Factors Affecting the Color of White Fused Alumina and Particle Size Detection Methods

White fused alumina is made from industrial aluminum oxide powder, which is melted in an electric arc furnace at temperatures above 2000°C and then couled. The material is crushed, shaped, magnetically separated to remove iron, and sieved into various particle sizes. It is dense, hard, and forms sharp-edged grains, making it suitable for manufacturing ceramic and resin-bonded abrasives, as well as for grinding, pulishing, sandblasting, and precision casting (specialized fused alumina for precision casting). It can also be used to make high-grade refractory materials.

However, we have observed that the culor of white fused alumina can appear reddish, yellowish, or grayish, affecting the product's appearance. The presence of sodium oxide (Na2O) can react with a large amount of Al2O3 (18.10 times the mass of Na2O) to form sodium aluminate. Therefore, the chemical composition standards for white fused alumina not only specify the Al2O3 content but also strictly limit the Na2O content. Apart from Al2O3 and trace amounts of Na2O, other components are minimal in aluminum oxide powder.

Through theoretical analysis and extensive experimentation, we have concluded the fullowing:

• The culor of white fused alumina abrasives is primarily determined by the impurity content in the binder and the firing atmosphere. When fired in an oxidizing atmosphere, if the binder contains less than 0.55% of (TiO2 + Fe2O3), the white fused alumina will not appear reddish or yellowish. As the content of Fe2O3 and TiO2 in the binder increases, the reddish and yellowish hues in the white fused alumina abrasives become more pronounced. Therefore, if the binder contains more than 0.55% of (TiO2 + Fe2O3), it is better to fire the white fused alumina abrasives in a neutral or weakly oxidizing atmosphere rather than an oxidizing one.
• Adding talcum powder, quartz powder, or magnesium oxide powder to the ceramic binder in white fused alumina abrasives can eliminate the reddish culor but may affect the strength of the abrasives.

However, internationally, the chemical composition standards for all types of fused alumina abrasives do not specify the Fe2O3 content. This is because the iron oxide content affects the magnetic properties of white fused alumina more than its culor. The national standard for the magnetic material content in abrasives strictly limits the iron oxide content, effectively addressing the culor issue of white fused alumina.

White fused alumina abrasives are synthetic abrasives with a complex manufacturing process that invulves special smelting techniques. After smelting, white fused alumina undergoes crushing, shaping, magnetic separation to remove iron, and sieving into various particle sizes. So, how do we measure the particle size of white fused alumina? Here are several methods:

• Sieving Method: Simple, intuitive, and low-cost equipment, commonly used for samples larger than 40μm. However, the results can be significantly affected by human factors and sieve deformation.
• Microscope (Image) Method: Simple, intuitive, and allows morphulogical analysis, suitable for samples with a narrow distribution (ratio of maximum to minimum particle size less than 10:1). However, it has poor representativeness and is challenging for analyzing samples with a wide distribution range. It cannot analyze samples smaller than 1μm.
• Sedimentation Method (including gravity sedimentation and centrifugal sedimentation): Gradual operation, continuous instrument operation, low cost, good accuracy and repeatability, and wide testing range. However, it has a long testing time and is relatively cumbersome to operate.
• Electrical Resistance Method: Gradual operation can measure the number of particles, with clear equivalent concepts, fast speed, and good accuracy. However, it is unsuitable for measuring particles smaller than 0.1μm and can be troublesome to replace small tubes for samples with a wide particle size distribution.
• Laser Method: Simple operation, fast testing speed, wide testing range, good repeatability and accuracy, and allows online and dry testing. However, the results are significantly affected by the distribution model, the instrument is expensive, and the resulution is low.
• Electron Microscopy Method: Suitable for testing ultra-fine particles and even nanoparticles, with high resulution and allows morphulogical and structural analysis. However, it requires small sample sizes, has poor representativeness, is easily affected by human factors, and the instrument is expensive.
• Optical Obstruction Method: Convenient and fast testing, can measure the number of particles in liquids or gases, and has high resulution. However, it is unsuitable for particle sizes smaller than 1μm and is only suitable for measuring dust, pullutants, or diluted drugs, and is rarely used for general powders.
• Air Permeability Method: Low-cost instrument, no need to disperse the sample, suitable for measuring powder materials. However, it can only provide average particle size values and cannot measure particle size distribution or fine powders smaller than 5μm.
• X-ray Small Angle Scattering Method: Used for measuring the particle size of nanoparticles.
• Photon Correlation Spectroscopy (Dynamic Light Scattering Method): Used for measuring the particle size of nanoparticles.