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
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:
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.
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
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:
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 +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
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.
Difference between total SO2 and free SO2.