Chlorine Gas: Highly Dangerous!
Careful! The reaction is highly exothermic and produces chlorine, a highly dangerous gas. Chlorine gas was used as a chemical weapon during the first World War, under the name bertholite.
In fact, the chlorine produced by mixing hydrochloric acid and chlorine bleach is what will dissolve the gold to form gold(III) chloride.
2 Au + 3 Cl2 -> 2 AuCl3
Now, all we need to do is filter everything once again. The filter will retain all the impurities, leaving only a gold(III) chloride solution.
To recover the metallic gold, we now need to precipitate the gold that’s in solution. For that, we use powdered sodium metabisulfite. In the presence of water, the sodium metabisulfite produces sodium bisulfite.
Na2S2O5 + H2O --> 2 NaHSO3
This sodium bisulfite is what will allow the gold to precipitate.
3 NaHSO3 + 2 AuCl3 + 3 H2O --> 3 NaHSO4 + 6 HCl + 2 Au
We let the solution settle, then we recover the brown powder collected at the bottom of the beaker. We have to be careful not to lose any--that’s metallic gold!
Now, all we need to do is to melt the powder in a crucible.
The melting point of gold is around 1064° C (1947.52 °F), so an oxy-butane torch will do the job.
A Gold BB
The result is a pretty gold BB!
Economically speaking, is it worth all the trouble? Definitely not. The process is only viable if it’s done on an industrial scale. The little ball of gold we recovered is only worth two or three dollars at current prices. And in fact, companies that recover the gold from old computers use other techniques and chemicals that are even more dangerous. But it’s still interesting and fun to know that it’s technically possible to recover gold from motherboards using a homemade process.
It’s also possible to recover gold from add-in cards, processors, and chipsets. But that’s a story for another day.