Silica particles for heat treatment

Abstract

The present invention is 2 / g~500m 2 BET surface area / g, number average particle size d of 350 μm to 2000 μm 50 , particle size distribution span (d 90 -d 10 ) / d 50 , bulk density greater than 0.35 g / mL, 1.5 cm 3 The present invention provides fumed silica granules having a pore volume of pores greater than 4 nm of 10 ...

Description

Technical Field

[0001] The present invention relates to fumed silica granules, their preparation and use for producing heat-treated silica granules.

Background Art

[0002] Fumed silica-based granules can be used in a variety of different applications, for example, as a catalyst support, i.e., a support for liquid substances, in cosmetic applications, for thermal insulation materials, as a pharmaceutical excipient, and the like.

[0003] In some applications, such as the production of catalyst supports or abrasive materials, the silica granules need to undergo a heat treatment step. In this step, typically carried out at temperatures above 300 - 500 °C, all volatile residues are removed, and the number of free silanol groups on the silica surface and the overall porosity are significantly reduced. Such heat-treated silica granules usually have a higher bulk density than their precursors. Agglomeration and clogging into larger, often undesirable silica fragments generally occur during the heat treatment step.

[0004] Numerous methods are known for preparing fumed silica granules from fumed silica powder dioxide that can be used as precursors for subsequent heat treatment. This preparation usually involves the agglomeration of fumed silica particles. This can be done by wet granulation. In wet granulation, an aqueous colloidal fumed silica dispersion is usually prepared under constant mixing. The solvent is then gradually removed to leave a dry residue, which can be further milled and classified.

[0005] US20170008772A1 describes a method for preparing synthetic amorphous silica powder, which includes the following steps: (a) forming a fumed silica slurry and then drying it, for example, in a rotary kiln to obtain silica powder; (b) subjecting the obtained silica powder to a primary firing at 900 - 1200 °C; (c) pulverizing and subjecting it to a secondary firing at 1100 - 1400 °C; (d) pulverizing and washing; (e) filtering to obtain the target sintered silica powder. The unsintered silica granules obtained by the evaporation of water from the silica slurry in step (a) of this method are not mechanically compressed and thus have a relatively high porosity and a low bulk density. In contrast, the sintered silica particles obtained after step (b) of the method have little porosity and a very small BET surface area.

[0006] US4042361 discloses a method for preparing silica granules using pyrogenic silicon dioxide. This is incorporated into water to form a dispersion, then the water is heated and evaporated, and the residue is pulverized into fragments with a size of about 1 millimeter to several centimeters, and this is further calcined at 1150 °C - 1500 °C.

[0007] One special type of wet granulation method is based on spray - drying a silica dispersion to obtain spherical granules with a narrow particle size distribution.

[0008] EP1717202A1 discloses the preparation of a silica - based sintered material with an average particle diameter of 10 - 120 μm, which involves the steps of spray - drying a dispersion containing fumed silica and then heat - treating the silica granules.

[0009] It is also possible to obtain silica particles by dry-compressing silicon dioxide. Exothermic silicon dioxide is very dry, and capillary forces cannot bring about particle bonding, so the compression of dry exothermic silicon dioxide is difficult. Exothermic silicon dioxide is characterized by extreme fineness, low bulk density, high surface area, very high purity, a substantially spherical primary particle shape, and no pores. Exothermic silicon dioxide often has a high surface charge, which makes aggregation difficult due to electrostatic interaction.

[0010] WO2009007180A1 discloses a method for producing silica particles having a BET surface area of less than 1 m 2 ² and a medium particle diameter of 10 - 140 μm, which comprises: (a) compressing exothermic silicon dioxide powder into slag; (b) grinding these slags and removing slag fragments less than 100 μm and greater than 800 μm; (c) treating the obtained slag fragments having a tamped density of 300 - 600 g / L at 600 - 1100 °C in an atmosphere suitable for removing hydroxyl groups; and (d) sintering at 1200 - 1400 °C.

[0011] US20160082415A1 relates to providing a functionalized silica carrier material having improved desorption properties, for example, for enzyme adsorption. US20160082415A1 discloses a method for producing such functionalized granular silica, which includes forming sedimentary silica or fumed silica into slag by dry compression, its screen granulation or screening at a screen size of 3000 μm, screening out fines, and subsequently reacting the obtained granules with a surface modifier. Only specific examples of sedimentary silica particles are shown in this patent application.

[0012] A method for preparing fumed silica particles having a pore volume of 0.1 to 2.5 mL / g, a bulk density of 0.5 to 1.2 g / cm 3 50 and an average particle size d in the range of 150 to 300 μm is disclosed, which involves spray-drying an aqueous dispersion and then melting the silica particles.

[0013] The selection of a suitable silica precursor for preparing the heat-treated silica article is of utmost importance. Many types of silica particles known from the prior art are not optimal for performing such a heat treatment process, resulting in performance degradation, clogging, and inferior product characteristics of the heat-treated silica product.

Prior Art Documents

Patent Documents

[0014] U.S. Patent Publication No. 2017 / 0008772A1

[0015] U.S. Patent No. 4042361

[0016] European Patent Publication No. 1717202A1

[0017] International Publication No. 2009 / 007180

[0018] U.S. Patent Publication No. 2016 / 0082415A1

[0019] U.S. Patent Publication No. 2019 / 0053150A1

[0020] U.S. Patent Publication No. 2019 / 0062193A1

Summary of the Invention

Problems to be Solved by the Invention

[0021] ​The object of the present invention is to provide a particulate silica-based material optimized for producing heat-treated silica granules having an increased bulk density, especially by a continuous method.

[0022] Such a silica-based particulate material should be suitable for continuous heat treatment in a rotary kiln or similar device, with high throughput and without clogging.

Means for Solving the Problems

[0023] Fumed silica granules The present invention provides 20 m 2 / g to 500 m 2 / g of BET surface area, a number average particle size d of 350 μm to 2000 μm determined by laser diffraction method 50 , a span of particle size distribution of 0.8 to 3.0 (d 90 -d 10 ) / d 50 , a bulk density of more than 0.35 g / mL determined by mercury intrusion method, a pore volume of pores larger than 4 nm of 1.5 cm 3 / g or less determined by mercury intrusion method and provides fumed silica granules having the same.

Mode for Carrying Out the Invention

[0024] In the context of the present invention, the terms "particulate material", "granular material", and "granules" are used interchangeably and are understood to mean a granular, free-flowing particulate solid material that can be easily poured. The granules of the present invention can be in the form of granular particles having a spherical or any other shape, such as pellets, rings, etc., or in the form of irregularly shaped fragments of any ground compact having a statistical particle size distribution, etc.

[0025] The granules of the present invention are made from fumed silica. Fumed silica can be prepared by flame hydrolysis or flame oxidation. This generally involves oxidizing or hydrolyzing a hydrolyzable or oxidizable starting material in a hydrogen / oxygen flame. Starting materials used in the pyrogenic process include organic and inorganic substances. Silicon tetrachloride is particularly preferred. The hydrophilic silica thus obtained is amorphous. Fumed silica is generally in the form of a mass. By "mass" it is understood to mean that the primary particles, which are formed first during production, subsequently bind strongly to one another in the reaction to form a three-dimensional network. The primary particles are substantially pore-free and have free hydroxyl groups on their surfaces.

[0026] The granules according to the invention consist essentially of fumed silica, i.e. they contain at least 85% by weight, preferably at least 90% by weight, more preferably 95% to 100% by weight of fumed silica. Apart from fumed silica, the granules may contain water and some trace impurities.

[0027] The total metal impurities in the granules of the present invention are preferably less than 500 ppm, more preferably less than 200 ppm, more preferably less than 100 ppm, more preferably less than 50 ppm.

[0028] Trace impurities can include Ca, Cu, K, Li, Mg, Mn, Na, Ti, V, Zr, Al, B, Cr, Ni, P. To determine the metal content, the silica granules are dissolved in a solution containing hydrofluoric acid. The silicon tetrafluoride formed evaporates and the remaining residue is analyzed by inductively coupled plasma mass spectrometry (ICP-MS).

[0029] The particle size distribution of the granules of the present invention can have a significant impact on the applicability of such silica granules for processes involving heat treatment.

[0030] The granules according to the present invention have a number average particle size d of 350 μm to 2000 μm, preferably 350 μm to 1800 μm, more preferably 350 μm to 1600 μm, more preferably 400 μm to 1400 μm, more preferably 400 μm to 1200 μm, more preferably 500 μm to 1150 μm, more preferably 550 μm to 1100 μm, even more preferably 600 μm to 1050 μm, and most preferably 700 μm to 1000 μm. 50 The number average particle size of the granules can be determined by laser diffraction particle size analysis according to ISO13320:2009. Using the obtained measured particle size distribution, the average value d that reflects the particle size not exceeding 50% of all particles is defined as the number average particle size. 50

[0031] The granules according to the present invention preferably have a d value of 100 μm to 1000 μm, more preferably 120 μm to 900 μm, more preferably 150 μm to 850 μm, more preferably 200 μm to 800 μm, more preferably 250 μm to 750 μm, and more preferably 300 μm to 700 μm. The preferred d value is 800 μm to 2500 μm, more preferably 900 μm to 2000 μm, more preferably 950 μm to 1900 μm, and more preferably 1000 μm to 1800 μm. The d and d values can be determined by laser diffraction particle size analysis according to ISO13320:2009. Using the obtained measured particle size distribution, the values d and d that reflect the particle size not exceeding 10% or 90% of all particles are defined respectively. 10 90 10 90 10 90

[0032] The granules of the present invention preferably have a particle size of 2000 μm or less, more preferably 1700 μm or less, more preferably 1500 μm or less, and more preferably 1300 μm or less. The absence of particles having a particle size exceeding a specific range can be achieved, for example, by sieving the granules through an appropriate sieve.

[0033] ​​​​​​​ The proportion of particles having a particle size of 100 μm or less in the granules of the present invention is preferably less than 30% by weight, more preferably less than 20% by weight, more preferably less than 15% by weight, and still more preferably less than 10% by weight. This proportion can be calculated from the results of particle size analysis by the laser diffraction method.

[0034] The granules of the present invention have a relatively narrow particle size distribution characterized by the value of span (d 90 -d 10 ) / d 50 which is in the range of 0.8 to 3.0, preferably 0.9 to 2.0, more preferably 1.0 to 1.8, still more preferably 1.1 to 1.7, and even more preferably 1.2 to 1.6. Granules having such a narrow particle size distribution have been found to be particularly suitable for carrying out heat treatment in a continuous manner. The narrow particle size distribution of the silica granules of the present invention results from the relatively low proportions of both fine powder and larger silica granules present. Silica fine powder can cause clogging during a continuous heat treatment process. Larger silica granules can be mechanically unstable during a continuous heat treatment process, for example, in a rotary kiln, and can break apart to form fine powder.

[0035] The term "pore volume of pores larger than 4 nm" is related to the cumulative pore volume of pores larger than 4 nm, which can be determined by the mercury intrusion method in accordance with DIN ISO 15901-1. The principle of this method, first described by H. L. Ritter and L. C. Drake in Ind. Eng. Chem. Anal. Ed. 17 (1945) pages 782 - 786 and 787 - 791, is based on the measurement of the volume of mercury pressed into a porous solid as a function of the applied pressure. Only pores into which mercury can penetrate at the applied maximum pressure, for example, 417 MPa, i.e., pores generally having a pore diameter larger than 4 nm, are detected. Liquid mercury does not wet the surface of the probe porous solid and penetrates the pores only under pressure. The pressure to be applied is inversely proportional to the opening width of the pore opening, and in the case of cylindrical pores, the relationship between the pore radius r p and the pressure p is given by Washburn's equation: rp = -(2×σ / p)×cosθ [wherein r p is the pore radius, p is the pressure, σ is the surface tension of mercury (0.48 N / m), θ is the contact angle of mercury (140 °C)] is represented by.

[0036] The pore volume of pores larger than 4 nm corresponds to the cumulative pore volume of all pores that can be determined by the mercury intrusion method in accordance with DIN ISO 15901-1 up to the determination limit at a maximum pressure of 417 MPa.

[0037] The pore volume of pores larger than 4 nm of the particles of the present invention determined by the mercury intrusion method in accordance with DIN ISO 15901-1 is 1.5 cm 3 / g or less, preferably 0.1 cm 3 / g to 1.5 cm 3 / g, more preferably 0.2 cm 3 / g to 1.45 cm 3 / g, more preferably 0.3 cm 3 / g to 1.4 cm 3 / g, more preferably 0.5 cm 3 / g to 1.3 cm 3 / g, more preferably 0.6 cm 3 / g to 1.2 cm 3 / g, more preferably 0.7 cm 3 / g to 1.1 cm 3 / g, more preferably 0.8 cm 3 / g to 1.0 cm 3 / g.

[0038] The term "pore volume of pores less than 4 μm" relates to the cumulative pore volume of pores less than 4 μm that can be determined by the mercury intrusion method in accordance with DIN ISO 15901-1 and corresponds to the cumulative pore volume of all pores less than 4 μm that can be determined by this method.

[0039] The pore volume of pores less than 4 μm of the particles of the present invention determined by the mercury intrusion method in accordance with DIN ISO 15901-1 is preferably less than 1.4 cm 3 / g, more preferably 0.05 cm 3 / g to 1.4 cm 3 / g, more preferably 0.1 cm 3 / g to 1.3 cm 3 / g, more preferably 0.2 cm 3 / g to 1.25 cm 3 / g, more preferably 0.3 cm 3 / g to 1.2 cm 3 / g, more preferably 0.4 cm 3 / g to 1.2 cm 3 / g, more preferably 0.4 cm 3 / g to 1.1 cm 3 / g, more preferably 0.4 cm 3 / g to 1.0 cm 3 / g, more preferably 0.4 cm 3 / g to 0.9 cm 3 / g, more preferably 0.5 cm 3 / g to 0.9 cm 3 / g, more preferably 0.6 cm 3 / g to 0.9 cm 3 / g.

[0040] The percentage of the pore volume of pores less than 4 μm with respect to the cumulative pore volume of pores more than 4 nm of the particles of the present invention is determined by the mercury intrusion method in accordance with DIN ISO 15901-1 for both pore volumes, and is preferably more than 35%, more preferably more than 40%, more preferably more than 50%, more preferably 55% to 95%, more preferably 60% to 90%, more preferably 65% to 85%, more preferably 70% to 80%. The percentage of the pore volume of pores less than 4 μm with respect to the cumulative pore volume of pores more than 4 nm can be obtained by dividing the former by the pore volume of the latter and multiplying the result by 100%.

[0041] The porosity of the granules determined by mercury intrusion according to DIN ISO 15901-1 is preferably less than 77%, more preferably 10% - 75%, more preferably 20% - 70%, more preferably 30% - 75%, more preferably 40% - 72%, more preferably 50% - 70%, more preferably 52% - 67%, more preferably 55% - 65%.

[0042] The granules according to the present invention are characterized by a limited porosity and pore volume determined by mercury intrusion when compared to similar silica materials known from the prior art. Without wishing to be bound by any theory, such a reduction in porosity is thought to be correlated with a decrease in the evaporation rate of water and the removal of free silanol groups from the silica surface under heat treatment. As a result, if the porosity and pore volume are too high, plugging may be caused when rapid sintering of the heat-treated silica granules occurs, and the performance may be reduced during the sintering process.

[0043] The granules of the present invention have a BET surface area of 20 m 2 / g to 500 m 2 / g, preferably 30 m 2 / g to 450 m 2 / g, more preferably 40 m 2 / g to 400 m 2 / g, more preferably 50 m 2 / g to 380 m 2 / g, more preferably 60 m 2 / g to 350 m 2 / g, more preferably 70 m 2 / g to 320 m 2 / g, more preferably 80 m 2 / g to 320 m 2 / g, more preferably 80 m 2 / g to 220 m 2 / g. The specific surface area, also simply called the BET surface area, can be determined by nitrogen adsorption in accordance with the Brunauer - Emmett - Teller method according to DIN 9277:2014.

[0044] The granules according to the present invention preferably have a tapping density of more than 200 g / L, more preferably 200 g / L to 1000 g / L, more preferably 230 g / L to 800 g / L, more preferably 250 g / L to 700 g / L, more preferably 280 g / L to 650 g / L, more preferably 300 g / L to 600 g / L, more preferably 320 g / L to 550 g / L.

[0045] The tapping density of various powdery or coarse-grained particulate materials can be determined in accordance with DIN ISO787-11:1995 ‘‘General methods of test for pigments and extenders--Part11:Determination of tamped volume and apparent density after tamping’’. This involves measuring the apparent density of the bed after agitation and tamping.

[0046] The bulk density of the granules according to the present invention measured by mercury intrusion porosimetry is more than 0.35 g / mL, more preferably 0.35 g / mL to 1.20 g / mL, more preferably 0.40 g / mL to 1.1 g / mL, more preferably 0.45 g / mL to 1.0 g / mL, more preferably 0.47 g / mL to 0.95 g / mL, more preferably 0.50 g / mL to 0.90 g / mL, more preferably 0.55 g / mL to 0.85 g / mL, more preferably 0.60 g / mL to 0.80 g / mL. The bulk density of the granules of the present invention can be determined by mercury intrusion porosimetry in accordance with DIN ISO15901-1 at the minimum mercury pressure applied by this method, for example, a pressure of less than 0.01 MPa, for example, 0.0031 MPa.

[0047] The bulk density determined by mercury intrusion porosimetry is considered to be a more appropriate value for defining the material density of the particulate material, for example, as compared with the tapping density of the same material. Therefore, the bulk density determined by mercury intrusion porosimetry excludes the large interparticle spaces into which mercury can penetrate at the minimum applied pressure, while the tapping density defines the density of the material including all interparticle spaces.

[0048] The relatively high bulk density of the granules of the present invention is correlated with their increased mechanical strength, which is highly beneficial for further handling or further processing, especially heat treatment, of such silica materials. When such heat treatment is carried out continuously, for example, in a rotary kiln or a similar device, the mechanical strength of the silica granules is increased, so that the formation of unnecessary fines and clogging in this process are suppressed.

[0049] Method for preparing granules The present invention further provides a method for preparing the granules of the present invention, comprising the following steps: a) Compressing fumed silica having a water content of 0.1% to 10% by weight to obtain compressed silica fragments having a tapping density of at least 200 g / L; b) Crushing the compressed silica fragments obtained in step a) while isolating the crushed fragments having a size of 2000 μm or less using a sieve having a maximum mesh size of 1000 μm to 2000 μm; c) Separating fine particles from the crushed fragments having a size of 2000 μm or less obtained in step b) using a sieve having a maximum mesh size of 200 μm to 600 μm to obtain granules; d) Optionally, using the fine particles having a particle size of 600 μm or less separated in step c) in step a). The present invention provides a method comprising the above steps.

[0050] The method of the present invention preferably includes all of steps a) to d). In this case, silica fines having a particle size of 600 μm or less, more preferably 500 μm or less, and even more preferably 400 μm or less, separated in step c) can be mixed with fresh fumed silica and reused in step a) of the method. This reduces the amount of wasted silica material and increases the overall yield of the method.

[0051] Steps a) to d) of the method according to the invention are preferably carried out sequentially, i.e., step a) is followed by step b), then c), and finally step d).

[0052] The method of the present invention can be carried out batchwise, semi - continuously, or preferably continuously.

[0053] In step a) of the method of the present invention, fumed silica having a number - average particle size d of 600 μm or less, preferably 5 μm to 500 μm, more preferably 10 μm to 400 μm, more preferably 15 μm to 300 μm, more preferably 20 μm to 200 μm 50 can be used. The number - average particle size d of the fumed silica 50 can be determined by laser diffraction particle size analysis in accordance with ISO13320:2009.

[0054] In step a) of the method, fumed silica having a water content of 0.1 wt% to 10 wt%, preferably 0.3 wt% to 8.0 wt%, more preferably 0.5 wt% to 5.0 wt%, more preferably 1.0 wt% to 3.0 wt% is used as the starting material for producing silica granules. Surprisingly, it has been found that this selected water content of the fumed silica enables the production of silica granules with improved mechanical properties and at a higher production rate. To achieve the required water content of the fumed silica, any suitable technique or device may be used to add water before or during step a) of the method of the present invention. Water can be sprayed, for example, onto the fumed silica to achieve its uniform distribution.

[0055] The fumed silica used in step a) of the method of the present invention can have a tapping density of 15 g / L to 190 g / L, preferably 20 g / L to 150 g / L, more preferably 30 g / L to 100 g / L, more preferably 40 g / L to 80 g / L.

[0056] In step a), compress the fumed silica to obtain compressed silica fragments having a tapping density of at least 200 g / L, preferably 200 g / L to 1000 g / L, more preferably 230 g / L to 800 g / L, more preferably 250 g / L to 700 g / L, more preferably 280 g / L to 650 g / L, more preferably 300 g / L to 600 g / L, more preferably 320 g / L to 550 g / L. The tapping density of the fumed silica and the compressed silica fragments can be determined according to DIN ISO787-11:1995.

[0057] The relatively high tapping density of the compressed fragments obtained in step a) of the method of the present invention makes its mechanical strength higher, reduces the formation of fine powder, and ultimately improves the sintering performance of such silica granules in the continuous heat treatment of such silica granules.

[0058] The compression in step a) of the method is preferably understood to mean mechanical consolidation without the addition of any binder. The compression should ensure a uniform press of the fumed silica powder to obtain compressed silica fragments having a substantially equal tapping density.

[0059] Step a) of the method of the present invention can be realized by any suitable device, for example, by compressing the fumed silica in a roller compactor, whereby the compressed silica fragments are in the form of strip-shaped intermediates. The properties of such compressed silica fragments are affected by process parameters such as the selected process control mode, the compression force, the width of the gap between the two rollers, and the pressure holding time established by appropriate changes in the rotational speed of the press rollers.

[0060] The compression of the compressed silica fragments can be achieved by two rollers, and one or both of these rollers may have a ventilation function simultaneously.

[0061] Preferably, two compression rollers can be used, which may be smooth or profiled. The profile may be present on only one of the compression rollers or on both. The profile may consist of an axially parallel waveform or of any arrangement of recesses (dents) of any configuration. In a further embodiment of the invention, at least one of the rollers can be a vacuum roller compressor.

[0062] For the compression in step a) of the method according to the invention, a suitable method is, in particular, a method in which the fumed silica to be compressed is compressed by two compacting rollers. One of such rollers can be arranged to be rotationally driven. Alternatively, it is also possible that both compression rollers cannot be driven. The specific pressure applied between the two compression rollers can be 5 kN / cm to 50 kN / cm, preferably more than 12 kN / cm, more preferably more than 12 kN / cm and less than 30 kN / cm, and even more preferably more than 12 kN / cm and less than 18 kN / cm.

[0063] After the compression in step a) of the method, the obtained compressed silica fragments are ground in step b) while isolating the ground fragments having a particle size of 2000 μm or less, preferably 1500 μm or less, using a sieve having a maximum mesh size of 1000 μm to 2000 μm, preferably 1000 to 1500 μm. Step b) of this method can be carried out using any suitable device having a sieve and capable of grinding the silica fragments, for example, in a screen granulator.

[0064] For the grinding of the compressed silica fragments in step b) of the method, a device comprising two counter-rotating rollers or spiked rollers having a defined gap can be used.

[0065] In step c) of the method of the present invention, the crushed fragments having a particle size of less than 2000 μm isolated in step b) are further subjected to separation of fine particles having a particle size of 600 μm or less, preferably 550 μm or less, more preferably 500 μm or less, using a sieve having a mesh size of 200 μm to 600 μm, preferably 250 μm to 600 μm, more preferably 300 μm to 600 μm, more preferably 350 μm to 600 μm, more preferably 400 μm to 600 μm.

[0066] The relatively small difference between the mesh sizes applied in steps b) and c) of the method of the present invention narrows the particle size distribution of the resulting silica granules, which is particularly beneficial for enhancing the sintering performance of a continuous sintering process using such silica granules.

[0067] Step c) of the method of the present invention can be carried out by any suitable device having a sieve, for example, a shifter, a screen, or a classifier. The shifter used can be a cross-flow shifter, a counter current deflection sifter, etc. The classifier used can be a cyclone.

[0068] The particles having a particle size of 600 μm or less separated in step c) of the method of the present invention are optionally used in step a) of the method.

[0069] The method according to the invention may further comprise an optional purification step e). In step e) of the method, the granules obtained in step c) of the method are exposed to an atmosphere containing one or more reactive compounds suitable for the removal of hydroxyl groups and impurities at a temperature of 400 °C to 1100 °C, preferably 600 °C to 900 °C. These may preferably be chlorine (Cl2), hydrochloric acid (HCl), sulfur halides such as SCl2, S2Cl2, SCl4, and / or halides of sulfur oxides such as SOCl2, SO2Cl2, hydrogen, or mixtures thereof. More preferably, chlorine, hydrochloric acid, disulfur dichloride, or thionyl chloride can be used. Usually, the reactive compound is used as a gas mixture with air, oxygen, helium, nitrogen, argon, and / or carbon dioxide. The proportion of the reactive compound in such a gas mixture can be 0.5% to 20% by volume.

[0070] Use of the granules The granules according to the invention can be used for various different applications, for example as a catalyst support, as a support for liquid substances, in cosmetic applications, for thermal insulation materials, as a pharmaceutical excipient, as an abrasive, as a component of silicone rubber, etc.

[0071] The granules of the present invention are particularly suitable for producing heat-treated silica granules, especially in a continuous process.

[0072] Such heat treatment can be carried out in any suitable apparatus, for example, in a suitable rotary kiln. In order to produce heat-treated granules with particularly high purity, especially with a low metal content, potential contamination during the heat treatment step should be avoided. For this purpose, the material of the heat treatment device should be selected accordingly.

[0073] The heat treatment is preferably carried out at a temperature of at least 300 °C, more preferably at least 500 °C, more preferably at least 700 °C, more preferably at least 900 °C, more preferably at least 1000 °C, more preferably at least 1100 °C, more preferably at least 1200 °C.

[0074] In some cases, it may be beneficial to perform the heat treatment of the granules of the present invention in several steps, for example, 2, 3, or more steps, using different successively increasing temperatures individually adjusted in each heat treatment step. Surprisingly, it has been found that the overall throughput of such a heat treatment process can increase and no unnecessary blockage of the heat treatment device occurs. Preferably, the heat treatment of the granules of the present invention is carried out at a first heat treatment temperature and then at least at a second heat treatment temperature, and the second heat treatment temperature is at least 10 °C, preferably at least 20 °C, more preferably 30 °C, more preferably at least 40 °C, more preferably at least 50 °C higher than the first heat treatment temperature.

[0075] Particularly preferably, the granules according to the present invention can be used as a precursor for producing heat-treated high-density silica granules, for example, in the form of a catalyst carrier, in the form of a carrier for a liquid substance, as a component of a cosmetic formulation, as a component of a heat-insulating material composition, as a pharmaceutical excipient, as an abrasive, as a component of a silicone rubber, etc.

Examples

[0076] The particle size (d 10 , d 50 , d 90 ) of the particles was measured in the dry state using a laser diffraction analyzer Beckman Coulter LS.

[0077] Using an AutoPore V9600 device (Micomeritics), the bulk density, porosity, and cumulative pore volume of pores larger than 4 nm were determined by mercury intrusion porosimetry according to DIN ISO 15901-1. Only the pore volume of pores with a pore diameter of more than 4 nm, i.e., pores into which mercury can penetrate at the maximum applied pressure (417 MPa), was detected.

[0078] BET specific surface area [m 2 / g] was determined by nitrogen adsorption in accordance with the Brunauer - Emmett - Teller method according to DIN9277:2014.

[0079] Preparation of silica granules (Example 1) (According to the present invention) Fumed silica powder AEROSIL® 90 (BET = 90 m 2 / g, manufacturer: Evonik Resource Efficiency GmbH) is placed in a storage tank and treated with deionized water in a mixing unit (target value 1.5 wt% H2O). Then, in this unit, fines obtained from screening in one of the subsequent process steps are added and homogenized. From there, the material flows into a hopper where a stuffing screw rotates without supply, i.e., based only on its mass. The hopper is under negative pressure from the outside. Its walls are made of sintered metal covered with cloth. While the material is being aerated by vacuum, the fumed silica powder is conveyed to the roll by the stuffing screw. Between rolls having a corrugated profile (6 mm), the material is compacted at a specific pressure of more than 12 kN / cm and less than 18 kN / cm. The corrugated profile forms "rods" of compacted, i.e., compressed, fumed silica. Then, these rods are crushed in a screen granulator. The mesh size of the screen granulator is 1250 μm. The mesh size of the screen granulator limits the larger particle size. The smaller size is defined by subsequent screening.

[0080] In a screen with ultrasonic cleaning, the material crushed in the screen granulator is screened to separate those that do not meet the size. The mesh size is 500 μm. The fines are returned to the storage container by a vacuum cycle conveyor.

[0081] (Comparative Example 1) In the subsequent screening, it was carried out in the same manner as in Example 1, except that a sieve having a mesh size of 100 μm was used instead of the sieve having a mesh size of 500 μm used in Example 1.

[0082] (Comparative Example 2) Silica granules were prepared from an aqueous dispersion containing 20% by weight of AEROSIL® 90 by spray drying technology (atomization by nozzle, pressure of the dispersion: 8 bar) at an inlet temperature of 350 °C and an outlet temperature of the spray dryer of 100 °C. Drying was carried out in countercurrent mode. The product was post-treated in a fluidized bed to further increase the size of the agglomerates. The finished product was separated by a filter.

[0083] The silica granules of Example 1 and Comparative Examples 1 and 2 have the physicochemical properties summarized in Table 1.

[0084] Heat treatment of silica granules The silica granules of Example 1 and Comparative Examples 1 and 2 were continuously heat-treated in a rotary kiln with a diameter of about 140 mm and a length of 2 m under the same conditions (maximum temperature = 1350 °C). The feed rate of the silica granules was continuously increased in each case until the first signs of overload and residence became apparent. In this way, the maximum sintering performance [kg / h] was determined and compared for different granules (Table 1).

[0085] It was found that the silica granules prepared in Example 1 have a much higher maximum throughput rate than the silica from Comparative Examples 1 and 2 without any residence (Table 1).

[0086] All three types of granules have a similar BET surface area. As a result of their preparation, the granules of Example 1 have a higher average particle size, higher bulk density, lower porosity, and pore volume of pores larger than 4 nm than the other granules (Table 1).

[0087] The silica particles from Comparative Example 2 have a much higher fluidity (data not shown in Table 1) and a narrower particle size distribution than the particles from Example 1. Nevertheless, the silica particles from Example 1 achieve a higher maximum sintering performance, which cannot be explained purely by the particle morphology or size, but can be explained by a particularly favorable combination of such particles' relatively large average particle size, relatively low porosity, and high bulk density.

[0088]

Table 1

Claims

1. 20 m ２ / g to 500 m ２ / g of BET surface area, The number average particle size d of 350 μm to 2000 μm determined by the laser diffraction method ５０ , The span of the particle size distribution (d ９０ -d １０ ) / d ５０ , Fumed silica particles having a bulk density of more than 0.35 g / mL as determined by the mercury intrusion method, Pore volume of pores larger than 4 nm determined by mercury intrusion method is 1.5 cm ３ / g or less and having a bulk density of more than 0.35 g / mL.

2. Particle d １０ The particle according to claim 1, wherein d determined by the laser diffraction method is from 100 µm to 1000 µm.

3. The particles according to claim 1, wherein the proportion of particles having a particle size of 100 μm or less in the particles is less than 20% by weight.

4. The span (d ９０ - d １０ ) / d ５０ is from 0.9 to 2.0, the particle according to claim 1.

5. The particles according to claim 1, wherein the tapping density of the particles is 300 g / L to 600 g / L.

6. The particles according to claim 1, wherein the particles have a porosity of less than 77% as determined by the mercury intrusion method.

7. A method for preparing the particles according to claim 1, comprising the following steps: a) Compressing fumed silica having a water content of 0.1% by weight to 10% by weight to obtain compressed silica fragments having a tapping density of at least 200 g / L; b) Crushing the compressed silica fragments obtained in step a) while isolating crushed fragments having a size of 2000 μm or less using a sieve having a mesh size of 1000 μm to 2000 μm; c) Separating fine particles from the crushed fragments having a size of 2000 μm or less obtained in step b) using a sieve having a mesh size of 200 μm to 600 μm to obtain particles; d) Optionally, using the fine particles having a particle size of 600 μm or less separated in step c) in step a); and including the above steps.

8. The method according to claim 7, characterized in that it is carried out continuously.

9. The method according to claim 7, characterized in that in step a) of the method, fumed silica having a water content of 0.5% by weight to 5.0% by weight is used.

10. The method according to claim 7, characterized in that the mesh size of the sieve used in step b) of the method is 1000 μm to 1500 μm.

11. The method according to claim 7, characterized in that the mesh size of the sieve used in step c) of the method is 400 μm to 600 μm.

12. The method according to claim 7, further comprising step e) of exposing the particles obtained in step c) of the method to an atmosphere containing one or more reactive compounds selected from the group consisting of chlorine, hydrochloric acid, sulfur halides, halogenated sulfur oxides, hydrogen, or mixtures thereof at a temperature of 400 °C to 1100 °C.

13. The method according to claim 7, wherein step a) of the method is carried out by two compression rollers, and the specific pressure applied between the two compression rollers is more than 12 kN / cm.

14. Use of the granules according to claim 1 as a catalyst support, as a support for liquid substances, in cosmetic applications, for thermal insulation materials, as a pharmaceutical excipient, in the manufacture of heat-treated silica granules, as an abrasive, as a component of silicone rubber.

15. Use of the granules according to claim 14 in the manufacture of silica granules heat-treated at a temperature of at least 500 °C.

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