Some science behind the scenes

Aluminium environmental problems

"Red mud" storage facility in Stade, Germany. The aluminium industry generates about 70 million tons of this waste annually.

Because of its strong affinity to oxygen, Aluminium is almost never found in the elemental state; instead it is found in oxides or silicates. Feldspars, the most common group of minerals in the Earth's crust, are aluminosilicates. Thus, although aluminium is an extremely common and widespread element, it is trapped and the common aluminium minerals are not economic sources of the metal. Almost all metallic aluminium is produced from the ore bauxite (AlOx(OH)3–2x).

 

Large deposits of bauxite occur in Australia, Brazil, Guinea and Jamaica and the primary mining areas for the ore are in Australia, Brazil, China, India, Guinea, Indonesia, Jamaica, Russia and Suriname.

One of the problems then is that most of this ore then has to be transported from the country in which it is mined to the country in which it is being processed.  This is often the country that will consume the aluminium, but not always.  Iceland, for example, has a very large smelter plant and it has this plant because it has an abundant supply of geothermal energy, as has New Zealand.  Once processed the aluminium is then shipped. 

 

Thus in reality the energy needed to smelt aluminium is extremely high - a large number of 'sea-miles' are needed, with the added disadvantage that the countries with renewable energy sources are often extremely beautiful and relatively pollution free.

An aluminium plant is not a pretty thing and the fumes can cause considerable pollution.  Given the toxicity of Aluminium, this is not a problem to be brushed over lightly.

The process

In order to produce aluminium, Bauxite is first converted to aluminium oxide (Al2O3) via the Bayer process.

Al2O3 + 2 NaOH → 2 NaAlO2 + H2O

2 H2O + NaAlO2 → Al(OH)3 + NaOH

The intermediate sodium aluminate, given the simplified formula NaAlO2, is soluble in strongly alkaline water, and the other components of the ore are not. Depending on the quality of the bauxite ore, twice as much waste ("red mud") as alumina is generated.

 

The conversion of alumina to aluminium metal is achieved by the Hall-Héroult process. 

Aluminium electrolysis with the Hall-Héroult process consumes a lot of energy.

The worldwide average specific energy consumption is approximately 15±0.5 kilowatt-hours per kilogram of aluminium produced (52 to 56 MJ/kg). The most modern smelters achieve approximately 12.8 kW·h/kg (46.1 MJ/kg).

Electric power represents about 20% to 40% of the cost of producing aluminium, depending on the location of the smelter. Aluminium production consumes roughly 5% of electricity generated in the U.S.  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. Nevertheless, the world's largest smelters of alumina are located in the People's Republic of China, Russia and the provinces of Quebec and British Columbia in Canada.

 

 In this energy-intensive process, a solution of alumina in a molten (950 and 980 °C (1,740 and 1,800 °F)) mixture of cryolite (Na3AlF6) with calcium fluoride is electrolyzed to give the metal:

Al3+ + 3 e− → Al

At the anode, oxygen is formed:

2 O2− → O2 + 4 e−

The aluminium metal then sinks to the bottom of the solution and is tapped off, usually cast into large blocks called aluminium billets for further processing. To some extent, the carbon anode is consumed by subsequent reaction with oxygen to form carbon dioxide.

In effect, if we summarise, the production of aluminium is highly energy dependent and is polluting, both in terms of the red sludge produced and the carbon dioxide from the process itself as well as that produced in producing the needed energy.  To this can be added the sea-miles needed to transport ore and metal, as well as the polluting effects of aluminium on the environment.

Given these figures there is every reason to encourage recycling of aluminium and in very careful selection of its use.  Beverage cans may not be the most eco-friendly way of using this metal.

Recycling

cheap 'bling' jewellery may not be the most eco-friendly way
to use aluminium

Aluminium is theoretically 100% recyclable without any loss of its natural qualities.

According to the International Resource Panel's Metal Stocks in Society report, the global per capita stock of aluminium in use in society i.e. in cars, buildings, electronics etc. is 80 kg (180 lb). Much of this is in more-developed countries (350–500 kg (770–1,100 lb) per capita) rather than less-developed countries (35 kg (77 lb) per capita).

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). An aluminium stack melter produces significantly less dross, with values reported below 1%. The dross can undergo a further process to extract aluminium.

 

In Europe aluminium experiences high rates of recycling, ranging from 42% of beverage cans, 85% of construction materials and 95% of transport vehicles.

These problems become of paramount importance once one realises that Aluminium is the most widely used non-ferrous metal.  Global production of aluminium in 2005 was 31.9 million tonnes. It exceeded that of any other metal except iron (837.5 million tonnes). Forecast for 2012 is 42–45 million tonnes, driven by rising Chinese output.

Uses

Aluminium is remarkable for the metal's low density and for its ability to resist corrosion. About the only substances that attack aluminium in nature are chlorides such as common sodium chloride [salt].  Because salt is a well-known sources of corrosion of aluminium, household plumbing is never made from this metal and cars have to be carefully manufactured so that the salt from ‘gritting’ in winter does not set up a galvanic reaction between the steel chassis and aluminium body. 

not exactly eco-friendly

Structural components made from aluminium and its alloys are vital to the aerospace industry and are important in other areas of transportation and structural materials.  Sustainability is in some senses dependent on aluminium, as if cars do not rot, if structures do not rot and if airplanes do not rot, then they are more likely to last longer and be used longer, saving the vital energy needed for constant manufacture of new products.

Given the problems of pollution and energy consumption, some of the uses to which aluminium has been put in the past might be better re-evaluated.  The real value of the metal is in its ability to make things last longer, coupled with its low weight, thus to use it for drinks cans, tacky jewellery or ornaments, body decoration and so on [which are rarely recycled] seems unnecessary.  Some of the many uses for aluminium metal are as follows, we have put a * by those applications where longevity results:

  • * Transportation (automobiles, aircraft, trucks, railway cars, marine vessels, bicycles, spacecraft, etc.) as sheet, tube, castings, etc.
  • Packaging (cans, foil, frame of etc.)
  • * Construction (windows, doors, siding, building wire, etc.).
  • A wide range of household items, from baseball bats to watches to jewellery.
  • *Street lighting poles
  • Sailing ship masts.  The salt water corrodes the masts
  • misuse ............
    Outer shells of consumer electronics, also cases for equipment e.g. photographic equipment, MacBook Pro's casing
  • *Electrical transmission lines for power distribution
  • *MKM steel and Alnico magnets
  • Super purity aluminium (SPA, 99.980% to 99.999% Al), used in electronics and CDs.
  • Heat sinks for electronic appliances such as transistors and CPUs.
  • * Substrate material of metal-core copper clad laminates used in high brightness LED lighting.
  • Powdered aluminium is used in paint, and in pyrotechnics such as solid rocket fuels and thermite.
  • Aluminium is used to make food containers, because of its resistance to corrosion, but then so does glass.
  • *Aluminium with magnesium [alloy] is used to make the body of aircraft.
  • Aluminium is used to make cooking utensils, because it is resistant to corrosion, and light-weight, but then so is steel.
  • A variety of countries, including France, Italy, Poland, Finland, Romania, Israel, and the former Yugoslavia, have issued coins struck in aluminium or aluminium-copper alloys.
  • Some guitar models sport aluminium diamond plates on the surface of the instruments, usually either chrome or black. Kramer Guitars and Travis Bean are both known for having produced guitars with necks made of aluminium, which gives the instrument a very distinctive sound.
  • Several sulfates of aluminium find applications. Aluminium sulfate (Al2(SO4)3·(H2O)18) is produced on the annual scale of several billions of kilograms.
    • About half of the production is consumed in water treatment.
    • The next major application is in the manufacture of paper.
    • It is also used as a mordant,
    • in fire extinguishers
    • as a food additive (E number E173)!
    • in fireproofing, and
    • in leather tanning.
  • Aluminium ammonium sulfate, which is also called ammonium alum, (NH4)Al(SO4)2·12H2O, is used as a mordant and in leather tanning. Aluminium potassium sulfate ([Al(K)](SO4)2)·(H2O)12 is used similarly. The consumption of both alums is declining.
  • Aluminium chloride (AlCl3) is used in petroleum refining and in the production of synthetic rubber and polymers.
  • Aluminium chlorohydrate has fewer and very different applications, e.g. as a hardening agent and an antiperspirant. It is an intermediate in the production of aluminium metal.
misuse ......................

There are further uses made of aluminium which also would not seem to align with the goals of longevity.  One important structural limitation of aluminium alloys is their fatigue strength. Unlike steels, aluminium alloys have no well-defined fatigue limit, meaning that fatigue failure eventually occurs, under even very small cyclic loadings. This implies that engineers must assess these loads and design for a fixed life rather than an infinite life.  In other words, aluminium alloys are not best suited to any load bearing application.

Another problem with aluminium alloys is their low melting point, which make them more susceptible to distortions from thermally induced stress.