The purpose of digesting is to use alkaline chemicals to remove lignin from the fiber raw materials as much as possible, and retain cellulose and hemi-cellulose, and dissociate the fibers into the pulps according to the requirements of different pulps. Digesting is a complex chemical process. Generally, the basic operation flow includes loading, liquor feeding, heating, temperature holding, steam discharge and unloading.

The digestion equipments can be divided into two types: batch type and continuous type. The batch digesting equipments includes spherical boiler, pulp digester, and the like; the continuous digestion equipments can be divided into the horizontal tube type, the inclined tube type, the vertical type, and the like.

pulp digester

Spherical Boiler

Spherical boiler is mainly used for small or medium-sized straw pulp mill who adopts the equipments by the alkaline method, the sulfate method or neutral salt method. The structure includes a sphere, a base, and transmission system.

Pulp Digester

Pulp digester is a vertical type of fixing equipment. Due to different corrosiveness and process characteristics of digesting liquors, their structures, forms and volumes are also different. Therefore, it is divided into sulfate digester and sulfite digester. It is generally large in volume and is used in large and medium-sized paper mills. Its corollary equipments include loader, blow tank, and the like.


Digestion Equipment

Continuous digestion equipments can be divided into tower (or vertical) type continuous digester, also known as kamyr continuous digester; pandia horizontal tube type or inclined tube type continuous digester. Tower continuous digester is suitable for large and medium-sized wood pulp mills and is most commonly used worldwide. It can be divided into single tower type and double tower type; the horizontal tube type can be divided into one, two and three tube type; the inclined tube type digestion equipments can be divided into single inclined tube type and double inclined tube type.

The key equipments of the horizontal tube type continuous digester include: screw feeder, and double pin drums, double pulsator or double helix gauges, T-shaped tube, pressure impregnator and unloader.


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Pulp is a lignocellulosic fibrous material prepared by chemically or mechanically separating cellulose fibres from wood, fiber crops, waste paper, or rags. Many kinds of paper are made from wood with nothing else mixed into them. This includes newspapers, magazines and even toilet paper. Pulp is one of the most abundant raw materials.

A pulp mill is a manufacturing facility that converts wood chips or other plant fibre source into a thick fibre board which can be shipped to a paper mill for further processing. Pulp can be manufactured using mechanical, semi-chemical or fully chemical methods (kraft and sulfite processes). The finished product may be either bleached or non-bleached, depending on the customer requirements.

A paper machine (or paper-making machine) is an industrial machine which is used in the pulp and paper industry to create paper in large quantities at high speed. Modern paper-making machines are based on the principles of the Fourdrinier Machine, which uses a moving woven mesh to create a continuous paper web by filtering out the fibres held in a paper stock and producing a continuously moving wet mat of fibre. This is dried in the machine to produce a strong paper web.

The pulp produced up to this point in the process can be bleached to produce a white paper product. The chemicals used to bleach pulp have been a source of environmental concern, and recently the pulp industry has been using alternatives to chlorine, such as chlorine dioxide, oxygen, ozone and hydrogen peroxide.

Chemical pulp is produced by combining wood chips and chemicals in large vessels called digesters. There, heat and chemicals break down lignin, which binds cellulose fibres together, without seriously degrading the cellulose fibres. Chemical pulp is used for materials that need to be stronger or combined with mechanical pulps to give a product different characteristics. The kraft process is the dominant chemical pulping method, with the sulfite process second. Historically soda pulping was the first successful chemical pulping method.