# SO2 and platinum, palladium recovery



## overdriv (Mar 24, 2012)

I use SO2 to recover my gold from AR and find it very preferable. 
1. My question is would using SO2 make recovering platinum, palladium, etc more difficult to recover? 
2. Should the platinum be recovered first or after the gold?

Usually, the platinum group would be separated from the gold by inquartation and cementing with copper. But sometimes you get a surprise. Testing with stannous on a pregnant solution of gold gives such a dark (black) color that seeing any platinum would be difficult. It would only be after precipitating the gold that the platinum would be noticed. 

It would be great if Harold could chime in, but if not is there anyone else that uses SO2?

3.My thoughts would be to use zinc to precipitate the platinum group metals from solution (AR) if I thought they were of too little amounts to properly work on. Collect and wash the precipitates and store them for future (larger) amounts to accumulate. 

4.However should the AR solution be reduced to a concentrate before trying to get the platinum group out, or will it come out in gold's more preferred dilute solution?

Any thoughts?

Nick


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## Harold_V (Mar 25, 2012)

overdriv said:


> 1. My question is would using SO2 make recovering platinum, palladium, etc more difficult to recover?


I am unaware of any problems that may arise from the use of SO2. I used it routinely, but I rarely precipitated PGM's from such solutions, due in part to the level of concentration being quite low. I typically sent such solutions to the stock pot. 



> 2. Should the platinum be recovered first or after the gold?


A great deal depends on the level of concentration of each of the metals, but my logic was that gold powder is easy to wash, while platinum recovered with ammonium chlorate is not. It dissolves too easily, so my procedure was to recover gold first. Do keep in mind, it would have been quite unusual for any of my solutions to have had a great abundance of platinum or palladium in solution, so that may not have been true were that not the case. Gold recovered from such a solution will have contamination, and may not wash fully. A second refining may be needed to bring up the quality, but it should serve perfectly well in eliminating the PGM's, at least to acceptable levels. 



> Usually, the platinum group would be separated from the gold by inquartation and cementing with copper. But sometimes you get a surprise. Testing with stannous on a pregnant solution of gold gives such a dark (black) color that seeing any platinum would be difficult. It would only be after precipitating the gold that the platinum would be noticed.


True. You can test for the PGM's before precipitating gold by placing a drop of solution in a cavity of a spot plate, then inserting a crystal of ferrous sulfate. That will precipitate the gold, at which time you can then test the solution with stannous chloride, or even DMG. That way you can make decisions on which method you prefer to use. 



> 3.My thoughts would be to use zinc to precipitate the platinum group metals from solution (AR) if I thought they were of too little amounts to properly work on. Collect and wash the precipitates and store them for future (larger) amounts to accumulate.


I'd agree, assuming you don't have a great deal of copper present, but then it would have to be eliminated in any case, so that's likely the best approach. In my case, I sent all of that type of material to the stock pot, as I already mentioned, but my intention, from the outset, was to process the waste material by furnace, secure in the knowledge that I had considerable silver chloride present, plus I knew that any base metals that got included would act as collectors for greater values. I was well rewarded for the decisions I made, and by the processes I elected to use---but you may not enjoy the same success, in particular if you have no intentions of using a furnace to recover values. 



> 4.However should the AR solution be reduced to a concentrate before trying to get the platinum group out, or will it come out in gold's more preferred dilute solution?


Because the level of concentration is generally very low, if you hope to recover any of the platinum group chemically, you most likely will be required to evaporate the solutions. That's the reason I finally abandoned any attempts. The level of concentration was generally too low to achieve even a trace of recovery. That, of course, would not be true if you recovered with copper, or zinc. I think you will come to understand that traces of platinum and/or palladium will not precipitate, and when they will, rarely will they do so totally. Recovery in the stock pot is pretty much a requirement (which includes the use of zinc). 

Hope this helps. 

Harold


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## Lou (Mar 25, 2012)

SO2 is well known to cause drag down of palladium.


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## Barren Realms 007 (Mar 25, 2012)

Lou said:


> SO2 is well known to cause drag down of palladium.



As refrenced by the table that Steve posted.

http://www.goldrefiningforum.com/phpBB3/viewtopic.php?f=49&t=6026&start=140


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## Lou (Mar 25, 2012)

No, as referenced from experience.

A few modifications need to be made to the chart, namely that some of the reagents are not in fact reducing agents, but merely selective precipitants.
I never looked at the chart until you linked to it just now. I'll see about making a more complete chart.

I realize Steve knows the difference but others might not.


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## Harold_V (Mar 26, 2012)

Lou said:


> No, as referenced from experience.


Yeah, same with me, although I didn't consider it a problem because very little came down, and it was eliminated in the second refining. Yet another reason to inquart, where palladium chiefly follows the silver. 

My response was more in keeping with the idea that the use of SO2 did not complicate further processing of a solution. That was the question. 

Harold


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## Lou (Mar 26, 2012)

One issue with SO2 is that under some circumstance it can reduce Pt(IV) to Pt(II) and make that precipitation all the more difficult.


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## freechemist (Mar 26, 2012)

Hi overdriv,

From my own practice with dental alloys, often rich in Pt and/or Pd, here some answers to your questions.

1.) _"My question is would using SO2 make recovering platinum, palladium, etc more difficult to recover?"_

SO2 definitely makes recovery of platinum more difficult, because of reduction of Pt(IV) to Pt(II), like Lou did mention it already. Personally I always used sodium sulfite (Na2SO3), which in these strongly acid solutions reacts immediately to SO2.

2.) _"Should the platinum be recovered first or after the gold?"_

Recovery of platinum before gold makes sense, if the dissolved PM-scrap contains at least 5-10% Pt, and only if it is precipitated together with palladium, eventually also present in solution. A disadvantage of this method lies in the fact, that the precipitated Pt and/or Pd entrains some gold from the solution.

3.) _"My thoughts would be to use zinc to precipitate the platinum group metals from solution (AR) if I thought they were of too little amounts to properly work on. Collect and wash the precipitates and store them for future (larger) amounts to accumulate."_

If only minor amounts of remaining Pt and/or Pd are dissolved, you can absorb them on a strongly basic anion exchange resin (quaternary ammonium, like Dowex or Amberlite) as their hexachloro-anions, especially after addition of an oxidant like NaOCl or NaClO3. These resins absorb up to 10% of their dry weight in PM, until much more than ca. 1 mg Pt and/or Pd per liter remains in solution. The loaden resin can be washed, dried, slowly burnt down and finally calcined to the metals at about 800oC.


Hi Lou,

You write: _"SO2 is well known to cause drag down of palladium."_

In my practice I never was confronted with this phenomenon, not even working up multikilogram-batches of dental-alloy-scrap with AR.


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## Lou (Mar 26, 2012)

Hi freechemist,

Chances are you are also washing your gold well and are keeping the solution strongly acidic.
If the pH is allowed to rise overmuch, palladium will surely contaminate the gold. This is plainly evident in the gold by its color and persistent oxidation that no polishing flux will remove. It will also show up in ICP.

It's been my experience mostly in very high palladium concentration solutions, from which I remove the gold first and then follow after with removal of the PGMs. The gold can be cleaned up adequately well by a prolonged simmer in 50 % w/v sulfuric acid.
Reference:

T. A. Wright. Metals and Alloys 3, 146 (1932)


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## Harold_V (Mar 27, 2012)

freechemist said:


> You write: _"SO2 is well known to cause drag down of palladium."_
> 
> In my practice I never was confronted with this phenomenon, not even working up multikilogram-batches of dental-alloy-scrap with AR.


I had the same experience, but was inclined to think that palladium would be dragged down regardless of the precipitant used. 

Thanks to both of you for enlightening me as to the difficulties of recovering platinum after the use of SO2. That may well be one of the reasons I had poor results from dilute solutions, and resorted to the stock pot. 

Harold


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