Sulfamic acid for denoxing solutions

[Topher_osAUrus]

Just rode hard and put away wet. :mrgreen:

Lol.

Thats my life story summed up in 7 words

 

[Geo]

Sulfamic acid is needed for the beginner who can not source nitric acid reasonably and must use a solid nitrate substitute. It is really very hard to to calculate the amount needed when the starting weight in metals is impossible to determine. This often is the case when working with electronic scrap. Even foil weight can be misleading and hard to calculate. I have learned that it's better to add an excess of oxidizer by a small amount and then deplete the overage of nitrate with sulfamic acid. This ensures that all the base metal is consumed which is the most prolific reason for misplacing your gold as a beginner.

 

[solar_plasma]

Göran wrote:

- Will it remove nitric salts even if there is no other acid presents?

Would be easy to test: Then it would precipitate sulfates with Pb(NO3)2 or Ba(NO3)2 . I doubt. If I don't forget it, I'll try this next time in the lab.

I tested it quickly in the cold, - no precipitate. Next time I have more time, I test it with heating and then with an addition of HCl.

 

[lazersteve]

- So, post links to any good sulfamic acid threads, tips on how to use it... Lazersteve apparently uses it in dry form. Is that better than making a solution and add it in liquid form?

The others have covered the sulfamic threads that I know of, so I'll skip this one.

- Should it be used in a hot or cold solution, is it working fast enough to notice in cold diluted nitric acid.

I use mine, as you correctly quoted from my previous post, in dry crystal form directly into the AR or nitric containing solution. I never check the temperature, but try to add it as soon as the AR finishes it's work on the scrap/Au powder (still warm). I also use it to test my waste for the presence of nitric/nitrates to determine which one of my waste stream drums the waste goes into. I do this because I reuse the solutions containing nitric until they no longer have active acid that can be used for dissolving base metals. I use the left over nitric solutions for digesting high base metal content scrap. Additionally, Copper nitrate solutions can be converted back to usable nitric acid with electricity and graphite/copper electrodes as discussed here : Copper Nitrate to Nitric Acid. Typically silver and palladium reactions produce recyclable solutions after the metals are removed. Leftover AR digestion solutions generally do not get reused/recycled.

- Will it remove nitric salts even if there is no other acid presents?

In my experience it remove nitric acid and nitrous acids. I've never experimented with pure nitrate salt solutions to see if it actually works, but I'm pretty sure it only works if there is a free acid in the solution. Try dissolving sodium nitrate in distilled water and add sulfamic acid, if you get a fizzing reaction then it likely works with all neutral salt solutions too.

- Is it vital or even needed to stir the solution?

I feel stirring is a must. If you do not stir then the sulfamic crystals are slow to dissolve and react. Stirring also exposes the solution to the sulfamic acid as it dissolves. There have been many instances where I have added sulfamic acid without stirring and waited for the initial fizzing reaction to cease, then I stirred and the reaction kicked up again. On this subject: It is important that you perform your sulfamic and SMB reactions with pregnant solutions in a larger catch vessel so that you do not have a loss due to vigorous fizzing which causes overflows. A vigorous sulfamic reaction can easily quadruple the volume of a pregnant AR solution. I use a clean 5 gallon bucket as a catch basin for 4 Liter and smaller flasks. I use a large 25 gallon rectangular plastic tub as a catch basin for 5 gallon bucket reactions.

- How much is needed for each ml of 70% nitric acid.

I do not measure my sulfamic acid additions as I have no idea how much nitric acid is free in the pregnant AR. Instead I use visual cues to guide me.

1) Add the dry sulfamic: For solutions less than 100mL in volume i start with a pinch between my forefinger and thumb. I start with approximately 1/2 to 1 teaspoon of sulfamic for pregnant solution from 100ml to 2 Liters. For 2 to 4 liters of solution I start with one table spoon. For 3-5 gallons of pregnant solution I start with approximately a 1/4 to 1/2 cup of sulfamic crystals. The solution can be hot or cold, but hot tends to react quicker. I never actively heat the solution, instead I add the sulfamic to the unfiltered pregnant solution that is freshly prepared and hence still warm from the heat of reaction from the digestion step.

2) Allow the initial reaction to cease:The initial reaction can be quite vigorous as stated above so be prepared. Spray down any over spray from the initial reaction once it dies down. With very clean (little to no dissolved base metals) solutions that have a high free nitric acid content, you may see a color shift from red to orange. This is likely the free NOx being consumed by the sulfamic reaction.

3) Stir vigorously to confirm the crystals are all fully reacted with the solution: At this stage you will have a good idea how much free nitric acid is in the mix, by the secondary reaction. The more fizzing/foaming/red fumes at this stage, the more free nitric remains. Check the bottom of the reaction vessel for sulfamic crystals. if you have no fizzing/foaming/ red fumes, and left over crystals in the bottom, then do not add any more crystals as you are likely done, or very close. Skip to step 5 below to confirm. Note: With very dirty solutions (high base metal content with dissolved values) you will need to add a little water (10 to 30% of the original volume) to get the sulfamic to fully react. As a rule of thumb: if you can not see thru the solution due to it's dark color (typically green or blue), dilute it with some water.

4) Repeat steps 1-3 as required:With each addition, follow all of the steps above in order and follow the visual clues as to how much sulfamic to add. Add less sulfamic as the reaction gets weaker and weaker (First foam and red fumes, then fizzing, then slight bubbling). A note here is that sulfamic crystals will produce a very slow stream of bubbles on there own, this is not considered fizzing. Fizzing produces a distinct foaming on the surface of the solution. After performing these steps several times, you will get a feel for the free nitric acid. The less excess nitric you add in the digestion stage, the less sulfamic you will need.

5) Coarse/Fine filter the pregnant solution: To remove the excess sulfamic crystals you will want to coarse filter the solution thru a few layers of coffee filters. For dirty solutions (green, blue, or brown) with base metals present, this removes the trash (plastic, paper, ceramic, fake stones, etc) from the digestion plus the undissolved sulfamic crystals. It will not remove trace AgCl, other insoluble chlorides (eg.: lead) and other super fine particulate. For the second/third refining digestion (bright orange colored solutions) this may clear everything up if you have done the first digestion correctly (good filtration). Repeat the filter process for fine particulate if the solution is bright orange/yellow. Proceed to step 6 below.

6) Testing for Remaining free nitric acid:Test the solution from step 5 above with a small pinch of dry SMB and wait 5 minutes. If the SMB produces Au powder that dissolves with a puff of red fumes, then you will need to continue the sulfamic additions and retest. Once the SMB produces Au that does not dissolve, finish out the SMB process as usual. If the Au powder partially dissolves, but then begins to settle, the last of the nitric was removed by the precipitated Au powder and you can continue. Be sure to do this step in a catch basin as well.

- Any drawbacks?

Foam over is the primary concern I have when using sulfamic acid.

- Describe how you are using it and cues to when you have added enough.

See my process description above.

- I have bought a small package on eBay, any tips on where you can get it and how much it should cost in various package sizes?

I buy it in 50# bags from chemistrystore.com . This last me about 1 year with my workload lately.

Thank you for posting this thread. Using sulfamic is a vital skill to learn for the refiner.

Steve

 

[Topher_osAUrus]

So, I must ask.

What can one expect to happen if TOO much sulfamic has been added to the solution.

What negative side effects may this have overall?

 

[Geo]

Other than a boil over depending on the amount of free nitric in solution, not much, really. Just like any liquid, you can only add so much salt until it is saturated. When it becomes saturated, no more will dissolve. I've never witnessed any adverse effects on the gold recovery, no matter how much I added. Too, you can always add a bit more water. If you evaporate the solution, you can get some crystals formed. Adding water will cause these to dissolve.

 

[Topher_osAUrus]

Other than a boil over depending on the amount of free nitric in solution, not much, really. Just like any liquid, you can only add so much salt until it is saturated. When it becomes saturated, no more will dissolve. I've never witnessed any adverse effects on the gold recovery, no matter how much I added. Too, you can always add a bit more water. If you evaporate the solution, you can get some crystals formed. Adding water will cause these to dissolve.

Thanks Geo. That indeed does help.

But, I tend to always dissolve my sulfamic crystals in hot hot hot water. Then, while both solutions are hot, I add them together. (usually, sometimes i add it after the sulfamic acid[aq] has cooled to ambient temp)

I was curious if the excess sulfamic would raise any problems while trying to reduce the gold with a certain (any of the..) precipitant. Or if it's overall a safe method, and having excess isn't an issue during precipitation.

I have taken some chemistry (wayyyy back when...like 14 years ago) but, i slept mostly, and have yet to begin my college courses yet on the matter, so I am far too rusty on my stoichiometry to even begin trying the balanced equations...but, even if I was a chemistry all-star, there are sooo many variables that it would be nothing more than conjecture(for me).

Hopefully one of our resident chemistry-gods can hit me with some knowledge!

Thanks again Geo
-topher

 

[kurtak]

Hmmm - There was just a discussion on Kens RPM forum about denoxing as well - to which I also replied & personally I don't get it -- I mean why mess around with evaporation (as described in Hokes) or chemical denoxing like urea (which we all know works - but not well) or sulfamic when there is a much simpler method (thank you Harold)

As has been pointed out (many times) in the first place we want to try using "just enough" nitric (or nitrates) as is needed to dissolve the metal being dissolved --- but as we also know it is near impossible to hit the amount of nitric right on the nose so most always will end up with at least "some" (hopefully very little) free nitric - & in some cases (deliberately) lots of free nitric

In other words if the goal in the first place is to use just enough nitric to dissolve the metal --- why not use metal - to use up the over shooting of nitric that results in having free nitric

We basically have two situations concerning free nitric in our AR - Situation (1) is a lot of free nitric because we have no idea how much we need so we deliberately "over shoot" it --- Situation (2) we have a pretty good idea of just how much nitric we need but still over shoot - even if its just a drop or two

Situation (1) We deliberately use to much nitric because we are leaching gold (&/or base metal) from something like Chip ash, bench sweeps or CPUs going directly to AR & because we have no real idea of how much metal we need to dissolve we deliberately over shoot the nitric

This is a recovery process & it just does not make sense to me to denox (with sulfamic - or by evaporation) & then chemical precipitate (SMB or FeSO4) & then send the solution to the stock pot to "recover" any "trace" gold that didn't come down with the SMB or FeSO4 --- IMO you are spending "way" to much time playing "I am Mr. chemist" to recover what is going to be dirty gold that has to go for a second refine anyway (this is of course all after you have filtered the original leach solution)

In other words - IMO - all the time you are spending playing Mr. chemist here - could be spent doing something else (& there is "always" something else to be done)

This situation is much easier to deal with & it gives you time to do something else "while recovering your gold"

After you have filtered you original leach solution (if you suspect lead - add "a little" sulfuric before filtering --- it doesn't take much) put in a copper bus bar & cement the values out --- you have just killed three birds with one stone (1) the solution is denoxed as the free nitric dissolves copper (2) ALL your values are recovered (3) the solution does not have to go to the stock pot as you just did what the stock pot recovery does anyway --- AND - once you put the copper bus bar in to cement your values you can walk away & do something else

In other words - the end result is exactly the same - you have recovered your gold that needs to go to a second refine anyway - you just didn't spend time playing Mr. chemist to recover it - you spent the time getting something else done

Situation (2) This is a no brainer - here you are dissolving a known amount of metal so you should come darn close hitting the nitric to only what you need & therefore should have very little free nitric to denox --- with your AR on low heat - drop in a small gold button (I always keep a few small buttons just for this purpose) walk away (go do something else that's needing done) come back & when you see no more reaction working on the button your solution is denoxed

As the nitric is used up the reaction slows away down - so it can take awhile - but that's ok because as I say there is always something else to do while waiting --- however if your in more of a hurry then use a little gold powder instead of a button (again I always keep just a few grams of powder around just for this purpose) add 1 - 2 "tenths" gram (a pinch) powder to your AR (on low heat) give it a half hour (go do something else) if all the powder dissolves add another pinch - when it doesn't dissolve the pinch of powder its denoxed & the un-dissolved powder just becomes part of your gold drop

The credit of using gold to denox AR (whether a button or powder) actually goes to Harold - I read about it (as advised by Harold) back when I first joined the forum - I have been using it ever since - I have tried all the other methods (evaporation, sulfamic & yes even urea) & I have come to the conclusion that using metal to consume the free nitric in my AR (whether it is situation 1 or situation 2) serves me much better then any other method --- simply because I can put the metal in (which ony takes a couple seconds) & let it do it own work on consuming the free nitric & while that is taking place I can be doing other things

To be honest I really don't understand why in the case of situation (1) members don't "simply" cement there value back with copper & in the case of situation (2) members don't just consume the "little" free nitric with the addition of a little more gold

So - I am open to comments as to why

Kurt

 

[Geo]

It can be hard to know when all the values are cemented. Aside from leaving the copper in solution for a prolonged period of time, there's really no good way to gauge the completed process. After a solution is completely saturated with copper, a stannous chloride test is useless. If the solution is black to start with, how can you tell if there's a color change. I agree with Kurt that in a recovery, cementation is a very effective way of reclaiming the values from a mixed metal solution. I have read where some people use it exclusively to precipitate their gold as purity is not an issue. If you don't want to wait the length of time needed to be sure you have cemented all of your gold out of solution, you need to be able to remove the free nitric quickly. Again, looking at it from the beginners point of view, the process with the least amount of steps is probably the best. Cementation is a "set it and forget it" type of operation because it needs no attention to do it's work but it takes so long to be sure. By removing the free nitric and precipitating in one steady process lets you test the solution after the drop to make sure the solution is barren. If you are working on your own material and time is not an issue, it would be appropriate to use cementation as long as you have the copper metal on hand for it. I suppose it's really a matter of personal preference. I still get in a hurry sometimes and add too much nitric acid. I guess it's because I have gotten so used to using the sulfamic acid that exact measurements is something I rarely practice.

 

[jeneje]

Evaporation, evaporation, evaporation!! :idea:

Ken

 

[Anonymous]

Evaporation, evaporation, evaporation!! :idea:

Ken

Or you keep your solution at exactly 85 degrees Celsius until it stops any brown fumes and begins to fume white. At that point your Nitric has all gone and the white is the HCl coming off. Nothing else required. Again credit to my Australian friend for that one.

Jon

 

[Topher_osAUrus]

Great post as always Kurt!

And I agree, BUT (there's always that big-hairy "but" isnt there? :lol: ) I think Goran wanted the spirit of this thread as a composed sideline of information to be accessed when people are in a rush and dump in the nitric, or use metal nitrates, or (as you said) are doing stuff like sweeps, chips, incinerated stuff, filters, whatever.. Where excess is a necessary evil. I only like sulfamic (I cant lie, i use it...but maybe once or twice a week, if that) because it serves multiple functions..
-If aqueous, it will dilute the solution precipitating any silver chloride.
-it will remove excess nitric
-it will convert to sulfuric and ppt lead sulphate

So, sometimes it is just a good way to go about it. Way way way better than urea. But, you are correct, the best way to go about it is to be stingy with the nitric. But right now I just am finishing my 2nd leach(AR) of a whole truckload of filters i foolishly used last year when first starting. The first time i did it, i thought i got all the PMs out, but apparently I wasnt even close. So, frustrated (foolishly), I just threw in a 3/1 ar this time. Did i go overboard with the nitrix this time? Most likely...ill know in a couple hours.. But, sulfamic will save me, because coming up in a few hours, i have to hang up my mr. Chemist hat, and get out my daddy daycare apron. So, with its ability to multitask for me, it has opened a door for me to get finished with that today, so i can melt after bedtime.

And hey... Whats with all the "time to do other things"... You tryin to say i need to get off my ass (still) and finish my fume hood!?! Lol (i know i do, dont rub it in more! I need a truck to get my supplies, and mine is being lent to family whose vehicle bit the dust..)

Thanks again for the thought provoking post!-part of why i love this forum.

-topher the-tardy

 

[solar_plasma]

It always depends on set and setting. I need processes that are sort of drop&forget or much better done within a 45 or 90 minutes time window inclusive explanation, cleaning and putting things a safe place. Sulfamic acid is a tremendous boon to me without any disadvantages. Cementing high values would be possible, but it is not fun to work with in my world.

 

[4metals]

Kurt,

I agree with everything you have said and it shows that you have a good understanding of the entire refining / recovery process and you can work it to maximize your time. I believe that you, for the most part, do all of the refining yourself. Harold was also a one man operation. So from your perspective, and Harold's, you are able to maximize your time and effort and produce recovery yields that you have confidence in.

As far as the majority of members of this forum are concerned your methods will serve them well.

But I come from a world where a refiner has multiple employees, and multiple customers who all want good results in a timely manner. Rarely is it possible to batch different lots into bulk process lots without going through an extensive process of quantifying each lot content beforehand.

As a forum member operating a commercial refinery in India on gold from the mining industry learned, sometimes you have to know what is in every job before you give it out to employees. Since you are the employee/employer/cash cow/and likely the one who cleans the bathroom too, you don't have those issues. Don't think I am being critical, I have been there done that myself and it is a different world.

One example is chemical stone removal. High karat work, totally unassayable before processing, and usually a customer looking over your shoulder. Not exactly the type of process lot where you want to be adding nitric with an eyedropper or where you want to be adding fine gold to the lot. (Because they will swear it isn't yours when you deduct it later from the finished bar weight) Granted these are larger lots but they are done by refiners day in day out and it is always prudent to add nitric acid in excess so you can kick off a reaction one afternoon and have it done the next morning. So the next morning comes, the customer is there already looking at his watch, and you have a batch of cold aqua regia that has finished reacting. The quickest thing you can do is use a chemical means to eliminate the free nitric acid. I prefer sulfamic acid because it is the fastest, and with karat that may have lead it eliminates a small sulfuric acid addition. I generally do not use urea, although I did for some years back in the day. I always preferred to gas off the excess free nitric acid with sulfur dioxide gas if time was not an issue.

The excess nitric added to the reaction allowed me to have as complete a reaction as possible and dissolve as much gold as possible in the first run. It didn't cause the reaction to die out for lack of an oxidizer and come in in the morning to a partially digested lot, it was ready to rock. But, and there is always a but, there was excess nitric to deal with.

So my point is any process in the refining world has to be carefully weighed between how to best spend your time, how to best enlist the help of chemistry, and how to be assured that what you end up with in your hand is all of what was in that particular lot. Believe me, I enjoy hearing the perspective of refiners who work in different situations than I normally see, and I do learn from it, but those in other situations should also realize there is an entirely different world out there handling the same or similar materials in an entirely different way. The good thing is nobody is right or wrong it's just different.

 

[g_axelsson]

I think Goran wanted the spirit of this thread as a composed sideline of information to be accessed when people are in a rush and dump in the nitric, or use metal nitrates, or (as you said) are doing stuff like sweeps, chips, incinerated stuff, filters, whatever.. Where excess is a necessary evil.

Almost spot on, I wanted this thread to be a place to collect information about using sulfamic to denox solutions. But for that sake it isn't wrong to mention that there are alternatives to sulfamic. What is the best option for you all depends on your situation.

I'm thankful for all posts so far. There has been a lot of interesting information and descriptions of how to use sulfamic. It seems like it is a very forgiving chemical who doesn't come back and bites you when you least expect... except from boil over.
Any chemical that Lazersteve uses 50 pounds per year must be good for something in refining. :mrgreen:

What I'm still lacking is a bit more links to old threads... you would think that with close to a quarter of a million posts there should be more than three threads with useful information.
If that was all there was this thread was surely needed. :lol:

Göran

 

[Lou]

It comes back to bite you during waste treatment as it is a chelator.

 

[4metals]

It comes back to bite you during waste treatment as it is a chelator

.Sulfamic acid is used widely in descaling operations for rust removal and hard water scaling. I have used it in electroplating cleaning lines and never had any significant hold up of metals due to chelation when using classic destruct treatment systems. I always separate streamed EDTA rinses for that reason but never sulfamic rinses. Maybe the concentration or mixture was too dilute to effectively chelate the waste and it may be different with higher concentrations. (as in de-NOxing reverse AR solutions)

Can you be more specific Lou?

 

[Geo]

https://en.wikipedia.org/wiki/Sulfamic_acid
https://en.wikipedia.org/wiki/Thiourea

This is the two ingredients of TarnX brand silver cleaner. Just a little tidbit to add to the discussion.

 

[solar_plasma]

If you only add a nearly stoichiometric ammount to remove nitric, I can't see, where it could form chelates in the waste treatment, if it isn't there. Further at least silver sulfamate has been shown to cement on zinc. But if it would not cement out on copper, this would in deed be a problem for the stock pot treatment.

Sulfamic acid derivates of Ag, PGM and other metals

 

[g_axelsson]

If you only add a nearly stoichiometric ammount to remove nitric, I can't see, where it could form chelates in the waste treatment, if it isn't there. Further at least silver sulfamate has been shown to cement on zinc. But if it would not cement out on copper, this would in deed be a problem for the stock pot treatment.

Sulfamic acid derivates of Ag, PGM and other metals

Interesting text, although 1 page in English and 238 in German feels a bit heavy to read in one go. :lol:

I copied the link to this thread as a reference. Initial supplied by solar_plasma.
"Sulfatderivate zum Aufbau von Funktionsmaterialien"
http://oops.uni-oldenburg.de/925/1/miesul09.pdf

This is the English summary.

Summary

This work deals with the syntheses and investigations of complex compounds with derivatives of the sulphate ion. The work focuses mainly on compounds with benzenepolysulphonates.
Conversion of tetrachloroplatinates with sulfamic acid leads to A2[PtCl2(NH2SO3)2]·xH2O (A = K, Rb, NH4; x = 0 or 2). The coordination of the platinum ions occurs both in cis- and trans-configuration. K2[Pt(NH2SO3)4]·2H2O is synthesised by reaction of K2[PtCl2(NH2SO3)2] with silver chloride.
The reactions of diverse benzenepolysulphonic acids and metallic salts under modified reaction conditions lead to a variety of complex compounds. Dimerisation occurs in 0∞{[Cd(m-BDS)2/2(H2O)4]2} (m-BDS = 1,3-benzenedisulphonate), chains are build up in 1∞[Cu(p-BDS)2/2(H2O)4] (p-BDS = 1,4-benzenedisulphonate), 1∞[Cu(p-BDSCl4)2/2(H2O)4] (p-BDSCl4 = 2,3,4,5-Tetrachloro-1,4-benzenedisulphonate) and 1∞[Zn(p-BDSCl4)2/2(DMF)4]. Layers exist in 2∞[Cu(BTS)2/3(H2O)4]3⋅4H2O (BTS = 1,3,5-benzeneltrisulphonate), 2∞[La(BTS)4/4(H2O)5] and 2∞[Zn(p-BDS)4/4(DMF)2]. Using 1,2,4,5-benzenetetrasulphonic acid (BtetraSH4) leads to a ladder structure for 1∞[Cu(BtetraS)2/4(H2O)4]2⋅0,5H2O. The thermal analyses of the water or DMF free compounds show high thermal stability up to 400°C in most cases. The thermal stability of the DMF free compound of [Zn(p-BDS)(DMF)2] is especially noteworthy, it is stable up to 560°C. Additionally, the structural characterisation of the hydrous 1,3-benzendisulphonic acid was possible. The p-benzenedisulphonate ions are deprotonated and occur with oxonium ions as counterions: [p-BDS2-][H3O+]2.
Conversion of sulfanilamide with copper nitrate yields [Cu(NO3)2(H2O)(H2Sulf)2]. Monomeric units are existent in the crystal structure. Quite unusually, the copper ions occur with a sevenfold coordination sphere. This coordination is build up of the monodentate coordination of two sulfanilamide molecules, one water molecule and the bidentate coordination of two nitrate ions.

Göran