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Recycling silver in fixer for alt processes purposes?

Philippe Gauthier , Feb 19, 2004; 11:44 a.m.

I've noticed that silver tends to precipitate as flakes in old film fixer left to sit for a couple of weeks. Has anyone here tried to recycle it? I'm not thinking of getting metallic silver, for which I have no use, but to get silver nitrate, which is rather expensive and used in small quantities in several cool alt processes.

I know that you must dissolve silver in nitric acid to get silver nitrate, but pure nitric acid is nasty stuff; does anyone knows of the lowest concentration that would work? Are the flakes pure enough to be turned in sufficiently high grade silver nitrate? Are there other safety considerations?

My assumption here that the silver which precipitates spontaneously will be purer than the mixed silver/iodine sludge that you get with the usual "iron sponge" trick to remove the silver from the fixer. Is this a reasonable hypothesis? I'm not trying to make anything that makes sense on a business basis, just to find a cool way to prepare some of my own chemicals.

Responses

Donald Qualls , Feb 20, 2004; 11:16 a.m.

The "mixed silver-iodine sludge" obtained by using steel wool to precipitate dissolved silver from used fixer should work very well for reprocessing into silver nitrate -- the trick is to take up the iodine in some manner before reacting with the silver nitrate. One simple way to do this would be to first react the sludge with a slight excess of sodium hydroxide (very hazardous stuff, but not much more so than nitric acid). The solution need not be strong, it just needs to replace the hydroxide in solution with iodide, and 0.1 Molar should suffice. The sludge can be washed with hydroxide solution until the solution remains alkaline, and the resulting sodium iodide solution can, if desired, be evaporated to give the iodide in crystal form for easy disposal.

The remaining material should be pretty pure silver, probably pure enough to work well with nitric acid to produce silver nitrate. I've considered this process myself, though I don't have any exhausted fixer yet to start with (and the quantity obtainable is tiny). The nitric acid need not be concentrated; 0.1 Molar solution ought to be more than strong enough (though the resulting concentration of silver nitrate in solution will be low, it can be concentrated by evaporation -- boil off the excess water), and is safe enough to be used in first chemistry courses in high school, probably no more hazardous than 28% acetic acid used for making stop bath. Just add nitric acid to the washed sludge, with stirring, until no more sludge dissolves (you may leave some iron oxide behind -- it won't react with the nitric acid and you won't want it anyway), leaving a slightly acid solution (which can be neutralized with a little of the sodium hydroxide solution not used for the wash step).

Key here is that neither hydroxide or acid needs high concentration -- weak solutions are much safer to work with. Even so, wear chemical rated gloves (either solution under a nail can cause a nasty burn) and eye protection (even weak hydroxide can cause serious eye damage, possibly without telltale irritation until after the damage is done; nitric acid will stain the cornea, though it's not as bad as hydroxide).

Philippe Gauthier , Feb 20, 2004; 03:09 p.m.

Thank you, it looks a bit less like suicide that way... Do you know anything about electrodes? A contact tells me that one of his relation uses a copper and an iron electrode to separate the silver; the silver desposits on the copper electrodes and he later dissolves it in nitric acid. It looks a bit cleaner, but I wonder if the reaction is spontaneous (fueled by the difference of potential between iron and copper) or if it needs some external electric power. Would you happen to know anything about this?

This guy also says that with the electrode thing, he gets up to 10 grams of silver per liter of fixer, that he later dissolves in nitric acid. This sounds way too much to me, unless the fixer is absolutely 100% spent and exhausted - and even then...

Last thing: I would suppose that the evaporation of silver-saturated nitric acid emits toxic fumes, is that right?

Donald Qualls , Feb 20, 2004; 09:53 p.m.

Well, let's see -- the copper and iron electrodes are probably just wired together. There is electricity involved, but it's generated by the dissimilar metals in an electrolyte solution (since most fixer is slightly acidic). You should see silver precipitating as a flock on the copper electrode, from which it can be washed with distilled water and filtered from the wash for collection. The figure of 10 g/L seems a little high, but that's presumably for exhausted fixer, which would have fixed as many as twenty films per liter; that would amount to removing only half a gram of silver (as the chloride, bromide, and iodide, of course) from 80 square inches of film and might not be off by a huge factor -- especially if the film involved is something like X-ray film, as it often is when people talk about silver recovery devices they've seen. Or it might be 1/10 g/L, instead, which would still be nearly free compared to the price of silver nitrate. The silver produced this way is probably better for use in making silver nitrate, as long as it can be adequately separated from the copper -- copper and nitric acid react spectacularly, with evolution of nitric oxide gas (which is very, very bad to breath, even in tiny quantities). There wouldn't be much gas from weak acid, but a little is still too much.

Evaporating nitric acid to concentrate it is generally a bad idea -- at some concentration, the acid comes off as rapidly as the water, and not only doesn't the acid get stronger, but the vapor is very corrosive and quite dangerous. Fortunately, if you use only a small excess of 0.1 M acid (just enough to be sure all the silver has reacted, added slowly with agitation), there will be little if any nitric acid in the solution, which will evaporate down to silver nitrate crystals (the hydrated form, of course). You'll want to dry these crystals, and weigh them, so you know how much water to add to make your silver nitrate solution -- but the amount of acid left, if you added the acid slowly until the silver just dissolved, should be too little to cause trouble. Working under a fume hood is still a good idea, of course, but a kitchen exhaust fan is probably adequate for this process using dilute acid.

Jacques Augustowski , Feb 20, 2004; 11:10 p.m.

Stainless steel anode and carbon cathode or the way around I don't remember but it works, giving a pure silver deposit on the stainless steel, monitor the current so that a sludge doesn't form, by separting the anode from the cathode or reducing the voltage. a simple wall wart of low current works perfectly, the fixer should a little acidic Jacques

Philippe Gauthier , Feb 21, 2004; 12:33 p.m.

Thank you Donald - the whole scheme looks relatively easy. I'll start collecting my fixer and I'll see how much silver I can gather when I use these electrodes. What about increasing the acidity of the fixer before using the electrodes, adding some acetic acid, for instance? Is it worth the trouble?

Dan Schwartz , Feb 21, 2004; 07:53 p.m.

Philippe,

I just stumbled across this thread, since I only venture over here about once or twice a week. I never considered converting recovered silver into silver nitrate: Instead, I was planning on sending it in for beer money! :)

Anyway, do it electrically, with stainless steel electrodes -- I don't recommend carbon because it's fragile. Then, just hook up a DC power supply to the electrodes, and sit back & wait

Incidentally, I'm in the process of building a silver recovery unit to keep my fixer(s) "clean" to extend their' life, a la inline silver recovery. Check out this thread (opens in new window) with an overview of my design. One important consideration is to make sure you use glass, stainless steel, or a polyolefin (polyethylene) for the construction.

Cheers!
Dan Schwartz

Dan Schwartz , Feb 27, 2004; 07:41 a.m.

By the way, I'm using a 33 gallon Rubbermaid (high density polyethylene (HDPE)) tote, which I bought for $9 at K-Mart. I work part time at a commercial color print lab lab, and they are all too happy to give me their used RA4 bleach-fix and C41 bleach-fix and stabilizer -- About 2-3 gallons per day!

Basically, with this kind of volume I'll be doing a 2 step process: I'll be recovering about 95% of the silver by electrolysis, which plates pure silver on the electrodes. Then, for the last 5% I'll react it out with a wad of steel wool, then sewer the cleaned liquid.

Cheers! Dan

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