Silver Nitrate (Nitric Acid + Peroxide) Patent Application

[Elemental]

BLUF: I found a patent application (1993) for generation of Silver Nitrate via Nitric Acid + Peroxide which I thought would be worth sharing

As I spend this winter looking for more raw precious metal stock, I've begun rebuilding my Silver Cell and as part of the research I saw someone had mentioned by adding peroxide to their nitric acid while dissolving silver it would reduce the amount of NOx fumes that are generated/lost during the reaction by re-constituting the NOx as nitric acid. Is anyone doing this? Does it have any impact upon a silver cell? I'm going to do a deeper dive into the electro-chemistry regarding having the peroxide present in the electrolyte. Thanks for any insights regarding this process.

Elemental

 

[Martijn]

I've heard about it being used in AR to reduce the nitric fumes and regenerate into HNO3.
As long as the H2O2 does not oxidize any metals you want to keep out if solution it's a good way to reduce NOx fumes.

 

[orvi]

It certainly work to nearly eliminate the fumes generated (tryied), but i do not know exactly about safety consequences. Mixing two strong oxidizers (nitric and H2O2) together maybe isnt the best thing from safety point - mainly on larger scale (I am just assuming here, as I hear it).
As i am thinking about it right now, no immediate danger comes to my mind, but for first hearing, it just does not feel OK
Way in the past, when chemplayer has his videos on YouTube, i remember one mixing nitric and peroxide together. Also NurdRage made nitric acid by bubbling NO2 through 30% H2O2 - to improve the efficiency of the process in comparison to just water as absorption medium.
I personally tried it once when dissolving some stuff in nitric, and it worked pretty nicely in means of minimising NO2 fumes. But the H2O2 degraded quickly in the mixture, and thus it should be dosed frequently.
We abandoned this approach mainly due to cost of peroxide vs. nitric, and also due to increased volume of water in the soup because we could only search 30% H2O2 (nitric is 65% and have significant density). Also more paperwork with peroxide here, compared to nitric.

 

[Elemental]

@Martijn and @orvi, thank-you for your responses. I managed to find some safety documentation in regards to concentrations, I've attached it. Orvi, I think you have a great point dilution of the solution using this process. Having to dilute the Nitric to less than 35%, plus the additional water from the peroxide means a lot of volume. I may test it out at the test-tube sized scale, but will stick with just straight nitric to make my electrolyte for my silver cell. When I get a chance to experiment, I'll update this post. Thank-you for your thoughts on it.

Elemental

 

[orvi]

Maybe for process consideration it will be OK to start with plain concentrated nitric, and gradually add peroxide to "refresh" it - simultaneously diluting it with water from peroxide and creating bit more from peroxide reduction. This could be calculated pretty nicely I think. Silver is relatively calm in point of reaction with nitric, so slowly also concentrated stuff could be somewhat used... Should be tested tho. Maybe starting with 50 % stuff, then gradually adding H2O2 etc.
Just thoughts. If the safety side is given a green tick, then why not to try it out.

EDIT: based on later findings, this assumption was clearly wrong and potentially very dangerous. Should not be attempted.

 

[orvi]

In addition to previously posted SOLVAY pdf, this is original reference from 30s. Good times, when experiments were very nicely and closely described
Shortly said, mixtures above cca 35% of nitric and 30% H2O2 tend to decompose spontaneously, reaction setting up on its own even at room temperature. If concentrations are higher, mixtures undergo violent runaway reactions.
Authors also measured the conductivities of these mixtures and found out that conductivity of such mixtures decrease significantly. This could tell that some peroxide-type compound could be generated in process, which is of low conductivity (typical for weakly ionizing compounds). Or other unknown species.
And in this chemistry, espetially with larger quantities, i would not stand a chance creating "unknown possible short lived peroxides" on large scale with plenty of oxidizers in the mixture.
Guys said that when concentrated mixture boiled out, it could ignite the cupboard - very nice to occur when refining in the DIY garage fume hood

When dosed continuously (maybe via some nice peristaltic pump or addition funnel) it should work safe (my opinion). As long as the concentration of peroxide did not rise up.
Also watching the fumes could give you good view of what is going on - more brown NO2 = less re-oxidation of the NOx-es is happening = less peroxide inside

 

[nickvc]

I can’t honestly see any advantage to this in a silver cell as fumes are of very little consequence, the biggest problem is the fouling of your electrolyte with too much base metal in your solution that can be remedied by adding nitric to your existing electrolyte and dissolving more silver, I did this as I mainly ran sterling with gold plated jewelry which was melted into bars and put into the cell to recover the gold from the slimes the fine silver recovered was a bonus.

 

[Elemental]

@nickvc, the type of cell you're describing is what was recommended to me to run instead of doing a full dissolve with nitric and then then plate on copper for my sterling silver refining. This should hopefully cut down on my nitric usage considerably. I'm currently building a new framework for a three/four liter Moebius silver cell and hope to test it out this spring. I just purchased a new 750gram steel split mold for casting anode bars.

It's been an interesting project, it has a pump for electrolyte circulation and draining (we'll see how long that lasts, I need to find a peristaltic pump), easily removable cathode bars for removing fine silver and the ability to remove the cathodes for when as your recommended, I add more nitric is to the electrolyte (this way it only dissolves silver out of the anode bar). It also has lights to shine beneath the polycarbonate, so I can see the crystal growth and better see the electrolyte color change as it becomes fouled.

 

[nickvc]

I dissolved some of the already refined silver in the cell which allowed more of the sterling to go into solution as I was after the gold in the slimes , I used color as the indicator that the electrolyte was fouling , it started to take on a green hue not that blue you normally see when you start running.

 

[Lou]

At big silver nitrate production facilities, the practice is to sparge O2 through the reaction mix and mop up any NOx with H2O2 or even HNO3 as the first scrubber medium. This only works because it is chloride deficient (Cl- will eventually decompose the peroxide).

At the old RMC, there was a pressure reactor that was designed to run on an oxygen overpressure of 50 psi. Silver dissolves in two fashions according to the reaction order and the concentration of nitric acid:

Ag+ 2 HNO3 = H2O + AgNO3 + NO2 (at higher concentration nitric acid)
3 Ag + 4 HNO3 = 2 H2O + 3 AgNO3 + NO (a more efficient reaction, at lower concentration).

Under the pressure conditions, you get substoichiometric nitric acid usage where every mole of nitric acid inevitably goes to making a mole of silver nitrate.

 

[Elemental]

I have a UV-VIS spectrophotometer (ThermoSpectron Genesys 2) showing up today, it needs a little TLC, but I was hoping to use this track/test concentrations of Copper Nitrate a bit more scientifically.

 

[orvi]

I have a UV-VIS spectrophotometer (ThermoSpectron Genesys 2) showing up today, it needs a little TLC, but I was hoping to use this track/test concentrations of Copper Nitrate a bit more scientifically.

Very very nice ! Colourimetry is sufficient for this, one Cu wavelength, one callibration plot in Excel, one smart "equation" into the Excel document, and checking is made conveniently in matter of minute

 

[Elemental]

I was quite lucky, I expected it to show up with burnt out bulbs or the optical pathing system to be off track. Turns out the only thing that doesn't work is an ancient linear flash card for storing data. Since I only need to do a single wavelength, I can do without the card. I'm pretty happy as I only paid $11.00 for it.

 

[FrugalRefiner]

Excellent!

Dave

 

[orvi]

I was quite lucky, I expected it to show up with burnt out bulbs or the optical pathing system to be off track. Turns out the only thing that doesn't work is an ancient linear flash card for storing data. Since I only need to do a single wavelength, I can do without the card. I'm pretty happy as I only paid $11.00 for it.

What a bargain at my university, they were also "disposing" some older machines, but these were bit too old for use. And newer stuff was all broken in some way. I just salvaged and repaired two old magnetic stirrers, but bigger machines, I never have the luck for it.
Nephelometer - broken apperture. FTIR - dead. Analytical scales - way too off etc.