CA1210979A - Process of producing fiberboard according to the wet method - Google Patents

Abstract

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 de-fibration step is used for impregnation of the chips prior to the defibration, whereby said substance is bound in the pulp.

Description

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This invention relates to a process for producing fiberboard according to the wet method in a closed backwater system, in which process chips of ligno-cellulose-containing material are preheated in an atmosphere of saturated steam at increased temperature and underincreased 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.
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 ;nto pulp at increased temperature and under increased pressure in an atmosphere of steam. `~
The temperature can be kept between 125 and 200C
depending on the type of starting material, pulp qual-ity 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 7~ -during the defibration is in mos-t cases kept in the range between 150 and 170C 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 K~lh 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 170C and coniferous wood is used as the star-ting material, about 8 percent of the dry substance of the wood is 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 thorougly 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 conse-quently in the fiberboard. In order to obtain a low content of soluble substance in the backwater9 it has been usual hitherto to draw off a portion of the back-water and -to replace it with fresh wa~er. 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 3 the drawback of making the process and therewi-th the final product more expensive.
The present invention provides a process to reduce the content of released organic sub-stance in the backwater in the production of pulp for fiberboard and this object is attained by the process being characteri~ed in that the pulp ob-tained hy the defibration is dewatered and that the 3~ q~

resultant aqueous solu-tio~ of organi~ sub-tance released in the defibration s-tep ;s returned into -tne process by impregnation of the chips prior to the deFibration of the same.
According to the inven-tion it has been established that when the entering c~ip~ are impregnated with an aqueous solution ofsubstance released during the de-fibration and thereafter deFibrated at 170C~ -the dissolved substance is bound practically completely in the pu1p and cannot be remove~ 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 substan-tially of polysaccarides, which can undergo bothpolymerisation and condensation processes and be con-verted to substances difficult to dissolve in water.
It is also possible that the polysaccarides are ab-sorbed 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 impreg-- nation 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 pro-cess 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 100C and thereupon com-pressed and relieved from excess water and in com-pressed state transferred into the impregnation sol-ution, within which the chips then expand and absorb solution. This is in some cases simultaneously sub-jected to pressure in order to ~acilitate the ab-sorption of liquid.

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~low much of released material must be rendered harmless, depends upon, how much of released sub-stance can be allowed to remain in the fiberboard and determined by the concent:ra-tion of dissolved substance in the backwater. As long as the con-centration in the backwater can be allowed to be as high as 5.~ percent, the problem often can be solved by directly dewatering the final pulp to a dry con-tent 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 quan~ity which the chips are capable to ~bsorb.
If one is compelled to reduce the backwater con-centration 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 dis-solved substance to said value, it may be necessary to add so much ~ater during the dewatering of the pulp that the quantity of squeezed-out solution ex-ceeds that which the chips are capable o~ absorbing during the impregnation, and in such a case the sol-ution must be reduced by evaporation to a quantity suitable for the impregnation.
In some cases, it may de desirable also to render harmless contaminated water coming from other parts of the system, e.g. wood water squee~ed 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 dewater;ng thereof for removal o~ re-leased substance. In such cases so much squeezed-out solution is obtained usually as to render necess-ary evaporation to that volume which the chips are capab-le o~ absorbing.

5 ~ S~979 Evapora-tion of the squeezed-ouk solution can be effected in any known method of evaporation, but should suitably be done at reduced pressure in order t~ avoid trouble due to incrustation, or by evaporation of heated aqueous solution with air, the water then es caping 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 con-veyor 10 to a steaming vessel 12 wherein they are mois-tened 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 con-taining released substance of polysaccarides etc., which aqueous solution has been obkained by mechanical s eg ~ ~
A dewatering of the pulp in a ~r 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 3D 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 in-soluble state, is blown through a blowduct 40 to the cyclone 16. The pulp is fed from the cyclone 16 by means of screw conveyor ~6 to receptacle 48, which is , 6 ~ 7 ~

equipped with a stirring device. A pipe 42 ~rom 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 resultant aqueous solution 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 ex-changer 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 imprègnation vessel 24 ~or 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 ma-chine 84, from which outflowing backwater via pipe 86 is collected in the tank 74. The wet sheet is con-veyed to hot press 88 wherein it is dewatered mech-3U anically and finally dried under heat and pressure inusual manner to end ~roduct.

The present invention, will be further described by way of the following Example.

7 ~ 7 Example The incoming chips of coniferous wood, which are assumed to amount per hour to 8 000 kgs T.S. + 8 000 kgs of wa-ter, are fed into the steaming vessel 12, and simultaneously 1 925 kgs of steam are supplied through the pipe 14, whereupon the chips holding 100 percent of moisture are conducted down into the screw press 18 where they are dewatered to a T.S. of 55 percent, 3 380 kgs of H20 of 100C flowing down into the vessel 22 and the compressed plug of chips being forced into the impregna-tion vessel 24 and allowed to expand there and suck up 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 170C and is supplied with steam through the pipe 32.
The chips heated to 170C with their content of water and organic substance absorbed by the chips are con-veyed 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 in-soluble state, is blown through the duct 40 into thecyclone 16.
Pulp from the cyclone is fed by the screw 46 into the receptacle 48, into which at the same time 3 380 kgs 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 pressure screw press 52 and dewatered to 55 percent dry substance. Here-under,10 400 kgs of water with 400 kgs of soluble sub-stance from a total 640 kgs of substance released fromthe chips flows down into the vessel 56. 245 kgs of dissolved substance remain in the pulp. This ~uantity of dissolved substance is caused to remain the fiber-board in order that all released material shall be 8 lz~ g rendered harmless. Since the fiberboard uni-ts in the hot pressing operation after mechanical squeezing out of the backwater to a dry content of 60 percent hold about 5 200 kgs of water to be evaporated and re-moved, the backwater may be permitted to hold 245 kgsof dissolved substance per 5 200 kgs of water or 4.5 percent. As the solution flowing down into the vessel 56 comprises 10 400 kgs of water + 400 kgs of released substance, this is more -than the chips normally are capable to absorb, a quantity which usually does not exceed 8 000 kgs of water per 8 ooo kgs of chips T.S.
Therefore, 2 400 kgs of water must be evaporated in the tower 64, which is effected by prehea-ting of the solution. The preheating is done with steam coming from the cyclone 16 through the pipe 14a. In the tower 64,2 400 kgs 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 kgs of water and 400 kgs soluble substance, is pumped into the impregnation vessel 24 through the pipe 26.
The whole quantity of released wood substance can he rended harmless in such a way that 400 kgs are bound within the pulp during the defibration and 245 kgs are bound in the fiberborad. 5 200 kgs 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 suffi-ciently low level to avoid any discoloration of the fiberboard units in the hot pressing operation~
Furthermore, all squeezed-out wood water had been rended harmless.

Claims (4)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A process for producing fiberboard from ligno-cellolose-fiber-containing chips according to the wet process in a closed backwater system having reduced concentration of organic substances in the backwater, which system includes in sequence the steps of preheating the chips in a steam environ-ment, defibrating the preheated chips in a vapor phase of saturated steam, suspending the resultant fiber slurry in backwater from a subsequent fiberboard-forming station, forming wet sheets from the resultant pulp suspension, re-moving moisture from said wet sheets for use as backwater in forming said pulp solution and drying the wet sheets, said process being characterized by removing aqueous solution of organic substances resulting from the defibrating step from said fiber slurry before it is passed to the pulp suspension step, and using said removed solution of organic substances to impregnate said chips before being passed to said pre-heating step.

2. The process according to claim 1, including the additional steps of presteaming said chips and then com-pressing them with consequent removal of chemical-containing moisture before being subjected to said impregnating step.

3. The process according to claim 2, including the step of passing said chemical-containing moisture to said fiber slurry to form part of the removed impregnating solution.

4. The process according to claim 1, in which fresh water is added to the defibrated pulp slurry in an amount sufficient to impart to the removed solution of organic substances a predetermined concentration.