Different Processes For Making Wood Pulp

Trees are the primary raw material for making wood pulp used by the paper and board industry. It is made of cellulose fibers bound together with linen. For making it the fibers are separated from each other and converted into a mass in a mill. The separation of the fibers is done mechanically where the fibers are separated using certain chemicals. The linen is dissolved away with the help of the chemicals. After the process of separation is completed the fibers are washed and screened to remove the remaining bundles.

The pulp is used for making unbleached papers or for bleached papers used for making bright paper. It is directly fed into a paper machine for drying and pressing into bales. This is the raw material used in the paper mills all over the world. Since the trees vary from each other, no two trees product the same grade of fiber. Even if the same species of the trees are grown in different parts of the world, the fibers produced are different from each other.

For example, the slow growing pines found in Northern Europe have long fibers that provide strength and long life required for high grade printing papers and packaging. The shorter fibers produced from the pines growing closer to the Equator have low strength and no other properties necessary for making other products. Sometimes, the properties depend on the method used for separating the fibers.

Mechanical pulping is used these days, especially for newsprint. Currently, the mechanical pulping uses new techniques with the help of refiners. The wood chips are subject to intense shearing forces between a fixed plate and a rotating steel disc. This is followed by the pre-softening of the chips using heat and a mild chemical treatment. The chemical used for the treatment is sodium bisulphite.

With the help of mechanical pulping about 90% of the material is obtained because it uses every part of the log except the bark. The energy requirement for this process is very high and can be only partially compensated by using the bark. The investment costs for this type of mechanical mills are relatively low when compared to other types. The product obtained in this process can be used for making bulk grades of paper like packaging and newsprint etc.

For use in higher value products the pulp can be bleached and used. It has low strength when compared to the chemical pulping because it retains the linen that reacts with ultra violet light when exposed to sunlight. Another property is that it can become pale when exposed to light.

The material produced using the chemical pulping process is based on two types of procedures, sulphate and sulphite. The pulp produced by the sulphate process uses sodium sulphate and caustic soda to cook the chips. This results in the production of a strong and dark brown pulp that can be bleached for getting the required brightness. The sulphite mills produce brightly bleached pulp using hydrogen peroxide.

These materials meet the requirements of the disposable sectors of chlorine free products. They are also used as writing papers and in printing. The yield from the chemical pulping is less compared to the mechanical pulping because the linen in fully dissolved and separated from the fibers. The waste linenis used as fuel. Modern chemical wood pulp mills are net energy producers that supply power to local heating plants.

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