For the extraction of Lead when impurity content is very high, then during roasting of galena, the formation of which compound (s) is/are prevented by the usage of lime is__
 

By filling the lime and when air gets diffused and PbS particles come under limited supply of air therefore formation of ${\mathrm{PbSO}}_4$ is prevented. 

$\mathrm{PbS}+\frac{3}{2}{\mathrm{O}}_2\to \mathrm{PbO}+{\mathrm{SO}}_2$

$$\begin{gathered} \mathrm{PbO}+{\mathrm{SiO}}_2\to {\mathrm{PbSiO}}_3 \\ G\mathrm{angue} \\ {\mathrm{PbSiO}}_3+\mathrm{CaO}\to {\mathrm{CaSiO}}_3+\mathrm{PbO} \\ \mathrm{More} \mathrm{basic} \mathrm{less} \mathrm{basic} \end{gathered}$$