Case hardening is a process of heating steel in an atmosphere of:

Which process is used for the purification of aluminum?

The reaction for obtaining the metal $(M)$ from its oxide $(M_{2}O_{3})$ ore is given by
$M_{2}O_{3(s)} + 2 Al(\ell) \xrightarrow{\text{Heat}} Al_{2}O_{3}(\ell) + 2 M_{(s)}$, $(s = \text{solid}, \ell = \text{liquid})$
In this case, $M$ is

$A$ layer of coke is spread over bauxite during the electrolytic reduction of alumina by the Hall-Heroult process. This layer acts as a/an:

$Al_{2}O_{3}$ was leached with alkali to get $X$. The solution of $X$ on passing of gas $Y$,forms $Z$. $X$,$Y$ and $Z$ respectively are

Gold is extracted by a hydrometallurgical process based on its property: