Kraft paper process

The principle uses of Na2S04 are in the manufacture of paper, soaps, and detergents. These accounted for 65% of U.S. consumption from 1990 to 1995, representing a significant shift from 1980 when paper production alone consumed 67%. Pulp (qv) and paper consume only 25% (2). The kraft paper process uses a mixture of sodium sulfide and sodium hydroxide to digest wood chips. Both the sulfide and hydroxide are generated, starting with sodium sulfate as the raw material. 

Kraft chemical pulp process, 13 94 Kraft lignins, 15 19-20 applications of, 15 19-20 Kraft paper process... 

When softwood (pine, fir, spruce) is converted into pulp in the Kraft paper process, the water-insoluble liquids which were present in it are freed and can be removed by physical separation from the process water. This material is known as crude sulfate turpentine (CST). Fractional distillation of CST gives a number of products as shown in Table 4.2. The residue is known as tall oil and contains diterpenoids such as abietic acid. 

For thousands of years, turpentine has been obtained from conifers by a process known as tapping. A cut is made in the bark which prompts the tree to exude an oil which can be drained off into vessels attached to the tree. Turpentine thus obtained is referred to as gum turpentine. Nowadays, another form of turpentine is available in large quantities as a by-product of paper manufacture. When softwood (pine, fir, spruce) is converted into pulp in the Kraft paper process, the water insoluble liquids which were present in it are freed and can be removed by physical separation from the process water. This material is known as crude sulfate turpentine (CST). Fractional distillation of CST gives a number of products as shown in Table 9.2. (Dipentene is the name given to racemic limonene.) The residue from the distillation is known as tall oil and contains diterpenes such as abietic acid. 

Turpentine is obtained by tapping of pine trees and this product is known as gum turpentine. However, a much larger commercial source is the so-called crude sulfate turpentine, which is obtained as a by-product of the Kraft paper process. The major components of turpentine are the two pinenes with a-pinene (65) predominating. Turpentine also serves as a source of/ -cymene (83) and, as mentioned earlier, the shikimate anethole (53) (Zinkel and Russell, 1989). 

The most important natural feedstock is turpentine. It can be produced by tapping suitable conifers, a process that involves making an incision in the bark and collecting the exudate in cups. Such turpentine is called gum turpentine. Wood turpentine is extracted mainly from tree stumps but the major source of turpentine is known as crude sulphate turpentine (CST) as it is a by-product of the Kraft paper process. 

The most commonly used reinforcement for high pressure decorative and industrial laminates is paper (qv). The strong substrate layers, or filler, are kraft paper. Kraft is a brown paper made from a sulfate pulp process (8). It consists of both short cellulose fibers from hardwoods and long fibers from conifers. The long fibers impart most of the wet strength required for resin saturation processes. 

The type of varnish used in the process depends on the kraft paper manufacturer and basis weight of the papers the machine, temperature, and control (scraper bars, squeeze roUs) used the method of cutting the paper to size the laminate being produced (post-forming or regular) and the press-cure cycle (see Laminated materials, plastic). 

In 1980, >1 one million ts of sodium sulfate were consumed in the United States, but this had declined to <600, 000 t by the end of 1994. The decline is partly a result of higher energy prices and more efficient use of Na2S04 by the paper industry. At one time the kraft paper industry consumed two-thirds of sodium sulfate production. Pressures on paper producers to clean effluent streams and reduce energy forced improvements in internal processes and recycling of sodium sulfate (11,12). 

The decorative plastic laminates widely used for countertops and cabinets are based on melamine—formaldehyde resin (see Laminates). Several layers of phenohc-saturated kraft paper are placed in a press and a sheet of a-ceUulose paper printed with the desired design and impregnated with melamine—formaldehyde resin is placed over them. Then a clear a-ceUulose sheet, similarly impregnated with the resin, is placed on top to form a clear, protective surface over the decorative sheet. The assembly is cured under heat and pressure up to 138°C and 10 MPa (1450 psi). A similar process is used to make wall paneling, but because the surfaces need not be as resistant to abrasion and wear, laminates for wall panels are cured under lower pressure, about 2 MPa (290 psi). 

Tall oil rosin is obtained from crude tall oil obtained from the Kraft (sulphate) pulping of various coniferous trees in the paper manufacturing industry. During the Kraft pulping process the fatty acids and the resin acids from the coniferous wood are saponified by the alkaline medium. On concentration of the resulting pulping liquor, the sodium soap of these mixed acids rises to the surface from where they are skimmed out. By acidification of this material with sulphuric acid, the crude tall oil is obtained. Fractional steam distillation of the crude tall oil allows the separation of the tall oil fatty acids and the tall oil rosins [21]. 

FIGURE 21.4 The kraft pulping process (with chemical recovery). (Taken from U.S. EPA, Profile of the Pulp and Paper Industry, 2nd ed., report EPA/310-R-02-002, U.S. EPA, Washington, November 2002.)... 

During the kraft pulping process, the first step in making hundreds of paper products, crude tall oil is obtained from the alkaline material by acidifying it with sulfuric acid. The crude tall oil is then converted through dehydration, dry distillation, and finally the fractionation of the vaporized tall oil compounds. Fatty acids, rosin acids, and neutral materials make up tall oil. 

Kraft-mill black liquor Kraft paper Kraft process... 

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