Pulp Bleaching

09.09.2023
What is bleaching?

• Bleaching is the treatment of wood (and other

lignocellulosic) pulps with chemical agents to increase
their brightness.
• Lignin removal leads to greater fiber-fiber bonding
strength in paper, but the strong chemical used in
bleaching chemical pulps decreases the length of
cellulose molecules, resulting in weaker fibers.
• Bleaching can be categorized into 2 classes:
mechanical & chemical.
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: Thomas.Hu@fpinnovations.ca
 COMPOSITION OF WOOD

Softwoods Hardwoods

Cellulose Cellulose
~ 42% ~ 45%

Hemi- Hemi-
celluloses celluloses
~ 27% ~ 27%
Lignin Lignin
~ 28% ~ 20%
WHAT IS LIGNIN?

Lignin is a 3-dimensional, random polymer of

coniferyl & p-coumaryl or sinapyl alcohols

CH OH CH OH CH OH
2 2 2

OMe MeO OMe

OH OH OH

coniferyl p-coumaryl sinapyl

STRUCTURE OF SOFTWOOD LIGNIN
COLOR OF WOOD
KRAFT PULPING VS. MECHANICAL PULPING
EFFECTS OF LIGNIN ON MECHANICAL PULPS

High yield & opacity

EFFECTS OF LIGNIN ON MECHANICAL PULPS

Low strength, achievable brightness & brightness stability

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)
MECHANICAL PULP BLEACHING

Objective
to remove color from, and thus increase the
%ISO brightness of mechanical pulps while
retaining their high-yield characteristics
MECHANICAL PULP BLEACHING
- current technologies -

 Sodium hydrosulfite (dithionite) bleaching

 Reductive removal of lignin chromophores

 Alkaline hydrogen peroxide bleaching

 Oxidative removal of lignin chromophores
MECHANICAL PULP BASED PULP & PAPER
GRADES

Grade ISO Brightness Bleaching

Process
BCTMP market pulp 75 - 85
Tissue, absorbent products 60 - 80
LWC (coated) 70 - 72 Peroxide
Supercalendered A (SCA) 68 - 72
SCB 62 - 68
Highbrights 62 - 72
Rotonews 57 - 62
Newsprint 55 - 60
Hydrosulfite
Directory 55 - 60
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
HYDROSULFITE BLEACHING REACTIONS

 Reduction of carbonyl groups in lignin

H O H OH
H
Na2S2O4
+ NaHSO3
MeO MeO
OLig OLig

Lig Lig
Na2S2O4
+ NaHSO3
O OH
O OH
HYDROSULFITE DECOMPOSITION

1. Hydrolysis
2 Na2S2O4 + H2O 2 NaHSO3 + Na2S2O3
bisulfite thiosulfate
2. Oxidation with limited air
2 Na2S2O4 + O2 + 2 H2O 4 NaHSO3

3. Oxidation with excess air (entrapped in pulp)

Na2S2O4 + O2 + H2O NaHSO3 + NaHSO4

bisulfate
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 – 60C, 0.5 – 2 hours and up-flow tower

Consistency = weight pulp / (weight of pulp + weight of water)

EFFECT OF AIR AND HYDROSULFITE
CHARGE
12
11
Brightness gain (%ISO points)

10
9 Without air

8
7
With air
6
5
4
3
2
1
0
0 0.5 1 1.5 2 2.5

Na2S2O4 (% on pulp)
 EFFECT OF TEMPERATURE AND pH

67

66
%ISO Brightness

65

64

63
60 oC

50 oC
62
80 oC

61
2 3 4 5 6 7 8 9

Bleaching pH
 EFFECT OF CONSISTENCY

70

65
%ISO Brightness

60
5% Consistency, 60 min
10% Consistency, 30 min

55

50
0 0.5 1 1.5 2 2.5

Na2S2O4 (% on pulp)
FORMATION OF THIOSULFATE

2 Na2S2O4 + H2O 2 NaHSO3 + Na2S2O3

sodium 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
 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
 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)
SOURCES OF HYDROSULFITE IN PULP
MILLS

 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

NaBH4 + 8 NaOH + 8 SO2 4 Na2S2O4 + NaBO2 + 6 H2O

hydrosulfite borate
FLOWSHEET FOR HYDROSULFITE
BLEACHING IN AN UP-FLOW TOWER

Up-flow tower
Steam Temp. control
Unbleached
stock storage

Bleach
solution

Cs. control
White water
Ratio
control
Level control
Bleached stock
Level chest to paper machine
 HYDROSULFITE REFINER BLEACHING

 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
- continue

16
9 - 14 points
14
ISO Brightness gain

12 8 - 10 points

10

8 4 - 6 points

0
Tower (single stage) Refiner (single stage) Refiner and Tower
(two-stage)
 HYDROSULFITE BLEACHING OF
RECYCLED FIBERS

 Regain brightness of deinked, recycled newspapers

which may have been lost through reversion (yellowing)

Lig-TMP Lig-Newsprint Lig-ONP

Na2S2O4
Light

O OH O
O OH O

ONP = old newsprint

 HYDROSULFITE BLEACHING OF
RECYCLED FIBERS - continue

 Remove color introduced into pulps/papers in the form

of dyes and other coloring agents
PhCONH SO3H N N

N N Red dye
SO3 H
OH
Na2S2O4
H H
PhCONH SO3H N N

N N
SO3 H
OH H H

Na2S2O4
PhCONH SO3H NH2 H2N

NH2 H2N SO3H

OH
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
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)
PEROXIDE BLEACHING CONDITIONS
- continue

 Temperature
 50 - 85C, typically 60 C

 Time
 1 – 3 hours, typically 2 hours

 Vessel
 Down flow tower
PEROXIDE BLEACHING REACTIONS

1. HOOH + OH- = HOO - + H2O

H O
H O
H O
2. HOO- + +
MeO H O
MeO OLig
OLig

Reactions 1 & 2 result in oxidative removal of lignin chromophores,

giving a brighter pulp while contributing to yield loss & COD in effluent
 PEROXIDE BLEACHING REACTIONS
- continue

CH2OH CH2OH
O O
OR OR
3. HOO- + OH
O O

OH O O

CH2OH
OH
Other degradation +
products O
OH
O
Reaction 3 results in oxidative degradation of carbohydrates,
contributing to yield loss and effluent with high COD
PEROXIDE DECOMPOSITION REACTIONS

1. Alkaline decomposition during bleaching

HOOH + OH - HOO- + H2O

H2O + OH - + O2 H2O2

2. SO2 Neutralization at the end of bleaching

H2O2 + H2SO3 H2SO4 + H2O

Reaction 1 is catalyzed by transition metals such as Mn2+ & Fe3+, thus

efficient removal of transition metals is critical for peroxide bleaching
EFFECT OF WOOD SPECIES & PULPING
CONDITIONS

90
85
Aspen CTMP
80 Spruce CTMP
Spruce TMP
%ISO Brightness

75
70
Hemlock - Balsam Fir SGW
65
60
55
50
Douglas Fir SGW
45
40
0 1 2 3 4 5 6 7

H2O2 (% on pulp)
EFFECT OF CAUSTIC AND PEROXIDE
CHARGE

77

H2O2 (4% on pulp)

75
%ISO Brightness

73

71
H2O2 (3% on pulp)

69
1 2 3 4 5 6
NaOH (% on pulp)
METAL CONTENTS IN EASTERN
SPRUCE/BALSAM GROUNDWOOD & CTMP

90
CTMP
80
Metal content (ppm) of the pulps

70
70 65
CTMP
60 55
50
50

40

30
CTMP
20
8 10
10

0
Mn2+ Fe3+ Cu2+
CHELATING (METAL SEQUESTERING)
AGENTS

 DTPA (diethylenetriamine pentaacetic acid)

 most effective, supplied as pentasodium salt
 EDTA (ethylenediamine tetraacetic acid)
 cheaper, less effective, as tetrasodium salt
O O O

OH OH OH
N N O N
N N
O OH O
O OH O OH O OH
OH OH
EFFECT OF DTPA, Na2SiO3 & MgSO4 ON
Mn2+ CATALYZED H2O2 DECOMPOSITION

2+
20 g/L H2O2, pH 10.5, 40 ppm Mn (Bambrick, 1984 Tappi Pulping Conf. 345-351)
100

80
Residual H2O2 (%)

60

40

Control
20 DTPA (0.2%)
Na2SiO3 (3%) + MgSO4 (0.08%)
DTPA + Na2SiO3 + MgSO4

0
0 0.5 1 1.5 2
Time (hours) (at 60oC)
EFFECT OF DTPA, Na2SiO3 & MgSO4 ON
Fe3+ CATALYZED H2O2 DECOMPOSITION
3+
20 g/L H2O2, pH 10.5, 40 ppm Fe (Bambrick, 1984 Tappi Pulping Conf. 345-351)
100

80
Residual H2O2 (%)

60

40
Control
DTPA (0.2%)
Na2SiO3 (3%) + MgSO4 (0.08%)
20
DTPA + Na2SiO3 + MgSO4

0
0 0.5 1 1.5 2
Time (hours) (at 60oC)
EFFECT OF DTPA, Na2SiO3 & MgSO4 ON
H2O2 DECOMPOSITION

40 ppm Mn2+, 40 ppm Fe3+ & 2 ppm Cu2+ (Bambrick, 1984 Tappi Pulping Conf. 345-351)
100
Control
DTPA (0.2%)

80 Na2SiO3 (3%) + MgSO4 (0.08%)

DTPA + Na2SiO3 + MgSO4
Residual H2O2 (%)

60

40

20

0
0 0.5 1 1.5 2
Time (hours) (at 60oC)
 EFFECT OF CHELATING AGENT

78
Pretreatment with
0.16% DTPA
74
%ISO Brightness

70 Without pretreatment

66

62

58

54
0 1 2 3 4 5 6

H2O2 (% on pulp)
 EFFECT OF SODIUM SILICATE

72
0.15% DTPA pretreatment

68
%ISO Brightness

0.15% DTPA added

to bleach liquor without DTPA

64

60

1.5% H2O2, 0.1% MgSO4

56
0 1 2 3 4 5 6

Na2SiO3 (% on pulp)
EFFECT OF CONSISTENCY

16
Brightness Gain (% ISO points)

High consistency (20%)

14

12
Medium consistency (10 -15%)
10

6 Low consistency (3 - 6%)

0
0 0.5 1 1.5

H2O2 (% on pulp)
H2O2 CONCENTRATION AT VARIOUS
CONSISTENCIES

15

(4% H2O2 on pulp)

H2O2 concentration (g/L)

10
10

5 4.44

2.55

0
6% consistency 10% consistency 20% consistency
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
 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
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
MAKE-UP SYSTEM FOR H2O2 BLEACH
SOLUTION

50% H2O2

To process
NaOH solution

Na2SiO3 & Static Mixers

MgSO4

H2O
Continuous peroxide bleach solution make-up system
 FLOWSHEET FOR ONE-STAGE, MEDIUM
CONSISTENCY, PEROXIDE BLEACHING PROCESS

DTPA

~20% Cs. H2O2 Bleach Solution

Steam (peg) mixer

Pulp
Pulp Thickener SO2 or H2SO4

10 -15% Cs.
Bleached stock
to paper machine
Dilution H2O

Bleach Tower
 FLOWSHEET FOR ONE-STAGE, HIGH
CONSISTENCY, PEROXIDE BLEACHING PROCESS

H2O2 Bleach Solution

~4% Cs.
~35% Cs.
~20% Cs. Bleach
DTPA Tower
25% Cs.

Pulp SO2
HC Pulp
Thickener HC
Bleached
Mixer
Pulp
~4% Cs.
Stock Chest
 FLOWSHEET FOR TWO-STAGE, MEDIUM-HIGH
CONSISTENCY, PEROXIDE BLEACHING PROCESS

Medium Consistency High Consistency

Bleached Tower Bleached Tower
Make -up Fresh Bleach
Chemicals Solution

Pulp

Recycling of H2O2
 FLOWSHEET FOR TWO-STAGE, PEROXIDE-
HYDROSULFITE BLEACHING PROCESS

H2O2 Bleach Solution Upflow Tower

DTPA

4% Cs.
Pulp

Na2S2O4

SO2
10-15% Cs.

Dilution H2O Ratio

control Bleached stock
to paper machine
Bleach Tower
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
 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
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
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
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)]
72

71
ISO Brightness (%)

70

69

68
Y
67
"0.1%THPS + Y"

66
Hu, James, et al. Pulp Paper Can. 37-42 (2009)
65
0.0 0.2 0.4 0.6 0.8 1.0 1.2
Hydrosulfite (Y) (% on pulp) (pulp = SGW)
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)]
82
Control TMP
80
In-refiner "Y + DTPA"-bleached TMP
78 78.1

76
ISO Brightness (%)

76.5
75.0
74 74.3
73.8
73.5
72

70 70.5

68

66

64
2.0 3.0 4.0 5.0 6.0 7.0
H2O2 (on pulp) (pulp = TMP from beetle-killed LPP)
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
YELLOWING OF BLEACHED TMP UNDER
OFFICE FLUORECENT LIGHT EXPOSURE

90

85
Brightness, ISO(%)

80 Spruce Bleached Kraft

75

70

65

60

55 Spruce Bleached TMP

50
0 10 20 30 40 50 60

Office light exposure (days)

YELLOWING OF MECHANICAL PULPS
- continue

 Cause
 due to the oxidation of lignin

Lignin Free radicals Yellow products

 Consequence
 Limiting wider use of BCTMP in high-quality, fine
papers such as printing and writing papers
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
YELLOWING INHIBITION BY ATTACHING
INHIBITOR TO PULP

Hu, et al. J. Pulp Paper Sci. 31(3): 109-115 (2005)

74

72 Bleached TMP
Inhibitor-attached, Bleached TMP
70
Brightness, ISO (%)

68

66

64

62

60

58

56
0 10 20 30 40 50
Office light exposure (days)
 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
REFERENCES SUGGESTED FOR FURTHER
READING

1. C.W. Dence and D.W. Reeve, Editors, “Pulp Bleaching,

Principle and Practice”, Tappi Press, 1996, p.457 - 512.
2. J. Kappel, “Mechanical Pulps: from Wood to Bleached
Pulp”, Tappi Press, 1999, p.263 - 322.