ABSTRACT
Introduction ............................................................................................................ 350 Pulping in Alkaline Media ................................................................................ 350 The Kraft Process .............................................................................................. 351
Overview ................................................................................................................ 351 Different Pulping Phases and Kinetics .............................................................. 351 Topochemical Effects ........................................................................................ 354 Kappa Number Signicance ............................................................................. 354
Pulping Chemistry ................................................................................................. 355 Nature of Reactions at 170°C ............................................................................ 355 Quinone Methide Chemistry ............................................................................. 356 Fragmentation via Nucleophilic Chemistry: Soda and Kraft Pulping .............. 358 Fragmentation by AHQ: Adduct vs Electron Transfer Chemistry .................... 362 Condensation Reactions .................................................................................... 365 Competing Reactions: Vinyl Ethers and Stilbenes ............................................ 367 Oxidation and Reduction Reactions .................................................................. 368 Lignin Reduction Reactions .............................................................................. 368
Kinetics .................................................................................................................. 371 Initial and Bulk Delignication ......................................................................... 371 Competing Reactions ........................................................................................ 372
Structure of Residual and Dissolved Lignin .......................................................... 374 Components in the Cooking Liquor .................................................................. 374 The β-O-4 Structure ......................................................................................... 375 Phenolic Hydroxyl Groups ................................................................................ 376 Polydispersity, the Presence of Lignin-Carbohydrate Complexes .................... 377 “Condensed” Lignin Structures ........................................................................ 377 Lignin Condensation ......................................................................................... 379
Improving Pulping Efciency ................................................................................380 Changing the Process ........................................................................................380 Changing the Tree ............................................................................................. 381
References .............................................................................................................. 382
By far, the most important chemical pulping process is the kraft process. Its advantages include excellent paper strength and permanence, relatively low energy requirements, chemical recycle and, therefore, low chemical costs, and insensitivity to wood species. The disadvantages are relatively low pulp yields (~40-55% from wood, depending on grade and whether bleached), high capital investment, brown unbleached pulp color, high bleach chemical costs, signicant amounts of organic components in the bleaching efuent, and strong odors. An alternative alkaline chemical process is soda/anthraquin one (AQ) pulping. While low in odor, the process requires higher levels of NaOH than the kraft process in order to achieve roughly the same delignication rate and its higher alkali level leads to more carbohydrate damage [1]. Thus, in general, the soda/AQ pulping process leads to lower pulp strength, especially with softwoods.
