Understand High-purity Microcrystalline alumina ceramic granulation powder in One article

Pressing molding, as an important forming method in advanced ceramic production, has been applied more and more widely. Due to the increasingly fine requirements for raw materials in its production, in order to improve the fluidity of the porcelain material, enhance the sintering performance and lower the sintering temperature, it is necessary to make the alumina powder into false particles, that is, granulation powder, so as to evenly fill the mold, thereby increasing the forming density of the green body and ensuring the sintering density after the porcelain is formed. Granulation is one of the most fundamental and crucial steps in the production process of alumina ceramic products.

Common granulation methods

1. General granulation method

This method involves directly adding plasticizers to the powder, followed by mixing and sieving to obtain pellets. The advantage of the general granulation method is that it is easy to operate, but the problem lies in the poor quality of the pellets, the size of the spheres varies, and the sphericity is not good. 

2. Pressure Granulation Method

 (1) Dry Pressure Granulation Dry pressure granulation refers to the process of shaping powder materials through molds, followed by crushing and spheroidization. The specific steps of dry press granulation are: pre-pressing and conveying → rolling and forming → crushing and granulation. Dry press granulation has the advantages of high granulation efficiency and low production cost. In particular, compared with some granulation methods, such as spray granulation, dry press granulation requires a very low binder content, which can reduce the problems of low sintering density and numerous pores caused by binders.

(2) Cold isostatic pressing granulation Cold isostatic pressing granulation is similar to dry pressing granulation. Both involve subjecting the powder to a certain pressure to form the shape and then crushing and spheroidizing it. The difference lies in that the ceramic powder is placed in a specific mold and then put into a cold isostatic pressing device. Cold isostatic pressing takes advantage of the incompressibility of liquid media and the feature of uniform pressure transmission. It can achieve uniform pressurization of the sample from all directions, ensuring that the pressure on the powder in all directions is uniform and of constant magnitude. The powder is pressed into a green body by cold isostatic pressing process, then crushed by a crusher and sieved to complete the granulation process.

3. Spray granulation method

 The spray granulation method involves mixing the raw material with a plasticizer to form a slurry with a certain solid content. Then, the slurry is sprayed into the granulation tower using a sprayer for atomization, resulting in spherical particles with good fluidity. This method has a high output and is suitable for industrial production.

 4. Freeze-drying method

This method involves spraying a metal salt solution into an organic liquid, which can be immediately frozen. The frozen material is then placed in a low-temperature and low-pressure environment to sublimate, dehydrated and thermally decomposed, and finally the required powder is obtained. The particles obtained by this method have a uniform composition and excellent forming and sintering properties. However, this method is not suitable for mass production in industry and is mostly used for granulation in laboratories.

The shape of granulated powder particles and the reasons for their formation

The quality of the particle shape of alumina ceramic granulation powder will directly affect the fluidity and bulk density of the product. The changes in fluidity and bulk density will cause changes in the pressing density of the green body, which in turn will affect the changes in the firing density and shrinkage rate of ceramics. Therefore, how to ensure that the granulated powder has a good particle shape is a problem that must be solved in the production process. The particle shape with the best fluidity should be spherical. However, during the production process, due to various reasons, the granulated powder often forms non-spherical shapes. The typical shapes of granulation powder particles are as follows:

(1) Medium solid type: Spherical in shape: This is the best form. The apple-shaped shape with a shallow pit on the ball, although not as ideal as the round one, is still a good shape in large-scale industrial production.

 (2) Broken type: The particles are in the form of broken blocks. 

 (3) Adhesive type: In mild cases, one large particle is surrounded by one or more small particles; in severe cases, multiple particles of different sizes stick together into a ball. 

 (4) Hollow type: A hollow shape with a deep pit on the sphere depending on the degree. A abacus bead with a perforated hole in the middle.