The Manufacturing process

Pulp is the generic term for a wide range of products resulting from the manufacturing processes that involve the chemical and/or mechanical treatment of various types of plant material.

Wood pulp, like other types of pulp, is manufactured by separating the wood fibres which are held together by a material called lignin. The fibres can be separated by either mechanically tearing them apart or by chemically dissolving them. Over the years, research and development programmes have given rise to new systems that combine aspects of both processes, blurring the boundary between the two families of pulp.

Mechanical pulps (high yield pulps)

These are characterised by the fact that a very high yield is produced, in the region of 85 to 95 percent. The yield simply means the amount of original wood retained in the final product. The most common use for mechanical pulps is newsprint and ground wood printing-writing papers.

Chemical pulps (low yield pulps)

One of the main objectives of chemical pulping is to remove as much as possible of the lignin and extractives. The result is a much lower yield, in the region of 40 to 55 percent. The pulps produced are substantially stronger, easier to whiten and less likely to lose their brightness over time when compared to their mechanical counterparts. One of the most common uses for chemical pulps is white writing paper.

Regardless of the process used, one of the most important determining factors on the properties of a particular pulp yield is the characteristic of the wood that starts the process, e.g. the species, age, density, moisture content and the time that has elapsed since it was cut.

Both mechanical and chemical pulps, although different, go through the same initial process.

Stages of Pulp Processing

• Stage 1 :: Defibration. This is where undesirable fibrous and non-fibrous materials are removed, e.g. knots in wood pulp.
• Stage 2 :: The screening process. The remaining pulp is then screened for a dual purpose. One is to totally separate the fibres and the other is to remove any unwanted material from the fibres.
• Stage 3 :: Cleaning. This is where any small particles of dirt and grit that remain in the fibres are removed. This stage also marks the end of the mechanical pulp process.
• Stage 4 :: Unbleached pulp. At this stage, much of the unbleached pulp can be used for products that do not attach importance to colour, eg boxes, bags and newsprint. For higher grades (quality) of paper, such as writing paper, treatment is necessary in order to whiten the pulp. This treatment is known as bleaching. Bleaching involves heating or cooking the pulp as well as oxygen being blown over it in order to whiten the dark colour of the pulp (shown in the diagram below).
• Stage 5 :: Finishing. At the next stage, if the pulp is to be transported either for direct retail or to a paper or board mill, it will need to be drained of any remaining moisture. If the pulp mill has an integrated paper mill, the draining process is not required.

Date: 2015-02-28; view: 967