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used for removing the gangue from the ore are based on the differences
between the physical or chemical properties of the gangue and the ore.
Different separation techniques are accordingly employed.

3.4.3 Extracting Metals Low in the Activity Series

Metals low in the activity series are very unreactive. The oxides of these
metals can be reduced to metals by heating alone. For example, cinnabar
(HgS) is an ore of mercury. When it is heated in air, it is first converted
into mercuric oxide (HgO). Mercuric oxide is then reduced to mercury
on further heating.
Heat
2HgS(s) + 3O (g)  2HgO(s) + 2SO (g)
→
2 2
Heat
2HgO(s)  2Hg(l) + O (g)
→
2
Similarly, copper which is found as Cu S in nature can be obtained
2
from its ore by just heating in air.
Heat
2Cu S + 3O (g)  2Cu O(s) + 2SO (g)
→
2
2
2
2
Heat
→6Cu(s) + SO (g)

2Cu O + Cu S 
2 2 2
3.4.4 Extracting Metals in the Middle of the Activity Series
The metals in the middle of the activity series such as iron, zinc, lead,
copper, are moderately reactive. These are usually present as sulphides
or carbonates in nature. It is easier to obtain a metal from its oxide, as
compared to its sulphides and carbonates. Therefore, prior to reduction,
the metal sulphides and carbonates must be converted into metal
oxides. The sulphide ores are converted into oxides by heating strongly
in the presence of excess air. This process is known as roasting. The
carbonate ores are changed into oxides by heating strongly in limited
air. This process is known as calcination. The chemical reaction that
takes place during roasting and calcination of zinc ores can be shown
as follows –
Roasting
Heat
→
2ZnS(s) + 3O (g)  2ZnO(s) + 2SO (g)
2 2
Calcination
Heat
→
ZnCO (s)  ZnO(s) + CO (g)
3 2
The metal oxides are then reduced to the corresponding metals by
using suitable reducing agents such as carbon. For example, when zinc
oxide is heated with carbon, it is reduced to metallic zinc.
ZnO(s) + C(s) → Zn(s) + CO(g)
You are already familiar with the process of oxidation and reduction
explained in the first Chapter. Obtaining metals from their compounds
is also a reduction process.
Besides using carbon (coke) to reduce metal oxides to metals,
sometimes displacement reactions can also be used. The highly reactive
metals such as sodium, calcium, aluminium, etc., are used as reducing

Metals and Non-metals 51

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