GranuHeap instrument as new tool for a fast and easy determination of the flow ability and the cohesion of a granular material (clay, flour, lactose, polymeric and metallic powders)

When grains are poured on a flat support, they form a heap. Fig. 1 shows two heaps of sugar. While the granular sugar forms a classical conical heap, the powdered sugar forms a strongly irregular heap. Contrary to the granular sugar, the powdered sugar is strongly cohesive due to the small size of the grains. This example clearly shows that the heap shape strongly depends on the grain properties. In particular, a cohesive granular material gives a high value of the repose angle and strong deviations from the conical shape. Therefore, a precise measurement of the heap shape gives some useful information about interactions between the grains. The angle of repose test is very sensitive to the method used to create the heap. Therefore, an initialization protocol has to be defined. Moreover, after the heap formation, the measurement of the repose angle is not obvious. As shown in Fig. 2, the shape of the heap has to be analyzed carefully.

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Therefore, the classical method which consists in measuring the heap height h on a circular support of diameter ø and in calculating the angle with the relation tan(α)=2h/ ø is subject to caution. For measurements with GranuHeap instrument, an initialization tube with an internal diameter equal to the support diameter is placed on the support. After filing manually the tube with the sample of powder, the initialization tube goes up at a constant speed of 5 mm/s. Therefore the powder flows from the tube to form a heap on the cylindrical support (see Fig. 2a) and the handling of the powder by the operator is erased.

This support can rotate slowly around its axis. Then, a CCD camera is able to take pictures of the heap for different orientations. To obtain the results presented in the next sections, 16 images separated by a rotation of 11,25° were recorded. In this way, even if the heap shape is complex and asymmetric (clay, lactose, flour powders with high cohesion), we are able to extract all the geometrical information. From each picture of the heap, a dedicated algorithm finds the position of the interface powder/air by image analysis (fig.2b). The repose angle αr is the angle of the isosceles triangle which has the same surface area as the heap. The isosceles triangle corresponds to the ideal heap shape. In addition to this parameter, a cohesive index σr is measured. This index σr corresponds to the deviation between the heap interface and the ideal heap shape. For a non-cohesive powder, σr is close to zero. Table 1 gives the empirical relation between the flow properties and the repose angle.

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We can observe in table 2 that a large range of powders (free flowing to high cohesive) can be analyzed by using GranuHeap instrument. This technique can be used for having good and rapid information on the rheological behavior of your material. GranuHeap instrument can be used for quality control due to its fastness (less than 1 minute for one complete analysis) and its high degree of automation (easy to perform). Moreover, analysis report is generated to ensure the traceability of your test. This new instrument can also be used in R&D process for developing new formulations or to study the effect of an additive (nature, amount…) in your powder. At every time, you are able to observe in live your measurements (heap rotation) and after the analysis, you can recover the images linked to these analysis (fig.3).

Repose-angle-Powder-Flow-Ability-Cohesion-instrument-measurement-GranuHeap-APTISTable 2: experimental values obtained for different materials tested with GranuHeap instrument.

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(1) Measuring the flowing properties of powders and grains

G. Lumay, F. Boschini, K. Traina, S. Bontempi, J.-C. Remy, R. Cloots, N. Vandewalle

Powder Technology, Volume 224, July 2012, Pages 19-27