(743g) Investigating the Moisture Content Impact on the Flow Behaviour of a Wet Glass Beads Powder

Granular materials are well known to exhibit a broad range of intricate behaviours and might vary in size and shape; in addition, moisture can be present. In the chemical and process industries, where those materials are widely handled and produced [1], studying the flow behaviour of such materials is of critical importance. Indeed, understanding the impact of internal (e.g., particles size and shape) and external (e.g., applied stresses, moisture content) physical properties on the granular flow behaviour will help industrial practitioners handle and produce particulates in an efficient and less costly way.

In this work, we investigate the impact of moisture (at various concentrations) on the flow behaviour of a powder of wet glass beads using the shear cell of the automated powder rheometer (FT4, Freeman Technology). Shear testers are well known for the relative simplicity of their procedures used to evaluate the stresses needed to generate shear; the shear can lead to either compaction or dilation states of the studied material, under a given applied normal stress. The experimental procedure of the FT4 shear test is as follow: the bladed rotational shear head moves downwards to apply vertical consolidation stress (normal stress σ) on the flat powder sample placed in a steady vessel. Once the desired normal stress is reached, the shear head starts to rotate inducing a shear stress (τ) that increases until the powder bed fails. This procedure is usually repeated at different consolidation stresses in order to build up the yield loci: σ=μτ+c, where m is the angle of internal friction and c is the so-called cohesive strength. The Mohr circle analysis applied on the yield loci allows the evaluation of the flow function coefficient FF which provides information on the flow behaviour of the studied powder, where high FF corresponds to a free flowing powder (Jenike [2]).

In this study, a powder made of monodispersed (500 mm diameter) glass beads is wetted at different concentrations ω (from 0.1% to 5%) and tested with the FT4 shear cell. The FT4 automated conditioning blade is used to prepare each wetted sample using the same automated mixing procedure. This method allows also an in-situ test, where the shear test is performed directly on the mixed sample. The angle of internal friction and the cohesive strength and the flow function coefficient were then evaluated at different initial consolidation stresses and water contents. The first results showed that, at a fixed water concentration, the angle of internal friction is independent of the initial consolidation stress. The values of flow function showed that, according to Jenike classification [2], a wet powder with a fixed w can be classified as free flowing (FF>10) if the powder is highly consolidated or as cohesive (2<FF<4) at low consolidation stresses. This might be explained by the fact that the liquid bridges created by the water content can be broken at high consolidation stresses. In addition, for a fixed initial consolidation stress, the angle of internal friction appears to be independent of the water content with a value equal to the one obtained in a dry case. Furthermore, it appears that the cohesive strength c increases when increasing the water content and reaches an asymptotic value. However, it should be noted that even if this final observation is in good agreement with the one found in the literature [3], it was obtained using one initial consolidation stress. Therefore, further measurements, including different consolidation stresses, will be performed in order to confirm the first measurements and observations.

References

[1] F. J. Muzzio, A. Alexander, C. Goodridge, E. Shen, and T. Shinbrot, “Solids mixing part A: fundamentals of solids mixing,” Handb. Ind. Mix. Sci. Pract., pp. 887–985, 2004.

[2] A. W. Jenike, Storage and flow of solids, Rev. ed. Salt Lake City, Utah : University of Utah, 1970.

[3] V. Richefeu, M. S. El Youssoufi, and F. Radjai, “Shear strength properties of wet granular materials,” Phys. Rev. E Stat. Nonlin. Soft Matter Phys., vol. 73, p. 051304, May 2006.