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 collection of 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 less than 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 Aluminium 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”. (For information and illustrations, see Environmental Management - Emission Control).

Each pot is fitted with a microprocessor (a small computer) which continuously monitors the pot to maintain optimum operation conditions. Each pot room has a number of special multipurpose 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. The liquid metal is then sent to the Casthouse where impurities are removed and alloying elements such as magnesium, silicon or manganese are added according to its final use.

Casthouse

Molten metal is carried from the potline to the Casthouse in transport ladles on specially designed vehicles. Overhead cranes lift the ten-tonne ladles and pour the molten aluminium into holding furnaces. Here it is mixed with specific amounts of alloy materials to produce the prescribed chemical composition from the customer. Properties such as strength, corrosion resistance, and surface quality can be affected by chemical composition and other processes.

The aluminium is then cast into rolling slabs, extrusion billets, remelt ingots or tee shapes in the Casthouse.

Tomago Aluminium Casthouse supplies semi-finished products to customers who convert the shapes into finished products using a variety of processes. Some of these processes include:

• Casting: Pouring molten metal into moulds (eg wheel rims, cylinder heads)
• Forging: Hot metal is hammered or squeezed into the shape of a die, used extensively in highly stressed structural parts. (eg nuts, bolts)
• Drawing or Pressing: Forcing a flat metal blank into a die under press from a metal ram, mainly used to manufacture 'holloware' (eg cans, cookware)
• Spinning: A circular metal blank is rotated at high speed and pushed into the shape required by the pressure from a tool (eg pots, pans)
• Extruding: Pushing the aluminium through a die to form tube, rod, or complex shapes.(eg window frames, yacht masts)
• Rolling: Squeezing the aluminium between two rollers to form a flat plate sheet or foil product. Thickness can vary from thick plates (50mm) to very thin foil (as thin as .0006mm).

The Anode Production Line

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 pot room. 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 pot room where they are slowly consumed in the aluminium process. They are replaced on a rotating schedule every 3-4 weeks (every 72 shifts).

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 to its maximum potential 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.