Spray evaporation of a liquid raw material for preparation of silicon dioxide and metal oxides

Abstract

The present invention relates to a process for preparing a metal oxide, comprising a) spraying a liquid raw material comprising at least one metal compound by mixing it with a gas to form an aerosol; b) forming a gaseous reaction mixture from the aerosol obtained in step a) by complete evaporation thereof; c) converting the gaseous reaction mixture obtained in step b) to metal oxide in the presence of oxygen.

Claims

1. A process for preparing silicon dioxide and/or a metal oxide, comprising the following steps: a) forming an aerosol by spraying a liquid raw material comprising at least one silicon compound and/or a metal compound into a gas stream; b) forming a gaseous reaction mixture by complete evaporation of the aerosol obtained in step a); c) converting the gaseous reaction mixture that was formed by complete evaporation in step b) to silicon dioxide and/or a metal oxide by reacting the gaseous reaction mixture in a flame at an adiabatic flame temperature of more than 500 C. in the presence of oxygen; wherein the process takes place in a reactor comprising reaction zones A and B, and wherein: step b) takes place entirely in zone A; step c) takes place entirely in zone B.

2. The process of claim 1, wherein evaporation in step b) comprises heating the aerosol and/or reducing the partial pressure of the evaporated liquid in the gas stream.

3. The process of claim 2, wherein evaporation in step b) comprises preheating the gas to a temperature of 50 to 400 C.

4. The process of claim 2, wherein evaporation in step b) comprises preheating the gas to a temperature of 80 to 350 C.

5. The process of claim 4, and wherein the adiabatic flame temperature during step c) is 1000-2500 C.

6. The process of claim 1, wherein the aerosol formed in step a) comprises liquid droplets having a numerical average particle size of not more than 2 mm, wherein the aerosol is a biphasic liquid/gas mixture with the liquid droplets finely distributed in the gas.

7. The process of claim 1, wherein the silicon compound is used for preparation of silicon dioxide.

8. The process of claim 7, wherein the silicon compound is a non-halogenated compound selected from the group consisting of: tetraalkoxyorthosilicates; silanes; silicone oils; polysiloxanes and cyclic polysiloxanes; silazanes; and mixtures thereof.

9. The process of claim 7, wherein the silicon compound is a chlorinated compound selected from the group consisting of: silicon tetrachloride; dichlorosilane; trichlorosilane; methyltrichlorosilane; dimethyldichlorosilane; methyldichlorosilane; dibutyldichloro-silane; ethyltrichlorosilane; propyltrichlorosilane; and mixtures thereof.

10. The process of claim 9, wherein evaporation in step b) comprises preheating the gas to a temperature of 80 to 350 C.

11. The process of claim 10, and wherein the adiabatic flame temperature during step c) is 1000-2500 C.

12. The process of claim 11, wherein the aerosol formed in step a) comprises liquid droplets having a numerical average particle size of not more than 2 mm.

13. The process of claim 11, wherein the ratio of gas volume in standard cubic metres used in total in steps a) and b) to the amount of the liquid raw material used in kilograms is from 0.1 to 100 m.sup.3 (STP)/kg.

14. The process of claim 1, wherein a metal oxide is prepared comprising at least one of the elements Al, Ce, Fe, Mg, In, Ti, Sn, Y, Zn and/or Zr as the metal component.

15. The process of claim 14, wherein evaporation in step b) comprises preheating the gas to a temperature of 80 to 350 C.

16. The process of claim 15, and wherein the adiabatic flame temperature during step c) is 1000-2500 C.

17. The process of claim 16, wherein the aerosol formed in step a) comprises liquid droplets having a numerical average particle size of not more than 2 mm.

18. The process of claim 16, wherein the ratio of gas volume in standard cubic metres used in total in steps a) and b) to the amount of the liquid raw material used in kilograms is from 0.1 to 100 m.sup.3 (STP)/kg.

19. The process of claim 1, wherein the liquid raw material used in step a), prior to performance of step a), has a pressure of at least 1.5 bar and the gas mixture obtained in step b) has a pressure of not more than 1.2 bar.

20. The process of claim 19, wherein the aerosol formed in step a) comprises liquid droplets having a numerical average particle size of not more than 2 mm and the ratio of gas volume in standard cubic metres used in total in steps a) and b) to the amount of the liquid raw material used in kilograms is from 0.1 to 100 m.sup.3 (STP)/kg.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a schematic diagram of a reactor suitable for carrying out the process of the invention. The figure includes reference letters denoting reactor zones and components, and reference numerals identifying functional elements involved in the process.

DETAILED DESCRIPTION OF THE INVENTION

(2) The liquid raw material used in the process according to the invention forms an aerosol through spraying. The liquid raw material can be mixed with a gas simultaneously with the spraying, in step a), or directly after the spraying, in step b). The aerosol formed at first can be distributed further and diluted in a gas stream.

(3) The ratio of total gas volume used in steps a) and b) in standard cubic metres to the amount of the liquid raw material used in kg is preferably from 0.05 to 200, more preferably from 0.1 to 100, more preferably from 0.5 to 50, m.sup.3 (STP)/kg.

(4) The gas used in step a) and/or b) preferably comprises oxygen, preferably in the form of a mixture comprising oxygen and nitrogen, particular preference being given to using air as gas in step a).

(5) In step b) of the process according to the invention, a gaseous reaction mixture is formed from the aerosol obtained in step a).

(6) More preferably, relatively easily evaporable liquid silicon compounds and/or metal compounds which remain stable on evaporation, i.e. are not subject to thermal breakdown, are used in the process according to the invention. It is particularly advantageous when the silicon compounds and/or metal compounds used have a boiling point of less than 300 C. at 10 mbar, more preferably of less than 300 C. at 100 mbar, most preferably of less than 300 C. at 500 mbar. Unless explicitly stated otherwise, the values for absolute pressures are always reported in relation to the present invention.

(7) In order to ensure that no liquid droplets containing silicon compound and/or metal compound have to be converted to silicon dioxide or metal oxide in the reaction flame, complete evaporation and conversion of all liquid components of the aerosol obtained in step a) to the gas phase takes place in step b) of the process according to the invention.

(8) Such an evaporation of the liquid raw material after the aerosol has been formed can be achieved by different means. In principle, additional energy can be transferred to the aerosol by an appropriate heat source and/or the partial pressure of the evaporated liquid in the gas stream can be reduced after the aerosol has been formed. Accordingly, one possible implementation of steps a) and b) of the process according to the invention can be effected through the use of a preheated gas. The gas used in step a) and/or b), preferably air, can be preheated to a temperature of 50 to 400 C., more preferably of 80 to 350 C. The liquid raw material may also be preheated prior to performance of step a). The liquid raw material used can also be superheated, i.e. heated to a temperature higher than boiling point at standard pressure. Preferably, the liquid raw material, prior to performance of step a), is preheated to a temperature up to 500 C., more preferably 100 to 400 C., more preferably 150 to 350 C. The preheating and possible superheating of the raw material used reduces the amount of energy needed for evaporation thereof that has to be provided via the preheated gas.

(9) The energy needed for evaporation of the liquid raw material can be provided either via the preheating of the raw material used or via the preheated gas into which the raw material is sprayed. Thus, the conditions can be optimized to the respective raw material, and it is also possible to use critical raw materials, for example those having low breakdown temperatures.

(10) Since the adiabatic flame temperature during the performance of step c) of the process according to the invention usually reaches more than 500 C., preferably from 1000 to 2500 C., the above-described preheating of the reaction mixture promotes reduction of the temperature gradient in the flame and hence formation of silicon dioxide and/or metal oxide particles with homogeneous sizes.

(11) In a preferred embodiment of the invention, the liquid raw material used in step a), prior to performance of step a), has a pressure of at least 1 bar, more preferably at least 1.5 bar, most preferably at least 2 bar. The gas mixture obtained in step b) can preferably have a pressure of not more than 1.2 bar, preferably not more than 1.1 bar, more preferably not more than 1 bar.

(12) Preferably, in step b) of the process according to the invention, a higher temperature of the gaseous reaction mixture than that corresponding to the dew point of this mixture is attained. It is thus ensured that a completely gaseous reaction mixture is used in step c) of the process according to the invention. Preferably, the gaseous reaction mixture used in step c) has a temperature higher by at least 10 C., more preferably by at least 30 C., most preferably by at least 50 C., than the dew point temperature of this mixture.

(13) Steps a) and b) of the process according to the invention may take place in direct succession or at least partly simultaneously. Preferably, steps a) and b) take place at least partly simultaneously, meaning that the evaporation of the liquid takes place at least partly even as the liquid raw material is still being sprayed.

(14) The oxygen needed for the conversion of the silicon compound and/or metal compound to silicon dioxide and/or metal oxide can be supplied during at least one of steps a) to c). It is possible here to use oxygen, for example in pure form or as a mixture with other gases, especially air. When air is introduced during the performance of steps a) and/or b), it is still referred to as primary air. It may also be advantageous when secondary air is additionally used during step c). In general, the amount of secondary air will be such that the ratio of secondary air to primary air is from 0.1 to 10.

(15) It is particularly advantageous when oxygen is present in excess compared to combustible constituents of the reaction mixture. The index (lambda) is the ratio of the amount of oxygen present in the reaction mixture divided by the amount of oxygen needed for the complete combustion of all combustible constituents of the reaction mixture, each in mol/h. Preferably, is set at greater than 1.2; more preferably, of 1.4 to 5 is chosen.

(16) During at least one of steps a)-c) of the process according to the invention, a gaseous fuel can be used. Examples of such a fuel include hydrogen, methane, ethane, propane, butane and/or natural gas.

(17) The reactor for conversion of the liquid raw material comprising at least one silicon compound and/or metal compound to silicon dioxide and/or metal oxide by the process according to the invention may comprise at least two reactor zones A and B, each of which may be the parts of a common reaction chamber, may overlap with one another or may be spatially separated from one another. The functional role of the reaction chamber A is mainly to convert the liquid raw material used to a gaseous reaction mixture by means of at least one gas. In the reaction zone B, by contrast, a chemical conversion of the gas mixture formed beforehand, containing silicon compound and/or metal compound, to silicon dioxide and/or metal oxide takes place.

(18) Preferably, reactor zone A is above reactor zone B. More preferably, the silicon compound and/or metal compound is introduced in the upper part of reactor zone A.

(19) The reactor zone A may contain those elements which can improve the mixing of the liquid silicon compound and/or metal compound introduced into reactor zone A and the gas. For example, reactor zone A may contain various baffles or static mixers.

(20) The invention is elucidated in detail hereinafter with reference to FIG. 1, which shows a specific embodiment of the present invention. This greatly simplified drawing is intended to give a complete overview of the process steps according to the invention. There follows a detailed description (Table 1) of the fundamental reactor parts (A-G) and of the corresponding reactant and product streams (1) to (7).

(21) TABLE-US-00001 TABLE 1 Reference numbers/letters and explanations of FIG. 1 A Reactor zone A B Reactor zone B C Optional filter upstream and/or downstream of the raw material preheating (D) D Optional preheating of the liquid raw material E Fine distributor of the liquid raw material in the gas (nozzle) F Optional mixing elements (static mixers) G Mouth of burner (1) Liquid containing a silicon and/or metal compound for preheating D (2) Primary air to the distributor E (3) Gaseous silicon compound and/or metal compound to reactor zone A (4) Optional core fuel (for example hydrogen) to reactor zone A (5) Optional peripheral fuel (for example hydrogen) to reactor zone B (6) Secondary air to reactor zone B (7) Product mixture comprising silicon dioxide and/or metal oxide

(22) In the reactor shown in FIG. 1, the liquid raw material, a liquid comprising silicon compound and/or metal compound, is preheated if required in an apparatus intended for the purpose (D). Before and/or after the preheating, it is optionally possible for one or more filters (C) to be installed in order to free the raw material used of any solid particles present therein. In the particular embodiment shown in FIG. 1, the reactor zones A and B are positioned one on top of the other. In the upper part of the reactor zone A, the liquid raw material (1) is introduced via a fine distributor (E) and finely distributed. The primary air (2) likewise introduced in the upper part of the reactor zone A ensures that the finely distributed liquid raw material containing silicon compound and/or metal compound and gas are mixed with one another, and a gaseous reaction mixture or an aerosol which becomes fully gaseous further on in the reactor zone A is formed. Better mixing of the gaseous components in the reactor zone A is achieved with the mixing elements (F) installed therein, for example static mixers. An identical gaseous silicon and/or metal compound (3) to that present in (1) or another can additionally be fed to the reactor zone A. If a silicon and/or metal compound other than that used in (1) is used here, the corresponding mixed oxides can be prepared as product. A fuel gas, for example hydrogen, can be supplied both to the reactor zone A (core fuel, 4) and to the reactor zone B (peripheral fuel, 5). In the latter case, peripheral hydrogen can contribute to stabilization of the flame produced in the reactor zone B. Optionally, it is also possible to supply an additional amount of air (secondary air, 6) to the reactor zone B, in which the conversion of the reaction mixture to the product mixture comprising silicon dioxide and/or metal oxide (7) takes place.

(23) The metal oxide obtainable by the process according to the invention preferably contains at least one of the elements aluminium (Al), cerium (Ce), iron (Fe), magnesium (Mg), indium (In), titanium (Ti), tin (Sn), yttrium (Y), zinc (Zn) and zirconium (Zr) as metal component, more preferably Al and/or Ti.

(24) Metal oxides in the context of the invention also include mixed metal oxides and doped metal oxides, also including silicon dioxide doped with metal oxides, metal oxides doped with silicon dioxide, or mixed oxides containing metal oxides and silicon dioxide.

(25) A mixed metal oxide is understood to mean a metal oxide in which intimate mixing of mixed oxide components takes place at the level of primary particles or aggregates. The primary particles in this case may have oxygen-bridged metal components in the form of M1-O-M2 bonds. In addition, it is also possible for there to be regions of individual oxides M1O, M2O, M30, . . . in the primary particles.

(26) A doped metal oxide is understood to mean an oxide in which the doping component is present predominantly or exclusively at a lattice site of the metal oxide lattice. The doping component may be in metallic or oxidic form. One example of a doped metal oxide is indium tin oxide, where tin atoms occupy sites in the lattice of the indium oxide.

(27) The silicon and metal compounds in the context of the present invention may be organometallic and/or inorganic in nature. Examples of inorganic starting materials may especially be silicon tetrachloride, metal chlorides and metal nitrates. Organometallic compounds used may especially be silicon alkoxides and/or metal alkoxides and/or metal carboxylates. The alkoxides used may preferably be ethoxides, n-propoxides, isopropoxides, n-butoxides and/or tert-butoxides. The carboxylates used may be the compounds based on acetic acid, propionic acid, butanoic acid, hexanoic acid, oxalic acid, malonic acid, succinic acid, glutaric acid, adipic acid, octanoic acid, 2-ethylhexanoic acid, valeric acid, capric acid and/or lauric acid. Particularly advantageously, it is possible to use 2-ethylhexanoates and/or laurates.

(28) The silicon compounds and/or metal compounds used in the process according to the invention may, according to their nature, be dissolved, for example, in water or in organic solvents. Accordingly, liquid raw material containing a silicon compound and/or metal compound, in the context of the present invention, may be the solution of a silicon compound and/or metal compound, or even of a solid. The term liquid raw material relates to the state of matter thereof under conditions that exist in the case of use in step a) of the process according to the invention.

(29) Organic solvents, or constituents of organic solvent mixtures, may preferably be alcohols such as methanol, ethanol, n-propanol, isopropanol, n-butanol or tert-butanol, diols such as ethanediol, pentanediol, 2-methylpentane-2,4-diol, dialkyl ethers such as diethyl ether, tert-butyl methyl ether or tetrahydrofuran, C1-C12 carboxylic acids such as acetic acid, propionic acid, butanoic acid, hexanoic acid, oxalic acid, malonic acid, succinic acid, glutaric acid, adipic acid, octanoic acid, 2-ethylhexanoic acid, valeric acid, capric acid, lauric acid. In addition, it is possible to use ethyl acetate, benzene, toluene, naphtha and/or benzine. It is possible with preference to use solutions that contain C2-C12 carboxylic acids, especially 2-ethylhexanoic acid and/or lauric acid.

(30) Preferably, the content of the C2-C12 carboxylic acids in the solution is less than 60% by weight, more preferably less than 40% by weight, based on the total amount of solution.

(31) In a particularly preferred embodiment, the solutions of the silicon compounds and/or metal compounds simultaneously contain a carboxylate and its parent carboxylic acid and/or an alkoxide and its parent alcohol. More particularly, the starting materials used may be the 2-ethylhexanoates in a solvent mixture containing 2-ethylhexanoic acid.

(32) In a particularly preferred embodiment of the process according to the invention, a silicon compound is used for preparation of silicon dioxide.

(33) The silicon compound used in the process according to the invention may be a non-halogenated compound selected from the group consisting of tetraalkoxyorthosilicates, silanes, silicone oils, polysiloxanes and cyclic polysiloxanes, silazanes and mixtures thereof. Tetraalkoxyorthosilicates used may, for example, be tetraethoxyorthosilicate (TEOS) and tetramethoxyorthosilicate (TMOS). Silanes used are preferably alkoxysilanes, alkylalkoxysilanes, arylalkylalkoxysilanes, for example tetramethoxysilane, tetraethoxysilane, methyltrimethoxysilane, methyltriethoxysilane, dimethyldimethoxysilane, dimethyldiethoxysilane, trimethylmethoxysilane, trimethylethoxysilane, diethylpropylethoxysilane. Polysiloxanes and cyclic polysiloxanes, for example octamethylcyclotetrasiloxane, decamethylcyclopentasiloxane, dodecamethylcyclohexasiloxane, hexamethylcyclotrisiloxane and silazanes such as hexamethyldisilazane may likewise be used as silicon compound in the process according to the invention. Particular preference is given to using octamethylcyclotetrasiloxane.

(34) The silicon compound used in the process according to the invention may likewise be a chlorinated compound selected from the group consisting of silicon tetrachloride, dichlorosilane, trichlorosilane, methyltrichlorosilane, dimethyldichlorosilane, methyldichlorosilane, dibutyldichlorosilane, ethyltrichlorosilane, propyltrichlorosilane and mixtures thereof. Particular preference is given to using silicon tetrachloride.

(35) For preparation of an aluminium oxide by the process according to the invention, aluminium chloride in particular is suitable as the corresponding metal compound. The aluminium chloride, a compound which is solid under standard conditions, may be used in the form of a melt or a solution in a suitable solvent.

(36) For the preparation of titanium dioxide, it is possible to use titanium tetrachloride, for example, as the corresponding metal compound.

EXAMPLE 1

(37) Octamethylcyclotetrasiloxane (D4) is initially charged in a 200 litre vat and conveyed with a gear pump at a constant conveying rate of 12.5 kg/h to a pipe coil heated with thermal oil (FIG. 1, D), in order to preheat D4 to 150 C. The octamethylcyclotetrasiloxane that has been preheated in this way is guided to a one-phase nozzle (FIG. 1, E) from SCHLICK (Hollow-Cone Mod. 121) with a bore diameter of 0.7 mm, which in this case generates a backpressure of about 2.8 bara. The filter (FIG. 1, C) installed upstream of the nozzle ensures that the nozzle cannot become clogged by any solid particles present. The preheated D4 finely distributed by means of the one-phase nozzle is mixed with an air stream preheated to 295 C. (FIG. 1, 2). The spraying of the liquid octamethylcyclotetrasiloxane into this preheated air results in complete evaporation of D4 in the downstream pipeline (burner tube) (FIG. 1, A) having the diameter of 80 mm and a length of 4.2 metres, and forms a gas mixture. In a downstream static mixer (FIG. 1, F) from Sulzer (Mischer CompaX with metered addition), 6.25 m.sup.3 (STP)/h of hydrogen (FIG. 1, 3) are mixed in (primary H.sub.2). Good mixing of all components promotes complete and homogeneous conversion of the raw materials in the downstream reaction zone (FIG. 1, B). The gas mixture thus produced is fed to the burner and, with a calculated exit velocity of 51 m/s (under standard conditions) or 99 m/s (under operating conditions), exits from the mouth of the burner (FIG. 1, G) having diameter 32 mm into the reaction zone (FIG. 1, B). For stabilization of the flame, what is called a peripheral flame is generated. For this purpose, an additional 3 m.sup.3 (STP)/h of hydrogen flows out of a concentric annular gap with gap width 1.5 mm, and burns in a diffusive pilot flame. The hot reaction products are drawn into the reaction zone with 55 m.sup.3 (STP)/h of externally introduced air (FIG. 1, 6). The gas/solids mixture produced after the reaction is cooled down to <200 C. and then supplied pneumatically to a filter system. The pyrogenic oxide formed (10 kg/h) is separated here from the main gas stream and conveyed into a bunker. The further details for performance of this experiment can be found in Table 2.

(38) By contrast with the preparation process for pyrogenic silicon dioxide described in EP 0471139 A2, in the process according to the invention, the raw material to be processed (octamethylcyclotetrasiloxane) is not converted to the gas phase in an external evaporator via heating at a hot wall surface, but via spraying, i.e. via direct transfer of a preheated, finely distributed liquid into the gas phase by mixing with the air stream in the reactor zone A intended for the purpose.

(39) The process described here was operable successfully for several months without clogging of the nozzle or other apparatuses by solid particles or deposits in gel form.

EXAMPLE 2

(40) Analogously to Example 1, silicon tetrachloride (tetrachlorosilane, SiCl.sub.4) is used as raw material for preparation of pyrogenic silicon dioxide. The details for performance of this experiment can be found in Table 2.

EXAMPLE 3

(41) Analogously to Example 1, titanium tetrachloride (TiCl.sub.4) is used as raw material for preparation of pyrogenic titanium dioxide. The details for performance of this experiment can be found in Table 2.

(42) TABLE-US-00002 TABLE 2 Example 1 Example 2 Example 3 Raw material D4 SiCl.sub.4 TiCl.sub.4 Raw material boiling point [ C.] 171-175 57 136 Raw material feed temperature 150 30 30 [ C.] Raw material throughput [kg/h] 12.5 80 530 Primary air rate [m.sup.3 (STP)/h] 128 70 1545 Primary air feed temperature [ C.] 295 270 230 Primary H.sub.2 rate [m.sup.3 (STP)/h] 6.3 26 180 Primary H.sub.2 feed temperature [ C.] 35 30 30 Reaction mixture temperature 255 63 138 upstream of the reaction zone [ C.] Pressure upstream of the reactor 995 985 990 zone [mbar] One-phase nozzle diameter [mm] 0.7 (1) 1.4 (1) 2.3 (3) (number of nozzles) Burner tube diameter [mm] 80 80 200 Burner tube length [m] 4.2 4.2 3.0 Burner tube volume [l] 15 15 377