project 2.2: effect of particle concentration on break-up ...domino.bhrgroup.com/portals/0/meeting...

www.bhrgroup.com © BHR Group 2017

Project 2.2: Effect of Particle Concentration on Break-up Kinetics using different Bead Fill Ratios on Milling Clusters of Nanoscale Silica Particles

Dr Nigel Heywood; Brian Perkins [email protected] Mobile: +44 (0) 7847 627044

Direct dial: +44 (0) 330 119 19 87

24-25 May 2017 DOMINO Spring Meetings Confidential to DOMINO Members

© BHR Group 2017 2

Introduction- Background Stirred bead mills have been used in industry quite widely for fine and ultrafine grinding of high concentration slurries:

Inkyo et al. (2006), Journal of Colloid and Interface Science 304, 535-540

• minerals

• ceramic materials

• pigments

• chemical products

© BHR Group 2017 3

Operation modes

Series of mills

Re-circulation

Batch

Re-circulation

Mills in series

Schwedes and Bunge, Advanced Powder Technology,1992

© BHR Group 2017 4

Background

The milling process (when recirculation mode is used) is affected by several factors:

• Stirrer (accelerator) speed

• Grinding bead size

• Grinding bead filling volume (“Fill Ratio”)

• Solids concentration

• Continuous phase viscosity

• Recirculation flowrate

• Residence time and number of tank turnovers (determined by flowrate and volume)

© BHR Group 2017 5

WAB SBM

© BHR Group 2017 6

WAB DYNO Mill operated in re-circulation loop of a stirred tank

Three accelerators; diameter: 0.064 m

Gross dispersion volume in the chamber measured at 495 ml

Actual effective volume is less due to presence of beads

DYNO®-Mill ML WAB brochure DYNO®-Mill ML WAB brochure

Experimental Setup

© BHR Group 2017 17-218/05/2017 7

Previous DOMINO work with a stirred bead mill

The stirred bead mill has been used for the following systems:

Aerosil 200V-in-water

► Solid concentration: 1 – 15 wt.%

Zinc oxide in water

► Solid concentration: 5 wt.%

Nanoclay in polyol

► Solid concentration: 3 wt.%

© BHR Group 2015 8

Previous DOMINO work with Aerosil 200V in Water using a SBM to 2013 (from DOM 65 report)

© BHR Group 2017 9

Previous Recent DOMINO work with 15 % Aerosil 200V in Water using a SBM in 2014

(tip speed 8 m/s, dispersion volume 2 litres; 42.5 litre/h)

Bead size Fill ratio

1.0 mm 65%

0.8 mm 65%

1.0 mm 50%

1.0/0.3 mm 65%

© BHR Group 2017 10

2015/2016: Aerosil 200V in Distilled Water using a (tip speed 8 m/s, dispersion volume 4 litres; 1.0 mm bead size)

Aerosil 200V

Concentration

Fill ratio

Flowrate

through mill,

l/h

2014/2015

Work

(all at 42.5 l/h)

10% 45% 40 -

10% 55% 40 -

10% 65% 40 -

15% 45% 40 -

15% 55% 40 50%

15% 65% 40 65%

(15% 65% 20) -


Objectives of current study and scope of work

To investigate the effects of nanocluster concentration and bead fill ratio on mechanism and kinetics of break-up using a stirred bead mill • 5% w:w Aerosil 200V was dispersed in distilled water

• Using a tip speed 8 m/s, dispersion volume 4 litres; 1.0 mm bead size, 40.9 l/h flowrate.

• Results for 10% and 15% w:w presented at May 2016 SCM • Results for 5%, 10% and 15% compared here to see if fines and

coarse particle generation at 5% concentration is a function of fill ratio (found not to be at 15%, but possibly at 10%)


Definition of “Bead Ratio” or “Fill Ratio”

Net volume of bead mill, Vm, = total volume inside mill minus volume occupied by accelerators

“Volume of beads”, Vb, is volume of beads and air when beads poured randomly into a graduated cylinder

Bead or Fill Ratio = Vb/Vm x 100%


Experimental- Protocol and operating conditions

Pre-dispersion preparation:

► Aerosil 200V was weighed at a concentration of 5% w:w and hand-mixed into distilled water, giving a total volume of 4 litres

► This blend was then mixed at 120 rpm for 30 min in a baffled tank equipped with a pitched blade turbine

Samples were taken at regular intervals once milling started.


Experimental- Protocol and operating conditions

Aerosil 200V concentrations : 5%, 10% and 15%

Fill ratios: 45% , 55% and 65%

Experiments at accelerator tip speed of 8 m/s

A peristaltic pump was used providing a flow rate of around 40.9 l/h

Tank volume: 4 litres

1 mm TOSOH beads


Materials- Dispersed phase ► Aerosil® 200V is a fumed hydrophilic silica manufactured by Evonik Ind.

► The primary particle is 12 nm

► Electron microscopy and particle size measurements have shown smallest aggregates of around 50-60 nm


Dispersion Analysis (Particle Size Distribution)

Beckman-Coulter LS 230

Laser diffraction (0.4 – 2000 mm) + PIDS (Polarization Intensity Differential Scattering) (0.04 – 0.4 mm)

40 nm – 2000 mm

Refractive index of silica: 1.46 + 0.01 i

Samples were taken from the stirred tank


Break up of nanoparticle clusters

Primary Particles

Aggregates

Agglomerates Erosion

Rupture

Shattering

0

5

10

15

20

25

0.01 0.1 1 10 100 1000

Particle Size, µm

Vo

lum

e %

Erosiont=0t>0t>>0t→∞

0

5

10

15

20

25

0.01 0.1 1 10 100 1000

Particle Size, µm

Vo

lum

e %

Erosiont=0t>0t>>0t→∞

t=0t>0t>>0t→∞

0

5

10

15

20

25

0.01 0.1 1 10 100 1000

Particle Size, µm

Vo

lum

e %

Rupturet=0t>0t>>0t→∞

0

5

10

15

20

25

0.01 0.1 1 10 100 1000

Particle Size, µm

Vo

lum

e %


t=0t>0t>>0t→∞

Shattering

0

5

10

15

20

25

0.01 0.1 1 10 100 1000

Particle Size, µm

Vo

lum

e %


t=0t>0t>>0t→∞

Ozcan-Taskin, N. G., et al., Chem Eng Res Des (2009), doi:10.1016/j.Cherd.2008.12.012


Evolution of PSD- 5% Aerosil 200V : 45% Fill Ratio

Evolution of PSD indicates erosion as the predominant break up mechanism


Evolution of PSD- 10% Aerosil 200V : 45% Fill Ratio (2016)




Evolution of PSD indicates erosion as the predominant break-up mechanism


Analysis of Particle Size Distribution (PSD)

“Fines” always defined as particles < 1 µm

“Coarse” always defined as particles > 1 µm

Sauter mean diameter, d32, µm

► Defined as the diameter of a sphere that has the same volume/surface area ratio as a particle of interest


Evolution of d32 for fines – 5%, 10% and 15% Aerosil in water

After 20 mins, Sauter mean diameter of “fines” is of the order of 120- 140 nm (0.12 to 0.14 micron) in agreement with previous results with Aerosil 200V in water


Fines generation rate- 10% Aerosil in Distilled Water

At 10%, break-up kinetics seems slightly greater at higher fill ratios after 20 mins



At 15% Aerosil, break-up rate appears independent of fill ratio



As the fill ratio is increased from 45% to 65%, break-up rate appears to be largely constant over time at 5% Aerosil 200V, same as 15%


Number of Tank Turnovers, Nt, Effective Mill Volume, Veff and Cumulative Residence Time (RT), Tcr

Number of Tank Turnovers (dimensionless), Nt

► Nt = Volume flowrate Q (litre/min)*time (min)/Tank Volume, Vt (litre)

► Volume flowrate, Q = 40/60 (litre/min)

► Tank volume, Vt = 4 litre

Effective Mill Volume (litre)

► Veff= Vm-0.62*Vb

► Vb/Vm * 100% - Fill Ratio

Cumulative Residence Time (min)

► Tcr = Veff/Q*Nt = Veff*time/Vt


Effective (or “Free”) Volume in Bead Mill, Veff

Fill Ratio Veff

(litre)

45% 0.357

55% 0.326

65% 0.296

0% 0.495



At 10% Aerosil, break-up kinetics greater at higher fill ratios after RT of 2 mins



At 15% Aerosil, break-up rate confirmed as independent of fill ratio when related to cumulative residence time within bead mill



Break-up kinetics in terms of RT shows no Fill Ratio dependence


Fines Generation Rate : 5%, 10%, 15% Aerosil in Distilled Water

Fines generation rate appears to be slightly faster at 15% Aerosil concentration


Evolution of d32 coarse particles at 10% Aerosil 200V

At 10%, rate of reduction in coarse d32 greater at higher fill ratios after 30 mins



At 15%. rate of reduction in coarse d32 seems independent of fill ratio



At 5%, no effect of Fill Ratio on Coarse Particle Size Development


Evolution of d32 coarse particles at 10% Aerosil 200V, RT Plot

At 10%. rate of reduction in coarse d32 seems faster at higher fill ratios



At 15%. rate of reduction in coarse d32 seems independent of fill ratio



At 5%. RT plot shows rate of reduction in coarse d32 independent of fill ratio


Evolution of d32 coarse particles at 5, 10, 15% Aerosil 200V, RT Plot

Rate of reduction in coarse d32 appears to be greater for the 5% up to 3 residence times compared with 10% and 15%. Is the difference significant?


Conclusions

Deagglomeration of 5% w:w Aerosil 200V in distilled water studied using a stirred bead mill to

► establish effect of bead fill ratio on break-up kinetics and mechanisms

► compare break-up behaviour with 10% and 15% data obtained in DOMINO year 2015/2016. Fill ratios were 45%, 55% and 65%.

Predominant mechanism of break-up was found to be erosion, irrespective of Aerosil concentration and bead fill ratio.


Conclusions

Sauter mean diameter d32 of “fines” (aggregates) was of the order of 120 to 140 nm for 5% Aerosil concentration and fill ratio combinations after 20 minutes for a 4-litre dispersion

► in agreement with previous results (2015/2016) with Aerosil 200V in distilled water using 10% and 15% w:w 200V.

As bead fill ratio is increased from 45% to 65%

► break-up rate (as defined through the % volume of “fines” in total PSD) appears independent of fill ratio

► confirmed when fines % expressed in terms of cumulative residence time (RT) in mill

Fines % is possibly faster at higher Aerosil concentrations


Possible Future Work with WAB Mill

Suggestions for Members?

www.bhrgroup.com © BHR Group 2017

The experimental work was performed by Brian Perkins

Thank you for your attention

Dr Nigel Heywood

[email protected] Mobile: +44 7847 627044

Direct dial: +44 (0) 330 119 19 87

http://www.bhrgroup.com/

project 2.2: effect of particle concentration on break-up ...domino.bhrgroup.com/portals/0/meeting...

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