There are three methods of silver recovery from waste fixer solution and other silver-rich processing solutions. These methods are:
- Metallic replacement
1) Electrolytic Recovery
Electrolytic recovery is the most widely used silver recovery method for fixer solution and photographic processing solution. 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 the other 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.
2) Metallic Replacement
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, chemical recovery Cartridges, or silver recovery cartridges. As the recovery cartridge is used, the active surface area is reduced and small channels will begin to develop in the iron substrate.
A properly designed and maintained Metallic Replacement CRC is capable of recovering more than 95% of the silver from silver-rich solutions when used in accordance with manufacturer specified flow rates.
3) Precipitation Method
Precipitation method involves metering silver-rich solutions or washes water into a reaction vessel and adding a precipitant to separate the silver from the solution. The chemical TMT (trimercapto-s-triazine, trisodium salt) is used 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 Precipitation 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, Precipitation TMT is so reliable that silver concentrations can be easily predicted and compliance can be consistently met. Often, this will justify the additional cost.
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.
If you interested in any of these Silver Recovery Methods, Hallmark Refining offers these three technologies.