1. Mining and milling
Uranium is usually mined by either surface (open cut) or underground mining techniques, depending on the depth at which the ore body is found. The mined uranium ore is sent to a mill which is usually located close to the mine. At the mill the ore is crushed and ground to a fine slurry which is leached in sulfuric acid:
UO3 + 2H+ ====> UO22+ + H2O
UO22+ + 3SO42- ====> UO2(SO4)34-
to allow the separation of uranium from the waste rock. It is then recovered from solution and precipitated as uranium oxide (U308) concentrate. The UO2 is oxidised to UO3.
With some ores, carbonate leaching is used to form a soluble uranyl tricarbonate ion: UO2(CO3)34-. This can then be precipitated with an alkali, eg as sodium or magnesium diuranate. Alkaline leaching is not undertaken in Australia at present.
Because uranium needs to be in the form of a gas before it can be enriched, the U308 is converted into the gas uranium hexafluoride (UF6) at a conversion plant in Europe or North America.
The mixed uranium oxide concentrate U3O8 received by the refinery is dissolved in nitric acid. The resulting solution of uranium nitrate UO2(NO3)2.6H2O is fed into a countercurrent solvent extraction process, using tributyl phosphate dissolved in kerosene or dodecane. The uranium is collected by the organic extractant, from which it can be washed out by dilute nitric acid solution and then concentrated by evaporation. The solution is then calcined (heated strongly) to produce pure UO3.
Most nuclear reactors require uranium to be enriched from its natural isotopic composition of 0.7% U-235 (most of the rest being U-238) to 3.5-4% U-235. The enrichment process removes 85% of the U-238 by separating gaseous uranium hexafluoride into two streams: One stream is...