Assertion – The non-fusible impurities present in iron ore are removed by CaCO3 .
Reason: CaCO3 which decomposes into CaO and CO2. The CaO formed reacts with SiO2 to remove silica
Both assertion and reason are correct and the reason is the correct explanation for the assertion.
Limestone, which is added as part of the feed gets decomposed just above 1000 degree celcius as
CaCO3 → CaO+CO2
The lime thus formed, reacts with silica to form fusible slag. Do note that silica is a refractory material and does not melt easily.
CaO+SiO2 → CaSiO3
This slag does melt near the zone of fusion and forms a protective liquid layer preventing molten iron from getting oxidized. Now, which would be lighter - the slag or the molten metal?