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To my knowledge, metalic ions only precipitate from a solution as salts & and a process that could possibly produce monocristalline metal would be very interesting indutrially - I've never heard of one, so I don't think it is possible.

But: is it not conceivable to dissolve a metal in an acid (or maybe base), and then (later) neutralize the acid, without resulting ions that form a salt with the metal?

What am I missing?

As an aside, I arrived at this question while thinking if there could be a biological, and thus wet-chemical, pathway from ores to pure metals (in metallic form). The last step in a such a process would be the precipitation/crystallization as a metal.

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2 Answers 2

up vote 3 down vote accepted

The easiest way to produce big crystals of metal from water solution is reactions like this

$\ce{Fe_{(s)} + CuSO_4_{(aq)}\to Cu_{(s)} + FeSO_4_{(aq)}}$

performed at very low concentration of the salt. The lesser know method is usage of reducer, like hydrazine

$\ce{CuSO_4_{(aq)} + N_2H_4_{(aq)} \to N_2_{(g)} + H_2_{(g)} + H_2SO_4_{(aq)} + Cu_{(s)}}$

This kind of reactions usually produces fine powder (when I tried it, the resulting powder oxidezed with air in less then half of hour), but it is possible to produce visible crystals this way, working in gel with reagent transfer by diffusion.

For industrially valuable path from ores to metals it not absolutely necessary to get metal directly as result of the process. For example, iron oxalate when heated slightly decomposes, releasing metallic iron

$\ce{FeC_2O_4_{(s)} \to Fe_{(s}) + 2 CO_2_{(g)}}$

The process gives very fine powder, like in previous example.

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thx - just to get this straight: getting pure metal is not a very big challenge, but crystal growth in metals works such that the crystals are very small/it's very hard to grow large crystals and the binding that happens between monocrystallines in a macroscopic lump of iron does no arise in a situation as described here? –  martin Jan 25 '13 at 15:03
    
@martin Well, the first path easily gives crystals 1-5 mm in size. Crystals with size 1-10 cm may be obtained, if molted metal is cooled slowly. Sure, they are tied together in metal slab, but they exists. Actually, many processes are focused on making this crystals smaller. –  permeakra Jan 25 '13 at 15:59

I think you've just described the general concept of Electrowinning?

There are a large number of ways to recover metals from solutions or molten salts.

Producing monocrystalline metals is a different matter - crystal grains in metals tend to be microscopic, but there are techniques that permit growing large crystals of metals such as the Czochralski process or zone melting. Monocrystalline metals sometimes have attractive mechanical properties not achievable in polycrystalline metals - for example, Callister Materials Science and Engineering 6e p. 225 has a nice photo of a single-crystal jet turbine blade that is highly resistant to creep failure with respect to conventional blades.

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