Transcript The importance of fibre flocculation in flotation deinking

The importance of fibre flocculation in
flotation deinking
P. 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
Background
•
Influence of pulp concentration on flotation efficiency
• (pilot flotation trials – VOITH facility)
Tot. (%)
conc.
flot. cell #
Fibres (%)
At same brightness
At same cleanliness
Losses (%)
Accepts brightness
concentration
Concentration (%)
The higher the concentration,
the lower the ink removal and the better the yield
Britz, H., Peschl, A. (1994) , Wochenblatt für Papierfabrikation, n°15: 603-608, 1994.
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Background
•
Influence of concentration on air content
• (industrial flotation trials)
Air content
Ink removal
Flotation efficiency of free ink (%)
90
TABLE III.
Regression model and t-test statistics of a regression model of the form: Air content = a 0 + a1
• consistency + a2 • feed brightness.
88
Regression Coefficient R2 = 0.751
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Variables
Range
Intercept [a0]
84
Coefficient
σ
t-Student
8.574
0.216
39.76
Total effect
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
82
80
12
13
14
15
16
17
Air content in primary cells (%)
Figure 4. Flotation efficiency of free ink in three parallel flotation lines
each operated at different air contents.
The higher the air content,
the better the ink removal
The higher the concentration,
the lower the air content
Dorris, G.M., Pagé, N., Gendron, S., Murray, T. & Ben, Y. (2006) Prog. Pap. Recycling, 16 (1), pp.31-40.
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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
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Background
•
Influence of concentration on fibre flocculation
• (laboratory trials with various pulps)
Example with TMP fibres
•Same results with BKP (HW, SW, mix), DIP,
etc.
The higher the concentration,
the higher the fibre flocculation
Huber, P., Carré, B., and Petit-Conil, M. (2008). BioRes. 3(4), 1218-1227.
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Background
•
Influence of crowding factor on pulp flocculation
• (flocculation varied by changing concentration and pulp
mixtures HW/SW)
crowding sphere
Kerekes (1985)
crowding
•
N= nb fibres in the crowding sphere
(non dimensional concentration)
fibre crowding determines fibre
flocculation (at given turbulence)
Huber P., Roux J.C., Mauret E., Belgacem N., and Pierre C. (2003), J. Pulp & Pap. Sci. 29(3):77-85.
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Background
•
Influence of crowding factor on gas hold-up
• (Column bubbling of virgin pulp)
Gas hold-up
(crowding)
The higher the fibre crowding,
the lower the air content
Tang, C. & Heindel, T.J. (2006) The Canadian Journal of Chemical Engineering, 84
(2), pp.198-208.
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Motivations & objectives
influence of pulp flocculation on flotation efficiency ?
How to vary flocculation ?
By changing concentration
1.2
flocculation (relative)
•
By adding dispersants
Non surface-active fibre
dispersants :
1
0.8
0.6
0.4
• Guar gum
• CMC
0.2
0
8
10
12
14
16
concentration (g/L)
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Outline
•
•
•
Background
Methods : flocculation sensor, gas hold-up sensor…
Results
•
Mechanisms
• Effect of concentration
• Effect of dispersants
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Materials and Methods
Flocculation measurement
Assessment of fibre flocculation
•
•
• 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
Pilot flow-loop
On-line
thickness =
3.5 mm
overflow
tank
Floc. sensor
IMAGE
ANALYSIS
flow
meter
FLASH
TRANSPARENT
CHANNEL
CCD
pulp
tank
1 m3
surface
•
General methods
CCD
CAMERA
•
Si
Di
…
Flocculation index:
FI
diameter
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Flocculation sensor principles
Flocculation index :
•
surface
•
binary morphology  floc size distribution
•
Flocculation index:
Si
Di
…
diameter
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Reduced sensitivity to light diffusion
•
•
Problem : filler diffuse light
Fibres only
Fibres+20% filler
RMS = 0.323
FI = 2.63 mm²
RMS = 0.069
FI = 2.65 mm²
(no filler)
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
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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 aeration
 Includes both hydraulic and physico-chemical effects
 relevant parameter that affects flotation efficiency
Air content = %air / unit volume
collection
flotation rate efficiency
constant
Fpulp
Fair
Air ratio = Fair/Fpulp
bubble surface
area flux
Ek Sb
k
4
 Sb is proportional to air content
Leichtle (1998)
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Flotation monitoring :
On-line measurement of air content
•
Dorris et al. (2006)
Paprican sensor
• Based on pressure difference
between immersed gauges
• Apparent pulp density varies with air
content
P
Figure 1. Installation of air content probe in a Voith cell.
•
Installation
• Installed on reject side, across the
hatch, at an angle of 60°
• probes installed in pre-flotation 1ry
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Materials and Methods
ii) Bubble size measurement
• 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)
Viewing
chamber
CCD
camera
Halogen
light source
PC for image
analysis
Bubble size
distribution
D.Beneventi,
Pilot verticell
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Flocculation
Effect of concentration
•
Effect on fibre flocculation (flotation cell inlet)
flocculation index (mm²)
8
8 g/L
7
16 g/L
6
5
4
3
2
Fibre fraction = 56%
1
0
0
5
10
15
20
concentration (g/L)
•
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
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Effect of concentration: industrial trials at Aylesford
Air content
100
• Large variations of air content over time (in 1ry)
• Higher air content contributes to better ink removal
air %
95
yield index (%)
efficiency
• Higher air impairs flotation yield
• Higher air content is caused by concentration
decrease
90
conc.
85
80
%
75
70
52
54
56
58
60
35
85
80
25
75
20
70
15
10
65
Air 1ry, 3rd cell
5
Huber, P., Rousset, X., Zeno, E. and Vazhure, T.
(2011) Ind. Eng. Chem. Res. 50(7) :4021-4028
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55
ink removal
01/12/2008
08/12/2008
15/12/2008
40
35
air content (%)
30
ink removal (%)
air content (%)
30
0
24/11/2008
(But take care, it is a question of
compromise: a too high consistency will
induce a too high decrease in ink
removal efficiency)
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Air 2ry, 1st cell
To maximise the flotation
yield, work at highest possible
concentration while
maintaining ERIC target
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ink removal (%)
50
22/12/2008
1.7
1.5
1st stage
cleaner feed
concentration
1.3
1.1
25
0.9
20
0.7
15
10
0.5
0.3
Air 1ry, 3rd cell
5
0
24/11/2008
0.1
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01/12/2008
08/12/2008
15/12/2008
-0.1
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22/12/2008
concentration (%)
•
Flocculation
Effect of dispersants
•
•On the fibre flocculation
flocculation index (mm²)
4.5
4
Selected dispersants (Guar gum
and CMC) effectively deflocculate DIP
-14 to 19 %
3.5
+1% guar
+2% guar
+1% CMC
+2% CMC
3
2.5
2
reference
1.5
guar gum
1
CMC
0.5
0
0
1
2
3
% additive
(Flotation cell inlet)
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Effect of dispersants : pilot trials
•
Selected dispersants (guar gum or CMC) clearly improved
flotation selectivity
Ink removal (%)
• Better ink removal + lower losses at the same time
60
58
56
54
52
50
48
46
44
42
40
•Increased air content thanks to
pulp de-flocculation (at the fibre
level)
•Lower entrainment of fine elements
thanks to depressing mechanism
(from adsorbed dispersants)
Guar gum
2% CMC
CMC
1% CMC
1% guar
2% guar
ref
0
10
20
30
40
50
(high overall losses because of
low froth height)
Total Losses (%)
Zeno, E., Huber, P., Rousset, X., Fabry, B. and Beneventi D. (2010).
Ind. Eng. Chem. Res., 2010, 49 (19), pp 9322–9329
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Effect of dispersants : pilot trials
Better selectivity : the link is fibre flocculation
flocculation directly influenced the pulp aeration : Gas hold-up when
flocculation
6
air content (%)
5
4
3
2
guar gum
CMC
1
0
0
1
2
3
4
5
flocculation index (mm²)
Zeno, E., Huber, P., Rousset, X., Fabry, B. and Beneventi D. (2010).
Ind. Eng. Chem. Res., 2010, 49 (19), pp 9322–9329
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Effect of dispersants : pilot trials
Bubble size ?
Rising velocity in Newtonian fluid :
Drift flux model :
Limited bubble size decrease
Not sufficient to explain increased gas hold-up
Higher 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
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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)
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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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