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Converting wood into pulp for paper-making

Wood is made up of two basic substances: cellulose (fibres) and lignin (which is essentially the glue that holds the fibres together). When the fibres are separated, the result is wood pulp.

The process of converting wood into pulp for paper-making is remarkably simple, there are two common processes for separating the fibres: mechanical pulping and chemical pulping. Mechanical pulping grinds the wood into individual fibres, while chemical pulping dissolves the lignin. The most common chemical pulp is kraft pulp, made through an alkaline cooking process that breaks the lignin down with the help of sodium sulphate. Bleached kraft pulp is known for its strength, absorbency and brightness.

The lopings, logs, or slabs, up to 11 inches square, are put into a hopper or feeder and guided under the cutter; this consists of a discplate, which, revolving at great speed, disintegrates the laminations of the wood. The wood falling on horizontal rollers. By packing in this manner, 60 cwt. of wood can be treated at one time in place of a limit of 32 cwt. of other material, and as nearly the same quantity of alkali or soda ash is required in dealing with either the large or the small weight. After boiling the wood about three hours, a pulp is produced from which paper can be made ,without the admixture of rags esparto, or any other material.

The basic process for making paper is simple. Pulp fibres are combined with water and other ingredients, forming a mixture that is similar in consistency to orange juice. The mixture is strained through a fine screen, which serves as a bed for the mat of wet fibres. The mat is dried with the help of heat, gravity, pressure and vacuum technologies, and the result is paper.

Modern papermaking machines can produce more than 700 tonnes a day at speeds of over 1,400 metres a minute. Equipment is customized to make many paper products, with different levels of quality. The machines and processes vary, but they do have things in common. They are all complex and they require the dedication of highly skilled workers.


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