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Calcined Alumina
Alumina oxide
Chemistry
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Links to Other Materials
- Zircon - Related
- Alumina Hydrate - Related
- Tabular Alumina - Related
- Alumina - Parent
- Cache
Hazards
- Aluminosis
Miscellaneous
- Family: Alumina
- Region: North America
- Mined At: Unspecified
- Raw Mineral: No
- Generic: Yes
Notes
Calcined alumina is generally used in the manufacture of high-grade ceramic shapes, refractories and fused alumina abrasives. Calcined alumina can be compressed to produce a fired density of 3.8 or more.
Calcined (or alpha) alumina is made by calcining a source alumina powder at 1200-1300C to convert it to pure Al2O3. This is the densest and most stable crystalline form of alumina. It is insoluble in water but is soluble in hydrofluoric acid and potassium bisulfate. When nearly 100% of the material converts to the large hexagonal, elongated tablet shaped crystals associated with the alpha phase, the product is referred to as "Tabular Alumina". The calcining process increases crystal size (and thus decreases surface area), the crystals can be as large at 15 microns in diameter. These crystals are extremely hard (9.0 Moh).
Unground calcined aluminas are typically 100-300 mesh, but finer grades (generally "Ground Alumina") are produced by milling. Calcined aluminas are available in numerous grades based on the heat treatment applied, crystal size, soda content, and degree of thermal conversion to alpha phase. Soda content is a major factor in determining the final use (low soda materials are used for electronic applications, medium soda for electrical insulation and porcelains, high soda for glass, glaze, fiberglass and electrical porcelain).
Some exceptionally fine 'super ground' grades are available which can be made into casting slurries of very high specific gravity and which cast well with very low shrinkage (even though alumina powder is not a plastic material). Deflocculation can be achieved using a low pH (3.5-4.5) positive anion mechanism employing hydrochloric or nitric acid, a high pH (11-12) cation mechanism with alkali hydroxide salt additions, or with the addition of standard alkali polyelectrolyte dispersants. With the addition of organic binders, alumina bodies can be cast and pressed into a wide variety of shapes requiring heat and abrasion resistance. Alumina parts are then sintered to permit discrete crystals to react with each other to form larger ones.
Calcined alumina can be substituted for silica filler in porcelain bodies (325 mesh). It reduces shrinkage, increases thixotropy, provides strength in the kiln minimizing warping, benefits glaze fit, and adds fired strength. The book "Clay Bodies" by Robert Tichane has more information on this.
Finely ground alumina and silica can be substituted for part of the kaolin in a recipe (i.e. to reduce glaze shrinkage and therefore crawling). It is very important that the material not be too coarse in order to participate in glass building the way kaolin does (325 mesh is too coarse).
Unlike hydrated alumina, the calcined material has no loss in weight on firing. Thus it produces no gases of decomposition.Properties
Body Thermal Expansion -
A very low expansion material.
Data
- Melting Point (MP): 2040C
- Specific Gravity: 3.75-3.90
- Hardness (Moh): 9.0
- Index of Refraction: 1.765
- Surface Area (m2/gm): 0.5-25
- Bulk Density g/cc: 1.0-1.3
- Density (loose packed): 0.7-1.0
- Hardness (Moh): 9.0
- Specific Gravity: 3.75-3.90
- Melting Point (MP): 2040C
- Index of Refraction: 1.765
- Surface Area (m2/gm): 0.5-25
- Bulk Density g/cc: 1.0-1.3
- Density (loose packed): 0.7-1.0
Suppliers
- Generic
Authors
- Tony Hansen (Owner)
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