Pulping and Bleaching PSE 476/Chem E 471

1. Pulping and BleachingPSE 476/Chem E 471 Lecture #11 Anthraquinone/Polysulfides

2. Kraft Pulping AdditivesAgenda • Anthraquinone • Reactions with lignin and carbohydrates • Use information • Polysulfides • Formation chemistry • Reaction chemistry • Use information

3. Pulping Additives:Anthraquinone • Bach and Fiehn first reacted anthraquinone with hydrocellulose under alkaline conditions in 1972 • Degradation of cellulose was slowed • When this additive was added to an alkaline cook, not only was the degradation of carbohydrates reduced, the rate of lignin removal was increased.

4. Pulping Additives:Reactions of Anthraquinone • It was discovered that anthraquinone (AQ) acts as a catalyst under pulping conditions. AQ oxidizes carbohydrates while the reduced form AHQ reacts with lignin.

5. Pulping Additives:Anthraquinone and Lignin +OR • AHQ reacts with quinone methides; this requires a free phenolic hydroxyl group. This is a rapid reaction. • Slow alkali cleavage of non phenolic groups follows.

6. Pulping Additives:Anthraquinone and Carbohydrates • AQ oxidizes the reducing end of carbohydrate to an aldonic acid. • AQ is reduced to AHQ. • Carbohydrate are stabilized against peeling.

7. Pulping Additives:Anthraquinone • There are a large number of quinone chemicals which will function as catalysts. • In United States, AQ is the most cost effective. • AQ added at catalytic levels. • 1987 FDA allows max addition of 0.11%. • AQ works better with hardwoods than softwoods. • AQ compensates for reduction in: • EA, Sulfidity, H factor.

8. Pulping Additives:Polysulfides • Sulfur polymers: Na2Sx • Polysulfides oxidize carbohydrate reducing end groups to carboxylic acid thus slowing peeling. • Can be formed through the reaction: Na2S + So ® Na2S2 Na2S2 + So ® Na2S3 • The addition of elemental sulfur to white liquor forms polysulfides. • Resulting increase in total sulfur causes sulfidity of liquor to rise.

9. Pulping Additives:Polysulfides • It is best to generate polysulfides through the oxidation of white liquor forming elemental sulfur which will further react with Na2S. 2Na2S + 2O2 + H2O ® 2So + 4NaOH* Na2S + (x-1)So ® Na2Sx *This reaction can be catalyzed using activated carbon • The oxidation of Na2S to thiosulfate is a competing reaction under these conditions: 2Na2S + 2O2 + H2O ® Na2S2O3 + 2NaOH

10. Pulping Additives:Polysulfides • Polysulfides are unstable at temperatures > 130°C 2Na2S3 + 6NaOH ® 4Na2S + Na2S2O3 + H2O • The conditions used in a kraft cook must therefore be modified in order to gain a yield benefit from the reaction of polysulfides. • Use thin or short chips. • Allow for ample impregnation time. • Use a slow rate of heat up.

11. Pulping Additives:Polysulfides • The use of polysulfides gives a yield gain by reduction of peeling reactions. • Most of the gain through reduced degradation of glucomannans. • In 1987, a 3.2% yield increase in a kraft mill corresponded to a 1.7 million dollar savings.

12. White Liquor Oxidation System Chiyoda polysulfide process was developed by Chiyoda and Mitsubishi Paper

13. Chiyoda polysulfide process (2) • This process is a very simple. Major equipment comprises the filter and the reactor. • The unique up flow type filter can remove substantially all the suspended solid in feed white liquor. • This process does not affect the operation of existing equipment. • The catalyst has three to four-year life, and it maintains high polysulfide concentration during the life. • An activated carbon with specific pore structure is used as the catalyst. The catalyst maintains optimum conversion to maximize polysulfide production and long catalyst life.

14. Chiyoda polysulfide process (3) • Oxidized carbohydrate becomes stable compound for alkaline liquor and the amount of dissolved carbohydrate in the black liquor is reduced. As a result; • Pulp yield is increased with insignificant changes to pulp properties. • Black liquor viscosity is decreased. • Recovery boiler load is reduced.