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.
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 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.
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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