Liquid-liquid solvent extraction processes have been widely used for aqueous radioactive material separations. These processes employ two immiscible liquid phases, aqueous and organic, to preferentially extract from one phase to the other the desired components of the feed. A solvent extraction cycle consists of up to four operations: extraction, scrubbing, stripping and washing, which are illustrated in Fig. 3.11 and exemplified by the reprocessing of UNF.

Reprocessing entails the separation of plutonium and unused uranium from the fission products and other transuranics in UNF. In this reprocess­ing application, the UNF is dissolved in nitric acid and the aqueous solution is mixed with an immiscible organic liquid or solvent to form a dispersion of aqueous or organic phase drops in the continuous other-phase. The solvent comprises the extractant, tri-butyl phosphate that will preferentially extract uranium and plutonium, dissolved in a diluent, which is typically a kerosene. The aqueous and organic phases are separated after adequate time mixing to ensure sufficient extraction of the desired species. A small quantity of fission products and other transuranics are inevitably extracted but these are separated from the organic phase in the scrub step by contact­ing it with a relatively small volume of dilute nitric acid. Uranium and plutonium are then stripped into dilute nitric acid by further contacts with the solvent. The solvent is finally washed of acidic degradation products by contacting it with sodium carbonate and dilute nitric acid to make it ready for further extraction.

In industrial practice, each step of the solvent extraction cycle step is repeated a number of times to achieve the desired separation of uranium and plutonium. The liquid phases are mixed and separated in process

3.11 Typical solvent extraction cycle.

equipment collectively known as contactors. The cycle is additionally con­figured so the aqueous phase flows from contactor to contactor in the opposite direction to the organic phase, or counter-currently, to increase efficiency.

A wide range of contactors for solvent extraction have been used in general industry and these fall into two overall types: stage-wise and dif­ferential. Stage-wise contactors, typified by mixer-settlers, are composed of a number of discrete stages in which the two phases are mixed together and brought to chemical equilibrium before being physically separated and passed counter-currently to the adjacent stages. In differential contactors, the two phases do not reach equilibrium at any point in the contactor. Instead, mass transfer occurs throughout the unit driven by a continuous concentration gradient as the phases flow counter-currently. The phases are only separated as they exit the contactor.

For successful nuclear facility application, contactors must:

• develop sufficient interfacial area between the two phases to promote the desired transfer of the extractable components;

• facilitate counter-current flow of the two phases, while avoiding exces­sive entrainment of one phase in the other as they leave the contactor;

• have flexibility to operate over a range of phase flow rates, ratios and feed concentrations;

• be reasonably compact with low hold up of the process solutions;

• be mechanically reliable and/or easy to maintain.

Reliability is particularly important and a maintenance-free design with no moving parts within the hot cell is preferable. A low holdup of process solutions is important for nuclear applications, since this reduces the aque­ous-solvent contact time and hence the degradation of the solvent by the radioactivity of the aqueous phase. It also facilitates designs that are safe by geometry from nuclear criticality at higher than very dilute concentra­tions of fissile material.

A summary of solvent extraction contactors that are currently in use, or that have been proposed, is shown in Table 3.1. It can be seen that, in the nuclear industry, only three types of contactor are in current use: mixer — settlers, pulsed perforated plate columns and centrifugal contactors.