Optimization of Pretreatment of Kraft Pulp with Nitrogen Oxides
Modified kraft pulp subjected to oxygen bleaching following pretreatment with 2% nitrogen dioxide for 140 min with the addition of oxygen resulted in low selectivity after subsequent oxygen bleaching. In contrast, pretreatment in the absence of oxygen removed more than 80% of the lignin after oxygen bleaching and still maintained high selectivity (kappa number less than 5 at viscosity 950 dm3/kg).
Nitric acid which is produced and added during pretreatment is consumed in lignin reactions, which favor delignification and give rise to NO2. The presence of hydrogen ions is a key-parameter for the extensive generation of NO2, and also nitrate ions promote the generation of nitrogen dioxide.
By applying a programmed temperature profile and decreasing the acidity level during pretreatment of kraft pulp with NO2 it is possible to suppress the depolymerization of the polysaccharides and achieved higher selectivity under certain conditions after oxygen bleaching. Additional precaution against depolymerization of the cellulose is a stepwise dilution and a decreased temperature.
Pulp with kappa number 3.5 and intrinsic viscosity of 950 dm3/kg was produced from industrial kraft pulp, (softwood) by oxygen bleaching following pretreatment with 2% NO2. After treatment with nitrogen dioxide at 12% consistency without the addition of oxygen at modest temperature, (e.g. 40°C), the pulp was diluted with acid sodium nitrate solution to 5% consistency and kept at a high temperature, (e.g. 90°C) for 90-180 min. in stage two (ripening stage). This process is denoted as S3. The generation of NO2 from HNO3 is a very important reaction at ripening stage. Reactions between HNO2 and the lignin explain the effects on the pulp of dilution, temperature and oxygen addition.
Kraft pulp impregnated with different spent liquors and pretreated according to the S3-process resulted in high selectivity after oxygen bleaching. A kappa number of 3.5 at viscosity 950 dm3/kg was obtained even when the pulp contained 43 kg of solids from the black liquor per 1000 kg pulp.
Transition metal compounds like mangan and magnesium presence in the spent liquors affect the delignification and depolymerization of cellulose during oxygen bleaching. When no magnesium protector was applied before the oxygen bleaching, an addition of 300 ppm of manganese decreased the depolymerization rate of the cellulose. With 60 ppm Mn a net catalytic or retarded effect was found which depended on the bleaching conditions. An increased ration of Mg:Mn suppressed the depolymerization effectively during oxygen bleaching.