process” is one of the most dominating pulping technologies worldwide. Kraft
process involves digestion of wood chips in an alkaline solution of sodium hydroxide (NaOH) and sodium sulfide (Na 2 S) for removal of hemicelluloses and lignin
components. Sulfite process is another common popular pulping process which is
based on acidic or neutral cooking with salts of sulfites (SO 3
2À
) or bisulfates
(HSO 4
À ) and bases like calcium (Ca
2+ ), magnesium (Mg
2+ ), ammonium (NH 4
+
),
sodium (Na
+
), etc. (Hultman 1997). Heat and pressure is used for the pulping
process. The reaction continues to a certain predefined degree of delignification,
and the products receive a good strength. About 40–50% of fiber yields from
chemical pulping, and the remaining is considered as by-products or burned in
recovery boiler (Hultman 1997).
3.3 Bleaching
The residual lignin (5%), which remained in pulp after pulping process, is removed
by multistage bleaching process to give the paper a specific brightness (Ljungberg
and Brannvall 2011). Chlorine (Cl 2 ), chlorine dioxide (ClO 2 ), hydrogen peroxide
(H 2 O 2 ), caustic, oxygen, ozone, hypochlorite, and sodium bisulfite are the major
used bleaching agents.
3.4 Chlorine Bleaching
In papermaking process bleaching is an important process in which chlorine
bleaching with elemental chlorine is the most common technology, which generates
adsorbable organic halides (AOX). In this process residual lignin is removed
between the range of 5% and 10%. In this process, for whitening the pulp, many
steps are performed with the help of chlorine dioxide or hypochlorite.
3.5 Elemental Chlorine-Free (ECF) Bleaching
In large-scale paper mills, the brightness of pulp is achieved by ECF bleaching
where it uses oxygen delignification (ODL), followed by ClO 2 and other chemical
agents. The sequence of the chemicals includes chlorine dioxide, caustic soda,
oxygen, and hydrogen peroxide. The ECF bleaching is based on the utilization of
ClO 2 , in which only small portions of AOX are formed.
340
I. Haq and A. Raj
process involves digestion of wood chips in an alkaline solution of sodium hydroxide (NaOH) and sodium sulfide (Na 2 S) for removal of hemicelluloses and lignin
components. Sulfite process is another common popular pulping process which is
based on acidic or neutral cooking with salts of sulfites (SO 3
2À
) or bisulfates
(HSO 4
À ) and bases like calcium (Ca
2+ ), magnesium (Mg
2+ ), ammonium (NH 4
+
),
sodium (Na
+
), etc. (Hultman 1997). Heat and pressure is used for the pulping
process. The reaction continues to a certain predefined degree of delignification,
and the products receive a good strength. About 40–50% of fiber yields from
chemical pulping, and the remaining is considered as by-products or burned in
recovery boiler (Hultman 1997).
3.3 Bleaching
The residual lignin (5%), which remained in pulp after pulping process, is removed
by multistage bleaching process to give the paper a specific brightness (Ljungberg
and Brannvall 2011). Chlorine (Cl 2 ), chlorine dioxide (ClO 2 ), hydrogen peroxide
(H 2 O 2 ), caustic, oxygen, ozone, hypochlorite, and sodium bisulfite are the major
used bleaching agents.
3.4 Chlorine Bleaching
In papermaking process bleaching is an important process in which chlorine
bleaching with elemental chlorine is the most common technology, which generates
adsorbable organic halides (AOX). In this process residual lignin is removed
between the range of 5% and 10%. In this process, for whitening the pulp, many
steps are performed with the help of chlorine dioxide or hypochlorite.
3.5 Elemental Chlorine-Free (ECF) Bleaching
In large-scale paper mills, the brightness of pulp is achieved by ECF bleaching
where it uses oxygen delignification (ODL), followed by ClO 2 and other chemical
agents. The sequence of the chemicals includes chlorine dioxide, caustic soda,
oxygen, and hydrogen peroxide. The ECF bleaching is based on the utilization of
ClO 2 , in which only small portions of AOX are formed.
340
I. Haq and A. Raj