Black liquor, from Kraft process

Black liquor from kraft or soda mills pulping hardwoods may present problems during its evaporation and combustion by conventional methods. These problems have been encountered with some eucalypt species, particularly with wood from trees of old age, and special measures may be needed to overcome the problems caused by extractives. Apart from the formation of deposits and scale, which reduces heat transfer in the evaporators, the extractives increase the viscosity (so that the evaporation process is affected) and reduce the swelling properties when incinerated, resulting in incomplete combustion (130). These difficulties with black liquor from the soda process can be avoided with the use of a wet oxidation process conducted under high pressure (97). 

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]. 

FIydroxybutanoic acid is a major organic component of kraft black liquors from hardwood pulping, but is formed only in trace quantities from the degradation of glucose polymers. It seems likely that it arises from the xylan by a process along the lines of that illustrated in Figure 6.13. 

Fig. 6.3.2 Technological platform for the reclamation and upgrading of black liquor from the Kraft process...

Dimethyl sulfide (DMS) is recovered from the waste black liquor from the kraft pulping process by flash drying of the liquor after the DMS content has been increased by additions of sulfur. DMS is an odorant for natural gas and a solvent. DMS is oxidized to give dimethylsulfoxide (DMSO), an important solvent and a medicinal of interest in the treatment of arthritis. 

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... 

Black liquor is 13 to 17% strength, rinsed extract from washed and cooked woodchip pulp, produced in the Kraft pulping process. This... 

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. 

Two other important side products of the kraft process are sulfate turpentine and tall oil. The turpentine is obtained from the gases formed in the digestion process. From 2-10 gal of turpentine can be obtained per ton of pulp. Tall oil soap is a black viscous liquid of rosin and fatty acids that can be separated from the black liquor by centrifuging. Acidification gives tall oil. These side products will be discussed later. 

Hagglund and Enkvist (6) developed a laboratory scale method for manufacturing methyl sulfide from kraft black liquor by pressure heating after addition of sodium sulfide. This process was later taken over by Crown-Zellerbach in the United States and developed in pilot plant and full scale. However, the yield is only about 7% of the initial lignin utilized in the process. 

An essential factor in the kraft process has been the recovery of the spent liquor from the cooking process. The black liquor removed from the... 

Tall oil is the generic name for the oil obtained upon acidification of the black liquor residue from kraft pulp digesters. Kraft processing dissolves the fats, fatty acids, rosin, and rosin acids contained in pinewoods in the form of sodium salts and when the black liquor is concentrated to make it possible to recover some of its chemical and heating value, the soaps become insoluble and can be skimmed off. The brown, frothy curd thus obtained is then made acidic with sulfuric acid, converting the constituents to a dark-brown fluid (tall oil). 

Dimethyl sulfide (DMS) is recovered from the waste black liquor in the kraft pulping process by flash drying of the liquor after the DMS content has been increased by additions of sulfur. 

The recovery of phenol, cresols, and guaiacol by extraction from kraft pulping process black liquor. 

The production of wood pulp from lignocellulosic materials by treatment with various chemical liquors, particularly the neutral sulfite semichemical process and the kraft or sulfate process, gives residual black liquors. These contain salts that carry acetic acid and formic acid equivalent to 5% or more of the dry weight of the wood. 

Liquors from Neutral Semichemical Pulping and from Kraft Pulping. In making neutral sulfite semichemical pulp, the black liquors may have about 10 parts of water to 1 part of total solids, of which about one-third is sodium acetate and sodium formate. After evaporation to about 1 part solids to 1 part water, sulfuric acid is added to the concentrate to free the acetic and formic acids. When the concentrate is extracted with acetone, the mixed acids are obtained, the acetone is separated for recycle, and the acids are concentrated and refined. The raffinate is stripped and is passed to the usual furnace to be burned for recovery of the inorganic salt values. This process gives a smelt of sodium sulfate, which may be used in the kraft process as chemical makeup. The loss of the fuel value of the acetic and formic acids is practically negligible. 

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