Alumina

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Calcined Alumina - Related
Alumina Hydrate - Related
Tabular Alumina - Related
Boron Carbide - Alternative
Boron Nitride - Alternative
Silicon Carbide - Alternative
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Miscellaneous

Family: Alumina
Region: None
Mined At: Unspecified
Raw Mineral: No
Generic: Yes

Notes

Alumina (properly called aluminum oxide) powder as used in ceramics can be a white granular material (like table salt) or an exceptionally fine white and dense powder (depending on the type and processing method). The Bayer refining process of turning Bauxite into alumina used by alumina refineries worldwide involves four steps - grinding and digestion, clarification, precipitation and calcination. Alcoa describes the process like this:
"To turn bauxite into alumina, we grind the ore and mix it with lime and caustic soda, pump this mix into high-pressure containers, and heat it. The aluminum oxide is dissolved by the caustic soda, then precipitated out of this solution, washed, and heated to drive off water. What is left is the white powder called alumina, which is transformed into aluminum metal in the smelting process."

There are three general types of alumina: Hydrated, calcined and tabular. Within each of these there are many grades. Aluminas vary in the amount of soda (Na2O), ultimate crystal size, chemical purity and the physical properties of the powder or granules. Calcined aluminas are generally used in porcelain and whiteware bodies, low soda for electronic applications, and high purity grades for optical glass. High purity aluminas (99.99%) for optical and electronic applications are made using non-Bayer processes such as ammonium aluminum sulfate, aluminum chloride or aluminum alkoxide. Typical bodies for use in electronic applications may contain 95% or more alumina.

Alumina oxide ceramics have high melting temperatures and hot and cold mechanical strength and are good for abrasion and corrosion resistant applications where heat resistance is also important (compressive strength may average 250,000 psi but high purity mixes can be up to 500,000 psi!). Alumina ceramics can be extremely hard, exceeded only by silicon carbide, boron carbide and diamond. They also have outstanding electrical and thermal properties (spark plugs, for example, are made using a high alumina porcelain (about 90%) for its insulating properties coupled with its strength, heat and thermal shock resistance). High alumina ceramics (99%+) can provide such good resistance to chemical attack that they can resist hydrofluoric acid and molten alkalis and alkali vapors. The chemical inertness of these same bodies make them ideal for making valves and seals exposed to severe corrosive and abrasive conditions. Alumina ceramics also resist the effects of radiation that can destroy other materials. Alumina ceramic can have very high dielectric strength, high resistivity and low dielectric loss (thus its use in insulators and electronic components).

High alumina bodies are often lacking in forming properties so organic and inorganic lubricants, binders, electrolytes and plasticizers are used.

Since alumina is by nature refractory, alumina ceramics and alumina refractories might seem like redundant terms. However the former refers to alumina containing bodies that have a fine grain structure and dense matrix and whose purpose is more than just resistance to temperature. The term 'high alumina ceramics' generally refers to mixes containing 85+% alumina. Some high demand applications such as furnace tubes and lab ware cross the boundaries of both alumina ceramics and alumina refractories, these are often made from 99.8% alumina mixes. 99.9% aluminas are used in super duty applications like nuclear ceramics and cutting tools.

Alumina and alumina mixes can be dry pressed, isostatic pressed, hot pressed and extruded, tape cast and inject or compress molded. However most processing and firing methods involving alumina have to be adjusted compared to those used for more traditional ceramic mixtures.

While greater quantities of alumina often improve the properties of mixes in which they are being used alumina is expensive and greater quantities require higher firing. Thus a compromise between performance and cost must be reached.

The mineral corundum yields native alumina while the hydrated minerals gibbsite, diaspore, and boehmite are also found in nature. Alumina occurs as silicates in clays, feldspars, kyanite, and many other minerals. However, the principle sources of purified and hydrate alumina are native bauxite and laterite deposits.

(Richard Willis)

Aluminum tri-oxide, of the ideal form Al₂ O₃ Melts at 2030ºC Water insoluble

The most common molecular form of aluminum comprising clays and glazes, the form which compounds with silica and water to create a hydrous alumina-silicate (Al₂ O₃ , SiO₂ , H₂ O — see kaolin ), a sufficient ingredient whereby a given earth can be called a clay. It is not a necessary ingredient since occasionally, albeit rarely, the aluminum has been re
placed by iron and/or magnesium. Alumina is usually the principal refractory of a clay or glaze formula. When aluminum is called for in a recipe for clay, glaze, engobe, slip or whatever it is best found and used via its natural sources — rocks and minerals of substantial alumina content — and formulated to a recipe accordingly to what is apportioned in the source material. Pure alumina vis a vis a refined powder is usable to proportion aluminum to a recipe but caution must be taken in considerati
on of its resistances to blending and fusion: wet-mixes must be frequently stirred lest the alumina falls to a sediment, and unblended pockets or veins of it in body or covering will result in pockets or veins of un-fused dry powder after firing, just as do natural-clay calcium deposits.

Most recipes when calling for aluminum are indeed calling for this aluminum oxide, and more likely yet are calling for an aluminum oxide in the company of silicon, which usually translates as an aluminum silicate (known as “ alumina-silicate”) of the ideal form Al₂ O₃ , SiO₂ ; and when an alumina-silicate is being called for one normally formulates it to the recipes in question via the alumina-si
licates of kaolin and its varieties or the alumino-silicates of alkaline clays because they contain built-in, homogeneously blended, proportions of alumina and its fusibles in ready-to-use forms for most purposes.

On the beneficial side, as a clay or glaze ingredient, alumina contributes viscosity, toughness, hardness, refractoriness, matte-ness, opacity, surface tension, density, etc. Also, once fused, it turns solubles to insolubles, such as lead, borax, lithium, soda, etc. Also, alumina is the major stabilizer of silicon in clays and glazes, preventing the contained “glass” from crystallizing and thereupon falling apart into a “sand” when cool.

Overabundance of alumina is normally the major cause of a clay or glaze not fusing, and so not maturing, at its theoretical temperature range. Meanwhile, it is the pillar which supports reclining vitrifying materials (see kaolin and porcelain ) . Alumina’s major ore is bauxite.

Alumina minerals

(major mineral sources of alumina and their typical apportioning of Al₂ O₃ )

albite NaAlSi₂ O₈ 19 diaspore Al₂ O₃ , H₂ O 85

alumbre KAl(SO₄ )₂ 12H₂ O 10 dumortierite Al₈ BSi₃ O₁₉ (OH) 64

alunite KAl₃ (SO₄ )₂ (OH)₆ 37 hydrargilite Al(OH)₃ 65

andalusite Al₂ SiO₅ 63 kaolin Al₂ O₃ , 2SiO₂ , 2H₂ O 39

anorthite CaAl₂ Si₂ O₈ 36 kaolinite Al₄ (Si₄ O₁₀ )(OH)₈ 39

bauxite Al₂ O₃ 2H₂ O 61 leucite K(AlSi₂ O₆ ) 23

boehmite Al O(OH) 85 millsite NaKCaAl₆ (PO₄ )₄ (OH)₉ 3H₂ 36

cyanite Al₂ SiO₅ 63 nepheline NaAlSiO₄ 32

corundum Al₂ O₃ 100 orthoclase KAlSi₂ O₈ 18

crandalite CaAl₃ (PO₄ ) ₂ (OH) 5H₂ O 35 sillimanite Al₂ SIO₅ 63

cryolite Na₃ AlF₆ 24 spinel MgAl₂ O₄ 71

dawsonite NaAl(OH)₂ CO₃ 35 wavellite Al₃ (PO₄ )₂ (OH)₃ 5H₂ O 37

(Note: Though bauxite and kaolin are not minerals, they are listed here for convenient comparison)

URLs

Calcination of Alumina - http://www.qal.com.au/a_process/process4.html
Gladstone Alumina Refinery - http://www.qal.com.au/

Suppliers

Alcan Chemicals
Alcan Chemicals Europe
Alcoa
Aluminium Pechiney
Pichiney

Authors

Tony Hansen (Owner)

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<notes>
<note>Alumina (properly called aluminum oxide) powder as used in ceramics can be a white granular material (like table salt) or an exceptionally fine white and dense powder (depending on the type and processing method). The Bayer refining process of turning Bauxite into alumina used by alumina refineries worldwide involves four steps - grinding and digestion, clarification, precipitation and calcination. Alcoa describes the process like this: 
"To turn bauxite into alumina, we grind the ore and mix it with lime and caustic soda, pump this mix into high-pressure containers, and heat it. The aluminum oxide is dissolved by the caustic soda, then precipitated out of this solution, washed, and heated to drive off water. What is left is the white powder called alumina, which is transformed into aluminum metal in the smelting process." 
 
There are three general types of alumina: Hydrated, calcined and tabular. Within each of these there are many grades. Aluminas vary in the amount of soda (Na2O), ultimate crystal size, chemical purity and the physical properties of the powder or granules. Calcined aluminas are generally used in porcelain and whiteware bodies, low soda for electronic applications, and high purity grades for optical glass. High purity aluminas (99.99%) for optical and electronic applications are made using non-Bayer processes such as ammonium aluminum sulfate, aluminum chloride or aluminum alkoxide. Typical bodies for use in electronic applications may contain 95% or more alumina. 
 
Alumina oxide ceramics have high melting temperatures and hot and cold mechanical strength and are good for abrasion and corrosion resistant applications where heat resistance is also&nbsp;important (compressive strength may average 250,000 psi but high purity mixes can be up to 500,000 psi!). Alumina ceramics can be extremely hard, exceeded only by silicon carbide,&nbsp;boron carbide and diamond.&nbsp;They also have outstanding electrical and thermal properties (spark plugs, for example, are made using a high alumina porcelain (about 90%)&nbsp;for its insulating properties coupled with its strength, heat and thermal shock resistance). High alumina ceramics (99%+) can provide such good resistance to chemical attack that&nbsp;they can resist hydrofluoric acid and molten alkalis and alkali vapors. The chemical inertness of these same bodies make them ideal for making valves and seals exposed to severe corrosive and abrasive conditions. Alumina ceramics also resist the effects of radiation that can destroy other materials. Alumina ceramic can have very high dielectric strength, high resistivity and low dielectric loss (thus its use in insulators and electronic components). 
 
High alumina bodies are often lacking in forming properties so organic and inorganic lubricants, binders, electrolytes and plasticizers are used. 
 
Since alumina is by nature refractory, alumina ceramics and alumina refractories might seem like redundant terms. However the former refers to alumina containing bodies&nbsp;that have a fine grain structure and dense matrix and whose purpose is more than just resistance to temperature. The term \'high alumina ceramics\' generally refers to mixes containing 85+% alumina. Some high demand applications such as furnace tubes and lab ware cross the boundaries of both alumina ceramics and alumina refractories, these are often made from 99.8% alumina mixes. 99.9% aluminas are used in super duty applications like nuclear ceramics and cutting tools. 
 
Alumina and alumina mixes can be dry pressed, isostatic pressed, hot pressed and extruded, tape cast and inject or compress molded. However most processing and firing methods involving alumina have to be adjusted compared to those used for more traditional ceramic mixtures. 
 
While greater quantities of alumina often improve the properties of mixes in which they are being used alumina is expensive and greater quantities require higher firing. Thus a compromise between performance and cost must be reached. 
 
The mineral corundum yields native alumina while the hydrated minerals gibbsite, diaspore, and boehmite are also found in nature. Alumina occurs as silicates in clays, feldspars, kyanite, and many other minerals. However, the principle sources of purified and hydrate alumina are native bauxite and laterite deposits.
</note>
<note>Aluminum tri-oxide, of the ideal form Al2

O3

Melts at 2030ºC Water insoluble 
 
The most common molecular form of aluminum comprising clays and glazes, the form which compounds with silica and water to create a hydrous alumina-silicate (Al2

O3

, SiO2

, H2

O — see kaolin


), a sufficient ingredient whereby a given earth can be called a clay. It is not a necessary ingredient since occasionally, albeit rarely, the aluminum has been re 
placed by iron and/or magnesium. Alumina is usually the principal refractory of a clay or glaze formula. When aluminum is called for in a recipe for clay, glaze, engobe, slip or whatever it is best found and used via its natural sources — rocks and minerals of substantial alumina content — and formulated to a recipe accordingly to what is apportioned in the source material. Pure alumina vis a vis a refined powder is usable to proportion aluminum to a recipe but caution must be taken in considerati 
on of its resistances to blending and fusion: wet-mixes must be frequently stirred lest the alumina falls to a sediment, and unblended pockets or veins of it in body or covering will result in pockets or veins of un-fused dry powder after firing, just as do natural-clay calcium deposits. 
 
Most recipes when calling for aluminum are indeed calling for this aluminum oxide, and more likely yet are calling for an aluminum oxide in the company of silicon, which usually translates as an aluminum silicate (known as “

alumina-silicate”) of the ideal form Al2

O3

, SiO2

; and when an alumina-silicate is being called for one normally formulates it to the recipes in question via the alumina-si 
licates of kaolin and its varieties or the alumino-silicates of alkaline clays because they contain built-in, homogeneously blended, proportions of alumina and its fusibles in ready-to-use forms for most purposes. 
 
On the beneficial side, as a clay or glaze ingredient, alumina contributes viscosity, toughness, hardness, refractoriness, matte-ness, opacity, surface tension, density, etc. Also, once fused, it turns solubles to insolubles, such as lead, borax, lithium, soda, etc. Also, alumina is the major stabilizer of silicon in clays and glazes, preventing the contained “glass” from crystallizing and thereupon falling apart into a “sand” when cool. 
 
Overabundance of alumina is normally the major cause of a clay or glaze not fusing, and so not maturing, at its theoretical temperature range. Meanwhile, it is the pillar which supports reclining vitrifying materials (see kaolin

and porcelain

)
. Alumina’s major ore is bauxite. 
 
 
 

 
 
 
 
Alumina minerals 
 

(major mineral sources of alumina and their typical apportioning of Al2
O3
) 
 
 
 
 
albite NaAlSi2
O8 
19 diaspore Al2
O3
, H2
O 85 
 
alumbre KAl(SO4
)2
12H2
O 10 dumortierite Al8
BSi3
O19
(OH) 64 
 
alunite KAl3
(SO4
)2 
(OH)6 
37 hydrargilite Al(OH)3 
65 
 
andalusite Al2
SiO5 
63 kaolin Al2
O3
, 2SiO2
, 2H2
O 39 
 
anorthite CaAl2
Si2
O8 
36 kaolinite Al4
(Si4
O10
)(OH)8 
39 
 
bauxite Al2
O3
2H2
O 61 leucite K(AlSi2
O6
) 23 
 
boehmite Al O(OH) 85 millsite NaKCaAl6
(PO4
)4 
(OH)9
3H2 
36 
 
cyanite Al2
SiO5 
63 nepheline NaAlSiO4 
32 
 
corundum Al2
O3 
100 orthoclase KAlSi2
O8 
18 
 
crandalite CaAl3
(PO4
) 2
(OH) 5H2
O 35 sillimanite Al2
SIO5 
63 
 
cryolite Na3
AlF6 
24 spinel MgAl2
O4 
71 
 
dawsonite NaAl(OH)2
CO3 
35 wavellite Al3
(PO4
)2 
(OH)3
5H2
O 37 
 
(Note: Though bauxite and kaolin are not minerals, they are listed here for convenient comparison) 
 
 
 
</note>
</notes>
</material>

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