Method for producing high specific surface area tungsten metal powder and tungsten metal powder

Abstract

The present invention is 2 The present invention relates to a method for producing a tungsten metal powder having a BET specific surface area of ​​more than 100 nm / g, and to the tungsten metal powder obtained by this method.

Description

[Technical field]

[0001] The present invention is 2 The present invention relates to a method for producing a tungsten metal powder having a BET specific surface area of ​​more than 100 nm / g, and to the tungsten metal powder obtained by such a method. [Background technology]

[0002] Tungsten metal has been used primarily for the preparation of tungsten carbide for hard metals, or as a metal powder for heavy metal applications. For this purpose, loosely agglomerated tungsten metal powder is preferred, which typically ranges from 0.01 to 6 m 2 / g. To develop new fields of application of tungsten metal powder, for example as a catalyst support in catalytic reactions or as a starting material for further synthesis, tungsten metal powders with a larger specific surface area than those currently available are needed.

[0003] From the above viewpoint, EP 2 933 040 describes a method for preparing tungsten fine powder, in which the powder is first classified into a fraction having a relatively small average particle size and a fraction having a relatively coarse average particle size, the fraction having a relatively coarse average particle size is subjected to an oxidation treatment for forming an oxide film on the particle surface, and then an alkaline aqueous solution is used to remove the oxide film formed by the oxidation treatment, and an alkaline treatment is performed to form a natural oxide film on the fine powder. 2 It is believed that tungsten metal powder with a BET specific surface area of ​​1000 nm / g can be obtained by this method. This method has the disadvantage of using metallic tungsten as the starting material rather than the more readily available and less expensive tungsten oxide.

[0004] RU 2 558 691 is a compound in which an alkaline salt of tungsten is mixed with magnesium or calcium as a reducing agent at a concentration of 0.95T. melt ≦T≦0.85T boil The present invention discloses a method for preparing tungsten metal, which is heated to a temperature T in the range of Tmelt and T boil are the melting and boiling points of the reducing agent, respectively. After cooling, the reaction mass is subjected to an acid leaching and washing process. By this method, the specific surface area of ​​the tungsten metal powder can be increased up to 21.1 m 2 / g. However, this document does not specify how to prevent the oxidation of the tungsten metal powder obtained during acid leaching.

[0005] DE 1 245 601 relates to a process for producing metal powders from volatile fluorides of metals of groups IIIb, IVa, Va, VIa, VIIa or VIII of the periodic table, in which two separate gas streams, on the one hand fluorine and on the other hand a stoichiometric excess of hydrogen, are introduced into a reaction space through a nozzle and a concentrically surrounding ring nozzle, where at least one gas stream acts as a carrier gas for the selected metal fluoride, then the hydrogen and the fluorine are ignited to form a hydrogen-fluorine flame, the powder formed in the flame is recovered and the residual gases are subsequently discharged from the reaction zone. The specific surface area of ​​the powder thus obtained is between 8 and 14 m2. 2 / g. However, in this process, elemental tungsten must exist as a gaseous compound, which is highly toxic and therefore difficult to handle.

[0006] US 2016 / 0322169 describes a sintered quartz crystal with an average particle size of 180 μm and a specific surface area of ​​8.8 m2, containing 0.5% germanium by mass, 0.3% oxygen by mass, 300 ppm carbon, 100 ppm phosphorus, and 350 ppm or less of other contaminants. 2 / g of tungsten metal granules are described.

[0007] In their paper "Evolution of the reduction process of tungsten oxide to ultrafine tungsten powder via hydrogen" published in High Temperature Materials and Processes 2021; 40: 171-177, Y. Wang et al. 2.9 The analysis of the reduction of 9m to W is presented here under the conditions considered.2 Tungsten metal was obtained having a BET specific surface area of ​​1.0 μm / g.

[0008] CN 100357050 discloses a furnace for reducing WO3 in the presence of H2, which produces a WO3 with a BET specific surface area of ​​19-23 m 2 It is possible to prepare metallic tungsten having an average SAXS particle size of 30 to 35.5 nm.

[0009] CN 109014231, US 2003 / 0121365, CN 106623960 describe multi-stage reduction processes for preparing tungsten metal powder.

[0010] According to practical studies, 6m 2 It has been shown that tungsten metal powders with high specific surface areas of > 1000 nm / g are difficult or impossible to prepare in classical large-scale industrial production furnaces for tungsten metal production, such as rotary kilns and push furnaces. [Prior art documents] [Patent documents]

[0011] [Patent Document 1] European Patent Application Publication No. 2933040 [Patent Document 2] Russian Patent Application Publication No. 2558691 [Patent Document 3] DE 1245601 A1 [Patent Document 4] US Patent Application Publication No. 2016 / 0322169 [Patent Document 5] China Patent Publication No. 100357050 [Patent Document 6] China Patent Application Publication No. 109014231 [Patent Document 7] US Patent Application Publication No. 2003 / 0121365 [Patent Document 8] China Patent Application Publication No. 106623960 [Non-patent literature]

[0012] [Non-Patent Document 1] Y. Wang et al.,High Temperature Materials and Processes 2021;40:171-177 Summary of the Invention [Problem to be solved by the invention]

[0013] Therefore, an object of the present invention is to provide a process for producing tungsten metal powder having a large specific surface area, which can be carried out on an industrial scale. [Means for solving the problem]

[0014] Surprisingly, it has been found that this aim is achieved by specific reaction conditions, under which undesirable side reactions are suppressed and tungsten metal powders with a large specific surface area are formed. Moreover, the particles produced have the same morphology, in particular shape, size and shape, as the oxide employed. This makes it possible to selectively influence the produced metal powder for the subsequent application. Different shapes are possible, for example, cubes, rhomboids, rectangular prisms, acicular bodies, ellipsoids, spheres, etc. Also, the particle size can vary, with narrow to broad particle size distributions. It is also possible to individually adjust the particle habitus and the specific surface area, which has further advantages.

[0015] The present invention therefore first relates to a method for determining the 8m 2The present invention relates to a method for producing a tungsten metal powder having a specific surface area of ​​more than 100 nm / g, in which a powdered tungsten source is first heated to a first temperature T1 at a first heating rate HR1 in a hydrogen flow, and then heated to a second temperature T2 at a second heating rate HR2, where T1<T2およびHR1> HR2, and the dew point temperature τ of the process exhaust gas does not exceed +10°C. [Brief description of the drawings]

[0016] [Figure 1] 1 is a FESEM micrograph of a tungsten metal powder according to the present invention having a specific surface area of ​​20 m2 / g or more. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0017] In the present invention, the "dew point temperature" is the temperature below which the actual temperature must be at a given pressure in order for the moisture contained in the process exhaust gas to precipitate in the form of dew, mist or ice crystals. The more moisture there is, the higher the dew point temperature. The dew point temperature is therefore a measure of the partial pressure of water vapor and can be measured directly with a cooled mirror dew point hygrometer or indirectly with other hygrometric methods. Commercially available alternative methods for measuring the dew point temperature include, for example, the use of dew point meters, capacitance probes or laser measurement devices.

[0018] The industrial production of tungsten metal is usually achieved by the reduction of its oxide. The typical reactions that take place during reduction vary depending on the type of furnace employed, but can be summarized in general terms as follows: (1) WO3 → W 24 O 68 →W 18 O 49 →WO2→W (2) WO3 → W 24 O 68 →WO2→W, Here, depending on the reduction conditions, different suboxides may also be formed, characterized by a W:O molar ratio of less than 1:3.

[0019] Within the scope of the present invention, it has surprisingly been found that in order to form tungsten metal powders with a large specific surface area, it is advantageous to suppress the formation of WO2 as an intermediate phase. Moreover, it has been found that the direct conversion of higher oxides of tungsten in particular leads to the desired metal powders with a high specific surface area.

[0020] Therefore, the powdered tungsten source may be selected from the group consisting of ammonium paratungstate, ammonium metatungstate, tungstic acid, WO3, and 2 <x<3のWO x Preferably, the powdered tungsten source is selected from the group consisting of WO3, or <x<3のWO x It is.

[0021] Within the scope of the present invention, it has further surprisingly been found that by suitable temperature control, especially in the reduction reaction, the formation of undesired WO2 as an intermediate can be suppressed. A two-stage process control, in which each temperature is reached by different heating rates, has proven to be particularly advantageous. Thus, in a preferred embodiment, the first temperature T1 is selected to be between 400 and 500°C, preferably between 430 and 460°C. Preferably, this temperature is reached by a heating rate HR1 of less than 10 K / min, preferably less than 5 K / min.

[0022] Furthermore, it has proven advantageous to continuously remove the water vapor generated as a reduction reaction product. In particular, effective evacuation has been achieved when a certain dew point temperature in the process exhaust gas is not exceeded, in particular when a very low heating rate is used to reach the second temperature. Therefore, an embodiment is preferred in which the second temperature T2 is selected such that T2 is between 500 and 650 ° C, preferably between 550 and 590 ° C. Preferably, this temperature T2 is reached by a heating rate HR2 of less than 2 K / min, preferably less than 1 to 1.7 K / min.

[0023] The process according to the invention is controlled such that the dew point temperature τ of the process exhaust gas does not exceed +10° C. In a preferred embodiment, the dew point temperature τ is less than or equal to 0° C. In a further preferred embodiment, the dew point temperature τ is less than or equal to −5° C. Under such conditions, a particularly fine powder is obtained. Preferably, the process according to the invention is characterized in that the dew point temperature τ of the process exhaust gas satisfies −40° C.<τ<+10° C. The dew point temperature can be determined as described above and can be controlled, for example, by the reaction temperature, the hydrogen flow rate or other process parameters such as the general reaction rate, the proportion of inert gas in the hydrogen flow rate or the amount of material in the process space.

[0024] In order to obtain a particularly homogeneous product, it has proven to be advantageous if the hydrogen used in the hydrogen stream already has a low dew point temperature before being introduced into the reaction space. The freshly fed hydrogen therefore preferably has a dew point temperature τ below 0° C., more preferably below −40° C. Further preferred is an embodiment of the process according to the invention, in which the hydrogen stream is preheated to a temperature between 350 and 600° C.

[0025] In a preferred embodiment, the present invention relates to a method for producing a viscoelastic material having a viscosity of 8 m, determined by the BET method according to DIN ISO 9277. 2 The present invention relates to a method for producing a tungsten metal powder having a specific surface area of ​​more than 100 μm / g and a particle size of 10-1000 μm as determined by a laser diffraction method, the method comprising the steps of: a powdered tungsten source is first heated to a first temperature T1 at a first heating rate HR1 in a hydrogen flow; and subsequently heated to a second temperature T2 at a second heating rate HR2 in a hydrogen flow;<T2およびHR1> HR2, T1 is a temperature between 400 and 500 °C, T2 is a temperature between 500 and 650 °C, HR1 is 10 K / min, HR2>2 K / min, and the dew point temperature τ of the process gas does not exceed +10 °C.

[0026] To promote the direct conversion to higher oxides, it has proven to be advantageous to introduce a powdered tungsten source into the process space and heat it there. The embodiment in which the powdered tungsten source is introduced into the process space and heated there is therefore preferred. In some cases, it has proven to be advantageous to change the reaction conditions during the reduction in order to further increase the homogeneity of the product. Thus, for example, the hydrogen flow can be temporarily replaced by an inert gas flow, for example an argon or nitrogen flow. In this way, the fineness of the product is surprisingly improved, when the reaction is supposed to be stopped by introducing an inert gas flow. If fresh dry hydrogen is introduced again into the reaction space, a nucleation shower is generated, which is supposed to increase the fineness of the product. Thus, a preferred embodiment of the process according to the invention is characterized in that the hydrogen flow is temporarily replaced by an inert gas flow, said inert gas being preferably argon or nitrogen.

[0027] Thus, in a preferred embodiment, the process according to the invention comprises the following steps: a) providing a powdered source of tungsten; b) heating the powdered tungsten source in a flow of hydrogen at a first heating rate HR1 to a first temperature T1; c) heating in a flow of hydrogen at a heating rate HR2 to a second temperature T2; d) replacing the hydrogen flow with an inert gas flow, preferably nitrogen; e) replacing the inert gas flow with a hydrogen flow; f) optionally repeating steps d) and e) to obtain tungsten metal powder; g) passivating the tungsten metal powder; Here, T1<T2およびHR1> HR2, and the dew point temperature τ of the process exhaust gas does not exceed +10°C.

[0028] The process according to the invention is preferably carried out under dry conditions, i.e. under low (gas) humidity. It has also been found that the hydrogen flow acting as reducing agent can support the evacuation of process off-gases, in particular water vapor, as long as a sufficiently high flow rate is selected. Therefore, embodiments are preferred in which the gas flow has a Reynolds number Re between 60 and 600, preferably between 75 and 300. In this way, it is prevented that pulverulent material is blown out of the reaction vessel and carried out of the process space.

[0029] The Reynolds number Re, which is used to express the flow inside a pipe dimensionlessly, can be calculated according to the following formula, taking into account the volumetric flow rate. The material properties of the gas are based on a temperature of 20°C and an absolute pressure of 1013 mbar.

number

[0030] Due to the large specific surface area of ​​the tungsten metal powder produced by the process according to the invention, adsorption of oxygen and moisture takes place on the surface of the powder after the product leaves the reaction space. This process is exothermic and can lead to self-ignition, especially if the amount of powder is large. To avoid this, it is necessary to passivate the powder immediately after the reduction treatment. Therefore, in a preferred embodiment, the process according to the invention further comprises a step of passivating the tungsten metal powder. More preferably, such passivation is carried out in stages, and said tungsten metal powder can be treated, for example, with a mixture of air and an inert carrier gas, preferably nitrogen, optionally with moisture in the form of water vapor further mixed into the mixture. The gas mixture thus produced is preferably passed through the process space at room temperature before the tungsten metal powder produced is removed. The proportion of atmospheric oxygen and / or moisture in the mixture can be slowly increased until it corresponds to the typical composition of ambient air. When this state is reached, the tungsten metal powder is passivated and can be safely removed from the process space.

[0031] The large specific surface area of ​​the tungsten metal powder according to the invention is achieved in particular by carrying out the reduction in an atmosphere that is as dry as possible. It has been found that the specific surface area can be increased even further if the reduction is carried out using a specific reaction vessel for receiving a powdered tungsten source through which the evacuation of the gaseous reaction products, currently water vapor, from the bulk powder can be improved. The process according to the invention is therefore preferably carried out in a reaction vessel for receiving a powder, in particular a powdered tungsten source, for the production of tungsten metal powder, said reaction vessel having a gas-permeable bottom. Preferably, said gas-permeable bottom of the reaction vessel is in the form of a mesh fabric or in the form of a permeable plate, such as a sintered porous material, preferably with a mesh size of 25 μm to 5 mm. If the process according to the invention is carried out in a reaction vessel with a gas-permeable bottom, the diffusion processes in the reactants can proceed faster, since the gaseous reaction products can escape not only upwards but also downwards. This doubles the exchange surface area available for the discharge. Preferably, these gaseous components are water vapor, CO2, argon, gaseous hydrocarbons, CO, Cl, NO x , SO2.

[0032] The present invention further relates to a tungsten metal powder having a high specific surface area, i.e., 8 m2, determined by the BET method according to DIN ISO 9277. 2 / g, the tungsten metal powder having a particle size of 10 μm to 1000 μm as determined by laser diffraction. In a particularly preferred embodiment, the tungsten metal powder has a specific surface area of ​​15 μm to 1000 μm as determined by the BET method according to DIN ISO 9277. 2 / g, preferably 20 to 40m 2 / g. More preferred is an embodiment in which the tungsten metal powder according to the present invention has a particle size of 30-300 μm as determined by laser diffraction method.

[0033] Preferably, the tungsten metal according to the present invention is obtained or prepared by the process according to the present invention.

[0034] Tungsten metal powder has a layer of oxygen naturally adsorbed on its surface. The amount of adsorbed oxygen depends essentially on the proportion of the powder surface accessible to the atmosphere, in addition to the ambient conditions, and it is expected that a powder with a large specific surface area that is handled in the atmosphere will have a correspondingly large amount of residual oxygen. Surprisingly, the powder according to the present invention has a relatively low oxygen content, despite its large specific surface area. Thus, the tungsten metal powder according to the present invention has an oxygen content of 900-1500 ppm / m3, determined by the carrier gas thermal extraction method (LECO method). 2 / g, preferably 950 to 1050 ppm / m 2 It is characterized by its oxygen content in g / g.

[0035] In particular, the tungsten metal powder according to the invention is characterized by its structure, which is in particular the result of the process according to the invention. The tungsten metal powder according to the invention is preferably in the form of porous particles consisting of crystallites. The crystallite size of the formed tungsten metal primary crystals can be determined by X-ray diffraction method (Scherrer method). Preferably, the crystallite size decreases with increasing BET specific surface area.

[0036] The present invention will be further illustrated by the following examples, which should not be construed as limiting the scope of the invention in any way. EXAMPLES

[0037] Working Example: The following powders were prepared by the process according to the invention using a reaction vessel with a gas permeable bottom:

[0038] 1. Example 1 The reaction vessel according to the invention is fitted with a reactor having a specific surface area of ​​0.7 m2, measured by the BET method according to DIN ISO 9277. 2 / g WO3 was filled and placed in the reaction space. The reaction temperature was set to 570 °C, the heating rate was 10 K / min up to 450 °C and then reduced to 1.5 K / min. The volumetric flow rate of the introduced hydrogen stream was controlled by feedback control so that the Reynolds number Re(H2) was 109 based on the ambient conditions (absolute pressure 1013 mbar, temperature 20 °C) and the hydrogen dew point temperature was below -40 °C. The dew point temperature of the process exhaust gas was adjusted to +5 °C. After 40 hours, the reaction was stopped and the dew point temperature of the process exhaust gas was reduced to -35 °C. Before the material was removed from the reaction space, tungsten metal powder was mixed with the following gas mixture for passivation: 1.800L / h nitrogen and 200L / h air for 30 minutes 2. 30 minutes with 600L / h nitrogen and 400L / h air 3. 400L / h nitrogen and 600L / h air for 30 minutes 4. 200L / h nitrogen and 800L / h air for 30 minutes 5. 1000L / h air for 30 minutes

[0039] After the reaction was completed, samples (1a, 1b, 1c) were taken from each part of the reaction vessel and the specific surface area was measured. The specific surface area of ​​the tungsten metal powder measured by the BET method according to DIN ISO 9277 is summarized in Table 1. [Table 1]

[0040] As can be seen from Table 1, the process according to the invention ensures obtaining a powder with a high specific surface area.

[0041] 2. Example 2 A reaction vessel with a gas-permeable bottom was fitted with a specific surface area of ​​0.7 m2, measured by the BET method according to DIN ISO 9277. 2 / g WO3 was filled and placed in the reaction space. The reaction temperature was 570 °C, the heating rate was 10 K / min up to 450 °C and then reduced to 1.5 K / min. The volumetric flow rate of the introduced hydrogen flow was such that the Reynolds number Re(H2) was 109 and the dew point temperature of hydrogen was below -40 °C. After some time, the hydrogen flow was replaced by a nitrogen flow with a Re number of 313. The calculation of the Re number was based on the following values: In the case of nitrogen ρ(N2)=1.25kg / m 3 η(N2)=16.6μPa·s In the case of hydrogen ρ(H2)=0.089kg / m 3 η(H2)=8.4μPa·s

[0042] After 30 minutes, the hydrogen flow was resumed at a Re(H2) number of 109. This process was repeated several times, and restarting the hydrogen flow caused a sudden increase in the dew point temperature. The dew point temperature of the process exhaust gas was adjusted to +5°C. After 40 hours, the reaction was stopped and the dew point temperature of the process exhaust gas had dropped to -35°C. Before the material was removed from the reaction space, tungsten metal powder was mixed with the following gas mixture for passivation: 1.800L / h nitrogen and 200L / h air for 30 minutes 2. 30 minutes with 600L / h nitrogen and 400L / h air 3. 400L / h nitrogen and 600L / h air for 30 minutes 4. 200L / h nitrogen and 800L / h air for 30 minutes 5. 1000L / h air for 30 minutes

[0043] After the reaction was completed, samples (2a, 2b, 2c) were taken from each part of the reaction vessel and the specific surface area was measured. The specific surface area of ​​the tungsten metal powder measured by the BET method according to DIN ISO 9277 is summarized in Table 2. [Table 2]

[0044] As can be seen from Table 2, by varying the gas flow rate, the homogeneity of the product can be further improved.

[0045] The particle sizes of all samples were within the range according to the invention.

[0046] Figure 1 shows a surface area of ​​20 m 2 1 is a FESEM micrograph of tungsten metal powder according to the present invention having a pore size of over 100 nm.

[0047] As can be seen from the data provided, the process according to the invention allows for a simple and efficient production of tungsten metal powder having a large specific surface area which can be used for further applications.

Claims

1. 8 m determined by the BET method according to DIN ISO 9277 ２ 1. A method for producing a tungsten metal powder having a specific surface area of ​​greater than 1 / g, wherein a powdered tungsten source is first heated at a first heating rate HR in a hydrogen flow. １ At the first temperature T １ followed by a second heating rate HR ２ At the second temperature T ２ where T １ <T ２ And HR １ >HR ２ and the dew point temperature τ of the process exhaust gas does not exceed +10°C.

2. T １ is 400 to 500°C, preferably 430 to 460°C, and / or the heating rate HR １ 2. The method according to claim 1, characterized in that the saturation temperature is <10 K / min, preferably <5 K / min.

3. T ２ is 500 to 650°C, preferably 550 to 590°C, and / or the heating rate HR ２ 2. The method according to claim 1, characterized in that the saturation temperature is < 2 K / min, preferably 1 to 1.7 K / min, more preferably 1.5 K / min.

4. 2. The method according to claim 1, wherein the dew point temperature τ of the process exhaust gas satisfies −40° C.<τ<+10° C.

5. 2. The method of claim 1, wherein the hydrogen stream is preheated to a temperature between 350°C and 650°C.

6. 2. The method according to claim 1, characterized in that the hydrogen used has a dew point temperature τ of less than −20° C., preferably less than −40° C.

7. 2. The method according to claim 1, characterized in that the method is carried out in a heated reaction space, the temperature of the reaction space being below 900°C, preferably between 550 and 700°C.

8. 2. The method of claim 1, wherein the hydrogen flow is temporarily replaced with an inert gas flow.

9. The hydrogen flow has a Reynolds number Re(H ２ 2. The method of claim 1, comprising:

10. 10. The method of claim 1, wherein said method includes the additional step of passivating said tungsten metal powder.

11. 10. The method of claim 1, wherein the method is carried out using a reaction vessel having a gas-permeable bottom.

12. Tungsten metal powder having a particle size of 10 μm to 1000 μm determined by the laser diffraction method and a specific surface area of more than 8 m ２ / g determined by the BET method in accordance with DIN ISO 9277, The tungsten metal powder has a tungsten content of 900-1500 ppm / m as determined by carrier gas thermal extraction. ２ / g oxygen content.

13. 13. Tungsten metal powder according to claim 12, characterized in that the tungsten metal powder is obtained or prepared by a method according to at least one of claims 1 to 10.

14. The tungsten metal powder has a tungsten content of 950-1050 ppm / m as determined by carrier gas thermal extraction. ２ 14. Tungsten metal powder according to claim 12 or 13, characterized in that it has an oxygen content of 0.1g / g.

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