Metals occupying low positions in the electrochemical series can be extracted by hydrometallurgy because the metal ions (Mn+) of such metals can be easily reduced by treatment with some more electropositive metal. The metals occupying higher positions in the electrochemical series cannot be extracted by hydrometallurgy because the metal ions of such metals are difficult to be reduced.
Copper can be extracted by hydrometallurgy because it occupies quite lower position in the electrochemical series. On the other hand, zinc cannot be extracted by hydrometallurgy because it occupies higher position in the series and has large negative reduction potential.