That best silver recovery equipment for you depends on the following factors:
1) Your processing volume, whether or not you want to reuse your fixer or bleach-fix, and environmental concerns, such as strict limits on effluent discharge.
2) The training and technical knowledge of your personnel,
3) The price of silver,
4) The operating and refining costs.
The cost of silver recovery equipment is closely related to the volume of materials you process. If your volume is low, chemical recovery cartridges are a reasonable choice for silver recovery.
If your processing volume is high, you probably don't want to make frequent cartridge replacements or provide a large storage area for spent and replacement chemical recovery cartridges.
Although initial capital investment for electrolytic silver recovery cells is higher than for chemical recovery cartridges, you don't have the recurring cost of equipment replacement. If you use an electrolytic cell, your refining costs for the recovered silver will be much lower than with other methods, because the silver you recover is usually more than %95 pure.
For a large-volume operation, in-situ ion exchange is also an option. You can use this method for primary treatment, or use it to tail an electrolytic unit if you dilute the discharge from the electrolytic cell with wash water. This method will let you recover the maximum amount of silver and minimize the amount of silver discharged. It requires a greater capital investment and more chemical handling than chemical recovery cartridges or electrolytic cells.
The silver concentration that can be discharged to a treatment plant or to a receiving body of water is often regulated even though silver in photographic effluent is in a form that is not harmful. Therefore, cost may not be the primary consideration in choosing your silver recovery method. If you do not know the code limits for silver, check with your local regulatory agency. You may need to consider a method that will reduce the silver to an acceptable level. In some cases a regulatory agency may take into account the non-toxic nature of the silver in photographic effluent and allow a variance.
Chemical recovery cartridges can achieve silver recovery efficiencies greater than %90. However, it is difficult to maintain this level of recovery, so cartridges are an unreliable choice if you need to observe low discharge limits. Another problem with chemical recovery cartridges is that as silver is recovered, the steel wool becomes soluble, producing iron levels in the effluent as high as 3000 mg/L. Iron is regulated to levels well below those concentrations by many sewer codes.
It is also difficult to maintain very low silver concentrations in effluents with electrolytic recovery cells. The concentration depends on how low the current density can be set with your unit. As the silver concentration gets lower, the current density must be set lower to prevent silver sulfide from forming. With low current densities, a large cathode area is needed to achieve the necessary recovery rate. If your electrolytic cell is not capable of reducing the silver concentration in the effluent to the level required, you can simply feed the effluent from the electrolytic cell through chemical recovery cartridges to recover additional silver.
The greatest recovery efficiency is possible with an ion exchange recovery method. In-situ precipitation ion exchange can reduce silver concentrations in the effluent to a level as low as 0.1 mg/L. However, it is critical that you dilute the concentrate with the proper amount of wash water before treatment; too high a thiosulfate concentration in the solution will cause silver to leak through the column.
You need to do small study to know which silver recovery equipment is the best for you.
Source: Technical Documents
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