Technical Information about white corundum & white fused alumina
Technical sheet of white corundum
|Properties||Chemical Classification||Chemical Composition|
|hardness, abrassive, refractory, polishing and grinding||Oxide||Al2O3|
Properties of white corundum
The toughness of brown corundum is greater than that of white corundum (this can be even further enhanced by thermal retreatment and special surface treatments). It owes its toughness to its titanium oxide content which reduces crystal size.
Beta alumina is the main impurity phase in white corundum and it migrates to the lowest fusion point part of the ingot. Adding 2% of chromium oxide, which substitutes Cr3+ for Al3+, increases the toughness of the white corundum structure. The volatization of soda at 2100°C creates a porosity in the white corundum that can be beneficial.
In 1961, after 44 years of gestation, zirconia alumina abrasives (AZ) appeared on the alumina market. It is obtained by fusion of a mixture of bauxite, zircon, coke, scrap iron, with solidification by quenching.
It is 5 times stronger than brown corundum. Concerning morphology, brown corundum grains are massive (isometric) and white ones are acute and sharp. For a given particle size distribution fraction, the higher the apparent density, the more isometric the grains are.
White corundum phases
For the solid corundum phases, this includes the mixing of the raw materials on one hand, and the self-lining of the furnace on the other. The raw materials float and cover the liquid corundum restricting thermal loss by radiation, which is about 900KW per square meter for the uncovered liquid surface at 2000°C. The lining, which is an essential element of an arc furnace, is formed in two phases:
- The solid corundum in thermal balance with the liquid corundum that is being developed to make up the sides of the crucible
- A compound containing titanium carbide in chemical balance with the titanium ferrosilicon to make up the bottom of the crucible.
The liquid phases are made up of the liquid corundum being reduced in the center of the furnace and the titanium ferrosilicon that coalesces at the bottom of the crucible due to its density. After several “pure” corundum castings, the ferrosilicon that has accumulated at the bottom of the crucible has to be drained. This is done by overturning the crucible. The gaseous phases which results from the reaction of various oxides with carbon are mainly CO and SiO.
Balance of white fused alumina
This aspect is of great importance to the process and for safety reasons. If titanium ferrosilicon from the reduction process becomes included in the corundum after solidification, it needs to be separated magnetically. In order for ferrosilicon to be ferromagnetic, its silicon content must be less than 21% by mass. However in order for it to be easily ground,its silicon content must be more than 13% by mass.
Therefore, the aim is for silicon content within the two limits. Following this, attention needs to be paid to Ti content. Beyond a threshold value, the excess precipitates on the bed in the form of TiC; the consequence being a reduction of the useful depth of the crucible. On the other hand, if the Ti content is less than that needed for thermodynamic balance of the concerned reagents, the TiC on the bed will be dissolved in the ferrosilicon liquid.
The threshold for balance in titanium depends on the targeted TiO2 content in the brown corundum that is to be produced and on the necessary level in the ferrosilicon so that it is in thermodynamic balance with the bed at the bottom of the crucible. Although chemistry governs the balance at the bottom of the crucible, the vertical sides of the self-induced lining are dependent on the thermal balance at an unconventional temperature higher than 2000°C.
To ensure the safety and the stability of the furnace requires skill from the operator. He must maintain a lining thickness of between 30 and 50 cm by controlling the temperature gradient between the center, the zone where the liquid corundum can be found, and the external side of the metallic tank that is being cooled by permanently running water.
The global thermal balance is more easily obtained. It is based on adapting the rate of filling the furnace to the fusion capacity of the furnace to ensure that the liquid is covered, thereby reducing loss by radiation.