Aluminium smelting

Aluminium smelting

Production of aluminium from alumina is based on the Hall-Heroult process, invented simultaneously and independently by Charles Hall (US) and Paul Heroult (France) in 1886. Aluminium smelting involves reduction of alumina to aluminium by electrolysing molten alumina in electrolytic cells (pots). The Hall-Heroult process is power intensive, and critically depends on a continuous supply of power. In case of an interruption in power supply, the pots could freeze and would require an elaborate process of restarting.

Typically, a smelter comprises potlines and potrooms (housing electrolytic cells/pots), gas cleaning facilities (scrubbers), cryolite re-generation facilities, and anode carbon plant. Electrolytic cells (pots) are rectangular steel shells lined with thermal insulation. Pots also have an inner lining made of carbon (which acts as cathode), and suspended anode blocks. The electrolytic bath, which dissolves alumina, consists of molten cryolite and aluminium fluoride (as additive).

Anodes are made from calcined petroleum coke, which is crushed and mixed with hard pitch in mixers heated by steam. The paste is pressed in hydraulic presses to the required shape and size, and baked in order to improve conductivity and strength. Coal tar pitch (a residual product from the steel industry) is required as a binder in the manufacture of anodes. Anodes are required to be replaced periodically, as carbon is consumed to form carbon dioxide and carbon monoxide. The spent anodes are recycled.

There are two types of technologies used in making anodes: Soderberg, and pre-bake technology. Soderberg technology uses a single and continuous anode, which is fed into the pot in the form of a paste through a suspended steel shell. The anode gets baked in place due to the heat generated in the pot during electrolysis.

In pre-bake technology, multiple anodes are used, which are prepared separately in an anode plant. Since the past 15-20 years, pre-bake technology has been used for all greenfield smelters and expansion projects. In addition, several Soderberg technology based smelters have been converted to pre-baked technology smelters.

The carbon lining of electrolytic cells acts as cathode. Cathodes are made by ramming a mixture of anthracite coal and 12 per cent soft pitch. Calcined anthracite coal is crushed and mixed with soft pitch in mixers heated by steam to form a paste. For monolithic type of linings, the paste is taken to potrooms in special boxes, and rammed into position with pneumatic reamers. In case of pre-baked carbon block linings, the paste is suitably pressed to form blocks, which are then baked and used for lining the electrolytic cells.

In the electrolysis process, alumina is dissolved in the electrolytic bath (a mixture of molten cryolite and aluminium fluoride). The cryolite bath is maintained at an operating temperature of 920-970 degree centigrade, and dissolves around 10 per cent of alumina. A high intensity direct current is passed through the mixture, resulting in the breaking up of alumina (an oxide of aluminium) into aluminium and oxygen ions. The positively-charged aluminium ions get deposited at the cathode in the form of molten metal, and negatively-charged oxygen ions react with anode carbon to form carbon dioxide and carbon monoxide. The molten metal is tapped from the electrolytics cells periodically and cast into ingots.

The most important operating parameters that have an impact over the product quality, rate of production, efficiency and costs are current rating and current density. 

In the Hall-Heroult process, the most important input is electrical energy. Energy accounts for around 25-35 per cent of the total cost of primary aluminium production. Energy efficiency of the smelting process is around 35-45 per cent, and largely depends on cell design, current rating, anode current density, thermal insulation, and other operating parameters. The most significant impact of technological development has been a continuous decline in the consumption norms for electricity, largely due to the increase in the intensity of the electric current supplied to the electrolytic cells. The current rating of pots increased from 4 kA in 1886 (when the electrolytic process was invented) to 300 kA in 2000. Power consumption has declined from 40,000 kWh per tonne in 1886, and 21,000 kWh in 1950, to 15,000 kWh in 2000 (in the new smelters). However, the rate of improvement in power consumption has declined. For instance, during the 1980-81 to 1999-2000 period, power consumption norms declined by 3 per cent.

Another crucial aspect of technological development in aluminium smelting technology is reduction in consumption norms for various raw materials, such as carbon (used in anodes and cathodes), cryolite and aluminium fluoride. Around 450 kg of carbon is consumed for every tonne of primary aluminium produced. Carbon consumption accounts for around 10 per cent of the cost of production. Improvements in consumption norms have been achieved largely through better control of process parameters (such as alumina concentration in bath, temperature, and voltage across the pots) and recycling of inputs (such as cryolite, and spent anodes and cathodes).