To make corundum products, it is necessary to understand these indicators and properties of corundum refractory materials

um, but the density is higher than that of white corundum. Both high alumina corundum and brown corundum are produced from bauxite. The purity of brown corundum is lower than that of high-alumina corundum, but its true density is higher than that of high-alumina corundum.

 Sintered corundum is a refractory raw material made of industrial alumina as raw material, which is ground and balled, and then sintered at an ultra-high temperature of about 1800 °C. The density of sintered corundum is very high, the apparent porosity is low, its bulk density is >3.50g/cm³, and the apparent porosity is

 Brown corundum

 1. Raw material characteristics

 Brown corundum is made of bauxite, anthracite and iron filings, smelted in an electric arc furnace, smelted, desiliconized, iron-removed, titanium-reduced, and cooled.

 When smelting brown corundum, the bauxite melts and floats in the upper layer of the melt due to its low density; after melting, the iron filings settle to the lower part of the melt due to its high density. At the same time, the carbon in the anthracite reduces the SiO₂, Fe₂O₃, and TiO₂ in the bauxite into metals. These metals are mixed with a metal melt formed from iron filings to form a denser ferrosilicon alloy that settles to the bottom of the furnace. Subsequently, the corundum melt is cooled and crystallized to form corundum crystals. Due to the inclusion of impurities, the corundum obtained by this method turns brown and is called brown corundum. The alumina content of brown corundum can reach more than 97%, and the grinding capacity is 0.1 (the grinding capacity of diamond is 1). .

 High alumina corundum

 High alumina corundum, also known as sub-white corundum, is essentially a product made by improving the process of brown corundum. High alumina corundum has higher purity and lower price, which provides a new choice for refractory materials. However, the production process of high alumina corundum is complex, and the variable range of process parameters is narrow. If not controlled properly, the quality of high alumina corundum is more easily affected than other corundum raw materials.

 The main points of manufacturing high-alumina corundum: one is to carry out deep reduction, to remove impurities to the maximum extent, and to form only a small amount of carbides such as Al₄C₃; second, after the reduction is completed, carry out oxidation, blow oxygen to the molten pool, or add Fe₂O₃ , SiO₂ and other oxidants, destroy the formed carbides.

 The advantages of high alumina corundum are mainly reflected in the cost performance. The alumina content of high alumina corundum can reach more than 98%, but the price is similar to that of brown corundum. The price of high alumina corundum is about 12% higher than that of brown corundum, which is only about 55% of the price of white corundum.

 white corundum

 White corundum is made from industrial aluminum oxide and smelted at high temperature. Its Al₂O₃ content is more than 98%, white in color, large in grain size, and its main crystal phase is elongated or diamond-shaped. Its apparent porosity is 6% to 10%. Due to more pores, white corundum is brittle and hard, and its grinding capacity is 0.12, which is slightly higher than that of brown corundum (0.10).

 GB/T2479-2008 limits the content of harmful substance Na₂O in white corundum too wide. In Table 4, the Na₂O content is 0.35%, reaching 0.9%. Therefore, when purchasing, we must pay attention to selecting the manufacturer, specify the content of Na2O when signing the contract, and detect the index of Na2O when delivering.

 dense corundum

 1. Raw material characteristics

 Dense corundum is fused corundum made from industrial aluminum oxide and adding a small amount of additives. Dense corundum has two characteristics: 1. The total amount of impurities is slightly higher; 2. It is produced in a reducing atmosphere.

 Dense corundum is gray-white, alumina content ≥98%, bulk density ≥3.8g/cm³, apparent porosity ≤4%. The SiO₂ content of dense corundum is higher than that of white corundum, but the content of NaO is significantly lower than that of white corundum. For details, see the comparison of technical indicators of the two corundums.

 Sintered corundum

 Sintered corundum is a granular corundum material made of industrial alumina as raw material, ground and pressed into pellets or blanks, and sintered at 1750~1900℃. General requirements, Al₂O₃ content of sintered corundum>99.3%, true density>3.98g/cm³, bulk density>3.4g/cm³, apparent porosity

 "Tabular corundum" is named because the corundum crystals look like a plate under the two-dimensional cross-section of the microscope. But if restored into a three-dimensional image, the corundum crystals are not necessarily plate-like. In fact, "tabular corundum" is just a high-purity corundum raw material sintered at high temperature. In "tabular corundum", α-Al₂₃ crystals are fully developed, the grain size is 40~200um, and there are many closed pores of 5~15um in the crystal. In terms of performance, "tabular corundum" has better thermal shock resistance than fused corundum.