PULPING AND BLEACHING
Paper industry uses wide variety raw materials employing different type of pulping and bleaching processes depending on the type of raw materials and requirement of pulp furnish for final paper making. Both unbleached and bleached pulp is produced to meet the requirement of paper. Requirement of bleaching chemicals are also varying depending on the quality of paper and brightness and environmental considerations for cleaner and greener paper. Various steps in pulping and bleaching process is given in Table M-III 2.1. Pulping and bleaching play important role in providing strength to paper depending on fibre length, cellulose content. Commonly used term in pulping is given in Table M-III 2.2.
Table M-III 2.1: Puling and Bleaching Process
Table M-III 2.2: Commonly Used Terms in Pulping
Cooking Liquor obtained after causticising green
liquor containing NaOH, Na2S, and small quantity
Liquor obtained after washing of pulp after cooking
which is further concentrated for burning in furnace to recovered chemicals
Liquor obtained after dissolving smelt from furnace in
weak liquor from causticising section
Total titratable alkai
NaOH/ NaOH +Na2S
NaOH/ NaOH +Na2S )] x 100
Na2S/( Na2SO4 + Na2S )
Over all recovery
Recovery ± black liquor stock/ froth liquor consumed
No. of 0.1 N KMnO4 consumed by 1 gm of moisture
Dilution factor (D.F.)
DF = W -E
W= Water added per ton of mass
E=Water going in pulp per ton of pulp
Wood to liquor ratio
B.D. pulp/ total pulp weight (100gms ) i.e. wt of BD
pulp in 100 gm of pulp+ water mixture
No of milligrams of metallic copper which is reduced
from cupric hydroxide to cuprous oxide in alkaline
medium by 100 gm of pulp
837+.0323*40 ml KMnO4 No.
Chipping quality plays important role in the overall quality of pulp. Efficiency of debarking process, chip size- length, thickness, uniformity of size, removal of dust and fines are important factors affecting pulping efficiency and quality of pulp. In case of bagasse depithing is very important in overall quality of pulp and paper. In case of agricultural residue size of the straw sand removal of dust also is important. In chipping of bamboo and wood drum chipper and disc chipper are used
KRAFT PULPING PROCESS
Kraft pulping involves Pulping, Washing, Screening, Bleaching. Pulping involves separation of fibre for chemical or mechanical pulping, removal of knots, washing of pulp for recovery of chemical, rejects uncooked material, sand and other foreign material. Bleaching of pulp involves bleaching of washed and screened pulp with bleaching chemicals – chlorine, caustic, hypochlorite, chlorine dioxide, ozone, hydrogen peroxide, sodium hydrosulfite. Conventional used bleaching sequences were (CEHH, CEH, CEHDED, CEDED). Many of the mills in India have also gone for oxygen delignification and newer bleaching sequences.
Cooking of raw material is done batch digester, continuous digester. Spherical batch digester is commonly used for cooking agricultural residues. Cooking liqour used in kraft pulping process is sodium sulphide, sodium carbonate. Cooking cycle may be around 3-5 hr depending of raw material and extend of cooking in batch digester. Typical continuous digester consists of Upper heating zone, upper cooking zone, lower cooking zone, and washing zone. Sequence of operation in coking in digester are chip filling, presetting, liquor charging, heating up and pressure up period, cooking and blowing
Bulk density: 130-150 kg/m3.
Bath ratio: 1:3, Sulphidity 15%
Cooking temp: 170-175 oc.
Temp to rise: 135 oC 2 hr.
Time at: 135-140 oC 2hr.