Rare and expensive a century ago, aluminium has since been identified as the mostcommon metal on earth, forming about eight percent of the earth's crust. It is the third most plentiful element known to man. Only oxygen and silicon (sand) exist in greater quantities.
Cryolite is a white translucent, sodium aluminium fluoride material component found in its natural state only in Greenland . Most of the cryolite used in aluminium production today is synthetically produced. Held at 1030°C, the molten cryolite dissolves up to 20% of alumina readily. The electrolytic cell holding the molten cryolite is a tank lined with carbon which serves as one electrode. Large carbon blocks inserted from the top of the bath act as the anode, or other electrode, and a heavy electrical current is passed between these two sets of electrodes through the solution. This current breaks down the alumina into aluminium and oxygen. The molten metallic aluminium collects at the bottom of the cell and is drained off every few days as sufficient metal accumulates. The oxygen combineswith the carbon at the anodes and is given off as carbon dioxide gas. This became the first industrially applied method of making the metal aluminium from alumina, and is the one still in use today.
The immediate effect of the discovery of this process was to send the price of aluminium tumbling from $18 to $4.50 per kg, the first step in a downward course which has today established the selling price in terms of under two dollars per kg. But the discoveries bringing about low-cost production did not lead directly to the widespread use of aluminium. Alumina is produced in a totally separate first stage process from Bauxite ore. This (Bayer) chemical process starts by immersing crushed bauxite into a caustic soda solution which dissolves the alumina to form sodium aluminate liquor. After filtering, the impurities are left behind as a "red mud" and the liquid is treated to precipitate the aluminium content out of the solution which is now in the form of aluminium hydroxide. This material is then separated from the liquor and changed to alumina, which resembles course granulated sugar, by heating in kilns at 1000o
The production of primary aluminium is a young industry - just over 100 years old. But it has developed to the point where scores of companies in some 35 countries are smelting aluminium and thousands more are manufacturing the many end products to which aluminium is so well suited. For its first half century the aluminium industry pursued the dual role of improving and enlarging production processes to reduce the price of the metal and, at the same time, proving the worth and feasibility of aluminium in a wide range of markets. Such was the dynamic approach of the industry to this problem that the consumption of aluminium gained the remarkable record of doubling every ten years. The strong demand for aluminium stimulated the rapid expansion of productive capacity to meet it.
The first World War had a dramatic effect on aluminium production and consumption. In the six years between 1914 and 1919 world output soared from 70,800 tonnes to 132,500 tonnes a year and it is a striking testimony to the adaptability of the metal that after the very large expansion occasioned by war the ground was held. Once the changeover to civilian production had been carried through the increased capacity was occupied before very long in supplying the normal demands of industry. And this happened again, on a much larger scale, as a result of the second World War.
Constant research and product development throughout the 1950's, 60's and 70's led to an almost endless range of consumer goods incorporating aluminium. Its basic benefits of lightness, strength, durability, formability, conductivity and finishability made it a much sought after product. The necessity for the industry itself to pioneer the use of aluminium led to an integrated structure in the major companies from the mining of bauxite to, in some cases, the finished consumer product. As the total world production soared, countries with raw materials and especially those with cheap energy resources, began to enter the market with primary metal for others to further the process. Today a significant proportion of metal is marketed in this way.
Electrical Conductivity & Resistivity
The electrical conductivity of 99.99% pure aluminium at 200C is 63.8% of the International Annealed Copper Standard (IACS). Because of its low specific gravity, the mass electrical conductivity of pure aluminium is more than twice that of annealed copper and greater than that of any other metal. The resistivity at 200C is 2.69 microohm cm.
The electrical conductivity which is the reciprocal ofresistivity, is one of the more sensitive properties of aluminium being affected by both, changes in composition and thermal treatment. The addition of other metals in aluminium alloys lowers the electrical conductivity of the aluminium therefore this must be offset against any additional benefits which may be gained, such as an increase in strength.
Heat treatment also affects the conductivity since elements in solid solution produce greater resistance than undissolved constituents. The very good electrical properties of aluminium have made it an obvious choice for applications in the electrical industry, particularly in power distribution where it is used almost exclusively for overhead transmission lines and busbars. The first major aluminium transmission line was completed in 1898 in the USA : a 46-mile, three-phase installation for the Standard Electric Company of California , from Blue Lakes to Stockton .
Its use later became much more general when it was found possible to reinforce the cable (usually alloy 1350) with galvanised steel wire which increased the spans without too much sag. Although this product is still used, high strength (6061 type) all aluminium multi-strand cables are now preferred for some installations because higher line tensions can be achieved which can be applied to increase the distance between the pylons or alternatively reduce their height.
The melting point of aluminium is sensitive to purity, e.g. for 99.99% pure aluminium at atmospheric pressure it is 6600C but this reduces to 6350C for 99.5% commercial pure aluminium. The addition of alloying elements reduces this still further down to 5000C for some magnesium alloys under certain conditions. The melting point increases with pressure in a straight line relationship to 9800C at 50 kbar.
The difference between the melting points of two alloys of aluminium is used to advantage in the manufacture of aluminium heat exchangers, where the fins are made from aluminium-manganese (3103) or (3003) alloy clad with 5, 7.5% or 10% silicon alloy. The assembled heat exchanger is heated to the temperature which will just melt the cladding while allowing the core to remain solid; this causes the molten cladding alloy to flow by capillary action to the joints which become structural on cooling. The highly controlled heating necessary in this brazing process is done using either a vacuum furnace, controlled atmosphere furnace, or flux bath.
Recycled or Secondary Aluminium
- Light Weight : Aluminium is a very light metal with a specific weight of 2.7 g/cm3, about a third that of steel. For example, the use of aluminium in vehicles reduces dead-weight and energy consumption while increasing load capacity. Its strength can be adapted to the application required by modifying the composition of its alloys.
- Corrosion Resistance : Aluminium naturally generates a protective oxide coating and is highly corrosion resistant. Different types of surface treatment such as anodising, painting or lacquering can further improve this property. It is particularly useful for applications where protection and conservation are required.
- Electrical and Thermal Conductivity : Aluminium is an excellent heat and electricity conductor and in relation to its weight is almost twice as good a conductor as copper. This has made aluminium the most commonly used material in major power transmission lines.
- Reflectivity : Aluminium is a good reflector of visible light as well as heat, and that together with its low weight, makes it an ideal material for reflectors in, for example, light fittings or rescue blankets.
- Impermeable and Odourless : Aluminium foil, even when it is rolled to only 0.007 mm thickness, is still completely impermeable and lets neither light aroma nor taste substances out. Moreover, the metal itself is non-toxic and releases no aroma or taste substances which makes it ideal for packaging sensitive products such as food or pharmaceuticals.
- Recyclability : Aluminium is 100 percent recyclable with no downgrading of its qualities. The re-melting of aluminium requires little energy: only about 5 percent of the energy required to produce the primary metal initially is needed in the recycling process.
Aluminium is relatively unique in being highly economic to recycle. Metal can be reclaimed and refined for further use at an energy cost of only 5 per cent of that required to produce the same quantity of aluminium from its ore. There has been a healthy secondary metal industry for many years and as refining techniques improve the use that can be made of reclaimed aluminium will increase from its present usage in Europe of 40% of all metal currently processed.
The most dramatic example of recycled metal is in the United States . In the USA of the one million tonnes of aluminium sheet used annually for beer and beverage cans, over 50% is supplied from used can scrap. Europe is now following this example with the building of dedicated aluminium can recycling plants.
Aluminium recycling is the process by which scrap aluminium can be reused in products after its initial production. The process involves simply re-melting the metal, which is far less expensive and energy intensive than creating new aluminium through the electrolysis of aluminium oxide (Al2
), which must first be mined from bauxite ore and then refined using the Bayer process. Recycling scrap aluminium requires only 5% of the energy used to make new aluminium. For this reason, approximately 31% of all aluminium produced in the United States comes from recycled scrap. Used beverage containers are the largest component of processed aluminium scrap, with most UBC scrap manufactured back into aluminium cans.
Lightness is the outstanding and best known characteristic of aluminium. The metal has an atomic weight of 26.98 and a specific gravity of 2.70, approximately one-third the weight of other commonly used metals ; with the exception of titanium and magnesium. As with most metals the density decreases with increasing temperature. The addition of other metals in the amounts commonly used in aluminium alloys does not appreciably change the density (plus 3%, minus 2%), except in the case of Lithium alloys where the density of the alloy is reduced by up to 15%.
Weight is important for all applications involving motion. Saving weight results in more payload or greater economy of operation. Saving weight also saves energy, reduces vibration forces, improves the performance of reciprocating and moving parts, reduces tiredness when using manually operated equipment, offers lower shipping, handling and erection costs. Low weight combined with the high strength possible with special alloys has placed aluminium as the major material for aircraft construction for the past sixty years. Although purchased on a weight basis, metals are generally used on a volume basis, it is therefore important to compare the cost of aluminium with other materials on this basis.