Effect of Particle Size and Cohesion on Powder Yielding and Flow

Abstract

The bulk properties of powders depend on material characteristics and size of the primary particles. During storage and transportation processes in the powder processing industry, the material undergoes various modes of deformation and stress conditions, e.g., due to compression or shear. In many applications, it is important to know when powders are yielding, i.e. when they start to flow under shear; in other cases it is necessary to know how much stress is needed to keep them flowing. The measurement of powder yield and flow properties is still a challenge and will be addressed in this study. In the framework of the collaborative project T-MAPPP, a large set of shear experiments using different shear devices, namely the Jenike shear tester, the ELE direct shear tester, the Schulze ring shear tester and the FT4 powder rheometer, have been carried out on eight chemically-identical limestone powders of different particle sizes in a wide range of confining stresses. These experiments serve two goals: i) to test the reproducibility/ consistency among different shear devices and testing protocols; ii) to relate the bulk behaviour to microscopic particle properties, focusing on the effect of particle size and thus inter-particle cohesion. The experiments show high repeatability for all shear devices, though some of them show more fluctuations than others. All devices provide consistent results, where the FT4 powder rheometer gives lower yield/steady state stress values, due to a different pre-shearing protocol. As expected, the bulk cohesion decreases with increasing particle size (up to 150 μm), due to the decrease of inter-particle cohesion. The bulk friction, characterized in different ways, is following a similar decreasing trend, whereas the bulk density increases with particle size in this range. Interestingly, for samples with particle sizes larger than 150 μm, the bulk cohesion increases slightly, while the bulk friction increases considerably-presumably due to particle interlocking effects-up to magnitudes comparable to those of the finest powders. Furthermore, removing the fines from the coarse powder samples reduces the bulk cohesion and bulk density, but has a negligible effect on the bulk friction. In addition to providing useful insights into the role of microscopically attractive, van der Waals, gravitational and/or compressive forces for the macroscopic bulk powder flow behaviour, the experimental data provide a robust database of cohesive and frictional fine powders for industrially relevant designs such as silos, as well as for calibration and validation of models and computer simulations.

Effect of milling and sieving processes on the physicochemical properties of okra seed powders

“…milling speeds, the best flow properties on the basis of the SE was found for the highest granulometric class. This is consistent with the fact that larger particles are expected to flow more easily (Petit et al , ; Shi et al , ). Also, the better flowability of the 180–315 µm granulometric class for samples milled at 6 000 r.p.m.…”

Designing high‐performance colour cosmetics through optimization of powder flow characteristics

“…Sample 1 is mostly consisting of Titanium Dioxide, with a smaller particle size, compared to Talc, which is the main ingredient of sample 5. Smaller particle size usually means better cohesion between the particles , which in our case means that the particles from sample 1 would be more probable to stick with each other when the brush is introduced, compared to sample 5. This could also connect to the way the powder flows, insisting that when the cohesive forces are stronger, it would require more energy for the powder to flow.…”

“…Performance test: includes the cake strength and pay‐off which were tested as described below: Pay‐off: Two types of brushes were used for this research. The same amount of force was applied to the brush whereas picking up the sample and then the mass of the brush and sample were weighed using a balance. Cake strength: Ogami et al designed the Shock Resistance test, which is widely used in the cosmetic industry. First, pressed powder was dropped from a controlled height of 30.5 cm, and then, the damage of pressed powder was observed.…”

Stretching the limits of dynamic and quasi-static flow testing on cohesive limestone powders

“…Each grade of the Eskal series is milled and then sieved to ensure a certain particle size distribution. Three grades of Eskal are chosen specifically from the experience in a previous study [6]: fine/cohesive Eskal300 (d 50 = 2.22 µm), slightly cohesive Eskal15 (d 50 = 19 µm) and coarse/free-flowing Eskal150 (d 50 = 138 µm). The details of their physical properties are summarized in Table 1.…”

“…In the same figure, we have also indicated the values of angle of repose and the extrapolation of the flow angle from the rotating drum at 0 rpm for both Eskal150 and Eskal300. Note that here we have also included the data at larger pre-shear stresses (σ > 2 kPa) from the previous study [6] for the sake of completeness and validation.…”