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
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
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