Influence of the Gas Phase on Pretreatment of Kraft Pulp with Nitrogen Oxides
While nitric oxide (NO) is virtually inert against water-impregnated kraft pulp it participates in important reactions when reactive nitrogen compounds, e.g. NO2 and HNO2, have initiated lignin reactions. Experiments with kraft lignin (Indulin AT) showed that nitric oxide was both produced and consumed during treatment with NO2. One reaction path in which NO was consumed was its reaction with NO2 to nitrogen(III)compounds, e.g. HNO2 and N2O3. These compounds were consumed in reactions with the lignin. The introduction of oxygen into the reactor suppressed the formation of nitrogen(III)compounds and favored the formation of nitrogen compounds of oxidation states IV and V.
The influence of the gas phase on the pulp properties depended on the conditions during the pretreatment. During pretreatments at 65-70 oC the kappa number was higher when oxygen was added compared to pretreatments in which the oxygen addition was omitted. The influence of the oxygen addition on the kappa number was reversed when the temperature was increased to 80-90 oC. The drop in viscosity during the pretreatment was smaller in treatments without the addition of oxygen than in those with added oxygen. This contributed significantly, especially at high temperatures, to the superior selectivities (viscosity at a given kappa number) obtained after the subsequent oxygen bleaching of pulps pretreated without the addition of oxygen. This can be mainly ascribed to a pH-buffering effect of HNO2, promoted by high partial pressure of NO, resulting in a suppressed acid hydrolysis of the polysaccharides.
The detrimental effect of oxygen on the viscosity after pretreatment and on the selectivity after the subsequent oxygen bleaching decreased when the oxygen was introduced during a late stage of the pretreatment.
The importance of efficient generation reactions of NO2 (including other reactive nitrogen compounds) was confirmed in experiments with consecutive treatments with NO2 and washing with water after each treatment. Superior selectivity was obtained when the pulp was pretreated once (2 % NO2) under favorable conditions compared to nine consecutive treatments with a total of 18 % NO2 under conditions unfavorable for the generation reactions.