ETHANOLAMINE PULPING-AS A NOVEL OPPORTUNITY TO OVERCOME RICE STRAW PULPING CHALLENGES

摘要

Soda pulping is the dominant process for annual plants, but it is faced with severe drawbacks. Strongly alkaline cooking liquors dissolve carbohydrates to a great extent with negative impact on pulp yield. Most annual plants have a high content of silica, which is dissolved to a high extent in the strongly alkaline cooking liquor and thus creates serious problems in the evaporators, the recovery boilers and in the caustizising plant. These are the main reasons why soda pulping black liquor handling and recovery of chemicals is still problematic. The situation is completely different when monoethanolamine (MEA) as the main delignifying agent was investigated. Apart of high selectivity of MEA which results in pulps with high yields, low kappa numbers and acceptable strength properties, the most important advantage of MEA pulping of annual plants is the direct MEA recovery by distillation. This could be highlighted for rice straw as the richest silica owner among the different annual plants. Therefore, in this study pulping of rice straw with MEA was investigated in comparison to conventional soda pulping as reference. Rice straw can be effectively delignified with MEA. At a maximum cooking temperature of 150°C, time at temperature of 90 min. with pure MEA a sufficient delignification was achieved. Pure MEA cooking yielded in 55% screened yield and an extremely low kappa number of 8.6. The main focus also was laid on reduction of the MEA charge by partial substitution with water. Even a MEA/water ratio of 25/75 can be successfully applied. Under these conditions a kappa number of 20 was attained which means the pulp is still bleachable. Another focus of this research was laid on the reduction of temperature. The results show cooking until 130°C has not any significant effect on delignification rate and the produced pulps have the kappa numbers under 20. Only around 105 °C the pulps attained with kappa numbers higher than 20. Also, the strength properties of MEA pulps are comparable to that of soda pulps. The pulps obtained in the MEA/water and soda cooks were selected for bleaching. ECF bleaching (DEpD) was applied. A similar brightness of 80% ISO was reached at a kappa number below 2 for both pulps.