NO145921B - PROCEDURE FOR THE PREPARATION OF DRY PAPER - Google Patents

Description

The present invention relates to a method for the production of dry paper pulp (hereinafter referred to as pulp) suitable for e.g. as absorbent fiber or for dry paper, cardboard or plate production.

In known pulp production, logs are ground into pulp with grindstones. Cooling water is used to reduce the temperature during the grinding process and the product is a diluted slurry of pulp in water. The process uses large amounts of water and produces large amounts of aqueous waste water that must be treated.

Another known method for shredding wood

to pulp is thermomechanical treatment. In this method, wood chips are first preheated and then introduced between two opposing grinding surfaces, usually rotating discs. The pre-treated wood chips are introduced at or near the center of the machine and must pass through the outlet's grinding zone. The heat generated by the grinding converts the water in the wood chips into steam.

Water is introduced during the thermomechanical treatment in such a large quantity that the moisture content of the tiles is kept approximately constant. The tile is thus kept in a high-pressure steam zone at 100°C or higher for a certain time.

The pulp produced by thermomechanical digestion usually has a moisture content approximately the same as the original timber. If the pulp is to be dried as such, it is usually sieved to remove twigs and treated to relax the pulp fibers and make them more easily felted together. The relaxation treatment consists of suspending the fibers in warm water. To carry out the screening satisfactorily, the mass is diluted to a ratio between water and dry fibers of at least 30:1 on a weight basis. The process as a whole therefore still uses

large amounts of water.

Norwegian patent application no. 760592 describes a method in which the wood material is defibrated and then dried with hot air in a flash dryer to a dry content of 80-95% by weight without the use of wet digestion or mechanical compression.

According to the present invention, a method is provided for the production of dry wood pulp by thermomechanically digesting wood chips and drying the pulp thus formed, the weight ratio between moisture and dried solid wood material being kept below 5:1 throughout the process, and this method is characterized by the mass is dried by bringing it into contact with a gas at a temperature of 300-600°C in or at the inlet of a high-turbulence mixer through which the mass and hot gases pass and which comprises relatively movable mechanical elements that exert a shearing effect on the mass for breaking up of fiber bundles in the pulp.

It is preferred that the ratio of water/dry fibers in the mass is kept below 3:1 and that the moisture content in the tile is not significantly increased at any stage. The water content of the dry fiber of a living tree is usually between 2:1 and 3:1. Wood delivered to the paper mill typically has a water/fiber ratio of between 1.5:1 and 2.5:1, and this moisture content is suitable for thermomechanical digestion.

The high-turbulence mixer used as a pulp dryer is described in US patent no. 3,069,784, but here it is only suggested to use the dryer with pulp produced by chemical digestion or by mechanical digestion at a very low consistency such as 93-95% moisture. It has been found that the use of this type of dryer has particular advantages when drying thermomechanical pulp which has not been diluted. If the pulp is to be used for paper production, the present method gives substantial increases in the paper's breaking strength and bursting strength without the need for a relaxation treatment of the pulp with hot water as described above. When the mass is to be used in absorbent products, the present method produces a mass that requires significantly less energy for mass in the form of a bale or sheet to be torn up into separate pulp fibers.'

It has been found that the present method produces a dry wood pulp which is particularly suitable as absorbent fibers for use in water-absorbing articles such as disposable diapers, sanitary pads, night pads, etc. The dry wood pulp produced according to the invention is also suitable for use in the dry production of paper , boards or cardboard.

The thermomechanical digestion process can take place in a single defibrating machine (which is usually a double disc refiner) or two or more machines can be used in succession. The wood chips are preheated before being fed into the double disc refiner, e.g. when treating the tile with steam. The amount of water introduced into the tile during this steam pre-treatment is small and usually less than 15%. The preferred method for thermomechanical wood pulping consists of preheating the chip with steam that maintains 105-135°C followed by a single defibration step in a double disc refiner at a temperature lower than during preheating, of between 100 and 110°C. The steam emitted in the double disc refiner can be used as part of the steam used to preheat the chip. If desired, a bleaching agent can be introduced into the double disc refiner to bleach the pulp during digestion.

The drying process is carried out in a highly turbulent mixture. The pulp, which usually maintains the same moisture content as at the outlet of the thermomechanical refiner and a gas, usually air, which maintains 300-600°C, is fed into the mixer and brought into contact in or at the entrance to the mixer. The mixer is designed in such a way that it exerts a shearing force on the mass. It can e.g. be equipped with hammers that work against a surface and/or spikes or teeth that engage each other and. rips up the mass. The shearing force tears up fiber bundles in the pulp. An example of a suitable high-turbulence mixing machine has the trade name "Atritor".

The highly turbulent mixture also causes a fairly even dispersion of the wood fibers in the air that flows through the mixture. The heated air preferably acts so quickly that its temperature drops to 100°C within 1 second of contact with the mass. This fiber suspension in air can, if desired, be used for immediate subsequent treatment, e.g. for depositing the fibers on a cloth.

When pulp produced according to the present invention is to be used for the production of paper or the plate material, the dry pulp fibers can be stored for later use, but it is preferred that the high-turbulence mixer used as dryer directly supplies the papermaking or platemaking process.

In the paper machine or plate machine, a gas stream, usually air, carries the fibers through a cloth where the fibers are deposited as a surface layer. Various dry-forming paper machines and cardboard machines are known. With known methods, the fibers are usually fed to these machines which tear up dried pulp such as e.g. can be dried pulp processed in a hammer mill to give it a suitable form for suspension in air flow. The air flow with the fibers is passed through a wire cloth that is permeable to the air, but on which the fibers are deposited as a layer.

The openings in the sieve that the air flow must pass through before it reaches the wire cloth are of such a size that it lets through separated wood fibers of a suitable size for incorporation into the manufactured paper or cardboard, but which do not allow the passage of agglomerated fiber bundles or twigs. The dry forming machine preferably has a stirring or scraping device which ensures that the sieve is not blocked by fiber bundles or twigs and preferably has a drain so that these waste products are removed from the machine without stopping the paper production. In a coordinated operation, twigs and the like that are separated at the outlet are recycled to the incoming thermomechanical uprooting.

The air stream containing the wood fibers after passing the sieve falls against the paper—the wire cloth. This can be a fine metal cloth or a textile cloth.

The flow of gas used to transport the fibers through the sieve to the paper wire can be supplied in a known manner, e.g. by means of a suction box under each dry forming machine. Alternatively, or in addition, the air flow from the dryer can carry the fibers completely through the forming cloth. However, it may be beneficial to separate the fibers from the moist air from the drying apparatus, e.g. in a cyclone.

The air flow with the pulp fibers from the dryer feeds one or more dry forming machines. Typically, a double disc refiner and high turbulence dryer can feed multiple dry forming machines. These can be arranged in a row, especially when producing cardboard.

When the fibers from one machine are deposited on the fiber layer from the previous machine, cardboard can be produced consisting of a number of fiber layers supplied by a double disc refiner and dryer.

After deposition on the forming cloth, the paper or cardboard undergoes various finishing treatments. Usually the web is treated with liquid binder, e.g. when showering. Examples of binders are starch and starch derivatives or polymer latex emulsions. Then the products are often pressed to fix the web and dry it and remove moisture introduced with the binder. The product can also be glued, coated, embossed and/or calendered.

Paper and cardboard made from wood pulp produced according to the present invention need not consist exclusively of thermomechanically produced fibres. If desired, one or more outer layers can consist of covering fibers such as e.g. chemical pulp, as these then cover one or more layers of wood pulp produced according to the present invention. For the production of cardboard, e.g. the first fiber layer that is carried on the forming cloth is chemical pulp fed into a dry forming machine. Several successive layers can then be placed on this outer layer from different dry forming machines which all receive input from the same thermomechanical refiner and dryer. The last layer to be applied can be a further cover layer. The combined layers can then be treated with binder and pressed as mentioned above.

The described method provides a method for the production of paper or cardboard which requires little or no water and which consequently emits very little polluted water. Such a facility does not need to be located near any major water supply. Furthermore, the method can be used economically on a much smaller scale than conventional wet digestion and wet papermaking. For example, the factory may be located near a medium-sized forest of e.g. 40,000 acres, while a normal paper mill for economic operation requires a forest at least 10 times as large.

When the mass produced by thermomechanical refining and high-turbulence drying is to be used as absorbent mass, it can be collected loosely for packing in bales or sheets.

According to a preferred method, however, the mass from the dryer is deposited on a gas g.j impermeable surface as a layer and takes the form of a continuous sheet or web of torn or absorbent mass.

The pulp comes from the dryer as a suspension of wood fibers in air. The mixture is brought to impinge on a permeable surface which may be a rotating drum or a flat belt. A suction device is usually used to apply a vacuum to the underside of the permeable surface so that the wood fibers are held firmly against the surface. In a preferred embodiment, the permeable surface is part of a vacuum drum. The pulp fibers are initially held firmly against the drum by suction, but as the layer of fibers builds up on the drum, the air flow through the cloth is reduced. The suction device advantageously covers only part of the drum and the layer of wood pulp fibers can be rubbed off the drum, e.g. down on a flat conveyor belt at a point where suction is not applied, e.g. approximately diametrically opposite the area where the fibers were first deposited on the drum.

The layer of wood fibers will preferably have a weight of 500-5000 g/m 2. The layer therefore requires pressing, e.g. with a pressure of approx. 3 50 kg/cm 2 to produce a durable continuous sheet that can be processed without falling apart. The pressing can take place in a plate press and in this case the fiber layer is preferably cut into separate sheets before being pressed or processed in press rollers where a continuous web or separate plates can be formed.

The dry wood mass that is formed by thermomechanical uprooting and high-turbulence drying according to the invention has not undergone relaxation treatment in water and easily forms a voluminous absorbent mass. The sheets or plates of pressed pulp are particularly easily broken down into wood fibers by shredding equipment such as e.g. a hammer mill. Furthermore, the pulp can be pressed to a relative density of 0.5 - 1 and could still be broken up easily, while pulp produced in known ways cannot be pressed to a relative density above 0.5 if they are to be broken up satisfactorily. This is a particular advantage if the pulp sheets are to be transported from the wood pulp factory to a factory that manufactures absorbent products.

In the following, the invention will be described, for example, with reference to the attached drawings, where Fig. 1 schematically shows an elevation view from the side and partially in section of apparatus suitable for the production of dry wood pulp according to the present invention and for the production of the pulp for cardboard, and Fig. 2 is a schematic elevation, partly in section, through apparatus suitable for the production of absorbent fibers using the method of the invention.

The apparatus shown in fig. 1 generally comprises a preheater 1, a thermomechanical defibrating machine 2, a dryer 3, a fiber distributing device 4, a dry forming machine 5 and a press plant 6.

Wood chips enter the preheater 1 through the inlet 11. The preheater has the form of a screw conveyor. The residence time for the wood chips in the pre-heater can be regulated by regulating the speed of the feed screw, and is preferably from 0.5-3 min. Steam is fed into the preheater 1 through the inlet 15.

Wood chips coming from the preheater 1 pass through a rotary valve 17 and a chamber 18 to a feeding machine 19 which is a screw conveyor. It feeds the wood chips into the thermomechanical defibrating machine 2.

The machine 2 is a double disc refiner which has two counter-rotating discs 31, 32 on shafts 33, 34 which rotate respectively about a common axis. The discs 31 and 32 have opposite grinding surfaces 35 and 36, separated by a narrow gap 41. Wood chips are fed by the feeder 19 into the chamber 37 behind the disc 31.

From this chamber 37, the wood chips pass through two inlet openings 38 and 39 in the disk 31 to the middle area 40 in the double disk refiner. To get out of the refiner 2, the chip must pass through the gap 41 from the middle area 40 to the edge 42. The wood chip is ground into pulp in the gap 41.

Water is supplied to the double disc refiner through the inlet 43 and into the chamber 44. The water goes to the middle area 40 of the double disc refiner through the openings 45 and 46 in the disc 32. The heat generated during the grinding causes some of the water in the wood chips and the water introduced through the openings 45 and 46 is converted into steam. The steam can pass through the openings 38 and 39, the chamber 37 and the feeding system 19 to the chamber 18. The chamber 18 is provided with an air tube 22 with a valve 23 so that when an excess of steam builds up in the chamber 18 the steam can escape from the apparatus . The amount of water introduced through the inlet 43 is regulated so that the moisture content in the wood chips is kept approximately constant. The temperature in the thermomechanical digestion machine 2 is thereby prevented from rising too high.

Wood pulp from the machine 2 falls onto a screw conveyor which leads the pulp to the flash dryer 3 through the pipe 47. Hot air with a temperature of e.g. 400-500°C is produced in an air heater 48 heated by a burner 49, and is fed to the dryer through the pipeline 50. The hot air comes into contact with the mass at the inlet funnel 51 to the dryer.

The dryer 3 is a high-turbulence mixer where hot air and wood pulp are mixed rapidly, agglomerated bundles of pulp fibers are broken apart and the dried wood fibers are carried away in a stream of hot air. The dryer 3 consists of a rotating disc 52, a fixed disc 53 and a suction fan 54. The rotating disc 53 is provided on one side with hammers 55 which strike against the dryer wall and on the other side has spikes 56. The spikes 56 engage between spikes 57 on the fixed disk 53. Wood pulp and air entering the dryer 3 through the inlet funnel 51 must pass the hammers 55 which break up the fiber bundles and then between the spikes 56 and 57 which distribute the fibers evenly in the air flow. The air and fibers then pass through the opening 58 in the middle of the stationary disc 53, the opening of which is regulated by the valve 59. From there, they are sucked by the fan 54 which is mounted on the same shaft 60 as the disc 52 by the valve 59 to the outlet 61 from the dryer 3.

The air flow which transports the pulp fibers goes from the dryer 3 through the pipeline 62 to the distributor 4. The air pipe 62 feeds six inlet pipes of the type 63a leading to six cyclone separators 64a - 64f, respectively. The inlet, e.g. 65a to each pipe 63 is regulated e.g. with a damper so that approximately equal amounts of fibers are supplied to each of the cyclone separators 64a - 64f. The cyclone separators 64 are a conical rotary separator which discharges moist air through the outlet at the top, 66a, and fibers through the outlet at the bottom, 67a. The cyclone separators 64a - 64f are mounted above a dry forming cardboard machine 5. The cyclone separators 64a - 64f feed pulp fibers through the outlet in the bottom to the fiber distributors 68a - 68f, respectively.

The fiber distributors 68a - 68f may be as described in British patent 1,207,556. Thus, each distributor, as shown in case 68a, consists of a housing 69, the underside of which is a flat screening cloth 70 with openings large enough for fibers suitable for the manufacture of cardboard to pass unhindered, but sufficiently fine to prevent agglomerated fiber bundles and twigs go through. Two or more stirrers 71 rotate over the cloth 70 in planetary motion.

The fibers enter the distributor 68a through the inlet 72 on one side of the housing 69 and the stirrer 71 distributes the fibers evenly over the screen 70. Twigs and lumps of fiber that do not pass through the cloth 70 are led over the cloth towards an outlet 73 on the opposite side of the house in relation to the inlet 72. From the outlet 73, the twig passes through the pipe 74 to the conveyor belt 75 where the material comes together with material returned from the distributors 68b - 68f and is fed back to the inlet 11 in the preheater 1. The feed rate of fibers to the distributors 68a - 68f, compared with the flow rate for fibers through the cloth 70, is chosen so that approximately 1/3 of the fiber weight is returned through the outlet 73.

The fiber distributors 68a - 68f are mounted above the dry-forming cardboard machine 5. The same applies to two further distributors 79 and 80 of the same construction as the distributors 68a - 68f. Distributors 79 and 80 receive fibers from cyclone separators 81 and 82 respectively. The inlet pipes 83 and 84 for the cyclone separators 81 and 82 are not supplied from the air pipeline 62 but from another pipe 85. The cyclone receives air suspension of wood fibers from a separate supply of tire pulp (not shown) made from shredded chemical pulp. The covering compound is used for the production of cardboard with a white surface and the distributor 79 and/or 80 does not need to be used if you want a white surface or no white surface.

The dry forming machine 5 consists of an endless belt 86 of air-permeable textile which runs under the fiber distributors 79, 68a - 68f and 80 and around the rollers 87 and 90. The belt forms the paper forming surface on which the fibers are laid down. The first fibers to be placed come from the distributor 79. A suction box 91 is mounted behind the belt 86 below the distributor 79 and ensures a constant air flow through the belt 86. The fibers from the distributor 79 are laid down as a tangled layer on the belt 86. The fibers from the distributor 68a are placed on in a similar way on top of the layer just formed. Air from the distributor 68a passes through the first layer of fibers from the distributor 79 and through the belt 86. The fibers from the distributor 68a are infiltrated both with each other and with the fibers laid down from the distributor 79. In a similar way, several layers of fibers are applied from the distributors 68b - 68f and 80. Suction boxes 92a - 92f and 93 are placed below the distributors 68a - 68f and 80, respectively.

After all fiber layers have been deposited on the belt 86, they go under a shower head 94 which stands above a suction box 95 and which applies binder such as e.g. watery starch on the fibers.

At the roller 87, the formed layer of cardboard 96, which contains a binder but is not fixed or pressed, is pulled off from the belt 86. The cardboard web 96 mainly consists of fibers from the distributors 68a - 68f, i.e. fibers that have been refined or wound up and dried according to to the invention.

The cardboard has a white covering layer of chemical pulp added to the distributors 79 and 80.

The cardboard web 96 goes to the press plant 6 where the web first goes around a heating drum 97. This dries the cardboard web to a certain extent and pressing rollers 98 and 9 9 which are against the drum 97 press the cardboard. From the drum 97, the cardboard web goes to the glue press 100, where glue is applied to the web 95 in the nip between two rollers. The cardboard web 95 goes to a final dryer 104 which comprises six heating drums and a calender roll 105 before it is rolled up onto the sleeve 106.

The apparatus shown in fig. 2 generally comprises a preheater 201, a thermomechanical refiner 202, a dryer 203 and sheet forming drum 204. The preheater 201, the thermomechanical refiner 202 and the dryer 203 are the same as the preheater 1, the machine 2 and the dryer 3 shown in fig. 1 and described above; the parts with the reference numbers 211 - 261 in fig. 2 are the same as the parts with respective reference numbers 11-61 in fig. 1.

The air flow which transports the separate wood fibers goes from the outlet 261 of the drum 203 through the pipeline 262 to the sheet forming drum 204. The pipeline 262 has a converging part 263 with a fishtail shape. For example, the cross-section can be reduced from 20 x 20 cm in the tube 262 to 5 x 60 cm at the narrowest point 264. From 264, air and fibers enter the inlet box 265 to the sheet forming drum 204.

The sheet forming drum 204 consists of a drum 266 which rotates in the direction shown and has a surface in the form of an air-permeable membrane 267. Inside the drum 266 there is a suction box 268 which is connected through the pipe 269 to the suction fan 270. The suction box 266 is behind it part of the permeable surface 267 which at all times passes the inlet box 265. The inlet box 265 is closed against the drum 266

in the edges using flexible closing rollers 271 and 272.

A belt-skirt conveyor 273 passes over the rollers 274, 275, 276 and comes into contact with the drum 266 over a part of it which lies approximately diametrically in relation to the inlet box 265 and the suction box 268. Wood fibers that enter the inlet box 265 are deposited on the porous surface 267 above the part behind which the suction box 268 lies. The fiber layer on the permeable surface 267 gradually thickens as it moves from the closing roller 271 to the closing roller 272. The layer of deposited fibers 272 is then transported from the porous surface 267 to the conveyor belt 273 where no suction is applied to this surface in this area.

The conveyor belt 273 transports the fiber layer 272 to pressure rollers 278, 279, which press sufficiently to attach the fibers to a continuous sheet or web. The web is transported away from the pressure rollers on a conveyor belt 280 and collected at 281.

Claims (2)

1. Process for the production of dry wood pulp by thermomechanically digesting wood chips, and drying the pulp thus formed, the weight ratio between moisture and dried solid wood material being kept below 5:1 throughout the process, characterized in that the pulp is dried by bringing it into contact with a gas at a temperature of 300-600°C in or at the inlet of a high-turbulence mixer (3) through which the pulp and hot gases pass and which comprises relatively movable mechanical elements that exert a shearing effect on the pulp to break up fiber bundles in the pulp . 2. Method as stated in claim 1, characterized in that the water/dry fiber ratio is kept below 3:1. f