Although aluminium can be produced by several methods, only one is commercially used. In the late 19th century, the Deville process, which involved a direct reaction of aluminium chloride with metallic sodium, was the main method of aluminium production. However, this process has now been replaced with an electrolytic process which is more economical.
What is Poltine?
A pot is a large rectangular cell lined with insulating bricks and carbon blocks. This lining forms the cathode. Typically, a potline is a long building or group of buildings that houses a series of large electrolytic cells where aluminium is made.
Aluminum Smelting Process using Potlines
Aluminium is now produced in modern smelters which help to reduce the final costs. The Chinese aluminium producer, Chinalco is the largest aluminium manufacturer in the world with 6.6 million tonnes of aluminium production recorded in 2020. Hongqiao, Rusal, and Xinfa also ranked among the largest aluminium producers globally.
Alumina is dissolved in deep, rectangular steel shells known as reduction pots which are striped with carbon. These pots are filled with a molten electrolyte which mostly comprises cryolite, a compound of fluorine, alumina, and sodium. Direct current, utilizing carbon anodes, is passed to the carbon cathode bottom cell lining through the electrolyte. Now, a crust will develop on the molten bath surface and alumina is added to this crust surface where it is preheated at a temperature of about 950°C (1,750°F) using heat from the cell. The moisture gets eliminated and periodically when the crust is broken, alumina is fed into the molten bath. Alumina is directly fed into the molten bath in newer cells through automated feeders.
The process of electrolysis results in molten aluminium deposition of the cell and the development of carbon dioxide on the carbon anode. For every kilogram (2.2 pounds) production of aluminium, there is a consumption of about 450 grams (1 pound) of carbon. You will need around 2 tonnes of alumina to produce around 1 tonne of aluminium. The aluminium smelting process using potlines is continuous. However, additional alumina is periodically added to the bath to replace the alumina that is consumed during the process. The electric current helps to generate heat which maintains the molten condition of the bath so that the additional alumina which is freshly added gets dissolved. However, during the process, the cryolite electrolyte loses some fluoride which is why it is essential to add fresh aluminium fluoride to restore the chemical composition of the molten bath.
Potline operation
In actual practice, potlines or long rows of aluminium reduction pots are connected in a series electrically. The current loads of these potlines range from 30,000 to 300,000 amperes and normal voltages range from four to six volts. A single potline is usually formed using 50 to 250 pots having a total line voltage of over 1,000 volts. Power is one of the most expensive ingredients for producing aluminium but with technological advances, the power requirement has been reduced to about 13 kilowatt-hours for efficient cells. Molten aluminium is siphoned into large crucibles from the cells. Now, the metal can be directly poured into molds to generate foundry ingot from where it may be transported to holding furnaces to further refine the metal or form alloy with other metals. The aluminium extracted is around 99.8% pure. Moreover, computer control and mechanized carbon plants have automated the outline operations.
Summing up
The process of producing aluminium is automated. Computer process systems now control the voltage in electrolysis cells by moving up and down the anodes. China ranks as the world's largest aluminium producer.
