How Aluminium is Produced
MINING.
Immediately prior to mining any deposit,
the land is cleared and the top soil, to a minimum of 6 inches, removed
and stockpiled for later replacement when mining is completed.
The surface occurrence of the ore (usually
less than 100 feet) makes the deposits suitable for mining by simple
open-cast methods. Due to the soft, earthy nature of the ore, no
drilling or blasting is generally required.
Deposits are located in areas varying
from gentle undulating to rugged, hilly terrain involving major capital
expenditures in establishing a system of ore transportation.
Bauxite contains about 75 per cent hydrated alumina (A12O3 o 3H2O and A12O3
o H2O). It is crushed and grounded into a powder and mixed with a solution
of caustic soda. This paste is mixed with further amounts of caustic soda solution
in autoclaves or digesters. There, under pressure and at a high temperature,
the caustic soda combines with the hydrated alumina to leave a solution called
sodium aluminate. Any impurities remain as un-dissolved residue.
Na2OoAl2O3 + 4H2O + residues
The residue or "red mud", mainly
the oxides of iron, silicon and titanium, is removed by sedimentation
and filtration. The inert red mud is washed to recover the chemicals
and is disposed of by a "wet stacking" technique over a
specially prepared land area.
The sodium aluminate solution is then pumped into precipitator tanks where
very fine and pure alumina trihydrate is added as "seed".
Dissolution
Dissolution of the alumina occurs at elevated temperatures The exact procedure
required for digestion, depends on the nature of the ore deposits.
The following Table outlines a number
of minerals commonly found in bauxites:
| Gibbsite (hydrargillite) |
alpha-Al2O3.3H2O |
| Boehmite |
alpha-Al2O3.H2O |
| Diaspore |
beta-Al2O3.H2O |
| Hematite |
alpha-Fe2O3 |
| Goethite |
alpha-FeOOH |
| Magnetite |
Fe3O4 |
| Siderite |
FeCO3 |
| Ilmenite |
FeTiO3 |
| Anatase |
TiO2 |
| Rutile |
TiO2 |
| Brookite |
TiO2 |
| Halloysite |
Al2O3. 2SiO2.3H2O |
| Kaolinite |
Al2O3. 2SiO2.2H2O |
| Quartz |
SiO2 |
Anatase TiO2
Rutile TiO2
Brookite TiO2
Halloysite Al2O3. 2SiO2.3H2O
Kaolinite Al2O3. 2SiO2.2H2O
Quartz SiO2
In order to remove the iron oxides and
most of the silicon oxides present, the ore is first treated with
sodium hydroxide. Under agitation and with gradual cooling the alumina
trihydrate contained in the solution precipitates on the "seed".
The trihydrate solids are then separated from the caustic soda solution
by settling and vacuum filtering. The caustic soda solution is recovered
and returned to the start of the process to be reused in autoclaves.
The digestion process takes advantage of the solubility of amphoteric
Aluminium oxides to form a solution of aluminate ions, whilst the
basic iron oxides which form do not dissolve and are separated by
filtration.
Complete extraction from diasporic bauxite
requires stronger caustic solutions, in addition to higher temperatures
and pressures. In general the reaction equilibria above move to the
right with increases in caustic soda concentration and temperature.
In practice this means that for deposits containing the more easily
recovered Gibbsite only, production costs are much lower than when
Boehmite or Diaspore are present.
The control of silica in the conventional
Bayer Process is most important and in fact ores having reactive
silica greater than 7% cannot be economically processed.
Unlike quartz, which is considered virtually non-reactive at Gibbsite extraction
temperatures, some minerals, including kaolins, dissolve rapidly and the reaction
of the silica can give rise to appreciable loss of caustic soda and Aluminium.
The control of silica is generally carried
out during, or prior to, the digestion step, and generally involves
dissolution, eg for kaolin
Al2O3.2SiO2 + NaOH ---> NasSiO3
and desilication via precipitation.
Na2SiO3 + NaAlO2 ---> Na2O.Al2O3.2SiO2
Dissolution is necessary to supersaturate
the liquid to a point where the sodalite formed acts as a seed to
precipitate more sodalite. The rate of precipitation is found to
increase with temperature, however at 135-150 C it is significantly
slower than is required for complete Gibbsite extraction which occurs
within minutes. The need for desilication therefore means that the
material must be held at the digestion temperature long enough to
allow the silica to precipitate.
The trihydrate solids are then passed
through high-temperature (900-1100°C) calciners that extract
the chemically combined water that they contain. Al2O3 o 3H2O + heat
Al2O3+ 3H2O
The Aluminium oxide (Al2O3) that results is a white powder, like table salt
in appearance, known as calcined alumina.
Smelting
An Aluminium smelter comprises three main sectors: carbon, potlines and casthouse.
Carbon Sector
This is where anodes are produced. Anodes are designed to be hung over electrolytic
pots to carry the required electrical current. These anodes are a mixture of
coke and petroleum pitch. Coke is crushed to a very precise granulometry, mixed
with pitch to form a paste which is moulded in vibro-compactors to produce
raw anodes or "green" anodes. Raw anodes are then transferred to
gas or oil-fired furnaces where they are baked for several days at high temperature
(1100° C).
Once baked, anodes are rodded, that is attached to Aluminium stems from which
they will hang over electrolytic pots.
Anodes burn and must be replaced about every twenty days; the carbon sector
is also responsible for the recovery of carbon content in spent anodes (or
butts) for recycling as well as for the cleanup of stems which will be reused.
The smoke given off by the anode baking process is thoroughly treated in highly
sophisticated systems.
Potlines
Aluminium is obtained from alumina by electrolytic reduction - a chemical term
meaning the removal of oxygen atoms from Aluminium oxide. The calcined alumina
is reduced to Aluminium metal in electrolytic cells, or "pots", connected
in series to a direct current power source. The cells are rectangular steel
pots lined with refractory bricks and carbon blocks acting as the cathode.
The pot contains a molten electrolyte, called "bath" in which alumina
is dissolved. The electrolyte is a mixture of cryolite (Na3A1F6), a molten
salt, and certain additives to give it appropriate density, conductivity and
viscosity. The principal additive is Aluminium fluoride (A1F3) which must be
replaced from time to time due to losses through evaporation and a chemical
reaction converting it into more cryolite. The emitted fluorides are collected
and treated.
Suspended in the electrolyte are a number of anodes (positive electrodes) which
act as electrical conductors for a high intensity direct current.
Electrical current passing from the anode through the electrolyte to the pot,
which acts as a cathode, reduces the alumina molecules into Aluminium and oxygen
at a temperature of approximately 950°C. This process is called electrolysis.
The oxygen is released on the carbon anode, where it combines with the carbon
to form carbon monoxide and carbon dioxide (CO and CO2). The Aluminium, being
heavier than the bath, settles to the bottom of the pot. Considerable electrical
energy, between 13 and 17 kilowatt-hours per kilogram of Aluminium, is consumed
in the process.
Each pot is tightly closed to achieve greater energy efficiency and to capture
the pollutants emitted; gas treatment centres provide a very effective environmental
protection.
At regular intervals, the molten Aluminium is tapped from the bottom of the
pot into large laddles and transferred to holding furnaces for casting.
Casthouse
Molten metal from potlines is transferred to huge holding furnaces with a capacity
of up to 60 tonnes, where it is refined and optionally mixed with metal additives
to produce Aluminium alloys.
While Aluminium is sometimes used as is
in its commercially pure form, most applications involve the addition
of small quantities of other metals to create alloys with special
properties. Certain alloying elements will increase strength or corrosion
resistance, while others enhance such properties as machinability,
ductility, weldability and strength at high temperature.
Recyclable Aluminium beverage cans, for
example, are made from an alloy containing manganese and magnesium
that provide strength and formability. Magnesium is added to Aluminium
to create an alloy that provides the added strength required for
the can top.
Alloys containing magnesium and silicon are very corrosion-resistant and suitable
for use in window frames, doors and pleasure boats. Copper and zinc are added
to produce alloys of the highest strength, while chromium, manganese or titanium
are added for grain-size control. Recent research with metal matrix composite,
a combination of Aluminium and ceramic particles, promises stronger materials
that are more cost-effective, stiffer and provide better resistance to wear.
Aircraft components are made from high-strength
alloys containing copper, magnesium, silicon and zinc as the principal
alloying elements. The aerospace industry employs the new Aluminium-lithium
alloys which provide significant weight savings over alloys of similar
strength. Aluminium-magnesium-silicon alloys are used in architectural
applications where pleasing, corrosion-resistant surface finishes
are required. Automotive castings, strong and machinable, are made
from Aluminium-copper-silicon alloys.
Once the exact content of the molten Aluminium
has been analysed, it is either cast into ingots, slabs or billets
using the DC casting (direct chill) method, or cast directly into
semi-finished products .Aluminium is cast into shapes that vary depending
upon the type of equipment that will be used to process the metal.
For example, very large ingots of rectangular shape, also called
slabs, are intended for hot-rolling to produce plate, sheet and foil.
Cylindrical ingots, also called billets, are for extrusion while
metal for re-smelting can be cast into large blocks called sows,
as well as tri-lock shapes or T-ingots, depending upon the shape.
Each ingot can weigh up to 25 tonnes.
Working the Aluminium
1. Rolling
The process of flattening the ingot or slab is carried out by either hot- or
cold-rolling the metal. In hot-rolling, the ingot is preheated so as
to soften and/or homogenise it, and is then passed back and forth through
massive rolls that reduce the ingot's thickness while increasing its
length. The width remains unchanged. Hot-rolling improves the metallurgical
qualities of the metal without appreciable work-hardening. Subsequent
cold-rolling gives the strength characteristics that result from work-hardening,
and the metal can be rolled to tighter dimensional tolerances.
Plate, which is hot-rolled, is 6.30 millimetres or more in thickness, while
sheet, which is hot-and-cold-rolled, varies in thickness from 6.30 down to
0.15 millimetres. Foil is also cold-rolled to gauges below 6 microns. A continuous
length of foil, 450 kilometres long, could be rolled from a single slab. The
Aluminium beverage can market is the primary end-user for Aluminium sheet.
2. Extruding
The extrusion process consists of pushing a pre-heated cylindrical Aluminium
ingot through a steel die. The ingot is formed into a continuous length
of uniform cross-section by forced flow through the steel die. The outline
of the die opening is reproduced in the extruded Aluminium in much the
same way as decorative icing is forced from a pastry pouch. Extruded
tubing and hollow shapes are formed by placing a steel mandrel inside
the die opening. The Aluminium is forced to flow between the mandrel
and the die, reproducing the shape of the mandrel on the inside of the
section and the shape of the die on the outside.
Extruded tubing is used in the manufacture of such items as doors, window frames,
building wall cladding, highway lighting standards and garden furniture. Larger
extrusions are also used in the manufacture of railcars, truck trailers, aircraft
and ship super-structures.
3. Other Methods
Aluminium may be cast into various shapes by pouring molten metal into moulds.
The methods used are die casting, permanent mould casting or sand casting.
Forging - In this process, the desired
part is formed in a confined die from a hot metal slug. This is achieved
by applying force which causes the metal to flow and fill the die
cavity.
Drawing - All Aluminium wire and some
tubing and rod products are manufactured by a cold-rolling process
called drawing; a starter stock is pulled through a die in which
it is both shaped and reduced. In the production of tubing, lengths
of extruded, thick-walled tube are drawn through progressively smaller
dies until reduced to the desired diameter.
Impacting - Also known as impact extruding, impacting is a combination of both
extruding and forging. A disc-shaped slug of metal is placed in a die and struck
by a punch; part of it is forged into a base, flange or hub, and the remainder
is extruded upwards, downwards or sideways from the forged section.
Anodising - Aluminium, particularly when
intended for architectural purposes, may be anodised. Anodising is
an Electro-chemical process whereby the natural oxide film on Aluminium
is thickened by passing an electric current through certain acid
electrolytes with the Aluminium part as the anode. Anodising also
provides a means of colouring the metal with dyes. This process also
increases aluminium's hardness and corrosion resistance.
See also Powder
Coating
© 2007 H.Snelson Engineers - Aluminium
Extrusions Aluminium Fabricators
Nat Lane, Wharton Industrial Estate, Winsford, Cheshire, CW7 3BS. UK.
Tel: +44 (0) 1606 553580 Fax: +44 (0) 1606 861084
