WHY RECOVER SILVER?
 
There are three good reasons to recover silver from lithographic, photographic, and x-ray film processing solutions:  conservation of a precious metal, economic return, and environmental concerns.
 
Silver is a precious metal used in many industrial products.  A prime example is photographic film that uses silver because of its unmatched quality as a light-sensitive material for creating a photographic image.  Unlike many other natural resources, silver is not destroyed in the photographic process and can be recovered and re-used.
 
Because silver is a valuable element, recovery can be economically profitable.
 
Increased public awareness of the consequences of environmental pollution has given rise to more stringent regulations governing the discharge of many materials.  As a result, a reduction in the amount of silver discharged in photographic effluent is required to comply with federal, state, and local codes.  Often, this can be achieved using a silver recovery system.

 

SOURCES OF SILVER?
 
The primary source of silver in film processing is the photographic and x-ray film itself.  The emulsion on the plastic film base contains a light-sensitive silver compound.  When the emulsion is exposed to light or x-rays, a photochemical reaction occurs creating a latent image.  A developer solution changes the exposed silver to metallic silver, which forms the final image.  Formation of the image uses only a portion of the total silver available.  The remaining silver must be removed in order to make the exposed image permanent.  To remove the unneeded silver, the film is placed in a fixer bath, a solvent that washes away unexposed silver compounds from the film.
 
With black-and-white positive films, the amount of silver removed in the fixing bath can be as much as eighty percent (80).  With color film, the amount of silver removed can be almost  one hundred percent (100%).  On the other hand, a lot of black-and-white negative film with a high percentage of exposed area will retain most of the silver on the film, leaving little in the fixer bath. In general, when the fixer is exhausted it will contain somewhere between 1/2 and 1 troy ounce of silver per gallon of solution.

 
 
METHODS OF SILVER RECOVERY

FROM SOLUTIONS:
The two most widely used methods for recovering silver dissolved in spent fixer (or hypo) are electrolytic plating and metallic replacement.  X-Rite silver recovery systems utilize the electrolytic principle for metallic silver recovery.  It is technically the most advanced and fastest growing method of recovery.
 
1) Electrolytic Silver Recovery
 
In the electrolytic method of recovery, silver is removed from fixing baths by passing a controlled electrical current between two electrodes:  a negatively charged cathode and a positively charged anode, which are suspended in the solution.  As fixer flows into the unit, the positive silver ions are repelled by the anode and attracted to the cathode, where they are deposited to build up into a layer.  For optimum silver recovery, the solution is agitated by a power-driven, rotating cathode.  The drum- or disc-shaped cathode is designed of stainless steel to avoid corrosive reactions with the fixer solution and for ease of silver removal.  The recovered silver flakes normally run 85 to 97 percent in purity.
 
2) Metallic Replacement Silver Recovery
 
This method uses canisters packed with steel wool.  There are other canisters filled with Multi-Media technology. 
 
The following is an explanation of a simple ion-exchange that takes place in the canister.  The more active iron ions in the steel wool replace the less active silver ions in the solution that's entering the canister.  As a result, the silver that remains in the canister as the iron is picked up and washed out in the solution, leaves a residue known as silver sludge.  Although often less expensive than electrolytic recovery systems, metallic replacement units are not as efficient or effective.  It is much more difficult to refine the silver sludge, which can reduce the economic rewards of recovery.  Often the units are heavy and cumbersome, causing handling and shipping problems.  Because the metallic replacement unit is expended in the recovery process, it must be replaced periodically.  Moreover, once it's activated the metallic replacement unit will decay whether silver is being collected or not.