The main minerals of gibbsite in bauxite are (al203.3h20), boehmite (al203.h20), and diaspore, which are regarded as the same composition of boehmite, but with density and difficulty. Pure aluminum, alumina (Al203) anhydrous oxide, containing 52.9% aluminum and 47.1% oxygen. Bauxite can be as hard and soft as stone or mud, and it may occur in compacted soil (including fragile, re cemented), (pisolites) pellets, or hollow, branched material (tubes). It can be light yellow, pink, yellow, red, or white, or any combination.
Type
Element (Minerals/Ores of)
Mineral Classification
Oxide
Chemical Formula
Al(OH)3 (Gibbsite), γ-AlO(OH) (boehmite), α-AlO(OH) (diaspore)
Streak
White (boehmite)
Mohs Hardness
6.5-7 (diaspore), 3.5 (boehmite)
Crystal System
Orthorhombic
Color
White, colorless, pale greyish brown; yellowish or reddish when impure. Bauxite ore is reddish-brown, white, tan, and tan-yellow.
Luster
Vitreous, pearly, adamantine
Fracture
Conchoidal (diaspora), Uneven (boehmite)
Description
Aluminum is the most abundant metal element in the Earth’s crust. Bauxite ore is the main source of aluminum and contains the aluminum minerals gibbsite, boehmite, and diaspore. Aluminum is used in the United States in packaging, transportation, and building. Because it is a mixture of minerals, bauxite itself is a rock, not a mineral. Bauxite is reddish-brown, white, tan, and tan-yellow. It is dull to earthy in luster and can look like clay or soil.
Relation to Mining
Because aluminum metal reacts with water and air to form powdery oxides and hydroxides, aluminum metal is never found in nature. Many common minerals, including feldspars, contain aluminum, but extracting the metal from most minerals is very energy-intensive, and expensive. Therefore, bauxite is the primary source of the world’s aluminum and it supplies 99% of metallic aluminum. It is also used in the production of synthetic corundum and aluminous refractories.
Excavate
Aluminum extraction takes place in three main stages – bauxite mining, ore recovery, and aluminum smelting.
Bauxite mining is by surface method (open pit mining), in which the topsoil cover is removed and bulldozed and scraped, and then the area is replanted and returned to the original state sometimes better than or converted to agricultural land use. The bottom layer of bauxite, if necessary, is broken by explosives before mining by endloaders, electric shovels, or hydraulic excavators. Sometimes bauxite is crushed, washed, some clay and sand removed, and then dried in a rotary kiln. Others may just be crushed or dried bauxite. The ore is then loaded into trucks, train cars, or put on conveyor belts, transported by ships or refineries.
In almost all commercial operations, alumina is extracted from bauxite by Bayer refining process. This process, discovered by Karl Bayer in 1888, consists of four stages.
(1) Digestion – fine grinding, in which the bauxite is divided into steam heating devices, fed called biogas digesters. Here, it is mixed, under pressure, with a hot solution of caustic soda. The bauxite and alumina silica react with sodium aluminosilicate soda precipitates formed from caustic soda solution Baijiu or green liquor.
(2) Clarification – where the green wine or alumina containing solution is separated from the waste (undissolved iron oxide and silica, which are part of the original bauxite, are now made from sand and red mud waste). This stage consists of three steps: first, coarse sand size waste is removed and washed away to restore caustic soda; Second, the red mud is separated out, and third, the rest of the green wine is pumped through a filter to remove any remaining impurities. Sand and mud pumped together into the lake, green wine pumped into the hot place, is cooled from 1000 degrees Celsius to about 65o ¡ ¡ – 790 triple heat exchanger
(3) Precipitation – at this stage the alumina precipitates as the crystal of alumina hydrate. To do this, the green liquor solution is mixed with a small amount of fine-grained alumina to stimulate the solid precipitation of alumina hydrate in a high dust collector container as a solution for cooling. When the completion of the solid alumina hydrate is passed to the next stage, the rest of the wine, which contains caustic soda and some alumina, returns to the biogas digester.
（4） Roasting – in the final stage of alumina hydrate cleaning to remove any remaining wine and dry. Finally, it is heated to about 1000 ° C to drive away the crystal water, leaving the alumina, which is a dry, pure white, sandy material. A portion of the alumina can be left in hydrate form or further treated by the chemical industry.
All commercial production of aluminum is based on the holcherut smelting process in which aluminum and alumina oxygen are separated by electrolysis. This includes an electric current through a molten solution of alumina and natural or synthetic cryolite (sodium fluoroaluminate). The melting solution is in the aluminum reduction cell lining or belongs to the pot with carbon (cathode) at the bottom and connected in a series of electrical appliances called the reduction cell. A carbon anode is inserted into the top of each tank, which is immersed at the bottom in a molten solution.
An electric current passes through the reason that combines carbon dioxide from alumina to form the anode gas carbon dioxide oxygen. The rest of the molten metal aluminum gathered at the bottom of the pot cathode. Every once in a while, it issiphoned and transferred to the large holding furnace. Remove impurities, add alloy elements and liquid aluminum to cast ingot.
The smelting process is a continuous process. As a cryolite bath, the decrease of alumina content is more new. The heat generated is passed by an electric current to keep it molten so that it dissolves the alumina cryolite bath. A lot of energy consumption in the smelting process, from 14-16 million kwh of electric energy, need to produce a ton of aluminum from two tons of alumina. Cheap power supply, therefore, is crucial to economic production.
Aluminum ingots are produced in various shapes and sizes depending on their end use. They may be rolled into plates, sheets, foils, rods, or rods. They may be drawn into the cables that are stranded in the power transmission lines. Press hundreds of ingots into different forms of beneficial and decorative, or fabricate plants that may make them into large structural shapes.
Many factors in the number of aluminum production cycles relate to the environment, a large number of resources are allocated to minimize the impact of mining, refining, and smelting on the surrounding environment. The mine has been rehabilitated and every effort has been made to restore the area to at least its original state. Take extreme care in the treatment and disposal of red mud in refineries. This is usually injected into some dams sealed with impervious material to prevent contamination around the countryside. Strict measures have also been taken to minimize fluoride emissions from smelters and refineries with dust or corrosive substances[ 1]
Bauxite is the name for a mixture of similar minerals that contain hydrated aluminum oxides.These minerals are gibbsite, diaspore, and boehmite. Bauxite forms when silica in aluminum-bearing rocks (that is, rocks with a high content of the mineral feldspar) is washed away (leached). This weathering process occurs in tropical and subtropical weathering climates. This means that many countries with current tropical climates, or that were once tropical, have the largest reserves of bauxite ore, such as Brazil, Jamaica, Guinea, and Australia.
Alternative sources of aluminum might someday include kaolin clay, oil shales, the mineral anorthosite, and even coal waste. However, as long as bauxite reserves remain plentiful and production costs are low, the technologies to process these alternative sources into alumina or metallic aluminum will likely not progress beyond the experimental stage.
Surface Bauxite Mining: IMAR 7th Edition
Approximately 85%–90% of total world bauxite mining involves open pit mining methods, principally in China, eastern and southern Europe, and Russia. In open pit operations, bauxite is normally extracted from strata typically 4–6 m thick under overburden that may range up to 10 m thick, covered by a thin layer of topsoil with its associated vegetation. At some nonmetallurgical mining sites, the overburden thickness may reach 70 m or more.
Open pit bauxite mining operations may use backhoes, bucket wheel excavators, bulldozers, draglines, power shovels and scrapers for the stripping of overburden from the ore. In addition, draglines, front-end loaders, and power shovels are commonly used to excavate and load the crude ore. Loading is usually into haul trucks, or directly into rail cars or onto conveyor systems, for transport to bauxite processing facilities or stockpiles.
Underground Bauxite Mining: IMAR 7th Edition
Depending on the nature of the bauxite, underground operations typically use a basic array of standard underground mining techniques, which include block caving combined with shrinkage stoping, sublevel stoping, top slicing, longwall, and room-and-pillar methods. Excessive water inflow is a significant problem at most of these mines, particularly in workings developed below the karst water table. In these cases, dewatering shafts are often drilled to lower the active water level at the mine site.
Uses
About 85% of all the bauxite mined worldwide is used to produce alumina for refining into aluminum metal. Another 10% produces alumina which is used in chemical, abrasive, and refractory products. The remaining 5% of bauxite is used to make abrasives, refractory materials, and aluminum compounds.
The lightness, strength, and corrosion resistance of aluminum are important considerations in its application. Metallic aluminum is used in transportation, packaging such as beverage cans, building construction, electrical applications, and other products.
Aluminum, the third most abundant element at the Earth’s surface, is apparently harmless to plant and animal life.
| General Properties | |
|---|---|
| Symbol: | Al |
| Atomic Number: | 13 |
| Standard atomic weight (Ar): | 26.9815386(8) |
| Electron configuration: | [Ne] 3s2 3p1 |
| Atomic Properties | |
|---|---|
| Electronegativity (Pauling scale): | 1.61 |
| Atomic Radius: | 118 pm |
| Ionic Radius: | 53.5 pm (+3) |
| 1st Ionization energy: | 578 kJ/mol |
| 1st Electron affinity: | -43 kJ/mol |
| Oxidation States: | 1,3 |
| Physical Properties | |
|---|---|
| Standard State: | solid |
| Bonding Type: | metallic |
| Melting Point: | 933 K |
| Boiling Point: | 2792 K |
| Density: | 2.7 g/cm3 |
| Metal/Non-Metal: | metal |
| Main isotopes of Aluminium | ||||
|---|---|---|---|---|
| Isotope | % in Nature | Half Life | Decay type | Decay product |
| 26Al | trace | 7.17×105y | β+ | 26Mg |
| ε | 26Mg | |||
| 27Al | 100% | stable |
