• Universitat de València

There are different crystalline phases (polymorphs) of aluminium oxide. Among them, the alpha polymorph or corundum stands out due to its high technological interest, given its high hardness, wear and chemical attack resistance, thermal stability and high melting point. The conventional process for obtaining corundum for non-metallurgical uses consists of the slow crystallisation of alumina smelters obtained from bauxites by the Bayer method. In general, it is possible to obtain corundum under ordinary pressure by sol-gel methods, which are based on obtaining intermediate polymorphs of alumina from hydroxide or oxy-hydroxide, these intermediate polymorphs being subsequently transformed into alpha-polymorphs (corundum) by undergoing an additional heat treatment. Current methods for obtaining corundum require pre-treatments of the raw material such as melting, dissolution or treatment with acids or bases and usually produce intermediate polymorphs of alumina, thus additional heat treatments are required to obtain the alpha polymorph or corundum. Simpler corundum production processes would therefore be desirable.

Researchers from the Department of Geology at the Universitat de València have developed a new procedure for obtaining low crystalline corundum (nanocrystalline) that uses natural or synthetic raw materials in solid state, without prior pre-treatment. This process only requires two stages, one of firing at a suitable temperature under ordinary pressure, and the other of sudden cooling of the firing product. The method of the invention produces nanocrystalline corundum, obviating the formation of the low-temperature polymorphs of alumina, so that the product is porous in nature, which facilitates its subsequent disaggregation, avoiding the hardness and difficult grinding of compact corundum.

The main application of the technology is in the chemical industry sector, to obtain corundum for various uses (refractories, abrasives, functional fillers for plastic polymers, production of ceramics, glass and ceramic glazes incorporating reactive alumina, etc.) and nano-corundum for special applications (catalyst supports, implants, aerospace applications, etc.).

The main advantages provided by the invention are:

• The use of natural or synthetic raw materials in solid state.
• Simplicity of the process which allows corundum to be obtained both on a small scale and industrially.
• The corundum obtained is easy to disaggregate, as it is a porous product.

• Patent granted