Unbleached pulps exhibit a wide range of brightness depending on the pulping process and fibrous raw material used. The sulfite process produces the brightest pulp, up to 65% whereas Kraft, soda and semi-chemical may produced pulp of only 15% brightness.
Holocellulose (Cellulose + hemi-cellulose) are inherently white and do not contribute to color. It is the chromophoric group in lignin which are responsible for color.
Two approaches are used in the chemical bleaching of pulps. One approach utilizes selective chemicals that destroy some of the chromophoric group but do not attack lignin. The other approach is to almost totally remove all lignin. The first approach provide around 70% brightness and retain high pulp yield, while second approach provide 90% + brightness but reduces pulp yield.
|A||Acid Wash||To remove metal element from pulp|
|B||Boron Hydride NaBH4|
|C||Chlorination||5 -7% on pulp||40 min||2||3 - 4%||20 - 25 0C||Elemental chlorine (Cl2) is an effective de-lignifying agent. As it breaks lignin bonds, it adds chlorine atoms to the lignin degradation products, thus producing significant amounts of chlorinated organic material.|
|D||Chlorine Dioxide||0.6-1.0% on pulp||180 min||3.5 - 4.0||10 - 12%||60 - 80 0C||Chlorine dioxide (ClO2) is a highly selective chemical that can both de-lignify and brighten pulp. It oxidizes lignin, but does not add chlorine atoms onto lignin fragments; however, small amounts of elemental chlorine and other chlorine compounds formed during the chlorine dioxide bleaching process react with degraded lignin to form chlorinated organic compounds.|
|CD or C/D or D/C||Chlorine and chlorine dioxide added together|
|E||Alkaline Extraction||3 -4% on pulp||120 min||12||10 - 20%||45 - 95 0C||To remove colored components from partially bleached pulps that have been rendered soluble in dilute warm alkali solution.|
|EO||Alkali extraction reinforced with oxygen|
|EP||Alkali extraction reinforced with hydrogen peroxide|
|EOP||Alkali extraction reinforced with oxygen and hydrogen peroxide|
|F||Formamidine Sulfinic Acid|
|H||Sodium Hypo-chlorite||1% on pulp||11-11.5||3 - 15%||30 - 60 0C||Sodium hypochlorite (NaOCl) is an inexpensive de-lignifying agent formed by mixing elemental chlorine with alkali at the mill.|
|M||Chlorine Monoxide or Hypochlorous|
|O||Oxygen||1-5 - 2.0% on pulp||60 min||>7||10 - 15%||85 - 95 0C||Oxygen removes lignin and modify other coloring components. In the oxygen delignification/bleaching stage the pulp is treated with oxygen in a pressurized vessel at elevated temperature in an alkaline environment.|
|P||Peroxide||2 -3 % on pulp||1 -2 hr||10 - 30%||65 - 80 0C||Hydrogen peroxide (H2O2) is mainly used to brighten pulps in the final bleaching stages. Peroxide is often used at the end of a conventional bleaching sequence to prevent the pulp from losing brightness over time. Preferred for mechanical and recycled fiber.|
|Paa||Peracetic Acid, CH3COOOH|
|Q||Chelatin||To control the brightness restricting and reversion effects of iron salts and other heavy metals in the pulp.|
|W||Wash||Pulp is washed almost every bleaching stage to remove reactants of preceding stage.|
|X||Xylanase||Xylanase-based enzymatic pretreatment, in a TCF (Totally Chlorine Free) sequence, results in easier bleaching and delignification of the pulp, causing a bleach-boosting effect.|
|Y||Sodium Hydrosulfite||0.5 - 1.2% on pulp||35- 60 min||5.5||5 - 8%||60 - 75 0C||Reductive bleaching. Good for recycled fibers.|
|Z||Ozone||0.1 - 1.0% on pulp||2.5||5 - 15%||<650C||Ozone (O3) is also an effective de-lignifying agent. It also brightens the pulp as well. Ozone attacks the cellulose fiber as well as the lignin.|
|ZD||Ozone and Chlorine Dioxide added sequentially in same stage|
Bleaching sequences can be any combination of these stages. Lately selection of bleaching sequence has largely being guided by environmental considerations. In USA, Canada and Europe, use of elemental chlorine for bleaching is almost stopped. Two categories of bleaching sequences used are; ECF (Elemental Chlorine Free) and TCF (Total Chlorine Free). ECF uses Chlorine Dioxide while TCF exclude Chlorine Dioxide. Read more about ECF from Alliance for Environmental Technology, a group lobbying for ECF.
Other issue to consider in deciding bleaching sequence are
- Type of Pulp: The high yield pulp which retain most of their lignin are bleached by modifying the lignin instead of removing it. Chemical pulp where most of the lignin is already removed bleaching is performed by removing the remaining lignin. Recycled fiber are de-inked before any bleaching.
- Degree of Brightness: The bleaching sequence and number of stages are mostly governed by the requirement of brightness levels. A simple CEH or DEH is enough to achieve 70 or below brightness. 4 to 5 stages are required for 80 -90 brightness and more stages for >90 brightness.
- Classical Bleaching Sequences
H CEH CEHH CEHEH CEHHD CEHDH CEDED CEHDED CEDH
DED DEDED (ZD)EopDD ODEOPDD
OAPP OZEPY OXZEPY