# Remove Silicon Carbide from Platinum Lemel



## platinumsmith (Apr 16, 2011)

Hi there, I wondered if anybody knows a way to remove the silicon carbide I get from my emery paper that gets mixed up with Platinum filings and scrap?
When I melt up my Platinum scrap (lemel) I get a glass type residue that often does not completely precipitate out of the molten metal.
It would be great if there was a way to dissolve it prior to melting?
Maybe acids or caustic solution is possible?
Or maybe there is a way to separate it by specific gravity.
Anyone else have this problem?
cheers
j


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## nickvc (Apr 16, 2011)

Some platinum alloys here in the UK contain cobalt which is magnetic so you can use a magnet to pull the platinum out and leave the residue. The only other method I can think of would be to use hydrofluoric, I'm not keen on using it in any circumstances due to it's highly dangerous nature,but it would dissolve the glass.


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## Lino1406 (Apr 24, 2011)

Platinum is very dense - 21.4
silicon carbide - 3.2
use this difference!


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## Lou (Apr 25, 2011)

Too bad sodium polytungstate saturated solution can't be used. It's density is 2.9 max from what I remember off the brochure they sent me.


Lou


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## HAuCl4 (Apr 26, 2011)

platinumsmith said:


> Hi there, I wondered if anybody knows a way to remove the silicon carbide I get from my emery paper that gets mixed up with Platinum filings and scrap?
> When I melt up my Platinum scrap (lemel) I get a glass type residue that often does not completely precipitate out of the molten metal.
> It would be great if there was a way to dissolve it prior to melting?
> Maybe acids or caustic solution is possible?
> ...



Smelting with lead has worked for thousands of years. Base metals float to the surface, along with silica and other slags.


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## Harold_V (Apr 27, 2011)

HAuCl4 said:


> Smelting with lead has worked for thousands of years. Base metals float to the surface, along with silica and other slags.


That sounds wrong. Very, *very* wrong. Can you provide cites?

Harold


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## HAuCl4 (Apr 27, 2011)

Yes I can. And probably you could too. Half of the process is described in Hoke as well, re. separating iridium from platinum.

Since it is so old and obscure, I'm going to claim the details of the process, as adapted and improved by me, as *proprietary*. :lol:


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## Harold_V (Apr 28, 2011)

The part that gives me doubts is the claim that base metals float to the top. Unless they are well oxidized, that's certainly not going to happen---and if lead is present, it, too, must oxidize. That is generally not the case---litharge is used, which liberates oxygen to oxidize other substances, but the lead that results becomes the collector of values. It would be desirable that it not oxidize, at least not in that operation. Something doesn't add up.

Do take note---I stand to be corrected if I'm wrong. 

Harold


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## HAuCl4 (Apr 28, 2011)

As you drop the temperature slowly from 1150 C ish down to 950 C ish... the copper crystallizes and floats to the top...dragging all the other base metal stuff up...can be removed as a "dross"...the iridium, insoluble in lead, settles to the bottom of the crucible...with accurate treatment with dilute nitric of the lead-silver-gold-PGM alloy...followed by dilute aqua regia on the nitric residues...the iridium is quantitatively separated from the other precious metals...provided there is no Rh, Ru, Os in the mix...

Simple really. :lol:


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## Platdigger (Apr 28, 2011)

What was your source? Sounds like it could be a good read.


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## Lou (Apr 28, 2011)

Oh the method works well for separating iridium from platinum quantitatively. You just have lead fumes and a bunch of lead waste to deal with after parting.


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## HAuCl4 (Apr 28, 2011)

Lou said:


> Oh the method works well for separating iridium from platinum quantitatively. You just have lead fumes and a bunch of lead waste to deal with after parting.



Lead fumes are minimized using a crucible lid, that is opened only to dross the melt at lower temperatures, and for pouring. A good fume extractor or vaccuum cleaner processor works wonders.

Lead nitrate, after extracting all PM values is converted to lead sulphate (re-claiming the nitric acid as well), which is re-cycled by mixing with sodium carbonate and charcoal onto the next batch to purify.

I take the above over putzing around endlessly with ammonium chloride and platinum iridium salts and absorbing them in the operator's lungs and skin. It works well for the common jewellery alloys of platinum that use iridium as hardener.

For the ruthenium hardened alloys the plan is a different one. :shock:


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## nickvc (Apr 28, 2011)

Most casting alloys these days seem to contain cobalt as the hardener I don't know if that would cause problems using lead.
The other simple idea that occurred to me was to sieve the lemel and remove the larger pieces and then to use a gold pan to separate the platinum from the carbide. I think our mining and prospecting members could tell you if that would work, and the lighter material could go into your sweeps just in case any values got through.


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## HAuCl4 (Apr 28, 2011)

Well if you do not want the platinum refined and just want to take the non metal stuff out before re-melting the platinum, probably mixing some borax and fluorite, and heating to 800 C would clean up the filings without even melting the platinum. I'd try that. It may work and only takes 5 minutes. :roll:

Never heard or read of cobalt as hardener, but it should be straightforward to dissolve in AR and then cement it. If there are no other PGMs in there it should be the easiest thing in the world to refine. Recovering the cobalt shouldn't be too hard either cementing it too.

edit to add: I mean dissolve the alloy in AR, cement the platinum, thinking cobalt itself is the best option here to cement, then recover the cobalt by cementing with zinc or magnesium. 8)


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