In Pictures: Tom's Hardware Recovers Gold And Silver From CPUs

The Formation Of Metallic Silver

After a few hours, metallic silver becomes noticeable. We also see that the silver chloride precipitate has begun to blacken in the light. The zinc has reacted with the hydrochloric acid to form dihydrogen (and zinc chloride).

Zn + 2 HCl -> H₂ + ZnCl₂

The dihydrogen will then reduce the silver chloride; metallic silver will then form.

H₂ + 2 AgCl -> 2 Ag + 2 HCl

Filtering And Rinsing

We filter the solution and rinse everything. The result is metallic silver! Let’s let the powder dry...

Melting The Silver

Now, all we need to do is melt the silver in a crucible with an oxy-butane torch.

A Silver Coin

And voilà: a pretty little silver coin! Of course, it’s not 100% pure, which is impossible with this kind of do-it-yourself process, but it’s reasonably close. We suspect the presence of platinum, palladium, and rhodium. Let's continue.

After The Silver, We Go For The Gold

We haven't finished with our CPUs yet, though. We still need to get them to give up their gold!

After a short rinsing with distilled water, we put them in a bath of hydrochloric acid and 3% hydrogen peroxide, in a proportion of two to one.

Another Unpleasant Bath

You can see the very fine gold wires around the processor die separating. We’ll leave the processors in the bath for a few hours...

Filtering, Yet Again

After a few hours, we filter the solution to recover the gold flakes and various waste substances.

Danger: Chlorine Gas

What’s left behind in the filter is a mixture of various metals and impurities. We put everything in a mixture of 35% hydrochloric acid and 5% chlorine bleach, in a proportion of two to one. The reaction is highly exothermic and produces extremely dangerous chlorine gas (Cl₂)!

2 HCl + NaClO -> Cl₂ + NaCl + H₂O

The chlorine gas is what attacks the gold to form gold(III) chloride.

Filtering Again

We filter everything one more time. The filter retains all the impurities, and nothing is left in the solution except gold(III) chloride.

Now We Precipitate

To recover the metallic gold, we need to get the gold that’s in solution to precipitate. For that, we use powdered sodium metabisulfite. In the presence of water, the sodium metabisulfite produces sodium bisulfite.

Na₂S₂O₅ + H₂O -> 2 NaHSO₃