Production of Aluminum by Molten Salt Electrolysis

Overview of Aluminum Production

Aluminum is produced on an industrial scale almost exclusively by molten salt electrolysis of aluminum oxide (alumina) dissolved in a molten mixture of salts. This process is known as the Hall–Héroult process. It converts chemically bound aluminum in alumina into metallic aluminum using electrical energy at high temperature.

This chapter focuses on the chemistry and process principles of this molten salt electrolysis, not on the mining and refining steps that prepare alumina, and not on the general theory of electrolysis (covered elsewhere).

Raw Materials and Feedstocks

Alumina as the Aluminum Source

The immediate feedstock for aluminum electrolysis is alumina, $ \mathrm{Al_2O_3} $, a white, high–melting solid obtained from aluminum ores (chiefly bauxite) by chemical refining.

Key requirements for alumina used in electrolysis:

• High purity (low contents of $\mathrm{Fe_2O_3}$, $\mathrm{SiO_2}$, $\mathrm{TiO_2}$, alkali metals, and water)
• Controlled particle size and flow properties (for smooth feeding into cells)
• Low sodium content (to limit contamination of the metal and electrolyte)

Cryolite and Other Flux Components

Pure alumina melts at over $2000^\circ\mathrm{C}$, which would make direct melt electrolysis impractical. Instead, alumina is dissolved in a molten salt mixture based on cryolite, reducing the working temperature to about $950$–$980^\circ\mathrm{C}$.

• Cryolite: Natural mineral $\mathrm{Na_3AlF_6}$, used industrially as synthetic cryolite.
• Roles of cryolite:
• Solvent for alumina
• Melting point depressant (lowers the liquidus temperature of the electrolyte)
• Provider of sufficient electrical conductivity in the melt

Common additions to adjust the properties of the molten bath:

• $\mathrm{AlF_3}$ (aluminum fluoride): to adjust the cryolite ratio and lower melting temperature
• $\mathrm{CaF_2}$ (calcium fluoride): improves fluidity and conductivity
• Small quantities of $\mathrm{LiF}$ or other additives (in some modern cells) to optimize conductivity and energy efficiency

The cryolite ratio $R$ is often defined as
$$
R = \frac{n(\mathrm{NaF})}{n(\mathrm{AlF_3})}
$$
and is typically in the range 2.2–3.0 in industrial cells. Adjusting $R$ tunes melting point, viscosity, and solubility of alumina.

Electrolysis Cell Design and Operating Conditions

Basic Cell Configuration

The Hall–Héroult cell is essentially a large, rectangular electrolysis cell with:

• Cathode: the carbon-lined steel shell (bottom and sides). The carbon lining becomes the current-carrying cathodic surface on which aluminum is deposited.
• Anodes: thick carbon blocks suspended from above, dipping into the molten electrolyte. Traditionally, these are consumable (prebaked or Søderberg type).

Inside the cell:

• A bottom layer of molten aluminum (cathode product) with density about $2.3–2.4\ \mathrm{g\ cm^{-3}}$.
• Above it, a layer of molten cryolite-based electrolyte with density about $2.1\ \mathrm{g\ cm^{-3}}$.
• The denser aluminum stays at the bottom as a separate liquid layer, from which it is siphoned off periodically.

Operating Conditions

Typical industrial parameters (approximate ranges):

• Temperature of electrolyte: $950$–$980^\circ\mathrm{C}$ (varies with composition)
• Cell voltage: about $4$–$5\ \mathrm{V}$ (only a fraction is the thermodynamic decomposition voltage; the rest is due to ohmic and overvoltage losses)
• Current intensity: several hundred kA (e.g. $200$–$600\ \mathrm{kA}$) in modern cells
• Current density at anodes: a few $\mathrm{A\ cm^{-2}}$
• Electrolyte composition: cryolite with $2$–$5\%$ (by mass) dissolved alumina

The relatively low alumina concentration demands continuous or intermittent feeding of alumina to keep its concentration within a narrow operating window. Too low an alumina concentration leads to disturbances (see below).

Electrochemical Reactions in Molten Salt Electrolysis

Overall Stoichiometry

The net chemical transformation is the reduction of alumina to aluminum metal with simultaneous oxidation of carbon:

Overall cell reaction (using consumable carbon anodes):
$$
\mathrm{Al_2O_3 + 3C \rightarrow 2Al + 3CO}
$$
or, including $\mathrm{CO_2}$ formation:
$$
\mathrm{Al_2O_3 + 3/2\,C \rightarrow 2Al + 3/2\,CO_2}
$$
(In practice, both CO and $\mathrm{CO_2}$ form; the simplified overall stoichiometry is often written using $\mathrm{CO_2}$.)

Cathodic Reaction (Reduction of Aluminum Ions)

Alumina dissolves in molten cryolite, where it forms complex oxide and fluoride species; for the stoichiometric description, it is convenient to express the reaction as if $\mathrm{Al^{3+}}$ ions were present:

At the cathode (molten aluminum layer):
$$
\mathrm{Al^{3+} + 3e^- \rightarrow Al(l)}
$$
Thus, aluminum ions are reduced to metal, which joins the molten aluminum pool at the bottom of the cell.

Anodic Reaction (Oxidation of Oxide Ions and Carbon)

The oxide component of alumina is oxidized at the carbon anodes. Again in simplified ionic form:

At the anode:
$$
\mathrm{2O^{2-} \rightarrow O_2(g) + 4e^-}
$$

However, the anodes are made of carbon, which reacts with the nascent oxygen:

• Formation of carbon monoxide:
  $$
  \mathrm{C(s) + \frac{1}{2}O_2(g) \rightarrow CO(g)}
  $$
• Formation of carbon dioxide:
  $$
  \mathrm{C(s) + O_2(g) \rightarrow CO_2(g)}
  $$

Combining the cathodic and anodic half-reactions with carbon consumption yields the net process involving $\mathrm{Al_2O_3}$ and carbon as shown above.

Faraday’s Law and Aluminum Yield

Each $\mathrm{Al^{3+}}$ ion needs three electrons to be reduced. To calculate theoretical production from an electric charge $Q$:

• Charge per mole of electrons: Faraday constant $F \approx 96485\ \mathrm{C\ mol^{-1}}$
• For 1 mol of Al:
  $$
  Q_\text{theor} = 3F
  $$

The current efficiency is the ratio of actual aluminum produced to the theoretical amount determined by the passed charge and stoichiometry. Industrial cells achieve current efficiencies typically around $90$–$95\%$.

Role of the Molten Salt Electrolyte

Why Molten Salt and Not Aqueous Electrolysis?

Aluminum is very reactive and has a very negative standard electrode potential. In aqueous solution, water is reduced preferentially at the cathode instead of $\mathrm{Al^{3+}}$, so metallic aluminum cannot be deposited from water-based electrolytes.

A molten salt electrolyte based on cryolite provides:

• A medium where aluminum species can be present in an oxidized state and be reduced without competing water reduction.
• Ionic conductivity at elevated temperatures.
• A much lower operating temperature than pure molten alumina.

Solubility and Transport of Alumina

Alumina dissolves in molten cryolite forming fluoroaluminate and oxyfluoroaluminate species. While the detailed speciation is complex, the important process points are:

• Maintaining alumina concentration typically in the range of about 2–5% by mass.
• Providing sufficient solubility to ensure ion transport, but not so high as to cause precipitation or sludge.
• Preventing anode effects, which are gas evolution disturbances occurring when alumina concentration in the electrolyte becomes too low.

Anode Effect

When alumina concentration drops below a critical level:

• The oxide ion supply to the anode becomes insufficient.
• Instead of smooth gas evolution, a gas film (mainly $\mathrm{CF_4}$ and $\mathrm{C_2F_6}$ from anode–electrolyte reactions) forms under the anode.
• This gas film increases the electrical resistance dramatically, so the cell voltage spikes (anode effect).
• Anode effects produce greenhouse gases (perfluorocarbons) and reduce current efficiency.

Operators counteract anode effects by:

• Monitoring cell voltage and currents.
• Feeding alumina when the voltage rises in a characteristic way.
• Adjusting anode positions and bath composition.

Materials and Electrode Technology

Carbon Anodes

Most industrial cells use consumable carbon anodes:

• Made from calcined petroleum coke and coal tar pitch, baked at high temperature.
• Attached to anode rods and suspended in the electrolyte.
• Gradually consumed by oxidation to CO and $\mathrm{CO_2}$.

Two main technologies:

1. Prebaked anodes:
• Anodes are baked in separate furnaces, then mounted in the cell.
• When an anode is sufficiently consumed, it is replaced individually.
• Offers good control of anode quality and emissions.
2. Søderberg (self-baking) anodes:
• A continuous mass of carbon paste is fed, which bakes in situ by the cell heat.
• Simpler in equipment but more difficult to control emissions and anode quality.
• Less common in new smelters due to environmental considerations.

The consumption of carbon is directly related to the current passed and the overall stoichiometry. As the anode is consumed, it must be lowered or replaced to maintain the correct inter-electrode spacing.

Cathode Liner

The cathode consists of carbon blocks forming the cell bottom and sometimes lower sidewalls:

• Serves as the current collector and the surface on which molten aluminum rests.
• Must withstand high temperature, fluorides, molten aluminum, and thermal gradients.
• Gradually wears out due to chemical attack and mechanical stresses; after several years, cells require relining.

Some advanced designs explore inert or wettable cathodes (e.g. TiB₂-based materials) to:

• Reduce voltage drops.
• Modify cell hydrodynamics.
• Extend cell life and efficiency.

Inert Anodes (Concept)

Research and development aim to replace consumable carbon anodes with inert (non-consumable) anodes made of ceramics or metal-ceramic composites that are stable in the highly corrosive fluoride melt.

If completely inert anodes were used, the anodic reaction could become:
$$
\mathrm{2O^{2-} \rightarrow O_2(g) + 4e^-}
$$
and the net reaction:
$$
\mathrm{Al_2O_3 \rightarrow 2Al + \tfrac{3}{2}O_2}
$$
This would:

• Eliminate direct carbon consumption.
• Reduce direct $\mathrm{CO_2}$ and perfluorocarbon emissions.
• Change process economics and environmental footprint.

However, large-scale industrial implementation is challenging due to material stability and cost.

Process Control and Energy Considerations

Energy Use and Cell Efficiency

The Hall–Héroult process is energy-intensive. Energy is consumed in:

• Providing the decomposition energy to reduce $\mathrm{Al^{3+}}$ to Al.
• Overcoming internal resistances and overvoltages.
• Compensating for thermal losses to maintain the high temperature.

Key measures for energy efficiency:

• Optimizing electrolyte composition to reduce resistivity and operating temperature.
• Minimizing anode–cathode distance (without shorting) to reduce voltage drop in the bath.
• Improving thermal insulation of the cell.
• Using high-conductivity busbar systems to reduce electrical losses.

Heat balance is critical: the cell’s resistive heating must roughly match heat losses to keep the operating temperature stable.

Alumina Feeding and Bath Composition Control

Automated systems measure cell voltage and aluminum fluoride concentration to control:

• Alumina feeding:
• Small amounts are fed periodically or continuously from hoppers.
• Feeding events are often triggered by characteristic voltage changes.
• Bath composition (NaF/AlF₃ ratio):
• Adjusted by adding $\mathrm{AlF_3}$ or sometimes NaF.
• Ensures the desired melting temperature, viscosity, and alumina solubility.

Incorrect composition or feeding leads to problems such as:

• Anode effects (too little alumina).
• Sludging or crust formation (too much alumina or improper temperature).
• Increased energy consumption and reduced current efficiency.

Tapping and Metal Handling

Molten aluminum is periodically removed:

• Tapped by vacuum siphons or ladles from the aluminum pool at the bottom.
• Sent to holding furnaces for alloying and casting into ingots, slabs, or other forms.

Care is taken to avoid:

• Excessive disturbance of the electrolyte.
• Inclusion of bath material (cryolite) in the metal.
• Contamination by impurities.

Environmental and Safety Aspects

Emissions

Main emissions from aluminum electrolysis include:

• $\mathrm{CO_2}$ from carbon anode consumption.
• Fluoride-containing gases (e.g. HF, particulate cryolite dust).
• Perfluorocarbons (PFCs) such as $\mathrm{CF_4}$ and $\mathrm{C_2F_6}$ during anode effects.

Modern plants employ:

• Dry or wet scrubbing of off-gases using alumina, which absorbs HF and returns fluorides to the process.
• Tight control of anode effects to reduce PFC emissions.
• Enclosure and ventilation systems to protect workers and the environment.

Solid and Liquid Wastes

Wastes include:

• Spent pot linings (containing carbon, fluorides, and cyanides) that require specialized treatment and disposal or recycling.
• Dusts and sludge from gas cleaning systems.

Management strategies:

• Recovery of fluorides for reuse.
• Neutralization and secure landfilling where necessary.
• Development of recycling routes for spent pot lining components.

Workplace Safety

Safety considerations in molten salt electrolysis:

• Handling of hot, molten materials at ~1000°C.
• Risk of explosions if water contacts molten metal or electrolyte.
• Exposure to emissions if ventilation is inadequate.
• Electrical hazards due to high currents and busbar systems.

Protective measures:

• Strict procedures for tapping and maintenance.
• Personal protective equipment (PPE) for heat and chemical exposure.
• Continuous monitoring of gas concentrations and cell conditions.

Process Developments and Trends

Ongoing developments seek to improve the Hall–Héroult process and reduce its environmental footprint:

• Higher amperage cells with improved busbar design for reduced specific energy consumption.
• Advanced cell control systems using real-time monitoring and predictive models for alumina feeding and voltage control.
• Inert anode research and wettable cathodes to reduce carbon use and emissions.
• Integration with low-carbon electricity sources (hydro, wind, etc.) to lower the overall carbon footprint of aluminum production.
• Recycling of aluminum: although not an electrolysis process, extensive recycling reduces the need for primary (electrolytic) aluminum and drastically lowers energy demand per unit of metal.

These trends illustrate how understanding the chemistry and electrochemistry of molten salt aluminum production underpins technological advances and environmental improvements in this important industrial process.