Note: Descriptions are shown in the official language in which they were submitted.
21279~2
WO93/1~263 PCTtF193/00019
Process for producing paper
The present invention relates to a process for producing paper
by adding to an aqueous fiber suspension, which possibly con-
tains a filler, auxiliary agents to improve retention and/or
dewatering, the auxiliaries being a cationic long-chain poly-
acrylamide and an aluminum salt, and by dewatering the fiber
suspension during the sheet-forming stage.
The invention thus relates to improving retention and dewater-
ing in connection with the production of paper. By means of
retention agents, dispersed or emulsified substances present in
the pulp, such as fillers, resin dispersions, fines, etc., are
flocculated, whereby they are caused to adhere to the paper
web. Owing to the high water content of the pulp it is impor-
tant that the agents used for improving retention also improve
dewatering in the wire section of the paper-making machine.
High dewatering and high retention are indeed often achieved
simultaneously. Dewatering can further be divided into free
dewatering and dewatering produced by means of reduced pres-
sure. These may be contradictory, and therefore a precise bal-
anc~ is required between these properties. Since the dewatering
of the paper web is most expensive in the drying section of the
paper-making machine, maximal dPwatering at as early a stage of
the process as possible is advantageous. The aim in selecting
the retention agent is to obtain a maximally dry paper web both
after the wire section and after the press section.
It is known that many advantages can be gained by combining, in
a suitable manner, polymeric organic and inorganic components
when forming a paper web. Advantage is taken of this commer-
cially by com~ining a cationic starch and a silica sol in a
system called Compozil. According to the Hydrocol combination,
a cationic polymer and an anionic swelling bentonite are added
to the pulp. In patent application SE-8700058-4, a cationic
WO93/11263 pcr/Fls3/oools
2127~92 2
long-chain polysaccharide, mainly starch, is first added to an
alkalized pulp and then an aluminum source, whereupon polymeric
aluminum compounds are formed. It is stated that a synergistic
effect is produced in this manner.
In patent application SE-8501652-5 it is claimed that, by
adding to the pulp first a cationic polyacrylamide instead of a
cationic starch or guar gum and subsequently an anionic silica
sol, a clearly improved synergistic effect is achieved, espe-
cially in a pulp which contains large amounts of interfering
substances.
The object of the present invention is to provide a paper
production process wherein paper or board is made fro~ an
aqueous suspension cont ining cellulose fiber and possibly an
inorganic filler by using a chemical combination and batching
method which improve retention and dewatering.
It is also an object of the invention to provide economical and
well-controlled web formation by the process according to the
invention, in particular in a neutral and alkaline paper pro-
duction process. The other objects are a clean machine and good
compressibility. Furthermore, the quality properties of the
paper must be good.
These objects have been achieved by the process according to
the invention, the principal characteristics of which are gi~en
in the accompanying patent claims.
The invention is based on the fact that by using a long-chain
polyacrylamide and an aluminum salt, a synergistic effect is
achieved by adding to an aqueous fiber suspension, which pos-
sibly contains a filler, first a cationic lon~-chain polyacryl-
amide and then, directly before sheet formation, a polyaluminum
salt or a combination which comprises an aluminum salt and a
base or an acid which form _n situ aluminum hydroxide particles
WO93/1~263 212 7 9 ~ ~ PCT~F193/00019
having anionic surface charges, in which case the pH before
sheet formation should be within the range 7-9 in order to
produce the anionic surface charges of the aluminum hydroxide.
According to the invention, it has been observed that a syner-
gistic effect is produced by a suitable dosage.
The present invention provides a number of advantages over the
commercial systems and inventions mentioned above. By using a
long-chain cationic polyacrylamide, the process is not tied to
polysaccharides, for example starch, which need to be used in
large amounts. Therefore there is the danger that, when passing
into the cycled waters, they cause problems, since they in-
crease the consumption of oxygen in the water and load the
waste water treatment plant. Furthermore, they deteriorate
dewatering in certain conditions. Polysaccharides often also
contain anionic substituents, even though they are cationized.
For this reason there may arise interaction with many different
pulp components. At the same time the pH dependency also in-
creases. Also, it is not possible to control sufficiently well
the constancy o the quality of the polysaccharides, since they
are derived from vegetable raw materials. In a cationic poly-
acrylamide, it is possible to produce, within very wide limits,
the desired chain length and charge density.
The known system based on a colloidal silica sol is in general
very expensive rompared with the system according to the inven-
tion.
The knowm-system made up of a polymer and bentonite involves
certain disadvantages. It has been noted that bentonite in-
creases the linting and porosity of paper. Its handling re-
quires precise and rather expensive equipment. Controlling the
constancy, i.e. the formation, of paper with such a system is
problematic, and variations in basis weight may be great.
WOs3/14263 PCT/F193/00019
~ 27~92 4
According to the invention, it is also possible to add to the
fiber suspension cationic auxiliary chemicals, which may also
be polymeric, before the adding of the cationic polyacrylamide.
According to the invention, the cationic long-chain palyacryl-
amide is first added to the stock, which is thereafter sub-
jected to shear forces. The aluminum salt is added according to
the invention after the shearing stage.
According to the invention, very good retention and dewatering
are achieved without the formation suffering to the same extent
as when conventional retention agents are used. This is due to
the fact that the cationic flocs formed by the cationic long-
chain polyacrylamide are comminuted by shearing forces in~o
"microflocs", which are then, before web-forming, bound to-
gether with the help of aluminum hydroxide particles which have
anionic surface charges. Although these bonds will open in the
headbox, they are largely re-formed on the wire, whereupon the
"microflocs" of the web provide good formation, and the small
even-sized pores of the web, which are not clogged owing to the
good retention of fines, provide good dewatering, especially in
the press section and the drying section, and often also im-
proved dewatering at the suction boxes of the wire.
In the invention it is possible to use the cationic long-chain
polyacrylamide in amounts which are much larger than when
batching the retention agents in the conventional manner, i ust
before web forming. Overdosage leads in the latter case even to
a situation in which retention is no longer improved or to a
situation~in which strong flocculation deteriorates paper for-
mation. According to the invention it is possible to use a l-
to lO-fold excess of cationic long-chain polyacrylamide as
compared with normal use. The amount depends, for example, on
the filler content of the pulp and on the cationic matter con-
tained in the pulp. The amount of long-chain polyacrylamide is
prefer~bly about 0.01-0.2 % of the dry weight of the pulp.
WO93/1~263 2 ~ ~ 7 9 ~ 2 PCT/F193/00019
Normally the amount is over 0.02 %.
The cationic auxiliary chemical added to the fiber suspension
before the cationic polyacrylamide may be, for example, a dry-
strength agent, such as-a cationic or amphoteric starch or guar
gum or a cationic or amphoteric short-chain polyacrylamide. It
may also be a wet-strength agent, such as a polyamidamine-
epichlorohydrine resin or polyamine-epichlorohydrine resin. It
may also consist of cationic substances, so-called fixer chemi-
cals, which neutralize and/or bind anionic interfering sub-
stances, such as polyethylene-imines, quaternary polyamines or
alum, or polyaluminum chloride.
These cationic chemicals enhance the action of the cationic
long-chain polyacrylamide, since they reduce the anionic quali-
ty of the pulp suspension and prevent interfering substances
from consuming the cationic long-chain polyacrylamide intended
for the flocculation of the fiber suspension. Thus the said
ca~ionic chemicals ensure that the shearing of the flocs in,
for example, the pressure sieve or the feeding pump will result
in stable microflocs in the headbox, since they contain a suf-
ficient amount of cationic polyacrylamide and the surface
charge of the microflocs is sufficiently cationic in order that
they react with aluminum hydroxide particles having anionic
r charges.
The amount of these cationic chemicals is preferably approx.
O.Ol - l % of the dry weight of the pulp.
,
Examples of the cationic long-chain polyacrylamides used in the
invention include the following~ Especially advantageous are
the copolymers of acrylamide and one or two cationic un-
saturated monomers. Suitable cationic monomers include dialkyl-
amino(met)acrylates or -(met)acrylamides, in the form of acid
salts or quaternary ammonium salts. The alkyl groups may each
contain 1-4 carbon atoms, and the amino alkyl group may contain
WO 93/1~263 PCI/F193/00019
2127932
1-8 carbon atoms. Dialkylaminoethyl~met)acrylates, dialkyl-
aminomethyl(met)acrylamides and N, N-dialkylamino-propyl(met)-
acrylamides and their quaternary salts are preferred monomers.
Other suitable cationic monomers include diallyldialkylammonium
chlorides. The polymer may be either linear or cross-linked or
partly cross-linked. In this context, cationic polyacrylamides
also include the homopolymers of cationic acrylic monomers and
the mixed polymers of two or more cationic monomers, at least
one of the monomers being acrylic-based.
The aluminum salts used in the invention are water-soluble, and
they may be aluminum su~fate, aluminum chloride, aluminum
nitrate, or acid aluminum hydrophosphates in which P:Al = 1.1:1
- 3:1.
When these aluminum salts or their mixtures are used, a base is
added to form aluminum hydroxide having anionic surface
charges. The base used may be, for example, sodium or potassium
hydroxide, sodium or potassium carbonate, sodium or potassium
metasilicate, sodium or potassium waterglasses, sodium or
potassium phosphate or borate, or sodium or potassium alumi-
nate, or mixtures of these.
Aluminate compounds such as sodium aluminate or potassium alum-
inate can also be used as the water-soluble aluminum salts. In
this case, acid is added in order to form, within the pH range
7-9, an aluminum hydroxide having anionic surface charges. The
acid used may be mineral acids such as sulfuric acid, hydro-
chloric acid, nitric acid or phosphoric acid, or organic acids
such as oxalic acid, citric acid or tartaric acid. The acid
used may also be acid aluminum salts such as aluminum sulfate,
aluminum chloride, aluminum nitrate, or various water-soluble
aluminum hydrophosphates.
According to the invention it is also possible to use water-
soluble polymeric aluminum salts, i.e. p~lyaluminum salts, so-
W093/1~263 2 1;? 7 9 ~ 2 PCT/F193/00019
called basic aluminum salts, which are also called polyaluminumhydroxy sal.s or aluminum hydroxy salts. According to the in-
vention it is possible to use as these salts, for example poly-
aluminum sulfate, polyaluminum chloride and polyaluminum chlor-
ide sulfate..The polyaluminum salt may, in addition t~ thechloride and/or sulfate ion, also contain other anions, e.~.
phosphate, polyphosphate, silicate, citrate, oxalate, or
several of these.
Commercially available polymeric aluminum salts of this type
include PAC (polyalumin~m chloride), PAS (polyaluminum sul-
fate), UPAX 6 (silicate-containing polyaluminu~ chloride), and
PA~S (polyaluminum sulfate silicate).
The net formula of the water-soluble polyaluminum salt may be,
for example
n[A12(0H)m/cl~6-m]
and its alkalinity may vary so that the m-value ranges from l
to 5 (alkalinity is respectively 16 - 83 % according to the
formula (m:6) x l00). In this case the ratio Al/OH is 2:l -
l:2.5. n is 2 or higher.
When a polyaluminum compound is used, it is also possible to
add a base in order to optimize the Al/OH ratio, even if all of
the polyaluminum compounds in accordan~e with the invention do
work as such.
The said ~ase or acid which forms in situ an aluminum hydroxide
with the aluminum salt may be added to the fiber suspension,
for example before the adding of the cationic long-chain poly-
acrylamide, or just before the aluminum salt, or after it, or
simultaneously with it.
- The aluminum hydroxide may also be formed before the moment of
WO93/14263 PCT/F193/00019
21279~2 8
adding, for example in the adding tube, or in advance in sol
form.
The amount of the aluminum salt, calculated as Al2O3, is pref-
erably approx. 0.0l-l.0 % of the dry weight of the pu~p.
The paper pulp used may be bleached or unbleached sulfate or
sulite pulp, semichemical pulp, refiner mechanical pulp,
groundwood pulp, or mixtures of these. If a filler is present,
it is preferably ground or precipitated calcium carbonate, but
also other fillers such as kaolin, talc or titanium oxide are
possible.
The invention is described below in greater detail with the
help of examples.
In the tests described, the pH is approx. 8-8.5, normally
approx. 8 when a polyaluminum salt + CaCo3 or alum + a base are
used (the Al:OH ratio being approx. 4.5).
ExamPle 1
Using a Britt Dynamic Jar as the tester, tests were carried out
on a neutral pulp which was made up of bleached birch pulp and
bleached pine pulp at a ratio of 60:40. The pulp components had
been ground to SR values of 20 and 2S. The filler was calcium
carbonate, DX-40, 20 % of the dry weight of the pulp. The pH of
the pulp was approx. 8. In the tester the pulp was of a typical
headbox consistency, i.e. approx. 0.8 %. After the adding of
the retention agent, thé pulp was filtered for 30 s, and the
ash content was determined.
Tests were performed in this example by using the following
systems:
System (I):
500 ml of a dilute pulp was placed in the tester, at l000 rpm.
WO93/14263 PCT/Fl93/000l9
2127~2
After 10 s, polyacrylamide A was added for 5 s. After 10 s,
100 ml of filtrate was filtered for approx. 30 min.
System (II):
Pulp was added as in I, but a base had been added to it for
controlling the Al/OH molar ratio approx. 30 min before the
pulp was placed in the tester. After the polymer addition, the
rotation speed was increased to 1500 rpm for a period of 20 s,
whereafter it was returned to 1000 rpm, and alum A12(SO4)3 x
14H2O was added. After 5 s, a filtration was performed as in
System I.
System (III):
As System II, but without the addition of a base.
System (IV):
As System II, but without the additions of a base and alum.
System (V):
Was performed in accordance with System II, but without the
addition of a base. Instead of the polyacrylamide, a cationic
starch, Raisamyl 135, havin~ a degree of substitution of 0.035,
was added and was mixed in the same manner as the polymer in
System II. Silica sol BMA (Eka Nobel) was used instead of alum.
System (VI):
Was performed as System II, but without the addition of a base
to the pulp. Alkali-treated bentonite was added instead of alum
(Hydrocol method).
. ~. . .
The polyacrylamides A and B in the examples are copolymers of
acrylamide and methyl-chloride quaternized dimethylaminoethyl
acrylate. Their charge densities and molecular weights are (A)
1 mequiv./g:7 106 and (B) 1.5 mequiv./g:6-10~.
Systems I-VI are compared in Table 1.
WO93/1~263 . PCT/Fl93/000l9
21~7~2 lo
Table I
Test System Polymer/ Alum Al/OH BMA Bento- Filler
No. batch kg/t (100%) nite reten-
kg/t kg/t tion
%
.
1 0 test 3
I A 49
300 g/t
3 II (ac- A 5 1:3 63
cording 1000 g/~
to inv.)
4 II (ac- " 5 1:4.5 61
cording
to inv.)
II (ac- " 10 1:3 68
cording
to inv.)
6 II (ac- " 10 1:4.5 81
cording
to inv.)
7 II (ac- " 10 1:6 58
cording
to inv.~
8 III " 5 1:0 56
9 III " 10 l:Q 64
II (ac- B 10 1:3 70
cording 1000 g/t
to inv.)
11 II (ac- " 10 1:4~5 83
cording
to inv.)
12 II (ac- B 10 1: 6 64
cording
to inv ~ )
13 III " 10 1:0 66
14 IV A - 58
1000 g/t
WO93/14263 212 7 9 .~ 2 PCT/F193/00019
Test System Polymer/ Alum Al/OH BMA Bento- Filler
No. batch kg/t (100%) nite reten-
kg/t kg/t tion
%
_
IV B - 52
l000 g/t
16 V Raisamyl 2 48
135
5 kg/t
17 V Raisamyl 2 59
135
10 kg/t
18 VI Hydrocol 1 50
~62
500 g/t
l9 VI Hydrocol 2 57
~62
1000 g/t
xample 2
This example shows that the proceiss according to the invention
improves retention in a paper pulp which contains a cationic
pulp starch. The pulp composition is in other respects similar
to that in the previous example. The test series was performed
in a Britt Dynamic Drainagei Jar. The batching methods comply
with the methods described in Example 1. The degree of sub-
~titution of the cationic starch was 0.03S. The starch was
added 15 min before the polyacrylamide, and the NaOH for pre-
liminary alkalization S min before the polyacrylamide. In this
example, the same polyacrylamides A and B were used as in
Example 1.
WO93/1~263 PCT/F193/00019
2127992 12
T~st Polymer/ Aluminum OH:Al Starch Ash re- Batching
No. batch sulfate kg/t tention method
kg/t kg/t %
l A/0.3 52
2 A/0.5 40 IV-
3 A/0.5 4 37 IV
4 A/0.5 5 4 42 III
A/0.5 5 4.5 4 48 II (accord-
ing to
invention)
6 A/0.5 5 2.25 4 46 II (accord-
ing to
invention)
7 A/0.5 3 4.5 4 47 II (accord-
ing to
invention~
8 A/0.S 3 2.25 4 44 II (accord-
ing to
invention)
9 B/0.5 39 IV
B/0.5 5 4 43 III
ll B/0.5 5 4.5 4 50 II (accord-
ing to
inven~ion~
12 B/0.5 3 4 ~2 III
13 B/0.5 3 4.5 4 48 II (accord-
in to
invention)
Example 3
Further retention tests were performed as in the above ex-
amples. The aluminum salt used was aluminum sulfate or a poly-
aluminum chloride product. The chemical formula of polyaluminum
c~loride (PAC) is ~ln(OH)mCl~3n-m). It is made up of a number
of aluminum nuclei. The pulp was similar to that in the pre-
vious examples. The polyaluminum chloride was batched in a man-
WO93~1`1263 212 7 9 ~ 2 PCT/F193/00019
ner similar to that of aluminum sulfate. The difference wasthat the pre-alkalization was omitted. The ratio OH:Al in the
following table indicates, in addition to the degree of pre-
alkalizati~n, also the alkalinity of the polyaluminum product.
The polyacrylamide used was the same polyacrylamide A as in
Example l. The batching methods were as in Example l.
Test Polymer/ Aluminum source OH:Al Ash re- Batching
No. batch l:Al sulfate tention method
kg/t 2:PAC %
~g/t
l 0.3 59
2 l.0 57 IV
3 l.O l/l0 4.5 8l II (according
to invention)
4 l.0 2/5.5 l.3 79 III (according
to invention)
l.0 2/5.l 2 84 III (according
to invention)
Example 4
This example shows that the action of polyacrylamide can be
enhanced by batching before it another polymer for binding
interfering substances. In this case a short-chain cationic
polymer (QPOL) was added as a so-called fixing agent before the
long-chain polyacrylamide. The product concerned had a par-
ticularly high charge density. It was added l0 min before the
polyacrylamide, by stirring slowly. The polyacrylamide was the
same as in Example 2. The batching methods were as in Example
1.
: ~ .
WO93/1~263 PCT/F193/00019
~ 7 9~ 2 14
Test Polymer/ Aluminum OH:Al QPOL Ash re- Batching
No. batch sulfate kg/t tention method
kg/t kg/t %
l l.0 l0 4.5 - 52 II taccord-
in~ to
- invention)
2 l.0 l0 4.5 l.0 68 II (accord-
ing to
invention)
Example 5
The process according to the invention works also when poly-
aluminum chloride (PAC) is used as the fixing agent before the
polyacrylamidè. In this case the test conditions are similar to
those in Example 4, except that, instead of a quaternary poly-
mer (QPOL~, the polyaluminum chloride product used in Example 3
was batched. The batching method was according to Example 3.
Test Polymer/ Aluminum OH:Al PAC Ash re- Batching
No. batch sulfate kg/t tention method
kg/t kg/t %
l I/l.0 l0 4.5 - 52 II (accord-
ing to
invention)
2 I/l.0 l0 4.5 2.5 63 II (accord-
ing to
invention)
ExamPle 6
By the process according to the invention, good dewatering
properties are achieved with wood-free fine-paper pulp. The
pulps and batching methods were in accordance with Example l.
The dewatering rate was measured by means of a cylindrical
tube. At the other end of the tube there was a wire through
which the dewatering took place. Before filtration, this tube
was used for adding the chemicals to the pulp in the manner
described in ths previous example, by using a Britt Jar Tester.
Thereafter the pulp was poured into a dewatering cylinder and
was filtered. The removed filtrate was measured as a function
of the time. The pulp was of a type similar to that~in the
WO93/1~263 212 7 9 ~ 2 PCT/Fl93/00019
15 ~
previous examples. In the filtrations, 500 ml of pulp per test-
ing point was used.
Test Polymer/ Aluminum OH:Al BMA Bento- Batching Dewater-
No. batch sulfatea) nite method ing time
kg/t (1) kg/t b~ s/250 ml
PAC (2)kg/t
1 A/0.3 I 45
2 A/1.0 IV 38
3 A~1.0 (1)10 III 36
4 A/1.0 (1~10 3 II (ac~ 33
cordin~
to inv.)
aa)C+A~1 (1)10 4.5 II (ac- 27
cording
to inv.)
6 bb)D+A/1+1 (2)10 4.5 II (ac- 31
cording
to inv.)
7 A/1.0 ~2)5.1 2 III (ac- 30
cording
to inv.)
8 D+A/1+1 (2)5.1 2 III (ac- 30
cording
to inv~)
9 C/10 2 V 36
E/1.0 l VI 30
a) silica sol, a commercial product
b) alkali-treated bentonite, a commercial product
aa) starch added 10 min before the polyacrylamide
bb) quaternary polymer added 10 min before the polyacrylamide
Polymer A: Polyacrylamide A, see Example 1
C: Cationic potato star h D.S. 0.035
D: Quaternary polyamine
~ B: Hydrocol 862
.
WO93/1~263 PCT/F193/OOOlg
21 279~2
16
Example 7
It is shown that the process according to the invention works
also when certain other aluminum salts are used. In this
example, polyaluminum salts were used which contained silica
groups in addition to chloride, or sulfate instead of chloride.
The fiber composition in the pulp was similar to that in
Example 2. The calcium carbonate concentration was 30 %. A
cationic polyacrylamide was added to the pulp in a Britt Jar
Tester, and it was mixed for 20 s at 1500 min 1. Thereafter the
aluminum salt was added and was mixed for 10 s at 1000 min 1.
In the filtering stage the rotation speed was 750 rpm. The ash
retention was calculated on the basis of the ash contents of
the pulp and the filtrate. The molecular weight of the cationic
polyacrylamide was approx. 7 million gtmol and its charge
density 1 meguiv./g. Compound A is a silicate-containing poly-
aluminum chloride and B is polyaluminum sulfate.
Test Polymer/ Aluminum Compound Ash retention
No. batch salt %
g/t ~g/t
1 1000 - 47
2 1000 2 A 56
3 1000 3 A 58
4 1000 5 A 71
1000 2.5 B 74
6 1000 5.0 ~ 77
. ~
