Causticization process is used for the preparation of 

Causticizing process (Gossage process) it is an old process and involves heating of 10% solution of $N{a}_{2}C{O}_3$ with a little excess of milk of lime $Ca{\left(OH\right)}_2.$

${\mathrm{Na}}_2{\mathrm{CO}}_3+\mathrm{Ca}(\mathrm{OH}{)}_2\to {\mathrm{CaCO}}_3\downarrow +2\mathrm{NaOH}$