Abstract:
Disclosed is a method for producing nano-sized titanium dioxide (TiO 2 ) ultrafine powder from titanium tetrachloride (TiCl 4 ) in the vapor phase by the gas phase oxidation reaction using flames, in which the method comprises: simultaneously introducing titanium tetrachloride (TiCl 4 ), vapor, argon, oxygen, hydrogen and air into a five-piped flame reactor to form a flame having a temperature of greater than 1,000° C.; and producing nano-sized titanium dioxide ultrafine powder having an average particle size of less than 50 nm.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a method for producing ultrafine powder, and more particularly, to a method for producing nano-sized titanium dioxide (TiO 2 ) ultrafine powder from titanium tetrachloride (TiCl 4 ) in the vapor phase by the gas phase oxidation reaction using flames.  
           [0003]    2. Description of the Related Art  
           [0004]    Nano-sized ultrafine powder, which refers to a powder having a particle size of less than 50 nm, is widely used as a new material due to its large specific surface area per unit weight and high activity.  
           [0005]    For example, nano-sized titanium dioxide ultrafine powder is used as a high-quality pigment and a photocatalyst, as well as cosmetics, medicines and coating material for transparent sound proof plates.  
           [0006]    There are two conventional methods to produce nano-sized titanium dioxide (hereinafter, referred to as “TiO 2 ”) ultrafine powder: a physical method that comprises vaporizing a metal by heating and condensing the metal vapor into ultrafine powder, and a chemical method that involves the chemical reaction of metal compounds.  
           [0007]    The physical method for producing nano-sized TiO 2  ultrafine powder requires great energy consumption for vaporization of the metal with the result of high production cost and low productivity but makes it possible to product high-purity powder. On the other hand, the chemical method provides low-purity powder at a low production cost with high productivity, and includes a gas phase method and a liquid phase method.  
           [0008]    Now, a description will be given as to a method for producing nano-sized TiO 2  ultrafine powder by the chemical method related to the present invention.  
           [0009]    In the preparation of nano-sized TiO 2  ultrafine powder by the gas phase chemical reaction method, it is necessary to provide a high temperature of at least 1000° C. and large gas flow rate. For this purpose, an approach for providing reaction conditions in the gas phase and high temperature using flames is known, wherein flame temperature, gas flow rate, concentration of reactants, and additives are critical reaction parameters that control the size and crystal form of the primary particles in the preparation of ultrafine powder.  
           [0010]    The conventional approach for producing ultrafine powder by the gas phase chemical reaction using flames is disclosed in U.S. Pat. No. 5,698,177 under the title of “Process for producing ceramic powders, especially titanium dioxide useful as a photocatalyst” as filed on Jun. 8, 1995; and U.S. Pat. No. 5,861,132 under the title of “Vapor phase flame process for making ceramic particles using a corona discharge electric field” as filed on Sep. 4, 1997.  
           [0011]    U.S. Pat. No. 5,698,177 suggests various methods in regard to the control of reaction parameters, the use of additives and the effect of the corona electric field formed over the burner of the reactor to produce TiO 2  powder for photocatalyst in the reaction system composed of TiCl 4 , air, and hydrocarbon-based fuel gas. The claims of this patent define the preparation of TiO 2  powder by the gas phase reaction of TiCl 4  and oxygen, the flame reactor used in the preparation, how to inject the sample, the added amount of sample and air, the voltage of the electric field, and the amount of additives.  
           [0012]    In addition, U.S. Pat. No. 5,861,132 discloses a process for producing powders of various metal oxides (e.g., silica, alumina, zirconia, etc.) including TiO 2  powder with various flame reactors (e.g., pre-mixed flame reactor, turbulent flame reactor, or larminar diffusion flame reactor) while providing the corona electric field over the reactor.  
           [0013]    In the preparation of nano-sized powder using the gas phase chemical reaction, the concentration of the sample in the reaction gas has to be considerably low and an excess of gas has to be introduced into the reaction region (flame) with increasing the added amount of the reactant in order to increase the yield of the nano-sized powder per unit time.  
           [0014]    However, the reaction system of the combustion gas composed of TiCl 4 , air and hydrocarbon introduced into the three pipes as suggested in U.S. Pat. No. 5,698,177 has a problem that the linear velocity of the gas increases in the burner due to an excess of air to reduce the retention time of the reactant, thus resulting in existence of non-reacted materials and incomplete combustion of the fuel gas.  
         SUMMARY OF THE INVENTION  
         [0015]    It is, therefore, an object of the present invention to solve the problem and to provide a method for producing nano-sized TiO 2  ultrafine powder from TiCl 4  using the reaction system of TiCl 4 -argon-hydrogen-oxygen-air, TiCl 4 -argon-hydrogen-air-air, or TiCl 4 -argon-hydrogen-oxygen/air-air with a five-piped turbulent diffusion flame reactor, the method comprising: maintaining a low concentration of the sample in the reaction gas, minimizing the amount of non-reacted materials and completely combusting the fuel gas (hydrogen) to increase the yield of the nano-sized powder per unit time.  
           [0016]    To achieve the above object of the present invention, there is provided a method for producing nano-sized TiO 2  ultrafine powder through oxidation with a mixed gas system passed through flames having a high temperature, the mixed gas system being composed of TiCl 4 -argon-hydrogen-oxygen-air, TiCl 4 -argon-hydrogen-air-air, or TiCl 4 -argon-hydrogen-oxygen/air-air and obtained from vaporization of a liquid reactant, TiCl 4 . In the preparation method, the TiO 2  ultrafine powder can have optimum particle size and crystal form by varying critical parameters such as the TiCl 4  content in the reaction gas, the flow rate and composition of the gas, and the like. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]    [0017]FIG. 1 is a schematic diagram of an apparatus for producing ultrafine powder as used in the present invention;  
         [0018]    [0018]FIG. 2 is an electron micrograph of the TiO 2  ultrafine powder produced with various concentrations of TiCl 4  in the reaction gas;  
         [0019]    [0019]FIG. 3 is a diagram showing the analysis results for the crystal structure of the TiO 2  ultrafine powder produced with various flow rates of oxygen; and  
         [0020]    [0020]FIG. 4 is a diagram showing the change in the average particle size and the crystal form of the TiO 2  ultrafine powder produced with various flow rates of oxygen. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0021]    Hereinafter, a description will be given in detail with reference to the accompanying drawings as to a method for producing nano-sized TiO 2  ultrafine powder while controlling the amount of TiCl 4  vapor, hydrogen, oxygen, air and argon injected into the flame reactor.  
         [0022]    [0022]FIG. 1 is a schematic diagram of an apparatus for producing the TiO 2  ultrafine powder as used in the present invention, in which the apparatus includes a sample vaporizer  10  for vaporizing liquid TiCl 4  as a reactant, and a five-piped burner  20  for producing flames through which the TiCl 4  vapor is passed to form nano-sized TiO 2  ultrafine powder by the oxidation reaction.  
       EXAMPLE 1  
       [0023]    This example is to control the particle size of the TiO 2  ultrafine powder produced by varying the concentration of TiCl 4  in the reaction gas.  
         [0024]    TiCl 4  (99.9%) as a liquid sample was injected into a vaporization container  11  of the sample vaporizer  10  with a syringe pump  12  while maintaining the temperature of a vaporization tank  13  at 180° C. After vaporization of TiCl 4 , the TiCl 4  vapor together with argon (Ar) as a carrier gas was injected into a first pipe  14  disposed in the center of the burner  20 . Argon, hydrogen, oxygen and air were injected into the five-piped burner  20  via second to fifth pipes  15 ,  16 ,  17  and  18  at the rate as presented in Table 1 to form flames, and Reynolds numbers at each tube are also presented in Table 1.  
         [0025]    The concentration of TiCl 4  in the gas injected into the burner  20  was controlled in the range of 1.13×10 −5  to 4.54×10 −5  mol/l. The flow rate of the gas injected into the five-piped burner was checked with eyes to maintain the stable flame state and controlled as presented in Table 1.  
                           TABLE 1                       Division   Gas   Flow Rate (λ/min)   N Re                     First Pipe   Argon and TiCl 4  (g)   2   16,500       Second Pipe   Ar   5   41,200       Third Pipe   Hydrogen   6    5,000       Fourth Pipe   Oxygen   15    90,600       Fifth Pipe   Air   60    272,500                   
 
         [0026]    The temperature distribution of the flame thus formed was measured with a R-type thermocouple. The flame temperature was constant at about 850° C. in the center of the burner and maximum (1700° C.) at around 7 mm from the center of the burner in the radius direction.  
         [0027]    The powder formed by changing the concentration of TiCl 4  in the reaction gas under the combustion conditions as shown in Table 1 was measured in regard to particle size and crystal structure. The average particle size was determined from the results of a BET analysis performed to measure the specific surface area of the particle per unit weight assuming that the particles are non-porous globular particles, according to a reduced equation (d p =6/(ρ p ·A, wherein ρ p  is the density (g/cm 3 ) of TiO 2 ; and A is the specific surface area (m 2 /g)).  
         [0028]    The average particle size of the TiO 2  ultrafine powder was increased from 19 nm to 28 nm with an increase in the concentration of the sample.  
         [0029]    [0029]FIG. 2 shows an electron micrograph of the nano-sized TiO 2  ultrafine powder produced by the above-described method (initial concentration of TiCi 4 : (a) 1.13×10 −5  mol/l, (b) 2.27×10 −5  mol/l, (c) 3.45×10 −5  mol/l, and (d) 1.54×10 −5  mol/l). As apparent from the figure, the particle size was increased with an increase in the concentration of the reactant in the almost same manner as the results of the BET analysis. An XRD analysis was performed to determine the crystal form of the TiO 2  powder, showing that about 45% of the powder was of the anatage type under the experimental conditions of the present invention.  
       EXAMPLE 2  
       [0030]    This example is to produce TiO 2  powder by reducing the flow rate of oxygen injected into the reactor to lower the temperature of the flame. For experimental conditions, the flow rate of oxygen injected into the fourth pipe  17  was reduced from 15 to 5 l/min from the gas injection conditions presented in Table 1. Subsequently, the flow rate of air injected into the fifth pipe  18  was increased to maintain the total flow rate constant so that the concentration of TiCl 4  in the reaction gas was maintained at a constant level (2.27×10 −5 mol/l). The maximum temperature of the flame in this case was lowered from 1700° C. to 1400° C.  
         [0031]    TiO 2  powder was produced under the experimental conditions, in which case the average particle size of the TiO 2  fine powder was reduced from 23 nm to 14 nm with a decrease in the flow rate of oxygen. Such a reduction of the average particle size resulted from the decreased growth rate of the particles due to agglomeration as the maximum temperature of the flame was lowered.  
         [0032]    The analysis results for the crystal form of the TiO 2  powder produced by the above-described method are illustrated in FIG. 3 (the flow rate of oxygen: (a) 15 l/min, (b) 10 l/min, and 5 l/min). It is apparent from the results of FIG. 3 that the anatage content hardly changed with a decrease in the flow rate of oxygen to 10 1/min but sharply increased at the flow rate of oxygen dropped to 5 l/min.  
         [0033]    A quantitative analysis of the anatage content revealed that the anatage content was 41%, 45% and 80% with a decrease in the flow rate of oxygen.  
       EXAMPLE 3  
       [0034]    This example is to analyze the particle size and the crystal form of TiO 2  powder produced under constant reaction conditions with various flow rates of oxygen injected into the reactor in the range from 4 to 8 l/min.  
         [0035]    For experimental conditions, the concentration of TiCl 4  was maintained at a constant level of 2.27×10 −5  mol/l, and the amount of the gas except for hydrogen was 5 l/min at the second pipe  15  for argon (Ar), 10 l/min at the fourth pipe  17  for air, and 65 l/min at the fifth pipe  18  for air.  
         [0036]    In this example, the gas injected into the fourth pipe  17  of the burner was air instead of oxygen in order to minimize the amount of oxygen. At this time, the temperature of the flam thus formed was measured with a change in the flow rate of hydrogen and the maximum temperature of the flame was varied from 1,300° C. to 1,000° C.  
         [0037]    TiO 2  powder was produced under the conditions, in which case the average particle size of the TiO 2  fine powder was reduced from 29 nm to 14 nm with a decrease in the flow rate of hydrogen from 8 l/min to 4 l/min. But, the average particle size was constant when the flow rate of hydrogen was less than 5 l/min.  
         [0038]    As for the crystal size of the TiO 2  powder, the anatage content was increased from 27% to 75% (FIG. 4) with a decrease in the flow rate of hydrogen from 8 l/min to 4 l/min.  
       EXAMPLE 4  
       [0039]    This example is to produce TiO 2  powder by mixing air with the oxygen gas injected into the fourth pipe  17  under conditions of Example 2. As for the experimental conditions, the concentration of TiCl 4  and the total flow rate were maintained as described in Example 2. The flow rate of hydrogen was 5 l/min in the third pipe  15 , while a mixture of oxygen having a flow rate of 4 l/min and air having a flow rate of 6 l/min was injected into the fourth pipe  17 .  
         [0040]    The TiO 2  powder thus obtained had an average particle size of 15 nm and an anatage content of 77%.  
         [0041]    As described above, the present invention uses a five-piped reactor in preparation of nano-sized TiO 2  ultrafine powder by the gas phase chemical reaction using flames, in which the TiO 2  ultrafine powder is produced in the reaction system of TiCl 4 -argon-hydrogen-oxygen-air, TiCl 4 -argon-hydrogen-air-air, or TiCl 4 -argon-hydrogen-oxygen/air-air, thereby providing a design data for large-scaled production.