Question

Cementite is:
A. Predominant content of cement
B. A mineral of calcium
C. A carbide of iron in steel
D. None of the above

Answer 1

We know that, metallurgy is a process of technical and scientific separation of
metals from their ores. There are many unwanted particles present in ore along with metals. They are termed as gangue. These are to be removed by different processes such as magnetic separation, hydraulic washing etc.

Complete step by step answer:
Let’s discuss cementite in detail. It is a compound of carbon and iron. It is a transition metal carbide whose formula is ${\rm{F}}{{\rm{e}}_{\rm{3}}}{\rm{C}}$. The compound consists of 6.67% of carbon and 93.3% of iron by weight. It is a carbon impurity which is produced on cooling of pig cast along with white cast iron as an impurity. So, we can say that cementite is a carbide of iron in steel.
Hence, the correct answer is option C.

Additional Information:
Let’s understand the extraction of iron. The extraction of iron is done in a huge container known as a blast furnace. One ore of iron is haematite ore whose chemical formula is ${\rm{F}}{{\rm{e}}_{\rm{2}}}{{\rm{O}}_{\rm{3}}}$(Iron (III) oxide). The removal of oxygen is done by the reduction reaction. As carbon is more reactive than iron, it displaces iron from the iron(III) oxide to form iron and carbon dioxide.
$2{\rm{F}}{{\rm{e}}_{\rm{2}}}{{\rm{O}}_{\rm{3}}}\left( s \right) + 3{\rm{C}}\left( s \right) \to 4{\rm{Fe}}\left( l \right) + 3{\rm{C}}{{\rm{O}}_{\rm{2}}}$

Note:
Cementite possesses the structure of an orthorhombic crystal. It is a brittle and hard material. It is classified as ceramic in pure form and it is found as an important constituent in ferrous metallurgy. It exists in most cast iron and steels. It is thermodynamically unstable as it is converted to graphite (high level of carbon) and austenite (low level of carbon) at higher temperature.