ELECTROCHEMICAL CELLS

Description

An electrochemical cell is capable of removing metals from aqueous solution and plating them out as a solid deposit on a cathode. In this way, the amount of metal and, most importantly, the amount of heavy metal released to the environment is reduced. Where the concentrations are sufficiently high, simple plate cells can be employed. Where the concentrations are low, from about 500 mg/l down to less than 1 mg/l, high mass transfer electrochemical cells employing fluidised beds are necessary to recover the metal economically. A fluidised bed cell uses mesh electrodes immersed in a bed of inert glass beads. The quality of the plated metal is excellent - it is pure and compact, and can often be recycled without further refining. Another method for the successful removal of low concentrations of heavy metals utilises charged graphite particles or carbon felt electrodes.

An electrochemical cell also performs oxidative reactions at the anodes. In this way, some toxic organic compounds can be broken down and the amounts released to the environment reduced.

Application

A principal use of electrochemical cells is for the recovery of metallic copper from aqueous solution in the printed circuit board industry. They are also used for the recovery of nickel, zinc, lead-tin, gold and silver from aqueous solutions in metal plating operations and in photographic applications. Cadmium is a red list substance used to produce a corrosion-resistant coating. Fluidised bed cells can recover metallic cadmium successfully, and in combination with ion exchange systems can reduce the concentration of cadmium to extremely low levels sufficient to satisfy the very strict discharge standards pertaining to this toxic metal.

After plating, metal objects are washed in drag-out baths, and these become contaminated with metal in solution. Electrochemical cells can be positioned in parallel to the first drag-out bath to maintain the metal concentration at 20-500 mg/l. A further rinse of the plated workpieces would reduce the metal concentration in the effluent to about 05-20 mg/l, sufficient to meet some discharge consents. In this way over 99 % of a toxic metal can be recovered and recycled. If necessary, the metal concentration of the effluent can be reduced to 005 mg/l by the application of a second electrochemical cell. Efficient systems have also been developed for application with running rinses.

Electrochemical cells are also used in the metal plating industry to oxidise cyanide to cyanate, which has a much smaller environmental impact. The organic complexants used in metal finishing can also be broken down, allowing the metal to be recovered, and the organic compounds to be treated by biological action.

Sizing

A typical electrochemical cell would have a 15 kW pump, with the power consumption for electrolysis being up to 2 kW.