Solids Characterization

Characterization of macroscopic, physical powder properties

The determination of the macroscopic physical properties of a powder sample is an essential part of its overall characterization. It provides a fundamental understanding of the behavior of a powder in its entirety and in particular with regard to processing, packaging, transport, storage and use of the powdered material. Since the macroscopic behavior of a powder depends very much on the respective conditions – e.g. the bulk density or the angle of repose of a powder accumulator is sensitive to the drop height and the resulting compaction – the measurement conditions for the determination of such parameters are defined in detail, depending on the material type, in different standards. The most commonly and widely used standard for macroscopic powder characterization is the ASTM D6393. Whereas many norms only define the measurement of one parameter (e.g. ISO 3953 – Metallic powders – Determination of tap density, ISO 3923 – Metallic powders – Determination of apparent density), the ASTM norm D6393 specifies the criteria for determining 10 different parameters, so-called Carr indices:

  • Measurement of Carr Angle of Repose
  • Measurement of Carr Angle of Fall Carr Angle of Collapse
  • Calculation of Carr Angle of Difference
  • Measurement of Carr Angle of Spatula
  • Measurement of Carr Loose Bulk Density
  • Measurement of Carr Packed Bulk Density or Tap Density
  • Calculation of Carr Compressibility
  • Measurement of Carr Cohesion
  • Measurement of Carr Uniformity
  • Measurement of Carr Dispersibility

Depending on the application, each of these parameters can be used for an evaluation of the powder or a statement of the powder quality can be made in the subsequent process. For example, the angle of repose and the angle of collapse should be taken into account when designing a conical storage silo. For the design of packaging bags and barrels or for the production of tablets from powder raw materials, tapping and bulk densities are decisive parameters.

Innovative, state-of-the-art instruments, such as the PowderPro A1 make it possible to determine the Carr indices fully automatically. This becomes possible thanks to the angle measurements by means of a CCD camera and an image processing routine, automatic control technology and state-of-the-art software with implemented standardized operating procedures (SOPs) as well as the direct data communication with an electronic balance, so that the weighing data are transmitted automatically and directly to the measuring system.

In addition to the individual determination of the Carr indices, the PowderPro A1 also allows the calculation of the resulting parameters, which make the characterization and evaluation of a powder sample perfect. In addition to the parameters defined in ASTM norm D6393, the PowderPro A1 can be used to determine the following parameters:

  • Voidage (powder porosity)
  • Flowability index
  • Floodability index

The flowability and floodability indices are important when powdery raw materials or products, for example, are conveyed in pipelines by air flow or when dust propensity has to be assessed from an occupational safety point of view.

Instruments

Definition of parameters for the determination of macroscopic powder properties

Carr Angle of Repose

The angle of repose is the angle between a horizontal plane and the free surface of a conical powder pile (in static equilibrium) accumulated by powder falling from a defined height.

It is determined by the height (h) and the radius (r) of the conical powder pile:

Carr Angle of Repose = tan-1 (h/r)

Carr Angle of Fall or Carr Angle of Collapse

The angle of collapse corresponds to the angle of repose of a powder heap to which a defined impulse shock has been given.

Carr Angle of Difference

The angle of difference is the difference between angle of repose and the angle of collapse.

Carr Angle of Spatula or Flat-Plate-Angle

The flat-plate angle is the angle between the surface of a powder pile and a defined spatula on which it was piled up after burying the spatula under a powder bed and extracting it.

Carr Loose Bulk Density

The bulk density is determined from the volume and mass of a loosely packed powder.

Carr Packed Bulk Density or Tap Density

The tap density is determined by the volume and mass of the powder after it has been compressed, which is done through repeated dropping of a measuring cylinder from a defined height.

Carr Compressibility

The compressibility is calculated from the ratio of the difference between the bulk density (L) and tap density (P) to the determined tap density:

Carr Compressibility [%] = 100 (P – L)/P

Carr Cohesion

The cohesion is a measure for interparticular interactions and is determined by defined sieving of the powder sample (at a given sieving time).

Carr Uniformity

Uniformity is a measure for the width of the volume-based particle size distribution determined by sieving analysis.

Carr Uniformity = d60/d10

Carr Dispersibility

The dispersibility is determined by dropping 10.0 g of the powder to be characterized from a defined height into a bowl and subsequently determining the mass (m) of the powder remaining in the bowl.

Carr Dispersibility = 100 (10 g – m)/10 g

Voidage

The voidage or so called powder porosity indicates the percentage value of the interparticle volume to the total volume of the powder compacted to volume constancy.

Flowability index

The flowability index is calculated from the weighted sum of the angle of repose, flat-plate-angle, compressibility, cohesion and uniformity indices.

Floodability index

The floodability index is calculated from the weighted sum of the indices of the flowability index, the angle of collapse, the angle of difference and the dispersibility.

Literature and norms

/1/ ASTM D6393 – Standard Test Method for Bulk Solid Characterization by Carr Indices

 

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