A pretreatment method with nitrite, nitric acid and oxygen for oxygen bleaching of kraft pulp is proposed as one of the possible methods for the reduction of organic chlorine compounds from bleaching process. The effects of nitrous acid pretreatment is compared with those of nitrogen dioxide pretreatment, and its bleaching mechanism is discussed.
(1) Pulp at medium consistency is pretreated with nitrite (as NO2- 0.3% on pulp) and nitric acid (4% on pulp) at 90°C for 3 hours in the atmosphere of oxygen, and then is oxygen bleached.Kappa number of oxygen-bleached pulps can decrease to 34 for both beech and ezo spruce pulp pretreated.
(2) The relationship between kappa number and viscosity of oxygen-bleached pulp after the nitrous acid pretreatment is similar to that after the nitrogen dioxide pretreatment which is correspond to Samuelson's S 3 process. Although Samuelson's S 3 process improves oxygen bleaching, it requires the two-stages pretreatment and gas-phase nitrogen dioxide. The nitrous acid pretreatment is free from these defects.
(3) In the Samuelson's S 3 process, nitrous acid which is a reasonable by-product from the first-stage treatment with nitrogen dioxide is thought to be a key compound in decreasing kappa number of pulp in the second-stage treatment.
(4) Coexistence of oxygen shows more effective kappa number reduction during the nitrous acid pretreatment. Veratryl alcohol, a non-phenolic lignin model compound, gives methanol under the nitrous acid pretreatment conditions with oxygen. It also gives more veratraldehyde in the presence of oxygen. These results indicate that oxygen in this pretreatment should play an important role in degradation and modification of lignin.
(5) Four-stages (NODP) bleaching of beech pulp with nitrous acid pretreatment (N), oxygen (O), chlorine dioxide (D), and hydrogen peroxide (P) gives pulps with the same brightness (87%) and viscosity (20 cP) level as a conventional bleaching (OCEHD).
