6. Copper extraction by case-hardening

The cementation or iron precipitation process is an old one, used by the Spanish on Rio Tinto mine waters as early as 1670, and by the English at County Wicklow in Ireland in 1880. The process is applicable to dilute solutions containing 0.3 to 3.5 g/L Cu. The iron acts as a reductant on the copper and passes into solution, precipitating copper onto the iron surface according to the following two redox half-reactions: $\begin{array}{cc}{\text{Fe}}^{\text{0}}\to {\text{Fe}}^{2+}+{\text{2e}}^{-}& \left(E^0=-\text{0}\text{, 440 V}\right)\end{array}$