Silver Recovery

[Ryuji]

When you add a less noble metal to a solution of a silver salt, an oxidation-reduction reaction takes place. The less noble metal is oxidized into solution and the silver salt is reduced to silver metal, which appears as a black sludge.

I've done this many times by adding steel wool to fixer. After a day or so, the steel wool has partially dissolved and there is silver present as a black suspension or sometimes a "mirror" on the wall of the container. The overall reaction is 2Ag(I) + Fe -> 2Ag(0) + Fe(II). There may also be some Fe(III) present. The principle with aluminum is similar, in that case the reaction goes as 3Ag(I) + Al -> 3Ag(0) + Al(III). Depending on the pH of the solution, your Al(III) will precipitate spontaneously and mix into the Ag sludge.

This is an easy way to remove some silver out of the solution, but the efficacy of such treatment is known to vary hugely depending on the exact composition of the steel, shepe of the cartridge, and the forced flow mechanism. The regulation for residual silver content in many areas is very low, and the steel wool method often does not meet the standard. In small scale amateur darkrooms, steel wool method is better than doing nothing, but don't assume this is the satisfactory solution in all situations.

Best silver removal method in terms of efficacy and cost is electrolytic system using stainless steel and moving carbon electrodes. The SS electrode will corrode very unevenly if the carbon electrode is not moving circularly around the SS disk.

It is not a great idea to ingest huge amounts of aluminum salts, but small amounts are okay. Aluminum hydroxide is one of the ingredients of Mylanta, and it dissolves in the stomach to produce soluble Al(III). Aluminum hydroxide flocculation is a major method of wastewater treatment.

Although I don't like to heat acidic food in non-anodized aluminium cookware, the whole "aluminium-pan can cause Alzheimer's disease" thing was a malicious rumor made by stainless steel industry in the past. In reality, alum is used in pickling (tho I don't use it), and some baking powders contain aluminium compounds as well. (Note that baking powder is a mixture containing baking soda, but not by itself.)

 

[Ryuji]

Suppose that I wanted to electroplate out the silver onto a cathode; could I put the used fixer solution in a plastic tub, attach metallic electrodes to either side of the container, and connect an appropriate DC current source?

As I wrote in my previous post, this is not sufficient. The electrode should move around so that all parts get used evenly.

Also, are there any gasses produced in the reaction that I would need to be concerned about, such as hydrogen?

The electrodes are made from stainless steel and carbon. The most advanced electrolytic system maintains the cathode potential at or upward (more positive) of -0.55V with reference to saturated calomel electrode. It's also best to adjust the pH to about 7 to 8 range before starting silver recovery (the pH of Clearfix is ideal for silver recovery system without any adjustment, and most color fixers are not far from it either).

A recent report on electrolytic silver recovery system was published by Mina and Chang of Kodak Rochester Lab in 1982. Mina, R. and Chang, J. C. 1982. Electrolytic silver recovery from spent fixing solutions---an electroanalytical study. Photo. Sci. Engr., 26, 223-7.

 

[Ryuji]

If you're talking about recovering silver from fixer, the excess thiosulfate in solution prevents the formation of insoluble silver chloride. (Just like fixing film.)

Agreed. Precipitation of silver halide in spent fixer is impractical unless thiosulfate is fully decomposed. But such an approach will have to deal with elemental sulfur, and will lead to another kind of sludge.

To the best of my knowledge, all methods of silver recovery involve reducing the silver somehow (either electrolytically or chemically).

A very simple, easy and fast way to remove silver out of spent fixer is to form any of a few insoluble silver compounds, such as silver sulfide. If you add sodium sulfide or liver of sulfur in exhausted fixer, you can easily see that silver sulfide is forming. There are other chemical agents that can precipitate insoluble silver compounds or metallic silver in the spent fixer bath. HOWEVER, the difficulty is that any of these methods will make particles smaller than 1 micron, and no inexpensive filtration method works to separate sludegs from a large quantity of treated solutions. (Most laboratory paper filters will let the particles go through, and the fine mesh filters will clog very fast.)

Ryuji

 

[trudee yama]

silver recovery unit devise

much to my surprise and delight, i found in "Porter's" a silver recovery unit. Does this really work? If i use this device, can I pour the silver free fixer down the drain?

 

[bob100684]

it depends on your local regulations. At one lab I worked at that was indeed the case, at another, one town over even our recovered waste had to be taken out by truck.

 

[Fulvio]

Metallic Silver recovery from fixing bath?

I don't know where to post this, but maybe fits this forum area.

Is there a "cheap" way to recover metallic silver from the fixing bath? (from any fixing bath? - I use hypo most of the time, though)

How about some electrolysis process? But how?

It would be even nicer if it becomes possible to recover the fixer at the same time too...

If you know some method it could be interesting to try out... as long as it's not
deadly insane as making silver nitrate!

 

[Photo Engineer]

Steel wool. There are lots of comments on this on APUG and Photo Net.

PE

 

[Fulvio]

Steel wool. There are lots of comments on this on APUG and Photo Net.

PE

thanks PE,

I should have realized this is a FAQ...

do you have a link where several recovery techniques are described in full detail?

thanks

 

[Photo Engineer]

Nope, sorry. This is so well known to me it is the kind of thing that often goes under my radar so I don't remember the specific links.

PE

 

[Fulvio]

I have found this:

http://www.pic.uk.net/coppice/coppice_pages/silver_recovery.htm

but does not go in deep detail

as far as I understood with the steel wool method you replace silver with iron in the fixing bath, making it more "environment-friendly" to dispose... And you get a silver sludge in the bottom of the bucket. But it has to be refined, right? What about the fixing solution? is it gone forever?

 

[panastasia]

I don't know where to post this, but maybe fits this forum area.

Is there a "cheap" way to recover metallic silver from the fixing bath? (from any fixing bath? - I use hypo most of the time, though)

How about some electrolysis process? But how?

It would be even nicer if it becomes possible to recover the fixer at the same time too...

If you know some method it could be interesting to try out... as long as it's not
deadly insane as making silver nitrate!

I'm curious, are interested in silver recovery because you think it's an environmental hazard, or do you intend to sell-back the silver for cash?

If it's to save the environment, that concern was nearly beaten to death a few months ago on this forum. I'll offer no further comment in that regard.

 

[Photo Engineer]

Well, there are references here on APUG.

Throw in steel wool and let the fix sit. After a day or so, collect the steel wool and the sludge and throw away the liquid as it is now unusable. The sludge and steel wool now contain silver. The steel wool is crumbly due to part of the matrix now being silver.

A photo finisher can send it out for silver recovery.

PE

 

[panastasia]

The steel wool goes into solution, replaced by the silver (replacement process at the molecular level). The silver is taken out of solution (the sludge) and is a rather pure form with some monetary value.

 

[Ian Grant]

I agree steel wool, but if you want to get good silver to make silver nitrate then just plate it out, that@s the easiest.

Assuming it's small scale then just rig up a rechargeable battery around 5 to 6 volts with silver wire for the anode and a carbon rod cathode.

PIC - a company I worked for used to be a member of PIC, we installed both Electro plating units and wire wool systems. With steel or wire wool systems the resulting slugde isn't very pure usually often about 60% silver, whereas plated silver will be over 98% pure, often far higher.

Ian

 

[Photo Engineer]

The steel wool goes into solution, replaced by the silver (replacement process at the molecular level). The silver is taken out of solution (the sludge) and is a rather pure form with some monetary value.

The fabric of the steel wool is gradually replaced by silver and the steel wool also rusts, therefore the entire mess, sludge plus crumbly wool residue, is a mixture of silver, iron and iron salts. This mass (mess) is what can be recovered, but is by no means 'rather pure'. It does have monetary value.

PE

 

[panastasia]

The fabric of the steel wool is gradually replaced by silver and the steel wool also rusts, therefore the entire mess, sludge plus crumbly wool residue, is a mixture of silver, iron and iron salts. This mass (mess) is what can be recovered, but is by no means 'rather pure'. It does have monetary value.

PE

PE

I had a description (a report) on the various processes of silvery recovery some years ago and remember the recovered silver was described as a "relatively pure form". But I may have read it wrong and I trust your knowledge on the subject.

Paul

 

[Photo Engineer]

Paul;

"Relatively pure form" is a slippery phrase. But depending on the silver content of the hypo and the type of hypo solution, yes it can be 'relatively pure'. The other ingredients are iron and iron salts primarily. And the silver is rather unusable until it undergoes a rather lengthy repurification such as described by Ian. This involves dissolving it again, electrolysis and then dissolution in nitric acid and recrystallization.

PE

 

[Ian Grant]

Ron, the smelted silver from iron sludge's is relatively pure after smelting, usually over 90% and up to about 96%, that is why there can be confusion, as we both know the sludges have a much lower silver content.

Ian

 

[panastasia]

Thanks Ron and Ian for clearing that up!

Paul

 

[Photo Engineer]

Ron, the smelted silver from iron sludge's is relatively pure after smelting, usually over 90% and up to about 96%, that is why there can be confusion, as we both know the sludges have a much lower silver content.

Ian

Ian;

Yes, but the sludge before smelting can vary from low to medium or high depending on a huge variety of factors, and that is all I was pointing out.

The smelted silver at 90% - 96% is sill too impure for photographic use due mainly to iron contamination. The photo grade silver ingot must analyze to about 99.999% pure with a very very tiny fraction of iron, lead, mercury, zinc, copper.. well you get the picture.

PE

 

[Photo Engineer]

I should add a afterthought.

Since many trace heavy metals are added at 6x10^-6 moles / mole of silver, this means that some impurities must be present at well below ~0.00000x grams / 108 grams of silver. The x denotes 6 x the molecular weight of the material. This gives you an idea of how pure photo grade silver must be.

PE

 

[Ulrich Drolshagen]

AFAIK you will have to use difficult to get *iron*wool rather than usual steelwool as you can buy easily in your local home improvement store. I have been told, using ironwool is a rather messy operation. The better way seems to be using sodium dithionite. Use 2 tablespoons per litre and put the (open!) containment in the garden far away from noses for some days. It will outgas hydrogen sulfide. After the reaction has finished the metallic silver particles sink down to the bottom of the containment and the liquid can be siphoned off with a small tube.

Ulrich

 

[Photo Engineer]

Ulrich;

In the US, a product called Steel Wool comes in 8 grades. It is dull gray and is produced as fine threads in various sizes according to these grades. It is sold in hardware stores. All of the grades appear to work in desilvering hypo. Another product comes as shiny ribbons as a kitchen product for cleaning pots and pans. It will not (AFAIK) work in hypo. It is sold in grocery stores.

PE

 

[Ian Grant]

Ulrich, the cheapest wire wool* is used for silver recovery from fixer and has been for a great many years, the sludge is normally just left to dry before smelting. *From your local home improvement store.

When I worked in precious metal recovery our factory plated out as much silver as it could overnight then the residual silver was recovered by passing the fixer through normal wire wool. Commercial cartridges for bleach fix use a different grade because bleach fix rots the wire wool, it's this that can be very messy.

Ron, my comments were really an addition to yours to a previous poster.

Ian

I

 

[Photo Engineer]

Ian;

Yes, I understood that. I felt that my comment needed further clarification in the following two posts of mine. Thanks. I would add that Wire Wool in the UK, would probably be what we call Steel Wool in the US.

I have found that acid fix rots the 'wool' as well as the silver replaces the iron and the whole thing is a real mess to me.

PE