Kraft lignin is presently available from MeadWestvaco Corp. (USA) and Borreg — aard Lignotech (about 10 thousand t/year at Backhammar plant), accounting for a total annual production around 1 million t [5]. This is a rather low production value considering that a mill with capacity for 500 thousand t of kraft pulp can produce about 200 thousand t of lignin in black liquor. The potential for lignin production in the existing pulp and paper industry is more than 50 million t/year [35].

Currently, few kraft pulp mills recover the lignin. The main utilization of black liquor is energy production in the recovery boiler, allowing the simultaneous recovery of cooking chemicals to reintroduce in the digesters. This is economically advantageous, unless the recovery boiler becomes the bottleneck of the process.

In this case, separation of lignin could be one solution to increase the pulp production capacity. Alternatively, the deviation of a fraction of lignin could become sustainable by the upgrading lignin for materials and specialty chemicals with high added-value [36].

Commercial kraft lignins are usually modified to their increase it solubility in aqueous solutions by means of oxidative sulfonation, carboxylation, and sulf- omethylation. The major final application of these lignins is as dispersant: the use of sulfomethylated kraft lignin was patented in 1954 [37]. MeadWestvaco Corp. and LignoTech Sweden produce lignosulfonates by sulfonation of kraft lignin, but the product has a much lower molecular weight than the lignosulfonates produced from sulfite pulping [38]. Other end uses are asphalt emulsions, lead-acid storage battery industry and products for cement and concrete industries [39]. This lignin, after chemical modification, competes with the lignosulfonates coming from the sulfite pulping industry.

12.2.1.2 Characteristics

In general, hardwood kraft lignin presents lower weight-average molecular weight (around 1 kDa) than the respective wood lignin (2-3 kDa) [10, 20, 40]. Com­paratively to hardwood, softwood kraft lignin Mw, in general, is higher (2-3 kDa) [23]. Other characteristics of kraft lignin are the higher contents of phenolic hydroxyl groups and condensed structures than the respective wood lignin [20, 41]. The predominant inter-unit linkage is still the fi-O-4, although in lower absolute amount than in wood lignin. The extension of lignin reactions depends fundamentally on kraft pulping conditions and wood species [10]. Some infor­mation about the composition and chemical structure of lignin recovered from kraft pulping streams can be found in literature [10, 20, 42] and is presented in Table 12.2.