Kraft lignin degradation products

Suparao, O., Covington, A.D., and Evans, C.S., Kraft lignin degradation products for tanning and dyeing of leather, J. Chemical Technol BiotechnoL, 80, 44, 2005. 

The most noteworthy limitation to this approach is the complexity and multi-functionality that exist in these waste streams. For example, a bleached kraft pulp mill is estimated to produce greater than 400 compounds including tannins, resins, and lignin degradation products (5). However, as only a relatively small number of chemical transformations offer benefits to an end-user, i.e. polymerisation, selective oxidation or reduction, C-H bond activation, and carbon-halogen bond cleavage, the problem is simplified to some extent. 

Recovery of inorganic chemicals is crucial to the cost effectiveness of the Kraft process. The black liquor which is obtained from delignification is rich in solubilised lignin and carbohydrate degradation products and, after concentration, is combusted in a recovery furnace. The Carbon dioxide which is produced during combustion converts unused sodium hydroxide into sodium carbonate. In addition, the sodium sulfate is converted, under the reducing atmosphere of the furnace, to sodium sulfide. 

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 consumption of effective alkali in a kraft cook corresponds to about 150 kilogram sodium hydroxide per ton of wood. As a result of the alkaline degradation of polysaccharides, about 1.6 equivalents of acids are formed for every monosaccharide unit peeled from the chain. Of the charged alkali, 60-70% is required for the neutralization of these hydroxy acids, while the rest is consumed to neutralize uronic and acetic acids (about 10% of alkali) and degradation products of lignin (25-30% of alkali). 

The color of unbleached pulp ranges from cream or tan for the sulfite process to dark brown for the kraft process. Although about 75-90 percent of the lignin has been removed by the pulping process, the remainder, along with other colored degradation products, must be removed by bleaching. 

Phenolic degradation products of lignin in Kraft black liquors were extracted with chloroform after acidification and separated by CE with UVD at 214 nm. Simple CZE was insufficient for the separation of low molecular mass phenolic and neutral degradation products of lignin. Enhanced separation was attained on turning to the MEC technique, where the analytes interact with micelles present in the carrier buffer solution . 

From wood, the reaction gives a high yield of low molecular mass degradation products (monomers to trimers, with traces of higher products) and, after treatment, no lignin can be detected in the remaining wood material. Thioacidolysis, when applied to kraft pulps, on the other hand, in addition to monomers-trimers also results in the presence of polymeric material formed as a consequence of pulping reactions [20]. 

The isolation of chemicals from Kraft lignin, the polymer by-product of the Kraft pulping process for paper production, is an established industrial process [219]. There have been several studies on alternative degradation processes such as hydro-genolysis [220], biodegradation with white-rot fungi [221], and chemical oxidation [121,122,222]. These processes can in principle be used with residual lignin obtained from vine shoots employed for paper pulp production. 

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