# Au, Ag, Pd contaminated with 316 stainless



## stant (Feb 26, 2020)

Hello.
I recover a brazing alloy that is mainly gold, silver and palladium from 316 stainless steel bits and bobs. While I can recover probably 95% of the precious metals via physical methods I'm left with tiny bits of base metal that have small blebs of this alloy, and no realistic way of separating them. The refiner I deal with for the precious metals doesnt want to touch the material with the stainless present. Over time I have built up a few Kgs of this low grade scrap and even if it contains a few percent precious metals, with the current sky high cost of Pd, it would a good little chunk of change.

I have zero need to separate the precious metals. If I could hand my refiner a lump of metal that was just the PGMs they could x-ray it and be happy. 

Obviously AR would digest the whole lot. Is there a dummy proof way of precipitating at least just the au and pd? I assume I'd need a huge quantity of AR to put a few Kgs of stainless into solution. Are there any other options? something that would dissolve the stainless and leave the base metals would be my ideal solution.

Thanks in advance for any help.

BTW, yes I did already ask my refiner. They sent me to someone who deals with low grade material, but they werent interested if 'only' a few ounces of au and pd would be the result...


----------



## Lino1406 (Feb 26, 2020)

The large difference in melting points calls for a 1st stage in melting which will leave most stainless and part of palladium as solids. The continuation seems obvious


----------



## FrugalRefiner (Feb 26, 2020)

I believe hydrochloric acid will dissolve the stainless, or at least most of it, while leaving the precious metals untouched. If any of the palladium does dissolve, which you can check with stannous chloride, it can be cemented back out of solution with some copper.

If the precious metals have alloyed with the stainless, the remaining material can be dissolved in a much smaller amount of aqua regia, and then all the precious metals can be cemented out with copper.

Dave


----------



## snoman701 (Feb 26, 2020)

This job is only straightforward to a person who does it regularly.

Contact Lou, he moderates this subforum.


----------



## sena (Feb 26, 2020)

Hi 
Can you share the pic of the material


----------



## anachronism (Feb 26, 2020)

FrugalRefiner said:


> I believe hydrochloric acid will dissolve the stainless, or at least most of it, while leaving the precious metals untouched. If any of the palladium does dissolve, which you can check with stannous chloride, it can be cemented back out of solution with some copper.
> 
> If the precious metals have alloyed with the stainless, the remaining material can be dissolved in a much smaller amount of aqua regia, and then all the precious metals can be cemented out with copper.
> 
> Dave



No. You don't want to dissolve a boatload of SS if you don't need to.


----------



## FrugalRefiner (Feb 26, 2020)

anachronism said:


> No. You don't want to dissolve a boatload of SS if you don't need to.



What would you suggest?

Dave


----------



## stant (Feb 26, 2020)

To answer two questions with one stone - I dont have any photos handy, but the vast majority of the material is very small bits of stainless (<1cm) with small amounts of PMG alloy, maybe a few cubic mm. The alloy is located in depressions and in other nooks and crannies so it would not be realistic to try to melt the material off. 

I dont want to dissolve a boat load of ss if I dont have to, but could someone guesstimate how much acid would be required to do so?


----------



## Yggdrasil (Feb 26, 2020)

Hi!
Can you easily grind the parts close to the PMs and make it thinner?
Are the boundary between the PMs and base metal defined enough to easily spot it? 
For each gram of base metal you can get rid of mechanically the less acid will be needed.
This may give you a small kayak instead of a boatload :lol:
HCl will easily dissolve most Stainless Steels, and it don't need to be concentrated.
Necromancers thread about an acid resistance chart comes into mind :wink:

*Added reference to boats :wink: *


----------



## stant (Feb 26, 2020)

Yggdrasil said:


> Hi!
> Can you easily grind the parts close to the PMs and make it thinner?



easily? no. remember these parts are <1cm max in the biggest dimension and smaller than that int he other 2 dims. We are talking easily 1000s of little tiny fiddly bits of stainless. If the question was between a gallon of AR and 5 gallons, I'm pretty sure I'd go with the 5 gallons. When you say a kayak full, just how literal are you being?


----------



## rickzeien (Feb 26, 2020)

Maybe tumble in a vibratory bowl with the right media. 

https://youtu.be/dx7n164apvs

Sent from my SM-G950U using Tapatalk


----------



## Yggdrasil (Feb 26, 2020)

Sorry about that, just failed attempt on humor, a remark pointed on the boatfull, 
since a kayak is smaller than a boat.

HCl will take most SS, I dont know about 316, it will take 304 though.
And as I said, no need to use concentrated.
It will quickly get quite dark green and murky. Heating will speed things up.
If the alloy has been heated (which is likely since it is braze?), it is important to test any solution for Pd.
PdO can be dissolved in HCl, and if it tests positive, it is important to cement out any dissolved Pd with copper before treating the solution as waste.


----------



## Shark (Feb 26, 2020)

I am not sure this will help with 316 SS but try heating a few to a dull red, then cooling them quickly in cold water. It may make the acids attack them quicker.


----------



## snoman701 (Feb 26, 2020)

Trying to cement Pd out of HCl that has dissolved stainless sounds like a lesson in frustration. I have a feeling that the iron is going to drop back out before all the Pd will. 

Trying to test HCl for the presence of Pd with stannous will be the same.

Further, dissolving stainless in iron takes a LOT of HCl without using an oxidizer. 

Like I said, contact Lou....strange matrices like this are what he does for a living.


----------



## Yggdrasil (Feb 26, 2020)

If you get the 316 dissolved and there is Pd dissolved alongside, the reaction series should be quite clear.
You use iron to drop out the Copper from a general waste stream, so the Pd should drop as usual with copper.
Concentration issues may make it slow though.
This waste stream will be a bit special since the Iron will drop the Copper, Nickel and maybe drag some Chromium down as well.
A stannous test may indicate if there is dissolved Pd.


----------



## anachronism (Feb 26, 2020)

It all depends on the ratios. 

To answer Dave's question. If a small percentage can't be leached then it's not worth dissolving the SS because how long and how much does it cost to recover that material. Not withstanding how much of that SS is there to dissolve. 

There is no data as to material in terms of the ratios apart from guesswork and a little bit of info from the kind OP in the form of "little pieces." 

You all need to stop offering random advice and ASK QUESTIONS before doing so because it clouds the issue. 

Slap me for being blunt but I am not incorrect. 

Why cannot we start asking for hard data before chucking in suggestions- is it really that hard? 

Jon


----------



## snoman701 (Feb 26, 2020)

Yggdrasil said:


> If you get the 316 dissolved and there is Pd dissolved alongside, the reaction series should be quite clear.


But it won't be. I know how the reactivity series works.

When you hang a piece of copper, with a stirrer and walk away, you end up with a mess. 

Yes, by the time the iron starts to come out, yes, the Pd should, and likely will be on the bottom of the barrel...but you'll be inside thinking about how successful you were today.

Then the reaction will continue. As the acid churns round and round, it will start to be consumed...and the pH will come up. And then the iron drops. I've made this mistake more times than I care to admit.

I also find stannous extremely difficult to interpret when you have dissolved stainless.


----------



## stant (Feb 26, 2020)

anachronism said:


> It all depends on the ratios.



unfortunately I do not have much in terms of hard facts. I would guesstimate that the material is between 5 and 10% precious metals based upon volume. The silver content should be a max of 15-20% with gold and Palladium making up the balance. Likely heavy on the pd side.


----------



## nickvc (Feb 27, 2020)

The easiest way to recover the PMs is to use reverse AR where you start by adding nitric acid to cover the material and then slowly add small amounts of Hcl, because the stainless does not react to the nitric but the PMs will with the additions of Hcl, this requires a fume hood and if you are not proficient with the process I’d suggest small sample runs before doing the lot.


----------



## Yggdrasil (Feb 27, 2020)

As Jon says we should get our facts first.  

I still like Nicks advice though, since Nitric won't do much to the SS and it will dissolve whatever 
Silver and Pd is accessible. 
When the Nitric reaction stops, add a small amount of HCl to dissolve the Au and more Pd.
If this is balanced correctly one may get the PMs without touching the SS.

Or contact our main guru on the subject, Lou


----------



## snoman701 (Feb 27, 2020)

stant said:


> The silver content should be a max of 15-20% with gold and Palladium making up the balance. Likely heavy on the pd side.



With that silver content, if it's tied up in the braze alloy, there is a reasonable chance it's just acid resistant....in which case you are melting the stainless in to copper. 

And yes, Nick is correct on the "easiest" way. But there is nothing easy about it. Especially given that the original poster is not an experienced refiner. Getting the metals into solution is one thing, getting them back out, another.


----------



## stant (Feb 27, 2020)

snoman701 said:


> With that silver content, if it's tied up in the braze alloy, there is a reasonable chance it's just acid resistant....in which case you are melting the stainless in to copper.



The silver is definitely alloyed. it is not silver bits brazed with pd/au.

What do you mean by melting the stainless into copper?

If you are going to suggest electrorefining that actually *sounds* pretty easy. I have a e-beam vacuum evaporator. I should be able to alloy the stainless and PGMs with copper fairly easily (or would I even need to do this under vacuum?). I assume everything I put into the copper will come out as anode mud, I could remove the ni and fe with a magnet. I'd still have a good bit of cr in the mix, but the result should be a much more concentrated button of my desired metals. no?


----------



## snoman701 (Feb 27, 2020)

Thing is, I've never encountered that braze, and after a cursory search of industrially available Au/Pd/Ag brazes, it would be relatively uncommon. It's usually Ag/Pd or Au with a little bit of Ag or Pd, but not all three. And with your reluctance to describe the material as anything other than bits and bobs, it's hard to give much more advice. Don't take my statement of "reluctance to describe" as an insult...I fully understand the reluctance. Threads like this often dance around the materials origins. 

As for melting in to copper, I mean exactly that, melting the stainless in to copper. Copper does not alloy with iron to any great degree, a few percent on either end of the phase diagram. The iron will largely slag off as a matte. Separation of copper from the rest of the metals can be done in a multitude of ways, largely dependent upon your plans for refinement of the final metals. But electrolytic refining of copper, to produce slimes, will require an oxide free copper dore that is 90-95% Cu....95% will work much better than 90%. Oxide free is important, and as copper oxide will end up in the slimes as it's non-conductive, but more than that, will cover the anode in a non-conductive scum that slows the dissolution considerably. I would NOT use this method unless I found the above method to be acid resistant, there is a greater chance for PM loss in the matte and in the slags. 

This forum is a great resource, but your material is difficult to refine....I wouldn't want to refine it as a first project. If you want to learn refining, start with a much easier material, like clean karat gold. Then move on to a tertiary system with Pd. Don't use the high price of Pd as an excuse to lose at least as much as you gain, fumbling through a difficult refine. Start at the beginning and learn the fundamentals.


----------



## stant (Feb 27, 2020)

Oh, dont get me wrong, I wasnt trying to protect sources. I simply didnt describe the material any better only because I dont see how additional information could provide help here. We are talking chemistry to separate out the materials, and chemistry doesnt really care what physical form the stuff is in, all other things being the same.

The material in question is little bits of machines stainless steel components, or cut sections thereof where I have already done any physical trimming that was realistic. The braze is either a bleb of precious metals stuck to the surface of the stainless, either out in the open or somewhat shielded, ie a bleb of braze in a corner where 2 faces, or even 3 faces meet, or worst case scenario, inside a milled in pocket. I process a large amount of this stuff, and have already removed probably 98% of the precious metals that were originally there, this is the last little remaining difficult bit that doesnt want to be dealt with in a mechanical means.

To be honest I dont really want to be a refiner. But I do want to process this material. I am not without experience when it comes to the physical isolation of precious metals from various scrap streams before. In the past ive alloyed iridium with manganese to separate it from ceramic, then digested the manganeese in acid much like inquartation for gold. But alas ive not really done any chemistry.

back to melting into copper.... if the iron will slag off, then is there any reason why I cant just dissolve the copper in hcl and melt the remainder? I dont need pure metals, I dont need to separate metals and I'm a-ok with treating the silver as waste. I'm just worried about the gold and palladium. I see no need to electrorefine the copper if 95% of the iron is going to go to slag. My only concern is the braze that is stuck in shielded areas that I mentioned before. let's pretend I have a cube of stainless with a 1:10 aspect ratio hole drilled in one face (ie 10 times as weep as it is wide) it is filled with braze. I have a few kg of liquid copper and this cub weighs maybe a gram. whats going to happen to the braze if the iron turns to slag? will all of the PMG bearing scrap go to liquid and then the Fe will come out of solution and turn to slag, or will if slag off basically instantaneously, possibly physically carrying the PMGs with it?

Thanks again for everyone's help!


----------



## Lou (Feb 27, 2020)

Stant,

I responded in PM in similar fashion but also for forum's use:

Because this is small-sized material, it can lend itself to copper collection BUT stainless steel dissolving in copper is not a fast nor a pleasant process and no, it doesn't just magically slag off. There's usually a lance involved to oxidize it out, several hundred times more copper relative to stainless, a rocking/rotary furnace motion and a lot of hoping it's all dissolved and into molten copper solution.

Some members here do the suggested reverse-aqua regia process on gold fill. Depending on the Ag concentration, it may be more or less successful. Many members here then have to evaporate off the nitric acid component or otherwise neutralize the acid with caustic to get a precious metal hydroxide (note this doesn't exactly work well for Pt) which can be either smelted with Cu, cupelled with lead, or redissolved in HCl and freed of excess nitric and worked up in classical fashion as described in Hoke and many other places here on the forum.

As others have alluded to, the usual approach of reverse aqua regia, which would passivate the surface of the stainless and selectively draw in the Pd/Au has to contend with the somewhat unusual presence of Ag in your braze. This means the process, if run successfully to completion, would leave behind AgCl. Recall that silver doesn't dissolve to great extent in aqua regia, and instead the silver metal forms silver chloride which forms a protective plaque on the surface and slows mass transfer/creation of Au(III)/Pd(II/IV). This means it slows down your leaching. This mass transfer would be enhanced with the use of sonication (and heat) as it descales the surface of AgCl and allows the dissolution front to proceed. 

As a friendly reminder to the future reader, reverse aqua regia isn't a cure all for pulling value from material. It relies on the presence of chromium in material to passivate the surface, and if that surface happens to be in physical contact with something capable of depassivating it, then the potential for a runaway reaction exists. 

On the commercial scale, the precious metal extraction is best done with specially designed solvent extractants. One that can work for Au extraction from the highly oxidizing reverse aqua regia is n-tributylphosphate. In low chloride environments, it will not bring in iron and is reasonably selective for Au and is stripped with oxalic acid. The way we work up Pd and Pt is proprietary and I'd lose my job if I got into it. 

That said, I think you have enough to get started.

As I said in PM, your best bet is probably to just digest the SS away in hot oxygen/oxidant-free HCl and then filter and with the rest of whatever you're extracting. Be sure to check the solution for values!

Thanks everyone for recommending me for helping him. In all seriousness, without member goodwill here and the work (sometimes very unique stuff) they've provided over the years, I would not have been able to pay my bills, much less progress professionally and employ dozens of people, provide them benefits, etc. So thanks again.


----------



## FrugalRefiner (Feb 27, 2020)

Lou said:


> As I said in PM, your best bet is probably to just digest the SS away in hot oxygen/oxidant-free HCl and then filter and with the rest of whatever you're extracting. Be sure to check the solution for values!



Thank you Lou!

Dave


----------



## anachronism (Feb 27, 2020)

FrugalRefiner said:


> Lou said:
> 
> 
> > As I said in PM, your best bet is probably to just digest the SS away in hot oxygen/oxidant-free HCl and then filter and with the rest of whatever you're extracting. Be sure to check the solution for values!
> ...



The point I made was that it depends upon what's there and that point is valid. If there's 50Kg of SS and 5g of Pd then it's not worth it.


----------



## Martijn (Feb 27, 2020)

If you use nitric like nick suggested but without the HCL and apply some heat it might dissolve all the silver and Pd and leave a gold sponge and clean SS. Maybe try a small bit and see what happens?
Separate the silver and Pd by making silver chloride and washing. Cement Pd on copper. Collect gold sponge and wash. SS in the recycle bin. 
Or am I thinking too simple?

Edit: 
First collect and separate the gold and the SS of course before making chlorides. 
And the silver content should be high enough to make the silver completely yield to the nitric and release the gold powder. Like with inquarting 

Martijn.


----------



## Lou (Feb 27, 2020)

Oh absolutely, Jon.

As you are now very much well aware, most of recovery and refining, the really difficult stuff anyway, is understanding _when_ to use a sledgehammer vs a chisel, or perhaps in other words...

sometimes it is better to selectively extract your value from the bulk and leaving the bulk behind, increasing the value density, as in dissolving the Au away from stainless with RAR (the chisel)
sometimes it is better to selectively extract the bulk away from the value, leaving the value behind, as in dissolving away the stainless from the Au with HCl (the sledgehammer)

Which way is better is the really dependent on the context of the situation...what is available, what reagents cost, what pollution control equipment and local regulations/permits apply.

In all cases the value density must increase before it enters refining, where in every case, small amounts of many elements, are taken away from the majority constituent, the element being refined. 


*Martijn*

How well that will work will depend on how much Pd is there relative to the Au. Just like Ag interferes with the processing of Au by forming that AgCl layer, gold will do so too (form a plaque like surface) and interfere with the leaching of the Pd and Ag from the gold matrix. He said this is Pd heavy, so it may have success. If it does, his best bet is to boil in strong nitric, filter off the gold and stainless, then repeat until the brown Pd color is not seen. Then he can precipitate the silver with excess HCl, filter that off, reuse the liquor to leach the gold, then, when the pieces fall apart, filter off the liquor and neutralize the liquor with caustic, collect the PM hydroxides, redissolve in HCl, and then precipitate the gold, filter it off, then cement the Pd or recover however he likes. 

So many products are not made with the total lifecycle thought through and as such, they do not go right into refining so they have to go (literally in many cases around the world) to different places to be recovered before they fit into the black box of refining and can be reused again.


----------



## Martijn (Feb 27, 2020)

Lou, theoretically it might work then. A small test would give clarity. And his refiner might accept the dirty gold powder as is, I was thinking. That would save some messing around with chemicals. 
But dealing with PGM's is something I have no experence with. 
And PGM's in solution are extremely toxic as I understand so one should stay far away from that without the right experience. 
I stand corrected. 
And thanks for the info.


----------



## Lou (Feb 27, 2020)

It’s nice to see so many people in this thread offering great advice, even if inferred from first principles and not necessarily experience!! Heck, this reminds me of 4th grade when they taught us Bloom’s taxonomy!


----------



## stant (Feb 29, 2020)

I too would like to say thanks for everyone's suggestions in this tread!


----------

