The first step in most printing operations usually
involves photography of some type and the related photographic
Although silver halide films are the most common
type of photographic media, there are other media available. The substrates used are generally polyester, vinyl acetate, or
Silver halide films ? These are the most common.
coated with a photographic emulsion that typically includes silver
halide salts suspended in a gelatin.
Vesicular films. These
are coated with a thermoplastic resin
and a light-sensitive diazonium salt.. These films take longer to
develop but avoid silver disposal problems.
Photopolymer films ? Carbon black is used and the films
processed in a weak basic solution. These films also take longer
to develop but avoid silver disposal problems.
Electrostatic films ? An electrostatic charge makes the
light sensitive, and a liquid toner brings out the image after the
film is exposed to light. These
films develop at about the
same rate as silver halide films and offer a very high resolution. They also avoid silver disposal problems.
Developing Silver Halide Films
The development of silver halide films can be
simplified into the following general steps.
Developer--> (Stop Bath) -- > Fixer -- > Wash -- >
Use of a ?stop bath? containing a weak acid, usually
acetic acid, prior to the fixer can help to neutralize any alkaline
developer carried over on the film
Monitor silver concentrations in fixing baths. Above a certain concentration level (~2 grams/liter) insoluble
are formed that can not be readily removed from the photographic
emulsion. Use of ammonium
thiosulfate can double
this concentration limit (to ~4 grams/liter).
?Squeegees? can be used to wipe off excess solution
from the film before moving to the next step. It?s important that they are used only after the film image has
A multi-stage rinse with counter-current flow can be used
instead of single staging rinsing. Fresh water enters the final
rinse tank and then flows from there to the previous rinse tank.
Wastewater flow from the first rinse tank then goes to an approved
treatment system and then to sanitary sewers.
Processing chemicals can be poured into storage containers
until needed again. This is
especially useful for small, manual developing operations.
Silver Recovery From Photo-processing Solutions
The silver contained in waste-waters from
the on-site treatment of these chemicals an economical necessity
for many companies, as well as an environmental requirement. This silver
can be recovered in some form and then sold.
In Dade County the sanitary sewer standard limit
silver concentration is 0.4 mg/L.
Metallic Replacement ? A more active metal, usually
be used to displace the silver from solution. This can be as
simple as a plastic container with steel wool or as complex as
an intricate system that uses replacement cartridges and
monitors concentration levels. The silver is then recovered as
a sludge. However, the
effluent solution now has a high iron
Chemical Precipitation ? Sodium sulfide can be used to
precipitate out the silver as silver sulfide. However this must be carried out in an alkaline media to prevent
the formation of dangerous hydrogen sulfide gas (H2S) and subsequent
filtering can be difficult because the
precipitate is very fine. Sodium borohydride can also be used to
precipitate out silver as silver metal. Concentrations of silver as low
as 0.1 mg/L are possible with either method, but the solutions are
generally not reusable for developing.
Ion Exchange ? Silver in low concentration levels can be
removed by pumping the solution through an anion-exchange resin column. The silver is ?trapped? in the resin column, and then be
recovered on or off-site. Unfortunately
recovery of the silver can be difficult, but concentration levels as low
as 0.1 ppm are achievable.
Electrolytic Recovery ? A controlled direct electric
passed between two electrodes suspended in the solution.
Silver then plates out n the cathode as an almost pure metal.
In some cases the silver bearing solution can actually be reused. Unfortunately very low levels of silver concentration can not
be achieved with this method alone, and it is often used in
conjunction with one of the other treatment systems.
Off-site ? Silver bearing wastes can be shipped off-site
by anapproved hauler for treatment and recovery at an approved
Silver Recovery From Photographic Film or Paper
Silver in scrap film or paper can be removed by treating
these materials with a sodium hypochlorite solution to oxidize
any silver metal. This
solution can then be added to the fixer
solution and the silver can be recovered in the same manner
as from conventional photo-processing waste solution. Some
facilities may choose to use fixer solution that they are
preparing FOR disposal anyway.
Scrap film can be sold to recyclers for silver
Scrap film or paper can be disposed of in the trash if
based on the Toxic Characteristic Leaching Procedure indicates levels
are below permitted standards for landfill disposal.
Page 1 Page 3