THE MAKING OF ALUMINIUM

Aluminium is obtained by the electrolysis of alumina using the process discovered simultaneously in 1886 in France by Paul Héroult and in the USA by Charles Hall.
Aluminium production is a continuous process, which extracts pure aluminium metal from alumina, the powdery white oxide of aluminium. The chemical equation for aluminium smelting is 2Al203 +3C = 4Al + 3C02.


WHAT IS A POTLINE

A potline is a long building or two buildings which contain a series of "pots", or large electrolytic cells, in which aluminium is made. Each pot is a large rectangular cell, lined with carbon blocks and insulating bricks. The pots are connected electrically in a series so that direct electric current flows through one pot, then on to the next and so on, to the end of the line. Inside the pot, alumina is dissolved in a "bath" of molten cryolite (sodium aluminium fluoride) and other materials. As the electric current is passed through the bath it generates the heat to keep the bath molten and causes the alumina to separate into two constituent elements, aluminium and oxygen.

Just under two tonnes of alumina and about half a tonne of carbon products are needed to make one tonne of aluminium.

A special feature of the pots at Tomago is that they have been designed to allow alumina to be added to the pot from feed hoppers mounted above each pot.

This means that the pot can be replenished with alumina without the pot hoods having to be opened, thus ensuring a highly effective collection of pot fumes.

These fumes are drawn off and treated in a "dry scrubber" - See Environmental Management - Emission Control - for information and illustrations.

Single Ladel Direct Tapping

Each pot is fitted with a microprocessor (a small computer) which continuously monitors the pot to maintain optimum operation conditions.
Each of the pot rooms has a number of special multi-purpose pot-tending machines used for tapping molten metal from the pot, for changing anodes, for replenishing alumina feed hoppers and other production operations.
The molten metal is regularly siphoned from the pots using a method called tapping (Single Ladle Direct Tapping here at Tomago).
The liquid metal is then sent to the Casthouse where impurities are removed and alloying elements such as magnesium, silicon or manganese may be added, depending on its final use.

CASTHOUSE

Molten metal is carried from the potline to the Casthouse in transport ladles on specially designed vehicles.
Overhead cranes are used to lift the ten tonne ladles and pour the molten aluminium into holding furnaces where it is mixed with specific amounts of alloy materials to produce special qualities and strength according to customer requirements.
The aluminium is then cast into slabs, billets or ingots. Some of the different aluminium processes are:

  • Casting: Pouring molten metal into moulds.
  • Forging: Hot metal is hammered or squeezed into the shape of a die, used extensively in highly stressed structural parts.
  • Drawing or Pressing: Forcing a flat metal blank into a die under press from a metal ram, mainly used to manufacture 'holloware'.
  • Spinning: A circular metal blank is rotated at high speed and pushed into the shape required by the pressure from a tool.
  • Extruding: Pushing the aluminium through a die to form longer tube, rod, or wire products.
  • Rolling: Squeezing the aluminium between two rollers to form a flat plate sheet or foil product at very fine gauges (as thin as .0006mm).

RODDED ANODES

Carbon anodes, made from petroleum coke and pitch, are manufactured on site. They are used to conduct electricity into the smelting cells/pots in the potroom.
Anodes are consumed in the smelting process and the remaining portions (known as butts) are recycled.
Petroleum coke is crushed, mixed with liquid pitch and vibrated into a rectangular block weighing more than 1400kg.
These anode blocks are baked in a natural gas fired baking furnace for several weeks to improve electrical conductivity characteristics.
Anodes are attached to rods and suspended into the electrolytic cells in the potroom where they are slowly consumed in the aluminium process. They are replaced on a rotating schedule about every few weeks.


Anode Production Line

PLANT SERVICES

The entire plant operates on a computerised planned maintenance system under which preventative maintenance and repairs to equipment are organised in order to reduce production delays, to maintain the equipment in good and efficient condition, and to concentrate all resources on maintenance efficiency.

This planned maintenance system involves strong teamwork among maintenance planners, maintenance foremen and tradesmen, and where necessary, engineering, in order to design problems out of the equipment. This teamwork encourages positive coordination with production personnel to ensure product quality and production commitments.