process of paper production

Paper is defined as the sheet material made by felting together the vegetable cellulose fibres which are contained in almost all plant materials. The fibres are first converted into pulp and subsequently treated with a myriad of speciality chemicals which serve as strengtheners, coatings and impregnations. The paper industry, therefore, employs a vast amount of natural resources, equipment, energy and synthetic materials. Obviously, it produces environmental problems too. As paper is ubiquitous, it is generally taken for granted. However, in recent years several pressure groups are targeting the paper industry to change its productions methods, its chemical ingredients and its energy consumption so as to reduce its impact on the environment. Paper Manufacture Paper manufacture involves the following five stages Stage-1: Raw Materials Raw materials for making paper are grouped in two broad categories: Fibrous and non-fibrous. Fibrous Raw Materials The primary source of fibrous raw materials for paper making is wood, the supporting and conducting tissue for the trees. Wood is composed of strong, relatively thick-walled, long cells. These, when separated from each other, are fine fibres, suitable for paper making. Chemically, the cell wall tissue of wood is made up of two groups of polymers: The polysaccharides and the lignin. The polysacharides constitute 70-80% of the woody tissue, with lignin making up the remainder. The polysaccharides of wood are collectively known as holocellulose. The major constituent of holocellulose is cellulose, a high molecular weight, linear polymer of condensed glucose units. The minor components of helocellulose include linear and branched chain condensed polymers of pentose and hexose sugars. Lignin is a complex polymer of condensed phenylpropane monomeric units. It serves as the adhesive material of wood, cementing the fibres together to form the anatomical structure of wood. Lignin is susceptible to degradation. Therefore, in paper making it is removed from wood, leaving the separated cellulose fibres in the form of pulp. Apart from wood, cereal straws, hemp, jute and bamboo may also be used as sources of fibres in paper manufacture. In order to increase the strength and tear resistance of paper, a small percentage of synthetic fibres like rayon or nylon are added. Non-fibrous Raw Materials For making paper for high grade applications, chlorine and chlorine dioxide are the common bleachiing agents. Sizing agents like rosin are used for the purpose of decreasing the water absorption characteristics of the paper. This water repellency is a useful property for printing with water-based inks. Rosin in generally added as soluble sodium salt and then precipitated by introducing a solution of aluminium sulfate--the so called paper-maker’s alum. In order to provide opacity, the wood fibres are mixed with fillers, like calcium carbonate and pigments, like titanium oxide. Coating materials like wax or latex are added not only to increase the water resistance of paper, but also to impart a glossy appearance. In this form the paper is commonly used for printing magazine. Stage-II: Pulping The purpose of pulping is to separate out the cellulose fibres of the wood from unwanted lignin. This must be done in such a way that the cellulose fibres are undamaged or are damaged to a minimum extent. Pulp produced from hardwoodsources like deciduous trees tends to have shorter, weaker fibres as compared to that obtained from softwood sources like coniferous trees. Wood is converted into pulp by mechanical, chemical or semi-chemical processes. Mechanical Pulping Mechanical pulping consistsof subjecting wood to the action of a grindstone in a stream of water. Previously, natural sandstone was used for grinding. However, modern grindmills employ either silicon carbide or aluminium oxide grits. Mechanical pulp is quite weak. As a result, paper made from groundwood pulp soon loses strength and turns yellow. Therefore, mechanical pulp is used in relatively impermanent papers, such as a newsprints. The advantages of this type of pulp is that it possesses excellent printing qualities and ink absorption characteristic. Moreover, it is produced inexpensively and utilizes the entire wood, giving essentially 100% yield. Chemical Pulping Chemical pulping involves the removal of lignin by chemical treatment and separating out of cellulose fibres. Generally, the pulp obtained by chemical methods is tan to dark brown in colour. In this form, it may be used for making grocery bags, corrugated containers and wrapping sheets. However, if white paper is required, a suitable bleaching agent is added to the fibres liberated chemically from lignin. Chemical pulping methods are carried out either under acidic conditions or under alkaline conditions. The most significant chemical-related change in recent years has been a move from acid to alkaline paper making. Alkaline paper is valued more because it lasts longer and is less susceptible to yellowing. The acid process--also called the sulfite process--is based on the fact that ligniin reacts with bisulfite ions to form lignosulfonates that are soluble in water and hence can be drained off. Sodium, magnesium or ammonium bisulfites are generally used. The wood chips are pressure-heated with bisulfite for about 6 hours, at a temperature of 400-440 K, while the pH of the ingredients is maintained at 2..7-3.1. The lignosulfates are removed by filtration. The alkaline process initially used sodium hydroxide for digestion. The resin acids were converted into soluble sodium salts, which were filtered off. However, the amount of alkali had to be carefully monitored. If sodium hydroxide was present in excess, it degradedthe cellulose fibres too, resulting in weak pulp. This problem was overcome by replacing a part of sodium hydroxide by sodium sulfate. The modified method is called sulfate or Kraft process. In this process, the wood chips are digested under pressure with a mixture of sodium hydroxide and sodium sulfate. A part of sodium sulfate is reduced to sodium sulfide by the organic constituents present in wood. Likewise, some of the sodium hydroxide is converted into sodium carbonate. Sodium hydroxide is regenerated by treatment of residual liquor with slaked lime. The spent liquor now contains a mixture of sodium sulfide and sodium hydroxide. To this sodium sulfate is added and the liquor is reused for digestion, this operation has several advantages. Firstly, sodium sulfide speeds up saponifications of resin acids and accelerates pulping without damaging cellulose fibres. Secondly, the expensive sodium salts are conserved for reuse. Thirdly, the pollution problem is alleviated to quite some extent. Last, but far from least, the pulp produced has an exceptionally good strength (Kraft in German means strength) and, therefore, can be used for high grade applications. Semi-chemical pulping The semi-chemical pulping method, as the name suggests, is a combination of chemical and mechanical procedures. In this technique, firstly a mild chemical treatment is given to affect delignification and then a mechaical treatment is given to segregate the fibres. The wood is digested with sodium sulfite, buffered with sodium bicarbonate or sodium carbonate to maintain a slightly alkaline pH. The digestion is followed by groundmill treatment, as described in mechanical pulping. Stage-III: Stock Preparation Stock preparation involves two operations: Furnishing and beating. Furnishing During this step, the pulp is dispersed in water using a washing machine type of equipment called hydrapulper. The pulp, at consistency of 4%, is suspensed in water maintained at about 230K and agitated for about 1 hour. This operation ensures that segregation of individual fibres of cellulose. The suspension of fibres is called paper slurry. Beating Beating--also called refining-- is meant to enhance the bonding capacity of fibres amongst themselves. As the fibres are brought into close contact, a large number of hydrogen bonds are formed between their surfaces. Beating is carried out by pressing the paper slurry between the two rotating surfaces of a beater. The greater the extent of beating, the stronger is the quality of paper. After beating, the fibres are resuspended in the hydrapulper and such additives as sizing agents, fillers and pigments are added to the paper slurry. Stage-IV: Paper Making Proper The paper slurry is fed into a head box from where it is passed to a rotating loop of wire guaze. The head box controls the rate at which the slurry is introduced on the wire guaze. The slurry is pressed by table rolls to drain off water. As the slurry moves further, vacuum suction rolls remove much of the residual water. The fibre mat then passes through a series of steam-heated cylinders which bring about the final drying of the paper web. Finally, the web is pressed by a series of heavy rollers to produce a compact sheet. This step is called calendering. If a glossy surface is required, the web is ubjected to super-calendering. In this operation the fibre mat is alternately pressed by steel and hard-rubber rollers. Since the rollers are made from different materials, the paper sheet is subjected to a varying degree of friction. This differential friction has a polishing effect and produces a fairly high degree of glazing. Stage-V:Finishing Finishing step includes such surface tratments as coating, impregnations and lamination of paper. These operations confer special properties to paper, such as glossy appearance and water repellency. Above all, these increase the strength of the paper, so that it can withstand corrosive environments. Water pollution By far, the greatest environmental problem associated with paper mills is that of water pollution. The discharge of spent water from pulp and paper making introduces two broad categories of pollutants into the natural aquatic systems. These are: Suspended matter and dissolved organic substances. The suspended matter present in paper mills is in form of wood particulates, fibre debris and filling and coating materials, such as china-clay, kaolin and calcium carbonate. The suspended particles are detrimental since these can form bottom deposits in receiving streams. These are not only inimical to aquatic life, but also unsightly. On decomposition these produce odoriferous components. These also render a stream opalescent, as a consequence of which the penetration of sunlight into the water is greatly reduced. This, in turn, reduces photosynthesis and thus adversely affects the aquatic life forms. The dissolved organic compounds present in spent liquors too cause a variety of effects in streams. These pollutants are slowly acted upon by microorganisms and are transformed into simple, harmless, inorganic comppounds. But in this transformation oxygen, dissolved in water, is consumed and some of the aquatic organisms are choked to death. Some of the dissolved compounds like polysaccharides and protein stimulate the growth of slime organisms, causing biological imbalance to occur. Others, such as lignins and tannins, discolour water and impart to it an unaesthetic appearance. Daftar pustaka: SODHI, G.S. 2005. Fundamental Concepts of Environmental Chemistry Second Edition. Alpha Science International Ltd. Harrow, U.K.