Homebrew PCBs

--- In Homebrew_PCBs@yahoogroups.com, Stefan Trethan
<stefan_trethan@g...> wrote:
> On Fri, 16 Apr 2004 10:22:49 +1000, Adam Seychell 
> <adam_seychell@y...> wrote:
> 
> > They use an ion exchange membrane in the regeneration cell. I'm
> > not exactly sure what the membrane is doing in this situation.
> > Obviously its separating ions from anode to cathode.
> > Unfortunately the image showing the electrolytic cell is missing
> > from the web page. I'm sure PVC membranes can be purchased but I
> > figure the control system necessary to make this work would be
> > prohibitively complicated for a hobby. I'm not saying it couldn't
> > be done, but attempting to build such a system could end up being
> > a whole hobby in itself.
> 
> i fear you might just be right with that..
> 
> However i did a quick experiment yesterday.
> 
> first i used two screws and passed current through.
> (immersed in "old" etchant with much copper)
> the iron blackens very fast but one could see copper plating on one
screw.
> 
> 
> then i used motor brushes (carbon) - you could see copper plating quite 
> good,
> but the brushes seem to dissolve into the etchant.
> i thought carbon wouldn't but then maybe only graphite doesn't.
> 
> I also noticed that i get clorine gass with any substantial current..
> no idea if it can plate withot gas (with low enough voltage).
> 
> 
> It seems not so easy to do after all.
> But you could solve the whole disposal problem with it.


The problem with producing chloride gas is the process mechanically
destroys the graphite electrode (anode, positive). Small bubbles
slowly break away at the surface and you end up with graphite power
settling to the bottom of the cell. The cathode (negative) survives ok
because it plates with copper. 

The Anode:
Its interesting that chlorine gas is sometimes uses to regenerate CuCl
etchants, and it can also regenerate FeCl3 etchants. Sometimes
electrodes can be attached to a motor and spun so the reactions at the
surface are rapidly mixed in the bulk of the solution. Chlorine gas is
not what we wish to produce and have escape into the air, because it
means we loose it. So by using a spinning anode and careful selection
of current density, it may be possible to get the regeneration
reaction happening without evolving chlorine gas. 

The Cathode:
	The problem of the copper at the cathode is that its trying to be
etched at the same time being plated out and will make the reaction
less efficient. The article you pointed to had mentioned the PVC
membrane separated the cathode and anode compartments. The cathode
compartment was kept at low copper content of 20g/l instead of the
usual etching strength 150g/l, in order to reduce etching rate of the
copper being plate out. From my experiments with alkaline ammonia
sulfate electrolytic regeneration, I found that the solution near the
surface of cathode must be kept stagnate as possible for the best
results. If the solution was vigorously agitated then the competing
rate of etch would override the rate of plating and therefore no
copper recovery was possible. Effectively the solution just warms up.
The advantage of alkaline ammonia sulfate regeneration is that the
anode reaction does not have problems with gas evolution , and
ordinary 316 stainless still anodes are used. The current density at
the cathode should be high as possible to maximize the ratio of
plating rate to etching rate. The upper limit of current density is
determined by you don't get water electrolysis and evolve hydrogen gas. 

At first you might like to setup a test jig with large ratio of anode
to cathode current density in a still solution and both electrodes
still. Set the voltage just before you smell chloride gas or see any
bubbles at the anode. Cheap source of carbon rods is from welding
suppliers. They are coated in thin foil of copper, but that can easily
be etched away.
 
Adam