Question

The main purpose of roasting and calcination is to convert:
A. Ore into metal
B. Ore into metallic carbide
C. Ore into oxides.
D. Ore into sulphides.

Answer 1

Hint: In calcination, an ore is heated below its melting point in absence of air. In roasting, an ore is heated below its melting point in excess of air.

Step-by-step solution: An ore is usually converted to oxide because oxides are easy to reduce. Sulphides and carbonates are not that easy to reduce hence they are first converted to oxides to make the process of obtaining metals from ores easy.
There are mainly two methods of converting ore to its oxide form. They are-
Calcination-In this process an ore is heated below its melting point in absence of air. First the moisture is driven out and the ore becomes dry .The hydrated ores become anhydrous and carbonate ores are converted into oxides. For example, Calcium carbonate is heated in absence of air to form calcium oxide(quick lime). The reaction is as follows-
$CaC{O_3}\xrightarrow{{heat}}CaO + C{O_2}$
Roasting-In this process an ore is heated below its melting point in excess of air. First the ore becomes porous and the reduction of any organic matter, if present, gets destroyed. Then Impurities of non-metals such as sulphur, arsenic are converted into their volatile oxides which get removed. So in Roasting, sulphide ores are converted into oxides .For example, Sulphur is oxidized to sulphur dioxide. The reaction is as follows-
${S_8} + 8{O_2}\xrightarrow{{heat}}8S{O_{2\left( g \right)}}$

Hence correct answer is C.

Note: The ores are heated to a temperature below its melting point so that the ore cannot change into liquid state and then vapour state. The reason behind this is that the ore in its vapour state will get mixed with the volatile impurities.