Metallic Replacement Silver Recovery Units. MRUs use a chemical process to recover the silver in the fixer solution.
This process involves the use of canisters ranging in sizes from 5 to 20 gal containing either steel wool or wound screen.
These units do not need a power source and are only connected with hose attachments. There is a bypass tube that is connected at the top of the cartridge, which allows the fixer solution to go directly to the drain instead of through the cartridge if the cartridge becomes plugged.
These systems are extremely efficient, with recovery of silver up to 95% if the units are maintained correctly. If the units are not used frequently, the filler material will oxidize and be unable to complete the chemical reaction needed to recover the silver.
Also, near the end of the cartridge’s life cycle, between 3 and 6 months, the efficiency of the unit decreases rapidly. When this happens, the silver-containing fixer is deposited into the drain without complete recovery. To prevent this from happening, the unit needs to be monitored by testing the discharge for silver content. A simple way to do this is by using silver-estimating test strips. Color comparison gives a rough estimate of how much silver is present.
Once the cartridge is no longer useful, it is either shipped or picked up to be taken to a refiner where the sludge at the bottom of the container is removed and processed to yield between 30 and 50% silver. The silver does not return any profit to the company that owns the unit because it is so difficult to remove the silver from the steel wool inside the canisters.
Martin Harper & Julie M. Siegel (2003) Comparison of Discharge Silver
Concentrations from Electrolytic Plating and Metallic Replacement Silver Recovery Units, Journal of the Air & Waste Management Association, 53:4, 434-441, DOI: 10.1080/10473289.2003.10466175