制浆造纸专业单词9

1、49Pulping refers to any process by which wood (or other fibrous raw material) is reduced to a fibrous mass. Basically, it is the means by which the bonds are systematically ruptured within the wood structure. The task can be accomplished mechanically, thermally, chemically, or by combinations of the

2、se treatments. Existing commercial processes are broadly classified as mechanical, chemical or semichemical. The general characteristics of these processes are summarized in Table 4-1.Current annual pulp production by each major process is shown in Table 4-2. Chemical pulping accounts for 70% of Nor

3、th American production, of which 95% is produced by the dominant kraft process. The tabulation shows that semichemical pulping accounts for only about 5% of production; however, high-yield kraft and high-yield sulfite tonnage now included in the totals for unbleached kraft and sulfite should more ap

4、propriately be included under semichemical.table 4-2. North American pulp production 1990 (000 000 short tons). Sources: API & CPPA.USA CanadaDissolving 1,293 221Chemical pulp, paper gradesTotal sulfite 1,561 1,603Bleached hardwood kraft 1,764Bleached softwood kraft 7,006Total bleached kraft 27,562

5、8,770Unbleached kraft 22,188 1,503Total kraft 49,750 10,273Semimechanical 4,219 514Mechanical 6,452 11,637Total 63,275 25,234A tally of North American forest-based pulp mills situated in the principal pulp-producing states and provinces (Table 4-3) helps to provide a geographical perspective. It is

6、of interest to note that most of the sulfite mills are located in traditional pulp-producing areas (e.g., Washington. Wisconsin. Ontario, Quebec), while more-recently exploited areas (e.g., Alabama. Georgia, Louisiana. British Columbia) are relying principally on the kraft process for chemical pulpi

7、ng.table 4-3. Geographical distribution of pulp mills, 1990 (Lockwoods Directory)., Semi- Mech-. Kraft Sulfite chem anical TotalUnited StatesAlabama 14 - 2 4 20Georgia 12 - 1 2 5Louisiana 10 - 4 3 17Maine 7 1 - 10 18 Oregon 7 - 3 11 17 Washington 7 6 3 7 23 Wisconsin 4 6 1 11 22Canada Brit. Columbia

8、 18 1 - 15 34Ontario 9 4 2 15 30Quebec 10 8 2 41 61A complete listing of pulp mills would also include the category, secondary fiber. Under this heading are the operations that recover usable pulp fiber from various waste paper sources. This fiber is ultimately recycled into a wide range of paper an

9、d paperboard products where strength and quality are adequate to replace more expensive virgin pulps. 4.1 INTRODUCTION TO VARIOUS PULPING METHODSMechanical PulpingThe oldest and still a major method of mechanical pulping is the groundwood process, where a block (or bolt) of wood is pressed lengthwis

10、e against a wetted, roughened grinding stone revolving at peripheral speeds of 1000-1200 m/min. Fibers are removed from the wood, abraded and washed away from the stone surface with water. The dilute slurry of fibers and fiber fragments (Figure 4-1) is screened to remove slivers and oversize particl

11、es, and is subsequently thickened (by removal of water) to form a pulp stock suitable for papermaking. The process is simple in principle, but the efficient production of uniform, good-quality pulp requires careful control of stone surface roughness, pressure against the stone, and shower water temp

12、erature and flow rate.A more recent development in mechanical pulping involves shredding and defibering chips of wood between the rotating discs of a device called a refiner; the product is known as refiner mechanical pulp (RMP). RMP typically retains more long fibers than stone groundwood and yield

13、s stronger paper.The basic RMP process has undergone extensive development in the past two decades. Most new installations now employ thermal and/or chemical presoftening of the chips to modify both the energy requirement and the resultant fiber properties. For example, when the chips are given a pr

14、essurized steam pretreatment. the resultant product, called thermomechanical pulp (TMP), is significantly stronger than RMP and contains very little screen reject material.Mechanical pulping processes have the advantage of converting up to 95% of the dry weight of the wood into pulp, but require pro

15、digious amounts of energy to accomplish this objective. The pulp forms a highly opaque paper with good printing properties, but the sheet is weak and discolors easily on exposure to light. To achieve adequate sheet strength, it is often necessary to add long-fibered chemical pulp to the mechanical p

16、ulp. Newsprint traditionally was made up of about 75% groundwood and 25% chemical pulp: now some newsprint is made from 100% TMP.Mechanical pulps are commonly produced from softwood species. The smaller, thinner hardwood fibers are more severely damaged during conventional mechanical pulping and yie

17、ld a finer, more flour-like pulp (Figure 4-2) that forms an exceedingly weak sheet. In spite of the obvious strength deficiencies, some exceptionally bright hardwood mechanical pulps (e.g., aspen and poplar) have sometimes been blended with softwood pulps to improve optical properties. Now. the rece

18、nt development and application of the chemi-thermomechanical pulping process has finally enabled the industry to exploit certain hardwood species such as aspen and eucalypt for the production of relatively strong, short-fibered pulps that are suitable for blending into a variety of papermaking furni

19、shes.Chemical PulpingIn chemical pulping, the wood chips are cooked with appropriate chemicals in an aqueous solution at elevated temperature and pressure. The objective is to degrade and dissolve away the lignin and leave behind most of the cellulose and hemicelluloses in the form of intact fibers

20、(Figures 4-3 and 4-4). In practice, chemical pulping methods are successful in removing most of the lignin; they also degrade and dissolve a certain amount of the hemicelluloses and cellulose so that the yield of pulp is low relative to mechanical pulping methods, usually between 40% and 50% of the

21、original wood substance.The two principal chemical pulping methods are the (alkaline) kraft process and the (acidic) sulfite process. The kraft process has come to occupy the dominant position because of advantages in chemical recovery and pulp strength. The sulfite process, which was more common up

22、 to the late 1940s, appears to be in an irreversible decline. No new sulfite mills have been built in North America since the 1960s. Nonetheless, sulfite pulping still has itsfigure 4-3. Softwood kraft pulp (red pine)figure 4-4. Hardwood kraft pulp (aspen). adherents, and process modifications have

23、been proposed which would appear to make the process more competitive. Table 4-4 provides a summary of chemical and semichemical pulping methodology. Each of these methods is considered briefly in this chapter. Kraft ProcessThe kraft process involves cooking the wood chips in a solution of sodium hy

24、droxide (NaOH) and sodium sulfide (Na2S). The alkaline attack causes fragmentation of the lignin molecules into smaller segments whose sodium salts are soluble in the cooking liquor. Kraft is the German word for strong, and kraft pulps produce strong paper products: but the unbleached pulp is charac

25、terized by a dark brown color. The kraft process is associated with malodorous gases, principally organic sulfides, which cause environmental concern.table 4-4. Major chemical and semichemical pulping methodsKraftAcid SulfiteBisulfiteNSSCChemicals:NaOHNa2SH2S03M(HSO3)(M = Ca. Mg, Na, NH4) M(HSO3)(M

26、= Mg, Na, NH4) Na2SO3Na2C03Cooking Time: 2-4 h4-20 h2-4 h1/4-1 hLiquor pH: 13+1-23-57-9Cooking Temp: 170-180C120-135C140-160C160-180CThe kraft process evolved over 100 years ago as a modification of the soda process (which utilizes only sodium hydroxide as the active chemical) when Carl S. Dahl intr

27、oduced sodium sulfate into the cooking system. The subsequent conversion of sulfate to sulfide in the cooking liquor produced a dramatic improvement in reaction kinetics and pulp properties when cooking softwoods. Because sodium sulfate has been the traditional makeup chemical, the kraft process is

28、sometimes referred to as the sulfate process. Hardwood pulping is less affected by the presence of sodium sulfide in the alkaline cooking liquor, and some hardwood pulp is still produced by the soda process.A number of different kraft pulp grades are produced. Unbleached grades for packaging applica

29、tions are cooked to a higher yield and contain more lignin than pulps that are subsequently bleached and made into white papers.Sulfite ProcessIn the sulfite process, a mixture of sulfurous acid (H2SO3) and bisulfite ion (HSO3-) is used to attack and solubilize the lignin. The sulfites combine with

30、the lignin to form salts of lignosulfonic acid which are soluble in the cooking liquor, and the chemical structure of the lignin is left largely intact. The chemical base for the bisulfite can be ionic calcium, magnesium, sodium or ammonium. Sulfite pulping can be carried out over a wide range of pH

31、. Acid sulfite denotes pulping with an excess of free sulfurous acid (pH 1-2), while bisulfite cooks are carried out under less acidic conditions (pH 3-5).Sulfite pulps are lighter in color than kraft pulps and can be bleached more easily, but the paper sheets are weaker than equivalent kraft sheets

32、. The sulfite process works well for such softwoods as spruce, fir and hemlock, and for such hardwoods as poplar and eucalyptus; but resinous softwoods and tannin-containing hardwoods are more difficult to handle. This sensitivity to wood species, along with the weaker strength and the greater diffi

33、culty in chemical recovery, are the major reasons for the decline of sulfite pulping relative to kraft. The trend toward whole-tree chipping puts sulfite at a further disadvantage because of its intolerance to bark. The relative advantages of the kraft and sulfite processes are summarized in Table 4

34、-5. A comparison of kraft. sulfite and bisulfite pulp strengths is shown graphically in Figure 4-5.table 4-5. Relative advantages of the two major chemical pulping processes (kraft vs. sulfite).Advantages of Kraft Process* produces highest strength pulp.* utilizes proven technology for efficient che

35、mical recovery.* handles wide variety of wood species.* tolerates bark in the pulping process.Advantages of Sulfite Process* produces brighter unbleached pulp.* pulp is easier to bleach to full brightness.* produces higher yield of bleached pulp.* pulp is easier to refine.Several grades of sulfite p

36、ulp are produced. depending on end use. Higher-yield grades are usually prepared using a cooking liquor low in free sulfurous acid, i.e., with bisulfite ion as the predominant active chemical. figure 4-5. Comparison of tear and tensile strengths for unbeaten pulps and pulps beaten to approximately 2

37、50 CSF (data of Hartler, et al).Semichemical PulpingSemichemical pulping combines chemical and mechanical methods. Essentially, the wood chips are partially softened or digested with chemicals; the remainder of the pulping action is then supplied mechanically, most often in disc refiners. A typical

38、process flow sequence is shown in Figure 4-6. Semichemical methods encompass the intermediate range of pulp yields between mechanical and chemical pulping, i.e., 55 to 85% on dry wood. The pulps have a number of end uses and some unique properties. As the prime example, pulps at about 75% yield exhi

39、bit exceptional stiffness, making them ideally suitable for the center fluted layer in corrugated container board.Strictly speaking, any mechanical pulping process that is modified by incorporating a chemical treatment of the chips prior to or during refining should qualify as semichemical pulping.

40、In practice, if the chemical treatment is relatively modest, the pulp is still considered to be a mechanical pulp. The general rule of thumb is that all pulps with a yield of 85% or higher are nominally considered to be mechanical pulps. As a result of this rather arbitrary classification, some cate

41、gories of modern pulping processes actually overlap between mechanical and semichemical pulping. For example, so-called chemimechanical pulps and very-high-yield sulfite pulps both cover the yield range from 80% to 92%.Included under the classification of semichemical pulping are the high-yield kraf

42、t and high-yield sulfite processes. In both instances the cooking is limited to partial delignification; the actual defibering is done mechanically. The degree of cooking controls the yield; as yield is increased, a greater amount of energy is required for defiberization.The neutral sulfite process,

43、 applied mainly to hardwood chips, is the most widely used semichemical process. Usually abbreviated to NSSC (for Neutral Sulfite SemiChemical). this process utilizes sodium sulfite cooking liquor which is buffered with sodium carbonate (soda ash) to neutralize the organic acids liberated from the w

44、ood during cooking.To get away from sulfur in the process, some semichemical cooking is now carried out with mixtures of sodium carbonate and sodium hydroxide (1). Other mills, especially where associated with a kraft mill, are using green liquor (sodium carbonate plus sodium sulfide) for cooking. A

45、 1978 survey of North American corrugating, medium mills (2) showed the following breakdown:figure 4-6. Flowsheet for semichemical pulping mill utilizing continuous digestion.Semichemical Process Number of MillsNSSC 21Green Liquor 8No-Sulfur 10Other 3Spent liquors from semichemical pulping operation

46、s are relatively concentrated sources of chemical and organic matter, and consequently, must be subjected to some type of chemical recovery-operation. Such processing can be readily accomplished in cross-recovery with an existing kraft or sulfite mill system. Where cross-recovery is not possible, fl

47、uidized bed incineration is the most common method used for combustion of organics and recovery of chemical.A good comparison of spruce semichemical pulps produced by five different methods and covering the entire yield range is provided in Table 4-6. The approximate behavior of selected strength pa

48、rameters over the full yield range for spruce kraft pulp is illustrated in Figure 4-7.figure 4-7. Effect of kraft pulp yield on selected strength parameters (at 500 CSF).4.2 MARKET PULPSPapermills can be broadly classified as either integrated (i.e. with their own pulp supply) or non-integrated. The

49、se distinctions are not always clearcut, and some qualification may be required in a given instance. For example, certain integrated mills must purchase a portion of their pulp requirement in the open market. As well, there are non-integrated mills that actually have a captive pulp supply, often fro

50、m another mill within the same company or from an affiliate.In order to appreciate the role of market pulps, it is useful to take a broad overview of the world situation as illustrated in Figure 4-8. The figures shown are order-of-magnitude: the situation is blurred because the tonnage contribution

51、of mineral loadings and coatings in overall paper and paperboard production is not known accurately. Also, note that a small percentage of the secondary fiber tonnage should be included in the total for market pulps.With respect to overall paper and paperboard production, market pulps make up a rela

52、tively small portion of the world figure 4-8. World pulp and paper production (000 000 metric tons per year).industrys fiber requirement, about 12%. Much of the market pulp is in the form of high-quality bleached softwood kraft, a vital component in many papermill furnishes. Nonetheless, because mos

53、t paper production capacity is independent of outside fiber supply, demand for market pulp is especially sensitive to the cyclical ups and downs of the paper industry.Nonpaper pulps comprise a small portion of overall virgin pulp production, under 5%; but they constitute over 20% of market pulp prod

54、uction. Generally, the end-uses are high value-added products, and the pulps serving these markets have special characteristics and command higher prices. Nonpaper pulps are marketed under three broad categories. The largest segment is dissolving pulp which has mature and stable markets. The fastest

55、 growing segment is fluff pulp. The balance, consisting of small and disparate uses (e.g. filters, inner shoe soles, laminates, etc.), are lumped together as specialty pulp.Dissolving PulpDissolving pulps are chemical pulps that are suitable for subsequent chemical conversion into such products as r

56、ayon, cellophane, cellulose acetate, cellulose nitrate and carboxymethyl cellulose. Production of chemical cellulose pulp grades represents a substantial industry in its own right.Dissolving pulps can be manufactured by either a modified kraft or sulfite process. In each case, the objective is a rel

57、atively pure and uniform cellulose product with a controlled weight-averaged degree of polymerization. Both lignin and hemicelluloses are considered contaminants and are removed. Softwoods are the major raw material, but some hardwood is used. The highest-quality pulps (99% alpha cellulose) are manufactured from cotton linters.The viscose-rayon process is the largest user of dissolving pulps. In the viscose process, the chemical cellulose is steeped in caustic solution to form alkali-cellulose