Introduction
Historically, the most commonly encountered flowchart for uranium processing consisted of conventional comminution followed by atmospheric leaching, solid/liquid separation, solvent extraction purification and finally ammonium diuranate (ADU) or yellow cake precipitation.
Today a number of alternative technologies are considered. The choice of technology is often driven by the uranium mineralogy and the lithology and nature of the host rock, subject of course to the normal financial, technical and marketing criteria imposed on projects.
Impact of Mineralogy and Lithology
Tetravalent uranium has low solubility in both dilute acid and carbonate solutions and oxidation to the hexavalent state is necessary to achieve economic recovery. Tetravalent minerals include uraninite, uranothorite and coffinite (as found at Dominion Reefs in South Africa), whilst carnotite (as present at Langer Heinrich in Namibia) is an example of a hexavalent uranium mineral. Typical oxidants include pyrolusite, sodium chlorate, hydrogen peroxide, SO2/air and ferric ion.
Multiple oxide minerals, such as brannerite, are more complex and refractory and may require fine grinding, prolonged leaching in hot acid, or leaching at elevated pressure and temperature in autoclaves. Such minerals are the focus of recent exploration in Zambia. Euxenite is another example of a refractory rare earth oxide and one that was present in the Dominion Reefs feed material that was subjected to pressure leaching. Uranium can also be associated with elements, such as carbon and phosphorous, that in turn affect process selection.
The nature of the host rocks plays a major part in process route selection. The presence of carbonate minerals at levels that result in uneconomically-high acid consumption, for example, would likely favor the selection of alkali leaching.
Potential to Pre-Concentrate
Screening can be an effective means of pre-concentration where uranium minerals preferentially report to certain size fractions of run-of-mine or primary crushed material.
Radiometric sorting is applicable to certain deposits particularly where the radioactive mineralized ore is sufficiently liberated from low-grade material and gangue. During the 1980s, radiometric sorting was incorporated into a number of plants and more recently Rossing Uranium (Namibia) has installed a demonstration plant.
Leaching
The numerous leaching options that present themselves include heap leaching, in-situ leaching, resin in pulp leaching/adsorption and leaching in agitated vessels or autoclaves under acid or alkali conditions.
Heap leaching was successfully employed on a number of earlier projects. It would still be applicable on low-grade ores where capital and operating costs need to be minimized. Traditionally, acid heap leaching was practised but consideration can also be given to alkaline heap leaching, as presently being implemented at Trekkopje (Namibia).
In-situ leaching (ISL) finds application where the ore and host rock structure as well as the surrounding aquifers permit its use. Current examples where ISL is employed or being considered include Uranium One’s Akdala mine plus their South Inkai and Kharasan projects in Kazakhstan and their Honeymoon project in Australia.
Resin in pulp (RIP) was used extensively in the USA and Russia in the past for uranium extraction. A number of suppliers are developing new and improved resins and RIP is likely to be a contender in future projects.
Sulphuric acid leaching is probably the most widely applied technology. See Figure 1.1 for a typical acid leaching flowchart.
Alkali leaching is the second major leach option considered. A typical alkali leaching flowsheet is shown in Figure 1.2.
Purification steps ahead of ADU precipitation might include solvent extraction, ion exchange or direct precipitation. In the case of both acid and alkali leaching, the purification steps need to be customized according to the levels of other contaminants, such as vanadium, molybdenum, arsenic and silica that need to be removed ahead of ADU precipitation.
Conclusion
In order to identify the optimum uranium extraction flowchart, it is clearly necessary to have a good understanding of the ore mineralogy and host rock lithology, supported by a well-designed metallurgical test program. Engineering trade-off studies can assist in selecting the optimum process route.
