the importance of fibre flocculation in flotation deinking
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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
4COST FP1005 – Trondheim 24-26 oct 2012
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
5COST FP1005 – Trondheim 24-26 oct 2012
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
6COST FP1005 – Trondheim 24-26 oct 2012
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.
7COST FP1005 – Trondheim 24-26 oct 2012
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)
8COST FP1005 – Trondheim 24-26 oct 2012
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)
9COST FP1005 – Trondheim 24-26 oct 2012
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
10COST FP1005 – Trondheim 24-26 oct 2012
Outline
• Background
• Methods : flocculation sensor, gas hold-up sensor…
• Results
•Effect of concentration
•Effect of dispersants
• Mechanisms
11COST FP1005 – Trondheim 24-26 oct 2012
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
12COST FP1005 – Trondheim 24-26 oct 2012
Flocculation sensor principles
• Flocculation index :
• binary morphology floc size distribution
• Flocculation index:Si
Di diameter
surf
ace
…
13COST FP1005 – Trondheim 24-26 oct 2012
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
14COST FP1005 – Trondheim 24-26 oct 2012
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
15COST FP1005 – Trondheim 24-26 oct 2012
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
18COST FP1005 – Trondheim 24-26 oct 2012
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,
19COST FP1005 – Trondheim 24-26 oct 2012
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
22COST FP1005 – Trondheim 24-26 oct 2012
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. %
23COST FP1005 – Trondheim 24-26 oct 2012
•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)
25COST FP1005 – Trondheim 24-26 oct 2012
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)
26COST FP1005 – Trondheim 24-26 oct 2012
Better selectivity : the link is fibre flocculation
flocculation directly influenced the pulp aeration : Gas hold-up when flocculation
Effect of dispersants : pilot trials
0
1
2
3
4
5
6
0 1 2 3 4 5
flocculation index (mm²)
air
con
ten
t (%
)
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
27COST FP1005 – Trondheim 24-26 oct 2012
Bubble size ?
Effect of dispersants : pilot trials
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
Zeno, E., Huber, P., Rousset, X., Fabry, B. and Beneventi D. (2010). Ind. Eng. Chem. Res., 2010, 49 (19), pp 9322–9329
28COST FP1005 – Trondheim 24-26 oct 2012
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)
29COST FP1005 – Trondheim 24-26 oct 2012
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)
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