Leaching with hydrochloric acid

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HansW
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Leaching with hydrochloric acid

Post by HansW » June 30th, 2020, 5:11 am

Hi there. I have a lot of gold filled jewellery scrap. I have gone the nitric acid leach, followed by AR route in the past. Mixed results. Lots of stuff doesn't dissolve in nitric and then messes up the AR step.
My question: Can you start with one or more hydrochloric acid leaches? That should get rid of copper, zinc, iron, tin etc. Especially tin, which messes up the nitric acid leach, and the iron, which doesn't seem to dissolve in nitric.
I have looked through the forum but couldn't find anything.
I am aware that there must not be any oxidants present in the HCl leach, otherwise gold will go into solution as the chloride complex. A small amount of gold will dissolve, but that will be recovered later from the stock pot by cementation onto copper. Silver might give problems, but I don't expect much silver to be around.
Thank you!

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goldenchild
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Re: Leaching with hydrochloric acid

Post by goldenchild » June 30th, 2020, 8:33 am

Are you carefully going through your scrap to remove the steel, tin and lead? What about incineration? Scrap containing those metals should be separated and processed separately. A magnet will find your iron and incinerating will find your tin and lead. The tin and lead should be attempted to be found by eye first.

That being said some will use an HCL bath(s) first. This process is painfully slower though. Especially on copper. You'll need to find a way to keep the acid warm and not easily boil/evaporate off. A crockpot has been used in the past. Rejuvenating the acid will be necessary as well. So if you have the time and patience to spare then you could certainly do it. Look around on the forum where people discuss this.

Note: Iron will certainly dissolve in nitric. Oxidation is what causes rust! Stainless steel is another story.
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HansW
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Country of Origin: nz

Re: Leaching with hydrochloric acid

Post by HansW » June 30th, 2020, 5:27 pm

Thank you. Very helpful advice!
If time and a bit of heat are the main drawbacks of hydrochloric acid leach, then I would prefer the HCl route. It is quite a bit cheaper than nitric and the NO2 fumes are really bad. Mechanical sorting is not my forte, but I will spend more time on magnetic sorting.

DylanDownright84
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Re: Leaching with hydrochloric acid

Post by DylanDownright84 » July 3rd, 2020, 5:51 pm

Magnetic sorting is definitely something you dont want to skip over when dealing with gold filled recovery and refining of the foils. Once you have the magnetic pieces separated from the non magnetic pieces they need to be incinerated to a dull glow at the very least. The incineration is arguably the most important part preparing the scrap for acid treatments. It will allow you to remove any tin or lead that made it through your sorting process as well as burn off any oils, dirt, and organics. Also, be extra vigilant in identifying plated pieces as they will eat up alot of acid and can throw yields off greatly.

You can start the process for the magnetic pieces similar to how the acid peroxide/copper II chloride works. The magnetic base metals will dissolve pretty easily in just straight HCL on sustained heat freeing up the gold foils. The not magnetic scrap can be treated with dilute nitric leaches. Make sure you filter off and save the solutions from your non magnetic scrap as they will most likely contain silver.
Edit:
Also, I wouldn't suggest a new member start off with processing gold filled materials as it can be a bit "tricky" and does have a bit of a learning curve involved with it. I mean no offense as I am not sure of your level of experience with this material, just talking from my first hand experience of jumping head first into it without being ready and learning the hard way on how to do certain things. And being kindly told to chill out and study. *thanks Dave* :D
"Nothing ventured, nothing gained"

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