Evaluation of bleachability on pine and eucalyptus kraft pulps (original) (raw)
Related papers
We investigated the bleaching efficiency of the final chlorine dioxide (D2) stage in the D0(EP)D1D2 bleaching sequence, focusing on the effect of pH in relation to the bleaching history of pulp samples. The samples used were unbleached kraft Eucalyptus grandis pulps with kappa no. 14.8 and the same pulp oxygen-delignified to kappa nos. 12 and 9.8. The samples were bleached according to the D0(EP)D1 sequence to a brightness of about 86% ISO and then submitted to the final D2 stage under identical conditions (e.g., chlorine dioxide charge, time, tempera¬ture, and final pH). The target final brightness was 90.5% ISO. Changes in the kappa number, brightness, viscosity, and contents of hexenuronic acid), 4-O-methylglucuronic acid, and the total amount of carboxylic acid (COOH) groups in pulps were monitored during the bleaching sequence. The final brightness of eucalyptus kraft pulps increased when the terminal pH of the D2 stage was raised from 3.0 to 6.5. The 90.5% ISO target brightness in the D2 stage was achieved for all pulps within a pH range of 4.5–6.5, but this required adjusting final pH for individual sam¬ples. The optimal pH value with respect to pulp viscosity was between 3 and 5. Despite dissimilar conditions applied in previous bleaching stages, the samples after the D1 stage revealed similar residual lignin contents as shown by kappa number analysis. The content of hexenuronic acid in the samples, however, varied broadly, from 2 to 26 mmol/kg. Conductometric titration showed different amounts of carboxylic acid groups in pulps after the D1 stage, of which hexenuronic acid accounted for only a minor part. The variations in the fiber charge resulted from the different bleaching conditions applied before the D2 stage. The fiber charge affected the alkali demand in the final D2 stage, whereas variations in the alkali demand affected the initial pH and associated process kinetics. Lower total fiber charge was found to be beneficial for improved final brightening and viscosity when bleached at higher final pH.
Progress in eucalyptus kraft pulp bleaching
… sobre celulosa kraft …, 2005
Bleached eucalyptus kraft pulps are largely used for manufacturing tissue and print & writing (P&W) paper grades. High final brightness (92 + % ISO) and brightness stability (<2% ISO) are rather significant for P&W for it affects optical brighteners demand. Bleaching chemical demand, bleaching yield, water consumption, effluent load & treatability, pulp OX, brightness stability, refinability and strength are drivers for choosing bleaching technology. This work critically reviews the state-of-the-art processes for oxygen delignification, first stage, second stage and final bleaching of eucalyptus kraft pulp in the light of the aforementioned drivers. The potential of new bleaching technologies such as the P Mo stage and the use of formaldehyde in D-stages are also presented. Implementation of single or double-stage oxygen delignification is determined by the true pulp lignin content (discounted the HexA´s). The high pulp HexA´s content and poor oxygen stage selectivity limits dropping kappa number under 9-10 in single or double Ostage. Application of Mo-catalyzed acid peroxide delignification after O-stage allows further reduction of kappa number to 3-4. Efficient post-oxygen washing is the key for low cost bleaching, with a kg of COD/odt consuming the equivalent to 0.085% active chlorine. A D-(EP)-D type three stage sequences suffices for bleaching eucalyptus kraft pulps. The inclusion of a fourth stage is desirable for production of high brightness/ low reversion pulps. Bleaching chemical consumption is strongly influenced by brown pulp origin, with variations of 3.2 to 7.7% active Cl 2 demand depending upon the pulp type. The type of ECF Bleaching technology, based on chlorine dioxide, affects chemical consumption only slightly. Hot acid/hot chlorine dioxide bleaching technology saves small amounts of active chlorine for high bleachability pulps but none for low bleachability ones. Atmospheric extraction (EP) suffices for eucalyptus kraft pulp bleaching. Formaldehyde saves more chlorine dioxide when used in D 1 than in D 0 /D HT stages. A final peroxide stage significantly improves pulp brightness stability. The production or organically bound chlorine decreases by 30% with hot chlorine dioxide bleaching but this gain disappears after effluent biological treatment.
The impact of kappa number composition on eucalyptus kraft pulp bleachability
Brazilian Journal of Chemical Engineering, 2007
Consumption of chemicals during ECF bleaching of kraft pulp correlates reasonably well with kappa number, which measures with KMnO 4 the total amount of oxidizable material in the pulp. However, the method does not distinguish between the oxidizable material in residual lignin and other structures susceptible to oxidation, such as hexenuronic acids (HexAs), extractives and carbonyl groups in the pulp. In this study an attempt is made to separate the main contributors to the kappa number in oxygen -delignified eucalyptus Kraft pulps and evaluate how these fractions behave during ECF bleaching using chlorine dioxide as the sole oxidant (DEDD sequence). Residual lignin and HexAs proved to be the main fractions contributing to the kappa number and chlorine dioxide consumption in ECF bleaching. Pulp bleachability with chlorine dioxide increases with increasing HexAs content of the pulp but chlorine dioxide per se does not react with HexAs. Reduction of pulp with sodium borohydride under conditions for removing carbonyl groups has no impact on bleachability. No correlation was found between the pulp of the extractive content and pulp bleachability. The removal of HexAs prior to ECF bleaching significantly decreases the formation of chlorinated organics in the pulp (OX) and filtrates (AOX) as well as of oxalic acids in the filtrates.
Holzforschung, 2000
Summary Residual lignins were isolated from unbleached and oxygen-bleached Eucalyptus kraft pulps by acid hydrolysis. The structural changes and degradation of residual lignin occurring during kraft pulping and oxygen bleaching were followed and identified by elemental analysis, residual carbohydrate analysis, molecular mass distribution, as well as qualitative and quantitative solution 13C NMR. The dissolved lignins in the kraft cooked and oxygen bleached liquors were also studied and compared with the corresponding residual lignins. Milled wood lignin treated under acid hydrolysis conditions served as a reference for the structural comparison. The results show that etherified syringyl structures were quite resistant towards degradation in the oxygen bleaching, causing little depolymerisation in residual lignin and a small increase in carboxylic acid content, but producing appreciable amounts of saturated aliphatic methylene groups.
THE CHEMISTRY OF BLEACHING AND POST-COLOR FORMATION IN KRAFT PULPS
In this review, the chemical structure of the residual lignin in softwood and hardwood is presented as far as we know it. With this knowledge as a starting point, the basic principles in bleaching of kraft pulp with bleaching agents such as chlorine dioxide, oxygen and hydrogen peroxide is discussed with an emphasis on differences between the behaviour of guaiacyl (softwood) and syringyl-guaiacyl (hardwood) lignin. The importance of other pulp constituents such as hexenuronic acid on the outcome of bleaching operations and various possibilities for removing this structure from the pulp is discussed. In the final part of the presentation, the post-color formation by storage or heat treatment in fully bleached chemical pulp is described and possible means of reducing this nondesirable effect are suggested.
Improved Eucalyptus Pulp Bleachability Via High Temperature Acid Treatment Edvins Ratnieks
2006
and develop the state of the art technology of acid hydrolysis to decrease the kappa number of pulps The Center has been studying specifically this phenomen since 1 994 It takes part of a major development project to understand and improve the bleachability of the eucalyptus kraft pulp produced by the mill The driving forces of this project have been the economical aspects of chemicals consumption due to poor selectivity and reactivity increasing production costs chlorine gas elimination and especially the kraft cook heterogeneity and its associated consequences Our first perception of the opportunities in the acid hydrolysis field are associated with the studies of the unbleached pulp leaching presented as a mill case in the 1994 Latin American Delignification Conference E Ratnieks V Sacon C Zimmer C Foelkel The findings were further on extended and presented in the 1995 TAPPI Pulping Conference E Ratnieks C Foelkel V Sacon C Zimmer Both alkaline and acid effects were studied We fo...
Assessment of Q(OP)D(PO) bleachability of softwood kraft pulp
Nordic Pulp & Paper Research Journal
Bleachability is evaluated as how easily a pulp sample is bleached and it depends on the structure of residual lignin and carbohydrates. Also, the bleachability varies depending on the bleaching sequence. ECF light sequences have been improved significantly in the recent years. However, we still don’t fully understand how ECF light bleach plants are optimally run. This work studies the bleachability of softwood kraft pulp in an ECF light bleaching sequence, (OO)Q(OP)D(PO). Three pulp samples with brown stock kappa number 27, 32 and 35 were bleached and studied for residual lignin, hexenuronic acid and carbohydrate content. It was found that in the bleaching stages that are highly delignifying, it is beneficial with a higher kappa number for the delignifying bleachability. However, in the bleaching stages where the objective is brightness increase, the brightness gain bleachability is improved by a lower kappa number. We also intended to determine which of the three samples had the b...
Tappi Journal
ABSTRACT: We investigated the bleaching efficiency of the final chlorine dioxide (D2) stage in the D0(EP)D1D2 bleaching sequence, focusing on the effect of pH in relation to the bleaching history of pulp samples. The samples used were unbleached kraft Eucalyptus grandis pulps with kappa no. 14.8 and the same pulp oxygen-delignified to kappa nos. 12 and 9.8. The samples were bleached according to the D0(EP)D1 sequence to a brightness of about 86% ISO and then submitted to the final D2 stage under identical conditions (e.g., chlorine dioxide charge, time, tempera-ture, and final pH). The target final brightness was 90.5% ISO. Changes in the kappa number, brightness, viscosity, and contents of hexenuronic acid, 4-O-methylglucuronic acid, and the total amount of carboxylic acid (COOH) groups in pulps were monitored during the bleaching sequence. The final brightness of eucalyptus kraft pulps increased when the terminal pH of the D2 stage was raised from 3.0 to 6.5. The 90.5% ISO target ...