DECONVOLUTING CHROMOPHOREFORMATION AND REMOVAL

DURING KRAFT PULPING INFLUENCE OF METAL CATIONS

Thomas J. Dyer, Art J. RagauskasInstitute of Paper Science and Technology

School of Chemistry and BiochemistryGeorgia Institute of Technology

Research Objective

To contribute to our understanding of the fundamental nature of chromophore formation during kraft pulping

Characteristic red-brown color of pulp obtained by cooking wood with a liquor containing sodium hydroxide and sodium sulfide– Varies in intensity and shade

according:- Wood species- Cooking technology- Cooking parameters

The Problem…• Holzer/1934: Presence of sulfur darkens the color of

kraft pulp more than that of a comparable soda pulp

• Bard/1941: Color may be produced by adsorption or absorption of colored material from the black liquor

• Pigman and Csellak/1948: Among the first to pin-point lignin and its degradation products as responsible for the bulk of the color found in kraft pulps, possible carbohydrate contribution

• Hartler and Norrström/1960, 70’s: Overall, the contribution from carbohydrates is low throughout the cook

Kraft Pulp Color• Proposed sources

– Extractives• Tannins

– Lignin and its reaction products– Carbohydrate degradation products

• Lignin and its degradation products– Found to be most responsible for

color in kraft BL• Pigman and Csellak (1948)

• Non process elements– Calcium– Iron– Copper, Aluminum, Magnesium

Possible Chromophoric StructuresL

O

O

O

O

OCH3L OM

O

L

L

O

O

HO

L

OH

O

O

L

OH

OCH3

Ortho-Quinone Para-Quinone Catechol-Metal Complex

Hydroxy-Quinone

Stilbene or Enol EtherL O

OCH3

OHO

O

L

Alpha-Carbonyl Stilbene-Quinone Carbohydrate Derived

(Conjugated Carbonyl, Aromatic, Furan Derivatives)

Transition Metal Complexes

• Transition metals– May form complexes with

catechols• 6-7/100 C9 units in kraft

lignin• Trace in residual lignin

• Ferric Ion Complexes– λmax = 500-550 nm

Addition NPE Studies

Jameson and Wilson, 1972; Ghosh, A. and Y. Ni, (1997)

Gellerstedt, G. and W. W. Al-Dajani (2001); Sundin, J. and N. Hartler (2000)

O

M

O OH2

OH2

O

M

O

O

O

Experimental Design

• Two central composite designs– Objective 1

• Constant kappa number ∼ 30• 27.6 – 30.9

– Four variables • Extractives• % EA (14 - 21%)• % Sulfidity (23 - 57%)• Maximum temperature (162 - 178°C)

– 40 experiments

x3

x2

x1

Experimental Parameters

• 100 g of southern pine wood chips– Extracted vs. unextracted

• Pulped to H-factor– From equation

• Disintegrated• Washed thoroughly• Screened

Experimental Results – Total Pulp Color

• Total Color– % EA, % Sulfidity

• Significant parameters– Max. Temperature

• Not significant– Curvature

• Due to quadratic relationship

160 150 140 130 120 110

Pulp Color Measurements via diffuse reflectanceIntegrate k/s curve over visible region (400 – 700 nm)

Experimental Results – Total Pulp Color

• Total Color– Minimal color

• ↑ % EA, ↓ % Sulfidity– Maximum color

• ↓ % EA, ↑ % Sulfidity

• Brightness– Showed similar trends

160 150 140 130 120 110

Impact of Extractives

• Extracted vs. unextracted wood chips– 0.06% vs. 1.80% extractives

• Extracted vs. Unextracted– Statistically the same kappa pulps

at 95% CI

28.6629.42Average Kappa #

1.051.06S.D.

1.24t (calculated)

0.19p-value

ν = 10t > 2.228 withCritical region:

Extracted Wood Chips

UnextractedWood Chips

H0: µ1 = µ2H1: µ1 ≠ µ2

Impact of Extractives

• Total Color– Extracted vs. unextracted

• ANOVA– Indicates the two are

significantly different

• Brightness– Extracted vs. unextracted

• ANOVA– Indicates the two are

not significantly different80

90

100

110

120

130

140

150

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

Sample

Tota

lCol

or

Unextracted Wood Chips Extracted Wood Chips

Impact of Transition Metals

• Potential contributors– Fe, Mg, Al, Mn, Ca, etc.

• Which are important??– Measured metals

• ICP

• Most other metals had a significant amount of variation when compared against pulping conditions, except S and Ca.

• Higher cooking sulfidity higher S in pulp

600

800

1000

1200

1400

1600

1800

2000

0.50 0.55 0.60 0.65 0.70 0.75 0.80 0.85 0.90

Initial EA (mol/L)

Cal

cium

(mg/

kg p

ulp)

Higher EA Lower Ca• Consistent with Gustavsson et al

Nordic Pulp and Paper Research Journal 14(1): 71-81 (1999)

Ca2+ vs. EA

Relationship Between Pulp Color and Ca++

60

70

80

90

100

110

120

130

140

150

800 1000 1200 1400 1600 1800 2000

Calcium Content (mg/kg pulp)

Chr

omop

hore

Inde

x

no good correlation between the pulp chromophore index and the iron in the pulp

How much is due to metals vs. other components

Examining The Relationship Between Color and Pulp Metals

Experimental Procedure

pH 3.0, 4ºC

48 hour

Ca, Mg, MnAll reduced 77 - 88%

Fe, AlTypically 10 – 30%

OH

HO

OOH

HO2C

OHO Xylan

Decrease in HexA 2-5%

Relationship Between Pulp Color and Ca++

50

60

70

80

90

100

110

120

130

140

150

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

Sample

Chr

omop

hore

Inde

x

Before Treatment After Treatment• Overall 48% reduction in color

•Sample 2 high effective alkali (20%), moderate sulfidity (30%)

•Sample 7 low effective alkali (16%) and a high sulfidity (50%)47%

Ca: 1080 ppm

55%Ca:1430

Principle component analysis examined source of variation in chromophore index 98% of the variation in chromophore index could be accounted through Mn, Mg, Ca at constant kappa number

Charting Color Formation Through Kraft Pulping

– Two pulping conditions• High %EA, Low % Sulfidity

– Low Color• Low %EA, High % Sulfidity

– High Color

Color Formation vs. Time

50

100

150

200

250

300

50 70 90 110 130

Cooking Time (minutes after 100oC)

Chro

mop

hore

Inde

x

21.4% EA, 23.2% Sulfidity 14.6% EA, 56.8% Sulfidity

Brighter pulp

Color Formation vs. Lignin Content

50

100

150

200

250

300

0 5 10 15 20 25Klason Lignin Content (%)

Chro

mop

hore

Inde

x

21.4% EA, 23.2% Sulfidity 14.6% EA, 56.8% Sulfidity

Brighter pulp

ESCA

• Electron Spectroscopy for Chemical Analysis– Bombard surface with x-rays

• Substrate ejects electrons– Specific binding energy– Depends on type of atom

• Measures 2-9 nm into surface

• Treated paper samples– Mercuric acetate

• Specific for lignin– Westermark (1999)– Heijnesson et al. (2003)

X-raysource

Sample

Channeltrondetector

Electrons

Analyser

Surface Lignin vs. Bulk Lignin

R2 = 0.99

01020304050607080

0 5 10 15 20 25Klason Lignin Content (%)

Surfa

ce L

igni

n Co

nten

t (%

)

21.4% EA, 23.2% Sulfidity 14.6% EA, 56.8% Sulfidity

Conclusion: Color Differences are NOT Due to Difference in Surface Lignin Content

Other Parameters Must Be Involved!

Color Formation vs. Surface Lignin

50

100

150

200

250

300

0 10 20 30 40 50 60 70 80Surface Lignin Content (%)

Chro

mop

hore

Inde

x

21.4% EA, 23.2% Sulfidity 14.6% EA, 56.8% Sulfidity

Brighter pulp

Impact of Calcium

• Calcium– Studies by Sundin & Hartler

• Lignin precipitation

– Li and Reeve• Darker lignin• Precipitate on surface

0

5

10

15

20

25

900 1400 1900 2400 2900 3400 3900

Calcium Content (ppm)

Klas

on L

igni

n Co

nten

t (%

)

21.4% EA, 23.2% Sulfidity 14.6% EA, 56.8% Sulfidity

Ca Mg Mn Fe

D T P A

A c id

U n tre a te d

0

500

1000

1500

2000ppm

DTPAAcidUntreated

Kappa 30

Impact of Metals

1. Q (Acid or DTPA)

2. Kraft Pulping

Ca Mg Mn Fe

D T P A

A c id

U n tre a te d

0

100

200

300

400

500

600ppm

DTPAAcidUntreated

Wood

Impact of Metals1. Q

2. Kraft Pulping

134

136

138

140

142

144

146

148

150

152

154

Chromophore Index

Untreated Acid Chelated

Kappa 30

DECONVOLUTING CHROMOPHOREFORMATION

For the Pulps Examined• Non process elements are a

major contributor to color• Ca, Mg, Mn are key

contributors• Wood NPE pre-extraction is

important

But is it important??

Mill Pulp Properties

• Two pulp mills– Similar products– Similar pulping conditions– Same wood source

• Observations– Lower kappa pulp-Pulp B

Tappi Brightness: 18

- Higher kappa pulp-Pulp ATappi Brightness: 27

65.6Reel83.9Reel

64.9Blend Chest

79.7Refiner Outlet

59.8Reg. Box77.7Refiner Inlet

55.1Washer73.0Washer

Average Kappa

Number

Pulp BAverage Kappa

Number

Pulp A

Mill Pulps – Transition Metals

2470

2403

3033

Ca(mg/kg)

44

40

57

Mn(mg/kg)

513

439

490

Mg(mg/kg)

1517

1507

1202

Ca(mg/kg)

143

140

128

Mn(mg/kg)

331

355

285

Mg(mg/kg)

33Blend Chest

23Refiner Outlet

32Reg.Box

29Refiner Inlet

37Washer19Washer

Fe (mg/kg)

Pulp BFe(mg/kg)

Pulp A

Process parameters were implemented that changedNPE’s and brightness values of Pulp B were raised

Conclusions

Conclusions

Overall color of kraft pulp– Influenced by pulping parameters

• % EA, % Sulfidity are significant• Maximum temperature not significant within

experimental limitations

Chromophore content– Changes with pulping, depending on conditions– More surface lignin needed for light colored

pulp to obtain same chromophore content

Conclusions• Differences in optical properties can not be attributed to surface lignin

concentration for pulps studied

• Non Process Elements: Ca, Mg, Mn significant contributor to pulp color

BUT – Prior EWLP reported studies • Quinones, Condensed Phenolics, Aliphatic hydroxyls

– Appear to be contributors to the color difference of kraft pulps studied

• Aliphatic carbonyl, Noncondensed phenolics, Catechols– Do not Appear to be Important contributors to the color difference

of kraft pulps studied

Acknowledgements

DOE, USDAMember Companies

of IPST@GT