Tall soap, from black liquor

Black Liquor Soap Recovery. Black liquor 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 liquor by skimming at an intermediate stage, when the black liquor is evaporated to 25% solids (7). At this solids level, the soap rises in the skimmer at a rate of 0.76 m/h. At higher solids 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 controlled 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 liquor content of 15% (10—12). 

The tall oil soap is removed from black liquor due to density differences during the black liquor evaporation process (the optimum dry solids content is 28—32%), and the... 

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. 

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. 

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). 

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. 

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. 

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. 

Despite the electronic communications revolution, paper production continues to grow worldwide at a rate of 2% to 3% annually. Much of the incremental increase is based on pine because no other long-fibered softwood offers such high yield rates per hectare coupled with a short growth cycle. New kraft pine pulpmills and most existing mills will continue to practice tall oil recovery because non-removal of tall oil soap from kraft black liquor adversely affects evaporator and recovery boiler capacity (14). Increased tall oil production has been accompanied by new fractionator capacity in Europe and incremental expansion in the United States. Since the paper industry is the largest user of rosin size, it might be expected that the increased paper production would absorb the increased availability of tall oil rosin. 

Sterols are synthesized in both plants and animals. Sterols and their derivatives, such as hormones and vitamin D2, perform various important functions in living organisms depending on their structure. Consequently, sterol products can be derived from several sources. Plant sterols, or phytosterols, are obtained from the unsaponifiable fraction of vegetable oils and fats. The amount of sterol, as well as its composition including fraction of unsaponifiable portion of the respective oil and fats, depends on the raw materials and is characteristic of the particular base material (Table 1). Soya oil represents a widely available source for commercial production of phytosterols. The unsaponifiable portion is separated into a sterol fraction and a tocopherol fraction. A small amount of tocopherol is usually left in the sterol fraction (approximately 4%), which acts as a natural antioxidant in the final product [2]. Plant sterols are also obtained from the black-liquor soap skimming in commercial pulping of wood. The neutral fraction of this so-called tall oil is relatively rich in sterols (approximately 32% of the neutral fraction [3]), and the sterol... 

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),... 

---