Silver Problem from Aqua Regia Process

dennyhb94

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Hi Guys,

I've been refining gold for a while now using the AR method and my goal is to get 999.9 using the AR method. But I consistently get 999.6 - 999.8 result instead. I know that because I have been sending to external lab (E.g. Metalor & Perth Mint) for analysis and their ICP OES result shows 999.7 consistently with Ag being the major impurities.

For my process:
1) AR dissolution
2) Spray water to the AR solution to reduce the temperature of the AR
3) Filter the AgCl twice using 10 micron and 5 micron filter
4) Adding urea and precipitate using SMB
5) Wash the precipitated gold using Nitric Acid, water, Sodium Thiosulfate, water, Nitric Acid then water again. I use a mechanical agitator to improve the washing process.

I tried refining the gold twice in the AR solution but the result is the same, 999.7 gold.

Please help me on how to improve my process to get 999.9 gold or perhaps there's some steps that I'm missing.
 

Lino1406

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After 1st nitric wash, the accepted way is water; HCl boil; water; ammoniac wash; water
 

Martijn

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Try to cool with ice cubes to below 5 degr. C to push out more AgCl before filtering. Keep one ice cube in the filter.
Then follow the washing steps lino suggested.
https://goldrefiningforum.com/phpBB3/viewtopic.php?f=49&t=325
Martijn.
 

FrugalRefiner

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Once your AR digestion is complete, dilute it with about three times its volume using cold water and/or ice. Concentrated AR can hold a bit of silver in solution. Diluting it will help it precipitate before you filter.

Any silver that makes it through can be removed with ammonia washes.

Better to use HCl washes on the precipitated gold than nitric. If you wash with nitric, then HCl, you will probably create a tiny bit of AR and dissolve a bit of gold.

Dave
 

dennyhb94

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Thank you for the recommendations.

To reply on the suggestions:

1) Diluting & reducing the temperature of the AR before filter. For this, i already did it in step (2) of my process. Perhaps, i didn't dilute / reduce the temperature enough ? Im not too sure about this and have to test further.

2) Using Ammonia for the AgCl that gets through. For this, i use Sodium Thiosulfate. From my understanding, both ammonia and thiosulfate dissolve AgCl.

3) Using HCl instead of Nitric for the wash. For this, i doubt changing from Nitric to HCl will have any effect because my issue is with silver impurities. And HCl do not react with Silver / AgCl.

I have 3 suspicions:
1) Sodium thiosulfate is not very effective at dissolving trace amount AgCl.
2) Some of the AgCl that dissolve in the sodium thiosulfate get re-precipitated. But i'm not sure if it's even possible.
3) I use room temperature water and not Hot / boiling water for the sodium thiosulfate solution.

What do you guys think ?
 

dennyhb94

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or pehaps, using urea may cause the purity of the gold to drop. I'm not too sure about this though.
 

Lino1406

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While thiosulphate decomposes in acidic environment, thiosulphate in alkali can produce silver sulphide occasionally
 

nickvc

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Try rinsing your precipitated powder in hot water and then boil it in water and decant do again until the water is colourless then add HCl and boil for 30-45 minutes decant and boil in water again decant and repeat until water is colourless, this should remove all contaminants including silver.
 

g_axelsson

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dennyhb94 said:
3) Using HCl instead of Nitric for the wash. For this, i doubt changing from Nitric to HCl will have any effect because my issue is with silver impurities. And HCl do not react with Silver / AgCl.
Nitric acid doesn't dissolve silver chloride. Silver chloride is dissolved in solutions with a lot of chloride ions, like strong hydrochloric acid. So boiling hydrochloric acid does dissolve AgCl. Not much but enough to get that last trace.

Listen to Nick, he's an expert.

Göran
 

dennyhb94

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Will the washing procedure produce 999.9 consistently though?
Because my customer require the refined gold to be 999.9 consistently.

I spoke with some refinery and they told me to use electrolysis machine to produce 999.9 consistently.
 

Geo

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Not with a single refine. I've found that to achieve purity above .999 you have to dissolve at least twice and precipitate with two different precipitants. It will need to be refined twice.
 

dennyhb94

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Hi Guys,

After diluting my solution about 2 - 3 times the volume, how long do i then need to wait to ensure most of the AgCl to precipitate out?
 

Geo

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It should turn cloudy as soon as it's diluted. Filter until the solution is clear.
 
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