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USEPA, Profile of The Nonferrous Metals Industry. EPA 310 R 95 010. This document discusses uses, processes and pollution prevention opportunities associated with aluminum production.
Special equipment is used to cut one layer after another off the surface, with the rock then being transported elsewhere for further processing. However, there are places where aluminium ore has to be mined from deep underground which require underground mines to be built to get at it.
Aluminum is the most abundant metal in the earths crust and is a constituent of many minerals. However, the material which is highest in aluminum and most free from objectionable impurities is bauxite which, therefore, is practically the only source of the metal under present processes.
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Bauxite is the general name given to the hydrated oxides of aluminum. It contains varying amounts of combined water and several impurities of which ferric oxide and silica are usually predominant. It is essential that the Al2O3 be obtained substantially free from these impurities as otherwise the iron oxide and silica would be reduced and contaminate the metallic aluminum produced in the Hall Process.
The separation of the alumina from the impurities in the bauxite ore is usually accomplished by the Bayer Process. In this process the ground bauxite is leached with caustic soda to produce soluble sodium aluminate , leaving the impurities in the insoluble residue. The aluminate solution is then decomposed producing Al3 which is calcined to produce Al2O3. In the standard Bayer Process for refining bauxite to produce alumina, the dissolution of the alumina in the ore to form sodium aluminate is accomplished in autoclaves under high pressures varying from 70 to 200 pounds per square inch. However, some lateritic bauxite ores are readily soluble in caustic, making it possible for them to be leached, or digested, at atmospheric pressure.
This study deals with the treatment of one such lateritic bauxite ore. Batch laboratory tests showed that a high alumina extraction in caustic could be accomplished at a coarse grind, thus promising low grinding costs, and, of greater importance, a coarse sand residue which could be washed more efficiently, thus keeping soluble loss to only a trace. The optimum grind was found to be minus 3/16 with 51% plus 32 mesh. However, at this coarse grind and at the desirable pulp density for digestion of approximately 23% solids, the heavy specific gravity solids had a settling rate as high as 75 feet per minute. The problem resolved itself to finding some type of continuous mixer or agitator which would keep the coarse, fast settling solids in homogeneous suspension during the digestion operation so that all of the bauxite particles would be attacked by the caustic sodium aluminate liquor. In this way it would be possible to capitalize on the important advantages of the coarse grind. The flowsheet in this study illustrates only the following two steps in the modified Bayer Process: In this study the minus 1.5 bauxite ore is ground to minus 3/16 in a Steel Head Rod Mill operating in open circuit. The ore is ground wet with hot sodium aluminate liquor at a temperature of 110°C. The rod mill is equipped with water cooled bearings because of this high temperature.
The rod mill discharges to a Agitator equipped with a turbine type propeller. The agitator acts as a mixing tank to mix the rod mill discharge with caustic soda and sodium aluminate liquor ahead of the digestion circuit. The slurry is pumped from this agitator with a Sand Pump to the digesters for the dissolution of the alumina content of the ore. The slurry from each of these agitators is pumped to a series of nine 22 ft. diameter x 20 ft. deep Digestion Agitators, each of which is equipped with a 96 diameter turbine type propeller consuming 70 HP. Eight Agitators in each series are operational, with the ninth employed as a stand by. Each series of eight agitators handles 19,800 gallons per minute of slurry, and the reaction time is three hours.
The digestion process is accomplished in a series of several Agitators to prevent short circuiting. As the digestion process continues and the alumina is dissolved, the specific gravity of the solids is actually higher at the discharge point than it is when introduced to the circuit. The bauxite sodium aluminate slurry is maintained at a temperature of 107°C by means of steam coils around the inside periphery of the covered concrete tanks. The interconnecting piping between the agitators is arranged so that any one agitator acts as a standby and can be by passed for maintenance.
The overflow of the first spiral classifier joins the hydro classifier overflow, and these products flow to the fine red mud washing thickeners mentioned above. The sodium aluminate liquor which overflows the primary thickener is clarified by filtering and the filtrate is decomposed to aluminum hydroxide which is calcined to produce the final alumina product.
Results at this plant are excellent, enabling this plant to realize a high alumina extraction with a minimum soluble loss.
HCH ultrafine mill is professional and idea equipment for alumium ore processing.
Aluminum processing Aluminum processing Ores: Aluminum is the third most abundant element on Earths surface. Only oxygen and silicon are more common. Earths crust to a depth of 16 km contains 8 percent aluminum. Aluminum has a strong tendency to combine with other common elements and so rarely occurs in nature in the metallic form. Its compounds, however, are an important ...
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 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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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 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 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.
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 metals 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 exam...