Electrolysis, or more specifically electrowinning, is the most widely used and universally applicable method for silver recovery in the photoprocessing industry. An electrolytic silverrecovery cell consists of a cathode and an anode. Oxidation occurs at the anode (positive electrode) and reduction at the cathode (negative electrode). Silver deposits on the cathode during electrolysis when a direct current is passed through the silver-bearing photoprocessing solution. After sufficient silver has been plated, the cathode is removed from the system and the silver stripped off [91,92]. The primary reaction occurring at the cathode is
If the cathode voltage is allowed to become too high, thiosulfate could be reduced at the cathode as shown in the following equation:
The production of sulfide is undesirable, because it will react with the silver complex to produce insoluble silver sulfide (known as "sulfiding"). Although from a recovery standpoint a small amount of silver sulfide can be tolerated, too high a level will result in a poor plate . Additionally, if in-line fixer desilvering were being done, silver sulfide formation would contaminate the fixer and could damage the photographic product. Therefore, it is necessary to compromise on the voltage applied, to obtain optimum current efficiency while minimizing sulfide production.
Electrolytic silver recovery requires a larger capital expenditure than the use of MRCs and also necessitates an electrical connection. However, it has the advantage of yielding nearly pure silver, resulting in lower refining and shipping costs. A primary advantage from an environmental viewpoint is that it allows fixer reuse for many processes because it does not contaminate the fixer when properly controlled.
There are essentially two ways in which electrolytic silver recovery can be applied . One involves its use in a terminal manner, and one concerns its application for the in-line desilvering of fixer. When used in a terminal manner, the silver-bearing solution is passed through the electrolytic cell to recover the silver and the desilvered solution is slowly discharged to the drain, perhaps through a secondary metallic replacement cartridge or using chemical precipitation for additional low-level silver recovery. An alternate terminal approach is to mix the electrolytically desilvered solution with silver-containing washwaters and pass the mixture through an ion-exchange system for further silver recovery.
Finally, part of the desilvered fixer or bleach-fix, if not mixed with other solutions or otherwise contaminated or altered during silver recovery, may be reused in making fresh replenisher, thus minimizing the environmental impact.
It is also possible to operate electrolytic equipment for the in-line desilvering of the fixer solution. The equipment is set to function so that the silver in the fixer tank is constantly maintained in the 0.5-1 g/L range. (This compares with typical silver concentrations in fixer tanks of 3 g/L when well seasoned.) Although careful control is essential to preclude the formation of silver sulfide, this method offers several environmental benefits. Depending on the process, the fixer replenishment rate can be reduced from 50 to 70% compared to the standard rate. Additionally, the lower silver level in the tank means that significantly less silver (only about 5-10% as much) will carry over to the washwaters, thus assuring that, overall, more silver is recovered and less is lost.
Several factors are involved in choosing and operating an electrolytic silver recovery unit [94,95]. The amount of current that a device delivers is important: low current density units can be used for desilvering fixers, but high current density is needed for bleach-fixes. Some method of agitation is required to keep the fresh silver-containing fixer in contact with the cathode, but too much turbulence that produces a vortex will whip air into the solution, consuming sulfite preservative and promoting sulfiding. A rotating cathode unit provides its own agitation, whereas a pump or impeller may be needed for a stationary cathode.
Some method for controlling the current is also important. Several methods are available, including timers, selective ion electrodes for online monitoring, and constant voltage operation including the more complicated use of potentiostatic control with IR (voltage) compensation [96,97]. The current density relative to the solution silver concentration should be high enough to desilver the solution in a reasonable time, yet low enough to prevent sulfiding. Well-designed controls step the current down in stages as silver is depleted from the solution.
In addition to the time, voltage, and current, pH is an important factor affecting electrolytic silver recovery. Tests have shown that the optimum pH for desilvering fixers is approximately 6.2, whereas the optimum for desilvering bleach-fixes is approximately 8-8.5. How high a pH can be used for desilvering bleach-fixes is limited by the evolution of ammonia. As the pH of the bleach-fix increases, a side reaction involving the reduction of iron is inhibited and the electrolytic silver-recovery efficiency increased .
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