Kraft pulping cooking chemicals

Kraft pulping involves the cooking of wood chips at 340-350°F and 100-135 psi in liquor that contains sodium hydroxide, sodium sulfide, and sodium carbonate. This process promotes cleavage of the various ether bonds in lignin and the degradative products so formed dissolve in alkaline pulping liquor. The Kraft process normally incorporates several steps to recover chemicals from the spent black liquor [3]. 

The soda, kraft, and sulfite pulping processes are used to prepare full chemical pulps. The soda process, which uses sodium hydroxide as the cooking chemical for delignification purposes, has hugely been superseded by the kraft process, which is characterized by its use of sodium hydroxide and sodium sulfide as active delignification agenls in the chip-cooking phase of the process. 

From Table III, the similarities and differences among processes become apparent. Certainly the domination of sodium as a base, and sulfur as an active reagent, become obvious. The narrow range of cooking temperatures and yields also becomes apparent. What is not shown is the strength advantage of Kraft pulp. Also not shown are such process considerations as chemical and energy recovery. 

Figure 7-38 illustrates the recovery and conversion of the kraft cooking chemicals. In the case of sulfur-free pulping (soda process, soda-oxygen process, and anthraquinone-alkali pulping) only sodium carbonate is recovered and chemical losses are compensated by adding sodium carbonate. The hydroxide-carbonate and lime cycles are the same as for the kraft process. 

It turns out that often many complex heterogeneous reactions can be globally described by simpler models. For example, delignificatlon reactions In conventional pulping such as kraft pulping (which Involves NaOH and Na2S as cooking chemicals) are usually treated by a simple relationship of the form ... 

C Gustavsson, K Sjdstrdm, W Wafa Al-Dajani. The Influence of Cooking Conditions on the Bleachability and Chemical Structure of Kraft Pulps. Nordic Pulp Pap Res J 14 71-81, 1999. 

---