Reaction stopped when adding nitric

[bpcp7208]

So I have a 1,000mL beaker that I starter off at 900mL. I have a lot of Au in solution. It is all once refined, I melted a little piece and got 18k. So I'm refining it again. I've been adding small amounts of nitric all day. I add a few mL then wait for the reaction to slow to a crawl then add a few more. Can the solution get to a point where it's too pregnant then stop reacting? I have a guesstimate of about 350 +/- grams of Au. I figured I'd dissolve until it wouldn't dissolve anymore then filter and precipitate. Why is it not reacting with adding more nitric?

Correction, it is having a little reaction but not much at all!

Also, how long do I let it boil to reduce the nitric?

 

[butcher]

Without knowing details it is hard to give answers.
What form is the gold in one large piece, or small cut strips hammered flat, or cornflaked pieces? The larger the surface area the slower the attack, more chance of loss of acid from the evolution of reactive gases (that normally do the work of dissolving the gold).

Heating when the reactions slow or seem to come to a stop can normally get the reaction going again as the added energy can get those electrons excited.

Silver in the alloy can slow things down to a crawl, consume or waste acids as fumes, as the crust of silver chloride protects the gold from the acids, sometimes we can overcome the problem somewhat by adding some heat and vigorously stirring, beating the crust loose with a glass stir rod.

If the silver content is too high, making it too difficult to dissolve our gold, then we may need to in quarter the gold by melting with more silver, and then parting the gold from the silver using nitric acid, which also puts the gold in a form that easily dissolves (as powder, or a holy lump like a sponge of gold)

with more surface area exposed, more exposed gold atoms on the surface available to the acids and gases in solution willing to take electrons from the many exposed gold atoms the gold will dissolve much easier than a large chunk of metal or a hunk of gold with a silver chloride salt crust.

As a starting figure or a fair guess, you can figure somewhere around 3.8 ml to 4 ml of HCl and about 0.95 ml to 1 ml of HNO3 for each gram of gold.


So figuring for about 350 grams of gold and 4ml HCl each gram of gold comes out to around 1400ml HCl and somewhere around 350 ml or less of nitric as a starting point as to what may be needed.

Adding pictures or details to your post can give us a better idea of what you can see that we cannot see, the very things that may get you a better answer, if we could see what your seeing...

 

[nickvc]

My simple advice for everyone dissolving gold using AR is to cover the material with HCl and add small additions of nitric only adding more as the reaction stops, if after an addition of nitric you have no reaction add a small amount of HCl and check to see if the reaction restarts, it frequently will as HCl will deplete much quicker than nitric.

 

[Yggdrasil]

Am I missing something or are you trying to dissolve base metals/silver from 18Karat gold with nitric?
This may be impossible, even in AR, that is why one inquart the gold.
How did you arrive to the conclusion that it is 18k?
What other metals are in there?

 

[kurtak]

So I have a 1,000mL beaker that I starter off at 900mL. I have a lot of Au in solution. It is all once refined, I melted a little piece and got 18k. So I'm refining it again. I've been adding small amounts of nitric all day. I add a few mL then wait for the reaction to slow to a crawl then add a few more. Can the solution get to a point where it's too pregnant then stop reacting? I have a guesstimate of about 350 +/- grams of Au. I figured I'd dissolve until it wouldn't dissolve anymore then filter and precipitate. Why is it not reacting with adding more nitric?

Correction, it is having a little reaction but not much at all!

Also, how long do I let it boil to reduce the nitric?

We need much more information :!:

So I have a 1,000mL beaker that I starter off at 900mL.

If you put 900 ml of acid in a 1,000 ml beaker to start out with - you put WAY to much acid in your beaker to start with --- you are taking a BIG chance of having a boil over from the reaction of the acid reacting with the metal

In a 1,000 ml beaker I would start with 500 ml HCl & for sure "no more" then 600 ml

Always figure 4 ml HCl per gram gold --- the amount of nitric you will need is going to depend A LOT on the conditions under which you are dissolving the gold

I have a guesstimate of about 350 +/- grams of Au.

Is all that gold in your beaker ?

if not - how many grams are you trying to dissolve in this batch ?

Is the gold in the forum of precipitated powders - or like foils recovered from pins &/or fingers - or melted & poured into water to make corn flake/shot (do you know what I mean by this ?) - or in larger solid form like melted buttons - like the pics you posted here ----------

:arrow: Why does my gold look like this?

How much nitric did you "start" with & then how much do you add with each addition after reaction stops/slows ?

Are you heating the acid (beaker on a hot plate) ?

Please answer all those questions & we will get you headed in the right direction

And as Nick said - pics would be "very" helpful

Kurt

 

[kurtak]

Am I missing something or are you trying to dissolve base metals/silver from 18Karat gold with nitric?

He is trying to re-refine gold from an original gold drop that came down dirty

when he melted a sample it did not pass a 22K test but passed a 18K test

Kurt

 

[Yggdrasil]

Thanks Kurtak!
I could not see that information in this post, is there another thread going on?
It might be wise to find out what the impurities are, they are some 20-25% after all.

EDIT:
An afterthought, since this post has become what I see riddled by assumptions , at least from my side.
Can you describe excactly what you have done?

Are you now trying to dissolve powders or the melted "18k lump"?
Are you using nitric to dissolve base metals or AR to dissolve all, for a second precipitation?

Can you please elaborate in detail.