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07/22/2018

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[ Home > Compliance ]
POLLUTION PREVENTION FOR PRINTERS AND PHOTO PROCESSORS
 

PHOTO PROCESSING

The first step in most printing operations usually involves photography of some type and the related photographic developing processes. 

  • Using electronic pre-press systems in the preparation of final copies can help to reduce the need for pre-press proofs and thus the amount of photo-processing required.

Photographic Films

Although silver halide films are the most common type of photographic media, there are other media available.  The substrates used are generally polyester, vinyl acetate, or glass. 

  • Silver halide films ? These are the most common.  They are coated with a photographic emulsion that typically includes silver halide salts suspended in a gelatin. 

  • Vesicular films.  These are coated with a thermoplastic resin and a light-sensitive diazonium salt..  These films take longer to develop but avoid silver disposal problems.

  • Photopolymer films ? Carbon black is used and the films are processed in a weak basic solution.  These films also take longer to develop but avoid silver disposal problems.

  • Electrostatic films ? An electrostatic charge makes the film light sensitive, and a liquid toner brings out the image after the film is exposed to light.  These films develop at about the same rate as silver halide films and offer a very high resolution. They also avoid silver disposal problems.

Developing Silver Halide Films

The development of silver halide films can be simplified into the following general steps. 

Developer--> (Stop Bath) -- > Fixer -- > Wash -- > Dry 

  • Use of a ?stop bath? containing a weak acid, usually acetic acid, prior to the fixer can help to neutralize any alkaline developer carried over on the film

  • Monitor silver concentrations in fixing baths.  Above a certain concentration level (~2 grams/liter) insoluble compounds are formed that can not be readily removed from the photographic emulsion.  Use of ammonium thiosulfate can double this concentration limit (to ~4 grams/liter). 

  • ?Squeegees? can be used to wipe off excess solution from the film before moving to the next step.  It?s important that they are used only after the film image has hardened. 

  • A multi-stage rinse with counter-current flow can be used instead of single staging rinsing.  Fresh water enters the final rinse tank and then flows from there to the previous rinse tank. Wastewater flow from the first rinse tank then goes to an approved treatment system and then to sanitary sewers. 

  • Processing chemicals can be poured into storage containers until needed again.  This is especially useful for small, manual developing operations.

Silver Recovery From Photo-processing Solutions

The silver contained in waste-waters from photo-processing makes 
the on-site treatment of these chemicals an economical necessity
for many companies, as well as an environmental requirement. This silver can be recovered in some form and then sold.

In Dade County the sanitary sewer standard limit for 
silver concentration is 0.4 mg/L. 

  • Metallic Replacement ? A more active metal, usually iron, can be used to displace the silver from solution.  This can be as simple as a plastic container with steel wool or as complex as an intricate system that uses replacement cartridges and monitors concentration levels.  The silver is then recovered as a sludge.  However, the effluent solution now has a high iron concentration. 

  • Chemical Precipitation ? Sodium sulfide can be used to precipitate out the silver as silver sulfide.  However this must be carried out in an alkaline media to prevent the formation of dangerous hydrogen sulfide gas (H2S) and subsequent filtering can be difficult because the
    precipitate is very fine. Sodium borohydride can also be used to precipitate out silver as silver metal. Concentrations of silver as low as 0.1 mg/L are possible with either method, but the solutions are generally not reusable for developing. 

  • Ion Exchange ? Silver in low concentration levels can be removed by pumping the solution through an anion-exchange resin column.  The silver is ?trapped? in the resin column, and then be recovered on or off-site.  Unfortunately recovery of the silver can be difficult, but concentration levels as low as 0.1 ppm are achievable. 

  • Electrolytic Recovery ? A controlled direct electric current is passed between two electrodes suspended in the solution. Silver then plates out n the cathode as an almost pure metal. In some cases the silver bearing solution can actually be reused.  Unfortunately very low levels of silver concentration can not be achieved with this method alone, and it is often used in conjunction with one of the other treatment systems.               

  • Off-site ? Silver bearing wastes can be shipped off-site by anapproved hauler for treatment and recovery at an approved
    facility.

Silver Recovery From Photographic Film or Paper 

  • Silver in scrap film or paper can be removed by treating these materials with a sodium hypochlorite solution to oxidize any silver metal.  This solution can then be added to the fixer solution and the silver can be recovered in the same manner as from conventional photo-processing waste solution.  Some facilities may choose to use fixer solution that they are preparing FOR disposal anyway.

  • Scrap film can be sold to recyclers for silver reclamation. 

  • Scrap film or paper can be disposed of in the trash if testing based on the Toxic Characteristic Leaching Procedure indicates levels are below permitted standards for landfill disposal.

 

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