WO1995023021A1

WO1995023021A1 - Silica-based sols, preparation and use of the sols

Info

Publication number: WO1995023021A1
Application number: PCT/SE1995/000200
Authority: WO
Inventors: Hans Johansson
Original assignee: Eka Chemicals Ab
Priority date: 1994-02-28
Filing date: 1995-02-24
Publication date: 1995-08-31
Also published as: ZA951466B; SE9400690D0; SE502464C2; TW282504B; AU1906295A; SE9400690L

Abstract

Silica-based sols containing silica-based anionic particles having a high specific surface area and silica-based anionic particles having a low specific surface area. The sols can be prepared by mixing a sol containing silica-based anionic particles having a high specific surface area with a sol containing silica-based anionic particles having a low specific surface area. The sols are used as flocculating agents, in combination with amphoteric or cationic polymers, particularly in the production of paper and pulp but also for water purification.

Description

Silica-based sols, preparation and use of the sols The present invention relates to silica-based sols con¬ taining silica-based anionic particles having a high specific surface area and silica-based anionic particles having a low specific surface area. The invention also relates to a method for producing the sols and to the use thereof as flocculating agents, in combination with amphoteric or cationic polymers, particularly in the production of paper and pulp but also for water purification. Silica-based sols, which term is used herein for silica- based hydrosols, are aqueous systems with very small silica- based particles which can be used in several fields of appli¬ cation depending, inter alia, on the particle size. During recent years, silica-based sols with anionic silica particles of small particle size have gained an increased use in the production of paper. The silica sols are here used as addi¬ tives to the stock in combination with amphoteric or cationic polymers, primarily to improve retention and dewatering.

European Patent 41056 discloses, for example, the use of colloidal silica sols in combination with cationic starch in papermaking. European Patents 185068 and 218674 disclose com¬ binations of silica sols with particles where at least the surface groups contain aluminium, and cationic natural poly¬ mers and cationic polyacrylamides, respectively. The colloidal silica in these sols has a specific surface area of 50-1000 m²/g. Commercially, use is generally made of sols of this type with discrete particles having a specific surface area of about 400-600 m²/g, with an average particle size generally below 20 nm and most often ranging from about 10 down to about 1 nm. Sols with colloidal particles of the above-mentioned order of size have resulted in good retention and dewatering and have high stability. Quite generally, it has been desired for silica sols which are to be used commercially that they are as monodisperse as possible, i.e. that they have as narrow a particle size distribution as possible.

European Patent 491879 discloses silica sols having a degree of aggregate or microgel formation corresponding to an S-value of 8-45% containing silica-based particles with a spe¬ cific surface area of 750-1000 m²/g which are surface-modified with aluminium to a degree of 2-25%.

The present invention relates to new silica-based sols containing silica-based anionic particles having a high speci¬ fic surface area and silica-based anionic particles having a low specific surface area. It has been found that the sols according to the invention, in combination with cationic poly¬ mers, surprisingly give improved retention and dewatering results at the production of paper and similar products. A higher effect can be achieved with these sols than what could be expected from the included amount of the respective types of particles. With the sols according to the invention, silica-based particles having a comparatively low specific surface area, from about 50 to 400 m²/g, i.e. a comparatively large average particle size, corresponding to from about 54 to about 7 nm, can also be used with good results. Silica-based sols with these larger particle sizes have themselves not given sufficiently good results to be used commercially within the field of retention and drainage.

The present invention thus relates to silica-based sols as further defined in the claims. The invention also relates to a method for producing silica-based sols and to the use of the sols as further indicated in the appended claims.

Silica-based particles, i.e. particles based on Si0₂, having a high specific surface area which can be used in the present sols include colloidal silica, colloidal aluminium- modified silica, aluminium silicate and different types of polysilicic acid. The particles can have a specific surface area within the range of from 425 to 1700 m²/g, corresponding to an average particle size within the range of from about 6.4 to about 1.0 nm. The given specific surface area can be measu¬ red by titration with NaOH according to the method described by Sears in .Analytical Chemistry 28(1956) :12, 1981-1983. The average particle size of discrete spherical particles can be calculated from the relation 2720 divided by the specific surface area indicated in m²/g, giving the average particle diameter in nm, as described by Her in The Chemistry of Silica, pp 346-347, John Wiley & Sons, 1979. The specific surface area of the silica-based particles is suitably higher than 450 m²/g and preferably higher than 475 m²/g. The upper limit for the specific surface area of the particles is suit¬ ably 1400 m²/g and preferably 1200 m²/g. An especially suitable range for the specific surface area is 750-1000 m²/g, in particular 800-950 m²/g. Suitable silica-based particles having a high specific surface area are such particles which in silica-based sols result in an S-value within the range of from 8 to 90%, and preferably within the range of from 8 to 60%. The indicated S-value can be measured and calculated in accordance with what is described by Her and Dalton in J. Phys. Chem. 60(1956), 955-957. The S-value indicates the degree of aggregate or microgel formation, where a low S-value indicates a higher microgel content, and can also be seen as a measure of the Si0₂ content, in % by weight, in the disperse phase. The silica-based particles having a high specific surface area may be such particles which are described in the above-mentioned European Patents 41056 and 185068, which are hereby incorporated herein by reference, provided that the specific surface area lies within the indicated limits. Suit- able particles described in these patents have a specific surface area of 425-1000 m²/g. Particularly suitable silica- based particles having a high specific surface area originate from silica-based sols having an S-value within the range of from 8 to 45% containing silica-based particles with a specific surface area within the range of from 750 to 1000 m²/g which are surface-modified with aluminium to a degree of from 2 to 25%. This type of particles and sols are disclosed in European Patent 491879, which is incorporated herein by reference. Further sols containing suitable silica-based particles are disclosed in European Patent 502089 and PCT Application WO 94/05596, which are also incorporated herein by reference. Suitable silica-based particles are also included in sols based on polysilicic acid, which means that the silicic acid material is in the form of very small particles, of the order of 1-2 nm, with a very high specific surface area, above 1000 m/g and up to about 1700 m²/g, and with a certain degree of aggregate or microgel formation, which are described in European Patents 348366, 359552 and PCT Applica¬ tion WO 89/06637. Silica-based anionic particles having a low specific surface area which can be used in the present sols include colloidal silica, colloidal aluminium-modified silica and aluminium silicate. These particles are suitably substantially spherical and preferably discrete, i.e non-aggregated, in con¬ trast to microgel and aggregates, which contain aggregated particles, e.g. in the form of straight or branched chain-like formations. These silica-based particles can have a specific surface area within the range of from 50 to 400 m²/g, suitably from 70 to 375 m²/g, and preferably from 80 to 250 m²/g. Sols with suitable silica-based particles having a low specific surface area can have an S-value within the range of from 25 to 95%, suitably at least 50%, and preferably from 60 to 95%. Sols with suitable silica-based particles having a low speci- fie surface area are commercially available under the trade- name Bindzil™, produced by Eka Nobel.

The sols of silica-based particles according to the invention may include silica-based particles having the high specific surface area in an amount of at least 50% by weight, suitably at least 60%, and preferably at least 75%, based on the total amount of silica-based particles. The upper limit of the included amount of high specific surface area particles is 99%, suitably 98%, preferably 95% and more preferably 90% by weight, based on the total amount of silica-based particles. Thus, the silica-based sols according to the invention may include the low specific surface area particles in an amount of from 1%, suitably from 2%, preferably from 5% and more preferably from 10% by weight, up to 50%, suitably to 40% and preferably to 25% by weight, based on the total amount of silica-based particles.

Sols according to the invention suitably have a dry solids content, calculated as Si0₂, of from 0.1 to 30% by weight. The dry solids content suitably exceeds 1%, preferably exceeds 3% and more preferably exceeds 5% by weight. Suitably, the upper limit is 25% and preferably 20% by weight. A major advantage of the present sols is that they can be prepared with higher dry solids contents and that an improved floccula- tion and dewatering effect can be obtained as compared with sols containing only silica-based particles having a specific surface area within the range of 425-1700 m²/g. As a result of the higher dry solids content of the stable sols according to the invention, the costs for storage and transportation can be significantly reduced. In papermaking, the improved dewatering obtained with the sols according to the invention means that the speed of the papermaking machine can be increased and, moreover, less water have to be removed in the press and drying sections of the machine, resulting in an substantially improved papermaking process. The silica-based sols according to the invention can be prepared by mixing, suitably under agitation, a sol containing silica-based anionic particles having a high specific surface area as defined above with a sol containing silica-based anio¬ nic particles having a low specific surface area as defined above. Thus, the sols according to the invention can contain a mixture of a sol containing silica-based anionic particles having a high specific surface area within the range of from 425, suitably from 450, preferably from 475 and most prefer¬ ably from 750 m²/g, to 1700, suitably to 1400, preferably to 1200 and most preferably to 1000 m²/g, and a sol containing silica-based anionic particles having a low specific surface area within the range of from 50 m²/g, suitably from 70 and preferably from 80 m²/g, to 400 m²/g, suitably to 375 and preferably to 250 m²/g. In the mixture, the sol containing silica-based particles having the high specific surface area can be included in an amount of from 50%, suitably from 60% and preferably from 75%, up to 99%, suitably to 98%, prefer¬ ably to 95% and more preferably to 90% by weight, and the sol containing silica-based particles having the low specific sur- face area can be included in an amount of from 1%, suitably from 2%, preferably from 5% and more preferably from 10%, up to 50%, suitably to 40% and preferably 25% by weight, calcu¬ lated on the total amount of silica-based particles. Sols of the respective types of particles within the defined specific surface area ranges that can be used in order to produce the sols according to the invention are known in the art and can be prepared by the skilled person.

When producing the sols according to the invention by this method, the sol with particles having the high specific surface area suitably has a dry solids content, calculated as Si0₂, of 0.5-20% by weight, preferably 5-17% by weight, and the sol with particles having the low specific surface area may have a dry solids content of up to 60% by weight, suitably 3-60% and preferably 15-50% by weight.

The silica-based sols according to the invention may also contain hydrated particles of clays which are expandable in water and which are of the smectite type. Clays of the smectite type are thin flake-like, layered silicate minerals and comprise both naturally occurring materials and synthetic ones. The materials may be chemically treated, for example alkali-treated. The clays should be dispersible in water and thereby expand so that particles having a high surface area are obtained. Examples of clays of smectite type which are expandable in water and which can be used in the present sols are montmorillonite, bentonite, hectorite, beidellite, nontro- nite and saponite. Bentonite is preferred, and a suitable ben¬ tonite is, for example, the one described in European Patent 235893, which after swelling preferably has a surface area of from 400 to 800 m²/g. The weight ratio of silica-based partic¬ les to clay particles can be from 20:1 to 1:10, based on dry matter. Suitably, the weight ratio ranges from 10:1 to 1:5, and preferably from 6:1 to 1:3. Aqueous mixtures of silica- based particles and clay particles according to the invention are stable and may have a dry solids content of from 1%, suitably from 5% and preferably from 8% by weight, up to 40%, suitably to 30% and preferably to 25% by weight

Silica-based sols according to the invention containing hydrated particles of clays which are expandable in water may, for example, be prepared by first mixing the clay into water and then adding a silica-based sol before the clay has had time to expand in the water, whereupon the aqueous mixture is well dispersed. It is however preferred that these mixtures be prepared by mixing the clay into a sol of silica-based partic- les and dispersing it well therein, using high shear forces. The sols used in the above-mentioned methods may be a silica- based sol according to the invention or a sol intended for the preparation thereof, whereby the mixture of clay particles and silica-based particles after the dispersing process is mixed with additional silica-based sol in order to prepare the mix¬ ture according to the invention. Aqueous mixtures of silica- based particles and clay particles of the smectite type and the preparation thereof are disclosed in PCT Application WO 94/05595, which is hereby incorporated herein by reference. The present sols are suitable for use as flocculating agents, for example in the production of pulp and paper and within the field of water purification, both for purification of different types of waste water and for purification speci- fically of white water from pulp and paper industry. The sols are used as flocculating agents in combination with cationic or amphoteric polymers, which may be natural, i.e. based on carbohydrates, or be synthetic. As examples of suitable poly¬ mers can be mentioned cationic and amphoteric starch, cationic and amphoteric guar gum, cationic and amphoteric linear and branched acrylamide-based polymers, cationic polyethylene imines, polyamido amines and poly(diallyl dimethyl ammonium chloride) . The sols can be used with one or more polymers. As examples can be mentioned silica-based sol in combination with cationic starch and cationic polyacrylamide and sol in combi¬ nation with low molecular weight highly charged cationic or amphoteric polymer and high molecular weight cationic polymer, especially cationic starch or acrylamide-based polymer. The sols can also be used in combination with polymers of opposite charges, such as sol in combination with cationic polymers, especially cationic starch, and anionic acrylamide-based poly¬ mer. Even if an arbitrary order of addition can be used, it is preferred that the polymer or polymers be added to pulp, stock or water before the sol. A preferred field of use of the sols, in combination with polymer, is for improvement of retention and dewatering in papermaking. The terms "paper" and "papermaking" as used herein of course include not only paper and the production thereof but also other cellulose fibre containing sheets or web-like products, such as pulp sheets, board and paperboard and the production thereof. The sols are hereby suitably added to the stock in an amount of from 0.05 to 5 kg/ton, calculated as dry on dry stock system, i.e. fibres and optional fillers, and preferably in an amount of from 0.1 to 3 kg per ton. The dry solids content of the sols when added to the stock is suitably adjusted to 0.1-10% by weight. For synthetic cationic or amphoteric polymers, use is usually made of at least 0.01 kg of polymer, calculated as dry matter per ton of dry stock system, and amounts of from 0.01 to 3 are suitably used and preferably from 0.03 to 2 kg per ton. For carbohydrate-based cationic or amphoteric polymers such as starch and guar gum, use is usually made of amounts of at least 0.1 kg/ton, cal¬ culated as dry matter per ton of dry stock system. Suitably, for these polymers use is made of amounts of from 0.5 to 30 kg/ton, preferably from 1 to 15 kg/ton.

When using sols according to the invention in the production of paper from a stock or suspension of cellulose containing fibres, and optional fillers, the invention further encompasses forming and dewatering the suspension in the pre¬ sence of cationic or amphoteric polymers, e.g. those mentioned above, and a silica-based sol according to the invention com¬ prising the mixture, in weight ratios as defined above, of a sol containing silica-based anionic particles having the high specific surface area as defined above, and a sol containing silica-based anionic particles having the low specific surface area as likewise defined above. Thus, the invention also encompasses separately adding to the stock the sol containing silica-based particles having the high specific surface area and the sol containing silica-based particles having the low specific surface area, . in arbitrary order and at the same or different points.

The sols may, in combination with the polymers, be used in the production of paper from different types of stocks of cellulose-containing fibres such as stocks of fibres from chemical pulp, such as sulphate and sulphite pulp, chemo- thermomechanical pulp (CTMP) , thermomechanical pulp, refiner pulp or groundwood pulp from both hardwood and softwood, and can also be used for stocks based on recycled fibres. Of course, the stocks may contain mineral fillers of conventional types, such as kaolin, titanium dioxide, gypsum, talc and both natural and synthetic calcium carbonates. Good results have also been obtained for stocks which are normally considered difficult. Examples of such stocks are those containing mecha- nical pulp such as groundwood pulp, stocks based on recycled fibre pulp and stocks which due to the white water system contain high amounts of anionic impurities, such as lignin and dissolved organic compounds and/or high contents of electroly- tes. As is well-known for silica-based sols as such, an impro¬ vement of the retention and dewatering effect can also be obtained for the present sols by adding aluminium compounds to the stock. Any in paper production per se known aluminium com¬ pound can be used, such as alum, aluminates, aluminium chlo- ride, aluminium nitrate and polyaluminium compounds, such as polyaluminium chloride, polyaluminium sulphate and polyalumi¬ nium compounds containing both chloride and sulphate ions. The stock may of course also contain known hydrophobing agents, such as colophonium dispersions, or synthetic stock sizes, e.g. ketene dimer and alkenyl succinic anhydride dispersions.

The invention is further illustrated in the following

Examples which, however, are not intended to limit the same.

Parts and percentages relate to parts by weight and per cent by weight, respectively, unless otherwise stated. Example 1

In this Example, silica-based sols according to the invention were prepared.

The following sols with silica-based particles having a high specific surface area were used: A) A polysilicic acid of the type described in European Patent 348366 having a specific surface area of about 1400 m²/g and a dry solids content of 1% by weight.

B) .An anionic silica sol of the type described in European Patent 491879 having a comparatively low S-value, about 25, a specific surface area of about 900 m²/g and being aluminium- modified to a degree of 5% with a dry solids content of 8.4% by weight.

C) 1-1- anionic silica sol of the type described in European Patent 41056. The sol was alkali-stabilised to a molar ratio of Si0₂:Na₂0 of about 40, and the particles had a specific surface area of 500 m²/g, commercially available under the tradename BMA-0, Eka Nobel.

The following silica-based sols with particles having a low specific surface area were used: D) .An anionic silica sol having a specific surface area of 360 m²/g and a dry solids content of 30% by weight, sold by Eka Nobel under the tradename Bindzil™ 30/360.

E) An. anionic silica sol having a specific surface area of 220 m²/g and a dry solids content of 40% by weight, commer¬ cially available under the tradename Bindzil™ 40/220.

F) .An anionic silica sol having a specific surface area of 220 m²/g and being aluminium-modified to a degree of 5% with a dry solids content of 30% by weight, commercially available under the tradename Bindzil™ 305.

G) An anionic silica sol having a specific surface area of 130 m/g and a dry solids content of 50% by weight, commer¬ cially available under the tradename Bindzil™ 50/130.

H) An anionic silica sol having a specific surface area of 80 m²/g and a dry solids content of 30% by weight, commerci¬ ally available under the tradename Bindzil™ 30/80. I) An anionic silica sol having a specific surface area of 80 m²/g and a dry solids content of 50% by weight, commer¬ cially available under the tradename Bindzil™ 50/80. To a beaker equipped with a magnetic stirrer and contain¬ ing 1.0 kg of sol with silica-based particles having a high specific surface area was added sol with silica-based partic¬ les having a low specific surface area in varying amounts, based on dry matter, as defined below. The sol mixtures were agitated for 10 min. The following sols were prepared: Sol .AE 80% of sol A,. 20% of sol E, dry solids content 1.2% Sol AF 80% of sol A, 20% of sol F, dry solids content 1.2% Sol AG 80% of sol A, 20% of sol G, dry solids content 1.2% Sol AH 80% of sol A, 20% of sol H, dry solids content 1.2% Sol BE 80% of sol B, 20% of sol E, dry solids content 10.0% Sol BF 80% of sol B, 20% of sol F, dry solids content 9.9% Sol BG 80% of sol B, 20% of sol G, dry solids content 10.1% Sol BH 80% of sol B, 20% of sol H, dry solids content 9.9% Sol BI 80% of sol B, 20% of sol I, dry solids content 10.1% Sol BI90: 90% of sol B, 10% of sol I, dry solids content 9.4%

Sol BI95: 95% of sol B, 5% of sol I, dry solids content 8.8%

Sol CD: 80% of sol C, 20% of sol D, dry solids content 16.7%

Sol CI: 80% of sol C, 20% of sol I, dry solids content 17.4%

The sols according to the invention were stored for 1 week before being evaluated, and showed good stability. Example 2

In this Example, a silica-based sol according to the invention containing bentonite was prepared. A bentonite-containing sol having a dry solids content of about 9.0% was prepared from sol B according to Example 1 and sodium bentonite. The bentonite-containing sol was prepared from 152.5 g of sol B, 6.4 g of bentonite and 54.0 g of water. The ratio of silica particles to bentonite particles in this sol was about 2:1. The bentonite was added to sol B, and dispersion was carried out by means of an Ultra-Turrax at 10,000 rpm for 10 min.

To 100 g of the sol prepared as above was added sol I according to Example 1 in an amount of 7.9 g under agitation. The following sol was prepared in the indicated weight ratios, based on dry matter;

Sol Bib: 46.7% of sol B, 30.0% of sol I, 23.3% of bentonite, dry solids content 12.0% by weight The resulting sol was stored for 1 week before being eva- luated, and showed good stability. No bottom phase was formed, and the viscosity of the sol was substantially constant. Examples 3-5

In these Examples, the dewatering effect was evaluated by means of a Canadian Standard Freeness (CSF) Tester, which is the conventional method for characterising dewatering or drainage capability according to SCAN-C 21:65. All additions of chemicals were made at a mixing speed of 800 rpm in a baffled Britt Dynamic Drainage Jar with a blocked outlet for 45 sec and the stock system was then transferred to the Canadian Standard Freeness Tester apparatus.

The stock was based on a CTMP pulp. 0.3 g/1 of Na₂S0₄- 10H₂0 had been added to the stock having a pH of 7.5. The stock concentration was 5.0 g/1. The CSF-value when no chemi¬ cals had been added was 200 ml. In all the tests, alum was added to the stock in an amount of 1 kg/ton, based on dry fibres, before the addition of cationic starch and silica- based sol. The cationic starch employed was a conventional medium-high cationised starch, Raisamyl 142, having a degree of substitution of 0.042. The cationic starch was added to the stock in an amount of 8 kg/ton, based on dry fibres. The stock was agitated for 30 sec, whereupon silica-based sol was there¬ after added in an amount of 1.0, 2.0 and 4.0 kg/ton, respec¬ tively, calculated as dry on dry fibres. After another 15 sec, the stock was transferred to the freeness apparatus. Example 3 (Comparative tests)

The dewatering effect was examined for reference sols A,

B, E, F, G and H according to Example 1. Table 1 shows the dewatering effect (ml CSF) for the sols added in different amounts (kg/ton) .

Table 1

Amount Sol A Sol B Sol E Sol F Sol G Sol H kσ/ton ml CSF ml CSF ml CSF ml CSF ml CSF ml CSF

1.0 615 610 535 530 500 460

2.0 620 625 600 600 555 525

4.0 625 630 630 625 600 580

Example 4

In this Example, the dewatering effect was evaluated for sols AE, AF, AG and AH according to the invention prepared in Example 1. Table 2 shows the results. Table 2

Amount Sol AE Sol AF Sol AG Sol AH kσ/ton ml CSF ml CSF ml CSF ml CSF

1.0 630 625 625 625

2.0 635 640 640 640

4.0 640 645 635 630

As is clearly evident from Table 2, improved dewatering was obtained with the sols according to the invention compared with sols A, E, F, G and H, respectively (Table 1) , from which they are prepared. Example 5

In this Example, the dewatering effect was evaluated for sols BE, BF, BG, BH according to the invention prepared in Example 1. Table 3 shows the results. Table 3

Amount Sol BE Sol BF Sol BG Sol BH kσ/ton ml CSF ml CSF ml CSF ml CSF 2.0 640 640 635 635

4.0 645 645 650 635 As is evident, all the sols resulted in a substantially higher dewatering effect than sols B, E, F, G and H, respect¬ ively, according to Table 1. Examples 6-8 In these Examples, the dewatering effect was likewise evaluated by means of a Canadian Standard Freeness (CSF) Tester. The chemicals were added to a baffled Britt Dynamic Drainage Jar with a blocked outlet at 1000 rpm, and the stock system was then transferred to the freeness apparatus. The stock was a standard stock based on a pulp composed of 60% bleached birch sulphate + 40% bleached pine sulphate. 30% chalk was added to the pulp as a filler, whereupon the pulp was diluted to a concentration of 4 g/1. 0.3 g/1 of Na₂SO₄.10H₂O was thereafter added. The stock had a pH of 8.1. In the tests, alum was added first in an amount of 1 kg/ton, based on dry stock system (fibres + filler) , unless otherwise indicated.

The cationic polymer was a cationic starch of the same type as in Examples 3-5, which was added in an amount of 8.0 kg/ton of dry stock system. The stock was agitated for 25 sec, whereupon silica-based sol was added in amounts of from 0.5 to 1.0 kg/ton of dry stock system. After another 15 sec, the stock was transferred to the freeness apparatus. Example 6 In this Example, the dewatering effect was evaluated for reference sols B and I and inventive sols BI95, BI90 and BI prepared in Example 1. Alum was not used in this Example. Table 4 shows the dewatering effect (ml CSF) for the sols added in different amounts (kg/ton) . Table 4

Amount Sol B Sol I BI95 BI90 BI kσ/ton ml CSF ml CSF ml CSF ml CSF ml CSF

0.5 450 270 - 470 455

0.75 470 300 490 500 -

1.0 500 325 515 520 500

The results show that the sols according to the invention resulted in a higher dewatering effect than sol B and I, respectively, from which they are prepared. Example 7

In this Example, the dewatering effect was evaluated for reference sols C, D and I and inventive sols CD and CI prepa¬ red in Example 1. Table 5 shows the results. Table 5

Amount Sol C Sol D Sol I Sol CD Sol CI kσ/ton ml CSF ml CSF ml CSF ml CSF ml CSF 0.5 465 385 270 485 470 1.0 565 475 325 565 560 As is evident, improved dewatering was obtained with the sols according to the invention as compared with sols C, D and

1, respectively. Example 8

In this Example, the dewatering effect was evaluated for the silica-based sol containing bentonite prepared in Example

2. Table 6 shows the results.

Table 6

Amount Sol Bib kσ/ton ml CSF

0.5 370

1.0 470

2.0 510

Claims

1. A silica-based sol, c h a r a c t e r i s e d in that it contains silica-based anionic particles having a high specific surface area within the range of from 425 to 1700 m²/g and silica-based anionic particles having a low specific surface area within the range of from 50 to 400 m²/g, the silica-based anionic particles having the high specific surface area being included in an amount of at least 50% by weight, calculated on the total amount of silica-based particles.

2. A silica-based sol, c h a r a c t e r i s e d in that it comprises a mixture of a sol containing silica-based anionic particles having a high specific surface area within the range of from 425 to 1700 m²/g, and a sol containing silica-based anionic particles having a low specific surface area within the range of from 50 to 400 m²/g, the sol containing silica-based anionic particles having the high specific surface area being included in an amount of at least 50% by weight, calculated on the total amount of silica-based particles.

3. A silica-based sol according to claim 1 or 2, c h a r a c t e r i s e d in that the silica-based anionic particles having the high specific surface area have a speci¬ fic surface area within the range of from 450 to 1400 m²/g.

4. A silica-based sol according to claim l or 2, c h a r a c t e r i s e d in that the silica-based anionic particles having the low specific surface area have a specific surface area within the range of from 70 to 375 m²/g.

5. A silica-based sol according to claim 1, 2 or 4, c h a r a c t e r i s e d in that the silica-based anionic particles having the low specific surface area are substan¬ tially spherical, discrete particles.

6. A silica-based sol according to any of the preceding claims, c h a r a c t e r i s e d in that the sol contains silica-based particles having the high specific surface area in an amount of from 60 to 95% by weight, calculated on the total amount of silica-based particles.

7. A silica-based sol according to any of the preceding claims, c h a r a c t e r i s e d in that the particles having the high specific surface area originate from a silica- based sol having an S-value within the range of from 8 to 45% and containing silica particles which have a specific surface area within the range of from 750 to 1000 m²/g and which are modified with aluminium to a degree of from 2 to 25%.

8. A silica-based sol according to any of the preceding claims, c h a r a c t e r i s e d in that the sol further contains hydrated particles of clays of smectite type which are expandable in water.

9. A method for producing a silica-based sol, c h a ¬ r a c t e r i s e d in that it comprises mixing a sol contai¬ ning silica-based anionic particles having a high specific surface area within the range of from 425 to 1700 m²/g, with a sol containing silica-based anionic particles having a low specific surface area within the range of from 50 to 400 m²/g, wherein the resulting sol contains silica-based anionic particles having the high specific surface area in an amount of at least 50% by weight, calculated on the total amount of silica-based particles.

10. Use of a silica-based sol containing silica-based anionic particles having a high specific surface area within the range of from 425 to 1700 m²/g and silica-based anionic particles having a low specific surface area within the range of from 50 to 400 m²/g, the silica-based anionic particles having the high specific surface area being included in an amount of at least 50% by weight, calculated on the total amount of silica-based particles, as a flocculating agent in combination with cationic or amphoteric polymers in the production of pulp and paper and for water purification.

11. Use according to claim 10, whereby the silica-based sol is used as a flocculating agent for improving retention and dewatering in papermaking.

12. Use according to claim 10 or 11, whereby the silica- based sol is used in combination with cationic starch or cationic acrylamide-based polymer or a combination thereof.

13. A process for the production of paper by forming and dewatering a suspension of cellulose containing fibres, and optional fillers, on a wire, c h a r a c t e r i s e d in that the suspension is formed and dewatered in the presence of cationic or amphoteric polymers and a silica-based sol com¬ prising a mixture of a sol containing silica-based anionic particles having a high specific surface area within the range of from 425 to 1700 m²/g, and a sol containing silica-based anionic particles having a low specific surface area within the range of from 50 to 400 m²/g, the sol containing silica- based anionic particles having the high specific surface area being included in an amount of at least 50% by weight, calcu¬ lated on¹ the total amount of silica-based particles.

14. A process according to claim 13, c h a r a c t e ¬ r i s e d in that the cationic or amphoteric polymers are selected from cationic starch, cationic acrylamide-based polymer and a combination thereof.