Corundum Refractory Raw Materials

Corundum is a polycrystalline α-alumina material. Aluminum oxide is one of the main components of the Earth's crust and has the advantages of being abundant, relatively inexpensive, and easy to obtain. Corundum possesses excellent physicochemical properties, including high temperature resistance, high strength, high hardness, electrical insulation, and corrosion resistance. As a result, corundum has become an extremely versatile refractory material.

The melting point of corundum is as high as 2050 °C. The refractoriness of high-quality corundum products exceeds 1900 °C, the initial load softening temperature is about 1850 °C, the maximum service temperature is 1950 °C, and the long-term service temperature is around 1800 °C. However, the coefficient of thermal expansion of corundum is 8.6 × 10⁻⁶ / °C, which means it has medium to low thermal shock resistance. The hardness of corundum is 9 on the Mohs scale, second only to diamond and a few synthetic superhard materials.

According to different manufacturing methods, corundum can be divided into two categories. The first involves directly producing corundum raw materials from bauxite, such as brown corundum and high-alumina corundum. The second involves refining alumina from bauxite to produce industrial aluminum oxide, which is then used to make corundum raw materials such as white corundum, dense corundum, and sintered corundum. A comparison of the performance of various electrofused corundum types is shown in Table 1.

As shown in Table 1, in terms of purity and price, white corundum ranks highest, followed by dense corundum, high-alumina corundum, and finally brown corundum, which is the lowest. Both white corundum and dense corundum are produced from industrial aluminum oxide. Dense corundum has slightly lower purity than white corundum but a higher density. High-alumina corundum and brown corundum are both produced from bauxite. Brown corundum has lower purity than high-alumina corundum but a higher true density.

Sintered corundum is a refractory raw material made from industrial aluminum oxide, which is finely ground, formed into balls, and then sintered at an ultra-high temperature of about 1800 °C. The density of sintered corundum is very high. It has high bulk density and low apparent porosity, with a bulk density greater than 3.50 g/cm³ and apparent porosity below 4.5%. Therefore, sintered corundum exhibits properties similar to those of electrofused corundum: good volume stability at high temperatures, resistance to reducing atmospheres, and resistance to erosion by molten metal and glass.

1. Characteristics of Raw Materials

The main raw material of brown corundum is high-alumina bauxite (Al₂O₃ content ≥ 85%), supplemented by anthracite (as a reducing agent) and iron filings (as a regulating component). The specific proportions and functions are as follows:

High-alumina bauxite: Provides the main source of alumina (Al₂O₃), with a content of 95%–97%. It is the key factor determining the high-temperature and corrosion resistance of brown corundum.

Anthracite: Used as a reducing agent in the high temperature of the electric arc furnace. It promotes the crystallization of alumina and simultaneously reduces the impurity content.

Iron filings: React with silicon to form low-silicon iron, thereby removing silicon impurities from the raw materials and improving product purity.

Optimization of raw material ratios: Some manufacturers adjust the proportion of auxiliary materials according to the intended application of their products. For example, in the production of refractory materials, the proportion of bauxite may be increased to enhance the Al₂O₃ content, while in the manufacture of abrasive materials, the amount of iron filings is strictly controlled to optimize particle toughness.

2.Technical index

GB/T2478-2008 (general abrasive brown corundum) stipulates the technical conditions of brown corundum, see table 2.

The raw material properties of brown corundum give it the following advantages:

High Temperature Resistance

High melting point: Brown corundum has a melting point of up to 2050 °C and can maintain structural stability even at high temperatures around 1900 °C. It is therefore suitable for use in high-temperature zones such as the lower body and waist of blast furnaces.

Strong thermal stability: Its main crystal phase is corundum, which remains stable at high temperatures. Brown corundum exhibits minimal changes in bulk density and strength and is not easily softened or deformed under long-term high-temperature operating conditions.

Corrosion Resistance

Excellent chemical stability: It has strong resistance to acids, alkalis, and other corrosive agents at room temperature, allowing long-term use in chemical reactors and as a lining material for pipes and vessels.

Strong oxidation resistance: Brown corundum is not easily oxidized in high-temperature oxidizing environments, which extends service life and reduces maintenance costs.

High Strength and Wear Resistance

Mohs hardness of 9.0: Second only to diamond and silicon carbide, it has excellent abrasion resistance and can withstand intense friction and impact.

Good erosion resistance: It is not easily worn by high-temperature gases or solid particles, making it suitable for furnace linings and other components exposed to severe scouring conditions.

Optimized Particle Characteristics

Regular particle shape: Particles are mostly spherical or nearly spherical with a uniform size distribution, providing good packing density and improving the compactness and uniformity of refractory materials.

High flowability: Brown corundum particles can be evenly distributed during construction, improving workability and efficiency.

Low linear expansion coefficient: The material exhibits minimal volume change at high temperatures, reducing the risk of cracking caused by thermal expansion.

High-alumina corundum is a refractory material made from high-quality bauxite as the main raw material. It is smelted in a high-temperature electric furnace through a special process, resulting in a material with high-purity alumina content, excellent high-temperature resistance, chemical stability, and mechanical strength. It is widely used in various high-temperature industrial applications.

The following analysis is presented from four perspectives: raw materials, manufacturing process, performance, and applications.

1. Raw Material Characteristics: High-Purity Bauxite as the Core

Raw material composition:

Bauxite: Alumina (Al₂O₃) content ≥ 83%. It is the main raw material providing high-purity alumina.

Reducing agent: Petroleum coke (used instead of anthracite coal commonly employed in brown corundum) is used to enhance impurity reduction.

Electrode material: High-quality graphite electrodes with very low ash content are used to minimize the introduction of impurities.

Raw material screening standards:

Bauxite must be strictly screened to ensure the alumina content meets the required standard, while also controlling the levels of silica (SiO₂), titanium oxide (TiO₂), and other impurities.

The fixed carbon content of petroleum coke should be greater than 75% to ensure efficient reduction.

2. Performance Advantages: High Temperature Resistance, Corrosion Resistance, High Strength

Chemical composition:

Aluminum oxide content ≥ 98.5%, with very low total impurity levels (such as TiO₂ and Fe₂O₃) to ensure material purity.

Physical properties:

High temperature resistance: Melting point up to 2050 °C, service temperature up to 1400 °C, with high-temperature hardness maintained at HRA 80 or above.

Corrosion resistance: Excellent resistance to acids, alkalis, salts, and other chemicals, making it suitable for harsh environments.

Mechanical strength: Bulk density ≥ 3.8 g/cm³, high compressive strength, and particle toughness superior to that of ordinary brown corundum.

Thermal stability:

The coefficient of linear expansion increases with temperature, but the material exhibits excellent volumetric stability at high temperatures, reducing the risk of thermal shock damage.

The main advantage of high-alumina corundum lies in its price-performance ratio. Although the alumina content can exceed 98%, its price is roughly equivalent to brown corundum. Specifically, high-alumina corundum costs about 12% more than brown corundum but only about 55% of the price of white corundum.

White corundum is made from industrial aluminum oxide by melting at high temperature. Its Al₂O₃ content is greater than 98%, and it is white with large grains. The main crystal phase is elongated or rhombic, and the apparent porosity ranges from 6% to 10%. Due to the higher porosity, white corundum exhibits a brittle and hard nature. Its grinding capacity is 0.12, slightly higher than that of brown corundum (0.10). White corundum is used in the production of high-grade refractory materials and is also one of the main types of abrasives.

GB/T 2479-2008 "Ordinary Abrasive White Corundum" specifies the technical requirements for white corundum; see Table 3.

As shown in Table 3, GB/T 2479-2008 places a rather broad restriction on the Na₂O content among harmful substances in white corundum. The Na₂O content ranges from a minimum of 0.35% to a maximum of 0.9%. Therefore, when purchasing, it is important to carefully select the manufacturer, specify the allowable Na₂O content in the contract, and test the Na₂O content upon delivery.

1. Characteristics of Raw Materials

Dense corundum is an electrofused corundum made from industrial aluminum oxide with a small amount of additives. It has two main characteristics: 1) a slightly higher total impurity content, and 2) production in a reducing atmosphere.

Dense corundum is greyish-white, with an alumina content of ≥98%, bulk density ≥ 3.8 g/cm³, and apparent porosity ≤ 4%. The SiO₂ content of dense corundum is higher than that of white corundum, but its Na₂O content is significantly lower. See the comparison of the technical indexes of the two types of corundum.

2.Technical indicators

China's ferrous metallurgy standard YB/T 102-2007 specifies the technical requirements for dense electrofused corundum; see Table 4.

Sintered corundum is a granular material made from industrial aluminum oxide as the raw material. It is ground, pressed into balls or billets, and sintered at 1750–1900 °C. The general requirements for sintered corundum are: Al₂O₃ content > 99.3%, true density > 3.98 g/cm³, bulk density > 3.4 g/cm³, and apparent porosity < 5%.

"Plate corundum" is so named because its corundum crystals resemble plates when viewed in two-dimensional cross-sections under a microscope. However, when reconstructed in three dimensions, the crystals are not necessarily plate-like. In fact, "plate-like corundum" is simply a type of high-purity corundum raw material sintered at high temperature. In plate corundum, α-Al₂O₃ crystals are fully developed, with grain sizes ranging from 40 to 200 μm, and many closed pores of 5 to 15 μm within the crystals. In terms of performance, plate corundum has better thermal shock resistance than electrofused corundum.