Post In-Situ Uranium Leaching Site Restoration Numerical Analysis
Authors: Schechter Robert S. , Bommer Paul M. 
Published: December 2007

This paper presents the development of and results from a computer model of post in situ uranium leaching site restoration. 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 bounded or unbounded, layers, and/or anistropic reservoir.

From a well pattern and reservoir point of view the effects of areal sweep, layering, and anisotropy are shown. Also the effects of the reservoir's cation exchange capacity and affinity for the leaching cation are demonstrated. These variables along with the effects of varying types of clean up solutions are related to restoration schemes and cautions are outlined for not just restoration but leaching as well.

Introduction

Leaching solutions commonly used for in situ uranium extraction generally include an oxidant such as dissolved oxygen or hydrogen peroxide to oxidize uranium from its relatively insoluble U(+4) valence state to the more soluble U(+6) form. To enhance the solubility of the oxidized uranium, carbonates are often included in the lixiviant formulation. These form stable, highly soluble uranyl dicarbonate or uranyl tricarbonate ions which can be recovered in surface ion exchange facilities. At present most solution mining employs ammonium carbonate/bicarbonates as the carbonate source. These solutions have been found to be effective in South Texas sandstones. Ammonium carbonates have been preferred to sodium carbonates because many of the formations experience severe permeability reduction when contacted with sodium ions. Potassium carbonates are more expensive than ammonium carbonates.

These leaching operations are conducted in aquifers. The injection and production rates are adjusted to minimize lixiviant escape from the immediate vicinity of the mineralized zone. After the mineral values have been produced, the mined zone must be restored. Restoration is generally defined by the terms of a mining permit issued by a responsible governmental agency and approved by the mining company. Because of the technology of in situ uranium ring is in the developmental stages, standarized rules for restoration have not yet been developed. The practice to date has been to require that groundwater samples drawn from the aquifer in the mined area have a composition similar to that which existed prior to mining. For example, in ammonium carbonate is one component of the lixiviant, then the levels of ammonium allowed to remain in the groundwater are related to those initially present. Since these initial levels are often quite small, restoration may require that a large portion of the ammonia be recovered from the aquifer. The difficulty of accomplishing this goal and the best procedure to use will depend on a number of factors, including the reservoir dimensions and its characteristics, the type of cation used in the lixivant, the well pattern and the type and concentration of the eluting solution. The particular problem addressed in this paper is the particular problem addressed in this paper is the removal of the lixiviant cation; however, there may be, and in general are, other criteria which must be satisfied. Vanadium, arsenic and manganese must all be reduced to tolerable concentration. Dissolved solids and radioactivity must both be reduced to near baseline levels.

Lixiviant cation restoration is considered here because this is believed to be one of the more complex tasks, especially if the residual concentrations are set at very small values. The removal process considered involves ion exchange since commonly occurring minerals, especially clays, exhibit a considerable cation exchange capacity.