Detailed Description of a Cell and its Basic Functioning
Even though the basic elements in the aluminum production are still the same invented by Hall and Héroult more than 100 years ago, a modern industrial electrolysis cell is different from the first pots of the 19th century.
First of all, the size of the cells is changed to accommodate for the much larger current intensities used nowadays. Secondly, in a modern pot we find a series of elements used to reduce the energy consumption and the gas emissions.
The alumina reduction occurs in a vessel which is made of several parts designed altogether to:
- Act as a container for the molten bath and aluminum
- Resist to the high temperatures (around 950°C) of the molten liquids it contains
- Resist to chemical attacks brought especially by the molten electrolyte constituents
- Resist to wearing caused by alumina abrasive behavior
- Reduce heat losses to a technical and economical optimal minimum
- Be mechanically enough resistant, but also with sufficient elasticity in order to accommodate for the thermal and physical expansion of the materials it contains
- Collect the electrical current coming from the anodes with a minimum voltage drop
To achieve all these properties the vessel is made with a combination of different materials. First, we find an outer steel container, usually referred as potshell, which contains all the other elements of the cell. On the potshell bottom are deposited some layers of thermally insulating bricks with the aim of reducing heat losses from the bath. Above this, we find a refractory bricks layer which are very resistant to prolonged periods of exposition to the cell high temperatures. The carbon blocks that physically constitute the container of the molten aluminum and electrolyte stay above these bricks layers. The bottom blocks are called cathodes, even though electrochemically speaking is the metal pool which acts as cathode. They also collect the current exiting from the metal pad. The container sides are made with other carbon blocks. Cathodes and side blocks are joined together with a mix of pitch and carbon dust that is pressed inside the joints between them. At the bottom of every cathode we find a slot that accommodates an iron bar, called collector bar, with the purpose to transport outside of the cell the current collected by the cathode. Cathode and collector bar are joined together filling the space between them with molten cast iron, which subsequently freezes bonding together the parts.
The vessel described above contains, as said before, the molten aluminum and electrolyte. Due to the different densities, the molten bath stays above the molten metal, as the oil stays on the top of the water to give a practical example. In the molten bath occur all the chemical reactions for the alumina reduction. This reactions are driven by the electrical current transported inside the bath by the carbon anodes partially immersed into the molten bath.
The term “anode” refers not only to the carbon block. More in general an anode is made by a rod, a yoke and a series of stubs (1 to 6 generally) partially housed in rounded cavities obtained in the carbon blocks. Anode rods are made with copper or aluminum while yoke and stubs are made with iron. The carbon part of the anode is joined together with the metallic part of the anode assembly pouring molten cast iron in the space between the anode cavity and the rod. The molten cast iron freezes joining together the carbon block and the metallic part. In a modern pot we can find up to 40 anodes and because the carbon of the anodes participates to the chemical reactions, hence being consumed, they need to be replaced on a regular basis.
All the anodes of a cell are fixed to an aluminum structure, called “bridge”. The bridge transports the electrical current to the anodes and is also equipped with an electrical motor and a series of levers in order to raise or lower all the anodes of a cell. In this way it is possible to control the voltage at which the pot is operating.
Because of the high working temperatures of the anodes, on the top of them is put a protective layer of material made of a mix of crushed bath and alumina, in order to avoid their burning to the air. This protective layer is called “crust”.
The CO2 formed by the reaction of the anodes with the alumina oxygen escapes from the bath as gas bubbles, while the aluminum, being no more bonded with the oxygen is deposited in the metal pad inventory, increasing its height as the production goes on.
As the aluminum production proceeds, the alumina dissolved into the bath is depleted, and needs to be restored on a regular basis. Each cell, hence, is equipped with an alumina bin with a feeding system which delivers alumina to the electrolyte.
This feeding system is made of a crust breaker, basically made with a steel rod operated with a pneumatic cylinder, which opens a hole on the crust, and a corresponding alumina feeder, which dumps a certain amount of alumina into the molten bath from the hole opened in the crust by the crust breaker. The bath is then restored in its alumina content. Depending on the pot size, each pot can have 1 to 6 crust breakers and alumina feeders. On some pot technologies, crust breaker and alumina feeder are integrated.
The feeding operations are usually controlled by a computer control system following some algorithms.
To collect the fumes escaping from the pot crust, due to the bath evaporation, the pots are completely closed with removable pot covers, which collect the gases coming from the bath and direct them towards the gas treatment center.