The Mechanism And Controlling Of Oxalate Formation During Oxygen Delignification

Oxalate scaling appears to be a difficult problem during pulp and paper making process. It exists in multiple sections of the mill, and it is very difficult to remove once it is formed. Reducing oxalate formation is the crucial method to control oxalate scaling. However, numbers of studies have just focus on the oxalate formation during hydrogen peroxide and ozone bleaching using mechanical pulp and chemical pulp, respectively. It is limited to recognize the oxalate formation during oxygen delignification. Therefore, in this work the oxalate formation and controlling were investigated during oxygen delignification, using Eucalyptus kraft pulp. The results are significant to deepen the recognition of oxalate scale problems, control the oxalate formation, improve energy efficiency and ensure normal operation of production.For these reasons, extraction method of the total and soluble oxalate contents in several raw materials was investigated and the oxalate was determined by Headspace Gas Chromatography. It was found that soluble oxalate can be extracted completely by distilled water at 70°C within 180 min. Total oxalate can be extracted completely by 2.0 mol/L hydrochloric acid at 70°C within 210 min. The developed method can be used determine the soluble and total oxalate contents in pulping and papermaking raw materials. It is well-suited for use in the pulp and paper industry.The effects of different factors on the oxalate formation of Eucalyptus kraft pulp were investigated in this study. The results showed the oxalate content in the effluent was linearly increased with the increased caustic charge. And oxalate content increased with increasing temperature, time, and oxygen pressure, and then kept stable. Based on single-factor experiments, the response surface methodology was used to optimize the main factors. The optimized condition was caustic charge 1.5%, 60 min, 100°C, 0.8 MPa, MgSO4 0.2% and 10% pulp consistency, indicating that the amount of oxalate formation can be effectively suppressed during oxygen delignification. Furthermore, an empirical model of oxalate formation was developed based on pulp initial kappa number and influence factors. The good regression coefficients(R2=0.967) indicate that the models are justifiable for the oxalate prediction, and applicable to the kraft eucalyptus pulp during oxygen delignification.We also studied the oxalate formation during hydrogen peroxide reinforced oxygen delignification(OP) and two stage oxygen delignification(OO) of Eucalyptus kraft pulp. The oxalate formation from OP was similar to the single oxygen delignification, but the oxalate formation rate was higher than that of single oxygen delignification. Compared with second stage of oxygen delignification, caustic charge, temperature had a great impact on the first stage oxygen delignification. There is no big difference between reaction time on the first and second stage.Meanwhile, the mechanism of oxalate formation was studied during the oxygen delignification process. It was found that lignin and hemicellulose were the main sources for the oxalate formation except cellulose. Compared with non-phenol lignin, more oxalate was formed from phenol lignin. In addition, with the increasing of the residual lignin content of black liquor, the oxalate content was also increased. The contribution of residual lignin of black liquor in pulps to oxalate was 58g/kg.At last, we selected several methods for controlling oxalate formation during the oxygen delignification. Oxalate related scaling could be decreased by xylanase and sodium phytate pretreatment. With the pretreament of the xylanase, the oxalate and carbonate content of oxygen delignification effulent were dropped by 26% and 30%, respectively. Combining xylanse pretreatment with ultrasonic technique, the oxalate and carbonate content were dropped by 35% and 31%, respectively. With the pretreament of sodium phytate, the oxalate and carbonate formation was declined, which were almost dropped by 29% and 25%. Moreover, conditions of the degradation of oxalate by oxalate oxidase were investigated in this paper. The oxalate in the effulent can be enzymatic degraded by 58%. So oxalate oxidase treatment can be used as an effective method to control oxalate in the effluent.