Evolution of structural and physical properties of uranium ore agglomerates during heap leaching

Heap leaching is commonly used to process low grade ores. However, ores containing fine particles and clays cause often clogging problems within heaps. To solve this problem, copper, nickel and uranium extractive industries use agglomeration. This process consists in gathering fine particles in order to adjust the ore particle size distribution. This allows improving the heap permeability and reducing fine migration during leaching. As agglomerates quality influences heap leaching efficiency, many quality tests have been performed. However they mostly focus on a column of agglomerates, and the influence of agglomerates structure on heap leaching remains poorly understood. The current study is an attempt to characterise the structure and the porosity of agglomerates and their evolution during leaching. For this study, a low grade uranium ore from Niger has been used for agglomeration. The ore has been agglomerated using sulfuric acid as binder. Then, agglomerates have been leached during 10 days with sulfuric acid at 10g/L. Evolution of structural and physical properties of uranium-ore agglomerates during heap leaching has been investigated for different operating conditions: agglomerates size, flow rate or acid concentration. Prior to heap leaching, X-ray tomography and SEM analysis highlighted that agglomerates are composed by the coalescence and layering of micro-agglomerates. These are composed by phyllosilicates (especially muscovite and kaolinite) coating a nucleus (mostly quartz or feldspar). One can also observe an aluminous silicate matrix phase around some nuclei. This might come from the reaction between phyllosilicates and the binder. Moreover, such a structure is the cause of the initial low connected porosity of the agglomerates (between 3 and 7%) which can be measured by mercury intrusion porosimetry (MIP). A positive correlation between the porosity and the agglomerate sizes has been exhibited. A combined MIP–SEM analysis carried out on agglomerates after 10 days of leaching revealed that the process improved the connected porosity of agglomerates by creating new mesopores. These are mostly located around micro-agglomerates, as showed with SEM analyses and they may come from the leaching of the aluminous silicate matrix phase during the process. X-ray tomography corroborates these observations and the increase of connected porosity by about a factor 2. This allows a better diffusion of the leaching solution within the agglomerates.