GB2255349A - Process for producing chromium metal - Google Patents

Abstract

Process for the manufacture of metallic chromium which has a purity of at least 99 %, characterised in that it includes a first stage of mixing chromium oxide with carbon or a carbon compound, moulding the mixture into a compact product which has an apparent density of 0.5 to 3.0 g/cm<3>, and heating the compact product in a vacuum not exceeding 6.67 x 10<3> Pa (50 mm Hg) to a temperature of 1200 to 1500 DEG C in order to obtain a reaction product (crude chromium) which has an oxygen content not exceeding 7 % and a carbon content not exceeding 5.3%; and a second stage of pulverising the crude chromium to a particle size not exceeding 841 mu m (20 mesh), moulding the latter into a compact product which has an apparent density of 2.0 to 6.0 g/cm<3>, and then heating the compact product in a vacuum not exceeding 6.67 x 10<3> Pa (50 mm Hg) to a temperature of 1200 to 1500 DEG C.

Description

PROCESS FOR PRODUCING CHROMIUM METAL The present invention relates to a process for producing chromium metal. More particularly, it relates to a process for producing chromium metal from chromium oxyhydroxide or chromium oxide.

Heretofore, various methods have been known for the production of chromium metal. They include an electrolytic method wherein an aqueous solution of chromium and ammonium alum is reduced, a method wherein chromium oxide is reduced by silicon or aluminum, and a method wherein chromium oxide is reduced under vacuum by using carbon or a carbon compound as a reducing agent.

Among them, the electrolytic method requires time and costs for the production, whereby the production costs will be high. The reduction method by means of silicon or aluminum is a batch system, whereby the quality of the product varies substantially. It has further drawbacks that the reducing agent, the furnace material, etc., are likely to be included in the product, whereby the quality will be low and that the yield of chromium metal is rather low. Further, the carbon reduction method of chromium oxide has a problem that chromium metal thereby obtained contains substantial amounts of oxygen and carbon.

Under these circumstances, the present inventors have conducted an extensive research on the production of chromium metal. As a result, it has been found possible to obtain chromium metal of a high purity by reducing treatment in an extremely short period of time by using chromium oxyhydroxide as the starting material.

On the basis of this discovery, the present invention provides a process for producing chromium metal, which comprises incorporating carbon and/or a carbon compound to chromium oxyhydroxide, followed by heat treatment under at least one condition selected from the group consisting of a reduced pressure atmosphere, a reducing gas atmosphere and an inert gas atmosphere.

As a result of a further research on a method for preparing chromium metal by reducing chromium oxide with carbon, it has been found that the reduction treatment can be conducted in a short period of time, and an evaporation loss of chromium can be minimized by conducting the carbon reduction in two steps and specifying the reduction conditions.

On the basis of this discovery, the present invention provides a process for producing chromium metal having a purity of at least 99%, which comprises a first step of mixing chromium oxide and carbon or a carbon compound, molding the mixture to a compact having a bulk density of from 0.5 to 3.0 g/cm3, and heating the compact under a vacuum of at most 50 mmHg at a temperature of from 1,200 to 1,5000C to obtain a reaction product (crude chromium) having an oxygen content of at most 7% and a carbon content of at most 5.3%, and a second step of pulverizing the crude chromium to a particle size of at most 20 mesh, molding it to a compact having a bulk density of from 2.0 to 6.0 g/cm3, and then heating the compact under a vacuum of at most 50 mmHg at a temperature of from 1,200 to l,5000C.

Now, the present invention will be described in detail with reference to the preferred embodiments.

In the accompanying drawings: Figure 1 is a graph showing powder X-ray diffraction spectra of chromium metals obtained in Examples 1 to 4.

Figure 2 is a graph showing powder X-ray diffraction spectra of chromium metals obtained in Comparative Examples 1 and 2.

In the first aspect of the present invention, chromium oxyhydroxide is used as the starting material.

As such chromium oxyhydroxide, the one obtainable by the method disclosed in EP 450307 A, a method of treating an aqueous chromium chloride solution with an ammonium chloride-containing gas, or a method of treating chromium hydroxide for dehydration, may optionally be used.

Howe-v- r, preferred is the one obtainable by the method disclosed in EP 450307 A.

As type A, CrOOH obtainable by contacting an aqueous solution of sodium dichromate (Na2Cr207 2H2O) with a reducing agent such as carbon or carbon monoxide gas, may be mentioned. Otherwise, as type B, a mixture of CrOOH and C, which contains a substantial amount of C as a reducing agent, may be mentioned.

When the reduction reaction is conducted by means of type A, there is a method of obtaining chromium metal by carbon reduction as represented by the following formula: 1) 2CrOOH + 3C g 2Cr + H20 + 3CO or a method of obtaining chromium metal by gas reduction as represented by the following formula: 2) 2CrOOH + 3H2 ' 2Cr + 4H20 Further, as a combination thereof, there is a method wherein the majority of oxygen is reduced with carbon, and a trace amount of residual oxygen is reduced by hydrogen gas, as follows: 3) CrOOH + C/H2 > Cr + CO/H2O As a reducing agent for the method of carbon reduction, a carbon powder such as graphite, carbon black or a carbonaceous material such as oil coke, or a carbon compound such as Cr23C6, Cr7C3 or Cr3C2, is mixed by a suitable method.Such chromium oxyhydroxide (CrOOH) and the reducing agent are in powder forms. There is no particular restriction as to the particle size. However, in msSt cases, a fine powder is preferred.

In the case of the carbon reduction, the blending proportion is preferably from 90 to 110%, more preferably from 97 to 103%, of the stoichiometrical amount. If the amount is less than this range, the reduction reaction tends to be inadequate. On the other hand, if it exceeds this range, a substantial amount of carbon tends to remain in the product, such being undesirable.

Then, the powder mixture or a molded product obtained by molding chromium oxyhydrochloride by means of a binder such as polyvinyl acetate, polyvinyl alcohol or starch, is introduced into a heat treatment furnace and subjected to a gas reduction reaction while maintaining the temperature at a level of at least 1,2000C in a reduced pressure atmosphere under a pressure in the furnace of from 0.05 to 100 mmHg, in a reduced pressure reducing atmosphere by a reducing gas such as hydrogen or in a normal pressure atmosphere of an inert gas such as argon.

On the other hand, when the reduction reaction is conducted by means of type B, the carbon content is influential over chromium metal thereby produced. For example, if the carbon content exceeds the stoichiometrical amount, the reduction product will be a mixture of chromium metal and chromium carbide. On the other hand, if the carbon content is less than the stoichiometrical amount, reduction of chromium oxyhydroxide tends to be inadequate.

erefore, if the carbon content in the starting material for the reduction reaction is not the stoichiometrical amount, it is preferred to adjust the starting material to bring the carbon content to a level of the stoichiometrical amount by adding e.g. a carbon content such as graphite, coke or Cr23C6 or an oxygen content such as CrO3 or CrO3.

Then, such a starting material is molded by an addition of a binder similar to the one described with respect to type A and then subjected to a reduction reaction in the same manner as in the case of type A.

With either type A or type B, the molded product may be heat-treated in one step continuously to calcination at-a high temperature. However, heat treatment in a two step or three step system is also preferred in which a preliminarily heating operation is conducted at a temperature of from 300 to 5000C-to decompose an intermediate decomposition product, and further the binder is decomposed at a temperature of 800 to 1,0000C to complete the calcination, as the case requires. The treating time may be adjusted depending upon the relation and the temperature rise.

The calcination temperature in this reaction is not particularly restricted for the production of chromium metal of a desired grade. To obtainchromium metal of a high quality, the obtained reduction product may further be subjected to such treatment as pulverization and molding, followed by second reduction.

Thus, by virtue of a large specific surface area and fine particle size of CrOOH, it is possible to obtain chromium metal having high purity in an extremely short period of time as compared with chromium metal obtainable by the electrolytic method or conventional reduction of chromium oxide.

Now, the first aspect of the present invention will be described in further detail with reference to Examples. However, it should be understood that the present invention is by no means restricted by such specific Examples.

EXAMPLE 1 1,000 g of CrOOH (type A) powder and 187.5 g of coke powder were mixed by a usual powder mixer, and the mixture was further kneaded by means of a 15% polyvinyl alcohol aqueous solution. This kneaded powder was molded by a usual hydraulic press, and the obtained molded product was dried in air, then put into a heating furnace and maintained at 9000C for 30 minutes and then at 1,4000C for 90 minutes. During this operation, the vacuum in the furnace was maintained to be 5 mmHg. After cooling, the obtained calcined product was subjected to a powder X-ray diffraction analysis, whereby as shown in Figure 1, only the Cr peak was identified, and Cr03 and Cr23C6 were not identified. Further, the analytical values (LECO-CS244, TC136) of remaining C and 0 were 0.2so and 0.30%, respectively.

EXAMPLE 2 1,000 g of CrOOH (type A) powder and 187.5 g of coke powder were mixed in the same manner as in Example 1, and the mixture was further kneaded by means of a 10% polyvinyl alcohol aqueous solution. This kneaded powder was molded, and the molded product thereby obtained was dried, then put into a heating furnace and maintained at 9000C for 30 minutes and then at 1,4000C for 90 minutes.

During this operation, the vacuum in the furnace was maintained to be 10 mmEg. After cooling, the sample was further pulverized and molded, and the molded product was put into a heating furnace and treated at 1,4000C for 120 minutes under a vacuum of 5 mmEg.

The obtained calcined product was subjected to a powder X-ray diffraction analysis, whereby a Cr crystal monophase was identified, and as the same analytical result as in Example 1, the product was confirmed to be highly pure chromium containing little impurities at a level of 0.01% of C and 0.03% of O.

EXAMPLE 3 1,000 g of CrOOH (type B) powder containing a carbon content larger than the stoichiometric amount and 55.6 g of Cr2O3 containing an oxygen source required to remove the excess carbon content by a reaction formula of C + 0 CO, were mixed, and the mixture was kneaded by means of a 10% polyvinyl alcohol aqueous solution and molded.

The folded product thereby obtained was dried and then put into a heating furnace and maintained at 9000C for 30 minutes and then at 1,3500C for 60 minutes. During this operation, the vacuum in the furnace was maintained at a level of 5 mmHg. The obtained calcined product was subjected to a powder X-ray diffraction analysis, whereby only a Cr peak was identified, and from the same analytical results as in Example 1, C was 0.28%, and 0 was 0.35%.

EXAMPLE 4 100 g of CrOOH (type B) powder containing a carbon content substantially equal to the stoichiometric amount was kneaded with a 15% polyvinyl alcohol aqueous solution, followed by molding. The molded product thereby obtained was dried and put into a heating furnace; and maintained at 9000C for 30 minutes and then at 1,4000C for 40 minutes under a vacuum of 5 mmHg.

The obtained calcined product was subjected to a powder X-ray diffraction analysis, whereby only a Cr peak was identified, and as a result of the same analysis as in Example 1, C was 0.3%, and 0 was 0.3%.

COMPARATIVE EXAMPLE 1 1,000 g of commercially available Cr203 (for pigment) and 237 g of coke powder were mixed, and the mixture was further kneaded by means of a 10% polyvinyl alcohol aqueous solution, followed by molding. The molded product thereby obtained was dried, then put into a heating furnace and maintained at 9000C for 30 minutes and then at 4000C for 120 minutes under a vacuum of 5 mmEg.

The obtained calcined product was subjected to a powder X-ray diffraction analysis, whereby as shown in Figure 2, Cr and Cr23C6 peaks were identified, and as a result of the same analysis as in Example 1, C was 1.65%, and 0 was 1.76%.

COMPARATIVE EXAMPLE 2 1,000 g of commercially available Cr203 (for pigment) and 237 g of coke powder were mixed, and the mixture was further kneaded by means of a 10% polyvinyl alcohol aqueous solution, followed by molding. The molded product thereby obtained was dried, then put into a heating furnace and maintained at 9000C for 30 minutes and then at 1350C for 240 minutes under a vacuum of 5 mmHg.

The obtained calcined product was subjected to a powder X-ray diffraction analysis, whereby Cr and Cr23C6 peaks were identified, and as a result of the same analysis as in Example 1, C was 1.93%, and 0 was 2.23%.

As described in the foregoing, according to the first aspect of the present invention, chromium metal can be obtained extremely simply in a short period of time by the method of reducing chromium oxyhydrochloride.

Now, referring to the second aspect of the present invention, the chromium oxide as the starting material in this .llerhed may be an oxide of trivalent chromium such as chromium tri or dioxide or chromium oxyhydrochloride.

However, it is preferred to use chromium oxyhydrochloride. The starting material chromium oxide may contain volatile components which can be removed during the process of the present invention, such as carbon, water and low volatile substances or decomposable organic substances, so long as they can be removed.

As carbon or a carbon compound to be used as the carbon source for reduction in the process of the present invention, a carbonaceous material such as graphite, carbon black or oil coke, or a chromium carbide such as Cr23C6, Cr2C3 or Cr3C2, or other carbon compounds can be used.

The starting material chromium oxide and carbon or a carbon compound are preferably in powder forms, and fine powders are particularly preferred.

As the binder to be used in the present invention, polyvinyl alcohol, polyvinyl acetate, polyvinyl butyral, starch, dextrin or a resin may be mentioned.

The mixing ratio at the time of mixing the carbon source to chromium oxide in the first step of the process of the present invention, may be from 90 to 110%, preferably from 97 to 103% of the stoichiometrical amount represented by the formula (1) or (2): Cr203 + 3C = 2Cr + 3CO (1) 2CrOOH + 3C = 2Cr + 3CO + H2O (2) Tf the ratio is less than 90%, the reduction reaction tends to be inadequate, and if it exceeds 110%, carbon tends to remain in the product, such being undesirable.

A binder is added to the powder mixture, and the mixture is then kneaded. The amount of the binder varies depending upon the particle sizes or the physical properties of the chromium oxide and the carbon source.

It is usually from 0.1 to 5% by weight, based on the total amount of the powder mixture. For the kneading, a kneader or a mixer which is commonly used, may be employed.

Molding in the first step is intended to mold the kneaded mixture into a brick-like or cog-like block, or into briquet. in the case of the brick-like or clog-like block, molding is conducted under a molding pressure of 2 from 0.1 to 5 ton/cm2, preferably from 0.2 to 3 ton/cm2, and in the case of the briquet-like block, molding is preferably conducted under a pressure of not higher than 10 ton/cm2.

Further, the bulk density of the compact thus obtained, is preferably from 0.5 to 3.0 g/cm3.

The compact thus obtained is subjected to a reduction reaction under a vacuum of at most 50 mmHg, preferably from 1 to 10 mmHg at a temperature of from 1,200 to 1,5000C, preferably from 1,350 to 1,4500C. In this case, heat treatment may be conducted in one step continuously to the reaction temperature. However, a two step or three step heat treatment is also preferred in which a preliminary heating operation is conducted at a temperature of from 300 to 5000C to decompose the majority of the decomposable substances, and then the binder is completely decomposed at a temperature of from 800 to 1,0000C, as the case requires, and then the reaction is conducted. There is no particular reason for limiting the time for the temperature rise and the maintenance, and the time may be prolonged as the case requires.

In the process of the present invention, the oxygen and carbon contents in the reaction product (crude chromium) obtainable by the reduction in the first step are required to be at most 7% and at most 5.3%, respectively. This can be accomplished by controlling the reduction time.

Then, to supply crude chromium obtained in the reduction reaction in the first step to the second step, the crude chromium is pulverized to a size of at most 20 mesh, and when the ratio of the remaining carbon to oxygen in the pulverized product is substantially equal to the stoichiometrical amount to form CO gas which will be evaporated, the pulverized product is molded by an addition of a binder, and when the ratio does not correspond to the stoichiometrical amount, a chromium oxide or a carbon source containing oxygen or carbon in an amount of the stoichiometrical amount + (5 g/1 kg of crude chromium) is added and mixed to the pulverized crude chromium product, and the mixture is molded by an addition of a binder.

The amount of the binder is preferably from 0.1 to 2.0% by weight, based on the crude chromium. Molding is preferably conducted under a molding pressure of from 1 to 10 ton/cm2 to form a briquet-like molded product. The bulk density of the molded product is closely related to the bulk density of resulting chromium metal.

Accordingly, the density of the molded product is usually from 2.0 to 6.0 g/cm3, preferably from 3.0 to 6.0 g/cm3.

The reaction conditions for the second step may be within the same ranges of conditions as defined for the first step.

Chromium metal obtained by the method according to the second aspect of the present invention is highly dense with a purity of at least 99%, and a non-metallic component such as carbon or oxygen is extremely little.

Thus, it is extremely excellent in its quality.

Further, when chromium oxyhydrochloride is used, the reaction time can be substantially shortened, whereby no substantial evaporation of chromium is observed during the reaction.

Now, the second aspect of the present invention will be described with reference to Examples, but it should be understood that the present invention is by no means restricted by such specific Examples.

EXATThE I 1,000 g of Cr2O3 powder having an average particle size of 2 Fm and 238.8 g of coke powder having a size of at most 200 mesh were mixed by a powder mixer, and 100 ml of a 10% polyvinyl alcohol aqueous solution was further added, and the mixture was kneaded. This kneaded product was molded by a hydraulic press under a pressure of 0.2 t/cm2 into a brick-like block of 200 x 100 x 25 mm.

After drying, the obtained molded product having a bulk density of 2.5 g/cm3 was put into a heating furnace and maintained at 9000C for 30 minutes and then at 1,4000C for further two hours. During this operation, the vacuum in the furnace was 5 mmEg. After cooling, the obtained' reaction product was 687 g. The reaction product was subjected to a powder X-ray diffraction analysis, whereby in addition to a Cr peak, Cr203 and Cr23C6 were detected, and the analytical values of the remaining carbon and oxygen were 0.98% and 1.65%, respectively.

65 g of this reaction product was pulverized to a size of at most 100 mesh. Then, 1.81 g of coke powder and 30 ml of a 40% polyvinyl alcohol aqueous solution were added thereto, and the mixture was kneaded. This kneaded product was molded by a briqueting machine under a molding pressure of 4 t/cm2 to obtain seven almond-like briquets with the maximum length, the maximum width and the maximum thickness being 40, 35 and 20 mm, respectively. After drying, the briquets having a bulk density of 4.5 g/cm3 were put into a heating furnace and maintained at 9000C for 30 minutes and then at 1,4000C for 4 hours. During this operation, the vacuum in the furnace was 5 mmHg. After the reaction, 627 g of chromium metal was obtained.

The obtained chromium metal was subjected to a powder X-ray diffraction analysis, whereby only a Cr peak was detected. The carbon and oxygen amounts in the chromium metal were 0.0098 and 0.039%, respectively. The chromium evaporation loss was 0.5% in the first step and 1% in the second step.

EXAMPLE 2 1,550 g of Cr203 powder, and 370 g of coke powder as used in Example 1 were processed and molded in the same manner as in Example 1 to obtain one clog-like block (upper portion: 200L x 100W x 30Hmm, leg portion: 30w x 30H x 100Lmm). After drying, the molded product having a bulk density of 2.5 g/cm3 was reacted under the same conditions as in Example 1. The reaction product was 1,059 g. The reaction product was subjected to a powder X-ray diffraction analysis, whereby in addition to a Cr peak, CrO3 and Cr23C6 were detected, and the analytical values of the remaining carbon and oxygen were 0.89% and 1.18%, respectively. This ratio was close to the stoichiometrical ratio, and no adjustment of carbon or oxygen was conducted.

The reaction product obtained in the first step was pulverized to obtain a powder having a particle size distribution such that particles of from 20 to 100 mesh were 10% and particles of at most 100 mesh were 90%.

1,000 g of such powder and 43 ml of a 40% polyvinyl alcohol aqueous solution were added, and the mixture was molded under the same conditions as in Example 1 to obtain ten briquets having the same shape as in Example 1. After drying the briquets having a bulk density of 4.7 g/cm3 were reacted under the same conditions as in Example 1 to obtain 977 g of chromium metal.

The obtained chromium metal was subjected to a powder X-ray diffraction analysis, whereby only a Cr peak was detected. Further, carbon and oxygen were 0.007% and 0.010%, respectively. The chromium evaporation loss was substantially the same as in Example 1.

EXAMPLE 3 1,550 g of Cr203 powder, and 376 g of coke powder as used in Example 1 were processed and molded in the same manner as in Example 1 to obtain a molded product having the same shape as in Example 2. After drying, the molded product having a bulk density of 2.5 g/cm3 was reacted under the same conditions as in Example 1. The obtained reaction product was 1,073 g. The reaction product was subjected to a powder X-ray diffraction analysis, whereby in addition to a Cr peak, Cr2C3 and Cr23C6 were detected, and the analytical values of the remaining carbon and oxygen were 1.76% and 1.65%, respectively.

;-his reaction product was pulverized to obtain a powder of at most 100 mesh. To 950 g of this powder, 22 g of Cr2O3 and 41 ml of a 40% polyvinyl alcohol aqueous solution were added, and the mixture was treated in the same manner as in Example 1 to obtain ten briquets.

After drying, briquets having a density of 4.5 g/cm3 were reacted under the same conditions as in Example 1 to obtain 918 g of chromium metal.

The obtained metal was subjected to a powder X-ray diffraction analysis, whereby only a Cr peak was detected. Carbon and oxygen in the chromium metal were 0.0118 and 0.019%, respectively. The chromium evaporation loss was substantially the same as in Example 1.

EXAMPLE 4 1,000 g of water-containing CrOOH fine powder containing 53.18 by weight of chromium and 180 g of coke powder of at most 200 mesh were mixed by a powder mixer, and 100 ml of a 15% polyvinyl alcohol aqueous solution was further added, and the mixture was kneaded. This kneaded product was molded by a hydraulic press under a pressure of 0.25 t/cm2 to obtain two molded products having the same shape as in Example 1. After drying, the molded products having a bulk density of 1.2 g/cm3 were put into a heating furnace and maintained at 9000C for 30 minutes and then at 1,4000C for further 30 minutes.

During this operation, the vacuum was 5 mmHg. After cooling the obtained products were 550 g. The reaction products were subjected to a powder X-ray diffraction analysis, whereby in addition to a Cr peak, Cr203 and Cr23C6 were detected, and the analytical values of remaining carbon and oxygen were 1.15% and 1.71%, respectively.

500 g of this reaction product was pulverized to a size of at most 100 mesh, and then 0.68 g of coke powder and 30 ml of a 40% polyvinyl alcohol aqueous solution were added, and the mixture was kneaded. The kneaded product was molded under the same conditions as in Example 1 to obtain five briquets having the same shape as in Example 1. After drying, the briquets having a bulk density of 4.5 g/cm3 were put in a heating furnace and maintained at 9000C for 30 minutes and then at 1,4000C for.3 hours. During this operation, the vacuum in the furnace was 5 mmHg. After the reaction, 480 g of chromium metal was obtained. The chromium metal was subjected to a powder X-ray diffraction analysis, whereby only a Cr peak was detected, and carbon and oxygen were 0.008% and 0.021%, respectively. Further, the chromium evaporation loss was substantially 0 in the first step and 0.7% in the second step.

As described in the foregoing, the method according to the second aspect of the present invention, carbon reduction is conducted in two stages and the conditions during the reduction are specified, whereby chromium meta can be obtained by carbon reduction with minimum evaporation loss of chromium and by a short period of reduction treatment.

Claims (9)

CLAIMS:

1. A process for producing chromium metal having a purify of at least 99%, which comprises a first step of mixing chromium oxide and carbon or a carbon compound, molding the mixture to a compact having a bulk density of from 0.5 to 3.0 g/cm3, and heating the compact under a vacuum of at most 50 mmHg at a temperature of from 1,200 to 1,5000C to obtain a reaction product (crude chromium) having an oxygen content of at most 7% and a carbon content of at most 5.3%1 and a second step of pulverizing the crude chromium to a particle size of at most 20 mesh, molding it to a compact having a bulk density of from 2.0 to 6.0 g/cm3, and then heating the compact under a vacuum of at most 50 mmHg at a temperature of from 1,200 to 1,5000C.

2. The process according to Claim 1, wherein in the second step, chromium oxide or a carbon source is added and mixed to the pulverized crude chromium in an amount corresponding to the stoichiometric amount + (5 g/l kg of crude chromium), where the stoichiometric amount is a theoretical amount required to convert all of carbon and/or oxygen in the pulverized crude chromium to carbon monoxide.

3. The process according to Claim 1, wherein the chromium oxide is an oxide of trivalent chromium.

4. The process according to Claim 1, wherein the chromium oxide is chromium oxyhydroxide.

5. The process according to Claim 1, wherein the compact in the first step is a brick-like block or a clog-like block.

6. The process according to Claim 1, wherein the compact in the first step is briquet-like.

7. The process according to Claim 1, wherein the compact in the second step is briquet-like.

8. A process for producing chromium metal, which comprises incorporating carbon and/or a carbon compound to chromium -oxyhydroxide, followed by heat treatment under at least one condition selected from the group consisting of a reduced pressure atmosphere, a reducing gas atmosphere and an inert gas atmosphere.

9. The process according to Claim 8, wherein the chromium oxyhydroxide is chromium oxyhydroxide having carbon and/or a carbon compound preliminarily incorporated.