written by: Willie Scott
• edited by: Lamar Stonecypher
• updated: 11/5/2011
Aluminum is one of the most used metals in today’s industry, having properties of strength, durability, conductivity, lightness, and corrosion resistance. Alumina, processed from the mined bauxite ore, is fed into a cell where it is mixed with cryolite, through which a DC electric current is passed.
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Bauxite is extracted through open cast mining and converted to alumina, usually close to the mine. The alumina powder is shipped to aluminum smelters worldwide where it is smelted and cast into aluminum billets, blocks, and ingots before being cut to manageable sizes using large circular saws. The aluminum blocks are then palleted for distribution to various aluminum extrusion and component manufacturing specialists.
This is another article in my series on metallurgy. Here we will examine the mining of bauxite, its processing to alumina, and then follow the smelting and casting of the molten aluminum.
We will begin with a look at the mining of bauxite and its processing to alumina powder.
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Bauxite Mining and Conversion to Alumina Powder
Where is bauxite found?
The main bauxite ore producers are Russia, Australia, and Brazil, with the ore being normally obtained from open cast mining relatively shallow strata up to six meters below ground. Workers engaged in this type of mining should always wear protective clothing as the dust from bauxite mine and marl (calcium carbonate or lime) quarrying contribute to air pollution in the form of small irritant particles.
How is bauxite processed to alumina?
The extraction of aluminum powder from bauxite is carried out using the Bayer process. This entails the washing and crushing of the ore before adding it to a vessel containing caustic soda and lime where steam is injected into the resultant liquor. This liquor is filtered before being dried and calcified in a rotary kiln from which it exits as alumina powder.
The alumina is then shipped to the various smelters where it is stored in large silos, ready for conveyance to the cells.
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Aluminum Smelting in Furnaces
The cells used to reduce the bauxite powder to aluminum are supplied by the aluminum smelting pot manufacturer Hall-Heroult. This cell consists of a rectangular steel box insulated with fire bricks along the bottom the sides. Carbon blocks containing conductor rods are attached to the bottom brick lining, with the rods protruding from the cell structure. The sides of the cell are lined with carbon on top of the firebricks.
Square anode blocks constructed from compressed petroleum coke and coal tar are fixed to rods and suspended from two beam-like bus-bars attached to the cell structure, which as well as supplying electric current can lower or raise the anode blocks. The alumina is provided to the cell through an ore bin located above the cell, and a portable fume extraction hood covers the cell, with the fumes being scrubbed before being emitted from high brick chimneys.
Anode blocks are held in place on the bus-bars by clamps, which enable them to be replaced as they get worn away in the reduction process.
An example of a cell is shown below.
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Alumina Electrolytic Process
In this process the oxygen in the alumina reacts with the carbon anodes and cathodes forming CO2 and aluminum.
The alumina is conveyed from the storage silos and fed into an ore bin centrally located over the cell. The bin then supplies the cell with alumina as required.
Cryolite powder (sodium fluoride) and alumina are loaded into the cell. A low voltage, high amperage DC electrical current is applied to the anode bus bar from where it is transmitted to the anode block, while the aluminum smelting DC bus temperature monitoring is carried out in the control room. The current passes from the anode block, through the mix of cryolite and alumina into the cathode, and in the process the alumina is reduced, becoming molten aluminum. The molten aluminum drops to the bottom of the cell from where it is removed by a siphon into a crucible and transported in its molten state to the cast house to undergo the next process.
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Casting of Aluminum
The molten aluminum in the crucible is poured into a reverberatory furnace, usually located in the cast house section of the smelter. This is a simple furnace consisting of a steel box lined with firebricks internally and externally, having two or three gas burners inset to the side walls. There is a loading door on the front, which can be lowered and raised as required and a tapping point at the rear to allow the metal to be supplied to the casting equipment.
The furnace temperature is held a little above 600C while samples of the molten aluminum are taken to evaluate the metal's properties. Any necessary measures are taken to remove impurities.
Once the required purity is achieved, the furnace is tapped and the molten aluminum cast into ingots, billet, or rectangular blocks depending on the clients requirements.
The billets and blocks are then cut to standard sizes, stamped with the cast details, and palleted for transportation to the various aluminum extruding facilities.
An example of a reverberatory furnace is shown below.