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1. Principles of the Hall-Héroult Process

1.1 Summary

The principles of the Hall-Héroult process to produce aluminum in an electrolytic cell are shown. Alumina, an aluminum oxide (Al203), is dissolved in molten cryolite (Na3AlF6). It is decomposed electrolytic between carbon and aluminum electrodes at about 950° C to aluminum and oxygen. The carbon anode is continuously consumed by reacting with the oxygen to give carbon dioxide (CO2). The typical features of the electrolysis cells in an aluminum plant are described. Technical cells are operated with a current intensity of 100 to 500 kA. They are equipped with devices to operate the cells in a highly automatic i.e. computer controlled way. The waste gases of the process are collected and thoroughly cleaned (scrubbed) before being exhausted to the atmosphere.
The discussion of alternative methods to produce aluminum in technical quantities mentions the Direct Carbothermic Reduction Process and the Alcoa Smelting Process which were developed to replace the classical Hall-Héroult process. Technical problems which could not be solved stopped however these projects.
Inert anodes must sustain the chemical attack of the anodic oxygen and of the electrolyte at electrolysis temperature. Several candidate materials were investigated however without con-vincing success so far. Using materials which are wetted by aluminum as cathode would avoid several problems essentially the magnetic effects and the chemical reactions of the carbon lining. Several patents and publications propose materials and arrangements to replace the aluminum metal pad as cathode.

1.2 Aluminum a Noble Metal

Friedrich Wöhler produced 1827 pure aluminum by reacting metallic potassium (K) with anhydrous aluminum chloride (AlCl3):

AlCl3 +3K = Al + 3KCl

(1.1)

and later (1854) Henri Sainte-Claire Deville replaced the expensive potassium (K) by sodium (Na) to produce aluminum industrially. At that time aluminum was considered as a precious metal: the price of one ounce (28.3 g) of silver and aluminum was one dollar. Similar to the golden top of the Egyptian obelisks the Washinton Monument received 1884 an aluminum capstone (Figure 1.1).

Washington Monument

1.1 Washington Monument.

Setting of the aluminium capstone on the top of the Washington Monument (1984).

1.3 Production by Electrolysis

In 1886 Hall of the USA and Héroult of France invented simultaneously and independently of each other the process to produce aluminum by electrolysis.

Photographs Hall Héroult

1.2 Photographs of Hall and Héroult.

Paul-Louis-Toussaint Héroult and Charles Martin Hall Setting invented nearly simultaneously the process to produce aluminum from aluminum oxide (Al2O3) by electrolysis. Aluminum oxide (alumina) is dissolved in cryolite (Na3AlF6) and electrolysed using a carbon anode and an aluminum cathode.

Alumina, an oxide of aluminum (Al2O3), is dissolved in molten cryolite (Na3AlF6) and decomposed electrolytically to give liquid aluminum. The anode of the electrolytic cell is made of carbon and the pool of already produced aluminum acts as cathode. The oxygen of the alumina is discharged at the anode where it reacts with the carbon anode to produce carbon dioxide (CO2).

Principle Hall Héroult

1.3 Schema of a Hall-Héroult Electrolytic Cell.

Figure 1.3 shows schematically such a Hall-Héroult electrolysis. A steel shell which is lined with carbon blocks and thermal insulation material contains the liquid cryolite electrolyte and liquid aluminum. The process uses electrical energy to reduce electrolytically aluminum oxide and to keep the electrolyte at a temperature of about 950° C. During aluminum production the chemical reactions consume continuously alumina and anodes which must be added respectively replaced to the electrolytic cell. Aluminum and anode gases (in essence carbon dioxide and carbon monoxide) are produced and removed from the cell.

1.4 Industrial Production of Aluminum

In Figure 1.4 you see a photograph of an industrial electrolysis cell (AP30-cell in St. Jean de Maurienne, France, Rio Tinto Alcan former Péchiney) arranged side by side e.g. the long sides of the pots are facing each other.

AP30 StJeanDeMaurienne

1.4 The AP 30 Electrolytic Cell (Rio Tinto Alcan former Péchiney).

This figure shows the AP 30 electrolytic cell of Péchiney (now Rio Tinto Alcan). This cell is operatated at 300 kA and is equipped with 20 double anodes and five side raisers that carry the electric current to the anode beam (not shown).
See some more Modern Electrolysis Cells.

Figure 1.5 shows the slide show of the schematic cross section for such a technical cell.

1.5 Slide Show of the schematic Cross Section for an Industrial Electrolysis Cell.

Anodes

The carbon anodes are attached to the iron yoke and aluminum rod with the anode clamp to the anode beam. After an anode is consumed the anode clamp is opened and the consumed anode is replaced by a new one. A hoist motor changes the position of the anode beam to control in this way the distance between the electrolytic active bottom side of the anode and the upper surface of the metal, the anode cathode distance.

Pot Shell, Lining, Bath and Metal Pad

The steel pot shell contains the liquid phases namely the electrolyte and the molten aluminum pad. The pot shell is lined with carbon bottom blocks (cathode blocks), carbon side blocks and ramming paste. In addition thermal insulating bricks (refractory and insulating bricks) of the cell lining control the thermal loss through the bottom and side walls. A gas barrier normally made of steel protects the insulating lining against corrosive electrolyte components.
The electrolyte freezes on the cell sides to a layer, the side ledge. This layer protects the side and bottom carbon blocks against corrosion and erosion by the molten electrolyte and moving metal. Electrolytic bath freezes also on its upper surface to a solid top crust. On the top crust a layer of alumina or crushed solid bath is piled up. This layer controls the thermal loss of the upper surface of the electrolysis cell. It protects also the carbon anodes against the reaction with the ambient air. The steel shell is the container for the pot lining, the electrolytic bath and the produced aluminum. It is surrounded with reinforcing girders (cradles) to resist the forces of the thermal and chemical expansion of the lining materials.

Electric Current Flow

On its way through the electrolytic pot the electric current is flowing through the risers to the anode beam, then to the anodes, crosses the anode cathode distance and is conducted by the collector bars and the aluminum bus bars to the following electrolysis cell. These busbars transport the electric current from one electrolytic cell to the next one either around the pot shell and/or under it.

Hooding

The top side of the cell is closed by a hooding to collect the waste gases. The cell gas exhaust conducts these gases to a scrubbing unit. The scrubber mixes the waste gases with alumina to adsorb e.g. remove the gaseous and particulate fluoride components before the waste gas is exhausted to the atmosphere.

Alumina

Since the electrolytic process consumes alumina at a nearly constant rate alumina must be added to the electrolytic bath. In modern cells automatic transport and feeding systems have replaced the vehicles which went from one pot to the other, brought alumina from the silos to the pots and crushed the top crust to push alumina into the bath. In patents and the literature several systems are described to transport alumina from the plant silo to the pots and to add a known amount of alumina to the electrolyte. The tubes which transport alumina are worn by the moving material and on the other hand alumina tends to stick together in blind angles or block the tubing if the speed is too small. To overcome these problems the conventional fluidized bed was modified especially to decrease the amount of compressed air to transport alumina.

Dense Phase Conveying System

Two parallel pipes are transporting alumina (Figure 1 5) and compressed air. The compressed air enters into the alumina pipe through openings which are filled with porous material. Plates release the air pressure in such a way that about the same amount of air enters along the alumina transporting pipe. The result is a smooth motion of alumina.

Dense Phase Conveying System

1.6 Dense Phase Conveying System.

Alumina in the lower pipe is fluidized by the compressed air of the upper pipe through openings filled with porous material. In this way the Dense Phase Conveying System solves some of the problems of conventional fluidized bed systems like high comsumation of compressed air and compacting of the alumina powder.

Hyperdense System

This system (Figure 1.7) uses the capacity of alumina to become fluidized under low pressure. When alumina is fed to the bath and the alumina level decreases in the pot hopper pressure is released. Alumina is fluidized locally to fill the void which moves along the tube to the silo. Because of using low pressure the hyperdense system is not able to transport alumina uphill, a disadvantage for the modernization of old pot lines.

Hyper Dense System

1.7 Hyperdense System.

When Alumina flows from the hopper to the electrolytic bath alumina becomes fluidized because the pressure is reduced and air is sucked into the system.

Point Feeder

The automatic closed transport system conveys alumina from the plant silo to alumina hoppers which are placed between the anode beams.

Alumina Point Feeder

1.8 Alumina Pointfeeder with Metering Device.

A crustbreaker opens the top crust and a defined amount of alumina flows from the metering device into the electrolyte. The pocess control system adds alumina according to a sophisticared algorithm to keep tha alumina concentration in the bath at the wanted value.

To design such a transport system one has to take especially into account the abrasive power of alumina and its tendency to plug narrow tubes. At the pots several pneumatic air cylinders activate several crust breakers to keep holes open in the top crust. A dosing system adds alumina in defined quantities to the electrolyte where it is dissolved. The process control system activates the point feeders (crust breakers and dosing devices) according to a sophisticated algorithm to keep the alumina content of the electrolyte at the target value.
A critical component of the system is the dosing device which measures a defined amount of alumina in a reproducible way.