I have a mess on my hands

[lekjaz]

Thanks to this board and sreetips vids, I have had many successful batches of karat gold scrap that have been refined to 99+ with inquartation with silver, boil in dilute nitric, dissolve in HCL (31.45% muriatic acid), dump gold out of solution with SMB. Unfortunately, this batch has me scratching my head.

I made the mistake of mixing a large amount of gold filled scrap I got cheap with some high quality karat scrap. All the karat gold was marked and tested with a Mizar M24. The GF was also marked and tested. I did my best to pick clean the GF and then it went into the hot dilute nitric. I boiled the karat scrap/GF scrap mix in dilute nitric repeatedly until there were no more red fumes and the acid was clear. My well water has high iron, so I always wash with distilled between steps.

After the last nitric boil, the solids were that nice purplish-brown color I'm used to seeing. I poured off the nitric, washed with distilled, and added about 300ml HCL and ±10ml fresh 68% nitric to make as I normally do. No heat - I just let it set overnight because it was late in the day. By morning, the brown blobs dissolved, but the solution is yellow-black although when I dip a plastic spoon into the beaker it comes out that familiar yellow. Stannous tests positive for gold.

I added another 300ml HCL, solution is still yellow-black. I ran through the Büchner funnel and got a tiny bit of black sediment, but the the solution is still yellow-black. So I try SMB, but it will not dump any gold. I just get a layer of whitish mud at the bottom.

Since SMB is not working, what are some strategies to get this gold out of solution? Is my concentration of gold in HCL too high? Would dilution help? Should I just cement out and run again? If so, what should I cement it out with? I have truckloads of clean copper and can easily get zinc billets. Tin is a little harder to get but I could come by some online if needed. There should be about 3 oz of 24k tied up in this batch - can't afford to lose it.

And as I press "Submit" on this thread, I'm swearing off gold filled scrap forever.

 

[Lino1406]

Decant the liquid, put aside;
Put a fresh AR on a metal sample - see if there's reaction

 

[lekjaz]

Thanks for your reply. How will this help? There is no metal to recover in this batch. The only thing at the bottom of the beaker is white slime, which I assume is undissolved SMB.

 

[Lino1406]

In this case, SMB is not SMB

 

[lekjaz]

I suppose that's possible, but I purchased a brand new package and it clearly says "Sodium Metabisulfite" on the canister.

 

[goldenchild]

Filter the solution and cement with copper until it tests negative for gold. Then proceed as normal and NEVER EVER mix GF with karated scrap again.

 

[Lino1406]

It is not 1st time Sulphite is messed with Sulphate

 

[lekjaz]

Filter the solution and cement with copper until it tests negative for gold. Then proceed as normal and NEVER EVER mix GF with karated scrap again.

Thank you for the response. This morning I divided off a small beaker and added a rod of clean copper. When I came back from work, there was a black crust on the copper. Not much of it has dissolved, and solution still tests positive for gold. Would low heat help this along?

And yes, I agree about GF scrap. Never again!

 

[goldenchild]

Did the reaction completely stop? If so you may need to acidify it with more HCL to get the reaction going again. You also have to observe if the gold is going back into solution once it cements out. If so that means there's nitric still in solution. If this is the case you'll need to just keep the copper in solution until the nitric is used up. At that point the gold will cement out and stay out of solution as a sponge. There's no need to heat.

 

[lekjaz]

Did the reaction completely stop? If so you may need to acidify it with more HCL to get the reaction going again. You also have to observe if the gold is going back into solution once it cements out. If so that means there's nitric still in solution. If this is the case you'll need to just keep the copper in solution until the nitric is used up. At that point the gold will cement out and stay out of solution as a sponge. There's no need to heat.

Goldenchild thank you for your thoughtful response. It seems the reaction has stopped. I checked the solution in it's current state (copper rod, heat turned off, been sitting at room temp for 24 hours now), and there is little change. There is a bit of black crust on the rod. It releases with a tap on the lip of the beaker.

Per your recommendation I will keep adding HCL until a reaction restarts.

The solution is a bit less black and more green, a slight bit more sediment on the bottom but still lots of white muck. Still tests positive for gold.

Thanks to you and all on this board for your support. I am grateful.

 

[FrugalRefiner]

When cementing, you need circulation of the solution.

This is a "replacement reaction". The gold in solution has been oxidized, meaning it has lost an electron, and has become an ion. Because gold is not a highly reactive element, it would very much like to get it's electron back. The copper metal you're putting in the solution is more reactive, so it's "willing" to give up an electron to the gold ions. When this happens, the copper metal is oxidized (they give up an electron) and becomes copper ions, and the gold ions are reduced (they gain an electron) and become solid, metallic gold. The copper replaces the gold in the solution.

But for this transfer of an electron to occur, the gold ions must be in very close proximity to the copper metal. They need to "touch" so to speak.

If the copper metal just sits in a still solution, the gold ions close to the copper react and create the black coating you see. But if the solution isn't being circulated, the solution close to the copper becomes depleted of gold ions, so the reaction slows and can grind to a halt. And yet, when you test some of the solution that is farther away from the copper, it still tests positive for gold.

The coating also adheres to the copper, which further restricts the ability of new gold ions to come in contact with any metallic copper.

So you need some circulation. A simple method is to use an aquarium air pump and tubing to bubble air into the pregnant solution to keep everything moving. It brings fresh solution into contact with the copper, and it also helps to knock the deposited gold off the copper, exposing fresh copper for the reaction.

4metals described a clever system using a piece of PVC pipe. Drill some holes near the bottom end to allow fresh solution in. Drill some more near the top a bit below the top level of the solution to allow the solution out. Attach the air hose at the bottom end of the PVC so the bubbles rise inside the pipe. Hang your copper inside the pipe above the air hose. Fasten the PVC pipe within your container of solution. The bubbling will constantly bring fresh solution in the bottom, cause it to pass over the copper (with the bubbles knocking the cemented gold off the copper), and allowing the solution to flow out the top, mixing all the solution in the container.

Dave

 

[lekjaz]

When cementing, you need circulation of the solution.

Dave,

Thanks for the detailed response.

I have set up an aquarium air pump bubbler with a new rod of clean copper, no heat. The solution is gurgling in my fume hood and seems to be well agitated. I will look into the PVC pipe idea.

It seems logical that I will need to run the reaction until the solution tests negative for gold.

Based on the karat of starting scrap, there should be just under 3 oz of 24k gold in this batch, so I should expect this reaction could be running for a while.

The SMB sludge on the bottom is water soluble, so after all the reaction is complete, a few washes in plain water should remove it.

It also seems logical that this technique will kick out the silver and PGMs along with the gold, so that resulting solids will need to be refined again. But this reaction should keep all the polluting metals from the GF scrap in solution.

Does all of this seem accurate to you?

 

[geedigity]

When cementing, you need circulation of the solution.

Dave,

Thanks for the detailed response.

I have set up an aquarium air pump bubbler with a new rod of clean copper, no heat. The solution is gurgling in my fume hood and seems to be well agitated. I will look into the PVC pipe idea.

It seems logical that I will need to run the reaction until the solution tests negative for gold.

Based on the karat of starting scrap, there should be just under 3 oz of 24k gold in this batch, so I should expect this reaction could be running for a while.

The SMB sludge on the bottom is water soluble, so after all the reaction is complete, a few washes in plain water should remove it.

It also seems logical that this technique will kick out the silver and PGMs along with the gold, so that resulting solids will need to be refined again. But this reaction should keep all the polluting metals from the GF scrap in solution.

Does all of this seem accurate to you?

Just make sure you do not dispose of any wash water when you are dissolving the white sludge. If the sludge is dissolving, put a copper rod in that solution also.

 

[lekjaz]

Just make sure you do not dispose of any wash water when you are dissolving the white sludge. If the sludge is dissolving, put a copper rod in that solution also.

Understood. Thanks to you all for your assistance!

 

[lekjaz]

I think I've cemented most of the precious metals out of the acid solution. It's been gurgling along all week, slowly at first but I rigged a simple PVC pipe and that seemed to pick up the pace.

The solution is that familiar reddish brown color, but appears to be in suspension and super fine, so fine that much of it passes right through a Medium filter paper in the Büchner funnel. I guess it is logical that the gold would be much smaller particles given how slowly it cemented out. I just wonder if anyone else has seen this.

 

[FrugalRefiner]

I think I've cemented most of the precious metals out of the acid solution.

You don't have to guess. What does a stannous chloride test show?

Dave

 

[lekjaz]

Regardless of if it's in solution or suspension, it will test positive for gold. I don't think a stannous test will answer my question.

 

[g_axelsson]

Regardless of if it's in solution or suspension, it will test positive for gold. I don't think a stannous test will answer my question.

That's wrong. Gold in an acid solution is testing positive when using stannous. Gold particles in suspension does not test positive using stannous.

What is the pH? If you have exhausted all the acid the pH can go up and iron drop as a hydroxide. If you have fine particles in suspension and almost neutral pH, then take a small sample and add a little HCl to see if the particles goes into solution again and then test with stannous.

Fine cemented gold usually looks like black ink and given time it will sink to the bottom.

If you suspect that you still have gold in suspension or even colloidal, just evaporate a sample to dryness and redissolve in aqua regia. If there were gold in the sample you can now test it with stannous again.

Göran

 

[lekjaz]

For the past few weeks I have had an air bubbler slowly dissolving copper rods and tubes inside a PVC pipe basket inside a large plastic pitcher. This setup is super slow but it's working. I clearly have a batch of colloidal gold on my hands. The gold is too small to be caught in any filter I own and won't settle out from the aqueous solution. I even put a drop on a slide and put it under a lab microscope but still could not see any individual gold particles.

I put in a copper rod last weekend and it has barely changed, so I think I've dropped all the gold I'm going to from this solution. There is some discussion of how to refine colloidal gold on this board, but little in the way of definitive steps. I saw mention of a book in the Library, but I couldn't locate it. Any links would be helpful.

My current plan is to boil off the water, melt whatever residue remains, and then return to standard nitric/AR refining. Gold is still hovering around $1900 a ounce so at least I won't take a bath on this bad batch. I've got 3 oz of 24k gold locked up in this goop.

 

[g_axelsson]

How do you know that you have colloidal gold? Did you do the test I suggested? Drying out a sample and dissolve it in AR to test with stannous?

Colloidal gold is easy to refine, just dissolve it in aqua regia or maybe even easier in HCl and bleach. It's super fine so it will go into solution in seconds. After that there is only gold in solution.

Maybe the reason there is so little written about colloidal gold is that it is uncommon that you have any large amount in solution. Most of the colloidal gold a refiner sees is the purple results of a positive stannous test, and that is easily turned back into solid gold by incineration and leaching the ashes.

Göran