High-brigthness Bleaching Of China Fir CTMP Pulp And Its Mechanism

China's paper industry is facing the pressure of fiberous-materials supply , which have great drawback impacts on the country's paper industry. Therefore, the central government of China has issued sets of policies and regulations to encourage development of fast-growing wood plantations and improvement of untilization efficiency of woods and efforts of exploring new pulp wood resources as well. In the southern part of China, China fir ( Cunninghamia lanceolata ) has been used as raw materials in pulp mill for kraft pulping and chemimechanical pulping. CTMP pulp from this kind of fir has a difficulty in being bleached for higher brightness level with an economical chemicals application amount, which impacts on the application of those pulps in some high-quality paper and paperboard grades.The following aspects of topics have be investigated by this thesis, including hydrogen peroxide degraduation behaviors and its inhabitation measures; new bleaching process at high temperature and reducing retention time; enhancement pulps'bleachabilities by oxidant activation of unbleached pulps; investigations on combination of bleaching chemical mixtures, such as peroxides-TAED; and applications of some new bleaching chemicals as FAS as well; the order of bleaching sequences for multi-stage bleaching were examined by testing the brightness gains of bleached pulps after those sequences, either.The mechanism for bleaching chemistry have been inspected and proposed, by characterization of lignin-structures of both unbleached and bleached pulp samples. All analytical events are carried out by means of modern instruments i.e. IR spectrometer, UV-Vis spectrometer, UV-confocal resonance spectrometer. The results can indicate the obvious changing of chromophores in bleached pulps. Basing on previous investigations, a proposed suitable bleaching sequence was recommended for high-brightness-bleaching of the fir CTMP pulp.The brightness ceiling of the fir CTMP pulps could be increased after bleaching with optimal conditions of single-stage hydrogen peroxide bleaching, i.e. chemical dosages as H₂O₂ 6%, NaOH 4.5%, Na₂SiO₃ 2%, DTPA 0.1%, and pulp consistency at 20%, temperature at 70oC, retention time for 180 min. The target brightness of bleached CTMP puls can be achieved as 79.7%ISO from the start-point of 45.6%ISO.Improvement of bleaching efficiency has been recognized by oxidant acitivation of unbleached pulp prior to bleaching operation. Comparing the effects on bleaching efficiency enhancement of several types of oxidants, the ascent order could be lined as ClO₂ > acidic H₂O₂ >alkaline H₂O₂ > NaClO. The brightness, 77.7%ISO, of bleached pulps could be achieved after activation by oxidant ClO₂ and one-stage hydrogen peroxide bleaching with a H₂O₂ dosage of 6%; in contrast, if the activation was excutivated by alkaline hydrogen peroxide, the taget brightness of bleached pulp was 76.5%ISO even using a higher H₂O₂ dosage level of 6.5%. Furthermore, the chlorine dioxide activation conditions were used for above trials, which were detailed as ClO₂ dosage of 0.5%, pulp consistency of 10%, reaction temperature at 70 oC, retention time for 30 min. with an initial pH of 4-4.5, at the end of activation reaction, the pH of stocks was adjusted to pH5-6 using 2N H₂SO₄ solution.The brightness values of three bleached pulps were compared after one-stage bleaching with different bleaching chemicals such as alkaline hydrogen peroxide, sodium perborate, sodium percarbonate, and mixed with activator TAED respectively, and the pulps with an initial brightness of 43.8%ISO were used for above trials, and target brightness of all above bleached pulps were 73.1 % ISO, 70.9% ISO and 69.8% ISO, respectively. As the result of TAED application, rentention time of bleaching could be shortened in contrast with a conventional alkalie hydrogen peroxide bleaching.The bleached pulp brightness can be improved with combination of different type chemicals as bleaching agent for their own stage. For instance, oxidizing bleaching ( P ) stage + reducing bleaching( F or T ) stage as bleaching sequence could provide bleached pulps with brightness of 78%ISO - 80.5%ISO. With comparison of target brightness after PF and FP bleaching, the result shows that, PF sequence does better than FP on brightness ceiling with 1-2 percentage units higher. FAS could also be in high temperature bleaching (90oC, 30min). A China fir CTMP pulp could be bleached to a brightness of 83.2%ISO by using the three-stage quences of P*PF, with a smaller bleached pulp yield loss and better stabilities, in terms of P.C. Fiber quality index, fiber curl and kink, could be improved by bleaching operations, with little changes in fiber length and wideness. Bleaching operations might move more fiber latencies, which could to lead to improve sheets forming and increase the pulp physical strength properties.China fir lignin is one of G structure type with a ceartain amount of H lignin structures. The following function groups were existed in China fir lignin—carbonyl group, methoxyl group, phenolic and non phenolic hydroxyl group (with non-phenolic being its main components), quinones structures.Interpreted by IR spectroscopy,α- andβ- carbonyl group in lignin could be destroyed by hydrogen peroxide bleaching, and also phenyl ring. The phenolic groups not etherifying by could be destroyed and removed easily. The phenolic groups were availably removed by FAS bleaching. Interpreted by UV-Vis spectroscopy, the chromophores of carbonyl groups conjugating with phenyl ring might be removed by P-TAED bleaching. And it might be removed by FAS bleachin as well, and also quinines structures. The absorption bands in UV-Vis spectrum were decreased greatly by PF bleaching, which said that FAS was suitable used in end bleaching stage. Interpreted by UV CRS spectroscopy, coniferaldehydes structures might be removed by bleaching, and the pulp bleaching stabilities be further improved.The probable reactions between the two main chromophores groups,α-carbonyl group and o-quinones, in the lignin structures of China fir and the reducing bleaching agent of FAS, were suggested in this paper.With the falling of market sale prices of FAS and TAED in future, it would become a promising bleaching technology of the combining three-stage sequences of P*PF.