EP3041631B1 - Chromium metal powder - Google Patents

Description

The present invention relates to a metal powder having a chromium content of at least 90% by mass and a process for its production.

The large-scale production of chromium metal powder from chromium oxides is currently only by aluminothermic (powder morphology see FIG. 1 ) and electrolytic (powder morphology see Figures 2 ) Procedure. However, powders produced in this way have a poor pressing and sintering behavior. In addition, due to the use of Cr (VI) compounds electrolytic processes are environmentally harmful. Increasingly stringent environmental regulations mean that this process is hardly acceptable economically and environmentally.

In addition to the processes already mentioned, the reduction of chromium oxides with hydrogen and / or carbon (see for example: Metallurgy of the Rare Metals - Chromium "; Arthur Henry Sully; Butterworths Scientific Publications (1954 ) GB 512,502 . JP 54013408 A . JP 07216474 A . JP 3934686 B2 and JP 06081052 A ).

To date, however, it has not been possible with the known processes to produce chromium metal powder which is suitable for demanding powder metallurgical processes, for example the production of thin-walled components or components of complex shape, in particular because the green strength of known powders is too low and their hardness is too high ,

The present invention has therefore set itself the task of providing metal powder having a chromium content of at least 90% by mass, which can be processed well powder metallurgy, in particular by pressing and sintering. In particular, a metal powder is to be provided with which complex-shaped and / or thin-walled components can be produced in a simple manner by powder metallurgy. The metal powder should further be produced in a high metallic purity, in particular a metallic purity comparable or better than metal powder, which is obtained by electrolytic route. Furthermore, it is an object of the invention to provide a method which is suitable for large-scale, cost-effective and environmentally friendly production of such metal powder.

The object is achieved by metal powder with a chromium content of at least 90% by mass, which is measured by a nanohardness of _{0.005 /} 5/1/5 EN ISO 14577-1 (2002 edition - Berkovich indenters and Oliver and Pharr analysis methods) of ≤ 4 GPa. The hardness value refers to a metal powder, which is preferably subjected to no further treatment, such as an annealing. The nanohardness _HIT is preferably _{0.005 /} 5/1/5 ≦ 3.7 GPa, more preferably ≦ 3.4 GPa. With very high requirements, for example for very thin-walled components, a nanohardness _{HIT 0.005 /} 5/1/5 of ≤ 3.1 GPa has proven itself. With very pure chromium powder, a nanohardness of _{0.005 /} 5/1/5 of about 1.4 GPa can be realized. The nanohardness is determined in the pure chromium phase. If there is no pure chromium phase, the nanohardness is determined in the chromium-rich (phase with the highest chromium content) phase. The metal powder according to the invention thus has a significantly lower nanohardness compared with the nanohards of metal powder according to the prior art. Since the powder according to the invention can be produced without a downstream milling process, the specified nanohardness can be achieved even with very fine-grained powder having a BET surface area of preferably ≥ 0.05 m ² / g. The information on the BET surface area in the context of this application relates to a BET measurement according to the standard (ISO 9277: 1995, measuring range: 0.01-300 m ² / g, device: Gemini II 2370, baking temperature: 130 ° C., heating time : 2 hours, adsorptive: nitrogen, volumetric evaluation by five-point determination).

The metal powder according to the invention is also characterized by a green strength measured according to ASTM B 312-09 at a compacting pressure of 550 MPa of at least 15 MPa, preferably of at least 20 MPa.

In the case of very pure, coarse-grained chromium powder with a comparatively high BET surface area, metal powders having a green strength of up to about 50 MPa can be produced at a pressure of 550 MPa. The ASTM B 312-09 leaves open whether a wax is used as pressing additive. According to the invention, a wax was used as pressing additive, namely 0.6% by mass of an amide wax, namely LICOWAX® Micropowder PM (supplier Clariant, product number 107075, CAS No. 00110-30-5 ).

Furthermore, the green strength preferably has the following values: at least 8 MPa, preferably at least 13 MPa, at a compression pressure of 450 MPa; at least 6 MPa, preferably at least 11 MPa, at a compression pressure of 300 MPa; at least 4 MPa, preferably at least 6 MPa, at one Compressing pressure of 250 MPa and at least 2 MPa, preferably at least 2.5 MPa, at a pressure of 150 MPa. Green strengths could be achieved at pressures of 450, 300 and 250 MPa of 18.5, 13.0 and 7.5 MPa and above.

The metal powder according to the invention can be processed in a simple manner by powder metallurgy, for example by pressing and sintering. In particular, the metal powder according to the invention enables the simple and cost-effective production of powder metallurgy components with thin-walled areas, complex shape or relatively unfavorable pressing ratio.

The properties with regard to nanohardness and green strength can be achieved if the chromium content is at least 90% by mass and thus the content of other substances of 10% by mass is not exceeded. The other substances are present in an advantageous manner separated from the chromium phase. Furthermore, the other substance can be deposited in metallic or non-metallic form, preferably via a diffusion bond. Such powders are referred to as composite powder. Shares (advantageously <5% by mass) of the other substance can also be dissolved in the chromium and form a chromium mixed crystal. Such powders are referred to as alloyed powders. The metal powder then comprises a pure chromium phase and / or a chromium mixed crystal phase.

La ₂ O ₃ (up to a maximum of 5 mass%) or Cu (up to a maximum of 10 mass%) may be mentioned by way of example, in the case of La ₂ O ₃ La (OH) ₃ and in the case of Cu CuO to Cr ₂ O. _{3 are} mixed and fed to the reduction. Of course, other metals or non-metals are possible.

The metal powder preferably has both a green strength at a compacting pressure of 550 MPa of at least 7 MPa, preferably at least 10 MPa, more preferably of at least 15 MPa, particularly preferably of at least 20 MPa, and a nano-hardness _{HIT of 0.005 /} 5/1/5 of ≦ 4 GPa, preferably ≦ 3.7 GPa, more preferably ≦ 3.4 GPa, particularly particularly preferably ≦ 3.1 GPa.

Furthermore, the metal powder according to the invention preferably has a sponge-like particle shape / morphology (division of the particle shape / morphology see Powder Metallurgy Science; Randall M. German; MPIF; Princeton, 1994, second edition, page 63 ). This has a favorable effect on the green strength.

The combination of sponge-like particle shape / morphology and low hardness allows comparatively high densities, but above all a very high green strength at a given density.

According to the invention, it is provided that the metal powder has a BET surface area without a surface-enlarging process of ≥ 0.05 m ² / g. Preferably, the BET surface area is 0,0 0.07 m ² / g. BET surfaces of 0.25 m ² / g and above could be achieved. In this context, without a surface-enlarging process, it can also be called "as produced" and means for the person skilled in the art that the metal powder was obtained directly from the process and in particular is no longer subjected to a grinding process. Such a grinding process can be recognized by the morphology of the metal powder, since during the grinding process smooth fracture surfaces are formed which can not be found in unmilled powder. According to the invention, only a deagglomeration is preferably provided.

In one embodiment, it is provided that the metal powder according to the invention has a metallic purity, i. a content of chromium based on other metals, of ≥ 99.0 Ma%, preferably ≥ 99.5 Ma%, more preferably ≥ 99.9 Ma%, particularly preferably of ≥ 99.99 Ma%. Metallic purity here means the purity of the metal powder without consideration of non-metallic constituents such as, for example, O, C, N and H.

The oxygen content of metal powder according to the invention is preferably not more than 1500 μg / g of chromium, more preferably not more than 1000 μg / g of chromium. In a particularly preferred embodiment, the oxygen content is not more than 500 μg / g chromium. The achievable carbon content can be set very low and is preferably not more than 150 μg / g chromium, more preferably not more than 100 μg / g chromium. In a particularly preferred embodiment, the carbon content is not more than 50 μg / g chromium.

In one embodiment, it can be provided that the metal powder is granulated. The granulation can be carried out by conventional methods, preferably by spray or build-up granulation (see also Powder Metallurgy Science; Randall M. German; MPIF; Princeton, 1994, second edition, pages 183-184 ). Under granules is the merger of individual powder particles to understand that are connected to each other, for example by means of a binder or by Sinterhalsbildung.

In one embodiment variant, the metal powder has a bulk density of ≦ 2.0 g / cm ³ . The bulk density is preferably 0.1 to 2 g / cm ³ , more preferably 0.5 to 1.5 g / cm ³ . Since a comparatively high bulk density is achieved for the achievable particle size or BET surface area (preferably ≥ 0.05 m ² / g), the powder exhibits good filling behavior during the pressing process.

Furthermore, the metal powder preferably has a compact density of ≥ 80% of the theoretical density at 550 MPa pressing pressure. This makes it possible to produce components without high sintering shrinkage near net shape

The metal powder according to the invention can be prepared by reducing at least one compound of the group consisting of Cr oxide and Cr hydroxide, optionally with a mixed solid carbon source, under at least temporary exposure to hydrogen and hydrocarbon. Preferred chromium oxide or chromium hydroxide are Cr (III) compounds in powder form, for example Cr ₂ O ₃ , CrOOH, Cr (OH) ₃ or mixtures of chromium oxides and chromium hydroxides. The preferred chromium source is Cr ₂ O ₃ . For a high degree of purity in the end product, it is preferably provided that the Cr ₂ O _{3 used has} at least pigment quality.

The compound of the group consisting of Cr oxide and Cr-oxide, optionally with a mixed solid carbon source, is heated to a temperature T _{R of} 1100 ° C ≤ T _R ≤ 1550 ° C and optionally maintained at this temperature. Temperatures <1100 ° C or> 1550 ° C lead to deteriorated powder properties, or to a more uneconomical process. The reaction proceeds particularly well for industrial purposes when temperatures T _R of about 1200 ° C to 1450 ° C are selected.

%red

Reduktionsgrad in %

Mred,O

Masse [g] O im reduzierten Pulver

Ma,O

Masse [g] O im Pulveransatz (vor der Reduktion)

While in the lower inventive temperature range very long holding times to T _{R are} required to set an advantageous degree of reduction of 90%, in the upper inventive temperature range, the holding time be chosen very short or omitted altogether. The degree of reduction R is defined as the ratio of the amount of oxygen in the chromium oxide or chromium hydroxide removed up to the time t, based on the total amount of oxygen present in the unreduced chromium compound: $$\%\mathit{red}=\left(\raisebox{1ex}{$\mathit{mred}.O$}\!\left/ \!\raisebox{-1ex}{$\mathit{Ma}.O$}\right.\right)x100$$

% red

Reduction rate in%

MRED, O

Mass [g] O in the reduced powder

Ma, O

Mass [g] O in the powder batch (before reduction)

Based on the examples, the skilled person can easily determine the optimum combination of temperature and time for his furnace (continuous furnace, batch furnace, maximum achievable furnace temperature, etc.). Preferably, the reaction over substantially at least 30%, more preferably at least 50% of the reaction time is maintained substantially constant (isothermal) on T _R.

The presence of hydrocarbon ensures that powder having the properties according to the invention is formed via a chemical transport process. The total pressure of the reaction is advantageously 0.95 to 2 bar. Pressures above 2 bar adversely affect the economics of the process. Pressures below 0.95 bar have an adverse effect on the resulting hydrocarbon partial pressure, which in turn has a very unfavorable effect on the transport processes via the gas phase, which are of great importance for adjusting the powder properties of the invention (for example hardness, green strength, specific surface area) are. In addition, pressures below 0.95 bar adversely affect the process costs.

How the hydrocarbon partial pressure can be adjusted easily can be seen from the examples. Advantageously, the hydrocarbon is present as CH ₄ . At least
during the heating process, at least temporarily, the hydrocarbon partial pressure is 5 to 500 mbar. A hydrocarbon partial pressure <5 mbar has an unfavorable effect on the powder properties, in particular the green strength. A hydrocarbon partial pressure> 500 mbar leads to a high C content in the reduced powder. The residual gas atmosphere is preferably hydrogen. The action of hydrogen and hydrocarbon takes place at least in the temperature range 800 ° C to 1050 ° C. In this temperature range, the hydrocarbon partial pressure is preferably from 5 to 500 mbar. The reaction mixture forming from the starting materials is preferably at least 45 minutes, particularly preferably at least 60 minutes. in this temperature range. This time includes both the heating process and any isothermal holding phases in this temperature range. With the inventive process conditions it is ensured that at temperatures preferably <T _R at least one compound selected from the group consisting of Cr oxide and Cr hydroxide at least partially converts to chromium carbide under the action of hydrogen and hydrocarbon. Preferred chromium carbides are Cr ₃ C ₂ , Cr ₇ C ₃ and Cr ₂₃ C ₆ . The partial formation of chromium carbide, which occurs via the hydrocarbon partial pressure, in turn has a favorable effect on the powder properties. With the inventive process conditions it is further ensured that the chromium carbide reacts with the Cr oxide / Cr hydroxide present in the reaction mixture and / or admixed to form Cr, this process dominating at T _R.

The hydrocarbon may be added to the reaction in gaseous form, preferably without admixing a solid carbon source. The at least one compound of the group consisting of Cr oxide and Cr hydroxide is preferably reduced under at least temporary action of an H ₂ -CH ₄ gas mixture. Advantageously, a H ₂ / CH ₄ volume ratio in the range 1 to 200, particularly advantageously selected from 1.5 to 20. The action of the H ₂ -CH ₄ gas mixture is preferably carried out at least temporarily during the heating phase to T _R , the influence on the formation of the powder form, in particular in the temperature range 850 to 1000 ° C is very low. If a temperature of about 1200 ° C is reached, is preferably switched to a pure hydrogen atmosphere, preferably with a dew point of <-40 ° C (measured in the gas supply). If T _{R is} below 1200 ° C, switching to the pure hydrogen atmosphere is preferred when T _{R is reached} . The isothermal phase on T _R and cooling to room temperature are advantageously carried out in a Wasserstoffatmoshäre. In particular, when cooling, it is advantageous to use hydrogen with a dew point <-40 ° C to avoid reoxidation.

In one embodiment variant, a solid carbon source is admixed with the Cr oxide and / or Cr hydroxide. Preference is given here per mole of oxygen in the chromium compound between 0.75 and 1.25 mol, preferably between 0.90 and 1.05 moles of carbon used. This refers to the amount of carbon available for reaction with the chromium compound. In a particularly preferred embodiment, the ratio of O to C is slightly substoichiometric at about 0.98. It is preferably provided that the solid carbon source is selected from the group of carbon black, activated carbon, graphite, carbon-releasing compounds or mixtures thereof. As an example of a carbon releasing compound, chromium carbides such as Cr ₃ C ₂ , Cr ₇ C ₃ and Cr ₂₃ C _{6 may} be mentioned. The powder mixture is heated to T _R in an H ₂ -containing atmosphere. The H ₂ pressure is preferably adjusted so that at least in the temperature range 800 ° to 1050 ° C, a CH ₄ partial pressure of 5 to 500 mbar results. The isothermal phase on T _R and cooling to room temperature are again advantageously carried out in a hydrogen atmosphere. During these process phases, the presence of hydrocarbon is not required. Hydrogen prevents reoxidation processes during this process phase and during the cooling phase. During the cooling phase, a hydrogen atmosphere with a dew point <-40 ° C. is preferably used.

Figur 3

zeigt eine REM Aufnahme von Cr₂O₃ (Pigmentqualität).

Figur 4;5a,b

zeigen REM Aufnahmen von nach dem erfindungsgemäßen Verfahren erhältlichen Metallpulvern.

Figur 6

zeigt die Grünfestigkeit von erfindungsgemäßem Pulver (CP-181) im Vergleich zu aluminothermisch hergestelltem Chrompulver (Cr-Std).

Figur 7

zeigt die relative Pressdichte von Pulver gemäß Erfindung im Vergleich zu aluminothermisch (A-Cr) und elektrolytisch (E-Cr) hergestelltem Cr unterschiedlicher Reinheit (Angabe in Gew.%) und Pulverpartikelgröße.

Figur 8

zeigt den zeitlichen Verlauf der Reduktion von Cr₂O₃ zu Cr bei unterschiedlichen Temperaturen gemäß Erfindung.

Figur 9

zeigt die spezifische Oberfläche von verschiedenen Chrompulvern gemäß Erfindung.

Further advantages and details of the invention are explained below with reference to examples and figures.

FIG. 3

shows an SEM image of Cr ₂ O ₃ (pigment grade).

4, 5a, b

show SEM images of obtainable by the process according to the invention metal powders.

FIG. 6

shows the green strength of powder according to the invention (CP-181) in comparison with aluminothermically produced chromium powder (Cr-Std).

FIG. 7

shows the relative compacted density of powder according to the invention compared to aluminothermic (A-Cr) and electrolytic (E-Cr) produced Cr of different purity (in wt.%) and powder particle size.

FIG. 8

shows the time course of the reduction of Cr ₂ O ₃ to Cr at different temperatures according to the invention.

FIG. 9

shows the specific surface area of different chromium powders according to the invention.

Example 1:

500 g of Cr ₂ O ₃ in pigment grade (Lanxess Bayoxide CGN-R) with a mean, measured by laser diffraction particle size d ₅₀ of 0.9 microns (powder morphology see FIG. 3 ) was in H ₂ (75vol.%) - CH ₄ (25vol.%) (flow rate 150 l / h, pressure about 1 bar) in 80 min. heated to 800 ° C. Subsequently, the reaction mixture was slowly heated to 1200 ° C, the reaction mixture 325 min. in the temperature range 800 to 1200 ° C was. Thereafter, the reaction mixture was in 20 min. T _R heated with T _R = 1400 ° C. The holding time to 1400 ° C was 180 min. Heating from 1200 ° C to T _R and holding on T _{R was} done by adding dry hydrogen with a dew point <-40 ° C, the pressure being about 1 bar. The furnace cooling was also carried out under H ₂ with a dew point <-40 ° C. A metallic sponge was obtained, which could easily be deagglomerated to a powder. The chromium metal powder thus produced is in FIG. 4 play. The degree of reduction was> 99.0%, the carbon content 80 μg / g and the oxygen content 1020 μg / g. An X-ray diffraction analysis provided only peaks for cubic body-centered (BCC) chromium metal. The specific surface area was determined by BET method (according to ISO 9277: 1995, measuring range: 0.01-300 m ² / g, device: Gemini II 2370, annealing temperature: 130 ° C., heating time: 2 hours, adsorptive: nitrogen, volumetric evaluation via five-point determination) and was 0.14 m ² / g, the bulk density 1.2 g / cm ³ . The nanohardness _{HIT 0.005 /} 5/1/5 was determined according to EN ISO 14577-1 and was 3 GPa. The green strength was determined according to ASTM B 312-09. As a compression additive, 0.6% by mass of LICOWAX® Micropowder PM (supplier Clariant, product number 107075, CAS No. 00110-30-5 ) used. At a compacting pressure of 550 MPa, the green strength was 23.8 MPa, at 450 MPa 18.1 MPa, at 300 MPa 8.5 MPa, at 250 MPa 7.2 MPa and at 150 MPa 3.0 MPa.

Example 2:

Pigment grade Cr ₂ O ₃ (Lanxess Bayoxide CGN-R) with a mean laser diffraction particle size d ₅₀ of 0.9 μm was well blended with amorphous carbon black (Thermax ultra-pure N908 - Cancarb). The carbon content of the mixture thus prepared was 0.99 mol / mol of O in Cr ₂ O ₃ . 12500 g of this mixture were in 80 min. to 800 ° C and then in 125 min. heated to 1050 ° C. The heating was carried out under the action of H ₂ , wherein the H ₂ pressure was adjusted so that in the temperature range 800 ° C to 1050 ° C, the measured mass spectrometry CH ₄ partial pressure> 15 mbar scam. The total pressure was 1.1 bar. Thereafter, the reaction mixture was in 20 min. T _R heated with T _R = 1200 ° C. The holding time at 1200 ° C was 540 min. Heating from 1000 ° C to T _R and holding on T _{R was} done by supplying dry hydrogen with a dew point <-40 ° C, the pressure was about 1 bar. The furnace cooling was also carried out under H ₂ with a dew point <-40 ° C. A metallic sponge was obtained, which could easily be deagglomerated to a powder. The thus produced chromium metal powder is in the Figures 5 a, b play. The carbon and oxygen contents are given in Table 1. X-ray diffraction analysis only yielded peaks for cubic body-centered (BCC) chromium metal. The green strength was determined according to ASTM B 312-09. As a compression additive, 0.6% by mass of LICOWAX® Micropowder PM (supplier Clariant, product number 107075, CAS No. 00110-30-5 ) used. As pressing pressures 550 MPa, 450 MPa, 350 MPa, 250 MPa and 150 MPa were used. FIG. 6 shows the measured green strength values compared to samples pressed with aluminothermically produced powder (Cr-Std). The powder according to the invention (CP181) shows a green strength which is at least 5 times higher.

The powder formulation (with 0.6 Ma% LICOWAX® Micropowder PM Press Additive) was further pressed at various pressures into pill-shaped samples. In FIG. 7 For example, the relative compacting densities are shown in terms of compacting pressure as compared to standard chromium metal powder (E-Cr: electrolytically produced, A-Cr: aluminothermically produced) having different particle sizes.

Furthermore, the BET specific surface area (ISO 9277: 1995, measuring range: 0.01-300 m ² / g, apparatus: Gemini II 2370, baking temperature: 130 ° C., heating time: 2 hours, adsorptive: nitrogen, volumetric evaluation over Five-point determination) and the nanohardness _{HIT 0.005 /} 5/1/5 according to EN ISO 14577-1. Table 1 lists these characteristics and compares them with the properties of electrolytically produced chromium powders. Striking is the significantly lower nanohardness of the powder according to the invention. The particle size calculated from the BET surface area was 8.3 μm. Table 1: Properties of chromium powder according to the invention in comparison with electrolytically produced chromium powder Powder type BET surface area [m ² / g] O [μg / g] C [μg / g] Nanohardness [GPa] Chromium powder according to the invention (Example 2) 0.10 1064 114 2.92 Electrolytically produced chromium powder, particle size <45 μm 0.11 736 87 5.32

Example 3:

In each case 20 g of a mixture according to Example 2 were in a molybdenum crucible in 80 min. to 800 ° C and then in 125 min. heated to 1050 ° C. The heating was carried out under the action of H ₂ , wherein the H ₂ pressure was adjusted so that in the temperature range 800 ° C to 1050 ° C, the measured by mass spectrometry CH ₄ partial pressure> 15 mbar. The total pressure was 1.1 bar. Thereafter, the reaction mixture was heated to T _R at a heating rate of 10 K / min. As T _R 1150 ° C, 1250 ° C, 1300 ° C, 1350 ° C, 1400 ° C, 1450 ° C and 1480 ° C were used. The holding times on T _R were 30 min, 60 min, 90 min, 120 min and 180 min. Heating from 1000 ° C to T _R and holding on T _{R was} done by supplying dry hydrogen with a dew point <-40 ° C, the pressure was about 1 bar. The furnace cooling was also carried out under H ₂ with a dew point <-40 ° C. The degree of reduction was determined as set out in the description. How out FIG. 8 it can be seen, an advantageous reduction of> 95% at 1400 ° C, 1450 ° C and 1480 ° C already at a holding time of 30 min. clearly exceeded. At 1350 ° C it takes about 80 min., At 1300 ° C about 160 min. At 1250 ° C and 1150 ° C it takes about 260 min. or 350 min. (extrapolated values). SEM investigations showed that the powders thus produced had a sponge-like morphology combined with a very high BET surface area (see FIG. 9 ) respectively.

Claims (16)

1. Metal powder having a chromium content of at least 90 Ma%, characterized by a nanohardness _{HIT 0.005/5/1/5} according to
   EN ISO 14577-1 of ≤ 4 GPa, a green strength measured according to ASTM B312-09 of at least 15 MPa at a compression pressure of 550 MPa and a surface area according to BET, measured according to ISO 9277:1995, preferably without surface-enlarging operation, of ≥ 0.05 m2/g.
2. Metal powder according to Claims 1, characterized in that the metal powder is chromium powder having a metallic purity ≥ 99.0 Ma%.
3. Metal powder according to any one of Claims 1 to 2, characterized in that the metal powder is provided as an alloyed powder or composite powder.
4. Metal powder according to any one of Claims 1 to 3, characterized in that the metal powder is granulated.
5. Metal powder according to any one of Claims 1 to 4, characterized in that the compression density at a compression pressure of 550 MPa is ≥ 80 % of the theoretical density.
6. Method for producing a metal powder according to any one of Claims 1 to 5 by reduction of at least one compound of the group consisting of chromium oxide and chromium hydroxide, optionally with an admixed solid carbon source, under at least temporary action of hydrogen and hydrocarbon, wherein the compound of the group consisting of chromium oxide and chromium hydroxide, optionally with an admixed solid carbon source is heated to a temperature TR with 1100°C ≤ TR ≤ 1550°C and optionally held at this temperature, wherein at least during the heating operation, the hydrocarbon partial pressure is at least temporarily 5 to 500 mbar and the action of hydrogen and hydrocarbon occurs at least in the temperature range of 800 to 1050°C.
7. Method according to Claim 6, characterized in that, at least in the temperature range of 800 to 1050°C, the hydrocarbon partial pressure is 5 to 500 mbar.
8. Method according to any one of Claims 6 to 7, characterized in that the sum of heating time and holding time in the temperature range of 800°C to 1050°C is at least 45 minutes.
9. Method according to any one of Claims 6 to 8, characterized in that the total pressure is 0.95 to 2 bar.
10. Method according to any one of Claims 6 to 9, characterized in that the compound of the group consisting of chromium oxide and chromium hydroxide is reduced under at least temporary action of a H₂-CH₄ gas mixture.
11. Method according to Claim 10, characterized in that the H₂/CH₄ volume ratio is 1 to 200, in particular 1.5 to 20.
12. Method according to any one of Claims 6 to 11, characterized in that a solid carbon source is admixed, which is at least one component selected from the group consisting of carbon black, activated carbon, graphite, carbon-releasing compound, and mixtures thereof.
13. Method according to Claim 12, characterized in that between 0.75 and 1.25 mol, preferably between 0.90 and 1.05 mol of carbon is used per mol of oxygen in the chromium oxide or chromium hydroxide.
14. Method according to any one of Claims 6 to 13, characterized in that at least one compound selected from the group consisting of chromium oxide and chromium hydroxide is at least partially reacted under the action of hydrogen and hydrocarbon to form a chromium carbide selected from the group consisting of Cr₃C₂, Cr₇C₃, and Cr₂₃C₆.
15. Method according to Claim 14, characterized in that the chromium carbide is at least partially reacted with at least one compound selected from the group consisting of chromium oxide and chromium hydroxide to form chromium.
16. Method according to any one of Claims 6 to 15, characterized in that the hydrocarbon is CH₄.

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