Modern inorganic chemistry

THE TRANSITION ELEMENTS 391

as a peroxodisulphate or lead* IV) oxide and concentrated nitric

acid, the purple colour of the manganate(VII) ion is seen.

IRON

THE ELEMENT

After aluminium, iron is the most abundant metal; and the fourth
most abundant of all the elements; it occurs chiefly as oxides (for
example haematite (Fe 2 O 3 ), magnetite (lodestone) (Fe 3 O 4 ) and as
iron pyrites FeS 2. Free iron is found in meteorites, and it is probable
that primitive man used this source of iron for tools and weapons.
The extraction of iron began several thousand years ago, and it is
still the most important metal in everyday life because of its abund-
ance and cheapness, and its ability to be cast, drawn and forged for
a variety of uses.
The process of extraction requires first smelting (to obtain the
crude metal) and then refining. In smelting, iron ore (usually an oxide)
is mixed with coke and limestone and heated, and hot air (often
enriched with oxygen) is blown in from beneath (in a blast furnace).
At the lower, hotter part of the furnace, carbon monoxide is produced
and this is the essential reducing agent The reduction reactions
occurring may be represented for simplicity as:

3CO 4- Fe 2 O 3 ^ 2Fe + 3CO 2 (13.4)

Fe 2 O 3 + CO -> 2FeO 4- CO 2 (13.5)

FeO + C -> Fe + CO (13.6)

Reaction (13.4) is exothermic and reversible, and begins at about
700 K; by Le Chateliers Principle, more iron is produced higher
up the furnace (cooler) than below (hotter). In the hotter region
(around.900 K), reaction (13.5) occurs irreversibly, and the iron(II)
oxide formed is reduced by the coke [reaction (13.6)] further down.
The limestone forms calcium oxide which fuses with earthy material
in the ore to give a slag of calcium silicate; this floats on the molten
iron (which falls to the bottom of the furnace) and can be run off at
intervals. The iron is run off and solidified as "pigs'—boat-shaped
pieces about 40 cm long.
Pig-iron or cast iron contains impurities, chiefly carbon (up to
5 %). free or combined as iron carbides. These impurities, some of
which form interstitial compounds (p. 113) with the iron, make it
hard and brittle, and it melts fairly sharply at temperatures between
1400 and 1500 K; pure iron becomes soft before it melts (at 1812 K).
Hence cast iron cannot be forged or welded.