Oxalic acid

[sayf]

its my first time using oxalic acid to precipitate gold
I firstly denoxed nitric acid , then i added oxalic acid powder to the solution (my solution contains copper in addition to gold)
Then i started adding NaOH to raise the PH and precipitation started directly , i kept adding the naoh until the color of the solution turned to brown and PH reached 4
But the amount of the precipitate after filtering it is very huge about 100 grams and there is a white-blue precipitate at the bottom of the beaker (i expect it is oxalic acid)
Isnt the oxalic acid sellective for gold?
How can i refine this precipitate?
I attached a photo for the precipitate

 

[Martijn]

Precipitates at Ph 4 are probably hydroxides. Mainly iron.
Filter and add a bit of hcl to dissolve the hydroxides.
Why raise to ph 4?

 

[Lino1406]

Oxalic acid is used for 2nd stage refining, hence the starting point should be almost pure gold solution

 

[sayf]

Here is what happened when i added hcl to the precipitate , every thing has changed into green color
i dont know what to do next honestly

 

[FrugalRefiner]

I'll say it again; when in doubt, cement it out.

I've never used oxalic acid, but I'm not sure you understand what's been discussed here.

Let's start with "i added oxalic acid powder to the solution". From what I've read, oxalic acid is used fully dissolved and at close to boiling temperature.

"Then i started adding NaOH to raise the PH and precipitation started directly , i kept adding the naoh until the color of the solution turned to brown and PH reached 4" Why? Again, I've never used it, but I don't recall recommendations to raise the pH with NaOH. Where did you find this procedure?

Dave

 

[sayf]

Many in the forum uses ammonia to increase the PH but i watched a video for owl tech using potassium hydroxide so i think sodium hydroxide is also fine
And yeah the solution was almost boiling when i added the oxalic and sodium hydroxide to it
Yeah i increased the ph to 4 and i think that was my mistake, it shouldnt exceed 2 or 3
And my other mustake that my aqua regia solution had other metals dissolve beside gold
And as i read that oxal8c is used with second drop
But i read a chart for lazer steve indicating that ixalic acid is sellective for gold precipitation
So if it is sellective then my first conclusion was to use it in a AR solution that contains other metals beside gold!

 

[Lino1406]

Oxalic acid redox potential 0.49v, implies it can reduce monovalent copper although it is close

 

[Geo]

The solution was much too contaminated for oxalic acid precipitation. As Lino said, oxalic acid is used when needing ultra-pure gold. SMB would have been okay and ferrous sulfate would have been better. Then you redissolve the resulting dropped gold powder. That solution would work for oxalic acid precipitation because the powder was relatively pure at the beginning. Also, using HCl and H2O2 or chlorine to dissolve the dropped gold powder gives the added benefit of not having to destroy excess nitric acid. Simply heat to drive off excess chlorine or oxygen whichever the case may be.