An integrated multiscale approach to heap leaching of uranium-ore agglomerates

In heap leaching, agglomeration of clay rich ores is commonly used and may prevent plugging phenomena within heaps, but is not always successful. In order to better understand the heap leaching process of uranium ore agglomerates, a multi-scale approach, based on 10-cm and meter-scale columns, was used to achieve a Representative Elementary Volume (REV) of packed agglomerates. Flow rate and agglomerate size distribution were found to have no impact on the ore leaching kinetics. An increase of the sulfuric acid content of the leaching solution caused a slight increase of uranium extraction. Meter-scale tests indicated that scaling up had no significant influence on extraction kinetics. Some tests have also shown that column porosity and permeability decreased during irrigation and with the heap depth. This led to the occurrence of plugging. X-ray tomography analyses on clogged sections of the column revealed that this phenomenon arises from agglomerate mechanical degradation. In addition, an integrative numerical model of the leaching of the meter scale column of agglomerates was built, combining flow and reactive transport equations where the effective reaction term was directly inferred from the REV experiments. An excellent agreement was observed with the column tests.