Powder is a state of matter, which is different from gas, liquid and solid. Under the action of external forces, the powder will show the deformation and fluidity - rheological properties that solids do not have. Among them, the deformation of powder is related to the friction. The application occasions include powder stacking, powder pressing and molding. The powder is required to be "as static as it is" after deformation, so as to maintain the state unchanged; The application of powder fluidity includes feeding, conveying and other links. It is required that the powder can "move like a rabbit" to improve production efficiency.
1 Friction of powder
The friction of powder is generally characterized by friction angle. The friction angle represents the ability of the powder to block internal failure or sliding, and can measure the difficulty of the powder from static rotation (deformation). In the powder team, the friction angle is just like the iron discipline in the army, which can restrict the behavior of "particle soldiers", and specify the conditions under which the powder can remain static and move. The friction angle of powder includes internal friction angle, angle of repose, wall friction angle and sliding friction angle.
2 Fluidity of powder
For the battlefield of positional warfare, the army with strong and lasting combat effectiveness is likely to win the final victory. In fact, it is not difficult to achieve this effect, as long as there are sufficient troops and can be replaced continuously; Here, the continuity of substitutes is crucial. The position of powder is often the silo. Only when the particles flow and replace continuously to form a continuous "combat power" can the tasks of feeding and conveying be completed successfully. Generally, the powder with small particle size, large specific surface area, rough surface, irregular shape and high moisture content has poor fluidity, which can be improved by powder granulation, powder surface modification, mechanical grinding and polishing, drying, etc. In addition, the fluidity of powder is not only related to the powder itself, but also closely related to the material and structure of the silo. The fluidity of the powder in the silo can be analyzed using the physical quantities in the table below.
3 Application of powder friction and fluidity
3.1 Prevention of powder arching
Arcging is a common problem during powder feeding and conveying operations, which will directly lead to the blockage of the discharge port and prevent the continuous flow of powder.
The reasons for the powder arching include: ① the internal friction and cohesion work together to form the consolidation strength of the powder and hinder the particle movement; ② The wall friction angle between powder and silo wall is large; ③ External factors lead to powder cohesion; ④ The discharge diameter is too small.
The measures to solve the powder arching include: ① correctly design the material and geometric structure of the silo according to the friction and fluidity of the powder; ② The arch is broken by external forces such as pneumatic, vibration and mechanical forces.
3.2 Design of integrated flow silo
The flow mode of powder in the silo is divided into funnel flow and integral flow. A conical channel is formed in the middle of the funnel flow bin, and the particles in the surrounding detention area are dense and show poor fluidity, which leads to the instability of the flow channel, material arch, and last-in-first-out; However, all powders of the integrated flow silo move along the silo wall and are in motion, avoiding the unstable flow of powders, forming a first-in, first-out flow, and minimizing the problems of agglomeration, deterioration and segregation during storage.
When designing the integrated flow silo, the hopper must be steep enough to allow the powder to flow along the hopper wall, and the opening must be large enough to prevent the formation of material arch; In addition, any discharge device must discharge uniformly on the fully open discharge port to avoid particle flow deflecting to one side of the discharge port.
According to the criterion of powder fluidity, the critical condition of powder arching is FF=ff, based on which the minimum discharge diameter of the silo B ＞ f · H can be deduced（ θ)/ρ， Where f is the critical open yield strength when arching, H（ θ) Is the half top angle of the hopper θ Function of, ρ Is the bulk density of powder. Therefore, when designing the silo, the half top angle of the hopper can be reduced θ， However, this will increase the height of the silo; The silo wall should be made of materials with small wall friction coefficient, such as glass, polytetrafluoroethylene, stainless steel, etc. When the surface of the hopper wall is smooth enough, the half top angle of the hopper can be appropriately increased, thus reducing the height of the entire hopper.