WO2025098830A1

WO2025098830A1 - Process for producing pyrogenic metal oxides and metalloid oxides

Info

Publication number: WO2025098830A1
Application number: PCT/EP2024/080559
Authority: WO
Inventors: Franz Schmidt; Alexander Lygin; Roland Schilling; Ronald Ihmig; Nuh Yilmaz
Original assignee: Evonik Operations GmbH
Current assignee: Evonik Operations GmbH
Priority date: 2023-11-08
Filing date: 2024-10-29
Publication date: 2025-05-15
Anticipated expiration: 2026-05-08

Abstract

Process for manufacturing pyrogenic metal oxide and/or metalloid oxide by means of a burner (200) with a burner head (100) comprising an outlet end (104), a central section (110) having at least one central channel (112), and a jacket channel (116) located concentrically around the central section, and wherein 5 the following process steps are included: providing at least one metal precursor or metalloid precursor, at least one fuel gas and oxygen through the central section (110); providing at least one fuel gas through the jacket channel (116); providing a flame (190) by burning the fuel gas at the outlet end (104), wherein the ratio R1 = VC / VJ is at most 6.0, wherein VC is the normalized gas exit velocity of all gases from the central section (110) and VJ is the normalized exit gas velocity of all gases from the jacket channel (116).

Description

Process for producing pyrogenic metal oxides and metalloid oxides

The invention is related to a process for manufacturing pyrogenic metal oxides and metalloid oxides by means of a burner.

Background

Pyrogenic processes, i.e. , flame hydrolysis or flame pyrolysis processes, are well-known methods for producing fumed (pyrogenic) metal and metalloid oxides such as SiC>2, AI2O3, TiC>2 etc. This process involves strongly exothermic burning of the hydrogen fuel to produce water (H2 + 0.502 -> H2O) and a simultaneous hydrolysis of a metal precursor, e.g., SiCk or AICI3 using thus obtained water (flame hydrolysis). In flame pyrolysis process, organic precursor is pyrolyzed in a flame obtained by burning a fuel such as hydrogen.

In general, products with relatively high BET surface areas, small particles and narrow particle size distribution are accessible by pyrogenic methods. Higher BET surface areas are usually achieved by using more air for cooling the flame. However disadvantageously, plant productivity is decreased in this case, as the same metal oxide amount is distributed in a higher gas volume.

In a typical pyrogenic process, a burner geometry like shown in Ind. Eng. Chem. Res. 2022, 61 , 21, 7235-7244 is used for producing metal oxides or metalloid oxides. In this process, metal or metalloid precursors, air and H2 fuel are pre-mixed and introduced via a central reaction channel (pipe) to the outlet end of the burner head. Simultaneously, a small amount of fuel gas (hydrogen) is led through another inlet into a jacket channel located around the central reaction channel and is ignited in the presence of secondary air surrounding the burner head. Thus, hydrogen fed through the jacket is burned at the reactor opening and maintains the flame in the pyrogenic process, while the major fuel portion, primary air and metal precursor are fed through the central reaction channel into this flame.

Producing metal oxides with a relatively high BET surface area typically requires reducing of the flame temperature. This is usually achieved by the following measures or combinations thereof: (a) using the minimal amount of hydrogen feedstock with respect of the reaction stoichiometry and/or (b) using additional air or water vapor for “diluting” and cooling the flame. A process with a minimal amount of hydrogen feedstock is e.g., described in US 2014/0030525 A1. This process for producing the silicon dioxide powder having a BET surface area of 400 to 600 m²/g comprises: igniting in a burner a gas mixture comprising an oxidizable and/or hydrolysable silicon compound, hydrogen and an oxygen-comprising first gas, and burning a resulting flame into a reaction chamber, introducing an oxygen-comprising a second gas into the reaction chamber, and separating the obtained solid material from a gaseous material, wherein in the burner a quotient formed from the supplied amount of hydrogen and a stoichiometrically required amount of hydrogen is from 0.70 to 1.30. According to EP 0015315 A1 , the use of the additional air in flame hydrolysis processes, and of the minimal additional amount of water vapor is described as beneficial with respect to the higher BET surface area and the particle size.

However, generally using more diluted streams, such as additional air or additional water vapor (steam), leads to inevitable capacity reduction of the production plant and higher production costs. Reducing the flame temperature and obtaining fumed oxides with higher BET surface areas without adding of diluting feedstock streams would be therefore beneficial.

The object of the present invention is to provide a method for producing metal and/or metalloid oxides with a relatively high BET surface area at a high throughput of the production plant, i.e., without diluting the flame with additional air, water vapor and other gases.

These requirements could be solved by a process for manufacturing pyrogenic metal oxide and/or metalloid oxide by means of a burner with a burner head comprising

- an outlet end,

- a central section having at least one central channel, and

- a jacket channel located concentrically around the central section, and wherein the following process steps are included:

- Providing through the at least the one central channel of the central section at least one metal precursor and/or metalloid precursor (jointly named “MP”), a first fuel gas, and an oxygen containing gas;

- Providing a second fuel gas through the jacket channel;

- Providing a flame by burning the first and the second fuel gas at the outlet end, wherein the ratio R1 = Vc / Vj is at most 6.0, and wherein Vc is the normalized gas exit velocity of all gases from the central section and Vj is the normalized exit gas velocity of all gases from the jacket channel .

The first fuel gas and second fuel gas are named jointly “fuel gas” or “fuel gases”. The first and the second fuel gas can be the same or different gases. The fuel gas may be a fuel gas mixture containing (additionally) another gas than fuel gas component(s), e.g. inert gases such as, but not limited to, nitrogen (N2) and/or carbon dioxide (CO2). Another (additional) gas than fuel gas component(s) may be an oxygen containing gas. The oxygen containing gas can be pure oxygen (O2), air or air enriched with oxygen (O2).

The burner head as part of a burner is typically mounted and located at least partially inside a housing of the burner. The burner can further comprise a combustion chamber, also named flame chamber, into which the flame at the outlet end of the burner head extends. The housing is typically connected to an outlet (line) or product line. This product line, and/or the burner housing may comprise a flame tube. Preferrable the flame tube forms the outlet part of the flame chamber as part of the housing and/or the first section of the product line. According to an advantageous solution, the flame tube is connected to a (subsequent) cooling pipe, whereby the cooling pipe may be also a section of the product line and/or forms a cooling unit, which can be connected to the product line.

The typical gas velocities at the exit of the burner head are described in US 2007/025388 A1. It is described as beneficial, to have overall exit velocities of the gases leaving the burner of above 10 m/s or even above 25 m/s. The gases exiting burner head and forming the flame are further cooled down to produce the fumed metal oxides or metalloid oxides. However, there is little knowledge about the influence of the gas velocity at the burner head on the fumed oxide product. Even less is known from the prior art to the influence of the exit gas velocity from the jacket channel.

The term “outlet end” refers in the context of the present invention to the outer part of the burner head i.e., of the respective outlet ends of the corresponding gas channels of the burner head, where the gases exit the corresponding channels of the burner head, and the flame is formed.

The term “channel” refers in the context of the present invention to any space suitable for supplying gases to the outlet end, such as, but not limited to, lines, pipes, and annular gaps.

The term “jacket channel” refers in the context of the present invention to the channel surrounding the central section, i.e., located around the central section e.g. so that the jacket channel and the central section form a system of concentric channels and can alternatively be described e.g. as a pipe-in-pipe system with the central section located at the inner (central) position of this system. The jacket channel is preferably the outermost part of the system of concentrical channels of the burner head.

In one preferred embodiment, the central section comprises several concentric channels, forming a subsystem of concentrical channels.

The central section includes at least one central channel for supplying MP, as well as the first fuel gas and oxygen containing gas, and optionally other gases.

According to one preferred embodiment of the invention, the central section consists of at least one channel or at least two concentric channels having round or non-round shapes, such as, but not limited to, oval, squarish, rectangular, and/or polygonal shapes. In another advantageous embodiment, the jacket channel and/or at least one concentric channel of the central section is formed by a group of smaller pipes, jointly as a group surrounding at least one another channel. The concentric channel formed by a group of smaller pipes can be the jacket channel and/or at least one of the channels of the central section. The small pipes forming a concentric cannel can have round or non-round shapes, such a but not limited to oval, squarish, rectangular, and/or polygonal shapes.

Preferably, the outlet end of each channel and/or smaller pipe of such channel is located in the same plane or close to one plane, advantageously up to +/- 10 mm close to one average (outlet) plane, more preferable up to +/- 5 mm.

In another preferred embodiment of the invention, the central section consists of a pipe bundle, i.e., a group of parallel pipes, located close to each other, each having a round, oval, squarish, rectangular, and/or polygonal shape. The outlet end of each such a pipe is preferably located in the same plane or close to one plane, advantageously up to +/- 10 mm close to one average (outlet) plane, more preferable up to +/- 5 mm.

The jacket channel is used for supplying at least one (second) fuel gas and optionally other gases, which can help maintaining the flame constantly burning and ensuring smooth operation throughout the whole production process.

According to a preferred embodiment of the invention, a single jacket channel in the form of a pipe having a round, oval, squarish, rectangular, and/or polygonal shape is present in the burner head. This (outer) jacket channel can also be described as an annular channel or grove having at the outlet end a circular orifice. In case of a round version, the circular orifice has the shape of a ring.

In one preferred embodiment, the jacket channel consists of a pipe bundle, i.e., a group of (smaller) pipes. This group of (smaller) pipes can be located close to each other around the central section each having a round, oval, squarish, rectangular, and/or polygonal shape. The outlet end of each of such a pipe is preferably located in the same plane or close to one plane, advantageously up to +/- 10 mm close to one average (outlet) plane, more preferable up to +/- 5 mm.

The term “normalized gas velocity” refers in the context of the present invention to the corresponding value determined or calculated at standard conditions i.e., 0 °C and 1 atm pressure. An appropriate unit for this value is e.g., Nm³/h with the letter “N” referring to “normalized” value. Such “normalized” values are used for the sake of simplicity for comparing of the flows or feed rates under varying reaction condition.

The term “gas exit velocity” refers in the context of the present invention to the average gas velocity of the respective gas(es) from the corresponding section and/or at least one channel. Thus, velocity “Vc“ refers to the normalized average velocity of all gases exiting from the central section of the burner head in the process. Analogously, velocity “ VJ“ refers to the normalized average velocity of all gases exiting from the jacket channel of the burner head in the process.

Ratio R1 = Vc / Vj is calculated by dividing

Vc calculated as

- the total volume in m³ at standard conditions of all feed gases exiting the central section per hour (= Nm³/h) divided by the area in m² of the total cross-section of all channels of the central section at the outlet end of the burner head by Vj calculated as the total volume in m³ at standard conditions of all feed gases exiting the jacket channel per hour (= Nm³/h) divided by the area in m² of the cross-section of the jacket channel at the outlet end of the burner head.

The burner head as such is usually placed and mounted at least partially inside a housing as part of a burner. The burner and/or the burner head may comprise and/or be connected to additional elements, such as, but not limited to, feed lines, outlet lines, maintenance elements, control and sensor equipment, and mounting elements. In a preferred embodiment of the invention, an inlet and/or a channel for secondary oxygen (secondary channel) containing gas is provided outside the jacket channel, preferably for supplying atmospheric air to the process flame. In an advantageous embodiment, this secondary channel is located between the jacket channel and the burner housing and/or formed at least for a partially length by the outer surface of the jacket channel and the inner surface of the housing of the burner. This secondary channel preferably surrounds concentrically the jacket channel.

Very surprisingly, it has now been found that adjusting the exiting gas velocity from the jacket channel of the burner head in relation to the exiting gas velocity from the central section of the burner head leads to significant changes in the obtained pyrogenic oxides. Specifically, it turned out that reducing the ratio of exit gas velocity from the central section to the exit gas velocity from the jacket channel leads to higher BET surface areas, without any decrease in plant productivity. Additionally, there is a positive impact on the particle size of the obtained metal oxide and metalloid oxide particles.

The term “oxygen-containing gas” means O2, air and/or O2 enriched air. The term “MP” means in the context of this invention the respective substance, i.e. the metal precursor and/or the metalloid precursor, and/or a mixture comprising the metal precursor and/or the metalloid precursor, present at least at the outlet end of the burner head in a non-solid, preferably gaseous form, such as gas, spray, steam, aerosol, or the like.

In one preferred embodiment of the process, the central channel of the central section is used for providing MP, the fuel gas and oxygen as the sole channel present in the central section. In this embodiment, the central channel of the central section may be connected upstream with a mixing chamber or any other mixing unit for the mixing the MP, the fuel gas, oxygen, and optionally other gases prior to supplying these into central section of the burner head.

In an alternative preferred embodiment of the invention, the central section has at least two central channels for separate supplying the MP, the fuel gas and /or oxygen.

According to a further preferred embodiment of the process, metal oxide or metalloid oxide is selected from the oxides of aluminum (Al), titanium (Ti), zirconium (Zr), yttrium (Y), lithium (Li), magnesium (Mg), lanthanum (La), cerium (Ce), iron (Fe), zinc (Zn), silicon (Si), and the mixtures thereof, preferably aluminum oxide (AI2O3), titanium dioxide (TiO2) and/or silicon dioxide (SiC>2), and the mixtures thereof, most preferably silicon dioxide (SiC>2).

According to a further preferred embodiment of the process, MP is selected from at least one of the following substances: aluminum chloride (AlCb), aluminum oxychloride (AIOCI), titanium tetrachloride (TiCk), titanium trichloride (TiCh), titanium oxychloride (TiOCh), tetraalkoxytitanate such as tetraethoxytitanate Ti(OC2Hs)4, tetraalkoxysilicate such as tetraethoxysilicate Si(OC2Hs)4, such as octamethylcyclotetrasiloxane, cyclic siloxane (R3Si-[O-SiR2]n-O-SiR3), acyclic siloxane (R3Si-[O-SiR2]n-O- SiRs) such as silicon oils, silicon tetrachloride (SiCk), trichlorosilane (HSiCIs), methyltrichlorosilane (CHsSiCb), dichlorosilane (H2SiCl2) and/or monochlorosilane (FbSiCI).

According to a further preferred embodiment of the process, the first and second fuel gas is selected from the following group of substances: hydrogen (H2), a hydrogen-containing gas mixture, carbon monoxide (CO), an organic compound having at least one hydrogen atom, such as hydrocarbons or alcohols, and the mixtures thereof. The hydrocarbon may be an alkane with a general formula C_nH2n+2, wherein “n” is preferably from 1 to 6, and most preferably “n” is a number from 1 to 4. The alcohol may be a primary alcohol, secondary alcohol and/or a tertiary alcohol. Most preferably, the fuel gas is hydrogen or a hydrogen-containing gas mixture.

In another advantageous embodiment, the second fuel gas provided through the jacket channel is selected from the group consisting of hydrogen (H2) or a hydrogen-containing gas mixture. Preferably the only substance acting as a burnable component (fuel gas) provided through the jacket channel is hydrogen. In one embodiment of the process, the second fuel gas provided through the jacket channel is a hydrogen-containing gas mixture containing nitrogen (N2), carbon dioxide (CO2) and/or another inert gas.

In an alternative embodiment of the process, the second fuel gas provided through the jacket channel can be different from the first fuel gas provided through the central section. Thus, the second fuel gas provided through the jacket channel can be hydrogen (H2) and the first fuel gas in at least one channel of the central section can be

- a hydrogen-containing gas mixture,

- carbon monoxide (CO), and/or

- an organic compound having at least one hydrogen atom. The suitable examples of such organic compounds are given above.

Most preferably, both the first and the second fuel gas is hydrogen or a hydrogen-containing gas mixture.

According to another advantageous embodiment of the process, the central section comprises the sole channel for supplying the MP, the fuel gas, and the oxygen containing gas.

The oxygen (O2) used in the inventive process can be supplied to the process in the form of air or air enriched with oxygen (O2) and is jointly named herein as oxygen enriched gas. The oxygen provided to the flame through the central section is typically named "primary” oxygen. By contrast, the oxygen provided to the flame through other routes, e.g. introduced through additional channels, feed lines or orifices in the combustion chamber, e.g. pressed or sucked into the flame, is typically called “secondary” oxygen. In a most typical case, the oxygen (in form of atmospheric air) is sucked from outside of the burner through a channel located between the jacket channel and the housing of the combustion chamber into the flame.

The normalized exit gas velocity (Vc) from the central section is preferably at least 15 m/s. In a more preferred version of this embodiment, the normalized exit gas velocity (Vc) from the central section is at least 20 m/s, more preferably at least 25 m/s, and most preferably at least 29 m/s. Vc of more than 100 m/s may be difficult to achieve and impractical in the real production plants.

In the inventive process, it can be advantageous to adjust the normalized exit gas velocity ( Vj) from the jacket channel to at least 4 m/s. In a preferred version of this embodiment, the normalized exit gas velocity (Vj) from the jacket channel is at least 6 m/s, more preferably at least 8 m/s, and most preferably at least 10 m/s. Vj of more than 100 m/s may be difficult to achieve and impractical in the real production plants.

In a preferred embodiment of the invention, the ratio R1 = Vc/Vj is in the range from 0.5 to 6.0, more preferably the ratio R1 is in the range of 1.0 to 5.0, more preferably R1 is in the range from 1.2 to 4.5, more preferably R1 is in the range from 1.5 to 4.0. These narrow ranges of R1 allow producing pyrogenic metal oxides and/or metalloid oxides with particularly high BET surface areas and small particle size.

According to a further preferred embodiment of the process, the produced pyrogenic metal oxide or metalloid oxide has a BET surface area of 10 m²/g to 600 m²/g, preferably 50 to 400 m²/g.

According to a further preferred embodiment of the process, the produced pyrogenic metal oxide or metalloid oxide has a numerical average particle size dso of at most 200 nm, preferably 20-200 nm, more preferably 50-180 nm, more preferably 100-170 nm.

According to a further preferred embodiment of the process, the produced pyrogenic metal oxide or metalloid oxide has a span of the particle size distribution defined as (dgo-dio)/dso of at most 0.35, preferably 0.25 - 0.35. The values of dio, dso and doo can be determined by static light scattering (SLS) method.

According to a further preferred embodiment of the process, the ratio (R2) of the total normalized feed rate of all gases introduced into the burner head in Nm³/h to the feed rate of metal precursor in Nm³/h is at least 5, more preferably between 5 and 20. Therein, the normalized feed rate of all gases is defined as volume flow at standard conditions (0°C, 1 atm) of all gases, such as, but not limited to, gaseous metal and/or metalloid precursor, O2, H2, CO, C_nH2n+2 and/or N2, fed into the burner head in m³ per hour.

In the inventive process, the fuel gas provided through the jacket channel is preferably hydrogen or a hydrogen containing gas. Additionally, it can be advantageous to also provide nitrogen through the jacket channel. In this embodiment it may be advantageous if the ratio (R3) of the normalized feed rate of the nitrogen (N2) to the jacket channel in Nm³/h to the normalized feed rate of the hydrogen (H2) to the jacket channel in Nm³/h is in the range of 0.10 to 1.0, preferably 0.15 to 0.6, more preferably 0.2 to 0.5. Surprisingly it was observed, that adding inert nitrogen, particularly in the above-specified ratio, to hydrogen fuel gas exiting the jacket channel leads to a smoother continuous operation of the production plant, more homogeneous flame formation and increased flame stability over long operation time.

In a preferred embodiment of the inventive process, the ratio (R4) of the normalized feed rate of the fuel gas through the central section in Nm³/h to the total normalized feed rate of the fuel gas through the jacket channel in Nm³/h is between 2 and 20, more preferably between 3 and 16, more preferably between 4 and 10. Thus, providing a relatively small portion of the fuel gas through the jacket channel, and the remaining part through the central section turned out to be the most efficient process mode ensuring both economical and smooth operation.

The pyrogenic metal oxides or metalloid oxides obtained according to one of the embodiments or versions of the process mentioned herein, can be used as constituent of paints or coatings, silicones, pharmaceutical or cosmetic preparations, adhesives or sealants, toner compositions, for modifying rheology properties of liquid systems, as anti-settling agent, for improving flowability of powders, for improving mechanical or optical properties of silicone compositions, as constituents of lithium-ion batteries.

The process of the present invention and suitable equipment (e.g. burners, burner head) shall be illustrated in more detail by means of the following figures. These figures only represent some exemplary suitable forms of e.g., the used burners, and should not be understood as in any way limiting the scope of the invention itself. Thus, it is shown as follows:

Fig. 1 : A first burner and burner head as two sectional views (I, II), the one (I) parallel to and including the longitudinal axis and another (II) perpendicular to the longitudinal axis;

Fig. 2: A second burner and burner head as two sectional views (I, II), the one (I) parallel to and including the longitudinal axis and another (II) perpendicular to the longitudinal axis;

Fig. 3: A third burner head as sectional view parallel to and including the longitudinal axis;

Fig. 4: Sectional views of three other types of burner heads (I, II, III) perpendicular to the longitudinal axis; and

Fig. 5: A diagram showing the BET surface areas and the average particle size of the fumed silica samples obtained adjusting various R1 ratios. The burner 200 is shown schematically in figure 1 in partial view I, having a burner head 100 with a longitudinal axis A, several feed lines 150, 152, 154, a housing 160 and one product line 170. The reference number 190 indicates the flame, shown in a very schematical way as dotted line, having a flame central axis parallel or identical with the longitudinal axis A. The general flow direction of the burner head 100 as shown in figure 1 (I) is indicated by the arrow B. The burner head 100 comprises an inlet end 102, an outlet end 104 and several (gas) channels 112, 116 each having outlet opening 140, 144 at the outlet end 104. Any additional equipment needed to run the burner and the described process, such as, but not limited to, further processing units, pumps, compressors, tanks, sensors, actors, igniting equipment, electrical supply etc. is not shown in the figures. In flow direction B and connected with the product line 170, there are further process units, such as, but not limited to, a cooling unit, deacidification unit and a separation unit for the metal oxide and/or metalloid oxide produced by the inventive process, which are not shown in the figures either.

According to the example of figure 1 shown in partial view I., the burner 200 comprises in radial direction a main section 110 having one single central channel 112 surrounded concentrically by a jacket channel 116. The jacket channel 116 is in radial direction the ultimate channel of burner head 100. The plane 148 aligned perpendicular to the longitudinal axis A defines in flow direction B the origin of the flame in the combustion chamber 162. In the shown embodiment of figure 1 , the outlet openings 140, 144 of the channels 112, 116 of the burner head 100 are located in this plane 148. The pyrolysis takes place inside the combustion chamber 162 located inside a housing 160 of the burner 200. The burner head 100 is mounted at least partially inside the burner 200 and/or the housing 160, whereby the mounting devices are not shown in detail. Furthermore, a flame tube may be part of the product line 170 e.g., as a first section and/or as a connecting part to the flame chamber 162 of housing 160 (not shown). Alternatively, a flame tube may be an integrated part of the flame chamber 162 and/or the housing 160 (not shown). Such a flame tube may have in axial direction a conical shape.

The “flame axis” is defined by the main or central axis of the gases and or the pyrolysis flame released form the burner head, the terms “flame axis” and “central axis” are herein used synonymously. Even though one single flame axis is mentioned, it is understood that a dynamic gas release or flame do define a corridor in which multiple axis are located rather than one single axis in a mathematical meaning. Thus, the central axis means the place and orientation of the axis with a highest statistical likeliness. Moreover, the axis is alternatively or additionally defined by the symmetry of the burner head and/or the outlet openings of the flame forming gases. Analogously, “central” shall mean radially close or overlapping to the central axis and/or close or overlapping to the flame axis. The terms “combustion chamber" and “flame chamber” are used herein synonymously.

The term “plane” means a surface area, projection plane, and/or direct surface in a technical, not in a mathematical meaning. It is used for explanation and illustration reasons. Thus, a “plane” may have a limited height or depth, formed as e.g. a narrow corridor. The burner 200 comprises a secondary channel 118, arranged radially outside jacket channel 116, as concentrical gas channel surrounding the channels 112, 116. Channel 118 is open to the atmosphere via the flange 158 and secondary air, as secondary oxygen containing gas, is sucked into the (secondary) channel 118 and subsequently into the combustion chamber 162 during the process. The secondary channel 118 can be defined as a channel of the burner 200 or as the transition spacing between burner head 100 and the housing 160 of the burner 200. According to an alternative embodiment (not shown), the secondary channel 118 for the providing of secondary air, may be a part of the burner head 100 as a concentrically channel of the burner head 100 surrounding the jacket channel 116.

The partial view II of figure 1 shows the concentrical orientation of the channels 112, 116 of the burner head 100, as well as the secondary channel 118 and the housing 160 of the burner 200. The D-D line indication shows the orientation of the cross section, being identically, analogously and/or parallel oriented to the plane 148 (partial view I).

During the process, the mixture of feedstock MP and primary air is provided by feed line 150 to the first, central channel 112 aligned equal to the longitudinal axis A. Additionally (first) fuel gas is lead to the central channel 112 by feed line 152. Secondary oxygen containing gas, such as atmospheric air, is sucked into the secondary channel 118 and released into the flame chamber 162 as soon as the flame is ignited.

The second fuel gas is lead through jacket channel 116 and released into the combustion chamber 162. The secondary fuel gas can be hydrogen or a fuel gas mixture, comprising hydrogen (H2) and an additional gas. The additional gas can be e.g. nitrogen (N2), a ^-containing gas. The additional gas has a diluting effect and can be also provided by the third feed line 154 to the jacket channel 116. The gas exiting the outlet opening 144 forms a jacket flame when ignited.

The burner 200 is shown schematically in figure 2 in partial view I, having a burner head 100 with a longitudinal axis A, several feed lines 150, 152, 154, a housing 160 and one product line 170. The burner head 100 according to figure 2 comprises three channels 112, 114, 116 each having outlet opening 140, 142, 144 at the outlet end 104. The burner 200 is usually connected to and controllable by a control unit 120, wherein the data connection is indicated by dotted data line 122. The product line 170 is the outlet line and leads to further process units (not shown).

According to the example of figure 2 shown in partial view I. and II., the burner 200 comprises in radial direction a central section 110 having two central channels 112, 114 surrounded concentrically by a jacket channel 116. The first central channel 112 is concentrically surrounded by the second central channel 114. The jacket channel 116 is in radial direction the ultimate channel of burner head 100. In longitudinal direction the burner 200 comprises three main sections 130, 132, 134 The main sections in axial direction are the following: the infeed section 130, the lead section 132, and the release section 134. The plane 148 aligned perpendicular to the longitudinal axis A defines in flow direction B the end of the lead section 132 and the beginning of the release section 134 and/or the flame chamber 162. In the shown embodiment of figure 2, the outlet openings 140, 142, 144 of the channels 112, 114, 116 of the burner head 100 are located in this plane 148. The release section 134 comprises the combustion chamber 162, also named “flame chamber”, wherein the hydrolysis and/or pyrolysis of the MP takes place. The infeed section 130 is defined as the section comprises at least partially incoming feed lines 150, 152, 154 each connected to at least one of the inner channels 112, 114, 116 of the burner head 100.

Analogously to the embodiment shown in figure 1 , the burner 200 comprises a secondary channel 118, being located between the jacket channel 116 and the housing 160. The secondary channel 118 is radially outside the jacket channel 116. At least one feed line 156, connected e.g., with at least one flange element 158, leads into the secondary channel 118. According to an alternative embodiment not shown in the figures, the burner can comprise no such secondary channel and secondary air or an (secondary) oxygen containing gas is provided directly into the flame chamber 162 by at least one feed line.

The partial view II of figure 2 shows the concentrical orientation of the channels 112, 114, 116, 18, and the housing 160 of the burner 200. The D-D line indication the orientation of the cross section, being identically, analogously, and/or parallel oriented to the plane 148.

The lead section 132 is in flow direction downstream to the infeed section 130, whereby the lead section 132 is defined by the completed infeed of any feedstock (educt) in at least one of the inner channels 112, 114, 116 of the burner head 100, and if applicable, air into the secondary channel 118 of the burner 200. In other words, in the lead section 132 no more (main) feed lines are connected and the final gas flow in flow direction B is formed inside the burner head 100. Channel 118 is open to the atmosphere via the flange 158 and secondary 02 is sucked in by means of an orifice and flange 158.

At the opposite (with respect to section 130) end of the lead section 132, the release section 134 is located. Terminal end of the lead section 132 is basically defined and limited in flow direction B by the outlet openings 140, 142, 144 of the channels 112, 114, 116 being located (approximately) within the plane 148. The plane 148 represents in flow direction B the inlet end of the inner combustion chamber 162 and the release section 134 in which the flame 190 extends.

During the process feedstock MP can be provided by feed line 150 to the first, central channel 112 aligned equal to the longitudinal axis A. In the process performed in burner 200 shown in figure 2, the feedstock MP may form a part of a mixture containing (primary) oxygen 02. Via feed line 152 the first fuel gas is lead to the second, inner channel 114, surrounding the first, central channel 112. Finally, secondary oxygen containing gas, such as atmospheric air, is sucked into the secondary channel 118 and released into the flame chamber 162 as soon as the flame is ignited. Secondary fuel gas lead through the jacket channel 116 into the combustion chamber 162 can be a secondary fuel gas mixture, comprising hydrogen (H2) and an additional gas, such as: nitrogen (N2) orN2-containing gas. The additional gas has a diluting effect and can be also provided by the third feed line 154 to the jacket channel 116. The gas exiting the outlet opening 1 4 forms a jacket flame when ignited.

In the embodiment shown in figure 3, the burner 200 is basically build analogously to the one shown in figures 1 and 2, so the missing components and reference numbers can be taken from e.g., figure 2. However, the burner head 100 provides the formation of the flame 190 by means of a conus section 136. The conus section 136 is part of the lead section 132. Additionally, the conus section 136 comprises an optional (short) ring section 138 at the outlet end (plane 148). The conus section 136 has an advantageous influence forming a stable and homogenous pyrolysis flame.

Different to figures 1 and 2, the burner head 100 according to figure 3 comprises beside the jacket channel 116 a central section 110 having three concentrical inner channels 112, 114.1 and 114.2. One central channel provides MP, the second provides oxygen containing gas and the third central channel provides the first fuel gas. Furthermore, a mixing chamber 180 is connected to the feed line 154 leading to the jacket channel 116, wherein hydrogen (H2) as secondary fuel gas and nitrogen are pre-mixed.

In figure 4 the concept of forming a concentrical channel by a group of smaller pipes 115, 117 is shown in three different embodiments, each in in one partial view l.-l 11. The term “smaller” refers to the diameter of these “pipes” compared to the channel formed by the respective group of pipes. The smaller pipes 115, 117 having a round sectional area. In the partial views I. -III. only some of the group of smaller pipes building one channel are drawn. The entirely channel is built and/or filled with the group of small pipes, as indicated by the respective dotted lines. However, the small pipes of one group of pipes may be in direct contact to each other or having a distance in circumferential direction to each other (not shown). Alternatively, small(er) pipes of one group may have non-round shapes (not shown).

In partial view I it is shown that the jacket channel 116 is built by a group of small pipes 117. In partial view II it is shown that the radial outer central channel 114 is built by a group of small pipes 115. Finally in partial view III. it is shown that both, the jacket channel 116 and one of the three central channels 112, 113, 114 can be built by a group of small pipes 115, whereby this concept of a group of smaller pipes 115 is shown for the radially outer central channel 114 and the jacket channel 116.

Figure 5 is discussed together with the experimental results, wherein a burner analogous to figure 1 was used. Additionally features and details regarding the process and apparatus are discussed together with the experiments and can be combined identically or analogously with the embodiments of the figure 2 to 4.

It is obvious for a person skilled in the art, that the burner 200 and/or the burner head 100 may have any orientation different to the orientation shown in the figures and any details provided herein in this regard needs to be understood analogously in connection with a different orientation of the burner or burner head. Moreover, the burner 200 and/orthe burner head 100 needn’t have the simplified elongate form as shown in the figures but any useful inner and outer curved and/or angulated geometry.

In general, any hint, advantage and detail provided regarding the burner and burner head, especially together with the discussion of figures 1 to 3, shall apply for the process identically or if necessary, analogously, and vice versa: the process embodiments and details can be applied in combination with the details described with the discussion of figures 1 to 3.

In the following, the effect of the invention is shown by a set of experiments, wherein a burner according to figure 1 was used during the experiments.

Experiments

Three test series (A-C) with varying feed rates (Table 1 ) and varying burner head geometries (Table 2) have been carried out:

In a burner 200 with a burner head 100 schematically shown in figure 1 , SiCk vapor pre-mixed with hydrogen, air and nitrogen (added additionally to N2 contained in the air) were introduced into a burner head through one single central channel, as previously described regarding figure 1. The central section 110 comprises only one single central channel 112 in communication with the feed lines 150 and 152. A mixture of hydrogen H2 and nitrogen N2 was supplied through jacket channel 116, located concentrically around the single central channel 112. As soon as ignited, the gas mixture exiting of the jacket channel 116 forms a jacket flame, denoted by the doted line 190. Specific amounts of all feed gases are shown in Table 1. The introduced feed gases were mixed and ignited at the outlet of the burner head 100 resulting in a flame spreading along the flame axis A into the combustion chamber 162 of the burner 200. The mixture of produced particle and off-gas was led out through line 170 and further cooled downstream to the burner 200. Subsequently, the formed fumed silica powder was separated from the gases and deacidified using water steam at 600 °C in a conventional deacidification unit. The properties of the obtained fumed silica samples are given in table 2 below. The figure does not show the subsequent treatment downstream of product line 170. T able 1 : Feed rates and ratios for test series A, B and C

STC* = silicon tetrachloride (SiCk)

All flow rates of the gases were measured by means of flow meters placed along the respective feed lines of the gases or gas mixtures. In all experiments and throughout the whole application text, the normalized feed rate is given in Nm³/h and corresponds to gas throughput (volume/time) at standard temperature (0°C; 273,15 K) and pressure (1 atm; 101,325 kPa).

In each test series A, B, C, tests were performed using burner heads with varying total diameters of the central section as given in table 2. The central section was built as one central channel, i.e., one central pipe, the outer diameter thereof is given in table 2. The outer diameter of the respective jacket channels given in Table 2, whereby the jacket channel is also built as one pipe surrounding the central channel. The thickness of the wall between the central channel and the jacket channel was ~ 0.5 mm.

The final normalized gas exit velocities at the outlet end of the burner head were calculated from the flow rates as measured by the flow meter at the source of the respective gas and/or the respective feed line in consideration of the respective cross section of the outlet openings of the corresponding channels or sections.

The test series were conducted at constant flow rates, while the cross sections of the outlet openings (in most cases, of the jacket channel) were varied. Thus, VJ could be decreased significantly by slight increasing the outer diameter (hence, the cross-section) of the jacket channel at the constant mass flow through the jacket channel.

Table 2: Burner geometries and calculation of R1 ratio

Table 3 shows dependence of varying R1 ratios on BET surface areas of the obtained silicas in test series A-C.

The results (Table 3, also shown as a graph in figure 5) exemplify that in all three series reducing the ratio R1 = VcA/j down to 5 - 7 and below leads to significant increase in BET surface areas of the produced fumed silicas. The lower the ratio R1 = Vc/Vj, (below 5-7) the higher was the BET. Importantly, these significant increases of BET surface area were achieved by simple adjusting of the two exit gas velocities and without any decrease in plant throughput. On the contrary, in conventional flame hydrolysis processes, BET surface area increase is typically achieved by diluting the flame gases with the air and thus reducing plant productivity.

In fact, certain increase in BET surface area of the obtained silicas was also observed in the region R1 > 10-15. However, such high R1 values mean very slow exit gas velocities from the jacket channel. An operation mode of a production plant with such slow exit gas velocities from the jacket channel would be a disadvantage and highly risky due to the possible recoil of the flame (back to the jacket channel). Thus, the ratio R1 below - 6 was identified as an advantageous limit, avoiding the disadvantages at low gas velocities as mentioned above.

The minimal value of R1 achieved in the presented lab scale experiments was 1.56, limited by a too narrow gap between the walls of the jacket channel and the central pipe which would be required to achieve faster exit gas velocities from the jacket channel and thus lower R1 values. However, lower R1 values such as R1 < 1.0 are achievable and very suitable for use in relatively large plants for producing fumed oxides. R1 values of < 0.5 are considered impractical even in the large-scale plants due to extremely high exit gas velocities from the jacket channel.

Table 3: BET of the obtained products

T able 4 shows particle size distribution of the samples after test series A.

Surprisingly it was found that adjusting the R1 ratio led to certain changes in particle size distribution of the obtained silicas (table 4, figure 5). Thus, particles with the smallest size (dio, dso, dgo) were obtained in the range Vc/Vj of 1.5-4.0, whereas the largest particles were obtained with R1 ~6.0 (table 4, figure 5).

Table 4: Particle size distribution of the samples after test series A:

These results of the tables 3 and table 4 are also partially shown in figure 5. The graph of figure 5 shows as x-axis the BET surface in m² per gram, as second x-axis (right) the particle size distribution dso of the samples after test series. Thus, adjusting the normalized exiting gas velocity from the jacket channel of the burner head in relation to the normalized exiting gas velocity from the central section of the burner head surprisingly leads to significant change in the properties of the obtained pyrogenic metal oxide and metalloid oxides, as shown on the example of flame hydrolysis of SiC k to produce fumed SiOg. Specifically, it could be demonstrated, that reducing the ratio R1 of exit gas velocity from the central section to the exit gas velocity from the jacket channel to under 6.0, preferably under 5.0 or even lower, leads to higher BET surface areas, without any decrease in plant productivity, while further reducing R1 to the range 1.5-4.0 additionally helps to produce fumed oxide particles with the smallest particle size.

Claims

Claims:

1. Process for manufacturing pyrogenic metal oxide and/or metalloid oxide by means of a burner (200) with a burner head (100) comprising

- an outlet end (104),

- a central section (110) having at least one central channel (112), and

- a jacket channel (116) located concentrically around the central section, and wherein the following process steps are included:

- Providing through the at least the one central channel (112) of the central section (110) at least one metal precursor and/or metalloid precursor (MP), a first fuel gas, and an oxygen containing gas;

- Providing a second fuel gas through the jacket channel (116);

- Providing a flame (190) by burning the first and the second fuel gas at the outlet end (104), characterized by the ratio R1 = Vc / Vj of at most 6.0, wherein Vc is the normalized gas exit velocity of all gases from the central section (110) and Vj is the normalized exit gas velocity of all gases from the jacket channel (116).

2. The process according to claim 1 , wherein the first and/or the second fuel gas is selected from the group consisting of hydrogen (H2), a hydrogen-containing gas mixture, carbon monoxide (CO), an organic compound having at least one hydrogen atom, such as hydrocarbons or alcohols, and the mixtures thereof.

3. The process according to claim 1 or 2, wherein the second fuel gas provided through the jacket channel (116) is selected from the group consisting of hydrogen (H2) and/or a hydrogen-containing gas mixture.

4. The process according to claim 3, wherein the second fuel gas provided through the jacket channel (116) is a hydrogen-containing gas mixture containing nitrogen (N2), carbon dioxide (CO2) and/or another inert gas.

5. The process according to any preceding claim, wherein the second fuel gas provided through the jacket channel (116) is different from the first fuel gas provided through the central section.

6. The process according to any preceding claim, wherein the central section (110) comprises the sole central channel (112) for supplying the MP, the first fuel gas and the oxygen containing gas.

7. The process according to any preceding claim, wherein the central section (110) comprises at least two central channels (112, 114) for separately supplying

- the metal precursor and/or metalloid precursor,

- the first fuel gas, and /or

- oxygen containing gas.

8. The process according to any preceding claim, wherein the metal oxide or the metalloid oxide is selected from the oxides of aluminum (Al), titanium (Ti), zirconium (Zr), yttrium (Y), lithium (Li), magnesium (Mg), lanthanum (La), cerium (Ce), iron (Fe), zinc (Zn), silicon (Si), and the mixtures thereof, preferably aluminum oxide ( AI2O3), titanium dioxide (TiC>2) and/or silicon dioxide (SiO2), and the mixtures thereof, most preferably silicon dioxide (SiO2).

9. The process according to any preceding claim, wherein the MP is selected from at least one of the following substances: aluminum chloride ( AICIs), aluminum oxychloride (AIOCI), titanium tetrachloride (TiCk), titanium trichloride (TiCh), titanium oxychloride (TiOCh), tetraalkoxytitanate such as tetraethoxytitanate Ti(OC2Hs)4, tetraalkoxysilicate such as tetraethoxysilicate Si(OC2Hs)4, cyclic siloxane ([O-SiR2]n) such as octamethylcyclotetrasiloxane (D4), acyclic siloxane (R3Si-[O-SiR2]n-O- SIRs) such as silicon oils, silicon tetrachloride (SiCk), trichlorosilane (HSICIs), methyltrichlorosilane (CHsSICb), dichlorosilane (l- SiCh) and/or monochlorosilane (HsSICI).

10. The process according to any preceding claim, wherein the normalized exit gas velocity (Vc) from the central section (110) is at least 15 m/s, more preferably at least 20 m/s, more preferably at least 25 m/s, more preferably at least 29 m/s.

11. The process according to any preceding claim, wherein the normalized exit gas velocity (Vj) from the jacket channel (116) is at least 4 m/s, more preferably at least 6 m/s, more preferably at least 8 m/s, more preferably at least 10 m/s.

12. The process according to any preceding claim, wherein the ratio (R1) of Vc/Vj is in the range from 0.5 to 6.0, more preferably from 1.0 to 5.0, more preferably from 1.2 to 4.5, more preferably from 1.5 to 4.0.

13. The process according to any preceding claim, wherein the ratio (R2) of the total normalized feed rate of all gases introduced into the burner head in Nm³/h to the normalized feed rate of the metal precursor in Nm³/h is at least 5, more preferably between 5 and 20.

14. The process according to any of the claims 4 to 13, wherein the ratio (R3) of the normalized feed rate of the nitrogen (N2) to the jacket channel (116) in Nm³/h to the normalized feed rate of the hydrogen (H2) to the jacket channel (116) in Nm³/h is in the range of 0.10 to 1.0, preferably 0.15 to 0.6, more

5 preferably 0.2 to 0.5.

15. The process according to any preceding claim, wherein the ratio (R4) of the normalized feed rate of the fuel gas through the central section (110) in Nm³/h to the total normalized feed rate of the fuel gas through the jacket channel (116) in Nm³/h is between 2 and 20, more preferably between 3 and 16,0 more preferably between 4 and 10.