Some Simple Observations on the Failure of Particle Networks
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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
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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. Suspending Medium. < gel. gel. Particles. Network Characteristics. - PowerPoint PPT Presentation
Transcript of Some Simple Observations on the Failure of Particle Networks
Peter ScalesParticulate Fluids Processing Centre
Department of Chemical and Biomolecular Engineering
University of MelbourneAustralia
Some Simple Observations on the Failure of Par-ticle Networks
Network Characteristics
Particles
Suspending Medium
< gel gel
Flow properties typified by:
• Yielding• Shear thinning behaviour
at concentrations above gel
• Strain hardening in compression
• Strain softening in shear
Yield Measurement
0
10000
20000
30000
40000
50000
60000
70000
80000
0 0.0005 0.001 0.0015 0.002
Tim
e, t
(s)
(Specific Filtrate Volume)2, V2 (m2)
S1 Py Run 2Pressure Step
10 kPa
20 kPa
40 kPa
80 kPa150 kPa
300 kPa
0.2 rpm
vaneslurry
Torq
ue
Time
Shear and Uni-axial Compression
Volume Fraction 0.1 1
0.1
1
10
100
1000
AKP-15AKP-20AKP-30AKP-50
pH = 8.9
Z. Zhou
Shea
r Yie
ld S
tress
/d2
Volume Fraction 0.1 1
0.1
1
10
100
1000
AKP-15AKP-20AKP-30AKP-50
pH = 8.9
Z. Zhou
Shea
r Yie
ld S
tress
/d2
Volume fraction 0.2 0.3 0.4 0.5 0.6 0.8.0.1 1
1e-1
1e+0
1e+1
1e+2
1e+3
1e+4
1e+5 pH = 9.0
Compression
Shear
Z. Zhou
Yie
ld S
tress
/d2
Volume fraction 0.2 0.3 0.4 0.5 0.6 0.8.0.1 1
1e-1
1e+0
1e+1
1e+2
1e+3
1e+4
1e+5 pH = 9.0
Compression
Shear
Z. Zhou
Yie
ld S
tress
/d2
Zhou et al., CES, 56: 2901-2920 (2001)
PROCESS SHEAR COMPRESSION MODELCoating ✔ ✔Mixing/pumping ✔ ✔Pipeline transport ✔ ✔ ✗Sedimentation ✔ ✔Thickening ✔ ✔ ✗Pressure filtration ✔ ✔Belt press filtration ✔ ✔ ✗Centrifuge (decanter) ✔ ✔ ✗Centrifuge (bowl) ✔ ✔Vacuum filtration ✔ ✔ ✗Drying ✔ ✔ ✗
MOTIVATION
Critical State Approach (von Mises) Supposes that the compressive and shear stresses co-operate to produce yield
Compressive stress
Max
imum
she
ar s
tress
Elastic
Plastic
Implication is that continuous stirring will cause the network to proceed to maximum density
pres
sure
Eq’m density
unstirredstirred
Channell, White & Zukoski saw 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
But how do we get the water out?
The rate of collapse is controlled by the rate of water escape
SummaryThe 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
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