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When an exposed x-ray film is processed, the exposed silver halide crystals are converted to metallic silver. The unexposed silver halide grains are removed in the fixer bath. The fixer solution thus becomes rich in silver that can be recovered by several means. Silver recovery is profitable as the price of silver remains high. The technique selected is determined by the number of films processed and whether a manual or automatic processing system is used (Table 16-1).
Silver estimating papers are available that indicate, by comparison with a color chart, the grams of silver present per liter. Silver test paper is made by impregnating unsized (non-glossy) paper with yellow chemical color patches, each of which is marked with the silver concentration in troy ounces per gallon and in grams per liter. You immerse one end of a short strip of paper in the fixing bath for two or three seconds, shake off the excess liquid, and place it on a clean white surface. After 10 seconds, compare the wet paper with the color scale by finding the closest match you can. The numbers on the color scale may look something like this.
Gram per liter |
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Troy oz. per U.S. gallon |
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The paper is most sensitive in the range of 2 to 5 grams per liter (g/1) (1/4) to 5/8 Troy oz. per gallon). If the test shows a value greater than 5 g/1, dilute a small portion of the fixer with an equal volume of distilled or deionized water and retest. Multiply the result by 2 for the final answer.
Each manufacturer has its own way of packaging the product. Most common is a booklet with 17 sheets, each consisting of 12 test strips. Another style binds 20 strips at a time into a small booklet. The third style is a 5-meter (16-foot) roll in a clear plastic dispenser.
A plastic case or dispenser offers several advantages. The first is protection if it is accidentally placed in water or processing solution in the darkroom. The other advantage is protection from darkroom fumes and vapors. Unprotected test paper should not be kept longer than one year. Protected paper will keep several years.
The silver content of used fixer is normally 2 to 7 g/1 (1/4 to 7/8 troy oz per gallon), so test paper is ideal for estimating how much silver is potentially recoverable from photographic processing. Compare with the amount you are actually recovering per month to get an idea of how efficient your system is.
This testing paper is available from a number of companies. One of them is the X-Rite Company, 4101 Roger Chaffee Dr. S.E., Grand Rapids, Michigan 49508 - (616-534-7663).
Systems for silver recovery include
The system of metallic replacement chemically replaces the silver in the fixer solution with another metallic ion, usually iron. A cartridge is loaded with steel wool and the silver precipitates in the bottom of the cartridge while the iron becomes an ion and remains in solution. This system has the advantage of :
Approximately 200 gallons of fixer can pass through the cartridge before replacement of the steel wool.
The system of electrolytic recovery utilizes an anode and cathode placed within the fixing tank or other collecting tank. As the current flows through the solution, the silver ion is plated on the cathode. The fixing bath can be reconstituted and reused with this system. Electroplating requires:
The technique of silver removal using a chemical precipitation is less common and relies on addition of a compound that precipitates the silver in the form of a sludge of silver sulfide plus other compounds. The sludge is filtered, dried and packed.
Models of silver recovery units are produced especially for use with automatic processing units. The type of overflow system incorporated in the processor determines whether or not a connection is possible. Processors that utilize a spillway in the fixer tank to the drain can be connected to a recovery unit by installation of a spillway basin. If the processor does not have an automatic fixer replenishment system, the recovery unit can be placed in the fixer tank. This may require some modification. Processors with a separate drain for overflow fixer can be connected to a recovery unit.
Often radiographs are of no value because of technical errors or because of age. The radiographs should be saved for their value due to silver content. Many companies purchase x-ray film.
Companies that purchase fixer solutions containing silver or discarded radiographs are usually listed in the yellow pages of the telephone directory under the heading of "Gold and Silver Refiners and Dealers."
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