Before you start your silver recovery process, you need to know the silver concentration in the solution. This step is most important to know whether silver recovery is profitable or not.
There are many factors affect silver concentration in solution.
1) Type of Film Being Processed:
The silver content of x-ray film or photographic emulsions varies with the type of film. For example, the thin-emulsion films used in the graphic arts industry contain less silver than double coated x-ray films and therefore will yield less recoverable silver.
In processing color films, all of the silver is removed by the fixer to yield a dye image, and so nearly all of the silver is available for recovery.
2) Exposure Level:
In processing black-and-white materials, the ratio of exposed and developed silver to soluble silver halide has a considerable influence on the amount of recoverable silver in the fixing bath. This ratio depends largely on subject matter. For example, it negatives of a line drawing on white paper are processed, the emulsion contains a large proportion of exposed and developed silver and a small proportion of soluble silver halide.
On the other hand, if positives of the same drawing are processed, the emulsion contains a small proportion of exposed and developed silver and a high proportion of soluble silver halide. The positive line image, therefore, yields much more recoverable silver than the negative image since less silver is retained in the film.
3) Processing Work Load:
The quantity of material processed in a solution is a major factor affecting the amount of silver in that solution. Processing work loads vary considerably in many operations, and the load often peaks at certain periods during the working day.
If the flow rate at these periods is greater than the recovery unit can handle, silver will be lost to the drain.
4) Replenishment Rate:
In processing systems that require replenishment, always observe the recommended replenishment rates. Over replenishment dilutes the silver bearing solution going to the recovery cartridge and so results in a lower silver yield than expected. The cartridge becomes exhausted by the passage of solution with a low silver content. Silver is not lost by over replenishment, but the cartridge requires more frequent replacement and replenisher solution is wasted as well.
5) Solution Carryout:
Varying amounts of silver-bearing solution are carried out of the processing tank on the surfaces of the photographic material. This solution is not available for silver recovery by the cartridge.
Factors that affect solution carryout are machine speed and film area, as well as the incorrect adjustment or absence of squeegees and drive belts. Under normal circumstances, the volume of solution that may be carried out varies between 3 percent and 15 percent of the replenishment rate. Use of squeegees is an effective means of minimizing solution carryout.
6) Obstruction of Solution Flow:
If the solution going to the cartridge contains an abnormal amount of solid precipitates or gelatinous matter, the steel-wool filler acts as a filter and eventually obstructs the passage of solution. The solution is then diverted through the bypass loop of the circulating unit and is discharged to the drain untreated.
By known these factors, you can analyze your silver concentration in your solution before you start silver recovery process.
Source: Technical Documents
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