# HCl alone



## powermetalx (Dec 13, 2015)

It seems there is some disagreement over the use of HCl H2O2 vs HCl alone. See this thread. http://www.goldrefiningforum.com/phpBB3/viewtopic.php?f=61&t=2578&start=0

I have both pins and fingers to process. I had intended to use acid peroxide method for both. Or maybe nitric for the pins. But I can use HCl by itself with heat? If this is true, then why would anyone add the peroxide?


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## MarcoP (Dec 13, 2015)

Please bear in mind that acid peroxide is used to speed up the initial reaction, oxidize copper to put it into solution creating a CuCl2 leach. This could be also achieved with air oxygen, thus with a very slow start up.
A CuCl2 leach needs little copper dissolved to kick off.

This is why we only need very little acid peroxide; heat, in any case, will simply speed up the reaction.

Marco


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## butcher (Dec 13, 2015)

Copper is below hydrogen in the reactivity series, it place there is because of the fact that copper will not react with the hydrogen of acids, copper will not react with HCl acid. 
But copper oxides will react with HCl acid, so in order to dissolve copper we have to oxidize the copper some how, boiling hot solutions of acids can pull in enough air as dissolved gas to do this oxidation of the copper to copper oxide, and HCl changing the copper oxide to copper II chloride, all it would take is enough air to dissolve a very small amount of copper into solution to form a little bit of copper II chloride, then it is not the HCl that continues to dissolve the copper but the copper II chloride acting on copper to form copper I chloride, which with the free HCl involved just forms even more copper II chloride to dissolve even more copper, as CuCl2 continuing to pull more copper ions into solution, as the copper atom gives up their electrons, Note the HCl is not what dissolves the copper.

HCl and an oxidizer will attack copper, HCl alone will not harm the copper, copper II chloride is the real culprit in dissolving the copper.

Like stated above a very small addition of 3% medical grade H2O2 hydrogen peroxide in the HCl is all that is needed to jump start the reaction to form a little copper II chloride, then bubbling in air with fresh HCl in solution to keep the reaction going.

For a better understanding, study the copper II chloride leach (or etching solution), a good starting place is on Laser Steve's web site where you can find a well written document on its reactions, and use as an etching solution, used to etch the thin copper sheets when forming traces on circuit boards...

Now iron alloy and some components of brass alloy or other materials that can be used in the electronic pins, are a different story, if above hydrogen in the reactivity series of metals the HCl can attack them , the more reactive the easier the HCl will dissolve them..


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## FrugalRefiner (Dec 13, 2015)

powermetalx said:


> It seems there is some disagreement over the use of HCl H2O2 vs HCl alone.


There are many ways to skin the cat. HCl peroxide is a great "set and forget" method, as it doesn't require much attention, but it's slow. Heating HCl requires a bit more attention, but it's faster. If you want faster, nitric may be the way to go. 

HCl and peroxide are recommended most often because it's one of the less dangerous, more forgiving processes we use. Hot HCl may not be as forgiving, and the fumes are more problematic. My advice is to thoroughly study every process. Learn it's pros and cons. Then try some small scale testing. You'll find what suits your situation.

Dave


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## Barren Realms 007 (Dec 13, 2015)

Not trying to start anything but to keep saying that HCL alone will not attack copper by itself is completely incorrect. 

There have been so many times that I have put pins or incinerated copper baring material in HCL on the hot plate and come back a few hours later or the next morning and the copper started being dissolved buy the HCL.

Do some more experiments on your material guys.

Sorry for the mini rant but this has gotten to be a pet peve of mine to keep hearing this that copper will not go into HCL solution on it own with no peroxide or bubbler used.

It doesn't always happen but more often than not it will happen.


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## g_axelsson (Dec 13, 2015)

Incinerated copper turns into copper oxide on the surface and that can be attacked by the hydrochloric acid. When you got some copper I chloride in HCl it is easily oxidized into copper II chloride and that is what etches the copper metal and the process is started.

Copper reacts with hot concentrated sulfuric acid and with both diluted and concentrated nitric acid. It does not react with hydrochloric acid.

This is a site that's been under development for over 20 years and I think they would have added copper and HCl reaction if there was any.
http://www.webelements.com/copper/chemistry.html

You can test it yourself by putting a piece of bright clean copper wire in a beaker and the same wire in another beaker but this time heat it to incineration temperature for a while first.

Göran


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## Geo (Dec 14, 2015)

This has been argued about before. Years ago the subject of whether HCl would put copper into solution without an outside influence other that the metal and acid in close contact. The consensus at the time was that the oxygen entered the solution at the surface from the air and that oxygen then initiated the first small amount of copper oxide. There was even graphs and charts presented to show how long it would take. It would take less time if the copper was partially exposed to the air or fully submerged. This is still adding oxygen to the solution so it's not just the HCl attacking the copper. If you try it for yourself, the copper will start being attacked by the acid within a certain amount of time depending on other variations such as temperature and surface area of the metal without adding anything else.


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## jason_recliner (Dec 14, 2015)

It's so easy to try it. This was one of my early acquaintance experiments.

About half a gram of very finely stranded, clean, copper wire in 10ml of straight HCl in a 50ml beaker.
Overnight in Melbourne winter: copper is gone, light green solution.


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## g_axelsson (Dec 14, 2015)

Just because you see a reaction doesn't mean it's only one reaction at work...

Step one... CuO + 2HCl -> CuCl2 + H2O

This is the etching step .... CuCl2 + Cu -> 2 CuCl

This is the regenerating step .... 4CuCl + 4HCl +O2 -> 4 CuCl2 + 2 H2O ... and that goes back to the etching step, each round doubling the amount of copper in solution. It is easily seen that the limiting factor is the oxygen dissolved in the liquid.

The copper is etched by copper chloride while oxygen and HCl is used up to oxidize Copper I chloride into Copper II chloride. There is no direct reaction between HCl and copper.

What do we learn from this? Well, you don't need peroxide to start a copper chloride leach. Many scientific papers actually starts preparing copper chloride from copper oxide and HCl. So can we too if we would like to.
It also explains why the copper chloride leach is slow at the beginning but picks up speed as soon as it gets some copper in solution. And we also know that we need to add oxygen somehow to keep the process running, if HCl would work directly on copper we wouldn't need to add a bubbler.

Interesting experiment to do... start by boiling some HCl to remove any dissolved gases. Fill upp a bottle to the top with the acid, put a piece of bright copper in it and seal the bottle with a rubber stopper loosely fitted. (Don't want to blow stuff up if there is gas released somehow.)
Watch what happens over time. Is the copper dissolving when there is no or very little oxygen present?

Göran


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## Anonymous (Dec 14, 2015)

True Goran that's the chemistry however the net effect is that copper will dissolve in HCl without adding anything else 8)


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## upcyclist (Dec 14, 2015)

spaceships said:


> True Goran that's the chemistry however the net effect is that copper will dissolve in HCl without adding anything else 8)


Just to put both statements in the same post (which are both correct):
_
1. Academic: Pure copper will not react with hydrochloric acid.

2. Practical: Copper and hydrochloric acid combined in a real-world* environment will eventually produce a copper chloride leach._

*Real-world environment: Unless your copper is perfectly clean and you have ensured an oxygen-free environment, there will be some copper oxide present, which will react with the HCl.


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## g_axelsson (Dec 14, 2015)

spaceships said:


> True Goran that's the chemistry however the net effect is that copper will dissolve in HCl without adding anything else 8)


... except oxygen...

The scientist in me cringes, it's like saying money gives milk just because you can buy milk at the store and ignoring that there is a farmer and cows working in the background that you don't see.

8) 

Göran


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## Anonymous (Dec 15, 2015)

Hehe Goran!

I completely 100% UTTERLY agree with you. 

You have hit the nail on the head mate. The scientist will view the chemistry behind what is happening in this reaction which correctly is a multi staged reaction meaning that something does need to be added, whereas the practical approach is that the oxygen that's being added in the real world reaction, without the physical addition of any more chemicals to provide said oxygen does the job.

Upcyclist really did get it right. The scientific approach varies from the real world approach and the scientific approach is chemically correct whilst the real world approach is practically correct in terms of what appears to happen. If I didn't twist myself round in loops there hopefully you get what I mean? 

Chemists like Lou, GSP, 4metals and others may agree with me (or not) when I say that in refining the real world "reality" has its place over the pure chemistry in many practical applications and successful chemists who refine have to take that into consideration. 

Have I been clear there or does it look like twisted logic?


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## Geo (Dec 15, 2015)

Text book chemistry is the authority. It's like the law of physics, it doesn't change to meet expectations. In theory, everything works exactly the way the book is written. In practice, unknown variants effect the outcome and even known variants that should not influence anything still plays a part. I could cite example after example of things that happen but shouldn't be happening or things that do not happen that should be happening. All of us has had that one batch that the gold refused to drop. Why? We did everything exactly like all previous batches but there's just something about this one that's different.


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## kurtak (Dec 15, 2015)

I had no problem understanding what you said Jon

Here is my twist on it

HCl alone while not dissolve copper - it needs an oxidizer - real chemistry tells us that is so

in the real world put copper in nothing but HCl - copper may dissolve - simply by taking on oxygen from the air

so the copper is dissolving not by the HCl alone but rather by the chemistry of oxygen taken in from the air --- the oxygen just wasn't "added" by you

Kurt


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## FrugalRefiner (Dec 15, 2015)

I once started a small test batch of CuCl2 (AP) with a capful of peroxide and copper. I left it for a week to be sure the peroxide had been consumed. I threw in a few fingers and saw the foils come free in a couple of days. Then I got busy, and didn't get back to it for some time. When I checked on it again, my foils were gone! Given time, the oxygen in the air is sufficient to dissolve even gold.

Dave


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## g_axelsson (Dec 15, 2015)

You are completely clear, Jon, and I don't disagree with you, especially since upcyclist wrote


upcyclist said:


> 2. Practical: Copper and hydrochloric acid combined in a real-world environment will eventually produce a copper chloride leach.



In real world applications we don't need hydrochloric acid to leach copper with CuCl2. In circuit board industry they use ammoniacal copper chloride etch as CuCl is also soluble in ammonia.

We can even dissolve CuCl in concentrated NaCl solution, quite a lot actually.

People that doesn't understand the steps in the copper chloride leach will probably run into difficulties some time. But if you understand what is happening behind the scene then the problems becomes easier to solve. To state that hydrochloric acid dissolves copper does not make it easier for those that tries to learn the chemistry.

With this knowledge we can come up with new ways to solve problems... Realizing that HCl has nothing to do with dissolving copper so how do we refresh the leach? One way is electrowinning the copper from the copper chloride but the problem here is that process creates chlorine gas which is bad both for you, your environment and your equipment. But copper chloride is a fantastic chemical. We can actually electrically oxidize CuCl into CuCl2 by carefully adjust the voltage and other variables in our electrowinning cell.
2 CuCl + e- -> CuCl2 + Cu
As long as we only take it one step we avoid any chlorine gas.
We still need to dissolve the CuCl that is formed and we usually use HCl for that, but we can just as well use NaCl-solution and thus avoiding any acid use at all.

The above process is just something I've been thinking about for a while but I haven't tested it yet, so there probably are a lot of unknown problems to solve.

Göran


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## upcyclist (Dec 15, 2015)

g_axelsson said:


> People that doesn't understand the steps in the copper chloride leach will probably run into difficulties some time. But if you understand what is happening behind the scene then the problems becomes easier to solve. To state that hydrochloric acid dissolves copper does not make it easier for those that tries to learn the chemistry.



I agree, and that's what I was trying to illustrate. It's not that someone is necessarily stating poor science when they say "HCl & Cu make copper chloride", it's that they're simply expressing multiple reactions in a single step (if you don't mind waiting a bit, hehe). The important part is to make sure people understand that distinction once in a while, without turning it into a giant "I'm better because I [quote science]/[think in terms of real world]" debate. A simple "keep in mind, for the sake of new readers, that..." will suffice.


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## powermetalx (Dec 16, 2015)

Whew! Thanks for all the replies. I opted for the set-and-forget method of AP method. I don't have a way of heating up acid and/or safely dealing with hot acid. And nitric looks to have too many fumes. I may have to use it on some fully plated pins, but would like to avoid it....

Follow-up: is there a consensus on the best way to rejuvenate the HCl--that is, to remove some of the copper II chloride so that it will continue to attack copper (when oxygen is present that is). 

EDIT. I would prefer a chemical method as opposed to electro-chemical. i.e. no cells


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## g_axelsson (Dec 16, 2015)

You don't want to remove the copper II chloride, that is what etching the copper... 8) 

A properly working copper chloride etch only needs fresh HCl and sometimes water if you get too much evaporation. If the volume becomes too large, just pour off the excess. Oxygen is easily added with a bubbler, or just diffusion from the surface, but that's really slow.

And be aware of the fumes. HCl isn't as nasty (but still nasty) as NOx from nitric acid, but it is poisonous to iron, anything close by will rust.

Göran


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## FrugalRefiner (Dec 16, 2015)

powermetalx said:


> Follow-up: is there a consensus on the best way to rejuvenate the HCl--that is, to remove some of the copper II chloride so that it will continue to attack copper (when oxygen is present that is).


powermetalx, lazersteve has a great document on his web site that explains the CuCl2 (aka AP) process in detail. You'll find a link to his Guided Tour of the Forum in the Tips for Navigating and Posting on the Forum thread. He has links to his site in his signature line. He also has some good videos on his site, including the CuCl2 process.

Dave


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## solar_plasma (Dec 17, 2015)

Also in the real world HCl does nothing to copper. CuCl2 will etch copper. The HCl and O2 do nothing but oxidizing CuCl to CuCl2. 

There is a difference between how thing ARE and how things LOOK like. Yes, in the real world it looks like HCl would etch copper - which it does not -because we do not see the O2 and HCl oxidizing CuCl while the CuCl2 is doing the work.

The difference between theory and practice though is, that in theory we look at small closed quite managable systems, so we can calculate the variables with high accuracy. In practice we often do not know all variables or we are not able to calculate all of them, but the applicated theory will give an approximation that in the best case will be close enough to reach the goal.

There may be very well occur other processes in that leach, in fact they do, but the CuCl/CuCl2-model is exact enough to describe and calculate the output, speed rate, consumption etc "close enough". This will absolutely not be the case, if we use a HCl/Cu-model. This model will not be sufficient to do any predictions, what and when something will happen.


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## butcher (Dec 17, 2015)

In many things we work with with the chemistry of metals, we may have a hard time of describing the reaction, some examples, solubility, saying something is soluble or not, there can be variables that can be hard to describe, saying a salt is insoluble does not mean that none of the salt will dissolve into solution, similar with the reactions of metals and acids,we may also say the metal is not attacked by an acid, because of passivization or for many other reasons, this does not mean the acid has no reaction at all on the metal or the alloy, the reactions of metals and acids may also change with other factors such as concentration of the acid, temperature of the reaction or other details...this is only a couple of examples.

Understanding what is going on, will help us to understand better the reactions, and we may be more able to understand when we say the salt is insoluble, we can understand that even though most of the metal salt precipitated as an insoluble salt, that a small part of that precious metal salt may still be in solution (which we will wish to send to our stock pot for recovery later).


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