cjevancich
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Interesting byproduct from aluminum foil and can scrap metal
Hello everyone. I'm a long-time lurker and finally decided to make an account since I think I'm stumped and I saw no other pages on this topic.
I had a bunch of aluminum metal scrap, mostly saved from foil lids, cola cans, and aluminum foil. I wanted to convert it into a useful intermediate
for eventual aluminum chemistry so I decided to try and synthesize Sodium aluminate by dissolving it in a fairly concentrated amount of aqueous Sodium
hydroxide. I used stoichiometric proportions of Sodium hydroxide with a slight excess.
Everything dissolved and I strained out the paint, plastic and waste. The interesting part was that I had an insoluble precipitate. Of course, you'd
assume it meant this was an aluminum alloy, and the insoluble product must be a transition metal. That's what I assumed. I looked up common alloys and
low and behold, manganese and iron are fairly common.
But it doesn't end there. I filtered all this sludge out of my aqueous solution, and decided to dissolve it with Hydrochloric acid, my plan was to
then mix it with Sodium bicarbonate, and thus precipitate out any of the typical transition metals while digesting any aluminum contaminants.
So here's the part that stumped me. After adding Hydrochloric acid, many of the impurities were dissolved as I wanted, but I was left with a mostly
white precipitate. If this alloy was the expected aluminum-manganese-magnesium composition I expected, then it should have all dissolved since these
all have very soluble chlorides.
http://www.eng-tips.com/viewthread.cfm?qid=103264
The above is where I found details on composition.
I read on wikipedia that some aluminum alloys have silicon, copper, iron, and chrome. The Hydrochloric acid mixture has a yellow-green aqueous layer
which I suspect is Iron chloride, or very possibly Chrome chloride since it too is green. Copper wouldn't readily dissolve, and it isn't white anyway.
I very much doubt I have silicon, since the amounts used are usually in such trace quantities and I have quite a large amount of it for about 100g of
source material (well over 1g).
The combination of possible alloy metals, their chlorides being soluble in water, and having an insoluble white precipitate sludge has me frankly
mystified. I was hoping someone here might have a few ideas I didn't think of.
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watson.fawkes
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These
different applications all use different alloys. For example, this page has some information on alloys for foils. Note at the end 0.1% - 0.7% silicon content, not exactly "trace quantities" in a ~1% residual
sludge, particularly after it has oxidized to silicate.
As for other content, aluminum gets alloyed with almost everything (to a limit mostly around cost). For example, in the list of alloying materials
that appear on the Wikipedia page for aluminum, you left out zinc. Given that you've got a mixed feedstock, you should likely assume you've got some
of everything.
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bbartlog
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Silicon still seems like the best explanation. It's about one half percent of the typical aluminum can alloys, so if we assume conversion to
SiO<sub>2</sub> you could at least assume one gram of silica per one hundred grams of raw materials. I realize you have more than that ...
I suppose it could be some sort of aluminum silicate, although those should really be soluble in strong base.
The less you bet, the more you lose when you win.
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deltaH
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Silica dissolves to form sodium silicate easily so I don't think it's that. You're white stuff is clearly not an acidic oxide.
Looking at wiki's table of aluminium alloys, many contain Ti in small amounts, so your white insoluble stuff may be TiO2 amongst other things,
however, TiO2 should dissolve in concentrated acid if I am not mistaken?
[Edited on 11-11-2013 by deltaH]
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Praxichys
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I would say it was some depolymerization/hydrolosys product from the plastic and paint waste. If there was PET in there, NaOH might hydrolyze that to
ethylene glycol and sodium terephthalate, for instance.
Try taking a small amount of it, dry it, and heat it strongly. Does it melt, carbonize, or vaporize? It's probably something organic if its solubility
hasn't been affected much by strong NaOH or HCl.
[Edited on 11-11-2013 by Praxichys]
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unionised
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I wonder if both of the last two posts are sort of right.
TiO2 is a very common pigment - it's fairly inert and at least some of it is coated with SiO2 to stop it catalysing photodegradation of polymers etc.
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cjevancich
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It's funny, because after I got some sleep and was thinking about it again, I was wondering if I thought about it the wrong way. That is to say, that
maybe I should be interested in the paint/plastic that was on the foil and cans since I used NaOH.
Your posts gave me some reassurance, I'll get write back with what I find out tonight or tomorrow.
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confused
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if you didn't remove the paint/coating from the aluminum cans, it seems very likely that the 'gunk' would be part of the coating
http://www.youtube.com/watch?hl=en-GB&gl=SG&v=WnPrtY...
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cjevancich
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It seems it took much longer than I expected to separate and filter the precipitate, but I was finally able to run a few tests.
On the HCl-washed and filtered precipitate, I first placed a sample on a watch glass and then let it heat on a hot plate to look for decomposition
into carbon. It did not decompose, and only dried into a brittle flake. I then placed a sample under a torch flame, and it only glowed yellow-hot and
the flame was colored yellow-orange probably from sodium impurity. Finally, I took a sample and tried to destroy it with Chromic acid cleaner, and
here is where it was interesting.
The Chromic acid caused some of the precipitate to fizz, but not all of it, and most worryingly it produced a thick gas which caused me to promptly
discard the sample.
I haven't yet looked at any tables of metal salts, but apparently it is an ionic compound, or non-carbon compound. Further, while HCl will not
decompose it, Chromic acid, which has an oxidizer, will decompose some of it and evolves a gas.
Edit: Upon doing some reading, the thick gas may have been Chromyl chloride, which is a highly volatile compound formed when Chromic acid and HCl mix.
There was probably traces of HCl formed when the Chromic acid was added, since it would be a fair bet that the product has some Chloride salts in it
that were separated into HCl.
Edit 2: If there is a chloride, there are two possibilities. The first is that NaCl was in the sludge, since it was a byproduct of using Sodium
hydroxide. Perhaps I did not wash all of it out, and it dried with the mixture I was testing. Alternatively, my sample may contain Lead, Silver or
Mercury. I'm hoping that this is not the case.
[Edited on 16-11-2013 by cjevancich]
[Edited on 16-11-2013 by cjevancich]
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cyanureeves
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hard drive platters dissolved in hydrochloric acid also gives a white precipitate that turns to a gel then hardens to a transparent oily solid when
dried.i swear they look like aluminum after all the skin is stripped off and it is actually plated copper first under that shiny chromium outer layer.
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