Mechanical pulping is classified according to different grinding equipment and pulp quality

Mechanical pulping According to the different grinding equipment and pulp quality, it is mainly divided into:

①Whetstone grinding wood pulp

Grinding equipment is various types of grindstone wood grinders, mainly including bag type (including Dabei type, Kaweier type, etc.), chain type (see figure) and ring type, the first two are more commonly used. The wood segment can be continuously added to the feed box. The wood segment is pressed by the downward rotating chain and is in close contact with the surface of the rotating grindstone. In the friction zone, the wood fiber interlayer is softened and torn by the friction heating to form a single The root fiber is washed by water from the surface of the grindstone to the pulp pit below. The characteristics of the surface of the grindstone have a great influence on the quality of the pulp, and can be decorated with a stone carving device. This kind of pulp generally uses spruce, fir, poplar and other wood as raw materials, the whiteness is about 60%, and has good opacity and ink absorption performance. It is generally used for the production of newsprint and low-end printing paper.

②Brown ground wood pulp

The grinding equipment used is similar to grindstone wood pulp, but the wood segment is pre-steamed and heated to 120 ° C or more before the grind. This pulp is brown, and the physical strength is higher than the grindstone wood pulp. Used to manufacture special cardboard.

③Disc mechanical pulp

The disintegration equipment is a disc mill (see chemical mechanical pulping). Pulping raw materials For wood chips or other fiber materials, disk grinding mechanical pulp generally adopts two-stage refining method, the first stage refining machine Grind the material sheet into fiber material, and then enter the second stage refiner for fine grinding to separate fiber bundles and split fine fibers to improve the bonding force between the fibers. The biggest advantage of the disc mill mechanical pulp is that the raw material does not need a complete wood segment, and it is suitable for a wide range of raw materials.

④Hot grinding mechanical pulp

The main equipment for grinding is the same as that of the disc mill mechanical pulp, but the tablets are pre-steamed and heated before entering the disc mill. The heating temperature should be kept below the condensation temperature of the lignin in the raw materials. The whiteness of this slurry is slightly lower than that of millstone and wood pulp, while the physical strength is higher, and the range of use is basically the same. Thermo-mechanical pulp has developed rapidly due to its good quality, but its energy consumption is more than 50% higher than that of grindstone wood pulp, which is the main problem being studied and improved.

⑤Pressure stone grinding wood pulp

The main grinding equipment is the same as the grinding stone grinding wood pulp, but the grinding area and the abrasive groove can be closed to make the grinding area pressurized, so it has a higher grinding temperature than the grinding stone grinding wood pulp. This kind of slurry has a higher physical strength close to that of thermal mechanical pulp, and the energy consumption is close to that of grinding stone pulp. However, due to the complex structure of the wood grinder, the required investment is high.

Pulping process

Preparation, grinding and purification are important processes of this method. After being crushed into pulp, mechanical pulp generally contains incompletely dissociated fiber bundles, a small amount of wood debris, sand and other impurities, and generally needs to be purified by screening, precipitation and hydrocyclone. The separated coarse fibers or debris can enter the grinding equipment for re-grinding. The whiteness of mechanical pulp is generally less than 60%. In order to prepare paper with higher whiteness requirements, it can also be bleached. Due to the high lignin content of mechanical pulp, bleaching agents generally use the more expensive hydrogen peroxide (hydrogen peroxide) and sodium dithionite (safety powder). After two stages of bleaching, a bleaching mechanical pulp with a brightness of about 80% can be obtained.