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More and more sophisticated forms of purification and recycling are being applied, to ensure as environmentally friendly production as possible. The challenge is to achieve this without increasing overall process costs. A good example of a win-win situation is the precipitation of precious metals from pickling, plating and developing solutions. Due to its high reactivity, iron powder is a natural and cost-effective way to remove gold, silver and other precious metals through a chemical reaction called cementation. Cementation takes place when a metal is extracted from an aqueous solution containing the metal as a soluble salt. When the solution comes into contact with a metal (in this case, pure iron) which has a lower electrochemical potential than the metal to be extracted, cementation occurs.
The following example describes the reaction when metallic copper is extracted from a copper salt solution, using metallic iron:

Cu²⁺ + Fe = Cu + Fe²⁺

The reaction can be split up into two steps:

Fe ----> Fe²⁺ + 2e^-
Cu²⁺ + 2e^- ----> Cu

The metallic iron gives off electrons and is oxidized to ferrous ions, while the cupric ions take up these electrons and are reduced to metallic copper. Hence, the total reaction can be looked upon as a redox system. Whether a certain metal is capable of extracting another metal from its salt solution depends on the relative electrochemical potential of the two reactions involved. The principle is that the metal to be extracted must have a higher electrochemical potential than the metal used as a cementation agent. The electrochemical potential of a number of metals is shown in the table: