Characterizing powders for blending processes: from micro and meso-scale measurements to macroscopic behavior
C. Gatumel, L. Legoix , H. Berthiaux
Université de Toulouse, centre RAPSODEE, Ecole des Mines d’Albi-Carmaux, campus Jarlard, 81013 Albi Cedex 09, France
The annual amount of powder mixed in the industry is high and the quality level required for the blends is increasing as far as formulations become complex. The connection between powder characteristics, process parameters, intensity and scale of segregation and energy consumption is generally accepted without being clearly attested. Insofar as there is no constitutive relation predicting the result of blending from any properties of the raw materials, the characteristics or the properties to be measured are often relevant from a default choice, depending on available characterization facilities. The goal of this communication is to highlight links between properties measured at micro and meso-scale and some macroscopic behaviors that are controlling blending processes. Three examples will be developed around agitation and segregation phenomena.
Micro-scale is considered at particle size. Microscopic observations, particle size distribution determination, packing structure are concerned. By meso-scale measurements we mean flowability assessment by shear cell method, rheometry FT4, Flodex index, Hausner and Carr determination from volumenometer or Hosokawa tester. Finally macro-scale is the process scale, that one at which agitation mechanisms are developed in the blender and induce powder mixing and segregation. The blender used to assess powder agitation through power consumption studies is a planetary Triaxe® mixer which combines a gyration and a rotation motions of an impeller across a spherical tank, equipped with torque-meters. The curve obtained by plotting a dimensionless power number as a function of Froude number give information on the agitation regimes involved (C. André 2014). Segregation experiments are conducted in a laboratory-made device where a powder mix flows from a cylindrical hopper in a 2_D box and forms a heap. Segregation mainly takes place when grains are rolling on the free surface of the heap. A specific method of sampling has been developed to assess mixture homogeneity at different locations in the heap (L. Devriendt 2013).
Targeting efficiently the measurement methods to be used is the key of success in predicting the mixing performances. That means considering the degree of compaction or the packing state of powder and the stress level at every location in the blender. Meso-scale characterizations have to be performed at stress levels consistent with those of the process.
C. André, JF Demeyre, C. gatumel, H. Berthiaux, G. Delaplace, Derivation of dimensionless relationships for the agitation of powders of different behaviours in a planetary mixer, Powder Technology, 256, 33-38, 2014
Laurent Devriendt, Cendrine Gatumel, Henri Berthiaux, Experimental Evidence of Mixture Segregation by Particle Size Distribution, Particulate Science and Technology, 31 : 653-657, 2013