Pownceby and Johnson of the CSIRO reported in 2013 that Australian uranium ores are often composed of complex mineral assemblages. Differences in ore compositions and textures are seen between deposits as well as within a single deposit, which can host a range of ore types. Such a wide variety of uranium ores make it impossible for a single extraction or treatment process to be developed that will accommodate all of the ores. From a mineralogical perspective, key issues confronting the Australian uranium mining industry include: the prevalence of low grade ores; a lack of detailed chemical and mineralogical information (uranium speciation, texture, grain size) for the various ore deposit types; and the presence of refractory uranium-bearing minerals and highly acid-consuming gangue minerals.
The authors reviewed some of the main controls on uranium geometallurgy by linking concepts relating to ore genesis and the resulting ore mineralogy, with the processing behavior of specific Australian uranium ore types. Emphasis is placed on the value of detailed ore mineralogical analysis and the insight this provides into the factors of importance when considering uranium extraction.
The geological record preserved by the Australian landmass has captured multiple uranium mineralizing events dating as far back as the Paleoproterozoic (~2500 Ma), such as the formation of unconformity related uranium deposits, to recent Paleocene (~66.5 Ma) — present mineralizing events that have formed surficial uranium deposits. Changes to environmental, atmospheric, terrestrial and tectonic conditions have resulted in a large number of different styles of uranium mineralisation and consequently, there is no single extraction method that can be applied to optimally process all uranium ores.
The number of minerals which can accommodate uranium is vast, although mined deposits are largely dominated by the tetravalent uranium minerals uraninite, coffinite, brannerite and davidite. [The metamict mineral, allanite also contains substantial uranium... see Mary Kathleen Mine occurrence]. Surficial calcrete deposits are unique in forming concentrations of the hexavalent uranium mineral, carnotite. The susceptibility of the uranium minerals to acid or alkaline leach solutions, temperature, oxidants and organic matter varies widely and it is essential to characterize the mineralogy of the ore in order to select the most efficient method of processing. For example, uraninite is considered relatively easy to leach via either acidic or alkaline solutions, while coffinite, brannerite and davidite are more refractory, usually requiring the addition of an oxidant and more aggressive acidic leach conditions.
Other factors such as the grain size of the uranium mineralisation and the associated gangue mineralogy also play an important role in the ore process selection. Uranium ores with fine mineralisation (e.g. IOCG-U deposits) or poor liberation (e.g. some sandstone-hosted deposits) may require fine-grinding in order to expose the uranium mineral to the leach solutions, while the amount and type of gangue minerals associated with the ore will dictate the basicity/ pH of the lixiviant used.
The authors contrast five of the most economically important Australian uranium ore deposit types (in terms of annual revenue generated from uranium exports) and demonstrates that valuable information on the probable metallurgical behavior of an ore can be obtained from a carefully directed mineralogical and analytical study. The study emphasizes that key data required includes; 1) uranium mineral identification and probable physical characteristics, 2) an appraisal of uranium mineral liberation size, and, 3) major associated minerals and gangue.
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