EXPERIMENTAL STUDY AND NUMERICAL MODELLING OF GEOCHEMICAL REACTIONS OCCURRING DURING URANIUM IN SITU RECOVERY (ISR) MINING

Uranium In Situ Recovery (ISR) today represents more than 25% of the uranium world production. ISR consists of ore mining by in situ chemical leaching with acid or alkaline solutions.

The leaching solution is injected in the ore body through a number of wells and the uranium-enriched leachate is pumped in one or more distant wells. As ISR takes place underground, controls are limited to the analysis of the pumped solution, eventually leading to adjustments of the leaching solution. Therefore ISR mines management is still rather empirical. A numerical modelling has been considered to come to a more efficient management of the process. The modelling will also bring up to predict and minimize environmental impacts, as well as to design aquifer rehabilitation after the completion of mining activities.

Three types of phenomena have to be considered for uranium leaching by ISR: (1) geochemical reactions occurring during the process; (2) kinetics of these reactions and (3) hydrodynamic transport with respect of the reaction kinetics.

Leaching tests have been conducted on ore samples from Tortkuduk (Chu-Saryssu - Kazakhstan) uranium mine where ISR is conducted by sulphuric acid leaching. Two types of leaching experiments were conducted: batch reactors and extraction in flow through columns. For each type of experiment, two acidity concentrations were tested, as well as the addition of an oxidizing agent for the column experiments.

Mineralogy of the ore and the host sediments has been conducted by XRD. A geochemical model (CHESS) was used to simulate the batch experiments in order to model the leaching solution and to calibrate the kinetic reaction laws. Kinetic reaction laws were considered for the following minerals: pitchblende, schoepite, carbonates, feldspath-K, albite, muscovite, pyrite, hematite, gypsum and kaolinite. Rate constants were taken from the literature and the reactive surfaces were adjusted to fit the experimental data. Then, the geochemical kinetic model was validated, by modelling the column experiments with a coupled hydrodynamic and geochemical model (HYTEC). It resulted in a 1D hydrogeochemical transport model of the ISR process at the laboratory-scale.

Further work will be to build a 2D and a 3D reactive transport model of the ISR at field-scale based on the 1D laboratory model.