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Pulp and Paper 261

extraction step or as an initial bleaching stage. Delignifica- tities of chlorinated dibenzo-dioxins and dibenzofurans,
tionisgenerallyheldtoaboutonehalfoftheligninpresent. particularly when chlorine is used as the bleaching agent.
Oxygen and peroxide bleaching have many commonal- The pulp and paper industry has demonstrated that sub-
ities because they both react with organic compounds stitution of chlorine dioxide for chlorine in the bleaching
in similar ways. However, oxygen is used for delignifi- process significantly reduces if not eliminates the poten-
cation whereas peroxide eliminates color without lignin tial for formation of such chlorinated dioxins and furans
removal. and the consequent emission of trace quantities of such
In semichemical or mechanical pulps, the aims of compounds in pulp mill effluents.
bleaching are to brighten the pulp while retaining as much With the removal of chlorine from the bleaching se-
of the yield as possible. Therefore, the bleaching chem- quence, the process is termed Elemental Chlorine Free
icals used on such pulps are those that alter, but do not (ECF) bleaching and usually an Oxygen (O) stage is now
remove, the light absorbing molecules. This is accom- substituted for the Chlorine (C) stage. Regulatory agen-
plished with oxidative or reductive reagents. The predom- cies in Europe, and particularly in Scandinavia, have im-
inant oxidative agents are sodium and hydrogen perox- posed even greater restrictions on emissions from pulp
ide. Reducing agents include zinc and sodium hydrosulfite mill bleach plants and another new approach has been de-
(dithionites), sulfur dioxide, sodium sulfite, and bisulfite veloped, namely, Totally Chorine Free (TCF) bleaching of
and sodium borohydride. Of these, hydrogen peroxide and pulps. For TCF more radical changes are necessary with
sodium dithionite, represent the greatest use. substitution of both (C) and (D) stages with ozone (O),
The important parameters in bleaching wood pulp are peroxide (P), and enzyme (X) stages in a sequence such
the concentration (consistency) of pulp and bleaching as OXZP.
chemicals, the reaction temperature and duration (resi- The use of enzymes is the newest development in
dence time), the mixing of pulp and chemical, and the bleaching technology. At least one enzyme based pro-
pH at which the reactions are carried out. Initial tem- cess developed in Finland has been applied commercially.
peratures and concentrations are usually selected based The process uses xylanase to make lignin more vulnera-
upon experience with a particular pulps needs. Control ble to oxidation by attacking the surrounding polysaccha-
is achieved by carefully balancing all these various fac- rides that protect the lignin. Another exciting application
tors to optimize bleaching with a minimum expenditure of would be to use of these and other enzymes for removal
chemical. of lignin pollutants from waste effluents. Biotechnology
The bleaching of pulp is done in a carefully balanced should lead to safer and cleaner methods for pulping and
series or sequences of treatments. The number and type bleaching.
of bleaching steps is governed by the type of pulp and Bleaching remains an energy intensive and costly part
the end-use requirements. The various bleaching agents of pulp and paper production. Studies are continuing on
discussed above are used to remove or alter the residual reducing the investment in the large facilities required and
lignin in the pulp. These steps are given letter designa- the water and energy usage. The bleach plant of the future
tions by the pulp and paper industry. These designations will consist of fewer stages to achieve the brightness levels
are chlorine (C), chlorine dioxide (D), hypochlorite (H), required of paper.
peroxide (P) and oxygen (O). A caustic extraction step
(E) is usually used at some point between some of the
bleaching sequences. If more than one type of bleaching VI. PAPERMAKING
chemical is used in any one step, the minor agent is usually
subscripted (i.e., E 0 for caustic extraction with oxygen). If Paper is a thin sheet of material which, under low-power
the agents are used in equivalent amounts such as chlorine magnification, appears as a network of very small fibers.
and chlorine dioxide, the step may be designated as C/D These fibers are generally much greater in length than in
or C + D. diameter, and this length to width difference is an impor-
Some classes of pulp such as the sulfites or bisulfites are tant factor in controlling sheet properties. In engineering
relatively easy to bleach. These pulps can be bleached with terms, paper is an orthotropic material (i.e., the mechan-
as few as three to five steps (e.g., CEH, CEHEH). Kraft ical and physical properties of paper vary in each prin-
softwood pulps are difficult to bleach to high brightness. cipal, orthogonal direction). When the fibers are first de-
From five to seven steps may be required (e.g., CEHEH, posited to make the sheet, the fibers are rarely oriented
CEHDEDP). Kraft hardwood pulps generally are regarded in a completely random manner. Instead, the long axis
as intermediate in difficulty. of the fiber is frequently biased in the direction of ma-
In recent years, the bleaching process has come under chine travel. Thus, paper is stronger in tension, but tears
some scrutiny because of the potential to form trace quan- more easily in the machine direction. Since most fibers