"Re-energising" FeCl3 etchant
WaveRider, Mon Nov 13 2006, 03:36PMIt seems that by adding about 200ml 29% HCl to 1 litre of a nearly exhausted FeCl3 etchant bath breathes new life into it! All the cloudy black iron floating around disappears and the solution again etches well (becoming yellow tinged with green, Iprobably from increasing CuCl2 content).
I'm not a chemist, but I presume the iron goes back into solution as FeCl3 where it does it's etching work. Any new iron eveolved from the etching process goes back into solution if there continues to be excess HCl in the solution... I don't see any hydrogen evolution taking place, tho'... Any ideas on the reaction?
Re: "Re-energising" FeCl3 etchant
Bored Chemist, Mon Nov 13 2006, 06:15PM
There's a lot of complex reaction go on in the etch bath but here's a rough guide.
If you are etching Cu
2 FeCl3 + Cu --> FeCl2 + CuCl2
and
CuCl2 + Cu --> 2 CuCl
If you are etching steel then
Fe+2FeCl3 --> FeCl2
And (anyway).
2FeCl2 +O2 +2 H2O --> 2Fe(OH)3 + 4HCl
( this is the one that produces the brown sludge)
Adding the HCl gets the Fe(OH)3 back into solution as more FeCl3
Bored Chemist, Mon Nov 13 2006, 06:15PM
There's a lot of complex reaction go on in the etch bath but here's a rough guide.
If you are etching Cu
2 FeCl3 + Cu --> FeCl2 + CuCl2
and
CuCl2 + Cu --> 2 CuCl
If you are etching steel then
Fe+2FeCl3 --> FeCl2
And (anyway).
2FeCl2 +O2 +2 H2O --> 2Fe(OH)3 + 4HCl
( this is the one that produces the brown sludge)
Adding the HCl gets the Fe(OH)3 back into solution as more FeCl3
Re: "Re-energising" FeCl3 etchant
TheMerovingian, Thu Nov 16 2006, 10:22AM
Adding HCl corrects a bit the pH and sequestrates some Cu as a complex as you have correctly stated and dissolves iron idroxydes, but it doesn't reverse back Fe (II) to oxidizing Fe (III) ion. For this you need an oxidizer with a reduction potential higher than the reduction potential of the Fe3+ (III) + e- ----> Fe2+ (II) reaction. On the net there are many tutorials how to regenerate the etchant
TheMerovingian, Thu Nov 16 2006, 10:22AM
Adding HCl corrects a bit the pH and sequestrates some Cu as a complex as you have correctly stated and dissolves iron idroxydes, but it doesn't reverse back Fe (II) to oxidizing Fe (III) ion. For this you need an oxidizer with a reduction potential higher than the reduction potential of the Fe3+ (III) + e- ----> Fe2+ (II) reaction. On the net there are many tutorials how to regenerate the etchant
Re: "Re-energising" FeCl3 etchant
Chris, Thu Nov 16 2006, 07:49PM
The only way to actually regenerate FeCl3 in the spent solution is to bubble chlorine gas through it, chlorinating FeCl2 to FeCl3. You would remove the CuCl2 with fractional crystallization I guess. All routes to FeCl3 involve chlorine gas. It is made industrially by first chlorinating scrap iron with HCl giving FeCl2, then chlorinating that to FeCl3 with chlorine gas. In any case just a mixture of one part HCl (32%) with two parts topical H2O2 (3%) etches several times faster than FeCl3 and is many times cheaper and less messy. I have etched boards in about 5 minutes with that solution, it attacks copper like nothing else.
Chris, Thu Nov 16 2006, 07:49PM
The only way to actually regenerate FeCl3 in the spent solution is to bubble chlorine gas through it, chlorinating FeCl2 to FeCl3. You would remove the CuCl2 with fractional crystallization I guess. All routes to FeCl3 involve chlorine gas. It is made industrially by first chlorinating scrap iron with HCl giving FeCl2, then chlorinating that to FeCl3 with chlorine gas. In any case just a mixture of one part HCl (32%) with two parts topical H2O2 (3%) etches several times faster than FeCl3 and is many times cheaper and less messy. I have etched boards in about 5 minutes with that solution, it attacks copper like nothing else.
Re: "Re-energising" FeCl3 etchant
Eric, Thu Nov 16 2006, 08:46PM
Electrolysis could also be used to plate out copper and reoxidize Fe back to ferric in a 'spent' FeCl3 bath. Never tried it but it seems like it would work.
Eric, Thu Nov 16 2006, 08:46PM
Electrolysis could also be used to plate out copper and reoxidize Fe back to ferric in a 'spent' FeCl3 bath. Never tried it but it seems like it would work.
Re: "Re-energising" FeCl3 etchant
Chris, Fri Nov 17 2006, 07:16AM
I suppose that would work as long as the electrode potential for the plating is not too high. It would have to be done in a cell where chlorine is not allowed to escape as a gas, like cells for electrolyzing salt to NaClO3/4. Overall it's much simpler than membrane cells and stuff and could be very easy.
Chris, Fri Nov 17 2006, 07:16AM
I suppose that would work as long as the electrode potential for the plating is not too high. It would have to be done in a cell where chlorine is not allowed to escape as a gas, like cells for electrolyzing salt to NaClO3/4. Overall it's much simpler than membrane cells and stuff and could be very easy.
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