Explain the process of aluminium smelting with the help of flowchart - Social Science - Manufacturing Industries

Question

Explain the process of aluminium smelting with the help of
flowchart

Tejasri Kumaar · Accepted Answer

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.

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.