Dissolving pulp, bleaching

Attempts to remove hemicellulose for production of dissolving pulps with very low hemicellulose contents have shown that complete enzymatic hydrolysis of hemicellulose within the pulp is difficult to achieve. The xylan content in delignified mechanical aspen pulp was reduced from approximately 20 to 10%, whereas in bleached hardwood sulphite pulp the xylan content was decreased from 4 to only 3.5% even at very high enzyme dosages (50). The complete removal of residual hemicellulose seems thus unattainable, apparently due to modification of the substrate or to structural barriers. 

Our experimental work started from the following original cellulose samples (a) acetate-grade, bleached cotton linters, DP 1800 (b) hot, refined, spruce, sulphite-dissolving pulp, machine dried, ca. 93% a-cellulose, DP — 750 (c) never-dried, normal, rayon-grade, beech sulphite pulp, ca. 90% a-cellulose, DP = 825 (d) commercial cellulose powders obtained by hydrolytic degradation and/or mechanical disintegration of cotton linters or spruce sulphite pulp. 

Dissolving pulp, a bleached sulfite pulp from spruce, MbDoCell H3430 from Mb och DomsjS AB, Omskoldsvik, Sweden. 

Grafting of dissolving pulp (MDC) and grease-proof pulp (GP) (both are bleached sulfite pulps from spruce) and low-substituted ethyl cellulose (EC) and hydroxyethyl cellulose (HEC), all samples in never-dried state, with acrylonitrile (AN) using Mn +ions, complexed with pyrophosphate ip n inin amr >s... 

Several samples of commercial dissolving pulp from softwood were used bleached pulp (whiteness 92 %), almost entirely free of lignin, containing about 94 % a cellulose (I) paper pulp (II) (whiteness 91,5 %) mechanical yellow pulp (III) (containing lignin), Kraft Laponia pulp (IV), whiteness 92 %). 

Marton (1967) found that, with the exception of bleached pine kraft pulp, bleached pulps dissolved incompletely in the acetyl bromide/acetic acid solution. Iiyama and Wallis (1988a) noted (Table 2.2.4) that several unbleached eucalypt pulps dissolved incompletely or only after prolonging the digestion time beyond the standard 30 min. 

However, the process is more complex than this, and the usual molar ratio of chlorine dioxide to chlorine produced is about 1 1. The process is integrated with an electrolytic process for making the sodium chlorate, such that the liquor from the reduction step is recirculated to the electrolytic step. The product gas, a mixture of chlorine with chlorine dioxide, is washed with water, which preferentially dissolves the chlorine dioxide. The resulting solution is used for pulp bleaching. 

Dissolving pulps are high-quality, intensely bleached pulps. As they are used as chemical intermediate product or source material for viscose rayon and... 

Sixta, H. (2000) Comparative evaluation of TCP bleached Harwood dissolving pulps. l enzingerBerichte, 79, 119-128. 

The wastewaters generated from the paper and allied products industry contain BOD, COD, suspended solids, dissolved solids, color, acidity or alkalinity, and heat. Chemical pulping processes may produce wastewaters with heavy metals (Cr, Ni, Hg, Pb, Zn). If pulp bleaching is part of the operation, the wastewaters may contain additional heavy metals (Hg) and dissolved solids (chlorides). Mercury may present in the caustic used in pulping and bleaching operations. Zinc is used in the bleaching of ground wood pulp. Chromium, nickel, and iron may be introduced from the corrosion of process equipment. 

Materials. TCF-bleached beech Mg-sulphite dissolving pulp (BS), with 93.8% R18,2.9% pentosan, and 1.7 copper number, and Eucalypt prehydrolysis kraft pulp (EK), with 96.6% R18, 3.6% pentosan and 0.2 copper number, were... 

In conventional pulp bleaching, chlorine, hypochlorite, and chlorine dioxide are used with or without intermediate alkaline extraction steps. The first step in conventional lignin-dissolving bleaching is the chlorination. Qilorine water is normally used for this chlorination. It converts the residual lignin to products, that are soluble in water and/or alkali. Chlorine and hypochlorite react primarily with the benzene or phenol rings of lignin, in which substitution and oxidation reactions take place. In a separate extraction step, the unwanted soluble substances are removed with water and/or diluted sodium hydroxide solution. 

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