Refining
Bauxite Refining
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Bayer Process
Bayer Process
Bauxite Refining
Bayer Process
The Bayer Process, a thermo-chemical digestion method invented in 1887 by Austrian scientist Karl Josef Bayer, is the most economic means of obtaining alumina from bauxite.
Ore is delivered to the refinery from the mine.
The bauxite is washed and crushed to increase its surface area.
A hot caustic soda (NaOH) solution is used to dissolve the aluminium-bearing minerals in the bauxite.
Undissolved solids are removed from the “pregnant” liquor as “bauxite residue”. NaOH is recycled to the digestion process.
Alumina is recovered by crystallisation from the pregnant liquor, seeded by the addition of alumina particles.
The spent liquor is heated and subsequently cooled in a series of flash tanks. The remaining caustic soda is washed and recycled back into the digestion process.
The hydrated alumina is roasted at temperatures of up to 1100°C to drive off free moisture and chemically-connected water, producing alumina solids
The alumina is shipped to smelters to produce aluminium.
Smelting
Hall-Héroult
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Hall-Héroult Process
Hall-Héroult Process
Hall-Héroult
Smelting Process
The Hall-Héroult process, the industrial method for the smelting of primary aluminium, involves passing a large electric current through a molten mixture of cryolite, alumina and aluminium fluoride to obtain liquid aluminium metal.
The cell in which the electrochemical reduction process takes place. Ranks of pots are connected in series in a smelter potline.
The electrolytic solution, or “bath”, is made up of molten cryolite and aluminium fluoride, maintained at a temperature of around
960 – 980°C.
960 – 980°C.
Alumina is fed into the pot, in the majority of smelters via a system that injects the powder directly into the bath.
An electric current, at an amperage of up to 600 kA, is passed through the bath. The reduction process requires significant energy, generated in power stations.
Large carbon blocks form the positive electrode in the cell, conducting electricity and reacting with the oxygen in aluminium oxide (alumina) to produce liquid aluminium.
The pot is lined with carbon, which forms the negative electrode. The electrical current passes from the carbon anodes via the bath to the cathode cell lining.
Liquid aluminium is deposited at the bottom of the cell and periodically “tapped” for casting into solid metal products.
Processing
Aluminium
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Rolling & Casting Processes
Rolling & Casting Processes
Aluminium
Semi-Fabrication
The benefits of aluminium, its low density, strength, ductility and corrosion resistance are realised through a number of processing methods, to produce semi-finished products, that will become window frames, electric cables, car parts, cans and foils.
Cast aluminium billets are heated to approx. 500°C and forced through a steel die. As the extruded profile emerges from the die it is cooled, before being cut to the desired length.
Liquid aluminium, at a temperature of over 700°C is poured into moulds. The moulds are cooled and the aluminium solidified before being packed and shipped to the customer.
Molten aluminium is poured or injected under pressure into a mould, which can be made of metal, sand, resin, plaster, graphite or even wax – some have destructible cores to form internal cavities. The aluminium cools in situ and the mould is removed or broken to retrieve the casting.
Cast aluminium slabs are heated to approximately 525°C and passed through a sequence of rollers until either the required plate thickness is obtained or until the metal is thin enough (3 mm) for cold rolling. Cold rolling can produce sheets as thin as 0.006 mm.
