T.O. 33B-1-1
6-85
6.7.17
Silver Recovery.
The unexposed and undeveloped silver bromide grains in the film emulsion are removed by the fixer solution.
Therefore, the exhausted fixer becomes rich in silver content. The value and scarcity of silver makes recovery
economically feasible. Approximately 80 percent of the silver in the film emulsion is transferred to the fixer solution;
the remaining 20 percent forms the radiographic image. There are three basic methods of silver recovery from the fixer
solution. These are by electrolysis, metallic replacement, and chemical precipitation.
6.7.17.1
Electrolysis Recovery Method.
When electric current is passed between two electrodes immersed in the silver-bearing fixer, the silver is electronically
deposited upon the cathode. This silver can be stripped from the cathode and refined. This method permits re-use of
the fixer.
6.7.17.2
Metallic Replacement.
This method consists of replacing the metallic silver with a less valuable base metal such as iron, zinc, or copper. As
an example, if steel wool is inserted into the exhausted fixer solution, the silver in solution is replaced by the iron, and
the silver accumulates on the bottom of the container in the form of sludge. The sludge is removed and refined to
reclaim the silver. The fixer must be discarded after silver recovery by this method.
6.7.17.3
Chemical Precipitation.
Silver can be reclaimed from fixer by the addition of certain chemicals to the exhausted fixer. The silver is precipitated
out of the solution in the form of a sludge that can be recovered and refined. The chemical reaction generates
obnoxious fumes and odors, and separate facilities are recommended for this method of silver recovery. The fixer must
be discarded.
6.7.17.4
Silver Recovery from Films.
There are two methods used to recover silver from obsolete films. One method is to strip the silver bearing emulsion
from the film base by using chemical or mechanical means. The emulsion is then refined to reclaim the silver. The
second method is by burning the film in an incinerator that controls the burning process and the fly ash. The residual
ashes are then processed to obtain the silver content. It is usually more economical to simply market used or obsolete
film than to attempt silver reclamation from film on a small scale. Detailed information on silver recovery is provided
in Air Force Regulation (AFR) 400-14, "Reclamation and Use of Silver."
6.7.18
Film Reproduction Technique.
Often duplicate radiographs are required. If it is known in advance that duplicate films are required, it is more
economical and quicker to expose two films simultaneously in the original exposure. If lead screen techniques are
being used, slight increases in exposure will be required.
6.7.18.1
If multiple copies of an existing radiograph are required, they can be reproduced by contact printing techniques. The
duplicate radiograph can be made on a direct-positive film that produces a duplicate-tone facsimile of the original. The
film gradient of the duplicating film is -1.0, which means that density differences in the original image are faithfully
reproduced in the duplicate image. Duplicating film cannot reproduce radiographic density ranges equivalent to
originals. But by varying exposure, the density differences can be recorded accurately.
6.7.18.1.1
If duplicating film is not available, it is possible to use medical film that is designed for use with fluorescent screens.
These duplicates are also produced by the direct printing method. However, these films have a special property. While
not a positive film, they do undergo reversal with large exposures. That is, they increase in density up to a saturation
point after which time they decrease in density with exposure, and thus reverse. It is necessary to expose these films
such that reversals occur, and the original image is duplicated. If the original radiograph has a high density, exposures
of as much as two minutes to a photoflood lamp may be required. These exposure requirements must be generated for
each specific situation; generalization here is not practical.
