# lost pgm



## Inter Refiner (Oct 17, 2020)

hello friends,
I had 195 grams of pgm black that have the following composition as per xrf reading(54%pd, 13%pt, 4%rh, and 24%pb).
I dissolved the 195 grams in about 6 liters of hcl/h2o2 solution, I added about 1 liter of sulfuric acid to precipitate the large quantity of lead as lead sulfate, then I reduced the platinum group metals by formic acid, but here I got only 85 grams   and I was supposed to have 148 gams of pgms. does anyone have any idea where my metals are gone??? the remaining solutions tested negative with stannous chloride, 
any help is very appreciated


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## Lino1406 (Oct 17, 2020)

XRF is precise for metallic nuget state, if homogenous, not for other state


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## Inter Refiner (Oct 17, 2020)

Lino1406 said:


> XRF is precise for metallic nuget state, if homogenous, not for other state


Ok but where do you think the 195 grams go?


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## Lino1406 (Oct 17, 2020)

Weigh the lead sulphate for a beginning


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## Inter Refiner (Oct 18, 2020)

Lino1406 said:
 

> Weigh the lead sulphate for a beginning


They weigh about 141 grams


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## Lino1406 (Oct 18, 2020)

So? It is much more than 24% lead


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## Inter Refiner (Oct 18, 2020)

Lino1406 said:


> So? It is much more than 24% lead


Of course it is, now how can I dissolve this stubborn sulfate, I tried Aqua regia but it didn’t dissolve them


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## Lino1406 (Oct 18, 2020)

For what reason?


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## Inter Refiner (Oct 18, 2020)

Lino1406 said:


> For what reason?


If they are more than the lead I have, then maybe my pgms are precipitated accidentally with lead, so I am planning to redissolve them to see if there are any pgms, do you agree?


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## nickvc (Oct 18, 2020)

As Lino has pointed out your xrf reading may well be off, on a powder you need to shoot it multiple times from different angles and average it to get a reasonable figure to base your expectations on.
The amount of lead sulphate shows your expectations were off and I can’t see how you could have any more values in it unless you added a precipitant or you cemented it, you could always xrf it to check.


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## Inter Refiner (Oct 18, 2020)

What would Excessive amounts of sulfuric acid added affect my pgm solution, how do I know that I added enough sulfuric acid?


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## Lino1406 (Oct 18, 2020)

No need to dissolve lead sulphate, as the balance now is about closed. You can mill it and wash to gain small amount of PM's


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## g_axelsson (Oct 19, 2020)

Inter Refiner said:


> How do I know that I added enough sulfuric acid?



When it stops precipitating lead sulfate.

Lead sulfate is very stable but it can be dissolved by several procedures. Most basic is to roast the sulfate, SO2 is given off and the lead oxide can be dissolved in acids.

In Hydrometallurgy 96 (2009) there is an article "Metallic lead recovery from lead-acid battery paste by urea acetate dissolution and cementation on iron"
They use urea acetate at 105 C to dissolve lead sulfate.

From the 911 metallurgist you get a process where "The sludge is leached with ammonium carbonate [(NH4)2CO3] and ammonium bisulfite (NH4HSO3) to convert the lead sulfate (PbSO4) and lead dioxide (PbO2) into lead carbonate (PbCO3), which is acid soluble." 

But I don't think the lead sulfate contains a lot of PGM:s. Proper washing and filtering should recover enough traces. Use the XRF to see if there are any larger amounts of pgm in the lead sulfate.

There might be several reasons the numbers aren't adding up. XRF doesn't see all the elements, some XRF is lacking knowledge about some metals and just ignores the unknown elements. For example lead sulfate would probably report almost pure lead and no oxygen or sulfur. Hydrogen in water is also invisible to an XRF. All this makes measurements with XRF a challenge and to get a quantitative result you need standards, a proper understanding of how XRF works and laboratory conditions to prepare your sample.

Weighing powder samples could be problematic. Water is easily absorbed on the surface of the metal grains and powder have an extreme surface area.

Göran


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