Pulping And Bleaching (Part - 3) Chemical Engineering Notes | EduRev

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Chemical Engineering : Pulping And Bleaching (Part - 3) Chemical Engineering Notes | EduRev

The document Pulping And Bleaching (Part - 3) Chemical Engineering Notes | EduRev is a part of the Chemical Engineering Course Chemical Technology.
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Bleaching of Mechanical Pulp


Zinc hydrosulphite and sodium hydrosulphite are commonly used for ground wood pulp and refiner pulp. Zinc hydrosulphite is prepared by reacting zinc dust with sulphuric acid. Sodium hydrosulphite can be generated by reacting sodium borohydride and bisulfite.

Peroxide: Hydrogen peroxide and sodium peroxide are recognized as effective economical bleaching agents, they don’t form chlorinated organic derivatives. They are particularly used in the bleaching of high lignin wood pulp and as a final stage in chemical pulp bleaching.

News Print: News print one of the major portion of paper produced and playing important role in information, cultural, economic growth .News print typical contains 70% chemi -mechanical pulp from eucalyptus and 30% chemical pulp from reed and bamboo. Eucalyptus wood is chipped screened and the chips are washed. The washed chips are treated with steam and caustic soda solution. The impregnated chips are pressed to remove spent liquor in refined in two stages. Then pulp is washed on brown pulp washer and bleached by two step hypochlorite treatment. The bleached pulp is again refined in a third step refiner, screened, center cleared and dewatered in a thickener drum and stored. Process flow diagram of news print manufacture is given in Figure M-III 2.5. Newsprint is a paper of printing category embossing in itself the qualities like runnability, opacity and low cost.

Advantage of Ground wood pulp in news print furnish are

  • Higher bulk-improves calendar action.
  • Improve formation because of time fraction
  • Decreased far-sidedness
  • Better and improved opacity inhibits show through.
  • Better ink absorptivity
  • Absence of print through.
  • Improved surface 

Pulping And Bleaching (Part - 3) Chemical Engineering Notes | EduRev

Figure M-III 2.5: News Print manufacture  


Runnability, good print ability, minimum two side uniformity. Pulp formation should need following requirements:

  • Low freeness50csf.
  • Specially good screening
  •  Good pulp strength property
  • Uniform pulp quality
  • Controlled bleaching system to maintain required brightness

Mechanical pulp has to have good wet end strength, good bonding

Chemical pulp used in newsprint hard wood pulp, Bagasse pulp chemical

Bagasse TMP, CTMP

BAGASSE NEWSPRINT: Bagasse newsprint is made for 50% bagasse having good printing quality, run-ability, opacity, rapid absorption

BAGASSE PULPLING: Due to availability of bagasse from sugar mills number of agro-based paper mills are using bagasse. About 10% of the bagasse is available from sugar mills.

  • Bagasse 60% fibre and 30% pith
    •  60% good quality fibre
    •  30% pitch  
    • 10% water
  • 0.25 ton of bagasse (dry) per tonne of sugarcane.
  • 3 tons of bagasse per ton of unbleached pulp Bagasse with 50% moisture & about 2-2.5% sucrose & water soluble material 


DEPITHING- Dry depthing, wet depthing, combined depthing Pith has high ash, difficult to bleach and has little value in paper making Pulping: batch cooking10-12% NaOH, 1600 C,6-7 kg/ash,2 hrs , bleaching requirement 5-6%

Problems in Utilization of Hard Wood 

  1. Debarking
  2. Chipping problem – high density chips difficult to chip
  3. Higher reject
  4. Problem in washing due to more fines and foam
  5. Bleaching of mixed hardwood and bamboo pulp creates problem due to varying bleach Requirement
  6. Shade variation , darker shade  
  7. Problem in evaporator
  8. Higher  percentage of hard wood affects runnability of machine lower strength

News Prints Pulp: News print pulp furnish consist of Kraft pulp and mechanical pulp


25-30%  Kraft Pulp

70-75% Mechanical Pulp

 Raw Material: Bamboo, Hardwood, Bagasse,  

Advantage of ground wood pulp:

  • Higher Bulk – Improves Calendaring Action.
  • Improve Formation because fine fraction.
  • Better and Improved capacity, inhibits show through better ink absorbency.
  • Absence of print through good bonding. 

In Bleaching of Mechanical Pulp, Zinc Hydrosulfide 1%,Hydrogen Peroxide. 


Sulphite process was earlier use for pulping, however it has been replaced with Kraft pulping. Table M-III 2.3 gives details of important terms used in sulphite process.  

Acid Sulphite: The process in which the cooking acid contains a high % of free SO2 pH 1.2-1.5. Not suitable for resinous wood. 

NaHSO3 + SO2 

Bisulphate: The process in which the cooking liquor contains a predominance of bisulphate ion in the 3.6 pH ranges with no free SO2.


Neutral Sulfite: Na2SO3 + Na2SO3pH =7-9 

SO2 + H2O → H2SO3

CaCO3 + 2 H2SO3 →Ca (HSO3) +CO2+H2O

CaO + H2O →Ca (OH) 2 

Ca (OH) 2 + H2SO3 → Ca (HSO3)2 +2H2

Mg (OH) 2 + H2SO3 → Mg (HSO3) + 2H2

Na2CO3 + 2H2SO3→NaHSO3 + H2

NH3 + H2O →NH4OH 

NH4OH + H2SO3 →NH4HSO3 +H2

Rate of pulping NH3 > Mg > Na

Pressure: 620-755 kpa.Temperature:125 -160 oC

Total cooking time 6-12 hr.

Cooking acid 7% -8% of total SO2

1 ton of pulp 175- 220 kg of SO2 55 -68 kg MgO 

Table M-III 2.3: Important terms in Sulphite Process 




Total sulfur dioxide:

Total SO2 as determined by an iodometric titration and expressed as % total SO2

Free sulfur dioxide

Free SO2 is the actual free SO2 plus half of the SO2 in the bisulfite of the base. It is also called available SO2.

Combined SO2

Difference between total SO2 and free SO2.

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