Alumina grinding materials

Figure 4. Microstructure of a single phase submicrometer alumina grinding material manufactured by powder processing (average crystallite size 0.4pm, hardness HVIO (ground) = 22.4 0.7GPa).

SiC, a hard substance(Moh s hardness 9.17), which can be obcained by heating sand and coke. with some sawdust in an electric furnace at high temp. The name carborundum is also applied to other hard materials, such as fused alumina. These materials are used as abrasives for cutting, grinding or polishing(Refs 2,3,5,6 7). 

Refractoriness (Melting Temperature). Instantaneous grinding temperatures may exceed 3500°C at the interface between an abrasive and the workpiece being ground (14). Hence melting temperature is an important property. Additionady, for alumina, sdicon carbide, B C, and many other materials, hardness decreases rapidly with increasing temperature (7). Fortunately, ferrous metals also soften with increasing temperatures and do so even more rapidly than abrasives (15). 

The major raw materials used at present for the production of alumina are bauxites, which are found in the following mineral forms gibbsite (Al(OH)3), boehmite (AlO OH), and diaspore (AlO OH). The major impurities are the oxides of iron, silicon, and titanium, and organic compounds, all of which must be removed before alumina is suitable for aluminum production. The process objectives are, therefore, separation of impurities and compound production in the present case. Bauxite is first dried to facilitate grinding, destroy organic matter, and oxidize the associated ferrous minerals to the ferric state. The temperature of drying is not allowed to exceed 150 °C, because at higher temperature a part of the combined water is expelled and the solubility is affected adversely. 

PEBBLE MILL. A jacketed steel cylinder rotating on a horizontal axis and containing flint or porcelain pebbles as the grinding medium. Its operation is similar to that of a ball mill. It is used for grinding and mixing of dry chemicals, pigments, food products, and the like. Pebble mills are usually lined with alumina, buhrsione, or similar material to protect the walls from wear. 

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