WARNING: This experiment handles boiling hydrochloric acid and sulfuric acid as well as toxic quantities of ionic copper.
Gloves should be worn and the experiment should be performed outside or in a fume hood.
Greetings fellow nerds.
A popular printed circuit board etchant is copper chloride in hydrochloric acid.
Its main advantage is that it’s easily regenerated by adding additional hydrochloric acid and bubbling air through it, creating more etchant the more you use it.
However eventually you’ll have far more than you’ll need.
In this video we’re going to fully recycle the etchant and recover the hydrochloric acid and copper that went into it.
As an added challenge, we’re going to try and do this without creating any other chemical waste at the end.
Let’s get started. Here is the copper chloride etchant solution already transferred to this flask.
We first need to recover the hydrochloric acid so we’re going to set up our distillation apparatus.
Once that’s ready, turn on the stirring, the water supply and finally the heating.
No before you start it’s a good idea to filter the etchant to remove any insoluble particles.
Also, if you have a very high acid concentration, greater than 20% by mass, you need to dilute the etchant so it’s lower than 20% before you distill.
This is so that during distillation you don’t release hydrogen chloride gas first and waste it to the environment.
If this doesn’t bother you and you’re just after the copper, then you can distill straight away.
If you have a very high concentration, but are unsure of much you need to dilute it by,
then adding an equal volume of water is usually enough as this brings most commercial concentrations of hydrochloric acid to below 20%.
You can then discard the excess water during distillation as the water will distill off first.
Similar to how we distilled sulfuric acid, you can tell when the distillation starts distilling acid by watching the receiver.
If you see the distortions of the drops sinking then you’re distilling hydrochloric acid and you should change out the receiver to collect it.
At this point distill all the way until the residue is dry.
And here is our dried residue of copper chloride.
It has this non-uniform appearance because there is a mixture of copper (II) chloride and copper (I) chloride present as well as various amounts of residual water and hydrochloric acid.
Okay, we can go to the next step with just this amount.
But since i have a lot more copper chloride etchant I need to recycle i’m going to distill the rest of this and collect all the copper chloride to do it all at once.
I’ll skip over the remaining etchant since it’s just a repeat of what you saw earlier.
Here we are with all the combined dry copper chloride and hydrochloric acid.
Weigh the copper chloride so we know how much of the chemicals we need in the next step. I have around 20 g.
For every gram of copper chloride measure out 7 mL of water and 1 mL of sulfuric acid.
Okay, now we need to convert the copper chloride into copper sulfate in order to recover the copper.
Since we have around 20 g of copper chloride, we’ll be needing 140 mL of water and 20 mL of sulfuric acid at 98% concentration.
Mix them up in an erlenmeyer flask for distillation starting with the copper chloride.
Then add the water, and add the sulfuric acid last. Shake up the mixture to dissolve as much of of the copper chloride as you can.
Now as you mix it, most of the copper chloride will dissolve into a green solution but occasionally there may be this white precipitate of copper (I) chloride.
In concentrated hydrochloric acid both copper (II) and copper (I) chloride are soluble
but since we removed all the hydrochloric acid by distillation and then added water only the copper (II) chloride is soluble and this copper (I) chloride is precipitating out.
If this is present then we’ll need to oxidize it into copper (II) ions before proceeding further.
To do this is very easy, we just get an aquarium pump and bubble air through it. This can take a very long time, up to a few days.
Since i’m rather impatient i’m going to speed things up by stirring the solution on my hotplate stirrer as well as heating the solution to 60 degrees Celsius.
This should be done in a few hours.
What’s happening at this point is that we’re oxidizing the insoluble copper (I) chloride into soluble copper (II) ions.
Anyway, here we are about three hours later with everything dissolved.
Take out the air tube.
And here is our solution containing a mixture of copper chloride, sulfates and sulfuric acid.
Now we put it back on distillation, and distill off hydrochloric acid. Keep going until the residue is pretty much dry.
What we’re doing here is converting the copper chloride into copper sulfate. The removal of hydrochloric acid drives the equilibrium to the right.
Now if you calculated the stoichiometry you’ll find we used an excess of sulfuric acid.
This is to ensure we drive off all the hydrochloric acid and ensure we have negligible chloride contamination in our copper sulfate for the next step.
Okay, we are done when we see this misting happen in the flask. This occurs when almost all the water is driven off and now the sulfuric acid is starting to vaporise.
At this point, turn off heating and let it cool completely.
Now we have our copper sulfate. It’s this very light color because we’ve driven off most of the water.
Now we need to add some water back in. For every mL of sulfuric acid we used, we add at least 4 mL of water.
Since we used 20 mL of sulfuric acid, we now add 80 mL of water. You can add more if you want.
Now thoroughly mix the solution to dissolve the copper sulfate. Here we get the famous deep blue color of copper sulfate.
Not all of it will dissolve but that’s acceptable for the next step. Transfer the mixture to a beaker including the undissolved copper sulfate.
Now we electrolyze the solution. First fashion a copper cathode. I’m using some copper wire.
And partially bury it in the copper sulfate at the bottom. Now just above that hang a platinum anode.
I’m using a platinum coated titanium anode.
Make sure they don’t touch. Attach a constant current power supply with the negative terminal connected to the cathode and the positive terminal connected to the anode.
Run the electrolysis with a current density of under 100 ma per square centimeter of exposed platinum.
This will take a very long time so be patient.
What we’re doing here is plating out the copper ions onto the cathode to get copper metal.
At the anode, we’re electrolyzing water to get oxygen. The sulfate remains in solution and we get sulfuric acid.
We’re essentially recovering copper metal and sulfuric acid in this step.
The undissolved copper sulfate will dissolve as the concentration in solution decreases from the copper metal plating.
That’s why we didn’t need to completely dissolve the copper sulfate before we started.
We’re using a platinum anode as it’s one of the few anodes that can withstand this sort of electrolysis.
You can also use a lead dioxide anode if you happen to have one.
We couldn’t electrolyze the copper chloride because the anode would release chlorine gas and waste the hydrochloric acid, not to mention that chlorine is also toxic.
That’s why we went through the trouble of converting it to copper sulfate.
Now in my particular run, I had to electrolyze for about four days so i’m going to skip ahead.
I also had to stir the solution on the second day since the copper sulfate wasn’t dissolving fast enough.
But anyway, here we are four days later after the solution has gone completely clear.
Lifting out the electrodes we can see the copper wire is much thicker than it was originally.
This is due to the copper plating out onto it.
An interesting point is that this copper is essentially the copper that originally came off the printed circuit board during etching.
It all ended up here.
The brown powder in the beaker is actually copper metal that fell off the cathode. You can filter it off and recover it.
The solution is sulfuric acid and water. You can purify it by distillation as shown in a previous video or use it in future electrolysis runs.
Anyway, there you have it. From a batch of spent copper chloride etchant we have recovered all of the hydrochloric acid by two distillations.
And we have recovered all the copper by electrolysis. We used sulfuric acid as a catalyst and completely recovered that as well.
We have fully recycled the etchant back into the starting materials without producing any chemical waste.
Thanks for watching.