This essay focuses on Chemical pulping processes .A cooking liquor is used to dissolve the lignin, which is then wash from the cellulose. This preserves the length of the cellulose fibres.

Chemical pulping processes

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To make pulp from wood, a chemical pulping process separates lignin from cellulose fibre.

A cooking liquor is used to dissolve the lignin, which is then wash from the cellulose. This preserves the length of the cellulose fibres. Paper made from chemical pulps are also known as wood-free papers. This is because they do not contain lignin, which deteriorates over time. The pulp can also be bleached to produce white paper, but this consumes 5% of the fibres. Chemical pulping processes are not use to make paper made from cotton, which is already 90% cellulose.

There are three main chemical pulping processes: the sulfite process dates back.

The 1840s and was the dominant method before the second world war. The kraft process, invented in the 1870s and first used in the 1890s. Now the most commonly practised strategy. One of its advantages is the chemical reaction with lignin produces heat. Which can be use to run a generator.

Most pulping operations using the kraft process are net contributors. The electricity grid or use the electricity to run an adjacent paper mill. Another advantage is that this process recovers and reuses all inorganic chemical reagents. Soda pulping is another specialty process used to pulp straws, bagasse and hardwoods with high silicate content.

There are two major mechanical pulps: thermomechanical pulp (TMP) and groundwood pulp (GW).

In the TMP process, wood is chipped and then fed into steam-heated refiners. Where the chips are squeezed and converted to fibres between two steel discs. In the groundwood process, debark logs are fed into grinders. They are press against rotating stones to be made into fibres.

Mechanical pulping does not remove the lignin, so the yield is very high, > 95%. However, lignin causes the paper thus produced to turn yellow and become brittle over time. Mechanical pulps have rather short fibres, thus producing weak paper. Although large amounts of electrical energy are required to produce mechanical pulp. It costs less than the chemical kind.