The bulk density of a powder changes due to the force exerted on a powder.

This article discusses:

  • The different types of density and corresponding formulas
  • How density is measured
  • How the Hausner ratio relates to density

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Particle density

Density is a measure of how much mass of the substance is present in a given volume and is calculated by dividing mass by volume. The density can also differ. These different densities have to do with the fact that a powder is actually a mixture of particles and gas with space between the powder particles, because many powders are porous. The particles contain pores that provide different density calculations.

Bulk density

The bulk density of a powder changes due to the force exerted on a powder. In general, a powder with good flow properties will give a small change in bulk density when pressure is applied. Cohesive powders show a large change in density when pressure is applied. The bulk density of a powder will therefore change with the pressure that is applied. The bulk density is best represented as a graph in which the bulk density is plotted against the applied force. The bulk density at the top of a silo is different than at the bottom of a silo. The bulk density can be determined with a powder flow tester.

$Bulk\ density\ =\frac{Massa\ poeder}{Volume\ poeders\ na\ uitoefenen\ van\ druk\ } in\ \left(\frac{Kg}{M^3}\right)$
Infant formula

A tap density tester must ensure that particles fall into the voids, reducing volume and decreasing bulk density

Bulk density

The bulk density is a unit that is widely used in the processing of powders and granulates. Grain that is ground with a hammer mill can be taken as an example. There are all kinds of spaces between the grains for grinding the grain. After grinding, the grains are ground into flour. These particles are smaller so that there are fewer spaces between the particles. As a result, it takes up less volume. As a result, the bulk density increases after grinding.

$Stortgewicht\ =\frac{Massa\ poeder}{Volume\ poeders\ } in\ \left(\frac{Kg}{M^3}\right)$

Tap density 

The tap density depends on how compact the powder bed is: the more compact, the greater the tap density. Normally a tap density tester is used to measure the tap density. In the tap density meter, a specified powder mass is poured into a graduated cylinder. The poured bulk density here equals the mass of the powder divided by the measured poured volume. The measuring cylinder vibrates a number of times, giving the powder a more compact structure and ultimately reducing the volume.

$Tap\ density\ =\frac{Massa\ poeder\ na\ trillen}{Volume\ poeders\ } in\ \left(\frac{Kg}{M^3}\right) $

The principle behind the tap density tester is the Hausner ratio. This is based on the fact that strongly cohesive powders also have strong mutual attraction forces. These help to counteract gravity, allowing particles to support themselves around empty spaces. This creates larger voids in the bulk. The vibration of a tap density tester must cause the coherent attractive forces to weaken and particles to fall into the voids. This reduces the volume and increases the bulk density. This principle can also be applied when filling big bags. By vibrating the big bags during filling, the bulk density increases and a larger mass can be filled within the available volume.

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 ratioFlow properties
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

In addition to a measure of the flow properties, the Hausner ratio can also be seen as a measure of the ability to compact powders by vibration. The better a powder can be compacted by vibrations when filling a big bag, the more this advantage is negated because it also makes it more difficult for the product to leave the big bag. This is caused by the deteriorated flow properties.

Juul Jenneskens

Name: Juul Jenneskens
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A tap density tester must ensure that particles fall into the voids, reducing volume and decreasing bulk density