# Oxalic acid precipitation



## elfixx (Sep 11, 2009)

I tryed to precipitate gold using oxalic acid, is it normal that after 10min at 80°C I saw no signs of gold precipitating?? Seeing it didn't worked I then precipitated using SMB and I noticed the gold that came out was of much better quality then anything I have ever seen. The powder was extremly uniform and very light in color plus it was aglomerating very easily. Could the oxalic acid present in the solution have played a role in precipitating higher quality gold?


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## 4metals (Sep 11, 2009)

Oxalic acid needs to be close to boiling when added for it to drop quickly, all of the nitric has to be gone, and it takes an ounce of oxalic acid to drop an ounce of gold. Plus it works better if you've added a little (5ml/oz) ammonium hydroxide to the oxalic solution. If you've done it all right, the gold comes right down. Your temperature at 80C was a little cool. 

Anyway the oxalic must have had something to do with the precipitating process as the physical description of the gold you gave is much like a successful oxalic drop, and not like a SMB drop.


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## lazersteve (Sep 11, 2009)

I use dry SMB pretty much exclusively to precipitate gold on a daily basis and as long as the solution is free of base metals the gold always precipitates as you described.

Steve


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## 4metals (Sep 11, 2009)

Elfixx
Was your gold inquarted first (as Steve's example) or was it karat gold with silver and base metals. 
Base metals make SMB drops a darker brown.

Oxalic acid works best from a solution containing only traces of impurities, which is why it is used to make proof gold.


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## dick b (Sep 11, 2009)

4metals said:


> Elfixx
> Was your gold inquarted first (as Steve's example) or was it karat gold with silver and base metals.
> Base metals make SMB drops a darker brown.
> 
> Oxalic acid works best from a solution containing only traces of impurities, which is why it is used to make proof gold.


4metals:
Does Oxalix acid have less drag down that using SMB, or is necessary to refine twice to remove impurities to use Oxaclic acid to get finer quality in your finish gold?

As opposed to refine twice, HCL-CL and then AR, using SMB to participate both times. Looking to achieve 3n5 or better.

Thanks in advance.
dickb


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## 4metals (Sep 11, 2009)

Dick,
Are you refining karat gold? Most contaminants come from poor rinsing, if you use a small aspirator, a small buchner funnel, and a vacuum flask you can filter your precipitated sponge and rinse it well. You shouldn't have to drop twice. 

A few tricks, if you inquart or not, dilute your aqua regia with cold water and add ice to it. The solubility of silver chloride drops way off with temperature. Cold is good. Then filter it while it is cold. This will remove the biggest contaminant, silver. 

You can get an aspirator from Shor International (hope I don't get struck by lightening for saying that!) for 10 bucks. 

Oxalic acid does drop gold of high purity but it is expensive, has to be used boiling hot, and it won't solve your issues if your filtration is inadequate. If you inquart, nitric dissolve, digest in AR, chill and filter well, and drop with SMB, and filter and rinse well again, your gold will be better.

If you do this and your gold still is impure, I'll go through it with you step by excruciating step until we find the issue.


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## elfixx (Sep 11, 2009)

Each time I refine gold I first inquart with silver, disolve base metal in nitric and then go trought the AR process twice. This week I wanted to try oxalic acid to precipitate the gold on the second AR refining. Since it didn't droped a single flake after 10min of close to boil temp I just cooled and dropped with SMB but the gold came out differently and better than usual. I wish I had pics of it... it didn't looked like typical SMB dropped mud it looked much more like a sponge. Next time I am going to refine I'm going to try the same thing again and see if I can get the same result and I'll post pics of it.


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## dick b (Sep 12, 2009)

Hello again:

I'm still confused with this statement. " which is why it is used to make proof gold." The main purpose of refining is to get the best product possible. .9999 fine. With the different choices to use to precipate. SO2. sodium sulfite, SMB, oxalic acid and others, why would oxalic acid give better results removing impurities that the others?

I guess I wasn't clear about what I was asking in the last post. I've read Harolds posts on getting fine gold product and understand the need to eliminate base metal impurities first. But why would oxalic acid be better than SMB or SO2 to get more pure gold. All I could figure was that there was less drag down when using it. I can see where using sulphur dioxide gas would not add impurities to the solution, but what does oxalic acid bring to the process that is giving better results than SMB.

I'm still not sure if I'm clear about the question that I'm asking, but the quote I posted is what is confusing me.

I always appreciate the help offered here and thanks in advance.

dickb


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## lazersteve (Sep 12, 2009)

Dick,

Oxalic acid and it's salts (e.g.: sodium or potassium oxalate) are more *selective* reducing agents towards gold because oxalic acid reduces pretty much exclusively gold chloride solutions ( with a few exceptions ). Of course drag done can still occur, so removal of the base metals is still required for greatest purity.

The SO2 based reducing agents can also react with copper II chloride and some nitrates of PGMs if the solution is not properly denoxxed. Ferrous Sulfate can also reduce trace nitrates if they are not properly converted to chlorides before precipitation.

The more reactive reducing agents like hydrazine are pretty much non-selective and reduce all metals below a certain level in the activity series.

Solid metal cementation displaces all metals below the displacing metal (ie copper) in the activity series. So copper would reduce all precious metals together.

Steve


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## 4metals (Sep 12, 2009)

Oxalic acid is used to make proof gold because it will not bring down any platinum. Platinum chloride is not reduced by oxalic acid. It may bring down traces of palladium but they can be removed by a nitric boil. 

It's important to realize here that we're talking minuscule amounts of contaminants which are dragged down by most precipitants, most can be rinsed away with good filtration and rinsing. The assayers love affair with oxalic acid is because it will leave the platinum behind, platinum is one of the contaminants that is particularly hard to drop completely so it can linger as a contaminant, oxalic solves that.


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## dick b (Sep 12, 2009)

Thanks to lazersteve, elfixx and 4metals:

Now I can see what is being said about the value in using oxalic acid for the second refining.

I figured that the remaining impurities were base metals but thought the PMG's were being removed by inquarting with silver. It now seems like what is left after redisolving the gold for the second time is a small trace of all of the impurities that were originally alloyed with the gold and the dragdown with oxalic acid is less than with the other precipitates. 

I hope I'm not still missing the point and really appreciate your efforts to clarify the reason for using oxalic acid as a precipitate.

Thanks to all!
dickb


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## Harold_V (Sep 13, 2009)

dick b said:


> I figured that the remaining impurities were base metals but thought the PMG's were being removed by inquarting with silver.


I processed a large amount of dental waste in my years, all by inquartation. While a preponderance of the platinum and palladium were eliminated in the nitric digest, along with silver, not all of it was. It always reported in my solution after precipitation of gold. 

Harold


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## dick b (Sep 14, 2009)

Harold-V:

Thanks for your input. Your instructions on refining and the help you offer has been a great help to me in understanding the process of refining. My hat goes off to you, lazersteve, Chris. Lou, 4metals and the others that so generously take the time to help those of us who want to learn how to refine as a hobby. The picture that you often post showing the cornflake gold is the goal I set as the standard to achieve. Right now I'm working on distilling nitric and building a safe lab and hood to use. I hope to post some pictures and of my efforts to help others with the same problems that I have.

I have Steves videos in karat gold and AP, and have been successful in running trial batches and have some nice powder. But with all of the reading and absorbing the lessons learned by your experience, it leaves plenty of questions still needing answers. 

I just wanted to let you guys know that I respect your teachings and someday I hope to achieve the same results that come so easily to your efforts. After all, experience is the best teacher.

Sincere thanks to all.

Dick b


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## Harold_V (Sep 15, 2009)

Thanks, Dick b., from all of us!

Harold

P.S.---

I'm a bit partial to that picture, too!


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## markqf1 (Sep 15, 2009)

Ok, If I got this right, oxalic is used to prevent pt contamination of the end product.
I assume that most everything but pt, could be washed out with Harold's excellent wash procedures.

I wish my biggest problem was pt contamination of my gold. :lol: 

Mark


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## Lou (Oct 5, 2009)

It's mainly palladium contamination, rather than platinum contamination that the use of the ammonia-oxalic acid system defeats.


Hate to say it, but I've covered this all before:
http://goldrefiningforum.com/phpBB3/viewtopic.php?f=39&t=3079


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## nickton (Jan 21, 2017)

okay here it goes:

I am still new to all this , and have been reading Hoke's book and this forum for a while, and I hope I don't waste anyone's time here, but I do have a few questions to ask. First of all, I managed to find some Oxalic acid in powder form, for woodworking, and payed about 15 dollars for it, which is around 7 dollars more than I would pay for a container of Stump out at Ace hardware. This does not seem significantly more expensive like the way Nitric is more expensive than Muriatic for instance, so I wonder why authorities claim it is so costly. Perhaps it is because one needs to use so much more of it, but I don't know.
This leads to my second question: How much Oxalic powder do I mix in with water before I boil it, and then adjust the PH level to 5 (as recommended)? I can't seem to find an answer to this, other than one ounce of gold requires one ounce of oxalic (?)..(hope I got that right), and I don't know how much gold I plan to precipitate.
Now my third question is: Can you use Oxalic after putting gold into solution using bleach and Hydrochloric acid? All the demonstrations seem to refer only to previous Nitric acid use. Does one have to evaporate the bleach out, or otherwise neutralize it in some way? ( I doubt it but decided I'd add the train thought anyway. :mrgreen: )

My efforts have been a dismal failures so far, but I am more hopeful after much reading and trial and error. At least I can actually read sections of the Hoke book without falling asleep anymore. 

thanks for the great posts by the way.


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## rickbb (Jan 24, 2017)

nickton said:


> okay here it goes:
> 
> 
> Now my third question is: Can you use Oxalic after putting gold into solution using bleach and Hydrochloric acid? All the demonstrations seem to refer only to previous Nitric acid use. Does one have to evaporate the bleach out, or otherwise neutralize it in some way? ( I doubt it but decided I'd add the train thought anyway. :mrgreen: )
> ...



No. After you have gold in solution with HCL/ bleach, use low heat to vaporize off any remaining chlorine gas, or put out in the sun for a few hours. Then drop your gold with SMB.


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## nickton (Jan 26, 2017)

aha.

thanks for the reply. That definitely clears up a couple of things.


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