Process for treating wood pulp

摘要

To prepare a pulp (P) of papermaking fibers with a wood content, from used paper materials (M), a suspension (S) is formed which is cleaned (2) of impurities and is formed into a high consistency (HC) suspension with a solid content of at least 15%. The high consistency (HC) material is ground (4) to reduce the thickness and/or stiffness of the wood fibers while retaining their length. The ground material is diluted and passed through a slit fractionating stage (6), with the overflow (7) returned to the high consistency grinder (4). The fraction passes (8) to a hydrocyclone fractionating station (9), with its overflow also linked to the grinder (4) and its usable material (11) carried off as pulp (P). The high consistency (HC) material is dispersed before dilution. During grinding (4), the high consistency (HC) material is worked with an energy transfer of at least 200 KWh/to and pref. 300-400 KWh/to. The gap between the moving grinding tools is at least 0.5 mm. The grinding is at a temp. of max. 80 degrees C. The specific work of the high consistency grinder (4) is set so that the fiber content of the usable material from the hydrocyclone (9) has a fiber thickness is less than 5% of the paper thickness produced from the pulp (P). The slits in the fractionating stage (6) have a width of max. 0.2 mm, working with a suspension density of 1-4%. Cleaners are used at the hydrocyclone (9) working with force fields of≥800 m/sec<2>, where the suspension has a density of 0.8-1.5%. The material delivered from the grinder (4) is cleaned of the major portion of any hydrophobic matter by flotation (12). The process includes at least one chemical bleaching stage (14'). The process can include a fine grinding action, where the mean fiber length is reduced by≥20%.