Bleaching of pulp for papermaking is roughly divided into bleaching with a peroxide, bleaching with a chlorine-containing oxidizing agent, and bleaching with a reducing agent, and these have been employed depending on the kind of pulp and purpose of use in consideration of their bleaching characteristics and effects.
The bleaching with a peroxide represented by H.sub.2 O.sub.2 is employed mainly for bleaching of mechanical pulp and for deinking/bleaching of waste newspaper because of its bleaching effect on pulp containing lignin.
Since the bleaching with a chlorine-containing oxidizing agent is effective for chemical pulp containing a slight amount of lignin, bleaching with Cl.sub.2, NaClO or ClO.sub.2 is mainly carried out in multistage bleaching of kraft pulp, and bleaching with NaClO in bleaching of waste wood-free or mechanical paper. For example, in the bleaching of waste wood-free or mechanical paper, a 12% NaClO solution is added to pulp in an amount of about 8% based on the amount of pulp and mixed therewith, whereby the bleaching is carried out.
The bleaching with a reducing agent is excellent in decolonization of dye type coloring materials, but it has a limited-bleaching capability for peroxide type chemicals and chlorine-containing chemicals and entails a high cost. For these reasons and the like, it has been employed mainly for bleaching of mechanical pulp demanding a low brightness grade to be incorporated into newspaper, post-bleaching after H.sub.2 O.sub.2 bleaching of mechanical pulp demanding a high brightness grade and post-bleaching after H.sub.2 O.sub.2 deinking/bleaching of waste newspaper. In general, Na.sub.2 S.sub.2 O.sub.4 or TUDO of the dithionite family has been used as the reducing agent. Of these, Na.sub.2 S.sub.2 O.sub.4 has been mainly used from the viewpoint of bleaching capability, chemicals cost, etc. However, TUDO bleaching has recently come to be spotlighted as a substitute for NaClO bleaching in bleaching of waste wood-free or mechanical paper and as a substitute for Na.sub.2 S.sub.2 O.sub.4 bleaching in post-bleaching after H.sub.2 O.sub.2 deinking/bleaching of waste newspaper, from the viewpoint of the problem of waste water pollution by organic chlorine compounds represented by dioxins, demand for energy conservation, decoloring effect on dye type coloring materials, etc.