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Extractive Metallurgy of Uranium
Uranium is an energetic source of electricity, heat, and radioactive elements which are used in medicine, agriculture, industry, etc. Uranium is located in the periodic table in actinides zone. His oxidation states are 3, 4, 5, and 6. A fresh surface of metallic uranium is white and lustrous, with a bluish tinge, but it soon tarnishes in air. There are three crystalline forms of metallic uranium; the usual form is orthorhombic, and is malleable and ductile. The objective of treating uranium ores is obtaining a uranium concentrate under certain conditions: high recovery, low costs of production, and concentrate of good quality.
The type of process depends on the reactivity of the ore, mineralogical composition, type of rock, and gangue. Uranium minerals can be classified in three groups: minerals with hexavalent uranium, minerals with hexavalent and tetravalent uranium, and minerals with tetravalent uranium. Minerals from the first group are easily leached by acids forming uranyl ion UO22+ which is very stable and soluble. The second group requires oxidant conditions for its leaching. The third group needs very special conditions because minerals are almost insoluble. For its treatment is required fine grinding, high dosage of leaching agent, oxidant conditions, high pressure and temperature.
Hydrometallurgical processes are employed in the treatment of uranium ores due to the following reasons: Uranium has high solubility during leaching and good selectivity for its recovery from liquors, and most the time, uranium is dispersed in the ore matrix, and sometimes uranium ores have low grades. This is a restriction for its recovery by physical operations. Although there are some restrictions, the possibilities of treating uranium ores are very ample. For that reason, is only necessary select the way according to the mineralogical composition, gangue, and liberation. The involved stages can be grouped as fallow: Ore preparation, Uranium leaching, chemical concentration, and Final product.
Leaching is the chemical attack in order to produce a uranium solution. This is the main stage and has direct influence in costs. The type of ore and gangue determine the leaching process to be used. Thus, the process can be acid or alkaline. Acid leaching is used more frequently because crushing and grinding promote good conditions for leaching, and also allows a better solid-liquid separation. This is not a selective process because there are impurities in the solution. The equipments employed must resist the acid attack. The leaching agents to be employed are usually sulphuric, nitric, and phosphoric acid.
Alkaline leaching is more selective for uranium and very useful in the treatment of uranium ores where limestone is the main impurity. This type of gangue is very reactive and consumes high dosages of acid such as 100 kg/t. The alkaline process has high cost and requires fine grinding and high temperature with residence time very long during leaching. The alkaline process is justified when the ore has a high grade of uranium.
Solutions from leaching have a content 0.3 to 1.0 g/l U3O8 and dissolved gangue. This content is low and be increased by ion exchange, and solvent extraction. The final solution is treated with ammonium to produce a precipitated called yellow cake which is sent to nuclear reactors.
