EP1317401A1 - Nanocrystalline metal oxide powders-production and application methods - Google Patents

Nanocrystalline metal oxide powders-production and application methods

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Publication number
EP1317401A1
EP1317401A1 EP01965503A EP01965503A EP1317401A1 EP 1317401 A1 EP1317401 A1 EP 1317401A1 EP 01965503 A EP01965503 A EP 01965503A EP 01965503 A EP01965503 A EP 01965503A EP 1317401 A1 EP1317401 A1 EP 1317401A1
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Prior art keywords
percent
fact
accordance
metal oxide
content
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EP01965503A
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German (de)
French (fr)
Inventor
Gerhard Auer
Werner Schuy
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Kerr-Mcgee Pigments & Co KG GmbH
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Kerr-Mcgee Pigments & Co KG GmbH
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Publication of EP1317401A1 publication Critical patent/EP1317401A1/en
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    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B82—NANOTECHNOLOGY
    • B82Y—SPECIFIC USES OR APPLICATIONS OF NANOSTRUCTURES; MEASUREMENT OR ANALYSIS OF NANOSTRUCTURES; MANUFACTURE OR TREATMENT OF NANOSTRUCTURES
    • B82Y30/00—Nanotechnology for materials or surface science, e.g. nanocomposites
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01G—COMPOUNDS CONTAINING METALS NOT COVERED BY SUBCLASSES C01D OR C01F
    • C01G23/00—Compounds of titanium
    • C01G23/04—Oxides; Hydroxides
    • C01G23/047—Titanium dioxide
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J21/00—Catalysts comprising the elements, oxides, or hydroxides of magnesium, boron, aluminium, carbon, silicon, titanium, zirconium, or hafnium
    • B01J21/06—Silicon, titanium, zirconium or hafnium; Oxides or hydroxides thereof
    • B01J21/063—Titanium; Oxides or hydroxides thereof
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B82—NANOTECHNOLOGY
    • B82B—NANOSTRUCTURES FORMED BY MANIPULATION OF INDIVIDUAL ATOMS, MOLECULES, OR LIMITED COLLECTIONS OF ATOMS OR MOLECULES AS DISCRETE UNITS; MANUFACTURE OR TREATMENT THEREOF
    • B82B1/00—Nanostructures formed by manipulation of individual atoms or molecules, or limited collections of atoms or molecules as discrete units
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B82—NANOTECHNOLOGY
    • B82B—NANOSTRUCTURES FORMED BY MANIPULATION OF INDIVIDUAL ATOMS, MOLECULES, OR LIMITED COLLECTIONS OF ATOMS OR MOLECULES AS DISCRETE UNITS; MANUFACTURE OR TREATMENT THEREOF
    • B82B3/00—Manufacture or treatment of nanostructures by manipulation of individual atoms or molecules, or limited collections of atoms or molecules as discrete units
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01G—COMPOUNDS CONTAINING METALS NOT COVERED BY SUBCLASSES C01D OR C01F
    • C01G23/00—Compounds of titanium
    • C01G23/04—Oxides; Hydroxides
    • C01G23/047—Titanium dioxide
    • C01G23/053—Producing by wet processes, e.g. hydrolysing titanium salts
    • C01G23/0532—Producing by wet processes, e.g. hydrolysing titanium salts by hydrolysing sulfate-containing salts
    • C—CHEMISTRY; METALLURGY
    • C09—DYES; PAINTS; POLISHES; NATURAL RESINS; ADHESIVES; COMPOSITIONS NOT OTHERWISE PROVIDED FOR; APPLICATIONS OF MATERIALS NOT OTHERWISE PROVIDED FOR
    • C09C—TREATMENT OF INORGANIC MATERIALS, OTHER THAN FIBROUS FILLERS, TO ENHANCE THEIR PIGMENTING OR FILLING PROPERTIES ; PREPARATION OF CARBON BLACK  ; PREPARATION OF INORGANIC MATERIALS WHICH ARE NO SINGLE CHEMICAL COMPOUNDS AND WHICH ARE MAINLY USED AS PIGMENTS OR FILLERS
    • C09C1/00—Treatment of specific inorganic materials other than fibrous fillers; Preparation of carbon black
    • C09C1/36—Compounds of titanium
    • C09C1/3607—Titanium dioxide
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01P—INDEXING SCHEME RELATING TO STRUCTURAL AND PHYSICAL ASPECTS OF SOLID INORGANIC COMPOUNDS
    • C01P2002/00—Crystal-structural characteristics
    • C01P2002/50—Solid solutions
    • C01P2002/52—Solid solutions containing elements as dopants
    • C01P2002/54—Solid solutions containing elements as dopants one element only
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    • C01P—INDEXING SCHEME RELATING TO STRUCTURAL AND PHYSICAL ASPECTS OF SOLID INORGANIC COMPOUNDS
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Definitions

  • the invention comprises nanocrystalline powdery materials with a TiO 2 content of more than 50 w/w percent, a method for the production of such nanocrystalline powdery materials as well as the application of such nanocrystalline powdery materials.
  • Nanocrystalline metal oxides by which metal oxides with an average diameter of less than 100 mm are understood in the following text, gained several application opportunities in numerous fields in the past years.
  • One of the economically most important fields of application for nanocrystalline metal oxides is their utilisation as a catalyst material, especially in the catalytic decomposition of nitrogen oxides.
  • the method for catalytic destruction of nitrogen oxides in waste gases is described in detail in GB 1 495 396.
  • a gas mixture consisting of nitrogen oxides, oxygen, and ammonia is brought into contact with a catalyst.
  • the catalyst consists predominantly of titanium dioxide and can be produced by several methods, which are described in GB 1 495 396.
  • the components' crystallinity on the one hand and an intimate mixing of the titanium dioxide component with the other components before the final calcination of the catalyst substrate (page 18, lines 17 to 34) on the other hand are of special importance: Both a too coarse-crystalline titanium dioxide component and an insufficient micro-homogeneity have disadvantageous effects on the catalytic characteristics of the catalysts produced thereof.
  • the final calcination of the catalyst substrate is effected by bringing together all components required for a period of several hours, e.g. in a muffle kiln (see examples in GB 1 495 396).
  • a muffle kiln see examples in GB 1 495 396.
  • Various modifications and/or further developments of this method describe the process step of calcination in a similar way, e.g., by usage of a rotary kiln or similar aggregates (EP 268 265 A2, EP 640 386 Al, WO 99/41200).
  • US-A 3,018,186 describes a process, in which a titanium sulphate solution is converted into scaly titanium dioxide (predominantly as a modification of rutile) at temperatures of 800-l,800°C and with a retention time of 0.01-0.5 seconds.
  • the pure titanium sulphate solution used for this process is a material which is not available on a large scale, and is quite expensive in production.
  • the scaly titanium dioxide formed through this process has a diameter of 1 to 20 ⁇ m and is thus not suited for the production of catalysts.
  • US-A 2,397,430 describes a process for the production of titanium dioxide pigments, in which hydrated titanium oxide undergoes calcination at temperatures of approximately 1,000°C and with a retention time of a few minutes.
  • the titanium dioxide obtained this way is characterised by a complete or partial rutile modification and a particle size which is equivalent to the typical size of titanium dioxide pigments, and is thus not suited for the production of catalysts, among others.
  • US-A 5,009,879 describes a process for the production of titanium dioxide, in which the hydrated titanium oxide originating from the conventional sulphate process TiO 2 production undergoes flash calcination at temperatures of 800 to 1,600°C and with a retention time between 0.1 and 60 seconds.
  • the titanium dioxide obtained this way has a BET surface of 1 to 4.5 sqm/g and is thus not suited for the production of catalysts, among others.
  • US-A 5,833,892 describes a flash calcination process of titanium dioxide for the purpose of pigment production, in which the hydrated titanium oxide originating from the conventional sulphate process TiO 2 production, or other titanium-containing compounds such as TiOCl 2 , TiOBr 2 , TiOSO 4 , or hydrolysates of titanium alcoholates are atomised and undergo flash calcination at a temperature between 900 and 1,200°C, with fuels being added if required.
  • the titanium dioxide produced this way has a particle size of 150 to 250 nm (which is equivalent to the typical size of titanium dioxide pigments) and is thus not suited for the production of catalysts, among others.
  • nanocrystalline metal oxides intended for catalyst materials or intermediate products for the production of catalysts, respectively.
  • the production process should be effective and efficient, should ensure a high and constant product quality, and consequently allow flexible and fast control if variations of the educt or product characteristics are required.
  • the task of this invention is, therefore, to provide a nanocrystalline metal oxide powder which allows to form an intermediate product for the production of chemico- technical products, especially of DeNO x catalysts, in a simple and cost-effective way, i.e. without the disadvantages of previous processes.
  • This task is solved by claim 1.
  • the invention thus covers a nanocrystalline metal oxide powder with a TiO 2 content of at least 50 w/w percent to start with and preferably at least 75 w/w percent of TiO 2 , further with an average primary particle size of less than 50 nm and preferably of less than 35 nm, with a BET surface of 25-250 sqm/g, preferably of 50-150 sqm/g and especially preferably of 75-100 sqm/g, with a radiographic anatase percentage of at least 95% and preferably of at least 99%, and with a brightening capacity of 25-70 or preferably of 30-50.
  • Another task was to provide a production method for such nanocrystalline metal oxide powders.
  • the invention thus covers a process for the production of a nanocrystalline metal oxide powder in addition, which is performed with the help of short-time calcination (flash calcination) with an average retention time of the particles within the reactor of less than 120 seconds, or preferably less than 10 seconds, or most preferably less than 2 seconds.
  • the invention covers the application of nanocrystalline metal oxides produced in compliance with the invention, or in a comparable way, as a starting material for the production of chemico-technical products, especially of catalysts, colour pigments, ceramic products, products for electroceramic applications, enamels, welding electrodes, cosmetic products, or UV protectants.
  • a nanocrystalline metal oxide powder produced from traditional starting products by short-time calcination has the characteristics which allow application as a starting material for the production of catalysts, among others, if suitable process parameters are set.
  • flash calcination first the water is vaporised, followed by the calcination step, i.e. by the growth of the primary particles, with a retention time of the particles within the reactor of less than 120 seconds, or preferably of less than 10 seconds, or most preferably of less than 2 seconds.
  • Any apparatus or equipment allowing an average retention time of the calcination material of less than 2 minutes, or preferably of less than 10 seconds, or most preferably of less than 2 seconds can be used as a short-time or flash calcination facility.
  • These include all usual apparatuses the operating parameters of which are suited and which usually work with a direct hot gas supply, such as hot gas atomisation reactors, atomisation roasters, fluid-bed reactors, reaction cyclones, or flash calciners for example. Indirect heating is, however, also possible.
  • Introduction into the reactor may be effected in the form of watery solutions via single-fluid or two-fluid nozzles. Another possibility is to feed the starting material in the form of dried or partially dried materials.
  • the separation of the nanocrystalline metal oxide particles from the gas phase is performed outside the reaction chamber and can be effected by traditional technologies, which the specialist is acquainted with, such as through cyclones and/or electrical or mechanical powder separators.
  • the preferred method is a flash calcination process performed in a directly or indirectly heated reaction chamber, with a gas temperature of 800 to 1,200°C at the reaction chamber's entry and a gas temperature of 500 to 800°C at the reaction chamber's exit.
  • the material to be calcined is preferably fed into the reactor in counterflow to the hot gas, with the material to be calcined being subsequently carried over by the hot gas.
  • the nanocrystalline metal oxide powders obtained this way have good rheologic characteristics: They can well be carried and do not tend to caking.
  • a decisive advantage of the flash calcination technology is that it allows to combine the two process steps of drying and calcination used by traditional technologies, into one single process step.
  • flash calcination technology Another special advantage of the flash calcination technology is that it is possible to react to changes in the educt or product characteristics in a fast and flexible manner, thanks to the short retention times within the reactor. With rotary kilns, which are traditionally used, it may take quite a long time until changes to the process parameters really show an effect on the characteristics of the product obtained. This may lead to quality problems, demand a more personnel-intensive operation, and a larger amount of intermediate products, for example.
  • the suspension obtained is filtered and washed in deionised water in order to reduce the content of undesired dissolved solids such as alkali compounds and SO 4 .
  • the intermediate result is a filter cake.
  • Additives which may be required to achieve the characteristics desired can be added before, during, and after the process steps of filtration or washing, respectively.
  • the suspension of hydrated titanium oxide is preferably only partially neutralised, i.e. in such a manner that the content of sulphate bonded in the filter cake or in the calcined product, respectively, amounts to up to 7 w/w percent, or preferably to 0.5 to 3 w/w percent.
  • a sulphate content of this scale is advantageous to the catalytic characteristics.
  • the filter cake obtained this way is then calcined in compliance with the invention for less than 2 minutes, or preferably for less than 10 seconds, or most preferably for less than 2 seconds.
  • the process in compliance with this invention allows to expose the filter cake directly to flash calcination. It is however also possible to suspend the filter cake first and then to introduce the material obtained this way into the flash calcination facility. Finally, the partially neutralised and washed filter cake can first be completely or partially dried, and then the material obtained this way be introduced into the flash calcination facility.
  • the preferred method is to first mix the filter cake with deionised water in order to obtain a pumpable suspension, then to blind it with specific additives, if required for the application intended, and finally to directly introduce the suspension into one of the aggregates described above, with the help of a nozzle for example, and to expose it to short-time calcination (flash calcination).
  • the advantage of this process variant is that the nanocrystalline metal oxide powder desired is obtained by one single thermal process step, with the suspension allowing to be dosed into the calcination aggregate in an especially simple manner, in contrast to filter cakes.
  • An alternative variant is first to mix the filter cake with deionised water in order to obtain a pumpable suspension, then to blind it with specific additives, if required for the application intended, to completely or partially dry the suspension, and finally to introduce the product obtained into one of the aggregates described above and expose it to short-time calcination (flash calcination).
  • drying aggregates such as spray dryers, spin flash dryers, conveyor dryers, for example, or other devices, which the specialist is acquainted with, may be used.
  • a liquefier is used, pumpable, low-viscosity suspensions with a substantially higher solids content can be produced than without liquefiers, such suspensions allowing substantial energy savings during subsequent drying and/or calcination on the one hand and an increased throughput within the respective aggregate on the other hand.
  • any inorganic or organic compound which increases the electrostatic repulsion of the particles by adsorption on the surface of the hydrated titanium oxide such as hydrochloric acid, polyelectrolytes, or organic acids for example, are suited as liquefiers.
  • the preferred liquefiers to be used are organic compounds, which are degraded during the subsequent process step of calcination without leaving any residues on the nanocrystalline metal oxides.
  • compounds are especially preferred, the decomposition products of which do not contain any other compounds than those usually occurring in burner gases of directly heated calcination facilities.
  • Compounds suited for this purpose are organic acids of the C ⁇ H y O z structure, i.e. in particular carboxylic acids, among which acetic acid and formic acid are preferred most.
  • These compounds allow a distinctly increased solids content of the suspension of hydrated titanium oxide without leaving any residues on the nanocrystalline metal oxides after the calcination step, or any undesired compounds during the gas phase, at a concentration of 0.01 to 5,0 w/w percent, preferably of 0.2 to 2.5 w/w percent already.
  • This increase in the solids content of the suspension of hydrated titanium oxide allows to achieve a throughput at (spray-) drying and/or flash calcination which is increased by more than 100%, and to reduce the energy consumption by over 50%.
  • Such material can also be produced in another way, for example by one of the variants described in GB 1 495 396, such as by hydrolyses of titanium alcoholates or other inorganic or organic titanium compounds.
  • An especially preferable application form of the invention is to add to the hydrated titanium oxide, originating from the conventional sulphate process production of titanium dioxide, a tungsten compound, e. g. ammonium para tungsten, and/or vanadium compounds and/or other compounds which are supportive for catalytic activity, before the process step of flash calcination.
  • the metal oxide powder has a tungsten oxide content (in the form of WO 3 ) of preferably 0 to 20 w/w percent, or especially preferably of 5 to 15 w/w percent.
  • Another especially preferred application form of the invention is to add to the hydrated titanium oxide a silicon and/or aluminium compound and/or other compounds which are supportive for the thermal stability of the catalyst substances produced thereof, before the process step of flash calcination. It is an especially preferred method to add finely dispersed colloidal SiO .
  • the metal oxide powder has an SiO 2 content of preferably 0 to 20 w/w percent, especially preferably of 2 to 8 w/w percent. These compounds may be added before, during, and after the steps of (partial) neutralisation and washing described above. It is also possible first to completely or partially dry the suspension partially neutralised, washed and blended with the compounds stated before, and then to introduce the material obtained this way into the flash calcination facility.
  • the product in compliance with the invention and produced by the method described above has excellent characteristics, especially as a starting material for the production of catalysts.
  • the average particle size of the primary particles (determined on the basis of their BET surface, and on the assumption that the particles have a spherical geometry and a density of 4.2, or established on the basis of electron microscopic photographs, respectively) amounts to approximately 20 nm.
  • the particle size of the primary particles is predominantly determined by the temperature and retention time during calcination.
  • the particle size of the primary particles corresponds to their high specific surface (BET surface), and is a prerequisite for a high activity of the catalysts to be produced from this nanocrystalline powdery material.
  • the BET surface is determined in accordance with DIN 66131 (carrier gas method, single-point method, ratio between carrier gas and adsorptive 90:10, measuring gas nitrogen, adsorption at the temperature of the boiling nitrogen, on the assumption that one nitrogen molecule requires an area of 0.162 nm 2 , pre-treatment: heating in the nitrogen flow at 140°C for 1 hour).
  • a particle size measurement performed with a laser particle analyser delivers an average particle size (mean volume value) of approximately 1 ⁇ m (with conventional ultrasonic dispersion). This value describes solid aggregates, the size of which can essentially be determined by the flocculation structure of the starting product (hydrated titanium oxide), and which allow to be deflocculated by proportionally large forces only, such as by grinding for example.
  • agglomerates in an order of magnitude between 10 and 200 ⁇ m could be detected under the light microscope, the size of which is essentially determined by the kind of their introduction into the flash calcination reactor (e. g. by the type of nozzle). These aggregates or agglomerates, respectively, ensure good processability characteristics of the nanocrystalline metal oxides used as a starting material for catalysts.
  • the tinting strength is determined in accordance with DIN 53192 (with Dataflash 2000 (d/8°), device A of Datacolor).
  • the tinting strength is determined by analysing the values obtained this way, in accordance with DIN 55982.
  • the C/2° (CIE 1931) type of light and standard observer are used.
  • TRONOX® R-KB-2 a commercially available micronised titanium dioxide pigment of Kerr-McGee in the form of the rutile modification, coated by aluminium and silicon compounds and an organic coating, is used as a reference material.
  • the tinting strength of TRONOX® R-KB-2 is defined to be 100.
  • the chemical composition of the nanocrystalline metal oxides allows to adapt them ideally to the requirements profile of the application desired. If the metal oxides are intended to be used as a starting material for catalysts, for example, a residual sulphate content and the addition of e. g. tungsten would be beneficial.
  • An Na, K, and Fe content of less than 1,000 ppm, preferably less than 100 ppm would be as advantageous to the catalytic characteristics.
  • nanocrystalline metal oxides are, however, intended to be used as a starting material for complex mixed oxide pigments, a sulphate content as low as possible and the addition of other elements would be beneficial.
  • the TiO 2 and WO 3 content is determined in accordance with DIN 55912, Part 2, the sulphate content is determined in accordance with DIN 24935, the Na and K content is determined by HF microwave digestion in accordance with DIN 38406, Part 14, and the Fe content is determined in accordance with DIN 51083, Part 6.
  • the titanium dioxide In the absence of any additives (such as W, V, Si, Al for example), the titanium dioxide is completely provided in the form of its anatase modification. If additives as stated above are contained, the titanium dioxide has an anatase-type structure. The anatase modification is more advantageous to the catalytic characteristics, than the rutile modification.
  • the material produced in compliance with the invention and by the method described can be used as a starting material for the production of a wide range of chemico-technical or cosmetico-pharmaceutical products, besides their application as a starting material for the production of catalysts. Thanks to its nanocrystalline structure, the material allows to be used directly or after being conditioned in a suitable manner, if required, for applications requiring titanium dioxide which contains materials that are finely divided to a high degree, such as sun screening agents, or UV absorbers in plastics and coatings for example.
  • nanocrystalline materials produced in compliance with the invention as a starting material for the production of colour pigments, ceramic products, electroceramic applications, enamels, welding electrodes, or other products, with the nanocrystalline structure allowing to be especially well mixed with other components at micro level, and showing an especially high reactivity during subsequent calcination or sintering processes.
  • the nanocrystalline titanium dioxide powders in compliance with the invention for applications the starting material of which would usually be a titanium dioxide with pigment characteristics, i.e. which has an average primary particle size of approximately 0.2 ⁇ m, and is exposed to a calcination step after mixing with other components.
  • This suspension is filtered and washed with deionised water until the Na 2 O content amounts to ⁇ 100 ppm (related to TiO 2 ) and the SO 4 content is 1.5-2% (related to TiO ).
  • the filter cake is then mixed with deionised water until a solids content of 22.8 w/w percent is achieved.
  • the viscosity measured as the run-out time in accordance with DIN 53211, is 13 seconds (with a 4-mm nozzle).
  • the solids content is determined with a commercially available IR dryer.
  • the suspension of hydrated titanium oxide with a solids content of 22.8 w/w percent and a sulphate content of 1.6 w/w percent (related to TiO 2 ) and a BET surface (after drying) of approximately 300 sqm/g is calcined in a high-temperature mixing chamber in an oxidising atmosphere at 700°C (temperature at the mixing chamber's exit) for 0.3 seconds.
  • the suspension is introduced into the high-temperature mixing chamber via a two-fluid nozzle.
  • the product obtained consists predominantly of isometric primary particles of 20 nm, which are predominantly agglomerated to spherulitic particles with an average diameter of 10 to 200 ⁇ m.
  • TiO 2 is stoichiometrically neutralised with NaOH at 80°C. This suspension is filtered and washed with deionised water until the NaO content amounts to ⁇ 100 ppm (related to TiO 2 ) and the SO 4 content is 1.5-2% (related to TiO 2 ). The filter cake with a solids content of 46 w/w percent is then mixed with deionised water and 1.08 w/w percent formic acid (related to TiO 2 ) as a liquefier until a solids content of 37.5 w/w percent is achieved.
  • the viscosity of the 37.5 percent suspension is comparable to the viscosity of the 22.8 percent suspension without any liquefier.
  • the viscosity, measured as the run-out time in accordance with DIN 53211, is
  • the solids content is determined with a commercially available IR dryer.
  • This suspension of hydrated titanium oxide is calcined in a high-temperature mixing chamber in an oxidising atmosphere at 650°C (temperature at the reaction chamber's exit) for 0.3 seconds.
  • the suspension is introduced into the high-temperature mixing chamber via a two-fluid nozzle. No increased organic residues could be detected in the product obtained this way, in comparison with the product of the 1 st example (both products contain less than 0.05 w/w percent of carbon, related to TiO 2 ).
  • This suspension is filtered and washed with deionised water until the NaO content amounts to ⁇ 100 ppm (related to TiO 2 ) and the SO 4 content is 1.5-2% (related to TiO 2 ).
  • the filter cake is then mixed with deionised water, without adding any liquefier, until a solids content of 37.5 w/w percent is achieved.
  • a high viscosity substance is obtained, which does not allow to be pumped or atomised.
  • the viscosity measured as the run-out time in accordance with DIN 53211, cannot be determined as the material does not leave the nozzle at all due to its high viscosity (with a 4-mm nozzle).
  • the solids content is determined with a commercially available IR dryer.
  • a suspension of hydrated titanium oxide as described in the 1 st example (ref. first paragraph) is predried in a spray dryer and then calcined in an electric rotary kiln at approximately 600°C (temperature at the kiln exit).
  • the retention time within the rotary kiln amounts to approximately 4 hours.
  • the product obtained consists predominantly of isometric primary particles of about 20 nm, which are predominantly agglomerated to spherolitic particles with an average diameter of 10 to 200 ⁇ m.

Abstract

There are disclosed nanocrystalline powdery materials with a TiO2 content of more than 50 w/w percent, a method for the production of such nanocrystalline powdery materials as well as the application of such nanocrystalline powdery materials as a starting material for the production of chemico-technical or cosmetico-pharmaceutical products, especially of catalysts, colour pigments, ceramic products, products for electroceramic applications, enamels, welding electrodes, cosmetic products, or UV protectants.

Description

NANOCRYSTALLINE METAL OXIDE POWDERS - PRODUCTION AND APPLICATION METHODS
The invention comprises nanocrystalline powdery materials with a TiO2 content of more than 50 w/w percent, a method for the production of such nanocrystalline powdery materials as well as the application of such nanocrystalline powdery materials.
Nanocrystalline metal oxides, by which metal oxides with an average diameter of less than 100 mm are understood in the following text, gained several application opportunities in numerous fields in the past years. One of the economically most important fields of application for nanocrystalline metal oxides is their utilisation as a catalyst material, especially in the catalytic decomposition of nitrogen oxides.
Among various methods discussed for the removal of nitrogen oxides from burner gases (W. Weisweiler gives an up-to-date overview in Chemie Ingenieur
Technik [Chemistry Engineer Technology] 72, 441-449 (2000)), selective catalytic reduction (SCR) with ammonia in the presence of mixed oxide catalysts gained most acceptance in large-scale production.
The method for catalytic destruction of nitrogen oxides in waste gases is described in detail in GB 1 495 396. A gas mixture consisting of nitrogen oxides, oxygen, and ammonia is brought into contact with a catalyst. The catalyst consists predominantly of titanium dioxide and can be produced by several methods, which are described in GB 1 495 396. The components' crystallinity on the one hand and an intimate mixing of the titanium dioxide component with the other components before the final calcination of the catalyst substrate (page 18, lines 17 to 34) on the other hand are of special importance: Both a too coarse-crystalline titanium dioxide component and an insufficient micro-homogeneity have disadvantageous effects on the catalytic characteristics of the catalysts produced thereof.
The final calcination of the catalyst substrate is effected by bringing together all components required for a period of several hours, e.g. in a muffle kiln (see examples in GB 1 495 396). Various modifications and/or further developments of this method describe the process step of calcination in a similar way, e.g., by usage of a rotary kiln or similar aggregates (EP 268 265 A2, EP 640 386 Al, WO 99/41200).
The short-time calcination (flash calcination) technology, which is quite interesting for the principle it is based on, is described in several publications for the purpose of titanium dioxide production for pigmentary applications:
US-A 3,018,186 describes a process, in which a titanium sulphate solution is converted into scaly titanium dioxide (predominantly as a modification of rutile) at temperatures of 800-l,800°C and with a retention time of 0.01-0.5 seconds.
The pure titanium sulphate solution used for this process is a material which is not available on a large scale, and is quite expensive in production. The scaly titanium dioxide formed through this process has a diameter of 1 to 20 μm and is thus not suited for the production of catalysts.
US-A 2,397,430 describes a process for the production of titanium dioxide pigments, in which hydrated titanium oxide undergoes calcination at temperatures of approximately 1,000°C and with a retention time of a few minutes. The titanium dioxide obtained this way is characterised by a complete or partial rutile modification and a particle size which is equivalent to the typical size of titanium dioxide pigments, and is thus not suited for the production of catalysts, among others. US-A 5,009,879 describes a process for the production of titanium dioxide, in which the hydrated titanium oxide originating from the conventional sulphate process TiO2 production undergoes flash calcination at temperatures of 800 to 1,600°C and with a retention time between 0.1 and 60 seconds. The titanium dioxide obtained this way has a BET surface of 1 to 4.5 sqm/g and is thus not suited for the production of catalysts, among others.
US-A 5,833,892 describes a flash calcination process of titanium dioxide for the purpose of pigment production, in which the hydrated titanium oxide originating from the conventional sulphate process TiO2 production, or other titanium-containing compounds such as TiOCl2, TiOBr2, TiOSO4, or hydrolysates of titanium alcoholates are atomised and undergo flash calcination at a temperature between 900 and 1,200°C, with fuels being added if required. The titanium dioxide produced this way has a particle size of 150 to 250 nm (which is equivalent to the typical size of titanium dioxide pigments) and is thus not suited for the production of catalysts, among others.
A general description of the various calcination technologies available is contained in UUmann's Encyclopaedia of Industrial Chemistry (5th Ed., VCH Verlagsgesellschaft mbH, Weinheim, Volume B2, Pages 4-1 to 4-35).
The most important requirements on nanocrystalline metal oxides intended for catalyst materials or intermediate products for the production of catalysts, respectively, are: Primary particles as fine as possible, i.e. with a specific surface as high as possible, a high catalytic activity, good processability characteristics, ageing resistance, and temperature resistance.
The most important requirements on the production method for nanocrystalline metal oxides for catalyst materials or intermediate products for the production of catalysts, respectively, are: The production process should be effective and efficient, should ensure a high and constant product quality, and consequently allow flexible and fast control if variations of the educt or product characteristics are required.
Especially the long time of retention within the respective aggregates, such as rotary kilns for example, required by the conventional calcination technology, and the associated problems in connection with system control and constant product quality represent quite important problems in practice. In spite of these disadvantages, alternative technologies for large-scale production of nanocrystalline metal oxides could, however, not yet really establish themselves. The reason therefor might be seen in the very sensitive dependence of important ceramic characteristics, such as slirinkage, shrinkage constancy, crack formation, temperature stability for example, and of the catalytic characteristics on changes in the starting materials or production parameters for the starting materials concerned, which may even be very small in some cases - a problem which experts are well acquainted with.
The task of this invention is, therefore, to provide a nanocrystalline metal oxide powder which allows to form an intermediate product for the production of chemico- technical products, especially of DeNOx catalysts, in a simple and cost-effective way, i.e. without the disadvantages of previous processes. This task is solved by claim 1. The invention thus covers a nanocrystalline metal oxide powder with a TiO2 content of at least 50 w/w percent to start with and preferably at least 75 w/w percent of TiO2, further with an average primary particle size of less than 50 nm and preferably of less than 35 nm, with a BET surface of 25-250 sqm/g, preferably of 50-150 sqm/g and especially preferably of 75-100 sqm/g, with a radiographic anatase percentage of at least 95% and preferably of at least 99%, and with a brightening capacity of 25-70 or preferably of 30-50.
Another task was to provide a production method for such nanocrystalline metal oxide powders.
The invention thus covers a process for the production of a nanocrystalline metal oxide powder in addition, which is performed with the help of short-time calcination (flash calcination) with an average retention time of the particles within the reactor of less than 120 seconds, or preferably less than 10 seconds, or most preferably less than 2 seconds.
Finally, the invention covers the application of nanocrystalline metal oxides produced in compliance with the invention, or in a comparable way, as a starting material for the production of chemico-technical products, especially of catalysts, colour pigments, ceramic products, products for electroceramic applications, enamels, welding electrodes, cosmetic products, or UV protectants.
In view of the large-scale application of similar materials based on traditional production technologies, which has widely spread, it was of special importance to develop a new process for the production of such materials which does without any losses in terms of product quality, i.e. equivalent or better catalytic characteristics and comparable or better processing conditions during the production of catalysts were essential prerequisites. Another aim was to achieve a higher constancy of the materials' characteristics. The catalytic characteristics and ceramic processability characteristics are usually highly dependent on the raw materials used, and often high efforts must be made to optimise the process or the parameters of the raw materials, respectively.
It turned out that a nanocrystalline metal oxide powder produced from traditional starting products by short-time calcination (flash calcination) has the characteristics which allow application as a starting material for the production of catalysts, among others, if suitable process parameters are set. With this flash calcination process, first the water is vaporised, followed by the calcination step, i.e. by the growth of the primary particles, with a retention time of the particles within the reactor of less than 120 seconds, or preferably of less than 10 seconds, or most preferably of less than 2 seconds. Any apparatus or equipment allowing an average retention time of the calcination material of less than 2 minutes, or preferably of less than 10 seconds, or most preferably of less than 2 seconds can be used as a short-time or flash calcination facility. These include all usual apparatuses the operating parameters of which are suited and which usually work with a direct hot gas supply, such as hot gas atomisation reactors, atomisation roasters, fluid-bed reactors, reaction cyclones, or flash calciners for example. Indirect heating is, however, also possible. Introduction into the reactor may be effected in the form of watery solutions via single-fluid or two-fluid nozzles. Another possibility is to feed the starting material in the form of dried or partially dried materials.
The separation of the nanocrystalline metal oxide particles from the gas phase is performed outside the reaction chamber and can be effected by traditional technologies, which the specialist is acquainted with, such as through cyclones and/or electrical or mechanical powder separators.
Process parameters could be found which allow production of nanocrystalline metal oxide powders with good catalytic characteristics and ceramic processability features, among others. The preferred method is a flash calcination process performed in a directly or indirectly heated reaction chamber, with a gas temperature of 800 to 1,200°C at the reaction chamber's entry and a gas temperature of 500 to 800°C at the reaction chamber's exit. The material to be calcined is preferably fed into the reactor in counterflow to the hot gas, with the material to be calcined being subsequently carried over by the hot gas. The nanocrystalline metal oxide powders obtained this way have good rheologic characteristics: They can well be carried and do not tend to caking.
A decisive advantage of the flash calcination technology is that it allows to combine the two process steps of drying and calcination used by traditional technologies, into one single process step.
Another special advantage of the flash calcination technology is that it is possible to react to changes in the educt or product characteristics in a fast and flexible manner, thanks to the short retention times within the reactor. With rotary kilns, which are traditionally used, it may take quite a long time until changes to the process parameters really show an effect on the characteristics of the product obtained. This may lead to quality problems, demand a more personnel-intensive operation, and a larger amount of intermediate products, for example. In order to obtain the product in compliance with the invention, preferably a washed and bleached suspension of hydrated titanium oxide (washed and bleached hydrolysate), which is formed as a result of the conventional sulphate process titanium dioxide production, is completely or partially neutralised. The suspension obtained is filtered and washed in deionised water in order to reduce the content of undesired dissolved solids such as alkali compounds and SO4. The intermediate result is a filter cake. Additives which may be required to achieve the characteristics desired can be added before, during, and after the process steps of filtration or washing, respectively.
The suspension of hydrated titanium oxide is preferably only partially neutralised, i.e. in such a manner that the content of sulphate bonded in the filter cake or in the calcined product, respectively, amounts to up to 7 w/w percent, or preferably to 0.5 to 3 w/w percent. A sulphate content of this scale is advantageous to the catalytic characteristics.
The filter cake obtained this way is then calcined in compliance with the invention for less than 2 minutes, or preferably for less than 10 seconds, or most preferably for less than 2 seconds.
The process in compliance with this invention allows to expose the filter cake directly to flash calcination. It is however also possible to suspend the filter cake first and then to introduce the material obtained this way into the flash calcination facility. Finally, the partially neutralised and washed filter cake can first be completely or partially dried, and then the material obtained this way be introduced into the flash calcination facility.
The preferred method is to first mix the filter cake with deionised water in order to obtain a pumpable suspension, then to blind it with specific additives, if required for the application intended, and finally to directly introduce the suspension into one of the aggregates described above, with the help of a nozzle for example, and to expose it to short-time calcination (flash calcination).
The advantage of this process variant is that the nanocrystalline metal oxide powder desired is obtained by one single thermal process step, with the suspension allowing to be dosed into the calcination aggregate in an especially simple manner, in contrast to filter cakes.
An alternative variant is first to mix the filter cake with deionised water in order to obtain a pumpable suspension, then to blind it with specific additives, if required for the application intended, to completely or partially dry the suspension, and finally to introduce the product obtained into one of the aggregates described above and expose it to short-time calcination (flash calcination). In order to dry the suspension, all types of drying aggregates such as spray dryers, spin flash dryers, conveyor dryers, for example, or other devices, which the specialist is acquainted with, may be used.
It is also possible to mix the filter cake with a liquefier, adding deionised water if required, in order to increase the solids content of the suspension formed thereof.
If a liquefier is used, pumpable, low-viscosity suspensions with a substantially higher solids content can be produced than without liquefiers, such suspensions allowing substantial energy savings during subsequent drying and/or calcination on the one hand and an increased throughput within the respective aggregate on the other hand.
Basically, any inorganic or organic compound which increases the electrostatic repulsion of the particles by adsorption on the surface of the hydrated titanium oxide, such as hydrochloric acid, polyelectrolytes, or organic acids for example, are suited as liquefiers.
The preferred liquefiers to be used are organic compounds, which are degraded during the subsequent process step of calcination without leaving any residues on the nanocrystalline metal oxides. In order to avoid expensive additional gas cleaning, compounds are especially preferred, the decomposition products of which do not contain any other compounds than those usually occurring in burner gases of directly heated calcination facilities. Compounds suited for this purpose are organic acids of the CχHyOz structure, i.e. in particular carboxylic acids, among which acetic acid and formic acid are preferred most. These compounds allow a distinctly increased solids content of the suspension of hydrated titanium oxide without leaving any residues on the nanocrystalline metal oxides after the calcination step, or any undesired compounds during the gas phase, at a concentration of 0.01 to 5,0 w/w percent, preferably of 0.2 to 2.5 w/w percent already.
This increase in the solids content of the suspension of hydrated titanium oxide allows to achieve a throughput at (spray-) drying and/or flash calcination which is increased by more than 100%, and to reduce the energy consumption by over 50%.
Instead of using a washed and bleached suspension of hydrated titanium oxide, originating from the conventional sulphate process production of titanium dioxide, as a starting material as described above, such material can also be produced in another way, for example by one of the variants described in GB 1 495 396, such as by hydrolyses of titanium alcoholates or other inorganic or organic titanium compounds. An especially preferable application form of the invention is to add to the hydrated titanium oxide, originating from the conventional sulphate process production of titanium dioxide, a tungsten compound, e. g. ammonium para tungsten, and/or vanadium compounds and/or other compounds which are supportive for catalytic activity, before the process step of flash calcination. These compounds may be added before, during, and after the steps of (partial) neutralisation and washing described above. It is also possible first to completely or partially dry the suspension partially neutralised, washed and blended with the compounds stated before, and then to introduce the material obtained this way into the flash calcination facility. The metal oxide powder has a tungsten oxide content (in the form of WO3) of preferably 0 to 20 w/w percent, or especially preferably of 5 to 15 w/w percent.
Another especially preferred application form of the invention is to add to the hydrated titanium oxide a silicon and/or aluminium compound and/or other compounds which are supportive for the thermal stability of the catalyst substances produced thereof, before the process step of flash calcination. It is an especially preferred method to add finely dispersed colloidal SiO . The metal oxide powder has an SiO2 content of preferably 0 to 20 w/w percent, especially preferably of 2 to 8 w/w percent. These compounds may be added before, during, and after the steps of (partial) neutralisation and washing described above. It is also possible first to completely or partially dry the suspension partially neutralised, washed and blended with the compounds stated before, and then to introduce the material obtained this way into the flash calcination facility.
The product in compliance with the invention and produced by the method described above has excellent characteristics, especially as a starting material for the production of catalysts. The average particle size of the primary particles (determined on the basis of their BET surface, and on the assumption that the particles have a spherical geometry and a density of 4.2, or established on the basis of electron microscopic photographs, respectively) amounts to approximately 20 nm. The particle size of the primary particles is predominantly determined by the temperature and retention time during calcination. The particle size of the primary particles corresponds to their high specific surface (BET surface), and is a prerequisite for a high activity of the catalysts to be produced from this nanocrystalline powdery material. The BET surface is determined in accordance with DIN 66131 (carrier gas method, single-point method, ratio between carrier gas and adsorptive 90:10, measuring gas nitrogen, adsorption at the temperature of the boiling nitrogen, on the assumption that one nitrogen molecule requires an area of 0.162 nm2, pre-treatment: heating in the nitrogen flow at 140°C for 1 hour).
A particle size measurement performed with a laser particle analyser (Mastersizer 2000 of Malvem Instruments Ltd.) delivers an average particle size (mean volume value) of approximately 1 μm (with conventional ultrasonic dispersion). This value describes solid aggregates, the size of which can essentially be determined by the flocculation structure of the starting product (hydrated titanium oxide), and which allow to be deflocculated by proportionally large forces only, such as by grinding for example.
Furthermore, agglomerates in an order of magnitude between 10 and 200 μm could be detected under the light microscope, the size of which is essentially determined by the kind of their introduction into the flash calcination reactor (e. g. by the type of nozzle). These aggregates or agglomerates, respectively, ensure good processability characteristics of the nanocrystalline metal oxides used as a starting material for catalysts.
A characterisation of the nanocrystalline metal oxides on the basis of their brightening capacity in accordance with DIN 55982, which is a common parameter in the field of pigment technology, shows substantially higher values for nanocrystalline metal oxides produced by flash calcination than those found with nanocrystalline metal oxides obtained by traditional calcination technologies. This proves that the characteristics of the nanocrystalline metal oxides have additional structural variations in the 0.2 μm range, which are characteristic of the production process. This allows the interpretation that the nanocrystalline metal oxides produced by flash calcination in compliance with the invention have a stronger structural coherence at meso level (agglomerates in a size range of approximately 200 nm) and comparable characteristics at micro level (primary particles, BET surface, micro porosity). hi order to determine the tinting strength, first the colour values L* (brightness), a* (red tone), and b* (blue tone), or the standard colour values Rx, Ry and Rz respectively, of a mixture consisting of the nanocrystalline metal oxide and a grey paste are determined in accordance with DIN 53192 (with Dataflash 2000 (d/8°), device A of Datacolor). The tinting strength is determined by analysing the values obtained this way, in accordance with DIN 55982. The C/2° (CIE 1931) type of light and standard observer are used.
TRONOX® R-KB-2, a commercially available micronised titanium dioxide pigment of Kerr-McGee in the form of the rutile modification, coated by aluminium and silicon compounds and an organic coating, is used as a reference material. The tinting strength of TRONOX® R-KB-2 is defined to be 100.
The chemical composition of the nanocrystalline metal oxides, the main component of which is titanium dioxide and which may optionally contain further constituents, allows to adapt them ideally to the requirements profile of the application desired. If the metal oxides are intended to be used as a starting material for catalysts, for example, a residual sulphate content and the addition of e. g. tungsten would be beneficial. An Na, K, and Fe content of less than 1,000 ppm, preferably less than 100 ppm would be as advantageous to the catalytic characteristics.
If the nanocrystalline metal oxides are, however, intended to be used as a starting material for complex mixed oxide pigments, a sulphate content as low as possible and the addition of other elements would be beneficial.
The TiO2 and WO3 content is determined in accordance with DIN 55912, Part 2, the sulphate content is determined in accordance with DIN 24935, the Na and K content is determined by HF microwave digestion in accordance with DIN 38406, Part 14, and the Fe content is determined in accordance with DIN 51083, Part 6.
In the absence of any additives (such as W, V, Si, Al for example), the titanium dioxide is completely provided in the form of its anatase modification. If additives as stated above are contained, the titanium dioxide has an anatase-type structure. The anatase modification is more advantageous to the catalytic characteristics, than the rutile modification.
The material produced in compliance with the invention and by the method described can be used as a starting material for the production of a wide range of chemico-technical or cosmetico-pharmaceutical products, besides their application as a starting material for the production of catalysts. Thanks to its nanocrystalline structure, the material allows to be used directly or after being conditioned in a suitable manner, if required, for applications requiring titanium dioxide which contains materials that are finely divided to a high degree, such as sun screening agents, or UV absorbers in plastics and coatings for example.
It is however also possible to use the nanocrystalline materials produced in compliance with the invention as a starting material for the production of colour pigments, ceramic products, electroceramic applications, enamels, welding electrodes, or other products, with the nanocrystalline structure allowing to be especially well mixed with other components at micro level, and showing an especially high reactivity during subsequent calcination or sintering processes. It is, in particular, possible to use the nanocrystalline titanium dioxide powders in compliance with the invention for applications the starting material of which would usually be a titanium dioxide with pigment characteristics, i.e. which has an average primary particle size of approximately 0.2 μm, and is exposed to a calcination step after mixing with other components. Mixing the nanocrystalline titanium dioxide powder with nickel, chromium, cobalt, zinc, antimony, niobium, tungsten, or other compounds and subsequently calcining the mix, for example, allows to produce high-grade complex metal oxide pigments, with the increased reactivity of the nanocrystalline titanium dioxide powder being especially advantageous in comparison with that of the titanium dioxide pigments conventionally used. The invention is explained in more detail through several application examples in the following text.
1st example
A washed and bleached suspension of hydrated titanium oxide (washed and bleached hydrolysate) with a TiO2 contents of 10 w/w percent, which is formed as a result of the conventional sulphate process titanium dioxide production, is partially neutralised in such a manner that the SO4 portion exceeding 1.6 w/w percent (related to TiO ) is stoichiometrically neutralised with NaOH at 80°C. This suspension is filtered and washed with deionised water until the Na2O content amounts to <100 ppm (related to TiO2) and the SO4 content is 1.5-2% (related to TiO ). The filter cake is then mixed with deionised water until a solids content of 22.8 w/w percent is achieved.
The viscosity, measured as the run-out time in accordance with DIN 53211, is 13 seconds (with a 4-mm nozzle).
The solids content is determined with a commercially available IR dryer.
The suspension of hydrated titanium oxide with a solids content of 22.8 w/w percent and a sulphate content of 1.6 w/w percent (related to TiO2) and a BET surface (after drying) of approximately 300 sqm/g is calcined in a high-temperature mixing chamber in an oxidising atmosphere at 700°C (temperature at the mixing chamber's exit) for 0.3 seconds. The suspension is introduced into the high-temperature mixing chamber via a two-fluid nozzle. The product obtained consists predominantly of isometric primary particles of 20 nm, which are predominantly agglomerated to spherulitic particles with an average diameter of 10 to 200 μm.
The characterisation of the product shows the following parameters:
TiO2 content 95.5 w/w percent
Sulphate content 1.6 w/w percent, related to TiO2 Fe, Na, and K content <100 ppm, related to TiO2
Anatase content 100%
BET surface 77 sqm/g Powder density 0.41 kg/1
Particle size distribution (laser diffraction) D[v,0.1] 0.6 μm
D[v,0.5] 1.0 μm D[v,0.9] 1.9 μm Tinting strength in ace. with DIN 55982 41
- Reference: TRONOX® R-KB-2
- Density of sample and reference: 4.05 g/cm3
- Pigment volume concentration (PVC): 0.5%
- Colour paste used: Black paste in ace. with DIN 53165 - Method: Constant colour paste concentration
- Standard colour values of the L* = 58.8 colour paste with sample (absolute): a* = -0.8 b* = -4.3
- In relation to TRONOX® R-KB-2: L* = -9.7 a* = +0.3 b* = -1.2
2nd example
A washed and bleached suspension of hydrated titanium oxide (washed and bleached hydrolysate) with a TiO contents of 10 w/w percent, which is formed as a result of the conventional sulphate process titanium dioxide production, is partially neutralised in such a manner that the SO4 portion exceeding 1.6 w/w percent (related to
TiO2) is stoichiometrically neutralised with NaOH at 80°C. This suspension is filtered and washed with deionised water until the NaO content amounts to <100 ppm (related to TiO2) and the SO4 content is 1.5-2% (related to TiO2). The filter cake with a solids content of 46 w/w percent is then mixed with deionised water and 1.08 w/w percent formic acid (related to TiO2) as a liquefier until a solids content of 37.5 w/w percent is achieved.
As formic acid is used, the viscosity of the 37.5 percent suspension is comparable to the viscosity of the 22.8 percent suspension without any liquefier. The viscosity, measured as the run-out time in accordance with DIN 53211, is
17 seconds (with a 4-mm nozzle).
The solids content is determined with a commercially available IR dryer.
This suspension of hydrated titanium oxide is calcined in a high-temperature mixing chamber in an oxidising atmosphere at 650°C (temperature at the reaction chamber's exit) for 0.3 seconds. The suspension is introduced into the high-temperature mixing chamber via a two-fluid nozzle. No increased organic residues could be detected in the product obtained this way, in comparison with the product of the 1st example (both products contain less than 0.05 w/w percent of carbon, related to TiO2).
This higher solids content of the suspension of hydrated titanium dioxide leads to an increase in the throughput at flash calcination of approximately 100%, and a reduction of the energy consumption by approximately 50%.
3 rd example ("for the purpose of comparison)
A washed and bleached suspension of hydrated titanium oxide (washed and bleached hydrolysate) with a TiO contents of 10 w/w percent, which is formed as a result of the conventional sulphate process titanium dioxide production, is partially neutralised in such a manner that the SO4 portion exceeding 1.6 w/w percent (related to TiO2) is stoichiometrically neutralised with NaOH at 80°C. This suspension is filtered and washed with deionised water until the NaO content amounts to < 100 ppm (related to TiO2) and the SO4 content is 1.5-2% (related to TiO2). The filter cake is then mixed with deionised water, without adding any liquefier, until a solids content of 37.5 w/w percent is achieved. A high viscosity substance is obtained, which does not allow to be pumped or atomised.
The viscosity, measured as the run-out time in accordance with DIN 53211, cannot be determined as the material does not leave the nozzle at all due to its high viscosity (with a 4-mm nozzle). The solids content is determined with a commercially available IR dryer.
Even if diluted up to a solids content of 27.5%, the material's viscosity is still so high that it cannot be determined by measuring the run-out time in accordance with DIN 53211 (with a 4-mm nozzle).
4th example (for the purpose of comparison') A suspension of hydrated titanium oxide as described in the 1st example (ref. first paragraph) is predried in a spray dryer and then calcined in an electric rotary kiln at approximately 600°C (temperature at the kiln exit). The retention time within the rotary kiln amounts to approximately 4 hours. The product obtained consists predominantly of isometric primary particles of about 20 nm, which are predominantly agglomerated to spherolitic particles with an average diameter of 10 to 200 μm.
The characterisation of the product shows the following parameters:
TiO content 95.7 w/w percent
Sulphate content 3.0 w/w percent, related to TiO2
Fe, Na, and K content <100 ppm, related to TiO2 Anatase content 100%
BET surface 87 sqm/g Particle size distribution (laser diffraction) D[v,0.1] 0.6 μm D[v,0.5] 1.0 μm D[v,0.9] 1.6 μm
Tinting strength in ace. with DIN 55982 9
- Reference: TRONOX® R-KB-2
- Density of sample and reference: 4.05 g/cm3
- Pigment volume concentration (PVC): 0.5%
- Colour paste used: Black paste in ace. with
DIN 53165
- Method: Constant colour paste concentration
- Standard colour values of the L* = 42.2 colour paste with sample (absolute): a* = +0.2 b* = -7.6
- In relation to TRONOX® R-KB-2: L* = -26.3 a* = +1.3 b* = -4.5

Claims

Claims:
1. Nanocrystalline metal oxide powder with at least 50 w/w percent of TiO2, preferably at least 75% w/w percent of TiO2, further with an average primary particle size of less than 50 nm, preferably of less than 35 nm, a BET surface of 25-250 sqm/g, preferably of 50-150 sqm/g, especially preferably of 75-100 sqm/g, an anatase content, as determined by x-ray analysis, of at least 95%, preferably of at least 99%, and with a tinting strength of 25-70, preferably of 30-50.
2. Metal oxide powder in accordance with claim 1, characterised by the fact that the metal oxide powder has a sulphate content of 0 to 7 w/w percent, preferably of 0.5 to 3 w/w percent, and an Na content, K content and Fe content of less than 1,000 ppm each, preferably of less than 100 ppm each.
3. Metal oxide powder in accordance with claim 1 and claim 2, characterised by the fact that the metal oxide powder has a tungsten oxide content (in the form of WO3) of 0 to 20 w/w percent, preferably of 5 to 15 w/w percent.
4. Metal oxide powder in accordance with one or several of the claims 1 to 3, characterised by the fact that the metal oxide powder has an SiO2 content of 0 to 20 w/w percent, preferably of 2 to 8 w/w percent.
5. Method for the production of a nanocrystalline metal oxide powder in accordance with one or several of the claims 1 to 4, characterised by the fact that it is produced by means of flash calcination technique with an average retention time of the particles within the reactor of less than 120 seconds, preferably of less than 10 seconds, most preferably of less than 2 seconds.
6. Method for the production of a nanocrystalline metal oxide powder, characterised by the fact that a nanocrystalline metal oxide powder with a TiO2 content of at least 50 w/w percent, a BET surface of 25 to 250 sqm/g, preferably of 50 to 150 sqm/g, especially preferably of 75 to 100 sqm/g, an average primary particle size of less than 50 nm, preferably of less than 35 nm, and an anatase content, as determined by x-ray analysis, of more than 95%, preferably of more than 99%, is obtained with the help of a flash calcination with an average retention time of the particles within the reactor of less than 120 seconds, preferably of less than 10 seconds, and especially preferably of less than 2 seconds.
7. Method in accordance with claim 5 or claim 6, characterised by the fact that the flash calcination is carried out within a directly or indirectly heated reaction chamber, with the gas temperature at the entry of the reaction chamber amounting to 800 to 1,200°C and the gas temperature at the exit of the reaction chamber being 500 to 800°C.
8. Method in accordance with claim 5 or claim 6, characterised by the fact that the introduction of the material to be calcined into the reactor is executed in counterflow to the hot gas, with the material to be calcined being subsequently carried over by the hot gas.
9. Method in accordance with one or several of the claims 5 to 8, characterised by the fact that a hydrated titanium oxide is used as the starting product for flash calcination as is formed as a result of the conventional sulphate process production of titanium dioxide pigments after the process step of hydrolysis or after the process step of bleaching.
10. Method in accordance with claim 9, characterised by the fact that the hydrated titanium oxide which is used as the starting product for flash calcination can be conditioned by additional specific process steps such as neutralisation, partial neutralisation, filtration and/or washing, adding of additives, drying, liquefaction, or other measures.
11. Method in accordance with claim 9 or claim 10, characterised by the fact that the hydrated titanium oxide for flash calcination is suspended and sprayed into the reaction chamber.
12. Method in accordance with claim 9 or claim 10, characterised by the fact that the hydrated titanium oxide for flash calcination is first completely or partially dried and then fed into the reaction chamber as a completely or partially dried product.
13. Method in accordance with claim 9 or claim 10, characterised by the fact that the hydrated titanium oxide is first suspended, then completely or partially dried, and fed into the reaction chamber as a completely or partially dried product for flash calcination.
14. Method in accordance with claim 11 or claim 13, characterised by the fact that the solids content of the suspension of hydrated titanium oxide is increased by adding liquefiers.
15. Method in accordance with claim 14, characterised by the fact that an amount of carboxylic acids of 0.01 to 5.0 w/w percent, preferably of 0.2 to 2.5 w/w percent, both related to TiO2, is added to the suspension of hydrated titanium oxide as a liquefier, in order to increase the suspension's solids content.
16. Method in accordance with claim 15, characterised by the fact that formic acid or acetic acid is used as a liquefier.
7. Application of the nanocrystalline metal oxide powder in compliance with one or several of the claims 1 to 4, or of a nanocrystalline metal oxide powder which was produced in compliance with one or more of the claims 5 to 16, as a starting material for the production of chemico-technical or cosmetico-pharmaceutical products, especially of catalysts, colour pigments, ceramic products, electroceramic applications, enamels, welding electrodes, cosmetic products, or UV protectants.
EP01965503A 2000-09-11 2001-09-11 Nanocrystalline metal oxide powders-production and application methods Withdrawn EP1317401A1 (en)

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