(246d) Flow Behavior of Non-Drained Ultrafine Limestone Particle Packings

In Mechanical Process Engineering, the surface forces and adsorption layer effects between the particles play a significant role in the treatment of ultrafine particles with sizes smaller than 1 ìm. There are metastable states to observe during the transition from the concentrated suspension state versus the high viscous flow of a paste up to the Coulomb-friction dominated particle flow of a liquid saturated packing. On the one hand, the flowing suspension/ paste can be supersaturated with particles, so that dominant Coulomb-friction could be expected. How-ever, the packing flows still viscous. Otherwise, the pores of a flowing packing can be super-saturated with liquid, so that a dominant viscous flow is expected. However, the packing flows dominantly by compensating the Coulomb-friction resistance in the particle contacts. In the present work a rheological model for the metastable pastes- and packing transition state is assumed. Electrolytes and flocculants were inserted to influence the surface forces between the particles. The aim of this work is to describe the flow function of the produced limestone particle packings (pastes) by using of normal stress dependent Coulomb-friction contribution and a viscous part. The experiments were carried out with a modified Press-Shear-Cell. Shear ring and bottom surface are profiled by a wafer pattern to obtain a thin shear zone. Wall friction is avoided by this pyramidal roughness pattern. For the shear experiments two different layer heights in the apparatus chamber (3 and 5 mm) and shear velocities from 25,2 to 252,00 mm/min were used to vary the shear rate in each case. For the experimental determination of the shear stress, normal stresses from 100 to 400 kPa for each paste layer height and shear velocity were used. The paste state is predefined by the packing density of a loose packing without particle con-tact displacement. The packing densities of the different packings were determined experi-mentally by means of shear experiments by fitting the compression function. This gives us the option for recalculation of the characteristic adhesion forces between the primary particles in transition between paste state and frictional flow of a pre-consolidated and drained particle packing.

Key words: cake properties, viscous friction, Coulomb friction, adhesion forces, surface forces