Table of Contents
Principles of the Hall-Héroult Process
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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.
Friedrich Wöhler produced 1827 pure aluminum by reacting metallic potassium (K) with anhydrous aluminum chloride (AlCl3):
AlCl3 +3K = Al + 3KCl
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).
1.1 Washington Monument.
Setting of the aluminium capstone on the top of the Washington Monument (1984).
In 1886 Hall of the USA and Héroult of France invented simultaneously and independently of each other
the process to produce aluminum by electrolysis.
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).
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.
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.
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).
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.
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.
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.
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.
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.
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.
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.
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
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.
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.
The automatic closed transport system conveys alumina from the plant silo to alumina hoppers which are placed between the anode beams.
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.
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