A&P Silver Recovery: For those interested

Discussion in 'Lifestyle' started by BlackWRX, Dec 20, 2004.

  1. BlackWRX

    BlackWRX Active Member

    Apr 25, 2002
    Likes Received:
    Bay Area, CA
    There are several reasons to be interested in the recovery of silver from photo-processing waste. Silver is a valuable natural resource of finite supply, it has monetary value as a recovered commodity, and its release into the environment is strictly regulated. In photo-processing, silver compounds are the basic light-sensitive material used in most of today's photographic films and papers. During processing, particularly in the fixing bath or bleach-fix, silver is removed from the film or paper and is carried out in the solution, usually in the form of a silver thiosulfate complex.

    Major sources of recoverable silver are: photo-processing solutions, spent rinse water, scrap film and scrap printing paper. As much as 80 percent of the total silver processed for black and white positives and almost 100 percent of the silver processed in color work will end up in the fixer solution. Silver is also present in the rinse water following the fixer or bleach-fix due to carry-over.

    Economic considerations include initial equipment cost, the amount and value of silver recovered, and the return on investment. Space and energy requirements, day-to-day attention required, maintenance and reliability are also important. It is necessary to know the amount of silver available for recovery, the total volume of fixer and bleach-fix solutions used in processing, and the expected performance of the recovery method under consideration.

    Several technologies exist for recovering silver on-site. The most common methods of on-site recovery from the fixer and bleach-fix processing solutions involve metallic replacement, electrolytic recovery and chemical precipitation. Ion exchange and reverse osmosis are other methods that can be used alone or in combination with conventional silver recovery systems. However, these are generally considered suitable only for dilute solutions of silver. A silver recovery system can be devoted to a single process line or can be used to remove silver from the combined fixer from several process lines in a plant.

    The most widely used silver recovery method for large operations is electrolysis, where the silver is recovered from solution by electroplating it on a cathode. A controlled, direct electrical current is passed between two electrodes suspended in the silver-bearing solution. Silver is deposited on the cathode in the form of nearly pure silver plate. The cathodes are removed periodically and the silver is stripped off for sale or reuse. While this method requires a substantially larger capital expenditure and needs an electrical connection, it does have the advantage over other methods in that it yields virtually pure silver. This results in lower refining and shipping costs and no contamination of the fixer, thereby permitting its reuse for some processes. When properly operated, 95 percent of the potential available silver can be recovered. Combining electrolytic silver recovery with in-situ ion exchange can result in more than 99.5 percent silver recovery efficiency.

    A recirculating electrolytic recovery system has advantages over systems that only remove silver. Silver is removed from fixer solution by the recovery cell which is connected "in-line" as part of a recirculation system. Fixer solution reclaimed by electrolytic silver recovery can have limited reuse in the photo process. By recirculating the desilvered fixer to the in-use process tank, less fresh fixer solution is needed to replenish the bath. Fixer replenishment can be reduced 20 percent or more without degradation of product quality. Chemical replenishment can be managed through the frequent and consistent use of test strips. A properly designed recirculating system can lower the silver in the fixer from a concentration of 1 ounce/gal. to 1 ounce/100 gals. The amount of silver carried over to the rinse water is similarly reduced.

    Metallic replacement requires little capital expenditure for equipment and requires only a few simple plumbing connections. The equipment consists of a plastic container, a plastic-lined steel or stainless steel drum filled with metal, usually steel wool, and some plastic hose and plumbing connections. Silver is recovered when the silver-bearing solution flows through the cartridge and makes contact with the steel wool. The iron goes into solution as an ion, and the metallic silver is released as a solid to collect in a sludge at the bottom of the cartridge or is deposited on the steel wool. The yield a user can expect is determined by the silver concentrations in solution, the volume of solution that is run through the cartridge, and the care with which the operation is managed. When silver is no longer effectively removed, the silver-bearing sludge is sent to a refiner who will refine it and pay the customer for the recovered silver.

    Good luck.

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