Bauxite Facts from grace's blog

Bauxite Facts

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.

Bauxite Formula

From mineralogy point of view, the bauxite formula and chemical composition are tabulated below.

OxideFormulaChemical composition (%wt)MineralogyAluminaAl2O335 to 65Gibbsite, boehmite and diasporeSilicaSiO30.5 to 10Quartz, chalcedony, kaoliniteFerric oxideFe2O32 to 30Geothite, hematite and sideriteTitaniaTiO20.5 to 8Rutile and anataseCalciaCaO0 to 5.5Calcite, 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.

Composition

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.

Synthetic Mullite

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.

Mechanical properties can be further optimized by creating composites. The addition of Zr2O and SiC yields a fracture toughness of 7 MPa m-1/2 at room temperature.

Mullite in Porcelain

Mullite is also one of the vital ingredients used for making porcelain. Clays with less than 60% Al2O3 turn into mullite. The amount of mullite yielded is directly associated with the calcining temperature and the amount of Al2O3.

• Good electrical resistivity

• Optimal high-temperature strength

• Superior thermal stability

• Good thermal shock resistance

• Resistance to abrasion

• Resistance to oxidation and attack, in furnace atmospheres

• Resistance to a range of chemical attacks; it exhibits outstanding stability in acid metal slags, and is insoluble in a majority of the acids.

Ironstones 07X and Flint Clays 09X

These two rock groups are easily identified by their color and texture. Unweathered specimens of both have distinctive red, orange, yellow, or brown colors.

Ironstone is very hard, and if you scrape a powder with a knife blade, it will effervesce weakly with dilute hydrochloric acid. Ironstones are usually composed of iron-carbonate minerals such as siderite and iron oxides such as goethite, hematite, and limonite, but the exact mineral composition is not required for classification. Most shales contain some ironstone as nodules or void fillings, and this class is restricted to rocks that have more than 50 percent ironstone.

Flint clays are highly variable in color, have a distinctive milky luster, and a conchoidal fracture. Flint clay is a variety of clay-rich rock in which the clay mineral is predominantly kaolinite.

Ironstones and flint clays have additional fabrics according to one of the following categories:

• 0X4 Massive: Homogeneous material with no obvious lines or marks

• 0X5 Mosaic: Divided into fragments; each piece maintains it original position relative to adjacent pieces

• 0X1 Brecciated: Divided into fragments; pieces have rotated relative to adjacent pieces

• 0X6 Nodular: Rounded or irregular masses floating in a matrix of different material

• 0X7 Oolitic (Pisolitic): Numerous, small rounded grains set in a matrix of different material

Kaolini

Kaolin is a type of clay found in nature. It can also be made in a laboratory. People use it to make medicine.

Kaolin is used for mild-to-moderate diarrhea, severe diarrhea (dysentery), and cholera.

In combination products, kaolin is used to treat diarrhea and to relieve soreness and swelling inside the mouth caused by radiation treatments. Some of these combination products are used for treating ulcers and swelling (inflammation) in the large intestine (chronic ulcerative colitis).

Some people apply kaolin directly to the skin in a wet dressing (poultice) or as a dusting powder. It is used to dry or soften the skin.

Kaolin is also used in laboratory tests that help to diagnose disease.

In manufacturing, kaolin is used in tablet preparation and to filter materials and remove color.

Kaolin is also a food additive.

How does it work?

Kaolin acts as a protective coating for the mouth to decrease pain associated with radiation-induced damage.

When it is applied to the skin, kaolin acts as a drying agent.