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


Objective

to remove color from, and thus increase the

%ISO brightness of mechanical pulps while

retaining their high-yield characteristics

14


- 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


Fe3+ CATALYZED H2O2 DECOMPOSITION

0

20

40

60

80

100

0 0.5 1 1.5 2


Res

idu

al H

2O

2 (

%)

Control

DTPA (0.2%)

Na2SiO3 (3%) + MgSO4 (0.08%)


20 g/L H2O2, pH 10.5, 40 ppm Fe3+


44


H2O2 DECOMPOSITION

0

20

40

60

80

100

0 0.5 1 1.5 2


Res

idu

al H

2O

2 (

%)

Control

DTPA (0.2%)

Na2SiO3 (3%) + MgSO4 (0.08%)


40 ppm Mn2+

, 40 ppm Fe3+

& 2 ppm Cu2+


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



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


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


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


Bri

gh

tness, IS

O(%

)

Spruce Bleached TMP

Spruce Bleached Kraft

65


- 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


- 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


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

69

REFERENCES SUGGESTED FOR FURTHER

READING

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

Principle and Practice”, Tappi Press, 1996, p.457 - 512.

J. Kappel, “Mechanical Pulps: from Wood to Bleached

Pulp”, Tappi Press, 1999, p.263 - 322.

mechanical pulp bleaching - ubc fibre lab · mechanical pulp bleaching dr. thomas q. hu...

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