| With the increasing demand of energy-saving and environmental protection in pulp andpaper industry, clean bleaching technologies, such as totally chlorine free (TCF) bleaching,has been developed to an important alternative for the traditional chlorine bleaching in orderto reduce environmental load of the pulp bleaching process. Since many advantages, i.e.,friendly to environment, excellent process and equipment adaptability, high pulp brightnessstability and easy chemical recycling, etc., were found to the hydrogen peroxide bleaching, ithas become one of the most important TCF bleaching technology. However, in practicalproduction, because of the accumulating and introducing of transition metal ions, e.g., Fe3+,Mn2+, Cu2+, etc., in raw material and pulping process, the hydrogen peroxide will usually bedecomposed to some extent due to the catalysis of metal ions, which will significantly makethe bleaching process less efficient and selective. Therefore, the addition of additives in thehydrogen peroxide bleaching will be of great importance. Furtherly, how to obtain themaximum efficiency of additives is a key issue to improve the hydrogen peroxide bleachingperformance and adaptability. In this paper, the fast-growing eucalyptus kraft pulp was takenas the raw material, and the main focuses were on the establishment of several rapid methodsfor determining the important parameters involved in the hydrogen peroxide bleaching andthe investigation of effect of the ways for additives introduction on the bleachingperformance. In particularly, rapid methods for determining residual hydrogen peroxide andoxalate content in bleaching effluent were established. And the effect of the ways forintroducing magnesium-based protective agents on the bleaching selectivity and the contentsthe main components in effluent were investigated. Meanwhile, the effect of the stabilizers(sodium silicate) and pH on the hydrogen peroxide bleaching at different introducing ways ofthe magnesium-based protective agent were also studied. The success of the present work canprovide great guidance for the best manner and pattern for adding additives in practicalproduction and further explore the protection mechanism of magnesium-based protectiveagents in hydrogen peroxide bleaching.Firstly, two methods for, respectively, determining the residual hydrogen peroxide andoxalate content in bleaching effluents was established based on the phase reaction conversion technique of headspace gas chromatography. The method of residual hydrogen peroxide isbased on the reaction of hydrogen peroxide and permanganate in an acidic medium (0.1mol/L), in which hydrogen peroxide is quantitatively converted to oxygen within10min at600C in a sealed headspace sample vial. The released oxygen is then determined by GCequipped with a thermal conductivity detector. The method is robust, sensitive, and accurate,with reproducibility characterized by a relative standard deviation of <0.5%, a sensitivitywhose limit of quantification (LOQ) is0.96μmol, and a demonstrated recovery ranging from98%to103%. The method of oxalate is based on the redox reaction between potassiumiodate and oxalate, from which the carbon dioxide is generated and measured by HS-GC. Theresults showed that the reaction in a sulfuric acid solution (0.05mol/L) was complete in35min at950C. The RSD of the method in the repeatability testing was less than4.5%, theLOQ was0.35μmol, and the recovery ranged from95%to103%. The present method issimpler, more reliable, and more practical compared to the methods reported previously.In this paper, we investigated the effect of Mg(OH)2-introducing modes on the hydrogenperoxide (H2O2) bleaching processes for the eucalyptus kraft pulps. There are three ways tointroduce Mg(OH)2into the bleaching system, i.e., the direct-addition, or fresh-formedMg(OH)2by adding magnesium sulfate into a pulp slurry pre-mixed with sodium hydroxide,or vice verse. The results showed that the particle size distributions of Mg(OH)2were quitedifferent in these cases, in which the average particle sizes of fresh-formed Mg(OH)2aremush smalls than that of the direct-addition. As a consequence, the processes withfresh-formed Mg(OH)2were proved more effective for the pulp bleaching, in which therewere more hydrogen peroxide found in the bleaching effluents and the favorable pulpqualities. The time-dependent pH profiles of the process effluents also indicated that thedecomposition of H2O2with the direct-addition or no addition of Mg(OH)2was verysignificant, especially during the early stage of the bleaching processes.After adding sodium silicate in the above optimal way, a cooperative action wasobserved. The results show that the sodium silicate is added effectively suppresses the invaliddecomposition of hydrogen peroxide, the residual hydrogen peroxide content in the bleachingeffluents increased significantly, the final pH is extremely stable, the kappa number andviscosity stability,the whiteness improvement. But in the absence of direct precipitation magnesium hydroxide, the sodium silicate did nothing for hydrogen peroxide. Therefore, inthe actual bleaching process using sodium hydroxide must be present in the directprecipitation conditions.In the final part of the paper, from the point of view of the oxygen generated to explorethe introduction of magnesium peroxide affect the bleaching. The results showed that in theabsence of direct precipitation of magnesium hydroxide, much more oxygen had generated,the amount of oxygen generated significantly reduced after the addition of magnesium ions;sodium silicate added after the optimal mode of magnesium hydroxide the generation amountof oxygen further reduced. The efficiency and selectivity of the bleaching performance ofhydrogen peroxide of visual the amount of oxygen generation, i.e., the more oxygengenerated the lower efficiency and selectivity. The MgSO4+NaOH+Na2SiO3(from left toright added, similarly hereinafter) group bleaching efficiency was significantly greater thanthe MgSO4+NaOH group; the MgSO4+NaOH group after prolonged bleaching selectivityincreases with time and then decreases, while the MgSO4+NaOH+Na2SiO3groupselectively is growing but before120min it is less than the MgSO4+NaOH group, later thetwo group have the same selectivity; there is almost no difference of the lignin removal ratebetween the two group. When the bleaching time is short (less than120min) the pulpbrightness equivalent, the sodium silicate was not added at this time; if the time greater than120min, the sodium silicate can effectively suppress the invalid decomposition of hydrogenperoxide. In this paper, we know that the sodium silicate was added in the hydrogen peroxidebleaching process conditions of magnesium hydroxide must be presented, the auxiliaryfunction is obvious; On the basis of evidence we deduced that the magnesium ions formstable metal-carbohydrate complexes with the carbonyl groups of the partially oxidizedcellulose and reduce the rate of cellulose chain cleavage by the β-elimination reaction. |
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