Oxalic acid process variations

[bswartzwelder]

I have been reading quite a bit on the forum lately about the proper way to precipitate gold when using oxalic acid and am a bit confused.

One method says to mix up oxalic acid in water and heat it to near boiling. Then add some ammonium hydroxide to change the pH of the oxalic acid solution to below 6.0. Then, very slowly, pour the (denoxxed) auric chloride solution (boiling or near boiling) into the oxalic acid (boiling or near boiling) solution.

Samuel-a says to pour the oxalic acid solution into the auric chloride solution. Bring this mixture to a boil then add sodium hydroxide until the solution changes color. Would it be correct to assume that you could use ammonium hydroxide in place of the sodium hydroxide and it should work as well since you are using it to adjust the pH?

Yet, another posting says you can add oxalic acid (powdered or crystalized) to the auric chloride solution. Seems this method would produce less waste to have to get rid of. My assumption here would be the resultant solution should be boiling or near boiling and then add the sodium hydroxide or ammonium hydroxide.

Previously, I was unsure of whether I should add the ammonium hydroxide to the auric chloride solution or the oxalic acid solution.

I purchased ammonium hydroxide at WalMart. The stuff found with the window cleaning supplies. It seems to have a very low (like 3%) ammonium hydroxide, yet I've seen where people suggest using a more concentrated (10% to 30%) ammonium hydroxide. Where do I find the more concentrated stuff?

I really dislike using sodium hydroxide. Last time I did, a couple of drops landed on my sneakers. Two days later, I had holes in my sneakers. Thank heavens I was wearing an apron, gloves and a face shield. All this thinking is making my head hurt. Put me out of my misery and ease the pain. Thanks in advance.

 

[mls26cwru]

If you want to precipitate using oxalic acid, I would advise against using such a dilute amount of ammonium hydroxide... I was using concentrated Sodium hydroxide and it still takes a considerable amount to adjust the ph... remember that you are trying to neutralize two considerably strong acids. I can post the process I used when I have more time, or you can pm me.

Edit: also, someone correct me if i am wrong, but i believe it is not a good idea to add anything ammonia to your stock pot.

Mike

 

[Lou]

It is best to keep ammonia out of gold and silver related processes if at all possible.

 

[dannlee]

Right, wrong or indifferent, here are some hobbyist notes from how I tried out using oxalic for the last small 2 or 3 gram weight lots I reclaimed, found out less than a gram is not worth the extra work just to see the sponge.

I got about ~90% by weight to drop 'immediately' during the oxalic plunge, the sponge coming out as bright blond & clean sponge (see picture) resembling a golden cat-poo that got decanted a few minutes minute after reaction activity had ceased. That washed/dried sponge pictured reduced in size to about 20% of its wet dropped size, the two or three pieces dropped breaking up during washing & handling to remain as seen in attached picture.

The remaining ~10% by weight looked like red/brown barn paint in the beaker, plus an even layer of bright metallic gold plated out on the glass. These dregs includes nearly colloidal silts and fines that take hours to settle plus whatever impurities present, proof how thoroughly one had cleaned the values up to the drop point, but luckily it's in a fine enough form it washes easily when it is full of other metals.

To get that last ~10% reclaimed (washed down the AuCl beaker into still hot but exhausted solution) I dropped with SMB, did the steaming water bath rinses, and required HCl and Nitric rinses to prepare those last dregs to drop w/ SMB and produce gold as blonde as the other 90%.

Have the auric chloride evaporated-condensed down thick but still runny liquid (just above gel) and well de-noxxed. You want it easy to pour without having to wash-down beaker into the drop pot during the main reaction.

I had to use 'clear' 3% household ammonia – I figured since it is so dilute I’d just measure out ammonia as the total volume of liquid I wanted to use and adjust pH with oxalic...

Heat 3% ammonia just to the start of boil in microwave using a tall form beaker and watch glass - then add oxalic while monitoring pH.

Aim for 7.0pH then let it stand a while & reheat, as the solids are dispersed on the molecular level the pH number will drift 2 or 3 numbers. Adjust accordingly, reheat, retest. Get 6.5pH stable after a reheat and it is nearly show time.

Use that same pre-heated tall form beaker and watch glass as precipitation vessel, note the reaction releases heat, you want everything hot but not at or just below boiling or else it can get ugly.

The sponge pictured reacted as violently as seeing a highway safety flare/fusee burn underwater - churned is not a strong enough word, slow explosion comes close. From an inch of solution (100ml) in a six-inch tall-form 500ml beaker it coated nearly half the height with red slime, splashed to the top many places and gold-plated the original inch-high solution height onto the glass (it rubs off).

I used about a 12:1 ratio of liquids, any more oxalic solution and it dilutes the non-dropped remainders, the initial sponge was the goal, I knew the rest for me would need more work.

I know I made extra work, dirtying extra glass and using up a slow filter before neutralizing & discarding the oxalic. Decant the oxalic into another beaker, tip & rinse sponge into a dish and return liquids & silts to the drop beaker, clean/rinse everything into the drop beaker, add a tiny bit of SMB and keep heated until the liquid settles out and turns clear – decant liquid to waste through filter, there will be near colloidal gold captured, rinse and save filter in long term catch-all jar. Now water & dilute acid wash steam the sponge, a touch of hydroxide then heat to dull red & acid-washes on the 10% dregs to prepare them for AR again and a SMB final drop.

Anyhow, I also found too small gold weight or too dilute of an AuCl solution pour will yield a beaker of red ‘barn paint’ that requires great patience to reclaim. I have not done this enough times, have not standardized weights/volumes or controlled temperatures accurately… all I wanted to see was ‘the golden cat-poo’ :twisted:

 

[rrantique]

I have a question. I want to know if "hair & grease drain opener" will work in this Oxalic Acid process. It's Lye in liquid form around 50%. Now here is the reason I ask it also has 3% POTASSIUM HYDROXIDE. This stuff is cheap and already mixed up. NaOH gets very hot when mixed with water and I don't like mixing it so I'm hoping this will work.
Thanks
Michael

 

[MarcoP]

It is best to keep ammonia out of gold and silver related processes if at all possible.

Even Hook advice to use Ammonia to adjust PH before mixing both solutions, but she doesn't mention any proper PH to reach which it should be about 5 but no more then 6. Are you saying we could use something else or are you against the entire oxalic acid procedure.
I'm asking because I'm "just" about to go with oxalic on a gold power precipitated with SMB trying to follow suggested procedures as to use different precipitants for each precipitation.

As far I've learned SMB could drop Cu if used in excess (and I'm pretty sure no Cu was present and I've correctly and patiently used SMB and SnCl2) but keeps PGMs in solution. Oxalic instead will also drop PGMs. Here, I'm only trying to get the hand of it while following the best possible procedures.

Also ammonia is advised to rinse re-refined gold if we believe silver traces are mixed with the gold powder. Sorry, but more I read more I get confused, which is kind normal I think.

 

[butcher]

rrantique,
I have never used oxalic acid to precipitate gold from solution.
oxalic acid is used in the second refining, and the reason to use it is to get a pure gold without some of the other trace metals, If there was a reason I could see to use oxalic acid (which so far I do not), I surely would not use a drain cleaner or "hair & grease drain opener" , to lower pH, when you can get a fairly pure form of sodium hydroxide...

Personally I see no use in using oxalic acid myself (except maybe to see the reaction first hand), as You can use two refining processes with different precipitating agents and get practically the same results in purity, as described in Hoke's book, or which has been discussed several times on the forum...

Ammonium compounds with metals can pose dangers in certain situations, these dangers can even show up in your waste streams if not properly dealt with, Lou gives good advice, why risk it when there are other chemicals that will adjust the pH without introducing a possible danger to your lab.

In the rinse of the refined gold to remove traces of silver chloride with the ammonia wash the waste from this wash solution should be acidified to precipitate any silver chloride it may have dissolved for safety reasons, never store an ammonium solution with silver chloride in solution (silver diamine - chloride complex) promptly acidify the solution to precipitate any silver chloride and always treat your ammonium waste solution separate from other types of waste solution..

There are other times where ammonia, or ammonium compounds is suggested for use, these will normally also include warnings to the dangers of its use, if this is confusing you, I would suggest spending some more time studying about it, so that you understand more and are not confused before working with these solutions or methods.

Sometimes the more we read we can begin to get more confused.
But by studying more, the confusion will normally clear up as we understand more about it.

 

[MarcoP]

As suggested I've searched for a replacement for ammonia and found NaOH and did worked pretty well! On the next batch I will try ferrous sulfate and SMB with HCl+HNO3 and HCl+NaClO, not sure yet about when and why for the different combinations and order, but falling back on Hoke and searching the forums will give me all detailed answers I need.

Most likely off topic, but could I replace ammonia to remove AgCl from gold powder with NaOH and table sugar followed by rinses and a nitric leach cementing it back with copper? This way I could potentially also get a cleaner gold powder. One more step but little safer I think.

 

[g_axelsson]

Most likely off topic, but could I replace ammonia to remove AgCl from gold powder with NaOH and table sugar followed by rinses and a nitric leach cementing it back with copper? This way I could potentially also get a cleaner gold powder. One more step but little safer I think.

Sounds messy compared to the easy step of just washing with ammonia.

First you usually don't get a 100% conversion of silver chloride. Then you need to wash out all the NaOH, sugar and chloride ions before the nitric treatment. If you still have some chloride ions when treating with nitric you will make some AR and dissolve some gold or create new silver chloride unless you incinerate first. And then if you feel you need to continue the HCl wash you need to incinerate again to not dissolve any gold.

At least that's what I think would happen.

Göran

 

[Lou]

Just an FYI, don't use oxalic on solutions that haven't been dropped once or aren't free of copper (copper oxalate precipitates).


Lou

 

[MarcoP]

I see your point, however incineration between HCl and nitric, and the way around, is a must I often forget to detail it but I know it has to be done. Reading a lot about it's importance and highly understand the benefits of it.
In this case, yes, it's way easier and less time consuming to just use ammonia rinsed and immediately acidify it, I'd be better get some for when I need it.

Thank you Göran.

 

[MarcoP]

Just an FYI, don't use oxalic on solutions that haven't been dropped once or aren't free of copper (copper oxalate precipitates).


Lou

It was a very dirty material coming from AP and various mistakes so: incinerated, HCL and water rinsed, AR, cold diluted, sulfuric acid, decanted and filtered, SMB, HCl and water rinsed, incinerated, diluted HCl, AR and finally Oxalic.

SMB precipitate was nice already, Oxalic ppt is settling overnight and tomorrow ill post a second picture into my "my first gold powder" thread under gallery forum.