# Never Recovered 100% gold through Aqua Regia



## rohaneha (Nov 2, 2010)

Hi... i run a gold casting unit. i generally have scraps of 916purity of gold of approx 350gms (i.e. 322gms of 999purity) of scrap (mostly Filing scrap) which i try to refine once in a month. I add 900ml of Hcl and 300ml of nitric acid. i run the process for 4 hrs. in between i heat the aqua regia twice beforing adding urea in it. and then i filter the solution. After filtering i add 1500 ml of reducing agent i.e. hydrazine hydrate acid(previously mixed with water). i keep the solution overnight to settle the gold. the next day i recover the gold(i.e. the muddy substance) by heating it in a crucible. but i fell short of recovering the fineness of the gold by around 15gms. I tried many things but could not ovrcome this problem. is my technique correct, or have i missed something.


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## Anonymous (Nov 2, 2010)

I never use premade AR.I use enough Hcl to digest the amount of material I have,then add small amounts of nitric until the material is comepletly dissolved.I don't believe it has ever been has high as 3-1(900ml-300ml).
Basic AR is 4-1 and mine usually never gets that high.Is it possible you were off on your measurements or calculations?I doubt you have excess nitric,if you did it would simply redissolve your powder.However if that is the case,a simple stannous test would show it.If it does come back positive,simply add more precipitant.


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## Anonymous (Nov 2, 2010)

I forgot to ask,why are you using hydrazine to precipitate?That is an expensive ,unstable precipitant.Can you not get copperas in india?


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## goldenchild (Nov 2, 2010)

First and foremost mic is right about testing with the stannous chloride. This way you will be sure there isnt still gold remaining is solution. The 900 to 300ml ratio of HCL to nitric sounds ok as AR can also be 3:1 but mic is also right about premixing it. You should just add the HCL and add the nitric incrementally. You may find that you dont even need the entire 900ml of HCL if the reaction is well controlled. If you test and find that there is no gold left in solution and are coming up short on your yeild it must simply be that your beginning estimate of .916 purity is incorrect. You should also notice that 350 grams of .916 purity gold is only 320.6 grams of 24k and not 322. The fact that most jewelry isn't plumb could also be a factor here.


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## qst42know (Nov 2, 2010)

Filings are notoriously dirty, from sandpaper grit, casting plaster residue, pieces of files or saw blades, and some of anything else you can think of. 

Harold has dealt with a bunch of this type of scrap if I needed a process to efficiently deal with it I would ask him. 8)


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## Anonymous (Nov 2, 2010)

goldenchild said:


> The fact that most jewelry isn't plumb could also be a factor here.


So true,I forgot about that.The U.S. government allows manufacturers to be (up to) 1/2 karat off on their hallmarks.And I assure you every single one of them takes advantage of that.18k=17.5k.....14k=13.5k...and so on.Good point goldenchild.


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## Harold_V (Nov 3, 2010)

I tend to differ where the ratio of HCl to nitric is concerned. Knowing that an excess of HCl does no harm, I'd be inclined to use a slightly greater ratio. Reason being, if adding a little nitric does nothing, it can be a sign of exhaustion of the HCl that was added in the beginning. It was my policy to use a 4:1 ratio for my AR. It's more than obvious that the correct ratio is unclear, and has been disputed. Concentration of each is likely one of the reasons. 

Bottom line--always use a little more HCl than you may think is required. It does no harm, and insures success. 

Harold


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## rohaneha (Nov 3, 2010)

hey thanx mic for ur information. I have read a lot of this stannous test, but never tried it. can u tell me how to do this test? Secondly, after precipitation in hydrazine hydrate, the next day when i try to recover the gold from the solution, i see my solution murky and not clear. is there any fault in the proportion of hydrazine hydrate. do u know how much of exact hydra hydrate should be added to AR to get the proper results.


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## rohaneha (Nov 3, 2010)

mic said:


> I forgot to ask,why are you using hydrazine to precipitate?That is an expensive ,unstable precipitant.Can you not get copperas in india?
> 
> copperas? whats that?
> is it a product name or any acid?


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## qst42know (Nov 3, 2010)

Copperas = Ferrous Sulfate FeSO4

This link for stannous testing is from lazersteves signature line.

http://goldrefiningforum.com/phpBB3/viewtopic.php?f=41&t=5271&p=45030#p45030


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## Lou (Nov 3, 2010)

You need to digest all of it in aqua regia, then filter the solution cold. After that, dilute with water and add the hydrazine hydrate (it'll produce much gas). It should go murky in front of you and drop within 2 hours at the top end. 

Alternatively, use sodium metabisulfite or sulfur dioxide. Safer than hydrazine and certainly cheaper.

Lou


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## qst42know (Nov 3, 2010)

> Aqua regia is a molar ratio of 1:3 nitric acid : hydrochloric acid. Equivalent to a volumetric ratio of 1:4.



I expect this may be at the root of the ratio debate. There are tons of references to AR being a 3:1 ratio, "molar ratio". 

For simplicity most measure by volume don't they? Has anyone calculated the volumetric ratio for the acid concentrations they use?


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