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Jan 09 2012

Pique the Geek 20120108: Aluminum Part II of II

Last week we discussed the production and uses of aluminum, and that piece got a lot of comment traffic and made the Kos Recommended List, which I value greatly.  Some of the comments asked questions and made me decide to write a follow up piece, because some of the questions were excellent in their own right, and some of them also caused me to think a bit further about our use of this material.

Tonight we shall concentrate on a few more uses of aluminum, why it is so unique, and less environmentally damaging ways to produce it.  It turns out that there is an experimental process to refine aluminum that does not produce nearly as much carbon dioxide as the Hall-Héroult process, and might be more energy efficient as well.

There is no time like the present, so let us get started!  That is unless you have an objection.

As we mentioned, aluminum is a marvelous structural material, and is easily welded with the proper equipment, notably inert gas processes.  I have only a basic arc welder, so I can not work with it, but those who have inert gas blanketing (and whole lot of folks do) can weld it quite easily.  There are also some pretty good solders that join aluminum pieces together, but they are not as strong as a proper weld.

One of the most important scientific uses of aluminum was left out, and we regret doing that.  Many, if not most, terrestrial telescopes use aluminum as the reflecting layer, being bonded to the front of the reflecting mirror.  Aluminum is very reflective in the UV, visible, and IR parts of the spectrum, and being noncorrosive, is just about ideal for a reflecting surface.  There are a few reflecting materials that are better in particular parts of the spectrum, but overall aluminum is just about without peer.  For example, the Hubble Space Telescope has a reflecting layer of about a 70 nanometer (nm) layer of reflective aluminum overcoated with a protective layer of around 20 nm of magnesium fluoride to keep the aluminum oxide from forming, because it absorbs light.  If I am not very mistaken, those were both laid down by vacuum deposition.

Speaking of vacuum deposition, that is the way that potato chip bags and such are made.  A roll of plastic film is brought into a vacuum chamber where a block of aluminum is heated until the vapor pressure of aluminum is great enough to condense to the metal on the plastic as it unwound from the original roll and rewound onto an empty roll.  Enormous quantities of aluminized plastic are made that way, the aluminum acting as an oxygen barrier and the plastic providing toughness since such a very thin aluminum film is quite fragile.

Let us consider the environmental part again.  We will not repeat what was said last week because the link is in the opening to the introduction, but we shall say that the environmental costs, at least for the holes in the ground and waste materials, than the process being used currently.  But, if it pans out well, the new process has several advantages.

This process uses carbon to reduce aluminum oxide (alumina) to aluminum in an indirect manner.  It is called indirect carbothermic reduction, and involves a rare intermediate, an aluminum carbide, Al4C3.  Here is a very detailed technical report for it, and please notice that the toxic element, fluorine, is not necessary.  Please note that this is a .pdf file.  But it looks good energetically, and may be the way for the future.  However, it will require massive retooling for the the industry, and industrial processes change very slowly unless there is a major cost savings involved because of the enormous infrastructure already in place.

There is a way around some of the environmental problems associated with aluminum, and that is to recycle it.  Aluminum is a very good material for recycling, and industry recycles LOTS of aluminum.  Drink cans are particularly good for recycling, since they are almost pure aluminum.  About a third of all aluminum production in the US is from recycled material (from all sources), and about two thirds of beverage cans are recycled.  The energy savings are huge.  Remember, was said last week that it takes about 15 kw-hr to produce a kilogram of aluminum from bauxite, and a kw-hr amounts to 3.5 megajoules.  Aluminum melts at around 660 degrees C, so taking them from 20 degrees to 660 requires 573 kJ (one kg of aluminum is 37 moles and 640 degrees times 24.2 J/mol K).  Then melting it requires another 396 kJ, for a total of 969 kJ.  This amounts to about 28% of the energy to refine it.  You hear the number of 5% of the energy cost, but the 15 kw-hr figure does not count the cost of the energy to heat up the pot, just to separate the aluminum from the oxygen.  In other words, the refining figure from bauxite is lowballed, but I think that 5% is really too low a figure.

Someone asked a question in the comments last week about the use of aluminum in rocket propellants, and that is a very good question.  For the same reason that it requires a lot of energy to refine it from aluminum oxide, a lot of energy is released when it is burnt.  For the space shuttle, the primary fuel is (well, was) 16% aluminum, a rubberlike binder (which also acts as a fuel, but not as energetic as aluminum) at around 12%, and ammonium perchlorate as the oxidizer at about 70%.  An epoxy curing agent and a little iron oxide are also added, the curing agent to harden the binder and the iron oxide to catalyze the reaction.

When aluminum burns in oxygen, 1676 kJ of energy are released for every mole (27 g) of aluminum.  This is a huge energy output, and you can compare it to the energy output when carbon burns to carbon dioxide of 394 kJ for every mole (12 g).  To even the playing field, when 27 g of carbon burns, 886 kJ are released, so an equal mass of aluminum releases almost twice as much energy as that mass of carbon.  By the way, ammonium perchlorate is pretty treacherous stuff itself, as witnessed by this video.  Notice the shock wave that radiates from the center when it detonates:

When I made pyrotechnics for the Army I used quite a lot of aluminum powder, and you have to treat it with respect.  I mentioned this last week, but it bears repeating.  It normally comes coated with mineral oil to reduce dusting and protect it from oxidation, and that also makes it much less tricky.  We would wash off the mineral oil with acetone, and after that is has to be handled extremely carefully, because a spark of only a few millijoules can cause the dust to ignite, and static charges on the human body can amount to several hundred millijoules.  We always wore conductive soled shoes and our floors were treated to be conductive, and we earthed and bonded everything.

That same aluminum powder is also used to make aluminum (and other metallic paints with added coloring agents) paint.  In this case there is no need to wash off the mineral oil, since it gets dissolved in the vehicle and the thinner added anyway.  Aluminum paint is good stuff for protecting steel objects, because the aluminum is more reactive that the steel, so it is a sacrificial coating, much like galvanizing.

Well, you have done it again!  You have wasted many more perfectly good einsteins of photons reading this shiny piece.  And even though all of the Republican candidates for the Presidential nomination realize that none of them will win the general election when they read me say it, I always learn much more than I could possibly hope to teach by writing this series, so please keep those comments, questions, and corrections coming!  Tips and recs are also always appreciated.  I shall stay around as long as comments warrant, and shall return around the same time tomorrow for Review Time.

Warmest regards,

Doc, aka Dr. David W. Smith

Crossposted at Daily Kos,

Docudharma, and

firefly-dreaming

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