Good flow properties are desirable for optimal processing of powders.

This article discusses:

  • The factors that contribute to good flow characteristics
  • Sizes for flow properties
  • Different types of currents

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The six factors that contribute to good flow characteristics

Good flow properties are desirable for optimal processing of powders. For example, the better the powders flow in and out of a hopper, the faster and easier the processing. Good flow properties largely depend on the friction between the powder particles. These are created by (static) charges on the surface of the powder particles between the particles and the sphericity. Minimal cohesion between the powder particles results in maximum flow properties.

The flow properties of an installation can be traced back to six different facets:

  • The flow properties of the powder,
  • Gravity,
  • Compression of powders in the installation,
  • Vibrations in the installation,
  • Aerodynamics in the installation,
  • Mechanical properties of the installation.

Focusing on a combination of these six facets can improve poor flow characteristics. In addition, adjustments in the powder properties such as shape and size cause a difference in the flow properties of a powder. Flow properties also change with the addition of other powders. By adding fine powders, the surface of powder particles can be covered, significantly reducing friction and improving the flow properties of the powder.

Sizes for flow characteristics

As indicated above, the flow properties of a powder are partly influenced by the compression. The graph below shows how a powder can behave due to compression. The x-axis shows the consolidating tension to compress the powder, also known as Major principal consolidation tension. The y parameter is the strength of the powder known as Unconfined failure strength. The ratio between these numbers is a measure of the flow properties which is equal to ff, flow function. This yields the following formula:

$ff\ =\frac{Major\ principal\ consoledating\ stress}{Unconfined\ failutre\ strenght\ } $

 

ff<1Not flowing
ff>1>2Very cohesive
ff>2<4Cohesive
ff>4>10Easy flowing
ff>10Free flowing

Another important measure of flow properties is the Hausner ratio. The Hausner ratio can be calculated using the following formula:

$Hausner\ ratio\ =\frac{Tap\ density}{Stortgewicht\ } \left(dimensieloos\right)$

The Hausner ratio gives an index of the flow properties of the powder.

The table below shows how the flow properties relate to the Hausner ratio.

Hausner ratioStroomeigenschappen
1.00 - 1.11Excellent
1.12 - 1.18Good
1.19 - 1.25Decent
1.26 - 1.34Acceptable
1.35 - 1.45Bad
1.46 - 1.59Very bad
>1.60Really bad

The smaller the particle, the greater the contact surface per unit mass and the greater the van der Waals attraction force

The two kinds of schools

Within the concept of flow properties, a distinction is again made between two terms: internal friction and cohesion. Internal friction refers to friction that takes place between two powder particles under normal pressure. When there is no pressure at a certain moment, there is still a certain attraction between the powder particles that resists the flow. This is cohesion. In the dry powders industry, this force is called the Van der Waals force. This is an electric and magnetic force that arises at the molecular level due to the polarization and results in positive or negative attractive forces between atoms on the surface of the powder particles. The smaller the particle, the greater the contact surface per unit mass and the greater the Van der Waals attraction.

In addition to the size of the powder particles, the presence of liquids is a second factor that influences cohesion. Liquids such as water or oil increase the contact surface between particles, creating liquid bridges between the parts and allowing a capillary force to eventually connect the dust particles. The Van der Waals force then decreases due to the presence of liquid.

Powders with good flow properties will come out of a bunker by gravity. When designing bunkers, the first step is therefore always to determine the flow properties of the powder with the help of a powder flow tester. The aforementioned ff is hereby determined. The powder flow tester can be used to analyze how the forces between the powder particles mutually and the powder particles with the wall change with a changing pressure. After all, at the top of a bunker less pressure will be exerted on a particle than at the bottom of a bunker.

To make a good design for a bunker, it is therefore important to know the flow properties of powders.

Stroomeigenschappen van poeders_Van Der Waals krachten bij water.jpg

Van der Waals forces in water