Insights into Eucalyptus Pulp Bleaching Technology

(1)

Insights into Eucalyptus

Pulp Bleaching Technology

Tapani Vuorinen

[email protected]

November 24-27, 2013 Colonia del Sacramento, Uruguay

(2)

Outline

• From present to future

• Maintaining fiber properties

• Does chemistry help?

(3)

Outline

• From present to future

• Maintaining fiber properties

• Does chemistry help?

(4)

Main targets of pulp bleaching

• To reach high brightness

– Low lignin chromophore content

• To reach high brightness stability

– Low residual lignin content

– Low hexenuronic acid (HexA) content – Low carbonyl content

• To remove most of chromophores, lignin and

HexA without oxidizing polysaccharides

(5)

Approximate composition of oxygen

delignified eucalyptus kraft pulp

• Cellulose + hemicelluloses (excluding HexA)

98 m-%

6,250 mmol/kg (anhydrosugar units) 17,500 mmol/kg (hydroxyl groups)

12,500 mmol/kg (accessible hydroxyl groups)

• Hexenuronic acid (chemically bonded to xylan)

1 m-%

65 mmol/kg

• Lignin (chemically bonded to polysaccharides)

1 m-%

(6)

Characteristics of high quality bleached

eucalyptus kraft pulp

• Cellulose + hemicelluloses (excluding HexA)

> 99.5 m-%

< 10 mmol/kg carbonyl groups high accessibility

• Hexenuronic acid

< 0.1 m-%

• Lignin

(7)

ECF bleaching sequences of oxygen

delignified eucalyptus kraft pulp

• A/D-E

_OP

-D-P or D/A-E

_OP

-D-P

– 20-25 kg active Cl/ton 110-140 mmol/kg (ClO₂) 280-350 mmol/kg (act. Cl₂) – 7-10 kg hydrogen peroxide/ton 200-300 mmol/kg (H₂O₂)

• A-E

_OP

-D-P

– 14-15 kg active Cl/ton 80 mmol/kg (ClO₂) 200 mmol/kg (act. Cl₂)

(8)

Inefficiency in current ECF bleaching

technology

• Long retention times – huge bleaching towers

• Excessive oxidation power needed in removal of

residual lignin and HexA

– 4-6 equivalents of oxidant per C₆C₃ + HexA

– 7-9 equivalents of oxidant per C₆C₃ + HexA (HexA hydrolyzed by acid calculated out)

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Outline

• From present to future

• Maintaining fiber properties

• Does chemistry help?

• Conclusions

(10)

Irreversible deuteration of cellulose in

kraft pulping of birch

Raili Pönni et al.

Carbohydrate Polymers 101 (2014) 792-797

1) Pulping in D₂O 2) Washing with H₂O 3) Detection of OD band

(11)

FTIR spectra of HW pulps cooked in D

₂

O

and H

₂

O

(12)

Irreversible deuteration of eucalyptus

pulps treated for 4 hours at 90

o

C

1) Treatment in D

₂

O

2) Washing with H

₂

O

(13)

FTIR spectra of eucalyptus pulp treated

for 4 hours in D

₂

O at 90

o

C

(14)

Factors affecting on hornification of pulp =

aggregation of cellulose microfibrils

• Cooking

: removal of hemicelluloses and high

temperature + long time

• Bleaching

: high temperature and long time

• Drying

: removal of water during long time at

high temperature

Using lower temperature and faster reactions

could reduce hornification in bleaching

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Outline

• From present to future

• Maintaining fiber properties

• Does chemistry help?

(16)

UV Raman spectra of bleached kraft pulps

Kristiina Kellokoski M.Sc. Thesis 2013

(17)

General reaction scheme of chlorine

dioxide bleaching

(18)

Oxidants in chlorine dioxide bleaching

• Chlorine dioxide (ClO

₂

)

– Oxidizes phenols and hydroxyquinones (2 equivalents ClO₂ per

phenol or hydroxyquinone)

– Produces ½ equivalents HOCl and ½ equivalents ClO₂

-• Chlorite (ClO

₂-

₎

– Oxidizes aldehydes (formed in situ) forming equivalent amount of HOCl

– Decomposes to chlorate (ClO₃-_{) and HOCl (catalysis by HOCl)} – May react with HOCl to regenerate ClO₂

• Hypochlorous acid (HOCl)

– Oxidizes HexA

– Chlorinates and oxidizes lignin

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Stoichiometry of overall reactions

• 2ArOH + HexA + 4ClO

₂

2Ox

_Lig

+ Ox

_HexA

+

ClO

₃-

+ 3Cl

-– In this simplified (but relevant) scheme HexA and lignin are oxidized in a 1:2 ratio!

• Formation of HOCl

in situ

– 0.75-1 equivalents per added ClO₂

– 80-140 mmol/kg pulp, when total active Cl charge is 20-25 kg/ton

• Formation of chlorate (ClO

₃-

₎

– 0-0.25 equivalents per added ClO₂

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Reactivity of HOCl

• Electrophilic reactions

– Primary oxidation of HexA - OK

– Chlorination of lignin – reduces reactivity of residual lignin (by factor of 10 per each substitution)

– Oxidation of lignin - OK

– Oxidation of cellulose and hemicelluloses – may decrease brightness stability

• Nucleophilic reactions

– Secondary oxidation of HexA – consumes oxidant without promoting removal of HexA

(21)

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Effect of carbonyl content on brightness

reversion of ECF bleached eucalyptus pulp

Zhen Zhou et al. Holzforschung 65 (2011) 289-294 0 10 20 30 40 50 60 70 80 90 0 2 4 6 8 10 12 14 16 P C , 4 8 h a g e in g

Carbonyl content, mol/g

reference pulp (xylan 12%)

hemicellulose-reduced pulp (xylan 7%)

Brightness reversion is only

caused by carbonyl groups!

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UVRR spectra of eucalyptus kraft pulps in

A-E

_OP

-D-P bleaching sequence

Leonardo Clavijo M.Sc. Thesis 2010 HexA Lignin Cellulose

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Derivatives of HOCl (Cl

₂

)

• General reaction (nucleophilic substitution on chlorine):

Nu

-

+ Cl

+

-Cl

-

Nu-Cl + Cl

-Nucleophile

Product

Name

H

₂

O/HO

-

_HOCl

_{Hypochlorous acid}

ClO

-

_Cl

2

O

Chlorine monoxide

ClO

₂-

_Cl

2

O

2

Dichlorine dioxide

RCO

₂

H

RCO

₂

Cl

Acyl hypochlorite

ROH

ROCl

Alkyl hypochlorite

ArOH

ArOCl

Aryl hypochlorite

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Outline

• From present to future

• Maintaining fiber properties

• Does chemistry help?

(26)

Conclusions

• Eucalyptus pulp bleaching could and should be

intensified from today’s BAT

• In the future bleaching can be dramatically

faster than today and consume much less

chemicals

• In addition to lower investment and production

costs the new technology may better maintain

beneficial fiber properties

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Acknowledgements

Long-term financers and idustrial collaborators:

TEKES (Finnish Funding Agency for Technology and Innovation) Finnish Bioeconomy Cluster Ltd (Strategic Centre for Science, Technology and Innovation)

Andritz, Metsä Fibre, Kemira, Stora Enso, UPM

Research collaborators during the past years:

Aalto University BOKU

INPG Pagora

Universidad de la República VTT