Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
1157305
mis invention relates to a method of improving
the heat economy in a process of making groundwood, according
to which method wood is ground with a rotating grinding
member in a pressurized grinding space, warm shower water
is sprayed into the grinding space, the groundwood pulp
that is obtained is thickened by removing water from the
pulp, and the thickened pulp is stored and~or bleached after
cooling.
It is known to grind under atmospheric pressure
blocks of wood which have been debarked and sawn to a certain
length. The blocks arebrought in batches on conveyors above
a grinding pocket, from where they are dropped into the grind-
ing pocket when the previous batch has been ground.
The wood is defibrated by pressing the batch of
wood against the grinding stone by means of a hydraulic
piston. The friction between wood and the grinding stone
warms the wood, and the lignin which binds the fibers is
softened, whereby the fibers are separated. The water
sprayed onto the grinding stone contributes to the defibration
and transfers away from the grinding zone the frictional
energy generated by the rotation of the grinding stone and
the wood being ground. The shower waters also flush the
separated fibers into the pulp chest positioned under the
grinding stone. Tests have proved that the temperature of
the shower water affects the defibration, when high temper-
atures of shower water are used, the separated fibers are
more uniform and longer. Due to the long fibers, the
strength characteristics of paper made of such fibers are
better than that of paper made of fibers produced using low
temperature shower water. Tests have proved that in normal
grinding the effect of warm shower water is relatively weak.
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Also pressurized grinding in a grinding space at
a pressure exceeding atmospheric pressure is known (U. S.
Patents Nos. 3,808,090, 3,948,449, 4,207,139, 4,207,703 and
4,274,600, and 9ritish Patent ~o. 1,519,848).
Wood is fed into the pressurized grinding space
for instance by means of gate operated pressure equalizing
chambers built above the grinding pockets of the grinder.
me grinding space, defined by the gates and the pulp chest,
is pressurized preferably by means of air or steam.
The defibratioh takes place as in normal grinding
by pressing wood blocks against the grinding stone by means
of a hydraulic piston. Vibration caused by the grinding
stone, heat caused by the friction, and the shower water
separate the fibers from the wood material.
It has been observed that in pressurized grinding
the temperature of shower water has greater influence on the
defibration than in grinding at atmospheric pressure, The
warmer the shower waters are, the longer and more uniform
are the fibers separated from the wood material, and the
20 stronger is the paper made of such fibers. Thus the warmer
the shower waters are when returned to the grinder, the
better for the pressurized grinding.
After defibration the pulp suspension flows out
from the grinding space through a pipe in which sticks and
bigger slabs of wood are made finer by a stick crusher
before adjusting the flow rate. The temperature of the
pulp discharged from the grinding space is normally more
than 100C. Under practical circumstances the pulp temper-
ature may rise up to 145C, which depends on the temperature
30 of the shower water and on the pressure of the grinding space.
The temperature of the shower water when sprayed into the
"~p
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11S7305
grinding space must be 130-135C and the pressure in the
grinding space must be 3 bar. Heat energy contained in the
pulp suspension is released in the form of steam in a cyclone
in which the pressure is reduced to atmospheric pressure.
From the cyclone pulp can flow directly into a thickener
where the hot shower water is separated from the pulp and
returned to the grinding space. From the cyclone the pulp
can also be discharged into a tank from which it can be
pumped to different kinds of screening steps, e.g. to a
pressuxized screen or a hydro cleaner, before it flows into
a thickener where the hot shower water is separated from
the pulp. Also the screen rejects can be processed in the
usual manner by thickening, refining and screening before
they are passed to the thickener for separating the shower
water from the pulp. From this thickener the pulp is dis-
charged with a consistency of 10-33%.
A disadvantage of this known method is that to-
gether with the discharged pulp the process loses 2-lOyo
of the warm shower water. The temperature of the replacement
water is often considerably lower, generally 50-60C. This
replacement shower water having a low temperature decreases
by 2-SC the total temperature of the shower water which is
fed into the grinding space.
In addition, the pulp chest must often have large
dimensions, whereby there is a large cooling surface and
with resulting large loss of heat. The temperature of
the pulp leaving the thickener is also too high, 90-100C
At this temperature the pulp is likely to get dark during
storage, and is difficult to bleach for instance with
peroxide.
115730S
The object of this invention is to provide a
method eliminating the above-mentioned disadvantages and
improving the heat economy in a process of making groundwood
under pressure, and to make the groundwood pulp discharged
from the thickener more suitable for storage and bleaching.
In accordance with the invention there is provided
a method of improving the heat economy in a process of making
groundwood. The method requires replacement water for water
contained in a groundwood pulp produced by the method.
According to the method, wood is ground with a rotating
grinding member in a pressurized grinding space while shower
water is sprayed into the grinding space. The groundwood
pulp is fed to a thickener in which (a) water at lower
temperature than the feed pulp is sprayed onto the feed
pulp and (b) water is removed from the groundwood pulp
which has been sprayed with the lower temperature water.
Thus, the groundwood pulp is cooled and the lower tempera-
ture water is heated. The cooled groundwood pulp is then
subjPcted to further treatment. Substantially only the water
removed from the groundwood pulp is utilized as the shower
water in the grinding space. The lower temperature water
is substantially the exclusive source for the replacement
water.
The invention is based on the idea of causing
heat transfer between the groundwood pulp to be thickened
and the cold replacement water added to the shower water,
so that heat is transferred from the groundwood pulp to
the replacement water. Thus the cold replacement for the
shower water becomes warmer and the temperature of the shower
water of the pressure grinding process remains high, at the
same time the pulp to be discharged from the thickener is
cooled.
l 157305
In the simplest embodiment of the method only
enough water is sprayed on the groundwood pulp to be thickened
as is required for providing the necessary amount of warm
replacement water, but it is preferable to spray on the
groundwood pulp sufficient water that the pulp is cooled
down to a temperature which is suitable for storage and/or
bleaching. In this way the groundwood pulp is cooled by
the same water spraying step to a temperature required for
further treatment.
In one embodiment of the method, water is sprayed
in two parts, one part substantially corresponding to the
amount of replacement water required for the shower water
for the grinding space, and the other part corresponding
to the amount of water which in addition to said first
part is required for cooling the groundwood pulp down to
said further treatment temperature. This facilitates an
independent adjustment of both the amount of replacement
water for the shower water for the pressure grinding process
and the amount of water required for cooling the groundwood
pulp.
The invention will be described in the following
in more detail with reference to the accompanying drawing
which shows schematically a pressure grinding process
according to the invention,
The drawing shows a grinder 1 comprising a grinding
stone 2 rotating in a pressurized grinding space 3. The
grinding space forms two grinding pockets 4 above which
there are well known pressure equalizing chambers 5 closed
by gates. On opposite sides of the grinding stone there
are hydraulic pistons 6 for pressing blocks of wood dropped
into the grinding pockets against the grinding stone, The
1157305
grinding space is provided with a number of shower pipes 7
for supplying warm shower water onto the grinding stone.
For collection of the groundwood pulp there is a pulp pit 8
in the lower part of the grinding space.
A pipe 9 for groundwood pulp A extends from the
pulp pit of the grinder via a stick crusher 10 to a cyclone
11 which is provided with a steam exhaust pipe 12. From
the cyclone the groundwood pulp B, which is relieved of
pressure, flows through a pipe 13 to a tank 14~ From this
tank extends a pipe 15 through a pump 16 and a pressure
screen 17 to a thickener 18 for the groundwood pulp C to
be thickened. The thickener is equipped with an outlet 19
for the thickened groundwood pulp D.
The thickener is provided with two shower pipes
20 and 21 for spraying cold water E on the pulp to be
thickened. The thickener comprises a vat 22 both for the
water F which is removed from the groundwood pulp and for
the water El which is sprayed from the first shower pipe,
and another vat 23 for water E2 which is sprayed from the
second shower pipe. From the vat 22 a pipe 24 extends to
a tank 25, and from the vat 23 a pipe 26 extends to a
cooling water tank 27. From the tank 25 a pipe 28 extends
through a pump 29 to the shower pipes 7 in the grinding
space for supplying warm shower water G to said shower
pipes.
When groundwood pulp is being made, shower water
circulates continuously through the circulation system
formed by the pipes 9-13-24-28, As shown in the drawing
substantially only water removed from the groundwood pulp
is utilized as shower water. A part of the shower water is
lost with the thickened groundwood pulp and due to other
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- 1157305
losses of water from the process, exhausting steam etc.,
more water must be fed into the circulation system than
escapes from the thickener with the pulp. The water El
which is sprayed on the groundwood pulp from the shower
pipe 20 is now used as a replacement water and as shown
in the drawing, is substantially the exclusive source for
the replacement water. This water gets warm at the same
time as it cools the pulp, Thus the replacement water is
heated by the heat energy of the groundwood pulp which
improves the heat economy of the process. By means of
cooling water E2 which is sprayed from the shower pipe 21
the groundwood pulp is cooled further to a temperature
which is low enough for storing and bleaching.
The groundwood pulp C enters the thickener at a
temperature of 99-100C, and the temperature of the outgoing
thickened pulp D does not exceed 70C. The temperature of
the water which is fed into the shower pipes 20, 21 is not
more than 60C.
If for instance the pulp enters the thickener at
a temperature of 99C and leaves it with a consistency of
15%, the need of replacement water is 6.7% of the total
shower water. If the temperature of this replacement water
is 50C, the total temperature of the shower water is
0.067 x 50C + 0,933 x 99C = 95.7C
If the method according to the invention is used
and replacement water at 50C is sprayed continuously on the
groundwood pulp in the thickener, and if there is one shower
pipe, the temperature of the outgoing pulp and the replace-
ment water will be:
99C + 50C = 74 5C
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1157305
:
If there are two shower pipes, the average temper-
ature of the replacement water is
99C + 0.5 x 50C + 0,5 x 50C
99C + 0 5 x 50C 1.5
( 1 5 1.5 ): 2 = 77.2C
and the temperature of the outgoing pulp is 71.7C.
Therefore, with the method according to the
invention the total temperature of the circulating water
will be
0.067 x 77.2C + 0.933 x 99C = 97.5C and the temperature
of the outgoing pulp is 71.7C.
The drawing and the description shall only
illustrate the principle of the invention. In its details
the method may vary within the scope of the claims.
