Silver Recovery Technologies

There are several ways to undertake in-house silver recovery.

Factors affecting the business case for silver recovery include:

• Capital and running costs.
• Silver value.
• Resources.
• Chemical usage.

A silver recovery system needs to be efficiently and carefully run to ensure that maximum levels of silver are removed, and that silver discharge limits are met.

Your photochemical supplier should be able to advise you on the equipment suitable for your operations.  The main methods are:

1. Electrolytic
2. Metallic Replacement
3. Ion Exchange
4. Reverse Osmosis
5. Chemical Precipitation

1. Electrolytic

This is the normal system used in laboratories.

A controlled direct current of electricity is passed between two electrodes.

High purity silver is deposited on the negatively charged cathode and can be removed periodically as silver flake.

Important factors which will determine the efficiency and practicality (maintenance and cost) of the unit include:

• Silver concentration of the solution.
• Plating current.
• System capacity.
• Tank size.
• Cathode design.
• Agitation.
• Level of automation.

The silver that is extracted will be of high grade and can easily be refined.

Problems can occur with pH and sulphite controls that severely affect efficiency.

To meet tight silver controls it is usually necessary to further treat the output of an electrolytic machine with metallic replacement or ion exchange cartridges.

2. Metallic Replacement

An iron material (typically steel wool) is packed into a bucket or similar container.

When iron comes into contact with the dissolved silver solution (fixer), a reaction takes place and the iron dissolves.

Silver is reduced to its metallic state and drops to the bottom of the bucket as sludge. 

The sludge can then be refined to produce pure silver.

• The process is simple.
• It requires no electricity.
• It is easy to install and maintain.
• It is extremely messy and not easy to control.
• It is normally used for "tailing", with two or three cartridges in line, as a method of removing low levels of silver from electrolytic output or wash water.

3. Ion Exchange and 4. Reverse Osmosis (RO)

These are normally only used for removal of low levels of silver in wash waters.

RO can also be used to "clean" rinse baths.

They are not used for first line silver recovery; high levels of silver will render them ineffective in a very short time.

5. Chemical Precipitation

Metallic silver and insoluble silver compounds can be precipitated from photo processing solutions. 

This is a comparatively cheap system.

It requires a high level of control that is not generally possible in a photo laboratory.

More recent technologies are fairly simple to operate and are better options for most users.

Silver refiners and waste treatment companies mainly use it.