CN100431971C - Method for preparing titanium dioxide powder by flame hydrolysis - Google Patents

Abstract

Flame-hydrolytically produced titanium dioxide powder that is present in the form of aggregates of primary particles, and has a BET surface of 20 to 200 m/g, a half width (HW) [nm] of the primary particle distribution of HW = a x BET<f> where a = 670x10<9> m<3>/g and -1.3 <= f <= -1.0 and the proportion of particles with a diameter of more than 45 mum lies in a range from 0.0001 to 0.05 % by weight. The powder is produced by a process in which a titanium halide is vapourised at temperatures of less than 200 DEG C, the vapours are transferred to a mixing chamber by means of a carrier gas of defined moisture content and, separately from this, hydrogen, primary air, which may optionally be enriched with oxygen and/or preheated, and steam are added to the mixing chamber, following which the reaction mixture is combusted in a reaction chamber sealed from the ambient air, secondary air is in addition introduced into the reaction chamber, the solid is then separated from gaseous substances, and following this the solid is treated with steam. The titanium dioxide powder may be used for the heat stabilisation of polymers.

Description

Method for preparing titanium dioxide powder by flame hydrolysis

The present invention relates to titanium dioxide powder prepared by flame hydrolysis and its production and use.

Titanium dioxide is known to be produced by a pyrogenic process. Fire methods are understood to include flame oxidation or flame hydrolysis. In flame oxidation, a titanium dioxide precursor, such as titanium tetrachloride, is oxidized with oxygen according to equation 1a. In flame hydrolysis, titania formation is accomplished by hydrolysis of titania precursors, the water required for hydrolysis being derived from the combustion of a fuel gas such as hydrogen and oxygen (Eq. 1b).

TiCl ₄ +O ₂ →TiO ₂ +2Cl ₂ (equation 1a)

TiCl ₄ +2H ₂ O→TiO ₂ +4HCl (Equation 1b)

EP-A-1231186 discloses titanium dioxide having a BET surface area between 3 and 200 m ² /g and a weight-related _D90 particle size of 2.2 microns or less. In its examples it is mentioned that the _D90 diameter is between 0.8 and 2.1 microns. In addition, titanium dioxide with a BET surface area between 3 and 200 m ² /g and a distribution constant n of 1.7 or more, calculated according to the formula R=100exp(-bD ⁿ ), where D represents particle diameter and b is a constant. The value of n is obtained from three values D ₁₀ , D ₅₀ and D ₉₀ , which are approximately linearly related to each other. Titanium dioxide is obtained by flame oxidation of titanium tetrachloride and an oxidizing gas, the raw materials being preheated to a temperature of at least 500° C. before the reaction. In a preferred embodiment, the velocity of the reaction mixture is 10 m/sec or higher and the residence time in the reaction space is 3 seconds or less.

In EP-A-778812 a process for the production of titanium dioxide by a combination of flame oxidation and flame hydrolysis is described. In this connection, titanium tetrachloride and oxygen in a vapor state are mixed in a reaction zone, and the mixture is heated in a flame generated by combustion of hydrocarbon as a fuel gas. Titanium tetrachloride is fed into the central part of the reactor, oxygen is fed into the jackets surrounding the central part, and fuel gas is fed into the jackets surrounding those pipes which carry titanium tetrachloride and oxygen.

Preference is given to using a laminar diffusion flame reactor. In this method, highly surface-active titanium dioxide powder can be produced mostly in the anatase modification. In EP-A-778812 no information is given on the structure and size of primary particles and aggregates. However, these amounts are important for many applications, for example in cosmetic applications or as abrasives in dispersions in the electronics industry. The formation mechanism of titanium dioxide according to EP-A-778812 includes flame oxidation (equation 1a) and flame hydrolysis (equation 1b). Although different formation mechanisms can control the proportion of anatase, they cannot achieve a specific distribution of primary particles and aggregates. Other disadvantages of this process, as mentioned in US-A-20002/0004029, are the incomplete conversion of titanium tetrachloride and fuel and the gray color of the resulting titanium dioxide.

According to US-A-20002/0004029 these problems are now eliminated by using five pipes instead of the three pipes described in EP-A-778812. For this purpose, titanium tetrachloride vapor, argon, oxygen, hydrogen and air are simultaneously metered into the flame reactor. The disadvantages of this method are the use of the expensive noble gas argon and the low yield of titanium dioxide due to the low concentration of titanium tetrachloride in the reaction gas.

Titanium dioxide powder produced by flame hydrolysis has long been sold by Degussa under the designation P25.

It is a fine-grained titanium dioxide powder with a specific surface of 50 ± 15 m ² /g, an average primary particle size of 21 nm, a compacted bulk density (approximately) of 130 g/l, an HCl content of less than or equal to 0.300% by weight, and Residue according to Mocker (4.5 microns) is lower than or equal to 0.050%. This powder has good properties for many applications.

The prior art has shown widespread interest in pyroproduced titanium dioxide. In this regard, the commonly used general term "fire process", ie flame hydrolysis and flame oxidation, is not found to be an adequate description of titanium dioxide. Due to the complexity of the fire method, only some material parameters can be adjusted specifically. In particular, titanium dioxide is used in catalysis (eg photocatalysis), cosmetics (eg sunscreens), abrasives in the form of dispersions in the electronics industry, or for thermal stabilization of polymers. In these uses, the requirements on the purity and structure of titanium dioxide are increasing. Thus, for example, when using titanium dioxide as an abrasive in a dispersion, it is important that the titanium dioxide has good dispersibility and is as free as possible of coarse particles that could scratch the surface to be polished.

The object of the present invention is to provide titanium dioxide powder which has high purity, is easily dispersed, and contains as little as possible of coarse particle parts.

The object of the present invention is also to provide a preparation method of the titanium dioxide powder. In this regard, the method should be able to be implemented on an industrial scale.

The invention provides titanium dioxide powder obtained by flame hydrolysis in the form of primary particle aggregates, characterized in that:

- it has a BET surface area of 20 to 200 m ² /g, and

- the half-width HW (in nm) of the primary particle distribution has the following values:

HW[nm]=a×BET ^f , where a=670×10 ^-9 m ³ /g and -1.3≤f≤-1.0, and

- The proportion of particles with a diameter greater than 45 microns is in the range of 0.0001 to 0.05% by weight.

The term primary particles in the context of the present invention is understood to mean particles which are formed first in the reaction and which are able to aggregate to form aggregates during the subsequent course of the reaction.

The term aggregate in the context of the present invention is understood to mean primary particles of similar structure and size that have been bound together and whose surface area is smaller than the sum of the individual, individual primary particles. Several aggregates or also individual primary particles can further combine together to form agglomerates. Thus, aggregates or primary particles are adjacent to each other in the form of point objects. Depending on the extent of their agglomeration, the agglomerates can be broken up by applying energy.

On the other hand, aggregates can only be broken up by a high energy input, or not even broken up at all. Intermediate forms exist.

The average half-width HW of the primary particle distribution (number distribution) is obtained by image analysis of TEM photographs. According to the invention, the average half-width is a function of the BET surface area and the constant f, where -1.3≤f≤-1.0. Preferably, the half-width is in the range of -1.2≤f≤-1.1.

For example, a high BET surface area, a narrow distribution of primary particles and a low proportion (i.e. between 0.0001 and 0.05% by weight) of aggregates with a diameter greater than 45 μm are associated with the front surface of powders according to the invention when the surface is polished. properties are relevant. In the prior art, there is no titanium dioxide powder produced by flame hydrolysis that exhibits these characteristics at the same time. Of course, for example, powders according to the prior art can be largely removed mechanically from aggregates with a diameter greater than 45 μm, but the resulting powder will not meet the requirements of the invention with regard to the BET surface area and the half-width value of the primary particles scope of protection.

The BET surface area of the titanium dioxide powder according to the invention is in the broad range of 20 to 200 m ² /g. It has proven to be advantageous if the BET surface area is in the range of 40 to 60 m ² /g. A range of 45 to 55 m ² /g is particularly advantageous.

For titanium dioxide powders according to the invention with a BET surface area between 40 and 60 m ² /g, 90% of the number distribution of primary particle diameters is distributed between 10 and 100 nm. Typically, 90% of the number distribution of primary particle diameters is distributed between 10 and 40 nm.

Furthermore, the equicircular diameter (ECD) of such titania powder aggregates may be less than 80 nm.

Titanium dioxide powders according to the invention having a BET surface area between 40 and 60 m ² /g may have an average aggregate area of less than 6500 nm ² and an average aggregate perimeter of less than 450 nm.

Additionally, the BET surface area of the titanium dioxide powder according to the invention may be in the range of 80 to 120 M ² /g. A range of from 85 to 95 m ² /g is particularly preferred.

For titanium dioxide powders according to the invention with a BET surface area of between 80 and 120 m ² /g, the 90% distribution of the number distribution of primary particle diameters may have values between 4 and 25 nm. Furthermore, such titanium dioxide powder may have an aggregate equicircular diameter (ECD) of less than 70 nm.

Titanium dioxide powders according to the invention having a BET surface area between 80 and 120 m ² /g may have an average aggregate area of less than 6000 nm ² and an average aggregate perimeter of less than 400 nm.

The proportion of titanium dioxide powder aggregates and/or agglomerates according to the invention having a diameter greater than 45 μm is in the range of 0.0001 to 0.05% by weight. A range from 0.001 to 0.01% by weight may be preferred, and a range from 0.002 to 0.005% by weight may be particularly preferred.

The titanium dioxide powder according to the invention contains rutile and anatase as crystal forms. In this regard, the anatase/rutile ratio may range from 2:98 to 98:2 for a given BET surface area. The range from 80:20 to 95:5 may be particularly preferred.

The titanium dioxide powder according to the invention may contain chloride residues. The chloride content is preferably below 0.1% by weight. Titanium dioxide powders according to the invention having a chloride content in the range from 0.01 to 0.05% by weight may be particularly preferred.

The compacted bulk density of the titanium dioxide powder according to the invention is not limited. However, it has proven to be advantageous if the compacted bulk density has a value of from 20 to 200 g/l. A compacted bulk density of 30 to 120 g/l may be particularly preferred.

The present invention also provides a method for preparing titanium dioxide powder according to the present invention, characterized in that:

- evaporation of a titanium halide, preferably titanium tetrachloride, at a temperature below 200° C., the vapor of said titanium halide being conveyed to the mixing chamber by means of a carrier gas in which the content of water vapor is in the range of 1 to 25 g/m ³ in, and

independently of this, feeding hydrogen, optionally oxygen-enriched and/or preheated primary air and water vapor into the mixing chamber,

- wherein said primary air has a water vapor content in the range of 1 to 25 g/m ³ ,

- the lambda value is in the range 1 to 9, and the gamma value is in the range 1 to 9, next,

- a mixture of titanium halide vapor, hydrogen, air and water vapor is ignited in a burner and the flame burns back into a reaction chamber isolated from the surrounding air, where

- a vacuum of 1 to 200 mbar is present in said reaction chamber,

- the outlet flow velocity of the reaction mixture from said mixing chamber to said reaction space is in the range of 10 to 80 m/sec,

- Additionally, secondary air is introduced into the reaction chamber, wherein

- the ratio of primary air to secondary air is between 10 and 0.5,

- followed by separation of solids from gaseous substances, and

- Subsequent treatment of the solid with water steam.

An essential feature of the method according to the invention is to vaporize the titanium halide at a temperature below 200° C. and to deliver the titanium halide vapor into the mixing chamber by means of a carrier gas, for example air or oxygen with a defined carrier gas moisture content. For example, it has been found that the quality of the product decreases at higher gasification temperatures.

In addition, it has also been found that when the content of water vapor in the gas or primary air is in the claimed range of 1 to 25 g/ ^m3 , there is no significant hydrolysis of the titanium halide in agglomerated form, while on the other hand the water vapor content affects the subsequent primary particle and aggregate structures. Powders according to the invention cannot be obtained outside the claimed scope. In a preferred embodiment, the water vapor content of the gas or primary air is between 5 g/m ³ and 20 g/m ³ .

Air can also be used as carrier gas. This allows a higher space-time yield of the reaction chamber than when using inert gases.

Furthermore, the outlet flow velocity of the reaction mixture from the mixing chamber into the reaction space is in the range of 10 to 80 m/sec. In a preferred embodiment, the outlet flow velocity is between 15 and 60 m/sec, and in a particularly preferred embodiment, between 20 and 40 m/sec. Below this value, a homogeneous powder is not obtained, but a powder comprising particles with a diameter of 45 micrometers or more in an amount above 0.05% by weight is obtained.

In addition, the reaction must be carried out so that the lambda value is in the range of 1 to 9 and the gamma value is in the range of 1 to 9.

Oxides produced by flame hydrolysis are generally obtained such that the gaseous starting materials are stoichiometrically relative to each other such that the added hydrogen is at least sufficient to react with the halogen X present in the titanium halide _TiX4 to form HX. The amount of hydrogen required for this is referred to as the stoichiometric amount of hydrogen.

The ratio of added hydrogen to that required for the stoichiometry specified above is termed gamma. γ is defined as:

γ = hydrogen added/hydrogen required stoichiometrically, or

γ = feed _H2 (moles)/stoichiometric _H2 (moles)

In addition, in the case of oxides produced by flame hydrolysis, generally an amount of oxygen (for example from air) is used which is at least sufficient to convert the titanium halide to titanium dioxide and any excess hydrogen which may still be present to water. This amount of oxygen is referred to as the stoichiometric amount of oxygen.

Similarly, the ratio of added oxygen to stoichiometrically required oxygen is termed λ and is defined as follows:

λ = Oxygen added/Oxygen required stoichiometrically, or

λ = feed _O2 (moles)/stoichiometric _O2 (moles)

Furthermore, in the method according to the invention, air (secondary air) is introduced directly into the reaction chamber in addition to the primary air in the mixing chamber. It has been found that the titanium dioxide powder according to the invention cannot be obtained without adding additional air to the mixing chamber. In this connection, it should be noted that the ratio of primary air to secondary air is between 10 and 0.5. This ratio is preferably in the range of 5 to 1.

In order to be able to meter the amount of secondary air precisely, it is necessary to burn the flame back into the reaction chamber which is sealed off from the surrounding air. This enables precise control of the process, which is necessary in order to obtain the titanium dioxide according to the invention. The vacuum in the reaction chamber is preferably between 10 and 80 mbar.

It is also an essential feature that the titanium dioxide powder after separation from gaseous substances should be treated with water vapor. This treatment is mainly used to remove halogen-containing groups from the particle surface. At the same time, this treatment reduces the number of agglomerates. The treatment can be carried out continuously, whereby steam is used to treat the powder convectively or cocurrently (possibly together with air), insofar as water vapor is always introduced into the vertical heatable column from below. Powder feeding can be done from the top or bottom of the column. The reaction conditions can be chosen so that a fluidized bed is formed. The temperature for the treatment with steam is preferably between 250 and 750°C, with values from 450 to 550°C being preferred. In addition, it is preferable to perform the treatment in a countercurrent manner so as not to form a fluidized bed.

Additionally, it may be advantageous to introduce water vapor into the mixing chamber together with the air.

Figure 1A schematically represents an arrangement for implementing the method according to the invention. In the figure: A=mixing chamber, B=flame, C=reaction chamber, D=solid/gas separation, E=post-treatment with water vapor. The substances used are identified as follows: a = mixture of titanium halide and carrier gas with specified moisture content, b = hydrogen, c = air, d = water vapor, e = secondary air, f = water vapor or water vapor/air . Figure 1B shows a portion of the layout of Figure 1A. In this figure, steam (d) is introduced into the mixing chamber together with air (c). FIG. 1C shows an open reaction chamber in which secondary air e is sucked in from the surroundings. With the layout according to FIG. 1C it is not possible to obtain titanium dioxide powder according to the invention.

The invention also provides the use of titanium dioxide powder according to the invention for heat protection stabilization of silicone resins.

In addition, the present invention also provides the use of the titanium dioxide powder according to the present invention in a sunscreen.

Furthermore, the present invention also provides the use of the titanium dioxide powder according to the invention as catalyst, catalyst support, photocatalyst and abrasive for the production of dispersions.

Example

analyze

The BET surface area is determined according to DIN 66131.

The compacted bulk density is determined based on DIN ISO 787/XI K 5101/18 (unscreened).

Bulk density is determined according to DIN-ISO 787/XI.

The pH value is determined based on DIN ISO 787/IX, ASTM D 1280, and JIS K 5101/24.

The proportion of particles larger than 45 microns is determined according to DIN ISO 787/XVIII, JIS K5101/20.

Determination of the chloride content: Weigh out approximately 0.3 g of the granules according to the invention precisely, add 20 ml of 20% sodium hydroxide solution (analytical grade), dissolve and transfer into 15 ml of cold _HNO3 with stirring. The chloride content in the solution was titrated with AgNO ₃ solution (0.1 mole/l or 0.01 mole/l).

The half-width of the primary particle distribution and aggregate area, perimeter and diameter were determined by image analysis. Image analysis was performed using Hitachi's H 7500 TEM instrument and SIS's MegaView II CCD camera. The image magnification used for the evaluation was 30000:1 at a pixel density of 3.2 nm. The number of particles evaluated was greater than 1,000. Prepared according to ASTM3849-89. The low threshold bound in terms of detection is 50 pixels.

Embodiment A1 (according to the present invention)

At 140°C, 160 kg/hr of TiCl ₄ was vaporized in the evaporator. The TiCl ₄ vapor was transferred into the mixing chamber by using nitrogen (15 Nm ³ /hr) with a moisture content of 15 g/m ³ carrier gas as the carrier gas. Independently of this, 52 Nm ³ /hr of hydrogen and 525 Nm ³ /hr of primary air were introduced into the mixing chamber. In the central tube, the reaction mixture is fed into a burner and ignited. The flame burns in a water-cooled flame tube. In addition, 200 Nm ³ /hr of secondary air was added to the reaction space. The powder formed was separated in a downstream filter and then convectively treated with air and steam at 520°C.

Examples A2 to A9 according to the invention were carried out analogously to example A1 . The parameters changed in each case are listed in Table 1.

The physicochemical data of the powders from Examples A1 to A9 are presented in Table 2.

Comparative Examples B1 to B3 and B5 to B8 were also carried out analogously to Example A1. The parameters changed in each case are listed in Table 1.

Comparative Example B4 was carried out using an open burner. The amount of secondary air was not determined.

The physicochemical data of the powders from Examples B1 to B9 are presented in Table 2.

Table 3 shows the half widths of the primary particles calculated from the BET surface area at f=-1.0, -1.05, -1.15 and -1.3. The factor 10 ^-9 is used to convert meters to nm. The factor f can only be negative, and the unit of BET ^f is g/m ² .

Fig. 2 shows the half-widths of the primary particles of the titanium dioxide powder produced in the examples. In this regard, titanium dioxide powders according to the invention (identified as ■) are within the claimed range, ie the half width HW[nm]=a×BET ^f , where a=670×10 ⁻⁹ m ³ /g and − 1.3≤f≤-1.0, while the comparative example (marked as +) is not within the claimed scope.

Table 3: Calculated Primary Particle Half Widths

Thermal Stabilization of Polymers

Example C1: no titanium dioxide powder (comparative example)

LSR 2040的两组分硅酮橡胶作为基础组分(加成交联)。在使用溶解装置均匀混合该两组分后，在180℃下硫化10分钟。生产出6毫米厚的样品板(大约10×15cm)。将样品板在加热炉中于80℃下加热至恒重(大约1天)。为了检查对加热的热稳定性，进行热贮存测试。为此，将尺寸为5×7cm的样品带保持在275℃的循环空气烘箱中。测量重量损失。Use the trade name Silopren purchased from Bayer

Two-component silicone rubber of LSR 2040 as base component (addition crosslinking). After uniformly mixing the two components using a dissolution apparatus, vulcanization was performed at 180° C. for 10 minutes. Sample panels (approximately 10 x 15 cm) of 6 mm thickness were produced. The sample plate was heated to constant weight (approximately 1 day) in an oven at 80°C. In order to check the thermal stability to heating, a thermal storage test was performed. For this, sample strips with dimensions 5×7 cm were kept in a circulating-air oven at 275° C. Measure weight loss.

Example C2: Addition of titanium dioxide powder according to the prior art (comparative example)

LSR 2040的两组分硅酮橡胶作为基础组分(加成交联)。使用溶解装置向这些组分之一中加入1.5重量％(以总批料计)的二氧化钛粉末P 25 S(Degussa AG)5分钟。接着，按照实施例1所述进行样品板的硫化和生产。Use the trade name Silopren purchased from Bayer

Two-component silicone rubber of LSR 2040 as base component (addition crosslinking). To one of these components was added 1.5% by weight (based on the total batch) of titanium dioxide powder P 25 S (Degussa AG) for 5 minutes using a dissolution apparatus. Next, the vulcanization and production of the sample panels was carried out as described in Example 1.

Sample strips of 5 x 7 cm were stored at 275°C. Measure weight loss.

Examples C3-5 were carried out analogously to C1, but in C3 titanium dioxide powder A1 according to the invention was used, in C4 A3 was used and in C5 A7 was used instead of P25 S. Table 4 shows the change in length of the samples after storage at 275°C for 1, 3 and 7 days.

The results show that an effective thermoprotective stabilization of the polymer is achieved by using the titanium dioxide powder according to the invention.

Table 4: Two-component silicone rubber

photocatalytic activity

Example D1: Titanium dioxide powder according to the prior art (comparative example)

To determine the photocatalytic activity, the sample to be measured was suspended in 2-propanol and irradiated with UV light for 1 hour. Then, the concentration of acetone was measured.

Approximately 250 mg (accuracy 0.1 mg) of titanium dioxide powder P 25S (Degussa AG) was suspended in 350 ml (275.1 g) of 2-propanol using an Ultra-Turrax mixer. The suspension was delivered by means of a pump thermostatically controlled to 24° C. with coolant into a glass photoreactor equipped with a radiation source and pre-filled with oxygen. By way of example, a TQ718 (Heraeus) type Hg medium-pressure immersion lamp with an output of 500 watts was used as the radiation source. The borosilicate glass protective tube limits the emitted radiation to wavelengths >300nm. The radiation source is externally surrounded by flowing water cooling tubes. Oxygen is metered into the reactor by means of a flow meter. The reaction begins when the radiation source is turned on. At the end of the reaction, a small amount of the suspension was immediately removed, filtered and analyzed by gas chromatography.

The measured photoactivity k is 0.68×10 ^-3 mole kg ^-1 min ^-1 . This value is taken as base value 1. The titanium dioxide powder according to the invention has a slightly lower photocatalytic activity of 0.8 to 0.9.

Claims (3)

1. prepare the method for titania powder by flame hydrolysis, it is characterized in that:

-evaporate halogenated titanium being lower than under 200 ℃ the temperature, by the content of water vapor wherein 1 to 25g/m ³Scope in carrier gas described steam is transported in the mixing section, and

-therewith independently, hydrogen, optional primary air and the water vapor that is rich in oxygen and/or preheating are transported in the described mixing section,

In-wherein said the primary air content of water vapor 1 to 25g/m ³Scope in,

The ratio lambda value of the oxygen that-oxygen that adds and stoichiometry are required is in 1 to 9 scope, and the ratio γ value of the required hydrogen of the hydrogen of interpolation and stoichiometry is in 1 to 9 scope,

Next,

-in burner, light the mixture of forming by halogenated titanium steam, hydrogen, air and water vapor, and flame returns in the reaction chamber that completely cuts off with ambient air, wherein

-in described reaction chamber, there is a vacuum of 1 to 200 millibar,

The exit velocity of-reaction mixture from described mixing section to described reaction compartment in 10 to 80m/sec scope,

-in addition, in described reaction chamber, introduce secondary air, wherein

The ratio of-primary air and secondary air between 10 to 0.5,

-then, from the gaseous mixture of described reaction chamber discharging, isolate solid, and

-use the described solid of steam-treated subsequently.

2. the method for claim 1 is characterized in that described halogenated titanium is a titanium tetrachloride.

3. the method for claim 1 is characterized in that described water vapor is introduced in the described mixing section with described primary air.

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