Bulk Density of a Powder

The bulk density of a powder is its mass divided by the bulk volume it occupies. The volume includes the spaces between particles as well as, of course, the envelope volumes of the particles themselves. The bulk density should not be confused with particle density which is dealt with in section 2.4. 

There is some confusion as to whether moisture should be included in the mass of the powder when testing the bulk density in most cases, it is appropriate to include the moisture whilst in some, the moisture has to be excluded. In order to avoid confusion, the basis should be clearly stated in either case. 

As to the value of bulk density, this very much depends on the 

The standardization here is, therefore, not so much concerned with the actual measurement, as with the sample preparation. The selection of the definition adopted for measurement depends on the application and the test conditions should simulate the actual conditions to which the powder is subjected in practice. 

Much the same applies to the simple measurement of the compacted bulk density the material is tamped, layer by layer, with a rod of some sort, according to a prescribed procedure. 

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The bulk density of a powder is calculated by dividing its mass by the volume occupied by the powder (Abdullah Geldart, 1999). Tapped bulk density, or simply tapped density, is the maximum packing density of a powder achieved under the influence of well-defined, externally applied forces (Oliveira et al., 2010). Because the volume includes the spaces between particles as well as the envelope volumes of the particles themselves, the bulk and tapped density of a powder are highly dependent on how the particles are packed. This fact is related to the morphology of its particles and such parameters are able to predict the powder flow properties and its compressibility. 

The solids density Ps is the density of fhe solid material from which fhe parficle is made and excludes any pore spaces within the particle. It can be measured using a specific gravity bottle and a liquid in which the particle does not dissolve. The envelope density of a particle is that which would be measured if an envelope covered the external particle surface, i.e. it is equal to the particle mass divided by the external volume. In most analyses the envelope and solids densities are assumed to be equivalent. The bulk density of a powder ps is the effective density of the particle bed defined by... 

The bulk density (/%) of a powder mixture is its mass divided by the bulk volume it occupies. The tapped density (p,) is the density of a powder mixture that has been compacted by tapping or vibration following a specified procedure. Carr57 reported that the more a material is compacted in a compaction or bulk/tapped density test, the poorer its flow properties. Carr index (i.e., compressibility), is defined as ... 

Common indices of flowability are the Hausner ratio and the Can index (compressibility). The increase in bulk density of a powder is related to the cohesiveness of a powder. So measurement of the bulk density of a powder is essential to define the flow characteristics. Ratios of poured-to-tapped bulk densities are expressed in two ways to give indices of flowability ... 

The bulk density of a powder is obtained by dividing its mass by the bulk volume it occupies. The volume includes the spaces between particles as well as the envelope volumes of the particles themselves. The true density of a material (i.e., the density of the actual solid material) can be obtained with a gas pycnometer. The bulk density of a powder is not a definite number like true density or specific gravity but an indirect measurement of a number of factors, including particle size and size distribution, particle shape, true density, and especially the method of measurement. Although there is no direct linear relationship between the flowability of a powder and its bulk density, the latter is extremely important in determining the capacity of mixers and hoppers and providing an easily obtained valuable characterization of powders. 

Tap density is the bulk density of a powder which has been compacted by tapping or vibration following a specified procedure. A large number of machines are available to measure tap density some use a fixed volume of powder and some a fixed weight of powder. The sample is dropped (tapped) a set distance at a set frequency for a fixed number of times. Vibrators of known frequency have also been used for compaction. A simple device, developed by Neumann in... 

The increase in bulk density of a powder is related to the cohesivity of a powder. Ratios of the poured to tapped bulk densities are expressed in two ways to give indices of flowability. 

The measurement of the bulk density of powders in no exception to the general situation outlined above, but it is so fundamental to their storage, processing, and distribution that it does merit particular consideration. The bulk density of a powder is its mass divided by the bulk volume it occupies. The volume includes the spaces between particles and the envelope volume of the particles themselves. The spaces between particles are denoted as porosity or voidage, and can be defined as the volume of the voids within the bulk volume divided by the total bulk volume. Bulk density and porosity are related by... 

The bulk density of a powder, which often is an important product quality aspect, depends, among others, on the inter-particulate porosity. The porosity of particle... 

The range of terms used to describe the general property of the bulk density of a powder is legion but can readily be subdivided into ... 

Tap density is the bulk density of a powder compacted by tapping or being subjected to vibration. A specified mass or volume of powder should be poured into a container or measuring cylinder and then tapped against a hard surface from a standard height (usually... 

A freshly poured powder bed can be initially consolidated by vibration so that the particles settle down into a more intimate contact. As the vessel containing the poured powder is tapped, the powder bed consolidates. Industrial standards have been set up to specify the amount of tapping, and the way in which the tapping is applied to the powder bed to achieve a consolidated bed and the result is known as the bulk density or the tap density of the powder. Commercial instrumentation is available for measuring the bulk density of a powder [1, 2, 3]. 

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