that the leach solution can be pumped through the ore without great loss to the surroundings, and environmental hazards must be avoided.The leaching process and its chemistry are basically simple. Uranium is generally found to have one of two oxidation states - oxidized, U (+6), or unoxidized, U (+4). In the oxidized +6 state, uranium forms many soluble ions, among them the uranyl ion UO2(), the uranyl dicarbonate ion UO2(CO3)(−2), and the uranyl tricarbonate ion UO2(CO3)3(−4). Hostetler and Garrels have investigated the equilibria of uranium minerals with natural solutions and found that under oxidizing conditions, stable soluble ions exist over a wide range of pH. The results suggest that to dissolve uranium minerals, one must provide an oxidizing agent to oxidize reduced uranium to the +6 state and a complexing agent that will form stable complex ions with U+6. A typical set of reactions is as follows:(1) (2) SPEJ P. 393