GranuTools instruments range for powder flowability measurement: The basic principles

Selected geometries

Different flow geometries are available to evaluate the rheological properties of a powder or a granular material.
A consortium of French research groups called GDR Midi has studied these geometries carefully.
In their publication dedicated to dense granular flows [1], the different geometries are analyzed: plane shear, annular shear, vertical-chute flows, inclined plane, heap flow, and rotating drum.

Among these powder flow instruments, GranuTools has selected three configurations: the rotating drum (GranuDrum), the heap flow (GranuHeap) and the vertical-chute flow (GranuFlow).
These particular geometries have been selected for two reasons.

Firstly, there are very well known fundamentally. Secondly, there are easy to use.
As a consequence, the instruments proposed by GranuTools are easy to operate and are based on a strong scientific knowledge to measure powder flow properties [2].

Moreover, two others instruments are also available: GranuPack to measure compaction dynamics (Hausner ratio, bulk density, tapped density and densification dynamics) & GranuCharge to measure powders electrosatic properties.

GranuDrum

The GranuDrum instrument is an automated powder flowability measurement method based on the rotating drum principle.
A horizontal cylinder with transparent sidewalls called drum is half filled with the sample of powder.

The drum rotates around its axis at an angular velocity ranging from 2 rpm to 60 rpm.  A CCD camera takes snapshots (30 to 100 images separated by 1s) for each angular velocity.

The air/powder interface is detected on each snapshot with an edge detection algorithm.

Afterwards, the average interface position and the fluctuations around this average position are computed.

Then, for each rotating speed, the flowing angle (also known in the literature as ‘dynamic angle of repose’) αf is computed from the average interface position and the dynamic cohesive index σf is measured from the interface fluctuations.

In general, a low value of the flowing angle αf corresponds to a good flowability.
The flowing angle is influenced by a wide set of parameters: the friction between the grains, the shape of the grains, the cohesive forces (van der Waals, electrostatic and capillary forces) between the grains.
The dynamic cohesive index σf is only related to the cohesive forces between the grains.
A cohesive powder leads to an intermitted flow while a non-cohesive powder leads to a regular flow.

Therefore, a dynamic cohesive index closes to zero corresponds to a non-cohesive powder.

When the powder cohesiveness increases, the cohesive index increases accordingly.

In addition to the measurement of both the cohesive index σf and the flowing angle αf as a function of the rotating speed, the GranuDrum allows to measure the first avalanche angle and the powder aeration during the flow.

GranuHeap

When a powder is poured onto a surface, a heap is formed.
It is well known that both the repose angle and the heap shape strongly depend on grain properties.
In particular, a cohesive powder forms an irregular heap while a non-cohesive powder forms a regular conical heap.

Therefore, a precise measurement of the heap shape provides useful information about the physical properties of the powder sample.

The GranuHeap instrument is an automated heap shape measurement method based on image processing and analysis.
A powder heap is created on a cylindrical support.

In order to obtain reproducible results, an initialization tube with an internal diameter equal to the circular support is installed on the support.

After filling the initialization tube by hand with a fixed volume of powder (typically 100 ml), the tube goes up at the constant speed.

Thereby, the powder is flowing from the tube to form a heap on the cylindrical support.
A controlled rotation of the support allows obtaining different heap projections corresponding to different heap orientations.
A custom image recognition algorithm determines the position of the powder/air interface.

The repose angle refers to the angle of the isosceles triangle with the same surface than the powder heap projected image.
This isosceles triangle corresponds to the ideal cohesiveness heap shape. The repose angle is computed for each image, i.e. for each heap orientation.

Afterwards, an averaged value is computed.

In general, the lower the repose angle is, the better the powder flowability is.
The deviation between the real heap interface and the isosceles triangular heap provides the static cohesive index.
The static cohesive index is computed for each image, i.e. for each heap orientation.

Afterwards, an averaged value is computed.
This static cohesive index is close to zero for a non-cohesive powder and increases when the cohesive forces inside the powder strengthen.

The next Table summarizes the empirical relation between the results obtained with the GranuHeap instrument and powder flowability.

FlowRepose Angle (°)Static cohesive index
Excellent25-30< 0.2
Good31-350.3 – 0.5
Fair36-400.6 – 0.8
Passable41-450.9 – 1.2
Poor46-551.3 – 1.7
Very poor56-651.8 – 2.4
Very very poor>66> 2.5

GranuPack

The bulk density, the tapped density and the Hausner ratio measurement (commonly named “taptap test”) is very popular for powder characterization because of both the simplicity and the rapidity of the measurement.

Moreover, the density and the ability of a powder to increase its density are important parameters for storage, transportation, caking, etc.
The recommended procedure is defined in the pharmacopeia.

This simple test has three major drawbacks.
First, the result of the measurement depends on the operator.

Indeed, the filling method influences the initial powder volume. Secondly, the volume measurements by naked eyes induce strong errors on the results.

Finally, with this simple method, we completely miss the compaction dynamics between the initial and the final measurements.

The GranuPack instrument is an automated and improved tapped density measurement method based on recent fundamental research results.
The behaviour of the powder submitted to successive taps is analysed with an automatized device.

The Hausner ratio Hr, the initial density ρ(0) and the final density after n taps ρ(n) are measured precisely.
The tap number is commonly fixed at n=500.

Moreover, a dynamical parameter n1/2 and an extrapolation of the maximum density ρ(∞) are extracted from compaction curves.

Additional indexes can be used but they are not presented in this report.

The powder is placed in a metallic tube with a rigorous automated initialization process.

Afterwards, a light hollow cylinder is placed on the top of the powder bed to keep the powder/air interface flat during the compaction process.
The tube containing the powder sample rose up to a fixed height of ΔZ and performs free falls.
The free fall height is generally fixed to ΔZ = 1mm or ΔZ=3mm.
The height h of the powder bed is measured automatically after each tap. From the height h, the volume V of the pile is computed.

As the powder mass m is known, the density ρ is evaluated and plotted after each tap.
The density is the ratio between the mass m and the powder bed volume V.

With the GranuPack method, the results are reproducible with a small quantity of powder (typically 35 ml).
The Hausner ratio Hr is related to the compaction ratio and is calculated by the equation Hr = ρ(500) / ρ(0) ,
where ρ(0)  is the initial bulk density and ρ(500)  the tapped density computed obtained after 500 taps.

GranuFlow

The GranuFlow instrument is a laboratory hopper associated with an electronic balance connected to a computer.

The flow rate is measured as a function of the aperture sizes D to obtain a flow curve.
The aperture size is modified easily with a rotating device.

Finally, the flow curve is fitted with a theoretical model to extract the main parameters: the minimum aperture size Dmin and a the Beverloo parameter C related to the flowability of the sample.

GranuCharge

Electrostatic charges are created inside a powder during a flow.
This apparition of electric charges is due to the triboelectric effect, which is a charge exchange at the contact between two solids.

During the flow of a powder inside a device (mixer, silo, conveyor, …), the triboelectric effect takes place at the contact between the grains and at the contact between the grains and the device.

Therefore, the characteristics of the powder and the nature of the material used to build the device are important parameters.

The GranuCharge instrument measures automatically and precisely the quantity of electrostatic charges created inside a powder during a flow in contact with a selected material.

The powder sample flows inside a vibrating V-tube and fall in a Faraday cup connected to an electrometer.
The electrometer measures the charge acquired by the powder during the flow inside the V-tube.

In order to obtain reproducible results, a rotating or a vibrating device is used to feed the V-tube regularly.

In a few words

The wide range of instruments proposed by GranuTools allows solving the diversified problems encountered in industries processing powders and granular materials.

As a function of the needs, GranuTools staff helps customers to select the right instrument, set up the appropriate powder flow measurement protocol and provide results interpretation.

Instrument assessment

Our experience has shown that in most cases, customers need to compare precisely the flowability of a selected sample with the flowability of their reference material.

In specific cases, a deep understanding of the sample is needed to improve a process or to solve a practical problem.

Both approaches (simple comparative powder measurements or deep rheological analysis) are possible with GranuTools powder flow instruments.
[1] On dense granular flows, Eur. Phys. J. E 14, 341–365 (2004)

[2] Measuring the flowing properties of powders and grains, Powder Technology, Volume 224, page 19, 2012

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