Summary of Powder Mixing-This is a scientific real-time operation process, and its purpose is to uniformly mix different kinds of powders together.
I. broad spectrum physical characteristics of powder
At present, no matter what the chemical composition of the food, chemical industry and metal powder we encounter, their external physical properties have the following important indexes.
1. Specific gravity of powder and its influence on mixing
The specific gravity of powder is also usually expressed by loose specific gravity and vibrated specific gravity. Generally, the specific gravity of the powder is between 0.2 and 2.0. Such powder is relatively easy to mix evenly with other powders, and it is relatively difficult to mix with other powders if it is lower than 0.5 or higher than 2.0. This is because too light powder floats easily and too heavy powder sinks easily, which makes mixing difficult.
For example, the loose specific gravity of the resin powder in the left of the above figure is 0.4g/cm3, and the vibrated specific gravity of the resin powder in the right of the above figure is 0.48g/cm3. It can be seen that the compaction density of some powders will vary greatly.
2. fluidity of powder and its influence on mixing
The fluidity of the powder can be expressed by the stacking angle (left in the above figure) or measured by a standard fluidity measuring instrument (right in the above figure). Powder with poor fluidity (stacking angle greater than 40) is difficult to mix due to the large surface friction of powder. But it also has the advantage that once the mixture is evenly mixed, secondary segregation and stratification of the mixture are not easy to occur. Powder with good fluidity (stacking angle is less than 30), because its powder is "smooth" and can be easily mixed evenly, with short mixing time and high efficiency. The other side of things cannot be ignored, that is, the powder with too good fluidity will produce new segregation and stratification during blanking and transportation after being mixed evenly.
3. Particle Size and Its Influence on Mixing
The particle size is usually expressed by mesh number, and the particle size suitable for mixing is 50 mesh to 300 mesh. Large powder with less than 50 mesh is not easy to blend into the main powder, and is often stored at the bottom. Small powders above 300 mesh have changed personality due to their small size. Usually, the smaller the powder, the more active it is, and the easier it is to float above the powder, causing mixing difficulties.
For example, the 20-mesh crystalline salt particles on the left in the above figure are easy to stay at the bottom of the container in the mixing process, and the 2000-mesh pigment powder on the right in the above figure is easy to float above the material and difficult to mix during mixing.
4. Homogeneous mixing
Homogenization means that several kinds of powders are prepared for mixing. Their density, particle size and fluidity are basically similar. This is an ideal mixing condition and the mixing work is efficient and thorough.
The upper left shows 800 mesh atomized copper powder with a specific gravity of 4.0, and the upper right shows 650 mesh reduced iron powder with a specific gravity of 3.8. The difference between the two powders is very small and belongs to homogeneous mixing. If they are mixed together evenly, the mixing is relatively simple.
5. Differential mixing
The mixing task we take over is often differential mixing, i.e. the density, particle size and fluidity of several powders to be mixed are different or quite different, which leads to many mixing process difficulties. Such as the difficulty in mixing light and heavy powders, the difficulty in mixing superfine powders, the difficulty in mixing fluidity, the difficulty in segregation due to excessive fluidity difference, etc.
On the left of the above picture is 800 mesh flour, and on the right of the above picture is 20 mesh salt particles. The particle sizes of the two powders are very different and belong to differential mixing, so it is difficult to mix the two powders evenly.
Usually, the difficulty of mixing the powder evenly is only directly related to the above physical characteristics.