The study of powder flow has a lot of implications in industry and powder behavior is non-conformist as far as flow properties are concerned! The reasons are manifold. For one, with the extremely small powder particles, gravitational flow is much less easy to predict. Then there is the loss of energy they undergo due to friction with other particles and the sides of the container. Powder flow is a vast field of study with many scientists having painstakingly decoded the behavior to the best of their ability providing us with information on how powders behave under different conditions.
What are powder flow properties?
In practical applications and difficulties like segregation, pneumatic conveying or caking, certain properties of powder flow are studied to understand the behavior of powders. These are:
- Cohesive strength
- Wall friction
- Internal friction
- Powder Compressibility
- Angle of repose
- Chute angles for powders.
In all these tests, actual plant or process conditions are simulated.
What is a powder rheometer?
A powder rheometer characterizes the flow properties or rheology of the powder. This is its primary function. But rheometers have been continuously refined so that they have become versatile powder flow tester with different accessories to measure wall friction and bulk properties like density, permeability, and compressibility etc.
Powder flow measurement.
The classic tests and associated measurements with a wide range of applications have been
- Flow through an orifice – measuring the rate at which powder flows through an orifice of tightly defined dimensions directly gives a value to the flowability of the powder.
- Angle of Repose – when powder is poured from a vessel, it forms a conical pile. The angle of the cone to the horizontal is directly indicative of the nature of the interaction between the particles and hence a measure of flowability.
- Tapped density tests. – A powder when tapped causes the particles to realign with each other which depends on the cohesivity of the particles, which also affects the flowability. These test values are expressed in ratios of the tapped density to the untapped density like the Hausner Ratio and the Carr’s Compressibility Index.
All the above tests provide parameters according to which the flowability of the powder can be classified.
These are simple tests and are manually done using simple and inexpensive equipment. They are the choice of most manufacturers in the powder industry.
The disadvantages of these simple tests are that, because of the element of manual implementation, they do not achieve consistence in repeatability. There is also little scope of the testing environment to reflect the actual manufacturing environment.