Recovering gold from dirty AP

[WaywardKnight]

Hi Everyone

I am just starting out and playing around with a few things. I've learned a lot about various reactions and basic processes by reading and experiencing.

I have a few questions for my current situation though.

Material: I'm playing around with pulverised fingers, <200g. I know chemcials are probably not the most effective way to refine poulverised scrap, but it was free material to play around with.

I mixed the powder in HCl and H202. I'm using 32% HCl and 50% H202. I'm using a 20-1 ratio. I started with 300ml of HCl and 15ml of H202.

The solution turned red when I first added the H202. I've since learned (thanks to this forum) that is due to impurities in the brand of HCl I'm using. I used an aquarium pump to agitate the solution. The solution went a dark black colour over a day, I assumed this was a result of it being saturated with copper. I filtered the powder. Rinsed it with hot water, let it dry (but was probably a bit impatient), and added the same amounts of HCl and H202.

Same results, so I figured I would do the same thing a third time. The solution went reddish again but this time didn't go dark. When I inspected the ppwder I could not longer see gold flakes in it.

I performed a stannous test and got the photo in the attached photo (yellow/orange colour being the original solution colour). It is important to note, I dont have a test solution for my stannous chloride. So I dont have anyway to test it is working. It seems like it is though.

My three questions are:
1) Should I use SMB or copper to drop the gold, noting there is likely many other metals in there still. How should I purify it? A HCl wash of all powders?
2) Why did the gold dissolve the thrid time but not the first two. After the first two times, I could still see the gold flakes, so it was not the case that the gold chloride was dropped from the solution once the copper needed space.
3) Should I just use HCl and the aquariam pump to dissolve copper rather than adding the H202?

Thanks for any advice you can provide.

Wayward

 

[butcher]

Wayward,

You are using too high of an oxidizing agent to recover the gold easily, and with any tin bearing solder in the solution, you could be locking up some of your gold as colloids.

The hydrogen peroxide is too strong to make an effective leach of the base metals and leaving the gold untouched, basically, the highly oxidized solution will dissolve gold as well as base metals, this is very similar to trying to use aqua regia to recover gold with (a process which normally does not work out all that well, it is better to dissolve base metals leaching gold untouched where possible in a recovery process.

3% H2O2 in the HCl is enough to get the copper started going into solution and making Cupric chloride CuCl2, after that, all you need is bubbled in air and some small additions HCl to rejuvenate the leach.

With the strong peroxide your dissolving gold and other base metals (including tin if involved all at the same time, and the gold is going into solution and precipitating back onto undissolved metals, the solution darkens with Cuprous (copper 1) chlorides or ferrous Iron Fe3) chlorides.

You can add more copper metal or more copper-based scrap (No Tin) to cement the gold from the solution which will be black powders from contamination, the gold can be refined Using the Strong H2O2 solution and HCl.

I suggest doing some studying on how the cupric chloride leach (AP chloride leach) (copper II chloride leach) works to gain a better understanding.

 

[WaywardKnight]

Hi Butcher

Thank you for the response. I was hoping that the 50% H202 with a 20-1 ratio would not cause the gold to dissolve (I was planning on using AR to dissolve the gold after other metals). Once the gold was gone, I suspected I was wrong. But good to have a second opinion to confirm it . Nothing teaches like failure or mistakes, and given I did not get injured in my experiment, and only lost a trivial amount of money in chemicals, I'm counting this as a win.

I'll do some more reading to about cupric chloride leaching as you have suggested.

In the meantime, I'll keep working towards recovering some gold from this batch to help focus my further reading. At this point, I'll add some copper to the solution and work on purifying or filtering the metals that drop. Then I'll use some soft iron to cement the copper in preparation for disposal. I know there are methods to extend the life of the HCl, but I don't want the scope of this first experiment to grow too broad. I want to end this experiment with safe disposal of the solution, and I'll play around with reusing it in the future.

Thanks again for the response Butcher.

Wayward

 

[butcher]

Sounds like a plan, looks like our Knight is finding the right path, coming out a little less wayward.

Gaining an understanding of the basic principles of how things work we spend less time trying to understand why things do not work as we thought they might and trying to recover from mistakes, it gives us an understanding that helps us understand and troubleshoot problems and avoid most problems, to begin with.

 

[WaywardKnight]

HI All

I successfully used copper to precipitate out the gold, as well as everything else lower than copper in the reactivity series. I learnt a good lesson, don't use copper pipe, it is painful to wash down the inside.

I filtered it out, and was left with a black sludge. This was what I was expecting and I was pretty happy with things at this point. I washed the black sludge with hot water than evaporated the water out. I put a bar of iron (soft iron) into my beaker of green, copper saturated HCl to prep for disposal.

For the gold, my plan was to dissolve it, filter out the last bits of plastic with it, then use SMB to precipitate it. I thought I would dissolve it with HCl and H202, I did it by accident initially, and it will avoid having to use nitric acid.

Here is where I have hit my two snags:
1) The copper is not cementing to the iron bar. After several days there is no real change. I've added small amounts of H202, HCl with and without a air pump. I'm still researching this, and will hopefully figure it out the problem soon.

2) The main issue...the HCl (34%) + H202 (50%) doesn't want to dissolve the gold. I started with a 10-1 ratio (40ml of HCl and 4ml of H202). Since then I have added another 20ml of HCl and 6ml of H202. I've read stiring helps, so I've got a magnetic stirrer going. The solution has gone a yellow colour, but the bulk of the powder has not dissolved.

The only thing I can think of regarding 2, is the lack of air pump in the solution . I don't have the pump going in it because the volume if liquid is so small. it'll just splutter and splash everywhere. I've tried searching the forum but without success. A many references to people accidentally dissolving their gold with H202 (like me), but nothing about doing it purposefully. I can revert to the AQ method and add a few drops of nitric (70%) to the solution, but I would like to understand the underlying issue before moving on.

Any advice would be appreciated.

Wayward

 

[butcher]

Has the iron been losing weight, or being attacked?
I do not have any answer, just some thoughts.

With excess oxidizer, it may have to dissolve some iron and use up some of the oxidizers and acids before it can begin to cement the copper.

I do not know how concentrated the solution is but adding some water may help, along with stirring or strong agitation.

Another thought is the strong oxidizer in the solution could be passivating the iron (rusting it with a protective coating) that is making it more difficult for the copper ions to come into contact with the elemental iron atoms under that layer of oxidized iron.

As far as the gold dissolving again some water may help, 50% H2O2 is a very strong oxidizer (be very careful of skin burns from handling it), diluting peroxide with water to around 30% would still be strong enough to attack gold (it will generate chlorine in solution from the acid) the diluting solution may also be helpful to hold more gold ions in the solution.

You need both HCl and chlorine in solution to dissolve the gold.
Some of the HCl is converted to chlorine from the H2O2 some HCl is needed to make chlorides of the metals
Another thought comes to mind the possibility of limited reagents in solution or running out of one or the other?
Too high of H2O2 and not enough HCl acid in solution will not work as effectively in dissolving gold, or if you do not have excess free HCl to make metal chloride once the gold is oxidized it will at some point stop going into solution.
Similarly, if you run out of chlorine in solution (or H2O2 to generate chlorine in solution) but have plenty of HCl in solution It may stop oxidizing the gold although you have plenty of HCl acid...

You can experiment with adding one or the other...

 

[WaywardKnight]

So my first attempt is complete. No gold button to show for my efforts, but I do have a half dozen tiny gold beads trapped 8n borax. From looking at other posts on this forum I should be able to get the gold out when I do my next run and can blob it all together.

I'm pretty sure I lost decent portions of gold throughout the process, but it was all learning. I'll try to get some more material and start my next attempt.

Thanks for all replies and enjoy the picture of my tiny gold beads