• TABLE OF CONTENTS
HIDE
 Title Page
 Copyright
 Dedication
 Acknowledgement
 Table of Contents
 List of Tables
 List of Figures
 Abstract
 Introduction
 Literature review
 UV-visible response of kraft lignin...
 Electrokinetic study of kraft...
 Isolation and analysis of kraft...
 Optical effects of lignin solution...
 Number averaged molecular weight...
 Experimental kraft pulping
 Pulping effects on kappa number,...
 Pulping effects on the molecular...
 Conclusions and recommendation...
 Appendix
 Reference
 Biographical sketch
 Copyright














Group Title: Characterization of kraft lignin and investigation of pulping effects on pulp yield, lignin molecular mass and lignin content of black liquor with a c
Title: Characterization of kraft lignin and investigation of pulping effects on pulp yield, lignin molecular mass and lignin content of black liquor with a central composite pulping design
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Title: Characterization of kraft lignin and investigation of pulping effects on pulp yield, lignin molecular mass and lignin content of black liquor with a central composite pulping design
Series Title: Characterization of kraft lignin and investigation of pulping effects on pulp yield, lignin molecular mass and lignin content of black liquor with a c
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Creator: Dong, Daojie,
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Table of Contents
    Title Page
        Page i
    Copyright
        Page ii
    Dedication
        Page iii
    Acknowledgement
        Page iv
        Page v
    Table of Contents
        Page vi
        Page vii
        Page viii
        Page ix
        Page x
    List of Tables
        Page xi
        Page xii
        Page xiii
    List of Figures
        Page xiv
        Page xv
        Page xvi
        Page xvii
        Page xviii
        Page xix
    Abstract
        Page xx
        Page xxi
    Introduction
        Page 1
        Page 2
        Page 3
        Page 4
        Page 5
    Literature review
        Page 6
        Page 7
        Page 8
        Page 9
        Page 10
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        Page 21
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        Page 23
        Page 24
        Page 25
        Page 26
        Page 27
    UV-visible response of kraft lignin in black liquor
        Page 28
        Page 29
        Page 30
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        Page 50
        Page 51
    Electrokinetic study of kraft lignin
        Page 52
        Page 53
        Page 54
        Page 55
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    Isolation and analysis of kraft lignin
        Page 78
        Page 79
        Page 80
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    Optical effects of lignin solution and determination of Mw of kraft lignin by low angle laser light scattering (lalls)
        Page 95
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        Page 132
    Number averaged molecular weight of lignin
        Page 133
        Page 134
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    Experimental kraft pulping
        Page 155
        Page 156
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    Pulping effects on kappa number, pulp yield and the lignin content of black liquor
        Page 169
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    Pulping effects on the molecular weight and molecular weight distribution of lignin
        Page 220
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    Conclusions and recommendations
        Page 274
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    Appendix
        Page 284
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    Reference
        Page 309
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    Biographical sketch
        Page 329
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        Page 331
        Page 332
    Copyright
        Copyright
Full Text









CHARACTERIZATION OF KRAFT LIGNIN AND INVESTIGATION OF
PULPING EFFECTS ON PULP YIELD, LIGNIN MOLECULAR MASS AND
LIGNIN CONTENT OF BLACK LIQUOR WITH A CENTRAL COMPOSITE
PULPING DESIGN












By

DAOJIE DONG


A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL
OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT OF
THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY

UNIVERSITY OF FLORIDA


1993

































Copyright 1993

by

Daojie Dong













To my mother, Ting-Xu Ding, and my wife, Wei.

To my motherland, China.













ACKNOWLEDGEMENTS


The author would like to thank his advisor Dr. A. L. Fricke for his guidance

and support during the course of this thesis work. He wishes to thank Dr. I. A.

Bitsanis for his friendship and willingness to review his extra papers other than

thesis material and to thank Dr. R. S. Duran for his inspiring questions and

enthusiastic altitude toward scientific work. He is also grateful to Dr. G. Hoflund,

Dr. C.W. Park and Dr. C. D. Batich for serving on his committee and for their

comments and critiques that improved the thesis manuscript. Many thanks are also

due to Dr. B. M. Moudgil of the Department of Materials Science and Engineering

for the use of his equipment and for reviewing Chapter 4 of this dissertation, to

Dr. A. W. Cumming of the Department of Physics for his timely help on light

scattering theory and fruitful suggestion on the modeling of the optical effects, to

Dr. H. Pendse of the University of Maine at Orono for his support in locating the

equipment needed, and to Dr. H. Johnson of the Goodyear Company for arranging

the author's trips and the experiments at his company, though that part of work was

not included in this dissertation. Without their generous support, this dissertation

would not have been so successful.








The author also wishes to thank the members of Dr. Fricke's research group,

the personnel and fellow graduate students in the Department of Chemical

Engineering for their friendship and support.

Finally, the author would like to thank his wife, Wei, for her love,

encouragement and endurance. Without her love and support, this work would not

have been possible. He feels that he owes his parents and his parents-in-law a great

deal, and that he and his wife are indebted to Xiu-Feng Huang, who raised the

latter. During the several years which have been required to complete this

dissertation, they missed the author's family so much, though they have always had

family reunion in their dreams. The author would also thank his gifted daughter,

since she makes life a lot brighter.














TABLE OF CONTENTS


Page
ACKNOWLEDGEMENTS....................................................................... iv

LIST OF TABLES.................................................. ................................ xi

LIST O F FIG U R ES......................................................................................... xiv

AB STRA CT....................................................... ...................................... xx

CHAPTERS

1 INTRODUCTION..................................................................... 1

2 LITERATURE REVIEW.................................................................. 6

2.1 Lignin............................................................................. ..... 6
2.1.1 Classification....................................... ................... 6
2.1.2 Difficulties and Developments........................... ..... 10
2.2 Isolation of Lignin from Kraft Black Liquor............................ 12
2.3 Characterization of Black Liquor and Purified Lignin.............. 14
2.3.1 Chemical Analysis of Black Liquor.............................. 14
2.3.2 Lignin Content in Black Liquor................................. 14
2.3.3 Surface Charge and Zeta Potential................................ 16
2.3.4 Molecular Weight of Lignin.......................... .......... 16
2.4 Kraft Delignification............................................................. 18
2.4.1 Process............................................ ...................... 18
2.4.2 Chemistry........................................ ..................... 19
2.4.3 Effects of Pulping Variables.......................................... 21
2.4.4 Kinetics.......................................... ....................... 22
2.4.5 Fundamentals................................... ..................... 26

3 UV-VISIBLE RESPONSE OF KRAFT LIGNIN IN BLACK
LIQ U O R ........................................................................ ......... 28








3.1 B ackground............................................................. ............... 28
3.2 Experim ental........................................................................... 30
3.2.1 Equipment and Parameters........................... ........... 30
3.2.2 Sample Preparation..................................................... 31
3.2.3 Molecular Weight and Chemical Analysis of Lignin..... 31
3.2.4 Solutions........................................ ....................... 32
3.3 Results and Discussion......................................................... 33
3.3.1 Spectrum and Characteristic Wavelength.................. 33
3.3.2 pH Effect............................................. ..................... 40
3.3.3 Molecular Weight Effect.............................. ........... 43
3.3.4 Corrected Extinction Coefficient................................ 44
3.3.5 Measurement of the Lignin Content of Black Liquor.... 47
3.4 Sum m ary.................................................... ........................... 50

4 ELECTROKINETIC STUDY OF KRAFT LIGNIN........................ 52

4.1 Background................................................. .......................... 52
4.2 Experim ental.............................................. ........................... 56
4.2.1 Materials and Preparation of Lignin Suspensions........ 56
4.2.2 Instruments and Microelectrophoresis Experiments....... 57
4.3 Results and Discussions...................................................... 59
4.3.1 Zeta Potential and IEP.................................. ........... 59
4.3.2 Origination of Surface Charge........................................ 63
4.3.3 Salt Effects and Adsorption............................................. 65
4.3.4 Charge Reversal and Surface Adsorption.................. 71
4.3.5 Stability............................................. ........................ 76
4.4 Sum m ary..................................................... .......................... 76

5 ISOLATION AND ANALYSIS OF KRAFT LIGNIN.................... 78

5.1 Background................................................. .......................... 78
5.2 Isolation of Lignin from Kraft Black Liquor ............................ 80
5.2.1 Implications of the Electrokinetic Results.................. 80
5.2.2 Procedure........................................... ....................... 81
5.2.3 Arbitrary UV Unit and The UV .................................. 85
5.2.4 Lignin Mass Balance................................... .......... 88
5.2.5 Y ield.............................................................................. 93
5.3 Sum m ary.................................................................................... 93








6 OPTICAL EFFECTS OF LIGNIN SOLUTION AND
DETERMINATION OF M, OF KRAFT LIGNIN BY LOW
ANGLE LASER LIGHT SCATTERING( LALLS)......................... 95

6.1 B ackground................................................................................ 95
6.2 Instrum entation....................................................................... 98
6.3 Theory( A Brief Summary )...................................... .......... 99
6.4 Correction for Optical Effects...................................................... 101
6.4.1 Fluorescence..................................................................... 101
6.4.2 A bsorption................................................................... 103
6.4.3 A nisotropy................................................................... 103
6.5 Experim ental........................................................................... 105
6.5.1 Sample Preparation.......................................................... 105
6.5.2 Specific Refractive Index Increment (SRII)................. 106
6.5.3 Rayleigh Factor and Depolarization Ratio.................... 108
6.5.4 Transmittance of Lignin Solution................................. 109
6.5.5 Correction Procedure.................................................... 109
6.6 Results and Discussion................................................................. 110
6.7 Sum m ary.................................................................................. 131

7 NUMBER AVERAGED MOLECULAR WEIGHT OF LIGNIN..... 133

7.1 B ackground................................................................................ 133
7.1.2 Theory(A Brief Summary)............................................ 133
7.1.2 Brief Review .................................................................... 137
7.2 Experim ental........................................................................... 139
7.2.1 M aterials........................................................................ 140
7.2.2 Instrum entation............................................................ 141
7.2.3 Chamber Temperature................................................. 142
7.2.4 Instrument Constant.................... ......................... 142
7.2.5 Measurement of the ,M of Lignin................................... 143
7.3 Results and Discussion................................................................. 146
7.3.1 Molecular Weight Effect............................................... 146
7.3.2 The M, of Kraft Lignin.......... ...................................... 149
7.4 Sum m ary.................................................................................. 154

8 EXPERIMENTAL KRAFT PULPING ............................................ 155

8.1 B ackground................................................................................ 155
8.2 Experimental Design............................................................ 157








8.2.1 Input and Output Variables........................................... 157
8.2.2 Variable Design.......................................................... 159
8.3 Experim ental........................................................................... 162
8.3.1 Kraft Pulping ............................................................... 162
8.3.2 Black Liquor Handling.................................................. 165

9 PULPING EFFECTS ON KAPPA NUMBER, PULP YIELD AND
THE LIGNIN CONTENT OF BLACK LIQUOR............................ 169

9.1 B ackground................................................................................ 169
9.2 Qualitative Analysis................................................................. 171
9.2.1 Relationship Between Pulp Yield and Kappa Number.. 173
9.2.2 Pulp Yield and Kappa Number....................................... 175
9.2.3 Lignin Concentration....................................................... 179
9.3 Response Surface Analysis.......................................................... 184
9.3.1 Models and Analysis........................................................ 184
9.3.2 Contour Plot Analysis.................................................... 188
9.4 Predictive Models .................................................................... 200
9.4.1 C riteria.............................................................................. 200
9.4.2 Variable Selections........................................................... 202
9.4.3 Predictive Models............................................................ 209
9.4.4 Residual Analysis........................................................ 212
9.5 Sum m ary.................................................................................. 217

10 PULPING EFFECTS ON THE MOLECULAR WEIGHT AND
MOLECULAR WEIGHT DISTRIBUTION OF LIGNIN.................. 220

10.1 B ackground................................................................................... 220
10.2 Pulping Effects on The Molecular Weight of Lignin................ 224
10.2.1 Weight-Averaged Molecular Weight.......................... 225
10.2.2 Number-Averaged Molecular Weight......................... 234
10.2.3 Molecular Weight Distribution.................................... 237
10.3 Response Surface Analysis..................................................... 240
10.3.1 Models and Analysis.................................................... 240
10.3.2 Response Surface and Contour................................... 243
10.4 Predictive Models......................................................................... 255
10.4.1 C riteria............................................................................ 255
10.4.2 Variable Selections....................................................... 256
10.4.3 Predictive Models........................................................ 262
10.4.4 Residual Analysis........................................................... 265








10.5 Sum m ary....................................................................................... 270

11 CONCLUSIONS AND RECOMMENDATIONS........................ 274

11.1 Fundamental Study of Kraft Lignin.......................................... 274
11.2 Statistical Study of Kraft Pulping Process................................ 277
11.2.1 Experimental Design and Modeling ........................... 277
11.2.2 Pulping Variable Effects.............................................. 279
11.3 Recommendations for Future Work........................................... 282

APPENDICES

A ELECTROPHORETIC EXPERIMENTAL DATA........................... 284

B LIGHT SCATTERING DATA....................................................... 287

C SUMMARY OF VPO DATA.......................................................... 300

D DATA FOR STATISTICAL MODELLING OF THE KRAFT
PULPING PROCESS..................................................................... 303

REFEREN CES.......................................................................................... 309

BIOGRAPHICAL SKETCH..................................................................... 329













LIST OF TABLES


Table Page

3-1 The Extinction Coefficients of Softwood Kraft Lignins of Different
M olecular W eights............................................................................ 45

3-2 Chemical analysis of Purified Lignins........................... ........... 46

3-3 Lignin Content of Kraft Black Liquors............................................. 49

4-1 Zeta Potential of Softwood Kraft Lignins at Various pH values....... 61

4-2 Iso-Electric Points of Kraft Lignin Caused by The Aluminum
Chloride Nucleated on the Lignin Surface and The Solubility
Product of Aluminum Hydroxide................................. ............ 74

5-1 Lignin Mass Balance(Experimental Liquor ABAFX6768)............... 89

5-2 Lignin Mass Balance(Experimental Liquor ABAFX7576)................ 90

5-3 Lignin Y ields................................................................................... 92

6-1 Solvent Indices of Refraction and the Scattering Angles................... 107

6-2 Specific Refractive Index Increment of Kraft Lignin Solutions
and the Corresponding Optical Constants........................................ 113

6-3 Weight-Averaged Molecular Weight of Kraft Lignins Measured in
Different Solvents and Corrected for Optical Effects....................... 129

7-1 VPO Instrument Constants Calibrated with Different Solutes at
V various Sensitivities........................................... ........................... 148








8-1 Experimental Kraft Pulping Design and A Summary of Sample
L abels............................................................................................. 160

8-2 Summary of Experimental Pulping Conditions................................. 164

9-1 Pulp Yield, Kappa Number and Lignin Content of Black Liquor
for Kraft Cooking of Slash Pine.......................................................... 172

9-2 Parameters in the Full Quadratic Models for Pulp Yield, Kappa
Number and the Lignin Content of Black Liquors........................... 186

9-3 Parameters in the Candidate Models for Pulp Yield, Kappa
Number and The Lignin Content of Black Liquor for Kraft
Pulping of Slash Pine........................................................................... 206

9-4 Predictive Models for Pulp Yield, Kappa Number and the Lignin
Content of Black Liquors......... ........................................................... 210

9-5 Predicted Values and Residuals for Pulp Yield, Kappa Number and
Lignin Content of Black Liquors.................................................... 213

10-1 Molecular Weights of Kraft Lignins and the Averaged Pulp
Yield, Kappa Number and Lignin Content of Black Liquors for
Kraft Pulping of Slash Pine................................................................. 226

10-2 Parameters in the Complete Quadratic Models of the Molecular
Weights and the Molecular Weight Distribution of Lignin for Kraft
Pulping of Slash pine........................................................................ 241

10-3 Parameters in the Candidate Models for the Molecular Weights 258
and Molecular Weight Distribution Index of Lignin.......................

10-4 Predictive Models of the Molecular Weights and the Molecular
Weight Distribution of Lignin for Kraft Pulping of Slash Pine......... 263

10-5 The Predicted Values and the Residuals for Molecular Weights
and Molecular Weight Distribution of Kraft Lignin......................... 266

A-1 Zeta Potential of Softwood Kraft lignins at Different pH Values..... 284








A-2 Zeta Potential of Lignin ABAFX2930 with the Presence of
Sodium Chloride and Calcium Chloride............................................ 285

A-3 Zeta Potential of ABAFX2930 with the Presence of Zinc Sulfate
and Alum inum Chloride....................................................................... 286

B-1 Light Scattering Data...................................................................... 288

B-2 Optical Effects of Lignin ABAFX1112 in Three different types of
Solvent................................................................................................. 299

C-1 VPO Calibration Data.......................................................................... 300

C-2 VPO Data for Lignin in DMF at 80C and at High Sensitivity.......... 300

C-3 VPO Data for Lignins in DMF at 80C and at Low Sensitivity......... 301

D-1 Parameters in the Full Quadratic Models for Pulp Yield, Kappa
Number, Lignin Content of Black Liquor, Molecular Weights of
Lignin and the Molecular Weight Distribution Index of Lignin........ 303

D-2 Canonical Analysis Results.................................................................. 304

D-3 Summary of the Statistics for Model Selection................................ 305

D-4 Predictive Models for Pulp Yield, Kappa number, Lignin Content
of Black liquor, Molecular Weights of Lignin and Molecular
Weight Distribution of Lignin.......... ............................................ 308













LIST OF FIGURES


Figure Pane


2-1 Lignin Monomers..................................................................... 9

2-2 A Structural Model for Softwood Lignin......................................... 11

3-1 UV-Visible Spectra of Softwood Kraft Lignins in Different
Organic Solvents............................................. ........................ 34

3-2 UV Spectra for Selected Kraft Black Liquors and A Purified
Softwood Kraft Lignin in Sodium Hydroxide Solutions.................. 35

3-3 UV Spectra of Sodium Hydroxide Solutions with Respect to D.
I. W ater...................................................................................... 37

3-4 UV Absorbance of Softwood Lignin in NaOH at 280nm and
at Different pHs................................................ ........................ 39

3-5 Effect of pH on the UV Absorbance of Softwood Kraft Black
Liquors and A Purified Softwood Kraft Lignin............................. 41

4-1 Schematic of the Potential in Solid-Liquid Interface Region........ 54

4-2 The pH Effect on the Zeta Potential of Softwood Kraft Lignins.... 60

4-3 Mechanism of the Origination of Surface Charges on Kraft
Lignin and the pH Effect on the Zeta Potential of Lignin............... 63

4-4 The Zeta Potential of Softwood Kraft Lignin as Function of
pH with the Presence of CaCI2 and NaCI................................... 66








4-5 Zeta Potential of A Softwood Lignin as Function of pH with
the Presence of Zinc Sulfate at Different Concentrations................ 68

4-6 Zeta Potential of A Softwood Lignin as Function of pH with the
Presence of AlC13 at Different Concentrations........................... 70

5-1 Schematic of Sequence for purification of Kraft Lignin form
Black Liquor................................................. ............................ 82

6-1 Refractive Index Increment at 632.8nm as Function of
Concentration for A Softwood Kraft Lignin in Different
Solvents................................................. ................................... 111

6-2 Refractive Index Increment at 632.8nm for Slash Pine Kraft
Lignins of Different Molecular Weights in DMF or in 0.1N
Sodium H ydroxide............................................................................. 112

6-3 Cabannes Factors for A Slash Pine Kraft Lignin at 632.8nm and
in Different Solvents............. ............................................................... 115

6-4 Cabannes Factors for Slash Pine Kraft Lignins of Different
Molecular Weights in Different Solvents at 632.8nm...................... 116

6-5 Apparent Cabannes Factors, Cu(0), and the Proposed Cu'(0) for
a Slash Pine Kraft Lignin at 632.8nm in Different Solvents......... 119

6-6 The Apparent and the Corrected Low Angle Light Scattering
Plots for A Slash Pine Kraft Lignin in 0.1N Sodium Hydroxide
S solution .............................................................................................. 12 1

6-7 The [1-(Re/R)] versus Lignin Concentration Plots show the
Fluorescence Effect on the Observed Rayleigh Factor of a Slash
Pine Kraft Lignin in different Solvents.......................................... 122

6-8 The Corrected Low Angle Light Scattering Plots for A Slash
Pine Kraft Lignin in Different Solvents......................................... 125

6-9 The Corrected Low Angle Light Scattering Plots for Three Slash
Pine Kraft Lignins and One Sugar Maple Lignin in 0.1N Sodium
H ydroxide .............. ......................................................................... 126








6-10 Dependence of the Second Virial Coefficient of Softwood Kraft
Lignins on the Molecular Weight in 0.1N Sodium 128
H ydroxide................................................. ................................

7-1 VPO Calibration Curves with Sucrose Octaacetate in Different
Solvents at Different Sensitivities..................................................... 144

7-2 VPO Plots for Polystyrene Samples of Different Molecular
Weights in DMF at 80C and at High Sensitivity........................... 147

7-3 VPO Plots for Softwood Kraft Lignins in DMF at 80C and at
H igh Sensitivity.................................................................................. 150

7-4 VPO Plots for Softwood Kraft Lignins in DMF at 80C and at
L ow Sensitivity................................................................................... 151

7-5 VPO Plots for Softwood Kraft Lignins in DMF at 80C and at
Low Sensitivity... .......................................................................... ... 152

8-1 Schematic of the Kraft Pulping Process with Four Input and six
O utput V ariables................................................................................. 158

8-2 Schematic of Path for Black Liquor Treatment and Handling........ 166

9-1 Pulp Yield Plot as Function of Kappa Number for Kraft
Pulping of Slash Pine......................................................................... 174

9-2 Kappa Number Plots as Function of H Factor at Various Fixed
Chemical Levels for Kraft Pulping of Slash Pine.......................... 176

9-3 Pulp Yield Plots as Function of H Factor at Various Fixed
Chemical Levels for Kraft Pulping of Slash Pine.......................... 177

9-4 Effect of H Factor on the Lignin Content of Black Liquor at low
Effective Alkali and Two Sulfidity Levels for Kraft Pulping of
Slash Pine........................................................................................... 180

9-5 Effect of H Factor on the Lignin Content of Black liquor at
Medium and High Effective Alkali for Kraft Pulping of Slash
P ine...................................................................................................... 18 1


xvi








9-6 Contour Plots of the Effects of Effective Alkali and Sulfidity
on Kappa Number at Constant Cooking Time and Temperature
for Kraft Pulping of Slash Pine....................................................... 190

9-7 Contour Plots of the Effects of Cooking Time and Temperature
on Kappa Number at Constant Effective Alkali and Sulfidity for
Kraft Pulping of Slash Pine............................................................... 191

9-8 Contour Plots of the Effects of Effective Alkali and Sulfidity on
Pulp Yield at Constant Cooking Time and Temperature for Kraft
Pulping of Slash Pine.................................................................... 193

9-9 Contour Plots of the Effects of Cooking Time and Temperature
on Pulp Yield at Constant Effective Alkali and Sulfidity for
Kraft Pulping of Slash Pine............................................................... 194

9-10 Contour Plots of the Effects of Effective Alkali and Sulfidity
on the Lignin Content of Black Liquor at Constant Cooking
Time and Temperature for Kraft Pulping of Slash Pine................ 197

9-11 Contour Plots of the Effects of Cooking Time and Temperature
on the Lignin Content of Black Liquor at Constant Effective
Alkali and Sulfidity for Kraft Pulping of Slash Pine................... 198

9-12 Statistics for Kappa Number Correlations with Pulping Variables
for Kraft Pulping of Slash Pine.................................................... 204

9-13 Statistics for Pulp Yield Correlations with Pulping Variables for
Kraft Pulping of Slash Pine............................................................... 207

9-14 Statistics for the Correlations of the Lignin Content of Black
Liquor with Pulping Variables for Kraft Pulping of Slash Pine...... 208

9-15 The Residual Plot for Pulp Yield................................................. 214

9-16 The Residual Plot for Kappa Number............................................. 215

9-17 The Residual Plot for the Lignin Content of Black Liquor............. 216


xvii







10-1 Dependence of the Weight-Averaged Molecular Weight of Lignin
on H Factor at Five Fixed Chemical Levels for Kraft Pulping of
Slash Pine....................................................................................... 228

10-2 Dependence of the Weight-Averaged Molecular Weight of
Lignin on Pulp Yield for Kraft Pulping of Slash Pine..................... 230

10-3 The Lignin Content of Black Liquor as Function of the Pulp
Yield at Five Fixed Chemical Levels for Kraft Pulping of Slash
P ine................................................................................................ 232

10-4 Dependence of the Number-averaged Molecular Weight of
Lignin on the Pulp Yield for Kraft Pulping of Slash Pine.............. 236

10-5 Dependence of the Molecular Weight Distribution Index of
Lignin on the Pulp Yield for Kraft Pulping of Slash Pine .......... 238

10-6 Contour Plots of the Effects of Effective Alkali and Sulfidity on
Lignin Weight-Averaged Molecular Weight at Constant Cooking
Time and Temperature for Kraft Pulping of Slash Pine................ 245

10-7 Contour Plots of the Effects of Cooking Time and Temperature
on Lignin Weight-Averaged Molecular Weight at Constant
Effective Alkali and Sulfidity for Kraft Pulping of Slash
P ine................................................................................................ 246

10-8 Contour Plots of the Effects of Effective Alkali and Sulfidity on
Lignin Number-Averaged Molecular Weight at Constant Cooking
Time and Temperature for Kraft Pulping of Slash Pine................ 249

10-9 Contour Plots of the Effects of Cooking Time and Temperature
on Lignin Number-Averaged Molecular Weight at Constant
Effective Alkali and Sulfidity for Kraft Pulping of Slash Pine...... 250

10-10 Contour Plots of the Effects of Effective Alkali and Sulfidity on
Lignin Molecular Weight Distribution at Constant Cooking Time
and Temperature for Kraft Pulping of Slash Pine.......................... 253


XVIII








10-11 Contour Plots of the Effects of Cooking Time and Temperature
on Lignin Molecular Weight Distribution at Constant Effective
Alkali and Sulfidity for Kraft Pulping of Slash Pine.................... 254

10-12 Statistics for Correlations of the Weight-Averaged Molecular
Weight of Lignin with Pulping Variables for Kraft Pulping of
Slash Pine........................................................................................... 257

10-13 Statistics for Correlations of the Number-Averaged Molecular
Weight of Lignin with Pulping Variables for Kraft Pulping of
Slash P ine........................................................................................... 259

10-14 Statistics for the Correlations of the Molecular Weight
Distribution of Lignin with Pulping Variables for Kraft Pulping
of Slash Pine....................................................................................... 260

10-15 Residual Plot for the Weight-Averaged Molecular Weight of
Kraft Lignin................................................................................. 267

10-16 Residual Plot for the Number-Averaged Molecular Weight of
K raft Lignin................................................................................... 268

10-17 Residual Plot for the Molecular Weight Distribution Index of
K raft L ignin...................................................................................... 269














Abstract of Dissertation Presented to the Graduate School
of the University of Florida in Partial Fulfillment of the
Requirements for the Degree of Doctor of Philosophy

CHARACTERIZATION OF KRAFT LIGNIN AND
INVESTIGATION OF PULPING EFFECTS ON PULP YIELD, LIGNIN
MOLECULAR MASS AND LIGNIN CONTENT OF BLACK LIQUOR WITH
A CENTRAL COMPOSITE PULPING DESIGN

By

Daojie Dong

December 1993

Chairperson: Arthur L. Fricke
Major Department: Chemical Engineering

This thesis consists of two parts. One part deals with a kraft pulping

process with four input variables and six response variables. Slash pine was cooked

with a 3.5ft3 circulation digester under a 2k+2k+l (k=4) central composite design.

Predictive models for the six response variables pulp yield, Kappa number, the

lignin content of black liquor (LcoNTEN), the weight-averaged molecular weight

(R,), the number-averaged molecular weight ( i, ), and the molecular weight

distribution of dissolved lignin have been statistically established. Pulping effects

on the response variables were studied with response surface methodology. It was

found that as delignification proceeds, the M~ increases slowly in the bulk phase








and sharply in the residual phase, the M, increases homogeneously in both phases.

Results and discussion on each response variable are presented in detail.

The second part of the thesis involves fundamental studies of kraft lignin.

The ultra violet(UV) absorbance of lignin was found to be independent of

molecular weight, but strongly dependent on pH. With a corrected lignin

absorptivity of 23.7(l/gm.cm) at 280nm and pH=13, a reliable method for

determination of the LCONTEN was developed.

Electrokinetic study revealed that kraft lignin has an isoelectric point(IEP)

of about 1.0. Mono-valent and divalent ions indifferently adsorb on lignin surface,

but trivalent ion of A13 does specifically adsorb on the lignin surface and shifts its

IEP to higher pH. Trivalent ions and some divalent ions strongly affect the zeta

potential of lignin and cause three IEPs in the pH range studied. Based on the

electrokinetic observations, a procedure for isolation of kraft lignin from black

liquor was also established.

The 9~ of lignin was determined by low angle laser light scattering, the

optical effects of lignin solutions were quantitatively studied. Fluorescence was

found to be the major error in the determination of the M, of lignin and the

quantitative modelling of results revealed that the anisotropy is mainly contributed

by the solvent used rather than the lignin molecules in solution. By using the

correction procedure proposed, the M, of lignin determined in three different

solvents differed by only 10%, while the apparent values differed by 57%.













CHAPTER 1
INTRODUCTION



Lignin is a macromolecular substance occurring in close association with

cellulose in trees and plants. Although cellulose and lignin are the two most

abundant natural polymers in the world and both originate from the same sources,

practical applications and theoretical studies of these two polymers are enormously

different. While cellulose has been a very important material with many

applications and has attracted most of the attention in the development of polymer

science of these materials, lignin has been studied much less and its primary use

has been as a fuel in the paper industry. From the structural and theoretical point

of view, lignin should be a valuable polymeric material. However, present

technology and economics do not allow a better use of lignin than as a fuel. Two

main reasons for this are the complex structure of lignin and the lack of knowledge

about this material.

Lignin is extracted from wood during pulping operations in the paper

industry and is the main component of the black liquor byproduct. Approximately

80 x 106 tons of black liquor solids [61] are produced annually in the US and

Canada, about 40% of which is lignin. A great deal of effort has been made to








2
find better applications for lignin since the energy crisis of the early 1970s such

as lignin based carbon fibers, lignin based plastics, lignin based surfactants, lignin

based elastomer reinforcing agents, lignin produced vanilla, etc. Characterization

of lignin would be of fundamental importance in these applications.

In recent years, both environmental impact and energy conservation

considerations result in an urgent need for a more efficient black liquor recovery

process. Recovery process improvements require development of means to predict

liquor output with respect to pulping variables, and development of knowledge of

liquor composition, liquor properties, and of characteristics of lignins contained in

the liquors, which is the direct incentive for this study.

The objectives of this study include the following: (1) develop a reliable

method to determine the lignin concentration of black liquor ; (2) improve

methods for isolation of lignin samples for chemical analysis and molecular

weight characterization ; (3) develop reliable techniques based on first principles

to determine the molecular weight of lignin ; (4) investigate the electrokinetic

properties of kraft lignin ; (5) conduct a multi-variable pulping experiment with

slash pine wood, and characterize lignin concentration and lignin molecular

weight for all black liquors from the experimental design ; and (6) investigate the

dependence of pulp yield, Kappa number, lignin content of black liquor, the

weight-averaged molecular weight, the number-averaged molecular weight, and the








3
molecular weight distribution of lignin on the pulping variables, and statistically

model the kraft pulping process.

Characterization of lignin is of theoretical and practical importance.

Accomplishment of these objectives will establish a basis for data reduction of

black liquor properties, such as viscosity, heat capacity, density, boiling point

elevation, the thermal transition temperatures, etc. In turn, it would help

industry improve the design and operation of recovery systems at higher final

liquor solids content, so that the efficiency of energy recovery can be increased.

It has been reported that[61] black liquor can be treated as a binary system of

water and of lignin that is adulterated with impurities. At low solids content, black

liquor behaves as a solvent continuous material; at high solids( above 42-48%

at all temperatures) it behaves as a polymeric continuous material. As the

temperature is increased, the solvent continuous boundary appears to shift to higher

solids content. Since the polymer appears to be the continuum in the high solids

region, the concentration and molecular weight of lignin in the liquor governs

transport properties of the system at higher solids.

Since black liquor consists of a main component of polar polymeric material

of lignin and many other organic and inorganic ions, the electrokinetic study of

lignin will help one in understanding the interactions between these components

and the stability of the system. In fact, it offers important information about surface

adsorption and molecular associations.








4
The four-variable, two-level, central composite experimental design used in

this work essentially covers the complete practically possible pulping range used

in industry. Also, the size of the digester (3.5 ft') offers results comparable to

industrial digesters for scale-up. Statistical models based on this experimental

design will build a database for kraft pulping process, and such a database would

be very valuable to the pulp and paper industry, since there is very little prior

information available.

The thesis is arranged in two major parts: fundamental study of kraft lignin,

which consists of chapter 3 to 7, and statistical modelling of the kraft pulping

process, which includes chapters 8, 9 and 10. The related topics reported in

literature are reviewed in chapter 2. In chapter 3, development of a reliable

method for determination of the lignin content in black liquor is described. In

chapter 4, studies of the surface zeta potential of lignin are reported and the

interactions between charged lignin surface and some salts are discussed. Chapter

5 reports a complex procedure for purification of kraft lignin from black liquors.

In chapter 6, quantitative investigations of the optical effects of lignin solutions are

discussed, correction methods for the optical effects in the determination of the

weight-averaged molecular weight of lignin are proposed. Chapter 7 describes

determination of the number-averaged molecular weight of lignin. A preliminary

kraft pulping design of four input variables and six output variables is reported in

chapter 8. In chapter 9 and 10, statistical models of kraft pulping of slash pine are








5
developed, and the complex pulping effects are analyzed by using response surface

methodology. Finally, overall conclusions and the recommendations for future

work are summarized in chapter 11.













CHAPTER 2
LITERATURE REVIEW


2.1. Lignin


Lignin is a macromolecular substance that occurs naturally in close

association with cellulose in plants and trees. It may be considered to be a random

three-dimensional network polymer comprised of phenylpropane units linked in

different ways[77,157] and it functions as a cementing matrix, "nature's

glue"[190], which holds the cellulose fibers together[9,50], if one views wood as

a composite material.



2.1.1 Classification



Lignin can be classified into two main categories, based on the purpose of

isolation. The purpose of one class of isolation is to obtain lignin in a form that

is as close as possible to the protolignin, which is the lignin in the original

lignocellulose material, or the lignin in situ. Among the first class of lignins, the








7

best known is Klason lignin, which is obtained by treating wood with sulfuric acid.

Although this method for lignin isolation has great utility as an analytical means

of determining lignin content[208] of wood the highly condensed and altered

Klason lignin is generally unsuited for chemical characterization[164]. The most

useful forms of isolated lignin are milled wood lignin(MWL) or Bjorkman

lignin[164]. Milled wood lignin is purified from an aqueous p-dioxane extract of

finely milled wood. Although milled wood lignin is considered to be appropriate

for most chemical and biochemical studies, it has not been rigorously proved that

MWL is representative of protolignin[164]. The other types of isolation worthy of

mention in this class are Braun's native lignin and Brown rot lignin; the former

is the ethanol or aqueous dioxane extract of wood and the latter is the ethanol or

aqueous dioxane extract of Brown-rotted wood.

The purpose of the second class of isolation is to obtain a high quality

cellulose pulp. Hence, lignin is produced as a by-product in a pulping process,

during which lignin is severely modified, and has a structure much different from

protolignin. There are a variety of different pulping processes. Depending on the

pulping method, these lignins may be further classified as kraft lignin(KL),

sulfonated lignin(SL), organosolv lignin, etc. Usually, lignin is recovered from

black liquor, the by-product of a chemical pulping process. Commercially, black








8
liquor is produced principally by two wood pulping processes, the sulfite process

and the kraft process. In the sulfite pulping process, lignin is solubilized from the

cellulosic portion of the wood pulp by direct sulfonation, while the kraft process

is based on an alkaline degradation mechanism that causes cleavage of alkyl-aryl

ether linkages in the polymeric lignin, which subsequently results in chemical

functions of phenolic and carboxylic types in the lignin dissolved in the black

liquor.

From chemical and structural points of view, lignin can be broadly classified

into two main categories, even though it is extremely versatile, -- hard

wood(angiosperm,or deciduous) lignin and soft wood(gymnosperm, conifer, or

evergreen) lignin. Lignin from soft wood is made up mostly of a single monomer

type. As a result, the lignins of soft woods do not differ much from species to

species[165]. However, the lignins of hard woods do vary considerably among

species, because these lignins are derived from two monomeric types that are often

present in different proportions [165]. The lignin monomers(see Figure2-l), which

differ only in their number of methoxyl substituents, are p-coumary alcohol or p-

hydroxylphenyl units(I), Coniferyl alcohol or guaiacyl units(II), and sinapyl

alcohol or syringyl units(III).











CH2OH








RI R2
OH

(I) R'=R2=H p-Coumary Alcohol
(II) R'=H, R2=OCH3 Coniferyl Alcohol
(1l) R1=R2=OCH3 Sinapyl Alcohol


Figure 2-1 Lignin Monomers. Source: Sakakibara[180]
Note: Three types of lignin monomers differ only in the two substitute groups, R' and
R2.In softwood lignin, R' = H and R2 = OCH3. Hardwood lignin contains two types of monomers
of type II and type III.


Soft wood lignins are all quite similar and they are classified as guaiacyl

lignins that contain a small amount of p-hydroxyphenyl units[165]. Hard wood

lignins are usually mixtures of syringyl and guaiacyl units, but the ratio varies

widely among species[165]. For the purpose of this investigation the lignin

employed is mainly soft-wood kraft lignin(SWKL), and slash pine is the primary

wood source. Hence, in the main chapters of this thesis, lignin stands for SWKL

unless otherwise specified.









2.1.2 Difficulties and Developments



As mentioned in Chapter 1, the main use of black liquor is as a fuel in the

pulp and paper industry. There are some features that have limited the application

of lignin for than use as a fuel. One is the heterogeneity of protolignin[180]

resulting from the heterogeneous and complex nature of the structure of wood. For

example, compression wood contains not only more lignin but also more

condensed-type units than normal wood lignin[154]; lignin in the middle lamella

region of wood fibers may be different from the lignin in the S2 layer. The second

is the chemical and molecular weight inhomogeneity of chemical lignin: different

kinds of wood, different pulping processes different cooking conditions, and

different purification procedures may greatly influence its chemistry and molecular

weight. The third is its lack of purity and complex structure: lignin is separated

from black liquor from the pulping process, and the lignin derivatives are thought

to have undergone major structural modifications compared with the native

biopolymer[70]. Lignin is commonly viewed as being an almost hopelessly

complicated mixture of degraded and partially "condensed" components[68]. The

fourth is the lack of molecularityy", and the fifth is the lack of familiarity with this

type of raw material.

A great deal of effort has been devoted to understanding the chemistry and

structure of lignin[180]. Freudenburg[58] constructed a structural formula for


































































I
OH 0



CAro CK C2
HC---CH
I I
,-,c _O--CO


HzCOH
I
CH
CH


H3COCH

0 CH2



LHco 3COH
OH O-


Figure 2-2 A Structural Model for Softwood Lignin. Source: Sakabibara[180].


On



HCOH
HCOH
I
H2COH


0-
N3CO,


HC-

4--0 4 CH

H2COH








12
softwood lignin in the middle 1960s. The formula, composed of 18 monomer units,

was later modified several times[180]. In the early 1980s, Glasser[71] proposed a

structural model composed of 81 phenylpropane units based on the results of

computer simulation. A structural model of softwood lignin of 28 units was also

proposed by Sakakibara[179](see Figure 2-2), based on the results obtained by

hydrolysis and hydrogenolysis. The composition of this structural model were

compared with those of Spruce MWL[180], and it was found that the formula for

this model was very close to that for Spruce MWL. Even though significant

advances have been made in understanding the chemistry of lignin, all of the

structural models proposed are tentative and uncertain at present time, because

there does not exist any technology to separate protolignin from lignocellulose

materials without modification of the lignin.



2.2 Isolation of Lignin from Kraft Black Liquor



Kraft black liquor is a very complex mixture of many chemical species,

among which lignin is the main organic component that accounts for about 35-45%

of the total black liquor solids[41]. Usually, the dissolved lignin is precipitated by

the addition of mineral acids[164], H2SO4[s3], HC1[79], or acetic acid[124], or by

the addition of C02[2]. In addition, the kraft lignin may also be purified through

solvent(pyridine:acetic acid:water) fractionation[140]. It has been reported that








13
fractionation by a gradual decrease of pH was not possible, as most of the lignin

was precipitated sharply at pH 4.4[79]. It has been observed that acidification of

kraft black liquor to a pH of about 2 causes precipitation of acid lignin in a slimy,

gelatinous form. It also has been reported that the physical nature of lignin makes

it difficult to separate it from the acidified aqueous phase by centrifugation, and

almost impossible to separate it by settling and decantation or filtration[208].

Whalen[207] reported that the titration of black liquor in the presence of about 1%

chloroform or certain other organic compounds, based on the total black liquor

solids, causes precipitation or agglomeration of a granular acid lignin. This lignin

settles rapidly and can be simply and quickly isolated by using standard separation

techniques, such as decantation or vacuum filtration[208].

Precipitated lignin still contains substantial impurities from the mother

liquor; hence, subsequent washes[181] and treatments[79,136] are important.

Alternatively, kraft lignin may be purified by repeated dissolution in 0.1N NaOH

and precipitated with acid[111,164]. A detailed study on the effects of washing

steps on lignin was also reported[61,111]. In addition, proper drying is very

important because lignin is sensitive to exposure at high temperature; thus, freeze

drying is employed instead of oven drying at high temperature to minimize

chemical degradation and/or irreversible physical association.








14
2.3 Characterization of Black Liquor and Purified Lignin



2.3.1 Chemical Analysis of Black liquor



Chemical analysis of black liquor is by no means an easy task, since it is

a complex mixture of many chemical species. Recently, a comprehensive project

"Physical Properties of Kraft Black Liquor" has been conducted by Fricke's

research group, and some sound methods have been established and

reported[59,61,62]. In order to concentrate on the main topics of the thesis, which

are very broad, issues such as determination of the total black liquor solids,

inorganic ions, and the low molecular weight organic compounds in black liquor

are not reviewed. The related methodologies and developments can be found in the

literature [59,61,62].



2.3.2 Lignin Content in Black Liquor



Lignin, owing to its aromatic nature, shows a strong absorption in the

ultraviolet (UV) region in contrast to the polysaccharides of the cell wall, which

are transparent in visible and near-ultraviolet light[74,180]. Both lignin and lignin

model compounds exhibit a characteristic maximum in the UV region near

280nm[65]. In general, soft wood lignin shows a maximum at 280 to 285nm, and








15
hard wood lignin at 274 to 276nm[180]. The concentration of lignin in kraft black

liquor is typically measured by UV absorption at 280nm or 205nm[61].

Though the total lignin in lignocellulosic materials can be conveniently

determined by the Klason method(Klason lignin or acid insoluble lignin), the

determination of lignin in black liquor is not an easy task. Since it is impossible

to recover the lignin completely from the complex mixture, gravimetric approach

fails. Secondly, because the lignin in black liquor has been heavily modified during

a cooking process, there are no chemicals that can serve as a model compound for

it. Hence, it has not been possible to determine a reliable extinction coefficient for

this material. The absorptivity values of lignin in black liquor are all empirical. In

addition, interference from 2-furaldehyde(2-F),5-(hydroxymethyl)-2-furaldehyde

(HMF) and common acidic degradation products of carbohydrates may also affect

the results when the characteristic 280nm maximum is used[101]. A large

discrepancy exists among the absorptivity values reported. For example, the

absorptivity of acid soluble lignin at 205nm has been reported variously from 88-

103(l/gm.cm)[15,169,196], and that of lignin in black liquor at 280nm has been

reported to have values of 22.6-24.3(l/gm.cm)[2], 16.2-20.9(l/gm.cm)[188], 18.3-

23.5(l/gm.cm)[133], 23.8(l/gm.cm)[41], and 25.0(l/gm.cm) [173], respectively.








16
2.3.3 Surface Charge and Zeta Potential



Electrokinetic study of kraft lignin is essentially an unexplored area. Very

few reports[149,160] could be found in literature, and these only reported some

zeta potential data for particular applications.The pH effect on the zeta potential

of lignin has never been studied, and there has been no iso-electric point

information reported.



2.3.4 Molecular Weight of Lignin



Although it originates from the same sources as lignin, cellulose has

attracted most of the attention and has played a key role in the development of

polymer chemistry, whereas lignin technology has been plagued with ambiguities

in its chemistry. The most noticeable reason is the lack of "molecularity"[217]. In

order to understand the structure/properties relationships of lignin materials, it is

a first priority to obtain information concerning its molecular weight and molecular

weight distribution. Hence, many methods, including vapor phase

osmometry(VPO)[72,73,120,189], light scattering photometry, low angle laser light

scattering photometry(LALLS)[42], ultracentrifugation[38,49,162,181], and gel

permeation chromatography(GPC)[72,73,120,136,181,153] have been employed

in attempts to determine the molecular weight of lignin. Because of the complex








17
structure and heterogeneity of lignin and some impurities associated with it, many

difficulties have been encountered in attempting to determine the molecular weight

of lignin. When light scattering photometry is employed, the optical effects of

lignin solution due to fluorescence, anisotropy and absorption severely affect the

results. Without correction, a large error can occur[42]. When GPC is used, the

molecular weight and molecular weight distribution typically obtained have only

been relative quantities, since polystyrene standards are often employed to calibrate

the column, which are structurally and topologically far different from lignin[189].

The suitability of GPC is still open to question[135]. In fact, the weight-averaged

molecular weight determined by GPC is much lower than that determined by

LALLS[185]. In addition, the interactions between lignin molecules and the column

material, and the acknowledged associations of lignin macromolecules often cause

problems and uncertainties[38,67,136,181,183,189]. The relatively recent advances

in GPC technology for the determination of molecular weight: GPC equipped with

dual detectors, differential refractive index detector(DRI) and UV absorption

detector[185]; GPC coupled with LALLS (GPC/LALLS) [120,189]; and GPC

equipped with on-line differential viscosity detectors(GPC/DV) [189], have all

been used for to determine molecular weight of lignin with little success.

In the present investigation, VPO is employed to determine the number-

averaged molecular weight and LALLS is used to characterize the weight-

averaged molecular weight, since both are primary methods. Instead of presenting








18
a review of all of the literature relating to all aspects of the molecular weight of

lignin presented in this thesis, pertinent literature is reviewed in each chapter. For

example, the papers related to the determination of the weight-averaged molecular

weight of lignin by LALLS and the methodology are reviewed in section 6.1; those

related to the determination of number-averaged molecular weight of lignin by

VPO, in section 7.1; and those related to the effects of pulping variables on the

molecular weight and molecular weight distribution of lignin, in section 10.1.



2.4 Kraft Delignification



2.4.1 Process



Separation of lignin from wood is known as delignification. The kraft

pulping process, which uses a mixture of sodium hydroxide, sodium sulfide and

some other chemicals, is one of the most popular chemical delignification

processes. The origin of the kraft process dates back to 1870s[118]. Because

chemical losses resulting from the recovery process can be compensated for by

addition of sodium sulfate to the combustion furnace, where the sulfate is reduced

to sulfide in the reduction phase of the recovery system, the kraft pulping process

is also called the sulfate process[118]. Since the introduction of the Tomlinson








19
recovery furnace in the 1930s, kraft delignification has become the predominant

commercial pulping process.



2.4.2 Chemistry



The purpose of this section is to briefly review the chemical aspects of kraft

pulping. Morphological and topochemical considerations and problems connected

with diffusion of chemicals and lignin fragments will be discussed in following

sections. Larocque and Maass[125] pointed out that there are three separate steps

to the reaction of delignification. The first step involves absorption of alkali by the

acidic phenolic groups of lignin at the liquor interface. The second step involves

the formation of a lignin-alkali compound, and the third step involves a chemical

hydrolysis and the separation of alkali-lignin complex from the lignin surface.

Alkali absorption is a very rapid process and reaches equilibrium condition after

saturation.

As shown in Figure 2-2, the aryl propane units in lignin are interconnected

by C-O-C and/or C-C bonds. Of the ether bonds, only those belonging to the aryl

ether type can be cleaved during kraft pulping, whereas diaryl ether bonds are

virtually stable[70,145]. Similarly, diary C-C bond remains unchanged during kraft

pulping. Aryl alkyl C-C bond is generally believed to be sable[145], but Gierer[70]

reported that small amount of aryl alkyl C-C bond may be ruptured in the final








20
phase of delignification. Figure 2-2 also indicates that both a- and 0- aryl ether

bonds may be present in phenolic units and non-phenolic units. The aryl ether

bonds present in non-phenolic lignin units are more stable than their counterparts

in phenolic lignin units, therefore needs more drastic conditions to be cleaved.

Generally, the P-aryl ether bond is more stable than the a-aryl ether bond.

Chemically, kraft pulping is believed to be a competition between

degradation reactions, mainly involving aryl ether cleavage with participation of

neighboring groups, and condensation reactions, comprising conjugate addition of

carbanions to quinone methide intermediates[70]. The initial phase of

delignification is first order with respect to lignin concentration and independent

of the hydroxide and hydrosulfide ion concentrations[166], and mainly involves the

cleavage of a- and P-aryl ether bonds in phenolic lignin units[70]. The bulk phase

of delignification is first order with respect to lignin concentration, almost linearly

dependent on the hydroxide, but only slightly dependent on the hydrosulfide

concentration[70]. The cleavage of P-aryl ether bonds in non-phenolic lignin units

is considered to be the rate-controlling reaction of the bulk phase[70]. The residual

phase may involve the rupture of C-C linkages and the cleavage of aryl ether

bonds without neighboring group participation[70]. This phase of delignification

is dependent on the hydroxide ion concentration, but independent of the

hydrosulfide ion concentration.









2.4.3 Effect of Pulping Variables



Because of the complexities of chemical pulping, the predominant approach

towards the control of cooking process was to ignore the kinetics of the reaction

and treat the digester as a "black box" with certain recognized input and output

variables. This approach uses various dynamic response techniques and statistical

analysis[10], gives statistical models, and can be used to optimize the operating

conditions. The introduction of the H factor, a means of expressing cooking times

and temperatures as a single variable by Vroom[201], resulted in more efficient

experimental design and analysis. Hinrichs[90] performed 72 cooks of Douglas-fir

in a jacketed 2.5 ft3 rotational digester in order to investigate the effects of six

pulping variables on the kraft delignification. He found that[90] increasing the

effective alkali, sulfidity or temperature, or decreasing the liquor-to-wood ratio

increases the cooking rate; and that for any designed degree of delignification,

there is a sharply defined minimum effective alkali and a corresponding minimum

black liquor pH that could be used. Bailey and coworkers[10] conducted a 25 full

factorial design in a 1 ft3 digester with a pump for recirculation of the pulping

liquor, and found that the active alkali in the cooking liquor is generally the most

significant single variable. Hatton and coworkers[84,85] used an incomplete 32x7

factorial design to study the effects of temperature, effective alkali, and time-at-

temperature in a precision digester assembly in the circulating line of a 1 ft3








22
Weverk research digester-heat exchange system with Western Hemlock. Our

previous work[224] reported the statistical study results of kraft pulping of Slash

Pine in a 2.3 ftR rotational digester and a 3.5 ft vertical digester with the 24 full

factorial data. It was found that[224] the forced circulation increases the rate of

bulk delignification, and the responses of the yield, rejects and lignin content of

black liquor are highly non-linear with respect to pulping variables.

Since the principle objective of kraft pulping is high quality pulp at high

yield, there are essentially no reports dealing with the effects of pulping variables

on the composition and properties of black liquor, or on the lignins contained in

the black liquor. Previous research in our group has led to determining the effects

of pulping conditions on the weight-averaged molecular weight of lignin[61] with

a 24 full factorial design of 16 runs conducted in a 2.3 ft3 rotational digester.



2.4.4 Kinetics



In addition to the "black box" approach, many efforts have been made to

develop mechanistic models for the pulping process that are based on fundamental

physicochemical principles involving the aspects of the structure of lignin, the

diffusion process, and the kinetic expressions. Much progress has been made since

the early 1980s(see section 2.4.5) [34-36,64,217,219,220].








23
It is generally agreed that kraft delignification is a first order reaction with

respect to lignin concentration[70,118] and that the entire process consists of three

phases: the initial, bulk, and residual(final) phases[118]. The lignin removal in the

initial phase is about 10 times as fast as would be expected on the basis of a

corresponding H factor[l 18]. The type of lignin removed in this phase is, therefore,

called extracted lignin[118] and corresponds to about 20 to 24% of the total lignin

of soft wood. The low activation energy of the initial delignification (61 kJ/mol)

indicates that the rate of the process is diffusion rather than chemically

controlled[166]. The bulk delignification proceeds at a practicable rate only at

temperatures above 150C and the rate of delignification is dependent on the

concentration of hydroxyl and hydrosulfide ions in the cooking liquor[ 16,211].

The activation energy of this phase of delignification was found to be 150kJ/mol,

a normal value for heterolytic chemical reactions. Thus, the rate of bulk

delignification appears to be chemically controlled[212]. The residual

delignification has the lowest reaction rate and an activation energy of about

120kJ/mol[159], and leads to a dissolution of roughly 10-15% of the lignin

originally present in wood[70]. In addition, the transition point between the bulk

and the residual phases is known to shift to lower lignin contents when either the

cooking temperature, the effective alkali, or the sulfidity of a liquor is

increased[116]. In the case of pine, the initial, bulk, and final phases correspond

to H factors of about 100, 100 to 1000, and higher than 1000, respectively[118].








24
Recently, Chiang and his colleagues[34-36,64] studied the isothermal

pulping kinetics under an instantaneous isothermal condition over a temperature

range of 120C to 175C with a liquor-to-wood ratio of 50. With a sufficient amount

of data points at each temperature, they found that [35] the kraft delignification

curve can not be resolved unambiguously by three intercepting straight lines. By

using nonlinear regression techniques, they regressed the data points of kraft

delignification of Douglas-fir into a single equation for the entire delignification

process, and found that[35] the amounts of lignin removed during the initial, bulk,

and final phases are 18.8, 71.4, and 3.8%, while the rest ( 6% ) was physically

dissolved by alkali prior to chemical reactions. Also, it was reported that[35] the

activation energies of the initial, bulk, and final phases are 85.8, 123.8, and

116.0(kJ/mol), respectively; the pre-exponential factors are 5.91 x 109, 1.76 x 1013,

and 1.45 x 1010(min'-), respectively.

It is obvious that the kraft pulping system is highly heterogenous. Hence,

energy and mass transport play an important role. It must be pointed out that there

are two types of heterogeneity for this peculiar system. One type is that the

reaction takes place at the interface of solid chips and the cooking liquor, the other

is due to the heterogeneity of the wood structure itself, which has been clearly

confirmed by the topochemical studies of delignification[173,178]. The diffusion

process has not been well treated so far in regard to the pulping kinetics, except

for the initial phase delignification[81]. Wardrop and coworkers [202,203] reported








25

that ray cells and vessels govern the initial flow of cooking liquor into wood, and

that the pits afford an inter-cell penetration path for the transport of liquor into the

wood. Also, liquor penetrates into cell walls by diffusion from lumen, so that[203]

the order of the liquor contact is: secondary wall, middle lamella, and finally the

cell corer areas.

Procter and colleagues[ 173], by using UV microscopic techniques, observed

that both kraft and sulfite delignification occurs topochemically. They found

that[173] in both kraft and acid-sulfite pulping of Spruce wood, lignin is preferably

removed first from the secondary wall. At about 50% delignification, the heavily

lignified middle-lamella and cell-corner areas are attacked strongly, and the lignin

therein is rapidly dissolved. Saka and coworkers[178], by using bromination and

SEM-EDXA techniques, found that this is also true for the delignification of

Douglas-fir in soda and kraft processes. However, not much topochemical

preference was found in the removal of lignin by neutral sulphite up to 50%

delignification[173]. In addition, Obst[163] reported that the delignification rates

in kraft pulping of Loblolly pinewood and of crill, a middle lamella-enriched

fraction, are the same and topochemical differences appear to be more a

consequence of physical than chemical factors in kraft pulping.









2.4.5 Fundamentals



In terms of the fundamental structure and size of lignin macromolecules,

there exist two alternative theories[45]: lignin is an infinite cross-linked three-

dimensional network or it is a branched molecule of relatively small size. The latter

treats delignification as a process of functionalization and subsequent dissolution

of the lignin molecule, for which the molecular weight and molecular weight

distribution have been predetermined by the process in the wood cell[45]. The

former, which is more popular, views delignification as a process of degradation

or cleavage of the infinite network, and the degraded lignin is dissolved as

macromolecules in a wide range of sizes[45]. Yan[217,219,220] developed a

molecular theory of delignification based on Flory[218] and Stockmayer's [192]

theory(F-S theory). In Yan's theory, delignification is treated as a degelation

process. It concludes that[217] both protolignin and degraded lignin may be treated

as a branched cross-linked macromolecule, and the primary chains cross-linked

have the most probable distribution, or quickly assume this distribution on

degradation; delignification is analogous to degelation by which specific linkages

in the linear chain and at the cross-links are cleaved; the basic unit in softwood is

the phenylpropane (C9) unit, the kraft process is specific for the liberation of PhOH

groups from the cleavable aryl ether bonds, and this specificity determines the rate

and extent of cleavage. In addition to the degelation concept, another theory








27
emphasizes the physical factors. This theory[1,193] visualizes the delignification

occurring in two steps: cooking chemicals first degrade and dissolve the

hemicellulose in the cell wall, thus creating a porous structure in the wood fibers;

and then lignin is degraded and diffuses out through these pores. Therefore, there

is a correlation between the pore size and the molecular weight of degraded lignin

according to this theory.













CHAPTER 3
UV-VISIBLE RESPONSE OF KRAFT LIGNIN IN BLACK LIQUOR


3.1 Background


Ultraviolet(UV) spectrophotometry has been a useful method of the

determination of lignin content for lignin-containing solutions, especially for black

liquors. Although a number of studies have been reported [2,20,133,200,206]

previously, determination of lignin in black liquors is still in a confused state [61].

The lignin content of black liquor has been determined by using UV absorbance

at a number of different wavelengths as being most appropriate, such as

280nm[21,79,100,150,173,188,200,213,206], 205nm[16,101,113,114,143], 281nm

[65,99], 282nm[206], 300nm[21], 315nm[99], 355nm[186] etc. In most instances,

ultraviolet absorption at 280nm has been used to determine lignin content in black

liquors, but Kleinert and Joyce [113] studied the UV absorption of various lignins

in sulfite and sulfate pulping liquors, and reported that absorbance at 205nm is

influenced least by the by-products of pulping in black liquor, and thus, it may be

a more accurate measure of lignin content in black liquors.

The solvents used or the pH values of aqueous solutions used by various

investigators were quite different. Lin[132] reported the derivative UV








29
spectroscopy of lignin at a pH of 5, Boutelje and Eriksson[21] reported results for

UV absorbance at pH 6 and 12 Alen et al[2] also studied UV absorption in

dilute sodium hydroxide at a pH of 12. Shimada et al[186] and Janshekar et

al[99] studied the effect of pH on UV absorption of lignin. Lindstrom[136]

reported an absorptivity of 27.1 [1/gm.cm] of dialyzed lignin in 0.1N sodium

hydroxide solution.

Because of the different conditions used, the reported extinction coefficients

of kraft lignin are quite different, as indicated in section 2.3. Also, Hill [88]

reported that the UV absorption is affected by the molecular weight of lignin and

may be time dependent at some wavelengths. Kleinert" observed that UV

irradiation appears to cause a decrease in UV absorbance of lignin.

As discussed in section 2.3, the main problem is probably the lack of a

reliable standard to estimate the extinction coefficient of kraft lignin. In this study,

efforts have been made to obtain as highly purified a lignin as possible to use as

a standard, and to carefully analyze for the impurities remaining. Subsequently, the

extinction coefficient of lignin was determined and correction for the impurities

remaining was made. Lignin samples of different molecular weights were

employed to study the effects of molecular weight on the absorptivity. The effect

of solution pH on the UV-response of kraft lignin was also investigated.



Note Chemical Abstracts: CA74(6):23747r.








30

3.2 Experimental



3.2.1 Equipment and Parameters



A Perkin Elmer Lambda 4C dual beam UV-visible spectrophotometer

connected with a Perkin Elmer System 7 digital control and data analysis system

was used for the UV experiments. The sample cell width was 1.O(cm). The

following parameters were selected for spectrum studies:

Scan speed 60(nm/min)

Response 3

slit 0.25

Wavelength 190-900(nm)

After a background correction, the lignin solution was scanned and the spectrum

was plotted automatically. To calculate the extinction coefficient of lignin, instead

of obtaining data from a spectrum, absorbance values were measured by the

Wavelength Programming method in the absorbance mode under the following

conditions:

Slit 0.25

Integration 8

Baseline off

Wavelength 280, 290, 300(nm)








31
Each measurement was repeated three times, and the average was taken and

reported.



3.2.2 Sample Preparation



The lignin samples used in this chapter were prepared by cooking of slash

pine in a 2.3(ft) rotational digester with a 2' full factorial experimental design'.

This 2' full factorial experimental design essentially has the same cooking variable

conditions as the first 16 runs in the 2k+2k+l(k=4) composite design as will be

discussed in Chapter 8. The lignins were isolated from black liquors by a acid

precipitation method as described in the literature[61].



3.2.3 Molecular Weight and Chemical Analysis of Purified Lignin



In order to study the effect of molecular weight on the ultraviolet

absorbance of kraft lignin, samples of different molecular weight were employed



Note The experimental work of the UV-response of kraft lignin in black liquor had been
performed in the early stage of this study before the 2k+2k+l circulation pulping experiments were
started. All the lignin samples used in this chapter were prepared with a 2.3 ft rotational digester.
Some of the conditions are different from the circulation pulping experiments as will be discussed
in Chapter 8.
Note b The lignin samples used in this chapter were purified and characterized by the
methods somehow different from those that will be reported in the subsequent chapters of this
thesis.








32
to compare their absorptivities. The number-averaged molecular weight(M) of

lignin was determined with a Corona/Wescan 232A Molecular Weight Apparatus

at 80(C) with dimethyl formamide (DMF) as solvent. This apparatus has a similar

structure to the Wescan 233 Molecular Weight Apparatus as will be described in

chapter 7. The weight-averaged molecule weight (M,) of lignin was measured in

DMF at 80(C) with a KMX-6 LALLSP, which is described in Chapter 6. The

detailed methodology and correction procedure for the optical effects of lignin

solution are developed in Chapter 6. The organic and inorganic impurities

associated with the purified lignin were analyzed with a Dionex Ion

Chromatography and ion-selective electrodes. The detailed methods for impurity

analysis can be found in the literature[61].



3.2.4 Solutions



The purified lignin solutions in organic solvents: DMF, tetrahydrofuran

(THF) and dimethyl sulfoxide ( DMSO), were prepared at a concentration of about

0.05(gm/l). These solutions were used only for spectra, which were obtained with

respect to a reference of the corresponding solvent. The extinction coefficient of

lignin was measured and calculated in dilute sodium hydroxide solutions with

known pH values. A dilute sodium hydroxide solution with the same pH value as








33
the lignin solution measured was always prepared and used as a reference during

each measurement.



3.3 Results and Discussion



3.3.1 Spectrum and Characteristic Wavelength



As shown in Figure 3-1, the absorbance spectra of lignin solutions in

organic solvents, DMF, THF and DMSO exhibit a sharp peaks at the same

wavelength of 283(nm). A maximum absorption peak occurs at 240nm on the

spectrum of lignin/THF solution, but no discernable peak appears near 205(nm)

or near 350(nm) in any of the three spectra. At wavelengths above 500(nm), kraft

lignin solutions in these organic solvents generally does not absorb. Although the

205(nm) position is still uncertain since it is below the cut-off wavelength of these

organic solvents, the 350(nm) region is clearly shown not to be the characteristic

wavelength of kraft lignin.

The absorbance spectrum of lignin solution in 0.1N sodium hydroxide was

also determined over the entire UV-visible region of 190(nm) to 900(nm) (not

shown). Again, no absorption occurs at wavelengths above 500(nm) for either

purified lignin solutions or the black liquors. As illustrated in Figure 3-2, the

spectrum of purified lignin solution at a pH of 13.0 exhibits a maximum near

















z
T 2812/DMSO, Reference DMSO

0 I
CI



S283 nm


200 400 600 800
WAVELENGTH, nm


Figure 3-1 UV-Visible Spectra of Softwood Kraft Lignins in Different Organic Solvents.


















LIJ r,* -C. *W
CHardwood Mill Liquor, pH=13
z


CO







I I I I I I I

200 240 280 320
WAVELENGTH, nm

Figure 3-2 UV Spectra for Selected Kraft Black Liquors and A Purified Softwood
Kraft Lignin in Sodium Hydroxide Solutions.








36
220(nm) and a plateau region from 270 to 300(nm). The low wavelength region

near 205(nm) is saturated at a very low lignin concentration of about 0.02(gm/l).

The position of the maximum absorbance near 220(nm) varies with a change of the

lignin concentration and is saturated at about 0.08(gm/l). Upon carefully checking

the relationship between the absorbance and the concentration of lignin solution

it was observed that no linear relation between absorbance and concentration

exists either at 205(nm) or at the maximum absorbance position. Bobier and

coworkers[16] reported that absorbance at 205(nm) might be more "characteristic

of the resonating system of the aromatic lignin building units". A complete review

of studies reported indicates that most probably the use of absorption at 205nm to

determine lignin concentration originated from Kleinert and Joyce's work [113] in

which "the ultraviolet absorption measurements were carried out on ethanol

solutions of the materials investigated using 10.0mm matched silica cells in a

Beckman dual beam ultraviolet spectrophotometer and a Warren Spectracord."

Unfortunately, ethanol itself exhibits a maximum ultraviolet absorbance at exactly

205(nm).

Dilute sodium hydroxide solution absorbs ultraviolet strongly in the low

wavelength range, as shown in Figure 3-3, and the absorption extends to higher

wavelength as the sodium hydroxide concentration increases. However, no

absorbance occurs above 240(nm) at sodium hydroxide concentrations up to L.ON.

Therefore, small changes in the sodium hydroxide concentration may strongly































200 220 240 260
WAVELENGTH, nm


Figure 3-3 UV Spectra of Sodium Hydroxide Solutions with Respect to D.I. Water.








38
affect the absorbance of lignin solution at wavelengths below 240nm. Obviously,

it is only possible to measure the lignin content of black liquor at a wavelength

above 240(nm) when dilute sodium hydroxide solution is used as a solvent.

The 270-300(nm) absorbance plateau(Figure 3-2) is saturated at a lignin

concentration of about 0.15(gm/l) in 0.1N sodium hydroxide. At concentrations

below 0.08 (gm/1), a very good linear relation(Figure 3-4) exists between the

absorbance and the lignin concentration in the plateau region. The absorbance

begins to deviate from the linear relation as the lignin concentration increases

further above 0.08(gm/l).

As shown in Figure 3-2, the absorption spectra of soft wood black liquors

from different sources have exactly the same shape as that of the purified lignin

at wavelengths above 260(nm). Therefore, the lignin concentration of soft wood

liquors may be accurately measured by using the extinction coefficient determined

in the plateau (270-300nm) region. In addition, Figure 3-2 indicates that hardwood

lignin does absorb differently from the softwood lignin. In this study, however,

there is no attempt to investigate the UV-response of hard wood lignin and to make

detailed comparisons between species.
















WIl pH=13
Z 1.50- pH=12


0
U) 1.00 -



0.50



0.00 --** -
0.00 2.00 4.00 6.00 8.00 10.00 12.00
CONCENTRATION x 102 gm/l]


Figure 3-4 UV Absorbance of Softwood Lignin in NaOH at 280nm and at
Different pHs.










3.3.2 pH Effect



Different solvents, such as Ethanol[113], Dioxane/water [99,140], dilute

sodium hydroxide solution [2,88], and dilute potassium hydroxide solution[206]

etc. have been used to dissolve lignin for UV absorption measurements. Obviously,

it is very inconvenient to transfer the lignin in black liquor to some other solvent

for UV measurement, and it is most convenient to determine the lignin content of

black liquor in dilute sodium hydroxide solution, since a kraft black liquor usually

has a high pH with sodium hydroxide already present.

The lignin solutions and the reference solutions for study of the pH effect

on the absorbance of lignin were prepared as follows. An original lignin solution

of 1.0(gm/l) was prepared at a pH of 13.0. This was diluted to a lignin

concentration of about 0.04(gm/l) and adjusted to various pH values with sodium

hydroxide or dilute sulfuric acid. For example, 4.0ml of the original solution plus

6.0ml 0.1N sodium hydroxide solution and 90ml D.I. water gave a lignin solution

of 0.04(gm/l) with a pH of about 13; 4.0ml of the original solution plus 4.0ml

0.1N sulfuric acid and 92ml D. I. water gave a lignin concentration of 0.04(gm/l)

with a pH of about 7.0. In the case of black liquor, an original solution was made

at a black liquor solids of 3.0(gm/l) with a pH of 13.0. Although the exact lignin

content is unknown in the original solution, the diluted solutions have the same

lignin concentration, since all of them were diluted from the same original solution.
































0


280nm


Experimental Liquor

Mill Liquor


S Purified Kraft Lignin


SI


pH


a I


11.00


13.00


15.00


Figure 3-5 Effect of pH on the UV Absorbance of Softwood Kraft Black Liquors and
A Purified Softwood Kraft Lignin.


1.1


pi


I


1.00 -


0.90 -


ni


0I


6.0
5.00


7.00


9.00


* I


I


I


'A








42
The reference solutions of various pH needed were prepared by using the same

procedure starting with a 0.1N sodium hydroxide solution. Before UV

measurement, both the lignin solution and the reference solution were carefully

checked for pH values, and the difference between the two was controlled to within

0.1 pH units or less to ensure accuracy.

The absorbance dependence(at 280nm) of black liquors and a purified

lignin solution on pH is illustrated in Figure 3-5. This indicates that the

absorbance of lignin solution increases monotonically as the solution pH increases.

Generally, the absorbance increases very slowly in the pH range from 7 to 10,

rapidly from pH of 10 to 12, and flattens out at pH 12 to 14. Hence, the pH value

of the lignin-containing solution may result in a large discrepancy on the extinction

coefficient of lignin. Figure 3-4 shows further evidence that there is even a little

difference between the absorbance of lignin solutions at pH 12 and 13 at the

lignin concentrations above 0.05(gm/l).

As reviewed in Section 2.3.2, it is the aromatic nature of lignin that results

in the absorption near 280nm. The author believes that the dependence of lignin

absorption on the solution pH is due to the different electron environments of the

phenolic structure in lignin molecule. Since a lignin molecule has a rather complex

structure (see Figure 2-2), some phenolic hydroxyl groups in the molecule may be








43
more easily ionized than the others. As the solution pH is increased, more phenolic

hydroxyl groups ionize and give up protons, hence, the absorbance changes with

the increase of pH. At pH above 13, it is very probable that all ionizable phenolic

hydroxyl groups have already ionized into negatively charged ions. Further increase

in pH does not affect the phenolic hydroxyl structure in the lignin molecule, so that

the absorbance of lignin at 280nm becomes independent of pH. In other words, pH

13 is the lowest pH that ensures a complete ionization of the phenolic hydroxyl

groups in lignin molecule.



3.3.3 Molecular Weight Effect



Table 3-1 shows the extinction coefficients of five purified lignin samples

with their molecular weight and molecular weight distribution information. These

extinction coefficients were determined at three different wavelengths in the 270-

300nm plateau region at pH 12 and 13, respectively. The five lignin samples were

selected from a 2' full factorial experiment of pulping of slash pine in a 2.3 ft3

rotational digester. The combination of pulping conditions in the design covers a

wide experimental space and gives 16 lignin samples of various properties. These

16 cooking runs cover a H factor from 472 to 2274. The weight-averaged

molecular weight of the purified lignin samples ranges from 14,000 to 48,300








44

(Dalton), and the corrected number-averaged molecular weight" ranges from 2000

to 6100(Dalton). The molecular weight distribution index covers a range from 3.12

to 17.3. The five lignin samples selected for the UV study generally cover the

molecular weight and the molecular weight distribution range of this 2' full

factorial experiment. It was observed that the absorbance of lignin solution was

linear with respect to lignin concentration at both pH 12 and 13 over the entire

plateau region. No dependence of the extinction coefficient on the molecular

weight of lignin was observed at either pH 12 or 13 over the entire plateau region.



3.3.4 Corrected Extinction Coefficient



It is known that ultraviolet absorption is a relative method for lignin

measurement, and the accuracy of this method depends on the purity of a lignin

standard adopted. Obviously, the lignin extinction coefficient may be

underestimated due to a presence of the non-absorbed impurities in the standard

which would result in an overestimation of the lignin content of black liquor.

Hence, it is important to analyze for the impurities contained in the lignin standard




Note: The number-averaged molecular weight of lignin has been corrected for the
impurities associating with the purified lignin samples. The observed number-averaged molecular
weight is lower than the corrected value [61].










Table 3-1 The Extinction Coefficients of Softwood Kraft Lignins of Different Molecular Weights.



Lignin Sample #2 #4 #16 #9 #1 Average

Molecular Weight Mw (Dalton) 1.40E+04 1.72E+04 2.15E+04 4.83E+04 3.90E+04
(Dalton) Mn (Dalton) 2380.0 4170.0 6090.0 4060.0 2250.0
Mw/Mn 5.9 4.1 3.5 11.9 17.3
pH Wavelength
Extinction 13 280nm 22.6 22.4 22.9 22.5 22.7 22.6
Coefficient 13 290nm 22.8 22.4 23.2 22.8 22.8 22.8
(1/gm.cm) 13 300nm 21.6 21.5 22.3 21.7 21.6 21.7
12 280nm 21.7 22.4
12 290nm 21.1 22.4
12 300nm 19.0 20.8
pH Wavelength
Corrected 13 280nm 23.7 23.5 24.2 23.6 23.7 23.7
Extinction 13 290nm 23.9 23.5 24.5 23.9 23.8 23.9
Coefficient 13 300nm 22.6 22.5 23.6 22.8 22.6 22.8
(1/gm.cm) 12 280nm 22.8 23.7
12 290nm 22.2 23.7
12 300nm 20.0 22.0











Table 3-2 Chemical Analysis of Purified Lignins.

Impurities Lignin Samples
#2 #4 #16 #1 #9
Inorganic Sulfite 0.32 0.42 0.60 0.35 0.36
Ions Sulfate 0.25 0.24 0.27 0.15 0.22
(weight%) S% 1.13 1.22 1.19 1.52 0.95
Subtotal 1.70 1.88 2.06 2.02 1.53
Organic Lactate glycolate 0.80 0.75 1.03 0.63 0.83
Species format 1.34 1.18 1.26 1.03 1.42
(weight %) Oxalate 0.07 0.10 0.13 0.04 0.11
Acetate 0.71 0.66 0.91 0.63 0.86
Subtotal 2.92 2.69 3.33 2.33 3.22
Metal Ions Ca' 73.5 68.7 49.8 65.7 68.6
(ppm) K+ 80.0 103.0 49.1 44.2 80.2
Mg+ 5.1 <5.0 <5.0 21.6 <5.0
Na + 260.0 316.0 233.0 136.0 219.0

Total (weight%/) 4.64 4.62 5.42 4.38 4.79


and to make corrections for their influence on the observed extinction coefficient.

Table 3-2 summarizes the impurities of the five lignin standards used. Generally,

the inorganic anions contribute about 2% of the total weight, metal ions contribute

very little, and the organic low molecular species contribute about 2.5 to 3%. To

the best of the author's knowledge, none of these chemical species listed in Table

3-2 absorb near 280(nm). Their influence on the extinction coefficient of lignin can

be corrected for gravimetrically. With the corrections, the extinction coefficient of

lignin is increased about five percent with respect to the observed value(Table 3-1).








47
3.3.5 Measurement of the Lignin Content of Black Liquor



From Table 3-1, the average corrected lignin extinction coefficient of

23.7(l/gm.cm) at 280nm was selected for use in the calculation of the lignin

concentration in black liquor in the present study. Notice that the pH value of the

working solution of black liquor has to be 13.0 or higher and the lignin

concentration has to be less that 0.08(gm/1) to ensure the accuracy of the

measurement and calculation. Based on this requirement and an assumption that

the lignin content of black liquors varies from 30-45% of the total black liquor

solids, a simple and reliable procedure for the determination of the lignin content

of black liquor is proposed as follows.

First, measure the total black liquor solids by Tappi Method T635 om-82,

and adjust it to a total black liquor solids of 10-15% and a pH of 13.0. Then,

weigh 1.0gm black liquor and dilute it in a volumetric flask to 50ml with 0.1N

sodium hydroxide solution to make an original solution. Subsequently, transfer

1.0ml and 2.0ml of the original solution to two 50ml volumetric flasks,

respectively, make dilution with 0.1N sodium hydroxide to prepare two working

solutions of different lignin concentrations for ultraviolet measurement, and choose

0.1N sodium hydroxide solution as the reference. Following this proposed

procedure, the lignin concentration of the two working solutions would be within

a range of 0.012(gm/l) to 0.054(gm/n), and the UV-response of these solutions








48
would be in the linear range. In routine work, the lignin content expressed as the

percentage of the total black liquor solids can be simply calculated by equation 3.1

to 3.3.


LZ=10.54x (3.1)
S



S

1
LigZ ~%TBLS)=- x[L +LJ (3.3)
2

Where A, and A2 are the 280nm absorbencies of the working solutions 1 and 2,

respectively; Liand L2 are the lignin content of the black liquor calculated from

solution 1 and 2, respectively; TBLS stands for the total black liquor solids; and

S is the quantity of the black liquor solids in grams contained in the original

solution, which can be calculated by multiplying the liquor weighted in the original

solution by the black liquor solids content.

All of the black liquors produced by kraft pulping of slash pine with a 2k

+ 2k +1 (k=4) experimental design in a 3.5 ft3 vertical circulation digester as

discussed in Chapter 8 were analyzed by using this procedure, the results are

reported in Table 3-3 for all 44 pre-evaporation black liquors and 25 combined

post-evaporation liquors. Table 3-3 indicates that 3 the lignin content of the 44










Table 3-3 Lignin Content of Kraft Black Liquors (% B.L.Solids)

Samples Pre-evaporation Post-evaporation
Solids Ugnin Solids Ugnin
(%) (% solids) (%) (% solids)


ABAFX11
ABAFX12
ABAFX13
ABAFX14
ABAFX15
ABAFX16
ABAFX77
ABAFX19
ABAFX20
ABAFX21
ABAFX22
ABAFX23
ABAFX24
ABAFX25
ABAFX26
ABAFX27
ABAFX28
ABAFX29
ABAFX30
ABAFX31
ABAFX32
ABAFX33
ABAFX34
ABAFX35
ABAFX36
ABAFX37
ABAFX38
ABAFX39
ABAFX40
ABAFX41
ABAFX42
ABAFX43
ABAFX44
ABAFX61
ABAFX62
ABAFX63
ABAFX65
ABAFX66
ABAFX67
ABAFX68
ABAFX69
ABAFX71
ABAFX73
ABAFX75
ABAFX76


8.59
8.50
6.34
6.01
6.45
10.92
12.50
8.71
9.28
7.45
6.11
11.94
11.50
15.50
14.78
9.95
8.63
9.86
9.42
9.86
8.20
10.74
12.62
11.54
missing
11.14
11.12
12.64
12.14
15.29
12.82
8.92
9.17
12.47
13.40
12.71
9.87
9.34
10.67
11.03
10.66
13.68
11.78
9.59
9.18


35.13
34.87
43.22
43.70
35.90
40.22
40.43
41.56
42.71
42.24
41.23
39.14
37.61
39.33
39.22
41.05
40.44
44.01
41.46
35.38
33.50
43.23
44.70
37.85
missing
42.30
42.24
40.69
40.54
40.56
40.69
40.84
42.72
39.45
39.82
39.40
39.33
40.34
37.60
39.15
33.95
43.73
43.00
36.60
35.33


36.64

23.64

35.75
41.96

30.26

32.60

40.13

41.32

35.52

36.78

37.62

32.62

44.22

49.29

47.20

47.83

35.72

33.93
28.10

37.85

34.74
30.77
20.14
44.66

38.99


36.55

44.35

39.70
40.90

41.57

40.06

39.13

38.98

41.37

42.95

39.74

40.25

39.02

43.27

40.58

41.27

42.61

42.67
42.14

43.72

35.86
32.15
43.52
41.27

36.48


38.48








50
experimental liquors produced by a variety of pulping conditions varies from

33.50% to 44.70%, and generally that the lignin content in black liquor solids

remains constant or increases slightly after evaporation. The lignin content of the

pre-evaporation liquors in Table 3-3 was used in Chapter 9 to investigate the

pulping variable effects on the lignin concentration of black liquors, and the results

for the combined post-evaporation liquors were used to calculate the lignin yield

of the isolation process, as will be discussed in Chapter 5.



3.4 Summary



It is possible to determine the concentration of lignin in soft wood kraft

black liquor accurately, provided that proper conditions are satisfied and provided

that absorbance at or near 280nm is used for analysis, based on the results for a

comparative study of lignin solutions and of kraft black liquors.

No discernable absorbance peaks were observed near 350nm for kraft lignin,

and absorbance near 205nm is not linear with respect to lignin concentration. The

pH value of the lignin solution strongly affects the UV absorption of a lignin

solution at pHs below 13. The UV absorption of lignin solution is independent of

pH at pH-13 and at low lignin concentrations of less than 0.08(gm/l).

At 280nm, UV-visible absorption of lignin solutions at pH > 13 is

independent of lignin molecular weight. The relationship between absorbance at








51
280nm and lignin concentration is linear up to a concentration of about 0.08(g/l),

and the extinction coefficient is 23.7(l/gm.cm). The absorbance by soft wood kraft

black liquor solutions is very similar to the absorbance by the solutions of purified

lignin near 280nm, and absorbance near this wavelength is least affected by

variations in concentration of other components of black liquor.













CHAPTER 4
ELECTROKINETIC STUDY OF KRAFT LIGNIN



4.1 Background


Electrokinetic study of kraft lignin has been an unexplored area. Chemical

Abstracts(CA) was checked electronically up to 1993 under rather broad key word

combinations, such as "lignin/zeta potential", etc. A total of three entries were

found. McKenzie's work[149] mainly focused on the cellulosic fines in a pulping

process. Only a single value of zeta potential for milled wood lignin and for

Klasson lignin was reported without specification of the pH. Nyman, Rose and

Ralston[160] reported the coagulation of kraft lignin or lignosulfonates with some

salts. Rusetskaya and colleagues' work" was an application of zeta potential in the

air flotation of sulfate slurry waste water Cross checking resulted in obtaining

Lauzon and Malven's paper[127] which was an application of the zeta potential of

lignosulfonate to monitor oil drilling muds. All of the studies mentioned above

only reported or utilized some zeta potential values of lignin in their applications.




Note: Chemical Abstract, CA104(16):135426e.

52








53
There has been no report on the dependence of lignin zeta potential on pH, and

no isoelectric point information could be found.

As described in Chapter 2, lignin is a polar macromolecular material.

Usually, it is present or is utilized in a system that accounts for a large interfacial

effect. A surface interaction will contribute to the properties of lignin or of the

lignin containing system significantly, since the interfacial region normally has a

very strong electric field of the order of 106 (V/cm) or higher[4,18,78]. Therefore,

a knowledge of the electrokinetic properties of kraft lignin is clearly of

fundamental and practical importance. For example, it can be used to predict the

stability of a lignin containing system, to suggest better conditions for the isolation

of lignin from black liquors, and to supply a knowledge of the impurities of a

purified product. In addition, electrokinetic study would be useful in the

fundamental study of molecular associations of lignin in solution, in the

understanding of adsorption, desorption, and coagulation of lignin with other

materials. Also, it may play an important role in the development of the surface

modification and utilization of lignin.

This chapter is written based on a general knowledge of the interfacial

science; especially, the electric double layer theory, and a lengthy literature review

is omitted. Practically, it is very difficult to measure the electric potential at the











POTENTIAL


Stem Plane

----- Fluid Phase -

11 -SURFACE POTENTIAL
I I
I Sehear plane




-ZETA POTENTIAL


DISTANCE FROM SURFACE


Figure 4-1 Schematic of the Potential in Solid-Liquid Interface Region.
(Stem's Model)








55
particle surface. However, zeta potential, the potential measured at the shearing

surface(see Figure 4-1), can be conveniently determined by electrophoretic

experiments. Electrophoresis, by definition, studies the movement of suspended

particles in a fluid under the influence of an applied electric field. Figure 4-1

shows the schematic of the electric potential in the solid-liquid interface region.

Generally, it is believed that the interfacial region consists of two layers, a Ster

layer and a diffuse layer. The former is composed of a layer of ions specifically

adsorbed at the surface, and the later usually consists of an excess of counter-ions

and a deficit of co-ions.

The aim of this work was to obtain electrokinetic data for purified kraft

lignin, with or without the presence of various metal salts, as a function of pH by

performing a microelectrophoresis experiment in aqueous medium. The dependence

of the zeta potential of kraft lignin on pH was studied and the iso-electric point of

kraft lignin is reported. Surface interactions of kraft lignin in aqueous medium

were also investigated by using monovalent, divalent and trivalent ions at different

concentrations, and the results were interpreted in terms of surface adsorption.

Experimental observations, such as the iso-electric point and surface charge

reversals, etc. are elucidated.








56

4.2 Experimental



4.2.1 Materials and Preparation of Lignin Suspensions



All of the experimental liquors used for work reported in this chapter were

prepared by cooking of Slash pine in a 3.5 ft' vertical batch digester with forced

liquor circulation. Detailed pulping conditions and liquor handling information can

be found in Chapter 8. The experimental lignins were isolated from black liquor

by an acid precipitation method followed by subsequent washes and drying. The

detailed isolation procedure is reported in Chapter 5.

Indulin AT, a commercial softwood kraft lignin manufactured by Westvaco

company(Charleston, South Carolina), was also studied for the purpose of

comparison. The reagents used in this chapter, such as sodium hydroxide,

hydrochloric acid, sodium chloride, zinc sulfate, calcium chloride, aluminum

chloride, were all A.C.S. grade. All water used was deionized water.

Kraft lignin was suspended in deionized water at a mass concentration of

0.05%. The suspension was then sonicated for 30 minutes to break loose

agglomerates into small particles. The suspensions with known concentration of salt

were also prepared at a lignin mass concentration of 0.05%. A constant lignin

concentration in the suspensions made the surface coverage comparable when a salt

effect was investigated. However, it was observed that part of the lignin








57
precipitated on the bottom at a concentration of 0.05% for both Indulin AT and

the experimental lignins. Hence, the lignin concentration in suspension, strictly

speaking, is not constant or consistent during the electrophoresis experiments in the

sense of the total surface per unit volume in suspension.



4.2.2 Instruments and Microelectrophoresis Experiments



A lignin suspension of 2% volume concentration in deionized water was

first tested on a Maytec NBS-8000 (Maytec Applied Science Inc, Hopkinton,

Massachusetts) equipped with a Maytec SSP-1 ESA sample cell assembly. This

instrument determines the zeta potential of suspended particles based on the

principle of accoustophoresis. It was found that this instrument was not suitable

for the determination of zeta potential of lignin, because of the low density and

relatively low surface charge density of lignin. It was observed that a 2% volume

suspension of the experimental lignin (ABAFX6768) in D.I. water had a

conductivity of only 87 1 (ps/cm), while a 2% volume suspension of Indulin AT

had a conductivity of 3047 1 (ps/cm). This indicates that the purified lignin in

the present study contained much less charged impurities than the technical lignin

of Indulin AT.

A Model 501 Laser Zee Meterm(Pen Kem Inc, Bedford Hills, New York)

with a 632.8nm He-Ne laser as light source and with a flat cell configuration has








58

been employed to carry out the microelectrophoresis experiments. Three lignin

samples were also examined with a System 3000 Automated Electrokinetics

AnalyzerTm (Pen Kem Inc, Bedford Hills, New York). It was observed that,

although the dependence of the zeta potential of lignin on the suspension pH

measured with two instruments follows the same trend, the magnitude of the zeta

potential measured with the System 3000 was consistently higher than that

measured with the Laser Zee 501. Hence, all zeta potential data reported in this

study were determined with the Laser Zee 501 for consistency.

Electrophoresis experiments were all performed at ambient temperature and

at an apparent lignin mass concentration of 0.05%. A Coming 220 pH meter was

used to measure the pH values of the lignin suspension. Due to the restriction

of the sample cell material used, zeta potential of lignin suspension was measured

in a pH range of about 1.0 to 10.5 ( The suspension could attack the sample cell

material at a higher pH ). An aliquot of prepared lignin suspension was taken, and

its pH value was adjusted with either dilute sodium hydroxide or dilute

hydrochloric acid immediately before a measurement of zeta potential was made.

After the measurement, the lignin suspension was withdrawn from the sample cell,

the pH was remeasured, and this pH value was recorded with the corresponding

zeta potential determined.








59

4.3 Results and Discussions



4.3.1 Zeta Potential and IEP



The electrokinetic behavior of three kraft lignins is shown in Figure 4-2 as

a function of the pH values of the aqueous suspensions. More zeta potential data

of purified lignins with quite different cooking conditions are given in Table 4-1

(see Table 8-1 for pulping conditions). Figure 4-2 and Table 4-1 demonstrate that

the pH strongly affects the zeta potential of kraft lignin, and that both the technical

lignin of Indulin AT and the experimental kraft lignins essentially have the same

trends with respect to the dependence of pH. Even though the zeta potentials of

different lignins have different magnitudes at high pH values as shown, the zeta

potentials of different lignins at low pH values are nearly equal. Generally, the zeta

potential of kraft lignin increases monotonically from about -43mV to zero as the

pH value is decreased from 10.5 to 1.0, and a pH of 1.0 is found to be the iso-

electric point (IEP) of a kraft lignin. That is, at a pH of about 1.0, the shearing

interface between the bulk liquid phase and the moving particle with the attached

materials is electrically neutralized.

Since there no similar work has been reported, kraft lignin was compared

with some inorganic compounds with respect to the dependence of zeta potential

on pH. The zeta potential dependence of kraft lignin on pH is quite similar to that











0.0 -
0.0 --- -------------------------------------------------

-+ -1. KRAFT LIGNIN
.. -10.0 -
S+ INDULIN AT
. 0 ABAFX2122
w -20.0 -
1- + ABAFX2930
0 +
,o
< -30.0 -
I-l
N +
-40.0


-50.0 I I I I I
0.0 2.0 4.0 6.0 8.0 10.0 12.0
pH


Figure 4-2 The pH Effect on the Zeta Potential of Softwood Kraft Lignins.















Table 4-1 Zeta Potential of Softwood Kraft Lignins at Different pH Values.


ABAFX2122


ABAFX2930


ABAFX4142


ABAFX71


ABAFX73


Indulin AT


pH Zeta Potential pH Zeta Potential pH Zeta Potential pH Zeta Potential pH Zeta Potential pH Zeta Potential
(mV) (mV) (mV) (mV) (mV) (mV)


0.90 0.0
2.00 -12.6
2.51 -23.2
3.33 -31.2
4.93a -39.7
5.44 -40.2
6.50 -42.1
7.72 -43.2
8.31 -41.6
9.41 -41.9
10.31 -41.0


0.91 -4.1
2.03 -15.5
2.61 -26.4
3.51 -32.1
4.36a -43.0
5.55 -44.5
6.70 -43.9
7.80 -42.8
8.93 -45.0
10.23 -46.3


1.09 -1.0
1.84 -15.2
2.58 -26.4
3.39 -31.0
4.34a -38.7
5.52 -41.4
6.73 -42.1
7.51 -42.9
9.10 -41.8
10.51 -43.2


0.99 1.5
1.92 -8.3
2.53 -15.2
3.43 -26.8
4.48a -38.2
5.42 -37.4
6.00 -39.9
7.13 -39.7
8.36 -40.5
9.49 -39.3
10.39 -38.6


0.92 5.0
1.95 -6.7
2.47 -10.3
2.91 -20.4
3.35 -27.5
4.62 a -36.3
5.45 -38.0
6.84 -39.9
7.70 -38.8
8.89 -40.3
10.25 -41.1


Note: a This is the pH of 0.05% lignin suspension in D.I. water.


0.98
1.95
2.39
4.14
4.50
5.15a
6.04
7.40
8.54
9.27
10.35


0.0
-7.1
-16.5
-25.3
-34.4
-36.4
-37.4
-38.8
-39.0
-38.6
-40.0








62
for silica (SiO0). The zeta potential of both kraft lignin and silica is always

negative over a wide range of pH values; however, the IEP of silica is 2.0[96] and

the magnitude of its zeta potential is much higher than that for kraft lignin. As

shown in table 4-1, when suspended in water at a nominal mass concentration of

0.05%, Indulin AT had a pH value of 5.15 (notice that this was measured after the

suspension was withdrawn from the sample cell) and a zeta potential of -36.4mV,

while the experimental kraft lignins had a pH of about 4.50 and a zeta potential of

-38.7 to -43.0mV (except a -36.3mV for lignin ABAFX73). Zeta potentials of

-29.4mV and -31.5mV for a milled wood lignin(MWL) and Klasson lignin of

eucalyptus, respectively, are the only available data that have been reported

previously[149]. Since no pH value was specified, a comparison of these data to

the results from this work is not valid.

It was indicated by data shown Table4-1 and Figure4-2 that the magnitude

of zeta potential of experimental kraft lignins, generally, was higher than that of

Indulin AT, except for ABAFX73. There are at least two possibilities for this

difference. Firstly, different separation procedures may result in different

functionality or different concentration of functional groups on the surface of

lignin. Secondly, higher impurities of electrolytes associated with lignin may

compress the electrical double layer and reduce the magnitude of the zeta potential.

The first possibility will be discussed in Section 4.3.2, and the second will be

addressed in Section 4.3.3.









4.3.2 Origination of Surface Charge



A strong dependence of the zeta potential of lignin on pH indicates that H+

and OH' are the potential determining ions of kraft lignin. This conclusion is

further supported by results shown in Figure 4-4 that an addition of sodium

chloride or calcium chloride does not affect the IEP of kraft lignin. Hence,

ionization is proposed to be the principle mechanism for the origination of surface

charges on kraft lignin, as illustrated in Figure 4-3, equation 4.1. Based on

equation 4.1, the mechanism for the dependence of zeta potential of lignin on pH

is elucidated in equations 4.2 and 4.3.




[R][OH],+xH0O-[O -][R][OHRI]-+x[H30O] (4.1)


[O-][R] [OHJ]-x+yOH --[O0i-] [R][OHJ,_) +4yHzO (4.2)


[O0-][RJ[OH-z,+zH'-[O-][KRJ[OHJ-x+z (4.3)



Figure 4-3 Mechanism of the Origination of Surface Charges on Kraft Lignin
and the pH Effect on the Zeta Potential of Lignin.

Note R is the three dimensional backbone of lignin molecule, OH is either a aryl
or alkyl hydroxyl group, n is the total number of OH groups.








64
As indicated by Figure 4-3, equation 4.1, when a lignin particle is brought

into contact with water ( either in liquid or in air), a number of HW ions may move

away from the surface, and this ionization results in a negatively charged lignin

surface. In an aqueous medium, as the pH increases, more H' ions may dissolute

from the lignin surface into water (Figure 4-3, equation 4.2); subsequently, the

surface charge density increases, resulting in a higher magnitude of negative zeta

potential. As the pH value decreases (Figure 4-3, equation 4.3), the W ion moves

toward the negatively charged lignin surface. Consequently, a lower magnitude of

negative zeta potential results. When the concentration of H+ increases to a point

at which all negative charges at the shearing surface are neutralized, the lignin

particle with the attached materials is then electrically balanced. This pH

corresponds to the IEP of a kraft lignin.

Figure 4-3 is only an oversimplified model. During a pulping or purification

process, more functional groups, such as -COOH, -SO3H, and -SH, etc., may

also be produced on the kraft lignin surface. Certainly, the acid groups will be

more easily ionized to give up HI ions and produce a negative charge on the

lignin surface when brought into content with water. Therefore, a higher

functionality of acid groups can increase the magnitude of zeta potential of kraft

lignin as mentioned in section 4.3.1. Secondly, a single hydroxyl group on the

lignin surface might not give up a HN ion completely; the x, y, and z in the

equation 4.1 to 4.3 are the total effect of interactions between the hydroxyl groups








65
on the lignin surface and the HI and OH" ions in water. They are not necessarily

integers. Moreover, electric double layer theory[87,177,195] predicts that, in the

vicinity of lignin surface, there exists a thin layer of ions specifically adsorbed at

the surface and a diffuse layer of ions in the bulk, due to the effects of Columbic

interactions and thermal motion. Generally, the shearing interface falls in the

diffuse layer. Hence, the magnitude of electrical potential at the real lignin surface

should be higher than the zeta potential measured.



4.3.3 Salt Effects and Adsorption



The influences of monovalent, divalent and trivalent salts on the zeta

potential of kraft lignin are shown in Figure 4-4 to Figure 4-6. While sodium

chloride or calcium chloride only affects the magnitude of the zeta potential of

lignin, zinc sulfate and aluminum chloride cause charge sign reversals on the lignin

surface and result in more than one IEPs in the pH range studied.

Figure 4-4 shows the influence of sodium chloride and calcium chloride on

the zeta potential of kraft lignin in a wide range of pH values. The presence of

sodium chloride reduces the magnitude of the zeta potential, and the magnitude of

the zeta potential of kraft lignin decreases as the concentration of sodium chloride

increases. Calcium chloride affects the zeta potential in the same way as sodium

chloride. This indicates that both Na+ and Ca+ are attracted toward the lignin










' I


S 1.OE-3 M CaCI2
O 6.4E-3 M NaCI
4.9E-5 M CaCI2
A 8.7E-4 M NaCI
I-] 5.0E-4 M NaCI
0 $ No Salt _-


10.0


0.0

E




w -20.0
I-
0
a-
< -30.0

N
-40.0


-50.0


4.0


6.0


8.0


10.0


12.0


pH



Figure 4-4 Zeta Potential of Softwood Lignins as Function of pH with the Presence of
Sodium Chloride or Calcium Chloride.


a I a I I I I a I -


0.0


2.0





I


I








67
surface as counter ions, with a resulting decrease in the magnitude of zeta

potential of lignin. However, the presence of calcium chloride reduces the

magnitude of the zeta potential of lignin more effectively than sodium chloride,

since a Ca" ion carries two positive charges, it has a stronger effect than a mono-

charged Na+ ion. For example, the curve for a concentration of 4.87xl0"5 M

calcium chloride is above the curve for a 8.66x104 M sodium chloride solution.

As mentioned in section4.3.1, the impurity of electrolytes associated with lignin

reduces the magnitude of zeta potential in the same way that these salts do. Also,

neither sodium chloride nor calcium chloride changes the sign of zeta potential

and neither shifts the IEP of kraft lignin.

It is very probable that specific adsorption of Ca" on the lignin surface

does not occur; the sign of the surface charge would be reversed, if it were

chemically adsorbed. In other words, at least one layer of water molecules must

exist between the Ca" ions and the lignin surface at the equilibrium of both

Columbic and thermal interactions.

As shown in Figure 4-5, the presence of zinc sulfate at low concentrations

only reduces the magnitude of the zeta potential of lignin. However, at a high

concentration of about 1.0x 103 M, charge reversal occurs on the surface of lignin.

Consequently, three IEPs appear on the zeta potential versus pH curve. The first

charge reversal (CR1) that occurs at a pH of 1.0 changes the sign of the zeta

potential of lignin from positive to negative the second charge reversal(CR2) at













0.0 --- --------------------------------- -------------
10.0 0
0.0 ----






1--
0
. ZnSO, A q
, -30.0 00oo
I- No Salt
I-
N 0 < 5.OE-5 M
-40.0 1.OE-4 M
O 9.8E-4 M
-50.0 I I
0.0 2.0 4.0 6.0 8.0 10.0 12.0
pH


Figure 4-5 Zeta Potential of A Softwood Lignin as Function of pH with the Presence
of Zinc Sulfate at Different Concentrations.








69
a pH of 8.40 changes the sign from negative to positive, and the third charge

reversal (CR3) at a pH of 8.70 changes the sign from positive to negative. In the

region of pH from 6 to 9, a pH buffering effect was observed when adding base

to increase the suspension pH during the experiment.

Interestingly, the IEP of lignin at 1.0 is unchanged in the presence of zinc

sulfate. If the effects of zinc sulfate(Figure 4-5) and calcium chloride(Figure 4-4)

are compared, the zinc sulfate reduces the magnitude of zeta potential of lignin

less effectively than the calcium chloride, except at the region of charge reversal,

because the double charged SO4"2 ion has a stronger effect than mono-charged Cl"

as a co-ion in the system.

The electrophoretic behavior of kraft lignin at an apparent lignin mass

concentration of 0.05% in the presence of aluminum chloride is shown in Figure

4-6. Over a wide range of pH, the zeta potential of kraft lignin changes to a

positive value due to the presence of aluminum chloride. At a low concentration

of 5x10-5 M aluminum chloride, the IEP of lignin is shifted from 1.0 to about 2.0,

and CR2 and CR3 take place at pHs of 4.5 and 6.60, respectively. With higher

concentrations of aluminum chloride, the first charge reversal disappears, since the

zeta potential changes to positive in a wide range of pHs. The charge reversals of

lignin substrate caused by the presence of different concentrations of aluminum

chloride are summarized in Table 4-2. As the concentration of aluminum chloride












* I


i I


* I


i I


2.0 4.0 6.0 8.0
pH


~~ I


AICb
* No Salt
* 5.0E-5 M
A 1.0E-4 M
* 5.0E-4 M


10.0


Figure 4-6


Zeta Potential of A Softwood Lignin as Function of pH with the Presence
of AICl3 at Different Concentrations.


40.0



E 20.0

h-

z
w 0.0
I--
O


LU -20.0
N


-40.0


0.0


12.0








71
is increased, the CR2 decreases and the CR3 increases so that a broader pH region

of positive zeta potential results.



4.3.4 Charge Reversal and Surface Adsorption



The study of the charge reversal of a substrate caused by the presence of

other materials is an interesting topic. Charge reversal of inorganic compounds has

long been observed63,142,146,147]. The general behavior of charge reversal of a

inorganic substrate caused by hydrolyzable metal ions has been studied by James

and Healy[96,98]. It is reported that[98] a substrate shows an isoelectric point if

HW and OH' are potential-determining ions. When they are not, the substrate will

have a zeta potential of the same sign over the entire pH range. In the presence of

hydrolyzable metal ions, a substrate may have two more charge reversals, CR2 and

CR3. It is believed that[96] the CR1 if any, is the characteristic isoelectric point

of the substrate, the CR3 reflects a coating of the metal hydroxide on the colloidal

substrate, and the CR2 is generally agreed to be the pH of surface nucleation of

metal hydroxide.

In the present study, the electrokinetic behavior of kraft lignin was compared

with some inorganic compounds like SiO2 [96] and TiO2 [215]. Generally, the

behavior of lignin is similar to that of silica. However, the following differences

were also observed. The divalent ion of Zn" reverses the sign of zeta potential








72
of lignin at a high concentration of 9.8x104 M; however, it affects the zeta

potential of kraft lignin at lower concentrations only slightly in the pH range of

the supposed charge reversal. The trivalent ion of Al' shifts the CRI of the

lignin substrate to a higher pH at a low concentration of 5.0x10" M; it reverses the

sign of zeta potential of lignin to positive at a higher concentration of I.Oxl04 M

in the pH range below CR2, and reverses the zeta potential of lignin to positive

at a concentration of 5.0x104 M in the entire region of pH below CR3.

Indifferent adsorption of free ions results in only Columbic interaction so

that the maximum possible effect is to reduce the magnitude of zeta potential to

zero. This can neither reverse the sign of zeta potential nor shift the IEP of a

substrate. Generally, electrophoresis experiments[96] do not indicate any significant

shift in the zeta potential of silica and clays by the metal ions. However, it has

been reported that[210] careful streaming potential studies on vitreous silica

capillaries do indicate some shift for trivalent ions of La(III), and that the La3

ions adsorb [210] specifically (chemically) at the vitreous silica/water interface. As

evidenced in Figure 4-6, therefore, the Al" free ion does adsorb specifically at the

kraft lignin/ water interface so that it shifts the IEP of lignin to higher pH at low

concentration and the strong chemical adsorption reverses the zeta potential from

negative to positive at higher concentrations in a wide range of pH so that the

CRI disappears. In fact, kraft lignin does have better ability to adsorb than SiO2

, TiO2 and clays etc. structurally and functionally. It is believed that the minimum








73
zeta potential between the CR1 and CR2 occurs at a pH for which the

concentration of hydrolyzed species is negligible. In fact, the concentration of these

hydrolyzed products are very low at this point. For example, at an aluminum

chloride concentration of 1.05x103 M and the corresponding pH of 3.7(Figure 4-6),

the maximum possible AI(OH)3 Al+(OH)2 and Al2+(OH) are 1.23x10-34 M,

2.64x10-21 M and 5.26x10'14 M, respectively. Therefore, it is evident that the

charge reversal on the lignin surface is caused by the specific adsorption of Al"

ion at the lignin/water interface below this minimum point.

After passing the minimum point between CR1 and CR2, the hydrolyzed

aluminum concentration increases as the pH increases. Surface nucleation of

aluminum hydroxide on the lignin surface starts when the pH value reaches a

value corresponding to the CR2. This coating process of the charge-induced surface

precipitation continues to the maximum point between CR2 and CR3. Thereafter,

a dual surface colloidal particle with coated and uncoated lignin surface area

predominates the electrokinetic behavior of the lignin particles as pH is increased

further.

Based on a solubility product of 3.7x10-" for aluminum hydroxide[204] and

the observations from Figure 4-6, some interesting results were calculated and

summarized in Table 4-2. The CR2 and CR3 values are directly taken from Figure

4-6; and Kso is the solubility product of aluminum hydroxide found in literature;

the critical pH is a calculated value at which aluminum hydroxide starts








74

precipitating in the bulk phase at the given aluminum chloride concentration; Kss

is an estimated solubility product of the aluminum hydroxide on the lignin surface.

As indicated in Table 4-2, the CR3 increases and approaches the critical pH as the



Table 4-2 Iso-Electric Points of Kraft Lignin Caused by The Aluminum Chloride
Nucleated on The Lignin Surface and The Solubility Product of
Aluminum Hydroxide.


A1C13 CR CR3 Ko Kso Critical pH


5.00E-5 M 4.50 6.60 1.58E-33 3.70E-15 10.62

1.00E-4 M 4.20 7.80 3.98E-34 3.70E-15 10.52

1.05E-3 M <4.00 8.60 1.05E-33 3.70E-15 10.18

Mean 1.01E-33


Note: K3o is the solubility product of aluminum hydroxide ; Kso is the solubility
product of the aluminum hydroxide on the lignin surface ; CR2 and CR3 are the charge reversal
points or the second and third isoelectric points.


aluminum chloride concentration is increased. This indicates that a greater fraction

of the lignin surface has been coated with aluminum hydroxide. If a lignin surface

is fully coated, the CR3 would approach the critical pH. On the other hand, as the

aluminum chloride concentration is increased, the CR2 decreases. However, the

calculated Kso remains almost constant. This indicates that a Ksso value of

1.01x10"33 is probably the characteristic solubility product for the aluminum








75
hydroxide on the kraft lignin surface, and that the aluminum hydroxide on the

lignin surface and the aluminum hydroxide in the bulk are two different materials.

Comparison of Kso and Kso gives a ratio of Kso / Ksso = 3.66x101 which is

much higher than that of inorganic compounds[96].

As discussed above, it is probably unlikely that a divalent ion of Zn+ is

specifically adsorbed on the lignin surface. If so charge reversal would occur at

a lower pH and the IEP of kraft lignin would be shifted (see Figure 4-5). The CR2

and CR3 at pH values of 8.40 and 8.70 and a zinc sulfate concentration of

9.8x10'4M are caused by the nucleation of zinc hydroxide on the lignin surface

and by the coated lignin surface, respectively. Based on a zinc hydroxide solubility

product of 1.8x10'14[204], a critical pH at zinc sulfate concentration of 9.8x10'

M is calculated to be 8.63. This critical pH is in agreement with the CR3 value.

It is probable that a low available lignin surface per unit volume of suspension

resulted in a lignin surface fully coated with zinc hydroxide. The fact that the zeta

potential of lignin does not increase much in the pH range of the supposed charge

reversal at lower concentrations of zinc sulfate was unexpected. Possibly, the

divalent ions of SO"4 in the system play a role, however, there is not sufficient

evidence to confirm this.









4.3.5 Stability



As shown in Figure 4-2, the maximum instability for a kraft lignin/water

system takes place at a pH of 1.0. The lignin in water may be precipitated in the

pH range of 1.0 to 2.5; however, in the presence of mono-valent or divalent ions,

which is the case for black liquor, the instability range becomes broader, depending

on a specific situation. If the lignin is precipitated from water at a high pH like 7

or 8, the product would be a coprecipitate of lignin with some positively charged

species ( see Chapter 5). In the presence of triple charged ions, the system

stability is more complicated, and it is very sensitive to pH and to the

concentration of ions present. Also, the products precipitated at different

conditions might be quite different.



4.4 Summary



Kraft lignin has a negatively charged surface and the negative charges

mainly originate from an ionization of hydroxyl and acid groups on the lignin

surface. When brought into contact with water, kraft lignin has an iso-electric point

of pH = 1.0 and a negative zeta potential over a wide range of pH values above

1.0. The HW and OH- ions are the potential-determining ions of kraft lignin.








77
Mono valent ion of Na+ and divalent ion of Ca+ adsorb indifferently onto

kraft lignin. Their presence only reduces the magnitude of the negative zeta

potential of kraft lignin. They neither shift the iso-electric point nor reverse the

sign of the zeta potential of kraft lignin.

Trivalent ions of Al3 adsorb specifically at the kraft lignin/water interface

at low pH values. This specific adsorption results in a shift of the iso-electric point

to a higher pH and a sign reversal of the zeta potential of kraft lignin.

The presence of some trivalent ions (e.g., Al3+) or divalent ions(e.g., Zn+)

causes three charge reversals on the kraft lignin surface, CRI, CR2 and CR3 in the

order of low to high pH. The CR1 is the iso-electric point of kraft lignin. The

CR2 is the pH at which a surface precipitation of metal hydroxide induced by the

surface charge effect begins The CR3 is the iso-electric point of a dual lignin

surface coated incompletely or completely with the metal hydroxide.













CHAPTER 5
ISOLATION OF KRAFT LIGNIN



5.1 Background



As mentioned in section 2.1, lignin isolation methods can be broadly

classified into two categories based on the purpose of the isolation. The aim of one

category of isolation is to obtain a lignin that is physically and chemically as close

as possible to the protolignin. The other category of isolations deals with the

degraded lignin or lignin derivatives, usually the by-products of a pulping process.

This chapter deals with the latter, and the working materials are the experimental

kraft black liquors produced in experimental pulping according to a statistical

design described in Chapter 8.

Usually, the dissolved lignin is precipitated by the addition of mineral

acids[164], such as sulfuric acid[124,164], hydrochloric acid[79,161,162], or acetic

acid[124,164] to lower the pH of a liquor. Industrially, the established techniques

are based mainly on coagulating acid-precipitated lignin by heating at temperatures

at or near the boiling point of the black liquor or at superatmospheric pressures to

prevent actual boiling[208]. In addition, carbon dioxide[2,11,55], barium








79
chloride[80] and calcium chloride [226] have also been employed to precipitate

lignin from black liquors. The precipitated lignin still contains substantial amount

of impurities from the mother liquor; therefore, subsequent washes[181] and

treatments[79,136] are necessary. Because lignin is sensitive to exposure at high

temperature, freeze drying[111,161,162] should be employed instead of using a

high temperature oven drying. Kim and Fricke[61,111] reported an isolation

procedure, in which a black liquor was precipitated, the lignin was isolated, and

then was redissolved and reprecipitated. Alen and colleagues[2] studied the

isolation of kraft lignin with carbon dioxide and found that high pressure shortened

the precipitation time and that the initial pH values of black liquor greatly affected

lignin yield. Whalen and coworkers[207,208] reported that a presence of about one

percent of chloroform or certain other organic liquids causes precipitation of

agglomerated and granular lignin[207]. This lignin settled rapidly and could be

separated easily by filtration and decantation. In the acid precipitation process, the

final pH values reported generally vary from one to three( 1.0[79], 2.0[111,208],

2.5[182],and 3.0[161,162]). However, Gupta and Goring[79] reported that lignin

precipitated sharply at a pH of 4.4, and that fractionation by gradual acid

precipitation was impossible. No rationale has been given for selection of the final

pH, nor for its effect on the purified lignin. In our previous work, it seemed that

a summation of the different lignin fractions from a black liquor might well be




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