Why is iron pyrite not the ideal choice of ore for the extraction of iron?

The process that involves the removal of sulphur from the ores is

Roasting of sulphides gives the gas $X$ as a byproduct. This is a colourless gas with a choking smell of burnt sulphur and causes great damage to respiratory organs as a result of acid rain. Its aqueous solution is acidic,acts as a reducing agent,and its acid has never been isolated.
The gas $X$ is

$A$ sulphide ore is first converted into its oxide before reduction. This is done because

Calcination is used in metallurgy for the removal of

When limestone is heated strongly,it gives off $CO_2$. In metallurgy,this process is known as: