EP0103577B1

EP0103577B1 - Process of producing fiberboard according to the wet method

Info

Publication number: EP0103577B1
Application number: EP82903275A
Authority: EP
Inventors: Ove Danielsson
Original assignee: Sunds Defibrator AB
Current assignee: Valmet AB
Priority date: 1981-10-28
Filing date: 1982-10-28
Publication date: 1985-09-18
Also published as: FI71804B; FI834308L; NO154316B; SE8106369L; FI834308A0; FI71804C; SE428813B; DE3266424D1; JPS58501825A; WO1983001637A1; NO832354L; EP0103577A1

Abstract

A Process of producing fiberboard according to the wet method in a closed backwater system. Chips of lignocellulose-containing material after defibration to pulp are suspended in backwater so as to obtain a pulp suspension for forming of wet sheets, which are liberated from water by compression under supply of heat, the squeezed-out water being returned to the suspension step. The pulp obtained in the defibration is dewatered mechanically and the obtained aqueous solution of organic substance released during the defibration step is used for impregnation of the chips prior to the defibration, whereby said substance is bound in the pulp.

Description

This invention relates to a process for producing fiberboard according to the wet method in a closed backwater system, in which process chips of lignocellulose-containing material are preheated in an atmosphere of saturated steam at increased temperature and under increased pressure and thereupon defibrated under atmospheric or increased pressure, the defibrated material is suspended in backwater so as to obtain a pulp suspension for formation of wet sheets which are liberated from water by compression under supply of heat, the water squeezed out being returned to the suspension step (US-A-4 012 279).
In the manufacture of fiberboard according to the aforesaid art the lignocellulose-containing material, which may consist of any kind of annual or perennial plants, such as coniferous or hardwood, straw or bagasse, etc., is usually prior to the defibration step disintegrated to suitable piece size such as chips or chaff. These pieces, which for the sake of simplicity hereinafter generally shall be referred to as "chips", are disintegrated and treated mechanically, defibrated into pulp at increased temperature and under increased pressure in an atmosphere of steam. The temperature can be kept between 125 and 200°C depending on the type of starting material, pulp quality and supplied grinding energy. The defibration is usually performed in disc grinders of various types, such as machines equipped with grinding members for single or double rotation. The temperature during the defibration is in most cases kept in the range between 150 and 170°C and corresponding steam pressure. Depending on the incoming fiber material, the grinding energy may vary between 100 and 300 KWh per ton, but is usually about 200 KWh per ton, of bone dry material, when wood chips are defibrated and ground finally in one step.
If the defibration is performed at a temperature of 170°C and coniferous wood is used as the starting material, about 8 percent of the dry substance of the wood are dissolved by hydrolysis as polysaccarides of varying composition in the water contained in the chips and escapes partly into the backwater and partly into the finished fiberboard. This results in that in a thoroughly closed backwater system the fiberboard units when subjected to hot pressing become dark-coloured and even can get dark spots, and also show a tendency to stick to the press plates.
In order to avoid the problems set forth above, it is necessary that the quantity of released organic substance is low in the backwater system and consequently in the fiberboard. In order to obtain a low content of soluble substance in the backwater, it has been usual hitherto to draw off a portion of the backwater and to replace it with fresh water. Drawn-off backwater must be expected to be rendered harmless which may be done e.g by evaporation in combination with combustion of the black liquor or by various biological treatments. The known methods have all the drawback of making the process and therewith the final product more expensive.
The main object of the invention is to provide a novel process to reduce the content of released organic substance in the backwater in the production of pulp for fiberboard and this object is attained by the process being characterized therein that the pulp obtained by the defibration is dewatered and that the aqueous solution obtained thereby of organic substance released in the defibration step is returned into the process by impregnation of the chips prior to the defibration of the same.
According to the invention it has been established that if the entering chips are impregnated with an aqueous solution of substance released during the defibration and thereafter defibrated at 170°C, the dissolved substance in bound practically completely in the pulp and cannot be removed by washing with water. The conversion of the dissolved substance to insoluble state is effected probably by cooperation of several processes of chemical as well as physical nature. The released substance consists substantially of polysaccarides, which can undergo both polymerisation and condensation processes and converted to substances difficult to dissolve in water. It is also possible that the polysaccarides are absorbed so firmly into and onto the fiber walls that they cannot be dissolved out by means of water. Since it is well known that it is possible in the impregnation of chips with water to cause one ton of chips, calculated as bone dry, of coniferous wood having a dry content of 50 percent to absorb one cubic meter of water, it is possible in a thoroughly closed process water system to return a substantial portion of the released substance to the chips and, according to the invention, be rendered harmless in the defibration process. The impregnation is hereunder carried out in such a manner that the chips initially are softened by treatment with steam at 100°C and thereupon compressed and relieved from excess water and in compressed state transferred into the impregnation solution, within which the chips then expand and absorb solution. This is in some cases simultaneously subjected to pressure in order to facilitate the absorption of liquid.
How much of released material must be rendered harmless, depends thereon, how much of released substance can be allowed to remain in the fiberboard and determined by the concentration of dissolved substance in the backwater. As long as the concentration in the backwater can be allowed to be as high as 5.0 percent, the problem often can be solved by directly dewatering the final pulp to a dry content of 50 to 60 percent in a mechanical way and using squeezed-out aqueous solution for impregnation, provided that the solution can be limited totally to the quantity which the chips are capable to absorb.
If one is compelled to reduce the backwater concentration to 3.0 percent or lower, additional water must usually be added and the dewatering performed in one or several steps with intermediate addition of water. In order to reduce the content of dissolved substance to said value, it may be necessary to add so much water during the dewatering of the pulp that the quantity of squeezed-out solution exceeds that which the chips are capable to absorb during the impregnation, and in such a case the solution must be reduced by evaporation to a quantity suitable for the impregnation.
In some cases, it may be desirable also to render harmless contaminated water coming from other parts of the system, e.g. wood water squeezed out after steaming of the chips, and this may suitably be done by using the contaminated water for diluting the finally defibrated fibrous material prior to the mechanical dewatering thereof for removal of released substance. In such cases so much squeezed-out solution is obtained usually as to render necessary evaporation to that volume which the chips are capable to absorb.
Evaporation of the squeezed-out solution can be effected in any known method of evaporation, but should suitably be done at reduced pressure in order to avoid trouble due to incrustation, or by evaporation of heated aqueous solution with air, the water then escaping as an air-steam mixture.
The invention will hereinafter be described nearer by a preferred embodiment of a plant for carrying out the process with references to the attached drawing which diagrammatically shows a flow sheet for the plant.
The entering chips are conveyed by means of a conveyor 10 to a steaming vessel 12 wherein they are moistened with steam which is drawn off through pipe 14 from a cyclone 16 belonging to a later part of the plant. The moist chips are discharged into a screw press 18 where water is squeezed out mechanically and drained off through pipe 20 to a vessel 22 situated below. The plug of chips compressed in the screw press 18 is forced into an impregnation vessel 24 where it is allowed to expand and absorb aqueous solution containing released substance of polysaccarides etc., which aqueous solution has been obtained by mechanical dewatering of the pulp in a later step in the plant and been conveyed to the impregnation vessel 24 via pipe 26. Through a feed screw 28 the chips are fed into preheater 30 which is subjected to steam pressure and to which steam is fed from a pipe 32, from which pipe a branch pipe 32a opens into the steaming vessel 12. The chips heated by means of steam are conveyed through screw feeder 34 into a defibrator 36 with motor 38 and are ground to pulp. The finished pulp, in which the organic substance supplied during the impregnation in the vessel 24 was changed into insoluble state, is blown through a blowduct 40 to the cyclone 16. The pulp is fed from the cyclone 16 by means of screw conveyor 46 to receptacle 48, which is equipped with a stirring device. A pipe 42 from the vessel 22 leads to the receptacle 48, through which pipe drained-off wood water from the vessel 22 is supplied to the receptacle by means of pump 44. The pulp is fed from the receptacle 48 via chute 50 down into high pressure screw press 52, wherein the pulp is dewatered mechanically. The aqueous solution obtained thereby of organic substance released in the defibration step is drained off from the screw press 52 via pipe 54 to vessel 56, from which it is pumped by means of pump 58 via pipe 60 through a heat exchanger 62 to a cooling tower 64, where part of the water is evaporated. The heating in the preheater 62 is performed with steam which is drawn off from the cyclone 16 via pipe 14a and which leaves the preheater via conduit 66. Part of the water thus is evaporated in the tower 64 as a steam-air mixture, the air being supplied by means of fan 68. The evaporated solution is pumped by means of a pump 70 through the pipe 26 into the impregnation vessel 24 for impregnation of a new charge of fresh chips, as described earlier.
From the high pressure screw press 52 the pulp flows down into pulp chest 72, into which at the same time backwater is fed from a tank 74 by means of pump 76 and pipe 78. The final pulp suspension is pumped by means of pump 80 via conduit 82 to a forming machine 84, from which outflowing backwater via pipe 86 is collected in the tank 74. The wet sheet is conveyed to hot press 88 wherein it is dewatered mechanically and finally dried under heat and pressure in usual manner to end product.
In order further to illustrate the invention, the same will be described in the following by an embodiment with statements about the various conditions which may exist in the process. It shall, however, be clearly understood that this is an example only of the invention and that the same is not limited to the presented values.

Example

The incoming chips of coniferous wood, which are assumed to amount per hour to 8 000 kg dry substance +8 000 kg of water, are fed into the steaming vessel 12, and simultaneously 1 295 kg of steam are supplied through the pipe 14, whereupon the chips holding 100 percent of moisture, calculated on dry chips, are conveyed down into the screw press 18, where they are dewatered to a dry content of 55 percent, calculated on moist chips. 3 380 kg of H₂0 of 100°C flow down into the vessel 22 and the compressed plug of chips is forced into the impregnation vessel 24 and allowed to expand there and absorb aqueous solution with released substance of polysaccarides to be fed via the screw 28 into the preheater 30, which is subjected to steam pressure at 170°C and is supplied with steam through the pipe 32. The chips heated to 170°C with their content of water and organic substance absorbed by the chips are conveyed to the defibrator 36 via the screw feeder 34. The finished pulp, in which the organic substance added in the impregnation step has been converted to insoluble state, is blown through the duct 40 into the cyclone 16.
Pulp from the cyclone is fed by the screw 46 into the receptacle 48, into which at the same time 3 380 kg of wood water from the vessel 22 are conducted by means of the pump 44 and the pipe 42. From the receptacle 48 equipped with a device 'for thorough stirring the pulp flows down into the high pressre screw press 52 and is dewatered to 55 percent dry substance. Hereunder, 10 400 kg of water with 400 kg of soluble substance from a total 640 kg of substance released from the chips flow down into the vessel 56. 245 kg of dissolved substance remain in the pulp. This quantity of dissolved substance is caused to remain in the fiberboard in order that all released material shall be rendered harmless. Since the fiberboard units in the hot pressing operation after mechanical squeezing out of the backwater to a dry content of 60 percent hold about 5 200 kg of water to be evaporated and removed, the backwater may be permitted to hold 245 kg of dissolved substance per 5 200 kg of water or 4.5 percent. As the solution flowing down into the vessel 56 comprises 10 400 kg of water +400 kg of released substance, this is more than the chips normally are capable to absorb, a quantity which usually does not exceed 8 000 kg of water per 8 000 kg of chips dry substance. Therefore, 2 400 kg of water must be evaporated in the tower 64, which is effected by preheating of the solution. The preheating is done with steam coming from the cyclone 16 through the pipe 14a. In the tower 64, 2 400 kg of water are evaporated as a steam-air mixture. The air is supplied by means of fan 68. The solution remaining after the evaporation, 8 000 kg of water and 400 kg soluble substance, is pumped into the impregnation vessel 24 through the pipe 26.
The whole quantity of released wood substance can be rendered harmless in such a way that 400 kg are bound within the pulp during the defibration and 245 kg are bound in the fiberboard. 5 200 kg of water evaporated in the hot press are replaced by supply of water to the backwater cycle. Any discharge of backwater need not be done, and the content of 4.5 percent of dissolved organic substance in the backwater is kept at a sufficiently low level to avoid any discoloration of the fiberboard units in the hot pressing operation. Furthermore, all squeezed-out wood water has been rendered harmless.

Claims

1. A process of producing fiberboard according to the wet method in a closed backwater system, in which process chips of lignocellulose-containing material are preheated in an atmosphere of saturated steam at increased temperature and under increased pressure and thereupon defibrated under atmospheric or increased pressure, the defibrated material is suspended in backwater so as to obtain a pulp suspension for formation of wet sheets, which are liberated from water by compression under supply of heat, the water squeezed out being returned to the suspension step, characterized in that the pulp obtained in the defibration step is dewatered mechanically and that the aqueous solution of organic substance obtained in the defibration step is used for impregnation of the chips prior to the preheating step.

2. A process as claimed in claim 1, characterized in that the impregnation of the chips is effected thereby that the chips are steamed and compressed, dewatered mechanically and thereafter under compression are fed into an impregnation solution containing organic substance released in the defibration step and allowed to expand in said solution and thereunder to absorb solution.

3. A process as claimed in any of the claims 1 and 2, characterized in that to the defibrated pulp is added so much water that the pulp upon dewatering contains a quantity of released organic substance suitable for the continued process.

4. A process as claimed in claim 3, characterized in that the water leaving during the compression of the chips is supplied to the backwater cycle.

5. A process as claimed in any one of the claims 1 to 4, characterized in that the solution squeezed out for impregnation of the chips is brought by evaporation to contain a quantity of water adequate for the impregnation.