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Description  |
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The present invention relates to a process for improved dewatering and
retention in the paper, where a retention agent containing anionic groups
and being based on a polysaccharide or being an acrylamide-based polymer,
and an alkaline solution of an aluminate are added to the stock containing
lignocellulose-containing fibres and optionally fillers. The pH of the
stock prior to the addition of the aluminate should be below about 7 to
obtain the desired cationic aluminium hydroxide complexes in the stock.
The present invention is cost effective and insensitive to the content of
calcium in the white water.
BACKGROUND
In the production of paper, a stock consisting of papermaking fibres, water
and normally one or more additives is brought to the headbox of the paper
machine. The headbox distributes the stock evenly across the width of the
wire, so that a uniform paper web can be formed by dewatering, pressing
and drying. The pH of the stock is important for the possibility to
produce certain paper qualities and for the choice of additives. A large
number of paper mills throughout the world have changed, in the last
decade, from acidic stocks to neutral or alkaline conditions. However,
this change sometimes requires expensive investments for which reason
several mills are still manufacturing paper under acidic conditions.
In the production of paper, improved dewatering and retention are desired.
Improved dewatering (drainage) means that the speed of the paper machine
can be increased and/or the energy consumption reduced in the following
pressing and drying sections. Furthermore, improved retention of fines,
fillers, sizing agents and other additives will reduce the amounts added
and simplify the recycling of white water.
Fibres and most fillers--the major papermaking components--carry a negative
surface charge by nature, i.e. they are anionic. It is previously known to
improve the dewatering and retention effect by altering the net value and
distribution of these charges. Commonly, starch where cationic groups have
been introduced, has been added to the stock because of its strong
attraction to the anionic cellulose-containing fibres. This effect has,
however, been reduced in mills where the white water is hard, due to the
competition for the anionic sites between the cationic starch and calcium
ions. For most effective results, it has been thought that there must be a
suitable balance between cationic and anionic groups in the starch.
Starches, where both cationic and anionic groups are introduced are termed
amphoteric and are well known in papermaking.
It is previously known to combine starch with aluminium compounds to
further improve the effect. In P. H. Brouwer, Tappi Journal, 74(1), pp.
170-179 (1991) alum is combined with anionic starch to improve the
dewatering as well as gloss and strength of packaging paper. In this case
the pH of the pulp as well as the white water is 4.4 and the addition of
alum 50 kg/ton of pulp.
THE INVENTION
The invention relates to a process for improved dewatering and retention of
fines, fillers, sizing agents and other additives in the manufacture of
paper, where a retention agent containing anionic groups and an aluminate
are added to the stock of lignocellulose-containing fibres.
The invention thus concerns a process for the manufacture of paper on a
wire by forming and dewatering a stock of lignocellulose-containing
fibres, and optional fillers, whereby a retention agent containing anionic
groups, where said retention agent is based on a polysaccharide or is an
acrylamide-based polymer, and an alkaline solution of an aluminate are
added to the stock, which stock prior to the addition of the aluminate has
a pH in the range of from about 3 up to about 7.
According to the present invention it has been found that by adding an
alkaline solution containing an aluminate to a stock with a pH below about
7, it is possible to get an interaction between the cationic aluminium
hydroxide complexes developed in the stock and the anionic groups of the
retention agent and cellulose fibres.
As stated above, conventionally starch where cationic groups have been
introduced is used in papermaking. It is advantageous, however, to use
starch containing anionic groups since it is much easier and less
expensive to introduce anionic groups, such as phosphate groups, than it
is to introduce cationic ones, such as tertiary amino or quaternary
ammonium groups. According to the present invention it has been found that
a retention agent containing anionic groups, which is suitably a starch
containing anionic groups, in combination with an alkaline solution
containing an aluminate, gives improved and cost effective dewatering and
retention in acidic stocks.
The components can be added to the stock in arbitrary order. Preferably the
cationic aluminium hydroxide complexes are developed in the presence of
lignocellulose-containing fibres. Therefore, the invention especially
relates to addition of a retention agent and an aluminate to a stock of
lignocellulose-containing fibres, where the addition is separated from the
addition of an optional filler. Preferably also, the addition of retention
agent to the stock is separated from the addition of aluminate to said
stock. A considerable improvement, in comparison with prior art technique,
is obtained when the retention agent containing anionic groups is first
added and then the aluminate. However, the best effect is obtained if the
aluminate is first added to the stock followed by the retention agent
containing anionic groups. When an anionic inorganic colloid is added to
the stock in addition to the aluminate and in this case a retention agent
also containing cationic groups, it is suitable to add said colloid after
the addition of aluminate. Preferably the aluminate is added first
followed by the retention agent and as the third component the anionic
inorganic colloid.
A retention agent used in the present process is based on a polysaccharide,
from the groups of starches, cellulose derivatives or guar gums, or is an
acrylamide-based polymer. The retention agent containing anionic groups,
has negatively charged (anionic) groups, optionally with positively
charged (cationic) groups. The cellulose derivatives are e.g. carboxyalkyl
celluloses such as carboxymethyl cellulose (CMC). Suitably the retention
agent based on a polysaccharide is a starch containing anionic groups.
The acrylamide-based polymers used in the process of the invention are
water soluble polymers which contain acrylamide and/or methacrylamide as
the main monomeric component. The acrylamide-based polymers contain
anionic groups and optionally cationic groups, i.e. the acrylamide-based
polymers are either anionic or amphoteric. Preferably the acrylamide-based
polymers are anionic. The acrylamide-based polymers suitably have an
average molecular weight of from about 10,000 up to about 30,000,000 and
preferably from 500,000 up to 20,000,000. The acrylamide-based polymers
can be produced by introduction of ionic groups in a polymer containing
(meth)acrylamide as the main component. In a polymer containing
(meth)acrylamide as the main component anionic groups can be introduced
for example by hydrolysis or sulfomethylation reaction, while optional
cationic groups can be introduced for example by Hofmann degradation and
Mannich reaction. Anionic acrylamide-based polymers can also be prepared
by copolymerization of (meth)acrylamide and anionic monomers. Examples of
anionic monomers are .alpha.,.beta.-unsaturated carboxylic acids and
monomers containing sulfonic acid groups or phosphoric acid groups.
Amphoteric acrylamide-based polymers can be prepared by copolymerization
of (meth)acrylamide and a monomer mixture containing both cationic
monomers and anionic monomers. The amphoteric polymers can also be
prepared by introduction of cationic groups into a copolymer of
(meth)acrylamide and anionic monomers or by introduction of anionic groups
into a copolymer of (meth)acrylamide and cationic monomers. The
acrylamide-based polymers can have an anionic degree of substitution (DS)
of from about 0.5 up to about 100%, suitably from 1.5 up to 90% and
preferably from 3 up to 80%.
Although the advantages of the present invention can be obtained with any
of the retention agents containing anionic groups and where the retention
agent is based on a polysaccharide or is an acrylamide-based polymer, the
present invention will be described in the following specification with
respect to the use of starch containing anionic groups.
The anionic groups of the starch, which can be native or introduced by
chemical treatment, are suitably phosphate, phosphonate, sulphate,
sulphonate or carboxylic acid groups. Preferably the groups are phosphate
ones due to the relatively low cost to introduce such groups. Furthermore,
the high anionic charge density increases the reactivity towards the
cationic aluminium hydroxide complexes. The cationic groups are suitably
nitrogenous groups, such as tertiary amino or quaternary ammonium groups.
The presence of cationic groups is necessary to obtain an increase in
dewatering and retention effect when adding an anionic inorganic colloid.
The amount of anionic groups, especially the phosphate ones, in the starch
influences the dewatering and retention effect. The overall content of
phosphorus in the starch is a poor measure of the anionic groups, since
the phosphorus is inherent in the covalently bonded phosphate groups as
well as in the lipids. The lipids are a number of fatty substances, where
in the case of starch, the phospholipids and especially the
lysophospholipids are important. The content of phosphorus, thus, relates
to the phosphorus in the phosphate groups covalently bonded to the
amylopectin of the starch. Suitably the content of phosphorus lies in the
range of from about 0.01 up to about 1% phosphorus on dry substance. The
upper limit is not critical but has been chosen for economic reasons.
Preferably the content lies in the range of from 0.04 up to 0.4%
phosphorus on dry substance.
The starch containing anionic groups can be produced from agricultural
products such as potatoes, corn, barley, wheat, tapioca, manioc, sorghum
or rice or from refined products such as waxy maize. The anionic groups
are native or introduced by chemical treatment. Suitably potato starch is
used. Preferably native potato starch is used, since it contains an
appreciable amount of covalently bonded phosphate monoester groups
(between about 0.06 and about 0.10% phosphorus on dry substance) and the
lipid content is very low (about 0.05% on dry substance). Another
preferred embodiment of the invention is to use phosphated potato starch.
The aluminate used according to the present invention is per se previously
known for use in papermaking. Any aluminate which can be hydrolyzed to
cationic aluminium hydroxide complexes in the stock can be used. Suitably
the aluminate is sodium aluminate or potassium aluminate. Preferably the
aluminate is sodium aluminate.
The effect of the addition of an aluminate is very dependant on the pH of
the stock as well as the solution containing the aluminate. According to
the invention, the addition of the aluminate at a pH of the stock in the
range of from about 3 up to about 7 increases the dewatering speed and
degree of retention markedly. Prior to the addition of the aluminate, the
pH of the stock lies suitably in the range of from 3.5 up to 7 and more
suitably in the range of from 3.5 up to 6.5. Prior to the addition of the
aluminate, the pH of the stock lies preferably in the range of from 4.0 up
to 6.5 and more preferably in the range of from 4.0 up to 6.0.
Depending on the buffering effect of the stock, the pH of the stock after
the addition of aluminate should be in the range from about 3.5 up to
about 7. Suitably, after the addition of aluminate the pH of the stock
lies in the range of from 4.0 up to 6.5. Preferably, after the addition of
aluminium compound the pH of the stock lies in the range of from 4.0 up to
6.0.
When the alkaline solution of aluminate is added to the acidic stock,
suitably the pH of the solution is at least about 11 and preferably the pH
lies in the range of from 12 up to 14 for the cationic aluminium hydroxide
complexes to be developed.
The cationic charge of the various aluminium hydroxide complexes developed
decreases with time, an effect which is especially pronounced when the
content of calcium in the white water is low. The loss of cationic
character especially influences the retention of fines and additives but
the dewatering is also influenced. Therefore, it is very important that
the aluminate is added shortly before the stock enters the wire to form
the paper. Suitably, the aluminate is added to the stock less than about 5
minutes before the stock enters the wire to form the paper. Preferably,
the aluminate is added to the stock less than 2 minutes before the stock
enters the wire to form the paper.
The added amount of a retention agent based on a polysaccharide, can be in
the range of from about 0.05 up to about 10 percent by weight, based on
dry fibres and optional fillers. Suitably the amount of a retention agent
based on a polysaccharide, lies in the range of from 0.1 up to 5 percent
by weight and preferably in the range of from 0.2 up to 3 percent by
weight, based on dry fibres and optional fillers.
The added amount of a retention agent being an acrylamide-based polymer,
can be in the range of from about 0.005 up to about 2 percent by weight,
based on dry fibres and optional fillers. Suitably the amount of an
acrylamide-based polymer, lies in the range of from 0.01 up to 1.5 percent
by weight and preferably in the range of from 0.02 up to 1.0 percent by
weight, based on dry fibres and optional fillers.
The amount of aluminate added can be in the range from about 0.001 up to
about 0.5 percent by weight, calculated as Al.sub.2 O.sub.3 and based on
dry fibres and optional fillers. Suitably the amount of aluminate added
lies in the range of from 0.001 up to 0.2 percent by weight, calculated as
Al.sub.2 O.sub.3 and based on dry fibres and optional fillers. Preferably
the amount of aluminate added lies in the range of from 0.005 up to 0.15
percent by weight, calculated as Al.sub.2 O.sub.3 and based on dry fibres
and optional fillers.
In paper mills where the content of calcium and/or magnesium ions in the
white water is high, it is often difficult to produce efficiently paper of
good quality. In papermaking, normally the content of magnesium is low,
reducing the problem to comprise the presence of calcium ions only. In the
case of white water these positive ions can have their origin in the tap
water, in additives like gypsum and/or in the pulp, e.g. if a deinked one
is used. The calcium ions are adsorbed onto the fibres, fines and fillers,
thereby neutralizing the anionic sites. The result is restricted swelling
of the fibres giving poor hydrogen bonding and thus paper of low strength.
Furthermore, the effect of cationic dewatering and retention agents added
is reduced since the possibility of electrostatic interaction has been
restricted.
The present invention can be used in papermaking where the calcium content
of the white water varies within wide limits. However, the improvement in
dewatering and retention of fines and additives compared to prior art
techniques increases with the calcium content, i.e. the present process is
insensitive to high concentrations of calcium. Therefore, the present
invention is suitably used in papermaking where the white water contains
at least about 50 mg Ca.sup.2+ /liter. Preferably the white water contains
from 100 mg Ca.sup.2+ /liter and the system is still effective at a
calcium content of 2000 mg Ca.sup.2+ /liter.
In paper production according to the invention, additives of conventional
types can be added to the stock. Examples of such additives are fillers,
sizing agents and anionic inorganic colloids. Examples of fillers are
China clay, kaolin, talcum, gypsum and titanium dioxide. The fillers are
usually added in the form of a water slurry in conventional concentrations
used for such fillers. An example of a sizing agent that can be used under
acidic conditions is colophony rosin.
In paper production according to the invention, also conventional anionic
inorganic colloids can be added to the stock. A prerequisite that such an
addition brings about an effect on dewatering and retention is the
presence of cationic groups in the retention agent used. The colloids are
added to the stock as dispersions, commonly termed sols, which due to the
large surface to volume ratio avoids sedimentation by gravity. The terms
colloid and colloidal indicate very small particles. The particles of the
anionic inorganic substances should suitably have a specific surface area
above about 50 m.sup.2 /g. Examples of such colloids are bentonite,
montmorillonite, titanyl sulphate sols, silica sols, aluminium modified
silica sols or aluminium silicate sols. Suitably, the anionic inorganic
colloids are silica based colloids. Particularly suitable silica based
colloids are the aluminium containing silica sols which are disclosed in
the European patent 185,068, which is hereby incorporated by reference in
this application. Preferably the silica based colloids have at least one
surface layer of aluminium silicate or aluminium modified silica, since
the aluminium-containing surface layer makes the colloids more resistant
under the acidic conditions of the present invention. Also the aluminium
modified silica sols disclosed in the PCT application WO 90/00689 are
suitable for addition to an acidic stock according to the invention. Here,
the aluminium modification of the particles is carried out to a surface
modification degree of from 2 up to 25 percent, where the modification
degree is the number of aluminium atoms which has replaced silicon atoms
in the particle surface.
The colloidal silica particles in the sols should preferably have a
specific surface area of from about 50 up to about 1000 m.sup.2 /g and
more preferably from 100 up to 1000 m.sup.2 /g. It has been found that the
colloidal silica particles should suitably have a particle size below 20
nm and preferably from about 10 down to about 1 nm (a colloidal silica
particle having a specific surface area of about 550 m.sup.2 /g
corresponds to an average particle size of about 5 nm). Silica sols which
fulfil the above given specifications are available commercially, e.g.
from Eka Nobel AB in Sweden.
Suitable sols can also be based on polysilicic acid, which means that the
material of silicic acid exists as very small particles, in the order of 1
nm and with a very large specific area, above 1000 m.sup.2 /g and up to
about 1700 m.sup.2 /g and with some degree of microgel formation. Such
sols are described in the Australian patent 598,416.
The amount of anionic inorganic colloid added can be in the range of from
about 0.005 up to about 1.0 percent by weight, based on dry fibres and
optional fillers. Suitably the amount of the anionic inorganic colloid
lies in the range of from 0.005 up to 0.5 percent by weight and preferably
in the range of from 0.01 up to 0.2 percent by weight, based on dry fibres
and optional fillers.
In paper production according to the invention, also conventional cationic
inorganic colloids can be added to the stock. Examples of such positively
charged colloids are aluminium oxide sols and surface modified silica
based sols. Suitably the colloids are silica based sols. These sols can be
prepared from commercial sols of colloidal silica and from silica sols
consisting of polymeric silicic acid prepared by acidification of alkali
metal silicate. The sols are reacted with a basic salt of a polyvalent
metal, suitably aluminium, to give the sol particles a positive surface
charge. Such colloids are described in the PCT application WO 89/00062.
The suitable amount of cationic inorganic colloid added and order of its
addition to the stock corresponds to what is given for the anionic
inorganic colloids.
The effect of anionic silica based colloids added is most pronounced where
the calcium content of the white water is limited, while the effect of
cationic silica based colloids is good even where the calcium content of
the white water is high.
The addition of the solution containing aluminate can also be divided
into-two batches, to counteract the influence of the so called anionic
trash. The trash tend to neutralize added cationic compounds before they
reach the surface of the anionic fibres, thereby reducing the intended
dewatering and retention effect. Therefore, a part of the solution
containing aluminate can be added long before the stock enters the wire to
form the paper, to have sufficient time to act as an anionic trash catcher
(ATC). The rest of the solution is added shortly before the stock enters
the wire, so as to develop and maintain the cationic aluminium hydroxide
complexes which can interact with the anionic groups of the retention
agent and cellulose fibres. For example, 30% of the amount of aluminium
compound in the solution containing the aluminium compound can be used as
an ATC and the remaining 70% of the amount of aluminium compound to form
the cationic complexes.
Production of paper relates to production of paper, paperboard, board or
pulp in the form of sheets or webs, by forming and dewatering a stock of
lignocellulose-containing fibres on a wire. Sheets or webs of pulp are
intended for subsequent production of paper after slushing of the dried
sheets or webs. The sheets or webs of pulp are often free of additives,
but dewatering or retention agents can be present during the production.
Suitably, the present process is used for the production of paper,
paperboard or board.
The present invention can be used in papermaking from different types of
lignocellulose-containing fibres. The retention agent and aluminate can
for example be used as additives to stocks containing fibres from chemical
pulps, digested according to the sulphite, sulphate, soda or organosolv
process. Also, the components of the present invention can be used as
additives to stocks containing fibres from chemical thermomechanical pulps
(CTMP), thermo-mechanical pulps (TMP), refiner mechanical pulps,
ground-wood pulps or pulps from recycled fibres. The stock can also
contain fibres from modifications of these processes and/or combinations
of the pulps, and the wood can be softwood as well as hardwood. Suitably
the invention is used in papermaking of stocks containing fibres from
chemical pulps. Suitably, also, the fibre content of the stock is at least
50 percent by weight, calculated on dry substance.
The invention and its advantages are illustrated in more detail by the
following examples which, however, are only intended to illustrate the
invention and not to limit the same. The percentages and parts stated in
the description, claims and examples, relate to percent by weight and
parts by weight, respectively, unless otherwise stated.
EXAMPLE 1
In the following tests the dewatering for stocks has been determined with a
"Canadian Standard Freeness (CSF) Tester" according to SCAN-C 21:65, after
the addition of the retention agent containing anionic groups and the
alkaline solution containing aluminate. Some tests were also carried out
after the addition of other or further components, such as an amphoteric
potato starch, a polyaluminium chloride, alum and/or an anionic silica
based colloid. The stock was agitated at 800 rpm when the components were
added and the residence time for each component was throughout 45 seconds
for the first one and 30 seconds for the second one. In the tests where
three components were used, the residence time for the last component was
15 seconds. The pulp consistency was 0.3% by weight of dry substance.
After addition of the two or three components the flocculated stock was
passed to the CSF tester and measurements made 35 and 20 seconds,
respectively, after the last addition. The collected water is a measure of
the dewatering effect and given as ml CSF.
The collected water was very clear after the addition of the components
showing that a good retention effect of the fines to the fibre flocks had
been obtained by the process according to the invention.
The stock consisted of fibres from a sulphate pulp of 60% softwood and 40%
hardwood refined to 200 ml CSF, with 30% of China clay as filler.
The pH of the solution containing sodium aluminate was 13.5, as read from
the pH meter.
The polyaluminium chloride (PAC) used was Ekoflock from Eka Nobel AB in
Sweden, with a basicity of about 25% and a sulphate and aluminium content
of about 1.5 and 10% by weight, respectively, where the content of
aluminium was calculated as Al.sub.2 O.sub.3. The pH of the solution
containing PAC was about 1.7, as read from the pH meter.
The starches used were prepared by cooking at 95.degree. C. for 20 minutes.
The consistency of the starch solutions prior to the addition to the stock
were 0.5% by weight in all experiments.
Table I shows the results from dewatering tests where sodium aluminate was
added to the stock followed by various amounts of native potato starch.
The amount of aluminate added, was 1.3 kg calculated as Al.sub.2 O.sub.3
per ton of dry stock including the filler. The additions of aluminate were
made at a stock pH of 4.2 and 5.0. For comparison, only native potato
starch was added to the stock at a stock pH of 4.2 and 5.0. For further
comparison, in two series of experiments polyaluminium chloride (PAC) and
alum were added at a stock pH of 4.2, followed by native potato starch.
The amount of PAC and alum added, were 1.3 kg calculated as Al.sub.2
O.sub.3 per ton of dry stock including the filler. The content of calcium
was 20 mg/liter. Prior to the addition of the additives, the dewatering
effect of the stock with filler was 295 ml CSF. The results in ml CSF are
given in Table I.
TABLE I
______________________________________
Starch, kg/ton of dry stock
Additives pH 5 10 15
______________________________________
NPS (comp.) 4.2-5.0 255 255 250 ml CSF
AlNa + NPS 4.2 355 435 455 ml CSF
AlNa + NPS 5.0 325 365 370 ml CSF
PAC + NPS (comp.)
4.2 265 265 260 ml CSF
Alum + NPS (comp.)
4.2 275 310 310 ml CSF
______________________________________
wherein NPS = native potato starch
AlNa = sodium aluminate
PAC = polyaluminium chloride
Alum = aluminium sulphate
As can be seen from Table I, the addition of sodium aluminate in
combination with native potato starch at a pH within the pH range of the
invention enhances the dewatering. The dewatering effect with aluminate is
improved when the added amount of starch is increased, especially at a low
pH. Furthermore, the use of aluminate and native potato starch is much
more efficient than combinations of PAC or alum with native potato starch.
Also, at a pH of 4.2 the addition of alum and native potato starch means a
reduced or essentially unaltered dewatering effect as compared to the
dewatering effect of the stock itself.
EXAMPLE 2
Table II shows the results from dewatering tests with the same stock as in
Example 1, where sodium aluminate was added to the stock followed by
native potato starch. The amount of sodium aluminate added, was 1.3 kg
calculated as Al.sub.2 O.sub.3 per ton of dry stock including the filler.
The amount of starch added, was 15 kg per ton of dry stock including the
filler. The additions of aluminate were made at a stock pH of 4.2. The
calcium content was 20 and 640 mg/liter of white water. For comparison,
only native potato starch was added to the stock at a stock pH of 4.2. The
results in ml CSF are given below.
TABLE II
______________________________________
Calcium content,
mg/liter of white water
Additives 20 640
______________________________________
Only stock 295 315 ml CSF
NPS (comp.) 250 280 ml CSF
AlNa + NPS 455 485 ml CSF
______________________________________
wherein NPS = native potato starch
AlNa = sodium aluminate
As can be seen from Table II, the addition of sodium aluminate in
combination with native potato starch at a pH within the pH range of the
invention enhances the dewatering at a calcium content of 20 as well as
640 mg/liter. The dewatering is more efficient at 640 mg Ca.sup.2+ /liter,
which is a very hard water.
EXAMPLE 3
Table III shows the results from dewatering tests, where sodium aluminate
was added to a stock followed by native potato starch. The stock was the
same as the one used in Example 1, except that 30% calcium carbonate was
used as filler. The amount of sodium aluminate added, was 1.3 kg
calculated as Al.sub.2 O.sub.3 per ton of dry stock including the filler.
The amount of starch added, was 15 kg per ton of dry stock including the
filler. The additions of aluminate were made at a stock pH of 6.5. The
calcium content was 20 and 640 mg/liter of white water. For comparison,
only native potato starch was added to the stock at a stock pH of 6.5. The
results in ml CSF are given below.
TABLE III
______________________________________
Calcium content,
mg/liter of white water
Additives 20 640
______________________________________
Only stock 320 325 ml CSF
NPS (comp.) 275 280 ml CSF
AlNa + NPS 390 415 ml CSF
______________________________________
wherein NPS = native potato starch
AlNa = sodium aluminate
As can be seen from Table III, the addition of sodium aluminate in
combination with native potato starch at a pH of 6.5 enhances the
dewatering at a calcium content of 20 as well as 640 mg/liter.
EXAMPLE 4
Table IV shows the results from dewatering tests where sodium aluminate,
amphoteric potato starch and an anionic silica based colloid were added to
a stock consisting of bleached fibres from a sulphate pulp of 50% softwood
and 50% hardwood refined to 360 ml CSF with 30% China clay as filler. The
anionic silica based colloid was an aluminium modified silica sol sold by
Eka Nobel under the tradename BMA-9, with a specific surface area of 550
m.sup.2 /g and a mean particle size of 5 nm. The amount of starch and
silica based colloid added, were 15 kg/ton of dry stock and 2 kg/ton of
dry stock, respectively. The amount of aluminate added was 1.3 kg
calculated as Al.sub.2 O.sub.3 per ton of dry stock including filler. The
amount of cationic and anionic, native groups in the amphoteric starch
were about 0.35% N and 0.08% P, respectively. The additions of aluminate
were made at a stock pH of 4.1. The calcium content was 20, 160 and 640
mg/liter of white water. For comparison, polyaluminium chloride,
amphoteric potato starch and the anionic silica based colloid were added
to the stock. The addition of PAC was made at a stock pH of 4.1. The
results in ml CSF are given below.
TABLE IV
______________________________________
Calcium content,
mg/liter of white water
Additives 20 160 640
______________________________________
Only stock 450 475 480 ml CSF
AlNa + APS 620 600 590 ml CSF
APS + AlNa 515 535 535 ml CSF
AlNa + APS + BMA 645 630 610 ml CSF
BMA + APS + AlNa 555 550 540 ml CSF
PAC + APS + BMA (comp.)
520 525 -- ml CSF
______________________________________
wherein AlNa = sodium aluminate
APS = amphoteric potato starch
BMA = anionic silica based colloid
PAC = polyaluminium chloride
As can be seen from Table IV, the addition of sodium aluminate and
amphoteric potato starch increases the dewatering effect considerably,
especially if the aluminate is added first. When the anionic silica based
colloid is added the effect is further increased, especially if the
colloid is added as the last component. Also, the use of aluminate with
amphoteric potato starch and silica based colloid is much more efficient
than combinations of PAC with amphoteric potato starch and silica based
colloid. The dewatering effect is only slightly reduced as the calcium
content is increased.
EXAMPLE 5
Table V shows the results of retention tests, where sodium aluminate,
amphoteric potato starch and an anionic silica based colloid were added to
the same stock as used in Example 4. The retention of filler was
determined with a retention sheet former, developed to determine the total
and filler retention within the paper industry at the Centre Technique de
l'Industrie des P | | |
