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4.3. Bleaching
The object of bleaching is to render the pulp whiter without excessive degradation of the cellulose.
Industrial bleaching of pulps first with hypochlorite and later with chlorine, partly in combination, and
with an intermediate extraction step with alkali began at the end of the 19 th century. The development
of pulp bleaching techniques in the 20 th century has led to a large number of bleaching chemicals
(Appendix 6A) applied in numerous and highly specific processes (Appendix 6B, C, D & E) today.
The principal aim of pulp bleaching is to increase brightness. The whiteness of pulps is generally
determined by measuring the reflectance of nearly monochromatic light (457 microns) by a standard
General Electric (GE) reflectance meter. A magnesium oxide plate of known reflectance is used as a
standard. The brightness values are expressed as percentage of light reflected by the sample as
compared with that reflected by a completely white surface. Unbleached pulps generally exhibit
brightness values ranging from 25-65 GE units. The sulphite process usually gives the brightest
unbleached pulps, whereas those produced by kraft, soda or semichemical processes can be quite
dark. Groundwood pulps have brightness values in between 40 and 60 units.
To produce white fibres from the brown or pale yellow pulps, treatment with a bleaching agent is
required. The nature of the bleaching operation depends on several factors: the type of raw material
used to make the pulp, the pulping process, the degree of whiteness desired, and the purpose for
which the pulp is to be used. Bleaching carries further the fibre purification accomplished in the
pulping process. As the light-absorbing chromophoric component in unbleached pulps are
predominantly functional groups of degraded and altered residual lignin, bleaching can be performed
either by converting and stabilizing chromophoric groups without loss of substance (ligninpreserving
bleaching) or by removing the lignin (lignin- removing bleaching). The traces of lignin and
other coloured substances (quinone like substances formed from the phenolic groups of lignin by
various oxidative processes are known to absorb visible light) are removed or converted to colourless
forms by bleaching. The extraneous constituents of wood also contribute to the colour of certain
pulps, especially the groundwood pulps. Along with the removal of the residual lignin and other
compounds, insufficiently delignified particles (shives, bark specks) are also partly removed.
Therefore, bleaching can additionally be regarded as a purification process, which is used especially
in the case of dissolving pulp production to obtain a pure pulp with high alpha-cellulose content.
Bleaching without delignification to brightness values above 70 is difficult to achieve. High
brightness is not, of course, the only important characteristic of bleached pulps. A good paper pulp
must also have good strength and good papermaking properties, and it is important that these
properties are not lost in the bleaching process.
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