Aluminum Flow-Sheet
The
Production of Aluminum:
The
figure shown below schematically describes Aluminum Production.
Aluminum is
produced from alumina by an electrolytic process that uses large
quantities of
electrical energy to separate aluminum from oxygen in the alumina. For
this
process, a modern smelter requires about 13,500 (DC) kilowatt-hours of
electricity to produce one ton of aluminum.
In
nature, aluminum is never found in its metallic state but is a common
constituent of many minerals where it is normally combined with silicon
and
oxygen. Bauxite is the only ore from which aluminum can be economically
retrieved.
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Once
the ore is mined, a chemical process is used to extract aluminum oxide,
or alumina, and an electrolytic process reduces the alumina to
aluminum. Some four to five tons of bauxite are required to produce the
two tons of alumina which yield one ton of aluminum.
The
photograph on left shows the production of aluminum, from red ore to
white powder to silvery metal.
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Aluminum Chemical
Process
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The
first step in Aluminum Production is to mix crushed bauxite in a
solution of hot caustic soda in digesters. This allows the alumina
hydrate to be dissolved from the ore. After the red mud residue is
removed by decantation and filtration, the caustic solution is piped
into huge tanks, called precipitators, where the alumina hydrate
crystallizes. The hydrate is then filtered and sent to calciners to dry
and, under very high temperature, is transformed into the fine, white
powder known as alumina. The alumina is transferred to the Electrolytic
Process for aluminum reduction.
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Aluminum Electrolytic
Process
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Alumina
is a compound of aluminum and oxygen. To obtain metal from the alumina,
these elements must be separated by electricity in the smelting
process. This reaction takes place in large, carbon-lined steel cells,
or pots, through which a direct electrical current is passed.
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The
bottom of each pot acts as a cathode, or negative electrode. Carbon
blocks are suspended in the pot to serve as an anode, or positive
electrode. Inside the pot, alumina is dissolved in a molten
electrolyte, composed mainly of the mineral cryolite. The electrical
current passing from the anode to the cathode causes the oxygen in the
mixture to react with the carbon anode to form carbon dioxide, while
the aluminum settles to the bottom of the pot to be siphoned off to
Casting and Fabricating.
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Molten Aluminum
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Aluminum Casting &
Fabrication
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Before
casting into ingot for fabricating, the molten aluminum is treated to
ensure cleanliness and purity. Alloying ingredients are usually added
to increase strength or provide special properties. Traditionally, the
metal is then cast into ingots of various shapes, sizes and
compositions for a number of uses.
Ingots
are converted into sheet, plate or foil products, as well as extruded
shapes for engineering and architectural applications.
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Aluminum Ingot
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In
an alternative technique, continuous casting, molten metal is cast
directly into semi-finished form, bypassing the ingot stage. This
fabrication method is becoming more widely used for sheet and foil
products, and particularly for rod, which is subsequently drawn into
many forms of electrical and mechanical wire.
The
photograph at left shows an aluminum rolling mill.
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Ingot and
Billet
Ingot and
billet play
an integral part in the production of almost all other aluminum
products.
Plate, sheet, foil, wire, rod, and bar products are all produced by
pressing or
rolling ingot and billet.
Furnace
Hall
Furnace
Ingot and
billet are
cast from molten aluminum. Alloying elements are then added. Before the
alloyed
metal can be cast, it must be purified by forcing mixtures of gases
through the
hot metal. Impurities (dross) come to the surface and are skimmed
off. 
Most
metal is cast by the direct-chill (DC) process, which produces
huge sheet ingot for rolling mills, round log like billet for extrusion
Alloys
An alloy
is a material
made up of two or more metals. Alloys are designed and produced to have
certain
specific, desirable characteristics, including strength, formability,
and corrosion
resistance.
Some of
the common
elements alloyed with aluminum include copper, manganese, silicon,
magnesium
and zinc. Typical applications and uses of aluminum alloys include
building
products (siding and structural), rigid and flexible packaging (foil,
food and
beverage cans) and transportation (automobiles, aircraft, and railcars).
Casting
Products
The
automotive industry
is the largest market for aluminum castings and cast products make up
more than
half of the aluminum used in cars. Cast aluminum transmission housing
and
pistons have been virtually universal in cars and trucks throughout the
world
for years.
Cast
aluminum is also
widely used in other forms of transportation, including aircraft and
marine
engines and structures. Parts of small appliances, hand tools,
cookware,
lawnmowers and other machinery use thousands of different aluminum
castings.
Extrusion
Extruding
aluminum is
the most innovative forming process for this versatile metal, allowing
designers almost unlimited creativity and imagination to specify
profiles to
meet their exact, specialized needs.
The
process be
gins with
creation of a metal die, which
precisely matches the profile of the shape specified by the designer.
Aluminum
billets or logs, produced from ingots, are heated and forced under
pressure
through the die. The variety of shapes is virtually endless, and
profiles are
produced to exact specifications, with very close tolerances.
Once the
profile is
extruded, it can be further fabricated - cut to length, machined,
drilled,
punched, notched, bent and assembled into a semi-finished product. An
extruded
tube even can be “stretched” to produce tubing of exact inside and
outside
dimensions. Profiles can be painted, anodized, brushed or polished,
depending on
the desired finish.
Foil
Foil, like
sheet and
plate, is produced by passing aluminum between rolls under pressure.
Foil is
the thinnest of these three products and is less than 0.006 inch thick.
Foil is
produced from sheet coils that are heated and then passed through
high-speed
foil rolling mills. (For a more thorough explanation of the sheet,
plate, and
foil production process.
Flexible
packaging and
foil containers account for about three-fourths of all foil. Aluminum
foil
capacitors are found in virtually all electrical equipment, from
television
sets to computers. Formed into fins, foil is the heat exchanger in some
air-conditioning units and baseboard space
heaters.
Forgings
The three
basic types
of aluminum alloy forgings are: open-die forgings, closed-die forgings,
and
rolled rings.
In
automotive
applications, forged components are commonly found at points of shock
and
stress. Forged automobile components include connecting rods,
crankshafts,
wheel spindles, axle beams, pistons, gears, and steering arms.
Forgings
are also used
in helicopters, piston-engine planes, commercial jets, and supersonic
military
aircraft. Many aircraft are "designed around" forgings and contain
more than 450 structural forgings as well as hundreds of forged engine
parts.
"Forged"
is
the mark of quality in hand tools and hardware. Pliers, hammers,
sledgers,
wrenches, garden implements, and surgical tools are almost always
produced
through forging.
Impacts
An impact
is a part
formed in a confining die from a metal slug, usually cold, by rapid
single
stroke application of force through a punch, causing the metal to flow
around
the punch and/or through an opening in the punch or die.
An impact
implies a
hammering action and can be differentiated from an aluminum extrusion
whereby
an ingot or billet is forced un
der
applied pressure through a die
opening to form an elongated shape or tube; and is, by a shade of
meaning,
different again from an impact extrusion, or back extrusion. The impact
extrusion process combines extrusion and forging in a single press
operation.
Aluminum
Powder and Paste
Aluminum powder and
paste
are used in a wide variety of applications, ranging from paints and
coatings,
chemical and metallurgical applications, to propellants and
explosives.
Sheet and
Plate
When
aluminum is passed
between rolls under pressure, it becomes thinner, and longer in the
direction
in which it is moving. It can be flat-rolled and re-rolled until it
reaches the
desired thickness or gauge. Where the rolling process is stopped
determines
whether the final product will be plate (a quarter-inch thick or more),
sheet
(.249 to .006 inch), or foil (less than .006 inch).
Plate is
used in
heavy-duty applications in the aerospace, machinery, and transportation
markets.
It is used for storage tanks and containers in many industries, and is
especially useful in holding cryogenic materials. It provides
structural
sections for rail cars and large ships, and armor protection for
military
vehicles and the trucks that carry the payroll.
Sheet is
found in all
of the aluminum industry’s major markets. In packaging, it is used for
cans and
closures. In transportation, it provides panels for automobile bodies.
It is
used in home appliances and cookware. In building and construction, it
forms
siding and gutters, down-sprouts and roofing, and awnings and carports.
Sheet
can be color anodized, etched to a "matte" finish or polished to a
sparkling brightness and textured to resemble wood or painted for
lasting
beauty.
Wire, Rod
and Bar
Wire is a
long, thin
string of aluminum that can carry electric current. It is made from rod
or bar,
and by definition wire is less that three-eighths inch in diameter,
while rod
and bar are larger. Rod is round and bar can have any number of flat
sides.
Electrical
transmission
lines are the largest users of aluminum rod/bar/wire products. Rod and
bar
become the rivets, nails, screws, bolts, and parts of machinery and
equipment. It
is also used as chain link fence material, lightning conductor,
non-rusting
nails etc. Drawn tube carries liquids in heat exchangers, food
processing
equipment, water treatment plants, and other industrial applications.
Properties
of Aluminum and Resulting End Uses
The
properties of aluminum that
contribute to its widespread use are:
·
Aluminum
is light;
its density is only one third that of steel.
·
Aluminum
is
resistant to weather, common atmospheric gases, and a wide range of
liquids.
·
Aluminum
can
be used in contact with a wide range of foodstuffs.
·
Aluminum
has
a high reflectivity, and, therefore, finds more decorative uses.
·
Aluminum
alloys can equal or even exceed the strength of normal construction
steel.
·
Aluminum
has
high elasticity, which is an advantage in structures under shock loads.
·
Aluminum
keeps its toughness down to very low temperatures, without becoming
brittle
like carbon steels.
·
Aluminum
is easily
worked and formed; it can be rolled to very thin foil.
·
Aluminum
conducts electricity and heat nearly as well as copper.
