Silver may cause a material to be classified as a RCRA hazardous waste by the Toxicity Characteristic Leaching Procedure (TCLP) (40 CFR 261.24). However, for a material to be a RCRA-hazardous waste, it must first fit the definition of a solid waste under RCRA (40 CFR 261).

Several technology categories are used for silver recovery, including precipitation, ion exchange, reductive exchange, electrolytic recovery, reverse osmosis, and electrodialysis. The specific technology to be applied for a particular waste stream will be based on the waste characteristics, volume, and treatment objectives. For example, if reduction of silver concentrations to meet wastewater effluent limitations is the primary treatment objective, then a technology which achieves extremely high silver recovery from the waste stream is probably not a cost-effective application. On the other hand, in cases where silver recovery is of primary importance, then application of a highly efficient system, such as reverse osmosis or ion exchange, makes sense. The following paragraphs provide brief descriptions of various technologies used for treating silver-laden waste streams. Although this discussion centers on the application of these technologies for silver treatment and/or recovery, these technologies are also effective in addressing additional waste constituents, as described herein.

• Waste Stream Heavy Metal Precipitation: Hydroxide precipitation of metal-laden wastewater is a common technology that has been proven effective for several decades. The concept of this technology includes adjustment of the waste stream pH to a level at which the targeted metal is least soluble and will readily precipitate from solution. Precipitated solids are agglomerated, allowed to settle out, and then withdrawn from the treatment unit as a metal hydroxide sludge. Since the optimal treatment pH for various targeted metal species ranges from about pH 7 to over pH 12, most hydroxide precipitation systems are only effective in co-precipitating two, or perhaps three, different metals. As a result, most metal wastewater treatment systems which deal with multiple metal species include some means to segregate the waste stream flow trains, to more effectively address specific metals. Hydroxide precipitation of silver is effective, although it is not widely utilized, due to unfavorable economics and lower recovery efficiencies than competitive technologies.
  
  Silver is frequently precipitated from metal wastewater as silver chloride, which is extremely insoluble. Thus, silver can be selectively removed from a mixed metal wastestream without prior segregation or co-precipitation interference. Should alkaline conditions exist in the silver treatment wastestream, resulting in co-precipitation of additional metal species, the precipitated metal sludges can be acid washed to leave the insoluble silver chloride compound. Alternative silver precipitation processes, such as the patented process developed by Eastman Kodak, are available. These processes rely on the use of magnesium sulfate and lime. The silver then precipitates as a mixed sulfate-oxide and is recovered from the settled sludge.
  
  Sulfide is also widely used and is one of the oldest technologies for silver recovery in the photographic film processing industry. Hydrosulfite precipitation results in formation of both free silver and sulfide, with a more compact precipitate with favorable settling qualities. Disadvantages of this method include relatively high chemical costs and the need for supplemental heat input.
  
  Precipitation is not commonly used to recover silver from film processing operations, nor is it commonly used to process liquid wastes to meet discharge limits.
  
  
• Ion Exchange: Ion exchange is commonly used for silver recovery from liquid waste streams. One patented process exists which involves silver recovery from dilute photographic processing wash water by passage through a mixture of basic ion exchange resins. Silver retained on the resin is recovered through either elution of silver salts from the resin bed or by direct resin incineration and pure silver recovery. An ion exchange system for silver cyanide rinsewaters has also been developed, which consists of a five-column unit. In the first column, a cation exchange resin converts the silver cyanide to a silver hydrogen cyanide complex. The second column contains an anion exchange resin, which removes the silver cyanide. The effluent from the second column is then treated with sodium or potassium cyanide to recover the silver as either sodium or potassium silver cyanide. The additional three columns are used to achieve additional cyanide removal.
  
  Another application of ion exchange technology for silver recovery utilizes a Type I, strong base gel anion resin to selectively remove silver from waste rinses. The dissolved silver is present as a negatively charged thiosulfate anionic complex which exchanges with sulfate ions on the anion resin. The resin bed is then rinsed with 2 percent sulfuric acid to precipitate the silver collected on the resin. The effluent from this rinse is then collected, neutralized, and discharged. This system is capable of reducing silver concentrations in the treated effluent to between 0.1 and 2.0 mg/l. After a sufficient number of cycles through the resin bed, the ion exchange capacity of the resin bed to remove silver will be exhausted. At this time, the resin is sent to a silver refiner for incineration and silver recovery.
  
  Automated ion exchange columns units generally cost several thousand dollars and are practical only for large processing facilities. An ion exchange column is not suitable for high concentrations of silver, but may work well for recovering silver from fixer bath overflows that are diluted with rinse waters.
  
  
• Metallic Replacement: This method utilizes iron metal (typically steel wool) to react with silver thiosulfate solution in photo processing fixer solutions and rinse waters. The iron replaces the silver in solution, while the silver settles out as a solid, for subsequent removal. The silver-laden waste solution is passed through a container filled with steel wool as a means of contacting the silver with the iron. The typical system consists of two chemical cartridges installed in series. The silver concentration in the treated effluent is generally below 5 mg/l. There are three disadvantages of this system: silver is recovered as a sludge, which is more difficult and expensive to process than alternative technologies; chemical recovery cartridges can not be reused and, thus, must be replaced when exhausted; and the cartridge effluent wastestream contains high iron concentrations. The advantage to this technology is that it is low cost, readily available, and requires no energy or special plumbing connections.
  
  
• Electrolytic Silver Recovery: In this method, silver-laden solutions are passed through a system containing an anode-cathode array, which applies a controlled current. As the solution flows through the system, silver in virtually pure form is plated on the cathode. Once a sufficient quantity of silver has been accumulated on the cathode, the silver is recovered. Two disadvantages of the electrolytic recovery method are the relatively high capital costs and the lower treatment efficiency (typical silver effluent concentrations are in the range of 100 to 200 ppm). In common practice, electrolytic and metallic replacement systems are used in series, whereby the electrolytic unit will remove up to 90 percent of the silver in the influent, with the metallic replacement system removing most of the remaining silver in solution. However, even when these two technologies are used in combination, they still are not capable of achieving silver effluent concentrations to well below 5 ppm. Ion exchange or other alternative technologies must be implemented if extremely low effluent silver concentrations are consistently required.
  
  Electrolytic recovery systems can also be used "in-line" for silver recovery in fixer solutions. Since much of the silver is recovered during the in-line process, often the only final treatment required is a metallic replacement cartridge system. Another benefit of in-line silver recovery is that less silver is carried into the wash water, since the concentration in the fixer bath is maintained at a lower concentration.
  
  
• Reverse Osmosis: This method uses high pressure to force a liquid solution through a semi-permeable membrane, which will separate larger molecule substances, such as salts and organic compounds, from smaller molecular substances, such as water. Reverse osmosis (RO) is capable of removing up to 90 percent of silver thiosulfate complexes (the most common silver compound present in most photo processing solutions) from wash water. Along with silver, RO is effective in removing almost all other chemicals in solution. On this basis, RO is used to recover such photo processing chemicals as color couplers and ferrocyanide. An additional benefit of RO treatment is that, due to its high removal efficiencies, treated wash water may be suitable for reuse in final rinses. Once silver has been removed from the wash water, it can be recovered using such means as chemical precipitation, metallic replacement, or electrolytic recovery. The primary disadvantage of RO compared to alternative silver recovery technologies is the high capital investment required. As a result, its principal application is for treating wash water solutions to reduce silver concentrations to acceptable levels for discharge.

The compliance benefits listed here are only meant to be used as general guidelines and are not meant to be strictly interpreted. Actual compliance benefits will vary depending on the factors involved, e.g., the amount of workload involved.

Consult your local industrial health specialist, your local health and safety personnel, and the appropriate MSDS prior to implementing this technology.

• Reduces the volume of hazardous waste generated from these processes.
• Reduces or eliminates hazardous waste handling/treatment/disposal costs.
• Allows reuse of silver for other purposes.

• These processes can be costly if not selected in conjunction with specific treatment plan or goal.

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