The Hall-Héroult process, developed in 1886 independently by American Charles Martin Hall and Frenchman Paul Héroult, is the sole industrial method for the smelting of primary aluminium.
It involves passing a large electric current through a molten mixture of cryolite, alumina and aluminium fluoride to obtain pure, liquid aluminium metal.
Direct current (DC), at an amperage of up to 600 kA, is fed into a line of electrolytic cells connected in series (a “potline”). While these cells, or “pots” and potlines vary in size and configuration from one plant to another, the fundamental electrochemical process is identical.
Each pot is a large carbon-lined metal container, forming the negative electrode (cathode) in the cell. Typically a cathode will last between 1000 and 2500 days before it needs replacing.
The cell contains an electrolytic bath of molten cryolite (Na3AlF6), maintained at a temperature of around 960 – 980°C, in which alumina powder (Al2O3) is dissolved. Aluminium fluoride (AlF3) is added to the solution to maintain optimal chemistry and lower the electrolyte’s freezing point. Large carbon blocks are suspended in the solution and serve as the positive electrode (anode).
The electrical current passes from the carbon anodes via the bath 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 dissolved alumina is split into molten aluminium (Al) and oxygen (O2). The oxygen consumes the carbon in the anode blocks to form carbon dioxide. The electrolytic reaction can be expressed as:
2Al2O3 + 3C → 4Al + 3CO2
The high reactivity of aluminium atoms means that significant energy, in the form of electricity, is required to split alumina into its constituent elements. However, the reactivity of aluminium is also the physical property that gives the metal many of its unique qualities, which are put to use in its final products – heat conductivity in cans and cookware; electrical conductivity in cables; durability in window frames; light weight and strength in transport applications.
The molten metallic aluminium sinks to the bottom of the cell, while the gaseous by-products form at the top.
The aluminium is siphoned from the pot in a process called tapping, done by rotation every day or so, and transported to dedicated casting operations where it can be alloyed and cast into ingots, billets and other products.
In addition to carbon dioxide, the aluminium smelting process produces hydrogen fluoride. Fume treatment plants are used to capture this gas and recycle it as aluminium fluoride, for use in the smelting process. These plants are also used to remove other gaseous by-products such as sulphur dioxide, thus reducing emissions to the environment.
There are two main types of aluminium smelting technology: Søderberg and prebake. The principal difference between the two is the type of anode used.
Søderberg technology uses a continuous anode which is delivered to the pot in the form of a paste, and which bakes in the cell itself.
Prebake technology uses multiple anodes in each cell, which are baked in a separate facility and attached to rods that suspend the anodes in the cell. New anodes are exchanged for spent anodes, with the remaining anode “butts” being recycled into new anodes. Most primary aluminium production facilities use a variant of prebake technology called point fed prebake, which uses multiple in-cell feeders and other computerised controls for precise addition of alumina to the pot, which improves energy efficiency while reducing emissions, dust and raw material use.
“Electrolysis is where it all comes together – technology, people, energy and alumina, to produce a metal that will be used and recycled and used again, almost infinitely.”