Electrochemistry Interesting idea in an old thread- dissolving gold in a Chlor-alkali cell

[haveagojoe]

While investigating ways to make Hydrochloric acid, I became interested in the Chlor-alkali cell, an electrolytic cell comprising of two compartments containing saline solution and separated by a membrane. On application of a DC current, the electrolyte is reduced to Hydrogen gas at the cathode, and oxidized to Chlorine gas at the anode. Where the electrolyte uses Sodium Chloride (NaCl, aka edible salt), it produces Sodium Hydroxide in the cathode compartment (NaOH aka caustic soda or lye), and the Chlorine produced in the anode compartment is dissolved in, and/or reacted with the water to form Hydrochloric acid. The membrane between the two compartments prevents the two solutions from mixing and neutralizing eachother, while still allowing ion exchange between the electrodes.

At the anode:
2Cl−→Cl₂(g)+2e−

Cl₂+H₂→2HCl

Cl₂+H₂O→HCl+HOCl
- note: HOCl is Hypochlorous acid, which is unstable and wants to lose its Oxygen to become HCl


At the cathode:
2H2O+2e−→H₂(g)+2OH−

This struck me as a simple and very useful proposition for the hobbyist, particularly in areas where HCl is not readily available. There are a few videos on YouTube which show the process using a pair of plastic cups connected with a tube which is stuffed with cotton wool to form the membrane.

Then I came across this little nugget of an idea from an old thread: https://goldrefiningforum.com/threads/do-someone-have-experience-with-this-process.14308/post-143666
@Palladium seems to be proposing to leach gold directly in the anode compartment of the cell, using the Chlorine as it is generated there. I found this a fascinating concept and worthy of revived discussion, I hope it will pique the interest of other members as it did mine.
Sadly @Palladium has not been active for a while so we may not be able to ask him about the idea.

 

[orvi]

This particular setup would be little bit hard to maintain due to that membrane requirement. Chlorine generator is a useful thing to have, if you want to go away from using nitric in bulk. You can very easily produce chlorine gas and use that as oxidant instead. This combo 2 in 1 is even more interesting, but there will be one big problem and that being cross-contamination of compartments. But if you can source membrane materials which are used in industrial membrane process of making Cl2/NaOH - then you probably won

I got an even better idea while reading this... I´ll test it and if it works, ... Okay chances are low, but this would be a big gamechanger

 

[haveagojoe]

if you can source membrane materials

There are a few DIY options used in the Youtube videos, one is vegetarian gelatin, set in the tube; but the most promising seems to be terracotta clay as used to make plant pots. That apparently allows the use of higher currents.

I have been pondering the practical setup:
Firstly how to connect the two compartments easily- I think an "up and over" layout like a salt bridge might be easier to make than a horizontal tube;
Secondly I was wondering if the anode compartment could be sealed, venting through a pipe to a third compartment where it would bubble through water, to recover and dissolve gas which escapes from the electrolyte, thereby producing more HCl.

Youtuber Scrap Science has made some interesting setups- though he is focussing primarily on the Sodium Hydroxide generation side, it's the same process.

 

[Yggdrasil]

There are a few DIY options used in the Youtube videos, one is vegetarian gelatin, set in the tube; but the most promising seems to be terracotta clay as used to make plant pots. That apparently allows the use of higher currents.

I have been pondering the practical setup:
Firstly how to connect the two compartments easily- I think an "up and over" layout like a salt bridge might be easier to make than a horizontal tube;
Secondly I was wondering if the anode compartment could be sealed, venting through a pipe to a third compartment where it would bubble through water, to recover and dissolve gas which escapes from the electrolyte, thereby producing more HCl.

I'm sure if you poke around you will find Muriatic acid around 21% in hardware,
chemical suppliers will carry 31-36% and a plain business license should do for that.
It sure will be cheaper than making it from scratch.
Nitric can also be made but also bought, check with the authorities what kind of licensing they need.
If you have all the papers in order there should not be any stress regarding inspections and such.

 

[haveagojoe]

I'm sure if you poke around you will find Muriatic acid around 21%

Unfortunately 10% is the legal limit now without a licence.

 

[Yggdrasil]

Unfortunately 10% is the legal limit now without a licence.

What are the requirements for a license?

 

[haveagojoe]

What are the requirements for a license?

I think I would need to register as a business. I'd love to do that one day but for now my funds are very limited so I'm just a backyard hobbyist.

 

[Yggdrasil]

I think I would need to register as a business. I'd love to do that one day but for now my funds are very limited so I'm just a backyard hobbyist.

Here, you can register as a one man personal company, no registration fee.
That was sufficient for my current supplier, but for the other chemical companies a larger company form is needed.
Check around, it may not be as hard as you believe.

 

[haveagojoe]

Here, you can register as a one man personal company, no registration fee.
That was sufficient for my current supplier, but for the other chemical companies a larger company form is needed.
Check around, it may not be as hard as you believe.

Yes I will look into it, I have owned a limited company before, but the chemicals licence is issued by the government not the chemical supplier, I think I would need do demonstrate my requirement, show business premises and COSHH compliance and so on, there's a bit of a leap from what I do now to the next level.

 

[HarryLee1506]

Someone has done this method before, link below. I tried this method but gold did not dissolve into solution, base metals like copper went into solution very quickly...
https://goldrefiningforum.com/threads/pd-pt-rh-mix-industrial-cat-refining-tips.29773/#post-318682

 

[haveagojoe]

Someone has done this method before, link below. I tried this method but gold did not dissolve into solution, base metals like copper went into solution very quickly...
https://goldrefiningforum.com/threads/pd-pt-rh-mix-industrial-cat-refining-tips.29773/#post-318682

Are you sure that's the right link? The thread is about PGMs from catalytic converters and doesn't mention a chlor-alkali cell

 

[Marcel]

There are a few DIY options used in the Youtube videos, one is vegetarian gelatin, set in the tube; but the most promising seems to be terracotta clay as used to make plant pots. That apparently allows the use of higher currents.

I have been pondering the practical setup:
Firstly how to connect the two compartments easily- I think an "up and over" layout like a salt bridge might be easier to make than a horizontal tube;
Secondly I was wondering if the anode compartment could be sealed, venting through a pipe to a third compartment where it would bubble through water, to recover and dissolve gas which escapes from the electrolyte, thereby producing more HCl.

Youtuber Scrap Science has made some interesting setups- though he is focussing primarily on the Sodium Hydroxide generation side, it's the same process.

The clay pot works great. We used to do this in the old days, even on these forums—myself included. It’s inexpensive and effective. After some time, the pores become cloaked with base metal, but that isn’t a major issue; you simply switch to a new one.

I even had a large pot in my garden where I sometimes disposed of spent AP solutions. I filled it with plain pressed compost and placed it inside a plastic container. I regularly checked on it, The solids and cemented metals accumulated in the pot. One day, however, it cracked due to the acid eating away at it. I brought it inside, dried the compost, and now it’s stored in bags to recover the metals one day.

It’s a “green” alternative to dumping your solutions down the drain. Plus, there’s still a lot of cementation going on in this earth—some plants even managed to grow there. So the membrane function of the clay pot is definitly there and it has the right screening size. Although I have not used it in an Chlor-alkali process.

 

[HarryLee1506]

Are you sure that's the right link? The thread is about PGMs from catalytic converters and doesn't mention a chlor-alkali cell

oh sorry... my mistake, here is the link to that article:
https://goldrefiningforum.com/threa...als-etc-from-ore-waste-etc.34901/#post-375999

 

[haveagojoe]

oh sorry... my mistake, here is the link to that article:
https://goldrefiningforum.com/threa...als-etc-from-ore-waste-etc.34901/#post-375999

Aha yes, thank you. The VIDEO seems to be using a method very similar to what Palladium describes- although he mentions adding Hydrochloric acid- perhaps this is just to get the process started more quickly. It's a pity he doesn't describe it in more detail, only its benefits.

Palladium's idea seems to be to dissolve gold directly in the anode compartment, using the HCl and Chlorine as it is generated- in the video it appears that the solution from the anode side is pumped to a separate leaching vessel. I can imagine that pumping the solution over the electrodes would increase efficiency.

I assumed this would be as a refining step rather than for recovery leaching from scrap or ore, so I didn't think about how other metals would behave, but yes I imagine you would be right- any other metals copper and above would probably go into solution first. However in the video he suggests the opposite- that Platinum and Gold go into solution first. I'm not clear about this.

If it works to dissolve gold, the gold in solution could presumably then be precipitated by reversing the current, or even by just pouring the solution from the cathode side into the anode side, either way would neutralize the acid and the gold should fall out. In the video he seems to use some kind of electrolysis- it's not clear to me how this would work unless the electrowinning cell was galvanically isolated from the main chlor-alkali cell.

I will ponder it some more as this is a very interesting proposal.

Here are a couple of videos showing a basic chlor-alkali cell to generate HCl and chlorine:
Video 1
Video 2
(Disclaimer- safety is not up to par in these videos, linked for demonstration of the principle only)

Edit to add: this type of cell is also known as a Nelson's cell.

 

[haveagojoe]

I just realized @electrochemgold is actually a member!

@electrochemgold could you comment on this thread? Is your proposal based on a Nelson's cell?

 

[haveagojoe]

In the video he seems to use some kind of electrolysis- it's not clear to me how this would work unless the electrowinning cell was galvanically isolated from the main chlor-alkali cell.

In the comments of the video someone asks " You send a solution containing electrolyzed Chlorine from the anode to flow through the ore, then the pregnant solution returns to the conical tank to precipitate the metal chloride on the cathode?" and he replies "Yep!"

Very clever, no need for a separate electrolytic recovery cell, it all happens in the main cell. The chlorinated solution continuously replenishes the balance of chlorides on the cathode side so the process is circular.

 

[haveagojoe]

Ok, here's vague artistic impression of how I think it could work:

 

[Yggdrasil]

Ok, here's vague artistic impression of how I think it could work:

View attachment 67538

I'm afraid the cathode side will produce large(ish)amounts of Hydroxides since all the metals will dissolve in the HCl/Cl chamber.

 

[haveagojoe]

I'm afraid the cathode side will produce large(ish)amounts of Hydroxides since all the metals will dissolve in the HCl/Cl chamber.

Yes, I'm not sure how he handles that. Perhaps returning the still-partly-chlorinated gold-bearing solution redissolves the NaOH? I can imagine it might be possible to establish a kind of equilibrium?

 

[Yggdrasil]

Yes, I'm not sure how he handles that. Perhaps returning the still-partly-chlorinated gold-bearing solution redissolves the NaOH? I can imagine it might be possible to establish a kind of equilibrium?

Depending on the conditions, Gold seem to precipitate between 4.5 to 13 pH.
So this is at least not suited for separation this way.
But since you already are planning a pump, maybe pump it past a Copper system to Cement the Gold in circuit?