Precipitation Questions

[warrendya]

I watched Steve's video on processing fingers in AP. He comments early on after putting the fingers in that some of the gold is going into the AP. His General Reactions list states that gold can be precipitated from AP with SMB. I watched the Using Sodium Metabiulfite video, would everything be the same using it with AP as with HCL-CL?

In the SMB video, Steve notes he has diluted the Au-CL 3x with H2O. I've read so many posts in the last week they're starting to blur together, but I recall someone (think it was Harold) stating that the solutions should be as concentrated as possible before precipitation. Is this a disconnect? That post may have been about AR, though, I don't know if that makes any difference.

Thanks!

 

[Harold_V]

A concentrated solution is mandatory for platinum and palladium, but gold will precipitate nicely from dilute solutions, or concentrated solutions. I worked with highly concentrated solutions as a matter of need.

One of the purposes of diluting gold chloride is for the solution to self precipitate traces of silver, which will hitch along with gold, and not precipitate completely. That's in spite of the fact that HCl is a part of the solution. Simply diluting the gold chloride adequately yields the desired results. It's a well documented phenomenon.

The problem is how much is enough? Three volumes has been commonly suggested, but that may, or may not, be enough. If the solution has been evaporated, it may well be down to only a few ml already. The "three times volume" formula actually has no value as a result.

The other reason to precipitate somewhat dilute (3 ounces/liter or less) is that the reaction is exothermic. When you precipitate large amounts of gold from small volumes, the solution gets too hot for comfortable handling, and may even stop precipitating. My experience comes from precipitating no less than 17 troy ounces from 1 liter (or less) of solution, plus adding 3 liters of ice. SO2 gas was the reagent. The end result is hot water, often too hot to handle.

Harold

 

[warrendya]

Thanks for the info. If by dilute you mean 3 troy ounces of gold per liter or less, I don't think I'll ever have to worry about having too concentrated a solution!

 

[Bernie Foley]

Hi Harold,
and friends,
Now i can see why you do not like the word dropped for precipitation!!!
One would not want to drop that beaker of auric chloride!!!!!!!!!!!
WOW!...........Bernie Foley.....Have a great Day!

 

[markqf1]

Harold,
Seventeen troy ounces per liter!?
Wouldn't one troy ounce per liter of pgm be great?
Of course, gold is a different animal all together.

Mark

 

[Harold_V]

Harold,
Seventeen troy ounces per liter!?
Wouldn't one troy ounce per liter of pgm be great?
Of course, gold is a different animal all together.

Mark

Given the choice, I wouldn't even consider an ounce per liter with the pt group. I'd have it in no more than 300ml given a choice, if even that. Frankly, I see no reason for diluting such solutions.

I have no way of knowing at what point it begins to precipitate less than totally when it is diluted, but the one real good experience I had was when I precipitated several ounces of platinum from about 1.5 liters. The reaction was instantaneous and virtually complete. It's the one and only time I can recall every achieving what I considered a good recovery on platinum. I also recall numerous attempts that yielded nothing because the solution was too diluted.

I concluded that concentration is ultra critical. I think Lou would agree.

Funny-----Hoke alludes to concentrated solutions for the pt. group metals. Could it be she was right? :roll:

Harold

 

[Lou]

The more concentrated the better! Want to know what I do?

I do the reaction at about 100F/39C. It will precipitate and begin to settle. Then I put it in the chiller and let it cool til it's nigh on 32F/0C. Guess what happens? It all sits on the bottom and it is clear. If I'm feeling extra paranoid or I'm doing a lot of Pt and the precipitate volume doesn't look ''right'', then I pull out a sample of that solution, boil it down in a test tube to a few milliliters in volume, cool it and seed it with a bit of my ammonium hexachloroplatinate precipitate. If anything shows up, well, I need to do another precipitation.

For every ounce of Pt I am expecting, I want about 150mL solution. This is because H2PtCl6, that nice red-orange money solution, is very soluble in HCl. To that warm solution is what you add the saturated ammonium chloride (you can boil water and add the ammonium chloride until saturated, then add it to your big beaker). Then you filter this using a wonderfully nice filtration set up--a fritted glass filter that separates into two pieces! You lay your filter paper upon the frit, put the top graduated ''funnel'' part on top, clamp it, put it in your filtering flask, attach vacuum hose, pour in solution, and you're good to go!

Of course, I only make that beautiful canary yellow (NH4)2PtCl6 if and only if I don't have any hydrazine. If I have N2H4, guess what happens? I add my H2PtCl6 to conc. ammonia, make the diammine complex and then precipitate with hydrazine. Works like a charm. However, you can't beat the ammonium hexachloroplatinate (or palladium salt) for cheapness of reduction...you just heat it!

Hope this helps!

 

[cerise]

He does the foils nicely in the AP video but do i do the chips the same way ?

 

[cerise]

I am just starting this whole process,I am so confused and im sorry if im not up to par but could someone tell me how to precip And what is smb?
Thanks

 

[lazersteve]

Cersie,

You can find the answers to many basic questions (like what is SMB), by following the Guided Tour Link in my signature line. There are four links there that will teach you the basics. Sounds like you are interested in the General Reactions list link found in the Guided Tour.

Welcome to the forum,

Steve

 

[cerise]

Thanks Steve
This is the best place i have seen yet
Thanks so much.
Lou When I heat my HCl to boil my honeycombs do I also use nitric.I dont have a mixer yet so i have been putting the whole honeycomb in and I stir it gently every so often.I have been using AR and precipitating with aluminum foil. So far i have a little grey mud .I was just wondering if I should change anything.
Thanks
Cerise

 

[cerise]

I watched Steve's video on processing fingers in AP. He comments early on after putting the fingers in that some of the gold is going into the AP. His General Reactions list states that gold can be precipitated from AP with SMB. I watched the Using Sodium Metabiulfite video, would everything be the same using it with AP as with HCL-CL?

.

Thanks!

No one answered this part ?

 

[lazersteve]

Here's the answer again:

AP and SMB

Steve

 

[goldsilverpro]

Originally, I was the one who suggested adding 3 parts water to 1 part aqua regia, in a gold solution, in order to precipitate most of the silver as silver chloride. The silver chloride can then be filtered out. About 40 years ago, I noticed that the literature gave contradictory ratios for this precipitation, so, I did some experiments and plotted the results, in order to discover the truth. As memory serves me, I think that the concentrated acid held about 50 times more AgCl, in solution, than the 25% acid. With further dilution, the dissolution of AgCl leveled out. In other words, a 20% acid solution held about the same amount of silver in solution as a 25% solution. Therefore, further dilution serves no purpose.

The reason this dilution of the aqua regia is important is that, when you precipitate and rinse the gold, you dilute the aqua regia during the process. When you do this, you contaminate the gold with silver chloride. You solve this problem by diluting and filtering out the silver, before dropping the gold.

 

[Lou]

You would think that the more HCl that is in solution (meaning more Cl-) there would be a tendency keep AgCl out of solution. That is not the case. Instead, it is the high concentration of chloride anion that dissolves the AgCl. It is similar to how Ag2O which is also quite low in solubility, will dissolve in the presence of excess OH- forming Ag(OH)2 complex.

If the solubility product is an equilibrium controlled system (think of it as a bound for what can go into solution).
Expressed as such for silver chloride:
Ksp = [Ag+] × [Cl−]
Ksp = 1.8 × 10^−10
which is quite insoluble.
The common ion and salt effect of an aqua regia solution containing lots of chloride should have AgCl dropping out. It doesn't because it complexes to AgCl2- which is highly soluble, but only with lots of chloride present!

I'm glad to learn from Chris about his experience in solubility versus dilution.

Lou

 

[Harold_V]

That phenomenon manifested itself with the gold I'd refine a second time. It was for that reason that I followed up my washes with ammonium hydroxide.

I precipitated from extremely concentrated solutions when I re-refined. It wasn't uncommon to have as much as 18 ounces in a liter. Because of the low volume of solution, it couldn't absorb the heat that was released upon precipitation, so I added nearly three liters of ice before precipitation. It wasn't uncommon to see a slight cloud form when the ice was added. I attribute that to the concentrated solution precipitating miniscule traces of silver chloride. The ice in question was made by an ice machine, not in a freezer of a refrigerator. Ice so made rejects impurities as it forms, yielding clear ice.

I fully agree with the dilution of solutions, assuming it doesn't present problems. For my operation, processing dilute solutions would have been a disaster----I had more work to do than I could accomplish. Increasing volumes by dilution wasn't in the cards for me, not only because of longer filtration times, but also because of the increased space required for processing. As it was, I operated at maximum capacity almost daily---so larger beakers would not have worked. When one has options, precipitating from dilute gold chloride solutions is not a bad idea.

Harold

 

[goldsilverpro]

Harold,

I know that you abhor the use of plastic buckets, but that's what I always used to precipitate gold in. Therefore, a larger volume caused no problems. You're right. I always got a layer of gold powder that stuck to the bucket, especially in the scratches. I solved this problem by scrubbing the bucket and loosening the gold with a little water and about a 1/2" square of a green ScotchBrite pad. The tiny bit that remained was picked up the next time. I had 3 or 4 very clean buckets that were only used for pure gold. I think the 3/1 dilution procedure helped enable me to get pure gold the first time, without having to redo the AR (usually).

 

[Irons]

Harold,

I know that you abhor the use of plastic buckets, but that's what I always used to precipitate gold in. Therefore, a larger volume caused no problems. You're right. I always got a layer of gold powder that stuck to the bucket, especially in the scratches. I solved this problem by scrubbing the bucket and loosening the gold with a little water and about a 1/2" square of a green ScotchBrite pad. The tiny bit that remained was picked up the next time. I had 3 or 4 very clean buckets that were only used for pure gold. I think the 3/1 dilution procedure helped enable me to get pure gold the first time, without having to redo the AR (usually).

The small amount of Gold remaining in the scratches provide nucleation sites for the next batch to begin precipitating, so I don't think a little residue is a problem.