The Dynamics of Fine Powders

flour, potato starch, cracking catalyst, sand, and gravel, one will probably agree that the first four materials definitely are powders and the last one certainly is not. Whether one would call sand a powder probably depends on the partiele size and on personal Vlews. When the astronaut Neil Armstrong returned to the Earth from his trip on the surface of the Moon, he stated: 'The surface is fine and powdery. I can kick it up loosely with my toe. It does adhere in fine layers like powdered charcoal to the sole and inside of my boots. I only go in a small fraction of an inch, but I can see the footprints of my boots and the treads in the fine sandy partieles. ' These words elearly show that the behaviour of powders de­ pends on the circumstances. In wh at respects are those on the Moon different from those on the Earth? (1) The gravitational force on the surface of the Moon is only one-sixth of that on the Earth. (2) There is no gas on the Moon. The latter aspect means that any water brought there would evaporate and disappear immediately, hence powders on the Moon will always be perfect1y dry so that cohesion between the separate partieles due to liquid bridges will be zero.

Table of Contents:
1 General Introduction.- 1.1 Powders: what are they?.- 1.2 Powder behavior.- 1.3 Powders in industry.- 1.4 Powder operations.- 1.5 Origin and production of powders.- 1.6 About this book.- 2 Particle and Powder Characteristics.- 2.1 Characterization of single particles.- 2.2 Particle size distribution.- 2.3 The powder packing.- 2.4 The coordination number.- 3 Mechanics of Powders at Rest.- 3.1 The stress tensor.- 3.2 Two-dimensional system.- 3.3 Stress analysis of three-dimensional systems.- 3.4 The yield locus.- 3.5 The angle of repose.- 3.6 The free arch.- 3.7 Incipient fluidization.- 4 Theoretical Derivation of Interparticle Forces.- 4.1 Introduction.- 4.2 The Hamaker theory.- 4.3 Effect of particle deformation.- 4.4 Effect of gas adsorption.- 4.5 Practical implications.- 4.5.1 Powder cohesion.- 4.5.2 Powder elasticity.- 5 Two-phase Systems. The Equations of Motion.- 5.1 Introduction.- 5.2 Averaging of variables.- 5.3 The mass balances.- 5.4 The momentum balances.- 5.5 Constitutive equations.- 5.6 The buoyancy.- 6 Stability and Perturbations.- 6.1 Introduction.- 6.2 Continuity waves.- 6.3 The continuity shock wave.- 6.4 Stability of a homogeneous gas-fluidized bed.- 6.5 Other theories.- 6.6 Channelling.- 7 Experiments with Homogeneous Fluidization.- 7.1 Introduction.- 7.2 Description of apparatus.- 7.3 Experimental procedure.- 7.4 Experimental results.- 7.5 The effect of gravity.- 7.6 Discussion and conclusion.- 8 A Model of Expanded Powders.- 8.1 Introduction.- 8.2 The relation between elasticity and porosity.- 8.3 The coordination number.- 8.4 The relation between cohesion and elasticity.- 9 Rheology of Fluidized Powders.- 9.1 Introduction.- 9.2 Literature survey.- 9.3 Description of the apparatus.- 9.4 Measurements.- 9.5 Conditions for stable powder flow.- 9.6 Evaluation of the measurements.- 9.7 Correlation of the wall velocity.- 9.8 Conclusions.- 10 Fluidization in Practice.- 10.1 Introduction.- 10.2 The freely bubbling bed.- 10.3 Single bubbles.- 10.4 The bubble-cloud mechanism.- 10.5 General proof of cloud mechanism.- 10.6 Correlation with interparticle forces.- 10.7 Overall circulation.- 11 Handling of Powders.- 11.1 Introduction.- 11.2 Milling experiments.- 11.3 Mixing and segregation.