The Paper Making Process, Part 4: The Pulping Process

The Pulping Process

Digestion, the first stage of the pulping process, is the process of removing lignin and other components of the wood from the cellulose fibers which will be used to make paper. Lignin is the “glue” which holds the wood together; it rapidly decomposes and discolors paper if it is left in the pulp (as in newsprint, which is usually made from groundwood* pulp with little or no chemical treatment).

Mechanical pulping. Approximately 15% of the world’s pulp is formed using a mechanical process (WRF 1997) whereby the raw material is broken down by attrition into its individual fibres by grinding. This process is not wholly satisfactory, as the fiber are broken into smaller pieces and relatively little lignin is released, resulting in a poor quality, ‘woody’ paper. The mechanical process is also energy intensive. Mechanical pulp is used for newsprint as the paper is highly absorbent and therefore soaks up ink and dries quickly.

Chemical pulping. Forty two percent of all the world’s pulping capacity uses a variety chemicals as part of a high temperature cooking, or digestion, process (WRF, 1997) which breaks down the lignin, freeing the cellulose fibers. This process produces a high quality product, although the type of chemical used will determine the properties of the final product:

  • Caustic soda or sodium sulphate will produce a pulp with coarse, strong fibres (known as Kraft) suitable for strong boxes.
  • Ammonia or calcium sulphate will produce a finer fiber suitable for high quality printing and writing paper.

For smaller mills the chemical process is usually used, as purchase of dedicated equipment for wood grinding is available only for larger scale operations and is expensive to purchase. Furthermore, suitable types of wood for paper production are seldom available in developing countries.

The prepared stock is fed into the top of a digester and mixed with the cooking chemicals, which are called “white liquor” at this point. Digestion may be carried out on a batch or a continuous basis. For small-scale mills of up to, say, 30 t.p.d., batch cooking is preferred. Batch digesters are able to cope with a variety of stock feeds, for example straw, baggase, cotton and wood, in the same mill. As the stock and liquor move down through the digester, the lignin and other components are dissolved, and the cellulose fibers are released as pulp. After leaving the digester, the pulp is rinsed, and the spent chemicals (now known as “black liquor”) are separated and recycled (see later). In a typical rotating spherical batch digester capable of handling 30 t.p.d., the complete process from filling to emptying takes approximately 5-7 hrs. (Small-scale Paper Making’, ILO, 1985).

Bleaching and Refining

At this point, the “brownstock” pulp is free of lignin, but is too dark to use for most grades of paper. The next step is therefore to bleach the pulp by treating it with chlorine, chlorine dioxide, ozone, peroxide, or any of several other treatments. A typical mill uses multiple stages of bleaching, often with different treatments in each step, to produce a bright white pulp. Chlorine bleaching generally provides the best performance with the least damage to the fibres, but concerns about dioxins and other by-products have led the industry to move towards more environmentally friendly alternatives.

At this point, the individual cellulose fibres are still fairly hollow and stiff, so they must be broken down somewhat to help them stick to one another in the paper web. This is accomplished by “beating” the pulp in the refiners, vessels with a series of rotating serrated metal disks. The pulp will be beaten for various lengths of time depending on its origin and the type of paper product that will be made from it. At the end of the process, the fibers will be flattened and frayed, ready to bond together in a sheet of paper.

Forming the Sheet

Once the pulp has been bleached and refined, it is rinsed and diluted with water, and fillers such as clay or chalk may be added. In the mechanized process, this “furnish*”, containing 99% water or more, is pumped into the flowbox of the paper machine. From the headbox, the furnish is dispensed through a long, narrow “slice*” onto the “wire*”, a moving continuous belt of wire or plastic mesh. As it travels down the wire, much of the water drains away or is pulled away by suction from underneath. The cellulose fibers, trapped on the wire as the water drains away, adhere to one another to form the paper web. From the wire, the newly formed sheet of paper is transferred onto a cloth belt (or “felt”) in the press section, where rollers squeeze out much of the remaining water. In smaller paper mills the newly formed sheet may be handled manually and stacked one layer on top of another and pressed using a hydraulic press to remove the excess moisture.

Coating, Drying, and Calendering

After leaving the press section, the sheet encounters the drying cylinders. These are large hollow metal cylinders, heated internally with steam, which dry the paper as it passes over them. The sheet will be wound up and down over many cylinders in the drying process. Between dryer sections, the paper may be coated with starch to improve the printing and strength characteristics. After another round of drying, the paper sheet is passed through a series of polished, close-stacked metal rollers known as a “calender” where it is pressed smooth. Finally, the sheet is collected on a take-up roll and removed from the paper machine.

Cutting and Packaging

In many cases, the new paper roll is simply rewound on a new core, inspected, and shipped directly to the customer. Other paper grades, however, may be further smoothed by passing them through a “supercalender” where the sheet is polished by passing between steel and hard cotton rollers (much like ironing fabric), or they may be embossed with a decorative pattern. The paper may also be cut into sheets at the mill, often by automatic equipment which accepts a roll of paper at one end and delivers packages of cut sheets at the other, already boxed and wrapped for shipping.

Case Study – TARA

Established in 1988, TARA (Technology and Action for Rural Advancement) has now become a major manufacturer of handmade paper. It is the production and marketing wing of Development Alternatives, an international network dedicated to sustainable development, and operates on four basic principles. It aims to create new, local jobs – particularly for unskilled women and currently employs 35 women and 7 men. It makes products for basic needs and conserves scarce resources, for example, wood and water, through using alternative materials and recycling. It tries to minimize pollution.

Production of handmade paper The basic principles of handmade papermaking are quite similar to those in large mills.

Sorting and dusting. The raw material, in this case mainly rags, is sorted manually to remove buttons, plastic, synthetic fibers and other foreign materials. It is also given a vigorous shake to remove the dust and dirt.

Rag chopping. The sorted material is chopped into small uniform sized pieces.

Digestion and beating. The raw material is mixed with water and inert chemicals, such as chalk, clay, alum or rosin, and beaten in a Hollander beater. This is an oval U-shaped trough, with a heavy roll; the face of the roll carries hard wearing metal bars and similar bars are set into the
plate below the roll, which cut the raw material to make the pulp (see figure2). There is also a washing drum, which cleans the pulp and removes the dirty water. The consistency of the pulp determines the quality of the paper which will be made.


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