IL311591A - Method for producing a tungsten metal powder having a high specific surface area - Google Patents

Description

Application No. PCT/EP2022/078517 SMB Publication No. WO2023/062130 Method for Producing a Tungsten Metal Powder Having a High Specific Surface Area The present invention relates to a method for producing a tungsten metal powder having a BET specific surface area of more than 8 m²/g, and a tungsten metal powder obtainable by such method.

Tungsten metal has been primarily employed for the preparation of tungsten carbides for carbide alloys, or as a metal powder in heavy metal applications. For this purpose, loosely agglomerated tungsten metal powders are preferred, which typically have a very low to low specific surface area within a range of from 0.to 6 m²/g. In order to explore new fields of application for tungsten metal powders, for example, in catalysis, as a catalyst support, or as a starting material for further syntheses, there is a need for tungsten metal powders that have a larger specific surface area than previously available ones.

In view of the above, EP 2 933 040 describes a method for preparing a fine tungsten powder, in which the powder is at first classified into a fraction having a relatively small average particle diameter and a fraction having a relatively coarse average particle diameter, the fraction having a relatively coarse average particle diameter is subjected to an oxidation process for forming an oxide film on the particle surface, and subsequently an alkaline treatment is performed for removing the oxide film formed in the oxidation process and forming a natural oxide film on the fine powder with an alkaline aqueous solution. Tungsten metal powders having a BET surface area of from 5 to 15 m²/g are supposed to be available in this way. The method described has the disadvantage that tungsten metal rather than the more easily available and less expensive tungsten oxide is employed as the starting material.

RU 2 558 691 discloses a method for preparing tungsten metal in which an - 2 - alkaline salt of tungsten is heated together with magnesium or calcium as reducing agents to a temperature T within a range of 0.95 Tmelt ≤ T ≤ 0.85 Tboil, where Tmelt and Tboil designate the melting and boiling temperatures, respectively, of the reducing agent. After cooling, the reaction material is subjected to acidic leaching and a washing step. The specific surface area of tungsten metal powder is supposed to be increased at up to 21.1 m²/g by means of this method. However, this printed document does not specify how the oxidation of the obtained tungsten metal powder during the acidic leaching can be prevented.

DE 1 245 601 relates to a process for the production of a metal powder from a volatile fluoride of a metal of Group IIIb, IVa, Va, VIa, VIIa, or Group VIII of the Periodic Table, in which two separate gas flows of fluorine on the one hand and hydrogen in stoichiometric excess on the other are passed through a nozzle and a ring nozzle surrounding it concentrically into a reaction space, wherein at least one gas flow serves as a carrier gas for a selected metal fluoride, then hydrogen and fluorine are ignited to form a hydrogen-fluorine flame, further the powder formed in the flame is collected, and the residual gases are subsequently discharged from the reaction zone. The thus obtained powders are said to have a specific surface area of from 8 to 14 m²/g. However, in this process, the element tungsten has to be present as a gaseous compound, which is difficult to handle because of its high toxicity.

US 2016/0322169 describes tungsten metal granules having an average particle diameter of 180 µm and a specific surface area of 8.8 m²/g that contains 0.5% by mass germanium, 0.3% by mass oxygen, 300 ppm of carbon, 100 ppm of phosphorus, and not more than 350 ppm of other contaminants.

In their article "Evolution of reduction process from tungsten oxide to ultrafine tungsten powder via hydrogen", issued in High Temperature Materials and Processes 2021; 40: 171-177, Y. Wang et al. describe an analysis of the reduction of WO2.9 to W, wherein a tungsten metal having a BET surface area of m²/g was obtained under the examined conditions.

CN 100357050 discloses a furnace for the reduction of WO3 in the presence of H2, in which tungsten metal having a BET surface area of 19 to 23 m²/g and an average SAXS particle size of 30 to 35.5 nm can be prepared. - 3 - CN 109014231, US 2003/0121365 and CN 106623960 describe multistage reduction processes for the preparation of tungsten metal powders.

Practical studies have shown that tungsten metal powders having a high specific surface area of more than 6 m²/g can be prepared in the classical large-scale industrial production furnaces for the production of tungsten metal, such as rotary kilns or pushing furnaces, only with difficulty, or not at all.

Therefore, it has been the object of the present invention to provide a process for the production of tungsten metal powders having a large specific surface area that can be implemented in an industrial large scale.

Surprisingly, it has been found that this object is achieved by specific reaction conditions, in which undesirable side reactions are suppressed, and tungsten metal powders having a large specific surface area are formed. In addition, the particles produced have the same morphology, especially shape, size and geometry, as the oxides employed. It thereby becomes possible to selectively influence the produced metal powders for later applications. For example, different shapes, such as cubic, rhomboid, cuboid, acicular, ellipsoid, or spherical shapes, are possible. These can also vary in particle size and have narrow or broad grain size distributions. It is also possible to adjust the habitus of the particles and the specific surface area thereof separately, which may offer an additional advantage.

Therefore, the present invention firstly relates to a method for producing a tungsten metal powder having a specific surface area of more than 8 m²/g as determined by the BET method according to DIN ISO 9277, in which a powdery tungsten source is heated in a hydrogen flow at first with a first heating rate HRto a first temperature T1 and subsequently with a second heating rate HR2 to a second temperature T2, where T1 < T2 and HR1 > HR2, and wherein the dew point temperature τ of the process exhaust gases does not exceed +10°C.

"Dew point temperature" within the meaning of the present invention is the temperature that the actual temperature must fall below at a constant pressure for the moisture contained in the process exhaust gas to precipitate as dew, mist, or in the form of ice crystals. The more moisture there is, the higher is the - 4 - dew point temperature. Thus, the dew point temperature is a measure of the water vapor partial pressure and can be measured directly by a chilled mirror dew point hygrometer, or indirectly by other hygrometric methods. Alternative commercial measuring methods for determining the dew point temperature include, for example, using a dew point meter, capacitive probes, or laser measuring devices.

The industrial production of tungsten metal is usually effected by reducing its oxides. Typical reactions that proceed during the reduction differ depending on the type of furnace employed, but can be summarized in a general form as follows: (1) WO3 → W24O68 → W18O49 → WO2 → W (2) WO3 → W24O68 → WO2 → W, wherein different suboxides that are characterized by a molar ratio of W:O of less than 1:3 may also be formed depending on the reduction conditions.

Within the scope of the present invention, it has surprisingly been found that in order to form tungsten metal powders having a large specific surface area, it is advantageous to suppress the formation of WO2 as an intermediate phase. It has further been found that especially the direct conversion of the higher oxides of tungsten leads to the desired metal powders having high specific surface areas.

Therefore, an embodiment is preferred in which said powdery tungsten source is selected from the group consisting of ammonium paratungstate, ammonium metatungstate, tungstic acid, WO3, and WOx with 2 < x < 3. More preferably, said powdery tungsten source is WO3, or WOx with 2 < x < 3.

Within the scope of the present invention, it has further been surprisingly found that the formation of undesirable WO2 as an intermediate can be suppressed, in particular, by an appropriate temperature control in the reduction reaction. A two-step process control, in which the respective temperatures are reached by different heating rates, has proven particularly advantageous. Therefore, in a preferred embodiment, the first temperature is selected in such a way that T1 is - 5 - from 400 to 500°C, preferably from 430 to 460°C. Preferably, this temperature is reached by a heating rate HR1 that is less than 10 K/min, preferably less than K/min.

Further, it has proven advantageous to continuously remove the water vapor occurring as a reaction product in the reduction. An effective discharge was achieved, in particular, when a particular dew point temperature in the process exhaust gas was not exceeded, and a very low heating rate was used, especially for reaching the second temperature. Therefore, an embodiment is preferred in which the second temperature T2 is selected in such a way that T2 is from 500 to 650°C, preferably from 550 to 590°C. Preferably, this temperature T2 is reached by a heating rate HR2 that is less than 2 K/min, preferably less than from 1 to 1.7 K/min.

The process according to the invention is controlled in such a way that the dew point temperature τ of the process exhaust gases does not exceed +10°C. In a preferred embodiment, the dew point temperature τ is below 0°C, or equal to 0°C. In a further preferred embodiment, the dew point temperature τ is –5°C or less. Under such conditions, particularly fine powders could be obtained. Preferably, the process according to the invention is characterized in that the dew point temperature τ of the process exhaust gases meets –40°C < τ < +10°C. The dew point temperature can be determined as described above, and controlled, for example, through the reaction temperature, the hydrogen flow, or other process parameters, such as the reaction rate in general, proportions of inert gas in the hydrogen flow, or the quantity of material in the process space.

In order to obtain a particularly homogeneous product, it has proven advantageous if the hydrogen employed for the hydrogen flow already has a low dew point temperature before it is introduced into the reaction space. Therefore, the freshly supplied hydrogen preferably has a dew point temperature τ that is < 0°C, more preferably < –40°C. Further preferred is an embodiment of the process according to the invention in which said hydrogen flow is preheated to a temperature of 350 to 600°C.

In a preferred embodiment, the present invention relates to a method for producing a tungsten metal powder having a specific surface area of more than 8 - 6 - m²/g as determined by the BET method according to DIN ISO 9277, and a particle size of from 10 to 1000 µm as determined by laser diffraction. in which a powdery tungsten source is heated in a hydrogen flow at first with a first heating rate HR1 to a first temperature T1 and subsequently with a second heating rate HR2 to a second temperature T2, where T1 < T2 and HR1 > HR2, wherein T1 is a temperature of from 400 to 500°C, T2 is a temperature of from 500 to 650°C, HR1 is 10 K/min, and HR2 > 2 K/min, wherein the dew point temperature τ of the process gases does not exceed +10°C.

In order to promote the direct conversion to the higher oxides, it has proven advantageous if the powdery tungsten source is introduced into a process space and heated together with it. Therefore, an embodiment is preferred in which the powdery tungsten source is introduced into a process space and heated therein. In some cases, it has proven advantageous to vary the reaction conditions during the reduction, and thus to further increase the homogeneity of the product. Thus, for example, the hydrogen flow can be temporarily replaced by an inert gas flow, such as an argon or nitrogen flow, for example. In this manner, the fineness of the product could be surprisingly increased, where it is considered that the reaction is brought to a standstill by introducing the inert gas flow. When fresh dry hydrogen is again introduced into the reaction space, it is considered that a nucleation shower occurs, which increases the fineness of the product. Therefore, 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, wherein said inert gas is preferably argon or nitrogen.

Therefore, in a preferred embodiment, the process according to the invention comprises the following steps: a) providing a powdery tungsten source; b) heating said powdery tungsten source to a first temperature T1 with a first heating rate HR1 in the hydrogen flow; c) heating to a second temperature T2 with a heating rate HR2 in the hydrogen flow; d) replacing the hydrogen flow by an inert gas flow, preferably a nitrogen flow; e) replacing the inert gas flow by a hydrogen flow; f) optionally repeating steps d) and e) to obtain a tungsten metal powder; - 7 - g) passivating said tungsten metal powder, where T1 < T2 and HR1 > HR2, and wherein the dew point temperature τ of the process exhaust gases does not exceed +10°C.

The process according to the invention is preferably performed under dry conditions, i.e., at a low (gas) humidity. It has been found that the hydrogen flow, which serves as a reducing agent, can also support the discharge of the process exhaust gases, especially water vapor, as long as a sufficiently high flow rate is selected. Therefore, an embodiment is preferred in which the gas flow has a Reynolds number Re of from 60 to 600, preferably from 75 to 300. In this way, powdery material is prevented from being blown out of the reaction container and to carry it out of the process space.

The Reynolds number Re for the dimensionless description of the flow within tubes can be determined according to the following formula, taking the volume flow rate into account. The material properties of the gases refer to a temperature of 20°C and an absolute pressure of 1013 mbar. , where V = volume flow rate [m³/s] ρ = density of the gas [kg/m³] η = dynamic viscosity of the gas [µPa*s] d = diameter of the reaction space [m].

The large specific surface area of the tungsten metal powder produced by the process according to the invention has the result that an adsorption of oxygen and moisture on the surface of the powder occurs after the product has left the reaction space. Since this process is exothermic, it may lead to self-ignition, especially for larger quantities of powder. In order to avoid this, the powder should be subjected to passivation directly after the reduction process. Therefore, in a preferred embodiment, the process according to the invention further includes a step of passivating the tungsten metal powder. More preferably, such passivation is effected in steps, wherein said tungsten metal powder can be treated, for example, with a mixture of air and an inert carrier Re4V      d  - 8 - gas, preferably nitrogen, wherein moisture in the form of water vapor may optionally be further admixed with the mixture. The thus produced mixture of gases is preferably passed through the process space at room temperature before the produced tungsten metal powder 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 the ambient air. When this state has been reached, the tungsten metal powder is passivated and can be safely removed from the process space.

The large specific surface areas of the tungsten metal powders according to the invention are achieved, in particular, by performing the reduction in as dry as possible an atmosphere. It has been found that the specific surface area can be further increased if the reduction employs a specific reaction container for receiving the powdery tungsten source, through which container the discharge of the gaseous reaction products, presently water vapor, from the bulk powder can be improved. Therefore, the process according to the invention is preferably performed in a reaction container for receiving a powder, especially a powdery tungsten source, for the production of tungsten metal powders, in which said reaction container has a gas-permeable bottom. Preferably, said gas-permeable bottom of the reaction container 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 from 25 µm to 5 mm. When the process according to the invention is performed in a reaction container having a gas-permeable bottom, diffusion processes within the reactants can proceed faster, since the gaseous reaction products can escape not only upwards, but also downwards. The exchange surface area available for the discharge is thereby doubled. Preferably, these gaseous components are selected from the group consisting of water vapor, CO2, argon, gaseous hydrocarbons, CO, Cl, NOx, and SO2.

The present invention further relates to tungsten metal powders having a large specific surface area. Therefore, the present invention further relates to a tungsten metal powder having a specific surface area of more than 8 m²/g as determined by the BET method according to DIN ISO 9277, wherein said tungsten metal powder has a particle size of from 10 µm to 1000 µm, as determined by laser diffraction. In a particularly preferred embodiment, said tungsten metal powder has a specific surface area of more than 15 m²/g, - 9 - preferably from 20 to 40 m²/g, as determined by the BET method according to DIN ISO 9277. Further preferred is an embodiment in which the tungsten metal powder according to the invention has a particle size of from 30 to 300 µm, as determined by laser diffraction.

Preferably, the tungsten metal according to the invention is obtainable, or has been prepared, by the process according to the invention.

Tungsten metal powders naturally have a layer of adsorbed oxygen on the surface. The amount of oxygen adsorbed depends, in addition to the ambient conditions, essentially on the proportion of the surface of the powder that is accessible for the atmosphere, wherein a correspondingly large content of residual oxygen can be expected for powders having a large specific surface area that are handled in the air. Surprisingly, the powder according to the invention has a comparatively low oxygen content despite its large specific surface area. Therefore, the tungsten metal powder according to the invention is characterized by an oxygen content of from 900 to 1500 ppm/m²/g, preferably from 950 to 1050 ppm/m²/g, as determined by means of carrier gas hot extraction (LECO method).

In particular, the tungsten metal powder according to the invention is characterized by its structure, which is a result of the process according to the invention, in particular. The tungsten metal powder according to the invention is preferably in the form of porous particles composed of crystallites. The crystallite size of the tungsten metal primary crystals formed can be determined by X-ray diffraction methods (according to Scherrer). Preferably, the crystallite size decreases as the BET surface area increases.

The present invention is further explained by means of the following Examples, which should by no means be understood as limiting the idea of the invention.

Examples: The following powders were prepared using a reaction container with a gas-permeable bottom by the process according to the invention. - 10 - 1. Example The reaction container according to the invention was filled with WO3 having a specific surface area of 0.7 m²/g, as determined by the BET method according to DIN ISO 9277, and placed into a reaction space. The reaction temperature was set to 570°C, wherein the heating rate was 10 K/min up to a temperature of 450°C, and was thereafter slowed down to 1.5 K/min. The volume flow rate of the introduced hydrogen flow was controlled by feedback control in such a way that a Reynolds number Re (H2) of 109 was obtained, based on the ambient conditions (1013 mbar of absolute pressure, and a temperature of 20°C), wherein the hydrogen had a dew point temperature of less than -40°C. The dew point temperature of the process exhaust gases was regulated to be +5°C. After hours, the reaction was stopped, wherein the dew point temperature of the process exhaust gases sank to -35°C. Before the material was removed from the reaction space, the tungsten metal powder was admixed with the following gas mixtures for passivation: 1. 30 minutes with 800 L/h of N2 and 200 L/h of air 2. 30 minutes with 600 L/h of N2 and 400 L/h of air 3. 30 minutes with 400 L/h of N2 and 600 L/h of air 4. 30 minutes with 200 L/h of N2 and 800 L/h of air 5. 30 minutes with 1000 L/h of air After the reaction was completed, samples (1a, 1b, 1c) were collected from the reaction container at different sites, and their specific surface areas were determined. The specific surface areas obtained of the tungsten metal powders as determined by the BET method according to DIN ISO 9277 are summarized in Table 1.

Table 1: Example 1 specific surface area [m²/g] a 22-b 18-c 16- - 11 - As can be seen from Table 1, the process according to the invention reliably yields powders having a high specific surface area. 2. Example A reaction container having a gas-permeable bottom was filled with WO3 having a specific surface area of 0.7 m²/g, as determined by the BET method according to DIN ISO 9277, and placed into a reaction space. The reaction temperature was set to 570°C, wherein the heating rate was 10 K/min up to a temperature of 450°C, and was thereafter slowed down to 1.5 K/min. The volume flow rate of the introduced hydrogen flow had a Reynolds number Re (H2) of 109, wherein the hydrogen had a dew point temperature of less than -40°C. After some time, the hydrogen flow was replaced by a nitrogen flow having an Re number Re (N2) of 313. The calculation of the Re numbers was based on the following values: For nitrogen: ρ(N2) = 1.25 kg/m³ η(N2) = 16.6 µPa·s For hydrogen: ρ(H2) = 0.089 kg/m³ η(H2) = 8.4 µPa·s After 30 minutes, the hydrogen flow was resumed with an Re (H2) number of 109. This process was repeated several times, wherein a surge of the dew point temperature occurred when the hydrogen flow was resumed. The dew point temperature of the process exhaust gases was regulated to be +5°C. After hours, the reaction was stopped, wherein the dew point temperature of the process exhaust gases sank to -35°C. Before the material was removed from the reaction space, the tungsten metal powder was admixed with the following gas mixtures for passivation: 1. 30 minutes with 800 L/h of N2 and 200 L/h of air 2. 30 minutes with 600 L/h of N2 and 400 L/h of air 3. 30 minutes with 400 L/h of N2 and 600 L/h of air - 12 - 4. 30 minutes with 200 L/h of N2 and 800 L/h of air 5. 30 minutes with 1000 L/h of air After the reaction was completed, samples (2a, 2b, 2c) were collected from the reaction container at different sites, and their specific surface areas were determined. The specific surface areas obtained of the tungsten metal powders as determined by the BET method according to DIN ISO 9277 are summarized in Table 2.

Table Example 2 specific surface area [m²/g] a 23-b 23-c 24- As can be seen from Table 2, the homogeneity of the product could be further enhanced by varying the gas flows.

All the samples had a particle size within the range according to the invention.

Figure 1 shows FESEM micrographs of a tungsten metal powder according to the invention having a specific surface area of from 20 m²/g. What can be seen are structures with sizes on the order of < 100 nm.

As can be seen from the data provided, the process according to the invention allows for the simple and efficient preparation of tungsten metal powders having a large specific surface area, which can be employed for further applications.

Claims (14)

- 13 - C L A I M S :

1. A process for producing a tungsten metal powder having a specific surface area of more than 8 m²/g, as determined by the BET method according to DIN ISO 9277, in which a powdery tungsten source is heated in a hydrogen flow at first with a first heating rate HR1 to a first temperature Tand subsequently with a second heating rate HR2 to a second temperature T2, where T1 < T2 and HR1 > HR2, characterized in that the dew point temperature τ of the process exhaust gases does not exceed +10°C.

2. The process according to claim 1, characterized in that T1 is from 400 to 500°C, preferably from 430 to 460°C, and/or the heating rate HR1 is < K/min, preferably < 5 K/min.

3. The process according to at least one of the preceding claims, characterized in that T2 is from 500 to 650°C, preferably from 550 to 590°C, and/or the heating rate HR2 is < 2 K/min, preferably from 1 to 1.K/min, more preferably 1.5 K/min.

4. The process according to at least one of the preceding claims, characterized in that the dew point temperature τ of the process exhaust gases meets –40°C < τ < +10°C.

5. The process according to at least one of the preceding claims, characterized in that said hydrogen flow is preheated to a temperature of from 350°C to 650°C.

6. The process according to at least one of the preceding claims, characterized in that the hydrogen employed has a dew point temperature τ of < -20°C, preferably < -40°C.

7. The process according to at least one of the preceding claims, characterized in that the process is effected in a heated reaction space, wherein the temperature of the reaction space is not more than 900°C, preferably from 550 to 700°C. - 14 -

8. The process according to at least one of the preceding claims, characterized in that the hydrogen flow is temporarily replaced by an inert gas flow.

9. The process according to at least one of the preceding claims, characterized in that the hydrogen flow has a Reynolds number Re (H2) of from 60 to 600, preferably from 75 to 300.

10. The process according to at least one of the preceding claims, characterized in that the process includes another step of passivating the tungsten metal powder.

11. The process according to at least one of the preceding claims, characterized in that said process is performed by using a reaction container having a gas-permeable bottom.

12. A tungsten metal powder having a particle size of from 10 µm to 1000 µm, as determined by laser diffraction, and a specific surface area of larger than 8 m²/g, as determined by the BET method according to DIN ISO 9277, characterized in that said tungsten metal powder has an oxygen content of from 900 to 1500 ppm/m²/g as determined by carrier gas hot extraction.

13. The tungsten metal powder according to claim 12, characterized in that said tungsten metal powder is obtainable, or has been prepared, by the process according to at least one of claims 1 to 10.

14. The tungsten metal powder according to at least one of the preceding claims, characterized in that said tungsten metal powder has an oxygen content of from 950 to 1050 ppm/m²/g as determined by carrier gas hot extraction.