Pulping Process

Raw Materials In the past, the traditional fibrous raw materials for the manufacture of paper were cloth and agricultural residues; today the vast bulk of cellulose pulp is made from wood. Although over half of this is produced from softwoods (long-fibered), an increasing amount is being now produced from hardwoods (short-fibered). Traditional raw materials such as linen, cotton waste, straw, and agricultural residues are still used in small quantities, particularly where the paper sheet requires special properties. These distinctions are important in the selection of pumps, for the liquors have different characteristics. For example, straw black liquor is much more viscous than wood black liquor, and the proper corrections must be made in calculating the pump performance and pipe fric-tional losses.

Many grades of paper include fiber recovered from waste paper—both pre- and postconsumer. Recently, because of environmental concerns, much emphasis has been placed on recycling post-consumer paper products. Specialized plants exist for the recovery and processing of recycled fiber before it is used for papermaking. Some of the different types of recycled fiber are ONP (old newsprint), OCC (old corrugated containers), and MOW (mixed office waste). Special care is needed in the selection of pumps to handle recycled stock because of the large amount of foreign matter present—rope, string, metal, synthetic fibers and adhesive materials—all of which can cause problems in the process and pumping. Deinking is also widely used in the processing of recycled fibers. Flotation cell deink-ing, a popular method, uses large amounts of entrained air, which has a dramatic effect on centrifugal pump performance and must be accounted for.

Groundwood Pulp This type of pulp is produced by simply grinding away wood by mechanical action. Almost all of the wood is used in the pulp, including many of the resins and other complex organic compounds. The fibers are bruised so the pulp has inferior strength.

Large amounts of water are required for cooling and for carrying away the ground-wood pulp; the latter is usually acidic (pH 4 to 5), and so corrosion-resistant materials must be used. The pulp is used primarily for newsprint and magazines. Depending on the end use, some mechanical treatment (refining) may be required to alter the characteristics of the pulp, particularly the viscosity. In some cases, a mild bleach may be used to improve the color.

Refiner Mechanical Pulp The refiner mechanical pulping process utilizes a disc refiner to reduce wood chips to fibers. It produces a longer fibered pulp than conventional groundwood, but not as long as the chemical pulps. The pulp is therefore somewhat stronger and freer than SGW, but not nearly as strong as kraft pulp. RMP is actually the first of many processes utilizing the disc refiner to produce pulp, The processes vary as to the number of refining stages, refining pressure, temperature, and pre-treatment of the chips (steaming, chemical pre-treatment and so on). Two of the more popular versions are TMP (thermomechanical pulp), in which chips are pre-softened with steam, and CTMP (chemithermomechanical pulp), in which they receive an additional chemical pre-treatment. RMP pulps do not require a rigorous bleaching process in contrast to chemical pulps.

Chemical Pulping Wood is a complex, nonuniform material containing about 50% by weight cellulose fiber, 30% lignins, and 18 to 20% carbohydrate. The remainder is proteins, resin, and other complex organic compounds that vary from one species to another. Cellulose resists attack from most chemicals, whereas the carbohydrates and other organic materials generally form compounds with the chemical cooking liquor. Some paper products can use the carbohydrate fraction to contribute bulk to the sheet, and for such papers groundwood and RMP and other mechanical pulps are used. Where high strength is required, cooking is necessary to separate the fibers completely from the remainder of the wood.

Most cooking of wood is done in a pressure vessel at high temperature and pressure in the presence of an acid or alkali.

There is considerable tradition in chemical pulping, and a number of different processes are used. For many years the traditional method of producing pulp for highgrade papers was the acid sulfite process. This has been largely superseded in recent years by an alkaline process using sodium-based liquors in the presence of a sulfur radical; this is known as the sulfate or kraft process. The main reasons for the change to the sulfate process have been lower corrosion rates, ease of chemical recovery, and a stronger pulp. The properties of the liquids pumped in the two processes are different, and the pumps require different materials of construction.

Typical Sulfate Process Pulpwood logs are first chipped to about 4 by 8 in (19 by 3 mm) and then charged into either a continuous digester or a series of batch digesters. Digester capacities range from approximately 100 air-dried tons per day to over 2000 air-dried tons per day, necessitating a wide range of hydraulic coverage for digester pumps. Cooking liquor (NaOH plus up to 30% Na2S) is then allowed to react with the wood chips for 2 to 22 h at a temperature up to 350°F (177°C) and a pressure in the digester of 80 to 100 lb/in2 (551 to 689 kPa). In many mills, the heating of the chips and cooking liquor is by direct steam injection to the digester. In others, some form of indirect heating is used with a closed liquor recirculation system. In the latter case, the digester circulating pumps are a critical item because they must handle hot caustic solutions and entrained solid matter in a closed, pressurized circuit. After cooking, the contents of the digester are discharged to atmospheric pressure into a vessel called the blow tank, where the sudden expansion causes the fibers to separate from the liquid, which is now known as black liquor.

At this point, the process splits into two streams—one for fiber processing and the other for chemical recovery. The fiber is washed and screened and then formed into a pulp or paper sheet. The black liquor is washed from the pulp and treated for chemical recovery. Because the most troublesome liquors are to be found in the recovery process and bleach plant, the selection of these pumps is critical for the successful operation of the process.

The chemistry of the recovery process is as follows: After concentration of the black liquor in multiple-effect evaporators to about 50% total solids, the final concentration to 60 to 65% is done by direct contact with hot flue gas from the waste heat or recovery boiler. The 65% concentration black liquor is mixed with salt cake (Na2SO4) before being sprayed into the furnace under pressure generated by high-pressure pumps. The furnace atmosphere is maintained with a minimum of excess air so the Na2SO4 is reduced to Na2S, and sodium carbonate (Na2CO3) is formed in the process. These molten chemicals run out as a smelt and are dissolved in a tank to form green liquor. This liquor is then causticized with lime to form caustic soda (NaOH), with the Na2S still present along with other residual chemicals, thus forming the regenerated cooking liquor known as white liquor. The calcium carbonate (CaCO3) formed is burned in a lime kiln for reuse in causticizing. Various lime slurries and residues are formed during this process. The white liquor it then clarified and reused in the digesters, completing the cycle, as shown in Figure 1.

There are a variety of other pulping processes in use, but the sulfate process offers so many advantages that almost all recent installations have been of this type.

Bleaching Bleaching may be considered an extension of the cooking process, the object being to remove the coloring matter, carbohydrate, and lignins to that the remaining pulp

FIGURE 1 The recovery cycle in the sulfate process.

contains a maximum percentage of alpha cellulose, which is the purest cellulose form and the one most resistant to attack from normal chemicals. Bleaching is carried out to reach a degree of reflectance of monochromatic light and called pulp brightness. Because of resistance to attack, special highly reactive chemicals must be used for bleaching. The traditional chemical used for bleaching is chlorine, which is highly selective at attacking the lignin while resisting attack of the cellulose. Chlorine has a tendency, however, to form pollutants in the bleach plant effluent and is therefore being replaced with other chemicals such as chlorine dioxide, hydrogen peroxide, and gaseous oxygen and ozone. Chlorine dioxide produces highly corrosive liquors in the bleach plant.

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Survival Treasure

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