Wood-pulping liquors, recovery

Refining of Sugar and Recovery of Acetic Acid from Wood-Pulping Liquors... 

Pulping liquors. Pulping liquor, also called black liquor, is a corrosive material used to dissolve wood chips for the manufacturing of paper and other materials. To promote waste minimization and recycling, U.S. EPA excluded pulping liquors from the definition of solid waste if they are reclaimed in a recovery furnace and then reused in the pulping process. If the liquors are recycled in another way, or are accumulated speculatively, they are not excluded. 

Chemical wood pulping is an important industrial process, with a relatively low yield. However, this process is attractive, because of the low requirements of wood quality and wood species. Moreover, it includes very short cooking times due to the easy penetration of alkaline solutions into wood. A well established processing of the spent liquor, generation of process heat, recovery of the pulping chemicals and production of valuable by-products such as turpentine and tall oil are also advantages of the kraft cooking. Major drawbacks are however, the odour and the dark colour of the unbleached pulp. 

Kraft pulping is a common process in the paper industry. Figure 8.4 shows a simplifled flowsheet of the process. In this process, wood chips are reacted (cooked) with white liquor in a digester. White liquor (which contains primarily NaOH, NaiS, Na2C03 and water) is employed to dissolve lignin from the wood chips. The cooked pulp and liquor are passed to a blow tank where the pulp is separated from the spent liquor weak black liquor which is fed to a recovery system for... 

The kraft process evolved from the soda process. The soda process uses an alkaline liquor of only sodium hydroxide (NaOH). The kraft process has virtually replaced the soda process due to the economic benefits of chemical recovery and improved reaction rates (the soda process has a lower yield of pulp per pound of wood furnish than the kraft process). 

CTO prices are closely tied to the cycles of the U.S. economy and the paper industry. They vary between 120 and 220 per metric ton. In 1995 they were close to 200/t (4). With 50% of pine wood being converted to linerboard valued at 400— 600 per ton, pulp manufacturers do not focus on optimum black liquor soap recovery, which only amounts of 60—70 kg/1 of southern pine pulp. This soap is converted to 30—35 kg of CTO, worth 6— 7 or less than a little over 1% of the pulp value (5). This recovery is only 45% of the CTO available in the pine tree. With more care and higher CTO prices, 10—15 kg of additional CTO could be obtained per ton of pulp (6). 

Since the prices of petroleum products have soared, valorization of natural resources has intensively attracted the attention of researchers in different fields. To fully utilize wood, which is a renewable raw material, one must make better use of its 25-30% lignin content which forms the principal dissolved material in the spent liquor of chemical pulping processes. The annual production of sulfite pulp in Canada is approximately 2.6 million tons (data based on 1979), thus, at least one million tons of lignosulfonate is produced, representing a potential resource of raw material for other uses. However, among 35 sulfite mills in Canada, only two or three of them possess a recovery system for spent liquor, that means a very small portion of lignosulfonate has been recovered. The recovered lignin has been... 

The process is conducted at the high pH in the presence of sodium sulfide, sodium hydrosulfide, and sodium hydroxide at 150-180°C for about 2 hours (Fig. 33.26). Along with the primary cellulose product, the process generates a solution containing dissolved lignin, most of the wood s hemicellu-loses, and spent pulping chemicals ( black liquor ). This solution is used in the kraft recovery boiler as a source of power for the mill and fuel for chemical recovery operations crucial to economic operation. The chemistry of pulping reactions has been the subject of much study, and summaries are available the literature.425 127... 

Calcium carbonate scale is even more commonplace in Kraft pulp mill systems such as heat exchangers, where, compared to paper machines, it often forms a thick deposit leading to loss of performance of the heat exchanger. Calcium comes into the system with either the wood or from poor recovery liquor clarification and carbonate is formed from degradation of sugars in a side reaction during pulping conditions. Dissolution of filler (calcium carbonate) will increase the concentration of both calcium and carbonate in the system. 

---