KR900004490B1 - Process for production of high purity zirconia powder from zircon powder - Google Patents

Abstract

A method for producing a zirconia powder comprises the steps of preparing a raw material mixture comprising a zircon powder and a powdery carbon-containing material; and heating the raw material mixutre at a temperature range from 1300 to 2000 deg.C in a nonoxidising atmoshpere of which the pressure is not higher than 0.7 bar to thereby decompose and gasify the silicon oxide component of the zircon powder.

Description

Process for producing high purity zirconia powder from zircon powder

1 is a graph showing the relationship between the C / SiO ₂ molar ratio in a zircon-carbon mixture and the amount of SiO 2 contained in the zirconia powder obtained by desilicon heat treatment under reduced pressure.

2 is a graph showing the relationship between the pressure of argon gas atmosphere and the residual amount of SiO ₂ contained in zirconia powder when the zircon-carbon mixture is heated.

The present invention relates to a method for producing a high purity zirconia powder which is heated by the zircon powder in the presence of carbon to be destabilized zirconia or stabilized and partially stabilized zirconia. Zirconia, or zirconium oxide, ZrO _2, has been used as a fireproof material in the steel and glass manufacturing industries because its melting point is more than 2,700 ° C. Recently, zirconia has been used in new and widespread applications in you. For example, it is used as an important raw material for manufacturing solid electrolytes, ceramic capacities or some other electronic devices useful as optical glass, new abrasives, piezoelectric elements, oxygen concentration sensors. Crystallographicly pure zirconia belongs to the monoclinic at normal temperature. However, it is not completely stable and reversibly converts into a tetragonal crystal structure at about 1,100 ° C with relatively large volume changes. Thus practically pure zirconia is usually destabilized zirconia. Due to the volume change accompanying the conversion, the sintered body of destabilized zirconia does not have high mechanical strength up to normal temperature. In order to solve this problem, it is known to add stabilized or partially stabilized zirconia having an equiaxed crystal structure by adding a stabilizing oxide such as CaO or Y ₂ O ₃ , which forms a solid solution with ZrO ₂ . In addition to the aforementioned uses of zirconia, stabilized or partially stabilized zirconia with high strength and toughness are expected to be used in the so-called ceramics industry. A common raw material for producing zirconia is zircon, a zirconium silicate mineral that decomposes into ZrO ₂ and SiO ₂ at about 1530 ° C. represented by the general formula ZrO ₂ · SiO ₂ or ZrSiO ₄ . Therefore, to produce zirconia from zircon, silica must be separated from zircon.

A known method for producing zirconia from zircon includes an arc melting method in which a stabilized zirconia is produced by adding a stabilized oxide to a mixture of zircon, carbon and iron scrap, and heating it in an arc. Si ₂ component is separated from ZrO ₂ component by decomposition and reduction reaction and solution reaction system, silicon oxide is lost in gaseous form or ferrosilicon is formed by reaction with iron. This method can produce zirconia at a relatively low price and is suitable for mass production. However, high purity zirconia cannot be obtained by this method, and the zirconia produced by this method is a hard block type, which consumes a large amount of energy to crush.

Another known method is the alkali dissolving method. In this method, a mixture of zircon and alkali is fused and reacted to produce alkali silicate and alkali zirconate. Silicates can be washed off. By the acid treatment method, the zirconate is converted to zirconium oxychloride ZrOCl ₂ , which is water-soluble, so it can be easily converted to zirconium hydroxide. The zirconium hydroxide thus obtained is heat treated to obtain zirconia in powder form. This method yields high-purity zirconia, but this method uses a rotary method that includes many types of reactions, resulting in low productivity and very high production costs. Alternatively, in Japanese Patent Application Publication Nos. 58-9808 and 58-15021, a granular mixture of zircon yarn and carbon powder to which a stabilizing oxide is added is placed in the presence of granular carbon located adjacent to granules of the raw material mixture. It is described to perform desilicon of zircon by heating in a non-oxidizing atmosphere. By such heat treatment, the silica component of zircon is vaporized in reduced form and then reacted with granular carbon. Thus, silicon carbide is obtained with zirconia. However, this method has a relatively low productivity, high energy value, and high purity zirconia because it is necessary to perform the reaction for a long time at a high temperature.

It is an object of the present invention to provide a method for the efficient and economical preparation of high purity zirconia powders which are destabilized zirconia or stabilized and partially stabilized zirconia from zircon powders. In addition, the present invention is to prepare a raw material mixture of zircon powder and powdery carbonaceous or carbonaceous material and to heat the raw material mixture at a temperature of 1,300-2,000 ° C. under a non-oxidizing atmosphere having a pressure of 0.7 atm or less, thereby minimizing oxidation of zircon powder. It provides a method for producing zirconia powder by decomposition or vaporization. In preparing stabilized or partially stabilized zirconia, the raw material mixture further contains at least one stabilized metal oxide or at least one metal salt which converts to such a metal oxide at a temperature below the temperature used in the aforementioned heating step.

The first feature of the present invention is the desilicon heat treatment of the zircon-carbon mixture under reduced pressure, the mechanism of the desilicon being represented by the following scheme.

In order to continuously proceed the reaction of Scheme (1) at atmospheric pressure, it can be seen that the reaction temperature should be maintained at a minimum of 1,750 ° C. by thermodynamic calculation in the reaction of Scheme (2). Difficulties in continuous reaction at low temperatures are mainly due to the saturation medium pressure of SiO being 6.3 × 10 ^-2 atm at 1,500 ° C and as low as 3.5 × 10 ^-3 atm at 1,300 ° C. When the desilicon heat treatment is carried out under reduced pressure, the thermodynamic lower limit of the temperature should not be more than 1,750 ° C. According to the calculation from the equation (2), when the reaction takes place at a pressure of 0.1 atm, the lower limit of the temperature should be 1,640 ° C., and also 1,540 ° C. or less at 0.01 atm and 1,450 ° C. or less at 0.001 atm. That is, the desilicon heat treatment temperature may be lowered to about 100 ° C. by first reducing the pressure. Therefore, the method according to the present invention can remove silica almost completely from zircon by heating at a relatively low temperature for a relatively short time. In the de-silicon heat treatment, the upper limit of the pressure is fixed to 0.7 atm in consideration of the method and actual operation convenience according to the amount of residual silicon in the produced zirconia, and heat treatment is performed at a pressure of 0.6 atm or less. In general, heat treatment at a temperature of 1,400-1,800 ° C. yields good results. This heating temperature range is preferable not only in terms of heat treatment efficiency but also in processing cost. In order to obtain a high purity zirconia powder, the mixing ratio of carbonaceous or carbon-containing to zircon powder is important and the optimum range is listed next. In other words, the molar ratio of SiO ₂ of C to zircon powder contained in the carbon content is 0.4-2.0. The carbon content contains several carbon compounds that vaporize at temperatures below about 1,000 ° C. and is ignored when calculating the carbon C / SiO ₂ molar ratio in these compounds. It is optional to use the raw material mixture in powder form prior to desilicon heat treatment or to mold the powder mixture into a suitable molded body such as pellets. In order to increase the efficiency of the desilicon heat treatment and improve the purity of the product, it is preferable to flow the at least part of the carbon content by appropriately heating the mixture or adding an appropriate amount of solvent in the step of preparing the raw material mixture.

In addition, the zirconia powder obtained by the desilicon heat treatment according to the present invention contains a small amount of ZrO or ZrC and a solid solution of such a compound, because the amount of slightly exceeded the theoretical amount is usually used to completely separate the silica from the zircon. This is because it is necessary to use carbon. Therefore, it is preferable to oxidize the zirconia powder obtained by heating the zirconia powder under an oxidizing atmosphere such as air at a temperature suitable for oxidizing ZrO and ZrC to ZrO ₂ . This oxidation treatment is effective to remove the carbon remaining in the zirconia powder in the gas phase.

As described above, zirconia powder with a purity of 98% by weight or more can be easily and surely obtained by the method of the present invention.

In the present invention, the raw material is not particularly limited. A zircon powder obtained by pulverizing zircon, which is usually used as a base material, may be used. In order to increase the de-silicon reaction ratio of the reaction formula (1), it is preferable to use a zircon powder made of fine particles. In order to obtain high purity zirconia, it is preferable to use a zircon powder having a low content of impurity and a carbon content having a low ash content. Carbonaceous or carbon-containing materials include almost pure carbon materials such as petroleum coke, coal coke and carbon black, incomplete carbon materials such as petroleum perch and coal pitch, and phenolic resins which are heated and carbonized in a non-oxidizing atmosphere. And oils such as polyethylene and polyvinyl alcohol. Mixing of the zircon powder and the powdery carbon content is carried out by a method in which mixing is well performed.

To prepare stabilized or partially stabilized zirconia, at least one stabilized metal oxide selected from MgO, CaO, Y ₂ O ₃ and CeO ₃ is added to the zircon powder and the carbon content. Instead of these metal oxides, salts of the same metal that turn into oxides when heated to a temperature below the temperature used for the next desilicon heat treatment can be used. Examples of suitable metal salts are MgCO ₃ , CaCO ₃ , Ca (OH) ₂ , YCl ₃ · 6H ₂ O and Ce (NO) ₃ · 6H ₂ O. The total amount of stabilized oxides is 0.5-2.0 mol% of ZrO ₂ contained in the zircon powder. If the amount of stabilized oxides is smaller, it is impossible to obtain complete stabilized zirconia. In addition, when the stabilizing oxide is 20 mol% or more of ZrO ₂ contained in the zircon powder, desilicon heat treatment of the raw material mixture forms a crystal phase of a complex oxide such as CaZr ₄ O ₁₂ or Zr ₃ Y ₄ O _{12 in} addition to the ZrO ₂ phase. When sintering zirconia powder containing such a hydroxide phase, the mechanical strength of the sintered body is not as high as expected.

The vacuum de-silicon heat treatment of the raw material mixture is performed by the method as described above. Non-oxidizing gas atmospheres are used to prevent oxidation of the carbon content. Usually nitrogen gas, argon gas or carbon monoxide gas is used. The heat treatment temperature range is 1,300-2,000 ° C, preferably 1,400-1,800 ° C. The heat treatment period is not limited, and in general, the higher the temperature, the shorter it becomes. If the temperature is in the preferred range of 1,400-1,800 ° C., the desilicon is usually in 0.5-10 hours.

As described above, it is desirable to increase the purity of the zirconia powder obtained by oxidative heat treatment in air, and the heating temperature range for the oxidation treatment is preferably 600-900 ° C, and it is usually sufficient to continue heating for 1-5 hours. Do.

Example 1

Using a ball mill, 100 parts by weight of zircon powder and 11.0 parts by weight of petroleum coke powder are mixed well. The zircon powder contains 98.9% by weight of ZrO ₂ · SiO ₂ , the average particle diameter is 0.97 μm, the petroleum coke powder contains 90% by weight of fixed carbon, and has a particle size of 44 μm or less. The mixture in powder form is heated to 1,700 ° C. for 2 hours under an argon gas atmosphere with a pressure adjusted to 0.7 atm. The zirconia powder obtained by desilicon treatment under reduced pressure is chemically analyzed and powder X-ray diffraction analysis is carried out to confirm the presence of the crystal phase.

The results are shown in the following Table 1a, where "ZrO ₂ (m)" represents monoclinic zirconia. Then, the zirconia powder was heated at 800 ° C. for 2 hours in the air to oxidize impurities, followed by chemical analysis. And powder X-ray diffraction is repeated.

The results are shown in Table 1a below.

Example 2-4

In these examples, the desilicon heat treatment is carried out in the same manner as in Example 1 except that the desilicon heat treatment is performed under other conditions as shown in Table 1a. The results obtained in each example are shown in Table 1A.

Comparative Example 1-5

Using the powder mixture described in Example 1, zirconia powder is prepared and purified by the method of Example 1, except that desilicon heat treatment is performed under various other conditions shown in Table 1a. In Comparative Example 1-4, heating is performed at a pressure of 0.7 atm or more, and in Comparative Example 5, the heating temperature is performed at 1,300 ° C or lower.

Comparative Example 6

Using the same raw material as in Example 1, zirconia powder is produced by the method described in the specification of Japanese Patent Application No. 58-15021 described above. That is, the powder mixture is granulated, the granules are heated together with the carbon granules, at which time the amount of coke added to the zircon powder is reduced to 7.3 parts by weight and heating is performed under the conditions shown in Table 1a.

The resulting zirconia powder is analyzed and then subjected to oxidation treatment as mentioned in Example 1 and analyzed again. The results are shown in Table 1a.

TABLE 1a

Comparing the data of Examples 1-4 and the data of Comparative Examples 1-5, it can be seen that the desilicon method of the present invention can completely remove silica from zircon even at a relatively low temperature for a relatively short period of time. The method of Comparative Example 6 shows a zirconia powder having a high content of ZrO ₂ , but it can be seen that a small amount of SiC is inevitably contained in the final product.

Example 5-9

In these examples, a stabilized or partially stabilized zirconia powder is prepared in a similar manner to that of Example 1. In each example, 100 parts by weight of zircon powder mentioned in Example 1 was mixed with 8.0 parts by weight of petroleum locus used in Example 1, and then an appropriate amount of stabilized oxide was mixed as shown in Table 1b, and in powder form. The mixture of is heated to 1,600 ° C. for 2 hours under an arcon gas atmosphere with a pressure of 0.01 atm. The resulting zirconia powder is analyzed and oxidized as in Example 1. The results are shown in Table 1b. In this table, "ZrO ₂ (c)" and "ZrO ₂ (m)" represent equiaxed zirconia and monoclinic zirconia, respectively, and purity represents the total ZrO ₂ and stabilized oxide component. The experimental data in Table 1b demonstrate that the desilicon heat treatment according to the invention is very effective when the raw material mixture contains a stabilizing oxide.

TABLE 1b

Experimental studies according to the present invention confirmed that the efficiency of the desilicon heat treatment depends on the ratio of carbon to silica of the raw material mixture and is in the optimum range of carbon and silica ratio. In other words, the best results are obtained when the raw material mixture is prepared using a molar ratio of C in the carbon material and SiO _{2 in} the zircon powder of 0.4-2.0. In these C / SiO ₂ calculations, carbon contained in hydrocarbons and other organic compounds, which vaporize at temperatures below about 1,000 ° C., is ignored because the amount of carbon is involved in silica decomposition at high temperatures. An experiment on the C / SiO ₂ molar ratio in the desilicon process according to the present invention is described below.

The zircon powder was 99.5% by weight and containing a ZrO ₂ · SiO _2, mean particle size used in this experiment is a 1 5㎛. Carbonaceous material is petroleum coke powder containing 90% by weight of fixed carbon and 0.3% of ash, the particle diameter is 10㎛ or less. These materials are mixed well in various proportions to produce various sample mixtures with a C / SiO ₂ molar ratio of 0.2-3.0. Using a press equipped with a metal die, each sample mixture is molded into pellets having a diameter of 20 mm and a depth of 20 mm. In an argon gas atmosphere where the pressure was reduced to 0.01 atm, after each sample pellet was heated at 1,750 ° C. for 1 hour, air oxidation was performed at 900 ° C. to purify the zirconia obtained by the desilicon heat treatment. Each sample is analyzed to determine the residual amount of SiO ₂ . The results of this experiment are shown in FIG.

As can be seen in FIG. 1, the residual amount of SiO ₂ in the resulting zirconia increases markedly when the C / SiO ₂ molar ratio is 0.4 or less, because Hanso is insufficient to completely decompose SiO ₂ contained in the zircon. However, the content of SiO _{2 in} the final product increases with a C / SiO ₂ molar ratio of 2.0. In this case, if the amount of carbon in the raw material mixture is sufficient, the total SiO ₂ of the zircon powder is reduced to SiO, but if the amount of carbon is excessively excessive, the reduction reaction is unnecessarily strong and zirconium silicides such as ZrSi and Zr ₅ Si ₃ Is formed. Zirconium silicide remains in the zirconia obtained by the vacuum treatment, which causes a significant amount of silica in the zirconia after oxidation treatment. In order to obtain zirconia powder with very low silica content, it is preferable to desilicon heat-treat the raw material mixture at a pressure of 0.6 atm or less, except that the C / SiO ₂ is constant at 1.3 and the pressure in the argon gas atmosphere for the heat treatment is wide. Experimental results similar to those described above are shown in FIG.

Example 10-15

These examples limit the C / SiO ₂ molar ratio to within 0.4-2.0 and illustrate the advantages of desilicon heat treatment at pressures of 0.6 atm or less.

The zircon powder used in these examples contains 99.5% by weight of ZrO ₂ .SiO ₂ and has an average particle diameter of 0.97 μm. The carbon material is petroleum coke powder containing 90% by weight of fixed carbon and 0.3% by weight of pollen, and the particle size is 44 μm or less. In each example, the zircon powder and the petroleum roux powder are mixed well under the ball in proportion as shown in Table 2 below to achieve the desired C / SiO ₂ molar ratio. The powder mixture is molded into pellets having a diameter of 20 mm and a length of 20 mm, and the pellets are heated under a reduced pressure argon gas atmosphere. In each example. Heat treatment conditions are as shown in Table 2. In each example, the zirconia powder obtained by the heat treatment under reduced pressure was subjected to ignition analysis and powder X-ray diffraction analysis to confirm the presence of the crystal phase. The zirconia powder is then heated at 900 ° C. for 2 hours in the atmosphere to oxidize the impurities, followed by repeated chemical analysis and powder X-ray diffraction analysis. The results are shown in Table 2. As can be seen in Table 2, when the molar ratio of silica in the petroleum forks powder is 0.4-2.0 in the fixed carbon zircon powder, a zirconia powder having a purity of 98% by weight or more and a SiO ₂ content of 1% by weight or less is obtained. Can be.

Comparative Example 7

Zirconia powder is prepared in the same manner as in Example 14, but the pressure of argon gas atmosphere for desilicon heat treatment is 1.0 atm. As shown in Table 2, the content of SiO _{2 in} the final product is significantly higher than in the case of Example 14 even if the C / SiO ₂ molar ratio in the raw mixture does not change. In the production of stabilized or partially stabilized zirconia powders, the addition of stabilizers to the starting material does not affect the C / SiO ₂ molar ratio limit of 0.4-2.0 and the choice of desilicon heat treatment at pressures below 0.6 atm.

TABLE 2

Example 16-24

In these examples, at least one stabilized oxide is added to the mixture of zircon powder and petroleum coke powder used in Examples 10-15. In addition, desilicon and oxidation treatments and analysis of the product are carried out as described in Examples 10-15. Table 3 shows the starting materials, the desilicon conditions and the analysis results of Examples 16-25. In Table 3, each purity is represented by total ZrO ₂ and stabilized oxide components. Each raw material mixture contained 100 parts by weight of zircon powder.

Comparative Example 8

As can be seen from Table 3, the starting materials and operating conditions of Comparative Example 8 were the same as in Example 20 except that the pressure of the argon gas atmosphere in the treatment in the desilicon treatment was 0.8 atm.

The difference in these results indicates that the SiO ₂ content in the final product is significantly increased.

TABLE 3

The zirconia powder obtained in Example 16-19 through oxidation treatment was press-molded into test pieces for bending test, respectively, and sintered at 1,500 ° C under air. The bending test results performed on these samples are shown in Table 4.

TABLE 4

The data in Table 4 demonstrates the effect of the presence of an appropriate amount of stabilized oxide in the zirconia powder by the strength of the zirconia sintered body.

It can be seen that it is important to uniformly mix the carbonaceous or carbonaceous material with the zircon powder in order to effectively separate the silica from the zircon by the reduced pressure heat treatment according to the present invention. If the carbon content is in the form of a fine powder, such as in the case of petroleum coke or carbon black, it is relatively easy to mix it uniformly with the zircon powder. However, using relatively coarse powders of carbonaceous materials such as coal pitch, polyvinyl alcohol or polyethylene, the mixing of these powders with zircon powders is simple, but a uniform mixture cannot be obtained, and zirconia powders obtained by desilicon heat treatment This results in a large amount of silica easily present. In this case, it is effective to flow at least a portion of the carbon content while mixing with the zircon powder. The flow can be accomplished by raising the temperature as appropriate or by mixing in the presence of a solvent for the carbon content. The two flow methods may be used in combination. The experiments on the effectiveness of the flow mixing method on the quality of the produced zirconia are described below.

The zircon powder used in this experiment containing ZrO ₂ · SiO ₂ of 99.5% by weight and average particle size has a 1.5㎛.

As the carbonaceous material, a petroleum roux powder containing 90% by weight of fixed carbon and having a particle size of 44 µm or less, a coal-permeable coal perch containing 70% by weight of fixed carbon and having a particle size of 50-200 µm, 100-200 µm Powdered polyvinyl alcohol having a particle size of and powdered polyethylene having a particle size of 100 to 200 µm are used. The amount of each carbon content is adjusted so that the C / SiO ₂ molar ratio in the resulting mixture is 1.3. In Examples Nos. 1,2,5 and 7, four kinds of carbon contents are mixed with zircon powder, respectively, without using a solvent at room temperature. In another embodiment using coal pitch, polyvinyl alcohol or polyethylene, mixing is carried out under heating or in the presence of a small amount of solvent for the carbon content. The results are shown in Table 5 below. The desilicon heat treatment of the raw material mixture in each run number is performed at 1,600 ° C. for 2 hours under an argon gas atmosphere having a pressure of 0.001 atm. The resulting zirconia powder is subjected to atmospheric oxidation at 800 ° C. for 2 hours. Each sample was analyzed and the residual amount of SiO ₂ was measured, and the results are shown in Table 5. As can be seen from this table, it can be seen that the flow effect of coal pitch, polyvinyl alcohol or polyethylene is excellent in terms of the efficiency of desilicon.

TABLE 5

When using the fluid mixing method, there is no change in that the C / SiO ₂ molar ratio in the blend mixture is limited to 0.4-2.0. The results of this experiment are shown in Table 6.

In this experiment, the zircon powder and the coal pitch powder used in the above-described experiments were mixed at various ratios at 150 ° C. so that the C / SiO ₂ molar ratio was 0.3-3.0. Each mixture was subjected to vacuum de-silicon under the same conditions as in the above experiments. Heat treatment and then atmospheric oxidation.

After de-silicon treatment, each sample was analyzed by powder X-ray diffraction to confirm the presence of a crystal phase.

After oxidation treatment, the sample is analyzed to determine the content of SiO ₂ .

TABLE 6

Example 25-32

The zircon powder used in these examples contains 99.6% by weight of ZrO ₂ .SiO ₂ and has an average particle diameter of 1.5 μm. The carbon content contains 90% by weight of fixed carbon, petroleum crude powder with a particle size of 44 μm, 66% by weight of fixed carbon, petroleum sebum powder with a particle diameter of 200 μm or less, and 75% by weight of carbon. It selects from the polyethylene powder which is 200 micrometers or less.

The gist of each example is shown in Table 7, where it can be seen that in some examples the mixing of zircon powder and petroleum pitch or polyethylene is carried out at elevated temperatures or in the presence of a solvent. In each case, the raw material mixture is molded into pellets having a diameter of 20 mm and a length of 20 mm, and the pellets are heated to 1,700 ° C. for 2 hours under an argon gas atmosphere having a pressure of 0.01 atm.

Desiliconed materials are analyzed to determine the purity of ZrO ₂ and to heat the atmosphere to 900 ° C. for 2 hours to oxidize the impurities. Finally, the obtained zirconia powder is analyzed to determine the SiO ₂ residual amount. The results are shown in Table 7 below.

Example 33-41

In these examples, stabilized zirconia powder was prepared by the same method as in Example 25-32. The zircon powder and carbon content used in Examples 25-32 are used in these examples and at least one stabilizing oxide selected from CaO, MgO, Y ₂ O ₃ and CeO ₂ is added as indicated in Table 8. In some embodiments, the mixing of the raw materials is carried out at elevated temperatures or with the addition of a small amount of solvent.

In each example, the raw material mixture is molded into the same pellets as in the above-described example, and subjected to reduced pressure desilicon heat treatment under the same conditions as in Examples 25-32.

After chemical analyzer powder X-ray analysis, the obtained zirconia powder is purified by oxidation treatment as described in Examples 25-32. Next, the remaining amount of SiO ₂ is analyzed. The results are shown in Table 8.

In Table 8, "ZrO ₂ (t)" is tetragonal zirconia, and ZrO ₂ purity is expressed as the total ZrO ₂ and stabilized oxide component.

TABLE 7

TABLE 8

Claims (13)

A raw material mixture of zircon powder and powdery carbonaceous material is prepared, and the raw material mixture is heated to a temperature of 1,300-2,000 ° C. under a non-oxidizing atmosphere having a pressure of 0.6 atm or less to decompose and vaporize micro-oxidized oxide in the zircon powder. Method for producing a zirconia powder characterized in that. The method according to claim 1, wherein the molar ratio of carbon contained in the carbon content and not evaporated at a temperature of 1,000 ° C. or lower in a non-oxidizing atmosphere and SiO ₂ contained in the zircon powder is 0.4-2.0. The method according to claim 2, wherein the pressure is 0.6 atm or less. The method according to claim 3, wherein the temperature is 1,400-1,800 ° C. The method according to claim 1, wherein the zirconia powder, which is a product of the reduced pressure heating step, is heated in an oxidizing atmosphere to oxidize unnecessary zirconium compounds contained in the product. The method according to claim 5, wherein the heating is carried out in an oxidizing atmosphere at a temperature of 600-900 ° C. The raw material mixture according to claim 1, characterized in that the raw material mixture consists of at least one metal oxide which forms a solid solution with zirconia and acts as a stabilizer of zirconia or a metal salt which converts to the metal oxide at a temperature below the temperature in a reduced pressure heating step. The manufacturing method to make. The method of claim 7, wherein at least one metal oxide is selected from CaO, MgO, Y ₂ O ₃ and CeO ₃ . 9. A process according to claim 8 wherein the total amount of at least one metal oxide is 0.5-20 mole% of ZrO ₂ contained in the zircon powder. The process according to claim 1, wherein the carbon content is selected from carbon black, petroleum roux, coal roux, petroleum pitch, coal pitch, phenol resin, polyvinyl alcohol and polyethylene. 11. A process according to claim 10, wherein the raw material mixture is prepared by leaving at least a portion of the carbon content in a fluid state. 12. The process of claim 11 wherein at least a portion of the carbonaceous material is flowed under heating. 12. The process according to claim 11, wherein at least a portion of the carbon content is flowed using a solvent for the carbon content.

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