# Nitric acid elimination issues



## 925live (Aug 13, 2014)

Hey guys. I've been reading Hoke's book and using the search function like crazy on this forum. I am having an issue with nitric acid elimination. I completely dissolve my gold in AR (note: I am using a 4:1 hcl to nitric ratio and I am careful not to add too much) and filter it many times. Then I evaporate it over a day or more until it is about one third of its previous volume. At this point it's a thin syrup and it has a noticeable difference in the viscosity from before. I add some sulfamic acid and get absolutely no reaction. So this leads me to believe that I have evaporated away all of my nitric acid and only have gold and base metal chlorides. After this is add ice to the syrup until I at least double the liquids volume. It gets so cold that ice forms on the outside of my beaker. I let this ice melt for about ten minutes with occasional stirring. Then I get my smb and start adding it while some ice is still left on top of the liquid. It usually takes me half an hour to add all the needed smb because I do not want to add too much at a time and have excess SO2 gas suspending my gold particles in the solution. While I am doing this I am constantly doing stannous chloride test until I am getting absolutely no gold content in the solution. I let the gold settle overnight and come back the next morning and I have bubbling from the settled gold redissolving and a stannous test reveals that I have gold chloride in the solution. I am completely lost on how any nitric acid could have been left in this solution. I have used urea before but I quit using that because of the possible dangers involved (and it's messy), I have recently started using the sulfamic acid but after I have evaporated the solution it isn't reacting. I am already waiting on this gold for over two days and I am still running into issues. Why is it redissolving? How long should a refining session take? (These are diamond removals in small lots so there is no quartering) How can I eliminate nitric acid the quickest way possible? I appreciate any help, I am sitting on valuables longer than I want to be and I've looked at Hoke's book and this forum and I don't know what I am doing wrong. If anybody has any advice or at least suggestions of what to search or where to look I would greatly appreciate it.


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## Lou (Aug 13, 2014)

The sulfamic should work. It takes a bit of heat to drive the reaction, but I've added it to cold nitric solutions and it produced N2O.

Without seeing it, I can't be sure.

How much re-dissolving are we talking here? Like vivid purple, or faint low ppms? That might just be oxygen getting back in to solution. 

Also bear in mind that sulfites destroy nitrates as well (great scrubbing solution for NOx is sodium sulfite/thiosulfate); so you might just want to add a little more sodium sulfite and then boil to get the particles to settle.

The quickest way to eliminate nitric is to just use less of it, that is add 75% of the calculated amount then slowly add the last in--you've heard the haste makes waste.

Alternatively, you can always melt and part, then dissolving HCl/sodium chlorate, have a quick boil, then precipitate. Gold should be really nice with that method.

Lou


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## Palladium (Aug 13, 2014)

How much gold do you have in solution? You should filter after you have added twice the volume in water (ice) and added the sulfamic so that any trash from the sulfamic and any silver chlorides that precipitate from the dilution step can be removed. If it's from settings you might want to add a touch of sulfuric to remove any lead. I would add the sulfamic after the dilution so the solution has extra water for the chemical process to spread out. Water is an elixir for chemical reactions sometimes.


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## Lou (Aug 13, 2014)

Ralph, 

the trick thing about the sulfamic acid is that it generates the sulfuric acid in situ from its oxidation reduction reaction with the nitric acid. 
That sulfate drops the lead, and you can collect it with the silver chloride.

Using an ORP meter will really help people out when it comes to using sulfamic acid and killing excess nitric acid.


Lou


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## Palladium (Aug 13, 2014)

I see said the blind man.  
I wondered about that when i was typing. It just sort of jumped in my head for a moment. Since i'm only familiar with urea i don't know the reactions of the process. The only real chemistry i can figure is what seem to make rational sense. When i hear you guys talk about it i can reference and retain it. For some reason my mind can't articulate the chemistry side like with equations and all that. Their was a time when i was in high school and college that all that made sense. Back then i was a math wizz and loved to do calculations. Hell i even thought about being a mathematician! Since then life has dealt me more than one blow to the head i guess you could say and the darn cancer thing with all the radiation and medication saved my life, but at the cost of my brain function in that scope of things. Funny thing is i still have all my other skills, i'm still the great business man and bullshitter i have always been, it's just seems that other portion is gone. It's real frustrating sometimes knowing that i know how to do this, but yet it's like standing on the outside looking in.


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## 925live (Aug 13, 2014)

I just finished melting the gold that didn't get redissolved, I have about 37 grams left in the solution. The solution is showing a dark purple on stannous test. I just came across a thread that mentioned nitrates forming with base metals that can reform nitric acid when rehydrated. Could this be causing the problem? I started to evaporate it some more and added some hcl to rehydrate and run off any nitric and all of a sudden it reacts to sulfamic acid again. hmmm, should I be converting the nitrates to free nitric acid with the hcl then try to neutralize with the sulfamic? All I can see is before there wasn't any reaction to sulfamic and now there is so that's leading me to believe I had some nitrates hiding in there that converted to nitric overnight and led to my gold being dissolved this morning? What do yall think? I also will try the using extra sulfite to eliminate any leftover nitrates and I will filter right after I am done with the sulfamic. Lou, Palladium, I greatly appreciate your advice and I will follow it. Do you think I might be having the correct idea here?


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## Lou (Aug 13, 2014)

You're probably using way too much nitric acid.

Just add sulfite till it's gone, or else heat it up and then add sulfamic acid solution (SLOWLY, it will foam). Make sure you have a watch glass and pipette it in....don't want to spatter out your values.

Lou


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## Anonymous (Aug 13, 2014)

Dude if you had read the forum in any depth you'd have seen that a 4:1 HCl:Nitric mix isn't the way to do it. There are countless references to this. As Lou said you're probably using way too much nitric. As Lou and others have also said- just use Sulfamic Acid to get rid of the excess in ten/fifteen minutes.


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## Professor (Aug 14, 2014)

For over a hundred years jewelers and refiners have had that problem. The presence of other oxidizers that release chlorine or other halogens like bromine combined with stabilizers like sodium chloride or other salts that cause an incomplete precipitation of gold. Mostly the loss is reduced by agitation while boiling (adding a little more reagent), but the remainder is usually recovered by a stock barrel that has some base metal like sheet iron or especially zinc chips to cement out the rest. If you are working electronic scrap, careful cleaning of the concentrate, removal of any ceramics and also tungsten metal prior to the leach will help a lot. 
If you are working gold ore, as Hoke has stated (but in other words) all bets are off. 8)


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## Palladium (Aug 14, 2014)

Welcome to the forum professor!


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## Harold_V (Aug 14, 2014)

Professor said:


> Mostly the loss is reduced by agitation while boiling (adding a little more reagent),



Uhhhhhhh-----
We don't use the "b" word here, not when discussing evaporation. It sends the wrong message, which appears to be an ongoing problem. 

Plain and simple. When one evaporates values, boiling is the source of losses. It's expensive and wasteful, and can create a mess that is difficult to undue (been there, done that). 

If you mean evaporate, that's the word you should use. I'll comment negatively every time you fail to do so, and you'll get on my bad side if it happens regularly. We have enough trouble with readers picking up bad habits that are hard to break without allowing this one to pass. 

Welcome to the board.

Harold


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## Charles Connor (Aug 14, 2014)

I have had that problem before, i do not use the heating for the deNOxing, so what i did is to drop the gold in small ammounts with twice the volume, of the gold chloride, maybe the solution was saturated and as it is an exothermic reaction the gold redissolved with some free HNO3, also it tends to boil over the beaker so you got to be really carefull... At the begining it was a nightmare when it happened at first, but thats how i figured to handle that out, also to keep in mind that since the deNOxing process is being done by getting the NO or NO2 out of solution to decompose the HNO3, adding ice right after the heating or the addition of sulfamic, causes the gases to dissolve in your solution because of the temperature, because gases do that in low temperatures...


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## Harold_V (Aug 14, 2014)

Here's the hard facts in regards to how I dealt with excess nitric, which I PREFERRED, although by just a slim margin. 
I evaporated all of my gold chloride solutions, a matter of choice. I wanted the solutions to be highly concentrated (several ounces per liter), to keep fluids at a low level, and to have more vessels on the burners. Keep in mind, I operated on a commercial basis, and had only one fume hood, so space was at a premium. I'd still operate that way today, however, assuming I was still refining. I'd do that even if I was just refining for myself. I'd do it because it works, if for no other reason.

I used a slight excess of nitric to ensure that all values were put in solution. That was especially true when processing filings from the jeweler's bench, or polishing wastes. 

Once my solutions were on the burner for evaporation, I introduced a button of pure gold, of known weight, so I could account for added gold to the batch. That consumed the traces of free nitric, yielding a solution that gave no problems in precipitation. I did that for years, all without issue.  I will never understand why this is such a big deal, as it's easily solved.

Harold


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## kurtak (Aug 15, 2014)

Harold_V said:


> Once my solutions were on the burner for evaporation, I introduced a button of pure gold, of known weight, so I could account for added gold to the batch. That consumed the traces of free nitric, yielding a solution that gave no problems in precipitation. I did that for years, all without issue. I will never understand why this is such a big deal, as it's easily solved.
> 
> Harold



This was one of the first things I learned (from Harold) when I first joined the GRF about 3 & a half years ago - I have been using this method ever since - it works every time without fail & is so simple & it eliminates the need of other chemicals for denoxxing

I can't even imagine ever going back to an evaporation/chemical addition method of denoxxing - its just not needed

Kurt


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## Geo (Aug 15, 2014)

If the solution already has mixed metals, you can use almost any metal to deplete the nitric. I have used small bits of copper even though it's not the prescribed method. I figured that the powder was going to be re-refined anyway. In my mind, refining is a simple rendering down of materials, condensing, concentrating. My thought processes work that way. I can equate refining to rendering lard from a hog, there's the right way and then there's everybody else's way. :lol:


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## g_axelsson (Aug 15, 2014)

That would work bad, Geo.

I think some gold and precious metals will cement out on the copper (or other metal) and unless you are lucky and hit that perfect spot when you have added exactly right amount of metal the gold can't be redissolved, there's no free nitric left.

If we just ignore that case and see what happens when we add some copper...
- If you add too little copper you would still have nitric left.
- If you add too much copper, gold and other precious metals would start cementing.
- If you add just exact the right amount it would take a long time, the metal pieces gets smaller and smaller and the surface area goes down while the acid gets weaker.

A gold button on the other hand will never cement any precious metals and the surface area stays large as you don't need to fully dissolve it.

Göran


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## Palladium (Aug 15, 2014)

I to have used the method Geo is talking about. It you're going for a second refining any gold that may precipitate on the copper will be trapped and passed on to the second refining. It's a balancing act but i have use it before.


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## Geo (Aug 15, 2014)

Yes of coarse, I apologize about not adding that I do not recommend anyone use copper in this way. I do it when I don't have a gold button on hand. The assumption is that the copper will cement a small amount of gold and this gold which is redissolved depletes more nitric. Like I said, I do not recommend anyone use it this way. The only time I use it like that is during something like processing CPU's or another material that the exact weight in metal is unknown. Also, I'm at the point where I can recognize cemented gold powder. As a non-professional, I find little things that help me but if it's not accepted here, I seldom make mention of it.


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## Harold_V (Aug 15, 2014)

Geo said:


> If the solution already has mixed metals, you can use almost any metal to deplete the nitric. I have used small bits of copper even though it's not the prescribed method. I figured that the powder was going to be re-refined anyway. In my mind, refining is a simple rendering down of materials, condensing, concentrating. My thought processes work that way. I can equate refining to rendering lard from a hog, there's the right way and then there's everybody else's way. :lol:


The real risk of using this procedure (if you're toll refining and not assaying, as I did) is that you may cement some gold that won't get back in solution. It is then lost to the rightful owner when the solution is filtered, prior to precipitation. 

I'm not suggesting it doesn't work---it does. It just offers risk of loss of accountability---and certainly adds to impurities by having more base metal present to be dragged down. 

Harold


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