Powder Mixing Principle and Mixing Quality Analysis
Source: internal company
Powder mixing (homogenization) refers to the process in which more than two kinds of solid powder materials reduce their unevenness under the action of external force. For example, the total mixing in the drug production process is to allow the components of the drug to be evenly distributed into the auxiliary materials to meet the requirements of product quality.
1. Mixing (homogenization) mechanism
Due to the different purposes of powder homogenization, the requirements and evaluation methods for homogenization are not complete, and the homogenization methods are also different, but the basic principle of the homogenization process is basically the same (here mainly talks about homogenization, mixing, and evaluation of solid powder materials). To sum up, there are mainly three types:
• Convective mixing: Clumps of material move from one place of material to another, similar to the convection of fluid.
• Diffusion and mixing: The separated particles are dispersed on the continuously displayed inclined plane, and blend and penetrate with each other like the same diffusion effect to obtain uniformity of mixture.
• Shear mixing: The relative movement between particles inside the material pile forms a number of sliding surfaces in the material, which are mixed and mixed with each other like thin-layer fluid samples.
2. The randomness of mixing
Taking solid A and solid B, two equal materials with the same particle size, as mixing examples, if the density of A and B is the same, it seems very simple to reach the complete mixing state in theory, as long as A and B are staggered with each other, the complete ideal mixing can be achieved. However, if A is a multiple of B, there must be two A particles and one B particle arranged at the beginning. If the density of A and B are different and B is twice that of A, one A must be juxtaposed with two B grains. Homogenization in this way is unlikely to occur in industrial production, so the mixing state is the disordered and irregular arrangement. It is generally believed that the mixing process is a "random process", also known as "probabilistic mixing", and the degree he can achieve is called random complete mixing. The actual mixing problem is much more complicated than the above situation. Not only are the particle sizes uneven and the densities different, but also the solid powder properties that affect the mixing of solid particles are far more than density and particle size, as well as the mixing effect in the mixer (in the pile) (referring to the state of complex mixing movement).
3. The factors affecting mixing
Due to the change of the properties of the mixed materials and the mode of movement in the mixing process, the mixing process cannot reach the mixing state, especially for finer particles. Due to the agglomeration of powder and the response of static electricity, the phenomenon of reverse mixing homogenization is called reverse mixing, also called segregation. Obviously, segregation will hinder random complete mixing. Therefore, it can also be said that the mixing state is the balance between material separation and mixing. The establishment of the balance is based on certain conditions. Appropriate change of these conditions can transform the balance into a favorable aspect for mixing, thus improving the mixing operation. The factors affecting mixing are as follows:
1. Properties of particles
It mainly includes particle size and particle size distribution, particle shape, roughness, particle density, loose bulk density, electrostatic charge, moisture content, brittleness, angle of repose, fluidity, agglomeration, and elasticity. For example, the shape of particles affects the fluidity of particles; The density difference of particles will cause density segregation in mixing. Several Points of Two-dimensional Motion Mixer and YYH Two-dimensional Motion Mixer in Preventing the Above Segregation Factors 2. Two kinds of equipment are arranged in the barrel of the middle mixer, and scrapers are arranged in the middle mixer to force mixing, which conforms to the mechanism of diffusion mixing and shear mixing.
2. Mixing process
Operating conditions, including the number of each component of the material in the container and its ratio to the volume of the mixer, the method, and sequence of each component entering the mixer, the stirring components, and the rotating speed of the mixer. Mixing time: It changes according to different mixing equipment and different mixing materials. The mixing time of two-dimensional and two-dimensional motion mixers is over 25 ~ 30 minutes.
3. Mixer performance and mixing mode
Mixer performance includes the body, size, and geometry of stirring parts, cleaning performance, feed position and the surface processing quality of structural materials and the performance of the unloading device affect the movement of particles in the mixer, such as flow mode and speed. The two-dimensional and two-dimensional mixing homogenization methods are mechanical homogenization methods, which are beneficial to improve the mixing uniformity.
4. Mixing (homogenization) quality evaluation
Qualified rate (average) Qualified rate refers to the percentage of several samples within the upper and lower limits of the specified quality standard, that is, the qualified rate within the specified range. Although this calculation method also reflects the fluctuation of samples within a certain range, it cannot reflect the fluctuation range of some samples and does not provide the distribution of various fluctuation ranges in some samples. If the content of a component in the two samples is between 0.95% and 1.05%, the qualified rate is 100%. The average group was 1.0112%; The second group is 1.0016%, and the qualified rate and average value of the two groups are similar. The fluctuation amplitude of the two groups is quite different by single comparison, which indicates that other comparison methods are needed.