Which reducing agent is employed to get copper from the leached low-grade copper ore ?

Explanation

Although zinc ` (E_(Zn^(2+)//Zn) = - 0*76V)`, iron ` (E_(Fe^( 2 + )//Fe) = - 0*44 V)` and ` H _ 2 (E^(@)_(H^(+)//H _ 2 ) = 0*00V)` all the three can be used to get copper from the leached low grade copper, yet scrap iron being much cheaper than zinc and ` H _ 2 ` gas is usually used.
`" " Cu^( 2 + ) (aq) + Fe (s) to Cu (s) + Fe ^( 2 + ) (aq) `