Pulp

Pulp fibrous cellulosic material used in the production of paper. Wood is the principal raw material for the production of pulp; but cotton, linen, rags, straw, bamboo, etc are also used. Any given grade of paper may include one or several types of pulp, depending upon the properties desired in the finished product. The demand of paper in Bangladesh surpasses the production. Consequently, some papers, mostly value-added papers, packaging and board papers, fine papers, etc, are imported.

Pulp industry The four state-controlled pulp mills are: Karnafuli Paper Mills (KPM), Sylhet Pulp and Paper Mills (SPPM), North Bengal Paper Mills (NBPM), and Khulna Newsprint Mills (KNM). These are run by Bangladesh Chemical Industries Corporation. Four other mills operate in the vicinity of Dhaka under private management. These are Bashundhura Paper Mills, Sonali Paper Mills, Magura Paper Mills, and Tongi Board Mills.

Among the state-controlled mills, KPM, NBPM and KNM are integrated pulp and paper mills but SPPM produces only market pulp. The four mills in the private sector use waste paper, and market and import pulp as the fibrous raw material. Waste papers are mostly imported, and only a small quantity is collected locally.

The KPM is an integrated pulp and paper mill, where the Kraft pulping process is used. Bamboo and hardwood are the fibrous raw materials. Bamboo, however, is now a scarce raw material. So the mill uses heterogeneous mixture of different hardwoods.

Pulping Generally pulping is grouped into three main categories, viz, mechanical, semichemical, and chemical. The mechanical pulping can be done directly from wood log, known as groundwood pulping or from chips. Mechanical pulping from chips is not used for papermaking in Bangladesh. Only groundwood process is used in the country. This process is used in newsprint mill at Khulna (KNM). Groundwood pulp is produced by pressing debarked wood bolts transversely against a revolving grooved grindstone in presence of water. The preferred wood species are light coloured and long fibred softwoods. No softwood is available in Bangladesh. Gewa wood (Excoecaria agallocha) with a mild chemical treatment before grinding is used. The yield in groundwood pulping may be upto 95%. The energy consumption varies from 1,000-1,500 kwh per ton of pulp depending on the sharpness of the grinding stone.

The semichemical pulping involves two stages: a preliminary treatment of chips with chemicals, which results in partial delignification and softening of the middle lamelia and a subsequent treatment to isolate the fibres from the softened chips. The yield range of such pulps is about 55-80%.

In chemical pulping, sufficient lignin is dissolved mainly from the middle lamelia to allow the fibres to separate with little, if any, mechanical action. However, a portion of lignin is retained in the fibre wall. Complete removal of lignin during pulping would result in excessive degradation of the pulp. For this reason about 3-4% lignin is normally left in hardwood chemical pulp and 4-10% in softwood chemical pulp. This is subsequently removed by bleaching if bleached pulp is desired. The different chemical pulping processes are the soda process, the sulphate or kraft process, and the sulphite process.

Pulpwood Besides wood about 5% of pulp originates from other sources mainly bamboo, bagasse, straw, etc. The coniferous woods or softwoods are the most preferred species. Although hardwoods are now increasingly being used, they are not preferred because they do not, as do many softwoods, give a uniform pulping. The chemical composition of a hardwood sometimes renders it less suitable for pulping, and sometimes its structure is so dense that pulping liquors cannot penetrate easily. Hardwoods have been considered less suitable for pulping than softwoods as they possess shorter fibre and produce less uniform pulp. Fibre length has for a long time been considered of primary importance for the quality of paper pulps.

In Bangladesh different hardwoods, viz, gamar (Gmelina arborea), shimul (Bombax ceiba), kadam (Anthocephalus chinensis), pitraj (Amora species), Koroi (Albizia species), etc, are generally used for chemical pulping. In groundwood pulping for newsprint, only gewa (Excoecaria agallocha) is used. Various exotic hardwoods, viz, akashmoni (Acacia auriculiformis), mangium (Acacia mangium), Eucalyptus, etc. are now being grown in Bangladesh. These are well-known species for pulp making.

The pulp produced in Bangladesh is not of good quality. Therefore the country has to import high strength conifer pulp for reinforcement. Experimental studies have shown that jute fibre may be an excellent raw material for pulping to substitute conifers. However, for this new process technique has to be developed. [AKM Akhtaruzzaman]

See also paper.