Hallmark Refining Corporation is known for its refining capabilities and for innovative design and manufacturing of silver recovery equipment. Hallmark operates as a secondary refiner (TSDR), with a special emphasis on silver from the photographic industry and gold from the electronics industry.
Silver Recovery Technologies
Hallmark Refining offers three technologies to recover silver from silver-rich photographic processing solutions:
- Metallic replacement
Considerable research over a period of years has gone into making these recovery technologies efficient, economical and user friendly.
The most widely used silver recovery method for large photo processors is electrolytic, where the silver is recovered from solution by electroplating it onto 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.
There are two basic types of electrolytic equipment: one in which the cathode rotates in the solution and one in which the solution is pumped around a stationary cathode. Either type is capable of recovering more that 90% of the silver from silver-rich solutions. The Hallmark systems use a combination of both in a single unit.
The use of "Closed Loop" fix solutions combined with Hallmark's in-line ElectroSilver Sensor can significantly reduce the amount of silver carried over into the final wash water and discharged to the drain. This method can reduce mixing and chemical usage by up to 50%, further increasing the cost effectiveness of this technology. This approach may not be feasible in all circumstances and is dependent on the imaging media and application.
Fix solutions from Black and White processes are easy to desilver electrolytically and require little, if any, pH adjustment. Recovery efficiencies of more that 95% can be achieved.
Bleach-fix and fix solutions from color processes that contain iron-complex oxidizing agents are not as easy to desilver electrolyticly. Higher current densities and longer times are required, with routine pH adjustments necessary. The optimum pH range for silver-rich solutions containing iron is 7.8-8.5. Under acidic pH conditions (<7.0), the iron-EDTA complexes used in bleaches and bleach-fixes will oxidize the silver plated on the electrode and re-dissolve it. As the pH becomes more basic, (pH>7.5), the oxidizing power of the iron-EDTA complex is reduced and the plating of silver onto the electrode becomes more efficient. With proper equipment and pH adjustments, silver recovery efficiencies approaching 90% can be achieved from these solutions. It is important to maintain the correct levels of plating time and current density. Over-extending either of these two factors in an attempt to achieve higher efficiencies will cause sulfating. This results in coating the cathode with a black Sulfide precipitate, rendering it unsuitable for continued silver recovery.
Metallic Replacement (CRCs)
Metallic replacement is a process where elemental iron undergoes an oxidation-reduction reaction with the silver thiosulfate to produce ferrous ions and metallic silver. The exchange reaction is very rapid, but dependent on the contact of the silver-thiosulfate complex with the iron surface. Flow rate, iron surface area, contact time, and the pH of the solution are the major variables influencing recovery efficiency.
To ensure effective and controlled contact, metallic replacement is accomplished by metering silver-rich solutions at a controlled rate through a container of steel wool, iron particles or an iron-impregnated resin. These containers are generally referred to as metallic replacement cartridges (MRCs), chemical recovery Cartridges (CRCs), or silver recovery cartridges (SRCs). As the recovery cartridge is used, the active surface area is reduced and small channels will begin to develop in the iron substrate. The EPA required that two CRCs be placed in a series so that a secondary cartridge will collect the silver that a first one can potentially pass in the event of a break through. When the primary cartridge is expired, the secondary column is moved to the primary position and a new cartridge is placed in the secondary position.
A properly designed and maintained CRC is capable of recovering more than 95% of the silver from a silver-rich solutions when used in accordance with manufacturer specified flow rates. Two CRCs connected in series, with a valve between them to allow for testing will prevent the discharge of silver-rich solutions to drain. Hallmark's patented two - cartridge systems, properly installed and maintained and used in conjunction with manufacturer specified flow rates are capable of recovering 99% of silver from silver-rich solutions.
The precipitation method involves metering silver-rich solutions or wash water into a reaction vessel and adding a precipitant to separate the silver from the solution. Hallmark Refining uses the chemical TMT (trimercapto-s-triazine, trisodium salt) as a precipitating agent.
TMT is a chemical that causes silver to settle out of photographic liquids. The rate of this settling is astonishing. When TMT is introduced into silver bearing solutions, settled silver can be seen and measured in a mixing graduate within minutes.
The cost of operating a TMT silver recovery system is somewhat higher than other silver recovery technologies. Precipitation chemicals and filtration costs will account for about 25% of the recovered silver for a large lab. However, TMT is so reliable that silver concentrations can be easily predicted and compliance can be consistently met. Often, this will justify the additional cost.
TMT sludge from the precipitation process is easily refined and usually assays at no higher than CRS's. The TMT sludge is not considered hazardous material in most states, and can be shipped without a manifest. If you are having a difficult time meeting your present or future environmental regulations, TMT is worth considering.