Process for manufacturing groundwood pulp while maintaining a high and uniform moisture content in the wood in the grinding zone

A process is provided for manufacturing groundwood pulp of uniform quality by maintaining a high and uniform moisture content in the wood in the grinding zone which comprises grinding debarked wood logs or wood chips in a closed pocket grinder under atmospheric or superatmospheric pressure while applying shower water both to the free surfaces of the grindstone and to the grinding zone where the logs or chips are in contact with the grindstone, such as by spraying the water at the end portions of the logs or chip mass, optionally together with applying shower water via the ram plates and a water trap arranged around the lower part of the log pockets, and preferably retaining shower water within the grinding zone by filling with shower water the hollows between the logs or chips located nearest the surface of the grindstone.

Groundwood pulp is normally produced from debarked logs. According to U.S. 
Pat. No. 4,247,363, patented Jan. 27, 1981, to Soma et al, it is also 
possible to produce groundwood pulp from wood chips. The wood material is 
thrust against a rotating grindstone where the abrasive action loosens and 
releases the wood fibers in the surface of the wood against the grindstone 
in a grinding mill, which may operate at atmospheric pressure or at 
superatmospheric pressure. 
The process generates heat, and the wood fibers tend to stick to the 
grindstone, so during grinding of the logs the grindstone surface is 
cooled and cleaned by spraying shower water onto the surface. This shower 
water may optionally contain bleaching chemicals. The shower water 
normally is applied directly onto the free surfaces of the grindstone, in 
the direction of rotation of the grindstone and/or in the opposite 
direction thereto, at an angle of about 90.degree. to the suspension and 
drive shaft of the grindstone. Since the logs brought into contact with 
the grinding surface lie parallel to the shaft of the stone, the shower 
water is applied at an angle of about 90.degree. to the long axis of the 
logs. 
The fibers freed from the logs during the grinding operation are collected 
together with shower water in the bottom pit or dam of the grinding mill. 
The pulp concentration in the resultant groundwood pulp suspension is 
normally between 0.5 and 2% calculated as bone dry pulp. 
When groundwood pulp is ground using stone grinders, the wood is dried 
locally by the frictional heat generally by contact of the wood with the 
grinding surface of the stone. If the wood has a low moisture content, the 
wood may even become overheated, and subjected to thermal degradation. As 
the moisture content of the wood decreases, the temperature at which the 
lignin softens increases, which results in a pulp having relatively short 
fibers. Such pulps when used for paper manufacture give a paper having, 
among other things, a low tear strength. The magnitude of these effects 
and therefore the quality of the resultant groundwood pulp varies with the 
moisture content of the wood. 
Moreover, the grinding of wood having a low moisture content can give such 
a high temperature at the grinding surface (the area in which wood and 
grindstone are in surface contact with each other) that the fibers become 
discolored, which results in a pulp of low brightness, and, furthermore, 
in a high consumption of chemicals in any subsequent bleaching. 
Thus, in the grinding of wood while spraying shower water tangentially to 
the grindstone, it is difficult to obtain a strong and light pulp of 
uniform quality, because it is difficult to obtain wood starting materials 
having a high and uniform moisture content. 
These difficulties are resolved by the process of the present invention, 
which relates to a method for manufacturing groundwood pulp from 
lignocellulosic material, in which wood pulp debarked in a known manner or 
wood chips are ground in a closed pocket grinder at atmospheric pressure 
or at a superatmospheric pressure by spraying shower water onto the free 
surfaces of the grindstone at an acute angle, and, in addition, applying 
shower water to the grindstone within the area where the wood and the 
grindstone are in surface contact with each other, i.e., to the grinding 
surface, and to portions of the wood charge located immediately above the 
surface of the grindstone, preferably at an angle within the range from 
about 0.degree. to about 60.degree.. These are collectively referred to 
herein as the "grinding zone". A "pocket grinder" is a grinder in which 
the wood is pressed against the grindstone from radially arranged wood 
pockets. 
This method of applying shower water to the grinding zone of the logs or 
chips has surprisingly been found to prevent undesirable drying of the 
wood in the course of the frictional heating resulting from its being 
thrust against the surface of the grindstone, and thus holds the moisture 
content of the wood at any desired high and uniform level. 
It has been found particularly advantageous to introduce the shower water 
to one end portion or to both end portions of the logs, and to direct the 
shower water in substantially parallel alignment with the longitudinal 
axis of the logs and at an angle within the range from about 0.degree. to 
about 60.degree.. In this way, the hollows between adjacent logs are also 
filled with shower water, which actively contributes to maintaining a high 
and uniform level of moisture in the wood in the grinding zone. 
When introducing the shower water to the end portions of the logs or to 
wood chips, it has also been found particularly suitable to arrange a 
water trap at the lowermost part of each wood pocket. Such a water trap 
may suitably comprise plates or like elements located at the two plane 
ends and sides of the grindstone, and other plates connected with the 
plane ends and side plates arranged around each wood pocket, in a manner 
such as to hold the shower water within the grinding zone of the logs or 
chips to the greatest possible extent, and thereby making it possible to 
fill with water into the hollows located between the logs or chips 
situated nearest the surface of the grindstone at each stage of a grinding 
operation. 
The shower water is suitably introduced into the grinding zone of the logs 
or chips via spray pipes and through holes made in said pipes or through 
nozzles fixedly mounted on the pipes, it having been found that the shower 
water should be maintained at a superatmospheric pressure within the range 
from about 0.5 to about 40 kp/cm.sup.2, preferably from about 5 to about 
30 kp/cm.sup.2. The shower water pipes, together with the aforementioned 
holes or nozzles, can be fixedly mounted on the grinder or can be arranged 
for oscillatory movement. The shower water can be applied continuously or 
intermittently at intervals of from five to ten seconds. The spacing 
between the peripheral surface of the grindstone and the nearest spray 
delivery point above that surface should be greater than 10 mm. 
When it is said that the shower water is applied "substantially in parallel 
with the longitudinal axis of the logs or chip mass" it is meant that the 
central portion of the spray projected straight out of the holes or 
nozzles strikes the surfaces of the logs or chips at an angle within the 
range from about 0.degree. to about 60.degree., preferably between 
0.degree. and 15.degree., to the longitudinal axis of the logs or chip 
mass. 
In addition to supplying shower water via spray pipes the shower water can 
also be applied to the logs or chips from above through openings or 
nozzles arranged in each ram plate. This actively contributes to filling 
the hollows between the lowermost adjacent logs or chips located nearest 
the surface of the grindstone, and assists in maintaining a desired high 
and uniform level of moisture in the wood in the grinding zone. 
The flow of shower water to the logs or chips from all sources is suitably 
so controlled that the water introduced directly into the grinding zone is 
at a rate within the range from about 50 to about 600 liters/minute, for 
each ton of pulp produced per hour. Totally, thus, the rate of application 
of shower water per ton of pulp per hour may vary between 100 and 2000 
liters/minute (including the shower water applied to the free surfaces of 
the grindstone). 
It has been found of particular advantage to use filtered shower water, by 
filtering the shower water through suitable means such as a curved screen, 
a drum filter, a centrifuge, or a special filter. Thus, for example, 
shower water which has been ultrafiltered can be used to particular 
advantage. 
The temperature of the shower water introduced to the grinding zone should 
be within the range from about 65.degree. to about 120.degree. C., 
preferably from 80.degree. to 105.degree. C., this latter range having 
been found particularly advantageous. 
The method of applying shower water in accordance with the invention makes 
it possible to produce groundwood pulp which is stronger and brighter and 
of more uniform quality than groundwood pulp produced in accordance with 
the conventional shower water application methods. A further important 
advantage is that high quality groundwood is produced from aged wood, 
which has been stored, and has a low moisture content. During the method 
of the invention drying of the wood during the grinding process is 
substantially entirely prevented, thereby greatly decreasing the risk of 
harmful overheating. This also greatly increases the useful life span of 
the grindstone, and above all reduces the risk of damage to the 
grindstone, which is of significant importance, since a damaged grindstone 
normally results in a drop in production with associated economic losses. 
Another advantage is that the pulp produced is brighter, due to the fact 
that the fibers are not discolored by local overheating in the grinding 
zone. 
A further advantage afforded by the invention is that the flow of shower 
water can be distributed within the grinder more uniformly which makes it 
possible to reduce the total amount of shower water required. This 
reduction in the flow of shower water results in a higher pulp consistency 
in the resultant groundwood pulp suspension. This is of particular 
advantage when the pulp is to be stored in a tower subsequent to leaving 
the grinder, since the tower can then be made relatively smaller. 
A further advantage afforded by a higher pulp consistency is that it is 
cheaper to dewater the pulp suspension prior to drying, as well as prior 
to any subsequent bleaching stage. If subsequent bleaching of the pulp is 
desired, bleaching can be effected to advantage in accordance with the 
method described in U.S. Pat. No. 4,160,693, patented July 10, 1979, to 
Lindahl et al. 
Groundwood pulp produced in accordance with the invention has a high 
content of long and flexible fibers, which produce a strong paper. 
Alternatively, this property can be utilized in the manufacture of paper 
having a lower grammage than normal while retaining good mechanical 
strength properties. Further, when mixed with chemical pulp, such as 
sulphate pulp, or sulphite pulp, the pulp manufactured in accordance with 
the invention can be utilized in greater quantities than normal, thereby 
reducing the cost of manufacturing paper. The groundwood pulp is also 
suitable for the manufacture of paper of a wider and more varying quality 
range than is normal for pulps with a yield within the range from 92 to 
98%. This is due to the high degree of brightness of the resultant pulp, 
and its high percentage of long fibers.

In the system illustrated in FIG. 1, debarked logs 1 having a moisture 
content of between 20 and 65% are pressed heavily against the grinding 
stone 3 by means of rams 2, at a ram pressure of from 4 to 40 kg/cm.sup.2. 
The grinder is enclosed and atmospheric pressure or a superatmospheric 
pressure up to 10 kp/cm.sup.2 above atmospheric is normally maintained. 
During the grinding operation, shower water is supplied via pipes 4,5,6 
and 7, and is applied to the free cylindrical surfaces A,B of the 
grindstone at acute angles, but at right angles to the suspension and 
driveshaft 8 of the grindstone. 
In the preferred embodiment of the invention shown in FIGS. 2 and 3, shower 
water is also sprayed onto both ends 9, 10 of the logs from spray pipes 
11, 12 having nozzle-equipped spray headers 13 and 14 (see FIG. 3). The 
shower water is directed substantially in parallel with the longitudinal 
axis of the logs, i.e., at an angle of 0.degree., but can be applied to an 
angle up to 60.degree., and preferably not exceeding 15.degree.. 
The shower water sprayed onto the free cylindrical surfaces A, B of the 
grinding stone normally is at a superatmospheric pressure within the range 
from about 0.5 to about 30 kp/cm.sup.2 above atmospheric, while the shower 
water introduced to the grinding zone is at a superatmospheric pressure 
within the range from about 0.5 to about 40 kp/cm.sup.2 above atmospheric, 
preferably between 5 and 30 kp/cm.sup.2 above atmospheric pressure. 
The process of the invention is applicable to any kind of wood. In general, 
softwood such as spruce and pine can be ground more easily than hardwood 
such as beech and oak, but both types of wood can be ground satisfactorily 
using this process. Exemplary hardwoods which can be ground include birch, 
beech, poplar, cherry, sycamore, hickory, ash, oak, chestnut, aspen, 
maple, alder and eucalyptus. Exemplary softwoods include spruce, fir, 
pine, cedar, juniper and hemlock. 
The wood can be in the form of logs, or large pieces, or even in small 
form, such as wood chips having dimensions that are conventionally 
employed in pulping processes. Wood slivers and splintors, wood granules 
and wood chunks, and other types of wood fragments can also be used. 
The groundwood pulp that is obtained in accordance with the process of the 
invention can easily be bleached in accordance with known methods by 
treatment with chlorine, chlorine dioxide, chlorite, hypochlorite, 
peroxide, peracetate, oxygen or any combinations of these bleaching agents 
in one or more bleaching sequences as described in, for example, U.S. Pat. 
No. 4,160,693, patented July 10, 1979 to Lindahl, particularly column 3, 
lines 32 to 48, inclusive. 
Preferred embodiments of the invention are illustrated in the following 
working Examples, in conjunction with controls carried out in accordance 
with the conventional spraying technique. 
EXAMPLE 1 
This Example illustrates the manufacture of unbleached groundwood pulp from 
debarked spruce logs, the method according to the invention being compared 
with a Control using the conventional method of applying shower water 
solely to the free surfaces of the grindstone (Control 1). 
In the Example according to the invention, shower water in addition was 
introduced to the grinding zone by spraying at the ends of the logs. This 
was done by fitting one of nine pocket grinders in a groundwood pulping 
plant with spray pipes supplying shower water at right angles to the end 
portions of the logs, in accordance with the method illustrated in FIGS. 2 
and 3. 
Debarked spruce logs having an average moisture content of 53% were charged 
to the grinder. The logs were pressed against the surface of the 
grindstone at a pressure of 9 kp/cm.sup.2. At this ram pressure, the mean 
power of the motor driving the grindstone was found to be 1950 kW. The 
logs were ground at atmospheric pressure, but the grinder was well sealed, 
to prevent the loss of water vapor formed by friction of the wood against 
the grindstone. The groundwood pulp suspension obtained was discharged 
from the grinder through a closed conduit. A suction fan was connected to 
the conduit in order to recover water vapor and other gases. The fan 
conveyed the hot vapors to a heat exchanger in which air was heated from a 
temperature of about +5.degree. C. to about 40.degree. C. This preheated 
air was used for flash-drying of the groundwood pulp. 
In the Example according to the invention, the temperature of the shower 
water was 80.degree. C. Shower water was applied to the free surfaces of 
the grindstone at a rate of 800 l/minute under a super-atmospheric 
pressure of 9 kp/cm.sup.2 above atmospheric. Shower water was applied to 
the ends of the logs in the grinding zone at a super-atmospheric pressure 
of 12 kp/cm.sup.2 above atmospheric and a rate of 300 l/minute. Thus, the 
total amount of shower water supplied was 1100 l/minute. Samples of the 
resulting groundwood pulp suspension taken from the closed conduit had a 
pulp consistency of 2.72%. These pulp samples were screened in a 
laboratory screen having a slot width of 0.15 mm, before being formed into 
sheets which were tested for paper properties. The average results from 
five samples are shown in Table I. In addition to pulp and paper 
properties, the Table also shows the energy consumed. 
Control 1 
The same grinder as that used in Example 1 was used in this test, but with 
the supply of shower water to the pipes 11 and 12, FIGS. 2 and 3 stopped. 
Thus, in this case shower water was supplied only to the free surfaces of 
the grindstone. Shower water was applied at a rate of 1600 liters per 
minute and the discharge pressure was 9 kp/cm.sup.2 above atmospheric. 
Samples of the resulting groundwood pulp suspension taken from the closed 
conduit had a pulp consistency of 1.87%. The pulp was treated in the same 
manner as that recited in Example 1, and the results are set forth in 
Table I. 
TABLE I 
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Control 1 
Example 1 
______________________________________ 
Energy consumed when 
1085 1085 
grinding, kWh/ton 
Canadian Standard Freeness, ml 
145 150 
Long fiber content according to 
15 24 
Bauer McNett (+30 mesh) % 
Tensile index, Nm/g 33 41 
Tear index, mNm.sup.2 /g 
3.6 5.1 
Density, kg/m.sup.3 382 370 
Brightness according to SCAN, % 
61 63 
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As seen from the Table, the pulp produced in accordance with the invention 
(Example 1) has a surprisingly high content of long fibers. The long-fiber 
content of the pulp produced in accordance with the invention is 60% 
greater than that of conventional groundwood pulp (Control 1). It is 
particularly surprising that pulp produced in accordance with the 
invention has a substantially higher tear index. This increase in tear 
index is probably due to the high percentage of long fibers. It is also 
very surprising that the pulp produced according to the invention has such 
a high brightness. The reason for this may be that in the method according 
to the invention the temperature in the grinding zone is somewhat lower 
due to the more effective supply of cooling shower water to the logs as 
they are ground against the hot surface of the grindstone. 
EXAMPLE 2 
This Example illustrates the manufacture of groundwood pulp from spruce 
logs in accordance with the invention, with the addition of shower water 
containing bleaching waste liquor from a peroxide bleaching step. By way 
of comparison, groundwood pulp was also produced in accordance with the 
conventional technique with the addition of shower water, using water 
containing bleaching waste liquor (Control 2), as described in U.S. Pat. 
No. 4,029,543. 
In the tests, the same grinder and grinding conditions were used as in 
Example 1, with the following differences. 
In the Example according to the invention, the temperature of the shower 
water was 85.degree. C. This shower water with its content of bleaching 
waste liquor had the following composition: 
______________________________________ 
Hydrogen peroxide 0.53 g/l 
Na.sub.2 SiO.sub.3 (water glass) 
2.12 g/l 
Diethylenetriaminepentaacetic acid (DTPA) 
0.08 g/l 
Acetic acid 0.94 g/l 
Resin and fatty acids 0.13 g/l 
Measured pH 8.4 
______________________________________ 
Shower water was applied to the grindstone at a superatmospheric pressure 
of 10 kp/cm.sup.2 above atmospheric and a rate of 900 liters/minute. 
Shower water was applied to the ends of the logs at a superatmospheric 
pressure of 12 kp/cm.sup.2 above atmospheric, and a rate of 200 
liters/minute. Thus the total volume of shower water supplied in this 
Example was 1100 liters/minute. Samples of the resulting groundwood pulp 
suspension taken from the closed conduit had a pulp consistency of 2.85% 
(average value of five samples). The pulp samples were screened in a 
laboratory screen having a slot width of 0.15 mm, after which hand-made 
sheets were produced and tested for paper properties. The results of the 
analysis and the energy consumed are set forth in Table II. 
Control 2 
The same grinder as in Example 2 was used, but with the supply of shower 
water to the pipes 11 and 12 (FIGS. 2 and 3) stopped. Thus, in this case 
only the free surfaces of the grindstone were sprayed with shower water 
containing bleaching waste liquor. The shower water was charged at a rate 
of 1700 liters/minute, under a discharge pressure of 10 kp/cm.sup.2 above 
atmospheric. The shower water had a temperature of 70.degree. C. Samples 
of the resulting groundwood pulp suspension taken from the closed conduit 
had a pulp consistency of 1.65%. The pulp was treated in the same manner 
as that recited in Example 2. The results are set forth in Table II. 
TABLE II 
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Control 2 
Example 2 
______________________________________ 
Energy consumed when grinding, 
1025 1025 
kWh/ton 
Canadian Standard Freeness, ml 
185 175 
Long-fiber content according to 
17 27 
Bauer McNett (+30 mesh), % 
Tensile index, Nm/g 38 43 
Tear index, mNm.sup.2 /g 
3.9 5.4 
Density, kg/m.sup.3 362 350 
Brightness according to SCAN, % 
68 72 
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As seen from the Table, the pulp produced in accordance with the invention 
(Example 2) has a very high brightness. Apparently a more effective 
bleaching is obtained with the aid of bleaching waste liquor, when the 
shower water is also supplied directly to the guiding zone. It may also be 
that the shower water supplied to the free surfaces of the grindstone has 
difficulty in penetrating between wood and grindstone in the grinding 
zone. Thus, the high brightness may have resulted from a slightly lower 
temperature in the grinding zone, and/or a more abundant supply of 
residual bleaching chemicals, originating from the bleaching waste liquor. 
A significant advantage afforded by the present invention is that the pulp 
suspension from the grinder has a relatively high pulp consistency. If the 
pulp is to be bleached further, it is possible to directly dewater the 
pulp further to a higher pulp consistency with the aid of a relatively 
simple dewatering apparatus. In order to avoid a high fiber content in the 
white water when grinding in the conventional way, it is necessary, 
because of its low pulp consistency, to first thicken the pulp suspension 
on a filter, which is space-consuming, and requires a high investment 
cost. 
EXAMPLE 3 
This Example illustrates the manufacture of groundwood pulp from spruce 
logs having a low moisture content. The shower water used contained 
bleaching waste liquor. The electrical power input required during the 
grinding operation was measured to be 2000 kW. Groundwood pulp was also 
produced in the same grinder in accordance with the conventional method 
(Control 3) and with the same electrical power input. 
The Example according to the invention was carried out in the following 
manner. Debarked spruce logs having an average moisture content of 35% 
were ground while supplying shower water containing the same chemical 
compounds as in Example 2 and Control 2. Shower water was applied to the 
free surfaces of the grindstone at a rate of 950 liters/minute under a 
superatmospheric pressure of 8 kp/cm.sup.2 above atmospheric. Shower water 
was applied to the end portions of the logs at a rate of 100 liters/minute 
and a superatmospheric pressure of 10 kp/cm.sup.2 above atmospheric. The 
temperature of the shower water was 90.degree. C. 
Pulp samples were taken from the closed conduit and screened in a 
laboratory screen having a slot width of 0.15 mm, prior to forming sheets 
and testing for paper properties. The results obtained from the tests and 
the amount of energy consumed are set forth in Table III. 
Control 3 
In this Control, the same grinder was used as in Example 3, with the same 
electrical power input. Spruce logs having the same moisture content were 
used. The supply of shower water to the pipes 11 and 12 (FIGS. 2 and 3) 
was stopped. Thus, shower water was applied solely to the free surfaces of 
the grindstone as conventional. Shower water was charged at a rate of 1800 
liters/minute under a discharge pressure of 8 kp/cm.sup.2. The temperature 
of the shower water was 70.degree. C. 
Pulp samples were taken from the closed conduit and treated in the same 
manner as the samples obtained from Example 3. The results are shown in 
Table III. 
TABLE III 
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Control 3 
Example 3 
______________________________________ 
Electrical energy consumed 
1080 1030 
when grinding, kWh/ton 
Canadian Standard Freeness, ml 
140 170 
Long-fiber content according to 
9 22 
Bauer McNett (+30 mesh), % 
Tensile index, Nm/g 28 40 
Tear index, mMm.sup.2 /g 
3.3 4.8 
Brightness according to SCAN, % 
66 71 
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As seen from the Table, the strength values and brightness of the pulp 
produced in accordance with the invention are surprisingly high. This is 
very surprising in view of the low moisture content of the wood. It is 
also seen from the Table that when producing a groundwood pulp from wood 
having a low moisture content, the pulp obtained is much weaker when the 
shower water is supplied as in Control 2, solely in accordance with the 
known technique, i.e. directly onto the free surfaces of the grindstone. 
Thus, an important advantage afforded by the present method is that 
groundwood pulp of high and uniform quality can be produced, even when the 
amount of moisture contained by the wood varies greatly.