Oxidative bleaching, pulping

J Polcin and WH Rapson. Effects of Bleaching Agents on the Absorption Spectra of Lignin in Groundwood Pulps. Part II. Oxidative Bleaching. Pulp Paper Mag. Can. 72 80-91, 1971. 

Further Preparative Reactions. When pulps are to be used in the production of materials that do not retain the original fiber stmcture, such as rayon or ceUulose acetate film, the lignin, hemiceUulose, and other components must be reduced to the lowest possible concentrations. A surfactant (ionic or nonionic) is often added during a hot, weakly alkaline extraction step after chlorination. Another approach, sometimes used in addition to the surfactant step, is to treat the pulp with 6—10% NaOH after most of the oxidative bleaching is finished. This treatment removes most of the hemiceUulose. In most purification plants the final stage includes use of sulfuric acid chelators are optional. 

Acedox [Acetic oxidation] A pulp-bleaching process using peracetic acid as the oxidant. Developed by Eka Nobel in 1994 and first commercialized, in combination with Lignox, in Sweden in 1995. 

DegOX [Degussa oxidation] A pulp-bleaching process developed by Degussa. The active species is peroxomonosulfuric acid (Caro s acid). The first full-scale commercial trial was held in 1994. 

Lignox [Lignin oxidation] A pulp-bleaching process using hydrogen peroxide as the oxidant, and a chelating agent. Developed in Sweden in 1990 by Eka Nobel. See also Acetox. 

Although pulp may be bleached to various brightness levels by oxidation or reduction, all bleached pulps diminish in brightness with age. Normally, the pulp develops a yellow color. It is believed that color... 

Oxidation of Pulp. In the experiment reported in Figure 2, the bleached pulp stock was chemically modified prior to the formation of handsheets. The stock was first oxidized by dichromate as follows After a short period of pretreatment in a 0.2 N solution of oxalic acid at a pulp consistency of 3%, the pulp was filtered and potassium dichromate was added to yield a 0.1 N solution. The pulp was allowed to react at room temperature for 30 min and then was filtered and washed. Thereafter the oxidized pulp was soaked overnight in oxalic acid and then was filtered and washed again. 

Figure 2. Change in 2/ DPV with time of exposure. Photochemical deterioration of bleached-pulp handsheets, both oxidized and SBH reduced, in...

By this means peripheral, color-inducing functionalities are oxidized and decolorized over a bleaching period of 2-3 hr while the bulk of the lignin macromolecules remain intact (insoluble). Thus a brightness improvement of 8-10 points is possible while the yield loss is kept low. Unfortunately peroxide-based bleaching systems tend to be expensive relative to other alternatives for groundwoods, and hence this method is focused on pulps destined for tissue papers, paper napkins, and some specialty papers, rather than for newsprint applications. However, the increased resistance to color reversion of peroxide-bleached pulps tends to offset the increased cost of this step. 

Mechanical pulps are bleached by procedures that preserve lignin, but eliminate most coloured groups. Most mechanical pulps can be bleached to brightness levels of 75-80%, that are suitable for uncoated and coated papers. The principal bleaching procedures for mechanical pulps are reductive bleaching with sodium dithionite (hydrosulphite), and oxidative bleaching with peroxide under alkaline conditions. 

Both DRIFT and IR-PAS [81] have been used to investigate chemical changes in pulp lignin caused by the reductive or oxidative bleaching of mechanical pulps. The results showed a decline in aldehyde and ketone C=0 band intensity (due to alkaline peroxide and sodium borohydride bleaching) and a reduction in contributions from conjugated carbonyl groups. 

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