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What is white fused alumina ?
Corundum, or alpha alumina, is the most thermodynamically stable form of alumina. In fact the term corundum is generally applied when the alpha alumina has been produced by a fusion process. Finally, the alumina source is melted, typically in an electric-arc furnace, then slow-cooled, with the objective of forming an alumina single crystal on a macroscopic scale. We divide Corundum into three main groups:
- White corundum or white fused alumina (WFA) - “pure” alumina, including globular or bubble alumina
- Brown corundum or brown fused alumina (BFA) - aluminas with 1.5% or 3% TiO2, including semi-friable(TiO2 1.5%)
- Solid solutions with corundum matrices for example, zirconia alumina corundum (10, 25 or 42% of ZrO2) and aluminium oxynitrides (AlON).
In nature, corundum is the hardest mineral after diamond. Moreover, chemically inert with friability depending on its preparation, it has always been a multipurpose abrasive and remains the most commonly used today.
Also, it is used as an aggregate in refractories for its high temperature mechanical resistance, its dimensional stability, its chemical inertness and its electric resistivity. Moreover, it can be used as filler when an electric resistivity or a high friction coefficient is required.
Typically, alumina based raw materials are melted in a triphased electric arc furnace with a pivoting base. For example for white fused alumina the raw material is calcined alumina. In addition, it’s important to know how to deal with this to optimize the operation of the furnace and the quality of the production. Then, to operate the arc furnace it is important to respect the following points :
- adapt the chemical composition of the melt to the specifications of the targeted product;
- adapt the electrical adjustments (power, voltage) to the targeted product and to the composition of the melt;
- seek chemical and electrical stability of the furnace
- adapt the flow and the spatial distribution of the products in the furnace to the injected electric power;
Bauxite is the essential raw material for the manufacturing of corundum. To optimize the electrical consumption, we use a mixture of several types of bauxite and alumina can be added mainly to dilute the titanium oxide content of the end product, but also to reduce the specific energy of the fusion-reduction.
Therefore, the reducing agent is cooked with a stable fixed carbon content and particle size distribution that can be adjusted to prevent too rapid consumption above 2000°C.
White corundum: Less tough and more angular than brown corundum, it works faster and creates less heating. It is often used as a free abrasive (glass polishing). Its applications are vitrified grinding wheels, precision milling, rectification, sharpening of cutting tools, surfacing and fine rectification, where the heating of the part must be avoided (thermo-sensitive steel and alloys) and refractories.
Single crystals: They have numerous sharp edges guaranteeing a good bond with the matrix. They are very tough and difficult to blunt. Applications: plunge cut grinding.
Zirconia alumina: is self-sharpening and has great cutting qualities in resin bonded grinding wheels and blasting applications. Production rates are double those of brown corundum, with triple the yield (kg of metal removed for kg of abrasive consumed) and it is possible to apply milling pressures that are 4 or 5 times greater and at greater operating temperatures. For this reason they are particularly suited for flame de-scaling and to dressing iron ingots.
And finally, Bubble alumina: Used to create porosity in high performance vitrified grinding wheel abrasives, agglomerated abrasives and high performance alumina refractories.