Which process is used for the extraction of copper from low-grade copper ores?

To prevent the corrosion of the anode in the Hall-Heroult process,which of the following action is done?

In the production of iron,limestone is added to the blast furnace. At the end of the process,the $Ca$ ion is in the form of.......

Impure silver ore $+ CN^{-} + H_2O \xrightarrow{O_2} [X]^{-} + OH^{-}$
$[X]^{-} + Zn \longrightarrow [Y]^{2-} + Ag \text{ (pure)}$
The coordination numbers of the metals in $[X]$ and $[Y]$ are respectively

Which of the following is not added during the extraction of silver by the cyanide process?

$FeCr_2O_4$ (chromite) is converted to $Cr$ by following steps:
Chromite $\xrightarrow{I} Na_2CrO_4$ $\xrightarrow{II} Cr_2O_3$ $\xrightarrow{III} Cr$
Reagents in $I, II$ and $III$ steps might be:
$I$ Step $-$ $II$ Step $-$ $III$ Step