Sodium Hydroxide -vs- SMB to Precipitate Gold

[Scott2357]

I read somewhere you can use Sodium Hydroxide (NaOH) to precipitate gold from Auric Chloride (AuCl3) in the form of Au(OH)3.
Anybody ever use this and what might the Pros/Cons be -vs- using SMB?
Does anybody know what form the gold drops out as using SMB?
Is it still Au(OH)3 as with Sodium Hydroxide?

 

[lazersteve]

Scott,

Long ago I processed some gold plated quartz crystals that produced a very clean gold solution. I used sodium hydroxide to precipitate the gold as a black powder very much like a black colored salt. It was easy to collect and wash. The black material was then melted directly with a torch and produced a shiny gold ball.

Aqueous solutions of AuCl3 react with aqueous base such as sodium hydroxide to form a precipitate of Au(OH)3, which will dissolve in excess NaOH to form sodium aurate (NaAuO2). If gently heated, Au(OH)3 decomposes to gold(III) oxide, Au2O3, and then to gold metal.

I'm sure it wasn't 4 9's pure, but it came out pretty clean. The key was that no other metals where in the solution to contaminate the gold.

SMB drops metallic gold from solutions, mixed with drag down metals and copper I chloride if the mother liquor is not clean.

Steve

 

[chemist]

...SMB drops metallic gold from solutions, mixed with drag down metals and copper I chloride if the mother liquor is not clean...
Steve

Is this why nitric is used to remove the base metals prior to AR? If a person processes an inquarted gold alloy directly with AR (not using nitric to remove the base metals), will the SMB precipitation drop the copper along with the gold?

 

[lazersteve]

The copper comes down as copper I chloride, which is not soluble in dilute acids. The dissolved copper is in the form of copper II chloride which is reduced to copper I chloride by the SMB.

This, as well as some other reasons, is why you remove all base metals prior to dissolving your gold in AR or HCl-Cl.

Inquarted gold will not dissolve in AR. The high silver content will react with the chlorine in AR to form a protective layer of silver chloride around the alloy. This layer stops all dissolving action on the alloy.

Steve

 

[chemist]

Steve --- Thanks for explaining that.

 

[Scott2357]

Steve,

So if I understand correctly... SMB drops just gold regardless of any other metals present and NaOH drops all metals?

 

[lazersteve]

Steve,
So if I understand correctly... SMB drops just gold regardless of any other metals present and NaOH drops all metals?

SMB will precipitate some of the PGMs along with your gold if nitric acid remains in the solution.

Sodium hydroxide drops pretty much everything below sodium the activity series.


Steve

 

[butcher]

I was under the impression with sodium hydroxide there could be different metals precipitated by adjusting the PH levels, and that selectivity would be somewhat hard to control, and if Ph is raised even more many metals would dissolve as hydroxides, do I misunderstand this?

 

[lazersteve]

Butcher,

You are correct, but as a general statement, lye will drop all metals below sodium as the pH approaches 14. Adding lye increases the pH.

The pH of the solution will affect when each of the metals fall. As the lye is added, the pH will swing up and the various metals will drop. By controlling the rate of addition of the lye, one may be able to precipitate the various metal in reverse order of their reactivity.

As with any reaction, the conditions of the reaction will dictate how the reactants behave. If other salts or organics are involved, one may get varied results using lye.

My main point is that using lye to precipitate gold is not a good idea, except when your solution is already free of other metals.

Steve

 

[Scott2357]

Thanks everybody, that explains a lot. 8)

 

[Harkey1]

Thank you for the kick in the pants. Short Cuts rarely work.
I have save the solution that resulted from my attempt at nitric acid and silver. i still believe that silver is in there some where.I was unable to filter it properly because the stuff was too thick. I will go and work with it when I have more experience under my belt. If I am reading correctly...My initial question was answered in this post about using nitric. I should not go straight for the AR process and skip the Nitric.
In the solution that I pulled out there was quite abit of gold flakes (small) floating in it. Is it normal? The gold connectors that were in there turned a dull copper color. I have not ruined the Gold have I?

 

[lazersteve]

The gold is fine, but you have not removed all of the base metals as indicated by the copper solids in the beaker.

You must dissolve all of the base metals first with AP or nitric before moving forward.

The best process to learn first is the AP reaction and a batch of fingers. Once you can crawl using this method, you can begin to walk using the AR method. In the meantime, read, read, read!

Don't forget the importance of Hoke and your testing solutions.

Steve

 

[butcher]

Short Cuts rarely work.
people get lost trying to take shortcuts.

I have save the solution that resulted from my attempt at nitric acid and silver. i still believe that silver is in there some where. I was unable to filter it properly because the stuff was too thick.
use copper to cement values

I should not go straight for the AR process and skip the Nitric.
do not skip any steps, thats one way people fall down.

In the solution that I pulled out there was quite a bit of gold flakes (small) floating in it. Is it normal?
this means gold was not dissolved, in a dirty copper and base metal solution this can make seperation of values harder as these flakes can stay in solution with base metals (and if they do not settle), will not be cemented with the values dissolved in solution , when using a metal higher in series like copper to cement values.

The gold connectors that were in there turned a dull copper color. I have not ruined the Gold have I?
no it is not ruined, will take more work to get it back.
the filtering trouble you mentioned makes me expect tin also, here inceneration of cemented values needed.