Antwain
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Purifying zinc dust
I recently acquired some zinc dust, but dust it is not.
Most of it is solid and there are a few small free clumps at the top, but I am convinced from the colour that it is mostly zinc. Firstly, is this
normal? The zinc dust at uni is dust, but unlike this it is not probably 20 years old. Secondly is there some way to clean it?
I was thinking that ethanol would complex with zinc oxide/hydroxide without reacting rapidly with the metal (at low temperature). Then decanting the
ethanol and rapidly drying would fix things...?
Will this work and is this worth doing?
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not_important
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Wash a small amount (accurately weight it if you can) with ether, or an ester like EtAc, or dry acetone if you don't have those. Let the wash
evaporate in a test tube to see if there is any organic type contamination.
Next extract that washed zinc powder with a little cold water, and test the washing for halides and sulfates (AgNO3 and BaCl2 solutions).
Finally dissolve the sample in some HCl or H2SO4 in such a fashion that you can capture and measure all the gas given off. That gas, taken as H2,
along with the starting weight should give you a good idea as to the percentage of metallic zinc remaining. You could do a bit better by measuring the
gas, then absorbing any CO2 with NaOH solution, leaving just H2.
If there's no organic crude and no soluble salts then it's likely OK. Zinc dust always has a fair amount of oxide in it, and there's no simple way to
remove it - if it's not soluble in water it's likely not to dissolve in EtOH. So long as the percentage of metallic zinc is known, and it's not too
low, it should be usable.
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woelen
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I would simply dry it above a heat radiator for a few hours and then repackage it. If necessary, grind it to powder again. First try this with a small
amount (1 gram or so) in a petri dish to see what happens. Some zinc powders are pyrophoric and you don't want a big amount to ignite accidently.
I also have more than 20 years old zinc powder, and it still is a perfectly free flowing powder. Apparently your material was not stored properly.
[Edited on 2-11-07 by woelen]
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chemrox
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I used to use this as rocket fuel. The mix, Zn & S would get "stale" and be highly unstable. Take precautions as my friend woelen advises.
"When you let the dumbasses vote you end up with populism followed by autocracy and getting back is a bitch." Plato (sort of)
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The_Davster
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I think it may just be wet. I have encountered both types of zinc before, the powder and the lumpy stuff. I would follow what woelen suggested.
Or if you want ultrapure zinc:
"The zinc is purified by stirring 1.2 kg. of commercial zinc dust with 3 l. of 2% hydrochloric acid for 1 minute. The acid is removed by filtration,
and the zinc is washed in a 4-l. beaker with one 3-l. portion of 2% hydrochloric acid, three 3-l. portions of distilled water, two 2-l. portions of
95% ethanol, and finally with one 2-l. portion of absolute ether, the wash solutions being removed each time by filtration. Then the material is
thoroughly dried and any lumps are broken up in a mortar."
http://www.orgsyn.org/orgsyn/pdfs/CV3P0073.pdf(in the notes section)
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