Recycling Silver

Michael,

You have it exactly, and your summary is very well put.

The use of a low constant current is sort of poor man's potentiostat. If the current is very low, the electrode potential will also be very low, and can be arranged to be above the silver reduction potential but below the sulfate reduction potential. As the silver in solution is depleted, the electrode potential will rise. By watching the cell voltage, it is possible to estimate the point where nearly all of the silver has been removed, and stop the process there. This is the principle on which most battery chargers are based, although usually not with silver chemistry

It is also possible to simply apply a voltage slightly above the reduction potential for silver, but there is always a certain amount of "overvoltage" required that may be difficult to estimate. This is the reason why really cheap battery chargers tend to ruin rechargeables.

I was delighted to see that last URL you posted---I've been telling myself for some time that I should be able to make a usable silver electrode, but couldn't quite get motivated to actually do it. There are lots of older circuits (pre-microprocessor era) for potentiostats built around integrated operational amplifiers, so I really should follow through with this. That having been said, I also want to work the kinks out of my zinc cementation process and maybe try sulfide precipitation followed by potassium nitrate fusion. Gosh, where does all the time go?

Again, excellent summary--if you are new to electrochemistry, you must be a pretty fast learner!

greybeard said:

if you are new to electrochemistry, you must be a pretty fast learner!

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Well, my contracting jobs for the past 30 years have been to hit the ground running in hundreds of different manufacturing processes. I take process descriptions from guys like you, and I figure out how to "do them" in the world. And I have seen a *LOT* of processes over the years.

MB

Ah, that explains it, then.

Me? I spend a lot of my time looking at things that have been done by guys like you, and trying to figure out just how it is that they actually work

Back in the day, I did a fair amount of process automation in an integrated circuits lab, and I can state without reservation that if you can get a computer to make it work robustly, you really understand it.

On topic again: it wouldn't surprise me to find out that someone makes a battery charger that could be turned into a silver electrolyzer by feeding in a reference potential (suitably buffered and scaled) at the point where a voltage reference diode is originally found. It might be easier to duplicate than a totally home-brew circuit engineered from the ground (so to speak) up.

greybeard said:

On topic again: it wouldn't surprise me to find out that someone makes a battery charger that could be turned into a silver electrolyzer by feeding in a reference potential (suitably buffered and scaled) at the point where a voltage reference diode is originally found. It might be easier to duplicate than a totally home-brew circuit engineered from the ground (so to speak) up.

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I'm not sure that's all so practical. I expect the volumes processed by home users, and the anticipated plate area for the cathode, would render the capacity of even a small battery charger mostly useless. I suspect it's just too big, and not worth the expense to modify.

I doubt anyone would tackle home brewing one without a fair amount of technical background, say a manufacturing plant technician. And in that case I suspect that the requisite parts are easily obtainable in small quantities or even from a well stocked junk box.

Now that I understand the theory better, I would wager that something which can handle all of my waste fixer can be made this way. As a "weekend warrior" I neither produce large quantities like a regular lab would do, nor am I constrained by the fact that slow reduction may take a long time. Turn it on on Sunday afternoon and walk away for a week, sometimes two. I think I'm fairly average for the APUG population.

While not up to industrial efficiencies, I suspect that I could recover 85% of the silver in my spent fixer using no more than a doorbell transformer, a couple of op-amp ICs, a few biasing resistors, a voltage comparator IC, whatever material is good for the anode and cathode, and the homemade silver reference cell in my last link.

Granted it would take a fair bit of time for me to get it all constructed, and it might very well be cheaper to buy the commercially available "Silver Magnet" device if one considers the labor. But if the silver is then available for wet plate use, it's all good isn't it?

How hard is constructing this silver reference cell that is in the last link?

michaelbsc said:

I'm not sure that's all so practical. I expect the volumes processed by home users, and the anticipated plate area for the cathode, would render the capacity of even a small battery charger mostly useless. I suspect it's just too big, and not worth the expense to modify.

I doubt anyone would tackle home brewing one without a fair amount of technical background, say a manufacturing plant technician. And in that case I suspect that the requisite parts are easily obtainable in small quantities or even from a well stocked junk box.

Now that I understand the theory better, I would wager that something which can handle all of my waste fixer can be made this way. As a "weekend warrior" I neither produce large quantities like a regular lab would do, nor am I constrained by the fact that slow reduction may take a long time. Turn it on on Sunday afternoon and walk away for a week, sometimes two. I think I'm fairly average for the APUG population.

While not up to industrial efficiencies, I suspect that I could recover 85% of the silver in my spent fixer using no more than a doorbell transformer, a couple of op-amp ICs, a few biasing resistors, a voltage comparator IC, whatever material is good for the anode and cathode, and the homemade silver reference cell in my last link.

Granted it would take a fair bit of time for me to get it all constructed, and it might very well be cheaper to buy the commercially available "Silver Magnet" device if one considers the labor. But if the silver is then available for wet plate use, it's all good isn't it?

How hard is constructing this silver reference cell that is in the last link?

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For scientific research it's probably a neat project, for actual silver recovery it's probably not that practical, considering that, yes you can get roughly 2g of silver per litre of fixer, at current silver prices that would be roughly $2.30 per litre, and you would probably spend close to $2 on the electric power to recover it. I think silver recovery is more about environmental impact, and at that, I would rather find a lab that is recovering their silver and ask if I could put the small amount of fixer I use through their process.

Hey, it's a hobby. It doesn't have to make economic sense. It just has to be interesting.

wogster said:

... considering that, yes you can get roughly 2g of silver per litre of fixer, at current silver prices that would be roughly $2.30 per litre, and you would probably spend close to $2 on the electric power to recover it.

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Your finger probably just slipped on the calculator. When I figure the cost of the electric power, I come in quite a bit lower. Let's just say that even the wizards of Wall Street would not be able to get it to round up to one cent.

For scientific research it's probably a neat project, for actual silver recovery it's probably not that practical, considering that, yes you can get roughly 2g of silver per litre of fixer, at current silver prices that would be roughly $2.30 per litre, and you would probably spend close to $2 on the electric power to recover it. I think silver recovery is more about environmental impact, and at that, I would rather find a lab that is recovering their silver and ask if I could put the small amount of fixer I use through their process.

Wow. And I thought that we Californians paid a lot for electricity.

The estimate that I gave was based on a Troy ounce of silver per gallon, which seems to be a reasonable guess (this is a good bit more than 2 grams per liter, but that won't change the conclusion). The total charge transfer of some 28,000 coulombs sounds like a lot, but it is only 7.7 ampere hours, at a cell voltage of around a volt. Assuming only ten percent efficiency, that is still only 77 watt-hours. The local rate for residential service is twelve cents per kilowatt hour, so those 77 watt-hours would cost me a bit less than one cent.

At the silver price mentioned, a Troy ounce of silver would be worth $35.77.

Hey, it's a hobby. It doesn't have to make economic sense. It just has to be interesting.

But if it is fun and makes economic sense, it's even better.

greybeard said:

Hey, it's a hobby. It doesn't have to make economic sense. It just has to be interesting.

But if it is fun and makes economic sense, it's even better.

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True, that would be a win/win. But like my hunting friends, I don't bother trying to cost justify a hobby any more.

How difficult would this reference cell be to get built properly?

http://www.consultrsr.com/resources/ref/agcl.htm

Yea, at 1mA it would take a LONG time to rack up even a pennies worth of kW-hrs. If you really want to be green, you could use a solar cell. If you were not too picky about how long it took wouldn't a 4.7k Ohm resistor and a 12v wall wart get-er-done. I was thinking cut Aluminum strips from a soda can would work good for the anode and cathode. They are mostly impervious to Nitric acid. Also, I found some mention that hydrogen peroxide could be used to keep the N2O2 down when cleaning the cathode and making the AgNO3.

Whatcha' guys Think?
Chris Maness

greybeard said:

For scientific research it's probably a neat project, for actual silver recovery it's probably not that practical, considering that, yes you can get roughly 2g of silver per litre of fixer, at current silver prices that would be roughly $2.30 per litre, and you would probably spend close to $2 on the electric power to recover it. I think silver recovery is more about environmental impact, and at that, I would rather find a lab that is recovering their silver and ask if I could put the small amount of fixer I use through their process.

Wow. And I thought that we Californians paid a lot for electricity.

The estimate that I gave was based on a Troy ounce of silver per gallon, which seems to be a reasonable guess (this is a good bit more than 2 grams per liter, but that won't change the conclusion). The total charge transfer of some 28,000 coulombs sounds like a lot, but it is only 7.7 ampere hours, at a cell voltage of around a volt. Assuming only ten percent efficiency, that is still only 77 watt-hours. The local rate for residential service is twelve cents per kilowatt hour, so those 77 watt-hours would cost me a bit less than one cent.

At the silver price mentioned, a Troy ounce of silver would be worth $35.77.

Hey, it's a hobby. It doesn't have to make economic sense. It just has to be interesting.

But if it is fun and makes economic sense, it's even better.

Click to expand...

A troy ounce is 31.1034768g film fixer should be kept to 2g or less per litre for archival use, paper fixer can hold more, IIRC it's about 5g, so at those rates, it takes about 15½L of film fixer. That's roughly 4 gallons. Paper fixer should be around 6L or roughly 1½ gallons. I don't think a lot of us use paper fixer to exhaustion, so it may be a lot less in a litre of fixer.

Wogster:

The concentration of silver in the bath has little bearing on the economics of the process, and none at all on it's entertainment potential. Faraday's constant is the ratio of coulombs to moles, and is independent of concentration. In fact, the lower the concentration of silver ion, the easier it is to obtain a dense (and easily handled) deposit.


kq6up:

You probably don't want to use aluminum--nitric acid doesn't dissolve it, but sulfate (as in "used fixer") does, and as soon as you have some silver plated out you will have a "local cell" (tiny, short-circuited battery) which will probably cause the aluminum to return to its native geological origins. Stainless steel works well for the cathode; you can dissolve away or just peel off the silver, and graphite is the traditional anode (but stainless steel would work well there, too). One of the easier configurations would probably be a pair of discount-store stainless steel bowls or pots that "nest" with a half inch or so gap for the electrolyte. If you put the outer container on an old record player turntable for agitation, with a sliding brush electrical contact, you would have a really nifty electrolyzer.


michaelbsc:

The reference cell at that website is somewhat more sophisticated than necessary. For our purposes, we just need a stable reference, not an exact potential, because we can offset it to get the chemistry right. Assuming you can get hold of some silver wire (jewelry-making suppliers sell it in small quantities, and sterling alloy is probably pure enough) all you have to do is pack the tip of an eyedropper with glass wool (try the aquarium department of a pet store, the stuff from your attic insulation may be a bit too brittle), stuff some silver chloride on top of that, insert the silver wire and then fill the rest of the space with saturated potassium chloride. Sealing the top of the eyedropper will make the assembly easier to handle, and if you use the original rubber bulb you can restore the potassium chloride as needed by using a hypodermic syringe. (I can hear the doomsayers now: if you accidentally stick yourself, you're gonna die. So just don't do that...)

Two things to keep in mind: for this to be a stable reference, you have to draw zero current, and the fixer will dissolve the silver choride if it gets into the electrode. An op-amp voltage follower will give you a sufficiently good approximation to infinite input resistance to satisfy the first condition, and because the current is negligible, it is usual to employ a "salt bridge" (a longish tube filled with the cell electrolyte) to limit the diffusion of working electrolyte into the cell electrolyte while making an electrically continuous path.

greybeard said:

...That having been said, I also want to work the kinks out of my zinc cementation process and maybe try sulfide precipitation followed by potassium nitrate fusion. Gosh, where does all the time go?

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I have said for years that should I somehow win the Mega-millions lottery I wouldn't possibly be able to throw away my alarm clock.

Even if I never had to work another day in my life and I live to be a hundred I can't possibly finish all the stuff on my plate.

What, you only have one plate?

greybeard said:

What, you only have one plate?

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OK, so I'm a slacker.

Not necessarily. Maybe my plates are all much, much smaller than your one!

(20x24 wet plates, anyone? )

I have a whole box of photo paper that I now have reason to believe it is all bad. It is over 30 years old, and I was hoping it was good. I gave some to a student, and it came back crap. I tested some, and it WAS crap, beyond even artsy crappy crap. I was thinking a couple of possibilities, burn it, and smelt the ashes in a crucible. Or try to find some cheap mining grade KCN and leach all of the silver out in a 5 gallon bucket. I would think that I could use more current with the CN- complex ion because I would not be generating H2S. Any thoughts? It is a lot of paper with I am sure at least several ounces of silver.

Chris

If it is decent paper there was a discussion a couple if years ago about turning it I to POP.

What is "POP"?

Chris

kq6up said:

What is "POP"?

Chris

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POP is "Print out paper" which develops the image as it exposes as opposed to developing an image when put in a developer.

How long does it take to print it out in the sun?

So just stick it in a printing frame in a neg sandwich and fix when toasty? correct?

Interesting, I have seen paper get a ugly tint sitting in the trash can of my darkroom. I did not think it would be any good for anything but recovering silver from.

Thanks,
Chris Maness

Yes. Stick it in a print frame and check periodically. Just like cyanotypes. You have to soak it in something first, but that escapes me for the moment.



I haven\'t tried it yet. The discussion started a few months after I had thrown out a box of fogged paper, and I followed it pretty keenly.



I just got a box of fogged paper from another member, but haven\'t had time to experiment.

The thread is (there was a url link here which no longer exists)


Anyway, I have about 8-10 gallons of spent chemistry to take to the hazardous material dropoff. I'd be cool to recover the silver before taking it in.

willrea said:

The thread is (there was a url link here which no longer exists)


Anyway, I have about 8-10 gallons of spent chemistry to take to the hazardous material dropoff. I'd be cool to recover the silver before taking it in.

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You might try evaporating it down a little. If you want to just have it dump Silver Sulfide you can add Hydrogen Peroxide in excess. I have not done this in bulk, and it is quite exothermic, so start out with smaller batches to make sure it doesn't start boiling and getting out of control. The Ag2S will take several days to settle out as a black sludge. You can do the same thing with stainless steel electrodes and a 12V source. However, do not do this inside as you will get rotten egg gas if you do it quick and dirty with high current. The precipitate you can strain off and dry. Ag2S will dissolve in aqua-regia to give insoluble AgCl. However, be careful as this evolves poisonous N2O4 as well.

Have fun,
Chris Maness

i have made sun prints like this .. for almost a year ... and ran into a big issue ...

the problem with using regular photo paper as a POP type paper is
you can't fix the image. it will clear to white when fixed
and it will turn black or grey soon after it is scanned a few times.

regular pop images were toned and then fixed ...
i haven't toned and then fixed the faux pop images i have made with
regular photo paper, but from what i understand because it isn't actually a
chemical reaction just a "sun-stain" it won't tone+stain ...
i haven't tried to tone + fix to test what i was told though ...
i just believed my trusted source.

john