Pulping and Bleaching PSE 476/Chem E 471 Lecture #11 Anthraquinone/Polysulfides

Kraft Pulping Additives Agenda Anthraquinone Reactions with lignin and carbohydrates Use information Polysulfides Formation chemistry Reaction chemistry

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.

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.

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.

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.

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.

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.

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

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.

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.

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

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.

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.