WO2015065185A1

WO2015065185A1 - Silica particles and method of preparation thereof

Info

Publication number: WO2015065185A1
Application number: PCT/NL2014/050751
Authority: WO
Inventors: John Wilhelm Geus; George QUERCIA BIANCHI; Alberto LÁZARO GARCIA; Jacobus Hoekstra
Original assignee: Caprigemini B.V.
Priority date: 2013-10-29
Filing date: 2014-10-29
Publication date: 2015-05-07

Abstract

The invention is in the field of silica products. More in particular, the invention is in the field of amorphous silica particles. The invention is directed to amorphous silica particles and related products including clusters of said silica particles, a suspension of said silica particles, and an aerogel of said silica particles; a method for the production of said silica particles from a silicate mineral, preferably olivine; a method for the preparation of a suspension of said silica particles; and, the applications of said silica particles, for instance, as adsorbents, fillers in concrete and silicone rubber, a reinforcement material in elastomer products, an additive in food products, and a thickener of liquid systems, such as paints, thermosetting resins and printing inks.

Description

Title: Silica particles and method of preparation thereof

The invention is in the field of sihca products. More in particular, the invention is in the field of amorphous sihca particles.

The invention is directed to amorphous silica particles and related products including clusters of said silica particles, a suspension of said sihca particles, and an aerogel of said sihca particles; a method for the production of said silica particles from a silicate mineral, preferably olivine; a method for the preparation of a suspension of said sihca particles; and, the applications of said silica particles, for instance, as adsorbents, fillers in concrete and silicone rubber, a reinforcement material in elastomer products, an additive in food products, and a thickener of liquid systems, such as paints, thermosetting resins and printing inks.

At present a wide range of sihca products are manufactured for various applications. In particular, silica particles having a size in the range of a few microns or less are frequently used in conglomerate materials. The material properties of the silica particles, such as surface area, porosity and the level of impurities (e.g. iron, magnesium and sulfur) influences the properties of the resulting conglomerate. One such application of silica is in the reinforcement of rubber, where it is either used alone or together with soot. For rubber employed in electrical applications, the purity of the silica used is of importance since it is essential that the silica has a low electrical conductivity. Another such application is in concrete materials, in which silica leads to an increase in the compressive strength and durabihty of concrete materials. In concrete material applications, it has been found that the flow behavior and the dusting of the sihca particles during handling is of importance for the manufacture of such conglomerate materials.

Amorphous silica conforming to food grade standards is also used as a food additive, in particular as an anti-caking agent, but also as absorbent for haze forming proteins to clarify beverages, to control viscosity, as a flow agent in powdered foods, and as an anti-foaming agent and dough modifier. It is also used as an anticaking agent and as an excipient in pharmaceuticals for various drug and vitamin preparations. Other uses include as an abrasive in tooth paste.

In addition, silica is used as an adsorbent for pesticides, cosmetics and personal care products, for water vapor adsorption, soap and in industrial processes. Large quantities of silica are also used in the paper industry, for instance, to prevent bleeding of printing ink.

Silica is also used as catalyst support, mainly in the production of polyolefines, such as, Ziegler-Natta chromia and metallocene catalysts. It is also employed as a binder for zeolites in catalytic cracking catalysts.

Presently, one of the most common commercial methods of preparing amorphous silica is the acidification of sodium silicate (e.g. water glass) to produce either a gel, a precipitate, or colloidal silica. Typically for producing silica gels and precipitates, viscous sodium silicate solutions are used. Typically a stoichiometric excess amount of acid is employed. The silica gel or precipitate is then washed and subsequently dehydrated to produce silica. The drying of the gel is highly important, since rapid removal of excess water can lead to differences in shrinkage rates of the gel, as well as fracture of the gel into silica lumps. Slow drying is frequently used as solution to these problems, as this enables a more even shrinkage rate of the gel with the overall shape of the silica gel being maintained. Additionally, the surface tension of the excess water can be lowered by, for example ethanol, which also results in a higher porosity of the silica. It is also possible to increase the porosity of the silica gel by using additives, such as adsorbents which result in the surface of the silica becoming hydrophobic. The disadvantage of producing silica by this method is the level of

impurities present in the silica making such silica unsuitable for a number of applications. The silica produced according to this method usually contains sulphate and alkali or alkaline earth metal ions in an amount of about 1000 ppm. An example of such an amorphous silica is sold by Evonik under the name of Sipernat®, where the lowest reported amount of sulfate and sodium oxide present in these products is 0.1 wt.% and 0.1 wt.%, respectively, for silica material with a relatively low specific surface area of (i.e. less than 200 m²/g). For silica material with a higher specific surface area of 475 m²/g, the amount of sulfate and sodium oxide is higher, being reported as 0.6 wt.% and 0.7 wt.%., respectively.

Colloidal silica is usually prepared from more dilute sodium silicate solutions (e.g. about 6 wt.%). Sodium may be removed by an ion exchanger loaded with hydrogen ions and the resulting colloidal silica solution next is stabilized by the addition of sodium or ammonium

hydroxide. Lastly, the excess water is removed by evaporation.

In spite of large volumes of silica being produced by this method, the properties of the silica produced are difficult to control. The silica produced by this procedure is also often not suitable for use as a catalyst support due to its sulfur content and for other catalyst applications the sodium content is unfavorable. Additional disadvantages in this process is that the dried silica particles are highly clustered and are difficult to disperse in solids and liquids, and that dispersion in water leads to undesired gelling.

To overcome many of these problems another commercial method was developed by Degussa, now known as Evonik, for the production of pyrogenic silica, which is another type of silica that is more easily dispersed. Pyrogenic silica, also known as fumed silica, is produced by reaction of silicon tetrachloride with water vapor in a hydrogen-oxygen flame (flame hydrolysis). The elevated temperature (e.g. typically above 1000 °C) used to produce the pyrogenic silica, results in the number of hydroxy! groups present on the silica surface being low, typically having a silanol number (i.e. the number of OH groups per nm² on the surface of the silica particle) of between 2 and 3 (e.g. Aerosil® fumed silica product of Evonik), since the breaking of siloxane bonds and the subsequent reaction with water to surface hydroxy! groups does not proceed rapidly. Pyrogenic silicas typically produced have surface areas which vary from about 80 to 400 m²/g. Also the primary particles of pyrogenic silica, unlike the silica produced by wet processes, typically have an outer surface and no internal pore structure.

An advantage of pyrogenic silica is that it can be easily dispersed in rubber, polymer and liquids. This is because of the pyrogenic silica's particle size, which, on the other hand, makes it prone to becoming airborne (i.e. dusting) and its low bulk density. The dusting of pyrogenic silica, which is well known in the technical utilization, makes the use of pyrogenic silica disadvantageous due to its inhalation risk. Extensive safety measures and processing in a closed apparatus are therefore required. Another

disadvantage of pyrogenic silica is that it is difficult to extrude or tablet into shaped bodies. A further disadvantage of this method is that the silica produced by this method is relatively expensive which makes application in large scale productions uneconomical.

Due to the disadvantages of these commercial processes, new methods have been developed for the production of silica. These new methods have been directed to obtaining silica from silicate minerals, such as olivine. Olivine is typically an isomorphic mixture of forsterite, Mg2SiO₄ and fayalite, Fe2SiO₄. The general formula for olivine is (Mg,Fe)2SiO₄.

Olivine is found in high concentrations in dunite, an igneous, plutonic rock, of ultramafic composition, with coarse-grained or phaneritic texture.

WO-A-86/00288 describes reacting olivine with waste sulfuric acid to form a silica gel and a Fe/Mg-containing liquid. The disadvantage of this method is that the resulting silica gel had high levels of impurities, having both an iron sulfate and magnesium sulfate content of 1 wt.%.

WO-A-95/07235 describes a method for manufacturing silica by treating olivine with hydrochloric acid or other mineral acids in a reactor, and then separating, washing, drying and possibly grinding up the resulting silica. The disadvantage of this method is that the silica produced is only about 91 wt.% silica. A further disadvantage of this method is that it would be difficult to scale up to an industrial scale, since any reactor used would need a highly effective cooling system to control the exothermic reaction.

NL-A- 1007131 describes the production of silica having a high number of surface hydroxyl groups by dissolving olivine in acid. NL-A- 1007131 describes using both siphoning and sedimentation to separate the silica from the non-dissolved olivine particles. NL-A- 1007131 does not describe that the silica is further treated by washing. Nor does NL-A- 1007131 describe the purity of the silica produced.

WO-A-02/48036 describes a process for extraction of silica and magnesium compounds, by treating olivine with sulfuric acid, and using a weight ratio of olivine to sulfuric acid of 1.0 to 0.3. The disadvantage of this method is that the silica produced has a low BET surface area of about 100 m²/g and a high magnesia content of 3.5 wt.%.

The document Chemical Engineering Journal 211-212(2012)112- 121 describes amorphous silica nanoparticles having a particle size between 10 and 25 nm, a BET surface area between 100 and 300 m²/g, a pore diameter of about 20 nm and an impurity content of less than 1 %. This document also describes that the nano-silica is agglomerated into clusters. However, this document does not describe silica nanoparticles having a sulfur content of at most 0.03 wt.%, instead the lowest amount reported was greater than 0.9 wt.%. Nor does this document describe an aerogel of said silica nanoparticles, or a method of preparation thereof.

The document 8^th fib International PhD Symposium in Civil Engineering (2010)1-6 describes silica nanoparticles having a specific surface area of 100-500 m²/g, a mean particle size of 5-20 nm and a specific surface area in the micropores of 20-200 m²/g. Also described is that the nano-silica is produced by dissolving olivine in sulfuric acid, separating the unreacted materials from the suspension by decantation and precipitation the silica by filtrating and washing steps. However, no mention is made of the purity of the silica nanoparticles produced. This document also does not describe the possibility of modifying its method by diluting the slurry prior to separating out the silica particles. Nor does this document describe an aerogel made from these silica nanoparticles.

US-A-2006/0051279 describes producing precipitated silica from olivine, wherein the olivine is contacted with a heated acid, optionally separating the undissolved olivine, filtering and washing the silica slurry, and then slurrying the silica filter cake in aqueous solution. US-A- 2006/0051279 describes that the remaining insoluble materials in the silica can be separated by gravitational sedimentation methods before drying the silica. The disadvantage of this method is that it is difficult to scale up as an industrial process.

US-A-2009/0263657 describes a similar process to that of US-A- 2006/0051279, with the exception that the removal of undissolved olivine is not optional and that an aluminate is added to the slurry of the silica filter cake. A further disadvantage to this process, in addition to the disadvantage already mentioned for US-A-2006/0051279, is that the use of an aluminate in this process introduces an additional impurity into the silica produced. This would make the silica unsuitable for use as a catalyst support, as the presence of alumina can result in undesired acid properties of the silica and lead to unwanted reactions, such as, oligomerization of unsaturated organic compound.

The document Journal of Nanomaterials 2012(2012) 1-15 describes the sol-gel synthesis of silica nanoparticles by hydrolysis and condensation of metal alkoxides or inorganic salts using a mineral acid or a base (e.g. NH3). This document does not describe the purity of the silica nanoparticles produced.

The terms "dissolution" and "dissolving" are used interchangeable herein with the term "leaching". The term "suspension" is used herein interchangeably with the term "dispersion".

It is thus desirable to produce amorphous silica particles with improved properties. It is also desirable to provide a method of preparing amorphous silica that overcomes at least some of the disadvantages of the above-mentioned known methods.

It was found that amorphous silica particles with improved properties can be made by the dissolution of a silicate mineral, preferably olivine using an improved method.

Thus in a first aspect, the present invention is directed to a silica particle comprising < 0.03 wt.% sulfur, < 0.05 wt.% iron and < 0.2 wt.% magnesium, and wherein said particle has a surface area of between 500 and 950 m²/g, as determined by the nuclear magnetic resonance (NMR) intensity of the surface hydroxyl groups.

The advantage of the silica particle according to the invention is that it has a high purity (i.e. at least 99.5 wt.% silica, calculated as S1O2, and, at most 0.03 wt.% sulfur, calculated as the atomic weight of sulfur, based on the weight of the silica particle) and is highly hydroxylated (i.e. the silica particle has at least 8 OH nm -²). The high purity enables the silica particle of the invention to be advantageously used in applications which have strict requirements about the level of impurities in the materials used. The increased number of hydroxyl groups, as indicated by both the surface area and the silanol number, as determined by the NMR, leads to an improved ability for the silica particle to be dispersed in a liquid, in particular in an aqueous solution, thus preventing gelling. In addition, the advantage of the silica particle having a high density of surface hydroxyl groups (i.e. at least 8 OH nnr² of the silica particle) makes it easier to chemically modify the silica particle, e.g. grafting with organic compounds containing disulfide and/or hydroxyl groups. The combination of the features in the silica particles of the invention advantageously improves the ease and number of applications in which these particles can be applied.

The surface area of the silica particle of the invention is typically between 500 and 950 m²/g, preferably between 600 and 900 m²/g, and more preferably between 650 and 800 m²/g, as determined by the NMR (nuclear magnetic resonance) intensity of the surface hydroxyl groups. This method for determining the surface area, as defined herein, is the value determined by measuring the number of surface hydroxyl groups of the silica particle by means of NMR spectroscopy and assuming that the number of OH groups on the surface of a fully hydroxylated silica is about 5 OH groups nnv² (D.J. Lieftink, The preparation and characterization of silica from acid treatment of olivine (PhD Thesis), Utrecht University, 1997; L.T. Zhuravlev, Colloids and Surfaces A: Physiochemical and Engineering Aspects 173(2000)1-38 and P.K. Jal, S. Patel and B.K. Mishra, Talanta 62(2004)1005-1028). The surface area (SANMR) can subsequently be calculated by the following equation (1):

SANMR = a. OH/ Q-FH X SABET (1) wherein aoH is the silanol number, that is the number of OH groups per nm² on the surface of the silica particle, as determined the NMR intensity of the surface hydroxyl groups; am is the silanol number of a fully hydroxylated silica particle which has a value of about 5 OH groups nm-²; and SABET is the BET surface area of the silica particle.

Typically, the silica particle of the invention has a silanol number of between 8 and 40 OH groups per nm², preferably between 10 and 36 OH groups per nm² and more preferably between 12 and 32 OH groups per nm², as determined by NMR. Typically, when the silanol number is high, the BET surface area is generally lower. Conversely, at lower silanol numbers the BET surface area is generally higher. NMR is a characterization technique that uses the magnetic properties of certain atomic nuclei to determine the physical and chemical properties of the atoms surrounding the magnetic nuclei. The NMR active isotope (the atomic nucleus) used in this research is ²⁹Si with a spin number of 1/2. The most important parameter determined by NMR is the chemical shift. The chemical shift (d) refers to the variations of nuclear magnetic resonance frequencies of the active isotope (in this case ²⁹Si) due to the distribution of electrons in the surrounding atoms. This parameter is known to be sensitive to the bonding angles and bond lengths of the silica

structure. Thus, the different number of siloxane bridges and of hydroxyl groups can be established by NMR. Q², Q³ and Q⁴ are the silicon atoms with different number of siloxane bridges and of hydroxyl groups. The chemical shift is related to a reference signal, which is typically of tetra-methylsilane. From the Q values, the silanol content, which is one of the main parameters of nano-silica particles, can be calculated following formulas described by Leonardelli et al. Journal of the American Chemical Society 114(1992) 6412- 6418:

¾ = Q²/( Q² + Q³) (2) f_s = (Q² + Q³)/(Q² + Q³ + Q⁴) (3) aoH = f_s <l+f_g)/(SA_BET) ·Ν_Α/(Μ_8ι02 + MOH f_s(l+f_g)) (4) wherein f_g is the fraction of Q² silanol sites, f_s the fraction of all silanol sites, NA is Avogadro's number, SABET is the BET surface area determined by the BET method, Msi02 the molar mass of silica (i.e. 60.06 g/mol), MOH the molar mass of hydroxyl groups (i.e. 9 g/mol) and aoH the silanol content (OH/nm²).

The silica particle comprises < 0.03 wt.% sulfur, preferably < 0.02 wt.% sulfur and more preferably < 0.017 wt.% sulfur; < 0.05 wt.% iron, preferably < 0.04 wt.% iron, and more preferably < 0.03 wt.% iron; and < 0.2 wt.% magnesium, preferably < 0.1 wt.% magnesium, and more preferably < 0.07 wt.% magnesium. All these weight percentages are calculated as the atomic weight of the sulfur, iron or magnesium, and are based on the weight of the silica particle.

The silica particle of the invention also preferably comprises at least 99.5 wt.% silica, more preferably at least 99.7 wt.% silica, and even more preferably at least 99.9 wt.% silica, calculated as S1O2 and is based on the weight of the silica particle.

The silica particle of the invention typically contains substantially no measurable amount of sodium, i.e. sodium in an amount of < 0.001 wt. %, calculated as the atomic weight of sodium and is based on the weight of the silica particle.

Surprisingly it was found that the silica particle of the invention also has a low sulfur content, typically < 0.03 wt.%. This improved property enables the silica particle of the invention to be advantageously used in applications which have strict requirements about the level of impurities in the materials used, such as a catalyst support used in the production of polyolefines and as an additive in food products.

The silica particle typically comprises about 10 wt.% or less of water, calculated as hydroxyl groups present on the surface of the silica particle as determined after drying preferably under evacuation at temperatures above about 120 °C and is based on the weight of the silica particle.

All weight % referred to herein are based on the weight of the silica particle, unless otherwise indicated.

We distinguish the size of the individual silica particles, and the size of the clusters (also known as aggregates) of the silica particles. The particle size of the individual silica particle of the invention typically is between about 2 and 30 nm, and preferably between 3 and 20 nm. The BET surface area of the silica particle is typically between 90 and 500 m²/g, preferably between 150 and 450 m²/g, and more preferably between 200 and 400 m²/g. The BET method (S. Brunauer, P.H. Emmett and E. Teller, Journal of the American Chemical Society 60(1938)309) used for determining the BET surface area, is defined herein, as the value that can be measured by determining the amount of nitrogen adsorbed at 77 K and P/Po of approximately 0.05 to 0.3, where P is partial vapour pressure of nitrogen in equilibrium with the surface at 77 K and Po is the saturation pressure of nitrogen, and assuming a nitrogen cross sectional area of 16.2

A², after degassing the sample at 180 °C on a Micromeritics ASAP 2420.

The average pore size of the silica particle of the invention is typically between 2 and 150 nm, and preferably between 10 and 100 nm as determined by the BJH method. This average pore size, is the value determined by dividing the total pore volume by the BET surface area, and assuming that the pores are cylindrical. The BJH method (E.P. Barrett, L.G. Joyner and P.P. Halenda, Journal of the American Chemical Society

73(1951)373) can be used to calculate the pore size distributions from experimental isotherms using the Kelvin model of pore filling. The presence of micropores could be established from a t-plot analysis.

The silica particle of the invention was also found to have pore sizes of less than about 0.5 nm, as determined using positron annihilation spectroscopy (David W. Gidley, Hua-Gen Peng, Richard Vallery, Christoffer L. Soles, Hae-Jeong Lee, Bryan D. Vogt, Eric K. Lin, Wen-Li Wu and

Mikhail R. Baklanov, Dielectric Films for Advanced Microelectronics (M. Baklanov, M. Green and K. Maex eds.) John Wiley & Sons (2007)85 - 136).

The range of pore sizes of the silica particle of the invention was found to be surprisingly large, ranging from less than 1 nm to about 2 μιη, as determined by the BJH method. This improved pore size distribution advantageously provides for a considerable increase of the adsorption capacity of the silica particle, making it particularly suitable for use as an adsorbent. The presence of some of the pores in the silica material of the invention, in particular in the silica clusters according to the invention, may thus be too narrow to enable molecular nitrogen to penetrate them, which typically results in a surface area as determined by the BET method to be lower than that as determined from the NMR intensity of the number of silicon atoms containing surface hydroxyl groups. It also typically results in the high silanol number of the silica particle, because the NMR technique determines the total amount of silanols, and thus also includes silanols in the very small pores.

In a further aspect, the invention is directed to a method for the production of one or more silica particles according to the invention, wherein said method comprises contacting and mixing olivine with an acid thereby producing a slurry comprising silica particles and having a pH value of less than 1, diluting the slurry with a liquid thereby producing a diluted slurry having a pH value of at most 3, separating the silica particles from the diluted slurry, washing the separated silica particles first with the liquid and then water, and drying the washed silica particles.

The surprising advantage of the method of the invention is that it enables the production of silica particles of high purity (i.e. at least 99.5 wt.% silica, calculated as S1O2, and, at most 0.03 wt.% sulfur, calculated as the atomic weight of sulfur, based on the weight of the silica particle) and which are highly hydroxylated (i.e. the silica particle has at least 8 OH per nm²).

Without wishing to be bound by theory, it is believed that during the dissolution of silicate mineral bodies/particles, preferably olivine bodies/particles, hydrated silicon dioxide species are produced initially as monomers or oligomers of silicon acid (H2SiO4) and that these subsequently condense to small silica particles. Once the pH is at a value below about 1 and the leaching process is essentially complete, the silica particles produced have a positive electrostatic charge. This positive charge is due to the uptake of additional protons at the surface hydroxyl groups. The silica particles released during the dissolution of olivine are believed to form linear chains due to the acid-catalyzed condensation of two hydroxyl groups on neighboring sihca particles thus forming oxygen bridges between the silica particles. To minimize the electrostatic repulsion upon addition of a subsequent silica particle to a chain, the charged silica particles form linear chains which leads to an increase in viscosity of the suspension.

By diluting the slurry it is believed that when the pH value increases to at most 3 the adsorbed hydrogen ions are neutralized and the sihca particles clusters, having initially reacted to form linear chains thus lose their electrostatic charge. This in turn allows the linear chains of the sihca particles to become flexible. When the electrostatic charge is removed, the Van der Waals attraction between the individual sihca particles dominates and the chains of sihca particles form essentially spherical clusters. These spherical clusters do not revert back into the linear chains of sihca particles. The maximum stability of the colloidal silica is typically exhibited as pH values from about 1 to 2. Whether growth of the sihca clusters proceeds during the increase of the pH value from 1 to 2 is not certain, nor is the strength of the individual silica particles in a cluster. However, it is important that no gelation takes place when the clusters of sihca particles are re-dispersed in a liquid, such as water. The absence of gelation may be due to the size of the clusters of silica or the surrounding of the silica clusters by strongly bonded water molecules. The advantage of diluting the slurry is that it prevents condensation of water from the hydroxyl groups present on the sihca particles when they contact each other to form aggregates. This is evident by the fact that no gelation occurs during the dissolution process. Another advantage of diluting the slurry is that it allows unreacted mineral solids to more easily settle out due to the decreased viscosity of the slurry. Further, the dilution of the slurry acts to advantageously prevent the crystallization of inorganic salts, such as magnesium sulfate, during filtration.

It has surprisingly been found that by controlling the pH level during the dissolution of silicate mineral bodies/particles, preferably olivine bodies/particles, according to the process of the invention that the properties of the silica particles can also be controlled so that an essentially

homogenous silica product is obtained (i.e. at least 99 % of the silica particles produced have the same properties).

A further advantage of the method of the invention include a low energy consumption, and hence a low CO2 footprint.

Typically the temperature used for leaching a silicate mineral, preferably olivine, with an acid is between 50 and 100 °C.

The method of the invention may further comprise a preprocessing step, wherein the silicate mineral, preferably olivine is

mechanically crushed into a suitable particle size, which is typically less than 1 mm.

Mixing may be carried out using common mixing equipment, such as by using stirrers, rotors and the like.

The pH value of the slurry produced by the dissolution of a silicate mineral, preferably olivine with an acid is typically less than 1, preferably less than 0.5 and more preferably less than 0.1.

Acids which may be suitably used in the method of the invention include hydrochloric acid, sulfuric acid, formic acid and combinations thereof.

The dilution of the slurry with a liquid typically results in an increase in the pH to a value of at most 3, and preferably to a pH value of between 1 and 2.

Typically the slurry is diluted with a liquid by a dilution factor of at least two. The dilution factor, as defined herein, is the volume of the diluted slurry/the volume of the undiluted slurry. Optionally the undissolved silicate mineral, preferably olivine can be separated from the slurry and/or the diluted slurry by suitable means, such as by sedimentation. Further, the separated undissolved silicate mineral, preferably olivine, may be added to the liquid used for diluting the slurry. The advantage of this step is that it enables any remaining

dissolvable silicate mineral, preferably olivine, a further opportunity to dissolve and to produce silica particles. Preferably, prior to diluting the slurry with the liquid, the undissolved silicate mineral, preferably olivine, is separated from the liquid. The advantage of such a recovery step is that it enhances the production of silica particles.

Suitable means which may be used for separating the silica particles from the diluted slurry include filtration or centrifugation means.

Typically, when filtration is used the resulting silica filter cake comprises at least 20 wt.% of solids, preferably at least 25 wt.% of solids and more preferably at least 30 wt.% of solids.

The remaining slurry liquid typically comprises water soluble iron (II) and magnesium salts. The iron(II) salts can be recovered by the addition of ammonium nitrate to the slurry liquid and a buffering agent which maintains the pH at a value of at most 6 thereby precipitating out the iron salts, preferably as magnetite (Fe3O₄). Preferably the buffering agent used to recover the iron is urea. Typically the temperature used for this

precipitation reaction is at least 80 °C. The precipitated iron salt, preferably magnetite, is then separated from the slurry liquid by suitable separating means, such as filtering or magnetically separating the precipitated iron salt. The iron(II) salts can also be precipitated by heterogeneous oxidation to the much less soluble hydrated iron(III) oxide and is a more rapid reaction. The heterogeneous oxidation of iron(II) can readily be performed at room temperature by the addition of a suitable oxidizing agent, wherein said oxidizing agent may include hydrogen peroxide and other inorganic peroxides, nitric acid, nitrate compounds and combinations thereof. Further, with the precipitation of hydrated iron(III) oxide by this method, which is difficult to filter, a flocculant may be added to the slurry liquid to enhance the sedimentation of the precipitated salts. The drop in pH due to the hydrolysis of the iron(III) by this method can be compensated by adding such compounds as solid magnesium hydroxide and magnesium oxide, instead of using urea. The precipitated iron obtained may also comprise other metals, such as Ni and Cr. The remaining aqueous salts, such as magnesium sulfate, can then be recovered from the treated slurry liquid by methods known in the art, such as drying and/or evaporation, and optionally pre-filtering to remove any solids present.

The separated silica particles are typically washed first with the liquid and then with water in at least one washing step, respectively.

Preferably two or more, three or more, four or more, five or more, and/or six or more washing steps are used for washing the separated silica particles with the liquid and/or water. When the electrical conductivity of the water used in the one or more washing steps does not increase upon passing through the silica particles, the washing of the silica particles is typically complete.

Preferably the liquid used for diluting the slurry and the initial washing of the separated silica particles is acidified water, which acidified water preferably has a pH value of about 2 or less.

The advantage of the washing steps is that dissolved salts are removed from the silica. A further advantage of first washing with a liquid, such as acidified water, is that iron present in a silicate mineral, such as olivine, is more easily removed. This is because oxidation of bivalent iron (Fe(II)), which is more soluble at pH values between 2 and 5 in an aqueous liquid, and subsequent precipitation of the oxidized Fe(II) to the trivalent iron (Fe(III)) is prevented and/or limited.

The washed silica particles are typically dried by means of air drying and/or under a partial vacuum at an elevated temperature. Suitable elevated temperatures which may be used typically range from about 100 to 190 °C, and preferably between 105 and 150 °C.

Surprisingly we found that capillary condensed water present in the narrow pores of the silica particles can be readily removed by drying under a partial vacuum at temperatures as low as 100 °C without

compression and shrinkage of the silica particles produced.

The silica particles of the invention may be in the form of powder, particles, clusters, or an aerogel.

In another aspect, the present invention is directed to a silica particle cluster (also known as a cluster of silica particles), which cluster comprises one or more silica particles of the invention, and which cluster has a size of between 40 nm and 60 μιη, preferably between 200 nm and 40 μιη and more preferably between 5 and 20 μιη or has a cluster size of between 30 nm and 300 nm, preferably between 50 nm and 250 nm, and more preferably between 70 nm and 150 nm.

The size of the silica particle cluster of the invention is measured using static and dynamic light scattering, and from the slope of the V_a-t-plot recorded at low relative pressures of nitrogen at 77 K, where V_a is the liquid volume of nitrogen adsorbed at 77 K and t is the thickness of the adsorbed layer of nitrogen (also known as the t-method) (B.C. Lippens and J.H. de Boer, Journal of Catalysis 4(1965)319-323 and B.C. Lippens, B.G. Linsen and J.H. de Boer, Journal of Catalysis 3(1964)32-37). The surface area per unit weight of silica calculated from the initial slope of the t-plot provides a surface-weight mean cluster size.

Surprisingly it was found that the dried silica particle clusters of the invention are mechanically strong materials, which may be in the form of bodies.

If it is desired to obtain a suspension of the silica particles in a liquid (also known as colloidal silica), this can be obtained by mechanically treating the silica particles using a suitable device. In further aspects, the invention is directed to a method for the production of a suspension of silica particles, wherein said method comprises mixing one or more silica particles according to the invention with a liquid, wherein preferably said mixing is by means of a rotor-stator mixer, a bead mill and/or an ultrasonic mixer; and, a suspension of silica particles obtainable by said method. Alternatively or additionally, a Y-jet type mixer and/or a high pressure dispersion mixer may be used for mixing the silica particles.

Typically the silica particles present in the suspension according to the invention are in the form of one or more clusters, which clusters typically have a size of between 40 nm and 60 μιη, preferably between 200 nm and 40 μιη and more preferably between 5 μιη and 20 μιη or have a cluster size of between 30 nm and 300 nm, preferably between 50 nm and 250 nm, and more preferably between 70 nm and 150 nm. The desired silica cluster size may suitably be controlled by the type of mixing means used.

Surprisingly it was found that utilization of a rotor-stator mixer to mix silica particles with a liquid produces a suspension of silica particles having clusters of silica particles with a size distribution typically with a peak maximum ranging from about 10 to 30 μιη.

Alternatively, when an ultrasonic mixer is used the suspension of clusters of silica particles produced typically has a bimodal peak

distribution with peak maximums between 40 nm to 80 nm and between 350 nm and 700 nm.

As a further alternative, when a bead mill is used the suspension of clusters of silica particles produced typically has a size of between 30 nm and 300 nm, preferably between 50 nm and 250 nm and more preferably between 70 nm and 150 nm.

A suitable pH range for the suspensions of the invention is typically between 5 and 9.5, and preferably between 7.5 and 9.5. The silica particles or colloidal silica of the invention exhibit a zeta potential which varies linearly within the pH value range of between about 1 and 9.5. The isoelectric point (IEP) for the suspensions of the invention is typically around a pH value of about 1-2. It was also found that the zeta potential values below -30 mV, which are generally considered as the value required to stabilize a colloid suspension electrostatically, may be obtained when the suspensions of the invention have a pH value of above 7.

The suspensions of the invention were found to be stable for typically up to a month. However, if long term stabihty of the suspensions is desired, then dispersing agents, such as citric acid, sodium hexafosfate and/or a suitable polymer, can be added to the suspensions. Optionally or alternatively, the pH value of the suspension is also adjusted to further improve the stability of the suspension. The pH of the suspension may be adjusted by the addition of a base, such as those selected from the group consisting of ammonia, sodium hydroxide, organic bases (such as ammines and nitrogen atom containing heterocyclic compounds) and combinations thereof, and preferably is ammonia or sodium hydroxide. Excellent stability of the suspension was found to be achieved by using sodium hexafosfate as the dispersing agent and adjusting the pH of the suspension to a value of between 8 and 10, preferably between 9 and 9.5 and even more preferably between 9.3 and 9.5, in particular by the addition of ammonia. Excellent results have also been obtained by adjusting the pH using NaOH, in particular when silica suspension of the invention is obtained by bead milling. When the presence of sodium or ammonium is not desired, the sodium or ammonium ions may be removed from the suspension by, for example, using an ion exchanger. Lastly, any excess liquid in the suspension may be removed by evaporation.

Suitable liquids which may be used for the suspension of silica particles include water, and non-aqueous liquids (such as ethanol, methanol, butanol and/or isopropanol) and combinations thereof. A disadvantage with using an ultrasonic mixer to produce the suspension according to the invention is that ultrasonification leads to the formation of necks between clusters of silica particles, as shown in Figure 1.

Surprisingly it was found that the formation of necks between silica particle clusters which occur when mixing the silica particles by ultrasonification could be avoided if a silica particle suspension, in

particular a silica particle suspension in water, is cooled to a temperature of less than 5 °C, preferably less than 2 °C more preferably less than 0.5 °C, and even more preferably less than 0 °C particle suspensions of the invention which may suitably be used is > -5 °C.

Typically the amount of silica particles in the suspension is between 5 and 40 wt.%, and preferably between 10 and 40 wt.%, and more preferably between 15 and 40 wt.%.

The silica particles of the invention may further be modified to comprise organic compounds containing disulfide and/or hydroxyl groups on the surface of said particles. This may be achieved by the addition of such organic compounds to a suspension of silica particles in a non-aqueous liquid. This modification makes the silica particles of the invention particularly suited for use in rubber based materials.

In a further aspect, the invention is directed to obtaining an aerogel by a method comprising the steps of mixing one or more silica particles according to the invention with a non- aqueous liquid thereby producing a suspension, and then heating the suspension to a temperature above the super critical temperature of the non-aqueous liquid.

Preferably a suitable mixing means which can be used is an ultrasonic mixer.

Suitable non-aqueous liquids which can be used include ethyl acetate, diethyl ether and the like.

The silica particles according to the invention may suitably be used in many applications, for instance, as adsorbents, fillers in concrete and silicone rubber, a reinforcement material in elastomer products, an additive in food products, and a thickener of liquid systems, such as paints, thermosetting resins and printing inks.

Preferably the silica particles according to the invention are used as a regenerable adsorbent, particularly for water vapour. The advantage of using the silica particles of the invention as an adsorbent is that after such an adsorbent is spent, it may be easily regenerated by heating at a

temperature of up to about 120 °C, and preferably under a partial vacuum.

For the purpose of clarity and a concise description, features are described herein as part of the same or separate embodiments, however, it will be appreciated that the scope of the invention may include

embodiments having combinations of all or some of the features described.

The invention is now elucidated on the basis of some examples, which are not intended to limit the scope of the invention.

Examples

Example 1 (a)

Silica particles according to the invention were produced in a double walled glass lined stainless steel stirred reactor of 80 liters at 100 °C. The reactor was initially filled with 40 L 3M sulfuric acid, the temperature of which was raised to 80 °C. Subsequently 9.6 kg of dunite was added. The ground dunite had a purity content of 89 wt.% of olivine with the

composition ((Mgi.84Feo. i53Nio.oo7)Si04). By employing the external heating (oil through the double wall) the temperature was increased to 100 °C. After one hour of reaction, a cooling system was used to decrease the temperature; however, the temperature did not drop substantially due to powerful exothermic reaction. The neutralization reaction was stopped by addition of 25 L of de-ionized water, after 2 hours when the concentration of [H⁺] has reached 0.5 mol/L. Then, the undissolved dunite in the resulting slurry was separated out by sedimentation and the slurry comprising a silica particle suspension was transferred into a separate vessel. To enhance the silica particles production, the separated undissolved dunite was diluted with 15 liters of 0.1 M sulfuric acid and the resulting silica particles were decanted into the above mentioned separate vessel thereby diluting the slurry.

Subsequently, the diluted slurry was filtered and the separated silica particles were washed using a filter press (P_abs = 3-5 bar). The washing liquid used was 0.1 M sulfuric acid. The silica particles were washed until the electrical conductivity was 30 mS/cm as measured with a conventional apparatus for measuring the electrical conductivity of liquids. Next demi- water was used to wash the separated silica particles until the conductivity was below 600 μ8/αη. Filtration and washing were performed using a filter press initially with 10 chambers and finally with 16 chambers. The BET surface area was determined as described herein above. The contents of iron, magnesium and sulfur in wt. % were determined with X-ray

fluorescence (XRF). The measured characteristics of the silica particles produced are shown in Table 1 below.

Table 1(a)

Characteristics of the silica particles of Example 1 (a)

The nano-silica process yield using this method is about 82 wt.%, calculated from the dunite consumed and the solid content of the silica was about 20 wt.%. After evacuation i.e. using a partial vacuum at 100 °C the solid content was about 90 wt.%.

Examples 1 (b) and (c)

Silica particles were produced according to the same method as described in Example 1 (a), with the exception that the filtration and washing were performed using a filter press with 10 chambers . The BET surface area was determined as described herein above. The contents of iron, magnesium and sulfur were determined in wt.% with X-ray

fluorescence (XRF).

The SANMR, f_g, f_s, aoH were determined, as described herein above, with the NMR analyses being carried out using a 400 Bruker Avance III spectrometer, equipped with a wide bore magnet of 9.4 T, and a 4 mm DVT probe, which operated at 79.5 MHz. The spinning rate was 10 KHz and the magic angle was adjusted using a sample of KBr. The ²⁹Si MAS spectra have been acquired at 300 K with a n/2 pulse of 4 μβ and a relaxation delay of 30 s. The sweep width was 64 KHz, and the numbers of scans 2900. No proton decoupling was applied.

The measured characteristics of the silica particles produced according to Examples 1 (b) and (c)are shown in Table 1(b) below. Table 1(b)

Characteristics of the silica particles of Examples 1 (a) and (b)

Characteristic Example 1 (b) Example 1 (c)

Chambers 10 10

Surface area BET (m²/g) 391 390

Surface area NMR (m²/g) 738 739

aoH (number of OH groups/nm²) 9.4 9.5

¾ 0.1 0.1

fs 0.3 0.3 XRF (wt.%)

Si0₂ 99.8 99.8

S0₃ <0.04 0.06

MgO 0.08 0.10

Fe₂0₃ 0.08 0.05

Example 2

2375 g of the silica filter cake with 20 wt.% of solids (BET surface area of 389 m²/g), produced according to example 1, was added to the same quantity of water and then mixed using of a rotor-stator type high energy mixer, viz. Silverson 5RL (U.K). The mixing was carried out at a rotor speed of 7,000 rpm for 30 min to produce a suspension. The pH of the resulting suspension was 5.74 the solid content was about 10 wt.% by mass. The particle size distribution of the silica particle clusters of the resulting suspension, without any filtration, was measured by a laser light diffraction device, viz, Malvern® Mastersizer 2000 (U.K.) in water. The suspension had clusters of silica particles ranging in size from about 1 μιη to 60 μιη, as shown in Figure 2, with an average surface weighting mean D[3,2] of 10 μιη. The dispersion in water was fairly stable for a period of 1 to 4 weeks after which some settling was observed. Agitation by the use of a high energy mixer was necessary to re-establish the suspension and maintained the same particle size distribution of the silica particles clusters.

Example 3

Other dispersion experiments were analogous to that of example 2, except that an ultrasonic device, viz. Hielscher Gmbh UP400S (Germany) coupled with a 22 mm diameter sonotrode (model H22) was used for dispersing the silica particles within water using ultrasound wave

generation at a power input of 139-200 W and a frequency of 24 kHz. The amplitude of the sound wave was adjusted from 80 to 100 % of the maximum (100 μιη). Using ultrasound in batch mode, 80 ml of slurry with 10 wt.% mass solid content were sonically treated in 5 min intervals to a maximum cumulative time of 20 min (total specific energy of 3 · 10⁶ kJ/m³). The suspensions were immersed in a bath filled with ice to maintain a temperature near 0 °C during each ultrasound treatment. The resulting colloidal silica was characterized by static light diffraction device, viz, Malvern® Mastersizer 2000 (U.K.) using water as dispersion agent, the results are listed in Table 2.

Table 2

Cluster size distributions and characteristics of ultrasonically treated clusters of silica particles in water (solid content 9.8-10 wt.%, pH value of 5.74).

After the ultrasound treatment the colloidal silica was kept in a static condition for 48 h, after which bottom sediments were observed. The sediments were separated from the resulting colloidal silica which was characterized by dynamic light scattering device, viz. Malvern® Zetasizer Nano ZS (U.K.). The stable resulting colloidal silica displayed a bimodal distribution of cluster sizes (see Figure 3). The distribution was peaking at 0.06 pm (low maximum) and 0.48 pm (high maximum). The final mass content of the resulting colloidal silica was 9.8 wt.%, as determined by oven- dried technique. The colloidal silica was stable, but some small sediment proceeded after about one month. Some agitation could re-establish the colloidal silica.

Example 4

The stability of the different colloidal silica produced was determined over a variable pH range by measuring the zeta-potentials using an electro-acoustic measurement device, viz. Malvern® Zetasizer Nano ZS (U.K.). Before this 20 g of the silica filter cake with 20 wt.% of solids, produced in example 1, was added to 396 g of distilled water and then mixed by the use of a rotor-stator type high energy mixer, viz. Silverson 5RL (U.K). The mixing was carried out at 7,000 rpm rotor speed for 30 min until a suspension of clusters of silica particles had been obtained. The pH of the resulting suspension was 6.5 with a solid content of about 1 wt.% by mass (see Figure 4, dashed circle). 80 ml samples of the resulting suspensions were used in the zeta-potential measurements. The pH values of the suspensions were adjusted by titration with either NaOH (see Figure 4, arrow pointing to the right) or HC1 (see Figure 4, arrow pointing to the left) to cover a pH range of between 1 and 13. At each pH point, disposable polystyrene cuvettes were filled with 1 ml of the prepared dispersion for five consecutive zeta-potential measurements. The resulting zeta-potential values in units of mV of the dispersion was plotted as a function of the pH and showed a minimum zeta-potential peak of -35 mV for an equivalent pH value of 9.68 (see Figure 4). The calculated isoelectric point (ΊΕΡ), where zero surface charge (0 mV) of the silica particles clusters is observed, is present at a pH value of 1.9. Example 5

Colloidal silica suspensions according to the invention was prepared by pre-dispersing a silica filter cake in water and subsequently breaking the resulting silica clusters by recirculation of the suspensions through a bead mill.

The pre-dispersion was performed on a suspension containing 10 wt.% solid silica, which was prepared by combining 1 kg of the silica filter cake in 1 1 demi-water. Different pH values of the silica suspensions were produced by adjusting the pH by titrating with NaOH.

The pre-dispersion was performed using a Turrax rotor-stator T25 equipment during 30 min at 22,000 rpm. Subsequently the colloidal silica suspensions were obtained by recirculation of the silica suspension through a WAB Dyno®-mill Multilab employing beads YTZ 0.3 mm with Dyno® Accelerators (Zr02) and a liner of silicon carbide. The volume of the milling device was 1905 ml. The flow through the milling equipment was 54.1 1/h. The stirring intensity in the vessel containing the suspension was varied by employing tip speeds of 7.8, 9.8, and 13.7 m/s. The pH values of the suspensions ranged from 5.7 to 10.5.

Figure 5, 6, and 7 show the results obtained for the colloidal silica suspensions using a static light diffraction device, viz. a Malvern®

Mastersizer 2000 (U.K.) in water.

Figure 5 shows the results obtained with a silica suspension having a pH value of 5.7 and a solid content of 10 wt.% by mass with tip speeds of the bead mill of 7.8, 9.8 and 13.7 m/s. At the left-hand side of Figure 5 are shown the volume percentages of particle-size distribution of the silica clusters, where D(0.9), D(0.5) and D(0.1) are referring to 90 vol.%, 50 vol.% and 10 vol.%, respectively.

At the right-hand side of Figure 5 is shown the surface weighted mean diameter (D[3,2]) and the volume of the silica clusters which are smaller than 1 μιη. The use of a tip speed of 7.8 m/sec leads after milling for about 6,000 s (100 min) to the complete disappearance of silica clusters larger than 0.5 μιη and silica clusters having a mean diameter of 118 nm. At a tip speed of 9.8 m/s the time of milling required to remove the larger clusters of silica is 3,000 s (50 min), while a tip speed of 13.7 m/s results in a milling time of about 2,300 s (about 38 min) to achieve the removal of the larger silica clusters greater than 0.5 μιη. The higher tip speed provides higher energy, for that reason less time is necessary to obtain a similar particles size.

The effect the pH value on the suspensions containing 10 wt.% by mass of solid material was determined at a tip speed of 9.8 m/sec, with the flow through the bead mill was 51.4 1/h, as shown in Figure 6. At pH values of 8.0 and 9.6 the milhng time to completely remove the silica clusters larger than about 0.5 μιη was about 1,000 s (about 17 min), which is significantly less than the milhng time required with suspension of a pH value of 5.7, viz. about 3000 s (50 min). This is due to the silica particles having a higher negative electrostatic charge at pH values of 8.0 and 9.8 than when the pH value is 5.7. When the suspension has a pH value of 10.5, larger silica particles remained in the suspension. The formation of the larger silica particles is also to be expected since it is well known that larger silica particles are produced by aging of silica at higher pH levels (Ostwald ripening).

The effect of the solid content by mass of the initial suspension of silica particles was also determined by comparing the effect of the milling time on the particle size of suspensions having 10 and 20 wt.% solids by mass, as shown in Figure 7. To prepare the suspension containing 20 wt.% solids 2 kg silica cake was brought in 1 1 water and 109 g NaOH of a concentration of 1 molar. The initial pH value of the suspension was 10.6 and the finale pH value was 9.3. The pre-dispersion was performed with a Saw type dissolver of an IKA stirrer with 60-500 rpm for 24 hrs. The tip speed in the bead mill was 9.8 m/s and the flow through the bead mill was 51.4 1/h. The slurry containing more solids exhibited an increase in the number of larger silica particles (> 0.5 μιη) due the mixing not being able to completely break up the silica clusters in such a concentrated suspension and also in part to the dissolution of smaller particles and precipitation onto larger particles via the process of Ostwald ripening.

The above-mentioned suspensions obtained by bead milling were also measured by laser light scattering, viz. Malvern® Matersizer 2000 (U.K.) in water. The period of time of milling was 1 h and 40 min. The effect of different tip speeds, pH values and solid content of the suspensions by mass are represented in Figure 8. It can be seen from Figure 8 that no silica clusters larger than about 80 μιη are present. The effect of the tip speed is small, as can be expected from the milling time employed, viz., 1 h and 40 min. The effects of the pH and the solid content by mass are shown in Figure 8 in the particle size distributions of the silica clusters smaller than 10 μ ηι.

Figure 9 shows the particle size distributions of the silica clusters after 4 days and removal of the small sediments. It can be seen that no significant changes in the silica clusters occur.

The silica cluster size distributions was also determined by dynamic light scattering, which can provide more accurately the

distribution of the clusters smaller than 1 μιη. The colloidal silica

suspensions produced at different tip speeds. pH levels and solid contents invariably displayed three peaks were measured after 4 days. The size and the volume percent's of the silica clusters shown in Table 3 (see below). From the data it can be concluded that a high tip speed and a pH level of about 9.3 leads to a high content of very small (< 500 nm) silica clusters.

Table 3

Cluster size distributions and characteristics of bead mill treated clusters of silica particles in water after 4 days. Tip speed pH Solids Z average Peak 1 Vol 1 Peak 2 Vol 2 Peak 3 Vol 3 (m/s) (wt.%) (nm) (nm) (%) (nm) (%) (nm) (%)

7.8 5.7 9.4 324 26 5.3 81 33.4 534 59.8

9.8 5.7 8.6 346 71 33.0 473 63.6 3691 3.4

13.7 5.7 8.7 311 457 66.6 38 32.8 4646 0.6

9.8 8.0 6.5 229 16 13 80 34.6 442 51.3

9.8 9.3 7.6 242 69 40.5 417 57.8 4554 1.6

9.8 10.4 6.6 236 82 31.8 326 47.6 678 19.9

9.8 9.4 7.6 229 16 13 80 34.6 442.4 51.3

9.8 9.4 14.8 299 56 22.8 453 76.8 4825 0.4

Example 6

The solid content of the silica suspension (18.2 wt.% by mass of S1O2 and a pH value of 9) was increased by the evaporation of water under reduced pressure in a rotary evaporator. The silica suspension (100 ml) was brought into a round bottom flask (250 ml). Evaporation of water by treatment in a rotary evaporator (pressure 1 mbar, temperature of 20 °C) resulted in stable silica suspensions of 30 wt. % and 40 wt.% by mass, respectively (see Figure 10).

Claims

1. Silica particle comprising < 0.03 wt.% sulfur, < 0.05 wt.% iron and

< 0.2 wt.% magnesium, and wherein said particle has a surface area of between 500 and 950 m²/g, as determined by the NMR intensity of the surface hydroxy! groups.

2. Silica particle according to claim 1, wherein said particle has a particle size between about 2 and 30 nm, and preferably between 3 and 20 nm.

3. Silica particle according to any of the previous claims, wherein said particle has a BET surface area between 90 and 500 m²/g, preferably between 150 and 450 m²/g, and more preferably between 200 and 400 m²/g.

4. Silica particle according to any of the previous claims, wherein said particle has an average pore size of between 2 and 150 nm, and preferably between 10 and 100 nm, as determined by the BJH method.

5. Silica particle according to any of the previous claims, wherein said particle has a surface area of between 600 and 900 m²/g, and more preferably between 650 and 800 m²/g, as determined by the NMR intensity of the surface hydroxyl groups.

6. Silica particle according to any of the previous claims, wherein said particle comprises at least 99.5 wt.% silica, more preferably at least 99.7 wt.% silica, and even more preferably at least 99.9 wt.% silica, calculated as S1O2 and is based on the weight of the silica particle; and, wherein said particle contains sodium in an amount of < 0.001 wt. %, calculated as the atomic weight of sodium and is based on the weight of the silica particle.

7. Silica particle according to any of the previous claims, wherein said particle has a silanol number of between 8 and 40 OH groups per nm², preferably between 10 and 36 OH groups per nm² and more preferably between 12 and 32 OH groups per nm², as determined by NMR.

8. Silica particle according to any of the previous claims, wherein said particle further comprises an organic compound containing a disulfide and/or hydroxy! group on the surface of said particle.

9. Method for the production of one or more silica particles according to any of the previous claims, wherein said method comprises contacting and mixing a silicate mineral, preferably olivine, with an acid thereby producing a slurry comprising sihca particles and having a pH value of less than 1, diluting the slurry with a liquid thereby producing a diluted slurry having a pH value of at most 3, separating the silica particles from the diluted slurry, washing the separated silica particles first with the liquid and then water, and drying the washed silica particles.

10. Method according to the previous claim, wherein the slurry has a pH value of less than 0.5 and preferably less than 0.1; and, wherein the diluted slurry has a pH value of between 1 and 2.

11. Method according to any of the claims 9-10, wherein the acid used is selected from the group hydrochloric acid, sulfuric acid, formic acid and combinations thereof; and, wherein the liquid used is acidified water.

12. Method according to claims 9-11, wherein the silica particles are dried by air drying and/or under a partial vacuum at a temperature of between 100 and 190 °C.

13. Method according to claims 9-12, wherein undissolved silicate mineral is separated from the slurry, wherein the separated undissolved silicate mineral is added to the liquid used for diluting the slurry, and wherein the undissolved silicate mineral is separated from the liquid and/or the diluted slurry.

14. Method according to any of the claims 9-13, wherein the separation of the silica particles from the diluted slurry produces a slurry liquid comprising water soluble iron and magnesium salts, wherein said slurry liquid is mixed with ammonium nitrate and a buffering agent which maintains the pH at a value of at most 6, thereby precipitating out the iron salt, and wherein the precipitated iron salt is then separated from the slurry liquid.

15. Silica particle obtainable by the method according to any of the claims 9-14.

16. Silica particle cluster comprising one or more silica particles according to any of the claims 1-8, wherein said cluster has a size of between 40 nm and 60 μιη, preferably between 200 nm and 40 μιη and more preferably between 5 and 20 μιη or said cluster has a size of between 30 nm and 300 nm.

17. Method for the production of a suspension of silica particles, wherein said method comprises mixing one or more silica particles according to any of the claims 1-8 with a liquid, wherein preferably said mixing is by means of a rotor-stator mixer, a bead mill and/or an ultrasonic mixer.

18. Method according to the previous claim, wherein a dispersing agent is added to the suspension of silica particles and optionally adjusting the pH value of the suspension, and wherein preferably the dispersing agent is sodium hexafosfate and the pH of the suspension is adjusted to a value of between 8 and 10 by the addition of ammonia; and/or, the suspension is cooled to a temperature of less than 5 °C, preferably to a temperature of less than 1 °C, more preferably to a temperature of less than 0.5 °C, and even more preferably less than 0 °C.

19. Suspension of silica particles obtainable by the method according to any of the claims 17-18.

20. Aerogel obtaining by the method comprising the steps of mixing one or more silica particles according to any of the claims 1-7 with a nonaqueous liquid thereby producing a suspension, and then heating the suspension to a temperature above the super critical temperature of the non- aqueous liquid.

21. Use of a silica particle according to any of the claims 1-8 as an adsorbent, particularly for water vapor, preferably as a regenerable adsorbent; a filler in conglomerate materials, such as concrete, and silicone rubber; a reinforcement material in elastomer products; an additive in food products; or, a thickener of liquid systems.