Note: Descriptions are shown in the official language in which they were submitted.
~v~
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.
(39'7~
~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.
