Powder mixing is a process of uniform distribution of particles of different physical and chemical properties through mechanical or fluid methods. In short, the inhomogeneity of two or more solid powder materials is reduced to the lowest under the action of external force. With the rapid development of new materials and industrialization, the homogeneous mixing of powder composites becomes more and more important.
One, mixing definition and mixing mechanism of powder
(1) mixing definition of powder
Powder mixing refers to the process of mixing two or more than two different components of the powder evenly. Mixing is also the first step in the whole process of preparing the product. The distribution of the components of the mixture is very important, because the unevenness of the mixture can not be adjusted in the subsequent process.
Classification of mixed forms of powder
(2) mixing mechanism
The mixing mechanism of powder is the diffusion, convection and shear between particles. The common mixing methods are three dimensional mixing, double cone mixing, ball milling mixing and advanced double motion mixing. The double motion mixing is to make use of the rotation of the barrel and the rotation of the inner blade of the barrel (2 times the rotation speed of the barrel), and force the powder to spread, convective and shear, and form a complex motion, so as to achieve the ideal mixing effect.
Two. Factors affecting the homogenization process of powder mixing
(1) particle size
The coarse powder particles have good compressibility, but the formability is poor, the fine particle size is good, but the compressibility is poor, so the proper matching of the particles can improve the filling type of the powder, improve the compressibility of the powder and improve the sintering properties of the powder. However, the larger the particle size difference is, the less likely it is to mix. Fine powders are easy to bridge and adhere to each other, which impede the movement of particles and are not easy to disperse evenly.
(two) the density of the loose assembly (specific gravity)
The bigger the density difference is, the less easy it is to mix. The lightweight powder with small bulk density is easy to float. When mixing, it always floats above, so it is difficult to mix with the heavier powder components and difficult to mix evenly.
The powders have poor fluidity and adhere to each other, which is not conducive to the interpenetration and convection of powders, and is not easy to mix well with other powders.
(four) the properties of powder
The particle properties include particle size and particle size distribution, particle shape, roughness, particle density, static charge, moisture content, fragility, repose angle, fluidity, mass and elasticity, etc. For example, the shape of particles affects the fluidity of particles, and the density difference of particles will result in density segregation.
Three, introduction of common powder mixing machine
At present, the commonly used powder mixer is mainly composed of three dimensional mixer, V mixer and double cone mixer.
(I) three dimensional mixer
The working principle: the barrel body of the material is driven by the active axis, and makes the complex movement of moving and rolling round and rolling around, making the material move along the barrel body as the ring, radial and axial three direction compound movement, so as to realize the mutual flow, diffusion, accumulation and mixing of various materials, and achieve the purpose of uniform mixing.
Three the advantage of dry powder mixer is that there is no material dead angle in the barrel.
The disadvantage is: low loading rate, generally not more than 60%; for ultra light powder, ultrafine powder is not easy to mix evenly, easy to float on the top of the barrel, it is difficult to achieve a full and uniform mixing between particles and particles on the micro.
(two) dry powder mixer
Working principle: using the two cylinder barrel length is not equal, the principle of asymmetrical line, when the mixing machine rotates the material from decomposition to combination, from combination to decomposition, to promote the material transversal communication, each rotation one circle of lateral force to make part material from one barrel to another, so that the material crosswise radial mixing, decomposition group Combined with each other, the material can achieve a uniform mixing effect.
The advantages of the V mixer are that when mixed materials with good fluidity and similar physical properties are obtained, a better mixing effect can be obtained.
The disadvantages are: when the material property gap is large, the ideal mixture can not be obtained; the energy consumption is much higher; the intergranular impact and the friction are large, which can easily change the basic characteristics of the powder component, such as the size of the particle and the shape of the particles.
(three) double cone mixer
Working principle: the powder is added to the biconical barrel, and with the continuous rotation of the barrel, the powder material has a complex impact movement in the container to achieve a uniform mixing.
The advantages of double cone mixer are: quick mixing between powders.
The disadvantages are: when the rotational speed is too slow, the material is easy to roll in layers, can not mix quickly and effectively, and easy to form spherical agglomeration.
Four. Application fields of different types of mixers
(I) three dimensional mixer
Three dimensional mixer is widely used in medicine, chemical industry, food, feed, pesticide and other industries. It is not only suitable for the mixing of ordinary powder material, but also used in the mixture of oil containing more, more sugar, large viscosity, flammable, explosive, heating and curing, heating softening material. Such as drugs, powder coatings (premix and finished products mixed), leveling agent, food additives, etc., and can be used with the production line.
(two) V type mixer
V mixer is suitable for mixing more than two kinds of dry powder and granular material, especially for small amount of additives. It is widely used in the fields of pharmaceutical, chemical, food, plastic, cosmetics, ceramics, metallurgy and so on.
(three) conical mixer
The conical mixer is suitable for mixing solid and solid, solid and liquid, liquid and liquid. It is widely used in chemical industry, dye, petroleum, metallurgy, building materials, rare earth and other fields. Thermal sensitivity of conical mixer
Raw material homogenization system for a large variety of alumina enterprises in China
For the large-scale raw material homogenization system, professional experts believe that:
Homogenization of alumina powder is also a reference to the advanced experience of foreign countries, Japan Zhao and 30 years ago has been in industrial application. The purpose of raw material homogenization is to ensure the stability of the production of ingredients and processes. Strictly speaking, the products of the factory should ensure the uniform stability of the quality. The quality assurance of the products of the same specification should be consistent with the quality assurance, whether in the current period or in different periods. The consistency requirement of powder material is even greater than that of the product itself.
The general principle of the large-scale homogenization system is to put the raw materials in different periods and different suppliers into different storage tanks, measure the material by weighing the multi point on the belt and add the mineralizer. Homogenization after the product comes out, Zhaohe approach is to classify the different products stored in a can, and then multi-point measurement of cutting, pneumatic mixing.