Page 205 - Chemistry ICSE Class X

Metallurgy 191
of aluminium by Charles M. Hall (1886), it has virtually become a versatile
metal in common use.
What is the position of aluminium in the Periodic Table

#NWOKPKWO KU C PQTOCN GNGOGPV 6JG GNGEVTQPKE EQPſIWTCVKQP QH CNWOKPKWO
K L M
is . Thus, there are three shells of electrons.
2 8 3
K L M
an atom of Al has three electrons in its outermost shell. So,
2 8 3
aluminium lies in Group 13 (or Group IIIA) and third period of Periodic
Table.
How is aluminium extracted from bauxite Fig. 7.8 Position of aluminium in the
periodic table
Purification of bauxite (Baeyer’s Process)
Bauxite contains iron oxide (FeO) and silica (SiO ) as impurities. Bauxite is
2
RWTKſGF D[ Baeyer’s process which consists of the following steps: dŚĞ ŵŽƐƚ ĐŽŵŵŽŶůLJ ƵƐĞĚ ŽƌĞ ŽĨ
ĂůƵŵŝŶŝƵŵ ŝƐ bauxite ; ů O ͘Ϯ, KͿ͘
Ϯ
ƒ Digestion of bauxite with sodium hydroxide. The powdered bauxite ĂƵdžŝƚĞ ƵƐƵĂůůLJ ĐŽŶƚĂŝŶƐ ŝƌŽŶ ŽdžŝĚĞ
Ϯ ϯ
is heated with concentrated solution of sodium hydroxide at 150°C. ;&ĞKͿ ĂŶĚ ƐŝůŝĐĂ ;^ŝK Ϳ ĂƐ ŝŵƉƵƌŝƟĞƐ͘
Ϯ
Aluminium oxide in bauxite dissolves in NaOH solution to form
sodium aluminate. The impurities are left behind undissolved and are
TGOQXGF D[ ſNVTCVKQP
150°C
Al O (s) + 2NaOH(aq) o 2NaAlO (aq) + H O
2
2
2
3
aluminium oxide sodium aluminate
(in bauxite) (soluble)
ƒ Precipitation of aluminium hydroxide. The solution of sodium
aluminate is diluted with water and then treated with a small quantity
of freshly precipitated aluminium hydroxide [Al(OH) ], or with a
3
dilute solution of HCl. Sodium aluminate undergoes hydrolysis to
give white precipitate of aluminium hydroxide.
Al(OH)
NaAlO (aq) + 2H O o NaOH(aq) + Al(OH) (s)p
3
3
2
2
sodium aluminium
aluminate hydroxide
(white precipitate)
6JG RTGEKRKVCVGF CNWOKPKWO J[FTQZKFG KU UGRCTCVGF D[ ſNVTCVKQP
ƒ Conversion of aluminium hydroxide to alumina. 6JG ſNVGTGF dŚĞ ƉƌŽĐĞƐƐ ŽĨ ĞůĞĐƚƌŽƌĞĚƵĐƟŽŶ ŽĨ
precipitate of aluminium hydroxide Al(OH) (s), is washed, dried and ĂůƵŵŝŶĂ ƚŽ ĂůƵŵŝŶŝƵŵ ǁĂƐ ĚĞǀĞůŽƉĞĚ
3
ignited to get pure alumina. ďLJ ŚĂƌůĞƐ DĂƌƟŶ ,Ăůů ŝŶ ϭϴϴϲ͘ dŚŝƐ
ŵĞƚŚŽĚ ŝƐ ŬŶŽǁŶ ĂƐ ,ĂůůͲ,ĞƌŽƵůƚ
heat
2Al(OH) o Al O (s) + 3H O ĞůĞĐƚƌŽůLJƟĐ ƉƌŽĐĞƐƐ͘
2
2
3
3
aluminium alumina
hydroxide (aluminium oxide)
Electroreduction of alumina to aluminium
#NWOKPKWO ECP DG QDVCKPGF HTQO CNWOKPC RWTKſGF
bauxite) by electrolytic reduction method. (Fig. 7.9).
The set-up involves the following:
ƒ Electrolytic bath. An iron tank lined from
inside with carbon-lining.
ƒ Operating temperature. The electrolytic bath
is maintained at about 950°C.
ƒ Voltage. About 5 – 6 volts. Fig. 7.9 Electrolytic extraction of aluminium from molten alumina
ƒ Cathode. The carbon-lining acts as cathode (–ve electrode).
ƒ Anode. Anode consists of a number of carbon rods suspended from
bus bars dipping into the electrolyte.