In situ leaching

Leaching consists in extracting mineral components by circulating an aqueous solution (leaching solution). This technique is generally used in mining activities to recover valuable metals (silver, gold, copper, uranium ...) from geological materials. The three leaching techniques used in mining industry are heap leaching, tank leaching and in situ leaching.

In situ leaching modelling is a strongly growing activity from the increase of the economic value of geological resources. The Reactive Hydrodynamics group, as its name suggests, is particularly efficient and well equipped to deal with leaching questions. Until now, most leaching studies have focused on uranium extracting, but the group has recently been involved in copper ore leaching on behalf of mining companies in Africa, in the "copperbelt".

In situ leaching

The aim of in situ leaching (or ISL for In Situ Leaching), sometimes named in situ recovery (or ISR for In Situ Recovery), is to dissolve metals such as copper or uranium directly into the deposit. With a series of injection and production wells, a flow of leaching solution is created to corrode the ore. The solution is afterwards recovered for being treated. Historically, in situ leaching was developed in the early 60s for uranium extraction and contributes today to 15% of the world uranium production. The main advantage of this technique is to recover metals or valuable minerals without resorting to traditional mining techniques involving drilling, blasting, stripping or expensive underground infrastructures. Therefore, there is a low surface coverage and no waste dump.

To apply in situ leaching, the deposit must be porous enough to enable a good circulation of the leaching solution. Typically, the minerals contained in porous sedimentary formations such as sandstones or highly fractured rocks can potentially be ISL exploited. Furthermore, the minerals must be confined between relatively impermeable layers, called aquitards, in order to canalize the leaching solution in the mineralized zone to avoid leakage. These leaks cause environmental contamination and reduce the effectiveness of the method since a part of the leaching solution is lost. The following figure shows a hydrogeological configuration frequently used to illustrate the ideal conditions of an ISL exploitation.

 

ISL

 

To sum up, the following conditions must be met :

1.   for mineralogy

kinetics in favour of the dilution of valuable minerals

low pH and Eh buffers, in particularly a low carbonate grade, reputed to be major acid consumers
 

2.   for the hydrodynamic environment

a confined environment, to avoid leakage and to have the leaching fluid flow through mineralization

a porous deposit, with an optimal accessibility of the grains

A perfect combination of all these conditions is rarely met. Each ISL exploitation has its mineralogical, geological and hydrogeological specificities. The mission of the Reactive Hydrodynamics group is to understand the specific behaviour of the system studied, from a geochemical as well as a hydrodynamic point of view, and to provide specific guidance for the improvement of the general process in an economic and / or environmental perspective. 

Application

Uranium in situ recovery (thesis of Jérémy Nos)

 Work issues

–    1D recovery study in a homogeneous environment, according to exploiting conditions (flow, acid concentration)

–    1D study of the influence of a uranium heterogeneous distribution

–    Model scaling