Alluminium Production

7/29/2019 Alluminium Production

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Alluminium Production

Aluminium ore, most commonly bauxite, is plentiful and occurs mainly in tropical and sub-tropicalareas: Africa, West Indies, South America and Australia. There are also some deposits in Europe.Bauxite is refined into aluminium oxide trihydrate (alumina) and then electrolytically reduced intometallic 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 ofalumina are required to produce one tonne of aluminium metal.


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Bauxite Mining

There are numerous bauxite deposits, mainly in the tropical and subtropical regions, but also inEurope. Bauxite is generally extracted by open cast mining from strata, typically some 4-6 metresthick under a shallow covering of topsoil and vegetation. In most cases the topsoil is removed and

stored.

Exporting bauxite mines generated about US$1.4m in revenue per hectare mined in 1998 and atypical mine employed about 200 people for each million tonnes/year of bauxite produced or about 11people per hectare. Usually mines offer relatively well-paid jobs and mining companies tend to provideassistance to their neighbouring communities.

There are attractive commercial and social reasons for the development of a bauxite mine. The miningcompany wants the ore to use or sell while the local inhabitants want the mine for employment andfor the community assistance that the mining company usually offers. National governments want thedevelopment for these social reasons and also for the revenue provided by a mining company

Alumina Refining

The aluminium industry relies on the Bayer process to produce alumina from bauxite. It remains themost economic means of obtaining alumina, which in turn is vital for the production of aluminiummetal - some two tonnes of alumina are required to produce on tonne of aluminium.

The Bayer Process


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The primary aluminium industry is dependent on aregular supply of alumina for four functions:

1. Basic raw material for aluminium production

2. Thermal insulator for the top of electrolytic cells

3. Coating for prebaked anodes

4. Absorbent filter for cell emissions

Alumina ProductionBauxite is washed, ground and dissolved in caustic soda(sodium hydroxide) at high pressure and temperature.The resulting liquor contains a solution of sodiumaluminate and undissolved bauxite residues containingiron, silicon, and titanium. These residues sink graduallyto 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 ofalumina are added to seed the precipitation of pure alumina particles as the liquor cools. The particlessink to the bottom of the tank, are removed, and are then passed through a rotary or fluidised calcinerat 1100C to drive off the chemically combined water. The result is a white powder, pure alumina. Thecaustic soda is returned to the start of the process and used again. More information about theChemistry of the Process is available.

Bayer Process Chemistry

1.8 million tonne alumina refinery at Gove,Northern Territory, Australia
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The process of producing pure alumina from bauxite (the Bayer Process) has changed very little sincethe first plant was opened in 1893. The Bayer process can be considered in three stages:

Extraction

The aluminium-bearing minerals in bauxite - Gibbsite, Bhmite and Diaspore - are selectivelyextracted from the insouble components (mostly oxides) by dissolving them in a solution of sodiumhydroxide (caustic soda):

Gibbsite: Al(OH)3 + Na+ + OH- ---> Al(OH)4

- + Na+

Bhmite and Diaspore: AlO(OH) + Na+ + OH - + H2O ---> Al(OH)4- + Na+

Depending on the quality of the ore it may be washed to beneficiate it prior to processing. The ore iscrushed and milled to reduce the particle size and make the minerals more available for extraction. Itis then combined with the process liquor and sent in a slurry to a heated pressure digester.

Conditions within the digester (concentration, temperature and pressure) are set according to theproperties of the bauxite ore. Ores with a high Gibbsite content can be processed at 140 oC. Processingof Bhmite on the other hand requires between 200 and 240oc. The pressure is not important for theprocess, as such but is defined by the steam pressure during the actual process conditions. At 240 oC

the pressure is approximately 35 atmospheres (atm).

Although higher temperatures are often theoretically advantageous there are several diadvantagesincluding corrosion problems and the possibility of oxides other than alumina dissolving into thecaustic liquor.

After the extraction stage the insoluble bauxite residue must be separated from the Aluminium-containing liquor by a process known as settling. The liquor is purified as much as possible throughfilters before being transferred to the precipitators. The insoluble mud from the first settling stage isthickened and washed to recover the caustic soda, which is then recycled back into the main process.

Precipitation

Crystalline aluminium trihydroxide (Gibbsite), conveniently named "hydrate", is then precipitated fromthe digestion liquor:

Al(OH)4- + Na+ ---> Al(OH)3 + Na

+ + OH-

This is basically the reverse of the extraction process, except that the product's nature is carefullycontrolled by plant conditions, including seeding or selcetive nucleation, precipitation temperature andcooling rate. The "hydrate" crystals are then classified into size fractions and fed into a rotary orfluidised bed calcination kiln. Undersize particles are fed back into the precipitation stage.

Calcination

"Hydrate", is calcined to form alumina for the aluminium smelting process. In the calcination processwater is driven off to form alumina:

2Al(OH)3 ---> Al2O3 + 3H2O

The calcination process must be carefully controlled since it dictates the properties of the final product.

Aluminium Smelting
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The basis for all modern primary aluminium smelting plants is the Hall-Hroult Process, invented in1886. Alumina is dissolved in an electrolytic bath of molten cryolite (sodium aluminium fluoride) withina large carbon or graphite lined steel container known as a "pot". An electric current is passed throughthe electrolyte at low voltage, but very high current, typically 150,000 amperes. The electric currentflows between a carbon anode (positive), made of petroleum coke and pitch, and a cathode(negative), formed by the thick carbon or graphite lining of the pot.

Molten aluminium is deposited at the bottom of the pot and is siphoned off periodically, taken to aholding furnace, often but not always blended to an alloy specification, cleaned and then generallycast.

A typical aluminium smelter consists of around 300 pots. These will produce some 125,000 tonnes ofaluminium annually. However, some of the latest generation of smelters are in the 350-400,000tonne range.

On average, around the world, it takes some 15.7 kWh of electricity to produce one kilogram ofaluminium from alumina. Design and process improvements have progressively reduced this figurefrom about 21kWh in the 1950's.

Smelter Energy Use

Aluminium is formed at about 900C, butonce formed has a melting point of only660C. In some smelters this spare heatis used to melt recycled metal.

Recycled aluminium requires only 5 per cent of the energy required to make "new" aluminium.Blending recycled metal with new metal allows considerable energy savings, as well as the efficient useof process heat. There is no difference between primary and recycled aluminium in terms of quality orproperties.

Aluminium smelting is energy intensive, which is why the world's smelters are located in areas whichhave access to abundant power resources (hydro-electric, natural gas, coal or nuclear). Many locationsare remote and the electricity is generated specifically for the aluminium plant.

The smelting process is continuous. A smelter cannot easily be stopped and restarted. If production isinterrupted by a power supply failure of more than four hours, the metal in the pots will solidify, oftenrequiring an expensive rebuilding process.

From time to time individual pot linings reach the end of their useful life and the pots are then takenout of service and relined.

Most smelters produce aluminium of 99.7% purity, which is acceptable for most applications. However,super purity aluminium (99.99%) is used for some special applications, typically those where highductility or conductivity is required. The marginal difference in the purities of smelter grade aluminium

La Grande Baie Smelter in Quebec, Canada
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and super purity aluminium results in significant changes in the properties of the metal.

Cell Chemistry and Processes

Alumina is reduced to aluminium metal in electrolytic cells known as pots, these are then organisedinto potlines.

The Potline

Pots are organised into "potlines" within an aluminium smelter. Modern potlines will tend to have potsarranged side-to-side and carry out almost all maintenance using overhead multipurpose cranes -several tasks have to be carried out regularly including replenishing alumina supplies, changing theanodes and removing the finished product, molten aluminium metal.

The molten metal which is removed (or "tapped") from the cell is then transferred to a holding furnaceprior to processing.

The Pot

A pot consists of two main parts:

1. A block of carbon which has been formedby baking a mixture of coke and pitch. Thisblock serves as an anode (or positiveelectrode).

2. Under the anode is a large rectangular steelbox lined with carbon made by baking amixture of metallurgical coke and pitch.This lining is the cathode(or negativeelectrode).

Between the anode and the cathode is a space filled

by electrolyte. This mixture must be heated toabout 980C, at which point it melts and therefined alumina is added, this then dissolves in themolten electrolyte.Smelter Technology TypesThis hot molten mixture is electrolyzed at a lowvoltage of 4-5 volts, but a high current of 50,000-280,000 amperes. This process reduces thealuminium ions to produce molten aluminium metalat the cathode, oxygen is produced at the graphiteanode and reacts with the carbon to produce carbondioxide.

2Al2O3 + 3C ---> 4Al + 3CO2However some of the metal, instead of being

deposited at the bottom of the cell, is dissolved inthe electrolyte and reoxidised by the CO2 evolved atthe anode:

2Al+ 3CO2 ---> Al2O3 + 3COThis reaction can reduce the efficiency of the celland increases the cell's carbon consumption.

The ElectrolyteThe electrolyte used is cryolite (Na3AlF6) which is thebest solvent for alumina. To improve theperformance of the cells various other compounds

A modern potline - this one can produce over

200 000 tonnes of aluminium per year

Tapping molten aluminium from a pot - themetal will now be transferred to a holdingfurnace
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are added including aluminium fluoride and calcium fluoride (used to lower the electrolyte's freezingpoint).The electrolyte ensures that a physical separation is maintained between the liquid aluminium (at thecathode) and the carbon dioxide/carbon monoxide (at the anode).

The AnodeAs we have seen, the carbon anodes used in the Hall-Hroult process are consumed during electrolysis. Twodesigns exist for these anodes; "Sderberg" and "Pre-Bake". Pre-Bake anodes are made separately, using cokeparticles bonded with pitch and baked in an oven. Pre-bake anodes are consumed and must then be changed.Sderberg anodes on the other hand are baked by theheat from the electrolytic cell, they do not need changingbut are "continuously consumed". For more informationsee the Technology Types section.

The CathodeThe cathode consists of a graphite shell embedded withsteel bars to minimise current resistance. During operationthe liquid aluminium itself begins to operate as the

cathode, a feature which can complicate cell design because of the inevitable magnetic effects of suchlarge currents. Typically a cathode will last between 1000 and 2000 days before it needs replacing.

Technology Types

There are two main types of aluminium smelting technology - Sderberg and Pre-bake. The principaldifference between the two is the type ofanode used.

Sderberg Cell:

Sderberg technology uses a continuous anode which is delivered to the cell (pot) in the form of apaste, and which bakes in the cell itself.

Pre-Bake carbon anodes
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Prebake Cell:

Pre-bake technology uses multiple anodes in each cell which are pre-baked in a separate facility andattached to "rods" that suspend the anodes in the cell. New anodes are exchanged for spent anodes -"anode butts" - being recycled into new anodes.

The newest primary aluminium production facilities use a variant on pre-bake technology called CentreWorked Pre-bake Technology (CWPB). This technology provides uses multiple "point feeders" andother computerised controls for precise alumina feeding. A key feature of CWPB plants is the enclosednature of the process. Fugitive emissions from these cells are very low, less than 2% of the generatedemissions. The balance of the emissions is collected inside the cell itself and carried away to veryefficient scrubbing systems which remove particulates and gases. Computer technology controls the


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process down to the finest detail, which means that occurrence of the anode effect - the conditionwhich causes small quantities of Perfluorocarbons (PFCs) to be produced - can be minimised. All newplants and most plant expansions are based on pre-bake technology.

Breakdown of technology types - IAI Members - 1997

Horizontal Stud Sderberg 5%

Vertical Stud Sderberg 11%Side Worked Pre-bake 8%

Centre Worked Pre-bake 75%

Figures taken from Anode Effect Survey 1994-1997.

Many IAI Members carry out continuous research into process improvements, particularly energyefficiency and emission levels. Some IAI Members market their smelting technology.

Aluminium Processing

Aluminium can be alloyed with other materials to make an array of metals with different properties.The main alloying ingredients are iron, silicon, zinc, copper and magnesium. Other materials are alsoused.

Aluminium can be rolled into plate, sheets, or wafer thin foils the thickness of a human hair. Therolling process changes the characteristics of the metal, making it less brittle and more ductile.

Aluminium can be cast into an infinite variety of shapes. The statue of Eros in London's PiccadillyCircus erected in 1893 is cast aluminium.

Aluminium can be extruded by heating it to around 500C and pushing it through a die at greatpressure to form intricate shapes and sections.

Aluminium can be forged by hammering to make stress-bearing parts for aircraft and internal

combustion engines.

Aluminium can be joined by welding, adhesive bonding, riveting or screwing. It can be formed bybending or superplastic moulding. It can be milled or turned on a lathe.

The properties of the metal can be modified through heat treatment or mechanical working.

The appearance can be modified by surface treatments such as anodising or powder coating.

Aluminium powder, flake and paste are formed by blowing gas under pressure at molten aluminium.This process forms droplets of different sizes. These aluminium products are used in explosives, rocketfuel, metallurgy, chemicals, inks, and decorative materials.

Aluminium Chemicals are important in water treatment, papermaking, fire retardants, fillers and

pharmaceuticals.

Aluminium's Properties
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Pure aluminium is a silvery-white metal with many desirable characteristics. It is light, nontoxic (asthe metal), nonmagnetic and nonsparking.

It is decorative. It is easily formed, machined, and cast. Alloys with small amounts of copper,magnesium, silicon, manganese, and other elements have very useful properties.

Strength depends on purity. 99.996 per cent pure aluminium has a tensile strength of about 49megapascals (MPa), rising to 700 MPa following alloying and suitable heat treatment.

Although not found free in nature, Aluminium is an abundant element in the earth's crust.

A key property is low density. Aluminium is only one-third the weight of steel.

Aluminium and most of its alloys are highly resistant to most forms of corrosion. The metal's naturalcoating of aluminium oxide provides a highly effective barrier to the ravages of air, temperature,moisture and chemical attack.

Aluminium is a superb conductor of electricity. This property allied with other intrinsic qualities hasensured the replacement of copper by aluminium in many situations.

Aluminium is non-magnetic and non-combustible, properties invaluable in advanced industries such aselectronics or in offshore structures.

Aluminium is non-toxic and impervious, qualities that have established its use in the food and

packaging industries since the earliest times.

Other valuable properties include high reflectivity, heat barrier properties and heat conduction. Themetal is malleable and easily worked by the common manufacturing and shaping processes.

Physical PropertiesDensity / Specific Gravity (g.cm-3 at 20 C) 2.70

Melting Point (C) 660

Specific heat at 100 C, cal.g-1K-1 (Jkg-1K-1) 0.2241 (938)

Latent heat of fusion, cal.g-1 (kJ.kg-1) 94.7 (397.0)

Electrical conductivity at 20C(% of international annealed copper standard)

64.94

Thermal conductivity (cal.sec-1cm-1K-1) 0.5

Thermal emmisivity at 100F (%) 3.0Reflectivity for light, tungsten filament (%) 90.0

These properties can be very significantly altered with the addition of small amounts of alloyingmaterials. Aluminium reacts with oxygen to form a microscopic (0.000000635cm) protective film ofoxide, which prevents corrosion.

Aluminium in massive form is non-flammable. Finely divided particles will burn. Carbon monoxide ordioxide, aluminum oxide and water will be emitted. This is a useful property for making rocket fuel.
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Casting

Applications

Cast parts are used in a variety of applications including:

Lightweight components for vehicles, aircraft, ships and spacecraft.

General engineering components where light weight and corrosion resistance are required.

Architectural fittings where light weight and good appearance are important.

High-tech products for office and home.

Casting falls into two main categories: Sand Casting and Die Casting.

Sand CastingThis technique is usually used for high production volume processes. Sand moulds are created usingvarious materials, the sand must be bonded together using either synthetic compounds or clay andwater and moulds must be rebuilt after each casting.The design of moulds is a very complicated process, however, in general, they are filled simply by

gravity without the need for any pressure differentials or mechanical action.Die CastingDie casting moulds are generally permanent and made of either cast iron or steel. There are threemain modes of die casting: high pressure, low pressure and gravity die casting.High pressure die casting is the most commonly used process, in which molten aluminium is injectedat high pressure into a metal mould by a hydraulically powered piston. The machinery needed for theprocess can be very costly and this high pressure die casting is only economic when used for highvolume production.Low pressure die casting uses a die which is filled from a pressurised crucible underneath. The processis particularly suited to the production of rotationally symmetrical products such as automobile wheels.Gravity die casting is suitable for mass production and for fully mechanised casting.For more information on casting processes visit the European Aluminium Association.

Rolling

Aluminium is first passed through a hot rolling mill and then transferred to a cold rolling mill.

Hot Rolling Mills

Prior to rolling the aluminium is in the form of an ingot which can be up to 600mm thick. This ingot isthen heated to around 500C and passed several times through the hot rolling mill. This graduallyreduces the thickness of the metal to around 6mm.

This thinner aluminium is then coiled and transported to the cold rolling mill for further processing.

Cold Rolling Mills
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There are various types of cold rolling mill, and theyproduce various types of rolled product, withthicknesses as low as 0.05mm. In general the typeof product depends on the alloy used, the rollingdeformation and thermal treatment used in theprocess as well as careful adjustments to the

mechanics and chemistry of the process. Rollingmills are controlled by very precise mechanisms andmeasuring systems.

Products

Rolled products can be divided into foil ,sheet and plate.

Foil is less than 0.2 mm thick and is used mainly in the packaging industry for foil containers andwrapping. Foil is also used for electrical applications, building insulation and in the printing industry.

Sheet is between 0.2 mm and 6mm in thickness and has a wide variety of uses in the constructionindustry including aluminium siding and roofing. Sheet is also used extensively in transportapplications such as automobile body panels, airframes and the hulls of boats.

Plate is any rolled product over 6mm in thickness. It also be found in a number of applicationsincluding airframes, military vehicles and structural components in bridges and buildings.

Extrusion

Aluminium extrusions are made from solid aluminium cylinders called billets, which are continuouslycast from molten aluminium. Billets are available in a wide variety of alloys, pretreatments anddimensions, depending upon the requirements of the manufacturer.

A plant producing rolled sheet aluminium
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The extrusion process involves aluminium metal being forced through a die with a shaped opening.This is made possible by preheating the billet to 450-500C and then applying a pressure of between500 and 700 MPa (equivalent to the pressure found at the bottom of a 60km high water tank!). Theheated and softened metal is forced against the container walls and the die by a hydraulic ram, theonly exit is the geometric cross-section of the die opening, and the metal is squeezed out.

The extrusion leaves the die at a temperature of around 500C and the exit temperature is carefully

controlled in order to achieve specified mechanical properties, a high quality surface finish and goodproductivity.

The Press

The press supplies the force necessary to squeeze the billet through the extrusion die. It consists of:

The container where the billet is put under pressure.

The main cylinder with the ram for pushing the billet into the container and through the die.

The front platen giving counter support to the die package.

The main columns fixing the front platen and the cylinder together.

The die is supported by a series of back dies or backers and bolsters for transferring the main pressload to the front platen.The principle of an extrusion press can be seen in the schematic diagram below:

ApplicationsAluminium extrusions are used throughout the construction industry, particularly in window and doorframe systems, prefabricated houses/building structures, roofing and exterior cladding and curtainwalling. Extrusions are also used in road and rail vehicles, airframes and marine applications.
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Aluminium Recycling

Value of Scrap

Anything made of aluminium can be recycled repeatedly not only cans, but aluminium foil, plates andpie moulds, window frames, garden furniture and automotive components are melted down and usedto make similar products again. The recycling of aluminium requires only 5% of the energy to producesecondary metal as compared to primary metal and generates only 5% of the green house gasemissions. Scrap aluminium has significant value and commands good market prices. Aluminiumcompanies have invested in dedicated state of the art secondary metal processing plants to recyclealuminium. In the case of beverage cans, the process uses gas collected from burning off the coatingto preheat the material prior to processing. The recycling of aluminium beverage cans eliminateswaste. It saves energy, conserves natural resources, reduces the use of city landfills and providesadded revenue for recyclers, charities and local town government. The aluminium can is thereforegood news for the environment and good for the economy.

Used beverage cans are normally back on supermarket shelves as new beverage cans in 6-8 weeks inthose countries which have dedicated can collecting and recycling schemes. The recycling rate foraluminium cans is already above 70% in some countries. Cans made from aluminium are worth 6 to20 times more than any other used packaging material.

In Europe, the aluminium beverage can meets the minimum targets set in the European directive onPackaging and Waste. Sweden (92 per cent) and Switzerland (88 per cent) are the European canrecycling champions. The European average is 40 per cent, a ten per cent increase since 1994.

Recycle Rates

Recycling rates for building and transport applications range from 60-90 per cent in various countries.Just over 11.6 million tonnes of old and new scrap were recycled in 1998 worldwide, which fulfilledclose to 40% of the global demand for aluminium. Of this total, 17% came from packaging, 38% fromtransport, 32% from building and 13% from other products. The aluminium industry is working withthe automobile manufacturers to enable easier dismantling of aluminium components from cars inorder to improve the sorting and recovery of aluminium. In 1997 over 4.4 million tonnes of scrap were

used in the transport sector and the use of aluminium in automobiles is increasing year upon year.Worldwide the future of scrap recycling certainly looks promising, especially with growth of packagingexpected in South America, Europe, and Asia, especially China.

Climate Change

The Kyoto Protocol in 1997 established greenhouse gas emissions reduction targets for the Annex Onecountries. Climate change presents both global opportunities and challenges for the aluminiumindustry worldwide. There exists a significant opportunity for the industry to contribute globally to thereduction of greenhouse gas emissions through the increased use of aluminium in transportation

applications. Every kilogram of aluminium that replaces traditional heavier materials in a vehicle todaysaves the equivalent of 20 kilograms of carbon dioxide emissions over the lifetime of the vehicle, alsothe industry can contribute to reduction of greenhouse gas emissions through the increased use ofrecycled aluminium - the recycling of aluminium saves up to 95% of the greenhouse gas emissionsgenerated by the production of the metal from bauxite.

The challenge for the industry lies in the relatively high-energy consumption and greenhouse gasemissions associated with the production of primary aluminium. The industrial processes of thePrimary Aluminium Industry were directly responsible in 1997 for emitting 110 million tonnes of CO2equivalents. 50 million tonnes (45%) of which originated from two perfluorocarbon compounds(PFCs); tetrofluormethane (CF4) and hexofluormethane (C2F6). On average the smelting process itself
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is responsible per tonne of aluminium for the production of 1.7 tonnes of CO 2 (from the consumptionof the carbon anodes) and the equivalent of an additional 2 tonnes of CO 2 from PFC emissions. PFCsare potent global warming gases as compared to carbon dioxide and have long atmospheric lifetimes.For example one kg of PFC (CF4) is equivalent to 6500 kg of CO2.

PFC Emissions

PFCs are not generated during normal smelting operating conditions. They are only produced duringbrief upset conditions known as "anode effects". These conditions occur when the level of thedissolved aluminium oxide (the raw material for primary aluminium) in the cell drops too low and theelectrolytic bath itself begins to undergo electrolysis. Measures to reduce the frequency and durationof anode effects not only reduce greenhouse gas emissions but they also benefit the producer byimproving energy and process efficiency.

Primary aluminium producers believe that the availability of adequate data on global PFC emissions isan essential preliminary step in responding to global warming concerns. The IAI has therefore justcompleted its report on the industry's PFC reduction programme from 1990-2000, integrating and

supplementing its previous two surveys, covering data from 1990-1997. To see a summary of thesurvey and for details of how to order a copy please see our publications page.

The graph shows a reduction of 47% between 1990 and 1997.

The survey questionnaires were sent to IAI correspondents around the world representing 104facilities. Unfortunately we were unable to cover producers in the Russian Federation, the Ukraine orChina.

Using the data from the 63% of world aluminium production that participated in these surveys, it canbe shown that an overall 60% reduction in the specific emission rate for CF 4 has occurred over the1990 to 2000 time period. This is one of the few examples of where the growth in global emissions ofa greenhouse gas from an industry sector are actually in decline. The declining rate of PFC emissions
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is the result of the industry's efforts to reduce the frequency and also to some extent the duration ofthe anode effects in pot line cells.

Voluntary agreements, between government and industry have played a significant role inencouraging this reduction in PFC emissions in many countries, such as Australia, Bahrain, Brazil,Canada, France, Germany, New Zealand, Norway and the UK. Together they represent around 50% ofworld production. For example under the US EPA's voluntary aluminium industrial partnership the US

aluminium industry achieved by 1998 a 46% reduction in PFCs (approximately 2.2 million metrictonnes of CO2 equivalents). Such reductions have been achieved through the use of computerizedanode effect suppression systems that reduce anode effect duration, as well as point alumina feedingsystems and computer feed control programmes that reduce anode effect frequency. There is also theongoing phasing out of older technologies and their replacement with more modern technology, wherethis is economically justified.

It is also noteworthy that a breakdown of survey results based upon the Kyoto protocols annex 1 andnon-annex 1 countries for calendar year 1997, showed that the performance within the worldwidealuminium industry on a PFC ( CF4) specific emission rate basis was virtually identical between Annex1 and Non-Annex 1 located plants, with Non Annex 1 performing slightly better.

Calendar year 1997

Annex 1

Countries

Non-Annex 1

CountriesTonnes of production capacity 14,500,000 7,300,000

Tonnes of production participating 9,310,312 4,372,188

% of production of participating 64% 60%

Weighted average kg CF4 per tonne 0.31 0.29

PFC Reduction Efforts

The greatest potential for reducing emission costs effectively are to be found in Asia (especially China)and in Eastern Europe and Russia. These regions tend to rely on older Sderberg technology, whichoffers significant greenhouse gas reduction potential. It is also important to note that over one third ofthe aluminium used worldwide, is produced from recycled aluminium scrap. The recycling ofaluminium saves up to 95% of the greenhouse gas emissions generated from the production of themetal from bauxite.

The aluminium industry is therefore encouraging more and more recycling of the metal with the resultthat some 40% (over 11 million tonnes) of the global demand for aluminium is already being met fromrecycling old products and process scrap. Older products made of aluminium, can be recycledprofitably and the metal can be used for new applications without loss of quality. The PFC surveyhowever also highlighted the considerable variation in performance between smelters using differenttypes of technology and even between smelters using the same technology.

Technologytype

Kilograms CF4 per tonne of aluminium produced (productionweighted average)

1990 2000

CWPB 0.42 0.21

PFPB 0.37 0.11

SWPB 1.37 1.06

VSS 0.52 0.36

HSS 0.54 0.51

The IAI is therefore introducing annual global PFC emissions reporting and will publish an annualsurvey report on the reduction of PFCs.

The IAI is also introducing a benchmarking programme based on the best performing smelters fromeach technology type. Seminars will be held to promote the spread of good practice throughout the


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industry. Each reporting smelter will receive a performance graph showing where it ranks in relation tothe performance of other de-identified plants with similar technology. The aluminium industry inadopting such measures is one of the first industries to adopt a truly global approach. This effort isdesigned to encourage continuing progress in reducing PFC emissions.

The IAI is also currently sponsoring the PFC related research projects at Portland State University,Oregon (USA). For instance the researchers are updating an atmospheric model to enable an analysis

of CF4 and C2F6 contribution to heat absorption relative to other concentrations of greenhouse gases.They are also conducting the analysis of a 20-year series of atmospheric samples for CF 4 and C2F6concentration changes. The analysis programme is designed to verify whether a slowing down in theaccumulation of PFCs in the atmosphere has occurred, which would be consistent with the level ofreduction recorded in the IAI survey.

Environmental Challenges

Environmental stewardship - the aluminium industry's approach to environmental

issuesThe aluminium industry is committed to good environmental stewardship:

The minimisation of any impact on the environment.

Research into energy and emission reduction.

Responsible, safe disposal or re-use of waste products

Maximising the use of recycled material.

Restoration of land to nature or to sustainable agriculture after mining or other industrialprocesses.

The aluminium industry's approach.The Aluminium Industry is continually seeking to reduce energy consumption and emissions throughmore efficient production and recycling and through collaboration with customer industries. Forexample PFC emissions per tonne of aluminium have diminished by 47% between 1990 and 1997.Energy consumption per tonne of production has fallen by 70% over the past hundred years.The Aluminium industry has also embarked on a rigorous life cycle analysis programme which isenabling it to establish the facts about aluminium's environmental performance. The first study is nowavailable and concludes that the use of aluminium in automobiles has the potential to save up to 20metric tonnes of CO2 equivalents for each tonne of additional automotive aluminium products used,due to enhanced vehicle fuel efficiency over the vehicle's lifetime.

Smelter Emissions

Most smelters operated by IAI members now have powerful scrubbing equipment which removes 96-99 per cent of all emissions from the pots and enables their re-use in the process. As a result, currentaverage levels of emission to atmosphere are as low as 1.1kg (0.5kg for the new modern plants) offluoride per tonne of aluminium. This compares with 3.9 kg per tonne in 1974.

There are two main types of fluoride emissions:

A mixture of the inorganic fluorides NaF, AlF3 and Na3AlF6 (as particulates) and HF (as a gas);
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The organic Perfluorocarbons (CF4 and C2F6) as gases.

Research from all around the world showed that vegetation quickly recovered when fluoride emissionswere reduced to current levels.Most aluminium smelters are surrounded by environmental control zones normally farmland, and theenvironment in these is closely monitored.

Polycyclic aromatic hydrocarbons (PAHs)These are produced during the manufacture of anodes for modern "pre-bake" aluminium smelters, andduring the electrolytic process itself in the older "Sderberg" type facilities. Current air emission levelsof PAH from pre-bake plants is 0.05 kg per tonne and 0.25kg per tonne from Sderberg plants. Inrecent years levels have been reduced considerably. Modern pre-bake plants emit less than 0.01kgper tonne

Pre-bake anodes are made from petroleum coke and pitch. These are by-products from the petroleumand steel process which are baked in either gas- or oil-fired ovens. Anodes are progressivelyconsumed during use and are eventually replaced. The butts are then recycled.PAH emissions from Sderberg facilities have been dramatically reduced by the introduction of "dryanode technology" at many locations as well as through other process improvements and alterationsin the raw materials used.

Sulphur DioxideGenerated from the sulphur content at fossil-fueled power stations, and other parts of the aluminiumproduction process - steam generation in alumina plants, ovens in anode plants and anodeconsumption in the pots.The remedy is to use low sulphur fuel and coke if available, and wet scrubbers to remove the particlesfrom the air.

Carbon DioxideCarbon dioxide is a feature of all metal processes which produce metal from ores containing oxides.The gas forms when the carbon in the anode combines with the oxygen in aluminium oxide during thesmelting process. It is therefore an unavoidable by product of the aluminium smelting process. Overthe last ten years the aluminium industry has reduced its carbon dioxide output by around 10 per centthrough better production techniques.

Inorganic FluoridesThese are compounds which have a local effect around a smelter (unlike PFCs which do not have anylocal effects but a global effect as a Greenhouse Gas). If the development in fluoride reductions isdivided into 3 "generations" we can describe the history in the following matrix:

Development in Fluoride Emissions from Aluminium Smelters kg Fluoride per tonneof Aluminium produced

1st Generation Plants 1940-1955 12 - 15 kg per tonne

2nd Generation Plants 1955-1975 2 - 6 kg per tonne

3rd Generation Plants 1975-today 0.3 - 1 kg per tonne

Perfluorocarbons (PFCs) - Tetrafluoromethane (CF4) and Hexafluoroethane (C2F6)These gases are chemically inert but have high global warming potential, they are produced in verysmall quantities during "anode effects" when the alumina concentration in the cryolite bath is reduced.The carbon anode then reacts directly with the fluoride in the electrolyte.Much of the world-wide aluminium industry is involved in national level voluntary programs related toPFC emissions. Some programs include research efforts on the development of a better understandingof process parameters related to PFC generation as well as emission reduction targets. It is clear thatthe more efficiently, the electrolytic process can be run, the lower the generation of PFCs.
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Alluminium Production

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