Pulp brightening

SGT St. Germain and DG Gray. Photoacoustic Fourier Transform Infrared Spectroscopic Study of Mechanical Pulp Brightening. J. Wood Chem. Technol. 7 33-50, 1987. 

If the lignin is not removed in the bleaching process, as is usually the case with pulps with a high lignin content, such as mechanical, ground-wood and NSSC pulps, either reductive or oxidative chemicals are used for pulp brightening. Sodium dithionite is the most common reductive chemical in use today, and peroxides are commonly used if the process is oxidative. When chemothermomechanical pulp is bleached, additional chemicals such as sodium silicate or DTPA (diethylenetriaminepenta-acetic acid) are also used. 

Fig. 29. CEDED pulp bleaching sequence A, delignifying, and B, brightening stages.

Kumiawan, P., "A Study of In-Situ Brightening of Mechanical Pulp via the Electro-Oxidation of Sodium Carbonate , M.A.Sc. Thesis, Dept, of Chemical and Bio-Resource Engineering, The University of British Columbia, 1998. 

Brightening - pLUORESCENT WHITENING AGENTS] (Volll) -of mechanical pulps pULP](Vol20)... 

Triazinyl derivatives of 4,4 -diaminostilbene-2,2 -disulfonic acid have been available since 1941 and remain the most important class of optical brighteners in terms of quantity. Some 75 derivatives have been put on the market [1], In the textile industry, they find use in brightening cotton, regenerated cellulosic fibers, and polyamides. They can also be included in detergent formulations and used to brighten paper, either in the pulp or in the coating. 

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