- Oxalic Gold Reduction

[NoIdea]

Thanks for the oxalate tip, HAuCl4.

Not sure what the reaction is exactly, but it isn't that one. The reaction is with ammonium oxalate which is what you get by raising pH to 6 with ammonia. fwiw.

Now i'm not sure either.
I'm just playing around now with these equations... maybe a chemist could advise?

HAuCl4 + Na2C2O4 = Au + CO2 + HCl + NaCl
HAuCl4 + C2H8N2O4 = Au + CO2 + HCl + NH4Cl

Nice dtectr, i haven't seen this video... i guess the keyword here is "hypnotic" :mrgreen:
It really is....
And you got this effect using SMB? Cool.

Without going into the maths, this would depend on how much of each you add and their end solubilities within your mix. Equillibrium is the solutions goal. I would say a mixture of both, how much each, pass. Someone with a better understanding of solubility products should be able to crunch some numbers for you. Just need the concentration of each of your reactants.

Deano

Hope this helps

 

[NoIdea]

Assuming everything you add reacts would help.

 

[Lou]

That gold is probably over 4N, easily.


Oxalic acid produces the highest quality gold of any precipitant.

Sulfur dioxide (thus sulfite, SMB, dithionite*, etc) drag down both copper and palladium. Copper (I) chloride is an especially common contaminant if the pH rises overmuch. Pd tends to hitch a ride.
It is considerably quicker than oxalic acid, but the quality of gold obtained is not as good. In any case, both are quantitative. Oxalic acid isn't expensive, it's something like $1.25/lb. SO2 is about $0.65/lb.

For what it's worth, it's quicker to dropwise add the gold solution to the hot, well-stirred almost saturated ammonium oxalate solution (make it strong so it buffers at 6.5, as indicator one may use bromocresol green and let it drip until blue-green goes to yellow at which point, add more ammonia). You must rinse more on the other end though.

 

[NoIdea]

That's good to know lou, it's funny how things play in your mind, i was thinking about those eqa's this afternoon, and if the NH4Cl was around it would drop palladium, would it not? Retorical question.

Deano

 

[samuel-a]

thanks for the input.

Thing is, Oxalic is very soluble in water, that we know, i was able to dissolve 80g in 100ml hot water.
Upon addition of NaOH, lots of sodium oxalate dropped. I seperated the oxalate powder from that solution, and got gold to precipitate with the addition of either one of them (aq & solid).

I think i do not fully understand the dynamics yet... will it be more practical to produce sodium/ammonium oxalate and adding it dry (like with SMB) to gold bearing solution with no pH adjustments...?


On other occasion (in the link at the starting post), i added the gold solution to a very dilute and hot oxalic solution where no sodium oxalate had precipitated from. The guys talked about how it looks like a Moral mushroom... thing is, that was only a few drop of gold solution that ware added, it was about an inch big lump and when melted down it weight 0.1g.

That's good to know lou, it's funny how things play in your mind, i was thinking about those eqa's this afternoon, and if the NH4Cl was around it would drop palladium, would it not? Retorical question.

Hi Deano
NH4Cl will not drop Pt at this low presence and dilution (assuming it's a second refining), let alone Pd that needs to be further oxidized with Cl to precipitate.

 

[HAuCl4]

Hi Sam:

re.
Wash gold powder: 3x water, 3x HCL, 3x D. water.
Dry gold powder
Melt

This is one case where one boil in concentrated nitric acid for 15 minutes would do better improving purity than 3 HCL washes. IMO.

 

[samuel-a]

This is one case where one boil in concentrated nitric acid for 15 minutes would do better improving purity than 3 HCL washes. IMO.

Sure HAuCl4. That's a decent suggestion. 8)

One would of course, needs to make sure no more chlorides are left before adding the nitric.

 

[macfixer01]

Very nice. That was neat to watch and reminds me of high school days. There was a local head shop called The Mouse House and besides posters, bongs, and rolling papers, one thing they sold was basically just large pill capsules that contained food coloring and aluminum bronzing powder. You'd dissolve the capsule in a Mason jar filled with water then shake up the jar to cause the aluminum to flow within the colored water. It looked just like in the video.

macfixer01

 

[Noxx]

That gold is probably over 4N, easily.

Oxalic acid produces the highest quality gold of any precipitant.

Sulfur dioxide (thus sulfite, SMB, dithionite*, etc) drag down both copper and palladium. Copper (I) chloride is an especially common contaminant if the pH rises overmuch. Pd tends to hitch a ride.
It is considerably quicker than oxalic acid, but the quality of gold obtained is not as good. In any case, both are quantitative.

Thank you for the input.

 

[samuel-a]

That gold is probably over 4N, easily.

Oxalic acid produces the highest quality gold of any precipitant.

Sulfur dioxide (thus sulfite, SMB, dithionite*, etc) drag down both copper and palladium. Copper (I) chloride is an especially common contaminant if the pH rises overmuch. Pd tends to hitch a ride.
It is considerably quicker than oxalic acid, but the quality of gold obtained is not as good. In any case, both are quantitative.

Thank you for the input.

Yes, i just like to point out how right lou is...

The batch was actually devided in two, one half was precipitated with Oxalic and the other, again with SMB.
Both ware later washed 3x water 3x HCl (1x boiling) 3x d.water.
The melt clearly showed the difference, where SMB drop showed some thin film of oxides upon removal of the flame, the gold the ware precipitated with the Oxalic showed absolutely no crusting of oxides film at the surface.

Either way, the gold is good enough for my buyer and his jewelry casting operation, so i don't know how much will i incorporate the use of Oxalic in my re-refining as it doesn't come cheap for me at the moment.

 

[Lou]

Sam,

You can always leave an excess of ammonium oxalate on the bottom of the reduction container, as more of it is chewed up to CO2, more will go into solution to maintain the concentration.


Lou

 

[Noxx]

Hey Lou,

Why are you using Ammonium Oxalate ? Wouldn't Potassium Oxalate or Oxalic Acid be a lot more soluble ?

Or is it because of the cost ?

 

[nickvc]

Sam
If your gold is going to be alloyed for jewellery so long as the contaminants in your fine gold are either silver or copper it shouldn't matter as both are usually used to make up the alloys anyway.
Check with your customer as to what his alloys are it might save you time and money and not be a problem for your customer so long as he knows the actual assay of your gold.

 

[samuel-a]

Hi nickvc

my buyer don't know the exact assay, as well as i.

he recognize fine gold by the wey it melts. I reckon after 30 somthing years on the job, he knows what to look for... anyway, he is OK even with 999.
So, usually i sell him gold that wes inquarted and refined once.

The last time i was able to xrf, i tested a once refined gold, it came up at above 999 but they wouldn't tell me exactly (for some unknown reason. The test was done for free so i couldn't complain though).

 

[alvaschein]

http://goldrefiningforum.com/phpBB3/viewtopic.php?f=67&t=10432&hilit=+hypnotic#p100818

It remembers me someting like the sci-fi film "Solaris" from Stanislav Lem (surface of the planet).
Very interesting behaviour of the solution (convection current).

Roy