Bleaching Agents, Pulp and Paper

Abstract

Abstract Worldwide, more than \documentclass{article}\usepackage{amssymb}\pagestyle{empty}\begin{document}${50\times 10^{6}{\hskip0.167em}{\hskip0.167em}{\rm{t}}}$\end{document} of pulp is bleached annually, making the pulp and paper industry one of the largest consumers of bleaching chemicals. The principal objective in bleaching any type of pulp is usually to increase the whiteness of the pulp, as measured by its brightness, which is defined as reflectance measured at a wavelength of 457 nm. Virtually all of the color of any pulp resides in its lignin component. Because concurrent cellulose damage would seriously weaken the fibers, pulping is therefore terminated when 5–8% of the original lignin remains. Bleaching effluent is typically contaminated with chloride ion and chlorine compounds, making it difficult or impossible to recycle it to the recovery system because of potential for corrosion. It is therefore necessary to treat and discharge the bleaching effluent. Associated environmental concerns are presently causing rapid changes in bleaching technology. With the exception of caustic, all important chemical pulp bleaching agents are oxidants, eg, chlorine, Cl 2 ; chlorine dioxide, ClO 2 ; calcium hypochlorite, Ca(OCl) 2 ; hydrogen peroxide, H 2 O 2 ; oxygen, O 2 ; and ozone, O 3 . Research in progress points to the possible commercialization of several other chemical pulp bleaching technologies: chemical pretreatments to improve the selectivity of oxygen bleaching, enzymatic pretreatments to facilitate subsequent delignification, and recycling of bleach plant effluents. Much new high yield pulping technology has been developed, including production of thermomechanical pulp (TMP) and chemithermomechanical pulp (CTMP). Because little material is lost in these processes, and the pulps have a very high lignin content, it is not feasible to bleach them by removing all of the lignin, as in chemical pulp bleaching. Instead, the lignin must be decolorized, a process sometimes referred to as brightening. Only two brightening agents, hydrogen peroxide and sodium hydrosulfite (sodium dithionite), are of commercial importance. Because pulp bleaching agents are, for the most part, reactive oxidizing agents, appropriate precautions must be taken in their handling and use. Hazards associated with pulp bleaching agents include their potential for damage resulting from contact with skin or eyes, their ability, as oxidizers, to cause fires or explosions. Additional information on health and safety aspects should be sought by consulting material safety data sheets. Most suppliers of bleaching chemicals provide on‐site training sessions by experts on the safe use and handling of their products, upon request.