Making metal powders using electrolysis - chosing the right electrolyte

Question

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.

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.

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...).

Answer 1

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.

Answer 2

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.