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Bauxite is a mineral with a dull, earthy luster which is usually white or gray, though sometimes can be found stained by yellow, orange, red, pink, or brown by iron or included iron minerals. It has no cleavage, a low specific gravity, and the Mohs Hardness Scale score is between 1 and 3. Its streak is usually white, but its iron stain can discolor. It has a variable chemical composition but is always rich in aluminum oxides and hydroxides. These are most of the identifying properties for bauxite and are useful for identifying the mineral; however, bauxite is most often processed into another material with properties much different than bauxite.
Interesting Bauxite Facts:
Bauxite is the principal ore of aluminum and crushing it is the first step in producing aluminum, and then purifying it using the Bayer Process.
Bauxite is washed in a hot solution of sodium hydroxide using the Bayer Process, which filters the aluminum from the bauxite.
Aluminum is usually produced where electricity costs are very low.
Bauxite is used as an abrasive. Sintered bauxite is often used as a sand-blasting abrasive produced by crushing bauxite to a powder, and then fusing it using a very high temperature into spherical beads.
The bauxite spherical beads are very hard and durable. They are sorted by size for use in sandblasting equipment and other sandblasting applications. Their spherical shape reduces wear on the delivery equipment.
Sintered bauxite can also be used as an oil field proppant, which ultimately allow for the flow of oil or natural gas out of rocks and into a well. A process known as hydraulic fracturing.
Bauxite resources are adequate throughout the world at current rates, but other materials could be used instead of bauxite include clay minerals, alunite, anorthosite, power plant ash, and oil shale, but at an increased cost.
Silicon carbide could also be used in place of the abrasives made from bauxite, and synthetic mullite may be used in place of bauxite-based refractories.
Small amounts of bauxite can be found in Arkansas, Alabama, and Georgia. Overall, though, there is very little mining of the mineral in the U.S., with 99% of it being imported from other countries.
Around the world, Australia, China, Brazil, India, Guinea, Jamaica, Russia, Kazakhstan, Suriname, and Greece are the top ten leading bauxite producing countries.
The top producers of bauxite have enough reserves for many years of continued production, with some having reserves lasting 100 years.
What is Bauxite?
Bauxite is primarily a metallic mineral though it is also used as an industrial mineral. It is the only ore used for large scale aluminium production. Although aluminium is the most abundant metallic element in the earth’s crust constituting about 8%, it usually occurs in clays, soil and rocks that cannot be utilized for its extraction.
Bauxite ore is soft and red clay, rich in alumina, and its name originates from Les Baux de Provence, It was a French geologist Pierre Berthier who first discovered bauxite near a village southern France in 1821. Later, a French chemist named Henri Sainte-Claire Deville officially termed the substance as “bauxite” in 1861. In any case, Bauxite is a form of sedimentary rock and is the principal source of the popular metal aluminium.
It is usually obtained from the topsoil in various tropical and subtropical regions. The ore is acquired through mining operations and currently, it is concentrated in most developed countries. More than 160 million metric tons of bauxite are mined each year.
From mineralogy point of view, the bauxite formula and chemical composition are tabulated below.
|Chemical composition (%wt)
|35 to 65
|Gibbsite, boehmite and diaspore
|0.5 to 10
|Quartz, chalcedony, kaolinite
|2 to 30
|Geothite, hematite and siderite
|0.5 to 8
|Rutile and anatase
|0 to 5.5
|Calcite, magnesite and dolomite
In terms of physical properties, Bauxite formula has a different composition. What it means is that bauxite contains a mixture of oxides like aluminium hydroxides, hydrous aluminium oxides, and minerals like gibbsite, boehmite and diaspore including some clay minerals. It also contains, insoluble materials namely magnetite, quartz, siderite, hematite and goethite. It is usually, yellow, white, beige, grey, reddish-brown, pink and brown.
Mullite - An Introduction
Mullite is the mineralogical name for the only chemically stable intermediate phase in the SiO2–Al2O3 system. The mineral rarely occurs in its natural form and can be found on the Isle of Mull off the western coast of Scotland.
Mullite is generally represented as 3Al2O3⋅2SiO2 (i.e. 60 mol% Al2O3). But in reality, it is a solid solution that has the equilibrium composition limits of 60–63 mol% Al2O3, below 1600 °C.
Different starting materials and preparation techniques are used to produce synthetic mullite ceramics. For instance, a mixture of sols, a mixture of solids, or a mixture of salt and sol can be used as the starting materials. Likewise, there is a range of preparation procedures, such as hydrothermal treatment of mixtures of sols, reaction sintering of mechanically combined powders, and chemical vapor deposition.
The properties of mullite are governed by the starting materials used and the preparation technique. Reaction-sintered mullite prepared from mechanically blended powders typically exhibit low fracture toughness (1–2 MPa m-1/2) and low strength (<200 MPa) because of the occurrence of amorphous grain boundary phases. On the other hand, gelation routes yield thoroughly mixed sub-micrometer particles that can be hot-pressed or sintered to make single-phase materials with excellent mechanical properties.