Reactive transport for uranium production by in situ leaching
Résumé
In situ Recovery, or solution mining, has been the first exploitation technique for uranium since 2009, with production volumes over 40% worldwide. The process involves a series of injector and producer wells (Fig. 1), and the circulation of a leaching solution (acidic, basic and/or oxidizing). The leaching solution interacts with the ore body, and solubilizes the desired element. It is particularly suited for deep, poor grade ores.
The production is organized around production blocks, each consisting in 20 to 40 injector wells and 6 to 10 producer wells (Fig. 2). The solutions are collected and sent to the plant to strip the uranium before recirculation in the ore. The plant specifications impose strict constraints on the total flow rate and mean uranium concentration, so that prediction of the production pattern of each block is key for planning. However, this task is complicated by the high variability of the ore body (ore grade, chemical and lithographic facies) superposed with the phenomenon at stake (hydrology, chemical reactions).
Reactive transport codes offer an interesting quantitative insight into the behavior of or body. Based on the actual description of the processes, they can simulate the production, using a limited set of parameters, most of which can be acquired during the ore characterization stage. Efficient use of these codes requires a good knowledge of the field parameters (e.g. permeability distribution) and the chemical system (initial state and chemical reactions involved). Although the reactive transport processes are responsible for the mobilization and migration of the uranium, the spatial distribution of the uranium bearing minerals (and possible enhancers or competitors) dictates the shape of the production curve. Therefore, a good qualification of the 3D spatial distribution is key.