We mentioned several technologies above. Three kinds of devices for liquid — liquid contact are generally used:

• Mixer-settlers

• Pulsed columns

• Centrifugal extractor

Mixer-settlers (Fig. 16.10) have been used for a long time in the nuclear industry and have some benefits (efficiency and flexibility) but have some drawbacks:

• Long contact times between the solutions (high residence time of the liquid phases) promote radiolysis of the solvent in highly active environments.

16.10 Mixer-settler principles (Source: AREVA, International Seminar on Nuclear Fuel Cycle, 19 October 2010, INSTN).

• Difficulty in achieving favourable geometry for high flow rates.

• Presence of impurities at the interfaces, which that tend to accumulate and are difficult to remove

Mixer-settlers are a series of mixing chambers, each with an impellor, combined with separation chambers for decanting.

A pulsed column (Fig. 16.11) is a several metres high vertical cylindrical or annular tube where liquid/liquid extraction is performed; two other volumes, again cylindrical or annular, located at both ends of the column are used for the separation of the two phases.

Pulsed columns are easy to use and efficient. The ability to continuously remove impurities from the interfaces is an essential advantage compared to a mixer-settler, as well as a lower residence time, which that reduces solvent degradation by radiolysis. Also, pulsed columns are prefabricated pieces of equipment and do not need maintenance. For these reasons, they are usually preferred over mixer-settler devices.

A centrifugal extractor (Fig. 16.12) employs a spinning rotor that first mixes and then separates the two phases inside the rotor where the centrifugal forces can be as high as 300 g, resulting in efficient and fast phase separation. The separated phases exit the contactor by overflow and underflow weirs, similar to a mixer-settler. The devices are compact and residence times are very low, which minimizes solvent degradation. They offer a wide range of capacities, and are geometrically safe up to significant capacity. On the other hand they are very sensitive to process disturbances such as the presence of crud or particles, which may settle and cause clogging. They are also mechanically complex, needing regular maintenance and significant heat is generated during mechanical mixing, which must be removed. Overall, the process advantages must be balanced against the greater complexity (Fig. 16.13).

16.11

Pulsed column photo and schematics (Source: AREVA, International Seminar on Nuclear Fuel Cycle, 19 October 2010, INSTN).

16.12 Centrifugal extractor schematics (Source: AREVA, International Seminar on Nuclear Fuel Cycle, 19 October 2010, INSTN).