Forestry And Environmental Science, SUST, Sylhet

Pulp & paper




Raw materials of Pulp & Paper:

Raw materials are generally two types:
a. Fibrous material
Woody fibrous
Soft wood spp. - Pine, Banspata
Hard wood spp. - Kadam, Gewa
Non wood fibrous - Bamboo,Cane, Straw

b. Non fibrous material
Sizing materials : Internal sizing materials: Starch, Gum, Resin.
Surface sizing materials: Wax emulsion,
Bitumin emulsion
Alum : Al 2(SO4)3. 18H2O
Fillers materials : CaCO3, Ca silicate, China clay
Coloring materials : Dye all types pigments.
Fungicides fire retardants: Borax, Borax acid.
Water :



Properties of paper

According to the TAPPI (TECHNICAL ASSOCIATION OF PULP & PAPER INDUSTRY) the properties of paper are widely tested as: Physical & Mechanical properties, common optical properties.

Physical & Mechanical properties are: Weight, Thickness, Tensile strength, Stress, Bursting strength, Tearing strength, Folding endurance, Moisture content, Water resistance.

Weight/ Grammage: It is measured gm\m

Thickness: It is the single sheet measurment under spacified conditions and is usually expressed in thousands of mm or inch.

Apparent Density:

Bulk:

Moisture content: It is determined by drying the sample to constant weight at 100x-105xCand expressed as percentages of the original weight of sample.

Water resistance: It is the properties of a sheet resisting the passage 0of liquid water into or through the sheet.

Tensile strength: It is the force parallel with the plane of the sheets required to provide failure in specimen width and length under specified comditions of loading.

Stress: It is the extension produced by loading to the point of rupture and results from the application of a tensile load supplied under specified conditions.

Bursting strength: It is the hydrostatic pressure required to rupture a circular area of the paper.

Tearing strength: It is the internal tearing resistance and average force in grams required to tear a single sheet of paper under standardize condition, in which the speciment is cut prior to actual tear.

Folding endurance: It is number of folds a paperwill withstand before failure when tested under specified condition in a specified instrument.





Pretreatment of wood & other raw materials:

a. Sawing & barking:
Pulp logs are sawn into boits of desired length either in the forests or in the mill. Next step is the removal of bark. It is essential as bark contains very little fibre of the type desired in the pulp. Bark is difficult to process & uses an excessive amount of chemical. A good part of bark lignin remains insoluble & its presence reduces the pulp yield as well as the brightness of the pulp.
Barking is done by mechanical or hydraulic barkers. Mechanical barkers separate barks either by the cleavage of the cambial layer- by crushing between rollers, hammering, abrasion with chains, rolling & tumbling in drums or by the cutting action of knives.
b. Knife Barker:
The knife barkers are the oldest type of barking equipment. The operation in wasteful as up to 25% of the wood may be lost along with the bark. It is also dangerous to operate. Therefore, knife barkers have been largely replaced by drum barkers.
c. Drum barkers:
Drum barkers utilize friction from tumbling or rotating action of a moving mass of pulpwood bolts, while in the stationery barker, mass of bolt are stirred up with continuous agitating cams. Water is sprayed in both types to keep the bark wet & flush it away.
d. Hydraulic Barker:
Hydraulic barkers employ water jets under a very high pressure, 100Kp/cm² or more to remove bark. Large quantities of water are required & power consumption is high for running huge pump. However, labour cost is less due to the high capacity of these barkers.
Generally logs are barked manually in the countries with low labour cost. In Bangladesh KPM uses a drum barker along with manual barking. KPM & SPPM have their log manually.
Chipping, crashing & Screening:
Wood must be chipped for all chemical pulping processes. The purpose of chipping is to reduce wood to a size that will facilitate penetration & diffusion of the chemicals needs for digestion. The chipper is a vertical steel disc with 4 to 12 radial slot in which knives are set. A chute or sprout feeds the bolts of pulpwood endwise at angle of 37x to 40x against the rapidly removing knives. Thin slabs of wood are shaved off which breaks up into chips of predetermined size, with little saw dust or fines.
The chip size for chemical pulping are generally 12 to 25mm along the grain, 6 to 50 mm across the grain & 1.5 to 3mm in thickness.
All chips are screened to remove fines & coarse materials. Fines are sent to the boiler & coarse materials are rechipped.