Key characteristics of the aluminium production process
Some of the key characteristics of aluminium production technology are
·
Economies of scale: Alumina refining and aluminium smelting operations are influenced by
economies of scale. The minimum economic size of an alumina refinery is around
1 mtpa, and that of a smelter unit is 250,000 tpa - 325,000 tpa. Refining operations
are influenced more by economies of scale, as refining is a continuous process.
·
High capital intensity: Alumina refining and aluminium smelting are highly capital intensive.
A new refinery and smelter, of a minimum economic size would require an
investment of over $1 billion each.
·
High power intensity: Aluminium smelting is highly energy intensive and requires
uninterrupted power supply. Interruptions in power supply could have a
significant impact on the cost of production. In India
·
Proximity to mines: Alumina refineries are located close to bauxite mines, as the cost
incurred in the transportation of alumina is lower. (One tonne of primary
aluminium requires around 6 tonnes of bauxite, or 2 tonnes of alumina.) In
addition, given that alumina refineries are designed for specific bauxite
varieties, the refineries need to have long-term contracts with specific
bauxite mines, in order to ensure uninterrupted supply of the required variety
of bauxite.
·
Continuous supply of low cost electric power: Smelters must be located close to a continuous
supply of low cost electric power.
·
Mining and refining, and smelting operations in
separate locations: In the aluminium
industry, mining and refining, and smelting operations are increasingly being
located at different places and managed by different entities, due to the
technological differences in the various stages of aluminium production.
Semi-fabrication
technology
Aluminium is consumed largely in the form of semi-fabricated products.
Semi-fabrications are produced from aluminium ingots, slabs and billets by
various processes, such as rolling, extruding, drawing, casting and forging.
Rolling
Rolling involves a reduction in the thickness of aluminium slabs by
passing them successively between heavy rollers. The slabs are heated, and
rolled to a thickness of 1-3 inches (2.5-7.6 cm). After cooling, the metal is
rolled again to form plates, sheets, or foil. Aluminium plates have a thickness
of over 0.25 inch (6.4 mm) and are largely used for manufacturing cooking
utensils, chequered floorboards for rail coaches and buses, ships and storage
tanks. Aluminium sheets can be further rolled to manufacture foils. Aluminium
foils have a thickness of less than 0.15 mm.
The intermediate products for rolling (slabs) can be produced by three
routes; conventional direct chill (DC) casting route followed by hot and cold
rolling; thick strip casting followed by hot and cold rolling; and thin strip
casting followed by cold rolling. The thin strip casting technology is
increasingly being preferred, due to lower energy consumption. Energy
consumption for the thin strip casting route is lower, as compared with that in
DC casting and thick strip casting, due to the elimination of hot rolling and a
reduction in the number of passes required to achieve the desired thickness.
Extrusion
Extrusion is a process, wherein an aluminium billet is forced through a
die to produce sections and profiles. The billet could be heated prior to
extrusion to produce hot extrusions. Extrusion presses of varying tonnage
(applied pressure) are used for producing extrusions, depending on the size and
complexity of the profiles. Extrusion process is used to make rods and tubing,
frames for doors and windows, heat sinks for electronic equipment (such as
printed circuit boards), and trims for automobiles.
Drawing
Drawing is used to produce aluminium wire and tubing. In order to
manufacture aluminium wire, an aluminium rod is pulled (drawn) through a series
of successively smaller dies. A wire is a rod of diameter less than 9.5 mm.
Stranding several wires into a single length makes an electrical conductor.
Bars, rods, and tubes can also be made by the drawing method directly
from stock produced by hot extrusion. An electrical conductor rod can also be
drawn and stranded directly from molten aluminium. Aluminium tubes are made by
pulling an aluminium rod through a die. A steel bar, called a mandrel and
placed at the centre of the die, removes the material from the centre of the
rod, and thus produces a tube.
Casting and forging
In the casting process, alloyed foundry ingots are melted and then
poured or forced into moulds of a desired shape. The aluminium is removed from
the moulds after it solidifies. One of the major applications of casting is
parts of automobile engines.
The three methods of casting are die casting, permanent mould casting
and sand casting. In the pressure die casting technology, wherein molten
aluminium is injected into the moulds under pressure, is increasingly being
used in applications, which require thin sections such as automobile wheels.
This method is normally used for high-volume production. Accurate parts,
requiring minimum of machining, are produced by die casting. Permanent mould
casting involves moulds and cores of steel or other metal. The most versatile
method of casting is sand casting. This process is more economical for small
quantities, intricate designs or for products where a very large casting is needed.
In forging, heated aluminium ingots or billets are pressed into the
desired shape using mechanical or hydraulic force. Forgings are used for
applications, which require strength, such as aircraft landing gear, automobile
axles and truck wheels.
The three basic types of aluminium alloy forgings are open-die forgings,
closed-die forgings and rolled rings. In open die forging, the work component
is not completely confined as it is being shaped by the dies. This process is
commonly associated with large parts such as shafts, sleeves and disks and the
weight of the item can range from 5 lbs to 500,000 lbs.
Impression die forging is another method of aluminium forging and
accounts for the majority of forging production. In impression die forging, two
dies are brought together and the work-piece undergoes plastic deformation
until its enlarged sides touch the die sidewalls.
In the ring rolling process, a pre-form is heated to forging temperature
and placed over the internal roll of the rolling machine. Pressure is applied
to the wall by the main roll as the ring rotates. The cross-sectional area is
reduced as the inner and outer diameters are expanded. Rings can be rolled into
numerous sizes, ranging from roller bearing sleeves to rings of 25 feet in diameter
with face heights of more than 80 inches.
Other processes
Aluminium is used in other forms, such as powders and pastes. Powders
and pastes consist of finely ground particles of aluminium. Aluminium powder is
used in explosives and inks. In the paste form, aluminium is used in paints and
for providing metallic finishes for automobiles.
Aluminium powder is also used to produce components, such as gears,
through powder metallurgy, wherein aluminium powder is pressed into the desired
shape and heated to bond the particles together. Powders of other metals may be
mixed with that of aluminium to enhance specific properties.
