Pulping And Bleaching (Part - 1) | Chemical Technology - Chemical Engineering PDF Download

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 

• Acquisition of Raw Material: Hard wood, soft wood, bagasse, wheat and Rice straw, sabai grass etc.
• Raw Material Preparation : Debarking, Chipping, Cutting, Screening
• Pulping  
• Chemical                 : Sulphate (Kraft), Soda Pulping, Sulphite Pulping
• Semi Chemical        :       Neutral Sulphite Semi-chemical (NSSC)
• Mechanical Pulping: Stone Ground Wood (SGW)     Thermo Mecha nical Pulp (T MP)      Refiner Mecha nical Pulp (R MP)      Cold Soda Refiner Mechanical Pulp (CRMP) 
• Sulfite:   Acid Sulfite higher % of free SO₂(Ca, Mg, Na, Ammonia base) Bisulfite: (little or no free SO₂ (Ca, Mg, Na, Ammonia base) 
•  Washing and Screening: 3-4 stage washing, screening and cenricleaning
• Bleaching: Bleaching chemicals: Oxygen, Ozone, Chlorine, caustic soda, chlorine dioxide, calcium hypochlorite Conventional bleaching sequence CEHH, CEH, CEHD, for chemical pulp Advance bleaching sequence Hydrogen peroxide, Sodium peroxide, Hydro sulfite for mechanical 

 Table M-III 2.2: Commonly Used Terms in Pulping  

 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.

Pressure:7-7.5 kg/cm²

Temp to rise: 135 ^oC 2 hr. 

Time at: 135-140 ^oC 2hr.