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Electrolytic Plating

Photography (Advanced) - Advanced manual for photography and other graphic techniques
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APPENDIX  I SILVER RECOVERY Recovering silver from the photographic process saves the government money. These savings can come from  two  sources:  the  monetary  value  of  recovered silver and avoiding fines from the Environmental Protection  Agency. Like most natural resources, silver is a valuable and diminishing resource. In photography, silver forms the image  on  most  types  of  photographic  films.  Unlike many other natural resources, silver is not destroyed in the photographic process. Much of this silver can be recovered, refined, and used again. In photography, silver is recovered from two main sources:  photographic  solutions  and  black-and-white scraps (film and paper). When most films and papers are processed, some of the silver they contain are removed in the fixing bath. With positive types of black-and-white film, as much as 80 percent of the silver that was in the emulsion may be removed during fixing. With color film and paper, close to 100 percent of the silver is removed in the fixer. When black-and-white negative film or black- and-white paper with a high percentage of exposed area is processed, most of the silver remains in the emulsion. Most of the silver that remains in film or paper can be recovered. ENVIRONMENTAL  ISSUES The  awareness  and  concern  for  polluting  our environment  has  brought  about  new  legislation  and stricter  enforcement  of  existing  environmental  codes. Silver is one of the heavy metals that is controlled by federal, state, and local legislation. These government agencies monitor the amount of silver that is discharged into the sewer system. Each Navy imaging facility is subject to environmental codes and restrictions of the state and local area. Each state has a similar set of codes that may differ somewhat, so it is important for you to become   familiar   with   them.   For   example,   the maximum  silver  concentration  limit  for  an  imaging facility in Gulfport, Mississippi, may be 5.0 mg/L (ppm), whereas an imaging facility in San Diego, California, may be 0.05 mg/L (ppm). Compliance  with  these  strictly  enforced  local  sewer code  ordinances  is  of  greater  concern  today  than economic gain. Violations can result in severe fines. Individual  violators  may  also  be  held  personally responsible for such fines. An excessive concentration of silver in the effluent of a photographic processor can cause an imaging facility to be closed until the silver concentration  is  within  acceptable  limits.  A  copy  of  the sewer codes for your local area may be obtained from the sewer authority or from Navy Public Works. SILVER  RECOVERY  TECHNOLOGY A  number  of  different  methods  for  recovering  silver from used photographic solutions are in use today. Two methods that are used commonly in Navy imaging facilities are metallic replacement and electrolytic plating. METALLIC REPLACEMENT The  metallic  replacement  method  uses  a  plastic cartridge packed tightly with steel wool. The cartridge resembles a 5-gallon bucket with tubes protruding from the cover. The system is inexpensive and well-suited for  the  small-volume  user. A silver recovery cartridge operates on the principle of metal ion exchange. When the fixer containing silver is passed through the cartridge, the iron in the steel wool or wire screen replaces the silver ions in the fixer. The silver then drops to the bottom of the cartridge as impure metallic silver sludge. The iron ions in the fixer are drained from the cartridge with the fixer into a drain or holding container (fig. AI-1). In time, the filter material in the cartridge dissolves and the cartridge must be replaced. After about 80 percent  of  the  steel  wool  is  dissolved,  the  cartridge becomes inefficient and silver passes through the system. The cartridge is actually exhausted before the filter is completely dissolved. A metallic-replacement unit is capable of desilvering to a lower level than an electrolytic plating unit; therefore, some imaging facilities use electrolytic plating unit first and then send the  solution  through  the  metallic-replacement  cartridge for final treatment. AI-l







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