Powder flowability at low stresses: ball indentation...Flowability measurement ØMore commonly...

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Powder flowability at low stresses: ball indentation Colin Hare Department of Chemical and Process Engineering, University of Surrey

Transcript of Powder flowability at low stresses: ball indentation...Flowability measurement ØMore commonly...

Page 1: Powder flowability at low stresses: ball indentation...Flowability measurement ØMore commonly measured with shear cells 3 σ n τ σ 1 σ τ σ c σ 1 [1} Schulze, D., 2007 “Powders

Powder flowability at low stresses: ball indentation

Colin Hare

Department of Chemical and Process Engineering, University of Surrey

Page 2: Powder flowability at low stresses: ball indentation...Flowability measurement ØMore commonly measured with shear cells 3 σ n τ σ 1 σ τ σ c σ 1 [1} Schulze, D., 2007 “Powders

Ø Uniaxial compression test

Powder flowability

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σ1 σc σ1 : pre-consolidation stress σc : unconfined yield stress = flow function, ffc

[1} Schulze, D., 2007 “Powders and bulk solids: behaviour, characterization, storage and flow”, Springer

Page 3: Powder flowability at low stresses: ball indentation...Flowability measurement ØMore commonly measured with shear cells 3 σ n τ σ 1 σ τ σ c σ 1 [1} Schulze, D., 2007 “Powders

Flowability measurement

Ø More commonly measured with shear cells

3

σn

τ

σ1

σ

τ

σ1 σc

[1} Schulze, D., 2007 “Powders and bulk solids: behaviour, characterization, storage and flow”, Springer

Page 4: Powder flowability at low stresses: ball indentation...Flowability measurement ØMore commonly measured with shear cells 3 σ n τ σ 1 σ τ σ c σ 1 [1} Schulze, D., 2007 “Powders

Ø Ball indentation2

Ø Elastically deforming region provides additional constraint:

Proposed method

4

Fmax

cσ.CH =

( )2maxmax

hRh2πF

AFH

-==

[2] Hassanpour, A., Ghadiri, M., 2007 “Characterisation of flow of loosely compacted cohesive powders by ball indentation”, Particle & Particle Systems Characterization 24, 117-123

Page 5: Powder flowability at low stresses: ball indentation...Flowability measurement ØMore commonly measured with shear cells 3 σ n τ σ 1 σ τ σ c σ 1 [1} Schulze, D., 2007 “Powders

Ball indentation

Ø Routinely used for characterising hardness of continuum solids Ø C is known for metals (~3)3

Ø C can be calculated from material properties for organics & polymers4

Ø Indentation procedure has been established5: Ø Indenter size to be used for a given particle size

Ø Minimum amount of powder needed Ø Bed diameter

Ø Bed height

Ø To what depth the indent should be made

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[3] Tabor, D., 1951 “The hardness of metals”, Clarendon Press [4] Johnson, K.L., 1985 “Contact mechanics”, Cambridge University Press [5] Zafar, U., Hare, C., Hassanpour, A., Ghadiri, M., 2017 “Ball indentation on powder beds for assessing

flowability: Analysis of operation window”, Powder Technology 310, 300-306.

Page 6: Powder flowability at low stresses: ball indentation...Flowability measurement ØMore commonly measured with shear cells 3 σ n τ σ 1 σ τ σ c σ 1 [1} Schulze, D., 2007 “Powders

Indenter size

Minimum:

6

00.5

11.5

22.5

33.5

44.5

5

0 5 10 15 20 25 30 35

Hard

ness

(kPa

)

Di/Dp

[5] Zafar, U., Hare, C., Hassanpour, A., Ghadiri, M., 2017 “Ball indentation on powder beds for assessing flowability: Analysis of operation window”, Powder Technology 310, 300-306.

Page 7: Powder flowability at low stresses: ball indentation...Flowability measurement ØMore commonly measured with shear cells 3 σ n τ σ 1 σ τ σ c σ 1 [1} Schulze, D., 2007 “Powders

Bed height

Minimum amount of powder

Bed diameter

7

01234567

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

Hard

ness

(kPa

)

Di/Dd

02468

101214

0 25 50 75 100 125 150 175

Hard

ness

(kPa

)

Bed height/Particle diameter

[5] Zafar, U., Hare, C., Hassanpour, A., Ghadiri, M., 2017 “Ball indentation on powder beds for assessing flowability: Analysis of operation window”, Powder Technology 310, 300-306.

Page 8: Powder flowability at low stresses: ball indentation...Flowability measurement ØMore commonly measured with shear cells 3 σ n τ σ 1 σ τ σ c σ 1 [1} Schulze, D., 2007 “Powders

Indentation depth

8

0

5

10

15

20

25

30

35

0 0.2 0.4 0.6 0.8 1

h/Ri

Hard

ness

(kPa

)

[5] Zafar, U., Hare, C., Hassanpour, A., Ghadiri, M., 2017 “Ball indentation on powder beds for assessing flowability: Analysis of operation window”, Powder Technology 310, 300-306.

Page 9: Powder flowability at low stresses: ball indentation...Flowability measurement ØMore commonly measured with shear cells 3 σ n τ σ 1 σ τ σ c σ 1 [1} Schulze, D., 2007 “Powders

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Ø Indenter diameter ≥ 17 dp Ø Bed diameter ≥ 1.5 DInd Ø Bed height ≥ 40 dp

Ø Dimensionless penetration depth (h/Ri) ≥ 0.4

Indentation procedure

Size d50 (μm)

Diameter (mm) Bed height (mm)

Bed Volume (mm3) Indenter Bed

50 0.85 1.7 2 5 100 1.7 3.4 4 40 200 3.4 6.8 8 300 500 8.5 17 20 4500

[5] Zafar, U., Hare, C., Hassanpour, A., Ghadiri, M., 2017 “Ball indentation on powder beds for assessing flowability: Analysis of operation window”, Powder Technology 310, 300-306.

Page 10: Powder flowability at low stresses: ball indentation...Flowability measurement ØMore commonly measured with shear cells 3 σ n τ σ 1 σ τ σ c σ 1 [1} Schulze, D., 2007 “Powders

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Ø Generally we want to know the unconfined yield stress Ø Need to know constraint factor

Ø How does constraint factor vary with particle properties? Ø Can shear cell data be reliably extrapolated to low stress?

Materials Ø Indentation tested for wide range of powders Ø Silanised glass beads (varying size distributions) Ø Food powders – sweetener, maize starch, pea protein Ø Inorganic powders – limestone, titania, talc, copper

Methods Ø FT4 shear cell Ø Ball indentation

Determining constraint factor

Page 11: Powder flowability at low stresses: ball indentation...Flowability measurement ØMore commonly measured with shear cells 3 σ n τ σ 1 σ τ σ c σ 1 [1} Schulze, D., 2007 “Powders

0

0.5

1

1.5

2

2.5

3

3.5

0 2 4 6 8 10 12

Unco

nfin

ed y

ield

stre

ngth

(kPa

)

Major principal stress (kPa)

45-53 μm 53-63 μm 63-75 μm 75-90 μm 90-106 μm

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Unconfined Yield Strength ffc=1 ffc=2

ffc=4

ffc=10

Page 12: Powder flowability at low stresses: ball indentation...Flowability measurement ØMore commonly measured with shear cells 3 σ n τ σ 1 σ τ σ c σ 1 [1} Schulze, D., 2007 “Powders

0

1

2

3

4

5

6

7

0 2 4 6 8 10 12

Hard

ness

(kPa

)

Major principal stress (kPa)

45-53 μm 53-63 μm 63-75 μm 75-90 μm 90-106 μm

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Hardness

Page 13: Powder flowability at low stresses: ball indentation...Flowability measurement ØMore commonly measured with shear cells 3 σ n τ σ 1 σ τ σ c σ 1 [1} Schulze, D., 2007 “Powders

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Constraint factor

Page 14: Powder flowability at low stresses: ball indentation...Flowability measurement ØMore commonly measured with shear cells 3 σ n τ σ 1 σ τ σ c σ 1 [1} Schulze, D., 2007 “Powders

0.0

0.5

1.0

1.5

2.0

2.5

3.0

0 2 4 6 8 10 12

Con

stra

int f

acto

r

Major principal stress (kPa)

63-75 μm 53-90 μm 45-106 μm 63-75 μm + 10% <25 μm

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Size distribution effect

Page 15: Powder flowability at low stresses: ball indentation...Flowability measurement ØMore commonly measured with shear cells 3 σ n τ σ 1 σ τ σ c σ 1 [1} Schulze, D., 2007 “Powders

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Adhesion to the piston

Page 16: Powder flowability at low stresses: ball indentation...Flowability measurement ØMore commonly measured with shear cells 3 σ n τ σ 1 σ τ σ c σ 1 [1} Schulze, D., 2007 “Powders

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Low stress behaviour

Page 17: Powder flowability at low stresses: ball indentation...Flowability measurement ØMore commonly measured with shear cells 3 σ n τ σ 1 σ τ σ c σ 1 [1} Schulze, D., 2007 “Powders

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Comparison of techniques

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Shearing at low stresses

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Food powders – shear cell ffc=1 ffc=2

ffc=4

ffc=10

Page 20: Powder flowability at low stresses: ball indentation...Flowability measurement ØMore commonly measured with shear cells 3 σ n τ σ 1 σ τ σ c σ 1 [1} Schulze, D., 2007 “Powders

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Constraint factor

Page 21: Powder flowability at low stresses: ball indentation...Flowability measurement ØMore commonly measured with shear cells 3 σ n τ σ 1 σ τ σ c σ 1 [1} Schulze, D., 2007 “Powders

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Slip-stick behaviour: pea protein

Page 22: Powder flowability at low stresses: ball indentation...Flowability measurement ØMore commonly measured with shear cells 3 σ n τ σ 1 σ τ σ c σ 1 [1} Schulze, D., 2007 “Powders

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Slip-stick behaviour: pea protein

Page 23: Powder flowability at low stresses: ball indentation...Flowability measurement ØMore commonly measured with shear cells 3 σ n τ σ 1 σ τ σ c σ 1 [1} Schulze, D., 2007 “Powders

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Slip-stick: strain rate effect

Page 24: Powder flowability at low stresses: ball indentation...Flowability measurement ØMore commonly measured with shear cells 3 σ n τ σ 1 σ τ σ c σ 1 [1} Schulze, D., 2007 “Powders

0

0.5

1

1.5

2

2.5

3

0 2 4 6 8 10 12 14

Unco

nfin

ed y

ield

stre

ngth

(kPa

)

Major principal stress (kPa)

maize starch indmaize starch

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

0 0.5 1 1.5 2

Unco

nfin

ed y

ield

stre

ngth

(kPa

)

Major Principal Stress (kPa)

shear cellball indentation

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Comparison of techniques - maize

Page 25: Powder flowability at low stresses: ball indentation...Flowability measurement ØMore commonly measured with shear cells 3 σ n τ σ 1 σ τ σ c σ 1 [1} Schulze, D., 2007 “Powders

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Shearing at low stresses

Page 26: Powder flowability at low stresses: ball indentation...Flowability measurement ØMore commonly measured with shear cells 3 σ n τ σ 1 σ τ σ c σ 1 [1} Schulze, D., 2007 “Powders

0

5

10

15

20

25

30

35

0 5 10 15 20 25

Hard

ness

(kPa

)

Major principal stress (kPa)

titaniatalclimestonecopper

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Inorganic powders

Page 27: Powder flowability at low stresses: ball indentation...Flowability measurement ØMore commonly measured with shear cells 3 σ n τ σ 1 σ τ σ c σ 1 [1} Schulze, D., 2007 “Powders

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Inorganic powders

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Ø Flowability cannot be easily measured at low stresses using shear cells Ø Target stresses often not obtained Ø Determined yield locus may be inconsistent

Ø Ball indentation can provide reproducible measurements of flowability at

low stresses Ø Requires constraint factor to be known (or determined)

Ø Constraint factor found to be independent of applied stress Ø Though varies (~ 2 – 6) for different materials Ø Reduces slightly with d50, though increases with fines addition

Ø Several powders exhibit sharp reduction in shear stress at lower stresses Ø Behaviour at higher stresses cannot be reliably extrapolated

Conclusions

Page 29: Powder flowability at low stresses: ball indentation...Flowability measurement ØMore commonly measured with shear cells 3 σ n τ σ 1 σ τ σ c σ 1 [1} Schulze, D., 2007 “Powders

Future work

Ø DEM will be used to analyse C at low stresses

Ø And influence of particle shape on C

Ø Experimentally assess dependency of C on surface energy

Ø A range of coatings applied to different batches

Ø Investigate indentation at high-strain rates

Ø Optimisation of shear cell procedure at low stresses Ø Number of pre-shear steps applied

Ø Defined end point – i.e acceptable deviation to define steady state

Ø Required agreement between consecutive pre-shears

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Page 30: Powder flowability at low stresses: ball indentation...Flowability measurement ØMore commonly measured with shear cells 3 σ n τ σ 1 σ τ σ c σ 1 [1} Schulze, D., 2007 “Powders

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Acknowledgements

Collaborators

Ø Dr Umair Zafar

Ø Dr Massih Pasha

Ø Mr Alex Stavrou

Ø Prof. Mojtaba Ghadiri

Ø Dr Ali Hassanpour

Ø Mr Tim Freeman

Ø Dr Marty Murtagh