`=>` Elements vary in abundance.
●Among metals, aluminium is the most abundant.
● It is the third most abundant element in earth’s crust (8.3% approx. by weight).
● It is a major component of many igneous minerals including mica and clays.
● Many gemstones are impure forms of `Al_2O_3` and the impurities range from `Cr` (in ‘ruby’) to `Co` (in ‘sapphire’).
● Iron is the second most abundant metal in the earth’s crust.
● It forms a variety of compounds and their various uses make it a very important element.
● It is one of the essential elements in biological systems as well.
`=>` The principal ores of aluminium, iron, copper and zinc have been given in Table1.
`=>` For the purpose of extraction, bauxite is chosen for aluminium.
`=>` For iron, usually the oxide ores which are abundant and do not produce polluting gases (like `SO_2` that is produced in case of iron pyrites) are taken.
`=>` For copper and zinc, any of the listed ores (Table1) may be used depending upon availability and other relevant factors.
`=>` Before proceeding for concentration, ores are graded and crushed to reasonable size.