Native silver metal forms a water-soluble complex with a dilute aqueous solution of $\mathrm{NaCN}$ in the presence of

In the extraction of aluminium:

Process $\mathrm{X}$: employed for red bauxite to remove iron oxide (main impurity).

Process $\mathrm{Y}$ (Serpeck's process): used for white bauxite to remove $\mathrm{Z}$ (main impurity) then,

select the correct option for the process $\mathrm{X}$ and impurity $\mathrm{Z}$.