Paper making using pulp wood

4.3 PULPING (DELIGNINFICATION).

Pulp is a lignocellulosic fibrous material prepared by chemically or mechanically separating cellulose fibers from wood, fiber crops or waste paper. Pulping is a process by which fibers are separated from wood or cellulose plant components and it can be done chemically, mechanically or the combination of the two methods i.e semi-chemical process. Wood and other plant materials used to make pulp contain three main components (apart from water), namely:

  1. Ligninè a 3-dimensional polymer that binds the cellulose fibers together.
  2. Celluloseè (desired for papermaking), a polysaccharide with 600 to 1500 repeated sugar units.
  3. Hemicellulose èa shorter branched carbonhydrate polymers.

Others include extractives (fats, alcohols, aromatic acids, essential oils and pigments), minerals and other organics, with the relative proportions of these components vary according to the fibre source.

The aim of pulping is to breakdown the bulk structure of the fibre source be it chips, stems or other plant parts into the constituent fibres.

 

4.4METHODS OF PULPING.

  • MECHANICAL PULPING.

Mechanical pulps are produced by grinding wood against a stone or between the metal plates, thereby separating the wood into individual fibers. The shearing action breaks cellulose fibers, so that the resulting pulp is weaker than chemically separated pulps. The lignin connecting cellulose to hemi-cellulose is not dissolved, it merely softens, allowing the fibers to ground out of the wood matrix. The yield is very high, greater than 85%. Some mechanical pulping methods also use chemicals (i.e semi-chemical pulps); their yields are lower since they remove more of the non-cellulose materials.

There are two major mechanical pulps

  1. Stone groundwood pulp (SGW).
  2. Thermo-mechanical pulp (TMP).

In the groundwood process, which is the oldest and most common mechanical pulp method, fibres are removed from short logs by pressing them against a rotating abrasive cylinder.

In the thermo-mechanical process, wood is chipped and then fed into steam heated refiners, where the chips are squeezed and converted to fibres between two steel discs.

  • CHEMICAL PULPING.

Chemical pulps are produced by chemically dissolving the lignen between the wood fibers, thereby enabling the fibers to separate relatively undamaged. In chemical pulping, chips and chemicals in aqueous solutions are cooked together in a pressure vessel known as the digested (at FIIRO  an improvised equipment called the AUTOCLAVE is used for to achieve same result). In large industries, this process is operated in batch or continuous basis. In batch digestion, the chips and digestion chemicals are fed together at elevated temperature and pressure. Once cooking is complete, pressure is released, blowing the deligninfied pulp out of the digested to a holding tank. The sequence is repeated. In continuous digestion, feeding of chips and chemicals is done at continuous rate.

There are three (3) main methods 0f chemical pulping:

  1. Soda Pulping èThis is a chemical process for making wood pulp with Sodium hydroxide (NaOH) as the cooking chemical. The soda process gives pulp with lower tear strength than other chemical pulping process. This was the first chemical pulping process.
  2. Sulpate or Kraft Pulping èThe sulphate process (also known as Kraft pulping process) describes a technology for conversion of wood into pulp consisting of almost pure cellulose fibres. The kraft process entails the treatment of wood chips (or other plant fibre source) with mixture of water, Sodium hydroxide (NaOH) and Sodium Sulphide. It is the dominant method for producing paper.

At FIIRO, Soda and Kraft process are commonly used in researches.

  1. Sulphite Pulping èThis process produces pulp using various salts of sulphorous acid to extract the lignin from wood chips in large pressure called digesters. The salts include Sulphites (SO32-) or bisulphites (HSO3).

4.5 PAPERMAKING PROCESS.

  • Wood Preparation.

This first step involves the debarking process of wood (removal of barks) either by passing it through drum debarker or being treated in a hydraulic debarker. After debarking, the logs are chipped by multi-knife chippers into suitable sized chips and then screened to remove over-large chips. The thickness of the chips is the most parameter as this determines the speed and the toughness of the impregnation of the cooking chemicals into the wood chip.

At FIIRO, the fibre source are mainly agro-waste, therefore debarking stage is not useful. The fibre source are chipped and then screened to remove or re-chip large ones.

In large Pulp and Paper industries, logs are debarked because bark contains little fibers, has a high extraction content, is dark and often carries large quantities of grit (small pieces of stone and sand).

Fig 4.1: Mid-ribs of Banana Stem.

Fig 4.2: Pineapple crown           Fig 4.3: Corn husks.

  • Cooking (Digestion).

The “cooking process” is where the main part of deligninfication takes place. This is also known as the pulping process whereby chemical pulps can be produced either by soda, sulphate or kraft, or sulphite process. At FIIRO, Soda and Kraft process are commonly used. Cooking process is carried out with the aid of digester or an improvised equipment known as an Autoclave.

  1. SODA PULPING: Say to obtain a pulp sample from corn husk, steps is given below.

Step 1:  Weigh the chipped sample (say 120g) and pour into a conical flask.

Step 2: Take little amount of the sample and determine the moisture content using a moisture analyzer. Say 25% was obtained.

Step 3: Calculate the required amount of water and caustic soda (NaOH) needed for pulping. This is calculated below:

Sample to liquor ratio = 1:5

... for 120g = 120:600

Moisture content = 25% è 25/100 x 120 = 30

... Total amount of water needed = 600-30 = 570ml of H2O.

Using 5% concentration of caustic soda è 5/100 x 600 = 30

... For the cooking process of 120g of corn husk,  30g of caustic soda is diluted in 570ml of water.

Step 4: Weigh the required amount of soda and water using a digital weighing balance and measuring cylinder respectively. Mix properly and pour into the flask containing the sample. Cover and tight properly the top with foil (to avoid the escape of water).

Step 5: Place the flask into the Autoclave and allow to cook for about one hour at a temperature of 121oC.

  1. KRAFT PULPING:

Step 1: Weigh the chipped sample (say 120g) and pour into a conical flask

Step 2: : Take little amount of the sample and determine the moisture content using a moisture analyzer. Say 25% was obtained.

Step 3: Calculate the required amount of water and caustic soda (NaOH) and Sodium Sulphide (Na2S) needed for pulping. This is calculated below:

Sodium hydroxide to Sodium Sulfide ratio = 1:3

Sample to liquor ratio =1:5

... for 120g = 120:600

Liquor weight = 20% è 20/100 x 600 =120g

... Na2S = ¼ x 120 = 30g

NaOH = ¾ x 120 = 90g

Moisture content = 25% è 25/100 x 120 = 30

... Total amount of water needed = 600-30 = 570ml.

... For the cooking process of 120g of corn husk,  30g of caustic soda and 90g of sodium sulfide is diluted in 570ml of water.

Step 4: Weigh the required amount of soda, sulphide and water using a digital weighing balance and measuring cylinder respectively. Mix properly and pour into the flask containing the sample. Cover and tight properly the top with foil (to avoid the escape of water).

Step 5: Place the flask into the Autoclave and allow to cook for about one hour at a temperature of 121oC.

Fig 4.4: Autoclave.

  • Pulp Washing.

Because of the high amount of chemicals used in cooking wood (or any other fibre source) in chemical pulping process, the recovery of chemical is of crucial importance. The process where the chemicals are separated from the cooked pulp is called pulp washing. At FIIRO, this is carried out with direct contact with running tap water with the pulp contained in a deckle.

In large pulp and paper industry where chemical pulping process is done, a stage known as chemical recovery is done after “cooking process” and each chemical recovery process differ according to the kind of chemical pulp process used. A good recovery of chemicals is important for some reasons which include:

  • The chemicals been expensive to replace.
  • The dissolved chemicals interfere with the processing of the pulp.
  • The chemicals been detrimental to the environment.

There are many types of machinery used for pulp washing, one of them is the Pulp Disintegration machine

  • Pulp Screening.

Apart from fibres, the cooked pulp also contains partially uncooked fibre bundles (shives) and knots. Pulp screening is the process whereby the pulp is seperated from shives (small bundles of incompletely cooked wood fibres), knots (incompletely cooked chips), dirts and other debris. The knots and shives are normally sent back to the digester and recooked so that their fibres are not wasted. They might end up as spots in the finished paper. Knots and shives are removed by passing the pulp over pulp screens equipped with fine holes. In the case of smaller scale or researches, they are handpicked.

Bleaching.

Bleaching is a multi-stage process that refines and brightens raw pulp. The objective is to dissolve (chemical pulps) or modify(mechanical pulps) the brown-coloured lignin that was not completely removed during pulping, while maintaining the integrity of pulp fibres. Though there are many kinds of bleaching agent and processes, at FIIRO hydrogen peroxide is diluted in water and heated at a graduated temperature for a particular period of time.

For the bleaching of 70g of mid-ribs of banana leaves, 70ml of hydrogen-peroxide is mixed in 30ml of water. The sample is poured into a beaker and heated using a hot plate with continuous stirring. At the end of each stage the pulp is washed off and checked if brightened quality has been achieved before proceeding to the next.

Other bleaching agents used in pulp and paper industry include:

Chlorine, Chlorine dioxide, Sodium hypochlorite, Oxygen, Ozone, Sodium dithionite (NaS2O4) etc.

  • Pulp Beating/ Stock Preparation.

This is where the cellulose fibres pass through a refining process which is vital in the art of papering. The word “beating” in paper-making process is also referred to as refining. This stage refer to the passage of the fibre slurry between a rotor or counter-rotating disks. This exposes the fibre to repeated compression and shearing forces. These forces gradually delaminate the fibre cell wall, increasing flexibility of the fibre surfaces. Before refining the fibres are stiff, inflexible and form few bonds. This process results in fibre shortening, fibre straightening and also increases interfibre bonding which increases paper strength. The properties of paper are directly related to the refining process. This process is carried out with the aid of Valley beater or Hollander beater in large processing industries.

Fig 4.6: Valley Beater.

A stage known as Stock Preparation is carried out simultaneously. It involves the addition of chemical additives during refining for imparting suitable properties in paper. Some paper-making additives include:

  • Starch (increases strength,increases surface tint, etc).
  • Titanium dioxide (as pigment for brightening sheet).
  • Talc (as filler, makes papers smoother and opaque).
  • Clay (Kaolin).
  • Latex (as adhesive to bind paper additives).
  • Paper Making.

This involves the conversion of pulp into a sheet of paper using a mould and/or spreading over a mesh surface, water removed by suction and resulting pad of cellulose fibres pressed and dried to form paper. At FIIRO, the refined pulp is spread over a deckle and dried using the hybridization oven or  by natural air.

                           Paper Calendering.

This involves the feeding of the paper sheet through rollers to increase density and promote density.

4.3 PULPING (DELIGNINFICATION).

Pulp is a lignocellulosic fibrous material prepared by chemically or mechanically separating cellulose fibres from wood, fibre crops or waste paper. Pulping is a process by which fibres are separated from wood or cellulosic plant components and it can be done chemically, mechanically or the combination of the two methods i.e semi-chemical process. Wood and other plant materials used to make pulp contain three main components (apart from water), mamely:

  1. Ligninè a 3-dimensional polymer that binds the cellulose fibres together.
  2. Celluloseè (desired for papermaking), a polysaccharide with 600 to 1500 repeated sugar units.
  3. Hemicellulose èa shorter branched carbonhydrate polymers.

Others include extractives (fats, alcohols, aromatic acids, essential oils and pigments), minerals and other organics, with the relative proportions of these components vary according to the fibre source.

The aim of pulping is to breakdown the bulk structure of the fibre source be it chips, stems or other plant parts into the constituent fibres.

 

4.4METHODS OF PULPING.

  • MECHANICAL PULPING.

Mechanical pulps are produced by grinding wood against a stone or between the metal plates, thereby separating the wood into individual fibres. The shearing action breaks cellulose fibres, so that the resulting pulp is weaker than chemically separated pulps. The lignin connecting cellulose to hemi-cellulose is not dissolved, it merely softens, allowing the fibres to ground out of the wood matrix. The yield is very high, greater than 85%. Some mechanical pulping methods also use chemicals (i.e semi-chemical pulps); their yields are lower since they remove more of the non-cellulosic materials.

There are two major mechanical pulps

  1. Stone ground-wood pulp (SGW).
  2. Thermo-mechanical pulp (TMP).

In the ground-wood process, which is the oldest and most common mechanical pulp method, fibres are removed from short logs by pressing them against a rotating abrasive cylinder.

In the thermo-mechanical process, wood is chipped and then fed into steam heated refiners, where the chips are squeezed and converted to fibres between two steel discs.

  • CHEMICAL PULPING.

Chemical pulps are produced by chemically dissolving the lignen between the wood fibres, thereby enabling the fibres to separate relatively undamaged. In chemical pulping, chips and chemicals in aqueous solutions are cooked together in a pressure vessel known as the digester (at FIIRO  an improvised equipment called the AUTOCLAVE is used for to achieve same result). In large industries, this process is operated in batch or continuous basis. In batch digestion, the chips and digestion chemicals are fed together at elevated temperature and pressure. Once cooking is complete, pressure is released, blowing the deligninfied pulp out of the digester to a holding tank. The sequence is repeated. In continuous digestion, feeding of chips and chemicals is done at continuous rate.

There are three (3) main methods of chemical pulping:

  1. Soda Pulping è This is a chemical process for making wood pulp with Sodium hydroxide (NaOH) as the cooking chemical. The soda process gives pulp with lower tear strength than other chemical pulping process. This was the first chemical pulping process.
  2. Sulphate or Kraft Pulping èThe sulphate process (also known as Kraft pulping process) describes a technology for conversion of wood into pulp consisting of almost pure cellulose fibres. The kraft process entails the treatment of wood chips (or other plant fibre source) with mixture of water, Sodium hydroxide (NaOH) and Sodium Sulphide. It is the dominant method for producing paper.

At FIIRO, Soda and Kraft process are commonly used in researches.

  1. Sulphite Pulping èThis process produces pulp using various salts of sulphorous acid to extract the lignin from wood chips in large pressure called digesters. The salts include Sulphites (SO32-) or bisulphites (HSO3).

4.5 PAPER-MAKING PROCESS.

  • Wood Preparation.

This first step involves the debarking process of wood (removal of barks) either by passing it through drum debarker or being treated in a hydraulic debarker. After debarking, the logs are chipped by multi-knife chippers into suitable sized chips and then screened to remove over-large chips. The thickness of the chips is the most parameter as this determines the speed and the toughness of the impregnation of the cooking chemicals into the wood chip.

At FIIRO, the fibre source are mainly agro-waste, therefore debarking stage is not useful. The fibre source are chipped and then screened to remove or re-chip large ones.

In large Pulp and Paper industries, logs are debarked because bark contains little fibres, has a high extractive content, is dark and often carries large quantities of grit (small pieces of stone and sand).

  • Cooking (Digestion).

The “cooking process” is where the main part of deligninfication takes place. This is also known as the pulping process whereby chemical pulps can be produced either by soda, sulphate or kraft, or sulphite process. At FIIRO, Soda and Kraft process are commonly used. Cooking process is carried out with the aid of digester or an improvised equipment known as an Autoclave.

  1. SODA PULPING: Say to obtain a pulp sample from corn husk, steps is given below.

Step 1:  Weigh the chipped sample (say 120g) and pour into a conical flask.

Step 2: Take little amount of the sample and determine the moisture content using a moisture analyzer. Say 25% was obtained.

Step 3: Calculate the required amount of water and caustic soda (NaOH) needed for pulping. This is calculated below:

Sample to liquor ratio = 1:5

... for 120g = 120:600

Moisture content = 25% è 25/100 x 120 = 30

... Total amount of water needed = 600-30 = 570ml of H2O.

Using 5% concentration of caustic soda è 5/100 x 600 = 30

... For the cooking process of 120g of corn husk,  30g of caustic soda is diluted in 570ml of water.

Step 4: Weigh the required amount of soda and water using a digital weighing balance and measuring cylinder respectively. Mix properly and pour into the flask containing the sample. Cover and tight properly the top with foil (to avoid the escape of water).

Step 5: Place the flask into the Autoclave and allow to cook for about one hour at a temperature of 121oC.

  1. KRAFT PULPING:

Step 1: Weigh the chipped sample (say 120g) and pour into a conical flask

Step 2: : Take little amount of the sample and determine the moisture content using a moisture analyzer. Say 25% was obtained.

Step 3: Calculate the required amount of water and caustic soda (NaOH) and Sodium Sulphide (Na2S) needed for pulping. This is calculated below:

Sodium hydroxide to Sodium Sulfide ratio = 1:3

Sample to liquor ratio =1:5

... for 120g = 120:600

Liquor weight = 20% è 20/100 x 600 =120g

... Na2S = ¼ x 120 = 30g

NaOH = ¾ x 120 = 90g

Moisture content = 25% è 25/100 x 120 = 30

... Total amount of water needed = 600-30 = 570ml.

... For the cooking process of 120g of corn husk,  30g of caustic soda and 90g of sodium sulfide is diluted in 570ml of water.

Step 4: Weigh the required amount of soda, sulphide and water using a digital weighing balance and measuring cylinder respectively. Mix properly and pour into the flask containing the sample. Cover and tight properly the top with foil (to avoid the escape of water).

Step 5: Place the flask into the Autoclave and allow to cook for about one hour at a temperature of 121oC.

Fig 4.4: Autoclave.

  • Pulp Washing.

Because of the high amount of chemicals used in cooking wood (or any other fibre source) in chemical pulping process, the recovery of chemical is of crucial importance. The process where the chemicals are separated from the cooked pulp is called pulp washing. At FIIRO, this is carried out with direct contact with running tap water with the pulp contained in a deckle.

In large pulp and paper industry where chemical pulping process is done, a stage known as chemical recovery is done after “cooking process” and each chemical recovery process differ according to the kind of chemical pulp process used. A good recovery of chemicals is important for some reasons which include:

  • The chemicals been expensive to replace.
  • The dissolved chemicals interfere with the processing of the pulp.
  • The chemicals been detrimental to the environment.

There are many types of machinery used for pulp washing, one of them is the Pulp Disintegration machine

  • Pulp Screening.

Apart from fibres, the cooked pulp also contains partially uncooked fibre bundles (shives) and knots. Pulp screening is the process whereby the pulp is separated from shives (small bundles of incompletely cooked wood fibres), knots (incompletely cooked chips), dirts and other debris. The knots and shives are normally sent back to the digested and recooked so that their fibres are not wasted. They might end up as spots in the finished paper. Knots and shives are removed by passing the pulp over pulp screens equipped with fine holes. In the case of smaller scale or researches, they are handpicked.

Bleaching.

Bleaching is a multi-stage process that refines and brightens raw pulp. The objective is to dissolve (chemical pulps) or modify(mechanical pulps) the brown-colored lignin that was not completely removed during pulping, while maintaining the integrity of pulp fibres. Though there are many kinds of bleaching agent and processes, at FIIRO hydrogen peroxide is diluted in water and heated at a graduated temperature for a particular period of time.

For the bleaching of 70g of mid-ribs of banana leaves, 70ml of hydrogen-peroxide is mixed in 30ml of water. The sample is poured into a beaker and heated using a hot plate with continuous stirring. At the end of each stage the pulp is washed off and checked if brightened quality has been achieved before proceeding to the next.

Other bleaching agents used in pulp and paper industry include:

Chlorine, Chlorine dioxide, Sodium hypochlorite, Oxygen, Ozone, Sodium dithionite (NaS2O4) etc.

  • Pulp Beating/ Stock Preparation.

This is where the cellulose fibres pass through a refining process which is vital in the art of paper-making. The word “beating” in paper-making process is also referred to as refining. This stage refer to the passage of the fibre slurry between a rotor or counter-rotating disks. This exposes the fibre to repeated compression and shearing forces. These forces gradually delaminate the fibre cell wall, increasing flexibility of the fibre surfaces. Before refining the fibres are stiff, inflexible and form few bonds. This process results in fibre shortening, fibre straightening and also increases interfibre bonding which increases paper strength. The properties of paper are directly related to the refining process. This process is carried out with the aid of Valley beater or Hollander beater in large processing industries.

Fig 4.6: Valley Beater.

A stage known as Stock Preparation is carried out simultaneously. It involves the addition of chemical additives during refining for imparting suitable properties in paper. Some papermaking additives include:

  • Starch (increases strength,increases surface tint, etc).
  • Titanium dioxide (as pigment for brrightening sheet).
  • Talc (as filler, makes papers smoother and opaque).
  • Clay (Kaolin).
  • Latex (as adhesive to bind paper additives).
  • Paper Making.

This involves the conversion of pulp into a sheet of paper using a mould and/or spreading over a mesh surface, water removed by suction and resulting pad of cellulose fibres pressed and dried to form paper. At FIIRO, the refined pulp is spread over a deckle and dried using the hybridization oven or  by natural air.        

  • Paper Calendering.

This involves the feeding of the paper sheet through rollers to increase density and promote density.

 

 

 



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