I am a fish
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Preparing pure metals by thermite reactions.
I am interested in isolating various metals (including manganese, vanadium and chromium) from their oxides by reacting the oxide with aluminium
powder. However, in my experience, thermite reactions always yield metal that is intimately mixed with the resulting Al2O3. What is the best way of
remedying this problem?
I have heard that CaF2, when added to a thermite mixture, acts as a flux, ensuring that the Al2O3 produced, floats to the top. Unfortunately, I have
no CaF2, and I don't want to use too much of my precious NaF (my only fluorine compound) in making it. How much would be required? Are there
any alternatives?
Another thing I've considered, is changing the scale of the reaction. When doing thermite, I usually react 5g of aluminium powder with a
stoichiometric mass of the metal oxide. Would working in smaller or larger quantities help?
1f `/0u (4|\\| |234d 7|-|15, `/0u |234||`/ |\\|33d 70 937 0u7 /\\/\\0|23.
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vulture
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You might try consulting pottery suppliers for CaF2, they usually have it cheap and plentiful. Don't ask any questions about purity though.
Also, you could make your reaction vessel of smaller diameter and both longer, to improve separation. I doubt this is going to bring major improvement
though.
One shouldn't accept or resort to the mutilation of science to appease the mentally impaired.
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Nick F
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How intimate are the mixtures after the reaction? You may be able to crush the product in order to powder the alumina, and sieve it to seperate out
the metal, if it's in large enough pieces.
Alternatively you might be able to concentrate the metal in the powder by taking into account the different densities, and melt the enriched powder in
a furnace (microwave furnace would be perfect for such small-scale work) for a period of time, allowing the alumina dust to float to the top. But
(domestic) microwave furnaces seem to have problems going over about 1200*C...
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Hang-Man
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This process is known as the Goldschmidt process, ie reducing the oxide with Al powder.
Any metal I get from thermite is usually very porous and contaminated. It almost looks sintered.
You could try the Kroll process if you have some Mg; reacting the chloride with Mg metal.
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Marvin
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I think the problem is simply the scale. You arnt doing enough of the mixture for the result to stay molten long enough to seperate. If thermite can
produce iron of quality enough for railway tracks I'm thinking it it can be quite pure. You could try sodium chloride instead of CaF2 and this
should decrease the violence of the reaction as well as increase the overall volume of material. Scaling this up might get you fairly pure metal,
though some Al dissolved in it will be unavoidable.
Given all the metals youve mentioned can be electroplated, Id be inclined to go that route instead. You could always reduce the oxide to the metal
with Al or carbon and then electrilytically refine but I dont see any real advantage to making the metal that way in the first place.
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chemoleo
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Aluminothermic chemistry
Well, quoting my favourite book, Jander & Blasius, Prep. Inorg. Chemistry (of which I scanned half, and of which all got lost during my hardware
crash  ) - and a fair bit of my own experience, the problem does indeed reside in
1. small quantities
2. Separation of the oxide from the metal
3. tuning the reaction so that it is neither too slow or too violent.
In the JB book, it specifically recommends to NOT to use smaller quanties than about 90-120 g in total, depending on the oxide (see below)
Also, it is recommended to do the reactions in a clay crucible, embedded in sand.
About 10 g of CaF2 is placed at the bottom of the crucible (i.e. the heavier metal will sink down anyway, and enable separation from the Al2o3.
If the reaction is too violent (such as with MnO2, where no residue is left with 200 mesh Al - i.e. my own experience), it is recommended to moderate
the reaction with surplus oxide (diluting the Al available for the reaction), or to use a lower oxide, such as Mn3O4.
If the reaction is not energetic enough to cause the full melting of the metal/oxide, or rather, to sustain itself; sulphur or other oxygen-rich
compounds are added, such as K2Cr2O7.
At last, the reaction is of course also controlled by the fineness of the reagents, the finer the Al, the faster the reaction. Coarse Al grains are
recommended for the isolation of the reduced metals.
Of course, as we all know (see the sodium threads), the aluminothermic process is not really ideal for metals with a high vapour pressure, such as the
alkali metals, Pb, Zn etc).
Most aluminothermic metals contain up to 1% of Al, and are often brittle.
From my own experience, I made a block of Fe a while ago, and that one is definitely not brittle.
Specifics:
Manganese:
MnO2 reacts too violently, so 80g of MnO2 is heated up to 800-900 deg C for 1 hr (no problem if you have a nice little furnace), and converted to
Mn3O4. This is mixed with 20 g of Al grains (9/10th of the stoichiometric amount). 10 g of CaF2 at the bottom of the crucible.
The Mn is obtained mechanically.
Using MnO2 with stoichiometric amounts of Al just results in a v. fast reaction, with no residue.. in case you wondered. I guess diluting it with MnO2
is a thing to try.
Chrom:
70 g of Cr2O3, 25 g of K2Cr2O7, and 32 Al grains. Again 10 g of CaF2.
I am sure though, once finer Al is used, the dichromate may not be necessary. Still have to try this...
Silicon, and Boron:
I believe I have posted instructions here: http://www.sciencemadness.org/talk/viewthread.php?tid=2030
At last - regarding the CaF2, I think it's the same mechanism as with cryolite, Na3AlF6, which is routinely used in the industrial production of
Al from Al2O3. There it acts as a eutectic. Probably the CaF2 acts in a similar way (it is stated to be a flux agent), i.e. it lowers the MP of Al2O3
so that the molten metal has a chance to sink to the bottom of the crucible and agglomerate.
[Edited on 31-7-2004 by chemoleo]
Never Stop to Begin, and Never Begin to Stop...
Tolerance is good. But not with the intolerant! (Wilhelm Busch)
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