After cooking process and washing process, the paper pulp is brown. Brown paper pulp presents no problem for certain uses, for example, sack paper, most corrugated boxes, some bag paper and so on. While, a major proportion of the Kraft pulp that is made is used for white or colored papers such as writing and printing papers, and then the pulp needs to be bleached, the process is called paper pulp bleaching process.
After several years of production practice, Anyang Machinery invented the disc filter, which is the latest generation of white liquor recovery equipment. Using the disc filter for pulp washing can save energy, reduce water consumption and fiber loss. Meantime protect the environment. For the superior appearance, disc filters are widely used in white liquor recovery system in paper mills. We can also produce gravity disc thickener and a series of paper pulp bleaching equipments. All of these equipments are up to the international advanced standards. Meantime, we can provide from bleaching equipment design to make installation and maintenance of professional one-stop service.
Bleaching involves removing virtually all of the lignin that still remains after cooking, as the lignin contains the chromophoric groups which make the pulp dark. Strictly speaking, bleaching and cooking are both delignification processes, and modern developments have tended to blur the difference between the two processes. However, traditionally the name ‘bleaching’ is reserved for delignification that is taking place downstream of the cooking process. In practice, there are two separate “bleaching” process steps: oxygen delignification and final bleaching.
To measure the lignin content in pulp, a number called the “Kappa number” is used. The Kappa number is directly proportional to the lignin content of the pulp. Pulp from the digester has a Kappa number of 20-35 for softwood and 15-20 for hardwood (hardwood contains less lignin and can therefore be cooked to a lower Kappa number). Oxygen delignification removes about half of the lignin remaining after the cooking process, so that the Kappa number of the oxygen delignified pulp is typically 12-18 for softwood. The final bleaching removes all remaining lignin and decreases the Kappa number to zero.