Performance And Application Of Tabular Corundum in Refractories

Tabular corundum is a dense, fully contracted, sintered α -al2o3 with grains of 50-400μm in aggregate structure. Tabular corundum gets its name because its grains are shaped like boards. Tabular corundum was prepared by fast calcination of ultrafine α -al2o3 pellets at slightly lower melting temperature. After heat treatment, the material balls of 18-20mm are crushed or milled to obtain tabular corundum of various sizes.

The low content of silicon oxide, iron oxide and titanium oxide in tabular corundum is important for excellent high temperature performance. The typical ultra-low content of ferric acid is less than 0.002%, which is very important for phosphate bonded refractories. When sintered tabular corundum is compared with other synthetic high alumina aggregates, such as fused white corundum, the biggest difference is found to be the impurity content with finer dimensions. This can make a huge difference in performance, especially at high temperatures. The higher impurity content introduced at a finer size will greatly reduce high temperature volume stability and creep resistance. A great difference can be seen by comparing the porosity of fused white corundum and tabular corundum . Although the total porosity of the two aggregates is the same, the grain porosity is significantly different.

The open porosity of electromelted grains is 2-3 times that of sintered grains. Most of the pores in fused alumina are composed of large open pores, while more than half of the pores in tabular corundum are closed pores. A high proportion of closed pores is necessary for high thermal shock resistance  this is typical of tabular corundum.

Tabular corundum exhibits high thermal shock resistance and high strength. Scanning electron microscopy shows that the surface of tabular corundum grains is not as smooth as that of fused corundum grains, but rather rough with shallow hemispherical pores. This surface structure improves the strength of refractories by promoting the reaction with matrix and mechanical interlocking.

The main performance of plate industry is as follows: 1, high purity Al2O3 concentration 99.4%; 2, crystal high hardness; 3, low open porosity and 2-3 times high closed porosity; 4, high particle packing density 3.55-3.6g/cm35, high melting point :2000℃6, chemical inertia; 7, good thermal shock resistance; 8, excellent volume stability; 9, the existence of microcracks; 10, high single grain strength.

Three, the main application fields of tabular corundum

The properties of tabular corundum have many applications in the field of refractories. Tabular corundum is suitable for ceramic, chemical and water-binding substrates. Tabular corundum can be used alone and in combination systems or with calcined and/or activated alumina. Because tabular corundum exhibits extremely high purity even in fine powder, it can be used to improve the properties of lower alumina aggregates. Examples include bauxite and fused brown corundum, using coarse particles of these aggregates and medium particles and fine powder of tabular corundum. It has already been mentioned that the increase in tabular corundum consumption is mainly due to continuous casting of steel. The use of tabular corundum is an industry standard, especially for sliding plates, dipping pipes and spouts .

New development of tabular corundum/spinel

It has been found in Japan that adding 20-30% aluminum-rich spinel to cement-bonded tabular corundum refractories can significantly improve slag resistance. The idea that aluminum-rich spinel is so common around the world has led to new applications for tabular corundum. In particular, the shapeless lining of ladles encouraged the use of tabular corundum in the future. Spinel can increase the thermal flexural strength of cement-bonded plate-shaped corundum castable to 22 N/mm2 at 1500℃. It is important to note that even using 15% cement did not reduce high temperature performance. Considering that the high-temperature flexural strength of castables with lower cement content is reduced by an order of magnitude  it is not clear that these findings will have a great impact on future refractory technology. It may be possible to use castables that have a higher cement content and thermal properties than ultra-low cement castables  and do not have the repeatability disadvantages known to ULCC. It is interesting that adding 2% silica powder into spinel tabular corundum castable can reduce the thermal flexural strength to less than 1 N/mm2 at 1500℃.

Because cement-bonded alumina systems have some limitations in silica-rich slag environments, Alphabond systems with pure alumina-based binders were developed. The main chemical component is alumina containing impurity CaO; 0.1%, SiO2. 0.2% Na2O; 0.5% and water ≤9% it can be treated like ordinary calcium aluminate cement. The system can be used alone or in conjunction with silicon micropowders.

New water-binding systems enable the refractory industry to use alumina castables where cement-binding systems do not perform optimally. It proves once again that tabular corundum can be applied flexibly in many different fields