Refining Process of Gold Powder

[marhaba01]

Hello there, hope you all are doing well.

I had a question regarding the aqua regia process. We receive impure gold bars having the following content - Gold 43%, Silver 52%, and the remaining other metals like copper, zinc, and so on.

I was curious about this one method and wanted your opinion on it. If this gold was melted down to a powder form of 200 mesh. And then taken for aqua regia:

1. Then would there be a possibility of losing out on gold particles in the fumes when the reaction takes place?
2. Should the ratio of HNO3 and HCL be different since it is powder and not in grains form?
3. What would be the ideal way of processing this powder using aqua regia?
4. Lastly, would 200 mesh be okay or a lower mesh size would be better?

I have attached a photo of the bar we usually receive and the type of xrf report we receive for your reference.

Thanks in advance

 

[nickvc]

If you can achieve small particle size then maybe aqua regia will not be best choice, you could dissolve the gold in HCl and hydrogen peroxide or bleach and recover the gold and any PGMs without having to worry about denoxing your solution and leaving the silver as a chloride.

 

[g_axelsson]

There was a discussion in the early days about an atomizer for high silver alloys so the fine powder could be directly digested in aqua regia. Too bad that most of the pictures are not there any longer. But maybe you can find something useful among the posts in that thread.

https://goldrefiningforum.com/phpBB3/viewtopic.php?f=39&t=6211

If I remember correctly it was a circular ring with nozzles pointing inwards and fed from a high pressure washer.

Göran

 

[Topher_osAUrus]

I think there is always losses in the dissolution process. Minute (minoot? Sp) traces of gold will go up and out with the steam. So, unless you have a complete reflux system, you would need an air filter system that catches the volatized gold. ...I just use a beaker, a watchglass, then a small plug of fiberglass insulation balled up, and stuck on he spout of the beaker, wedged underneath the watchglass (a 10 cent plate from a thrift shop)
-seems no matter how slow I take the reaction, the insulation always tests positive for gold.. presumably the small bubbles made during the process, pop a the top, then make a super fine mist/steam that carries off some of your crop.

The xrf looks like it could be digested in nitric, then the solids in AR. I imagine that would give a reasonably pure gold, without further refining (if you minded your p's and q's)
The material is almost inquartered for you... closer to "intripled" but, its close enough that I would give it a try. ...if it doesnt work out, (idk why it wouldnt..) then just move to AR.

Of course, pouring fine 200 mesh shot will make the dissolving process go faster. You would definitely want to add small bits of acid at a time, else it will be a very vigorous reaction. Which in turn could cause more losses in the effervescence....maybe even a boil over, if you were too gung-ho about it and just dumped nitric in

If thats not coherent enough, its because its
Time for coffee

 

[nickvc]

Goran the thread you and I remember was originally about dental scrap and 4metals got involved and I believe it was him that posted the idea of not using AR but HCl and hydrogen peroxide as it didn’t need denoxing and allowed all the values to be recovered within 24 hours and with less need for fume control, I’m sure there is a good reason 4metals went this route but I’m not a specialist in PGM refining so maybe someone else who is can chime in 8)

 

[Lou]

Nick,

From atomized karat/dental gold, 4N should be done in the course of one working day. Typically the fire assay lagging the gold out turn.

I'm not a huge fan of H2O2 unless it's done on much more dilute streams and dosed slowly into the reactor. Even then, most of the time it is decomposing and serving no other purpose than diluting the reaction.

Instead, he was using sodium chlorate. Also not a fan, as chlorate can in certain circumstances make a bunch of explosive popping from ClO2.

On atomized streams or fine powders, HCl/Cl2 (or even Br2, much preferably) is the best. Br2 is nice because it can be refluxed back into the reactor with a large enough condenser train and when done, one can recover the Br2 by oxidation with chlorine and switch from reflux to distillation/collection and save the Br2 for reuse.

 

[marhaba01]

@Nick - Thank you for this. After posting this question, I was going through the forum to see if I could find any similar post. Found a thread named gold sand. You had suggested the same there. I will definitely look into this.

@Topher_osAUrus - You got it right. Since the quantity we receive is large, we did want to speed up the dissolving process to finish the batches sooner. This is why we thought of getting the bar melted to 200 mesh-sized powder and then using aqua regia. As you mentioned about pouring in bit by bit acid, we will give definitely keep this in mind. Also what you said about the reaction, makes a lot of sense and it is exactly what we were fearing. An aggressive reaction leading to loss of particles. The air filter system - we do have a scrubber in place but a filter will help capture more of the particles I suppose, will surely have this placed before trying it all out.

The ultimate goal is to recover the gold and silver from this. But I was not sure of the idea of trying gold powder. Just something I am thinking of. Thank you all again. Will keep you guys posted on this.

 

[Deano]

You will always have some precious metal losses from wet chemistry fumes.

You can minimise these losses by passing all extractor fan fumes through an activated carbon scrubber and ashing the carbon to recover the gold.

It is generally not worth chasing values from small scale home setups but it is definitely worth it for commercial scale operations.

The carbon for a lab scale fume cupboard is contained in a perforated container, preferably plastic but 316 stainless will do if you can access large pieces of scrap perforated stainless cheaply.

You need residence time for the fumes in the carbon contactor so you usually go for a diameter of around one metre with a length of one metre for the above lab setup.

If your extractor fan is of the isolated type with high air flow rates then you will need a larger diameter contractor to slow the airflow sufficiently.

You can check the efficiency of the contactor by putting two of them in series and ashing some of the carbon in the second contactor every few months to check for leakage of values.

The best carbon for the contractor is standard gold mining commercial activated carbon, it needs to be changed every 12 months or so as the activation suffers in that type of environment.

Do not use activated coal based carbon, it has around 10% clay fraction solids and will interfere with recovery of the gold after ashing.

Deano

 

[nickvc]

Thanks Lou 8)
To the OP , you now have a process clarified by Lou that will work and quickly and probably cheaper as well.

 

[Lou]

The AgCl filter cake should be well rinsed and then converted to silver metal and run through a silver cell.