Aluminium smelting

The primary aluminium production process

The process of extracting a metal from its oxide is known as 'smelting'. Most smelting processes involve direct reduction of the ore by carbon to metal and carbon dioxide. The production of primary aluminium is a good example of such a process.

Bauxite - a rock composed of hydrated aluminium oxides - is the main ore of aluminium oxide (Al₂O₃), commonly known as 'alumina', which is used to make aluminium (Al). Mined bauxite is refined into alumina, which is then converted into metallic aluminium via an electrolytic process. Once aluminium is formed, the hot, molten metal is alloyed with other metals to make a range of primary aluminium products with different properties and suitable for processing in various ways to make end-user products.

It takes about 2 metric tonnes of bauxite to produce 1 metric tonne of alumina; and approximately 2 metric tonnes of alumina to produce 1 metric tonne of aluminium.

Aluminium smelting

In an aluminium smelter, direct current (DC) is fed into a line of electrolytic cells connected in series. These electrolytic cells are the nerve centre of the process. While the cells (or 'pots') vary in size from one plant to another, the fundamental process is identical and is the only method by which aluminium is produced industrially. It is named the Hall-Héroult process after its inventors.

Each cell is a large carbon-lined metal container, which is maintained at a temperature of around 960°C and forms the negative electrode (or cathode). The cell contains an electrolytic bath of molten salt called 'cryolite' (Na₃AlF₆), into which a powder of aluminium oxide (Al₂O₃) is fed and becomes dissolved to form a solution. Aluminium fluoride (AlF₃) is added to maintain the target bath chemistry. Large carbon blocks, made from calcined petroleum coke and liquid coal tar pitch, are suspended in the solution; and serve as the positive electrode or anode.

The electrical current passes from the carbon anodes via the bath, containing alumina in solution, to the carbon cathode cell lining. The current then passes to the anode of the next pot in series. As the electrical current passes through the solution, the aluminium oxide is dissociated into molten aluminium (Al) and oxygen (O₂). The oxygen consumes the carbon (C) in the anode blocks to form carbon dioxide (CO₂), which is released. The electrolytic reaction can be expressed as follows: 2 Al₂O₃ + 3 C → 4 Al + 3 CO₂

The hot, molten, metallic aluminium obtained in the process sinks to the bottom of the reduction cell, while the gaseous by-products form at the top of the cell. The aluminium is siphoned from the bottom of the cell in a process called tapping (done by rotation every 32 hours), and transported to dedicated casting operations where it is alloyed; then cast into ingots, billets and other products.

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In addition to carbon dioxide, the aluminium smelting process also emits hydrogen fluoride (HF) - an extremely toxic gaseous emission. Fume treatment plants (FTPs) are used to capture the hydrogen fluoride and recycle it as aluminium fluoride for use in the smelting process. During abnormal smelting conditions, known as anode effects, perfluorocarbon (PFC) gases are emitted. Two PFC compounds are released during anode effects, namely tetrafluoromethane (CF₄) and hexafluoroethane (C₂F₆), which have greenhouse gas warming potential of 6,500 and 9,200 times greater than CO₂ respectively.

The aluminium smelting process is extremely energy intensive, which is why most primary aluminium smelters are located where there is ready access to abundant energy/power resources. It is also a continuous process: a smelter cannot be stopped and restarted easily. To the contrary, if production is interrupted by a power outage for more than four hours, the molten aluminium in the cells will solidify. This is because metallic aluminium is formed at 900°C but, once formed, has a melting point of only 660°C. When cells 'freeze' in this way, the only recourse for recovery is to rebuild the smelter.

Aluminium smelting at DUBAL

DUBAL has 1,573 reduction cells arranged in eight potlines, giving an installed annual production capacity of 990,000 metric tonnes of molten aluminium (our annual production is approximately one million metric tonnes). The metal purity obtained is very high, averaging 99.80 per cent aluminium, with our high purity aluminium products containing more than 99.98 per cent aluminium.

On the environment front, very strict performance standards are applied to DUBAL's HF emissions. We are committed to meeting the International Aluminium Institute (IAI) Sustainable Development Initiative (SDI) target to reduce HF emissions to 0.5
kg/t aluminium. Our high-efficiency FTPs help contain our smelter's HF emission levels by capturing and treating more than 98 per cent of the fluoride emissions. Fluoride that is not captured escapes directly through roof vents in the potlines. Roof HF levels are closely monitored in the potlines, enabling rapid identification of any deviations and pro-active changes to the production process. A small amount of fluoride also escapes from the anode baking kilns, via the FTP stacks. Currently, a 38 per cent decrease in total fluoride emissions has been achieved against the 2000 benchmark.

DUBAL is also committed to meeting the IAI SDI target of reducing PFC emissions by 80 per cent by 2010 and 93 per cent by 2010 (from 1990 levels). By the end of 2008, a 77 per cent reduction had been achieved (confirmed by an IAI audit).

Driven by a quest for continuous improvement and ongoing innovation, DUBAL has over the past 25 years developed advanced reduction cell technologies that not only increase productivity but also reduce our operations' impact on the environment through improved energy efficiency and reduced emission levels. This has culminated most recently in DUBAL's proprietary DX Reduction Technology- a state-of-the-art technology performing among the most efficient reduction cell technologies currently available.

Operating stably at 375 kA and above, DX Reduction Technology cells offer several benchmark attributes that provide significant environmental advantages, notably:

• An energy-efficient design that enables specific energy consumption of 13.14 kWh/kg Al and 95.2 per cent current efficiency, together contributing to energy conservation and associated operating cost reductions
• Reduced environmental impact through lower fossil fuel consumption (a direct benefit of enhanced energy-efficiency) and reduced carbon consumption (anodes) of less than 0.408 kg C/kg Al. Moreover, DX Technology cells experience minimal anode effects (0.01 AE/pot/day), resulting in perfluorocarbon emissions of less than 0.01 mt CO_2eq/mt Al - a world-leading benchmark.

Complementary and auxiliary facilities

Several key operational areas are required to support the smelter operations, namely:

• Raw material handling facilities.
• A carbon plant to produce the anodes required for the electrolytic reduction process.
• Casting operations to manufacture finished aluminium products from the hot, molten aluminium.
• A power plant to produce the electricity required for the electrolytic smelting process (DUBAL has a captive power station, the waste heat of which is used to produce potable water through a desalination plant).

The production processes at DUBAL

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From operations to technology, environment protection to employee relations, DUBAL continually strives for excellence by benchmarking against the world's best.

External resources used to compile this page:
International Aluminium Institute (http://www.world-aluminium.org)
Rio Tinto Alcan (http://www.riotintoalcan.com)