Aluminum plays a key role in many aspects of our everyday lives but in such an unassuming fashion that we are often unaware of its presence. You find it in everything from beverage cans to power cables to car frames because its versatility allows it to take forms that give rise to an endless range of applications.
It is also the metal of choice in many advanced industries because its unique qualities - strength, durability, fl exibility, impermeability, conductivity, low density, resistance to corrosion, recyclability - make it highly functional and cost-effective while offering huge potential for research and innovation.
Even though aluminum has been produced commercially for just 150 years, it has now become one of the world's most used metals, just after steel. Demand for aluminum continues to grow across a vast range of sectors, as traditional uses are expanded and new ones are developed.
The metal exists in abundance in the Earth's crust, but only in its oxidised form in combination with other minerals. It has to be extracted from the ores, mainly bauxite, through a series of mechanical, chemical and electrometallurgical processes.
REFINING: After the bauxite is mined, it is sent for refi ning into aluminum oxide trihydrate, or alumina, the main raw material for primary aluminum production. The ore is washed, crushed and dissolved in caustic soda to unlock the alumina component, which is then precipitated and collected for further processing. Calcination – intense heating to remove the liquid – is applied to produce alumina.
Known as the Bayer process, this refi ning method has been in use since the 19th century because it remains the most economic means of extracting alumina. Typically, 2-3 tonnes of bauxite ore are needed to produce 1 tonne of alumina.
SMELTING: Alumina is reduced to metallic aluminum in an electrolytic cell called a "pot", which is made up of two main parts - one serves as the anode or positive electrode of the cell, and the other, as the cathode or negative electrode. The anode consists of a block of carbon formed by baking coke and pitch, while the cathode is found in the carbon lining of a large steel container located under the anode. This lining is formed by baking metallurgical coke and pitch.
The space between the anode and the cathode is filled with an electrolytic bath of sodium aluminum fluoride, or cryolite. The electrolyte is heated to about 980°C, at which point it melts and the alumina is added and dissolved. An electric current is then passed through the mixture to split the alumina into molten aluminum metal at the cathode and carbon dioxide at the anode.
This Hall-Héroult process was invented in 1886 and has since been steadily improved. Currently, it takes about 2 tonnes of alumina to produce 1 tonne of aluminum metal.
Smelting consumes huge amounts of electricity because of the high temperatures that must be generated. Typically, the electrolysing current is delivered at a high amperage and on average, it takes almost 14,171 kwh of electricity to produce 1 kg of aluminum from alumina compared with XinRen's average of 13,727 kwh in 2009. In addition, the smelting process is continuous as the metal in the pots cannot be allowed to solidify. Not surprisingly, most plants around the world are located in areas where there is ample access to inexpensive energy.
In China, coal is an abundant source of energy and vast reserves can be found in the provinces of Inner Mongolia, Shaanxi, Shanxi and Xinjiang. In fact, China has the world's third largest proved coal reserves as at end 2009 and Xinjiang alone holds about 40% of these. Xinjiang is also known to have the lowest cost of mining per tonne in China.
Today's plants use one of two technologies based on the Hall-Héroult process - Söderberg or pre-bake - which differ mainly in terms of the anode used. The Söderberg or self-baking technology uses a continuously created anode that is made by adding pitch to the cell that is baked in the cell itself. The pre-bake technology uses multiple anodes baked beforehand in a separate facility that are then suspended from rods in the cell. Once these anodes are consumed, new ones must be added.
Across the world, new plants and plant expansions are employing pre-bake cells. In China, the government is phasing out Söderberg cells as well, because they consume far more energy and produce more pollutive emissions than pre-bake cells do.
At XinRen, our plants employ a pre-bake variant called Centre Worked Pre-bake Technology (CWPB). Our 240kA system uses multiple-point feeders to introduce the alumina, as well as other computerised controls that increase feeding precision. The enclosed nature of the process means that less than 2% of the emissions generated escape the pots. The highamperage technology allows our plants to be far more energy-efficient and environmentally sustainable. Furthermore, the carbon anodes generally last longer, resulting in considerable savings on this front as well.
After the molten aluminum has been extracted at the smelting plant, it is sent to a holding furnace that is also run continuously. Once the furnace is full, the aluminum is flowed at carefully adjusted temperatures into moulds to produce ingots. After they have solidified, the ingots are cooled and packaged for delivery.
An extremely versatile metal, aluminum can be processed in vastly different ways depending on the end-application required. Its uses can be expanded further by alloying it with other materials such as iron, silicon or zinc to produce metals with different properties. An endless array of products can be achieved by rolling the aluminum into plates, sheets or foils, or by casting or extruding it into different shapes.
Potential For Growth
The metal's lightness, strength and workability make it widely used in the transportation industry, where its use in frames, parts and fittings results in improved performance, fuel savings and reduced emissions for the automobile, aerospace, rail and marine sectors. The building and construction industry provides yet another formidable market for aluminum, as do packaging and electrical applications.
With such a broad range of applications, global consumption of aluminum is expected to grow by around 8% in 2012. China's ongoing urbanisation and industrialisation are expected to continue boosting demand. Between 2004 and 2009, the country was the world's largest consumer of primary aluminum, accounting for around 40% of global consumption. It is likely to retain this lead, with consumption of primary aluminum expected to achieve a compound annual growth rate of 11% between 2010 and 2015.
Thanks to aluminum's unique properties, which make it the metal of choice in a vast swathe of sectors, the industry can enjoy sustained growth while also contributing to a more sustainable global economy.
In the transport sector alone, aluminum is invaluable in efforts to minimise fuel consumption by producing lighter vehicles and carriers, whether for land, air or sea. When used to replace heavier materials in a car, 1 kg of aluminum can eliminate up to 20 kg of carbon dioxide over the vehicle's lifetime.
Furthermore, aluminum is 100% recyclable, ie it can be reused again and again, which makes it an ideal material to minimise negative effects on the environment. Across the aluminum industry, efforts are underway to reduce greenhouse gas emissions from production while maximising the efficient use of energy, to further ensure a sustainable future for all.
At XinRen, we truly support these efforts, and have devoted considerable time and resources to improving and refining our processes in ways that will increase energy efficiency while limiting pollutive emissions.