1 grindstone wood pulp

The grinding equipment is of various types of grindstone grinding machines, 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 section can be continuously added to the material box, and the wood section is subjected to the pressure of rotating the chain downward, and is in close contact with the surface of the rotating grindstone. In the friction zone, the interfiber layer of the wood fiber is softened by friction and is softened and torn into a single sheet. The root fiber is washed by water from the surface of the grindstone to the underlying slurry pit. The properties 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. The pulp is generally made of wood such as spruce, fir, and poplar, with a whiteness of about 60%, and has good opacity and ink absorbing properties, and is generally used for production of newsprint and medium and low-grade printing paper.

2 brown groundwood pulp

The grinding equipment used is similar to the grindstone wood pulp, but the wood section is pre-steamed by steam to above 120 °C before being ground. The pulp is brown and the physical strength is higher than that of the grindstone. Used in the manufacture of specialty paperboard.

3 disc grinding mechanical pulp

The grinding equipment is a disc grinder (see chemical mechanical pulping). The raw material is wood chip or other fiber material. The disc grinding mechanical pulp generally adopts two-stage refining method. The first-stage refiner grinds the material into fiber material, and then enters the second-stage refiner for fine grinding to separate the fiber. The bundles and the fine fibers are split to increase the bonding force between the fibers. The biggest advantage of disc grinding mechanical pulp is that the raw material does not require a complete wood segment and is suitable for a wide range of raw materials.

4 thermomechanical pulp

The main equipment of the grinding is the same as the disc grinding mechanical pulp, but the web is preheated by steam before entering the disc refiner, and the heating temperature should be kept lower than the condensation temperature of the lignin in the raw material. The whiteness of the slurry is slightly lower than that of the grindstone, and the physical strength is higher, and the use range is basically the same. The thermomechanical pulp develops rapidly due to its good quality, but its energy consumption is higher than that of the millstone wood pulp by more than 50%, which is the main problem being studied and improved.

5 pressure grindstone grinding wood pulp

The main grinding equipment is the same as the grindstone wood pulp, but the grinding zone and the abrasive trough can be closed to bring the pressure in the refining zone, thus having a higher grinding temperature than the grindstone. This slurry has a higher physical strength close to the thermomechanical pulp, while the energy consumption is close to the grindstone. However, due to the complicated structure of the wood grinder, the required investment is high.

Pulping process

Preparation, refining and purification are important processes in this process. After mechanical pulp is ground into pulp, it generally contains incompletely disintegrated fiber bundles, a small amount of wood chips and sand dust and other impurities, which are generally purified by screening, sedimentation and hydrocyclone. The separated coarse fibers or chips can be passed to a refining device for re-moulding. The whiteness of mechanical pulp is generally less than 60%, and bleaching can also be carried out in order to dispense paper with high whiteness requirements. Due to the high lignin content of mechanical pulp, bleaches are generally more expensive hydrogen peroxide (hydrogen peroxide) and sodium dithionite (safety powder). After two stages of bleaching, bleached mechanical pulp with a whiteness of about 80% can be obtained.