In the cyanide process involving extraction of silver, zinc is used industrially as an

Formation of $Ni(CO)_4$ and subsequent its decomposition into $Ni$ and $CO$ (recycled) makes basis of Mond's process

$Ni + 4CO \xrightarrow{{{T_1}}} Ni(CO)_4 \xrightarrow{{{T_2}}} Ni + 4CO$

$T_1$ and $T_2$ are :

Froth floatation process used for the concentraion of sulphide one