Peroxide Bleaching of Eucalyptus CTMP

Using Mg(OH)2 as the Alkali Source

Z. He; Y. Ni

University of New Brunswick, Canada

Conventional Peroxide Bleaching of Mechanical Pulp

• Use NaOH as the alkali source

• High alkalinity, especially in the initial phase

• Peroxide decomposition and darkening

reactions

• High COD load of the effluent

• High anionic trash production

• Decreased light scattering coefficient

• Decreased pulp yield

Mg(OH)2: An Alternative Alkali for

Peroxide Bleaching

• Mild alkalinity

• Gradual release of HO- due to low

solubility

• One unit weight of Mg(OH)2 equals to

1.4 NaOH in the capacity of accepting

protons.

Mg(OH)2 - PM Bleaching

Process

Mg(OH)2

H2O2

PM MC

#1

MC

#2

DTPA/Na2S2O4

“QY”

PMg(OH)2 Versus PNaOH

PMg(OH)2 PNaOH

Lower pH Higher pH

Silicate decreased or eliminated Silicate needed

Longer time required Less sensitive to time

Less dissolved organics More dissolved organics

Lower COD Higher COD

Less anionic trash More anionic trash

Objectives

• To compare the Mg(OH)2-based and NaOH-

based peroxide bleaching of a eucalyptus

CTMP, in terms of

– Pulp brightness

– COD

– Anionic trash

– Pulp yield

Experimental

• Eucalyptus CTMP from a Brazilian mill

• Chelation: 0.2% DTPA (as 100%), 3%

consistency, pH 6.0, 70C, 30 minutes.

• Bleaching conditions:

– 2.5%, 4.0%, 6.2% H2O2;

– 1.5%-6.0% NaOH, or 0.5%-2.5% Mg(OH)2;

– 16% (MC), 30% (HC) pulp consistency;

– 80-90 C, 2-6 hours.

Medium-Consistency (MC) System

Effect of Peroxide and Alkali

Charge on Brightness (MC System)

Other bleaching conditions: 2.6% silicate, 0.13%

Epsom salt (NaOH-based process), 16% pulp

consistency, 80C, 3 hours

70

72

74

76

78

80

82

84

86

0 2 4 6 8

NaOH charge (%)

Bri

gh

tness (

% I

SO

)

2.5% peroxide

4.0% peroxide

6.2% peroxide

60

62

64

66

68

70

72

74

76

78

80

0 0.5 1 1.5 2 2.5 3

Mg(OH)2 charge (%)B

rig

htn

ess (

% I

SO

)

2.5% peroxide

4.0% peroxide

6.2% peroxide

Eucalyptus CTMP vs. Maple CTMP:

Mg(OH)2-based Peroxide Bleaching

64

66

68

70

72

74

76

78

80

2.5% peroxide,

1.0% Mg(OH)2

4.0% peroxide,

1.7% Mg(OH)2

6.2% peroxide,

2.5% Mg(OH)2

Bri

gh

tne

ss

(%

IS

O)

Eucalyptus CTMP

Maple CTMP

• The eucalyptus CTMP is less bleachable in the Mg(OH)2-

based process, compared with the maple CTMP.

Why Is the Eucalyptus CTMP Less

Bleachable in the Mg(OH)2-based Process?

• Wood properties

– Extractives

– Lignin

• Pulping process

– Chemical pretreatment (alkali dosage)

Effect of Acetone Extraction

70

72

74

76

78

80

82

84

86

Bri

gh

tne

ss

(%

IS

O)

4 6.2

Peroxide dosage (%)

Mg Process

Na Process

70

72

74

76

78

80

82

84

86

Bri

gh

tness (

% I

SO

)

4 6.2

Peroxide dosage (%)

Mg Process

Na Process

Without acetone extraction With acetone extraction

Other bleaching conditions: 2.6% silicate, 0.13% Epsom

salt (Na process), 16% pulp consistency, 80C, 3 hours (6

hours for the Mg process).

Alkaline Pre-treatment on Peroxide

Bleaching of A TMP

NaOH

%

Mg(OH)2

%

Silicate

%

DTPA

(40%) %

End

pH

Residual

H2O2, %

Bright.

% ISO

Without alkali pre-treatment

1.85 0 2.0 0 7.97 1.24 71.5

1.85 0 0 0.4 7.65 1.24 71.6

0 1.0 0 0.4 7.81 1.70 71.2

With alkali pre-treatment

1.0 0 2.0 0 9.20 1.43 67.6

1.0 0 0 0.4 9.39 1.22 67.3

0 1.0 0 0.4 8.90 1.83 62.7

Conductivity of Bleach Filtrate

(MC System)

0

0.5

1

1.5

2

2.5

3

0 2 4 6 8

NaOH charge (%)

filt

rate

co

nd

ucti

vit

y (

ms/c

m)

2.5% peroxide

4.0% peroxide

6.2% peroxide

0

0.1

0.2

0.3

0.4

0.5

0.6

0 0.5 1 1.5 2 2.5 3

Mg(OH)2 charge (%)F

iltr

ate

co

nd

ucti

vit

y (

ms/c

m)

2.5% peroxide

4.0% peroxide

6.2% peroxide

Other bleaching conditions: 2.6% silicate, 0.13%

Epsom salt (NaOH-based process), 16% pulp

consistency, 80C, 3 hours

Anionic Trash Formation

(MC System)

0

20

40

60

80

100

0 1 2 3 4 5 6 7

NaOH charge (%)

An

ion

ic t

rash

fo

rmed

(m

eq

/kg

)

2.50% peroxide

4.00% peroxide

6.20% peroxide

0

5

10

15

20

25

0 0.5 1 1.5 2 2.5 3

Mg(OH)2 charge (%)A

nio

nic

tra

sh

fo

rmati

on

(m

eq

/kg

)

2.5% peroxide

4.0% peroxide

6.2% peroxide

Other bleaching conditions: 2.6% silicate, 0.13%

Epsom salt (NaOH-based process), 16% pulp

consistency, 80C, 3 hours

Summary

• In the MC system

– The Na process was effective

– The Mg process gave ~ 5 units lower

brightness than the Na process at the same

peroxide dosage.

– But the Mg process produced less anionic

trash and conductivity

High-Consistency (HC) System

Effect of Temperature and Time on

Brightness (HC System)

NaOH-based Process Mg(OH)2-based process

• Other bleaching conditions: 2.6% silicate, 0.13% Epsom salt

(NaOH-based process), 30% pulp consistency.

72

74

76

78

80

82

84

86

2.5% Peroxide,

2.25% NaOH

4.0% peroxide, 3.0%

NaOH

6.2% peroxide, 4.0%

NaOH

Bri

gh

tne

ss

(%

IS

O)

70.0

72.0

74.0

76.0

78.0

80.0

82.0

84.0

2.5 peroxide,

0.75% Mg(OH)2

4.0% peroxide,

1.0% Mg(OH)2

6.2% peroxide,

1.5% Mg(OH)2

Bri

gh

tness (

% I

SO

)

80 degree C, 3 hrs

80 degree C, 6 hrs

90 degree C, 6 hrs

80C, 2 hrs

Comparison of the Bleaching Performance

at Optimized Conditions (HC)

Process NaOH-based (2.6% silicate, 0.13% Epsom

salt, 80 C, 3 hrs)

Mg(OH)2-based (2.6% silicate, 90 C, 6 hrs)

H2O2, % 2.5 4.0 6.2 8.0 2.5 4.0 6.2 8.0

NaOH, % 2.0 2.5 4.0 4.0 0 0 0 0

Mg(OH)2, % 0 0 0 0 0.75 1.0 1.5 2.0

Resid. H2O2, % 0.60 1.08 1.53 2.30 0.67 1.53 2.05 2.97

Bright., % ISO 78.5 82.9 85.8 86.9 76.9 80.8 83.2 85.0

For a very high-brightness CTMP

• Peroxide bleaching first

• Followed by the addition of OBA/FWA

Addition of Fluorescent Whitening Agent to

Pulps from the Mg(OH)2-based Process

FWA (Tinopal UP), % 0 0.2 0.4 0.8

Brightness, % ISO 83.2 85.5 87.2 87.8

Flu. comp., % ISO 0 2.76 4.00 5.17

CIE whiteness, % 59.0 66.9 71.5 72.9

L* 97.1 97.2 97.3 97.3

a* -2.12 -1.51 -1.21 -1.04

b* 7.41 5.71 4.77 4.46

Other bleaching conditions: 6.2 % H2O2, 2.6% silicate, 30% pulp

consistency, 90C, 6 hours.

Total Anionic Trash Formation in

the HC System

0

20

40

60

80

100

76 78 80 82 84 86 88

Brightness of bleached pulp (% ISO)

To

tal

an

ion

ict

trash

fo

rmati

on

(m

eq

/gkg

)

Na-based

Mg-based

Cationic demand of filtrate measured at pH 7.0

Sources of Anionic Trash

• Polygalacturonic acids (pectic acids):

polymer with uronic type of carboxylic

groups (pKa= ~3.4).

• Oxidized lignin: macromolecule with

lignin-bound carboxylic groups (pKa=

~5.4).

• Colloidal particles of extractive: aliphatic

type of carboxylic groups (pKa= ~5.5-

6.4).

De-methylation and Dissolution of

Pectins

O

COOCH3

O

OH

O

OH

*

*

n

O

COO-

O

OH

O

OH

*

*

n

Anionic trash

DissolutionDe-esterification

Pectic acids

pKa= ~3.4

Pectin

Lignin Oxidation and Dissolution

O

O

C

HC O Lignin

COOHCOOH

C

HC O Lignin

Oxidation

C

OCH3

OH

CH O Lignin

COD&

Anionic Trash

Dissolution

Oxidation

Dissolution

COD

pKa= ~5.4

Extractives

• Dissolved and dispersed extractives – Example: resin acids, fatty acids

COOH COO-

Resin acids

pKa= ~5.5-6.4

Anionic Trash Measured at Various pH

• The measured amount of anionic trash is pH-dependent.

• But the cationic demand of pectic acids is pH-independent

from 4.5 to 7.

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

3 4 5 6 7 8 9

pH

Cati

on

ic d

em

an

d (

meq

/l)

Sodium PolyGalA

Filtrate (alkaline

treatment)

Filtrate (peroxide

bleaching)

Estimation of the Contribution of Pectic Acids,

Lignin and Extractives to the Total Anionic Trash

The increase of cationic demand with pH is due to: • Extractives

•Oxidized lignin

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

3 4 5 6 7 8 9

pH of the DCS sample for CD titration

Ca

tio

nic

de

ma

nd

(m

eq

/l)

Alkaline treatment

4% H2O2

lignin

Extractives

Pectic Acids

Hemicellulose Type of Anionic Trash in

Peroxide Bleaching of the Eucalyptus CTMP

0

10

20

30

40

50

60

2 2.5 3 3.5 4 4.5

Pulp yield loss during bleaching (%)

Hem

icell

ulo

se t

yp

e o

f an

ion

ic t

rash

(meq

/kg

)

Na-based

Mg-based

Measured as the cationic demand of

filtrate measured at pH 4.5

Lignin and Extractives Types of Anionic Trash in

Peroxide Bleaching of the Eucalyptus CTMP

0

10

20

30

40

50

2 2.5 3 3.5 4 4.5 5

Pulp yield loss (%)

lig

nin

an

d e

xtr

acti

ves t

yp

es o

f

an

ion

ic t

rash

(m

eq

/kg

) Na-based

Mg-based

Difference of filtrate cationic demand measured

between pH 7.0 and 4.5

Comparison of Pulp Yield Loss at

Various Brightness Levels

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

76 78 80 82 84 86 88

Brightness (% ISO)

Pu

lp y

ield

lo

ss (

%)

Na-based process

Mg-based process

• Lower yield loss for the Mg- based process.

Correlation of Pulp Yield Loss

and COD

y = 0.0408x + 1.3516

R2 = 0.9951

0

1

2

3

4

5

0 20 40 60 80 100

COD of the bleaching effluent (kg/t)

Pu

lp y

ield

lo

ss (

%)

• The same correlation for both the NaOH-

based and Mg(OH)2-based processes

Dissolved Solid (HC system)

0

0.2

0.4

0.6

0.8

1

1.2

2.5% peroxide 4.0% peroxide 6.2% peroxide

Dis

so

lved

so

lid

in

fil

trate

(g

/L) Na-Based

Mg-based

Filtrate Conductivity (HC System)

0

0.2

0.4

0.6

0.8

1

2.5 4.0 6.2 8.0

Peroxide charge (%)

Fil

trate

co

nd

ucti

vit

y (

ms/c

m)

Na-based

Mg-based

Summary • Under the optimized conditions, the Mg process

in the HC system gives 1-2 units lower brightness than the Na process at the same peroxide dosages.

• The Mg process requires higher temperature and longer retention time to reach the brightness targets.

• At a similar brightness, the Mg process has – 50-80% less anionic trash

– 40-50% less COD,

– 20-30% less loss of pulp yield

– 50-60% less dissolved solid

– 50-60% less conductivity

Conclusions

• The Mg(OH)2-based process has been

successfully implemented in a number of mills

in N.A.

• In comparison with the NaOH-based process,

the Mg(OH)2-based process offers:

– Lower bleaching cost

– Similar or slightly lower brightness

– Higher light scattering coefficient

– Higher pulp yield

– Lower COD

– Less anionic trash

Acknowledgements

• Dr. Paulo Pavan, Melhoramentos Papéis

• Mark Wajer, Martin Marietta Magnesia

Specialties