Bleaching

3.2.1 Oxygen bleaching (O)In chemical pulp bleaching, an oxygen stage serve exclusively for removing residual lignin called delignification. The purpose of oxygen application in recovered pulp processing is to achieve an optical improvement of the pulp with an actual bleaching.While oxygen delignification in chemical pulp bleaching has been applied on a industrial scale since the middle 1960s, results of the oxygen bleaching of recycled fiber pulp were reported for the first time in the second half of the 1980s. the first tests with old corrugated containers (OCC) tried to improve pulp strength following the delignification that accompanies the treatment while simultaneously reducing the amount of wax introduced with the recovered paper. At a later stage, optical effects were also examined.Today, oxygen treatment is used occasionally in processing wood-free deinked pulp as a chemical pulp substitute or as market DIP for the production of writing and printing papers with high standards of optical quality. In most cases, mixed office paper largely consisting of wood-free, bleached fibers is used. Depending on the quality of sorting, this can be contaminated to some degree with unbleached fibers, colored papers, and other impurities. The oxygen bleaching of widely varying pulp compositions then levels the varying optical properties of the deinked pulp.Establishment of the oxygen stage for processing of deinked pulp for high quality graphical paper grades depends to a significant degree on economic criteria. The operation of the appropriate plants primarily in north America shows that their profitability links with the costs of cleaning, screening, and bleaching and the treatment of the resulting residues. Industrially, the efficiency is in doubt. The market situation of virgin hardwood pulp also has a major influence since both types of fiber stock compete as the raw material for the production of wood-free paper grades. The success of the process therefore connects to some degree to the political pressure for using recovered paper.chemistryThe complex reactions of the oxygen bleaching process are very fast radical chain reactions called auto-oxidation. With the participation of the oxygen and its radical reaction products, these reactions are responsible especially for degrading lignin and forming hydrogen peroxide. The brightness enhancements that accompanies bleaching is due to the subsequent, relatively slow reactions of the hydrogen peroxide with the chromophores. Figure 32summarizes the multitude of electrophonic (radical) and nucleophilic (ionic) reactions that can occur. They result in formation of hydro peroxides and many heterolytical and homolytical fragmentations. The hydroperoxides finally decompose. Although homolytical reactions for other oxygen chemical bleaching stages such as peroxide bleaching are unusable because the result in the decomposition of the bleaching agent, processes involving radicals are absolutely essential at the oxygen stage.Due to its extremely low reduction and oxidation potential, the electrophoniccharacter of oxygen is very weak. To start a reaction, raising the temperature or activating the system using a base such as sodium hydroxide is necessary. The activation uses an ionization of functional groups especially phenolic structures and structures with acidic hydrogen such as quinine methide in the lignin. To achieve practicable reaction times, temperatures in excess of 80℃are also necessary.In contrast with chemical pulp bleaching, reaction on the contaminants introduced with the recycled fibers in the oxygen treatment of deinked pulp besides those on the chromophores of the lignin occur. These give an improved pulp cleanliness. In this context, the following effects are important:Delignification of wood-derive dirt particlesColor stripping of water soluble dyesDetackification of stickiesWet strength resin removalImprovement of laser and xerographic ink removalFragmentation of adhesives such as hot melts and waxesSolubilization of binders that hold together flakes of coating, wet strength paper, and inksIndustrially, the success of an oxygen stage for deinking plants is under critical review.Process parametersAt atmospheric pressure, the low-cost oxygen dissolve only poorly in the bleaching solution. The process therefore uses pressure of 3-6 bar to increase the solubility. Originally conceived for the high consistency range, the oxygen stage is increasingly using pulp consistencies of 10%-15%. The precondition for application in this consistency range is an effective mixing of the oxygen into the recycled fiber pulp for which high-shear mixers are especially suitable. Usual reaction times are 30-60 min at about 100℃.A commercial process introduced in 1994 provides for the oxygen bleaching of wood-free pulp as a separate process stage and with conventional bleaching stages. Figure 33shows the process of the system. the cleaned and flotated medium consistency (10%) deinked pulp is heated by steam to 80℃-90℃and then introduced into a standpipe. With the exception of the oxygen. All chemicals are added before the pressure zone. Then the pulp slurry is pressurized in a MC pump with simultaneous degassing. After the addition of oxygen in a mixer, the deinked pulp is introduced into a pressurized upflow tower in which bleaching occurs. Depending on the optical characteristics required, the DIP is subjected to further deinking or bleaching stages after leaving the tower.To enhance the optical effect, peroxide is also frequently, peroxide is also frequently added to the bleaching solution. Figure 34 shows the interactions between the quantities of sodium hydroxide and peroxide and the brightness. For maximum brightness gains with low peroxide applications, only small amounts of sodium hydroxide may be used. With greater peroxide additions, the sensitivity of the brightness gain to the amount added will decrease. Despite this, large alkaliapplications should be avoided since they have a negative effect on the COD load of the process water. In contrast to the promising results achieved in laboratory scale, this process has not been successfully established in mill scale.Water glass exerts the strongest influence on the bleaching process. Additions of up to 4% considerably improve the brightness but may also cause negative carryover effects to the paper machine especially in integrated mills. Literature reveals different result for the influence of magnesium sulfate. Although no advantages for optimization of the commercial process described above occurred with addition of magnesium sulfate, test on the influence of additives at the oxygen stage indicate a stabilizing effect due to magnesium sulfate. If the treatment uses addition of peroxide, slightly better bleaching results occur when also adding DTPA.As the oxygen content increases, the optical effect improves. Greater brightness gains and an improved pulp cleanliness occur. Higher oxygen concentrations produce a stronger optical homogeneity especially for pulp mixtures having greatly fluctuating proportions of contaminants. Large oxygen quantities encourage the development of large gas bubbles particularly in medium consistencies. Some pulp is dragged along by their uncontrolled movement so the retention time is difficult to control. To keep the gas bubbles small, the pressure is often increased. The most effective method is to ensure an optimum distribution of the oxygen in the pulp with suitable mixing units. Oxygen levels of 5 kg/t pulp will then be sufficient to provide the desired bleaching effects.Besides the effects of the technical parameters just described, the success of oxygen bleaching depends considerably on the mechanical fiber content of the recovered paper. Proportions of 10% limit the increase in brightness. If no peroxide is added at the oxygen stage, a decrease in brightness in brightness must be expected depending on the amount of sodium hydroxide. Tests carried out by Ackermann et al. show that oxygen treatment can cause a dramatic loss in brightness in the bleaching of wood-containing deinked pulp due to yellowing of the mechanical fiber component. Only the addition of peroxide can prevent this brightness loss. Compared with a conventional peroxide bleaching without oxygen, better optical results are not possible.Suss was unable to determine any additional effects due to oxygen bleaching with the use of mixed office paper. Projects conducted by Patt et al.with wood-containing deinked pulp originating from old newspapers and magazines also produced no significantly greater brightness after a peroxide-supported oxygen stage compared with conventional peroxide bleaching. Advantages accruing from the addition of oxygen did occur for the removal of visible dirt specks. When evaluating this bleaching effect, one must note that dispersing effects are also due o the intensive mechanical influences in the high-shear mixer that cause a reduction of visible dirt specksOxygen treatment represent an effective process step within recovered paper processing especially in cases where unbleaching fiber negatively influence the optical cleanliness of wood –free deinked pulp as Fig. 35shows. With increasing contents of unbleached fiber in recycled pulp, the advantage of theperoxide-supported oxygen stage compared with peroxide bleaching grows in importance.SummaryTable 2 summarizes the characteristics of the oxygen treatment. The table shows that moderate bleaching conditions positively influence the result of the oxygen stage. Low alkali additions with the simultaneous application of peroxide produce the best bleaching results at temperatures of about 80℃and also reduce the COD load that is 5%-20% greater than with a conventional peroxide stage. The brightness gains are usually4%-8% and in exceptional cases even 12% ISO。