What is Auto Reduction Reaction?

1. The method of extraction of metal is called an Auto Reduction process or Self-Reduction process as no external reducing agent is used for the reduction process.
2. This method is useful for the extraction of Sulphide ores metals like Copper($\mathrm{Cu}$), Mercury($\mathrm{Hg}$), Cobalt($\mathrm{Co}$), etc., which are generally less electropositive.
3. These less electropositive metals and their sulfide ores are heated in presence of air to convert them into an oxide or sulphate form.$$\begin{gathered} 2{\mathrm{Cu}}_2\mathrm{S}\left(\mathrm{s}\right)+\hspace{0.17em}3{\mathrm{O}}_2\left(\mathrm{g}\right)\to 2{\mathrm{Cu}}_2\mathrm{O}\left(\mathrm{s}\right)+\hspace{0.17em}2{\mathrm{SO}}_2\left(\mathrm{g}\right) \\ \mathrm{Copper}\mathrm{Copper} \\ \mathrm{sulphide}\mathrm{oxide} \end{gathered}$$
4. Later the remaining part is heated in absence of air to give the desired metal and Sulphur dioxide gas. $2{\mathrm{Cu}}_2\mathrm{O}\left(\mathrm{s}\right)+\hspace{0.17em}{\mathrm{Cu}}_2\mathrm{S}\left(\mathrm{s}\right)\to 6\mathrm{Cu}\left(\mathrm{s}\right)+\hspace{0.17em}{\mathrm{SO}}_2\left(\mathrm{g}\right)$