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Name: David
Status: educator
Grade: 9-12
Location: TN
Country: N/A
Date: March 2007

Question:
We are using NaOH 20% and water 80% with aluminum soda cans to produce hydrogen in a stainless steel reaction tank. We send the resulting steam and gas to a bubbler to capture the water vapor and any suspended NaOH, then send the hydrogen gas to a burner. The question is after the reaction is complete there appears to be water, NaOH, and aluminum oxides left in the reaction chamber. Is it possible to reclaim the NaOH and aluminum through another chemical process?



Replies:
There should not be any metallic aluminum remaining. The aluminum oxides (typically a white gelatinous precipitate) can be filtered, centrifuged, or allowed to settle -- depending upon the configuration of your reactor. You would then have to replenish the NaOH remaining in solution. Alternatively, you can add more NaOH to form Al(OH)4[-1] which is soluble if the pH is up around 11-12 and continue to use the solution until it just becomes too messy, then clean the reactor.

Vince Calder


Not too easily, David. I suppose If you get usually familiar you might find a way to make Aluminum oxides/hydroxides precipitate and settle to the bottom without taking much NaOH with it.

Phase diagrams of H2O/Al2O3 and Al2O3/NaO might help.

Your handles are

- reaction feed rate (running, pausing, etc),

- temperature (cooling or boiling the bath during a pause)

- addition or removal of water

- neutralizing by adding acid (spends the NaOH, but precipitates the Al-hydroxide)

- mechanical processes like stirring, settling, filtering

You'll be lucky if you find something to get some NaOH back from the Al(OH)x.

Nothing gets aluminum back from it's oxidized state except the electrolysis the aluminum was manufactured with in the first place. It's a big energy cost. Changing aluminum oxide to metal takes an expensive amount of electric kw-hrs, accounting for much (most?) of the cost of new aluminum. I guess the chemistry pioneers reduced Al oxide with pure Calcium metal, which itself only gets made by electrolysis these days.

Even getting the last water out of Al2O3, converting Al2O2(OH)2-->Al2O3, can take over 1000C! I found that on a phase diagram. Maybe the toughest known substance to fully dry.

An alternate system (not better): A big enough running fire chamber will burn Aluminum cans quite easily. Then you will have nearly-pure(I think) Al2O3 powder ash on the bottom that can be used for ceramics. And you will have used (paid for) no NaOH. This monster should be big and have some thermal mass and insulation, and heat extraction which can be switched off, so it stays hot a long time between chucking in bunches of cans., and a gas flame injector to pre-heat it to get it started in the first place, and even then you want a relatively heavy stream of cans to keep it running. Another advantage is it probably processes more cans per minute from a given size plant.

Advantage of your setup is your hydrogen could feed a fuel-cell, which is 2-3x more efficient than a steam engine. Then you can let someone else use the electricity to make 0.8 new aluminum cans.

I kind of wonder if your Al(OH)x/NaOHy/H2Oz waste slurry can be reacted with alcohol to make Al(OC2H5)3 or similar. That's a clear liquid "sol-gel" percursor-chemical for coating things with Al2O3 by dip-coat or spin-coat or vapor-phase deposition. It's still oxidized aluminum, but now it has a reason for a premium value. Some scientists in my lab have used that, and there might be an industrial use now.

I am hoping it costs enough per pound new to pay you for your trouble.

Jim Swenson


It is in principle possible, but it probably would be more trouble than it is worth. It is certainly more trouble than the hydrogen-production process you employ.

You might be able to recover part of the sodium hydroxide if you get the residual liquor hot enough and concentrated enough. Your liquor is similar to the extracted liquor from the Bayer process that produces alumina for the aluminum industry. The Bayer process affords alumina from the hot, concentrated solution of sodium aluminate by simple crystallization. The liquor form this step is enriched in sodium hydroxide.

The crystals of hydrated alumina are then calcined to produce anhydrous alumina. See

http://www.world-aluminium.org/production/refining/chemistry.html.

You could then obtain aluminum metal from the alumina through the Hall electrolysis process;

http://www.world-aluminium.org/production/smelting/chemistry.html.

But this is heavy-duty, high-temperature, dangerous, industrial stuff!

Richard Barrans
Department of Physics and Astronomy
University of Wyoming



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