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Really fine metal powders are good for two things: making paints and pyrotechnics. For this purposes, I can see people usually use thing called ball mill.

ball mill

It's basically a barell stuffed with heavy balls and the thing to be powdered. You leave it for a while, rotating all the time, and the balls slowly crush your substance into smaller and smaller pieces.
There are many disadvantages to this:

  • I'd need to construct it
  • It would be running and making loud mess all the time
  • soft metals, like aluminium and copper don't even crush into pieces

So I have figured out a different way. More chemical one I'd say. I can dissolve metals in acid by forming salts out of them and then electrolyse those salts. Not so long ago, I complained that instead of copper plating I got copper powder. That's just what I want now. I'm really sorry that powder I created the other day is already flushed down the drain. It could make beautiful green flames.

copper powder

Now while the general idea is easy, I'm not so sure about choosing the right electrolytes. My theoretical knowledge of electro-chemistry unfortunately sucks. And I'm afraid that every metal will need different electrolyte.

For example I think using any acid for Aluminium could cause something nasty since it reacts with them little bit too quickly. For Iron, I tried hydrochloric acid and then I found out thet $\ce{HCl}$ dissolves iron to form iron chloride.

Are there general rules to figure this out? If not, what sould I consider to find electrolyte individually for these metals:

  • Aluminium
  • Iron
  • Copper
  • Magnesium

Powdered metals can be bought of course, but I don't want to waste good money for something, that can be easily made from trash (old nails, aluminium foils from chocolate...).

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  • $\begingroup$ Use sulfuric acid- sulfates don't give significant side reaction. A standard solution of say 30% sulfuric acid should corrode all of those metals. Chlorides will give some toxic risk with chlorine gas production. $\endgroup$ – user2617804 Sep 29 '14 at 8:52
  • $\begingroup$ I'm going to try it and post the results right away. $\endgroup$ – Tomáš Zato Sep 29 '14 at 9:11
  • $\begingroup$ Reaction with aluminium produces gas that is so irritating that I can't tell what does it smell like. It smells like breathing liquid sodium. $\endgroup$ – Tomáš Zato Sep 29 '14 at 9:37
  • $\begingroup$ So does reaction with iron. My nose really hurts now. And I was sniffing carefully. $\endgroup$ – Tomáš Zato Sep 29 '14 at 9:41
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Fine copper powder can be prepared using thermal decomposition of copper citrate (can be prepared by mixing dirt cheap citric acid, three equivalents of a base like potassium hydroxide and copper sulphate). I once made pyrophoric (!) copper powder using this technique. There are mentions of preparation of iron powder same way. Anyway, it is much more practical than electrolysis for big batches.

Frankly speaking, fine aluminium flakes are widely employed in various paints and are dirt cheap. They should be available in shops for artists. There is no reason to make them at home (and aluminium reacts with water once cleaned of oxide film)

Magnesium is probably the worst. It is quite costly and active and making fine powder is dangerous. It also reacts with water and alcohols, so making it employing electrolysis is out of question. There are mentions of home smelting of aluminium and magnesium, producing magnalium. Magnalium is brittle and can be easily milled, and it is active enough so fine milling is usually not needed. While fancy, proper pyrotechnics can be made without magnesium with easy, aluminium is usually much more preferable. Magnesium often produce huge volumes of white smoke, a habit aluminium doesn't have.

Regardless of above, I strongly encourage to be cautions when working with metal powders, especially magnesium.

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  • $\begingroup$ So what do I need? Citric acid? Potassium hydroxide? Copper sulphate? $\endgroup$ – Tomáš Zato Sep 30 '14 at 5:22
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    $\begingroup$ @TomášZato For copper powder, if you want to make it, you need copper sulphate or chloride and potassium or sodium citrate. If they are unavailable, they may be prepared before use in solution by mixing citric acid and potassium or sodium hydroxide. Precipitated copper citrate should be carefully heated under cotton jam, so gases could exit, but fresh air was blocked. I believe, that for iron similar procedure may be used, except instead of copper salt one should use iron (II) salt, but it is an educated guess, I didn't do it. $\endgroup$ – permeakra Sep 30 '14 at 9:07
  • $\begingroup$ After some research, I can't seem to find a way to get aluminium out of aluminium sulfate. Using baking soda, I can turn it into aluminium hydroxide. But what then? $\endgroup$ – Tomáš Zato Sep 30 '14 at 14:55
  • $\begingroup$ @TomášZato Forget about making aluminium or magnesium at home, using electrolysis or other methods. It is theoretically possible (not in water medium, of course), but requires too much work and involves significant risks. $\endgroup$ – permeakra Oct 7 '14 at 9:08
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    $\begingroup$ @TomášZato First, the amounts of reagents must be right (3 mol NaOH, 1 mol Cictric acid, 1.5 mol CuSO4), and second, the precipitation occurs after some time, not immediately. A deep blue solution is formed first and may be heated to quicken the precipitation. Yeah, it's a bit inconvenient. I heated the mixture to ~50-80 C for 10-15 min before precipitation occurred, some sources recommend merely leave the reaction mixture overnight. $\endgroup$ – permeakra Oct 18 '14 at 12:44
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Regarding the suggestion in the comments above: Concentrated sulfuric and nitric acids produce $\ce{SO2}$ and $\ce{NO2}$ gases, respectively, as they oxidize metals. This is a common way of dissolving many metals, but you need to do it somewhere where you won't breathe them in.

When you're actually electroplating, you'll want to dilute them with a different electrolyte. Potassium nitrate or citrate or phosphate buffers are common choices. Anything that doesn't complex or form an insoluble salt with the metal will do, though adding things like surfactants may change the morphology of the deposited metal structure. Depending on the metal, you might get precipitation if you raise the pH too high, but you don't want to electroplate in concentrated acid as you'll just redissolve your deposits.

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  • $\begingroup$ Can I use washing soda? $\endgroup$ – Tomáš Zato Sep 30 '14 at 10:55
  • $\begingroup$ Probably not a good idea. Many carbonate salts are insoluble (Cu and Fe) or unstable (Al) and it's going to neutralize your acid to make carbon dioxide. $\endgroup$ – Michael DM Dryden Sep 30 '14 at 16:26

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