First refining attempt of dentist lab materials, I need your input on something

[stefcrive]

Hello community,
I'm preparing myself for my first gold refining attempt. What I have to work with are filings, nuggets and raw material leftovers all coming from a dentist lab (please find a picture attached). I've XRF all these materials, they seem to be quite uniform in composition which is about 60% Au and 10% Pd, no silver apparently, and the rest is mainly copper and zinc.
The procedure I'm planning to follow is first a few boiling baths in diluted nitric acid, remove the base metal and Pd-rich nitric acid solution and then dissolve the remaining gold-rich residue in AR, denox the AR with sulphamic acid, and precipitate gold with a hot solution of oxalic acid which i found it seems preferable when dealing with material with PGMs present. The Pd should get dissolved in the nitric acid digestion and I'm planning to recover the palladium at a later stage by refining the stock pot solution using DMG or cementing it with a piece of copper.
I have a few question I would really like your input:

1. Although the XRF showed no silver present, and considered that it could be a false negative, I'm planning to test for silver by adding some drops of the nitric acid solution into some concentrated HCL to see if silver chloride forms. Is a proper way to test for silver?
2. If substantial Ag (30% or so) is present, even though some of the material is in powder form (ca. 10-30 micrometers powder grains of Au/Ag/Pd alloy), would I still have to inquart the the material with silver before the AR digestion, or the surface area of the powder is sufficient to dissolve all the gold anyway?
3. Inquarting is not necessary if the gold is alloyed with some metal other than silver, or if the silver content is less than about 20%, correct?
4. I expect the Pd to be dissolved in the nitric acid boils, can I then precipitate the Pd by using DMG directly to the diluted solution of nitric acid and Pd?
5. Is the use of oxalic acid appropriate in this case? or SMB would be easier and as effective to precipitate gold and leave any Pd behind?
6. Would you recommend anything else that you consider important for recovery the gold and the palladium in a case like mine, like any important tip or trick to prepare the material for digestion or to precipitate with oxalic acid?

Thanks a lot for your input, it is highly appreciated.
Best

Stefano

 

[Yggdrasil]

Hello community,
I'm preparing myself for my first gold refining attempt. What I have to work with are filings, nuggets and raw material leftovers all coming from a dentist lab (please find a picture attached). I've XRF all these materials, they seem to be quite uniform in composition which is about 60% Au and 10% Pd, no silver apparently, and the rest is mainly copper and zinc.
The procedure I'm planning to follow is first a few boiling baths in diluted nitric acid, remove the base metal and Pd-rich nitric acid solution and then dissolve the remaining gold-rich residue in AR, denox the AR with sulphamic acid, and precipitate gold with a hot solution of oxalic acid which i found it seems preferable when dealing with material with PGMs present. The Pd should get dissolved in the nitric acid digestion and I'm planning to recover the palladium at a later stage by refining the stock pot solution using DMG or cementing it with a piece of copper.
I have a few question I would really like your input:

1. Although the XRF showed no silver present, and considered that it could be a false negative, I'm planning to test for silver by adding some drops of the nitric acid solution into some concentrated HCL to see if silver chloride forms. Is a proper way to test for silver?
2. If substantial Ag (30% or so) is present, even though some of the material is in powder form (ca. 10-30 micrometers powder grains of Au/Ag/Pd alloy), would I still have to inquart the the material with silver before the AR digestion, or the surface area of the powder is sufficient to dissolve all the gold anyway?
3. Inquarting is not necessary if the gold is alloyed with some metal other than silver, or if the silver content is less than about 20%, correct?
4. I expect the Pd to be dissolved in the nitric acid boils, can I then precipitate the Pd by using DMG directly to the diluted solution of nitric acid and Pd?
5. Is the use of oxalic acid appropriate in this case? or SMB would be easier and as effective to precipitate gold and leave any Pd behind?
6. Would you recommend anything else that you consider important for recovery the gold and the palladium in a case like mine, like any important tip or trick to prepare the material for digestion or to precipitate with oxalic acid?

Thanks a lot for your input, it is highly appreciated.
Best

Stefano

First of all, sort what material can be sorted first.
Gold by itself and non gold by itself.
Do you know for certain there is no Mercury in there?
Since you are planning to dissolve all BM and Pd with Nitric first, the Silver will go into solution then.
Dissolve and refine that batch by itself, then go for the Gold, it is most likely alloyed so if you can test it with a XRF it would be good.
If it has more than 25% Silver Gold, you need to get it down to 25% by inquartering it either with Silver or Copper.

Edited to correct content.
.

 

[FrugalRefiner]

When you inquart, you want the gold content to be about 25%. That will allow the nitric leach to reach all the way into the center of the pieces to leach out all the silver.

If The silver content is over about 10%, it will interfere with the action of the AR.

Dave

 

[Yggdrasil]

When you inquart, you want the gold content to be about 25%. That will allow the nitric leach to reach all the way into the center of the pieces to leach out all the silver.

If The silver content is over about 10%, it will interfere with the action of the AR.

Dave

Oops, my mind was not present.
Of course you have to make sure the Gold content is below 25%.
Thanks Dave, I did not see that until now.

 

[stefcrive]

Hi Dave and Yggdrasil,
Thanks for your reply! Is inquarting necessary when the material is in powder form? (ca. 10-30 micrometers powder grains of Au/Ag/Pd alloy) or the surface area of the powder is sufficient to dissolve all the gold anyway in the AR step?
Is inquarting necessary if the gold is alloyed with some metal other than silver? or if silver is less than 20% lets say?
Thanks

 

[Yggdrasil]

Hi Dave and Yggdrasil,
Thanks for your reply! Is inquarting necessary when the material is in powder form? (ca. 10-30 micrometers powder grains of Au/Ag/Pd alloy) or the surface area of the powder is sufficient to dissolve all the gold anyway in the AR step?
Is inquarting necessary if the gold is alloyed with some metal other than silver? or if silver is less than 20% lets say?
Thanks

Maybe not, but you outlined a process where you will dissolve the Base metals in Nitric first.
In this case this will probably remove all Silver anyway, so for the powder it maybe ok to drop the inquarting.

 

[FrugalRefiner]

Several good questions.

Thanks for your reply! Is inquarting necessary when the material is in powder form? (ca. 10-30 micrometers powder grains of Au/Ag/Pd alloy) or the surface area of the powder is sufficient to dissolve all the gold anyway in the AR step?

In my opinion, probably not necessary. I think about the pieces of metal we dissolve like sponges in their structure. Silver is the main problem because it will not dissolve in AR. The silver will form a crust of silver chloride that prevents the AR from getting to the gold deep inside the pieces if it's over about 10%. The silver chloride becomes the sponge with lots of pores in it, but also the fibrous structure of the sponge. At a certain point, that fibrous structure stops any fresh acid from getting to the gold deep inside the sponge. If it's a great big sponge, the center will be untouched. But if it's a bunch of itty-bitty sponges, the AR can do its job.

Is inquarting necessary if the gold is alloyed with some metal other than silver?

If the other metal(s) will dissolve in AR, there's no need to inquart as everything will simply dissolve in the AR.

or if silver is less than 20% lets say?

10% is about the upper limit for silver content before it becomes a problem. At 20%, it would definitely cause issues.

There are, of course, always exceptions to the rules. Dental scrap can be one of them. Separating gold from platinum and palladium can be less than perfect. When they're all dissolved at the same time, and you try to precipitate one metal at a time to separate them, there will always be some drag-down of the metals you're hoping will stay in solution. It can be cleaned up with a second refining, but if you can keep from having all the metals in solution at the same time, the problem of drag-down can be minimized.

Inquarting can help. By adding silver to inquart, and leaching with nitric acid, much, if not all, of the palladium and platinum can be dissolved in the nitric leach, leaving nearly pure gold. When the gold is dissolved and dropped, it will be much cleaner.

Separating the palladium and platinum is a bit more of a chore, but it can be done. It is important to recognize the hazards of the platinum group metals, especially platinum itself. It is very toxic in solution or as dried salts. We have lost members to platinosis.

So, understand that refining dental scrap is considerably more dangerous than a lot of other types of scrap.

I also noticed in your pictures you seem to have a mix of fine dust with other more massive pieces, so the discussion above of not needing to inquart really only applies to the fines.

Dave

 

[nickvc]

You really have picked one the more difficult materials to refine as pointed out by the replies above , while you may remove the base metals and Pd in the powders by using hot nitric I would be tempted to inquart the lot using silver which will carry all the Pd and any Pt in the nitric treatment from which you can then remove the silver by adding HCl to form silver chloride and filter it off , the remaining PGM solution I would cement with copper with a bubbler or stirrer in the beaker to create metallic PGMs .
Separating those is far from easy and recovering all of them is near impossible without specialized equipment and a constant flow of material, for what it’s worth my view is melt assay and sell.

 

[orvi]

Go with inquartation - this will accomplish two things - homogenize the material, so you will exactly know what you are dealing with, and second - allow you to leach everything properly. Gold sponge would be fairly clean after the treatment with nitric, not to much PGM contamination.
Gold will be re-refined, and leaching liquor processed. Firstly, silver will be dropped with NaCl as AgCl, then the rest can go to precipitation of PGMs - either by copper cementation, or more quickly formate reduction. This will drop all the remaining PMs together, which could be then melted and sold directly as is. There is not much premium on PGMs in 99% form, so there is no interest in further refinement, otherwise you have someone to sell pure metals - and to clean them good, that is whole another league of refinement

 

[kurtak]

I've XRF all these materials, they seem to be quite uniform in composition which is about 60% Au and 10% Pd, no silver apparently, and the rest is mainly copper and zinc.

Per the bold print (no silver) --- there has been a lot of advice to inquart - this makes absolutely no sense to me considering XRF is indicating no silver

I think we can be quite confident that the XRF being used in this case is programed for PMs so we can also be quite confident that if the XRF says no silver then there is (little) or no silver

Why in the world would we want to complicate the refining process by adding a HUGE amount of silver & then part three metals (Ag - Au - Pd) with two acid processes (nitric & AR) plus the AgCl process that then needs a LOT of washing/filtering & then reduction of the AgCl

IMO - go straight to AR - dissolve Au Pd Cu Zn - drop Au with copperas - wash/filter Au sponge - cement or sodium/formic to recover Pd

Copperas should give you little or NO co-precipitation of Pd as long as AU sponge is WELL washed to get out all remaining AR dissolved Pd

Because I believe we can trust the XRF on this one - even if there is a trace of silver (not picked up by the XRF) that can certain be dealt with through going direct to AR



Inquartation is only needed when there is a high silver content to start with - that is NOT the case here ??????

Kurt

 

[kurtak]

when the material is in powder form

As Dave pointed out

I also noticed in your pictures you seem to have a mix of fine dust with other more massive pieces

So concerning the fine powders - the first thing I would do is get a 100 mesh classifier screen like this one

https://makeyourowngoldbars.com/pro...ssifier-sifting-rock-crusher-material-panning
in order to separate everything 100 mesh & smaller from 100 mesh & larger

Note; - 100 mesh screen is quite fine (very small holes) so it can be difficult getting the close to actual 100 mess size particles to actually go through the sifting screen

You can assist that by putting 3 or 4 large washers in the sifting screen & the washers will help drive the near 100 mesh particles through the very small screen holes --- also - only sift about one cup of material at a time

you posted ------

What I have to work with are filings

"Fillings" can have small bits of iron in them so the next thing I would do is spread the "fines" out in a thin layer & run a weak magnet over them to remove any fine iron particles

Then what we don't know is how much non metal (dirt) may (or not) be mixed in with the "fines"

So the next thing I would do is put 2 or 3 grams of the fine powder in my melting dish & see how it responds to torch melting

This will help determine if it can go to a straight up acid treatment - or needs smelting to slag off non metals &/or incineration before smelting

In other words - more then likely the "fines" are going have non metal dirt in them - the question how much dirt & what kind of dirt --- like ceramics/porcelain - oils/waxes - both

A small test melt of the fine powders will help determine the situation & the path forward for the fines

If the fines are "relatively" clean the path forward "may" be relatively simpler --- if they are dirty - the dirtier they are the more complicated it becomes

Or as Nick said -------

You really have picked one the more difficult materials to refine

If those fines turn out to be dirty

The larger then 100 mesh is a story of it's own to be determined by what it looks like after sifting

Kurt

 

[stefcrive]

It is important to recognize the hazards of the platinum group metals, especially platinum itself. It is very toxic in solution or as dried salts. We have lost members to platinosis.

Thanks for all your valuable information and insights. Thanks for the hazard notice as well, thankfully I have access to a fully equipped laboratory at my university.

I think we can be quite confident that the XRF being used in this case is programed for PMs so we can also be quite confident that if the XRF says no silver then there is (little) or no silver

So the XRF indeed showed no Ag present, but by looking at the old bags from the manufacturer of these raw materials (pictures attached), some of them indeed show Ag present. There is no way to trace the source of these materials and know exactly from which bag they are coming from. So I would have to do some tests the check the Ag content.

I know that If I put some drops of the diluted nitric acid solution after a digestion of some material, into a test tube of HCl I should see AgCl precipitating if Ag is present. Can this test also give you a rough estimate on the amount of silver present, to asses whether I should inquart or not?

A small test melt of the fine powders will help determine the situation & the path forward for the fines

I’m interested in this suggestion to test melt few grams of the powders to see whether they are clean enough for acid digestion, can you tell me what I should look for exactly?

Thanks again,
Stefano