MECHANICAL PULP BLEACHING - UBC Fibre Lab · MECHANICAL PULP BLEACHING Dr. THOMAS Q. HU...
Transcript of MECHANICAL PULP BLEACHING - UBC Fibre Lab · MECHANICAL PULP BLEACHING Dr. THOMAS Q. HU...
MECHANICAL PULP BLEACHING
Dr. THOMAS Q. HU
FPInnovations – Pulp and Paper Division
3800 Wesbrook Mall, Vancouver, BC V6S 2L9
Tel.: 604-222-3235
Fax: 604-222-3207
Email: [email protected]
2
CONTENT OF LECTURE
Introduction
Hydrosulfite bleaching (chemistry, process
parameters & flowsheet)
Hydrogen peroxide bleaching fundamentals
Hydrogen peroxide bleaching (process parameters
& bleach plant configurations)
Yellowing of mechanical pulps & its inhibition
Summary
3
Cellulose
~ 42%
Lignin
~ 28%
Hemi-
celluloses
~ 27%
Cellulose
~ 45%
Lignin
~ 20%
Hemi-
celluloses
~ 27%
Softwoods Hardwoods
COMPOSITION OF WOOD
4
WHAT IS LIGNIN?
OH
CH2OH
OMe MeO
OH
CH2OH
OMe
OH
CH2OH
p-coumaryl coniferyl sinapyl
Lignin is a 3-dimensional, random polymer of
coniferyl & p-coumaryl or sinapyl alcohols
5
STRUCTURE OF SOFTWOOD LIGNIN
OH OMe MeO
HO
OH O
OH OH O O
OH
OH HO
O OMe O
MeO
MeO
OH
OH HO
MeO O
O
OH OMe MeO O
OH
OH
OH
OH O O
O H
MeO OMe
OH
OH
OMe
O
O
OMe OH
MeO O
OH O OH
O O
O OH
O MeO
CHO OH
6
COLOR OF WOOD
Due to the presence of lignin chromophores
ortho-Quinones Coniferaldehydes
OLig MeO
O H
O
Lig
O
7
KRAFT PULPING VS. MECHANICAL PULPING
Softwood, ~ 28% Lignin
Kraft Pulp, ~ 5% Lignin Mechanical Pulp
~ 27% Lignin
Bleached Mechanical Pulp
~ 26% Lignin
Bleached Kraft Pulp
0% Lignin
Kraft Pulping
Mechanical
Pulping
Bleaching Bleaching
White pulp/paper Pale white/light yellow pulp/paper
8
WORLD MECHANICAL PULP PRODUCTION
- by year -
25
30
35
40
1975 1980 1985 1990 1995 2000 2005 2010 2015
year
Pro
du
cti
on
, m
illi
on
me
tric
to
ns
9
2009 MECHANICAL PULP PRODUCTION
- by country -
0
2
4
6
8
10
12
Pro
du
cti
on
, m
illi
on
metr
ic t
on
s
Canada
US
FinlandSweden
Norway
Rest of the world
10
EFFECTS OF LIGNIN ON MECHANICAL PULPS
High yield & opacity
0
20
40
60
80
100
Kraft TMP
Yie
ld (
%)
45 - 48%
92 - 96%
0
20
40
60
80
100
Kraft TMP
Op
acit
y (
%)
63 - 67%
95 - 97%
11
EFFECTS OF LIGNIN ON MECHANICAL PULPS
Low strength, achievable brightness & brightness stability
0
2
4
6
8
10
12
SW Kraft SW TMP
Breakin
g len
gth
(km
)
9 - 11
2 - 3.550
55
60
65
70
75
80
85
90
0 10 20 30 40 50 60
Office light exposure (days)
Brig
htn
ess, IS
O(%
)
Spruce Bleached TMP
Spruce Bleached Kraft
12
ISO BRIGHTNESS OF PULPS/PAPERS
Estimation of whiteness of pulps/papers
Directional reflectance of light at 457 nm
calibrated with MgO film (100% ISO) and a black
background (0% ISO)
deep brown kraft pulp (~ 30% ISO); fully bleached
kraft pulp/white office papers (~ 90% ISO);
unbleached mechanical pulps (45 - 60% ISO);
bleached mechanical pulps (55 - 85% ISO)
13
MECHANICAL PULP BLEACHING
Objective
to remove color from, and thus increase the
%ISO brightness of mechanical pulps while
retaining their high-yield characteristics
14
MECHANICAL PULP BLEACHING
- current technologies -
Sodium hydrosulfite (dithionite) bleaching
reductive removal of lignin chromophores
Alkaline hydrogen peroxide bleaching
oxidative removal of lignin chromophores
15
MECHANICAL PULP BASED PULP & PAPER
GRADES
Grade ISO Brightness
BCTMP market pulp 75 - 85
Tissue, absorbent products 60 - 80
LWC (coated) 70 - 72
Supercalendered A (SCA) 68 - 72
SCB 62 - 68
Highbrights 62 - 72
Rotonews 57 - 62
Newsprint 55 - 60
Directory 55 - 60
Bleaching Process
Peroxide
Hydrosulfite
16
SODIUM HYDROSULFITE BLEACHING
Principle uses
to increase ISO brightness of groundwood, TMP or
CTMP by ~8 points for use in newsprint or
telephone directory papers
to bleach deinked pulps and strip color from
recycled, mixed office waster papers
Limitation
Incapable of increasing mechanical pulp ISO
brightness by more than 10 points or so
17
HYDROSULFITE BLEACHING REACTIONS
Reduction of carbonyl groups in lignin
OLig MeO
O H
Na2S2O4
O
Lig
O
OLig MeO
OH H
H
OH
Lig
OH
NaHSO3 +
Na2S2O4 NaHSO3 +
18
HYDROSULFITE DECOMPOSITION
2. Oxidation with limited air
1. Hydrolysis
2 Na2S2O4 + H2O 2 NaHSO3 + Na2S2O3
2 Na2S2O4 + O2 + 2 H2O 4 NaHSO3
3. Oxidation with excess air (entrapped in pulp)
Na2S2O4 + O2 + H2O NaHSO3 + NaHSO4
bisulfite thiosulfate
bisulfate
19
HYDROSULFITE BLEACHING CONDITIONS
Bleaching chemicals % wt. on pulp
sodium hydrosulfite (Na2S2O4) 0.2 – 1.2%
Consistency (Cs.) and pH
4 – 10% Cs. and pH 5 – 7 (pH ~6.0 optimum)
Temperature, time and vessel
50 – 60C, 0.5 – 2 hours and up-flow tower
Consistency = weight pulp / (weight of pulp + weight of water)
20
EFFECT OF AIR AND HYDROSULFITE
CHARGE
0
1
2
3
4
5
6
7
8
9
10
11
12
0 0.5 1 1.5 2 2.5
Na2S2O4 (% on pulp)
Bri
gh
tne
ss
ga
in (
%IS
O p
oin
ts)
With air
Without air
21
EFFECT OF TEMPERATURE AND pH
61
62
63
64
65
66
67
2 3 4 5 6 7 8 9
Bleaching pH
%IS
O B
rig
htn
ess
60 oC
50 oC
80 oC
Poly. (80
oC)
22
EFFECT OF CONSISTENCY
50
55
60
65
70
0 0.5 1 1.5 2 2.5
Na2S2O4 (% on pulp)
%IS
O B
rig
htn
es
s
5% Consistency, 60 min
10% Consistency, 30 min
23
FORMATION OF THIOSULFATE
Thiosulfate is form during storage of hydrosulfite
solution if it is not chilled adequately and/or if the pH
drops below 9-10
Thiosulfate is formed during hydrosulfite bleaching,
particularly if pH is below 5.0
2 Na2S2O4 + H2O 2 NaHSO3 + Na2S2O3
sodium thiosulfate
24
THE THIOSULFATE PROBLEM
As little as 10 ppm thiosulfate (S2O32-) in paper machine
white water will cause severe corrosion of 304-
stainless steel in less than 15 days
Thiosulfate attack is the most serious form of
corrosion of Canadian newsprint machines
25
ESSENTIALS FOR HYDROSULFITE
BLEACHING
Pulp and bleach solution must be oxygen-free
Careful pH control of pulp and bleaching solution,
must avoid pHs below 5 or above 7
Sodium hydrosulfite should be freshly made, if stored,
should be at low temperature; storage tank should be
chilled (~ 5 oC)
26
SOURCES OF HYDROSULFITE IN PULP
MILLS
4 Na2S2O4 + NaBO2 + 6 H2O NaBH4 + 8 NaOH + 8 SO2
hydrosulfite borate
Solid sodium hydrosulfite and hydrosulfite bleach
solution containing pH buffer & other stabilizers
can both be purchased
Sodium hydrosulfite can also be produced on-site
using the Borol Process
27
FLOWSHEET FOR HYDROSULFITE
BLEACHING IN AN UP-FLOW TOWER
Unbleached
stock storage
Steam
White water
Level chest
Bleached stock
to paper machine
Up-flow tower
Bleach
solution
Cs. control
Temp. control
Ratio
control Level control
28
Hydrosulfite bleach solution added directly to
the dilution water line at the refiner
Opportunity to obtain higher brightness gain on
an intermittent basis
HYDROSULFITE REFINER BLEACHING
29
HYDROSULFITE REFINER BLEACHING
- continue
0
2
4
6
8
10
12
14
16
Tower (single stage) Refiner (single stage) Refiner and Tower
(two-stage)
ISO
Brig
htn
ess g
ain 8 - 10 points
4 - 6 points
9 - 14 points
30
Regain brightness of deinked, recycled newspapers
which may have been lost through reversion (yellowing)
O
Lig-TMP
O
OH
Lig-Newsprint
OH
Na2S2O4
O
Lig-ONP
O
Light
ONP = old newsprint
HYDROSULFITE BLEACHING OF
RECYCLED FIBERS
31
Remove color introduced into pulps/papers in the form
of dyes and other coloring agents
OH
N
SO3HPhCONH
OH
NH2
SO3HPhCONH
N
N N
SO3H
SO3H
H2NNH2
H2N
OH
N
SO3HPhCONH
N
N N
SO3HH H
H H
Na2S2O4
Na2S2O4
Red dye
HYDROSULFITE BLEACHING OF
RECYCLED FIBERS - continue
32
HYDROGEN PEROXIDE BLEACHING
Principle use
to increase ISO brightness of groundwood, TMP or
CTMP by up to 25 points for use in high brightness
specialty papers, lightweight coated papers, tissue &
in bleached CTMP production
Limitations
cannot bleach mechanical pulps to above 85% ISO
brightness; yield loss and high effluent COD
33
PEROXIDE BLEACHING CONDITIONS
Bleaching chemicals %weight on pulp
Hydrogen peroxide (H2O2) 1 - 5%
Caustic (NaOH) 0.6 - 5%
Sodium silicate (Na2SiO3) 2 - 6%
Magnesium sulfate (MgSO4) 0.01 - 0.2%
Chelating agent (DTPA) 0.1 - 0.6%
Consistency
10 - 40%, typically 12% (medium) or 25% (high)
34
PEROXIDE BLEACHING CONDITIONS
- continue
Temperature
50 - 85C, typically 60 C
Time
1 – 3 hours, typically 2 hours
Vessel
Down flow tower
35
PEROXIDE BLEACHING REACTIONS
OLig MeO
O H
OLig MeO
O H O H
H O
+ 2. HOO- +
1. HOOH + OH- = HOO- + H2O
Reactions 1 & 2 result in oxidative removal of lignin chromophores,
giving a brighter pulp while contributing to yield loss & COD in effluent
36
PEROXIDE BLEACHING REACTIONS
- continue
O
OH
OH
CH2OH
OR
O
O
O
CH2OH
OR
O
O
OH
CH2OH
O
O OH
Other degradation
products
3. HOO- +
+
Reaction 3 results in oxidative degradation of carbohydrates,
contributing to yield loss and effluent with high COD
37
PEROXIDE DECOMPOSITION REACTIONS
2. SO2 Neutralization at the end of bleaching
Reaction 1 is catalyzed by transition metals such as Mn2+ & Fe3+, thus
efficient removal of transition metals is critical for peroxide bleaching
1. Alkaline decomposition during bleaching
HOO- + H2O
H2O + OH- + O2
H2O2 + H2SO3 H2SO4 + H2O
HOOH + OH-
H2O2
38
EFFECT OF WOOD SPECIES & PULPING
CONDITIONS
40
45
50
55
60
65
70
75
80
85
90
0 1 2 3 4 5 6 7
H2O2 (% on pulp)
%IS
O B
rig
htn
es
s
Douglas Fir SGW
Hemlock - Balsam Fir SGW
Spruce TMP
Spruce CTMPAspen CTMP
39
EFFECT OF CAUSTIC AND PEROXIDE
CHARGE
69
71
73
75
77
1 2 3 4 5 6
NaOH (% on pulp)
%IS
O B
rig
htn
es
s
H2O2 (3% on pulp)
H2O2 (4% on pulp)
40
METAL CONTENTS IN EASTERN
SPRUCE/BALSAM GROUNDWOOD & CTMP
55
70
8
65
50
10
0
10
20
30
40
50
60
70
80
90
Mn2+ Fe3+ Cu2+
Meta
l conte
nt (p
pm
) of
the p
ulp
s
CTMP
CTMP
CTMP
41
CHELATING (METAL SEQUESTERING)
AGENTS
DTPA (diethylenetriamine pentaacetic acid)
most effective, supplied as pentasodium salt
EDTA (ethylenediamine tetraacetic acid)
cheaper, less effective, as tetrasodium salt
N
OH
NN
O
O
OH
O OHOHO
O
OH
N
OH
NO
O
O
OH
O OHOH
DTPA EDTA
42
EFFECT OF DTPA, Na2SiO3 & MgSO4 ON
Mn2+ CATALYZED H2O2 DECOMPOSITION
0
20
40
60
80
100
0 0.5 1 1.5 2
Time (hours) (at 60oC)
Res
idu
al H
2O
2 (
%)
Control
DTPA (0.2%)
Na2SiO3 (3%) + MgSO4 (0.08%)
DTPA + Na2SiO3 + MgSO4
20 g/L H2O2, pH 10.5, 40 ppm Mn2+
(Bambrick, 1984 Tappi Pulping Conf. 345-351)
43
EFFECT OF DTPA, Na2SiO3 & MgSO4 ON
Fe3+ CATALYZED H2O2 DECOMPOSITION
0
20
40
60
80
100
0 0.5 1 1.5 2
Time (hours) (at 60oC)
Res
idu
al H
2O
2 (
%)
Control
DTPA (0.2%)
Na2SiO3 (3%) + MgSO4 (0.08%)
DTPA + Na2SiO3 + MgSO4
20 g/L H2O2, pH 10.5, 40 ppm Fe3+
(Bambrick, 1984 Tappi Pulping Conf. 345-351)
44
EFFECT OF DTPA, Na2SiO3 & MgSO4 ON
H2O2 DECOMPOSITION
0
20
40
60
80
100
0 0.5 1 1.5 2
Time (hours) (at 60oC)
Res
idu
al H
2O
2 (
%)
Control
DTPA (0.2%)
Na2SiO3 (3%) + MgSO4 (0.08%)
DTPA + Na2SiO3 + MgSO4
40 ppm Mn2+
, 40 ppm Fe3+
& 2 ppm Cu2+
(Bambrick, 1984 Tappi Pulping Conf. 345-351)
45
EFFECT OF CHELATING AGENT
54
58
62
66
70
74
78
0 1 2 3 4 5 6
H2O2 (% on pulp)
%IS
O B
rig
htn
es
s
Without pretreatment
Pretreatment with
0.16% DTPA
46
EFFECT OF SODIUM SILICATE
56
60
64
68
72
0 1 2 3 4 5 6
Na2SiO3 (% on pulp)
%IS
O B
rig
htn
ess
without DTPA0.15% DTPA added
to bleach liquor
0.15% DTPA pretreatment
1.5% H2O2, 0.1% MgSO4
47
EFFECT OF CONSISTENCY
0
2
4
6
8
10
12
14
16
0 0.5 1 1.5
H2O2 (% on pulp)
Bri
gh
tne
ss
Ga
in (
% IS
O p
oin
ts)
Low consistency (3 - 6%)
High consistency (20%)
Medium consistency (10 -15%)
48
H2O2 CONCENTRATION AT VARIOUS
CONSISTENCIES
2.55
4.44
10
0
5
10
15
6% consistency 10% consistency 20% consistency
H2O
2 c
on
cen
tratio
n (
g/L
)
(4% H2O2 on pulp)
49
ESSENTIALS FOR PEROXIDE BLEACHING
Must use an optimal ratio of caustic/H2O2 to obtain
optimal brightness and maintain some H2O2 residual
(10-15% applied) at the end of bleaching to prevent
alkali-darkening of the pulps
Must control the decomposition of H2O2 by transition
metals (Mn2+, Fe3+, Cu2+) through the use of DTPA,
silicate & magnesium sulfate
50
BLEACH PLANT CONFIGURATIONS
One-stage, medium consistency (10-15% Cs.)
8-16 ISO brightness point gain, minimum capital costs, higher
chemical costs/unit of brightness gain
One-stage, high consistency (~25% Cs.)
8-20 ISO brightness points gain, moderate chemical cost,
higher capital cost
Two-stage, medium and high consistency
15-30 ISO brightness point gain, higher capital costs, lower
chemical costs/unit of brightness gain
51
DTPA PRETREATMENT IN THE MILLS
Carried out in the latency chest (3 - 5% Cs., 60 -
90oC and 30 - 60 min, with good agitation), followed
by a pulp thickening step
Carried out in the feed chest for the pulp thickener
(= or > 15 min., 40 - 50 oC and 3 - 5% Cs.), followed
by a pulp thickening step
52
MAKE-UP SYSTEM FOR H2O2 BLEACH
SOLUTION
Na2SiO3 &
MgSO4
H2O
NaOH solution
50% H2O2
To process
Continuous peroxide bleach solution make-up system
Static Mixers
53
FLOWSHEET FOR ONE-STAGE, MEDIUM
CONSISTENCY, PEROXIDE BLEACHING PROCESS
DTPA
H2O2 Bleach Solution
SO2 or H2SO4
Pulp
Pulp Thickener
Steam (peg) mixer
Bleach Tower
10 -15% Cs.
Dilution H2O
~20% Cs.
Bleached stock
to paper machine
54
FLOWSHEET FOR ONE-STAGE, HIGH
CONSISTENCY, PEROXIDE BLEACHING PROCESS
H2O2 Bleach Solution
HC
Mixer
HC Pulp
Thickener
Bleach
Tower DTPA
Pulp
~4% Cs.
~35% Cs.
25% Cs.
SO2
Bleached
Pulp
~4% Cs.
Stock Chest
~20% Cs.
55
FLOWSHEET FOR TWO-STAGE, MEDIUM-HIGH
CONSISTENCY, PEROXIDE BLEACHING PROCESS
Medium Consistency
Bleached Tower
Fresh Bleach
Solution
Make-up
Chemicals
Recycling of H2O2
High Consistency
Bleached Tower
Pulp
56
FLOWSHEET FOR TWO-STAGE, PEROXIDE-
HYDROSULFITE BLEACHING PROCESS
DTPA
H2O2 Bleach Solution
Bleach Tower
SO2 10-15% Cs.
Pulp
Upflow Tower
Ratio
control
Na2S2O4
Dilution H2O
4% Cs.
Bleached stock
to paper machine
57
OTHER EFFECTS PEROXIDE BLEACHING
Resin removal (resin & fatty acid >1% to <0.3%)
important for BCTMP, tissue & absorbancy products
Pulp strength improvement
bleaching with 3-4% H2O2 increases fibre flexibility
and thus tensile index and sheet density by ~25%
Improved pulp reslushing
important in reducing energy to disintegrate flash
dried bleached CTMP market pulps
58
OTHER EFFECTS PEROXIDE BLEACHING
- continue
Required an excessive amount of bleaching
chemicals
Aspen CTMP: NaOH (5%), H2O2 (4%), Na2SiO3 (3%),
DTPA (0.5%), MgSO4 (0.05%) = total 12.55% on pulp
Yield loss & effluent with high COD
bleaching with 3-4% H2O2 & 3-5% NaOH causes a
yield loss of 4-5% and consequently gives 60 to 75 kg
COD / tone of pulp
59
NON-CONVENTIONAL H2O2 BLEAHCING
Refiner bleaching
no need to have a bleach plant
use of sodium silicate not feasible (risk of silicate
scale on refiner plates)
Alternative alkali [MgO, Mg(OH)2] source
lower pH for bleaching, lower COD and higher yield
slower bleaching reaction, commercial application
yet to be demonstrated
60
NON-CONVENTIONAL H2O2 BLEAHCING
- continue
Alkaline peroxide chemithermomechanical
pulping
NaOH and H2O2 are added to the impregnator
no need to have a bleach plant
AP CTMP pulps possess properties similar to
BCTMP pulps
61
NEW REDUCTIVE BLEACHING AGENT - Discovered by Hu (FPInnovations) & James (UBC)
Tetrakis(hydroxymethyl)phosphonium sulfate (THPS), [P(CH2OH)4]2SO4
[Hu, James, et al. J. Pulp Paper Sci. 30(8): 233-240 (2004)]
65
66
67
68
69
70
71
72
0.0 0.2 0.4 0.6 0.8 1.0 1.2
Hydrosulfite (Y) (% on pulp) (pulp = SGW)
ISO
Bri
ghtn
ess
(%)
Y
"0.1%THPS + Y"
Hu, James, et al. Pulp Paper Can. 37-42 (2009)
62
REFINER “HYDROSULFITE + DTPA” BLEACHING
FOLLOWED BY CONVENTIONAL TOWER BLEACHING
Extremely effective for TMP made from darker chips such as beetle-
killed, blue-stained lodgepole pine [Hu, et al. Tappi J. 1: 25-31, (2011)]
70.5
74.3
75.0
73.5
73.8
78.1
76.5
64
66
68
70
72
74
76
78
80
82
2.0 3.0 4.0 5.0 6.0 7.0
H2O2 (on pulp) (pulp = TMP from beetle-killed LPP)
ISO
Bri
gh
tnes
s (%
)
Control TMP
In-refiner "Y + DTPA"-bleached TMP
63
YELLOWING OF MECHANICAL PULPS
Heat-induced yellowing
during papermaking peroxide & hydrosulfite
bleached mechanical pulps lose 1-2 and 3-4 ISO
points, respectively
Light-induced yellowing
when exposed to sunlight or office light, unbleached
or bleached mechanical pulps yellow (lose
brightness) rapidly
64
YELLOWING OF BLEACHED TMP UNDER
OFFICE FLUORECENT LIGHT EXPOSURE
50
55
60
65
70
75
80
85
90
0 10 20 30 40 50 60
Office light exposure (days)
Bri
gh
tness, IS
O(%
)
Spruce Bleached TMP
Spruce Bleached Kraft
65
YELLOWING OF MECHANICAL PULPS
- continue
Cause
due to the oxidation of lignin
Lignin Free radicals Yellow products Light O2
Consequence
Limiting wider use of BCTMP in high-quality, fine
papers such as printing and writing papers
66
YELLOWING OF MECHANICAL PULPS
- continue
Possible solutions
coating papers with yellowing inhibitors (UV
absorbers and/or radical scavengers/antioxidants)
chemically attaching yellowing inhibitors (radical
scavengers/antioxidants) to bleached mechanical
pulps
67
YELLOWING INHIBITION BY ATTACHING
INHIBITOR TO PULP
56
58
60
62
64
66
68
70
72
74
0 10 20 30 40 50
Office light exposure (days)
Bri
gh
tne
ss
, IS
O (
%)
Bleached TMP
Inhibitor-attached, Bleached TMP
Hu, et al. J. Pulp Paper Sci. 31(3): 109-115 (2005)
68
SUMMARY
Hydrosulfite bleaching
favored for bleaching mechanical pulps to ~ 60% ISO
brightness for use in short-life paper products
important to control hydrosulfite decomposition
Peroxide bleaching
favored for bleaching mechanical pulps to 75-85% ISO
brightness for BCTMP market pulp, tissue &
lightweight coated paper
important to control metal content of the pulps &
caustic/H2O2 ratio of the bleach solution