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Photography and medical imaging are major sources of silver-containing wastes in the Air Force. The Air Force has recovered silver from these wastes for many years. Silver is recovered because it is a valuable commodity and can be sold to off-set film processing costs and because silver is a regulated environmental contaminant.
This Fact Sheet addresses the principal types of silver-containing photographic wastes, applicable environmental regulations, available silver recovery technologies, and their relative advantages and disadvantages.
When photograph or X-ray film is exposed, a "latent" image (not visible to the naked eye) is formed. This image consists of elemental silver and is surrounded by silver halide crystals suspended in layers of gelatin coating on the film or paper support. Developing this film involves three basic steps:
When the film is moved from the fixer to the rinse, it carries a small amount of silver which is removed by the rinse water. Rinse waters contain low concentrations of silver ranging from less than 1 milligram per liter (mg/L) to greater than 5 mg/L. Although there is little economic benefit to recovering silver from rinse water, environmental regulations prohibit discharge of untreated rinse water if the silver concentration exceeds regulatory limits. Nearly all the silver in photographic wastes is bound up in silver thiosulfate complexes, which are highly stable. However, federal, state and local regulations governing silver-containing wastes do not distinguish between different forms of silver.
Excess or discarded photographic film is also a silver-containing waste. Since it is a dry solid waste, it is easily managed and is generally sold directly to a contractor for silver recovery. This fact sheet does not address technologies for recovering silver from film.
The regulation of wastes from photographic processing units can be very complicated. Regulation of the effluent from the developing equipment or the silver recovery unit depends on the method used to convey the waste to treatment or disposal. If the waste is discharged directly into a sanitary sewer as a process waste water, it is regulated as a point source under the Clean Water Act (CWA). If the waste is collected and stored in a container prior to treatment, it will be regulated as a hazardous waste under the Resource Conservation and Recovery Act if it contains silver in excess of 5 mg/L or if it exhibits other properties which may render the containerized waste hazardous.
Whether the treatment plant is a Publicly-Owned Treatment Works (POTW) or a Federally-Owned Treatment Works (FOTW) each must operate in accordance with the CWA. To insure compliance, a permit is issued by a state or by the Environmental Protection Agency (EPA) under the National Pollutant Discharge Elimination System (NPDES). The NPDES permit is issued to insure the discharge from the Treatment Works meets established standards.
The CWA strictly prohibits discharges of silver to the Treatment Works that would lower the pH of the waste water entering the Treatment Works to less than 5, or would interfere with the proper operation of the Treatment Works (stop biological activity). To insure waste is properly treated, the Treatment Works establishes a program to regulate discharges of industrial waste to the facility. Air Force installations, whether it operates its own Treatment Works or discharges to a POTW must insure its discharges are acceptable to the Treatment Works. The Treatment Works operator will determine the concentration of silver that can be discharged to the plant, based on the ability of that plant to treat the waste.
Many of the currently available technologies for silver recovery from wastewaters are most effective at a restricted range of silver concentrations. For this reason, some technologies are appropriate only for silver recovery from high-concentration fixer solutions, and others are more suited to low-concentration rinse water silver recovery. This section addresses the following silver recovery technologies for fixer solutions: metallic replacement, galvanic plating, electrolytic plating and precipitation. The costs, advantages and disadvantages of these technologies are summarized below:
The most commonly used technology for recovering silver from fixer solutions is metallic replacement. This technology is very simple in principle and operation. In most systems, a bucket or barrel filled with steel wool is connected to the fixer bath overflow and the excess solution flows through the unit under gravity. The silver thiosulfate complex in the fixer solution reacts with the iron in the steel wool. The iron goes into solution and the silver precipitates and settles to the bottom of the unit as a sludge.
Metallic replacement units require no electricity or special plumbing connections. These units take up little space and are readily available for $50 or less. The units must be replaced regularly as the iron in the steel wool is consumed. The size of the unit must be matched with the volume and silver concentration of fixer solution to optimize the amount of time the solution is in contact with the steel wool. When the unit is spent, it can be packaged and shipped directly to a silver reclaimer.
The silver sludge produced in the metallic replacement units is only 30 to 50% pure, and thus brings a lower price than silver recovered by other means. However, the low capital cost of metallic replacement is attractive for facilities which cannot justify more expensive technologies. The metallic replacement can achieve very low effluent concentrations (below 1 mg/L) if well maintained. Many facilities employ two units connected in series; the effluent from the first unit is treated further in the second unit. When the first unit is spent, the steel wool remaining in the second unit is often placed in the first unit to be further consumed. This configuration ensures low silver concentrations are attained on a consistent basis.
Galvanic plating units are very similar to metallic replacement units in many respects; however, the operating principle is electrolytic. Copper-clad iron screening chemically produces an electric charge in the fixer solution, causing silver to be plated onto the screen. These units require the same basic maintenance as the metallic replacement units. The higher capital cost of these units is offset somewhat by the higher quality of recovered silver obtained. These units function best at relatively high silver concentrations. A recent US Army study, which compared galvanic plating units with a number of alternative silver recovery technologies, found inconsistent recovery rates and effluents that, at times, exceeded statutory limits.
This technology operates by passing a controlled electrical current between two electrodes suspended in the fixer solution. Silver is deposited on the cathode (negative electrode) in nearly pure metallic form. Electrolytic plating is most efficient at high silver concentrations. Electrolytic units vary greatly in price from a few hundred dollars to more than $10,000. Since the effluent produced generally contains silver concentrations in excess of 100 mg/L, electrolytic plating units are only recommended in conjunction with some other silver recovery technology. Many large processing facilities use in-line recirculating electrolytic plating units to recover high-purity silver and maintain fixer quality, and use another technology to treat fixer solution overflow. This procedure has the advantages of producing a higher price for recovered silver and reducing the need for fixer solution replenishment. In addition, it produces a lower concentration of silver in the fixer solution overflow and rinse water, since over 95% of the silver can be recovered by the unit.
Chemical precipitation of silver from fixer solution employs sodium borohydride, sodium dithionate, or other chemicals to produce a low concentration of silver in the effluent. However, precipitation methods are generally done in batch mode and require separation of liquids and solids prior to discharge. There are also health and safety hazards associated with some of the chemicals used. For these reasons, precipitation is not commonly used to recover silver from film processing operations.
Although there is little economic benefit to silver recovery from rinse waters, the primary consideration is meeting effluent discharge standards. Effective technologies for silver recovery from low-concentration rinse waters include ion exchange, reverse osmosis, and metallic replacement. These technologies are also summarized in Tables 1 and 2.
| Ion Exchange | An ion exchange column recovers silver to form a concentrated solution. Silver ions in the rinse water are adsorbed onto a resin, then removed by a regenerating solution when the resin is saturated. The recovered silver solution is then trickled through a metallic replacement unit or an electrolytic plating unit to complete the recovery of the silver. Ion exchange columns can be automated, but must be regenerated every four to six months. 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. The effluent from a well maintained ion exchange column generally contains less than 1 mg/L of silver. |
| Reverse Osmosis | A reverse osmosis system separates silver from rinse waters through a selectively permeable membrane. The rinse water is pumped under high pressure over a membrane made of cellulose acetate or a similar material. The small pores of the membrane prohibit passage of most salts and organics, but permit the passage of smaller silver ions to produce a more concentrated silver solution. The concentrated silver solution can then be recovered by some other means. A reverse osmosis system is quite costly due to the specialized high-pressure pumping system. Such a system is also subject to fouling if the rinse water contains substantial amounts of foreign particles. The rinse water which does not pass through the membrane contains less than 1 mg/L of silver. |
| Metallic Replacement | Metallic replacement cartridges are suitable for both rinse water and fixer solution treatment. However, these cartridges work best only if the flow rate of the rinse water is low enough to provide adequate residence time in the unit. When used properly, these units can produce silver concentrations below 1 mg/L in the effluent at a low cost. |
| Technology | Relative Cost | Advantages | Disadvantages |
|---|---|---|---|
| Fixer Treatment | |||
| Metallic Replacement | Low | Cost | Low purity silver product Simple to operate Low concentration effluent |
| Galvanic Plating | Low | Cost | Effluent may not meet standards Simple to operate Higher purity silver product |
| Electrolytic Plating | Moderate to High | Higher purity silver product | Cost Reduced fixer requirements Complex setup/operation Effluent requires additional treatment |
| Precipitation | Low | Cost | Batch treatment High purity silver product Hazardous chemicals |
| Rinse Water Treatment | |||
| Ion Exchange | High | Low concentration effluent | Cost Complex setup Silver requires further recovery |
| Reverse Osmosis | High | Low concentration effluent | Cost Complex setup Silver requires further recovery |
| Metallic Replacement | Low | Cost | Low purity silver product Low concentration effluent Simple to operate |
Your installation can obtain silver recovery units from the Defense Reutilization and Marketing Office. To obtain information on their services, contact Ms. LaVona E. Remakel, DSN 246-6655.
PRO-ACT has completed several technical inquiries (TIs) concerning silver recovery materials handling and product information. To request a copy of any PRO-ACT TI, call us at DSN 240-4214. You may also contact 1st Lt. Timothy G. Bossetti, US Army Environmental Hygiene Agency, DSN 923-6205, for more information about the US Army study of silver recovery technologies.
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@MAST HEAD TEX = DSN 240-4214 (800) 233-4356
Last Updated: July 27, 1995