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Pulping and Bleaching PSE 476Chem E 471

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Bleaching chemicals are more ... Bleaching uses a combination of chemicals in series. ... Often, radical species are generated from the bleaching chemicals. ... – PowerPoint PPT presentation

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Title: Pulping and Bleaching PSE 476Chem E 471


1
Pulping and BleachingPSE 476/Chem E 471
  • Lecture 17
  • Introduction to Bleaching

2
Agenda
  • Brightness
  • General Bleaching Principles
  • Chemistry
  • Process
  • Chemicals
  • Description
  • Advantages/Disadvantages

3
Why 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).

4
Brightness 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)

5
Brightness 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.

6
Brightness 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).

7
General 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.

8
General 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.

9
General 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
10
General PrinciplesChemicals (1)
11
General PrinciplesChemicals (2)
12
General PrinciplesChemicals (3a)
13
General PrinciplesChemicals (3b)
14
General PrinciplesChemicals (3c)
15
Bleaching 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.

16
Bleaching 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.
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