Supercritical carbon dioxide extraction for uranium under development.

[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.

[amitten]

1. What I want to know is: Who is working on a way to eliminate

existing excess CO2 from the atmosphere? There has to be a way. Cutting oil consumption is akin to putting a bandaid on a huge gaping wound. No matter how much or how soon, it will not even begin to solve the problem.

And I'm not talking about adding special microorganisms to the oceans or any of that small, ineffective crap. Or planting a few more trees. I'm talking about a massive extraction of CO2 from the atmosphere, in a short amount of time. To at least put a dent in the problem while other slower measures take effect. I don't see any other way to make an impact that will have an effect in our lifetimes. Is anyone working on such an idea (far fetched though it is)?

[NNadir]

2. The extraction of carbon dioxide from the air requires energy.

There's the rub.

We know that it is possible to extract carbon dioxide from the air, since life on earth would not exist without that process. But as an industrial matter, it requires energy.

[amitten]

3. Well, it could require energy that ostensibly doesn't emit CO2.

And, what about finding a way to use CO2 itself as a form of energy?

:shrug: Nothing is impossible...

[Odin2005]

4. CO2 is a fully oxidized compound, so is worthless as a fuel source.

[phantom power]

5. You could apply nuclear, wind, or other non-fossil energy to do it, but...

then there is the question of what you do with it once you have it. Various sequestering schemes have been proposed, but they all seem dubious to me.

The most elegant sequestering is to simply allow plants to grow, but that assumes the overall living plant biomass grows over time. In our current world, the amount of living plant mass is decreasing, not increasing, and it's likely to stay that way until human population and resource usage comes under control.

[NNadir]

7. It is conceivable to industrially do what plants do with it, reduce it.

The potential industrial hydrogenation of carbon dioxide is well understood.

Carbon dioxide is, in fact, an important industrial material today, as is hydrogen.

It is theoretically possible, and in fact, demonstrable, that carbon dioxide can be removed from air. The most common procedures involve the use of membranes and or cyrogenic means, but there are others.

I think we do best industrially when we look at how nature does things. Nature removes carbon dioxide from the air in part from membrane like procedures, but also by equilibrium driven processes. The carbon dioxide never gets concentrated, but is continuously removed from the mixture (air) at the concentration at which it is found.

In a recent thread, I reported on the efforts of the famous chemist George Olah, who has been working on methanol fuel cells.

http://www.democraticunderground.com/discuss/duboard.php?az=view_all&address=115x47757

Although I was aware of his work in a general way, I didn't know that he had in fact, developed a fuel cell that is reversible.

It seems to me that this business is key. It is certainly imaginable that this type of system, an electrochemical reduction system, can be adjusted so that it is equilibrium driven.

Dr. Olah, postulates a series of transitions, as do I, beginning with the recovery of fossil fuel carbon dioxide emissions (which are relatively concentrated sources of carbon dioxide) to make, by reduction, storable fluid fuels. This may seem like a bit of a shell game, until one realizes, that one could, replace all of the oil products and natural gas products by capturing the CO2 of coal - so long as the energy from reduction came from some non-fossil fuel source.

Ultimately however, depending on the cost of energy, the degree to which energy from non-fossil sources is available, I can easily envision ways that one could make this an equilibrium driven process, similar to the hydride transfer process that takes place in photosynthesis. In this case, the source of the energy would be electricity, not necessarily light (as is used in photosynthesis). It would be very similar to the situation with heavier than air flight. We knew it was possible because birds and insects do it. Although we have never managed to fly using muscles, we have nonetheless learned to fly, indeed, better and higher than birds can do it.

I have read interviews with Dr. Olah over the years at which he expresses disgust with the whole charade over fuels. In his mind he has solved the matter of fuel cells. Maybe he has and maybe he hasn't, but he is a very brilliant man, a calibre who could be a Telsa, or a Haber, or other such genius whose schemes have vast industrial import. I note that he is not some garage scientist who claims to have found a secret way to get around the second law of thermodynamics. He is a winner of the Nobel Prize in Chemistry.

However, I very much doubt that any of this will happen in time. It may never happen. We must, if we are to survive, not rely on pie in the sky, could be, stuff, but instead, on what tools we already have and that we know already work.

Still, given the source, I cannot believe the reversible methanol fuel cell is not getting more attention. It should be a seriously investigated matter.

[jpak]

6. Supercritical CO2 is used to prepare samples for electron microscopy

It's a standard technique using ultra-pure CO2.

Unfortunately, the gas is not recycled is vented back to the atmosphere.

I suspect that's true for industrial processes that use it as well.

The only way that anthropogenic CO2 is going to be removed from the atmosphere is by chemical weathering of continental rocks and burial of carbonates in marine sediments.

Global warming will enhance crustal weathering rates and the rate of CO2 removal from the atmosphere....

But unfortunately, those processes are a lot slower than the rate of emission of anthropogenic CO2 to the atmosphere...

[NNadir]

8. I didn't know supercritical carbon dioxide was used for this purpose.

That is interesting.

Supercritical carbon dioxide is also widely used in chromatography. It is cool stuff.

Much of the world's carbon dioxide is now recovered from fossil fuel waste which is vented to the atmosphere. Unfortunately the world demand for this chemical is not nearly at the level of world production of fossil fuel waste.

[jpak]

9. It's also used to clean semiconductor chips...

http://www.reed-electronics.com/semiconductor/article/CA271877?pubdate=2%2F1%2F2003

and I believe its replaced certain CFC's for that purpose (CFC's were also used to clean samples for electron microscopy before SCCO2 came around)....

[NNadir]

10. That is also interesting.

It certainly is an interesting solvent. To the extent that it is used in these ways, it represents a type of sequestration.