# Dilute Aqua Regia and SMB



## DawgDen18 (Nov 26, 2018)

Hi, I'm new to the forum. I've been all over the internet searching for a process similar to mine, with no luck.

I have the following solution:
2 gallons [4:4:9 mix ratio] 37% HCl (SEMI grade): 68% HNO3 (SEMI grade): DI water

and in that solution I have dissolved:
approximately 45 grams 99.9% gold 
~100 PPM 99.999% copper
Trace PPM Ti 99.999%

The solution is a pale aqua blue-green color.

I have then added 10 g SMB incrementally, and the solution became dark, then became a very brilliant orange red color with no precipitant. I understand that at this point no matter how much SMB I add, the solution will not produce a precipitant. In addition, I have the thought that the excess of nitric acid in solution is causing re dissolution of my Au into auric chloride. 

I am planning to use 45% KOH to react with HNO3 in order to isolate the auric chloride. Am I missing something obvious or does this seem like a good plan?


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## FrugalRefiner (Nov 26, 2018)

You used way too much acid, and way,way too much nitric. The best practice is to use as much HCl as you think you'll need, then add nitric in increments, using just enough to dissolve the gold. You could have dissolved that much gold in a few ounces of acid.

The best option would be to use the acid to dissolve more gold, but unless you have a lot more gold to dissolve, you're beyond that.

Adding KOH, or anything else, is going to leave you with a lot of very salty solution.

Cementing on copper is a good way to recover from a mess, but you'll chew through pounds of copper.

As much of a pain in the rear as it may be, I'd consider evaporating it down in the classic method.

Dave


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## g_axelsson (Nov 26, 2018)

DawgDen18 said:


> In addition, I have the thought that the excess of nitric acid in solution is causing re dissolution of my Au into auric chloride.



Welcome to the forum!

Yes, you are correct. Look up denoxing and you will have your solution there. There are several ways to do it.
http://goldrefiningwiki.com/mediawiki/index.php/Denoxing

To neutralize the nitric acid with a base doesn't remove the nitrate. SMB doesn't work (my experience) in a neutral solution, it has to be acidic and when you get a acidic solution any nitrate salt will turn into nitric acid again.

Newbies often makes the mistake of hugely overdose the nitric acid. Usually you start with an excess of HCl and then add nitric acid bit by bit, so all the nitric is used up. The rule of thumb is to calculate 1 ml nitric acid and 4 or more ml HCl per gram of gold. Base metals will take different amounts. Then never start with more than half of the nitric, only add more if/when the reaction dies down and in small increments. It saves on acid, reduces the amount of waste and simplifies precipitating the gold in the end. I've managed to get down to 0.7 ml nitric acid/g of gold.

This is what I recommend, but there are many ways to get to the same result and every way is described somewhere on the forum but you have to search for it. :wink: 

Göran


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## DawgDen18 (Nov 27, 2018)

Hi and thanks for the reply. 

The reason that the acid is in this specific concentration is that it gives a much more controllable etch during process. 

The purpose of the process isn't to create a pregnant solution, it's a semiconductor process step. The solution is just a byproduct and I'm interested in converting it to a precipitate. If I can't do it with SMB I'll most likely just discard the waste and move on from trying to recover it. 

Am hoping for a very simple process since I don't have much time to commit.

Thanks for the link I'll look into it and see if I have any allowed chemicals in the lab that would work.


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## anachronism (Nov 27, 2018)

Nearly 45g of gold is certainly worth chasing if I read your initial post correctly.

Take your solution. Split it up into smaller batches, in vessels with plenty of room for foam.

The chemical you need is Sulfamic acid. 

Start cold because you have a LOT of Nitric to get rid of. Adding a very small amount of Sulfamic will make it foam and gas off NOX so do it in a cabinet.

You're going to need a lot of Sulfamic, and do it in stages avoiding foam overs.

The other way is to heat the solution until it starts gassing NOX brown fumes. Use a watchglass over the top- again in smaller batches. Over time the NOX will reduce and most of your Nitric will be gone. Then you can use Sulfamic to get rid of the rest. 

Only when all your Nitric is gone can you use a precipitant to any great effect. 

Having read this back I would suggest the second route over the first. 8) 8) 

Jon


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## g_axelsson (Nov 27, 2018)

That explains the extremely pure contaminants. :lol: 

An easy way to denox is to use sulfamic acid, just take it slow or it will foam over. Btw, use a catch pan under your beaker in case you have a foam over or the beaker breaks.

Göran


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## DawgDen18 (Nov 27, 2018)

All,

thanks for all your help.  

I have the amidosulfuric (sulfamic) acid in a ~90% assay. I'll work on using that to neutralize the bath.

I don't have approval to heat my solution and drive off NOx gases. We have a neutralization plant for chemical waste so I'm not going to worry about excess waste. 

Sorry if this thread isn't useful to the majority of people on the board. Hopefully some other MEMS labs are out there trying to make the most of their process.


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## anachronism (Nov 27, 2018)

DawgDen18 said:


> I don't have approval to heat my solution and drive off NOx gases. We have a neutralization plant for chemical waste so I'm not going to worry about excess waste.



Whether you have approval or not, you'll be doing that by using the Sulfamic acid. It will produce a lot of NOX and get hot anyway.


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## DawgDen18 (Nov 27, 2018)

Yeah, someone might want to check my math here but for the standard reaction

HNO3 + H3NSO3 --> H2SO4 + N2O + H2O
we have heat of formation DeltaH (kJ/mol)
-186.81, +16.42 | -811.3, +82.05, -285.8
for a final total reaction enthalpy of -844.7 kJ/mol (exothermic)

So yeah this looks like a strongly exothermic reaction.


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## nickvc (Nov 28, 2018)

Although now frowned upon on the forum another solution may be to use urea, again adding it in small amounts to avoid boil overs and use several beakers at most half full of the solution.


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## snoman701 (Nov 28, 2018)

That would take 3 lbs of sulfamic acid. It's an endothermic reaction and it is best done hot. I prefer to denox at 90 deg C. 

In addition, the addition of sulfamic very well could complicate waste treatment as it will chelate the copper in solution.

Just add a good piece of copper busbar. Even better would be atomized copper powder. 


Sent from my iPhone using Tapatalk


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## Lino1406 (Nov 28, 2018)

Sulphamic acid enthalpy of formation is much closer to sulfuric acid, namely around 700-800 kj/mol


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## anachronism (Nov 28, 2018)

snoman701 said:


> That would take 3 lbs of sulfamic acid. It's an endothermic reaction and it is best done hot. I prefer to denox at 90 deg C.
> 
> In addition, the addition of sulfamic very well could complicate waste treatment as it will chelate the copper in solution.
> 
> ...



And how much copper would you have to burn through to destroy that much Nitric? No mate- 3 pounds of sulphamic is far better.


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## g_axelsson (Nov 28, 2018)

snoman701 said:


> Just add a good piece of copper busbar. Even better would be atomized copper powder.


Copper powder in aqua regia with large amounts of acid left could create a boil-over as well as large amounts of NOx.

Göran


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## anachronism (Nov 28, 2018)

It would also add copper where there doesn't need to be copper. Why contaminate something if it's not necessary. 8)


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## snoman701 (Nov 28, 2018)

g_axelsson said:


> snoman701 said:
> 
> 
> > Just add a good piece of copper busbar. Even better would be atomized copper powder.
> ...



If you go slow, you can control it well. The trick is dispersing it so the reaction happens at the top of the beaker. You let chunks of atomized copper fall to the bottom of the beaker you'll boil over. 

A busbar is a much easier method, as the surface area can be used to control the speed of the reaction. 

If this is a frequently recurring disposal situation, there are much cleaner ways to go about it wherein the gold can be recovered without killing off the nitric at all. 

The only way I have ever used sulfamic is to add it to a hot solution. One would surmise that while the reaction occurs rapidly at 90 deg C, it is still occuring at 50...I just have no clue how much slower. 

Interestingly enough, I just screwed up and created a similar nightmare. I accidentally ended up with iron in a nitric digest and added HCl instead of H2SO4 to clean it up. Oops. Thankfully, it's only 200 mL in a 1 L beaker.


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## snoman701 (Nov 28, 2018)

anachronism said:


> It would also add copper where there doesn't need to be copper. Why contaminate something if it's not necessary. 8)



It already has copper in it. The cost to dispose of copper laden aqua regia doesn't change by concentration. Add a chelator and it goes up considerably.


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## Yggdrasil (Nov 28, 2018)

Hi!
I may be a bit off track, but I kind of get a bit bewildered when two esteemed members describe the same process as both exothermic and endothermic, which will be... quite "neutral" I guess??
I realize it may be just typo, but please can someone clarify my bewilderness here if possible. ;-)


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## anachronism (Nov 28, 2018)

snoman701 said:


> anachronism said:
> 
> 
> > It would also add copper where there doesn't need to be copper. Why contaminate something if it's not necessary. 8)
> ...



The copper is 100ppm if I read right Sno. That's 100g per tonne. Did you read that bit wrong? 8) 8)


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## anachronism (Nov 28, 2018)

Yggdrasil said:


> Hi!
> I may be a bit off track, but I kind of get a bit bewildered when two esteemed members describe the same process as both exothermic and endothermic, which will be... quite "neutral" I guess??
> I realize it may be just typo, but please can someone clarify my bewilderness here if possible. ;-)



I'm not sure if I'm one of the two you mean but when I add it and it reacts, it gets hot 8) 8)


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## Yggdrasil (Nov 28, 2018)

I'm not intending to point fingers here.
Bear in mind I have not used it myself yet, 
but all and any of the videos and descripions I have seen, points to the reaction to be exothermic.
I think it was Snoman who referred to it as endothermic, which I consider to be just an typing error, 
but it would be nice to get the nomenclature corrected for future reference.


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## g_axelsson (Nov 28, 2018)

Maybe in this case using a base to neutralize the acid is the right way to go.

When the solution becomes basic the metal ions will precipitate out as hydroxides. Then the solution can be filtered and washed to get rid of nitrates and chlorides.

Then the gold hydroxide can probably be calcined and turned back into metallic gold or dissolved back into gold chloride with HCl and precipitated with SMB.

This is just speculation from my side, this is nothing I have tested myself and I'm not sure it will work or even precipitate most of the gold. Calcining the gold hydroxide might lead to large losses as dust.
Try it on your own risk, but you seem to be good at theoretical calculations and you might have access to better tables of solubility constants so you can check it before testing.
I also suggest a couple of test tube sized trials before committing a larger lot.

Göran


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## snoman701 (Nov 28, 2018)

anachronism said:


> snoman701 said:
> 
> 
> > anachronism said:
> ...



No, I didn't read it wrong. 

100 PPM in solution might as well be 1000 when it comes to waste disposal. 

Now, as for endothermic or exothermic. The addition of sulfamic acid crystals to hot aqua regia with free nitric causes the temperature of the solution to go down. While the reaction is exothermic, the dissolution is endothermic.

Do you premix a sulfamic acid solution and add it?


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## g_axelsson (Nov 28, 2018)

g_axelsson said:


> Maybe in this case using a base to neutralize the acid is the right way to go.
> 
> When the solution becomes basic the metal ions will precipitate out as hydroxides. Then the solution can be filtered and washed to get rid of nitrates and chlorides.
> 
> ...


Just found this thread... proving that everything has already been discussed on the forum. :lol: 
https://goldrefiningforum.com/phpBB3/viewtopic.php?f=37&t=3959

Göran


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## snoman701 (Nov 28, 2018)

well, it was also sort of the OP's plan in their first post


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## Platdigger (Nov 28, 2018)

Another thing about using a hydroxide, at least in this situation, is that the nitrates are still recoverable.
That is of course if there is any interest in recovering them.


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## anachronism (Nov 28, 2018)

Well at 140 degrees C the Gold Hydroxide turns to gold oxide, so reducing that to gold metal is another step too. This is fun 8) 8)


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## Platdigger (Nov 28, 2018)

I found this post made by "SilverNitrate" The context was the original poster was asking if "Anybody on this forum ever use sodium hydroxide for a precipitant on A/R ?"

SilverNitrate posted: "I do, if only seeking gold from your AR, you must slowly add it (NaOH in water) to the AR until just before a color change (turn cloudy)."


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