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Which Method Is Best for Me ?

That depends on many factors, but the primary ones are your processing volume, whether or not you want to reuse your fixer or bleach-fix, and environmental concerns, such as strict limits on effluent discharge. Other factors that can affect your decision are the training and technical knowledge of your personnel, the price of silver, and operating and refining costs.

Consider Your Processing Volume

The cost of silver-recovery equipment is closely related to the volume of materials you process.

If your volume is low, chemical recovery cartridges are a reasonable choice for silver recovery. The cartridges are very efficient and very easy to monitor with KODAK Silver Estimating Test Papers (CAT No. 196 5466). They are also very simple to install. Your costs for purchasing, installing, operating, and monitoring this equipment are very low compared with other methods.

If your processing volume is high, you probably don't want to make frequent cartridge replacements or provide a large storage area for spent and replacement chemical recovery cartridges. Although initial capital investment for electrolytic silver recovery cells is higher than for chemical recovery cartridges, you don't have the recurring cost of equipment replacement. If you use an electrolytic cell, your refining costs for the recovered silver will be much lower than with other methods, because the silver you recover is usually more than ninety-five percent pure.

For a large-volume operation, in-situ ion exchange is also an option. You can use this method for primary treatment, or use it to tail an electrolytic unit if you dilute the discharge from the electrolytic cell with wash water. This method will let you recover the maximum amount of silver and minimize the amount of silver discharged. It requires a greater capital investment and more chemical handling than chemical recovery cartridges or electrolytic cells.

A comparison of the initial capital investment required for each method is shown in the chart below.

Consider the Amount of Silver Concentration That is Acceptable in Your Effluent

The silver concentration that can be discharged to a treatment plant or to a receiving body of water is often regulated even though silver in photographic effluent is in a form that is not harmful. Therefore, cost may not be the primary consideration in choosing your silver recovery method. If you do not know the code limits for silver, check with your local regulatory agency. You may need to consider a method that will reduce the silver to an acceptable level. In some cases a regulatory agency may take into account the non-toxic nature of the silver in photographic effluent and allow a variance.

Chemical recovery cartridges can achieve recovery efficiencies greater than ninety percent. However, it is difficult to maintain this level of recovery, so cartridges are an unreliable choice if you need to observe low discharge limits. Another problem with chemical recovery cartridges is that as silver is recovered, the steel wool becomes soluble, producing iron levels in the effluent as high as 3000 mg/L. Iron is regulated to levels well below those concentrations by many sewer codes.

It is also difficult to maintain very low silver concentrations in effluents with electrolytic recovery cells. The concentration depends on how low the current density can be set with your unit. As the silver concentration gets lower, the current density must be set lower to prevent silver sulfide from forming. With low current densities, a large cathode area is needed to achieve the necessary recovery rate. If your electrolytic cell is not capable of reducing the silver concentration in the effluent to the level required, you can simply feed the effluent from the electrolytic cell through chemical recovery cartridges to recover additional silver.

The greatest recovery efficiency is possible with an ion exchange recovery method. In-situ precipitation ion exchange can reduce silver concentrations in the effluent to a level as low as 0.1 mg/L. However, it is critical that you dilute the concentrate with the proper amount of wash water before treatment; too high a thiosulfate concentration in the solution will cause silver to leak through the column.

The table below gives a comparison of the recovery efficiencies of the various methods.

Is Fixer Reuse a Consideration ?

Desilvering, regenerating, and reusing fixer allows you to reduce effluent discharge and chemical costs. If this is an important factor in your operation, you may want to recover silver by electrolytic methods. Electrolytic silver recovery is the only method that permits fixer reuse.

Choosing a Method

The following table will help you select a silver-recovery method that's appropriate for your processing volume, level of effluent discharge, and need to regenerate and reuse fixer solutions.

Factors That Affect Silver Concentration in Solutions:

Type of Film Being Processed: the silver content of photographic emulsions varies with the type of film. For example, the thin-emulsion films used in the graphic arts industry contain less silver than doublecoated 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.

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 positiveline image, therefore, yields much more recoverable silver than the negative image since less silver is retained in the film.

Processing Work Load : The quantity of material processed in a solution is, of course, 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.

Replenishment Rate. In processing systems that require replenishment, always observe the recommended replenishment rates. Overreplenishment dilutes the silverbearing 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 overreplenishment, but the cartridge requires more frequent replacement and replenisher solution is wasted as well.

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.

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.

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