U.S. patent application number 10/007861 was filed with the patent office on 2002-10-10 for silica-based sols.
Invention is credited to Dahlgren, Maj-Lis, Johansson-Vestin, Hans, Persson, Michael, Tokarz, Marek.
Application Number | 20020147240 10/007861 |
Document ID | / |
Family ID | 27514522 |
Filed Date | 2002-10-10 |
United States Patent
Application |
20020147240 |
Kind Code |
A1 |
Persson, Michael ; et
al. |
October 10, 2002 |
Silica-based sols
Abstract
A process for the production of an aqueous sol containing
silica-based particles which comprises (a) acidifying an aqueous
silicate solution to a pH of from 1 to 4 to form an acid sol; (b)
alkalizing the acid sol at an SiO.sub.2 content within the range of
from 4.5 to 8% by weight; (c) allowing particle growth of the
alkalized sol for at least 10 minutes; or heat-treating the
alkalized sol at a temperature of a least 30.degree. C.; (d)
alkalizing the obtained sol to a pH of at least 10.0; and (e)
optionally concentrating the sol obtained according to (b), (c) or
(d) to provide an aqueous sol containing silica-based particles and
having a specific surface area of at least 90 m.sup.2/g aqueous
sol; as well as an aqueous sol containing silica-based particles
obtainable by the process. The invention also relates to an aqueous
sol containing silica-based particles which sol has a specific
surface area of at least 115 m.sup.2/g aqueous sol and an S-value
within the range of from 10 to 45% or contains silica-based
particles having a specific surface area of at least 550 and less
than 1000 m.sup.2/g SiO.sub.2. The invention further relates to the
use of the aqueous sol containing silica-based particles as a
drainage and retention aid in the production of paper as well as a
process for the production of paper from an aqueous suspension
containing cellulosic fibers, and optional filler, in which
silica-based particles and at least one charged organic polymer are
added to the cellulosic suspension.
Inventors: |
Persson, Michael; (Vastra
Frolunda, SE) ; Tokarz, Marek; (Kungalv, SE) ;
Dahlgren, Maj-Lis; (Nodinge, SE) ; Johansson-Vestin,
Hans; (Kungalv, SE) |
Correspondence
Address: |
Lainie E. Parker
Akzo Nobel Inc.
7 Livingstone Avenue
Dobbs Ferry
NY
10522-3408
US
|
Family ID: |
27514522 |
Appl. No.: |
10/007861 |
Filed: |
November 5, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10007861 |
Nov 5, 2001 |
|
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|
PCT/SE00/00822 |
Apr 28, 2000 |
|
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60132359 |
May 4, 1999 |
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60162445 |
Oct 29, 1999 |
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Current U.S.
Class: |
516/78 ;
516/81 |
Current CPC
Class: |
C01B 33/146 20130101;
C01B 33/141 20130101; C01B 33/143 20130101; D21H 17/29 20130101;
D21H 17/455 20130101; D21H 21/10 20130101; D21H 17/68 20130101;
C01B 33/1435 20130101; D21H 17/375 20130101 |
Class at
Publication: |
516/78 ;
516/81 |
International
Class: |
B01F 003/12; C01B
033/141 |
Foreign Application Data
Date |
Code |
Application Number |
May 4, 1999 |
EP |
99850074.8 |
May 6, 1999 |
SE |
9901687-5 |
Oct 29, 1999 |
EP |
99850160.5 |
Claims
1. Process for the production of an aqueous sol containing
silica-based particles which comprises: (a) acidifying an aqueous
silicate solution to a pH of from 1 to 4 to form an acid sol; (b)
alkalising the acid sol at an SiO.sub.2 content within the range of
from 4.5 to 8% by weight to a pH of at least 7; (c) allowing
particle growth of the alkalised sol for at least 10 minutes; (d)
alkalising the obtained sol to a pH of at least 10.0; and (e)
optionally concentrating the sol obtained according to (b), (c) or
(d) to provide an aqueous sol containing silica-based particles and
having a specific surface area of at least 90 m.sup.2/g aqueous
sol.
2. Process for the production of an aqueous sol containing
silica-based particles which comprises: (a) acidifying an aqueous
silicate solution to a pH of from 1 to 4 to form an acid sol; (b)
alkalising the acid sol at an SiO.sub.2 content within the range of
from 4.5 to 8% by weight; (c) heat-treating the alkalised sol at a
temperature of at least 30.degree. C.; (d) alkalising the
heat-treated sol to a pH of at least 10.0; and (e) optionally
concentrating the sol obtained according to (b), (c) or (d) to
provide an aqueous sol containing silica-based particles and having
a specific surface area of at least 90 m.sup.2/g aqueous sol.
3. Process according to claim 1 or 2, characterised in that it
comprises (e) concentrating the sol obtained according to (c) or
(d) to provide a sol having a specific surface area of at least 95
m.sup.2/g aqueous sol.
4. Process according to claim 1, 2 or 3, characterised in that the
alkalisation according to (b) and (d) is carried out by means of an
aqueous silicate solution.
5. Process according to any of claims 1 to 4, characterised in that
the particle growth and heat-treatment according to (c) is carried
out at a temperature within the range of from 35 to 95.degree.
C.
6. Process according to any of claims 1 to 5, characterised in that
the particle growth and heat-treatment according to (c) is carried
out for 20 to 240 minutes.
7. Process according to any of claims 1 to 6, characterised in that
the alkalisation according to (d) produces a sol having a molar
ratio of SiO.sub.2 to M.sub.2O, where M is alkali metal or
ammonium, within the range of from 15:1 to 30:1 and a pH of at
least 10.6.
8. Process according to any of claims 1 to 7, characterised in that
it further comprises addition of an aluminium-containing compound
and/or a boron-containing compound.
9. Process according to any of claims 1 to 8, characterised in that
the silica-based particles obtained have a specific surface area of
at least 550 m.sup.2/g SiO.sub.2.
10. Aqueous sol containing silica-based particles obtainable by a
process according to any of claims 1 to 9.
11. Aqueous sol containing silica-based particles, characterised in
that it has a specific surface area of at least 115 m.sup.2/g
aqueous sol and the silica-based particles have a specific surface
area of at least 550 and less than 1000 m.sup.2/g SiO.sub.2.
12. Aqueous sol containing silica-based particles, characterised in
that it has a specific surface area of at least 115 m.sup.2/g
aqueous sol and an S-value within the range of from 10 to 45%.
13. Aqueous sol according to claim 11 or 12, characterised in that
it has a molar ratio of SiO.sub.2 to M.sub.2O, where M is alkali
metal or ammonium, within the range of from 15:1 to 40:1.
14. Aqueous sol according to claim 12 or 13, characterised in that
the silica-based particles have a specific surface area of at least
550 m.sup.2/g SiO.sub.2.
15. Aqueous sol according to any of claims 11 to 14, characterised
in that it has an S-value within the range of from 25 to 35%.
16. Aqueous sol according to any of claims 11 to 15, characterised
in that it has a silica content of at least 10% by weight.
17. Use of an aqueous sol containing silica-based particles
according to any of claims 10 to 16 or produced by a process
according to any of claims 1 to 9 as a drainage and retention aid
in the production of paper.
18. Process for the production of paper from an aqueous suspension
containing cellulosic fibres, and optional fillers, which comprises
adding to the suspension silica-based particles and at least one
charged organic polymer, forming and draining the suspension on a
wire, characterised in that the silica-based particles are obtained
by a process according to any of claims 1 to 9 or present in an
aqueous sol according to any of claims 10 to 16.
19. Process for the production of paper which comprises: (a)
providing an aqueous suspension containing cellulosic fibres, and
optional fillers; (b) providing an aqueous sol containing
silica-based particles, the sol having a specific surface area of
at least 90 m.sup.2/g aqueous sol and the silica-based particles
having a specific surface area of less than 1000 m.sup.2/g
SiO.sub.2; (c) providing at least one charged organic polymer; (d)
adding the charged organic polymer and the silica-based particles
to the suspension; (e) forming and draining the obtained suspension
on a wire.
20. Process for the production of paper which comprises: (a)
providing an aqueous suspension containing cellulosic fibres, and
optional fillers; (b) providing an aqueous sol containing
silica-based particles having a specific surface area of at least
90 m.sup.2/g aqueous sol and an S-value within the range of from 10
to 45%; (c) providing at least one charged organic polymer; (d)
adding the charged organic polymer and the silica-based particles
to the suspension; (e) forming and draining the obtained suspension
on a wire.
21. Process according to claim 19 or 20, characterised in that the
sol has a specific surface area in the range of from 95 to 150
m.sup.2/g aqueous sol.
22. Process according to claim 19, 20 or 21, characterised in that
the silica-based particles have a specific surface area of at least
550 m.sup.2/g SiO.sub.2.
23. Process according to any of claims 19 to 22, characterised in
that the charged organic polymer is cationic starch or cationic
polyacrylamide.
24. Process according to any of claims 19 to 23, characterised in
that the aqueous sol is diluted to a silica content of from 0.05 to
5% by weight before adding the silica-based particles to the
suspension.
25. Process according to any of claims 19 to 23, characterised in
that the silica-based particles are added to the suspension in an
amount of from 0.005 to 0.5% by weight, calculated as SiO.sub.2 and
based on dry cellulosic fibres and optional fillers.
Description
[0001] The present invention generally relates to silica-based sols
suitable for use in papermaking. More particularly, the invention
relates to silica-based sots and silica-based particles, their
production and their use in the production of paper. The process of
this invention provides silica-based particles and sols containing
silica-based particles with high drainage and retention
performance, high stability and high solids contents.
BACKGROUND
[0002] In the papermaking art, an aqueous suspension containing
cellulosic fibres, and optional fillers and additives, referred to
as stock, is fed into a headbox which ejects the stock onto a
forming wire. Water is drained from the stock through the forming
wire so that a wet web of paper is formed on the wire, and the
paper web is further dewatered and dried in the drying section of
the paper machine. Drainage and retention aids are conventionally
introduced into the stock in order to facilitate drainage and to
increase adsorption of fine particles onto the cellulosic fibres so
that they are retained with the fibres on the wire.
[0003] Silica-based particles are widely used as drainage and
retention aids in combination with charged organic polymers like
anionic and cationic acrylamide-based polymers and cationic and
amphoteric starches. Such additive systems are disclosed in U.S.
Pat. Nos. 4,388,150; 4,961,825; 4,980,025; 5,368,833; 5,603,805;
5,607,552; and 5,858,174; and International Patent Application WO
97/18351. These systems are among the most efficient drainage and
retention aids now in use.
[0004] Silica-based particles suitable for use as drainage and
retention aids are normally supplied in the form of aqueous
colloidal dispersions, so-called sols. Commercially used
silica-based sols usually have a silica content of about 7 to 15%
by weight and contain particles with a specific surface area of at
least 300 m.sup.2/g. Sots of silica-based particles with higher
specific surface areas are usually more dilute to improve storage
stability and avoid gel formation.
[0005] It would be advantageous to be able to provide silica-based
sols and particles with further improved drainage and retention
performance and even better stability. It would also be
advantageous to be able to provide a process for preparing
silica-based sols and particles with improved drainage, retention
and stability properties. It would also be advantageous to be able
to provide a papermaking process with improved drainage and/or
retention.
THE INVENTION
[0006] In accordance with the present invention there are provided
silica-based sols and particles which are suitable for use as
flocculating agents in water purification and as drainage and
retention aids in papermaking. The silica-based sols and particles
according to the invention exhibit good stability over extended
periods of time, notably high surface area stability and high
stability towards gelation, and hence they can be prepared and
shipped at high specific surface areas and high silica
concentrations. The silica-based sols and particles have improved
capability to maintain the high specific surface area on storage at
high silica concentrations. The silica-based sols and particles
according to the invention further result in very good or improved
drainage and retention when used in conjunction with anionic,
cationic and/or amphoteric organic polymers. Hereby the
silica-based sols and particles according to the invention makes it
possible to increase the speed of the paper machine and to use a
lower dosage of additives to give a corresponding drainage and/or
retention effect, thereby leading to an improved papermaking
process and economic benefits. The invention thus relates to
silica-based particles and an aqueous sol containing silica-based
particles, herein also referred to as silica-based sol, and their
production, as further defined in the appended claims.
[0007] The present invention also relates to the use of the
silica-based sols and particles as drainage and retention aids in
papermaking, preferably in combination with organic polymers as
described herein, as further defined in the appended claims. The
term "drainage and retention aid", as used herein, refers to one or
more components (aids, agents or additives) which, when being added
to a papermaking stock, give better drainage and/or retention than
is obtained when not adding the components. The present invention
further relates to a process for the production of paper from an
aqueous suspension containing cellulosic fibres, and optional
fillers, which comprises adding to the suspension silica-based
particles and at least one charged organic polymer, forming and
draining the suspension on a wire. The invention thus relates to a
process as further defined in the appended claims.
[0008] The silica-based sols according to the present invention are
aqueous sols that contain anionic silica-based particles, i.e.
particles based on silica (SiO.sub.2) or silicic acid. The
particles are preferably colloidal, i.e., in the colloidal range of
particle size. The silica-based particles present in the sol
suitably have an average particle size below about 10 nm and
preferably in the range of from about 10 to about 2 nm. As
conventional in the chemistry of colloidal particles based on
silica, particle size refers to the average size of the primary
particles, which may be aggregated or non-aggregated.
[0009] The specific surface area of the silica-based sols is
suitably at least 80 m.sup.2/g aqueous sol, i.e., based on the
weight of aqueous sol, preferably at least 85 m.sup.2/g aqueous
sol, more preferably at least 90 m.sup.2/g aqueous sol and most
preferably at least 95 m.sup.2/g aqueous sol. In a preferred
embodiment of this invention, the specific surface area of the
aqueous silica-based sols is suitably at least 115 m.sup.2/g
aqueous sol, preferably at least 120 m.sup.2/g aqueous sol.
Generally, the specific surface area of the aqueous sol can be up
to about 200 m.sup.2/g aqueous sol, suitably up to 150 m.sup.2/g
aqueous sol and preferably up to 130 m.sup.2/g aqueous sol.
[0010] The specific surface area of the silica-based particles is
suitably at least 300 m.sup.2/g Si0.sub.2, i.e. based on the weight
of SiO.sub.2, preferably at least 400 m.sup.2/g SiO.sub.2 and most
preferably at least 550 m.sup.2/g SiO.sub.2. Generally, the
specific surface area of the particles can be up to about 1200
m.sup.2/g SiO.sub.2, suitably less than 1000 m.sup.2/g SiO.sub.2
and preferably up to 950 m.sup.2/g SiO.sub.2. In a preferred
embodiment of this invention, the specific surface area of the
particles is within the range of from 550 to 725 m.sup.2/g
SiO.sub.2, preferably from 575 to 700 m.sup.2/g SiO.sub.2. In
another preferred embodiment of this invention, the specific
surface area of the particles is within the range of from 775 to
1200 m.sup.2/g SiO.sub.2, preferably from 800 to less than 1000
m.sup.2/g SiO.sub.2.
[0011] The specific surface area can be measured by means of
titration with NaOH in known manner, e.g. as described by Sears in
Analytical Chemistry 28(1956):12, 1981-1983 and in U.S. Pat. No.
5,176,891, after appropriate removal of or adjustment for any
compounds present in the sample that may disturb the titration like
aluminium and boron species. When expressed in square metres per
gram of aqueous sol, the specific surface area represents the
specific surface area that is available per gram of aqueous
silica-based sol. When expressed in square metres per gram of
silica, the specific surface area represents the average specific
surface area of the silica-based particles present in the sol.
[0012] The silica-based sols usually have an S-value within the
range of from 10 to 45%, suitably from 20 to 40% and preferably
from 25 to 35%. The S-value can be measured and calculated as
described by ILer & Dalton in J. Phys. Chem. 60(1956), 955-957.
The S-value indicates the degree of aggregate or microgel formation
and a lower S-value is indicative of a higher degree of
aggregation.
[0013] The silica-based sols usually have a molar ratio of
SiO.sub.2 to M.sub.2O, where M is alkali metal ion (e.g. Li, Na, K)
and/or ammonium, of at least 10:1, suitably at least 12:1 and
preferably at least 15:1. The molar ratio of SiO.sub.2 to M.sub.2O
generally can be up to 100:1, suitably up to 40:1 and preferably up
to 30:1. Preferred ranges are thus from 10:1 to 100:1 and notably
from 15:1 to 30:1. The silica-based sols usually have a pH of at
least 8.0, suitably at least 10.0, preferably at least 10.5, and
more preferably at least 10.6. The pH can be up to about 11.5,
suitably up to 11.0.
[0014] The silica-based sols should suitably have a silica content
of at least 3% by weight but it is more suitable that the silica
content is within the range of from 10 to 30% by weight and
preferably from 12 to 25% by weight. In order to simplify shipping
and reduce transportation costs, it is generally preferable to ship
high concentration silica-based sols but it is of course possible
and usually preferable to dilute and mix the silica-based sols and
particles to substantially lower silica contents prior to use, for
example to silica contents within the range of from 0.05 to 5% by
weight, in order to improve mixing with the furnish components. The
viscosity of the silica-based sols can vary depending on, for
example, the silica content of the sol. Usually, the viscosity is
at least 5 cP, normally within the range of from 5 to 40 cP,
suitably from 6 to 30 cP and preferably from 7 to 25 cP. The
viscosity, which is suitably measured on sols having a silica
content of at least 10% by weight, can be measured by means of
known technique, for example using a Brookfield LVDV II+
viscosimeter.
[0015] The silica-based sols of this invention are preferably
stable. The term "stable silica-based sol", as used herein refers
to silica-based sols which when subjected to storage or ageing for
one month at 20.degree. C. in dark and non-agitated conditions
exhibit an increase in viscosity of less than 100 cP. Suitably the
viscosity increase, if any, is less than 50 cP and preferably less
than 30 cP when the sols are subjected to the above conditions.
[0016] In a preferred embodiment of this invention, the
silica-based sol is substantially free from aluminium, i.e. free
from added modifiers containing aluminium. In another preferred
embodiment of this invention, the silica-based sol is substantially
free from boron, i.e. free from added modifiers containing boron.
Minor amounts of such elements can however be present in the
starting materials used to prepare the silica-based sols and
particles. In yet another preferred embodiment of this invention,
the silica-based sols are modified using various elements, e.g.
aluminium and/or boron, which can be present in the aqueous phase
and/or in the silica-based particles. If aluminium is used, the
sols can have a molar ratio of Al.sub.2O.sub.3 to SiO.sub.2 within
the range of from 1:4 to 1:1500, suitably from 1:8 to 1:1000 and
preferably from 1:15 to 1:500. If boron is used, the sols can have
a molar ratio of B to SiO.sub.2 within the range of from 1:4 to
1:1500, suitably from 1:8 to 1:1000 and preferably from 1:15 to
1:500. If both aluminium and boron are used, the molar ratio of Al
to B can be within the range of from 100:1 to 1:100, suitably from
50:1 to 1:50.
[0017] Silica-based sols and particles according to the invention
can be produced starting from a conventional aqueous silicate
solution like alkali water glass, e.g. potassium or sodium water
glass, preferably sodium water glass. The molar ratio of SiO.sub.2
to M.sub.2O, where M is alkali metal, e.g. sodium, potassium,
ammonium, or a mixture thereof, in the silicate solution or water
glass is suitably within the range of from 1.5:1 to 4.5:1,
preferably from 2.5:1 to 3.9:1. Suitably a dilute silicate solution
or water glass is used which can have an SiO.sub.2 content of from
about 3 to about 12% by weight, preferably from about 5 to about
10% by weight. The silicate solution or water glass, which usually
as a pH around 13 or above 13, is acidified to a pH of from about 1
to about 4. The acidification can be carried out in known manner by
addition of mineral acids, e.g. sulphuric acid, hydrochloric acid
and phosphoric acid, or optionally with other chemicals known as
suitable for acidification of water glass, e.g. ammonium sulphate
and carbon dioxide. When adding a mineral acid, the acidification
is suitably carried out in two steps, a first step to a pH of about
8 to 9, whereupon a certain ripening, i.e., a particle growth, is
allowed to occur before further acidification to a pH of from about
1 to about 4. However, it is preferred that the acidification is
carried out by means of an acid cation exchanger which, among other
things, lead to more stable products. The acidification is
preferably carried out by means of a strongly acid cation exchange
resin, for example of sulfonic acid type. It is preferred that the
acidification is carried out to a pH of from about 2 to 4, most
preferably from about 2.2 to 3.0. The product obtained, an acid sol
or polysilicic acid, contains silica-based particles with a high
specific surface area, normally above 1000 m.sup.2g SiO.sub.2 and
usually around about 1300 to 1500 m.sup.2/g SiO.sub.2.
[0018] The acid sol is then subjected to alkalisation, herein
referred to as a first alkalisation step. The first alkalisation
can be carried out by addition of conventional alkali, e.g. lithium
hydroxide, sodium hydroxide, potassium hydroxide, ammonium
hydroxide and mixtures thereof, and/or an aqueous silicate solution
as defined above. Potassium and sodium water glass, particularly
sodium water glass, with a molar ratio of SiO.sub.2 to M.sub.2O as
defined above, is suitably used in the alkalisation step. The
SiO.sub.2 content of the water glass solutions used for the first
alkalisation is suitably within the range of from about 3 to about
35% by weight and preferably within the range of from 5 to 30% by
weight. The first alkalisation is usually carried out to a pH of at
least 6, suitably at least 7 and preferably at least 7.5, and the
pH is usually up to 10.5, suitably up to 10.0. The first
alkalisation is further suitably carried out to a final molar ratio
of SiO.sub.2 to M.sub.2O, M being as defined above, of less than
100:1, suitably within the range of from about 20:1 to about 80:1,
preferably from 30:1 to 70:1. In the preparation of a sol having a
low S-value as defined above the degree of microgel can be
influenced in several ways and be controlled to a desired value.
The degree of microgel can be influenced by salt content, by
adjustment of the concentration in the preparation of the acid sol
and in the first alkalisation step since in this step the degree of
microgel is influenced when the stability minimum for the sol is
passed, at a pH of about 5. By prolonged times at this passage the
degree of microgel can be directed to the desired value. It is
particularly suitable to control the degree of microgel by
adjustment of the dry content, the SiO.sub.2 content, in the first
alkalisation step whereby a higher dry content gives a lower
S-value. By keeping the SiO.sub.2 content in the first alkalisation
step within the range of from 4.5 to 8% by weight the S-value can
be controlled to the desired values of, for example, from 10 to
45%. To obtain sols with S-values within the range of from 20 to
40% the SiO.sub.2 content in the first alkalisation step is
suitably kept within the range of from 5.0 to 7.5% by weight.
[0019] The silica-based particles present in the alkalised sol
obtained in the first alkalisation step is then subjected to
particle growth so that particles with a lower specific surface
area and higher stability are obtained. The particle growth process
should suitably be carried out to provide silica-based particles
with a specific surface area of at least 300 m.sup.2/g SiO.sub.2,
preferably at least 400 m.sup.2/g SiO.sub.2, and most preferably at
least 550 m.sup.2/g SiO.sub.2, and up to about 1200 m.sup.2/g
SiO.sub.2, suitably less than 1000 m.sup.2/g SiO.sub.2, notably up
to 950 m.sup.2/g SiO.sub.2. In a preferred embodiment of this
invention, the particle growth process is carried out to provide
particles with a specific surface area within the range of from 550
to 725 SiO.sub.2, preferably from 575 to 700 m.sup.2/g SiO.sub.2.
In another preferred embodiment of this invention, is carried out
to provide particles with a specific surface area of within the
range of from 775 to 1200 m.sup.2/g SiO.sub.2, preferably from 800
to less than 1000 m.sup.2/g SiO.sub.2. The decrease in surface area
can be obtained by storage at room temperature during somewhat
longer times, a day up to about two days and nights, or,
preferably, by heat treatment. In the heat treatment, times and
temperatures can be adjusted so that shorter times are used at
higher temperatures. Even if it of course is possible to use fairly
high temperatures during very short times it is, from a practical
point of view, more suitable to use lower temperatures during
somewhat longer times. In the heat treatment, the alkalised sol
should suitably be heated at a temperature of at least 30.degree.
C., suitably from 35 to 95.degree. and preferably from 40 to
80.degree. C. The heat treatment should suitably be carried out for
at least 10 minutes, suitably from 15 to 600 minutes and preferably
from 20 to 240 minutes.
[0020] After the particle growth step, and optional cooling, the
obtained silica-based sol is again subjected to alkalisation,
herein referred to as a second alkalisation step, which further
increases the pH. The second alkalisation can be carried out by
addition of conventional alkali, e.g. lithium hydroxide, sodium
hydroxide, potassium hydroxide, ammonium hydroxide and mixtures
thereof, and/or an aqueous silicate solution as defined above.
Potassium and sodium water glass, particularly sodium water glass,
with a molar ratio of SiO.sub.2 to M.sub.2O as defined above, is
suitably used in the second alkalisation step. The SiO.sub.2
content of the water glass solutions used for the second
alkalisation is suitably within the range of from about 3 to about
35% by weight and preferably within the range of from 5 to 30% by
weight. The second alkalisation is suitably carried out to a pH of
at least 8.0, suitably at least 10.0, preferably at least 10.5 and
most preferably at least 10.6. The pH can be up to about 11.5,
suitably up to 11.0. The second alkalisation is further suitably
carried out to a final molar ratio of SiO.sub.2 to M.sub.2O, M
being as defined above, within the range of from about 10:1 to
100:1 and suitably from 12:1 to 40:1, preferably from 15:1 to
30:1.
[0021] In a preferred embodiment of the invention, the process also
comprises concentration of the silica-based sol. Concentration can
be carried out after the second alkalisation. Alternatively, or
additionally, the alkalised sol obtained after the first
alkalisation but before the particle growth or heat treatment step,
or the sol obtained after the particle growth or heat treatment
step but before the second alkalisation, can be subjected to
concentration. Concentration can be carried out in known manner
such as, for example, by osmotic methods, evaporation and
ultrafiltration. The concentration is suitably carried out to
produce silica contents of at least 10% by weight, preferably from
10 to 30% by weight, and more preferably from 12 to 25% by weight.
The concentration is further suitably carried out so that the
silica-based sol obtained in the process has a specific surface
area of at least 80 m.sup.2/g aqueous sol, i.e., based on the
weight of aqueous sol, preferably at least 85 m.sup.2/g aqueous
sol, more preferably at least 90 m.sup.2/g aqueous sol and most
preferably at least 95 m.sup.2/g aqueous sol. In a preferred
embodiment of this invention, the specific surface area of the
aqueous silica-based sols obtained is suitably at least 115
m.sup.2/g aqueous sol preferably at least 120 m.sup.2/g aqueous
sol. Generally, the specific surface area of the aqueous sol
obtained can be up to about 200 m.sup.2/g aqueous sol, suitably up
to 150 m.sup.2/g aqueous sol and preferably up to 130 m.sup.2/g
aqueous sol.
[0022] If desired, the silica-based sols and particles can be
modified by addition of compounds containing, for example,
aluminium and/or boron. Suitable aluminium-containing compounds
include aluminates like sodium aluminate and potassium aluminate,
suitably sodium aluminate. The aluminium-containing compound is
suitably used in the form of an aqueous solution. Suitable
boron-containing compounds include boric acid, borates like sodium
and potassium borate, suitably sodium borate, tetraborates like
sodium and potassium tetraborate, suitably sodium tetraborate, and
metaborates like sodium and potassium metaborate. The
boron-containing compound is suitably used in the form of an
aqueous solution.
[0023] When using an aluminium-containing compound in the process,
it is suitable to add it to the sol subjected to particle growth or
heat treatment, either before or after the second alkalisation
step. Alternatively, or additionally, the aluminium-containing
compound can be added to the silicate solution to be acidified, to
the acid sol or to the alkalised sol obtained in the first
alkalisation step before the particle growth or heat treatment
step. The aluminium-containing compound can be added in admixture
with acid in the acidification step and in admixture with alkali or
silicate solution in any of the alkalisation steps. The
aluminium-containing compound is suitably added in an amount such
that the obtained sol has a molar ratio of Al.sub.2O.sub.3 to
SiO.sub.2 as defined above.
[0024] When using a boron-containing compound in the process, it is
suitable to add it to the sol subjected to particle growth or heat
treatment, either before or after the second alkalisation step.
Alternatively, or additionally, the boron-containing compound can
be added to the silicate solution to be acidified, to the acid sol
or to the alkalised sol obtained in the first alkalisation step
before the particle growth or heat treatment step. The
boron-containing compound can be added in admixture with acid in
the acidification step and in admixture with alkali or silicate
solution in any of the alkalisation steps. The boron-containing
compound is suitably added in an amount such that the obtained sol
has a molar ratio of B to SiO.sub.2 as defined above. If both
aluminium-containing and boron-containing compounds are used, they
are suitably added in amounts such that the obtained sol has a
molar ratio of Al to B as defined above.
[0025] If the sol, before any aluminium and/or boron modification,
contains too high amounts of alkali metal ions or ammonium ions, it
is preferable to remove at least part of these ions, for example by
ion exchange, to provide silica-based sols with a final molar ratio
of SiO.sub.2 to M.sub.2O within the desired range as defined
above.
[0026] According to the present process, silica-based sols and
particles, notably stable silica-based sols and particles, having
the above characteristics can be prepared and the produced sols
exhibit good storage stability and can be stored for several months
without any substantial decrease of the specific surface area and
without gelation.
[0027] The silica-based sols and particles according to this
invention are suitable for use as flocculating agents, for example
in the production of pulp and paper, notably as drainage and
retention aids, and within the field of water purification, both
for purification of different kinds of waste water and for
purification specifically of white water from the pulp and paper
industry. The silica-based sols and particles can be used as
flocculating agents, notably as drainage and retention aids, in
combination with organic polymers which can be selected from
anionic, amphoteric, non-ionic and cationic polymers and mixtures
thereof, herein also referred to as "main polymer". The use of such
polymers as flocculating agents and as drainage and retention aids
is well known in the art. The polymers can be derived from natural
or synthetic sources, and they can be linear, branched or
cross-linked. Examples of generally suitable main polymers include
anionic, amphoteric and cationic starches, anionic, amphoteric and
cationic guar gums, and anionic, amphoteric and cationic
acryl-amide-based polymers, as well as cationic
poly(diallyidimethyl ammonium chloride), cationic polyethylene
imines, cationic polyamines, polyamidoamines and vinylamide-based
polymers, melamine-formaldehyde and urea-formaldehyde resins.
Suitably the silica-based sols are used in combination with at
least one cationic or amphoteric polymer, preferably cationic
polymer. Cationic starch and cationic polyacrylamide are
particularly preferred polymers and they can be used singly,
together with each other or together with other polymers, e.g.
other cationic polymers or anionic polyacrylamide. The molecular
weight of the main polymer is suitably above 1,000,000 and
preferably above 2,000,000. The upper limit is not critical; it can
be about 50,000,000, usually 30,000,000 and suitably about
25,000,000. However, the molecular weight of polymers derived from
natural sources may be higher.
[0028] When using the present silica-based sols and particles in
combination with main polymer(s) as mentioned above, it is further
preferred to use at least one low molecular weight (hereinafter
LMW) cationic organic polymer, commonly referred to and used as
anionic trash catchers (ATC). ATC's are known in the art as
neutralizing and/or fixing agents for detrimental anionic
substances present in the stock and the use thereof in combination
with drainage and retention aids often provide further improvements
in drainage and/or retention. The LMW cationic organic polymer can
be derived from natural or synthetic sources, and preferably it is
an LMW synthetic polymer. Suitable organic polymers of this type
include LMW highly charged cationic organic polymers such as
polyamines, polyamideamines, polyethyleneimines, homo- and
copolymers based on diallyl-dimethyl ammonium chloride,
(meth)acrylamides and (meth)acrylates. In relation to the molecular
weight of the main polymer, the molecular weight of the LMW
cationic organic polymer is preferably lower; it is suitably at
least 1,000 and preferably at least 10,000. The upper limit of the
molecular weight is usually about 700,000, suitably about 500,000
and usually about 200,000. Preferred combinations of polymers that
can be co-used with the silica-based sols of this invention include
LMW cationic organic polymer in combination with main polymer(s),
such as, for example, cationic starch and/or cationic
polyacrylamide, anionic polyacrylamide as well as cationic starch
and/or cationic polyacrylamide in combination with anionic
polyacrylamide.
[0029] The components of the drainage and retention aids according
to the invention can be added to the stock in conventional manner
and in any order. When using drainage and retention aids comprising
silica-based particles and an organic polymer, e.g. a main polymer,
it is preferred to add the polymer to the stock before adding the
silica-based particles, even if the opposite order of addition may
be used. It is further preferred to add the main polymer before a
shear stage, which can be selected from pumping, mixing, cleaning,
etc., and to add the silica-based particles after that shear stage.
LMW cationic organic polymers, when used, are preferably introduced
into the stock prior to introducing the main polymer.
Alternatively, the LMW cationic organic polymer and the main
polymer can be introduced into stock essentially simultaneously,
either separately or in admixture, for example as disclosed in U.S.
Pat. No. 5,858,174, which is hereby incorporated herein by
reference. The LMW cationic organic polymer and the main polymer
are preferably introduced into the stock prior to introducing the
silica-based particles.
[0030] In a preferred embodiment of this invention, the
silica-based sols and particles are used as drainage and retention
aids in combination with at least one organic polymer, as described
above, and at least one aluminium compound. Aluminium compounds can
be used to further improve the drainage and/or retention
performance of stock additives comprising silica-based particles.
Suitable aluminium salts include alum, aluminates, aluminium
chloride, aluminium nitrate and polyaluminium compounds, such as
polyaluminium chlorides, polyaluminium sulphates, polyaluminium
compounds containing both chloride and sulphate ions, polyaluminium
silicate-sulphates, and mixtures thereof. The polyaluminium
compounds may also contain other anions, for example anions from
phosphoric acid, organic acids such as citric acid and oxalic acid.
Preferred aluminium salts include sodium aluminate, alum and
polyaluminium compounds. The aluminium compound can be added before
or after the addition of the silica-based particles. Alternatively,
or additionally, the aluminium compound can be added simultaneously
with the silica-based sol at essentially the same point, either
separately or in admixture with it, for example as disclosed by
U.S. Patent No 5,846,384 which is hereby incorporated herein by
reference. In many cases, it is often suitable to add an aluminium
compound to the stock early in the process, for example prior to
the other additives.
[0031] The components of the drainage and retention aids according
to the invention are added to the stock to be dewatered in amounts
which can vary within wide limits depending on, inter alia, type
and number of components, type of furnish, filler content, type of
filler, point of addition, etc. Generally the components are added
in an amount that give better drainage and/or retention than is
obtained when not adding the components. The silica-based sols and
particles are usually added in an amount of at least 0.001% by
weight, often at least 0.005% by weight, calculated as SiO.sub.2
and based on dry stock substance, i.e. cellulosic fibres and
optional fillers, and the upper limit is usually 1.0% and suitably
0.5% by weight. The main polymer is usually added in an amount of
at least 0.001%, often at least 0.005% by weight, based on dry
stock substance, and the upper limit is usually 3% and suitably
1.5% by weight. When using an LMW cationic organic polymer in the
process, it can be added in an amount of at least 0.05%, based on
dry substance of the stock to be dewatered. Suitably, the amount is
in the range of from 0.07 to 0.5%, preferably in the range from 0.1
to 0.35%. When using an aluminium compound in the process, the
total amount introduced into the stock to be dewatered depends on
the type of aluminium compound used and on other effects desired
from it. It is for instance well known in the art to utilise
aluminium compounds as precipitants for rosin-based sizing agents.
The total amount added is usually at least 0.05%, calculated as
Al.sub.2O.sub.3 and based on dry stock substance. Suitably the
amount is in the range of from 0.1 to 3.0%, preferably in the range
from 0.5 to 2.0%.
[0032] Further additives which are conventional in papermaking can
of course be used in combination with the additives according to
the invention, such as, for example, dry strength agents, wet
strength agents, optical brightening agents, dyes, sizing agents
like rosin-based sizing agents and cellulose-reactive sizing
agents, e.g. alkyl and alkenyl ketene dimers and ketene multimers,
alkyl and alkenyl succinic anhydrides, etc. The cellulosic
suspension, or stock, can also contain mineral fillers of
conventional types such as, for example, kaolin, china clay,
titanium dioxide, gypsum, talc and natural and synthetic calcium
carbonates such as chalk, ground marble and precipitated calcium
carbonate.
[0033] The process of this invention is used for the production of
paper. The term "paper", as used herein, of course include not only
paper and the production thereof, but also other cellulosic
fibre-containing sheet or web-like products, such as for example
board and paperboard, and the production thereof. The process can
be used in the production of paper from different types of
suspensions of cellulose-containing fibres and the suspensions
should suitably contain at least 25% by weight and preferably at
least 50% by weight of such fibres, based on dry substance. The
suspension can be based on fibres from chemical pulp such as
sulphate, sulphite and organosolv pulps, mechanical pulp such as
thermomechanical pulp, chemo-thermomechanical pulp, refiner pulp
and groundwood pulp, from both hardwood and softwood, and can also
be based on recycled fibres, optionally from de-inked pulps, and
mixtures thereof. The pH of the suspension, the stock, can be
within the range of from about 3 to about 10. The pH is suitably
above 3.5 and preferably within the range of from 4 to 9.
[0034] The invention is further illustrated in the following
Examples which, however, are not intended to limit the same. Parts
and % relate to parts by weight and % by weight, respectively,
unless otherwise stated.
EXAMPLE 1
[0035] A standard silica sol was prepared as follows:
[0036] 762.7 g sodium water glass with a molar ratio of SiO.sub.2
to Na.sub.2O of 3.3 and SiO.sub.2 content of 27.1% was diluted with
water to 3000 g yielding a silicate solution (I) with a SiO.sub.2
content of 6.9% by weight. 2800 g of this silicate or water glass
solution was passed through a column filled with a strong cation
exchange resin saturated with hydrogen ions. 2450 g of
ion-exchanged water glass or polysilicic acid (II) with an
SiO.sub.2 content of 6.5% by weight and a pH of 2.4 was collected
from the ion exchanger. 1988 g of the polysilicic acid (II) was fed
into a reactor and diluted with 12.3 g water. 173.9 g of the 6.9%
silicate solution (I) was then added under vigorous agitation. The
resulting solution was then heated at 85.degree. C. for 60 minutes
and then cooled to 20.degree. C. The obtained silica sol (1a) had
the following characteristics:
[0037] Sol 1a (ref.): SiO.sub.2 content=7.3% by weight, molar ratio
SiO.sub.2/Na.sub.2O=40, pH=10.2, S-value=29%, viscosity=2.2 cP,
specific surface areas=530 m.sup.2/g SiO.sub.2 and 39 m.sup.2/g
aqueous sol.
[0038] Two further silica sols, Sol 1b and Sol 1c, were produced
which had the following characteristics:
[0039] Sol 1b (ref.): SiO.sub.2 content=7.3% by weight, molar ratio
SiO.sub.2/Na.sub.2O=63, pH=10.0, S-value=26%, viscosity=2.7 cP,
specific surface areas=500 m.sup.2/g SiO.sub.2 and 36.5 m.sup.2/g
aqueous sol
[0040] Sol 1c (ref.): SiO.sub.2 content=5.4% by weight, molar ratio
SiO.sub.2/Na.sub.2O=35, pH=9.8, S-value=32%, viscosity=1.6 cP,
specific surface areas=690 m.sup.2/g SiO.sub.2 and 37 m.sup.2/g
aqueous sol.
EXAMPLE 2
[0041] Six sols of silica-based particles according to the
invention were prepared from a polysilicic acid similar to the
polysilicic acid (II) produced with the same ion exchange process
and with an SiO.sub.2 content of 5.46% by weight. To 102.0 kg of
the polysilicic acid was added 1.46 kg of sodium water glass with a
ratio SiO.sub.2/Na.sub.2O of 3.3 under vigorous agitation resulting
in a solution with a molar ratio SiO.sub.2/Na.sub.2O of 54.0. This
solution was heat treated at 60.degree. C. for 2 h 20 min and
cooled to 20.degree. C. whereupon the product was concentrated to a
SiO.sub.2 content of 15.6% by weight. This intermediate sol product
was now divided into six separate samples, a to f. Samples a to c
were further alkalised with NaOH, samples d to f with water glass,
to achieve sols with a molar ratio SiO.sub.2/Na.sub.2O between 21.5
and 34.0 and a silica content of about 15.0% by weight. The
obtained sols of silica-based particles had the characteristics set
forth in Table 1:
1 TABLE 1 Vis- Molar ratio S-value cosity Specific Surface Areas
Sol [SiO.sub.2/Na.sub.2O] pH [%] [cp] [m.sup.2/g SiO.sub.2]
[m.sup.2/g aq. sol] Sol 21.5 10.7 31 17 720 108.0 2a Sol 28.0 10.3
30 29 710 106.5 2b Sol 34.0 10.0 29 40 690 103.5 2c Sol 21.5 10.7
31 20 680 102.0 2d Sol 28.0 10.3 29 34 670 100.5 2e Sol 33.0 10.0
29 38 680 102.0 2f
EXAMPLE 3
[0042] A polysilicic acid (II) produced with the above ion exchange
process and alkalised with water glass to a molar ratio
SiO.sub.2/Na.sub.2O of 54.0 as in Example 2 was heat treated at
60.degree. C. for 1 h. To 58 kg of this product was added 7.25 kg
of diluted water glass with a molar ratio SiO.sub.2/Na.sub.2O of
3.3 and silica content 5.5% by weight. The resulting sol of
silica-based particles, Sol 3, was concentrated to a silica content
of 15.2% by weight and had a molar ratio SiO.sub.2/Na.sub.2O=24,
pH=10.7, S-value=34, viscosity=9.0 cp and specific surface
areas=760 m.sup.2/g SiO.sub.2 and 115.5 m.sup.2/g aqueous sol.
EXAMPLE 4
[0043] 1000 g polysilicic acid (II) with an SiO.sub.2 content of
5.5% by weight was mixed with 14.5 g water glass solution with an
SiO.sub.2 content of 27.1% by weight and a molar ratio
SiO.sub.2/Na.sub.2O=3.3 under vigorous agitation resulting in a
product with a molar ratio SiO.sub.2/Na.sub.2O of 51 and a silica
content of 5.8% by weight SiO.sub.2, which was heat treated at
60.degree. C. for 1.5 h and then concentrated to a silica content
of 16.7% by weight SiO.sub.2, 283 g of the product obtained was
mixed with 33.0 g NaOH resulting in a sol of silica-based
particles, Sol 4, with SiO.sub.2 content=15.2% by weight, molar
ratio SiO.sub.2/Na.sub.2O=21, pH=10.6, S-value=32%, viscosity=14.2
cP and specific surface areas=720 m.sup.2/g SiO.sub.2 and 109.4
m.sup.2/g aqueous sol.
EXAMPLE 5
[0044] The general procedure according to Example 3 was followed
except that the heat treatment was carried for 1.25 h and
concentration was carried out to higher silica contents. Two sols
of silica-based particles were prepared; Sol 5a and Sol 5b. Sol 5a
had SiO.sub.2 content=18% by weight, molar ratio
SiO.sub.2/Na.sub.2O=18, pH=10.7, S-value=36%, viscosity=18 cP and
specific surface areas=700 m.sup.2/g SiO.sub.2 and 126 m.sup.2/g
aqueous sol. Sol 5b had SiO.sub.2 content=20% by weight, molar
ratio SiO.sub.2/Na.sub.2O=18, pH=10.7, S-value=37%, viscosity=31 cP
and specific surface areas=700 m.sup.2/g SiO.sub.2 and 140
m.sup.2/g aqueous sol.
EXAMPLE 6
[0045] Drainage performance was evaluated by means of a Dynamic
Drainage Analyser (DDA) available from Akribi, Sweden, which
measures the time for draining a set volume of stock through a wire
when removing a plug and applying vacuum to that side of the wire
opposite to the side on which the stock is present.
[0046] The stock used was based on a blend of 60% bleached birch
sulphate and 40% bleached pine sulphate to which was added 30%
ground calcium carbonate as a filler. Stock volume was 800 ml,
consistency 0.25% and pH about 8.0. Conductivity of the stock was
adjusted to 0.47 mS/cm by addition of sodium sulphate.
[0047] In the tests, silica-based sols were used in conjunction
with a cationic polymer, Raisamyl 142, which is a conventional
medium-high cationised starch having a degree of substitution of
0.042, which was added to the stock in an amount of 12 kg/tonne,
calculated as dry starch on dry stock system. Silica-based sols
according to Examples 1 to 4 were tested in this Example. In
addition, Sols 6a and 6b were also tested for comparison purposes.
Sol 6a is a commercial silica sol with an S-value=45%, SiO.sub.2
content=15.0% by weight, molar ratio SiO.sub.2/Na.sub.2O=40,
viscosity=3.0 cP, specific surface areas =500 m.sup.2/g SiO.sub.2
and 75 m.sup.2/g aqueous sol. Sol 6b is another commercial silica
sol with an S-value=36%, SiO.sub.2 content=10.0% by weight, molar
ratio SiO.sub.2/Na.sub.2O=10, viscosity=2.5 cP, specific surface
areas=880 m.sup.2/g SiO.sub.2 and 88 m.sup.2/g aqueous sol. The
silica-based sols were added in an amount of 0.5 kg/ton, calculated
as SiO.sub.2 and based on dry stock system.
[0048] The stock was stirred in a baffled jar at a speed of 1500
rpm throughout the test and chemical additions were conducted as
follows: i) adding cationic starch to the stock following by
stirring for 30 seconds, ii) adding silica-based sol to the stock
followed by stirring for 15 seconds, iii) draining the stock while
automatically recording the drainage time.
[0049] Drainage times for the different silica-based sols are shown
in Table 2:
2 TABLE 2 Dewatering time Silica-based sol [sec] Sol 1a (ref.) 12.0
Sol 1b (ref.) 11.1 Sol 1c (ref.) 12.0 Sol 2d 9.7 Sol 3 9.5 Sol 4
9.4 Sol 6a (ref.) 12.0 Sol 6b (ref.) 9.8
EXAMPLE 7
[0050] Drainage performance was evaluated according to the general
procedure of Example 6 except that the stock had a consistency of
0.3% and pH about 8.5. Retention performance was evaluated by means
of a nephelometer by measuring the turbidity of the filtrate, the
white water, obtained by draining the stock.
[0051] Silica-based sols according to Example 5 according to the
invention were tested against Sol 6a used for comparison. Table 3
shows the drainage time obtained at various dosages (kg/ton) of
silica-based particles, calculated as SiO.sub.2 and based on dry
stock system. The addition of only cationic starch (12 kg/tonne,
calculated as dry starch on dry stock system) resulted in a
drainage time of 15.8 sec.
3TABLE 3 Silica-based Drainage time (sec)/Turbidity (NTU) at
SiO.sub.2 dosage of sol 0.5 kg/t 1.0 kg/t 1.5 kg/t 2.0 kg/t 3.0
kg/t Sol 6a 11.1/-- 8.8/59 7.9/58 7.1/54 6.8/60 (ref.) Sol 5a
9.0/-- 7.1/52 6.3/50 5.2/52 5.7/53 Sol 5b 8.9/-- 6.9/-- 6.3/--
5.7/-- 6.0/--
* * * * *