China Powder Network News Press molding, as an important molding method in the production of advanced ceramics, has been more and more widely used. Since its production requires more and more finer raw materials, in order to improve the fluidity of the porcelain material, improve the sintering performance, and reduce the sintering temperature, it is necessary to make the alumina powder into false particles, that is, granulated powder, so as to uniformly fill the model and improve the blank. The molding density of the body ensures the sintering density after the porcelain is formed. In the production process of alumina ceramic products, granulation is one of the most basic and important links.
Commonly used granulation methods
1. General granulation method
In this method, plasticizer is directly added to the powder, and the pellets are obtained by mixing and sieving. The advantage of the general granulation method is that it is easy to operate, but the problem is that the quality of the pellet is poor, the size of the sphere is different, and the spherical shape is not good.
2. Pressurized granulation method
(1) Dry press granulation
Dry press granulation refers to the process of forming powder through a mold, and then crushing and spheroidizing. The specific steps of dry-pressing granulation are: pre-pressing conveying → rolling forming → crushing and granulating.
Dry pressing granulation has the advantages of high granulation efficiency and low production cost. In particular, compared with some granulation methods, such as spray granulation, the content of binder required for dry pressing granulation is very low, which can reduce the problems of low sintered density and many pores caused by the binder.
(2) Cold isostatic pressing granulation
Cold isostatic pressing granulation is similar to dry pressing granulation. The difference is that after the ceramic powder is placed in a specific mold, it is placed in a cold isostatic pressing device. Cold isostatic pressing takes advantage of the incompressible characteristics of liquid media and the characteristics of uniform pressure transfer, which can realize uniform pressure on the sample from all directions, and ensure that the pressure received by the powder in all directions is uniform and the size is unchanged. The powder is pressed into a green body through a cold isostatic pressing process, and then crushed by a crusher and sieved to complete the granulation process.
3. Spray granulation method
The spray granulation method is to mix the blank and the plasticizer to form a slurry with a certain solid content, and then spray the slurry into the granulation tower with a sprayer for atomization to obtain spherical particles with better fluidity. This method has a large yield and is suitable for industrial production.
(Spray granulation production process)
Inproduction, spray granulation can avoid re-agglomeration or sedimentation of each component in the slurry, so that the slurry can be kept uniform; at the same time, the slurry can be uniformly atomized, and spherical particles with uniform particle size distribution and good fluidity can be obtained. Such particles have excellent forming and sintering properties, which help to finally obtain a sintered body with high density and high flexural strength.
4. Freeze drying method
This method is to spray the metal salt solution into the organic liquid, which can be frozen immediately. The frozen material is then sublimated in a low temperature and low pressure environment, dehydrated and thermally decomposed, and finally the desired powder is obtained. The particles obtained by this method are uniform in composition and have good forming and sintering properties. However, this method is not suitable for mass production in industry, and is mostly used in laboratory granulation.
Granulated powder particle shape and formation reasons
The quality of the alumina ceramic granulation powder particle shape will directly affect the fluidity and bulk density of the product. Changes in fluidity and bulk density will cause changes in the compacted density of the green body, which in turn will affect the changes in the firing density and shrinkage rate of the 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 most fluid particle shape should be spherical. However, in the production process, due to various reasons, the granulated powder tends to form a non-spherical shape. There are four typical shapes of granulated powder particles:
(Four shapes of alumina granulated powder)
(1) Medium-solid type: spherical shape: the best shape; apple-shaped with a shallow pit on the ball, although not as ideal as a spherical shape, it is still a good shape in large-scale industrial production.
(2) Broken type: The particles are broken pieces.
(3) Bonding type: In the light case, one or more small particles are adhered around a large particle, and in the heavier case, many particles of different sizes stick together into a group.
(4) Hollow type: depending on the degree, there is a hollow shape with a deep pit on the sphere; there is a bead shape with a through hole in the middle.
Broken type, hollow type, and bonded type are all bad shape particles.
(1) There are two main reasons for the formation of the broken type: one is that the hard shell on the particle surface is formed too fast, and the internal pressure is too high, causing the sphere to explode and form fragments; Caused by rolling or squeezing, caused by external force. The degree of fragmentation is related to the mechanical strength of the particles, and the ones with low strength are easy to be broken.
(2) The reason for the formation of the hollow type: In the drying tower of spray drying, there is an intake temperature of 300-400 ° C, a slurry droplet is sprayed out by the spray gun, and falls at a certain height, and it is dried in the process. . Due to the high temperature, the surface of the slurry dries rapidly, and the binder and micropowder particles on the surface combine to form a hard shell, which closes the surface of the sphere, and the internal moisture continues to rapidly vaporize to form the air pressure in the sphere. When the air pressure reaches a certain level, the gas It burst through the sphere from the weak point, leaving a hole in the sphere.
(3) There are three reasons for the formation of the bond type: the spray gun is not well atomized; the air flow in the granulation tower is turbulent and irregular; the slurry is too thin or the spray amount is too large.
Nowadays, in the contemporary era of rapid development of science and technology, powder granulation technology, as a main method of powder and granule processing, is becoming more and more important with the emphasis on environmental protection, the improvement of the automation degree of the production process and the specific requirements of the production process. It shows that the granulation of powder products has become an inevitable trend of powder post-processing technology in the world.
Reference source:
[1] Zhang Benqing. Causes and prevention of bad shape of alumina ceramic granulation powder
[2] Liang Yue. Surface modification treatment of zirconia powder
[3] Wang Tengfei et al. Research progress of silicon nitride ceramic powder granulation
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