Transcript No Slide Title

Pulping and Bleaching
PSE 476
Lecture #3
Mechanical Pulp Bleaching
Agenda
• Mechanical pulp bleaching overview
• Reductive bleaching
» Chemistry
» Effect of variables
• Oxidative bleaching
» Chemistry
» Effect of variables
Mechanical Pulp Bleaching
• After the refining process, the resulting fibers
do not have the brightness required for most
paper applications. Bleaching is therefore
required.
• Goal of mechanical pulp bleaching:
» Brighten the pulp with little yield loss.
• There are two approaches to this problem:
» Reducing the colored structures.
» Oxidizing the colored structures.
Example of Chromophores Found
in Mechanical Pulp
CHO
HC
CH
CH
O
OCH3
O
OCH3
OH
O
O
Coniferaldehyde
10-20%
Adsorbed Light
C
OCH3
p-Quinones
O
OCH3
O
o-Quinines
30 - 65%
Adsorbed Light
CH CH
p-Quinone Methide
OH
O
O
H3CO
OCH3
OCH3
OH
o-Quinone Methide
p, p' Stilbene Quinone
O
Fe
Chelated Metals
Mechanical Pulp Bleaching
Reducing Bleaching Chemistry
• Reduction of chromophores (colored species) to
leucochromophores (uncolored species).
• Sodium dithionite (hydrosulfite Na2S2O4): most
commonly used chemical. Breaks down (dissociates)
into reactive species:
- Sulfur dioxide radical ion SO2-.
- Sulfur Dioxide SO2
- Sulfur Dioxide dianion (sulfoxylate) SO22-
• Dithionite is sold as a stabilized powder or produced
on site from sodium borohydride and sodium
bisulfite.
Text
Lignin Preserving Bleaching
Reducing
• The object is the reduction of chromophores, ie
unsaturated structures like the quinone shown below.
-
Na2S2O4 + 4HO
2Na2SO3 + 2H2O + 2e
Na2S2O4
O
OH
O
OH
Colored
Not colored
-
The last step
is photoyellowing
O2
Light
O
O
Colored
Mechanical Pulp Bleaching
Brightness Gain Using Hydrosulfite
• Reducing bleaching not as
efficient as oxidative
(H2O2).
• Factors influencing
brightness
BrightnessIncrease
Increase(%
(%MgO)
MgO)
Brightness
12
12
10
10
88
66
44
22
00
00
88
16
16
24
24
32
32
40
40
48
48
Pounds
Poundsof
ofBleach
Bleachper
perTon
Tonof
ofPulp
Pulp
Source: Pulp and Paper Manufacture, Volume 2 Mechanical Pulping, page 229
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Amount of bleach
Temperature
Time
pH
Pulp consistency
Chelating agents
• Color reversion a big issue.
• Wood species important.
Mechanical Pulp Bleaching
Hydrosulfite: Effect of Variables
Factor
Factor
Ranges
Ranges
Addition
AdditionRate
Rate
5-20
5-20lb/ton
lb/ton
NA
NA
Temperature
Temperature
>30
>30C
C
60C
60C
20-60
20-60min
min
4.5-6.5
4.5-6.5
Variable
Variable
Dependent
Dependent
4.5-5.5
4.5-5.5
3-5%
3-5%
??
2-5
2-5lb/ton
lb/ton
pulp
pulp
??
Retention
RetentionTime
Time
pH
pH
Consistency
Consistency
Chelating
ChelatingAgent
Agent
Optimum
Optimum Results
Results
7-10
7-10brightness
brightnesspoints
pointsatatstandard
standard
range
range
At
At30C
30Cno
nobrightness
brightnessobtained,
obtained,
higher
temps
than
60
C
higher temps than 60 Cmore
more
brightness
and
more
brightness
brightness and more brightness
reversion
reversion
Retention
Retentiontime
timedependent
dependentupon
uponother
other
variables:
variables: temp,
temp,charge
charge
Lower
LowerpH
pHresults
resultsininfaster
fasterbleaching.
bleaching.
3-5%
3-5%does
doesnot
notaffect
affectbrightness.
brightness.
Need
Needtotofind
findresults
resultson
onnewer
newerhigh
high
consistency
studies
consistency studies
Various
Variousagents
agentscan
canbe
beused
usedtoto
eliminate
eliminatethe
theeffect
effectofofmetals
metals
Mechanical Pulp Bleaching
Oxidative Bleaching Chemistry
• Peroxide oxidizes chromophores to uncolored
species.
• Reactive species: hydroperoxy anion HO2• Reactions very pH dependent.
» At pH 10.5, only 10% of H2O2 is the hydroperoxy anion.
» At higher pH’s, there is more of the anion but also more
decomposition of the peroxide to oxygen and water.
» NaOH and sodium silicate are used to control pH. Silicates
are added to stabilize peroxides.
• MgSO4 and chelating agents added to slow the metal
induced decomposition of H2O2.
Lignin Preserving Bleaching
Oxidative
• Unlike reductive bleaching, the colored compounds are
destroyed so they cannot reform (ring cleavage reactions).
• Hydrogen (mostly) or sodium peroxide are used.
• Active species: perhydroxyl (or hydroperoxy) anion HO2(pH 9-11)
» Reacts with carbonyl structures.
• Important to stabilize metals to lower radical formation.
» Magnesium silicates or chelating agents are added.
(-)
OOH
OH
O
O
O
O
(-)
O
(-)
OH
O
O
O
CO2H
CO2(-)
Mechanical Pulp Bleaching
Brightness Gain Using Peroxide
• Higher brightness reached
than with hydrosulfite.
• Factors influencing
brightness
16
16
Brightness
Brightness
12
12
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88
44
00
00
20
20
40
40
lb
lbperoxide/ton
peroxide/tonpulp
pulp
60
60
Amount of bleach
Temperature
Time
pH
Pulp consistency
Silicates/Chelating agents
• Color reversion a big issue.
• Wood species important.
Mechanical Pulp Bleaching
Peroxide: Effect of Variables
Factor
Range
Results
Bleach charge
1 to 2% on pulp
Amount charged effects amount of brightness.
Also depends on how much degrades
Too low pH = very slow bleaching, too high pH
= degradation of peroxides. Peak point
Higher temp increases rate. Most of the pulp
brightening in first portion of reaction
The higher the consistency, the better the
brightening
Rate dependent on the temperature
pH
9-12
Temperature
40-70 C
Consistency
20-30
Time
10-60 min
Silicates
1-5% wt%
Chelating agents
MgSO4 7H2O
0.25%
0.05%
Increased silicates increases brightness. From 1
to 5% silicates = 3 point brightness gain
DPTA, etc. slows the degradation of peroxide
Epsom salts stabilizes peroxide during storage
Photoyellowing
• This is the process your newspaper goes through on the
front porch in the sunshine.
• Yellowing initiated by lignin chromophores adsorbing UV
light (300-400nm).
» Oxygen essential to process.
• Radical are formed (both organic and oxygen radicals).
• Lignin is degraded, b-O-4 linkages broken, methoxyl
groups lost.
• Reaction products of these radical processes include
carbonyls, quinones, acids, and aldehydes.
» First 2 of this list can be very colored.