# Copper saturation in CuCl/AP solution - what to do about it



## bemate (Jun 2, 2017)

I currently have my big batch of pins marinating in CuCl (AP). The solution is doing its job, albeit slowly, and that is as expected. The foils are coming off and the pins are going into solution. So far so good.

The question is: what to do when the solution gets saturated with copper? Judging from pH and amount of copper dissolved, there is plenty of free acid available to continue the etching, but the Cu saturation stops the process. The miser in me does not care for wasting perfectly good acid just because it can't hold its Copper. And additionally, the more acid I can deplete on solubilizing Copper, the less I need to neutralize when treating it afterwards.

If I consider this as an equilibrium, where CuCl2 + Cu + H+ --> 2 CuCl +O2 + HCl- --> 2CuCl2 + H2O (Not balanced), then obviously the reaction stops when the liquid is unable to dissolve more CuCl2, it creates a negative feedback into the circular reaction where the formation of CuCl2 from CuCl stops, and we reach a situation where the solution is saturated with both ions (Cu1+ and Cu2+).

This saturation happens regardless of the concentration of H+ (residual acid), and so the reaction stops before acid is depleted. To me, the easiest and most obvious solution is to add more water to the reaction, lowering the concentration of all parts, which is what I am currently trying. I would guess it is possible to regenerate the AP into metallic Cu and acid through some kind of electrolysis as well, but that is not my point. I want to use the acid to do its job all the way. The question is; how far can it be diluted before the acid strength gets too low? Is there a critical threshold I need to consider?


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## g_axelsson (Jun 2, 2017)

My experience is that CuCl / CuCl2 can be a bit tricky with this and it takes a bit experimenting to get the feeling. If diluted too much the CuCl would become insoluble and precipitate as a white powder.

CuCl2 is helping dissolving CuCl as well, but when you have free copper in solution it will turn CuCl2 into CuCl, increasing the CuCl concentration and lowering the free Cl- concentration and CuCl will precipitate again unless the HCl concentration is high enough.

I would try to add some water and if you get a problem with too much CuCl precipitating then pour off the excess saturated solution and add some more HCl.

Göran


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## bemate (Jun 2, 2017)

Hmm, I will need to see how I go about this, but for now I just added another liter or so of water last night, we'll see how it looks over the weekend. It's not like this is a big problem, it just annoys me that I cannot work around it. Maybe some more controlled tests with this at some point would be an idea. It could be interesting to see this set up in a controlled environment with measured amounts to get proper numbers for molarity of saturation and so on. I don't have time currently, but after summer perhaps.


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## g_axelsson (Jun 2, 2017)

Exactly my view too.

Just a tip, even salt water (NaCl) dissolves CuCl, actually even better than HCl. There is a lot more to the copper chloride process if one would take the time to study it at depth.

Some of my notes...
http://goldrefiningwiki.com/mediawiki/index.php/Copper_chloride

An article on CuCl solubility.
http://pubs.acs.org/doi/abs/10.1021/je00028a027

Göran


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## Geo (Jun 6, 2017)

You can force copper(II) chloride to dissolve more copper than it can hold in solution. Once you turn off the air and the solution loses the extra oxygen, the excess copper will precipitate out as copper(I) chloride. Remove some of the saturated solution from the container as overage and add fresh HCl back in it's place. To give the copper more room in the solution, you can dilute the 32% HCl 50/50 with water before adding to the solution. You can add the two separately but when you add the water only, a cloud of CuCl solids will form and it takes awhile for it to dissolve again. I would dilute the original solution 75/25 with fresh chemicals/pregnant solution. As long as the solution is emerald green, it will dissolve copper and only fresh HCl is needed. If the solution is brownish, it needs to be diluted as above.


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## Topher_osAUrus (Jun 6, 2017)

g_axelsson said:


> ... even salt water (NaCl) dissolves CuCl, actually even better than HCl. ..
> Göran



That is an interesting tid bit Göran.
Have you tried any HCl/Brine/CuCl2 leaches?

Does it alter in anyway the etch rate of the leach?


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## g_axelsson (Jun 7, 2017)

Topher_osAUrus said:


> g_axelsson said:
> 
> 
> > ... even salt water (NaCl) dissolves CuCl, actually even better than HCl. ..
> ...


A couple of years ago I tested to dissolve CuCl in brine and it worked very well. But I never tested to use it as a leach, I don't see any advantages of using NaCl and HCl when HCl alone is enough.

There is one alternative to test though, to use low voltage electrolysis and by removing some copper from the CuCl turn it into CuCl2 and metallic copper, then you can reuse the solution and in theory just recycle it. Practically it would get fouled with zinc, iron, nickel and other base metals over time and would need to be replaced.
The trick is to use low enough voltage in the cell so the CuCl only get oxidized into CuCl2, too high voltage and you will release chlorine at the anode.
This would be a neutral pH copper chloride leach which doesn't release chlorine or HCl vapors. In other words, no harmful or corrosive emissions from the leach.

Göran


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