The PUREX process is a liquid–liquid extraction ion-exchange method used to reprocess spent nuclear fuel, in order to extract primarily uranium and plutonium, independent of each other, from the other constituents. The current method of choice is to use the PUREX liquid–liquid extraction process which uses a tributyl phosphate/hydrocarbon mixture to extract both uranium and plutonium from nitric acid. This extraction is of the nitrate salts and is classed as being of a solvation mechanism. For example, the extraction of plutonium by an extraction agent (S) in a nitrate medium occurs by the following reaction.
: + 4  + 2 S → []
A complex bond is formed between the metal cation, the nitrates and the tributyl phosphate, and a model compound of a dioxouranium(VI) complex with two nitrates and two triethyl phosphates has been characterised by X-ray crystallography. After the dissolution step it is normal to remove the fine insoluble solids, because otherwise they will disturb the solvent extraction process by altering the liquid-liquid interface. It is known that the presence of a fine solid can stabilize an emulsion. Emulsions are often referred to as third phases in the solvent extraction community.
An organic solvent composed of 30% tributyl phosphate (TBP) in a hydrocarbon solvent, such as kerosene, is used to extract the uranium as UO(NO)·2TBP complexes, and plutonium as similar complexes, from other fission products, which remain in the aqueous phase. The transuranium elements americium and curium also remain in the aqueous phase. The nature of the organic soluble uranium complex has been the subject of some research. A series of complexes of uranium with nitrate and trialkyl phosphates and phosphine oxides have been characterized.
Plutonium is separated from uranium by treating the kerosene solution with aqueous ferrous sulphamate, which selectively reduces the plutonium to the +3 oxidation state. The plutonium passes into the aqueous phase. The uranium is stripped from the kerosene solution by back-extraction into nitric acid at a concentration of ca. .