JPH025688B2 - - Google Patents

Description

[Detailed description of the invention]

[Technical Field] The present invention relates to a method for producing high-purity zirconia (zirconium oxide: ZrO ₂ ) powder from zircon powder. This is a method for efficiently producing high-purity zirconia by heat-treating a mixture of the zircon powder and a carbon-containing material under reduced pressure. Zirconia (ZrO ₂ ) is an oxide with a high melting point of 2700°C or higher, and is used as a fireproof material in the steel and glass industries, and as a solid electrolyte in oxygen concentration measurement sensors, abrasive materials, and application materials. It is widely used in many fields. Furthermore, recently, the high strength and toughness features of stabilized zirconia or partially stabilized zirconia have been adopted in the field of engineering ceramics. [Prior art and its problems] The main known zirconia production technologies at present include the carbon desiliconization arc furnace melting method and the alkali melting method. The above manufacturing method uses carbon CaO in zircon sand.
By adding stabilizers such as zircon and iron scraps, heating in an arc furnace, and reducing and melting, the SiO in zircon is volatilized into the gas _phase , or reacted with iron to form ferrosilicon, ZrO This method separates the _two components and simultaneously dissolves a stabilizer such as CaO in ZrO ₂ to obtain stabilized zircon. Although this method yields inexpensive zircon and is suitable for large-scale production, it has the problem of not being able to obtain highly pure zircon powder. Furthermore, melting in an arc furnace requires a considerable high temperature, which takes time, and energy is also required to crush the obtained zirconia block, which poses a problem from the viewpoint of energy saving. In the above manufacturing method, zircon sand and an alkali are melted and reacted to remove the SiO ₂ in the zircon as an alkali silicate, while the ZrO ₂ is converted into sodium zirconate, which is then subjected to processes such as acid treatment. via zirconium oxychloride (ZrOCl ₂ )
Make it. This zirconium chloride is soluble in water, and is converted into zirconium hydroxide through pH adjustment, followed by heat treatment to obtain zirconia. This method yields highly purified zirconia with a purity of 99% or more compared to the above-mentioned manufacturing method, but the disadvantage is that the manufacturing process is long, resulting in poor productivity and very high costs. [Objective and Summary of the Invention] The object of the present invention is to overcome the above-mentioned problems of the conventional technology with respect to the method of producing zirconia from zircon powder, and to provide a method of producing high-purity zirconia at low cost and efficiently. It's about doing.
For this purpose, the present inventors investigated the conventional carbon desiliconization method for producing zirconia from zircon, and found that it was difficult to efficiently remove SiO vapor during desiliconization of zircon. Therefore, they came up with a new technique of desiliconization by heat treatment under reduced pressure, and completed the present invention. That is, the gist of the present invention is, firstly, that the zircon powder and the carbon-containing material are mixed in a ratio such that the molar ratio of silica to carbon (C/SiO ₂ ) in the zircon powder is in the range of 0.4 to 2.0. A method for producing zircon powder, characterized in that the silica content in the zircon powder is volatilized and removed by heat-treating the mixed powder or its compact under reduced pressure of 0.6 atmospheres or less;
and secondly, the zircon powder and the carbon-containing material are mixed in a molar ratio of silica to carbon in the zircon powder (C/
SiO ₂ ) is mixed at a ratio in the range of 0.4 to 2.0, and the mixed powder or its compact is heat-treated under reduced pressure of 0.6 atmospheres or less to volatilize and remove the silica content in the zircon powder, and then This is a method for producing zirconia powder, which is characterized by subjecting it to oxidation treatment. [Conception and Structure of the Invention] When zircon powder and a carbon-containing material are mixed and heat treated at high temperature, a desiliconization reaction proceeds according to the following equation (1). SiO ₂ (l, s) + C (s) → SiO (g) + CO (g) ... (1) Thermodynamic temperature necessary for the reaction of equation (1) to proceed continuously under 1 atm. The conditions are a high temperature of 1750℃ or higher. However, if the vacuum desiliconization method described above is adopted, the thermodynamic lower limit of the temperature required to proceed with the reaction of equation (1) is 1750°C under 1 atm; 1640 under 0.1 atmosphere,
1540℃ under 0.01 atm, 1450℃ under 0.001 atm
Therefore, it can be seen that if the pressure is lowered by one order of magnitude, the lower limit of the heat treatment temperature required for desiliconization will be lowered by around 100°C.
This shows that if reductive desiliconization with carbon is performed under reduced pressure, zirconia can be efficiently produced from zircon powder by heat treatment at low temperature and for a short time. In the present invention, the desired effect aimed at in the present application can be obtained simply by mixing zircon powder and a carbon-containing material and heat-treating the mixed powder or its molded product under reduced pressure to desiliconize it. If heat treatment is subsequently carried out, a zirconia powder of higher purity can be produced. The zircon powder used in the method of the present invention may be pulverized zircon sand, but the finer the powder is, the better, in order for the reaction of formula (1) to proceed rapidly. In addition, in order to obtain high purity zirconia,
It is better to reduce the amount of impurity components other than ZrO ₂ and SiO ₂ as much as possible. The carbon-containing materials used in the present invention are not particularly limited, but include carbon materials such as coal coke and petroleum coke, as well as coal pitch, petroleum pitch, and organic resins such as phenol resins, which are carbonized at high temperatures. Carbon-containing materials may also be used. In the present invention, the above-mentioned zircon powder and carbon-containing material are mixed, and the mixed powder or the compact of the mixed powder is heat-treated for desiliconization under reduced pressure. It is necessary to do enough for it to progress completely. In the present invention, zircon powder and carbon-containing mixed powder can be subjected to depressurization and heat treatment for desiliconization as they are, or the mixed powder can be formed into a pellet body formed into a predetermined size. There is no difference in effectiveness even after heat treatment. The heat treatment temperature for the above-mentioned vacuum desiliconization is not particularly limited, but the reaction rate,
Considering the heat treatment cost, etc., the temperature is set in the range of 1400 to 1800°C.Furthermore, in the present invention, after the heat treatment for vacuum desiliconization, unreacted substances are completely oxidized to further increase the purity. Oxidation treatment is performed to make it into a material. That is, the purpose of this treatment is to oxidize lower oxide ZrO ₂ and carbide ZrC, etc. other than zirconia (ZrO ₂ ) generated after the desiliconization treatment to zirconia. The temperature of oxidation treatment is 600-900
A temperature range of 0.degree. C. is preferred. In the present invention, the mixing ratio of the zircon powder and the carbon-containing material is expressed as a molar ratio (C/ _SiO2 ) between the amount of silica in the zircon powder and the amount of carbon in the carbon-containing material, and is in the range of 0.4 to 2.0. The heat treatment for desiliconization is carried out under a reduced pressure of 0.6 atmospheres or less, and these ranges were determined by the experiments shown below. C/SiO ₂ is produced from ZrO ₂ +SiO ₂ with an average particle size of 1.5 μm and zircon powder containing 99.5 Wt% of SiO 2 and petroleum coke (90% fixed carbon, 0.3% ash) crushed to less than 10 μm.
Samples with different molar ratios were prepared, thoroughly mixed, and then molded into pellets of 20 mmφ x 20 mm using a molding machine. Samples with a C/SiO ₂ (molar ratio) in the range of 0.2 to 3.0 were prepared, desiliconized at 1750°C for 1 hour in a reduced pressure atmosphere of 10 ^-2 atmospheres (the remainder being Ar), and then oxidized at 900°C. The effect of C/SiO ₂ (molar ratio) in the mixture of zircon powder and carbon-containing material on desiliconization was investigated by analyzing the amount _of SiO 2 in the desiliconization process. It is involved in the reaction with SiO ₂ in
The amount of carbon contained in hydrocarbons that volatilize at low temperatures is excluded. Furthermore, C/SiO ₂ (molar ratio)
For the sample with a value of 1.3, the same heat treatment for desiliconization was performed at different pressures in the range of 1 atm to 10 ^-5 atm, and the effect of pressure on desiliconization was investigated. Figures 1 and 2 show the effects of C/C in the mixture of zircon and carbon-containing substances on the residual amount of SiO ₂ , respectively.
The influence of SiO ₂ (molar ratio) and pressure during desiliconization heat treatment is shown.
As is clear from Figure 1, C/SiO ₂ (molar ratio)
If it is smaller than 0.4, there will be insufficient carbon to completely desiliconize zircon, and zircon will remain even after the desiliconization heat treatment, resulting in an increase in the amount of SiO ₂ remaining. On the other hand, if the C/SiO ₂ (molar ratio) is greater than 2, the amount of carbon is sufficient to reduce the SiO ₂ in zircon and remove it as SiO, but the amount of carbon is too large and the zircon becomes too reducible.
This is not good because zirconia metal silicides such as ZrSi and Zr ₂ Si ₃ are generated, the amount of residual Si increases, and the amount of SiO ₂ in the final obtained ZrO ₂ increases.
Therefore, in order to maintain good purity of zirconia,
It is necessary to limit the amount of silica in zircon and the amount of carbon in the carbon-containing material to be within the range of 0.4 to 2.0, expressed as C/SiO ₂ (molar ratio). Also, regarding the pressure during de-siliconization under reduced pressure, as shown in Figure 2, if it exceeds 0.6 atmospheres,
Since the residual amount of SiO ₂ increases, it is necessary to limit the pressure to 0.6 atmospheres or less. The reduced pressure atmosphere used in carrying out the present invention is preferably a non-oxidizing gas atmosphere such as N ₂ , Ar, or CO in order to avoid burnout due to oxidation of carbon-containing materials. In the present invention, most of the crystals produced by the reduced pressure heat treatment are ZrO ₂ , and some ZrO, ZrC, etc. are produced depending on the mixing ratio of zircon and carbon. In such a case, after the heat treatment for desiliconization, oxidation treatment is performed to convert ZrO, ZrC, etc. into ZrO ₂ to achieve even higher purity. In the present invention, in order to completely desiliconize the silica in zircon, a carbon-containing material may be blended into zircon so that the amount of carbon is slightly in excess of the molar ratio corresponding to silica. In such a case, depending on the heat treatment conditions for desiliconization, ZrO, ZrC,
Furthermore, a solid solution of these is generated. In short, the oxidation treatment in the present invention is a treatment performed to oxidize ZrO and ZrC to ZrO ₂ . At the same time, if there is carbon remaining after the heat treatment, it is also oxidized and removed by volatilization into the gas phase. The temperature during the oxidation treatment is suitably in the range of 600 to 900°C. [Example] Average particle size with a total content of ZrO ₂ and SiO ₂ of 99.5%
Various mixtures with different C/SiO ₂ (molar ratios) as shown in Table 1 were prepared from 0.97 μm zircon powder and petroleum coke (90% fixed carbon, 0.3% ash) crushed to 44 μm or less. After thorough mixing in a ball mill, molded bodies of 20 mmφ x 20 mm were prepared using a mold forming machine. These molded bodies were subjected to vacuum desiliconization treatment under the heat treatment conditions shown in Table 1, and after the heat treatment, the existing crystal phase was identified by powder X-ray diffraction and the ZrO ₂ purity was analyzed. Furthermore, oxidation treatment was performed in the atmosphere at 900℃, and the crystals also present were identified.
The ZrO ₂ purity was analyzed, and the amount of SiO ₂ remaining in the zirconia powder was also analyzed. The results of these analyzes are also shown in Table 1. 1st
As is clear from the table, the molar ratio of silica in zircon powder to fixed carbon in petroleum coke (C/
SiO ₂ ) is 0.4 to 2.0, ZrO ₂ purity is 97 Wt% or more after oxidation treatment by performing heat treatment for desiliconization under reduced pressure within the range of 0.6 atm or less.
High purity zirconia powder of 98Wt% or more can be obtained,
In addition, in the final zirconia powder,
The residual amount of SiO ₂ was also 1.0 Wt% or less in the case of the example.

【table】

[Table] [Effects of the Invention] As described above, according to the present invention, inexpensive high-purity zirconia powder can be efficiently produced.

[Brief explanation of drawings]

Figure 1 is a graph showing the relationship between the molar ratio of SiO ₂ in the zircon powder to carbon in the carbon-containing material (C/SiO ₂ ) and the residual amount of SiO ₂ in the zirconia powder. 2 is a graph showing the relationship between the pressure during heat treatment and the amount of SiO ₂ remaining in zirconia powder.

Claims (1)

[Claims] 1. Zircon powder and carbon-containing material are combined in such a way that the molar ratio of silica to carbon (C/SiO ₂ ) in the zircon powder is
A zirconia powder characterized in that the silica content in the zircon powder is removed by volatilization by mixing the powder in a ratio ranging from 0.4 to 2.0 and heat-treating the mixed powder or its compact under reduced pressure of 0.6 atmospheres or less. Production method. 2 The zircon powder and the carbon-containing material are mixed in such a way that the molar ratio of silica to carbon (C/SiO ₂ ) in the zircon powder is
The silica content in the zircon powder is removed by volatilization by mixing the powder in a ratio ranging from 0.4 to 2.0, heat-treating the mixed powder or its compact under reduced pressure of 0.6 atmospheres or less, and then oxidizing the powder. A method for producing zirconia powder, the method comprising: