Centrifuge

Centrifuges can be sub-divided into two major categories, stationary devices and rotating devices. Both types of centrifuge work on a common principle, however. A collection of particles of different mass is set into motion around a common center. The faster these particles move, the greater will be the difference with which they tend to escape from their common center, and the more easily they will be separated from each other.

In a stationary centrifuge, a fluid (a gas or liquid) consisting of two or more components is sprayed into a cylindrical or conical chamber at a high rate of speed. As the fluid travels around the inside of the chamber, it separates into its components, the heavier substance(s) traveling to the outside of the container, and the lighter substance(s) remaining closer to the center of the cylinder.

One application of the stationary centrifuge is in the separation of the isotopes of uranium isotope from each other. Naturally occurring uranium consists of a mixture of uranium-235, which will undergo fission, and uranium-238, which will not. A sample of uranium is first converted into the gaseous compound uranium hexafluoride and then injected into a stationary centrifuge. As the rapidly moving stream of uranium hexafluoride travels around inside the centrifuge, it begins to separate into two parts. The heavier uranium uranium-235-hexafluoride concentrates along the outer wall of the centrifuge, while the lighter uranium-238-hexafluoride is left toward the center of the stream. The heavier isotope can then be drawn out of the centrifuge, leaving behind a sample of uranium hexafluoride slightly richer in the desired uranium-235 isotope. This sample can then be re-centrifuged and made still richer in the lighter isotope.

A centrifuge. Photograph by Charles D. Winters. National Audubon Society Collection/Photo Researchers, Inc. Reproduced by permission.