Process Steps, Conditions & Parameters

Step 1 — Alumina Feed Preparation

SGA (smelter-grade alumina) from the Bayer process is received, stored and fed continuously or semi-continuously to the electrolytic cells via point feeders (automated alumina dispensers). Alumina must meet strict specifications for particle size, surface area, α-Al₂O₃ content and moisture to ensure good dissolution and avoid crust formation.

Figure 1 — Schematic process flow of the Hall-Héroult Process (source)

Step 2 — Electrolytic Reduction (The Pot)

The core of the process is the electrolytic cell (pot) — a rectangular steel shell lined with carbon/graphite cathode blocks and refractory insulation. The molten bath fills the cell and is covered by a solidified crust of bath material and alumina. The carbon anodes (either Söderberg continuous self-baking anodes or prebaked carbon anodes) are suspended from an anode beam into the bath from above.

Figure 2 — Hall-Héroult electrolytic cell (source)

Direct current enters via the anodes, passes through the molten bath, and exits via the cathode lining at the base. Liquid aluminium, being denser than the bath, collects as a liquid metal pad on the cathode floor and is protected by it from re-oxidation.
 

Step 3 — Metal Tapping

Liquid aluminium (typically ~99.7% purity) is tapped (siphoned or vacuum-lifted) from the pot every 24–48 hours using a crucible-based vacuum tapping system. Metal is transferred in large crucibles to the cast house.

Step 4 — Anode Change

In prebaked anode (PBA) technology, individual carbon anode blocks (~1,000–1,500 kg each) are consumed over approximately 24–28 days and must be replaced regularly. Each pot carries 18–40 anode blocks simultaneously at different stages of consumption. Anode stubs (spent anode butts) are recovered and recycled in the anode baking facility.

In Söderberg technology, continuous anode paste is fed from above and bakes in situ using waste heat from the cell — eliminating the separate anode baking plant but producing more PAH (polycyclic aromatic hydrocarbon) emissions. Söderberg technology is being phased out globally.

Step 5 — Gas Treatment (Dry Scrubbing)

Each pot generates HF gas, CO₂, CO, SO₂ and PFC gases (CF₄ and C₂F₆ during anode effects). These are captured by a hooding and duct system and treated in a dry scrubbing plant where the gases pass through a bed of fresh alumina, which adsorbs the fluoride gases. The fluoride-laden alumina is then fed back to the pots as part of the regular feed — simultaneously treating the gas stream and recovering fluoride back into the process. Residual gases pass through bag filters before emission.

Step 6 — Casting

Liquid aluminium from tapping crucibles is transferred to the cast house, where it is:

• Held in holding/melting furnaces (gas-fired)
• Alloyed as required by adding silicon, magnesium, copper, manganese, etc.
• Degassed (nitrogen/argon purging or rotary degassing) to remove dissolved hydrogen
• Filtered through ceramic foam filters to remove inclusions
• Cast into standard product forms: T-bar ingots, sow ingots, rolling slabs, extrusion billets or wire rod via DC (direct chill) casting

Key Equipment & Devices

References

• Mandin P., Wüthrich R., & Roustan H. (2009). Industrial Aluminium Production: the Hall–Héroult Process Modelling. ECS Transactions, 19(26), 1–10. DOI: 10.1149/1.3247986
• Coursol P., Dufour G., Coté J., Chartrand P., & Mackey P. (Sep 25, 2012). Application of Thermodynamic Models for Better Understanding and Optimizing the Hall–Héroult Process. JOM, 64(11), 1326–1333. DOI: 10.1007/s11837-012-0426-x
• Sadighi S., Mohaddecy R.S., & Ameri Y.A. (2015). Artificial Neural Network Modeling and Optimization of Hall–Héroult Process for Aluminum Production. International Journal of Technology, 6(3), 480–489. DOI: 10.14716/ijtech.v6i3.887
• Wikipedia. Hall–Héroult process (Page version Dec 1, 2025)