Highly heat-insulating lightweight silica brick and method for producing same

Abstract

The invention relates to a fired lightweight silica brick, having a bulk density of at most 0.65 g / cm3 and a microporous structure, the microporous structure comprising a framework of interconnected, non-nanoporous particles of a basic silica component and pores having an average pore size of at least 1 µm, wherein nanoporous silica particles having an average pore size in the range from 1 nm to 100 nm are embedded in the framework of interconnected, non-nanoporous particles of the basic silica component.

Description

[0001] HIGHLY THERMALLY INSULATING SILICON FIREFLASK AND ITS MANUFACTURING METHOD

[0002] Description

[0003] The present invention relates to a silica flint, a method for its production, and its use.

[0004] Lightweight refractory bricks are shaped refractory products with a total porosity greater than 45% and an application temperature of at least 800 °C (G. Routschka and H. Wuthnow, Practical Handbook of Refractory Materials, Appendix, ISBN 978-3-8027-3168-6). A corresponding classification is based on the determination of the permanent change in length after exposure to temperature and a maximum bulk density (ASTM C155-97 (2022), ISO 2245:2006 / DIN EN 1094-2: 1998).

[0005] Lightweight firebricks exhibit low thermal conductivity and therefore high thermal insulation properties, making them particularly suitable for insulation applications. When specifying thermal conductivity values, it is necessary to state the test method used, as the results of the two standardized test methods relevant for lightweight firebricks – the so-called calorimeter test (ASTM C182 (2013)) and the so-called hot wire test (cross-wire method EN ISO 8894-1:2010) – differ significantly. The hot wire test shows significantly higher values ​​(Hagemann, L;

[0006] Peters, E.: Thermal Conductivity - Comparison of Methods: ASTM Method - Hot Wire Method and its Variations. Interceram 2 (1982), pp. 131 - 135).

[0007] Lightweight firebricks consisting of more than 90% by weight of silica (silicon dioxide, SiO2) are called "silica lightweight firebricks." The bulk densities of conventional silica lightweight firebricks range from approximately 0.40 to 1.25 g / cm³. 3The denser, less porous bricks exhibit higher thermal conductivity. Silica firebricks are used, among other things, in the glass industry. For example, the vaults of soda-lime glass melting furnaces, which are constructed of dense, highly refractory silica bricks on the hot side and operate at temperatures of up to 1600 °C, are lined on the cold side, usually in multiple layers, with silica firebricks to reduce heat loss. The individual layers can consist of bricks with different densities (i.e., bulk densities) or overall porosities. Particular attention is paid to the top one or two layers of bricks on the cold side, where, for energy efficiency reasons, the highest possible thermal insulation (and thus the lowest possible thermal conductivity) is optimal within the prevailing temperature range of approximately 200–900 °C.

[0008] Dense silica bricks are subject to corrosive wear, particularly in the joint area, which often leads to the formation of corrosion-induced channels (so-called "rat holes" or "blower holes"). Consequently, the underlying silica firebricks can also melt due to a reaction with foreign matter introduced from the furnace chamber. Although they have higher thermal conductivity values ​​than, for example, fireclay firebricks of comparable density, conventional silica firebricks are regularly used because, firstly, the melt fluxes generated during the process corrode the dense silica bricks significantly less, and secondly, due to their similar chemical composition, they have a lower potential for the formation of glass defects in the final product when dripping into the glass melt.

[0009] Conventional silica light firebricks are predominantly machine-pressed, using lightweight, naturally occurring, and possibly thermally treated raw materials and / or burnout materials to achieve the desired high porosity. The bricks are fired at temperatures above 1200 °C (DE 102012 219236 B4). Another, less commonly used manufacturing method is the so-called foaming process.

[0010] German patent DE 33 26271 describes a process for the production of silica lightweight bricks based on fine-grained amorphous quartz glass and / or quartz (SiÜ2) as raw material. The process involves shaping a mixture of a slip and a separately produced foam, drying it, and firing it at temperatures of 1000 °C to 1100 °C or 1100 °C to 1500 °C. The desired porosity or density is achieved by appropriately selecting the ratio of foam to slip. During production, magnesium montmorillonite in paste form is added as a thickening agent for the slip, and magnesium oxide caustic soda dispersed in water is added as a hardening agent.

[0011] A further development of this process is disclosed in GB 2 213 814 B, according to which a proportion of inexpensive, ultrafine, amorphous SiO2 fly ash with a primary particle size < 0.3 pm is used, which is obtained, for example, as a byproduct in the production of ferrosilicon or in the reaction of zirconium silicate to zirconium dioxide. Optionally, firing at temperatures of at least 800 °C can be carried out. However, thermal conductivity values ​​of fired silica lightweight bricks produced according to this process are not specified in GB 2 213 814 B. The silica dusts mentioned in GB 2 213 814 B consist of non-porous, spherical particles.

[0012] From DE 10 2015 202 277 B3, a multi-stage foaming process for the production of ceramic foams is known, wherein a slurry premix consisting of at least one particulate solid (mean particle diameter d50 0.1 - 5.0 pm and / or surface area 2 - 20 m²) 2 / g) with the suitability for setting, is mechanically foamed with the addition of at least one hydraulic binder. The dried slurry, which has undergone a shaping process, is then fired at temperatures in the range of 1400 °C to 2000 °C. Silicon dioxide is used as a particulate solid in the slurry premix as a mixture with other particulate oxides. The ceramic, highly refractory foams according to the invention are characterized by the fact that the pores have a highly homogeneous monomodal pore size distribution in the range of 20–200 pm.

[0013] US Patent 9,506,244 B2 discloses a solid foam produced by foaming a particulate mineral feedstock with submicrometer pores. A single type of silica particle is used as the feedstock, and the foam is not fired.

[0014] The thermal conductivity of lightweight refractory bricks depends not only on their overall porosity but also on their pore size, which for both pressed and foamed conventional silica lightweight refractory bricks ranges from the double-digit pm to the single-digit mm range. Regardless of the manufacturing process, conventional silica lightweight refractory bricks with low bulk density or high overall porosity are known to exhibit a minimum in thermal conductivity, which decreases with increasing temperature at lower porosity (i.e., higher bulk density). This results from the fact that radiative heat transfer in the pores increases more sharply with temperature than conductive heat transfer (W. Schulle, Refractory Materials, Chapters 2.4.2 and 3.4.3.3, ISBN 3-342-00306-5). It follows that for a specific application temperature, orWhen insulating against a temperature gradient, conventional silica firebricks already have an optimal bulk density, and simply reducing the bulk density does not further improve their thermal insulation properties. However, to minimize energy losses in high-temperature processes, such as glass manufacturing, it is desirable to provide firebricks with further improved thermal insulation properties.

[0015] The object of the present invention was therefore to provide a silica-

[0016] to provide lightweight firebricks suitable for use in the glass industry, exhibiting lower thermal conductivity values ​​compared to conventional silica lightweight firebricks, particularly in the temperature range of 200 °C to 900 °C.

[0017] This problem is solved by claim 1, according to which a fired silica light firebrick with a bulk density of at most 0.65 g / cm³ 3is provided, wherein the fired silica light flint has a microporous structure, the microporous structure comprising a framework of interconnected, non-nanoporous particles of a silica base component, and pores with a mean pore size of at least 1 pm, wherein nanoporous silica particles with a mean pore size in the range of 1 nm to 100 nm are embedded in the framework of the interconnected, non-nanoporous particles of the silica base component.

[0018] The lightweight firebrick according to the present invention is a "silica-based lightweight firebrick." Silica-based lightweight firebricks have a total silica (SiO₂) content of at least 90 wt.%. For some applications, particularly in the glass industry, a particularly high chemical purity of the silica-based lightweight firebrick is desired. In preferred embodiments, the total silica content in the lightweight firebrick is therefore at least 93 wt.%, and particularly preferably at least 95 wt.%.

[0019] The lightweight firebrick according to the present invention is a fired silica lightweight firebrick. This means that the brick has been subjected to a heat treatment ("firing") at a temperature of at least 400 °C, whereby the lightweight firebrick according to the invention has sufficient strength for use as thermal insulation material in high-temperature processes, for example in the glass industry, such as in a glass melting furnace dome.

[0020] According to the invention, the fired silica flint has a bulk density of a maximum of 0.65 g / cm³. 3 For the present invention, the bulk density is determined according to DIN EN 1094-4:1995. A bulk density of maximum 0.65 g / cm³. 3 This results in the silica firebrick according to the invention having a high overall porosity, which is advantageous for reducing the thermal conductivity of the silica firebrick.

[0021] The fired silica light flint according to the invention has a microporous structure, wherein the microporous structure comprises the following:

[0022] - a framework of interconnected, non-nanoporous particles of a silica base component, and

[0023] - Pores with a mean pore size of at least 1 pm.

[0024] Preferably, the microporous structure consists of a framework of interconnected, non-nanoporous particles of a silica base component, and of pores with a mean pore size of at least 1 pm.

[0025] The term "silica base component" refers to a silica-based material with a silica content of preferably at least 90 wt.%, particularly preferably at least 93 wt.%. The framework of interconnected particles of the silica base component can be obtained during the production of the lightweight firebrick from raw material particles of the silica base component. During firing of the lightweight firebrick according to the invention, these raw material particles sinter, forming a framework of interconnected particles of the silica base component. The raw material particles of the silica base component can, for example, be selected from the group consisting of quartz, tridymite, cristobalite, amorphous SiO2, and mixtures thereof.

[0026] According to the present invention, the interconnected particles of the silica base component are not nanoporous. For the purposes of this invention, the term "non-nanoporous" means that no, or a negligible number, of pores with a pore size between 1 and 1000 nm are present. Because the particles of the silica base component are not nanoporous, it is ensured that the lightweight firebrick possesses sufficient strength despite the presence of nanoporous silica particles. This allows for mechanical post-processing of the fired silica lightweight firebrick, e.g., by drilling, cutting, grinding, or milling, without the brick fracturing.

[0027] Particularly high strengths were achieved when the particles of the silica base component had a specific surface area of ​​less than 50 m². 2 / g, measured using the BET method according to DIN ISO 9277:2014. The particles of the silica base component particularly preferably have a specific surface area of ​​less than 40 m². 2 / g, preferably below 30 m 2 / g, preferably below 20 m 2 / g, measured using the BET method according to DIN ISO 9277:2014.

[0028] According to the present invention, the pores of the microporous structure have a mean pore size of at least 1 pm. Preferably, the mean pore size is in the range of 1 pm to 1 mm. Particularly preferably, the mean pore size is in the range of 20 pm to 200 pm.

[0029] Within the scope of the present invention, the mean pore size of the microporous structure is determined by scanning electron microscopy (SEM). This pore size determination can be carried out at an excitation voltage of 10–15 kV, a sample current of 1 nA, and using a secondary electron detector. For example, the JEOL JSM-7900F scanning electron microscope can be used.

[0030] According to the present invention, nanoporous silica particles are embedded in the framework of the microporous structure. This means that the nanoporous silica particles are present within and connected to the framework of the interconnected, non-nanoporous particles of the silica base component. Nanoporous silica particles are understood to be particles with a silica content of preferably at least 90 wt.%, particularly preferably at least 93 wt.%, and with a pore size in the nanometer range (1 nm to 1000 nm).

[0031] The embedding of the nanoporous silica particles into the framework of the microporous structure can be achieved by adding corresponding particles of a nanoporous silica raw material during the production of the lightweight firebrick. During the firing of the lightweight firebrick according to the invention, these nanoporous silica raw material particles combine with the raw material particles of the silica base component, so that a framework of interconnected particles of the silica base component with embedded nanoporous silica particles is formed.

[0032] Preferably, the nanoporous silica particles are homogeneously embedded in the framework of the microporous structure, i.e., the nanoporous silica particles are preferably homogeneously distributed in the fired light flint.

[0033] According to the invention, the nanoporous silica particles have a mean pore size of 1 to 100 nm. The mean pore size of the nanoporous silica particles can be determined by scanning electron microscopy (SEM). This pore size determination can be performed at an excitation voltage of 10–15 kV, a sample current of 1 nA, and using a secondary electron detector. For example, the JEOL JSM-7900F scanning electron microscope can be used.

[0034] Surprisingly, it was found that the presence of nanoporous silica particles with a mean pore size of 1 to 100 nm results in a lightweight firebrick with significantly lower thermal conductivity and therefore improved thermal insulation properties compared to conventional silica lightweight firebricks. This effect could be attributed to the fact that at pore sizes below the mean free path of air molecules, which is < 100 nm under standard conditions, the thermal conductivity of the pore gas is significantly reduced. This significantly improves the thermal insulation properties compared to conventional lightweight firebricks, which exclusively contain pores with a mean pore diameter in the pm range.

[0035] Preferably, the mean pore size of the nanoporous silica particles is in the range of 1 nm to 80 nm, even more preferably in the range of 1 nm to 70 nm, even more preferably in the range of 1 nm to 60 nm and even more preferably in the range of 1 nm to 50 nm.

[0036] Particularly good thermal insulation properties were found when the nanoporous silica particles had a specific surface area of ​​at least 50 m². 2 / g, preferably at least 100 m 2 / g, particularly preferably at least 500 m 2 / g, measured using the BET method according to DIN ISO 9277:2014. It was found that the thermal conductivity decreases with increasing BET surface area of ​​the nanoporous silica particles. According to the present invention, the nanoporous silica particles can have a BET surface area of ​​up to 1800 m². 2 exhibit / g.

[0037] Preferably, the volume fraction of the nanoporous silica particles relative to the total volume of the silica particles present in the lightweight firebrick (sum of the volumes of the non-nanoporous particles of the silica base component and the nanoporous silica particles) is in the range of 10 to 65 vol.%, preferably in the range of 15 to 55 vol.%.

[0038] Preferably, the nanoporous silica particles comprise silica aerogel, silica xerogel, silica cryogel, and / or mixtures thereof. Particularly preferably, the nanoporous silica particles consist of silica aerogel, silica xerogel, silica cryogel, and / or mixtures thereof. The terms "silica aerogel" (SiCh aerogel), "silica xerogel" (SiCh xerogel), and "silica cryogel" (SiCh cryogel) refer here to a solid gel consisting of highly porous silica (SiCh) whose dispersed phase is gaseous. Aerogels, xerogels, and cryogels differ from one another in the drying method used in their production. Aerogels are produced by supercritical drying of the gel, xerogels by convective supercritical drying, and cryogels by freeze-drying.

[0039] It has been found that lightweight firebricks with particularly advantageous properties (i.e., the lowest possible thermal conductivity) are obtained when the nanoporous silica particles consist of a hydrophilic silica aerogel. The use of hydrophilic silica aerogel is particularly advantageous when the lightweight firebrick according to the invention is produced using a foaming process employing a polar dispersion medium. In this case, a particularly stable dispersion and a particularly stable foam are obtained.

[0040] Silica aerogels are typically hydrophilic. However, hydrophobic silica aerogel is also produced on an industrial scale. Hydrophilic or at least partially hydrophilic silica aerogel can be obtained from hydrophobic silica aerogel by thermal treatment in air at temperatures in the range of 100 to 1000 °C (see, for example, EP 0810 975 B1).

[0041] The SiO2 aerogels used in this invention have a specific surface area of ​​100 - 1600 m². 2 / g, preferably 500 - 950 g / m² 2 (measured using the BET method according to DIN ISO 9277:2014), and preferably consist of SiO2 chains whose primary particles are approximately 1–10 nm in size, with the distance between the individual chains typically being between 10–100 nm. The porosity of the SiO2 aerogels is preferably greater than 90%. The pores are cylindrical and preferably have a diameter of 2 nm to 50 nm. The melting point of SiO2 aerogels is approximately 1200 °C. In a further preferred embodiment, the nanoporous silica particles consist of silica gel. Silica gel belongs to the group of xerogels and is also known as silica gel or silicic acid gel. It has a large specific surface area (approximately 150–900 m²). 2 / g, measured using the BET method according to DIN ISO 9277:2014). Typically, the porosity is over 50% and the pore size varies between 1 and 10 nm.

[0042] In further embodiments, the nanoporous silica particles can comprise or consist of precipitated silica, wherein the precipitated silica can preferably be produced according to the “Stöber process” known to those skilled in the art (as described, for example, in: Microporous and Mesoporous Materials, Volume 258, 2018, pp. 205-2010: “Tailoring the size and microporosity of Stöber silica particles, Microporous and Mesoporous Materials”).

[0043] In further embodiments, the nanoporous silica particles can comprise or consist of rice hull ash. Suitable thermal or thermochemical treatment can produce rice hull ash particles with a high SiCh content (at least 90 wt%), high nanoporosity, and a large specific surface area (BET value > 50 m²). 2 / g) are obtained (as described, for example, in: KR 2013 0071451 A).

[0044] Preferably, the lightweight firebrick according to the invention has a cold compressive strength of at least 0.5 MPa, preferably at least 1 MPa, determined according to DIN EN ISO 8895:2006.

[0045] Preferably, the lightweight firebrick according to the invention has a thermal conductivity of a maximum of 0.25 W / (m K) at a temperature of 800 °C, wherein the thermal conductivity is determined according to EN ISO 8894-1 :2010 (cross method).

[0046] Another aspect of the invention relates to a method for producing a fired silica lighthouse brick, in particular a fired silica lighthouse brick according to the present invention, wherein the method comprises the following steps in the specified order: a) providing a slurry comprising particles of a first silica raw material, particles of a second silica raw material, and a liquid dispersion medium, wherein the particles of the first silica raw material and the particles of the second silica raw material are dispersed in the liquid dispersion medium, and wherein the first silica raw material is a non-nanoporous silica raw material, and wherein the second silica raw material is a nanoporous silica raw material with a mean pore size in the range of 1 nm to 100 nm; b) foaming the slurry; c) subjecting the foamed slurry to a shaping process to obtain a shaped foam;d) Drying the shaped foam; e) Firing the dried foam at a temperature of 400 to 1100 °C, preferably at a temperature of 600 to 1100 °C, particularly preferably at a temperature of 800 to 1050 °C, to obtain the fired silica light flint.;

[0047] Within the scope of the present invention, the term "slurry" refers to a mixture of solids in a liquid dispersion medium.

[0048] According to the present invention, the slurry comprises at least the following components:

[0049] - Particles of a first silica raw material,

[0050] - Particles of a second silica raw material, and

[0051] - a liquid dispersion medium.

[0052] The particles of the first silica raw material and the particles of the second silica raw material are dispersed in the liquid dispersion medium, wherein the particles of the first silica raw material and the particles of the second silica raw material are preferably dispersed homogeneously.

[0053] In the process according to the invention, firing is carried out at a temperature of 400 to 1100 °C. It has been found that this temperature range is essential to maintain the nanoporosity of the particles of the second silica raw material during firing. In particular, it has been shown that pore growth occurs at higher firing temperatures, leading to an undesirable increase in the thermal conductivity of the resulting lightweight firebrick. Firing is preferably carried out at a temperature of 600 to 1100 °C. Firing at a temperature of 800 to 1050 °C is particularly preferred.

[0054] In the process according to the invention, the first silica raw material is a non-nanoporous silica raw material. This allows a lightweight refractory brick with sufficient strength to be produced despite the presence of the second, nanoporous silica raw material. Preferably, the lightweight refractory brick resulting from the process according to the invention has a cold compressive strength of at least 0.5 MPa, particularly preferably at least 1 MPa (determined according to DIN EN ISO 8895:2006).

[0055] Preferably, the specific surface area of ​​the particles of the first silica raw material is less than 50 m². 2 / g, measured using the BET method according to DIN ISO 9277:2014. The resulting low inherent porosity of the first silica raw material allows for the production of a lightweight firebrick with particularly high strength. The particles of the first silica raw material preferably exhibit a specific surface area of ​​less than 40 m². 2 / g, preferably below 30 m 2 / g, preferably below 20 m2 / g, measured using the BET method according to DIN ISO 9277:2014.

[0056] Preferably, the first silica raw material is selected from the group consisting of quartz, tridymite, cristobalite, amorphous SiO₂, and mixtures thereof. The second silica raw material is a nanoporous silica raw material with a mean pore size of 1 to 100 nm. The mean pore size of the nanoporous silica raw material can be determined by scanning electron microscopy (SEM). The measurement can be performed at an excitation voltage of 10–15 kV, a sample current of 1 nA, and using a secondary electron detector. For example, the JEOL JSM-7900F scanning electron microscope can be used.

[0057] A medium pore size in this range is advantageous – as described above – for obtaining a lightweight firebrick with particularly low thermal conductivities. Preferably, the medium pore size of the second silica raw material is in the range of 1 nm to 80 nm, even more preferably in the range of 1 nm to 70 nm, even more preferably in the range of 1 nm to 60 nm, and even more preferably in the range of 1 nm to 50 nm.

[0058] Preferably, the particles of the second silica raw material have a specific surface area of ​​at least 50 m². 2 / g, measured using the BET method according to DIN ISO 9277:2014.

[0059] A BET surface area of ​​at least 50 m² 2The high inherent porosity of the particles of the second silica raw material is promoted, which results in the lightweight firebrick obtained with the inventive process – as described above – exhibiting particularly advantageous thermal insulation properties. The BET surface area of ​​the nanoporous silica particles is particularly preferably at least 100 m². 2 / g, particularly preferably at least 500 m 2 / g, measured using the BET method according to DIN ISO 9277:2014.

[0060] Preferably, the second silica raw material comprises silica aerogel, silica xerogel, silica cryogel, and / or mixtures thereof. Particularly preferably, the nanoporous silica raw material consists of silica aerogel, silica xerogel, silica cryogel, and / or mixtures thereof. These materials exhibit the advantages described above. It has been found that silica lightweight firebricks with particularly advantageous thermal insulation properties are obtained using the process according to the invention when the nanoporous silica raw material consists of a hydrophilic silica aerogel. The use of hydrophilic silica aerogel is advantageous when using a polar dispersion medium, such as water or an aqueous alcoholic mixture, because this results in a particularly stable dispersion and, after foaming of the slurry, a particularly stable foam.

[0061] In a further preferred embodiment, the nanoporous silica raw material consists of silica gel, which belongs to the group of xerogels. In further embodiments, the nanoporous silica raw material can comprise or consist of precipitated silica, wherein the precipitated silica can be produced in particular according to the "Stöber process" known to those skilled in the art (as described, for example, in: Microporous and Mesoporous Materials, Volume 258, 2018, pp. 205-2010: "Tailoring the size and microporosity of Stöber silica particles, Microporous and Mesoporous Materials").

[0062] In further embodiments, the nanoporous silica raw material can comprise or consist of rice hull ash. Suitable thermal or chemical-thermal treatment can produce rice hull ash particles with a high SiCh content (at least 90 wt%), high nanoporosity, and a large specific surface area (BET value > 50 m²). 2 / g) are obtained (as described, for example, in: KR 2013 0071451 A).

[0063] In the process according to the invention, the first and second silica raw materials are present in the form of particles in the slurry. Preferably, the first and / or the second silica raw material is present as granules whose particle size distribution has a d50 value of 0.3 to 25 pm, preferably of 1.0 to 15 pm, determined by a particle size analysis using laser diffraction according to ISO 13320-1:2020. Preferably, the first and second silica raw materials are present in this particle size distribution.

[0064] The “d50 value” is known to specify a particle size (also “grain size”) for a particle mixture, where 50% of the mixture has a particle size corresponding to the d50 value and below, and 50% of this particle mixture has a particle size above the d50 value.

[0065] A d50 value of 0.3 to 25 pm, preferably from 1.0 to 15 pm, for the first and / or second silica raw material is advantageous in order to obtain a stable foam with homogeneous pore distribution and advantageous pore sizes in the process according to the invention after foaming the slurry, in particular pores with a mean pore size of at least 1 pm, preferably with a mean pore size in the range of 1 pm to 1 mm, particularly preferably 20 pm to 200 pm.

[0066] In a further embodiment, the second silica raw material can be used, at least partially, in a grain size of up to 3 mm, determined by sieve analysis (according to DIN EN 1402-3). This coarser grain size makes it possible to improve the mechanical properties, in particular the strength, of the resulting lightweight firebrick.

[0067] Preferably, the total content of the first and second silica raw material (sum of the first and second silica raw material), based on the total mass of the slurry, is between 33 and 79 wt.%, particularly preferably between 35 and 75 wt.%.

[0068] A preferred embodiment provides that the first silica raw material is present in a volume of 90 to 35 vol.%, preferably 85 to 45 vol.%, and the second silica raw material in a volume of 10 to 65 vol.%, preferably 15 to 55 vol.%, based on the total volume of the first and second silica raw materials. This volume ratio results in a lightweight firebrick with particularly good thermal insulation properties.

[0069] In the process according to the invention, the liquid dispersion medium can be selected from the group consisting of water and aqueous alcoholic mixtures, in particular mixtures of water with ethanol, methanol and / or isopropanol. Water is particularly preferred as the dispersion medium. The content of the liquid dispersion medium, based on the total mass of the slurry, is preferably 20 to 50 wt.%.

[0070] Preferably, the slurry comprises at least one dispersing agent, preferably selected from the group consisting of polyacrylates, polycarboxylates, mixtures thereof, lignosulfonates, oxalates, citrates, and in particular ammonium polyacrylates. If a dispersing agent is included in the slurry, it is preferably present in a quantity of 0.1 to 5.0 wt.%, and particularly preferably in a quantity of 0.3 to 0.6 wt.%, based on the total mass of the slurry.

[0071] In the process according to the invention, the slurry is foamed. This produces a liquid foam. To support the foaming, at least one surfactant (a so-called foaming agent) based on surfactants and / or proteins can also be added. Preferably, this foaming agent is present at a concentration of 0.5 to 10 wt.%, more preferably at a concentration of 1.0 to 5.0 wt.%, based on the total mass of the slurry.

[0072] Typically, in foaming processes, at least one inorganic binder such as calcium oxide (CaO) is added before and / or during foaming to stabilize the foam.

[0073] In the present invention, it was surprisingly found that the particles of the nanoporous silica raw material, such as hydrophilic or at least partially hydrophilic silica aerogel, stabilize the foam structure both after the molding process and during demolding, subsequent drying, and firing of the lightweight firebrick. The addition of a CaO-containing binder for further stabilization of the foam structure is therefore not essential. As a result, the process according to the invention can produce a lightweight firebrick with particularly high purity (i.e., a particularly high silica content).

[0074] According to one embodiment of the method according to the invention, at least one inorganic binder may be added before and / or during foaming. This binder is preferably selected from the group consisting of CaO-containing hydraulic binders, for example, calcium aluminate cements, Portland cements, and / or CaO-containing binders, e.g., lime milk, gypsum, and / or silica brines. When such an inorganic binder is used, it is preferably added in a weight fraction of 1.0 to 12.5 wt.%, more preferably 1.0 to 10 wt.%, particularly preferably 1.5 to 8.0 wt.%, and most preferably 2.5 to 6.0 wt.%, based on the total mass of the slurry.

[0075] Preferably, at least one organic binder can also be added to the slurry, preferably selected from the group consisting of polyvinyl alcohols, polyethylene glycols, polyvinyl butyral, polyvinylpyrrolidones, acrylic copolymers, and mixtures thereof. Polyvinyl alcohols are particularly preferred. The at least one organic binder may be present in a quantity of 0.1 to 2.0 wt.%, preferably 0.2 to 0.8 wt.%, based on the total mass of the slurry.

[0076] Preferably, the density of the slurry is reduced to 1.3 to 1.7 g / cm³ before foaming. 3 set.

[0077] Preferably, the foaming of the slurry takes place according to the invention.

[0078] The process is mechanical, e.g., using a suitable device (for example, a "dissolver"). In particular, foaming is achieved by generating alternating turbulent and laminar flow in the slurry. Alternatively, foaming can also be achieved by mixing with a separately produced foam.

[0079] According to a preferred embodiment, the foamed slurry contains 33.0 to 79.0 wt.%, preferably 35.0 to 75.0 wt.%, of the first and second silica raw materials, 0.1 to 5.0 wt.%, preferably 0.3 to 0.6 wt.% of the at least one dispersing agent, 0 to 12.5 wt.%, preferably 1.5 to 8.0 wt.% of the at least one inorganic binder, 0.1 to 2.0 wt.%, preferably 0.2 to 0.8 wt.% of the at least one organic binder, and 20.0 to 50.0 wt.% of the at least one dispersion medium.

[0080] According to a preferred embodiment, the foaming of the slurry is carried out with a foaming factor of 150 to 200%, wherein the foaming factor corresponds to the ratio between the volume of the liquid foam after and the volume of the slurry produced before foaming.

[0081] In the process according to the invention, the foamed slurry is subjected to a shaping process. This shaping process can be carried out by pouring the foamed slurry into a mold. In this way, a shaped foam (molded part) is obtained. After the shaping step, it is preferred that the shaped foam is set according to the shaping process, solidifies, and is removed from the mold before drying. The density of the dried foam can be between 0.4 and 0.8 g / cm³. 3 be.

[0082] If an inorganic binder is added before or during foaming, setting can occur during the drying of the formed foam. Depending on the size of the molded part, setting can take place over a period of 1 to 40 hours. Drying preferably takes place at a relative humidity of 30 to 70% and / or at a temperature of 25 to 80 °C, preferably 30 to 60 °C.

[0083] The dried foam is fired at temperatures of 400 to 1100 °C, preferably 600 to 1100 °C, and particularly preferably 800 to 1050 °C. Firing preferably takes place under an oxidizing kiln atmosphere. After firing, the silica light firebrick according to the invention can be mechanically processed, e.g., by cutting, drilling, grinding, or milling, without the brick breaking.

[0084] The present invention also relates to possible uses of the fired silica light refractory brick according to the invention, as well as of the fired silica light refractory brick obtained by the inventive method. This is preferably used as a thermal insulation material, in particular for the thermal insulation of a glass melting furnace dome.

[0085] The fired silica lightweight brick according to the invention can be supplied in standard brick format, for example as a rectangular brick measuring 230 x 114 x 64 mm. When using unmortared lightweight bricks in standard brick format for thermal insulation of curved surfaces, wedge-shaped joints and cavities form in the butt joints, which act as thermal bridges. Therefore, the fired silica lightweight brick according to the invention can also be manufactured with dimensions that deviate from the standard brick format, in particular with larger and / or more complex dimensions, such as a geometry adapted to the contour of the arch to be insulated, in order to avoid thermal bridges and to enable the arch insulation to be installed more quickly. The fired silica lightweight brick according to the invention can be designed such that at least one surface of the silica lightweight brick is concave or convex.

[0086] The present invention is described in more detail below with reference to exemplary embodiments, without limiting the scope of protection. An aqueous slurry was provided which, based on the total mass of the slurry, contained 35 to 75 wt.% of the first and second silica raw materials.

[0087] The first silica raw material used was fused silica particles. The slurry contained 15 to 55 vol% hydrophilic, particulate silica aerogel (second silica raw material), based on the total volume of the first and second silica raw materials. The slurry was homogenized by stirring using at least one commercially available organic dispersant and optionally other organic additives, and then mechanically foamed to a pourable consistency using a commercially available foaming agent.

[0088] The bulk density of the silica light firebrick according to the invention was adjusted by the volume ratio of slip and ready-processed, pourable foam.

[0089] The shaping process was carried out by pouring the foamed slip into a mold. After shaping, the formed foam was dried. Drying took place at a relative humidity of 30 to 70% and a temperature of 25 to 80 °C.

[0090] The added organic components evaporated during the subsequent firing of the lightweight firebrick under an oxidizing atmosphere, the firing being carried out at a temperature of 400 to 1100 °C, preferably at 800 to 1050 °C, in order to maintain the desired nanoporosity of the second silica raw material as described above.

[0091] The following specific examples are given for illustration. Table 1 shows the thermal conductivities (TGI) of fired silica lightweight firebricks according to the invention (Examples III and IV), determined according to EN ISO 8894-1:2010 (cross-type test) at temperatures of 400 °C and 800 °C, in comparison to conventionally produced (hydraulically pressed) fired silica lightweight firebricks (Example I) and to fired silica lightweight firebricks produced like the bricks according to the invention, but without the addition of the second silica raw material (i.e., without the addition of particles of a nanoporous silica raw material) (Example II). The firing temperature for the fired silica lightweight firebricks listed in Examples I to V was 1000 °C in each case. The proportion of silica aerogel is given in Table 1 in vol% based on the total volume of the first and second silica raw materials.The bulk density was determined according to DIN EN 1094-4:1995 and the cold compressive strength (CDF) according to DIN EN ISO 8895:2006. The SiO2 content was determined according to DIN EN ISO 12677. The mechanically foamed silica lightweight firebricks with SiO2 aerogel additive (Examples III and IV) were each produced without the addition of a CaO-containing binder.

[0092] Table 1 As can be seen from the values ​​in Table 1, the fired silica lightweight firebricks according to the invention, containing SiCh aerogel (Examples III and IV), exhibit a significantly lower thermal conductivity compared to the conventional bricks (Example I), both at comparable and higher bulk densities. In the case of the mechanically foamed bricks (Examples II - IV), it is clearly evident that even a small addition of nanoporous hydrophilic SiO2 aerogel results in a significant improvement in thermal insulation performance and a reduction in thermal conductivity.

Claims

Claims 1. Containing fired silica flint: 1.1 a bulk density of maximum 0.65 g / cm³ 3 ; and 1.2 a microporous structure, comprising the microporous structure: 1.2.1 a framework of interconnected, non-nanoporous particles of a silica base component, and 1.2.2 Pores with a mean pore size of at least 1 pm; characterized in that 1.3 Nanoporous silica particles with a mean pore size in the range of 1 nm to 100 nm are embedded in the framework of the interconnected, non-nanoporous particles of the silica base component.

2. Fired silica light flint according to claim 1, wherein the non-nanoporous particles of the silica base component have a specific surface area of ​​less than 50 m². 2 / g, measured using the BET method according to DIN ISO 9277:2014.

3. Fired silica light flint according to claim 1 or 2, wherein the nanoporous silica particles have a specific surface area of ​​at least 50 m². 2 / g, preferably at least 100 m 2 / g, particularly preferably at least 500 m 2 / g, measured using the BET method according to DIN ISO 9277:2014.

4. Fired silica light flint according to one of the preceding claims, wherein the nanoporous silica particles are present in a volume fraction of 10 to 65 vol.%, based on the total volume of the non-nanoporous particles of the silica base component and the nanoporous silica particles.

5. Fired silica flint according to any of the preceding claims, wherein the nanoporous silica particles are selected from the group consisting of: silica aerogel, silica xerogel, silica cryogel, precipitated silica, rice husk ash and mixtures thereof.

6. Fired silica light refractory brick according to one of the preceding claims, wherein the light refractory brick has a thermal conductivity of at most 0.25 W / (m K) at a temperature of 800 °C, determined according to EN ISO 8894-1 :2010.

7. A process for producing a fired silica light firebrick, comprising the following steps: a) providing a slurry comprising particles of a first silica feedstock, particles of a second silica feedstock, and a liquid dispersion medium, wherein the particles of the first silica feedstock and the particles of the second silica feedstock are dispersed in the liquid dispersion medium, and wherein the first silica feedstock is a non-nanoporous silica feedstock, and wherein the second silica feedstock is a nanoporous silica feedstock with an average pore size in the range of 1 nm to 100 nm; b) foaming the slurry; c) subjecting the foamed slurry to a shaping process to obtain a shaped foam; d) drying the shaped foam; e) Firing the dried foam at a temperature of 400 to 1100 °C, preferably at a temperature of 800 to 1050 °C, to obtain the fired silica flint.

8. The method of claim 7, wherein the particles of the second silica raw material have a specific surface area of ​​at least 50 m² 2 / g, preferably at least 100 m 2 / g, particularly preferably at least 500 m 2 / g, measured using the BET method according to DIN ISO 9277:2014.

9. Method according to claim 7 or 8, wherein the first silica raw material is selected from the group consisting of quartz, tridymite, cristobalite, amorphous SiO2 and mixtures thereof.

10. Method according to any one of claims 7 to 9, wherein the second silica raw material is selected from the group consisting of silica aerogel, silica xerogel, silica cryogel, precipitated silica, rice hull ash and mixtures thereof.

11. Method according to any one of claims 7 to 10, wherein the particles of the first and / or the second silica raw material have a particle size distribution with a d50 value of 0.3 to 25 pm, determined by laser diffraction according to ISO 13320-1 :2020.

12. Method according to any one of claims 7 to 11, wherein the total content of first and second silica raw material is 33 to 79 wt.%, preferably 35 to 75 wt.%, based on the total mass of the slurry.

13. Method according to any one of claims 7 to 12, wherein the first silica raw material is present in 90 to 35 vol.%, preferably in 85 to 45 vol.%, and the second silica raw material is present in 10 to 65 vol.%, preferably in 15 to 55 vol.%, based on the total volume of the first and second silica raw material in the slurry.

14. Method according to any one of claims 7 to 13, wherein no inorganic binder, in particular no calcium oxide (CaO), is added before and / or during foaming.

15. Use of the fired silica light flint according to any one of claims 1 to 6 as a thermal insulation material, preferably in a glass melting furnace vault.

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