common raw materials used to make paper include, esparto-grass, straw, wood,flax (linen), hemp, jute and rags-cotton and linen.
The process of paper manufacture consists of two main steps, namely, pulp making and the conversion of pulp into paper. During the first step, i.e. pulp making,fibers are extracted from the raw materials. In the second stage, the pulp in stage one is converted into paper proper ready for the market. Linen and cotton rags having already undergone a process of manufacture, consist of almost purefibers with the addition of fatty and coloring matters, which can be got rid of by simple boiling under a low-pressure steam with a weak alkaline solution. On the other hand, esparto, wood, straw, flax, hemp and jute as they are from the soil/wood contain all the intercellular matter in its original form, which should be dissolved by strong chemical treatment under a high temperature in other to free thefiber for subsequent use.
Wood-pulp has grown to be one of the most importantfibers for paper-making purposes. Scandinavia, Germany, the United States and Canada are the countries that mainly use wood as a material for paper-making, owing to their possession of large forest areas. These countries are the source of wood-pulp used by many other countries. Trees of medium age are usually selected, varying from 70–80 years’ growth and running from 8 to 12 in. in diameter. They are fell in winter and reach the mill in logs about 4 ft long. After being freed from the bark and the knots taken out by machinery, the logs are cut into small cubical chips about 2 in. in size by a revolving cutter. The chips are then bruised by being passed between two heavy iron rolls to allow a boiling solution to thoroughly penetrate them, and are conveyed to boilers over a screen of coarse wire-cloth, which separates out thefine sawdust as well as any dirt or sand. In the soda process, the wood is boiled in large revolving or upright stationary boilers to extract thefiber.
Most kinds ofstrawcan be utilized for making into paper, the varieties generally used arerye, oat, wheat and barley. Oat and rye are the most important, as they give the largest yield infiber. Germany and France are the two principal users of straw, which closely resembles esparto in its chemical constitution and is reduced to a pulp by a somewhat similar process. Fibers like jute, hemp and manila are chiefly used for the manufacture of coarse papers where strength is of more importance than appearance, such as wrapping papers, paper for telegraph forms, etc. The boiling processes for these are similar to those used for esparto and straw.
the hemicelluloses and celluloses intact, thereby increasing the yield of thefibers.
The exact processes of pulp production depend on the nature of the raw material used and the application to which the eventual paper is to be put. The separation of the fibers from the fiber-containing matrix may be achieved by chemical means, mechanical means or by a combination of both.
6.4.1 Chemical Pulping
During chemical pulping, the lignin binding thefibers is dissolved (broken down) by the use of a chemical. In the process, thefibers are released for further treatment and use. The particular chemical used gives rise to its own procedures. The most widely used chemical procedure is Kraft or sulfate pulping. Others are sulfite pulping and the NSSC (neutral sulfite semi-chemical) pulping. Another chemical used in the past but no longer used is soda (the process cooked wood with strong solution of NaOH and Na2CO3).
6.4.2 Kraft Pulping
Also called the sulfate pulping, this process cooks the raw material in a liquor to which sodium sulfate (Na2SO4) has been added (hence the name‘sulfate process’).
This separates the lignin and wood resins from the cellulosefiber pulp, which is then washed and, if necessary, bleached. Kraft pulping is the dominant method of pulp production in the world by virtue of its versatility and the high strength, long fiber, very low lignin content pulp it produces. The raw material most adopted to this process is coniferous wood (soft wood) even though the method can be used for all sorts of wood and non-wood materials. Most Kraft mills operate a closed loop system whereby 95–98% of the chemicals used in the process are recovered and reused. This means that the Kraft process is superior to other processes in terms of chemical usage per ton of wood-pulp produced. The Kraft process may be operated batchwise or in a continuous manner. Batch processes are easier to control. Con- tinuous processes, however, are more economical and are amenable to the instal- lation of pollution control facilities. Paper produced from Kraft pulp is very strong because of the longfibers it produces and the mild action of the chemicals on the pulp. Depending on the nature of the bleach, Kraft pulp comes in all manner of colors from dark color (for making sacks, wrappers and paper board) to light colors and white color pulps used for all manner of papers.
Aflowchart of the Kraft process is shown in Fig.6.1for the processing of wood pulp. The raw materials, the logs, when they arrive at the plant are first cut into convenient sizes and subsequently debarked. They are then conveyed to chippers where they are reduced to chips of preselected size. From the chippers, the chips are screened (rotating or vibrating screens are used) to separate the saw dust and oversized chips from the product (chips of required size). The oversized chips are returned to the chipper to reduce them to the required size. The product is then fed
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to a digester. The digester is made up of two zones. In thefirst, zone steam at a pressure of about 100 kPa is used to volatilize non-condensable gases and tur- pentine out of the chips. Subsequently, they pass on to a high pressure zone where they are dosed in cooking liquor. The temperature and pressure of this zone are kept at 170 °C and 900 kPa, respectively. The chips stay in this zone for about 1.5 h to cook. During the cooking, the lignin that welds the useful cellulose and hemicel- lulose compounds and other organic compounds is hydrolyzed and dissolved releasing these compounds into the solution or the atmosphere. The organic acids (rosin acid) form sodium soaps, and mercaptan and organic sulfides are released into the environment, which are the sources of foul odor associated with pulp mills.
Fig. 6.1 A simplifiedflowchart of the Kraft pulping process for wood
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The cooking reaction is stopped by quickly introducing aflow of cold cooking liquor into the digester. Before leaving the digester, the chips arefirst washed to reduce the chemical content of the chips and reducing the pressure of the dosing zone. The process producesflashing steam, which is used to pre-steam the chips entering the digester. The product from the digester is known as brown stock. The brown stock carries with it adhering liquor and an enormous heat energy load. The heat energy is tapped before further treatment of the brown stock. From the digester, the brown stock is washed and passed over screens to remove knots, unreacted chips, slivers and trash before subjecting it to the subsequent operations of thickening,filtering and bleaching. After bleaching, the pulp is washed and ready for use to make paper. Alternatively, it may be rethickened and made into laps (sheets of pulp dry enough to fold into a bundle) for storage and shipment.
6.4.2.1 Treatment of Black Liquor (Regeneration and Recycle of Cooking Chemicals)
An attractive and important feature of the Kraft process is the recovery and recy- cling of spent liquor from the cooking process. This spent liquor is commonly known as black liquor and is extracted from the cooked pulp. The black liquor is treated to recover its chemical content for reuse and its organic content as a source of provision of heat energy. The spent liquor from the washer contains between 95 and 98% of the total chemicals fed to the digester. The chemicals present in the liquor are organic sulfur compounds, sodium sulfide, sodium carbonate and small quantities of sodium sulfite, salt and silica. Traces of lime, iron oxide, alumina and potash are also found in the liquor. The total solids content of the black liquor is generally, on average, about 20%. To recover these chemicals, the black liquor is first concentrated in evaporators, burnt in a furnace andfinally limed.
In one such cooking liquor recovery process, the Thomlinson Kraft recovery process, the black liquor is first concentrated in multiple-effect evaporators to a concentration of about 35% solids. The resulting solution is sprayed into a Thomlinson furnace where it is burnt. In the process, sulfate is reduced to sulfide, steam is generated and a molten salt mixture (smelt) is produced. In the furnace, any remaining organic compounds are broken down, the carbon burnt away, and inorganic compounds are melted. The reduction of sulfate into sulfide takes place in the furnace according to the following reaction:
Na2SO4þ 2C ! Na2S þ 2CO2 ð6:1Þ The carbon (reducing agent) is supplied by the organics in the wood.
The smelt is allowed to fall and dissolve in a‘dissolving liquor’from a caus- ticizing plant to give a green liquor. Insoluble impurities in the smelt settle out and any carbonate is causticized by the addition of slaked lime to the green liquor. The reaction proceeds quickly as follows:
Na2CO3ð Þ þaq Ca OHð Þ2ð Þ !s 2NaOH aqð Þ þCaCO3ð Þs ð6:2Þ
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The slurry is separated in settlers with the aid of Monel metal screens as the filtering medium. Thefiltrate is the white liquor used in the cooking of thefibers. It contains sodium hydroxide, sodium sulfide and small quantities of sodium car- bonate, sodium sulfate, sodium sulfite and thiosulfate. These compounds are formed from the original sodium sulfate added to the digester. The calcium car- bonate sludge, also called mud, is sent to a lime kiln to regenerate calcium oxide for reuse.
Important by-products of the Kraft process are tall oil and turpentine (a mixture of organic compounds called terpenes, e.g.a-pinene). It is used as an antiseptic in detergent products. The tall oil is a black sticky, viscous liquid composed mainly of resin and fatty acids. It is recovered from weak black liquor by means of centrifuges or byfloatation from the concentrated liquor. The turpentine accompanies the relief gases from the digester from where it may be separated. About 11–40 L of tur- pentine may be won from every ton of pulp produced. Tall oil has a variety of uses in industry. It is used as a frothing agent in the flotation process for reclaiming low-grade metal-bearing ores (such as ores of copper, lead and zinc), and as a solvent or wetting agent in a variety of textile and synthetic fiber manufacturing processes. The distilled fatty acids of tall oil are used in soaps, detergents and disinfectants and as a base for lubricating greases, textile oils, cutting oils and metal polishes. They are also used as drying agents in paint. There are, however, a number of problems associated with the Kraft process, including:
i. Strong odoriferous materials released into air (air pollutants) that are difficult to control.
ii. Chemicals, mostly chlorine left in the bleach water are unfriendly to the environment.
iii. The bleaching process may hurt thefibers.
iv. The molten salt of the black liquor recovery process (is explosive and) can cause explosion when in contact with even small quantities of water.
6.4.3 Sulfite Pulping
The main chemicals used in this process are calcium bisulfate and sulfur dioxide.
Magnesium bisulfate may and is increasingly used in place of calcium bisulfate.
When magnesium bisulfate is used, the chemicals can be regenerated and recycled while this is not possible when calcium bisulfate is used. The process involves the cooking of an appropriately treated wood species in an aqueous solution containing calcium bisulfate (magnesium bisulfate) and an excess of sulfur dioxide. The main raw material used in this process is spruce wood, even though other wood species like hemlock and balsam are also used. The unit operations involved in the sulfite process are similar to the Kraft process. A simplified flowchart for the sulfite process (based on the use of magnesium bisulfite as the cooking chemical) is shown in Fig.6.2.
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As a first step in this process, the cooking liquor (Ca(HSO3)2, Mg(HSO3)2, NH4(HSO3)3) is prepared. The liquor charged to the digester contains 4.5% total sulfur dioxide and about 3.5% free sulfur dioxide. The liquor is prepared by absorbing a gas of sulfur dioxide in water in the presence of an appropriate cooking Fig. 6.2 A simplifiedflow chart of the sulfite pulping process for wood
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chemical in a counter-current absorber. The following reactions proceed to produce the cooking liquor;
2SO2þH2OþMgCO3 ! Mg HSOð 3Þ2þ2CO2 ð6:3Þ or
2SO2þMg OHð Þ2 ! Mg HSOð 3Þ2 ð6:4Þ When calcium bisulfite or ammonium bisulfite is used, the corresponding reactions proceed as follows:
2SO2þH2O þ NH3 ! NH4ðHSO3Þ3þCO2 ð6:5Þ 2SO2þH2O þCaCO3 ! Ca HSOð 3Þ2þCO2 ð6:6Þ The cooking liquor is charged to the digester together with appropriately pre- pared chips. The digester is either heated with direct steam or by heating the liquor outside of the digester in stainless steel tubes and subsequently circulating it through the chips with the help of pumps. The digester temperature is kept at 170– 176 °C for between 6 and 12 h. The digester pressure varies from 480 to 1100 kPa.
In the digester, two main reactions are believed to take place. The first is the sulfonation and dissolution of lignin with the bisulfite accompanied by a second reaction, which is the hydrolytic splitting of the cellulose-lignin complex. In the process, the hemicelluloses are hydrolyzed to simpler compounds and other extraneous wood components are acted upon.
From the digester, the reaction mixture is sent to a blow tank where the cooking liquor (a weak red liquor) is evaporated and burnt in a boiler during which mag- nesium oxide (MgO) and sulfur dioxide (SO2) are formed. The MgO is slaked and send to the absorption tower together with the SO2to produce the cooking liquor. In the boiler, steam is generated that is used in the process. The residual pulp is washed in the tank with fresh water and passed on from the blow tank to a series of screens where knots and large lumps are removed. Additional foreign matter is removed in a centrifuge. Subsequently, the pulp is concentrated in thickeners and sent to a bleacher where it is bleached with chlorine dioxide. After bleaching, the pulp mass is neutralized by milk of lime. It is then once again washed and thickened and sent to a machine stock chest from where it is formed into laps. The dry-fiber content of the pulp at this stage is 35%. The laps are dried in steam-heated rolls dryer and bailed. Thefinal pulp product has afiber content of between 80 and 90%.
6.4.3.1 Preparation of the Sulfite Cooking Liquor
The process involves first slaking burnt lime that contains a high proportion of magnesia (Mg(OH)2) with warm water to produce a suspension with a turbidity of 1oBe. The suspension is then treated with sulfur dioxide to produce the cooking
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liquor. The cooking liquor contains 4.5% total sulfur dioxide and about 3.5% free sulfur dioxide.
High-quality pulp is made from this process. The pulp is used in the manufacture of some of thefinest papers, e.g. bond paper, writing paper and book-paper. The pulp is also used as an additive of non-wood products such as plastic and synthetic fibers. The pulp is easy to bleach but the resultingfibers have the disadvantage of being weak.
6.4.3.2 Treatment of the Waste Liquor
Only the magnesium-based cooking liquor can be treated to regenerate the cooking liquor. The process for regeneration of the liquor involvesfirst sending the weak red liquor from the washer (after the blow tank) to multiple effect evaporators to be concentrated. The resulting heavy red liquor isfirst preheated and fed to a recovery furnace where it is burnt to produce magnesium oxide. Make-up magnesia is added to the product from the furnace and sent to an absorber where water and sulfur dioxide are added to produce the cooking liquor, which before being fed to the digester isfirstfiltered and kept in a storage tank.
Calcium-based sulfite liquor is not amenable to recovery and reuse of either the Ca or S content. Accordingly, when used as a cooking liquor, the waste liquor must necessarily be discarded into the environment, a decision that does not please an increasingly environmentally conscious society. Calcium-based sulfite cooking liquor is, therefore, increasingly not used in processing pulp. Ammonia-based spent cooking liquor, like the magnesium-based liquor, can be regenerated, however, the ammonia itself cannot be recovered.
6.4.4 Mechanical Pulping
In mechanical pulping, the extraction offiber from the wood is performed mostly by mechanical means without the use of any chemical. The pulp obtained by this process is of low quality in terms of color and strength. Because the pulp is not well reined, chemical decomposition of the non-cellulosic constituents sets in and results in the eventual brittleness and discoloration of thefiber. It is therefore used mostly for making cheaper grades of paper and board where permanency is not required. In practice, it is used in combination with pulp from other sources.
The mechanical pulping process uses mostly soft coniferous wood species to make pulp. The wood species commonly used are spruce and balsam. The wood log isfirst debarked and held at an acute angle against a rotating stone, which tears the fibers of the wood apart. The tilting of the log against the stone ensures that the fibers in the wood are not broken. The wood log is held against the stone by hydraulic pressure from hydraulic cylinders acting through pockets on the surface of the stone. Water is constantly poured onto the rotating stone to remove the heat due to friction between the stone and wood and also to carry the dislodgedfiber away. Thefibers arefirst carried to a stock sewer from where they are sent to be screened with a sliver screen. Finerfibers are collected through the screen into a
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