Patent EA-005240-B1 (EA)
Title: Method for bleaching paper pulp
Inventor: DELMAS MICHEL
IPC: D21C9/16
Filed: 2001-09-14
Abstract: 1. A method for bleaching raw paper pulps with organic peracids, characterized in that it is carried out in two steps at atmospheric pressure and at a temperature of less than or equal to 100 degree C, wherein: the first step is carried out by bringing the raw pulp into contact with a mixture of peracetic acid and performic acid so that the mixture obtained has a water content of less than or equal to 15% by weight; the second step is carried out by treating the bleached pulp obtained, after mechanical separation of the reaction medium obtained at the end of the first step, with a solution of sodium hydroxide and hydrogen peroxide. 2. The method as claimed in claim 1, characterized in that the peracetic acid and the performic acid are obtained by bringing a mixture of acetic acid and formic acid into contact with hydrogen peroxide having a concentration greater than 50% by weight. 3. The method as claimed in any one of claims 1 to 2, characterized in that the acetic acid + peracetic acid/formic acid + performic acid ratio is of the order of 9/1 by volume. 4. The method as claimed in any one of claims 1 to 3, characterized in that the quantity of peracids at the beginning of the first step is greater than or equal to 20% by weight. 5. The method as claimed in any one of claims 1 to 4, characterized in that the raw pulps are brought into contact with the mixture of peracids counter current wise with recirculation of the peracids. 6. The method as claimed in any one of claims 1 to 5, characterized in that the solution of peracids after the first bleaching step is enriched with peracids so as to bring the concentration of peracids to the chosen value. 7. The method as claimed in any one of claims 1 to 6, characterized in that the pulps treated during the first step are separated from the peracids by pressing. 8. The method as claimed in any one of claims 1 to 7, characterized in that the pressed pulps are deacidified by drying under vacuum, leaving residual hydrogen peroxide in the pulp. 9. The method as claimed in any one of claims 1 to 8, characterized in that the contact time between the peracids and the raw pulp is between 1 hour and 3 hours. 10. The method as claimed in any one of claims 1 to 9, characterized in that the treatment temperature is between 60 degree C and 90 degree C. 11. The method as claimed in any one of claims 1 to 10, characterized in that the second step is carried out by bringing the deacidified pulp into contact with a basic aqueous solution (pH between 8 and 10) containing 1 to 4% by weight of hydrogen peroxide (calculated relative to the dry pulp). 12. The method as claimed in any one of claims 1 to 11, characterized in that the contact time between the pulp and the hydrogen peroxide in a basic medium is between 1 h and 3 h. 13. The method as claimed in any one of claims 1 to 12, characterized in that the pulps obtained during the second step are, after pressing, washed with demineralized water. 14. The method as claimed in any one of claims 1 to 13, characterized in that the water introduced, by the raw pulp, on the one hand, and the hydrogen peroxide, on the other hand, is extracted from the cycle by distillation. 15. A paper pulp, obtained according to the method of claims 1 to 14 having a brightness value greater than 70. 16. A paper, manufactured using a pulp in accordance with claim 15.