Solvay process is used in the manufacture of sodium carbonate, $\mathrm{Na}_2 \mathrm{CO}_3$. This process involves following reactions.
$\begin{aligned}& \mathrm{NH}_3+\mathrm{H}_2 \mathrm{O}+\mathrm{CO}_2 \longrightarrow \mathrm{NH}_4 \mathrm{HCO}_3 \\& \mathrm{NH}_4 \mathrm{HCO}_3+\mathrm{NaCl} \longrightarrow \mathrm{NaHCO}_3+\mathrm{NH}_4 \mathrm{Cl} \\& \hline \mathrm{NH}_3+\mathrm{H}_2 \mathrm{O}+\mathrm{CO}_2+\mathrm{NaCl} \longrightarrow \mathrm{NaHCO}_3 \\&+\mathrm{NH}_4 \mathrm{Cl}
\end{aligned}$
The sodium bicarbonate obtained on heating gives sodium carbonate.
$\mathrm{NaHCO}_3 \stackrel{\Delta}{\longrightarrow} \mathrm{Na}_2 \mathrm{CO}_3+\mathrm{H}_2 \mathrm{O}+\mathrm{CO}_2$
Note: This process cannot be used in the manufacture of potassium carbonate as potassium bicarbonate is soluble in the solution of $\mathrm{NH}_4 \mathrm{Cl}$ and KCl.