What are the preparation methods of spherical alumina?

Alumina powder is one of the most important industrial powder materials. Spherical alumina powder, especially highly dispersed types, has excellent flowability due to its spherical structure. Its dispersibility, larger specific surface area, and physicochemical properties enhance its applications. With rapid industrial development, spherical alumina powder will see deeper development and broader applications.

The different crystal forms and unique properties of alumina and its hydrates enable broad applications. They are used in petrochemicals, electronics, refractories, ceramics, abrasives, pharmaceuticals, and aerospace. On the other hand, it is extremely difficult to improve the performance of alumina powder by directly processing it due to the stability of its physical and chemical properties. To achieve higher-performance alumina powder, advancements must start with synthesis technology.

Preparation of spherical alumina

At present, the preparation methods of spherical Al2O3 powder mainly include ball milling, homogeneous precipitation, sol-emulsion-gel, ball dropping method, template method and injection method.

Ball milling

Ball milling is the most common method for preparing ultrafine alumina powder. By rotating or vibrating, the ball mill grinds raw materials through impact, milling, and stirring. As a result, large particles are refined into ultrafine powder. Moreover, the size of the spherical alumina powder mainly depends on the raw material’s particle state and preparation process.

The ball milling method for spherical alumina is simple, cost-effective, and highly productive. However, the resulting powder has a relatively rough surface, increasing its specific surface area and reactivity. As a result, particle agglomeration occurs easily, making it unsuitable for high-quality spherical powder production.

Homogeneous precipitation method

The precipitation process in a homogeneous solution involves nucleation, growth, and eventual separation from the solution. Typically, this process is in a non-equilibrium state. However, if the precipitant’s concentration decreases or forms slowly in the homogeneous solution, numerous tiny nuclei can form uniformly. As a result, the fine precipitate particles disperse evenly throughout the solution and maintain equilibrium for a long time. This process is known as the homogeneous precipitation method.

The homogeneous precipitation method is mild, achieves a high sphericity rate, and produces particles ranging from 400 nm to 10 μm. It offers good dispersion but has low purity. However, we typically use aluminum sulfate as a raw material, which leads to harmful sulfide emissions during calcination. After sintering, agglomeration and porous structures often occur.

Sol-Gel Method

The sol-gel method uses alkoxides or inorganic salts to form a precursor sol through hydrolysis or polymerization.
We obtain alumina powder after alcohol washing, aging, and calcination. This method requires precise control of pH and reactant concentration. Spherical alumina produced by the sol-gel method has excellent uniformity and high chemical purity. However, the process is complex and costly.

Sol-emulsion-gel method

This method develops based on the sol-gel method. To obtain spherical powder particles, we use the interfacial tension between the oil phase and the water phase to create tiny spherical droplets. This confines the formation and gelation of sol particles within the droplets, ultimately producing spherical precipitated particles.

The sol-emulsion-gel method relies on large amounts of organic solvents and surfactants to form an emulsion. Separating the spherical powder from the emulsion requires a complex process. Additionally, drying and calcination often distort the powder’s spherical shape.

The droplet method

The droplet method is to drop the alumina sol into the oil layer (usually paraffin, mineral oil, etc.), and form spherical sol particles by the action of surface tension.

The sol particles are then gelled in an ammonia solution, and finally the gel particles are dried and calcined to form spherical alumina. This method is an improvement of the sol-emulsion-gel method, applying emulsion technology during the aging phase of the sol and keeping the oil phase stable, thus eliminating the need for separation of the powder and oily reagents. We typically use the droplet method to prepare larger spherical alumina particles, mainly for applications as adsorbents or catalyst supports.. However, the use of hot oil and the need to keep the sol dropping for extended periods are drawbacks of this method.

Template method

The template method uses spherical raw materials as agents to control the morphology during the process. The resulting products are typically hollow or have a core-shell structure.

Spray method

The essence of preparing spherical alumina by spraying is to achieve phase transformation in a relatively short time. We sphericalize the product by using the effect of surface tension. Based on phase transformation characteristics, we can divide it into spray pyrolysis, spray drying, and spray melting. The spraying method is suitable for producing micron- to nanometer-sized spherical alumina powders, just like the aerosol decomposition method. Although the reaction equipment is complicated, it is easy to realize industrialization.

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