Supercritical carbon dioxide extraction for uranium under development. [View All]

[NNadir]

One of the more exciting compounds in "Green Chemistry" is also one of the most problematic chemicals in the environment, carbon dioxide.

(Carbon dioxide, as many of us are aware, is destroying our atmosphere and represents a threat to life itself.)

Be that as it may, the use of carbon dioxide in industrial settings can be used to solve many thorny environmental problems. For instance, much of the water supply in the world is contaminated with dry cleaning fluid, perchloroethylene (PCE). Many people don't know this, but in theory much of this dry cleaning fluid could be (and in some places, has been) replaced by supercritical carbon dioxide as a dry cleaning solvent. This offers the opportunity for easy recycling of solvent - some, albeit small quantities - sequestered carbon dioxide, and the freedom from the transport and release of more dangerous solvents, of which PCE is just one.

A supercritical fluid is one that is high enough temperature and pressure that there is no distinction between liquid and gas phases. The temperature in question is called the critical temperature, and the pressure, unsurprisingly, is called the critical pressure.

The critical temperature for some materials is rather high. For water it is 373C. However for carbon dioxide, this temperature is just above 32C, a very accessible temperature that is the equivalent of a warm day.

Many people are aware of how strongly I support nuclear power, on the grounds that, while it is not risk free, it is risk minimized with respect to all other forms of scalable continuous energy. However, it is always attractive for further reduce risk. One of the more risky aspects of fuel recycling and uranium mining for that matter involves the use of solvents. In several processes that are currently used industrially (although better ones are probably available), uranium is extracted into organic solvents from nitric acid solutions. The elimination of these solvents would certainly be attractive.

Thus I was intrigued by this report which can be found in Ind. Eng. Chem. Res., Vol. 41, No. 9, 2002 pages 2282-2286. Apparently uranium can be extracted into supercritical carbon dioxide.

Here is an excerpt from the paper:

Extraction of uranium from solid matrixes is required for initial production of nuclear fuels, reprocessing of spent fuels, and decontamination of uranium wastes.1-3 Conventionally, leaching with strong acids such as nitric acid is widely used in these processes, but the treatment and recycling of waste acids are costly and their potential impact on the environment is a major concern. 1,2 Supercritical fluid extraction (SFE) techniques have been reported for recovering uranium from its oxides and from environmental samples using fluorinated â-diketones and tri-n-butylphosphate (TBP) as extractants.4-6 This new extraction technology appears promising for the effective processing of uranium with a marked reduction in waste generation because no aqueous solutions and organic solvents are involved and phase separation can be easily achieved by depressurization.
7,8

http://pubs.acs.org/cgi-bin/abstract.cgi/iecred/2002/41/i09/abs/ie010761q.html

I do realize that people may find this dorky, but I find this paper extremely interesting.