# De-Noxing



## Uljunky (Sep 2, 2013)

When evaporating an AR solution down do drive off the nitric acid I left it go too long and came back to a dry pan. I added water and there was a vigorous reaction, once that stopped I added HCl. I guessed there was no more nitric acid and added ammonium chloride to precipitate Pt and Ir. I did another batch of the same size, de-noxed properly and had the same yield from both batches. 
When de-Noxing why not just evaporate dry (without boiling) and go through the evap process just once?


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## lazersteve (Sep 2, 2013)

PGM ammonium chloride salts are volatile and will go up in smoke if left on a hot enough burner. By using gentle and controlled evaporation you can prevent any losses due to unwanted volatilization.

Steve


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## g_axelsson (Sep 2, 2013)

That is a really good question that I was going to ask some day but you beat me to it...

I'm using an old coffee cooker as a hot plate and it never goes above 110 C so I have never managed to boil any liquids in a beaker even if water drops turns to steam when it hits the plate directly. A couple of times I have evaporated gold chloride until dry. Will that create any gold or PGM salts that is hard to get into solution again? Do I have a fortune in my saved filters? :mrgreen: 

Göran


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## JHS (Sep 2, 2013)

check this out
http://goldrefiningforum.com/phpBB3/viewtopic.php?f=51&t=19089


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## Platdigger (Sep 2, 2013)

As to Uljunkys question. If you go all the way to dry, even without overheating, you are still left with nitrate salts. I believe this is the reason for adding hcl and evaporating again, to be able to drive off more nitric each time.


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## butcher (Sep 3, 2013)

I agree Platdigger, 
the base metals left in solution would also make nitrate salts, as well as chloride salts, these fused salts can also stick to glassware and become a bit harder to dissolve, even in HCl if they are fused hard, Calm Morrison Hoke addresses this in her book, stating a method to prevent this drying of the gold powders and salts (a little sulfuric acid added earlier) and also giving a method to dissolve the dried fused powders (which can be a bit dangerous if not performed properly).

Also gold chloride can be volatile, the gold at this point has not been reduced to metal, beside the splashing and popping of bubbling syrups which could splash out values, the gold chloride powders with high enough temperature could drive off some gold in the chloride fumes.

I see no benefit to drying at this point, most of the work to get the solution evaporated at this point has already been done, to re-wet the solution with a little HCl (which includes about 68 % H2O in the 32% HCl acid), to provide fresh excess HCl and keep the gold chloride in solution help drive off nitrates as NO2 gas, and give a bit more liquid to continue heating and driving off the excess nitric acid, and keep base metals in solution, add very little volume to the solution to dive off the second evaporation, as well as the third evaporation, which becomes fairly easy to finish the process at this point.

I also wonder if drying the solution to powders would cause problems later if PGM's were involved, as several of these metals have an appetite for gases or oxygen, I guess again temperature and how far the salts were dried may play a role here. I do not know?


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## Harold_V (Sep 3, 2013)

g_axelsson said:


> A couple of times I have evaporated gold chloride until dry. Will that create any gold or PGM salts that is hard to get into solution again? Do I have a fortune in my saved filters?


Evaporation to dryness can result in some gold precipitating, which is discussed by Hoke in her book. If memory serves, she recommends it be put back in solution by the careful addition of traces of sodium chlorate, not by adding nitric. 

If you are not concerned about the losses that may occur (most likely not lost, just tied up in the filter), there's nothing wrong with evaporating as you've suggested, then carefully adding a few drops of HCl to expel any potential nitrates. If there are none, so long as you re-process the filter and material, there should be no loss. 

You may recall that I used my spent filters as a savings plan, along with other waste materials that were generated (such as filters from my fume hood). Everything was incinerated and stored for future processing by furnace. I was handsomely rewarded for my plan. 

Harold


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## g_axelsson (Sep 3, 2013)

Thanks Harold, I feel better now. Yes, I remember your story about the filter from the fume hood, mostly silver if I recall correctly. I'm saving every filter I use, not only for the gold and pgm but also for silver chloride, and I have a stockpot that I've cleaned out twice. 1.5 g gold in the stock pot after processing 50 grams can look like a lot of losses but my batch sizes has been between 0.3-7 g and mostly between 1-3 g. As I have the stock pot I'm not that concerned about losses when dropping my gold or washing so I'm a bit careless when decanting during washes.

... but I'm ashamed that I haven't read Hoke straight through yet, even though I have the book right on the table.  

Göran


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## Uljunky (Sep 3, 2013)

Thank you all very much.


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## 9kuuby9 (Sep 5, 2013)

How do you actually confirm that a solution is completely De-Noxxed?

I know the method of adding silver to a nitrate solution and adding Gold to an AR solution; Deriving the observation from the fact that the added metals do not dissolve anymore.

But is their an other method to confirm that a solution is completely De-Noxxed?

Like for instance the ph? or an other test maybe? HCl is out of question because that will create AgCl and even more HNO3 ---> (AgNO3 + HCL = AgCl + HNO3)

I'm working whit a solution that contains Ag and Pd; Some leftovers did not dissolve This might however be Pt. But as for as I know Pt leaches into a nitrate solution when Ag is present.

And a question for freechemist or anyone who has a good grasp on the subject; Is it possible to precipitate Metallic Pd from a nitrate solution using Ferrous Sulfate?

Any help is kindly appreciated. :mrgreen:


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