Aluminium production & environmental impact


Key Points

• Aluminium was first produced in 1850

• After iron, aluminium is the second most-used metal in the world.

• After oxygen and silicon, aluminium is the third most abundant element in the earth’s crust.

• In Europe, after transport (39%), construction at 24% is the second largest application. 1

• In 2012, on average, each European citizen used 22kg of aluminium 1

• China is the largest producer of aluminium

• 90% of all aluminium recovered from demolition/end-of-life is recycled. 1

 

1 Mineral Extraction

Bauxite mining – Alumina production – Primary aluminium production – Semi-fabrication – Product manufacturing – Product use – Product Recycling

Bauxite mining

Aluminium doesn’t occur as a isolated ore (it’s too reactive with other compounds) but as one combined with other minerals, chief of which is bauxite. Bauxite is mined from open mines from locations mainly in a wide belt around the equator. Bauxite is a mixture of aluminium hydroxide, iron oxide, titanium dioxide and kaolinite. Large deposits of bauxite occur in Australia, Brazil, Guinea and Jamaica and the primary mining areas for the ore are in Australia (top producer), China,Brazil, India, Guinea, Indonesia, Jamaica, Russia and Suriname.

 

The establishment and exploitation of a bauxite mine in Suriname – the company’s PR film.

 

An Indian tribe in Orissa fights back against proposals for a bauxite mine on their land.

 

2 Bauxite Processing

Bauxite is processed into aluminium oxide (alumina) before it can be converted into aluminium through electrolysis. Alumina refineries employ the Bayer process that releases alumina from the bauxite in a caustic soda solution within a digester. Worldwide, 1 tonne of alumina is produced from around 2.9 tonnes of bauxite 2 The resulting bauxite residue (‘red mud’) is of insoluble particles.

 

Bauxite residue, Western Australia

Bauxite residue from the Bayer process has long posed a problem for storage and management. The industry generates some 120 million tonnes of residue per year from some 100 processors. Most of it is stored in holding ponds. Very little is reused. Research continues to look for suitable applications.

Potentially toxic metals and naturally occurring radioactive materials (NORMS) arise via the original bauxite, therefore, bauxite residues from different ores are likely to vary in type and concentration of respective metal species.

‘In October 2010, approximately one million cubic meters of red mud from an alumina plant near Kolontar in Hungary was accidentally released into the surrounding countryside in the Ajka alumina plant accident, killing ten people and contaminating a large area. All life in the Marcal river was said to have been "extinguished" by the red mud, and within days the mud had reached the Danube.’ (Wikipedia)

 

The Kolontar spillage disaster

 

3 Primary aluminium production

Primary aluminium is produced by electrolysis, usually in ‘pots’. Alumina is first dissolved in molten cryolite with calcium fluoride and then electrolytically reduced to aluminium at around 950 degrees C.

Cryolite is sodium aluminium fluoride. It occurs primarily in a large deposit in Greenland – although there are other sources including Spain and the USA. It is also used in the glass and enamel industries.

Molten aluminium is tapped from the pots and moved to the ‘cast house’ where it is alloyed in holding furnaces.  Depending on the end user, other metals are added to form aluminium alloys, and then cast into ingots called ‘aluminium billets’.

 

A short film about how aluminium is made (with funky music should you need to dance)

 

• Most aluminium is alloyed to improve its mechanical properties. The main alloying agents are copper, zinc, magnesium, manganese and silicon.

• Aluminium producers tend to locate smelters in places where electric power is both plentiful and inexpensive--such as the United Arab Emirates with its large natural gas supplies, and Iceland and Norway with energy generated from renewable sources. The world's largest smelters of alumina are China, Russia, and Canada.

 

4 Big energy

• Energy used is the industry’s biggest issue. So great is the consumption that aluminum is jokingly referred to as ‘congealed electricity’.

• The amount of energy needed to make aluminium is 211 GJ per tonne, compared with 22.7 GJ per tonne for steel.3

• The extent of the energy use owes to the strength of the chemical bond between aluminium and oxygen is significantly stronger than the same bond between iron and oxygen. As a result, much more energy is required to split the bond and form the metal.

 

5 Semi-fabrication and products

Just over half of aluminium used by the construction industry is in the form of aluminium extrusions; about 25% of the total becomes rolled aluminium products and around 8% of aluminium was used in castings (2008 figures).1

 

Extrusion

Heated billet is squeezed through an opening to form the cross-section of an aluminium profile.

 

Aluminium extrusion (and more funky music…)

 

Rolling

The process usually comprises of the hot rolling of ingots followed by cold rolling. Most sheet and plate is produced in a range of thicknesses between 3 to 25 mm, though thinner sheet can be cold-rolled down to 0.05mm.

Profiled sheets, typically used in roofing and cladding, is usually produced using cold-rolling.

 

Casting

Casting has three forms: Sand casting where molten aluminium is poured into the mold – this is useful for small runs; Permanent mould casting – as for sand but the molds are made from the more durable steel and iron; High pressure die casting where molds are loaded using gravity or low pressure until the aluminium solidifies – this process is appropriate for complicated shapes and low levels of dimensional tolerance.

 

Sand casting in China (volume warning)

 

6 Finishes

 

Anodising

Corrosion resistance can be excellent due to a thin surface layer of aluminium oxide that forms when the metal is exposed to air, effectively  preventing further oxidation.  Anodising thickens this layer and increases durability. Dyes can be added in the process to produce colours. Ultra violet degrades most of the colours over time, but red, black, blue and gold have the greater staying power.

Anodising is the most durable of finishes and so is popular in high traffic areas where the aluminum is subject to wearing and abrasion; It’s also generally less expensive than other coatings.

 

Anodising aluminum components

 

Anodising’s downside is where it can become subject to environmental pollution where the finish has been poorly applied.

Environmentally, the chemicals from anodising can be used by water company waste water treatment plants where the aluminum sulfate from anodising plants’ effluent can improve the solids settling efficiency.

Anodising doesn't emit VOCs

 

Organic coatings: paint and powder

Liquid paint is composed of pigment, resin, and solvent. Powder paint is simply pigment encapsulated in a powdered resin and is thus simply thought of as "Paint without the solvent." Powder coatings and liquid coatings made from the same resin and pigment will have practically the same performance characteristics.

Some resins are more easily manufactured in liquid coatings and some are more easily manufactured in powder coatings. A particular resin is usually manufactured in either powder or solvent-based coatings, but not both. Examples of this are epoxy, which is predominantly a powder coating, and PVDF which historically has been manufactured as a liquid coating. Many of the perceived advantages of powders over liquid coatings such as hardness and gloss are actually characteristics of the resin.

The performance of any organic coating (paints or powders) depends on the pretreatment, resin and pigmentation. With aluminium the pretreatment is of utmost importance. This is why organic coatings for aluminium should be factory applied. Resins are often the weak link in an organic coating system. Some resins, such as PVDF, have outstanding weatherability, while epoxy coatings are meant only for interior use.

There are many resins available for use such as urethanes, polyurethanes, aliphatic urethanes, polyesters, silicon polyesters, polyester TGICs, PVDF, etc. Only a few of these coating systems will last for more than five years in exterior architectural applications.

 

Polyvinylidene difluoride (PVDF aka PVF2)

PVDF is a speciality plastic material in the fluoropolymer family; It has a high level of resistance against atmospheric pollutants along with good gloss and colour retention. In its fine powder form it is the principal ingredient in paints for metals. PVDF is relatively chemically inert and will outlast anodizing in corrosive environments.

PVDF coatings are usually formulated as 70% PVDF and 30% other resins, acrylic usually predominating.

 

Powder coatings

For a given resin, the decision to use a powder or liquid coating is simply a question of application technique. The real advantage of powder is the reduction in air pollution compared to liquid coatings. When powders are cured in an oven they emit no VOCs. A disadvantage of powder is the large batch sizes that are typically required by powder coatings.

There are two main categories of powder coatings: thermosets and thermoplastics. The thermosetting variety incorporates a cross-linker into the formulation. When the powder is baked, it reacts with other chemical groups in the powder to polymerize, improving the performance properties. The thermoplastic variety does not undergo any additional actions during the baking process, but rather only flows out into the final coating.

The most common polymers used are polyester, polyurethane, polyester-epoxy (known as hybrid), straight epoxy (fusion bonded epoxy) and acrylics. (source Wikipedia)

 

Powder coating aluminium

 

7 Recycled aluminium (Secondary aluminium)

Recycled aluminium is known as secondary aluminium, but maintains the same physical properties as primary aluminium.

Recovery of the metal via recycling has become an important use of the aluminium industry. Recycling was a low-profile activity until the late 1960s, when the growing use of aluminium beverage cans brought it to the public awareness.

 

Recycling involves melting the scrap, a process that requires only 5% of the energy used to produce aluminium from ore, though a significant part (up to 15% of the input material) is lost as dross (ash-like oxide). (source Wikipedia)

 

8 Recycled content of products

The recycled content of products varies widely, but the overall world average is something like 30%. This proportion is only conditioned by the availability of scrap aluminium.

Some manufacturers demand high quantities of recycled aluminium for their products – but this has only the effect of diminishing the mix in other aluminium products.

It’s tempting as a Green designer to ask for high recycled content, but it actually makes no environmental sense at all.

 

9 Aluminium v Steel

 

Positive properties

+ High strength to weight ratio

+ High malleability

+ Easy machining

+ Excellent corrosion resistance

+ Good thermal and electrical conductivity

 

Negative properties

- Low modulus of elasticity

- Easily deforms

- Easily buckled

- Low fatigue strength

- Less resistant to fire (low melting point)

 

10 Environmental properties

Pro

Plentiful resource

Pro

Use of hydro-electricity can dramatically reduce fossil fuel energy

Pro

High durability

Pro

Low maintenance

Pro

European production highly regulated for pollution

Pro

Recycling uses a fraction of the energy used in primary production

Pro

Very high levels of recovery in construction and recycling

Con

Very high energy use

Con

Open-cast mining of bauxite can seriously damage local eco systems and communities

Con

Smelting uses large amounts of water

Con

Emissions to air and water from processing, some of which are hazardo

Con

Bauxite residue storage and management which demands land use and can sometimes fail (eg Ajka alumina sludge spill, Hungary, 2010)

Con

Long transportation routes in between extraction, processing and fabrication adds to embodied energy

Con

Environmental regulations governing production can vary greatly throughout the world

Con

Due to economics, the EU is currently a net exporter of aluminium scrap – which compromises the recycling industry in Europe

  

So, is aluminium a ‘sustainable’ material?

There isn't a straightforward answer. There's no doubting the wide range of uses of aluminium. It has the advantage of light weight combined with high strengh and durability, which, in many situations could be considered indispensible. But the hard reality of its soaring embodied carbon along with a currently disappointing recycling rate still make it a difficult material to use without some degree of consideration. Certainly the building designer should be  able to justify aluminium's function/application where specified.

 

 

References

1 European Aluminium Association (EAA), 2014

2 European Parliament briefing, 2013

3 Commonwealth Scientific and Industrial Research Organisation (CSIRO)

4 Wikipedia