Cost to refine silver braze (50% Ag) - Advice appreciated

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snoman701

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Oct 8, 2016
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Location
SE MI
I have a friend who has a large quantity of silver braze...ok, large quantity to me. More than 100 lbs.

50% Ag
35% Cu
Rest is Zinc, Nickel & others. Surprisingly haven't noted any cadmium...cadmium would change the plan.

I have no clue where to start in what a fair price to charge for braze refining is. I have heard prices for silver braze refining as high as 55% refine charge. Given the cost of refining, I'm looking at 35% charge, but want to make sure that is in the customers best interest.

My (untested) plan would be to inquart the braze with .99 silver (actually 1:4.5) which should then make an alloy pure enough to run it through a silver cell. That saves on digesting it all in nitric first, and considerably reduces the amount of waste that would have to be dealt with. I'd obviously buy a few lbs from him straight up first to make sure that the technique worked.

Buy price would be based upon an ingot melted, drilled, then scanned with xrf.
 
Nope...it's all dross. I don't mess with original product! It'd be worth considerably more, like you said!

I don't think I can send it directly to a cell...the cell wouldn't last very long.
 
I shouldn't call it dross....drippings would be more accurate. I've refined it before in a simple nitric digestion, it's clean.


Sent from my iPhone using Tapatalk
 
Is there tin in it? Often such silvaloy brazes have a tin component. Used to do a lot of A56T.
 
If you do the calculations, with the 4.5X added silver you will find that it would take much more nitric (I estimate about 3X more) to run it through the silver cell than it would to just dissolve it in nitric. You would also end up with 3 times more waste.

Dissolution in nitric is, in my judgement, the very best way to do it. With a lot of tin, the metastannic acid could cause problems, especially with filtering (my wick filter might work well in that case). With a small amount of tin, it would go much smoother and you probably wouldn't have to worry about it. You have about $12K in silver. If your percentages are true, it would take about 33 gallons of nitric diluted to 66 gallons with distilled water and 15 pounds of copper to cement the Ag. Economically, it sounds like a good deal to me. It shouldn't cost more than $300 for the chemicals and Cu. Getting rid of the waste is another expense.

Too bad it's not 50/50, Ag/Sn. You could then melt it into bars and run it directly through the silver cell, The tin, as metastannic acid, would stay on top of the filter cloth.
 
Thats a lot of fine silver to add to "alloy up" to the proper range!

Even then, how long would the cell electrolyte last with all the constituent elements stealing two (-NO3) ions compared to silver's one?
Copper contamination is relatively easy to pick up on, but is Zinc? Nickel? The others?

In an electrolysis book I have, the author talks about running coin silver, and low grade silver through a breakdown cell, and when the copper starts depositing, the silver anodes are swapped for a graphite with a "feeble current" to plate out the remaining silver, then upon completion of silver removal, cathodes change to copper ones and the power is raised so the copper deposits quickly and falls into a copper box(thats set up under the copper cathode, to catch the powder that falls).
With the base metals being removed and freeing up acid, very similar to lazersteve's "silver with no nitric" thread (or similarly named), they stop the process when the free acid begins dissolving the copper powder and/or copper box and replace the original electrodes. Starting it all over.

But, what does zinc and nickel contamination look like when it surpases the electrolyte saturation thresholds? Would it be as easy to tell when it is fouling the electrolyte and crystal?

It's probably best to listen to gsp. Dissolve, cement, refine.
-That man KNOWS silver
 
Too bad you don't have a reverbatory furnace so you can blow the tin and some of the copper out of it.

Then breakdown cell. Then finish cell. It still take silver to get pure silver.

Nitric would probably be less hassle, but a large scrubber will be required, or a superatmospheric pressure reactor, which happens to work quite well.

There are flocculants that make metastannic acid a nonissue.


Lou
 
So to my knowledge, no tin. At least it hasn't behaved as if there were tin in it in previous batches.

I know that at 65% of Ag value I could make money, but I'm not sure if that is good business for the customer. Honestly, 100 lbs was just a "big number"...there's probably more there.

But...flocculants you say. Lets talk about these. I imagine you have to have a solution that is devoid of excess nitric. We used a lot of long chain fatty acids to deflocculate mixtures to create colloidals (ink/polymer chemistry). Not quite sure what would be used to change surface charge in opposite direction, outside of metal ions and acids, which would seem to interfere with the PM recovery.
 
Nope...but it's good to know that there is a solution to the metastannic acid.

Are ANY of you knowledgeable on ORP probes? I need one for use with nitric acid / silver...please! Otherwise I'm going to end up at the mercy of a sales associate.
 
Lou said:
, or a superatmospheric pressure reactor, which happens to work quite well.

What are the advantages of pressurized reaction?
Can you slow the reaction by pressurizing it? Well controled with controlled titration of acid?
What kind of pressures?

I've likely got enough stainless pressure vessels that I could manage at least a two gallon 100 PSI reactor without issue.

Is the nitrogen dioxide reabsorbed by the water, or does it have to be vented off? (or, without looking up the phase diagram, it may never actively boil off, but immediately hydrolise a water molecule)

Once vented off, wouldn't you still need to scrub it, or is it then just much easier to vent off at a controlled rate?

My previous polymer experience had me doing a reasonable amount of pressurized work, some at supercritical pressures...so it's a set of tools (& and equipment) that I've already got a stubby handle on.
 
snoman701 said:
Lou said:
, or a superatmospheric pressure reactor, which happens to work quite well.

What are the advantages of pressurized reaction?
Can you slow the reaction by pressurizing it? Well controled with controlled titration of acid?
What kind of pressures?

I've likely got enough stainless pressure vessels that I could manage at least a two gallon 100 PSI reactor without issue.

Is the nitrogen dioxide reabsorbed by the water, or does it have to be vented off? (or, without looking up the phase diagram, it may never actively boil off, but immediately hydrolise a water molecule)

Once vented off, wouldn't you still need to scrub it, or is it then just much easier to vent off at a controlled rate?

My previous polymer experience had me doing a reasonable amount of pressurized work, some at supercritical pressures...so it's a set of tools (& and equipment) that I've already got a stubby handle on.

I wouldn't mess with this unless you have more than a stubby handle on working with pressurized systems and a very darn good idea of what ALL the reactions are. More than one accident has occurred with this technology, including one where a whole refinery had to be evacuated after a rupture disk blew and superheated NOx filled the facility. I've done a bit with SCF myself (CO2, H2O, C4H10, etc).

Many years back when I first ran the process, I was using a 75% Ag balance copper granulate charge and putting in a substoichiometric amount of nitrate to do the job (under ambient conditions). I was running it in a 316L (304L is better fyi) 4 liter Parr autoclave. My first experiment, if I recall correctly, I got up to some 100 psi or so pNO2/NO. As it was still climbing and my regulator only delivered to 200 psi g, I opened and quickly closed the valve to admit O2 into the chamber and noticed an immediate settling down of the pressure and a 20+Centigrade exotherm!

I have 40 some page paper I did on this process. It's in employ at several very large silver refineries where 100K ounce digestions are conducted. I used it for making silver nitrate for silver cells on much smaller scale.

The reaction runs under the conditions of extra oxygen and extra water. Water and oxygen oxidize formed NO2, much like in the latter part of the Ostwald process, to produce nitric acid. This happens at 50+ Centigrade and 50+ PSI O2 overpressure, which must be maintained at all times to prevent a loss of containment accident of the NO2.

4 NO2 (g) + O2 (g) + 2 H2O (l) → 4 HNO3 (aq) (Equation 1)

This then in turn reacts with more silver to generate more NOx, and at the end, heat is pumped into the system through the reactor jacket, whereas at first, cooling is required (both jacket and immersion coil). The end results is that almost 100% theoretical usage of nitrate can be had.

Now, there's one complicating fact that gets people into trouble. This reactions in equations 2 and 3.

3 Ag (s) + 4 H+(aq) + 4 NO3-(aq) → 3 Ag+(aq) + 3 NO3- (aq) + 2 H2O (l) + NO (g) (Equation 2)

which occurs when the nitric concentration in the system falls below about ~15.2 % (the order of the reaction changes, and NO is the product, not NO2). Now that NO will happily react to produce NO2 with excess oxygen, but it takes a while. Ask anyone who knows anything about NOx scrubbing. NO2 is the easy one to control. NO is the pain in the side and needs residence time. This can lead to overpressurization.

2 NO (g) + O2 (g) → 2 NO2 (g) (Equation 3)

That NO will also make nitric acid (per Eq 3), but the rate of Eq 3 is less than the rate of Eq 1. Of course there's also other side reactions (like HNO2, nitrous acid production).

Ag (s)+ 2 H+(aq) + 2 NO3-(aq → Ag+(aq) + NO3- (aq) + H2O (l) + NO2 (g) (Equation 4)

Equation 4 only happens so long as the nitric is in excess, relative to its rate of consumption. This is what you want to have happen. The whole thing became a nasty set of related equations, with some very nasty partials that related to the shrinking sphere problem. Anyway, we had to balance out the increase in surface area of the spherical shot with the decrease in acid concentration, and acid cycling back into the system. All that to hold some sort of steady state.

The only easy way to address the control issue is to add the silver to the nitric acid, under pressure, as spherical granules. That I would feel completely cool with doing. But no one wants to do that. That's expensive. That's complicated. The media serves as a reflux center for nitric acid every time the feed hopper is opened, so wash downs are needed, which means a pressure pump.
 
Snoman asked.

And because you're asking, Chris, I'll tell you:

It helps with compliance and lessens refining costs.
How?

It is against Federal law to release more than 10 pounds of nitrogen dioxide (the Reportable Quantity) into the atmosphere in a 24 h period without a permit to do so. That permit often requires that air quality impact studies be conducted. If one should have an incident, the person is required to notify the National Response Center within certain established time frames.

I have heard of several people who chose to violate the Clean Air Act and either ended up in jail or with heavy fines (much more than 100 lbs of silver!). One of them used to handle the material from a mutual acquaintance of ours in Alabama.

Another reason: say you spend the $1200 (varies), wait the 3-6 months and get a Permit to Install and Operate a packed bed wet scrubber as a pollution control device. Now you have to keep records (time and $), do periodic stack testing, maintain scrubber solution, do change outs and top offs which make another scrubber waste stream (which may be a D002 corrosive, D003 reactive waste, if sulfide based). Lot of work.

Oh, and a scrubber beastly enough to dissolve silver braze at 100 lbs/8h work day...probably $70K.

So, do all this pressure stuff correctly and you get mole for mole substoichiometric nitric consumption, little excess acidity. Lot's changed since the 70s, Chris.
 
Lou

I'm pretty sure that Chris knows things have changed since the 70s. Edit: He also has an incredible knack of explaining things in a manner that will be understood by the average man jack refiner.

This thread is one of a long line of threads asking technical details from the same author who as much as I like him would be best served nailing the basics and getting some results.
 
Jon...I've been working with silver on various scales for years. I used to save my wipes and scrap ink when I was screen printing defroster screens. I'd bring it home and smelt it back to semi-fine silver that I shotted out and sold to metalsmithing students in my art classes. They were happy to get silver for really cheap prices, even if it wasn't .9999.

Gold is entirely new, and yes, I need to work on the basics...it's a hobby, and it is gold fever. I also don't feel like it's sustainable. I've put away many, many processors, and some really rich gold plating...to the point that I've got a few ounces of gold in a very short period of time. But it's shear luck.

As Jon said, digesting silver or silver braze in acid is very literally the easiest thing that can be done. The only preparation that needs to be done is a quick incineration and then cleaning. Cementing it back out isn't especially difficult. I have a jug cementing right now.

My question for Lou was very sincere....as have been any questions I've asked. While I am naturally curious, and interested in the theoretical side as well, I am very project driven. It does exactly as I figured, it addresses my biggest concern with any silver refining, and that's the nitrogen dioxide.
 
Lou said:
I wouldn't mess with this unless you have more than a stubby handle on working with pressurized systems and a very darn good idea of what ALL the reactions are. More than one accident has occurred with this technology, including one where a whole refinery had to be evacuated after a rupture disk blew and superheated NOx filled the facility. I've done a bit with SCF myself (CO2, H2O, C4H10, etc).

I spec'd and built all of my pressurized systems from the ground up, including the SCF. I only have experience with CO2. I miss that work. What I really miss were DOD funds! I have sCO2 capabilities now, but not nearly as high of pressure.

The reactions will take more time to fully understand, since there's a lot of other elements involved.

Pressurized work is nothing to jump in to.

I am curious though, I had to vent my rupture discs outside. Why weren't there plans in place for when a disc does rupture? I don't imagine you can just vent a 100k ounce reactor into the atmosphere, but you also don't want to be filling up a refinery with a poisonous gas either.

Thank you for the primer either way!
 
In the case I was referring to, I'm sure there was a scrubber but it did not have the capacity to handle it all at once!
 
Is it just me, or does 10lbs of NO2 sound like a huge amount?
(I have no doubt in my mind thats not the correct limit.)

But, 10lbs of NO2... Man! I can only imagine that nitric digestion!
 
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