Title: Pulping and Bleaching PSE 476Chem E 471
1Pulping and BleachingPSE 476/Chem E 471
- Lecture 17
- Introduction to Bleaching
2Agenda
- Brightness
- General Bleaching Principles
- Chemistry
- Process
- Chemicals
- Description
- Advantages/Disadvantages
3Why Bleach?
- Improve brightness.
- Improve brightness stability.
- Clean up pulp (impurities).
- Wood based (bark, resins, sand, shives).
- Process based (carbon specs, rust, rubber).
- External sources based (plastics, grease, ash).
- Increase capacity of paper to accept printing.
- Remove impurities from pure cellulose (rayon).
4Brightness Determination (1)
Light shinning on a sheet of paper is either
transmitted, adsorbed, or reflected.
- Light is scattered by fibers at air/fiber
interfaces - Light is adsorbed by certain chemicals in the
fibers (lignin)
5Brightness Determination (2)
- Brightness is measurement of how much light is
reflected from a sheet of paper. - Whiteness does not mean brightness.
- Whiteness is a physical phenomena related to how
the eye views the paper. - A very white looking piece of paper may not have
high brightness. - Example blue dye added to a yellow tinged sheet
of paper will give a white sheet of paper with
low brightness.
6Brightness Determination (3)
- Brightness determination method
- Light reflectance measured and compared to light
reflectance from MgO. - MgO assumed to reflect 100 light.
- Brightness is reported as of MgO reflectance
(85 brightness is equivalent to 85 of MgO). - Variables
- Angle of light Light is applied to sheet at 45
angle. - Wavelength 457 nm (blue light most sensitive).
7General Principles
- Two types of bleaching
- Lignin removing chemical pulps.
- Lignin retaining mechanical pulps.
- Bleaching is used because at a certain point in
the pulping process, carbohydrate degradation
becoming greater than lignin removal. - Bleaching chemicals are more selective for
lignin. - Bleaching chemicals much more expensive than
pulping chemicals so they are not used in
pulping.
8General PrinciplesChemistry
- Pulping
- Pulping typically involves cleavage of ether
linkages and some substitution (sulfonation). - Bleaching
- Bleaching involves attacks on aromatic rings,
olefinic structures, and carbonyl groups. - Substitution reactions play a big role.
9General PrinciplesProcess
- Bleaching uses a combination of chemicals in
series. - One chemical alone will not remove residual
lignin. - Each step reacts with material modified in
previous step.
NaOH
NaOH
ClO2
ClO2
O2
O2
Unbleached
O
D
D
EO
Bleached Pulp
Pulp
10General PrinciplesChemicals (1)
11General PrinciplesChemicals (2)
12General PrinciplesChemicals (3a)
13General PrinciplesChemicals (3b)
14General PrinciplesChemicals (3c)
15Bleaching Generalities
- It is important to note that when bleaching with
a specific reagent, it will be converted into a
number of different reactive species which will
react with lignin and carbohydrates differently.
A simple example is when chlorine gas is added
to water both hypochlorous acid and/or
hypochorite is formed depending on the pH.
16Bleaching Generalities (2)
- Often, radical species are generated from the
bleaching chemicals. Let us use oxygen for an
example. Molecular O2 reacts with ionized free
phenolic groups generating a phenoxy radical and
a superoxide radical (under alkaline conditions).
The species is not terribly reactive with
lignin but under a variety of other reactions can
be reduced to hydroxide radical which reacts very
very rapidly with lignin and carbohydrates.