Question

Which of the following is not an example of roasting?
(A) $2ZnS + 3{O_2} \to 2ZnO + 2S{O_2}$
(B) $2PbS + 3{O_2} \to 2PbO + 2S{O_2}$
(C) $2C{u_2}O + 3{O_2} \to 2CuO + 2S{O_2}$
(D) $C{u_2}O + C{u_2}S \to 6Cu + S{O_2}$

Answer 1

Roasting is the process where ores are heated at high temperatures in the presence of air and converted into their oxides. It is done mostly for sulphide ores. This process leads to removal of moisture, toxic and acidic compounds and non-volatile gases like $S{O_2}$.

Complete step by step answer:
-First of all, we should know about the process of metallurgy.
Metallurgy is the process that is used for the extraction of metals in their pure form from their impure ores. We should know that compounds of metals are usually found in the earth mixed with soil, limestone, sand, and rocks; in this form are known as minerals. When there are one or more than one minerals present, that can be extracted, purified and sold at profitable rates, they are known as ores. Metallurgy is basically the process that deals with the purification of these metals and the formation of alloys.

-The metallurgical processes for extraction of pure metal occur in the following sequence:
(1) Crushing and grinding
(2) Ore concentration
(3) Hydrolytic separation
(4) Magnetic separation
(5) Froth floatation
(6) Roasting and calcination

-Now, we will talk about the roasting process in metallurgy.
Roasting is the process where an ore is converted into its oxide by heating it above its melting point in the presence of excess air. During roasting, moisture and non-metallic impurities are released from the mineral in the form of volatile gases. Roasting consists of a solid-gas thermal reaction which includes oxidation, reduction, sulphation and chlorination. This process may convert a metal sulphide to a metal oxide or to a free metal. This process is done mostly for sulphide ores like: ZnS (sphalerite), $C{u_2}S$ (chalcocite), etc.
-Now, coming back to our question, we will check which of the options is not a roasting reaction by checking the involvement of a sulphide ore.
Here we can see that option (A), (B) and (C) involve heating of concentrated ores to high temperatures in the presence of air and hence represent roasting reactions.
But option (D) $C{u_2}O + C{u_2}S \to 6Cu + S{O_2}$, is an auto reduction reaction where unoxidized copper sulphide gets reduced to Cu and $S{O_2}$. Hence it is not a roasting reaction.
So, the correct answer is “Option D”.

Note: Calcination is also a process of converting an ore into an oxide by heating it strongly. The difference between calcination and roasting is that: calcination occurs in limited supply of air and is done for thermal decomposition of carbonates and hydroxides; while roasting is done in excess air and mostly for sulphide ores.