Pulping And Bleaching (Part - 3) | Chemical Technology - Chemical Engineering PDF Download

Bleaching of Mechanical Pulp

Hydrosulphite

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 

Figure M-III 2.5: News Print manufacture  

QUALITY OF GOOD NEWS PRINT

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 

PROCESS: 

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

Furnish:

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 

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.

NaHSO₃

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 +2H₂O 

Mg (OH) ₂ + H₂SO₃ → Mg (HSO₃) + 2H₂O 

Na₂CO₃ + 2H₂SO₃→NaHSO₃ + H₂O 

NH₃ + H₂O →NH₄OH 

NH₄OH + H₂SO₃ →NH₄HSO₃ +H₂O 

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