In the $Down's$ process,why is $CaCl_2$ added to $NaCl$?

Is it true that under certain conditions,$Mg$ can reduce $Al_2O_3$ and $Al$ can reduce $MgO$? What are those conditions?

Extraction of lead by reduction methods is done by

Formation of metallic copper from the sulphide ore in the normal thermo-metallurgical process essentially involves which one of the following reactions?

In the manufacture of metallic sodium by the fused salt electrolysis $(Down's \ process)$,a small amount of calcium chloride is added to:

During the extraction of gold,the following reactions take place: $4Au + 8CN^{-} + 2H_2O + O_2 \longrightarrow 4[Au(CN)_2]^{-} + 4OH^{-}$ and $2[Au(CN)_2]^{-} + Zn \longrightarrow [Zn(CN)_4]^{2-} + 2Au$. $X$ and $Y$ are respectively: