Explanation

(a)
In the metallurgy of iron, when $CaCO_{3}$ is added to balst furnace, it removes impurities from ore and forms slag.
$CaCO_{3} \rightarrow CaO + CO_{2}$ (1070-1170 K)
$CaO + SiO_{2} \rightarrow CaSiO_{3}$ (1470 K)
$3CaO + P_{2}O_{5} \rightarrow Ca_{3}\left( PO_{4} \right)_{2}$