Kraft pulping, black liquors from

In addition to in situ soil applications, Fenton reagent has also been studied for treatment of industrial waste streams. For example, the treatment of residual Kraft black liquor from the pulp-and-paper industry has been shown to be effective [42]. Near-complete degradation of lignin (95-100%) and decolorization were achieved under optimized conditions. Basic oxygen furnace slag was evaluated as a source of iron for degradation of 2-chloro-phenol in industrial wastewater [43], and favorable results were achieved. 

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. 

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). 

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). 

The spent black liquor from the kraft pulping of pines contains the less volatile products of the wood resin in the form of sodium salts or soaps. The liquor first is concentrated in multiple-effect evaporators, and then the concentrate is sent to settling tanks. The soaps rise to the surface, are skimmed off, and then are acidified with sulfurous or sulfuric acid. The crude tall oil rises to the top and is mechanically separated. Crude tall oil from southern pines contains 40-60 percent resin acids and 40-55 percent fatty acids with 5-10 percent neutral substances. These components are separated by fractional distillation under vacuum. 

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

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. 

Derivation (1) From kraft pulping black liquor by heating with inorganic sulfur compounds (2) by interaction of a solution of potassium sulfide and methyl chloride in methanol. 

Black liquor from the kraft pulping of mixtures of Papua New Guinea hardwoods has poor swelling properties and sometimes high viscosities that could lead to burning difficulties in commercial recovery furnaces (130, 139-141). 

One way to assure low raw material costs is to have multiple use of the material. If wood chips, sawdust, or bark is to be extracted, the residue should be useful for another purpose. Examples of this approach are 1) Isolation of arabinogalac-tan gum from western larch Larix occidentalis) heartwood chips followed by kraft pulping of the extracted chips 2) isolation of mimosa tannins from bark of trees in which the peeled wood Acacia sp.) serves as raw material for chemical cellulose production (South Africa) and 3) tall oil separation from the concentrated black liquor derived from kraft pulping of southern pines. In each of these cases the brunt of the raw material cost is carried by some product other than the extractive. Assurance of low raw material costs virtually dictates extractive isolation and processing ancillary to a forest products processing plant. 

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... 

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]. 

A number of studies have evaluated the toxicity of pulping hquors, in particular the black liquors generated from Kraft mills. Table 8 shows a partial representation of toxicity data compiled by the NCASI (National Council of the Paper Industry for Air and Stream Improvement) and McKee and Wolf for Kraft mill pulping wastewaters [15,16]. The table indicates that hydrogen sulfide, methyl mercaptan, crude sulfate soap, and salts of fatty and resin acids are particularly... 

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. 

Condensation reactions of lignin have been mentioned 22) in connection with sulfite pulping as being responsible for color formation. However, most of the likely condensation reactions occurring during a kraft cook do not lead directly to chromophore formation. One potential condensation reaction which would give a chromophore is the formation of a chalcone (XIX) from vanillin and acetoguaiacone, both of which have been isolated by Enkvist 12) from black liquor. 

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