Fatty acids from black liquor

Black Liquor Soap Recovery. Black Hquor soap consists of the sodium salts of the resin and fatty acids with small amounts of unsaponifiables. The soap is most easily separated from the black Hquor by skimming at an intermediate stage, when the black Hquor is evaporated to 25% soHds (7). At this soHds level, the soap rises in the skimmer at a rate of 0.76 m/h. At higher soHds concentrations, the tall oil soap is less soluble, but higher viscosity lowers the soap rise rate and increases the necessary residence times in the soap skimmer beyond 3—4 hours. The time required for soap recovery can be reduced by installing baffles, by the use of chemical flocculants (8,9), and by air injection into the suction side of the soap skimmer feed pump. Soap density is controUed by the rate of air injection. Optimum results (70% skimmer efficiency) are obtained at a soap density of 0.84 kg/L (7 lb/gal). This soap has a minimum residual black Hquor content of 15% (10—12). 

Black Liquor Soap Acidulation. Only two-thirds of a typical black Hquor soap consists of the sodium salts of fatty acids and resin acids (rosin). These acids are layered in a Hquid crystal fashion. In between these layers is black Hquor at the concentration of the soap skimmer, with various impurities, such as sodium carbonate, sodium sulfide, sodium sulfate, sodium hydroxide, sodium Hgnate, and calcium salts. This makes up the remaining one-third of the soap. Cmde tall oil is generated by acidifying the black Hquor soap with 30% sulfuric acid to a pH of 3. This is usually done in a vessel at 95°C with 20—30 minutes of vigorous agitation. Caution should be taken to scmb the hydrogen sulfide from the exhaust gas. 

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. 

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). 

Yields of 50% and reduced energy consumption have been achieved by a history of innovation. Such innovation has included the Tomlinson black liquor recovery boiler, the Kamyr continuous digester and associated diffusion washer, multiple-effect evaporators, and low-odor concentrators. Economic advantages also have been gained by the development of systems for recovering extractives such as tall oil, fatty acids, and resin from the pulping liquor for sale as naval stores. Future innovations may focus on the lime kiln and other related systems. 

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. 

Derivation The spent black liquor from the pulping process is concentrated until the sodium salts (soaps) of the various acids separate out and are skimmed off. These are acidified by sulfuric acid. Composition and properties vary widely, but average 35—40% rosin acids, 50-60% fatty acids. 

Tall oil, derived from the Swedish tallolja meaning pine oil, is recovered from the black liquor of softwood pulping. It is taken out at an intermediate stage of the multiple-effect evaporation when the liquor contains about 30% total solids, after it is allowed to stand [21]. The soaps (sodium salts of fatty acids present) are insoluble, cream to the top of the vessel, and are skimmed off. The residual black liquor is returned to the evaporators to continue chemical recovery. The soap yield, which can range from 10 to 200 kg/tonne of pulp (or even higher for pine), is then acidified and the free fatty acids and resin acids obtained are separated by distillation. The fatty acids recovered consist mainly of oleic and linoleic acids and are employed in soap manufacture and as the drying oil components of paints and varnishes [22] (Chap. 19). Resin acids consist of terpene acids such as abietic acid and its positional and reductive variants, and are mainly employed in paper sizing. 

Tall oil is obtained from the concentrated black liquor. The soap on the surface of the blaek liquor eoncentrate is skimmed off, and after acidification yields a mixture of resin aeids, fatty acids and neutral eomponents. 

Crude tar oils are the cmde products of the sulfate soaps which are separated from the black liquor of paper pulping. The contents of fatty acids and resin acids are increased through the cmde processing. For instance, the components of domestic cmde tar oil are listed as follows ... 

Mercury will be revealed by a black precipitate produced when sulphuretted hydrogen is added to the liquor separated from the fatty acids, and may be estimated by filtering off this precipitate on a tared Gooch s crucible, which is then dried and weighed. 

The resin in the wood of resinous species is dissolved during pulping by alkaline processes with the formation of sodium salts of fatty and resin acids. The presence of these soaps in black liquors facilitates foaming, thereby reducing the efficiency of pulp washing and liquor concentration operations in the pulp mill. Removal of the soap for the production of tall oil reduces these problems. Foaming is encountered from time to time with other species but the causes are not known. 

The recovery of terpenes and fatty and resin acids from the kraft pulping of resinous species can improve the economy of the operations. During the pulping of Pinus spp., the gases released from the digester are condensed and the turpentine decanted from the aqueous distillate. The removal of the sodium salts of fatty and resin acids from the black liquor followed by acidification give tall oil, which is refined into a number of commercially important products. These aspects will be dealt with elsewhere in this volume (Sect. 10.1). 

These materials are by-products from the kraft pulping process (Fig. 10.1.3). As the pine chips are cooked in the alkaline liquor to produce pulp, the volatilized gases are vented and condensed to yield sulfate turpentine. As pulping proceeds, the alkaline liquor saponifies the fats and converts fatty and resin acids to the sodium salts. During the recovery of the pulping chemicals, the aptly named black liquor is concentrated in multiple-effect evaporators. The insoluble soaps can be skimmed from the surface of the black liquor, either from the black liquor as separated from the pulp and/or during/after concentration in the evaporators. The skimmed soap is acidified to yield a material known as crude tall oil (CTO). The term tall oil is derived from the Swedish word tallolja, which translates as pine oil. However, such literal translation would have caused confusion with the essential oil known as pine oil - thus, the simple transliteration to tall oil. 

When coniferous wood is pulped in the strongly alkaline Kraft pulping process, also called the sulfate pulping process , the predominant chemical pulping process today, the resin and fatty acids are converted into sodium soaps, and are after cooking recovered as so-called sulfate soap from the surface of the pulping liquor, the black liquor. The sulfate soap is a lamellar crystalline phase where the resin and fatty acids form the lamellae, and neutral components, such as sterols, are included between the lamellae [7],... 

Tall oil rosin [8052-10-0] is a by-product of the manufacturing of - paper pulp by the Kraft process. The so-called black liquor contains the sodium salts of ->ligninosulfonate and rosin acids as well as fatty acids. For details of further processing ->tall oil. The tall oil rosin is separated from the tall oil fatty acids by ->distillation. 

It is derived as a by-product from the Kraft (sulfate) process to make - paper pulp. Pinewood is the most common source. After separation of the pulp, the remaining black liquor is concentrated by evaporation, whereby the crude soap is salted out and collected. Acidulation with sulftiric acid and several refining steps gives crude tall oil. Production of one ton of paper pulp yields 30-40 kg of tall oil. Fractional steam-vacuum distillation (200-285 °C 13.3 kPa/lOOmm steam must be present to lower the b.p. and avoid decomposition) separates the oil from the pitch and low-boiling components. The oil is purified by additional fractionation procedures to separate the -+rosin and to produce various grades of purities of tall oil fatty acids (TOFA),... 

---