Hall-Héroult process - from alumina to aluminium

The Hall-Héroult process is a electrolytic process to extract aluminium from alumina consuming a lot of electricity. It involves the movement of electrons to drive the reduction of aluminium ions and the oxidation of oxygen ions. The process takes place in a large electrolytic cell, often referred to as a reduction cell or pot, which is deep, rectangular steel shell lined with carbon.

Reduction Cell 

The core of the reduction cell structure has anode and cathode, as well as electrolyte between the two:

Cathode (Negative Electrode): The carbon lining at the bottom and sides of the reduction cell acts as the cathode.

Anode (Positive Electrode): Carbon blocks suspended in the electrolyte serve as the anode.

Electrolyte: The electrolyte is primarily a compound of sodium, aluminium, and fluorine called cryolite Na3AlF6, which dissolves the alumina (Al2O3). 

Below are pictures of reduction cells. The carbon anodes are suspended in the electrolyte, and carbon cathode/lining is on the bottom and sides, with a molten electrolyte bath in the middle. Alumina is fed from the top. A crust is gradually formed on the top surface of the electrolyte. 

Dissociation of Alumina

When alumina is dissolved in the electrolyte (molten cryolite), it dissociates into aluminium ions (+3 positively charged) and oxide ions (-2 negatively charged), i.e. it breaks the bond between Al2 and O3 in the Al2O3 compound.

    Al2O3 -> 2Al(+3) + 3O(-2)

After the bond is broken, these ions Al(+3) and O(-2) are free to move within the molten electrolyte. The molten electrolyte is also called electrolytic bath.

Flow of Electric Current

A direct current (DC) is applied across the cell through the molten electrolyte. This current is crucial for driving the electrolytic reactions.

Electrons flow from the power source into the cathode and out through the anode.

The cathode attracts aluminium ions (+3), and the anode attracts oxide ions (-2).

Reduction at the Cathode (Negative Electrode)

Electron Gain: Aluminium ions Al(+3) in the electrolyte migrate towards the cathode because cathode attracts positively charged ions.

Reduction Reaction: At the cathode, each aluminium ion gains three electrons (reduction) to form aluminium metal (metallic aluminium). 

    Al(+3) + 3e(-1) -> Al

Result: The aluminium metal is formed in a molten state and collects at the bottom of the cell due to its density.

Oxidation at the Anode (Positive Electrode)

Electron Loss: Oxide ions O(-2) in the electrolyte are attracted to the anode because anode attracts negatively charged ions.

Oxidation Reaction: At the anode, each oxide ion loses two electrons (oxidation) to form oxygen gas. This oxygen further reacts with the carbon anode to produce carbon dioxide:

    2O(-2) -> O2 + 4e(-1)      

    2O2 + C -> CO2

Result: Carbon dioxide gas is released at the anode, and the carbon anode gradually wears down and needs to be replaced periodically, because the carbon is consumed to react with oxygen. Note, at the cathode, carbon is not consumed.

Overall Reaction

Combining the reduction and oxidation, the overall reaction in the electrolytic cell can be summarized as:

    2Al2O3 + 3C -> 4Al + 3CO2

Role of Electrons in the Reaction

At the Cathode: Electrons are supplied to the aluminium ions, reducing them to metallic aluminium.

At the Anode: Electrons are removed from the oxide ions, allowing them to form oxygen gas, which then reacts with carbon to form carbon dioxide.

Energy and Efficiency

The process is energy-intensive because a large amount of electrical energy is needed to drive these reactions. This energy overcomes the strong bonds in alumina and powers the electron transfer that is essential for the reduction and oxidation processes. This electrolytic process is the core of aluminium production, with the flow of electrons facilitating the transformation of alumina into pure aluminium metal.

Practical Operation details 

From  Britannica https://www.britannica.com/technology/aluminum-processing/Smelting

By means of carbon anodes (suspended in electrolyte, and hanged on a rod), strong direct current  (DC) is passed through the electrolyte to a carbon cathode lining at the bottom of the cell. 

Feed alumina

Initially Alumina  is added directly into the electrolyte bath. Gradually a crust forms on the surface of the molten electrolyte  bath (will explain how it forms later).  Alumina is then added on top of this crust, where it is preheated by the heat from the cell (about 950 °C) and its adsorbed moisture driven off. Periodically the crust is broken by crust breaker, and the alumina is fed into the bath. Again, note that initially in new cells, the alumina is fed directly into the molten bath by means of automated feeders, it's only after the crust is formed that alumina is fed onto the top of the crust.

Consumption of carbon and alumina

The results of electrolysis as described earlier are the deposition of molten aluminium on the bottom of the cell and the evolution of carbon dioxide on the carbon anode. About 450 grams of carbon are consumed for every kilogram of aluminium produced. About 2 kg of alumina are consumed for each kilogram of aluminium produced.

    450g of carbon, 2kg alumina -> 1 kg aluminium

The smelting process is continuous. Additional alumina is added to the bath periodically to replace that consumed by reduction. Heat generated by the electric current maintains the bath in a molten condition so that fresh alumina dissolves. Periodically, molten aluminium is siphoned off from the bottom of the cell.

Refill aluminium fluoride

Some fluoride ions F(-1) from the cryolite electrolyte is lost in the process due to evaporation at high temperature and side reaction, therefore aluminium fluoride (AlF3) is added, as needed, to restore the chemical composition of the bath. A bath with an excess of aluminium fluoride provides maximum efficiency. Maintaining the required concentration of fluoride ions are crucial for reducing the melting point of alumina and ensuring the electrolyte's conductivity .

Energy  ~13 kwh / 1kg aluminium

In actual practice, long rows of reduction pots, called potlines, are electrically connected in series. Normal voltages for pots range from four to six volts, and current loads range from 30,000 to 300,000 amperes. From 50 to 250 pots may form a single potline with a total line voltage of more than 1,000 volts. Power is one of the most costly ingredients of aluminium. Technological advances have reduced the amount of electrical energy necessary to produce 1 kg of aluminium from19 kwh in 1940 to 13 kwh in 1990 for the most efficient cells.

Outcome  ~99.8% pure aluminium

Molten aluminium is siphoned from the cells into large crucibles. From there the metal may be poured directly into moulds to produce foundry ingot, it may be transferred to holding furnaces for further refining or for alloying with other metals, or both, to form fabricating ingot. As it comes from the cell, primary aluminum is about 99.8% pure.

Automation and computer control have had a marked effect on smelter operations. The most modern reduction facilities use fully mechanized carbon plants and computer control for monitoring and automating potline operations.

How crust is formed?

The formation of the crust in the Hall-Héroult process is a gradual process that occurs as the electrolytic cell operates. 

1. Initial Setup

When the process starts, the cell is filled with the molten electrolyte, primarily cryolite, and the temperature is maintained around 950°C. At this stage, there is no crust—just the liquid electrolyte and alumina added to it.

2. Introduction of Alumina

Alumina is gradually added to the molten cryolite bath. This alumina does not instantly dissolve completely into the electrolyte. The rate of dissolution depends on the temperature, agitation, and concentration in the bath.

Some of the alumina remains suspended on the surface of the electrolyte for a while before dissolving, while other particles may not dissolve immediately due to local saturation or lower temperatures near the surface.

3. Solidification at the Surface

Cooling at the Surface: The surface of the molten electrolyte is exposed to the air, causing it to cool slightly, especially if the cell is not perfectly insulated. This slight cooling causes some of the cryolite and alumina at the surface to begin solidifying.

Crystallization: As the temperature at the surface drops slightly, some of the cryolite and undissolved alumina begin to crystallize and solidify, forming a crust.

4. Crust Formation and Growth

Gradual Build-Up: Over time, as more alumina is added and the surface continues to cool, the crust thickens. The crust is mainly composed of solidified cryolite mixed with undissolved alumina and other impurities.

Periodic Disruption: Operators or automated systems periodically break the crust to add more alumina to the bath. Each time the crust is broken, the process of cooling and solidification starts again, leading to a dynamic crust that continually forms, breaks, and reforms.

5. Function of the Crust

Insulation: The crust helps to insulate the molten electrolyte, reducing heat loss and helping to maintain the high temperature needed for the process.

Protective Layer: The crust also acts as a protective layer, minimizing the exposure of the molten bath to the atmosphere, which reduces oxidation and contamination of the electrolyte.

6. Steady-State Operation

Equilibrium: Once the cell reaches steady-state operation, a balance is established between the formation of the crust, its periodic breaking, and the addition of fresh alumina. The crust is thus a natural by-product of the operation and an integral part of the overall process.

This crust formation process is crucial to maintaining the efficiency and stability of the electrolytic cell during aluminium production.