PSE 476/Chem E 471 March 9, 2007
Study Guide for Lecture 19 and 20.
Final exam covers the whole material. Please use the old study guides
for the final. This one is for Lectures 19 and 20.
Bleaching Chemicals (Lecture 19-20)
1. For all of the chemicals discussed in these lectures, I expect that you will know the following information:
a) Advantages/Disadvantages of each bleaching agent.
b) Time: how long (roughly: 1 minute or 2 hours) each stage takes
c) Temperature (I know that there are temperature ranges used but some reactions need to be kept at lower
temperatures)
d) pH (acid/neutral/alkaline)
e) Selectivity
Oxygen Bleaching (Lecture 19)
1. Why has bleaching with oxygen become so prevalent? (several reasons)
2. Why is oxygen added to the pulp using a high shear mixture?
3. Why is oxygen bleaching done at an alkaline pH?
4. Chlorine dioxide is a very good delignification agent. Why would oxygen be used instead of ClO2?
5. What is typically the amount of lignin that can be removed using oxygen?
6. Do you know why there are so many oxygen species present during oxygen bleaching?
a) Can you name all of the species?
b) Can you tell which of the oxygen species is the most reactive?
c) Why are free phenolic hydroxyl groups on lignin needed in this process?
d) Do metals affect oxygen bleaching?
7. Higher temperature increases the rate of delignification with oxygen. Typical temperatures are 85°C to 110°C.
Why not bleach at higher temperatures?
8. Higher levels of NaOH also increase the rate of oxygen delignification. Why do mills control the levels of
NaOH used in this bleaching stage? (2 reasons)
Extraction (Lecture 21)
1. Ozone
a) If ozone is 106 times more reactive with lignin than with carbohydrates, why is there so much
carbohydrate degradation associated with ozone bleaching?
b) Why is the reaction time of ozone bleaching so short?
c) At what pH is ozone applied?
2. Hydrogen Peroxide
a) Why is hydrogen peroxide bleaching run at a pH between 9.5 and roughly 12.5-13?
b) What kind of lignin functional groups does hydrogen peroxide react with using brightening conditions?
c) If hydroperoxy anion only reacts with carbonyl and olefinic groups, how can hydrogen peroxide be used
as a delignifying agent.
d) Why is it so important to remove metals when bleaching with hydrogen peroxide?
i) What is added to remove the metals?
e) Is there carbohydrate degradation during peroxide bleaching?
3. NaOH extraction
a) What is the purpose of NaOH extraction? Is it used alone or can be utilized as a booster? When do we use
it in the sequence?
b) Why washing after NaOH is very important?
