MXPA01010726A - Silica-based sols - Google Patents

Abstract

The invention relates to an aqueous sol containing silica-based particles which has an S-value within the range of from 10 to 45%, a viscosity within the range of from 5 to 40 cP, and a molar ratio of SiO2 to M2O, where M is alkali metal or ammonium, within the range of from 10:1 to 40:1, or a silica content of at least 10%by weight. The invention further relates to a process for the production of silica-based particles comprising the steps of:(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 SiO2 content within the range of from 4.5 to 8%by weight to;(c) allowing particle growth of the alkalised sol for at least 10 minutes, or heat-treating the alkalised sol at a temperature of at least 30°C;and then (d) alkalising the obtained sol to a pH of at least 10.0. The invention further relates to silica-based particles obtainable by the process, the use of the silica-based particles as drainage and retention aids in the production of paper as well as a process for the production of paper from an aqueous suspension containing cellulosic fibres, and optional filler, in which silica-based particles and at least one charged organic polymer are added to the cellulosic suspension.

Description

SOLES DE SÍLICE The present invention relates, in general, to silica sols suitable for use in papermaking. More specifically, the invention relates to silica-based sols and silica-based particles, their production and their use in the production of paper. The process of this invention offers silica and sol particles containing silica-based particles with high performance characteristics in drainage and retention, high stability and high solids content.

BACKGROUND In papermaking techniques, an aqueous suspension containing cellulosic fibers and optional fillers and additives, known as pulp, is fed to a headbox that expels pulp over a forming wire. Water is drained from the pulp through the forming wire so that a wet paper web is formed on the wire, and the paper web is dehydrated further and dried in the drying section of the machine to manufacture paper. Auxiliary are usually introduced for drainage and retention in the paper pulp to facilitate drainage and to increase the adsorption of the fine particles on the cellulosic fibers so that these are retained with the fibers on the wire mesh. Silica particles are widely used as drainage and retention aids in combination with anionic and cationic polymer-based organic polymers based on acrylamide and cationic and amphoteric starches. Such additive systems are described in U.S. Patent 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 used today. Suitable silica-based particles for use as drainage and retention aids are usually provided in the form of aqueous colloidal dispersions called soles. Commercially available 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 mg2 / g. Silica-based particle sols with higher surface area areas are usually more dilute to improve storage stability and avoid gel formation. It would be desirable to be able to provide silica-based sols and particles with improved drainage and retention performance characteristics and even better stability. It would also be convenient to be able to provide a process for preparing suns and silica-based particles with better drainage, retention and stability properties. It would also be convenient to be able to provide a papermaking process with better drainage and / or retention.

The invention According to the present invention, sols and silica-based particles are provided which are suitable for use as flocculating agents in the purification of water and as drainage aids and retention in papermaking. The sols and silica-based particles according to the invention show good stability for long periods, remarkably high surface area stability and high stability to avoid gel formation, and can therefore be prepared and shipped with surface areas specific high and high concentrations of silica. Soles have better ability to maintain high specific surface area with storage at high concentrations of silica. The sols and silica-based particles also provide very good or better drainage and retention when used in conjunction with anionic, cationic and / or amphoteric organic polymers. By this means the present invention makes it possible to increase the speed of the papermaking machine and the use of lower doses of additives to obtain a drainage effect and / or equivalent retention, thus giving rise to an improved papermaking process and economic benefits. The invention thus relates to the silica-based particles and an aqueous sol containing silica-based particles, here also referred to as silica-based sols, and the production thereof, as further defined in US Pat. attached clauses. The present invention also relates to the use of sols and silica-based particles as drainage and retention aids in papermaking, preferably in combination with organic polymers as described herein, and as further defined in the attached clauses. The term "auxiliary for drainage and retention", when used herein, refers to one or more components (auxiliaries, agents or additives) which, when added to a pulp of paper to make paper, give better drainage and / or retention of what is obtained when the components are not added. The present invention also relates to a process for the production of paper from an aqueous suspension containing cellulosic fibers, and optional fillers, which consists in adding silica-based particles and at least one organic polymer to the suspension. with load, form and drain the suspension on a wire mesh. The invention thus refers to a process as further defined in the attached clauses. The silica-based sols according to the present invention are aqueous sols containing anionic particles based on silica, ie, particles based on silica (SiO2) or silicic acid. The particles preferably are colloidal, that is, in the colloidal range of particle size. The silica-based sols can have an S value in the range from 10 to 45%, conveniently from 20 to -40% and preferably from 25 to 35%. It is possible to measure and calculate the S value as described in Iler & Dalton in J. Phys. Chem. 60 (1956), 955-957. The S value indicates the degree of formation of aggregates or icrogel and a lower S value is indicative of a greater degree of aggregation. The silica-based silica sols can have a molar ratio of Sio2 to M20, where M is alkali metal ion (eg, Li, Na, K) and / or ammonium, within the range from 10: 1 to 40: 1, conveniently from 12: 1 to 35: 1 and preferably from 15: 1 to 30: 1. The silica-based sols can have a pH of at least 10.0, conveniently at least 10.5, preferably at least 10.6 and most preferably at least 10.7. The pH can be up to about 11.5, conveniently up to 11.0. The silica-based sols should conveniently have a silica content of at least 3% by weight, but it is more appropriate that the silica content be within the range of 10 to 30% by weight, and preferably from 12 to 20% by weight in weigh. In order to simplify the shipment and reduce transportation costs, it is generally preferred to send silica-based sols with high concentration, but it is certainly possible and it is usually preferred to dilute and mix the sols and particles based on silica. silica with water to substantially reduce the silica content before use, for example at silica contents of at least 0.05% by weight and preferably within the range of 0.05 to 5% by weight, in order to improve mixing with the raw materials components. The viscosity of the silica-based sols can vary depending on, for example, the silica content of the sol. Typically, the viscosity is at least 5 cP, usually within the range from 5 to 40 cP, conveniently from 6 to 30 cP and preferably from 7 to 25 cP. The viscosity, which is conveniently measured in the sols having a silica content of at least 10% by weight, can be measured by the known technique, for example using a Brookfield LVDV II + viscometer. The preferred silica-based sols of this invention are stable. This means that these silica-based sols, when subjected to storage or aging for one month at 20 ° C in dark conditions and without agitation, present only a small increase in viscosity, if at all there is any increase. The silica-based particles present in the sol conveniently have an average particle size below about 20 nm and preferably in the range from about 1 to about 10 nm. As is normal in silica chemistry, particle size refers to the average size of the primary particles, which may be aggregated or non-aggregated. The specific surface area of the silica-based particles is conveniently at least 300 m2 / g of SiO2, and preferably at least 550 m2 / g. In general, the specific surface area can be up to about 1050 m2 / g and conveniently up to 1000 m2 / g. In a preferred embodiment of this invention, the specific surface area is within the range of 550 to 725 m2 / g, preferably from 575 to 700 m2 / g. In another preferred embodiment of this invention, the specific surface area is within the range of 775 to 1050 m2 / g. It is possible to measure the specific surface area by means of titration with NaOH in the known manner, for example, as described by Sears in Analytical Chemistry 28 (1956): 12, 1981-1983 and in U.S. Patent No. 5,176,891, after of the adequate elimination of or adjustment of any of the compounds present in the sample that could hinder titration such as aluminum and boron species. The term "specific surface area", as used herein, represents the average specific surface area of the silica-based particles and is expressed as square meters per gram of silica (m2 / g SiO2). In a preferred embodiment of the invention, the silica-based sol thus has an S value in the range from 20 to 40%, a viscosity from 7 to 25 cP, a pH of at least 10.6, a molar ratio of Si02 to M20 within a range of 15 to 30 , a silica content of at least 10% by weight and contains colloidal, anionic, silica-based particles with a specific surface area within the range of 550 to 1050 m2 / g. The silica-based sols according to the invention with a silica content of 15 to 20% by weight usually contain particles with a specific surface area in the range from 550 to 725 m2 / g while the base sols. of silica according to the invention with a silica content of 10 to 15% by weight usually contain particles with a specific surface area in the range from 775 to 1050 m2 / g. In a preferred embodiment of this invention, the silica-based sols are practically free of aluminum, that is, free of added modifiers containing aluminum. In another preferred embodiment of this invention, the silica-based sol is practically boron-free, i.e., free of added modifiers containing boron. Minor amounts of these elements, however, may be present in the raw materials that are used to prepare the sols and silica-based particles. In still another preferred embodiment of this invention, the silica-based sols are modified using different elements, for example aluminum and / or boron, which may be present in the aqueous phase and / or in the silica-based particles. If aluminum is used, the sols can have a molar ratio of A1203 to Si02 within the range from 1: 4 to 1: 1500, conveniently 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 Si02 within the range from 1: 4 to 1: 1500, conveniently from 1: 8 to 1: 1000 and preferably from 1:15 to 1: 500. If boron and aluminum are used, the molar ratio of Al to B can be within the range from 100: 1 to 1: 100, conveniently from 50: 1 to 1:50. The sols and silica-based particles according to the invention can be produced starting from an aqueous solution of silicate, common type alkaline soluble glass, for example potassium silicate or liquid sodium, preferably liquid sodium silicate. The molar ratio of Si02 to M20, where M is alkali metal, for example sodium, potassium, ammonium, or a mixture of these, in the silicate or waterglass solution is conveniently within the range from 1.5: 1 to 4.5: 1, preferably from 2.5: 1 to 3.9: 1. Suitably a dilute solution of silicate or water glass is used which may have an SiO2 content of from about 3 to about 12% by weight, preferably from about 5 to about 10% by weight. The solution of silicate or water glass, which usually has a pH around 13 or above 13, is acidified at a pH from about 1 to about 4. The acidification can be carried out in a known manner by adding mineral acids, for example , sulfuric acid, hydrochloric acid and phosphoric acid, or optionally with other known chemical substances is suitable for the acidification of water glass, for example ammonium sulfate and carbon dioxide. When a mineral acid is added, the acidification is conveniently carried out in two steps, a first step at a pH of about 8 to 9, after which a certain maturation is allowed, that is, a growth of the particles before further acidification at a pH from about 1 to about 4. However, it is preferred that the acidification be effected by means of an acidic cation exchanger which, among other things, will result in more stable products. The acidification is preferably carried out by means of a strongly acidic cation exchange resin., for example of the sulphonic acid type. It is preferred that the acidification be carried out at a pH from about 2 to 4, 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, usually above 1000 m2 / g and usually around approximately 1300 m2 / g. The acidic sun then undergoes alkalization, in the present mentioned as a first step of alkalization. The first alkalization can be carried out by adding a normal alkali, for example lithium hydroxide, sodium hydroxide, potassium hydroxide, ammonium hydroxide and mixtures thereof, and / or an aqueous silicate solution as defined above. Sodium and potassium liquid silicate, particularly liquid sodium silicate, with a molar ratio of SiO2 to M20 as already defined, is conveniently used in the alkalinization step. The SiO2 content of the water glass solutions used for the first alkalization is conveniently 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 alkalization is usually carried out at a pH of at least 6, conveniently at least 7 and preferably at least 7.5, and the pH usually up to 10.5, conveniently up to 10.0. The first alkalization is further conveniently carried out at a final molar ratio of Si02 to M20, M being as defined above, within the range of from about 20: 1 to about 80: 1, preferably from 30: 1 to 70: 1. In the preparation of a sun as already defined, the degree of microgel can be modified in different ways and controlled to a desired value. The degree of microgel can be modified by the salt content, by the adjustment of the concentration in the preparation of the acid sol and in the first step of alkalinization since in this step the degree of microgel is modified when the minimum stability is exceeded. the sun, at a pH of about 5. For extended times in this step the degree of microgel can be directed to the desired value. It is particularly suitable to control the degree of microgel by adjusting the anhydrous content, the SiO2 content, in the first alkalinization step whereby an increased anhydrous content provides a lower S value. By maintaining the content of Si02 in the first alkalizing step within the range 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 from 20 to 40%, the content of SiO2 in the first alkalization step is suitably maintained within the range of 5.0 to 7.5% by weight. The silica-based particles present in the alkalinized sol obtained in the first alkalizing step are then subjected to particle growth to obtain particles with a lower specific surface area and greater stability. The particle growth process must be conveniently performed to provide silica-based particles with a specific surface area of at least 300 m2 / g and preferably at least 550 and up to about 1050 m2 / g and conveniently up to 1000 m2 / g. In a preferred embodiment of this invention, the particle growth process is performed to provide a specific surface area within the range of 550 to 725 m2 / g. In another preferred embodiment of this invention, the particle growth process is performed to provide a specific surface area within the range of 775 to 1050 m2 / g. It is possible to obtain a decrease in surface area by storing at room temperature for somewhat longer times, from one day to approximately two days and nights, preferably by heat treatment. In the heat treatment it is possible to adjust times and temperatures so that shorter times and higher temperatures are used. Of course, even if it is possible to use very high temperatures for very short times, from a practical point of view, it is more convenient to use shorter temperatures for somewhat longer times. In the heat treatment, the alkalinized silica solution should conveniently be heated to a temperature of at least 30 ° C, conveniently from 35 to 95 ° C and preferably from 40 to 80 ° C. The heat treatment should be carried out conveniently for at least 10 minutes, especially from 15 to 600 minutes, and preferably from 20 to 240 minutes. After the growth step of the particles, and optional cooling, the obtained silica sol is again subjected to alkalization, in this case, mentioned as a second alkalization step. The second alkalization can be carried out by the addition of a common alkali, for example, lithium hydroxide, sodium hydroxide, potassium hydroxide, ammonium hydroxide and mixtures thereof, and / or an aqueous silicate solution as defined above. Sodium potassium and sodium silicate, particularly sodium silicate, with a molar ratio of SiO2 to M20, as already defined, is suitably used in the second alkalizing step. The SiO2 content of the water glass solutions used for the second alkalization is conveniently 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 alkalization is suitably carried out at a pH of at least 10.0, conveniently at least 10.5, preferably at least 10.6 and more preferably at least 10.7. The pH can be up to about 11.5, conveniently up to 11.0. The second alkalization is further conveniently performed for a final molar ratio of Si02 to M20, M being as defined above, within the range from about 10: 1 to about 40: 1 and conveniently from 12: 1 to 35: 1, preference from 15: 1 to 30: 1. If desired, the process according to the invention can also comprise the concentration of the silica-based sol obtained after the second alcaiinization, otherwise, or additionally, the alkalinized sol obtained after the first alkalization but before the growth of the particles or the heat treatment step, or the sol obtained after the growth of the particles or the heat treatment step but before the second alkalization, may be subjected to concentration. The concentration can be carried out in a known manner such as, for example, by osmotic methods, evaporation and ultrafiltration. The concentration is conveniently carried out to obtain silica contents of at least 10% by weight, preferably from 10 to 30% by weight, and more preferably from 12 to 20% by weight. If desired, the sol and the silica-based particles can be modified by the addition of compounds containing, for example, aluminum or boron, or both. Suitable aluminum-containing compounds include aluminum aluminate type aluminates and potassium aluminate, conveniently sodium aluminate. The aluminum-containing compound is conveniently used in the form of an aqueous solution, suitable boron-containing compounds include boric acid, borate type scdio and potassium borates, conveniently sodium borate, tetraborate type sodium and potassium tetraborate, conveniently sodium tetraborate, and metaborate type sodium and potassium metaborate. The boron-containing compound is conveniently used in the phona of an aqueous solution. When an aluminum-containing compound is used in the process, it is convenient to add it to the sun subjected to particle growth or to the thermal treatment, before or after the second alkalization step. Otherwise, or in addition, the aluminum-containing compound may be added to the silicate solution to be acidified, to the acidic sol or alkalized sol obtained in the first alkalinization step before the growth of the particles or the passage of the heat treatment. The aluminum-containing compound can be added in admixture with acid in the acidification step and in admixture with the alkaline or silicate solution in any of the alkalization steps. The aluminum-containing compound is conveniently added in an amount such that the obtained sol has a molar ratio of A1203 to SiO2 as already defined. When a compound containing boron is used in the process, it is convenient to add it to the sun subjected to particle growth or heat treatment, before or after the second alkalinization step, otherwise, or in addition, the boron-containing compound can be added to the silicate solution to be acidified, to the acidic sol or alkalinized sol obtained in the first alkalinization step before the passage of the growth of the particles or of the thermal treatment. The boron-containing compound can be added in admixture with acid in the acidification step and in admixture with alkaline or silicate solution in any of the alkalization steps. The boron-containing compound is conveniently added in an amount such that the obtained sol has a molar ratio of B to SiO2 as already defined. If both aluminum-containing and boron-containing compounds are used, these are conveniently added in amounts such that the obtained sol has a molar ratio of Al to B conveniently as already defined. If the sun, before any modification with aluminum and / or boron, contains too high amounts of alkali metal ions or ammonium metal ions, it is preferred to remove at least part of these ions, for example, by ion exchange, to provide suns based on silica with a final molar ratio of Si02 to M20 within the desired range as already defined. According to the present process, the silica-based sols having silica contents of from 10 to 30% by weight, conveniently from 12 to 20% by weight, and viscosities of at least 5 cP, usually within the range from 5 to 40 cP, conveniently from 6 to 30 cP and preferably from 7 to 25 cP, after the optional concentration, can be prepared and the soles produced have good storage stability and can be stored for several months without substantial reduction of the area - of specific surface and without gel formation. The sols and silica-based particles of 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 the purification of different kinds of wastewater as for the purification of specifically white waters from the pulp and the paper industry. The sols and silica-based particles can be used as flocculating agents, conveniently as aids for drainage and retention, in combination with organic polymers that can be selected from anionic, amphoteric, nonionic and cationic polymers and mixtures thereof, herein also mentioned as "main polymer". The use of these polymers as flocculating agents and as aids for drainage and retention is well known in the art. The polymers can be obtained from natural or synthetic sources and can be linear, branched or crosslinked. Examples of the generally convenient main polymers include anionic, amphoteric and cationic starches, anionic, amphoteric and cationic guar gums and polymers based on anionic, amphoteric and cationic acrylamides, as well as cationic poly (diallyldimethylammonium chloride), cationic polyethylene imines, cationic polyamines, polyamidoamines and vinylamide-based polymers, melamine-formaldehyde resins and urea-formaldehyde. 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 these may be used individually, in mixtures with each other or together with other polymers, for example other cationic polymers or cationic polyacrylamide. The molecular weight of the main polymer is conveniently above 1,000,000 and preferably above 2,000,000. The upper limit is not crucial, it can be above 50,000,000, usually 30,000,000 and conveniently above 25,000,000. However, the molecular weight of polymers from natural sources may be higher. When the silica-based sols and particles are used in combination with the main polymer (s) as already mentioned, it is further preferred to use at least one low molecular weight (LMW) cationic organic polymer, commonly known and used as anionic waste collectors (ATC). TCAs are known in the art as neutralizing agents and / or fixatives for detrimental anionic substances absent in pulp and the use of these in combination with drain and retention aids often provide other improvements in drainage and / or retention. The cationic organic polymer LMW may come from natural or synthetic sources, and is preferably a synthetic LMW polymer. Suitable organic polymers of this type include highly charged cationic organic polymers LMW such as polyamines, polyamidoamines, polyethyleneimines, homo- and copolymers based on diallyldimethylammonium chloride, (meth) acrylamides and methacrylates. In relation to the molecular weight of the main polymer, the molecular weight of the cationic organic polymer LMW is preferably lower; conveniently it is at least 1000 and preferably at least 10,000. The upper limit of molecular weight is usually about 700,000, conveniently about 500,000 and usually about 200,000. Preferred combinations of the polymers that can be co-used with the silica-based sols of this invention include the cationic organic polymer LMW in combination with the 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. The components of the drainage and retention aids according to the invention can be added to the pulp in the usual manner and in any order. When drainage and retention aids containing the silica-based particles and an organic polymer, for example, a main polymer, are used, it is preferred to add the polymer to the pulp before adding the silica-based particles, even if You can use the opposite order of addition. It is further preferred to add the main polymer before a shearing step, which can be selected from pumping, mixing, cleaning, etc., and adding the silica-based particles after this shearing step. The LMW cationic organic polymers, when used, are preferably introduced into the pulp prior to introducing the main polymer. Otherwise, the LMW cationic organic polymer and the main polymer can be introduced into the pulp at about the same time, separately, or in a mixture, for example as described in U.S. Patent No. 5,858,174, which is incorporated by this medium as a reference. The cationic organic polymer LMW and the main polymer are preferably introduced into the pulp prior to the introduction of the silica-based sol. In a preferred embodiment of this invention, the sols and silica-based particles are used as drainage and retention aids in combination with at least one organic polymer, as already described, and at least one aluminum compound. The aluminum compounds can also be used to improve the performance of draining and / or retention of the pulp additives comprising the silica-based particles. Suitable aluminum salts include alum, aluminates, aluminum chloride, aluminum nitrate, and polyaluminum compounds, such as polyaluminium chlorides, polyaluminium sulfates, polyaluminium compounds containing chloride and sulfate ions, polyaluminium silicate sulfates, and mixtures of these. The polyaluminum compounds can also contain other ions, for example, anions from 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. Otherwise, or in addition, the aluminum compound can be added at the same time with the silica-based sol practically at the same point, separately or in admixture with it, for example as described in US Pat. No. 5 , 846, 384 which is incorporated herein by reference. In many cases it is usually convenient to add an aluminum compound to the pulp in the early stages of the process, for example before other additives. The components of the drainage and retention aids according to the invention are added to the pulp to be dehydrated in amounts that can vary within wide limits depending on, among others, the type and number of components, the type of raw materials, the content of the loading material, the type of loading material, the point of addition, etc. In general, the components are added in an amount that provides better drainage and / or retention than that which is obtained when the components are not added. The sun and the silica-based particles are usually added in an amount of at least 0.001% by weight, often at least 0.005% by weight, calculated as Si02 and based on the substance of the anhydrous pulp, is say, the cellulosic fibers and optional fillers, and the upper limit is usually 1.0% and conveniently 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 the anhydrous substance of the pulp, and the upper limit is usually 3% and conveniently 1.5 % in weigh. When an LMW cationic organic polymer is used in the process, it can be added in an amount of at least 0.05%, based on the anhydrous substance of the pulp to be dehydrated. Conveniently, the amount is in the range from 0.07 to 0.5%, preferably in the range from 0.1 to 0.35%. When an aluminum compound is used in the process, the total amount that is introduced into the pulp to be dehydrated depends on the type of aluminum compound used and other desired effects of this. For example, the use of aluminum compounds as precipitants for rosin-based sizing agents is well known in the art. The total amount added is usually at least 0.05%, calculated as A1203 and based on the anhydrous substance of the pulp. The suitable amount is in the range from 0.1 to 3.0%, preferably in the range from 0.5 to 2.0%.

Other additives that are common in papermaking, of course, can be used in combination with the additives according to the invention such as, for example, dry strength agents, wet strength agents, optical brighteners, colorants. , sizing agents as rosin-based sizing agents and sizing agents reactive with cellulose, for example alkyl and alkenyl ketene dimers and ketene multimers, alkyl and alkenyl succinic anhydrides, etc. The cellulose suspension, or paper pulp, may also contain mineral fillers of the common types such as kaolin, china clay, titanium dioxide, gypsum, talc and natural and synthetic calcium carbonates such as chalk, crushed marble and precipitated calcium carbonate. The process of this invention is used for the production of paper. The term "paper", when used in the present, of course includes not only the paper and the production of this, but also other sheet-like products or continuous material containing cellulose fibers, such as cardboard or paperboard and the production of these. The process can be used in the production of paper from different types of suspensions of cellulose-containing fibers and the suspensions should conveniently contain at least 25% by weight and preferably at least 50% by weight of these fibers, based on the anhydrous substance. The suspension can be based on fibers of the chemical pulp such as sulphate pulps, sulphite and organic solvents, mechanical pulp such as thermomechanical pulp, chemithermomechanical pulp, refiner pulp and crushed wood pulp, from hardwood and softwood , and can also be based on recycled fibers, optionally from de-inked pulps, and mixtures of these. The pH of the slurry, pulp, can be within the range of about 3 to about 10. The pH is conveniently above 3.5 and preferably within the range of 4 to 9. The invention is further illustrated in the following examples which, however, are not proposed to limit it. The parts and percentages refer to parts by weight and percentages by weight, respectively, unless otherwise mentioned.

Example 1 A standard silica sol was prepared as follows: 762.7 g of liquid sodium silicate with a molar ratio of Si02 to Na20 of 3.3 and SiO2 content of 27.1% was diluted with water to 3000 g to produce a silicate solution (I ) with a content of 6.9% by weight. 2800 g of this silicate or water glass solution was passed through a column packed with a strong cation exchange resin saturated with hydrogen ions. 2450 g of the water glass after the ion exchange or polysilicic acid (II) with an SiO2 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 to a reactor and diluted with 12.3 g of water. 173.9 g of the 6.9% silicate (I) solution was then added with vigorous stirring. The resulting solution was then heated to 85 ° C for 60 minutes and then cooled to 20 ° C. The silica sol (la) obtained had the following characteristics: Sol la (ref.): Content of Si02 = 7.3% by weight, molar ratio Si02 / Na20 = 40, pH = 10.2, value S = 29%, viscosity = 2.2 cP and specific surface area of the particles = 530 m2 / g. Two other silica sols were produced, sol Ib and Sun which had the following characteristics: sun Ib (ref.): Content of Si02 = 7.3% by weight, molar ratio Si02 / Na20 = 63, pH = 10.0, value S = 26 [sic], viscosity = 2.7 cP and specific surface area of the particles = 500 m2 / g. Sol lc (ref.): Content of Si02 = 5.4% in weight, molar ratio Si02 / Na20 = 35, pH = 9.8, value S = 32 [sic], viscosity = 1.6cP and specific surface area of the particles = 690 m2 / g.

EXAMPLE 2 Six sols of silica-based particles according to the invention were prepared from a polysilicic acid similar to polysilicic acid (II) produced with the same ion exchange process and having an SiO2 content of 5.46% by weight. To 102.0 kg of polysilicic acid was added 1.46 kg of liquid sodium silicate with a Si02 / Na20 ratio of 3.3 with vigorous stirring producing a solution with a molar ratio Si02 / Na20 of 54.0. This solution was treated with heat at 60 ° C for 2 hours 20 minutes and cooled to 20 ° C whereupon the product was concentrated to an SiO2 content of 15.6% by weight. This intermediate sun product was now divided into six separate samples, from a to f. Samples a through c were further alkalized with NaH, samples d through f with water glass to obtain sols with a molar ratio Si02 / Na20 between 21.5 and 34.0 and a silica content of about 15.0% by weight. The sols obtained from the silica-based particles had the characteristics established in Table 1: Table 1 Sun Relationship PH Value Viscosity Molar surface area S [%] [%] specific [Si02 / Na20] [m2 / g Si02] Sun 2a 21.5 10.7 31 17 720 Sun 2b 28.0 10.3 30 29 710 Sun 2c 34.0 10.0 29 40 690 Sun 2d 21.5 10.7 31 20 680 Sun 2e 28.0 10.3 29 34 670 Sun 2f 33.0 10.0 29 38 680 Example 3 A polysilicic acid (II) produced with the above ionic process and alkalized with water glass for a Si02 / Na20 molar ratio of 54.0 as in Example 2, was heat treated at 60 ° C for one hour. To 58 kg of this product was added 7.25 kg of water glass diluted with a molar dilution Si02 / Na20 of 3.3 and content of silica of 5.5% by weight. The sol resulting from the silica-based particles, sol 3, was concentrated to a silica content of 15.2% by weight and had a molar ratio Si02 / Na20 = 24, pH 10.7, value S = 34, viscosity equal to 9.0 cP and specific surface area of the particles = 760 m / g.

Example 4 1000 g of polysilicic acid (II) with an SiO2 content of 5.5% by weight was mixed with 14.5 g of a water glass solution with an SiO2 content of 27.1% by weight and a molar ratio SiO2 / Na20 = 3.3 with vigorous stirring giving rise to a product with a molar ratio Si02 / Na20 of 51 and a silica content of 5.8% by weight of SiO2, which was heat treated at 60 ° C for 1.5 hours and then concentrated to a content of silica of 16.7% by weight of SiO2. 283 g of the product obtained were mixed with 33.0 g of NaOH producing a sol of particles based on silica sol 4, with a content of SiO2 = 15.2% by weight, molar ratio Si02 / Na20 = 21, pH 10.6, value S = 32% viscosity = 14.2 cP and specific surface area of the particles = 720 m2 / g.

Example 5 The general procedure was followed according to Example 3, except that the heat treatment was carried out for 1.25 hours and the concentration was carried out for higher silica content. Two sols of silica-based particles were prepared; the sun 5a and the sun 5b. The sol 5a had a content of SiO2 = 18% by weight, molar ratio Si02 / Na20 = 18, pH = 10.7, value S = 36%, viscosity = 18 cP and specific surface area of the particles = 700 m2 / g. The sol 5b had content of SiO2 = 20% by weight, molar ratio Si02 / Na20 = 18.3, pH 10.7, value S = 37%, viscosity = 31 cP and specific surface area of the particles 700 m2 / g.

Example 6 The drainage characteristic was evaluated by means of a dynamic drainage analyzer (DDA), available from Akribi, Sweden, which measures the time to drain an established volume of pulp through a wire mesh when a stopper is removed and vacuum is applied to this side of the wire opposite to the side in which pulp is present . The pulp used was emptied into a mixture of 60% bleached birch sulphate and 40% bleached pine sulphate to which 30% crushed calcium carbonate was added as filler. The pulp volume was 800 ml, consistency 0.25% and pH approximately 8.0. The conductivity of the pulp was adjusted to 0.47 mS / cm by the addition of sodium sulphate. In the tests silica-based sols were used together with a cationic polymer, Raisamyl 142, which is a usual medium to high cationized starch with a substitution degree of 0.042, which was added to the pulp in an amount of 12 kg / ton, calculated as anhydrous starch on the anhydrous pulp system. The silica-based sols according to Examples 1 to 4 were tested in this example. In addition, soles 6a and 6b were also tested for comparison purposes. Sun 6a is a commercial silica sol with an S value of 45%, Si02 content 15% by weight, Si02 / Na20 = 40 molar ratio, viscosity = 3.0 cP, particle surface area = 500 m2 / g . The sun 6b is another commercial silica sol with a value S = 36%, content of SiO2 = 10.0% by weight, molar ratio Si02 / Na20 = 10, viscosity 2.5 cP, specific surface area of the particles = 880 m2 / g . The silica-based sols were added in an amount of 0.5 kg / ton, calculated as Si02 and based on the anhydrous pulp system. The pulp was stirred in a vessel with screens at a speed of 1500 rpm during the test, and the chemical additions were made as follows: i) the addition of cationic starch to the pulp followed by agitation for 30 seconds, ii) the addition of the silica-based sol to the pulp followed by agitation for 15 seconds, iii) draining the pulp with automatic recording of the draining time. The drainage times for the different silica-based sols are shown in Table 2: Table 2 Silica-based sun Dehydration time [seconds] Sol la (ref.) 12.0 Sol Ib (ref.) 11.1 Sol lc (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 Drain performance was evaluated according to the general procedure of Example 6 except that the pulp had a consistency of 0.3% and an approximate pH of 8.5. The retention performance was evaluated by means of a nephelometer measuring the turbidity of the filtrate, the white water, obtained by draining the pulp. The silica-based sols according to Example 5, according to the invention, were tested against the sun 6a used for comparison. Table 3 shows the drainage time obtained in different doses (kg / ton) of the silica-based particles, calculated as Si02 and based on the anhydrous pulp system. The addition of only cationic starch (12 kg / ton, calculated as anhydrous starch on the anhydrous pulp system) produced a draining time of 15.8 seconds.

Table 3 Sol based on Time of drainage Turbidity (NTU) at a dose of Si02 silica (sec) of 0.5 kg / t 1.0 kg / t 1.5 kg / t 2.0 kg / t 3.0 kg / t Sun 6a (ref.) 11.1 / - 8.8 / 59 7.9 / 58 7.1 / 54 6.8 / 60 Sun 5a 9.0 / - 7.1 / 52 6.3 / 50 5.2 / 52 5.7 / 53 Sun 5b 8.9 / - 6.9 / - 6.3 / - 5.7 / - 6.0 / -

Claims (21)

1. An aqueous sol containing silica-based particles is characterized in that it has an S value in the range from 10 to 45%, a viscosity in the range from 5 to 40 cP and a molar ratio of Si02 to M20, where M is alkali or ammonium metal, within the range from 10: 1 to 40: 1.

2. An aqueous sol containing silica-based particles is characterized in that it has an S value in the range from 10 to 45%, a viscosity in the range from 5 to 40 cP and a silica content of at least 10% by weight .

3. The aqueous sol according to claim 1 is characterized in that it has a silica content of at least 10% by weight.

4. The aqueous sol according to claim 1, 2 or 3, is characterized in that the silica-based particles have a specific surface area in the range from 775 to 1050 m2 / g.

5. The aqueous sol according to claim 1, 2 or 3, characterized in that the silica-based particles have a specific surface area in the range from 550 to 725 m2 / g.

6. The aqueous sol according to any of claims 1 to 5, characterized in that the S value is in the range from 20 to 40%.

7. The aqueous sol according to any of claims 1 to 6, characterized in that the viscosity is within the range from 7 to 25 cP.

8. The aqueous sol according to any of claims 1 to 7, characterized in that it has a molar ratio of SiO2 to M20, where M is alkali metal or ammonium, within the range of 15: 1 to 30: 1. "

9. The aqueous sol according to any of claims 1 to 8, characterized in that it has a pH of at least 10.6.

10. A process for the production of silica-based particles, characterized in that it comprises the steps of: (a) acidifying an aqueous silicate solution at a pH from 1 to 4 to form an acidic sol (b) alkalinizing the acidic sol to a content of Si02 within the range of 4.5 to 8% by weight at a pH of at least 7 (c) to allow the growth of the alkalinized sol particles for at least 10 minutes, and then (d) to alkalinize the obtained sol at a pH of at least 10.0.

11. A process for the production of silica-based particles, characterized in that it comprises the steps of: (a) acidifying an aqueous silicate solution at a pH from 1 to 4 to form an acidic sol (b) alkalinizing the acidic sol to a content of Si02 within the range of 4.5 to 8.0% by weight (c) heat treating the alkalinized sol at a temperature of at least 30 ° C, and then (d) alkalinizing the heat-treated sol to a pH of at least 10.0.

12. The process according to claim 10 or 11, characterized in that the alkalinization according to (b) and (d) is carried out by means of an aqueous silicate solution.

13. The process according to claim 10, 11 or 12, characterized in that the growth of the particles and the heat treatment according to (c) is carried out at a temperature in the range from 35 to 95 ° C.

14. The process according to claim 10, 11, 12 or 13, characterized in that the growth of the particles and the heat treatment according to (c) is carried out for 20 to 240 minutes.

15. The process with any of claims 10 to 14, characterized in that the alkalinization according to (d) produces a silica-based sol with a molar ratio of SiO2 to M20, where M is alkali metal or ammonium, within the range from 15: 1 to 30: 1 and a pH of at least 10.6.

16. The silica-based particles obtainable by a process according to any of claims 10 to 15.

17. The use of the silica-based particles according to any of claims 1 to 9 or 16 or produced by a process according to any of claims 10 to 15 as auxiliaries for drainage and retention in the production of paper.

18. A process for the production of paper from an aqueous suspension containing cellulosic fibers, and optional fillers, which consists of adding silica-based particles to the suspension and at least one organic polymer with charge, forming and draining the suspension in a wire mesh, characterized in that the silica based particles are present in an aqueous sol according to any of claims 1 to 9 or are produced by a process according to any of claims 10 to 15.

19. The process according to claim 18, characterized in that the charged organic polymer is cationic starch or cationic polyacrylamide.

20. The process according to claim 18 or 19, characterized in that before adding the silica-based particles to the suspension, the silica-based particles are diluted or mixed with water to form an aqueous sol with a silica content from 0.05 to 5% by weight.

21. The process according to any of claims 18 to 20, characterized in that the silica-based particles are added to the suspension in an amount from 0.005 to 0.5% by weight, calculated as SiO2 and based on the anhydrous cellulose fibers and optional loading materials. SUMMARY OF THE INVENTION The invention relates to an aqueous sol containing silica-based particles with an S value in the range from 10 to 45%, a viscosity in the range from 5 to 40 cP, and a molar ratio of Si02 to M20, where M is alkali metal or ammonium, within the range of from 10: 1 to 40: 1, or a silica content of at least 10% by weight. The invention further relates to a process for the production of silica-based particles comprising the steps of: (a) acidifying an aqueous silicate solution at a pH from 1 to 4 to form an acid sol; (b) alkalinizing the acid sol in a SiO2 content within the range of 4.5 to 8% by weight for; (c) allowing the growth of the alkalinized sol particles for at least 10 minutes, or treating the alkalinized sol with heat at a temperature of at least 30 ° C, and then (d) alkalinizing the obtained sol at a pH of at least 10.0. The invention also relates to silica-based particles that can be obtained by the process, the use of silica-based particles as drainage and retention aids in the production of paper as well as a process for the production of paper from of an aqueous suspension containing the cellulose fibers, and optional filler material, in which the silica-based particles and at least one organic polymer with charge are added to the cellulosic suspension.