HallHéroult

The Hall-Héroult process, named after American chemist Charles Martin Hall and French engineer Paul Héroult, is the dominant industrial method for producing aluminum metal. It electrolytically reduces alumina (Al2O3) dissolved in molten cryolite (Na3AlF6) to yield aluminum. The process was developed independently by Hall and Héroult in 1886 and quickly commercialized, transforming the aluminum industry.

In operation, a bath of molten cryolite with dissolved alumina is contained in a carbon-lined electrolytic cell. Graphite electrodes serve as both anodes and the connection to the external power supply, while the molten aluminum collects at the bottom of the cell and is periodically tapped. The bath temperature is typically around 950 to 1000°C. Alumina ions migrate to the cathode where each Al3+ gains three electrons to form metallic aluminum, while oxide ions are discharged at the anode, reacting with the carbon to produce carbon dioxide (and sometimes carbon monoxide). The overall reaction is commonly represented as 2 Al2O3 + 3 C → 4 Al + 3 CO2. The process is energy-intensive, and modern cells are designed for high current efficiency and continuous operation.

Significance and challenges: Hall-Héroult smelting is the primary method for producing nearly all commercially manufactured aluminum, supporting industries from construction to transportation. It relies on large amounts of electricity, contributing to its environmental footprint. Efforts to reduce emissions include research into inert anodes that would evolve oxygen instead of CO2 and improvements in energy efficiency and recycling.