Platinum Precipitation

I'm working on some platinum this week so I figured I would share some photos with the forum.

Here's a snapshot of a Pt solution to which I added a saturated solution of ammonium chloride:



Notice the distinct canary yellow color of the resulting salt.

I dissolved this Pt without using any nitirc acid or nitrate. The pregnant solution was a nice orange brown color before I added the 40 grams of ammonium chloride dissolved in 107 mL of water.

The yellow cloud formed instantly and is in the process of settling in the above photo.

I'll update this thread throughout the week.

Steve

Here's a shot of the precipitate after it has settled for 10 minutes.



Steve

After vacuum filtration we have the left over solution on the left and the pure platinum salt on the right. In the foreground is a stannous test of the left over solution indicating there is still a slight bit of Pt in the solution.



Tomorrow I will convert the Pt salt to metal using zinc and dilute HCl. I will also concentrate the green solution to harvest another crop of yellow Pt salt. Further testing will be required to determine what other metal(s) are in the left over solution.

Steve

Thanks Steve.

I must have missed some posts where can i read about dissolving plat without nitric

I would be very interested in seeing pictures of converting the salt to metal using zinc and HCL. Calcining is a pain.

shadybear said:

I must have missed some posts where can i read about dissolving plat without nitric

Click to expand...

Lou and myself have posted methods for doing so.

Search our posts referencing the keyword platinum, pt, peroxide, and bleach.

Steve

goldenchild said:

I would be very interested in seeing pictures of converting the salt to metal using zinc and HCL. Calcining is a pain.

Click to expand...

Your wish is my command:

Here's the next snap shot in the series:



Be sure you are wearing all of your proper safety gear (apron, gloves, goggles, and particulate filter mask) when working with PGM salts as they are hazardous to inhale and on contact with your skin. As an added safety I do not let my PGM salts dry out before I convert them to metals.

In the above photo you see the stannous chloride test swab of the water that was used to rinse the yellow salt into the beaker where it will be reduced to the metal using zinc turnings.

Here's the salt in a 5% solution of HCl with zinc turnings added just seconds before the photo:




and another snapshot only a few minutes later:



after 5 more minutes have passed with lots of stirring and rinsing of the beaker walls:



Note the distinctive 'sickly green' color as Lou so eloquently describes it. This is a sign the salt is beginning to convert to the metal. A little more acid is added to bump fizzing and a few more turnings of zinc are now added.

Less than 15 minutes (and no cloud of ammonium chloride ) later we have the converted platinum metal that settles very rapidly when stirred:



We still have to wait for all of the fizzing to stop and add a splash more HCl to be certain all of the zinc is gone.

I'll filter the solution after dinner.

Steve

Steve,

In the future, I'd recommend just calcining your ammonium hexachloroplatinate--the overall purity will be better and you won't use zinc and you'll generate less aqueous solution. Given the color of that precipitate, chances are that your platinum is quite pure.

To me, zinc is useful for cementing out the values in dilute solutions in a scavenging role.


Bear in mind that zinc works 100% effectively for all the platinum group except for Ir (hydrazine must be used).

Thanks for the advice Lou.

How bad of a hit in purity do you think the zinc will do to the Pt?

I've washed the resulting Pt in more dilute HCl and several hot water rinses after all of the fizzing ceased. I was considering a second round of washes with 20% NaOH. Do you think this is necessary?

I really dislike the clouds of ammonium chloride created when calcining, plus it takes four or five times longer than using zinc. Can you recommend an alternative to zinc or calcining thats as safe and also effective with good purity?

I'm in real need of getting my tube furnace up to speed so I can use hydrogen to convert the salts.

Thanks again for your input,

Steve

Calcination is the best because it is cheapest of all reduction methods and gives a great product texture. I understand what you say about the ammonium chloride smoke--I do it in a tube furnace so the smoke goes to the cooler end of the tube (it can actually be re-used!). Hydrogen is then pumped in to finish the job and then a vacuum and heat cycle to remove any adsorbed H2. This produces a platinum, palladium, or rhodium sponge that has few oxides present and melts without sputtering.

The purity should be good IF your zinc is high purity. If there are other metals present in the zinc powder, then they will manifest in your platinum. The zinc can be practically quantitatively removed from the Pt by using HCl or NaOH and rinsing really well.


There are really only these methods:

hydrazine and its salts (usually a kitchen sink reducer)
alkali/ammonium formates (good selectivity)
borohydrides (great selectivity, but can make various chalcogenides co-mingle)
zinc (poor selectivity)
pyrolysis (no selectivity, but the basemetals will oxidize)
aqueous hydrogen (this works well and with good selectivity because only the platinum metals are catalytic for it, but requires an autoclave and produces a fine product).
base or sulfide and then calcination (note, PdS is intractable as all hell!)

Lou,

That makes me feel better about using zinc as I am using high purity turnings that I make myself from known purity zinc blocks.

The aqueous hydrogen sounds interesting to me. I have recently acquired some autoclaves/furnaces that may fit the bill nicely for this function. Maybe when I get some time I'll look into the method or pick your brain for some pointers if you don't mind.

I also like the idea of using the tube furnace and recycling the ammonium chloride.

Steve

lazersteve said:

Thanks for the advice Lou.

How bad of a hit in purity do you think the zinc will do to the Pt?

I've washed the resulting Pt in more dilute HCl and several hot water rinses after all of the fizzing ceased. I was considering a second round of washes with 20% NaOH. Do you think this is necessary?

I really dislike the clouds of ammonium chloride created when calcining, plus it takes four or five times longer than using zinc. Can you recommend an alternative to zinc or calcining thats as safe and also effective with good purity?

I'm in real need of getting my tube furnace up to speed so I can use hydrogen to convert the salts.

Thanks again for your input,

Steve

Click to expand...

I believe that if you have really pure Pt in solution (after hydrolisis for example, or after scavenging any gold with oxalate and any Pd with DMG), you can skip the ammonium chloride and reduce directly with ammonium formate for high purity Pt. Perhaps Lou can further explain when this method is advisable. 8)

Another method, generally used in analysis, is reduction with H2S, a highly toxic and dangerous gas. I'd avoid this one.

Your photos are beautiful. How much Pt metal is in there approximately?.

H2S, although a reducing agent, usually drops sulfides which must be burned to get the metal.

Ammonium formate gives high purity Pt only if there's only Pt as the sole platinum group metal burden in solution.

I'm not a fan of formate or borohydride reduction because the metals have too much absorbed gas and make porous alloys which are no good to jewelers (they require vacuum melting and degassing).

Furthermore, if melting by torch, the melt loss can easily be up to 8%, with but 2% recoverable.

With sponge, it's rarely more than 2-2.5%, with about 0.5% recoverable.

The handful times I had the opportunity to melt 1-5 oz of Pt (lab-hobbyist scale), I used a discarded zirconia tube (those used for protecting thermocouples) closed at one end, in the manner of a test tube, heated with 2 oxyacetylene torches, protected by refractory bricks.
Kept it at 2,000 C for 15 minutes. Product looked good. I'd like to see an induction melt and pour by one of the big boys. No idea how they get the surface finish that some ingot photos show...

Nothing to see. Mostly you hear a humm. It's too bright to look at and you only pull the ingot at red heat.



Lou

Do they pour it on some ceramic ingot mold or what?. Tilting pour?. Must not be easy to get a smooth ingot.

Look at this pic referenced by goldenchild. The ingot has been through the rolls, and still looks like crap. Unless stamped I've never seen a photo of a nice looking Pt ingot!.

They guy apparently could get it to 99.99 (or so he says), but the cast is what it shows. :shock:

This is a photo of a good for delivery at NYMEX platinum bar that I found online. I guess that's as good as it gets without stamping.

Actually that bar looks struck, flattened before it was numbered.

Cast and drop forged, then stamped. Platinum freezes almost as soon as the heat is taken from it.

It doesn't oxidize. Palladium is another tough metal to cast because it is considerably more gassy.

Vacuum induction melting is the best way to melt these metals.