ALUMINIUM
Production Process
Aluminium can be produced via two different routes: primary aluminium production
from ore and recycling aluminium from process scrap and used aluminium products.
The production of primary aluminium consists of three steps: bauxite mining,
alumina production and electrolysis. The last two mentioned will be described
hereafter, bauxite mining is covered in the section Environment, Ecology &
Recycling.
Alumina production
Bauxite has to be processed into pure aluminium oxide (alumina) before it
can be converted to aluminium by electrolysis. This is achieved through the
use of the Bayer chemical process in alumina refineries. The aluminium oxide
is released from the other substances in bauxite in a caustic soda solution,
which is filtered to remove all insoluble particles. The aluminium hydroxide
is then precipitated from the soda solution, washed and dried while the soda
solution is recycled. After calcination, the end-product, aluminium oxide (Al2O3),
is a fine grained white powder.Four tonnes of bauxite are required to produce
two tonnes of alumina which in turn produces one tonne of aluminium at the primary
smelter. In 1998, 45 million tonnes of alumina were produced world-wide. The
main production areas are:
Alumina refineries are often located near to bauxite mines for logistics reasons.
Electrolysis
Aluminium primary smelting and casting
Primary aluminium is produced in reduction plants (or "smelters"),
where pure aluminium is extracted from alumina by the Hall-Héroult process.
The reduction of alumina into liquid aluminium is operated at around 950 degrees
Celsius in a fluorinated bath under high intensity electrical current. This
process takes place in electrolytic cells (or "pots"), where carbon
cathodes form the bottom of the pot and act as the negative electrode. Anodes
(positive electrodes) are held at the top of the pot and are consumed during
the process when they react with the oxygen coming from the alumina. There are
two types of anodes currently in use. All potlines built since the early 1970s
use the prebake anode technology, where the anodes, manufactured from a mixture
of petroleum coke and coal tar pitch (acting as a binder), are ‘pre-baked’
in separate anode plants. In the Soederberg technology, the carbonaceous mixture
is fed directly into the top part of the pot, where ‘self-baking’
anodes are produced using the heat released by the electrolytic process.
At regular intervals, molten aluminium tapped from the pots is transported to
the cast house where it is alloyed in holding furnaces by the addition of other
metals (according to the user’s needs), cleaned of oxides and gases, and
then cast into ingots. These can take the form of extrusion billets, for extruded
products, or rolling ingots, for rolled products, depending on the way it is
to be further processed.
Aluminium mould castings are produced by foundries which use this technique
to manufacture shaped components.
World-wide trends in production are shown in the following graph. Aluminium
output has increased by a factor of 13 since 1950, making aluminium the most
widely used non-ferrous metal. In 1998, world-wide production of primary aluminium
was about 22.7 million tonnes per year for and installed capacity of 24.8 million
tonnes.
References:
Web site: http://www.eaa.net/material/primary.asp
The Hall-Heroult method of aluminium production occurs in large refractory-lined steel containers called pots that are connected in series and housed in long buildings called pot rooms. Alusaf has seven such pot rooms producing over 670 000 tons of aluminium a year.
A. Suspended above each cathode are several closely arranged carbon blocks that serve as the anode (positive electrode). The anodes are suspended by rods in the bath of molten electrolyte in which the alumina is dissolved.
B. An electric current of up to 315 000 amps enters the pot via the anode blocks and reduces the alumina by electrolysis into aluminium and oxygen. The oxygen is deposited on the carbon anode where it burns the carbon to form carbon dioxide. The aluminium, being heavier than the electrolyte, collects at the base of the pot. The equation for the basic reaction is:
2Al2O3 + 3C = 4Al + 3CO2
C. Each pot consists of a steel shell that is lined with refractory and carbon blocks to serve as the cathode (negative electrode).
D. Cryolite, the predominant constituent of the electrolyte, is a sodium aluminium fluoride salt which, when held molten at a temperature of around 960°C, can dissolve alumina.
The electrolytic process of separating the Alumina atom into molten Aluminium and Carbon dioxide waste
To sustain the electrolytic
process, alumina is fed into the pots at regular intervals to maintain a sufficient
quantity of dissolved alumina in the bath. The process is controlled by a computer
that detects and interprets minute changes in electrical resistance and determines
when to feed alumina to the pot. As the carbon anode is gradually consumed,
it is periodically lowered to maintain the optimum distance of ±5cm between
the anode and cathode surfaces.
For each ton of aluminium
produced about 430 kg of carbon is consumed. A continuous supply of anodes is
manufactured at both smelters in dedicated carbon plants that comprise paste
plants, carbon bake furnaces and rodding shops.
1. In the paste plants, carefully crushed and graded fractions of calcined petroleum
coke and recycled anode butts are heated and mixed with molten pitch.
2. The hot mixture is then compacted into blocks called green (unbaked) anodes.
At Hillside, each anode weighs about 836 kg; at Bay side the anodes weigh about
624 kg. Approximately 400 000 anodes are produced each year for both smelters.
3. The green anodes are transferred to the carbon bake furnaces where they are
heated in deep brick-lined pits to around 1 100ºC over a period of 21 days.
This baking process calcines the binding pitch and ensures that the anodes have
good thermal and electrical conductivity. Exhaust manifolds collect waste gases
and carry them to the fume treatment centre.
4. After baking, aluminium rods are attached to the anodes and sealed with cast
iron. The rod suspends the anode in the pot and acts as an electrical conductor.
5. After the rods are attached, the anodes are delivered to the pot rooms for
positioning in the pots. Some 27 days later, the remains of the anodes, known
as butts, are returned from the pot rooms and recycled. The rods are also reused.
1. The molten metal is tapped
from each pot approximately once per day for transfer in special-purpose hot-metal
carriers to holding furnaces in the cast house. The furnaces are heated and maintain
the aluminium at the desired casting temperature of 700ºC.
2. After the aluminium is poured into cast house. furnaces, elements such as silicon,
magnesium, copper, iron, titanium or boron are added to meet requisite alloy
specifications. The metal surface is skimmed to remove the dross. The clean
alloy is then cast.
3. Forty-four 22-kg ingots are stacked in a configuration of interlocking bundles.
Each weighing one ton, they are strapped and trucked to the export stockyard
at the harbor in an around-the-clock road haulage super packs for easy handling.
The electrolytic production
of aluminium at the smelters is a complex 24-hours-a-day, 365-days-a-year process,
dependant on a regular supply of raw materials and huge amounts of energy.
Crucially important is the role of the ancillary departments that must ensure
an uninterrupted supply of quality raw materials, spares, consumables or services
to the three primary production areas - carbon, pot rooms and cast house. The
ancillary departments include maintenance, procurement, materials management,
laboratory, environment, finance, human resources, engineering, sales and marketing,
communication and information systems. The warehouses contain tens of thousands
of parts necessary for the continued functioning of the smelters. While certain
specialised parts are purchased abroad, Billiton Aluminium's purchasing policy
gives preference, on a competitive basis, to local suppliers and contractors.
Preventative maintenance is a key factor in the successful operation of the
smelter. Effective planning and organisation of maintenance work prevents production
backlogs, keeps equipment in top condition and ensures that productivity remains
high. Integrated information and technology systems are vital to Hillside's
success as the focus of organisations worldwide turns to knowledge as a prime
source of wealth creation. Designed for maximum effectiveness, the company's
three-level information network is fully integrated and encompasses programmers
to operate equipment, monitor performance and provide status reports. It also
manages the business systems for finance, maintenance and human resources.
References:
Web site : http://www.hillside.co.za/history/production.html
Aluminium ore, most commonly bauxite, is plentiful and occurs mainly in tropical and sub-tropical areas: Africa, West Indies, South America and Australia. There are also some deposits in Europe. Bauxite is refined into aluminium oxide tri hydrate (alumina) and then electrolytically reduced into metallic aluminium. Primary aluminium production facilities are located all over the world, often in areas where there are abundant supplies of inexpensive energy, such as hydro-electric power.
Two to three tonnes of bauxite are required to produce one tonne of alumina and two tonnes of alumina are required to produce one tonne of aluminium metal.
The aluminium industry relies on the Bayer process to produce alumina from bauxite. It remains the most economic means of obtaining alumina, which in turn is vital for the production of aluminium metal - some two tonnes of alumina are required to produce on tonne of aluminium.
The Bayer Process
The primary aluminium industry is dependent on a regular supply of alumina for
four functions:
Basic raw material for aluminium production
1.Thermal insulator for the top of electrolytic cells
2.Coating for pre baked anodes
3.Absorbent filter for cell emissions
Alumina Production
Bauxite is washed, ground and dissolved in caustic soda (sodium hydroxide) at
high pressure and temperature. The resulting liquor contains a solution of sodium
aluminate and un dissolved bauxite residues containing iron, silicon, and titanium.
These residues sink gradually to the bottom of the tank and are removed. They
are known colloquially as "red mud".
The clear sodium aluminate solution is pumped into a huge tank called a precipitator.
Fine particles of alumina are added to seed the precipitation of pure alumina
particles as the liquor cools. The particles sink to the bottom of the tank,
are removed, and are then passed through a rotary or fluidised calciner at 1100°C
to drive off the chemically combined water. The result is a white powder, pure
alumina. The caustic soda is returned to the start of the process and used again.
More information about the Chemistry of the Process is available.
The process of producing pure alumina from bauxite has changed very little since
the first plant was opened in 1893. The Bayer process can be considered in three
stages:
Extraction
The hydrated alumina is selectively removed from the other (insoluble) oxides
by transferring it into a solution of sodium hydroxide (caustic soda):
Al2O3.xH2O + 2NaOH ---> 2NaAlO2 + (x+1)H2O
The process is far more efficient when the ore is reduced to a very fine particle
size prior to reaction. This is achieved by crushing and milling the pre-washed
ore. This is then sent to a heated pressure digester.
Conditions within the digester (concentration, temperature and pressure) vary
according to the properties of the bauxite ore being used. Although higher temperatures
are theoretically favoured these produce several disadvantages including corrosion
problems and the possibility of other oxides (other than alumina) dissolving
into the caustic liquor.
Modern plants typically operate at between 200 and 240 °C and can involve
pressures of around 30atm.
After the extraction stage the liquor (containing the dissolved Al2O3) must
be separated from the insoluble bauxite residue and purified as much as possible
and filtered before it is delivered to the decomposer. The mud is thickened
and washed so that the caustic soda can be removed and recycled.
Decomposition
Crystalline alumina tri hydrate is extracted from the digestion liquor by hydrolysis:
2NaAlO2 + 4H2O ---> Al2O3.3H2O + 2NaOH
This is basically the reverse of the extraction process, except that the product's
nature can be carefully controlled by plant conditions (including seeding or
selective nucleation, precipitation temperature and cooling rate). The alumina
tri hydrate crystals are then classified into size fractions and fed into a rotary
or fluidised bed calcination kiln.
Calcination
Alumina tri hydrate crystals are calcined to remove their water of crystallisation
and prepare the alumina for the aluminium smelting process.
The mechanism for this step is complex but the process, when carefully controlled,
dictates the properties of the final product.
References:
Web site: http://www.world-aluminum.org/production/
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