# used for silver



## shyknee (Mar 6, 2011)

I know this is used for silver. Can anyone fill in more details on how it was used ?
Was it for stripping silver from fixer or was it just a nitric based silver cell?
http://cgi.ebay.ca/Film-Silver-Reco...428?pt=LH_DefaultDomain_0&hash=item2c5ae18e0c


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## goldsilverpro (Mar 7, 2011)

It's a typical unit designed for plating silver out of photo fixer, which is circulated through it. The stainless drums (cathodes) rotate and the silver deposits on the outside of them. The rotation is necessary to agitate and continually provide fresh solution at the cathode surface. Without this rotation, black, loosely adherent silver sulfide will form and you end up with a mess. Too bad no power supply comes with this unit. There should be a lot of these units available. They are not used much nowadays since most everyone has gone to dry developing or digital. A few years back, almost every large hospital and newspaper office had at least one of these units. I doubt if it's the best thing to use for a silver cell.


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## joem (Mar 7, 2011)

Yes this is for film fixative silver recovery.
Metafix units are still available. I got one free last fall and have another coming soon. A lot of smaller print shops still use film because they use older printing presses and the digital technologies do not match. In my industry we do not throw out older machines just because newer ones come along. We use them because they have been paid for long ago and every press run is very profitable. So it goes a lot of shops still use film and generate waste fix, as do dentists, hospitals, and photograhy. My reasearch has shown that my own teaching shop waste fix can produce at least 1/2 an ounce of high quality silver per 10 liters of fix. That's why these older recovery units are still being sold even without all the parts.


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## goldsilverpro (Mar 7, 2011)

A little more about why there is a rotating drum cathode on these units. A little electroplating 101.

I'm no chemist, but I'm pretty sure this is how it works. The silver in these solutions is in the form of a negative complex silver thiosulfate ion. Since this ion is negative, it is attracted to the anode and *repelled by the cathode*. All plating occurs only from the thin solution layer right next to the cathode and this layer's thickness is probably measured in nanometers. Once this layer is somewhat depleted, other reactions *must* occur, as long as there is current flow. In this case, the next cathode reaction, in the order of things, is the reduction of the thiosulfate ion to the sulfide ion. The sulfide ion precipitates silver sulfide, a black smelly material. 

The rotating cathode continually puts fresh solution into the cathode film and thus prevents the formation of sulfide. You can not plate out a sound silver deposit from a static fixer plating system. You must have some sort of agitation present that will continually refresh the cathode film. Other methods of agitation, such as ultrasonics or the impingement of solution on the cathode, can be successful.

Even with a rotating cathode, another thing that will cause sulfide to be produced is when the silver concentration in the overall solution becomes depleted beyond a certain level. In practice, they often plate, until the silver reaches a certain level, and then polish the solution by removing the remaining silver in a bucket of steel wool - a cementation reaction.

In most non-cyanide plating solutions, such as those used normally used to plate nickel or copper, the metal ion is positive and is attracted to the cathode. Therefore, agitation is not as critical in these cases. 

For some reason, although gold or silver form negative complex ions in most solutions, here again, agitation is usually not very critical. Probably because there is no secondary reaction (such as the sulfide production in the fixer solution), except for water splitting, that has a high enough cross-section (probability) to occur. The solution keeps plating gold, even though no agitation at all can cause the gold to plate at a lower rate. Water will then split to adjust the difference and maintain the set current flow.

For each valence form of each metal ion, there is a rate of deposition (plating) that will occur when 100% of the amps applied deposit metal, with no side reactions. This is called 100% "efficiency". For silver, which always has a valence (I realize this term is antiquated, but, so am I) of +1, it plates, at 100% efficiency, 4.02 (I think) grams per amp per hour. You can never plate silver faster than that. Normally, the lower the valence, the faster it will plate. In a cyanide solution, gold (valence of +1) will plate 3 times faster than out of a chloride system (valence of +3), at 100% efficiencies.

In practice, no metal plates at 100% efficiency. 98+% is about as high as you get from any metal. Bumper-type chrome plates at about 40% efficiency. The reduction in efficiency is always caused by the kicking in of a secondary reaction (or, is it the other way around?), of which water splitting at the cathode, (OH- ion and H2 gas are produced from the water) is the most common , which, in general, causes few problems. I could probably list 20 things that will effect plating efficiency - anode/cathode ratio, anode/cathode spacing, metal concentration(s) in solution, contamination (both organic and inorganic), condition of the cathode surface, agitation (or, lack of), etc., etc.

The anode efficiency (rate of dissolving) can also come into play. The rate of dissolving, at 100% efficiency, is exactly the same number as 100% efficiency plating, which is 4.02 g/amp-hour. For example, if the cathode efficiency is lower than the anode efficiency, the pH will climb. If the anode efficiency is lower, the pH will go down. This is due to the H+ ion (and O2 gas) produced from the water at the anode. If the efficiencies are equal, the pH won't change. The H+ and OH- neutralize each other and form H2O = water.

Sorry I got off on a tangent.

Chris


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## shyknee (Mar 7, 2011)

Thanks GSP and don't be sorry I love info.

thank you too Joem.

Must be a bit of a job to scrape that drum of is silver once in a while .


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## joem (Mar 7, 2011)

shyknee said:


> Thanks GSP and don't be sorry I love info.
> 
> thank you too Joem.
> 
> Must be a bit of a job to scrape that drum of is silver once in a while .


 No it flakes off very easy and no other refining is required , just melt it.


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## goldsilverpro (Mar 8, 2011)

The silver plated on these drums will usually run from about 85% (or, a little less) to 99% pure, depending on several variables used when plating the silver - current density, silver concentration, etc. 

The 85% material will be black, slimy, smelly, and loosely adherent. After it is removed from the drum (and/or the bottom of the tank), it usually contains a lot of liquid and it must be dried out over heat before weighing and/or determining the purity. After drying, it will contain silver sulfide (87% Ag), silver metal (maybe), and some dried salts from the fixer solution. 

The 99% silver is very white, dense, hard, brittle, and easily removed by tapping it with a hammer - since it's very brittle, it will easily break off in large (usually) pieces. I have seen the higher purity silver plated 1/4" thick, or more. 

Usually, the purity will be somewhere between these extremes.

In the trade, the silver is usually purified by melting with a lot of borax, a little soda ash, and rebar in a gas fired crucible furnace. The sulfur is removed by standing several pieces of rebar (long enough to protrude about 18" above the crucible) in the melt. The rebar is also used to stir the melt. The sulfur combines with the iron from the rebar and the resulting iron sulfide ends up in the slag. A large place I worked for ran tons of this material and was able to consistently get at least 99.90% silver (and, often, as high as 99.98%) using this method. Iron, of course, was the main contaminant in the final silver.

A characteristic of most all types of photo scrap is that the silver is pure to start with, in that it essentially contains no other metals. Therefore, unless you contaminate it with some metal (e.g., copper or zinc) while processing, it is usually not necessary to purify it further (e.g., in a silver cell).


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## shyknee (Mar 8, 2011)

That is so cool (the rebar to stir the melt), thanks for the insight GSP.

Hmm. I think it's time to experiment again. :twisted: 8)


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## joem (Aug 8, 2012)

I have now set up my silver recovery unit at work and am pumping fix in a circular pattern through the recovery unit. I set it to run at 3 pm today and will check it at 3 pm tomorrow to see how much silver has been recovered. Now since our dept has been hit by budget cuts, I will run the waste fix containers we have until the end of the month and then the whole dept is closed and I move onto to another job in the college. pics to come soon.


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## joem (Aug 9, 2012)

I checked my recover unit, to my surprise there was no silver inside.
then I realised I did not turn the valve in the right direction. :roll:  
so I'll update as it happens.
These are ten litres of used fix and the view of silver recovered after 6 hours.


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## joem (Sep 8, 2012)

End result of continous running on 30 liters over a few weeks resulted in over 10 grams of silver.
The unit is now for sale in the " For Sale" thread since I don't have any fix available to me.


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