Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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METHOD FOk DELIVERING PULP ONTO THE WIRE OF A PAPER
MACHINE
The prevent invention concern a method for delivering
pulp onto the wire of a paper machine, in which method
pulp is suspended into the white water into the form of a
suspension which is delivered onto the wire and in which
method the white water drained from the formed sheet
through the wire is returned to the suspension process.
An eQQential step in the papermaking process is the sheet-
forming proceQs, which take place by diluting the thick
pulp from pulp production into a low-consistency suspen-
sion and pumping this suspension via centrifugal cleaners
and deaeration to the headbox of the paper machine and
therefrom further onto a water-permeable wire, onto which
a homogeneous sheet is formed at the drainage of water.
The formed sheet i9 processed in the subsequent procesQing
tages into a finished paper.
The economicQ of papermaking presumes maximum retention,
which means that fibers carried along the suspension
should be retained as effectively as possible in the sheet
being formed onto the wire. For this purpose, the water
circulation of the process is closed by recirculating the
white water drained from the Qheet through the wire back
to the stock preparation phaQe, wherehy the fibers are
repetitively offered an opportunity to adhere to the
forming sheet. Furthermore, a closed water circulation is
important in the reduction of waste water effluents.
A problem of the closed water circulation in papermaking
is, however, that a continuously growing concentration of
- 20C~7~7~7~
constituentq dissolved or diqperqed from the stock being
prepared develops into the recirculated white water. These
constituents di3turb the papermaking by deteriorating re-
tention, preventing the effect of additiveq, causing accu-
mulations in the process machinery and occurring as trans-
ferred impurities in the paper being produced. A major
portion of the matter dissolved into the white water is
composed of carbohydrates, in particular of hemicellulose
having a molecular weight of approx. 10 000 ... 100 000.
In addition, the white water contains dissolved lignin and
extracted constituents that due to their lower solubility
and higher adsorption capability seek to precipitate on
the fibers, thereby avoiding enrichment into the white
water in the same manner as carbohydrates tend to do.
lS
The present invention aims to provide a method capable of
reducing the above-deqcribed problemq in papermaking which
arise from the closed water circulation. The invention is
characterized in that an enzyme is added to the white
water in order to disintegrate constituents such as
hemicellulose dissolved or dispersed from the pulp.
When the substances dissolved or dispersed into the white
water are advantageouqly disintegrated in accordance with
the invention into smaller fragments, water drainage from
the forming sheet on the wire it improved. In addition,
disintegration of hemicellulose has been shown to promote
the retention of fibers. Without any limitation on the
scope of this invention, the reason for this phenomenon is
aQsumed to be caused in that hemicellulose, which by its
adherence to fibers gives them an anionic characteristic
thus causing their mutual rejection, loses its adsorbing
capability during disintegration, whereby the fibers can
3 77~74
retain their neutral nature, thus improving their
retention in the sheet.
In relation to the qubject it recognized a prior art
method di~clo-~ed in FI patent application 874113 aiming at
improvement in water drainage from the forming sheet by
meanQ of enzyme. This method, however, relateq to the
addition of enzymes into the high-con~i~tency pulp Jo that
the enzymes effect the fiberY prior to the resuspension of
the thick pulp into a su~pen ion to be delivered onto the
wire. By contrast, the present invention as described
above concerns the addition of enzymes into the white-
water ~y~tem Qo that they are used in the white water for
the diQintegration of di~Qolved or dispersed constituents,
l particularly di~olved hemicellulo~e.
The enzyme applicable in the method are hemicellulaQe,
cellula~e, e~tera~e and pectina-~e a well a their
combined mixture. Cellula~e for inQtance with a CMC
activity of 5000 U/ml or ~-xylo~ida3e with an activity of
2000 nkat/ml are suitable. The enzyme can be metered by
0.01...1 l/m3 white water uQing a preferred metering
method in which the pure enzyme iQ first diluted into a
Qmaller quantity of water, in which it it then metered
into the white water. For the functioning of the enzyme,
water temperature mutt be 10...90 C, preferably
40...70 C, while the pH mutt be 3Ø..8.5, preferably
4Ø..6Ø
According to the method, the enzyme iQ primarily added to
the short circulation of rich white water in which white
water iQ recirculated from the sheet formation on the wire
back to the dilution of incoming stock. The problemQ
4 2Q~7~74
caused by constituents dissolved in the white water are
typically evident in this area. Since the fiber
consistency in the rich white water can be assumed low,
typically approx. 0.4 % maximum, the enzyme haA the
opportunity to attack the dissolved substances in the
white water effectively without interference from fibers,
therein purporting the actual objective of the invention.
A suitable location of enzyme addition is the wire pit,
into which white water drained from the sheet is routed
for mixing into the high-consistency pulp to be diluted.
According to the method, it is alQo possible to apply the
enzyme addition to the so-called long circulation of lean
white water in which white water drained from the sheet it
recirculated for reuse at locations situated closer to the
start end of the papermaking process. A suitable location
of enzyme addition here is for instance the couch pit,
which acts as a collecting well for the white water
drained from the sheet by suction. A portion of the
suction box white water can be routed from the couch pit
along with the short circulation to the wire well, while
the rest is added to the above-described long circulation
of lean white water. A particular advantage of this
application is that the white water entering the couch pit
haQ an appropriately lower consistency than that of the
white water éntering the wire well.
The maximum benefit of the invention iQ achievable in
applications to papermaking using groundwood pulp such as
TMP or CTMP. This is because the amount of deleterious
substances dissolving or dispersing into the white water
from the groundwood is particularly high. Nevertheless, an
advantageous result from the use of enzyme addition in
5 2Q~7774
accordance with the invention i9 alto achieved in
papermaking from chemical pulp.
Furthermore, the invention concern the use of enzyme5 in
the disintegration of organic matter dissolved or
dispersed from the pulp into the white water of the paper
machine. In accordance with the above description the
preferred enzyme are hemicellulase, cellulase, esteraYe,
pectina~e as well as their combined mixture.
The invention iQ described in detail in the following with
the help of the enclosed drawing which diagrammatically
illustrates a sheet forming procesY in accordance with the
invention and in which the short circulation of the rich
white water i8 shown as a whole.
As shown in the drawing, the thick stock received from
pulp preparation with a fiber consistency of approx. 5 %
is pumped via a line 1 to the bottom of a wire well 2,
where it is mixed with the white water. The obtained
suspension it routed via a fan pump 3 to a centrifugal
cleaner 4, whose accept fraction continues to a deaeration
chest 5. The reject fraction of the centrifugal cleaner 4
continues to the next centrifugal cleaner units 6, of
which only one is illustrated, while the number of the
units may in practice be higher in a so-called cascade
connection. The collective accept fraction from these
cleaners returns via a line 7 to the wire well 2 while the
reject fraction from the last cleaner unit is removed from
the process along a line 8. The cleaned pulp suspension
from the deaeration chest 5 is pumped by a feed pump 9 to
paper machine headbox 10, whose slice deliverq the pulp
onto a continuously circulating endless wire 11 thus
6 21~0~7~4
forming a homogeneous sheet on the wire. A major portion
of the white water contained in the pulp is drained
through the wire 11 at the horizontal section of the wire
following the headbox 10, and this white water fraction is
collected along a line 12 directly into a wire well 2.
Arranged after the above-described wire section against
the wire 11 is a top wire 13, whose drained white water is
routed to the wire well 2, as well as suction boxes 14,
whose drained white water is routed along a line 15 to a
couch pit 16. Part of the white water in this latter pit
is pumped along with the short circulation of rich white
water as required to complement the white water of the
wire well 2 in pulp suspension, while the rest is routed
along a line 17 to the so-called long circulation of the
lean white water in the paper machine.
The essential characteristic of the invention is the
enzyme addition to the white water 90 that the enzyme-
induced disintegration of dissolved and dispersed
substances in the white water promotes water drainage from
the sheet formed on the wire 11 and improve the retention
of fibers in the sheet and, above all, prevents the
influence of substances deleterious to the retention of
fibers. According to the invention, the enzyme can be
added to the above-described short circulation of rich
white water, whereby the recirculation line 18 can be
connected to the wire well 2 a shown in the figure.
Instead of or complementing this arrangement, the couch
pit 16 can be provided with an enzyme feed line 19,
whereby enzyme-treated white water is available in the
flows exiting the pit to the short circulation as well as
the long circulation of the paper machine.
7 2~C~7774
The effect of enzyme addition on Qheet forming and
achieved paper characteri~ticQ has been investigated in
testR which are deqcribed in the following examples.
Example 1
Hemicellulaqe enzyme with an activity of 2000 IU/ml was
added to a 1200 ml qample of rich white water of the short
circulation and to a 1200 ml sample of lean white water of
the long circulation. The enz.yme was metered in doqeR of
0 Al reference test), 3 Al and 30 Al per liter of white
water. The water pH was approx. 5.0 and temperature 50 C,
and in each sample the enzyme was allowed to function for
3 hour with intermittent mixing. After the enzyme
reaction, a pulp volume 300 ml of untreated TMoe waq mixed
to each white water sample, and the obtained su~penqions
were immediately subjected to freeness mea~urementQ
(according to Scan M 4:65) characterizing the water
drainage of the pulp as well as anion concentration
mea~urementq (according to Neimo, in KCL circular no. 18,
1983). The re~ultY are Rhown in Table 1 below.
Table 1
Water Enzyme Freeness Anion conc.
circ. dozing if (ml) (mekv/l)
________________ _______________________________________
Lean 0 49 0.418
white w. 3 ~5 0.037
57 0.032
Rich 0 40 0.133
white w. 3 40 0.070
49 0
8 ~a7~!7~
The TMP mixed with the rich white water qample~ from the
short circulation waq formed into keel according to Scan
M 45:76 (100 g/m2). The keel were teqted according to
Scan M 8:76 inqtructionq. The re~ult~ are qhown below in
Table 2.
Table 2
Enzyme Sheet Porosity Tensile Elonga- Tear
dozing bet w. GH strength tion factor
(~1/l) (g/m2) ~/100 ml) (Nm/g) (%)
_________________________________________ ______________
0 99.1 287 36.3 2.31 ~.16
0.3 101.0 345 37.4 2.33 5.54
3 102.9 306 36.7 2.18 5.78
The reqults in Table 1 Qhow that enzyme treatment of rich
and lean white water in the qhort and long circulation
reQpectively achieve an increase in freeneqq indicating
an improved water drainage and a Qignificant reduction in
anion concentration, which it indicative of improved fiber
retention and saving in the use of retention improver.
Further evidenced in Table 2 iq that the enzyme treatment
doe not deteriorate strength properties of the paper
produced from pulp qamples.
Example 2
Hemicellulaqe enzyme in accordance with Example 1 waq
added in concentrationQ 0, 0.3 and 3 ~1/l to a 1800 ml
sample of rich white water of the short circulation. The
water pH waq approx. 5.0 and temperature 50 C, and in
9 2~
each sample the enzyme way allowed to function for 3 hour
with intermittent mixing.
After the enzyme reaction, a pulp volume 300 ml of
untreated TMP was mixed to each white water sample, and
the obtained su3pen~ions were immediately subjected to
fiber retention mea~urement~ using the DDJ apparatus. The
resulte are ehown in Table 3 below.
Table 3
Enzyme dozing Retention
(,~1/1) (%)
_________________________________________________________
0 84.3
3 88.6
3~ 85.9
The results show retention improvement by the enzyme
treatment.
For those versed in the art it is obvious that the
different implementations of the invention are not
exhausted by thoqe described in the above but instead, can
be varied within the scope of the dieclosed claime.
