Page 129 - Membranes for Industrial Wastewater Recovery and Re-Use

104 Membranes for lndustrial Wastewater Recovery and Re-use

3.2.2 Pulping and paper manufacturing processes
Wood is a renewable resource that consists mainly of cellulose (45%),
hemicellulose (30%) and lignin (2 5%). Lignin is the resinous material that binds
the cellulose fibres together. In addition, wood has less than 5% of substances
like terpenes, resins and fatty oils and acids. Wood for papermaking has to be
debarked using a large, rotating drum that agitates the logs and allows the bark to
be striped by the abrasive action of the neighbouring logs. The logs are then
chipped using a large rotating blade prior to pulping.
The pulping process (Table 3.7) separates the cellulose and hemicellulose from
the lignin and removes other tree oils and resins. The remaining fibres are used to
produce the paper. There are two main pulping processes, chemical pulping and
mechanical pulping. Chemical pulping produces very pure cellulose fibres, and is
the most common form of pulping worldwide. In Kraft pulping, also called the
sulphate process, the wood chips are heated in sodium hydroxide and sodium
sulphide to produce a strong dark brown pulp. Because of the dark colour of the
resulting pulp, Kraft fibres require considerable bleaching to make them usable.
Kraft pulping is a highly efficient process for removing lignin and resins in
softwoods while still producing a high-quality pulp. Wood wastes are burned for
fuel and more than 95% of the pulping chemicals are recovered for reuse. In the
sulphite process, less commonly used for pulping than the Kraft process but still
widely used in Central Europe, the chips are heated in sulphuric acid. Recovery
of the pulping chemicals is less well developed than for the Kraft process.
In mechanical pulping the debarked logs are forced against a grinding stone or
rotating metal disks. As much as 95% of the wood resource is turned into pulp
though this action, since the lignin and tree resins remain in the pulp (unlike
Kraft pulping in which the yield is only 45-S0%). The high lignin content in the
pulp causes it to darken when exposed to sunlight. In mechanical pulping
the chips can also be steam treated to soften them prior to grinding, a process
known as thermomechanical pulping (TMP). In chemothermomechanical pulping
(CTMP) the wood chips are soaked with sulphur-based chemicals prior to
steaming to expedite the extraction of lignin and resin from the wood.
Mechanical pulp is used for lower grade papers, such as newsprint and telephone
directories, as well as for coated papers.
Bleaching is used to purify and clean the pulp. Bleaching removes lignin, which
affects the purity of the fibre. Kraft mills use mainly chlorine dioxide or other
strong oxidants, such as oxygen, ozone or hydrogen peroxide, to bleach pulp
while most mechanical pulp bleaching operations employ hydrogen peroxide.
Chlorine gas is still used in some old mills, particularly in USA, but due to its
tendency to form polychlorinated organic compounds it has been mostly
abandoned as a bleaching agent. Mechanical pulp is most often bleached with
hydrogen peroxide or with thiosulphate.
Recovered paper is used as fibre source in papermaking, particularly in regions
where the population density is very high. Worldwide, over 95 million tonnes of
paper are recovered each year to be made into recycled paper and paperboard,
contributing more than a third of the total fibre used to make the world’s paper