METHOD FOR PRODUCING METAL OXIDES BY MEANS OF SPRAY PYROLYSIS

Abstract

A process for producing a metal oxide powder proceeds by spray pyrolysis, in which a mixture comprising ammonia and an aerosol which is obtained by atomizing a solution containing a metal compound by means of an atomization gas is introduced into a high-temperature zone of a reaction space and reacted in an oxygen-containing atmosphere therein and the solids are subsequently separated off.

Claims

1. A process for producing a metal oxide powder by spray pyrolysis, said process comprising: introducing a mixture comprising ammonia and an aerosol which is obtained by atomizing a solution containing a metal compound by an atomization gas into a high-temperature zone of a reaction space, reacting said mixture in an oxygen-containing atmosphere in said reaction space, and subsequently separating the solids off.

2. The process according to claim 1, wherein the concentration of ammonia is 0.5-5.0 kg NH.sub.3/kg of the metal used.

3. The process according to claim 1, wherein the high-temperature zone into which the mixture is introduced is a flame which is formed by the reaction of an oxygen-containing gas and a combustion gas.

4. The process according to claim 3, wherein the flame and the mixture are at least partly spatially separated from one another within the reaction space.

5. The process according to claim 4, wherein the following applies to the ratio of mean velocity of the flame to mean velocity of the mixture: 2≦v.sub.flame/v.sub.mixture≦10.

6. The process according to claim 1, wherein at least one metal compound is a nitrate.

7. The process according to claim 1, wherein the metal component of the metal compounds is selected from the group consisting of Ag, Al, B, Ba, Ca, Cd, Co, Cr, Cu, Fe, Ga, Ge, Hf, In, Li, Mg, Mn, Mo, Nb, Ni, Pd, Rh, Ru, Sc, Si, Sn, Sr, Ta, Ti, V, Y and Zn.

Description

[0012] FIGS. 1, 2A and 2B show schematics of a possible arrangement for introduction of the feedstocks into the reaction space, where: 1=solution containing metal compound, 2=atomization gas, 3=ammonia, 4=air, 5=combustion gas, A=reaction chamber wall.

[0013] In a particular embodiment, the flame and the mixture are at least partly spatially separated from one another within the reaction space. FIG. 2B shows a schematic of such an arrangement, in which a bell jar B surrounds the mixture introduced into the reaction space. The metal oxide particles thus produced have particularly high homogeneity in terms of the particle size distribution.

[0014] The positive effect in terms of homogeneity can be enhanced further when, in this embodiment, the mean velocity of the flame, v.sub.flame is greater than the mean velocity of the mixture v.sub.mixture. More preferably, 2≦v.sub.flame/v.sub.mixture≦10; most preferably, 3≦v.sub.flame/v.sub.mixture≦5. The velocity figures are normalized velocities. They are found by dividing the volume flow rate having the unit m.sup.3 (STP)/h by the cross-sectional area.

[0015] In the process according to the invention, the solution(s) are introduced into the reaction space in the form of fine droplets. Preferably, the fine droplets have a median droplet size of 1-120 μm, more preferably of 30-100 μm. The droplets are typically produced using single or multiple nozzles.

[0016] In order to achieve solubility and in order to attain a suitable viscosity for the atomization of the solution, the solution can be heated. In principle, it is possible to use all soluble metal compounds which are oxidizable.

[0017] The metal component of the metal compound is preferably selected from the group consisting of Ag, Al, B, Ba, Ca, Cd, Co, Cr, Cu, Fe, Ga, Ge, Hf, In, Li, Mg, Mn, Mo, Nb, Ni, Pd, Rh, Ru, Sc, Si, Sn, Sr, Ta, Ti, V, Y and Zn. In principle, it is also possible to use a plurality of metal components, such that mixed oxides are obtained.

[0018] These may be inorganic metal compounds, such as nitrates, chlorides, bromides, or organic metal compounds, such as alkoxides or 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. From the group of the organic metal compounds, preference is given to using 2-ethylhexanoates or laurates. The solution may comprise one or more inorganic metal compounds, one or more organic metal compounds or mixtures of inorganic and organic metal compounds.

[0019] In a preferred embodiment, at least one metal compound is a nitrate. The metal oxide particles thus produced have particularly high homogeneity in terms of the particle size distribution.

[0020] The solvents can preferably be selected from the group consisting of water, C.sub.5-C.sub.20-alkanes, C.sub.1-C.sub.15-alkanecarboxylic acids and/or C.sub.1-C.sub.15-alkanols. Organic solvents used, or constituents of organic solvent mixtures used, 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, C.sub.1-C.sub.12-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. It is additionally possible to use benzene, toluene, naphtha and/or benzine.

[0021] Preference is given to using aqueous solutions, an aqueous solution being understood to mean a solution in which water is the main constituent of a solvent mixture or in which water alone is the solvent.

[0022] The concentration of the solutions used is not particularly limited. If only one solution containing all the mixed oxide components is present, the concentration is generally 1% to 50% by weight, preferably 3% to 30% by weight, most preferably 5%-20% by weight, based in each case on the sum total of the oxides.

EXAMPLES

[0023] The BET surface area is determined to DIN ISO 9277. The d.sub.50 results from the cumulative distribution curve of the volume-average size distribution. This is typically determined by laser diffraction. In the context of the present invention, a Cilas 1064 instrument from Cilas is used for this purpose. A d.sub.50 is the value at which 50% of the particles are within the size range indicated.

[0024] Metal compounds used are the respective nitrates. Examples without ammonia (suffix 0; comparative examples) and with ammonia (suffix 1; inventive examples) are conducted in each case.

Example Mn.SUB.0

[0025] 2 kg/h of a solution of manganese nitrate having a manganese concentration of 15.3% by weight are atomized with 5 m.sup.3 (STP)/h of air as atomization gas by means of a two-phase nozzle into a flame burning within a reaction space. The flame is formed by the reaction of 10 m.sup.3 (STP)/h of hydrogen and 30 m.sup.3 (STP)/h of air. After cooling, the metal oxide powder is separated from gaseous substances at a filter.

[0026] The examples Co.sub.0, Ni.sub.0, Zr.sub.0, La.sub.0, Al.sub.0 and Ce.sub.0 are conducted analogously. Amounts of feedstocks are shown in the table.

Example Mn.SUB.1

[0027] Like Mn.sub.0, except that a further 0.6 kg/h of ammonia are atomized into the reaction space as well as the solution and the atomizer air.

[0028] The examples Co.sub.1, Ni.sub.1, Zr.sub.1, La.sub.1, Al.sub.1 and Ce.sub.1 are conducted analogously. Amounts of feedstocks are shown in the table.

[0029] The metal oxide powders produced by the process according to the invention have lower values for BET surface area and mean particle size distribution.

TABLE-US-00001 TABLE Feedstocks and reaction conditions; physical properties Example Mn.sub.0 Mn.sub.1 Co.sub.0 Co.sub.1 Ni.sub.0 Ni.sub.1 Zr.sub.0 Zr.sub.1 La.sub.0 La.sub.1 Al.sub.0 Al.sub.1 Ce.sub.0 Ce.sub.1 Solution kg/h 2 2 2 2 2 2 4 4 3 3 4 4 4 4 Conc. of Metal % by wt. 15.3 15.3 14.6 14.6 14.4 14.4 6.5 6.5 9.6 9.6 4.0 4.0 6.0 6.0 Atomizer air m.sup.3 5 5 5 5 5 5 5 5 5 5 5 5 5 5 (STP)/h Ammonia kg/h 0 0.6 0 0.6 0 0.6 0 0.6 0 0.6 0 0.6 0 0.6 Ammonia/metal kg/kg 0 1.96 0 2.05 0 2.08 0 2.31 0 2.08 0 3.75 0 2.50 Hydrogen m.sup.3 10 12 10 12 10 12 12 12 12 12 12 12 12 12 (STP)/h Primary air m.sup.3 30 30 30 30 30 30 30 30 30 30 30 30 30 30 (STP)/h Lambda 1.68 1.40 1.68 1.40 1.68 1.40 1.40 1.40 1.40 1.40 1.40 1.40 1.40 1.40 V.sub.mixture Nm/s 0.33 0.40 0.32 0.39 0.32 0.39 0.41 0.46 0.39 0.43 0.44 0.48 0.44 0.48 V.sub.flame Nm/s 1.42 1.64 1.39 1.60 1.36 1.59 1.52 1.56 1.53 1.63 1.59 1.65 1.58 1.64 V.sub.flame/V.sub.mixture 4.3 4.1 4.3 4.1 4.3 4.1 3.7 3.4 3.9 3.8 3.6 3.4 3.6 3.4 T.sub.flame.sup.a) ° C. 646 742 623 712 611 708 636 659 663 700 683 708 674 700 BET surface m.sup.2/g 4.3 3.2 4.3 3.7 13.0 9.3 7.0 6.0 8.2 7.3 12.0 12.0 5.7 5.2 area d.sub.10 μm 0.10 0.09 0.24 0.09 0.21 0.07 0.41 0.36 0.36 0.31 0.78 0.75 0.20 0.16 d.sub.50 μm 0.25 0.21 0.38 0.18 0.59 0.42 3.53 3.01 2.29 1.68 6.80 5.64 1.22 0.95 d.sub.90 μm 0.10 0.09 0.24 0.09 0.21 0.07 0.41 0.36 0.36 0.31 0.78 0.75 0.20 0.16 .sup.a)flame temperature; measured 10 cm below the feed point of air and hydrogen into the reaction space