Some Simple Observations on the Failure of Particle Networks

1. Peter Scales Particulate Fluids Processing Centre Department of Chemical and Biomolecular Engineering University of Melbourne Australia Some Simple Observations on the Failure of Particle Networks

2. Suspending Medium  < gel gel Particles Network Characteristics • Flow properties typified by: • Yielding • Shear thinning behaviour at concentrations above fgel • Strain hardening in compression • Strain softening in shear

3. Yield Measurement Torque Time

4. Shear and Uni-axial Compression Zhou et al., CES, 56: 2901-2920 (2001)

5. MOTIVATION PROCESS SHEAR COMPRESSION MODEL Coating ✔✔ Mixing/pumping ✔✔ Pipeline transport ✔✔ ✗ Sedimentation ✔ ✔ Thickening ✔ ✔ ✗ Pressure filtration ✔ ✔ Belt press filtration ✔ ✔ ✗ Centrifuge (decanter) ✔ ✔ ✗ Centrifuge (bowl) ✔✔ Vacuum filtration ✔ ✔ ✗ Drying ✔ ✔ ✗

7. Critical State Approach (von Mises) • Supposes that the compressive and shear stresses co-operate to produce yield Implication is that continuous stirring will cause the network to proceed to maximum density f Plastic Maximum shear stress unstirred stirred pressure Elastic Compressive stress Eq’m density

8. Channell, White & Zukoskisaw a family of curves of the type one would expect for a series of shear stresses Shear rate 0 0.69 1.85 5.63 11.3 In other words, they saw that material undergoing shear flow could behave as if it were still a solid in compression. At the right shear rate, the system almost becomes incompressible

9. But how do we get the water out? The rate of collapse is controlled by the rate of water escape

10. Summary The challenge for hard to dewater materials is to find the right combination of shear and compression to exploit the material properties Getting the water out in a reasonable time is still an issue and pathways are needed The concept of a simple elasto-plastic transition (yield stress) for particle networks is too crude to be useful in multi-dimensional network yielding and we need a new fundamental description