A Method for Removing Ammonium Ions From a Subterranean Formation After In-Situ Uranium Leaching
Authors: Yan T.Y. 
Published: December 2007

Ammonium carbonate and bicarbonate are the preferred carbonate sources in alkaline in-situ leaching of uranium. The ammonium ion exchanges into the clay in the formation and is difficult to remove during restoration operations. A new process is proposed which holds the potential for rapid and effective reduction of ammonia in the formation and groundwater to acceptably low levels. The process employs ph-adjusted, chlorinated water to decompose the ammonia quantitatively. The operation involves flushing the formation with connate water or brine, injecting chlorinated water, and finally flushing with connate water. This process is effective in laboratory tests.

Introduction

In-situ leaching, or solution mining, is now emerging as a viable technique for recovering uranium from some low-grade ore bodies. As a result, several pilot and commercial in-situ leaching plants are in operation in south Texas - e.g., Mobil Oil Corp., Intercontinental Energy Corp., Wyoming Minerals Corp., Union Carbide Corp., and U.S. Steel Corp.Leaching systems are classified conveniently as either acidic or alkaline. For sandstone ores containing substantial quantities of carbonates, alkaline leaching is preferred. The alkaline leach solutions contain an oxidant (H2O2, NaClO3, or O2) and a mixture of carbonates. Ammonium carbonates have been the most popular source of carbonates. This type of leaching formulation has been used successfully in south Texas.Since the ore bodies generally contain up to 20% or more of cationic exchangeable clays, the NH4+ in the leach solution exchanges with the cations in the clay according to the following equations.

(1)(2)

The selectivity of clays for NH4+ is high, and the NH4+ ion exchange capacity of the ore may become quite high depending on the overall clay content. A typical NH4+ ion exchange isotherm is presented in Fig. 1, which shows, for example, that for a leach solution containing 10 g/L of NH4HCO3, the equilibrium concentration of NH4+ on the ore is 0.14 meq/g, equivalent to an NH3 concentration of 0.24% of NH3 in the ore body. At the conclusion of the leach operation, the formation is contaminated with this great quantity of NH4+ ion. If not removed, the NH4+ ion will release slowly by exchanging with the incoming cations in the aquifer, resulting in pollution of the groundwater. Government regulations require that water in the leached formation be restored substantially to its original quality. Furthermore, the current Texas Dept. of Water Resources permit procedures require groundwater restoration be completed immediately upon completion of mining of the site. Although baseline levels for NH3 are not set yet, levels in the 10-ppm range have been suggested. Since the ammonia is exchanged into the clays chemically, it cannot be flushed out readily and requires counter ions, such as Na+ or Ca++, for removal by ion exchange. Furthermore, the ion exchange equilibrium limitation makes the restoration of the desired low level difficult and time consuming.To speed up the restoration process, a restoration fluid with high salt content of NaCl or CaCl2 can be used.

JPT

P. 921^