CN1912242B - Paper making method - Google Patents

Abstract

A process for the production of paper which includes the steps of: (a) providing an aqueous suspension containing cellulosic fibers, and optional fillers; (b) an aqueous sol containing silica-based particle having a specific surface area of at least 90 m<2>/g aqueous sol, and the particles contained in the sol having a specific surface area less than 1000 m<2>/g SiO2; (c) at least one charged organic polymer; (d) adding the charged organic polymer and the aqueous sol containing silica-based particle to the suspension: and (e) carrying out formation of the obtained suspension on a mesh and drainage thereof.

Description

method of producing paper

This application is a divisional application of the Chinese patent application 00807147.0 filed on April 28, 2000 with the title of invention "Silica-based Colloidal Sol".

The present invention generally relates to a method of producing paper.

background

In the papermaking field, an aqueous suspension comprising cellulosic fibers and optional fillers and additives, called stock, is fed to the headbox and the stock is then sprayed onto the forming wire. Water is discharged from the stock through the forming wire, thus forming a wet paper web on the wire, which is then further dewatered and dried in the drying section of the paper machine. Drainage and retention aids are often added to the stock to aid in drainage and to increase the adsorption of fines to the cellulose fibers so that they remain on the web with the fibers.

Silica-based particles are widely used as drainage and retention aids in combination with charged organic polymers such as anionic and cationic acrylamide-based polymers and cationic and amphoteric starches. These additive systems are disclosed in US Patents 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 the most effective drainage and retention aids in use today.

Silica-based particles suitable as drainage and retention aids are usually supplied in the form of aqueous colloidal dispersions, so-called sols. Commercially used silica-based sols generally have a silica content of about 7-15% by weight and contain particles having a specific surface area of at least 300 ^m2 /g. Sols of silica-based particles with higher specific surface areas are generally too dilute to improve storage stability and avoid gel formation.

It would be advantageous to be able to provide silica-based sols and particles with further improved drainage and retention properties and even better stability. It would also be advantageous to be able to provide a way to prepare 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.

this invention

According to the present invention there are provided silica-based sols and particles suitable for use as flocculants in water purification and as drainage and retention aids in papermaking. The silica-based sols and particles according to the invention have good long-term stability, significantly high surface area stability and high gel stability, so that they can be prepared and transported with high specific surface areas and high silica concentrations. Silica-based sols and particles are more capable of maintaining a high specific surface area upon storage at high silica concentrations. The silica-based sols and particles according to the invention also have very good or improved drainage and retention when used in combination with anionic, cationic and/or amphoteric organic polymers. Thus, the silica-based sols and particles according to the present invention allow for increased paper machine speed and the use of lower additive dosages to obtain corresponding drainage and/or retention effects, thereby improving the papermaking process and economics. The present invention thus relates to silica-based particles and hydrosols comprising silica-based particles, also referred to herein as silica-based sols, and methods for their production, as further defined in the appended claims.

The present invention also relates to the use of silica-based sols and particles, preferably in combination with the organic polymers described herein, as drainage and retention aids in papermaking, as further defined in the appended claims. As used herein, the term "drainage and retention aid" refers to one or more components (aids, aids, reagents or additives). The invention further relates to a method for producing paper from an aqueous suspension comprising cellulose fibers and optionally fillers, comprising adding to the suspension silica-based particles and at least one charged organic polymer, the suspension The liquid forms and drains on the mesh. The invention thus relates to a method as further defined in the appended claims.

A silica-based sol according to the invention is an aqueous sol comprising anionic silica-based particles, ie particles based on silica (SiO ₂ ) or silicic acid. The particles are preferably colloidal, ie in the colloidal size range. The average particle size of the silica-based particles present in the sol is less than about 10 nm and preferably about 10-2 nm. As is common knowledge in silica-based colloidal particle chemistry, particle size refers to the average size of primary particles, which may or may not be aggregated.

The specific surface area of the silica-based sol is suitably at least 80 ^m2 /g hydrosol, i.e. based on the weight of the hydrosol, preferably at least 85 ^m2 /g hydrosol, more preferably at least 90 ^m2 /g hydrosol, most preferably at least 95 ^m2 /g hydrosol. In a preferred embodiment of the present invention, the specific surface area of the hydrous silica-based sol is suitably at least 115 ^m2 /g hydrosol, preferably at least 120 ^m2 /g hydrosol. Generally, the specific surface area of the hydrosol may be up to about 200 ^m2 /g hydrosol, suitably up to 150 ^m2 /g hydrosol, preferably up to 130 ^m2 /g hydrosol.

The specific surface area of the silica-based particles is suitably at least 300 ^m2 /g _SiO2 , ie based on the weight of _SiO2 , preferably at least 400 ^m2 /g _SiO2 , most preferably at least 550 ^m2 /g _SiO2 . In general, the specific surface area of the particles may be up to about 1200 ^m2 /g _SiO2 , suitably below 1000 ^m2 /g _SiO2 , preferably up to 950 ^m2 /g _SiO2 . In a preferred embodiment of the invention, the specific surface area of the particles is in the range of 550-725 m ² /g SiO ₂ , preferably 575-700 m ² /g SiO ₂ . In another preferred embodiment of the invention, the particles have a specific surface area in the range of 775-1200 ^m2 /g _SiO2 , preferably 800 to less than 1000 ^m2 /g _SiO2 .

After appropriate removal or adjustment of any compounds present in the sample that might disturb the titration, such as aluminum and boron species, the specific surface area can be determined as described, for example, in Sears' Analytical Chemistry 28 (1956): 12, 1981-1983 and U.S. Patent 5,176,891 In a known manner, it is determined by titration with sodium hydroxide. If expressed in square meters per gram of hydrosol, the specific surface area means the specific surface area per gram of hydrous silica-based sol. The specific surface area, if expressed in square meters per gram of silica, means the average specific surface area of the silica-based particles present in the sol.

Silica-based sols typically have an S value in the range of 10-45%, suitably 20-40%, preferably 25-35%. The S value can be determined and calculated as described in Iler & Dalton , Journal of Physical Chemistry 60 (1956), 955-957. The S value indicates the degree of aggregation or microgel formation and a lower S value indicates a higher degree of aggregation.

Silica-based sols typically have a molar ratio of _SiO2 to _M2O of at least 10:1, suitably at least 12:1, preferably at least 15:1, where M is an alkali metal ion (e.g., Li, Na, K) and/or or ammonium. The molar ratio of _SiO2 to _M2O may generally be up to 100:1, suitably up to 40:1, preferably up to 30:1. The preferred range is thus 10:1 to 100:1, especially 15:1 to 30:1. The silica-based sol typically has a pH of at least 8.0, suitably at least 10.0, preferably at least 10.5, more preferably at least 10.6. The pH may be up to about 11.5, suitably up to 11.0.

The silica-based sol should suitably have a silica content of at least 3% by weight, but more suitably the silica content is in the range of 10-30% by weight, preferably 12-25% by weight. To simplify and reduce shipping costs, it is generally preferred to ship high concentrations of silica-based sols, although it is possible and often preferred to dilute and mix silica-based sols and granules to substantially lower silica contents, such as 0.05-5 The silica content in the weight % range, which improves the mixing with the ingredients of the ingredients. The viscosity of silica-based sols can vary, for example, depending on the silica content of the sol. Typically, the viscosity is at least 5 centipoise, usually 5-40 centipoise, suitably 6-30 centipoise, preferably 7-25 centipoise. The viscosity, preferably measured in terms of sols having a silica content of at least 10% by weight, can be determined by known techniques, for example using a Brookfield LVDV II+ viscometer.

The silica-based sols of the invention are preferably stable. As used herein, the term "stable silica-based sol" refers to a silica-based sol that increases in viscosity by less than 100 centipoise when stored or aged for 1 month at 20°C in the dark and unagitated state. If the sol is subjected to the conditions described above, the viscosity increase, if any, is preferably less than 50 centipoise, preferably less than 30 centipoise.

In a preferred embodiment of the present invention, the silica-based sol is substantially free of aluminum, ie no aluminum-containing modifiers have been added. In another preferred embodiment of the present invention, the silica-based sol is substantially free of boron, ie no boron-containing modifiers have been added. However, small amounts of these elements may be present in the starting materials used to prepare silica-based sols and particles. In another preferred embodiment of the invention, the silica-based sol is modified with various elements present in the aqueous phase and/or in the silica-based particles, such as aluminum and/or boron. If aluminum is used, the molar ratio of _Al2O3 to _SiO2 of the sol is in the range of 1:4-1: ₁₅₀₀ , suitably 1:8-1:1000, preferably 1:15-1:500. If boron is used, the molar ratio of B to _SiO2 of the sol is in the range of 1:4-1:1500, suitably 1:8-1:1000, preferably 1:15-1:500. If both aluminum and boron are used, the molar ratio of Al to B may be in the range of 100:1 to 1:100, suitably 50:1 to 1:50.

The silica-based sols and particles according to the invention can be prepared starting from conventional aqueous silicate solutions such as alkaline waterglass, eg potassium or sodium waterglass, preferably sodium waterglass. In silicate solution or water glass, the molar ratio of _SiO2 to _M2O is suitably in the range of 1.5:1-4.5:1, preferably 2.5:1-3.9:1, wherein M is an alkali metal such as sodium, Potassium, ammonium or mixtures thereof. Preferably a dilute silicate solution or water glass is used, the _SiO2 content of which may be about 3-12% by weight, preferably about 5-10% by weight. A silicate solution or water glass, typically at a pH of about 13 or higher, is acidified to a pH of about 1-4. Acidification can be carried out in a known manner by adding mineral acids such as sulfuric acid, hydrochloric acid and phosphoric acid, or if desired using other chemicals suitable for the acidification of water glasses such as ammonium sulfate and carbon dioxide. If a mineral acid is added, the acidification is preferably carried out in two steps, the first step to a pH of about 8-9, where a certain maturation, ie, particle growth, and then further acidification to a pH of about 1-4 takes place. Preferably, however, the acidification is carried out with an acid cation exchanger which, inter alia, leads to a more stable product. Acidification is preferably performed using a strong acid cation exchange resin, such as a sulfonic acid type resin. Preferably, acidification is performed to a pH of about 2-4, most preferably about 2.2-3.0. The resulting product (acid sol or polysilicic acid) comprises silica-based particles with a high specific surface area, typically exceeding 1000 ^m2 /g SiO2 _and usually about 1300-1500 ^m2 /g _SiO2 .

The acid sol is subsequently basified, referred to herein as the first basification step. The first basification can be carried out by adding conventional bases, such as lithium hydroxide, sodium hydroxide, potassium hydroxide, ammonium hydroxide and mixtures thereof, and/or aqueous silicate solutions as defined above. Potassium and sodium waterglass, especially sodium waterglass, with the molar ratio of _SiO2 to _M2O as defined above, are suitable for this alkalization step. The _SiO2 content of the water glass solution used for the first alkalization is suitably in the range of about 3-35% by weight, preferably 5-30% by weight. The first basification is usually carried out to a pH of at least 6, suitably at least 7, preferably at least 7.5, and usually up to a pH of 10.5, suitably up to 10.0. The first alkalization is further suitably carried out to a final molar ratio of SiO ₂ to M ₂ O below 100:1, suitably in the range of about 20:1-80:1, preferably 30:1-70:1, wherein M is defined as above. In preparing sols with low S values as described above, the degree of microgel can be influenced and controlled to the desired value in several ways. The degree of microgel is influenced by the salt content by adjusting the concentration during the preparation of the acid sol and in the first alkalization step, since in this step the degree of microgel is lowest when the sol passes through a pH value of about 5 Stability is affected. It is particularly suitable to control the degree of microgelation by adjusting the dry content ( _SiO2 content) in the first alkalization step, such that a higher dry content results in a lower S-value. By keeping the _SiO2 content in the range of 4.5-8 wt% during the first alkalization step, the S value can be controlled to a desired value of eg 10-45%. In order to obtain sols with S values in the range of 20-40%, the _SiO2 content in the first alkalization step is suitably kept in the range of 5.0-7.5 wt%.

The silica-based particles present in the alkalized sol obtained in the first alkalization step are subsequently subjected to particle growth, which results in particles with a lower specific surface area and higher stability. The particle growth process should be suitably performed to obtain a specific surface area of at least 300 ^m2 /g _SiO2 , preferably at least 400 ^m2 /g SiO2 _, most preferably at least 550 ^m2 /g _SiO2 , and up to about 1200 ^m2 /g SiO ₂ , suitably below 1000 m ² /g SiO ₂ , especially silica-based particles up to 950 m ² /g SiO ₂ . In a preferred embodiment of the invention, the particle growth process is carried out so as to obtain particles with a specific surface area of 550-725 m ² /g SiO ₂ , preferably 575-700 m ² /g SiO ₂ . In another preferred embodiment of the invention, the particle growth process is carried out to obtain particles with a specific surface area of 775-1200 ^m2 /g _SiO2 , preferably 800-1000 ^m2 /g _SiO2 . The reduction in surface area can be achieved by storage at room temperature for somewhat longer periods (1 day up to about 2 days and nights) or preferably by heat treatment. During heat treatment, time and temperature can be adjusted so that shorter times can be used at higher temperatures. Even though it is of course possible to use higher temperatures for very short periods of time, it is practically more appropriate to use lower temperatures for somewhat longer periods of time. Upon heat treatment, the basified sol should suitably be heated to a temperature of at least 30°C, suitably 35-95°C, preferably 40-80°C. The heat treatment should suitably be performed for at least 10 minutes, suitably 15-600 minutes, preferably 20-240 minutes.

After the particle growth process and optional cooling, the resulting silica-based sol is here basified, here referred to as the second basified step, to further increase the pH. The second basification can be carried out by adding conventional bases, such as lithium hydroxide, sodium hydroxide, potassium hydroxide, ammonium hydroxide and mixtures thereof, and/or aqueous silicate solutions as defined above. Potassium and sodium waterglass, especially sodium waterglass, with the molar ratio of _SiO2 to _M2O as defined above, are suitable for the second alkalization step. The _SiO2 content of the water glass solution used for the second alkalization is suitably in the range of about 3-35% by weight, preferably 5-30% by weight. The second basification is generally carried out to a pH of at least 8.0, suitably at least 10.0, preferably at least 10.5, most preferably at least 10.6. The pH may be up to 11.5, suitably up to 11.0. The second alkalization is further suitably carried out to a final molar ratio of _SiO2 to _M2O in the range of about 10:1-100:1, suitably 12:1-40:1, preferably 15:1-30:1 , where M is defined as above.

In a preferred embodiment of the invention, the process also includes concentration of the silica-based sol. Concentration can be performed after the second alkalization. Additionally, the resulting alkalized sol after the first alkalization but after the particle growth or heat treatment step, or the resulting sol after the particle growth or heat treatment step but before the second alkalization may be concentrated. Concentration can be performed by known means, for example by osmosis, evaporation or ultrafiltration. Concentration is suitably carried out so as to obtain a silica content of at least 10% by weight, preferably 10-30% by weight, more preferably 12-25% by weight. Concentration is further suitably carried out such that the specific surface area of the silica-based sol obtained in the process is at least 80 ^m2 /g hydrosol, preferably at least 85 ^m2 /g hydrosol, more preferably at least 90 m2 ^/g , based on the weight of the hydrosol. grams of hydrosol, most preferably at least 95 ^m2 /gram of hydrosol. In a preferred embodiment of the present invention, the resulting suitable silica-based sol has a specific surface area suitably at least 115 ^m2 /g hydrosol, preferably at least 120 ^m2 /g hydrosol. Generally, the resulting hydrosol may have a specific surface area of up to about 200 ^m2 /g hydrosol, suitably up to 150 ^m2 /g hydrosol, preferably up to 130 ^m2 /g hydrosol.

Silica-based sols and particles can be modified, if desired, by adding compounds comprising, for example, aluminum and/or boron. Suitable aluminum-containing compounds include aluminates such as sodium aluminate and potassium aluminate, suitably sodium aluminate. The aluminum-containing compound is suitably used in the form of an aqueous solution. Suitable boron-containing compounds include boric acid, borates such as sodium borate and potassium borate, suitably sodium borate, tetraborates such as sodium tetraborate and potassium tetraborate, suitably sodium tetraborate, and metaborates such as Sodium metaborate and potassium metaborate. The boron-containing compound is suitably used in the form of an aqueous solution.

If an aluminum-containing compound is used in the process, it is preferably added to the sol subjected to grain growth or heat treatment either before or after the second alkalization step. Alternatively, the aluminum-containing compound can be added to the acidified silicate solution, to the acid sol or to the basified sol obtained in the first basified step before the grain growth or heat treatment step. The aluminum-containing compound may be added in admixture with the acid during the acidification step or with the base or silicate solution in any basification step. The aluminum-containing compound is added _in such an amount that the resulting sol has the molar ratio of _Al2O3 to _SiO2 as defined above.

If a boron-containing compound is used in the process, it is preferably added to the sol undergoing grain growth or heat treatment either before or after the second alkalization step. In addition, the boron-containing compound can be added to the silicate solution to be acidified, to the acid sol or to the basified sol obtained in the first basification step before the grain growth or heat treatment step. The boron-containing compound may be added in admixture with the acid during the acidification step or with the base or silicate solution in any basification step. The boron-containing compound is added in an appropriate amount such that the resulting sol has a molar ratio of B to _SiO2 as defined above. If both aluminum-containing compounds and boron-containing compounds are used, they are added in such amounts that the resulting sol has the molar ratio of Al to B as defined above.

If the sol contains too large amounts of alkali metal ions or ammonium ions prior to any aluminum and/or boron modification, it is preferable to remove at least some of these ions, for example by ion exchange, to obtain a final molar ratio of _SiO2 to _M2O in the above-mentioned desired range Silica-based sol inside.

According to the method of the present invention, it is possible to prepare silica-based sols and particles having the above characteristics, especially stable silica-based sols and particles, and the resulting sols have good storage stability and can be stored for several months without any significant specific surface area. drop without gelling.

The silica-based sols and particles according to the invention are suitable, for example, as flocculants in the manufacture of pulp and paper, in particular as drainage and retention aids, and in the field of water purification, for the purification of different kinds of waste water and for For purification of white liquor especially from the pulp and paper industry. Silica-based sols and particles can be used as flocculants in combination with organic polymers, also referred to herein as "primary polymers", selected from the group consisting of anionic, amphoteric, nonionic and cationic polymers and mixtures thereof, especially for drainage and Keep the aids. The use of these polymers as flocculants and as drainage and retention aids is well known in the art. Polymers may be of natural or synthetic origin, and they may be linear, or branched or crosslinked. Examples of generally suitable host polymers include anionic, amphoteric, and cationic starches, anionic, amphoteric, and cationic guar gums, and anionic, amphoteric, and cationic acrylamide-based polymers, and cationic poly(diallyldimethyl chloride ammonium), cationic polyethyleneimine, cationic polyamine, polyamidoamine and vinylamide based polymers, melamine-formaldehyde and urea-formaldehyde resins. Suitably, the silica-based sol is used in combination with at least one cationic or amphoteric polymer, preferably a cationic polymer. Cationic starch and cationic polyacrylamide are particularly preferred polymers and they can be used alone, together with each other or with other polymers such as other cationic polymers or anionic polyacrylamides. The molecular weight of the main polymer is suitably above 1,000,000, preferably above 2,000,000. The upper limit is not critical; it may be around 50,000,000, usually 30,000,000, suitably around 25,000,000. However, polymers of natural origin can have higher molecular weights.

If the silica-based sols and particles of the present invention are used in combination with the aforementioned primary polymers, it is further preferred to use at least one low molecular weight (hereinafter LMW) cationic organic polymer commonly known and used as an anionic waste catcher (ATC). ATC is known in the art as a neutralizing and/or immobilizing agent for deleterious anionic species present in raw materials, and its use in combination with drainage and retention aids can often further improve drainage and/or retention. The LMW cationic organic polymers may be of natural or synthetic origin and are preferably LMW synthetic polymers. Suitable such organic polymers include LMW highly charged cationic organic polymers such as polyamines, polyamidoamines, polyethyleneimines, based on diallyldimethylammonium chloride, (meth)acrylamide and Homopolymers and copolymers of (meth)acrylates. With regard to the molecular weight of the main polymer, the molecular weight of the LMW cationic organic polymer is preferably relatively low; it is suitably at least 1000, preferably at least 10000. The upper limit of molecular weight is usually about 700,000, suitably about 500,000, usually about 200,000. Preferred combinations of polymers that can be used with the silica-based sols of the present invention include combinations of LMW cationic organic polymers with host polymers such as cationic starch and/or cationic polyacrylamide, anionic polyacrylamide, and cationic starch and/or cationic A combination of polyacrylamide and anionic polyacrylamide.

The components of the drainage and retention aids according to the invention may be added to the stock in conventional manner and in any order. If using a drainage and retention aid comprising silica-based particles and an organic polymer, such as a primary polymer, it is preferred to add the polymer to the stock before adding the silica-based particles, even the reverse order of addition may be used. It is further preferred to add the primary polymer before a shearing step selected from pumping, mixing, cleaning etc. and to add the silica-based particles after this shearing step. If used, the LMW cationic organic polymer is preferably added to the feed prior to the addition of the main polymer. Alternatively, the LMW cationic organic polymer and the host polymer may be added to the feedstock substantially simultaneously (separately or in admixture), such as disclosed in US Pat. No. 5,858,174, which is incorporated herein by reference. The LMW cationic organic polymer and the host polymer are preferably added to the feedstock prior to the addition of the silica-based particles.

In a preferred embodiment of the present invention, silica-based sols and particles are used as drainage and retention aids in combination with at least one of the aforementioned organic polymers and at least one aluminum compound. Aluminum compounds can be used to further enhance the drainage and/or retention properties of feedstock additives comprising silica-based particles. Suitable aluminum salts include alum, aluminates, aluminum chloride, aluminum nitrate and polyaluminum compounds such as polyaluminum chloride, polyaluminum sulfate, polyaluminum compounds containing both chloride and sulfate ions, polyaluminum silicate- Aluminum sulfate, and mixtures thereof. The polyaluminum compound may also contain other anions such as ions of ionic phosphoric acid, organic acids such as citric acid and oxalic acid. Preferred aluminum salts include sodium aluminate, alum and polyaluminum compounds. The aluminum compound can be added before or after the addition of the silica-based particles. Alternatively, the aluminum compound may be added substantially simultaneously with the silica-based sol (alone or mixed therewith), such as disclosed in US Pat. No. 5,846,384, which is incorporated herein by reference. In many cases it is often appropriate to add the aluminum compound to the feedstock early in the process, eg before other additives.

The amount of addition of the components of the drainage and retention aid according to the invention to the raw material to be dewatered can vary within wide limits, depending inter alia on the type and number of components, the type of ingredients, the filler content, the type of filler, the point of addition ,wait. In general, the amounts of the components added result in better drainage and/or retention than would be achieved without the addition of the components. Silica-based sols and particles are generally added in an amount of at least 0.001% by weight, usually at least 0.005% by weight, calculated as _SiO2 and based on dry raw material, i.e. cellulose fibers and optional fillers, with an upper limit generally of 1.0%, Suitably 0.5% by weight. The primary polymer is generally added in an amount of at least 0.001%, usually at least 0.005% by weight based on dry raw material, and the upper limit is generally 3%, suitably 1.5% by weight. If an LMW cationic organic polymer is used in the process, it is added in an amount of at least 0.05% based on the dry matter of the feedstock to be dehydrated. Suitably, this amount is 0.07-0.5%, preferably 0.1-0.35%. If an aluminum compound is used in the process, the total amount added to the feedstock to be dehydrated depends on the type of aluminum compound used and the desired effect from it. For example, the use of aluminum compounds as precipitation agents for rosin-based sizing agents is well known in the art. The total amount added is usually at least 0.05%, calculated as Al ₂ O ₃ and based on dry raw material. The amount is suitably 0.1-3.0%, preferably 0.5-2.0%.

Other additives commonly used in papermaking can of course be used in combination with the additives according to the invention, for example dry strength agents, wet strength agents, optical brighteners, dyes, sizing agents such as rosin sizing agents and cellulose reactive sizing agents, Such as alkyl and alkenyl ketene dimers and ketene polymers, alkyl and alkenyl succinic anhydrides, and the like. The cellulosic suspension or raw material may also contain mineral fillers of the usual kind, for example kaolin, china clay, titanium dioxide, gypsum, talc and natural and synthetic calcium carbonates such as chalk, crushed marble and precipitated calcium carbonate.

The method of the invention can be used for the production of paper. The term "paper" as used herein of course includes not only paper and its production, but also other sheet or web-like products containing cellulose fibers such as board and paperboard, and their production. The method can be used for the production of paper from different kinds of cellulosic fibre-containing suspensions, and the suspension should suitably contain at least 25% by weight, preferably at least 50% by weight of these fibres, based on dry matter. The suspension can be based on fibers from chemical pulps such as kraft, sulfite and organosol pulps, mechanical pulps such as thermodynamic, chemo-thermodynamic, refinery and softwood pulps, and also optionally from deinking Recycled fibers of pulp, and mixtures thereof. The pH of the suspension, feedstock, may be in the range of about 3-10. The pH is suitably above 3.5, and preferably 4-9.

The invention is further illustrated by the following examples, but it is not intended to be limited thereto. Unless otherwise indicated, parts and % relate to parts by weight and % by weight, respectively.

Example 1

Standard silica sols were prepared as follows:

SiO ₂ and Na ₂ O molar ratio is 3.3 and SiO content is 27.1% 762.7 grams of sodium water glass diluted with water to 3000 g, to obtain a _SiO content of 6.9% by weight silicate solution (I ₎ . 2800 grams of the silicate or water glass solution was passed through a column packed with a strong acid cation exchange resin saturated with hydrogen ions. 2450 g of ion-exchanged water glass or polysilicate (II) with a _SiO content of 6.5% by weight and a pH of 2.4 were collected from the exchanger. 1988 grams of polysilicic acid (II) was charged to the reactor and diluted with 12.3 grams of water. Then 173.9 g of a 6.9% silicate solution (I) were added with vigorous stirring. The resulting solution was then heated at 85°C for 60 minutes and then cooled to 20°C. Gained silica sol (1a) has the following characteristics:

Sol 1a (reference): _SiO2 content = 7.3% by weight, molar ratio _SiO2 / _Na2O = 40, pH value = 10.2, S value = 29%, viscosity = 2.2 cps, specific surface area = 530 ^m2 /g SiO ₂ and 39 m ² /g hydrosol.

Two additional silica sols (sol 1b and sol 1c) were produced with the following properties:

Sol 1b (reference): _SiO2 content = 7.3% by weight, molar ratio _SiO2 / _Na2O = 63, pH value = 10.0, S value = 26%, viscosity = 2.7 centipoise, specific surface area = 500 ^m2 /g SiO ₂ and 36.5 m ² /g hydrosol.

Sol 1c (reference): _SiO2 content = 5.4% by weight, molar ratio _SiO2 / _Na2O = 35, pH value = 9.8, S value = 32%, viscosity = 1.6 cps, specific surface area = 690 ^m2 /g SiO ₂ and 37 m ² /g hydrosol.

Example 2

The 6 silica-based particles according to the invention were prepared from polysilicic acid similar to polysilicic acid (II) made by the same ion exchange process and having a _SiO2 content of 5.46% by weight. 1.46 kg of sodium water glass with a SiO ₂ /Na ₂ O ratio of 3.3 was added to 102.0 kg of polysilicic acid under vigorous stirring to obtain a solution with a molar ratio of SiO ₂ /Na ₂ O of 54.0. The solution was heat treated at 60 °C for 2 h 20 min and cooled to 20 °C, whereby the product was concentrated to a _SiO2 content of 15.6 wt%. This intermediate sol product is now divided into 6 separate samples af. Samples ac were further basified with sodium hydroxide and samples df were treated with water glass to obtain sols with a _SiO2 / _Na2O molar ratio of 21.5-34.0 and a silica content of about 15.0% by weight. The resulting sol of silica-based particles has the properties given in Table 1:

Table 1

sol Molar ratio [SiO ₂ /Na ₂ O] pH value S value [%] Viscosity [centipoise] Specific surface area [m ² /g SiO ₂ ] Specific surface area [ ^m2 /g hydrosol] Sol 2a Sol 2b Sol 2c Sol 2d Sol 2e Sol 2f 21.528.034.021.528.033.0 10.710.310.010.710.310.0 313029312929   172940203438 720710690680670680   108.0106.5103.5102.0100.5102.0

Example 3

Polysilicic acid (II) produced by the above ion exchange process as in Example 2 and basified with water glass to a molar ratio SiO ₂ /Na ₂ O of 54.0 was heat-treated at 60° C. for 1 hour. To 58 kg of this product was added 7.25 kg of diluted water glass having a molar ratio SiO ₂ /Na ₂ O of 3.3 and a silica content of 5.5% by weight. The resulting sol of silica-based particles was concentrated to a silica content of 15.2% by weight and a molar ratio _SiO2 / _Na2O =24, pH=10.7, S-value=34, viscosity=9.0 cps and specific surface=760 ^m2 /g SiO ₂ and 115.5 m ² /g hydrosol.

Example 4

1000 g of polysilicic acid (II) with a SiO content of 5.5% by weight and 14.5 g of a water glass solution with a _SiO content of 27.1% by _weight and a molar ratio _SiO / _NaO of 3.3 were mixed under vigorous stirring to obtain A product with a molar ratio SiO ₂ /Na ₂ O of 51 and a silica content of 5.8% by weight was then heated at 60° C. for 1.5 hours and then concentrated to a silica content of 16.7% by weight SiO ₂ . 283 g of the resulting product were mixed with 33.0 g of sodium hydroxide to obtain a sol of silica-based particles (sol 4) with _SiO2 content = 15.2% by weight, molar ratio _SiO2 / _Na2O = 21, pH = 10.6, S value = 32%, viscosity = 14.2 centipoise and specific surface area = 720 ^m2 /g _SiO2 and 109.4 ^m2 /g hydrosol.

Example 5

The general procedure as in Example 3 was repeated except that the heat treatment was carried out for 1.25 hours and the concentration was carried out to a higher silica content. Two silica-based particle sols were prepared; Sol 5a and Sol 5b. _SiO2 content of sol 5a = 18% by weight, molar ratio _SiO2 / _Na2O = 18, pH value = 10.7, S value = 36%, viscosity = 18 cps and specific surface area = 700 ^m2 /g _SiO2 and 126 ^m2 /g hydrosol. _SiO2 content of sol 5b = 20% by weight, molar ratio _SiO2 / _Na2O = 18, pH value = 10.7, S value = 37%, viscosity = 31 centipoise and specific surface area = 700 ^m2 /g _SiO2 and 140 ^m2 /g hydrosol.

Example 6

Drainage performance was assessed using a Dynamic Drainage Analyzer (DDA) from Akribi, Sweden, which measures the time for a set volume of material to drain through the wire when the plug is removed and a vacuum is applied to the side of the wire opposite that where the material is present .

The raw material used was based on a blend of 60% bleached birch kraft pulp and 40% bleached pine kraft pulp to which 30% ground calcium carbonate was added as filler. The stock volume was 800 ml, the consistency was 0.25% and the pH was about 8.0. The conductivity of the raw material was adjusted to 0.47 mS/cm by adding sodium sulfate.

In the test, the silica-based sol was used in combination with the cationic polymer Raisamyl 142, which is a conventional medium-high cationic starch with a degree of substitution of 0.042, at an amount of 12 kg/ton (based on the dry weight of the dry raw material system). Starch meter) was added to the raw material. Silica-based sols according to Examples 1-4 were tested in this example. Also, for comparison, sols 6a and 6b were tested. Sol 6a is a commercially available silica sol with S value = 45%, _SiO2 content = 15.0% by weight, molar ratio _SiO2 / _Na2O = 40, viscosity = 3.0 cps, specific surface area = 500 ^m2 /g SiO ₂ and 75 m ² /g hydrosol. Sol 6b is another commercially available silica sol, where S value = 36%, _SiO2 content = 10.0% by weight, molar ratio _SiO2 / _Na2O = 10, viscosity = 2.5 cps, specific surface area = 880 ^m2 / g SiO ₂ and 88 m ² /g hydrosol. The addition amount of silica-based sol is 0.5 kg/ton, calculated as _SiO2 and based on dry raw material system.

Throughout the test, the feedstock was stirred at 1500 rpm in a baffled tank and chemicals were added as follows: i) cationic starch was added to the feedstock followed by stirring for 30 seconds, ii) silica-based sol was added to the feedstock followed by Stir for 15 seconds, iii) drain the material while automatically recording the drain time.

The drainage times of different silica-based sols are given in Table 2:

Table 2

Silica based sol Drainage time [seconds] Sol 1a (Reference) Sol 1b (Reference) Sol 1c (Reference) Sol 2d Sol 3 Sol 4 Sol 6a (Reference) Sol 6b (Reference) 12.011.112.09.79.59.412.09.8

Example 7

The properties were evaluated following the general procedure of Example 6, except that the consistency of the stock was 0.3% and the pH was about 8.5. The retention performance was evaluated by measuring the turbidity of the filtrate (white water obtained by draining the raw material) using a turbidimeter.

The silica-based sol of example 5 according to the invention was tested against sol 6a for comparison. Table 3 presents the drainage times obtained at various dosages (kg/ton) of silica-based particles, calculated as _SiO2 and based on the dry raw material system. Only cationic starch (12 kg/ton, calculated as dry starch based on dry material system) was added, resulting in a drainage time of 15.8 seconds.

table 3

Silica based sol Drainage time (sec)/turbidity (NTU) at the following _SiO2 doses 0.5 kg/ton 1.0 kg/ton 1.5 kg/ton 2.0 kg/ton 3.0 kg/ton Sol 6a (reference) Sol 5a Sol 5b 11.1/-9.0/-8.9/- 8.8/597.1/526.9/- 7.9/586.3/506.3/- 7.1/545.2/525.7/- 6.8/605.7/536.0/-

Claims (7)

1. A method of producing paper, comprising: (a) providing an aqueous suspension comprising cellulosic fibers and optional fillers; (b) providing a substantially aluminum-free hydrosol comprising silica-based particles, said sol having a specific surface area of at least 90 ^m2 /g of hydrosol, and said silica-based particles having a specific surface area of less than 1000 ^m2 / grams of SiO ₂ ; (c) providing at least one charged organic polymer; (d) adding said charged organic polymer and said hydrosol comprising silica-based particles to said suspension; (e) Shaping and draining the resulting suspension on a wire. 2. A method of producing paper, comprising: (a) providing an aqueous suspension comprising cellulosic fibers and optional fillers; (b) providing a hydrosol comprising silica-based particles substantially free of aluminum, having a specific surface area of at least 90 ^m2 /g hydrosol, and an S value of 10-45%; (c) providing at least one charged organic polymer; (d) adding said charged organic polymer and said hydrosol comprising silica-based particles to said suspension; (e) Shaping and draining the resulting suspension on a wire. 3. The method according to claim 1 or 2, characterized in that the specific surface area of the sol is 95-150 ^m2 /g hydrosol. 4. The method according to claim 1 or 2, characterized in that the silica-based particles have a specific surface area of at least 550 ^m2 /g _SiO2 . 5. The method according to claim 1 or 2, characterized in that the charged organic polymer is cationic starch or cationic polyacrylamide. 6. The method according to claim 1 or 2, characterized in that, before adding the hydrosol comprising silica-based particles to the suspension, the hydrosol is diluted to a silica content of 0.05-5% by weight. 7. Process according to claim 1 or 2, characterized in that the hydrosol comprising silica-based particles is added to the suspension in an amount of 0.005-0.5% by weight, calculated as _SiO2 and based on dry cellulose fibers and optional fillers.