ALUMINIUM AND ITS METALLURGY

Aluminium is the most abundant metal in the earth’s crust. Aluminium does not occur free in nature, but its compounds are numerous and widely distributed.

The chief and important ore from which aluminium is exclusively and profitably obtained is Bauxite, AI₂O₃.2H₂O. The extraction of the metal from bauxite involves the three main steps.

Purification of Bauxite

Electrolytic reduction of Alumina, (AI₂O₃)

Purification of AI.

PURIFICATION OF BAUXITE

Bauxite is either white or brown in colour depending upon the presence of too much SiO₂; TiO₂ or Fe₂O₃ respectively. The purification is carried out as below

Hall’s dry process:

Bauxite is fused with sodium carbonate. Sodium meta aluminate, NaAIO₂, is produced which is soluble in water. It is extracted with water. The impurities present viz. ferric oxide and silica, are left behind.

AI₂O₃ + Na₂CO₃ → 2NaAIO₂ + CO₂

The water extract is heated to 50^o — 60^oC and a current of carbon dioxide is passed through it. Aluminium hydroxide is ignited to obtain pure alumina.

2NaAIO₂ + 3H₂O + CO₂ → 2AI (OH)₃↓ + Na₂CO₂

2AI(OH)₃ → AI₂O₃ + 3H₂O

Baeyer’s process

(For ores containing much Fe₂O₃): The calcined bauxite is digested with strong NaOH solution at 150^o under pressure when alumina (AI₂O₃) dissolves out leaving behind oxides of iron, titanium as residue.

AI₂O₃ + 2NaOH → 2NaAIO₂ + H₂O
Sod. aluminate

The mass is filtered and the filtrate is stirred with freshly precipitated AI(OH)₃. Sodium meta-aluminate in the filtrate is hydrolysed and is changed into aluminium hydroxide as precipitated.

2AI(OH)₃ AI₂O₂ + 3H₂O

Serpeck’s process (for ores contained much of silica, SiO₂):

The ore is heated with carbon at 1800^oC and nitrogen is passed over it. Aluminium nitride is formed which on hydrolysis with water yield AI(OH)₃. This is separated and calcined to get alumina.

AI₂O₃ + 3C+ N₂ → 2A/N + 3CO↑
Bauxite

A/N + 3H₂O → AI(OH)₂↓ + NH₃↑

2AI(OH)₃ → AI₂O₃ + 3H₂ O

SiO₂ + 2C → Si + 2CO

Silica is present as impurity is reduced by carbon to silicon, which volatilizes at the high temperature.

Electrolytic reduction of pure AI₂O₃ (Hall and Heroult’s process, 1886)

Aluminium metal has great affinity for oxygen and thus the reduction of its oxide by carbon is not possible under ordinary conditions. It does take place at very high temperature and AI metal formed reacts with carbon forming aluminium carbide (AI₄C₃).

The reduction is however, possible electrolytically, but is encountered with the following difficulties.

Pure AI₂O₃ is a bad conductor of electricity and has high melting point i.e., above 2000^oC.

If electrolysis of fused alumina is carried out, metal aluminium formed will vaporize off as its boiling point is about 1800^o
C.

PRINCIPLE OF EXTRACTION

Alumina is mixed with cryolite (Na₃AIF₃), fluorspar (CaF₂) in the ratio 20 : 60 whereby, it not only becomes good conductor but also fuses at about 900^oC which is much below the b.p. of aluminium.

The electrolysis of the fused mass is carried out in an iron box, which lined with gas carbon. The lining serves as the cathode, the anode consists of carbon rods dipped in the fused mass. The fused electrolyte is kept covered with a layer of powdered coke to prevent any action of air. The voltage employed in the electrolysis is 5.3 volts. The current passed (about 50,000 amperes) serves to purposes: (i) heating and (ii) electrolysis. Thus the fused mass is automatically kept at 900^oC during electrolysis.

Aluminium is obtained at the cathode and being heavier than the electrolyte sinks to the bottom and is tapped off periodically from the tap hole. Oxygen liberated at the anode attacks carbon rods and forms CO and CO₂. During electrolysis the concentration of the electrolyte goes on falling thereby increasing the resistance of the cell which is indicated by the glowing of a lamp placed parallel. Much of the alumina is then added and the process is made continuous.

The probable reactions may be given as
4A/F₃ → 4Al³⁺ + 12F^–

12F^– → 12F + 12e^–

12AI₂O₃ + 12F → 4AIF₃ + 3O₂

4C + 3O₂ → 2CO + 2CO₂

4AI³⁺ + 12e^– → 4AI At cathode At anode

PURIFICATION

Aluminium as produced by the electrolysis of AI₂O₃ is 90% pure. It can be refined further up to 99.9% purity by Hoope’s process.

The electrolytic cell consists of an iron tank lined with carbon. It is filled with three liquids differing in specific gravity. The upper layer is of pure fused aluminium and serves as cathode.

The bottom layer is that of impure metal in the fused state and serves as anode. The central layer is that of molten mixture of the fluorides of AI, Ba and Na and serves as an electrolyte.

On passing electric current, pure aluminium goes to the top layer from the central layer and an equivalent amount of the metal from the bottom layer passes into the central layer. There is thus gradual transference of aluminium from bottom layer to the top and the impurities are left behind. Crude aluminium is added from time to time.

Aluminium is not extracted directly from bauxite. Explain why?

PROPERTIES OF ALUMINIUM

Physical :

It is a bluish white metal with a brilliant luster.

It is a light metal and its density is 2.7.

Its melting point and boiling point are 658.7^oC and 1800^oC respectively.

It is good conductor of heat and electricity.

It forms alloys with other metals.

Chemical:

Action of air: It is not affected by dry air but in moist air a film of oxide is formed on the surface. It burns in oxygen producing oxide.

4AI + 3O₂ → 2AI₂O₃ + 77/2 kcals.

2AI + N₂ → 2AIN

Action of water: Pure Aluminium is not affected by pure water but impure metal is corroded by water containing some impurities.

It decomposes by boiling water evolving hydrogen.

2AI + 6H₂O → 2AI(OH)₃↓ + 3H₂

Action of acids:

(a) Dilute HCI and H₂SO₄ produce hydrogen with AI.

2AI + 6HCI → 2AICI₃ + 3H₂

2AI + 3H₂SO₄ → 2AI₂(SO₄)₃ + 3H₂

(b) Hot and concentrated H₂SO₄ produces sulphur dioxide.

2AI + 6H₂SO₄ → 2AI₂ (SO₄)₃ + H₂

(c) Concentrated HNO₃ does not react with AI as AI forms a self protecting coating of AI₂O₂ which renders HNO₃ unreactive towards AI. This is called passivity of AI.

Action of alkalies : When AI-powder is boiled with NaOH solution, H₂ is liberated and soluble aluminates are formed.

2AI + 2MOH + 2H₂O → 2MAIO₂ + 3H₂
meta aluminate

2AI + 6MOH → 2M₃AIO₃ + 3H₂ (M = Na, K)
meta aluminate

Action of halogens : When halogens are passed over heated aluminium halides are formed.

2AI + 3CI₂ → 2AICI₃

Action of nitrogen : Aluminium when heated with nitrogen gas gives aluminium nitride.

2AI + N₂ → 2AIN

Displacement of other metals : Aluminium displaces copper, zinc and lead from the solutions of their salts because these metals are less electropositive than AI. (see electrochemical series).

3ZnSO₄ + 2AI → AI₂(SO₄)₃ + 3Zn

USES OF ALUMINIUM

Aluminium, being very light, is used in householf utensils, aeroplane parts, precision and surgical instruments etc.

Since it is unattached by nitric acid, is used in chemical plants and also for transporting nitric acid.

Aluminium foil is used for packing chocolates, cigarettes etc.

Alums are used as mordents in dyeing and points.

Mixed with oil, it is used in steam piped and other metal objects.

It is used as a reducing agent for the production of certain metals such as chromium, iron, manganese etc.

Alumina is used for making refractory bricks and ultramarine.

ALUMINIUM ALLOYS

Aluminium forms a number of useful alloys, which are given as follow;

Alloy	Approximate composition	Uses
(1) Aluminium bronze	AI 10%, Cu 90%	For hard, non-corrodible vessels
(2) Duralumin	AI 95%, Cu3% Mn1% Mg1%	Aeroplanes and automobile parts
(3) Magnalium	AI 90%, Mg10%	Balance beams
(4) Y-alloy	AI 92.5%, Cu 4%, Ni 2%, Mg 1.5%	Aeroplanes, non-corrodible vessels.

GOLDSCHMIDTS’ ALUMINOTHERMIC PROCESS

The evolution of enormous quantities of heat in the oxidation of aluminium is used in thermite welding of metals. This process is also known as Goldschmidts aluminothermic process.

A mixture containing 3 parts of ferric oxide and one part of aluminium powder is placed in a crucible lines with magnesite and having a plug hole (see figure). This is covered with a layer of mixture of magnesium power and barium peroxide, with a magnesium ribbon inserted into it to act as a fuse. The broken ends of a rail or a girder etc. are brought nearer and thoroughly cleaned and surrounded by a
fire-clay mould. When the magnesium ribbon fuse is ignited, the reaction.

2AI + Fe₂O₃ → AI₂O₃ + 2Fe

(ΔH = 3230 kJ)

Starts producing tremendous amount of heat as a result of which the iron melts. This white hot molten iron is tapped from the crucible into the mould. The heated ends of the iron rods (to be welded) actually, melt and mix with the molten metal added, giving a firm and strong weld.