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DENSITY OF BULK MATERIALS

The bulk density(ρb0), (ρSch) is the ratio of weight to total volume including the voids and pores of the bulk material in the unconsolidated state. It is given in [kg/m³] or in [g/cm³].


TAP DENSITY

DIN EN ISO 787-11, ASTM B527

Mechanical stress may result in compaction or consolidation of the bulk material.

The Hausner ratio is a dimensionless physical index which describes the ratio of the unconsolidated bulk density volume divided by the tapped volume.

Compressibility Index is defined as the percentage change in volume by tapping a sample of fixed mass.

Compressibility Index
(%)
Hausner ratio Flow character
< 10 1.00 - 1.11 Excellent
11 - 15 1.12 - 1.18 Good
16 - 20 1.19 - 1.25 Fair
21 - 25 1.26 - 1.34 Passable
26 - 31 1.35 - 1.45 Poor
32 - 37 1.46 - 1.59 Very poor
> 38 > 1.60 Very, very poor

 

 

 

COMPRESSION DENSITY

ASTM-D6682-B

A bulk material is exposed to various external forces. When storing a bulk material in a silo, the pressure the bulk material is exposed to increases with increasing depth. The packing density of the particles also increases (i.e. the density of the bulk material increases).

The change in the packing density of the particles has a great impact on the behaviour of the bulk material. With smaller particle spacing, the contact surfaces between the particles are larger and the  forces between the individual particles are increased.

The bulk density (ρb) describes the ratio of the mass (m) of the bulk material to its volume (V). The density of a bulk material at a pressure of zero is called the bulk density (ρb0). With cohesive materials, the resulting bulk density is not completely reversible, and exhibits hysteresis.

When filling a container or when there is mass flow in the shaft and no shear movement has occurred in the bulk material, the material is subject to uniaxial compression. Through shearing a reorientation of the particles to one other is possible, which results in a higher density.

 

 

 

 

 

 

FLOW DENSITY

ASTM-D6682-A

During measurement of the yield locus, the density is measured simultaneously under the influence of pressure and shear stress. Even if the DIN only considers uniaxial compaction, both types of compaction can be used in automatic evaluations.

SHEAR STRESSES - MEASUREMENT ACCORDING ASTM D6682-A
σr Τrm ρrm Τrs ρrs σN Τm ρm
[Pa] [Pa] [g/cm³] [Pa] [g/cm³] [Pa] [Pa] [g/cm³]

14871 10758 1.499 9143 1.497 14871 10649 1.499
               
...              
               
14871 11132 1.501 9460 1.498 3103 2596 1.498

ρb0 = 1.286 g/cm³ bulk density
ρbr = 1.497 g/cm³ density(σr) = bulk density after consolidation

 

 

 

Example: Uniaxial compression and flow density of a bulk material

density measurement graph

Silo calculations according to EN 1991-4 or DIN 1055-6 yield locus

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