3.6 Total Chlorine-Free (TCF) Bleaching
TCF bleaching includes combination of ODL with ozone/peroxide which leads to no
formation of AOXs. There are some agents, such as pH, temperature, and reaction
time, which decide the bleaching stage. Before the progression to the next stage, the
pulp is washed with caustic to remove bleaching chemicals and dissolved lignin. At
the end of bleaching process, the pulp is pumped through a series of screens and
cleaners to remove contaminants. It is then concentrated and conveyed to storage.
The bleaching process can be enhanced by the use of enzymes, and a “chelating”
agent (ethylenediaminetetraacetic acid, EDTA) is added to bind the metal ions
contained in the pulp and prevent them from decomposing the hydrogen peroxide.
3.7 Hydrogen Peroxide Brightening
The bleaching of pulp with hydrogen peroxide is very effective in removing of high
content of lignin. Hydrogen peroxide alters the chemical structure of lignin by
oxidizing and remains with the pulp. Though hydrogen peroxide is environmentally
benign, it is expensive.
3.8 Paper Production
After the bleaching, the pulp is then converted to paper products such as writing
paper, books, and grocery bags. Sulfite pulp, which is primarily cellulose, is
converted into specialty paper, rayon, photographic film, plastic, adhesive, and
even ice cream and cake mixes. The fibers in pulp from recycled paper are usually
shorter, less flexible, and less water permeable and can therefore not be used for
high-quality paper products. Recycled pulp is therefore mainly used for the production of soft paper products like tissue paper, toilet paper, paper towel, and napkins.
To produce market pulp, the pulp consistency is adjusted to 4–10% before it is ready
for the pulp machine to make pulp sheet using plastic mesh or travelling metal
screen. The pulp sheet is passed through a series of rotating rolls that squeeze out
water and air until the fiber consistency is 40–45%. The sheet is then floated through
a multistory sequence of hot air dryers until the consistency is 90–95%. Further, the
obtained pulp sheet is cut into small portion and stacked into bales. Finally, the pulp
bales are compressed, wrapped, and packed into bunches for storage and delivery
(Ljungberg and Brannvall 2011).
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341
TCF bleaching includes combination of ODL with ozone/peroxide which leads to no
formation of AOXs. There are some agents, such as pH, temperature, and reaction
time, which decide the bleaching stage. Before the progression to the next stage, the
pulp is washed with caustic to remove bleaching chemicals and dissolved lignin. At
the end of bleaching process, the pulp is pumped through a series of screens and
cleaners to remove contaminants. It is then concentrated and conveyed to storage.
The bleaching process can be enhanced by the use of enzymes, and a “chelating”
agent (ethylenediaminetetraacetic acid, EDTA) is added to bind the metal ions
contained in the pulp and prevent them from decomposing the hydrogen peroxide.
3.7 Hydrogen Peroxide Brightening
The bleaching of pulp with hydrogen peroxide is very effective in removing of high
content of lignin. Hydrogen peroxide alters the chemical structure of lignin by
oxidizing and remains with the pulp. Though hydrogen peroxide is environmentally
benign, it is expensive.
3.8 Paper Production
After the bleaching, the pulp is then converted to paper products such as writing
paper, books, and grocery bags. Sulfite pulp, which is primarily cellulose, is
converted into specialty paper, rayon, photographic film, plastic, adhesive, and
even ice cream and cake mixes. The fibers in pulp from recycled paper are usually
shorter, less flexible, and less water permeable and can therefore not be used for
high-quality paper products. Recycled pulp is therefore mainly used for the production of soft paper products like tissue paper, toilet paper, paper towel, and napkins.
To produce market pulp, the pulp consistency is adjusted to 4–10% before it is ready
for the pulp machine to make pulp sheet using plastic mesh or travelling metal
screen. The pulp sheet is passed through a series of rotating rolls that squeeze out
water and air until the fiber consistency is 40–45%. The sheet is then floated through
a multistory sequence of hot air dryers until the consistency is 90–95%. Further, the
obtained pulp sheet is cut into small portion and stacked into bales. Finally, the pulp
bales are compressed, wrapped, and packed into bunches for storage and delivery
(Ljungberg and Brannvall 2011).
14 Pulp and Paper Mill Wastewater: Ecotoxicological Effects and. . .
341