The importance of fibre flocculation in flotation deinking

The importance of fibre flocculation in flotation deinkingP. Huber#, E. Zeno#, B. Fabry#, X. Rousset#, M.C. Angelier#, D. Beneventi*, T. Vazhure&

#:CTP, *: LGP2, &: Aylesford Newsprint

COST FP1005 “Fibre suspension flow modeling” 24-26 Oct. 2012, Trondheim, Norway

3COST FP1005 – Trondheim 24-26 oct 2012

• Influence of pulp concentration on flotation efficiency

•(pilot flotation trials – VOITH facility)

Britz, H., Peschl, A. (1994) , Wochenblatt für Papierfabrikation, n°15: 603-608, 1994.

Acc

epts

bri

gh

tnes

s

concentration

conc.

flot. cell #

Tot. (%) Fibres (%)

Lo

sses

(%

)

Concentration (%)

At same brightness At same cleanliness

The higher the concentration, the lower the ink removal and the better the yield

Background


Background

• Influence of concentration on air content

•(industrial flotation trials)

Dorris, G.M., Pagé, N., Gendron, S., Murray, T. & Ben, Y. (2006) Prog. Pap. Recycling, 16 (1), pp.31-40.

The higher the concentration, the lower the air content

TABLE III. Regression model and t-test statistics of a regression model of the form: Air content = a0 + a1 • consistency + a2 • feed brightness.

Regression Coefficient R2 = 0.751

Variables Range Coefficient σ t-Student Total effect

Intercept [a0] 8.574 0.216 39.76

Consistency (%) [a1] 0.964-1.15 -7.216 0.089 -80.82 -2.56

Feed brightness (%) [a2] 40.6-48.1 0.1729 0.0044 38.83 1.29

The higher the air content, the better the ink removal

80

82

84

86

88

90

12 13 14 15 16 17

Air content in primary cells (%)

Flo

tati

on

eff

icie

nc

y o

f fr

ee

in

k (

%)

80

82

84

86

88

90

12 13 14 15 16 17

Air content in primary cells (%)

Flo

tati

on

eff

icie

nc

y o

f fr

ee

in

k (

%)

Figure 4. Flotation efficiency of free ink in three parallel flotation lines each operated at different air contents.

Air contentInk removal


Mechanisms

• Hypothesis (Dorris et al. 2006)

•High concentration

(flocculation ?)

heterogeneous fibre suspension chanelling

air bubbles can travel faster, coalescence and rise faster to the top of the cell

Decrease of relative residence time air/pulp

air content is reduced

ink removal is impaired


Background

• Influence of concentration on fibre flocculation

•(laboratory trials with various pulps)

Huber, P., Carré, B., and Petit-Conil, M. (2008). BioRes. 3(4), 1218-1227.

The higher the concentration, the higher the fibre flocculation

Example with TMP fibres•Same results with BKP (HW, SW, mix), DIP, etc.


Background

• Influence of crowding factor on pulp flocculation

•(flocculation varied by changing concentration and pulp mixtures HW/SW)

Huber P., Roux J.C., Mauret E., Belgacem N., and Pierre C. (2003), J. Pulp & Pap. Sci. 29(3):77-85.

fibre crowding determines fibre flocculation (at given turbulence)

crowding

• N= nb fibres in the crowding sphere

(non dimensional concentration)

crowding sphere

Kerekes (1985)


Background

• Influence of crowding factor on gas hold-up

•(Column bubbling of virgin pulp)

Tang, C. & Heindel, T.J. (2006) The Canadian Journal of Chemical Engineering, 84 (2), pp.198-208.

The higher the fibre crowding, the lower the air content

Gas hold-up

(crowding)


Motivations & objectives

• influence of pulp flocculation on flotation efficiency ?

How to vary flocculation ?

By changing concentration By adding dispersants

0

0.2

0.4

0.6

0.8

1

1.2

8 10 12 14 16

concentration (g/L)

flo

ccu

lati

on

(re

lati

ve) Non surface-active fibre

dispersants :

• Guar gum• CMC


Outline

• Background

• Methods : flocculation sensor, gas hold-up sensor…

• Results

•Effect of concentration

•Effect of dispersants

• Mechanisms


Assessment of fibre flocculation

• General methods

• Pulp circulation on the flocculation pilot loop

• Fibre flocculation testing with the CTP FlocSens (image analysis)

• Constant flow speed : equivalent shear rate = 690 s -1 (medium speed)

• Flocculation sensor (+overflow) installed on Recycled fibres pilot plant, at flotation inlet

• Flocculation index:

IMAGE ANALYSIS

Si

Di diameter

surf

ace

…

Si

Di diameter

surf

ace

…

FLASHTRANSPARENT

CHANNEL

CCD CAMERA

thickness = 3.5 mm

FLASHTRANSPARENT

CHANNEL

CCD CAMERA

thickness =

FI

• Pilot flow-loop

flowmeter

1 m3

tank

overflowtank

pulp

CCD

Floc. sensor

flowmeter

1 m3

tank

overflowtank

pulp

CCD

Floc. sensor

CCD

Floc. sensor

On-line

Materials and Methods

Flocculation measurement


Flocculation sensor principles

• Flocculation index :

• binary morphology floc size distribution

• Flocculation index:Si

Di diameter

surf

ace

…


Reduced sensitivity to light diffusion

• Problem : filler diffuse light

• Even in presence of light diffusing filler :

Fibre flocculation measurement is possible

(independently on filler flocculation level)

Huber P., Roux J.C., Mauret E. and Carré B. (2006), APPITA Journal 59(1):37-43

(no filler)Fibres only Fibres+20% filler

RMS = 0.323FI = 2.63 mm²

RMS = 0.069FI = 2.65 mm²


Flotation monitoring : Assessment of pulp aeration

• Air content ≠ Air ratio

•Air ratio is a mechanical parameter only

•Air content is a true measurement of pulp aerationIncludes both hydraulic and physico-chemical effectsrelevant parameter that affects flotation efficiency

4k bE S

k

Sb is proportional to air contentLeichtle (1998)

flotation rate constant

collection efficiency bubble surface

area flux


Flotation monitoring : On-line measurement of air content

• Paprican sensor

• Based on pressure difference between immersed gauges

• Apparent pulp density varies with air content

• Installation

• Installed on reject side, across the hatch, at an angle of 60°

• probes installed in pre-flotation 1ry

Dorris et al. (2006)

Figure 1. Installation of air content probe in a Voith cell.

P


Pilot verticell

Materials and Methods

• Bubble collection via a sampling pipe and visualization in a glass window• Automated bubble count using a CCD camera and image analysis software (Sherlock 7)

ii) Bubble size measurement

Halogen light source

CCD camera

Viewing chamber

PC for image analysis

Bubble size distribution

D.Beneventi,


FlocculationEffect of concentration

• Effect on fibre flocculation (flotation cell inlet)

0

1

2

3

4

5

6

7

8

0 5 10 15 20

concentration (g/L)

flo

ccu

lati

on

in

dex

(m

m²) 8 g/L 16 g/L

Fibre fraction = 56%

• Pulp flocculation increases when increasing pulp concentration (8-16 g/L)

• Higher crowding

• More fibres interacting with each other (mainly governed by fibre concentration)

stronger flocculation


Effect of concentration: industrial trials at Aylesford

• Air content

• Large variations of air content over time (in 1ry)

• Higher air content contributes to better ink removal efficiency

• Higher air impairs flotation yield

• Higher air content is caused by concentration decrease

0

5

10

15

20

25

30

35

40

24/11/2008 01/12/2008 08/12/2008 15/12/2008 22/12/2008

air

con

ten

t (%

)

-0.1

0.1

0.3

0.5

0.7

0.9

1.1

1.3

1.5

1.7

con

cen

trat

ion

(%

)

1st stage cleaner feed

concentration

Air 1ry, 3rd cell

To maximise the flotation yield, work at highest possible concentration while maintaining ERIC target

(But take care, it is a question of compromise: a too high consistency will induce a too high decrease in ink removal efficiency)

0

5

10

15

20

25

30

35

24/11/2008 01/12/2008 08/12/2008 15/12/2008 22/12/2008

air

con

ten

t (%

)

50

55

60

65

70

75

80

85

ink

rem

ova

l (%

)

Air 2ry, 1st cell

Air 1ry, 3rd cell

ink removal

Huber, P., Rousset, X., Zeno, E. and Vazhure, T. (2011) Ind. Eng. Chem. Res. 50(7) :4021-4028

70

75

80

85

90

95

100

52 54 56 58 60 62 64

ink removal (%)

yiel

d i

nd

ex (

%)

air %

conc. %

70

75

80

85

90

95

100

52 54 56 58 60 62 64

ink removal (%)

yiel

d i

nd

ex (

%)

air %

conc. %


•On the fibre flocculation • Selected dispersants (Guar gum and CMC) effectively de-flocculate DIP

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

0 1 2 3

% additive

flo

ccu

lati

on

ind

ex (

mm

²)

guar gum

CMC

-14 to 19 %

reference

+1% guar +2% guar

+1% CMC +2% CMC

FlocculationEffect of dispersants

(Flotation cell inlet)


Effect of dispersants : pilot trials

• Selected dispersants (guar gum or CMC) clearly improved flotation selectivityBetter ink removal + lower losses at the same time

1% guar

ref

2% guar

2% CMC

1% CMC

ref

40

42

4446

48

50

52

5456

58

60

0 10 20 30 40 50

Total Losses (%)

Ink

rem

ova

l (%

)

Guar gum

CMC

Zeno, E., Huber, P., Rousset, X., Fabry, B. and Beneventi D. (2010). Ind. Eng. Chem. Res., 2010, 49 (19), pp 9322–9329

•Increased air content thanks to pulp de-flocculation (at the fibre level)•Lower entrainment of fine elements thanks to depressing mechanism (from adsorbed dispersants)

(high overall losses because of low froth height)


Better selectivity : the link is fibre flocculation

flocculation directly influenced the pulp aeration : Gas hold-up when flocculation


0

1

2

3

4

5

6

0 1 2 3 4 5

flocculation index (mm²)

air

con

ten

t (%

)

guar gum

CMC



Bubble size ?


Drift flux model :

Rising velocity in Newtonian fluid :

Limited bubble size decreaseNot sufficient to explain increased gas hold-upHigher drag on bubbles in de-flocculated pulp



Mechanisms

• Improved mechanism (this work)

•Lower (fibre) concentration or add fibre dispersants

de-flocculation

homogeneous fibre suspension

higher drag on air bubbles rise slowly to the top of the cell

increase of relative residence time air/pulp

air content is increased

ink removal is improved

• (limited coalescence (surfactants) limited chanelling)


Conclusions

• Relationship among flocculation, ink removal, turbulence and air content is not simple• Depends on hydraulic regimes in the flotation cells (turbulence pattern)

(lab cell ≠ pilot cell ≠ industrial cell ≠ various industrial flotation cells config.) Will affect interactions between air bubbles and pulp flocs

• Pulp flocculation does impact flotation efficiency• Mechanisms involved

Ink removal :– pulp de-flocculation (at fibre level) homogeneous fibre network higher

higher drag higher air content better ink removal Losses :

– With concentration : mechanism not clear– With dispersants : Lower entrainment of fine elements thanks to

depressing mechanism (from adsorbed dispersants)

• Selectivity ?• When increasing concentration :

poorer ink removal, lower losses But little impact on selectivity

• With added dispersants : clear selectivity increase (at least at lab and pilot scale)

Documents

The importance of fibre flocculation in flotation deinking