Homebrew PCBs

On Mon, 15 Jan 2007 23:54:43 +0100, William Carr <Jkirk3279@...>  
wrote:

>
> I bring this up because I wondered if the process could be used to
> recharge etchant.   If you put a copper rod in as the sacrificial
> scrap metal, you'd force copper into the solution.
> William Carr


Actually, you use a graphite rod as one electrode, or both, since you want  
to get copper out of it not into it. I've tried it, but it doesn't work  
very well. Chlorine gas is produced at the electrode (of course! you plate  
the Cu away from CuCl). This chlorine gas would need dissolving in the  
etchant, otherwise you just pollute the air.
One suggestion to do this was a rotating electrode. i am not impressed by  
that idea. I think one should try to make a downstream gas dissolver like  
used by aquarium people to dissolve CO2. Basically a vertical tube filled  
with obstructions like marbles, gas introduced at the bottom, and a pump  
circulating water (in our case etchant) from the top to the bottom, so  
there is a downflow. This slows down the gas dramatically, giving time for  
dissolution (it could be closed up on top so any chlorine reaching the top  
would accumulate for later dissolving instead of escaping).

Who knows it may even be suffucient to catch the chlorine gas in an  
upside-down container and keep it there presented to the etchant for slow  
dissolving without any pumps. I never tried.

A simple addition may be a magnetic stirrer to stir up the static  
gas/water for faster reaction.

The way this copper recovery is done industrially is by using a membrane  
between the electrodes that allows only ions through or something like  
that, i don't fully understand the process. But it works for them.

It seems lotsa effort for little gain to me. With the small number of  
boards i do excess CuCl has never been a problem.

ST