Basics of pulp bleaching

Bleaching Chemicals

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Basics of
Bleaching Chemical Pulps
Art J. Ragauskas
Institute of Paper Science and Technology
Georgia Institute of Technology

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Definition
Chemical treatment to:
¾
Increase brightness
¾
Improve cleanliness
¾
Improve brightness stability
¾
Remove hemicellulose
¾
Remove extractives

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Bleaching: A Working Definition
™
Removal of colored residual lignin from chemical
pulp (usually kraft) to increase its brightness,
cleanliness and other desirable properties, while
preserving the strength (cel lulose integrity) and
carbohydrate yield (cellulose and hemicellulose) of
the unbleached fiber, with due regard for potential
effects on the environment

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Bleaching vs. Pulping
‰
Compared to pulping, bleaching
¾
is also a delignification process
¾
is more selective in terms of yield and d.p.
¾
is more expensive per unit of lignin removed
¾
removes less lignin
¾
produces problematic effluent

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Bleaching vs. Pulping

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Bleaching Effluents: Processed in an
Environmental Acceptable Manner

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Relevant Statistics
¾70 million tons/year bleached
¾28 million tons in USA
¾12 million in Canada
¾Most used in grades needing strength
¾Proportion of pulp used on site is
¾74% in USA
¾30% in Canada

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Why Kraft Pulp Needs To Be
Bleached
ƒDark color of unbleached pulp is due to
residual lignin, which remains in the pulp
because of its
ƒhigh molecular weight
ƒhydrophobic nature
ƒchemical bonds to carbohydrates

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Bleaching Chemistry
™Bleaching chemicals are oxidizing agents
that
™break up the lignin molecule
™introduce solubilizinggroups into the
fragments
™disrupt lignin-carbohydrate bonds, allowing
fragments to dissolve

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Important Pulp Properties
Macroscopic ƒ
Kappa Number
ƒ
Brightness
ƒ
Opacity
ƒ
Tear Strength
ƒ
Viscosity
ƒ
Dirt Content
Microscopic ƒ
Residual Lignin
Content
ƒ
Light Absorption
Coefficient
ƒ
Scattering Coefficient
ƒ
Fiber Strength
ƒ
Cellulose Mol. Wt.

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Kappa Number
¾
Amount of oxidizing agent (KMnO
4
)
consumed by 1 gram of pulp
¾
Measures lignin content linearly
¾
Kappa No. = 6.7 times lignin content in %
¾
Successor to K no.
¾
K no. = 2/3 Kappa no.

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Light Absorption Coefficient ¾
Light passing through a
solution can be either
absorbed or transmitted
¾
Capacity of solute to absorb
light of given wavelength
given by Beer’s law
¾
Molar extinction coefficient
is characteristic of solute
¾
Different wavelengths are
absorbed differently
log
0
II
cl
⎛
⎝
⎜⎞⎠ ⎟=⋅⋅
ε

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Light Absorption by Pulp Sheets
¾
The product of molar extinction coefficient and
concentration is replaced by k, the absorption
coefficient
¾
The visible light absorbed by lignin is in the blue
range (400-500 nm), making it appear yellow.
¾
Bleaching removes lignin and therefore decreases the
absorption coefficient in this range. It is measured at
457 nm.

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Light Scattering by Pulp Sheets
¾
Pulp sheets are physically more complex than
solutions so they can scatterlight as well as
absorb or transmit it.
¾
The tendency of a sheet to scatter light can be
quantified in terms of s, thescattering
coefficient
¾
sdepends on free surface within sheet, and
therefore on degree of bonding

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Fate of Light Shining on Paper

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Fate of Light Shining on Paper
¾Light shining on a sheet of paper can
¾be specularlyreflected, to the extent that the
sheet is glossy
¾be reflected and scattered, to the extent that the
sheet is opaque
¾be transmitted, i.e., pass right through the sheet,
to the extent that the sheet is translucent (lacks
opacity)
¾be absorbed, to the extent that the sheet is
colored

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The Kubelka-MunkTheory
The fraction of light reflected by a thick sheet of
paper, called reflectance, can be calculated from 2
basic properties -absorption coefficient, k, (color)
and scattering coefficient, s (content of reflecting
surfaces within the sheet). The absorption coefficient
is a measure of how much light is absorbed and the
scattering coefficient a measure of how much is
reflected and scattered. Reflectance decreases as k
increases and increases as s increases.

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The Kubelka-MunkEquation The fraction of light reflected by a thick sheet of paper
can be calculated from 2 basic properties -absorption
coefficient, k, and scattering coefficient, s. Reflectance
decreases as kincreases and increases as sincreases.
R
k
s
k
s
k
s
∞
=+−
⎛
⎝
⎜
⎞
⎠
⎟+
⎛
⎝
⎜
⎞
⎠
⎟ 12
2

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Light Scattering and Absorption
ww
Incident light Incident light
Scattered light Scattered light
Transmitted light Transmitted light Absorbed light Absorbed light

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Brightness
¾
Reflectance of blue (457 nm) light from a thick
sheet
¾
Sheets having a low brightness appear yellow or
brown; those having a high brightness appear
white.
¾
Brightness may be increased by decreasing the
absorption coefficient (color intensity)
¾
Brightness may be increased by increasing the
scattering coefficient (e.g. snow vs. water)

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TAPPI Brightness
¾
TAPPI Test method T452
¾
45
o
illumination and 0
o
(perpendicular)
viewing
¾
Expressed as % of MgOreflectance

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TAPPI Brightness Measurement
Geometry

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ISO Brightness
¾
Diffuse illumination, 0
o
viewing
¾
Expressed as absolute reflectance
¾
Less dependent on surface characteristics

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ISO Brightness Measurement
Geometry

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Brightness Stability
¾
kincreases on exposure to heat or light
¾
Phenomena known as reversion, brightness reversion, color
reversion, thermal reversion, yellowing, photoyellowing
¾
Can be expressed as post color number, the change in k/s.
k
s
R
R
=
−
∞
∞
()1
2
2

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Kubelka-Monk Theory
iiixsxdw ixsxdw
ii--i(s+k)xdw i(s+k)xdw
s = scattering coefficient s = scattering coefficient
k = absorption coefficient k = absorption coefficient
Theory of light scattering. Developed for Theory of light scattering. Developed for
paint films. Assumes homogenous paint films. Assumes homogenous
material small particles material small particles
considers diffuse reflectance and considers diffuse reflectance and
transmittance of light transmittance of light
2 material properties describe 2 material properties describe
scattering of light scattering of light
i = intensity of light i = intensity of light

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Reflectivity
Reflectivity depends only on scattering and Reflectivity depends only on scattering and
absorption coefficients, s and k absorption coefficients, s and k
RR
∞
=Total reflectivity from a thick pile of paper =Total reflectivity from a thick pile of paper
Reflectivity is a measure of reflected light Reflectivity is a measure of reflected light
RR
∞
= 1 + k/s + [(k/s) = 1 + k/s + [(k/s)
22
+2k/s] +2k/s]
0.50.5

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Brightness
Brightness is the R Brightness is the R
∞
value measured using a value measured using a
blue light source having a wavelength of blue light source having a wavelength of
457nm 457nm
R=100% corresponds to reflectance off R=100% corresponds to reflectance off
magnesium oxide magnesium oxide

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Reflectivity
RR
0
=Total reflectivity from a single sheet with =Total reflectivity from a single sheet with
a black background a black background
Reflectivity depends only on scattering and Reflectivity depends only on scattering and
absorption coefficients, s and k absorption coefficients, s and k
RR
∞
= 1 + k/s + [(k/s) = 1 + k/s + [(k/s)
22
+2k/s] +2k/s]
0.50.5

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RR
0
and R and R
∞
are used to measure s and k are used to measure s and k
k/s = (1 k/s = (1--R R
∞
))
22
/2 R /2 R
∞
s= (1/b)[R s= (1/b)[R
∞
/(1/(1--R R
∞
22
)] ln[R )] ln[R
∞
(1(1--RR
0
R R
∞
)/(R )/(R
∞−
RR
0
)])]
b = grammage b = grammage

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Effect of Backing on Reflectance

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Opacity for Paper
When you Increase When you Increase
Opacity Will Opacity Will
Basis Weight Basis WeightIncrease Increase
density densityDecrease Decrease
% filler % fillerIncrease Increase
coating weight coating weightIncrease Increase

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Opacity Measurements
Backing plates Backing plates
Sheet Sheet
light lightdetector detector

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Non-Linear Relationship of Brightness to
ChromophoreConcentration

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Opacity
¾
Ratio of reflectance over black backing to
reflectance over white backing, expressed as a
percentage (TAPPI Methods T425 and T519)
¾
Increases as sincreases
¾
Increases as k increases
¾
Affected by bonding and fillers
¾
Bleaching affects k, usually not s

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Opacity
Opacity = Opacity =
Reflection factor of a sheet Reflection factor of a sheet
against a black backing against a black backing
Reflection factor of a large Reflection factor of a large
stack of sheets stack of sheets
Tappi Opacity = Tappi Opacity =
Reflection factor of a sheet Reflection factor of a sheet
against a black backing against a black backing
Reflection factor of a sheet Reflection factor of a sheet
against a standard backing against a standard backing

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Gloss
00ºº
9090ºº
7575º
Gloss Gloss
Intensity Intensity
Incident Light Incident Light

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Optical Testing

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Examples of Formation

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Fiber Wall Structure
¾
Wood cells, and therefore pulp fibers, are
hollow, elongated structures made up of
cellulose microfibrilsin a matrix of
hemicellulose and lignin
¾
Microfibrilseach consist of perhaps 36 parallel,
hydrogen bonded cellulose molecules
¾
Microfibrilsin the dominant wall layer, S2, are
helically wound at a low angle

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Fiber Wall Structure(2)
Warty
Inner Layer (S3)
Middle Layer (S2)
Outer Layer (S1)
Primary Wall
Middle Lamella

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Fiber Strength
¾
Tensile load on fiber is borne by microfibrils
and therefore by cellulose molecules
¾
Strength of structure decreases as the length of
its component molecules is decreased
¾
Cellulose chain cleavage by bleaching agents
can therefore reduce fiber strength
¾
The relationship between cellulose d.p. and
fiber strength is, however, nonlinear

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Fiber vs. Sheet Strength
¾
Fiber strength loss shows up first as loss in
tear strength
¾
Tear failure usually involves fiber breakage,
while tensile failure usually involves fiber
pullout

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Monitoring Fiber Strength
¾
Single fiber testing
¾
Zero-span tensile strength
¾
Tear vs. tensile curves
¾
Viscosity

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Pulp Viscosity
¾
Measured by dissolving pulp fibers in CED and
observing the time taken for the solution to pass
through a standard capillary (TAPPI Test Method
T230)
¾
Viscosity is expressed in centipoises (cp) or
millipascal-seconds (mPa.s), which are
numerically the same
¾
It is related to cellulose mol. wt. and indirectly to
fiber strength

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Non-Linear Relationship of
Strength to Viscosity
∆Conventional bleaching of conventional pulp
{Conventional bleaching O-delignifiedpulp
…Acid hydrolysis of bleached pulp

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Dirt
¾
Dark colored foreign matter
¾
Measured by T213 as equivalent black area
¾
From wood: bark, resin, sand, shives
¾
From process: carbon, sand, rust, rubber,
scale
¾
Other: plastics, grease, fly ash

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Bleaching Chemicals
Name Formula Symbol Chlorine Cl
2
C
Chlorine Dioxide ClO
2
D
Oxygen O
2
O
Hydrogen Peroxide H
2
O
2
P
Sodium Hypochlorite NaOCl H
Hypochlorous Acid HOCl M
Ozone O
3
Z
Sodium Hydroxide NaOH E

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Bleaching Chemical
Characteristics
¾
Equivalent Weight
¾
Efficiency
¾
Reactivity
¾
Selectivity
¾
Particle Bleaching Ability
¾
Environmental Implications

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Equivalent Weight
¾
Bleaching is an oxidation process
¾
Bleaching chemicals are oxidizing agents
¾
One equivalent weight of a bleaching chemical
is the weight of that chemical that is required
to do a specified amount of oxidation.
¾
Equivalent weight is therefore an inverse
measure of oxidizing power

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Equivalent Chlorine
¾
Equivalent chlorine is another way of expressing a
bleaching chemical’s oxidizing power
¾
It is defined as the number of pounds (or kg) of
chlorine that has the same oxidizing power as one
pound (or kg) of the bleaching agent in question
¾
Equivalent chlorine is therefore a direct measure of
oxidizing power

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Efficiency ¾
Some of the oxidizing power of a bleaching agent is
always wasted in side reactions
¾
Some bleaching agents are more prone than others to
undergo wasteful reactions; conversely, some use their
oxidizing power more efficiently than others
¾
Efficiency is a measure of the degree to which a
bleaching agent’s oxidizing power is used in desirable,
lignin-degrading reactions

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Equivalent Wt. and Efficiency L=Low M=Med. H=High
Chemical Equiv.
Weight
Equiv.
Chlorine
Efficiency
Cl
2
35.5 1.00 H
ClO
2
13.5 2.63 H
O
2
84.44 L
H
2
O
2
17 2.09 L
NaOCl 37.2 0.93 M
O
3
84.44 H

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Equivalent Chlorine Charge
9
The charge of chlorine or chlorine dioxide in the
first bleaching stage that employs either is often
expressed as kappa factor, sometimes also called
active chlorine multiple
KappaFactor
P
ercen
t
EqCl
KappaNo
=
.
.
2

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Reactivity and Selectivity ¾
Reactivity may be defined in terms of the
fraction of the residual lignin that the bleaching
agent is practically capable of removing
¾
Selectivity is the degree to which the bleaching
agent can remove lignin without dissolving or
damaging the other components of the fiber,
cellulose and hemicellulose

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Reactivity and Selectivity L=Low M=Med. H=High
Chemical Reactivity Selectivity
Cl
2
HH
ClO
2
MH
O
2
LM
H
2
O
2
LH
NaOCl M M
O
3
HM

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Particle/Dirt Removing Ability
and Environmental Implications ¾
Different bleaching agents differ in their ability to
remove dirt particles, a very important characteristic
¾
For good dirt removal, ch emical reaction with lignin
must be slow enough to allow time for diffusion of
chemical into particles
¾
Different bleaching agents engender different levels of
concern for the environment; whether the concern is
justified may be irrelevant

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Dirt and Environmental L=Low M=Med. H=High
Chemical Dirt
Removal
Environmental
Implications
Cl
2
HH
ClO
2
HM
O
2
ML
H
2
O
2
LL
NaOCl H H
O
3
LL