A method for preparing tungsten metal powder

Abstract

The present invention relates to a method for preparing tungsten metal powder by reducing tungsten oxide, characterized in that the properties of the obtained metal powder can be continuously monitored during the production process.

Description

Technical Field

[0001] The present invention relates to a method for preparing tungsten metal powder by reducing tungsten oxide, characterized in that the properties of the obtained metal powder can be continuously monitored during the production process. Background Art

[0002] Tungsten metal has the characteristics of high melting point and high boiling point, and is widely used in the fields of science, technology and medicine, such as manufacturing cemented carbide tools with tungsten carbide as a precursor. Depending on the application and field of use, there are different requirements and specification standards for tungsten metal, and the main form of use of tungsten metal is powder. However, all specification standards have one thing in common, that is, the powder must be reliably provided with constant high quality, and the provided powder should have a narrow or defined primary particle size distribution.

[0003] U.S. Patent 2006 / 0051256 describes a powder manufacturing device similar to a screw extruder, which can control the temperature through various heating and cooling elements. The screw unit used in the device can control the growth of the generated particles as needed.

[0004] German Patent DE3802811 relates to a method for preparing single-particle metal powder agglomerates, wherein more than 70% by weight of the metal powder agglomerates are composed of one or more metals selected from the group consisting of elemental molybdenum, rhenium or tungsten and binding metals selected from iron, cobalt, nickel, copper, silver, gold, palladium, platinum, rhodium, chromium and rhenium. Among them, the compounds of the metal and the binding metal are dissolved and / or uniformly suspended in an ionic or non-ionic liquid, and the residue obtained after drying this solution and / or suspension is calcined below 600 °C, and then reacted at a temperature of 600 - 1200 °C under reducing conditions to form metal powder.

[0005] WO2017 / 162048 discloses a method for reducing metal oxides, wherein, in a batch process, a strong oxidant or metal halide reacts with a reducing agent at a temperature below 580 °C to reduce the metal oxide.

[0006] To recover tungsten metal, tungsten-containing ore is calcined at 500 - 600 °C under aerobic conditions to remove any contained impurities. Na2WO4 is formed by reacting with an aqueous sodium hydroxide solution, and Na2WO4 is purified by a series of precipitations and crystallized into ammonium paratungstate by ion exchange or solvent extraction with ammonia. The obtained tungstate is filtered and dried, and converted into pure tungsten(VI) oxide by calcination at a temperature above 500 °C. Then, in a continuously operating furnace device, hydrogen is used as a reducing agent to extract metal powder from the oxide at a temperature above 650 °C. The process of converting tungsten oxide to tungsten metal can be represented by the following formula:

[0007] WO3 + 3H2 → W + 3H2O

[0008] Although there are various methods for preparing tungsten metal in the prior art, these methods cannot perform continuous quality monitoring on the produced metal powder. To detect the quality of the produced tungsten metal powder, samples are usually taken from the reaction stream and analyzed. Among them, the conversion rate of the reaction and the particle size of the obtained tungsten metal powder are used as important parameters. Especially in the case of further processing tungsten metal into tungsten carbide, the particle size of the tungsten metal powder is a key quality characteristic. Since the particle size of tungsten carbide highly depends on the particle size of the tungsten metal powder used, it must be controlled and adjusted very precisely. The conventional processes for controlling the quality of tungsten metal powder in the prior art have the following disadvantages: there is a time delay between sampling and obtaining the analysis results. Therefore, it is impossible to adjust the process parameters immediately during the production process. The said time delay is usually caused by the need for complex sample processing and analysis and possible transportation distances. Therefore, tungsten metal powder may not be produced with the desired quality within the required time (which may be several hours to several days depending on the available infrastructure), and defective products or inventory of unusable powder quality are produced due to the inability to adjust the process parameters accordingly during the production process. Such unusable products will result in high cost input and, in the worst case, a large loss of raw materials and resources. Therefore, to solve the quality monitoring delay and its related problems, it is desired to perform quality control in such a way that quality intervention can be carried out immediately during the ongoing production process.

[0009] In the production of tungsten metal powder, the oxygen content, average particle size, and specific surface area are usually used to evaluate the quality of the obtained powder. The said material properties need to be measured by different methods. Therefore, they cannot be measured during the ongoing production process or different instruments must be used for measurement. Moreover, in some of the analysis methods adopted in the prior art, complex and time-consuming sample preparation is required to measure the said material properties. Therefore, it is impossible to respond immediately to quality problems during the ongoing production process. Summary of the Invention

[0010] Therefore, the object of the present invention is to provide a method for producing tungsten metal powder, which can provide immediate and continuous quality assurance for the produced powder.

[0011] Surprisingly, it has been found that the content of tungsten(IV) oxide (WO2), an intermediate occurring during the preparation of tungsten metal, can be used as an indicator for the progress of the reaction and thus as an indicator for the quality of tungsten metal powder. Furthermore, it has surprisingly been found that the quality of tungsten metal powder is determined by the crystallite size of the tungsten metal powder obtained. In the present invention, it has also been found that both of these parameters can be determined "online" during the ongoing production process, thus eliminating complex sampling and sample handling and enabling immediate measurement results, and allowing for immediate intervention during the production process if required.

[0012] Accordingly, the first technical solution provided by the present invention is: A method for producing tungsten metal powder by reducing tungsten oxide, the method comprising the following steps:

[0013] a) Providing a reaction stream I containing tungsten oxide particles;

[0014] b) Treating reaction stream I with a reducing agent to obtain a reaction stream II containing tungsten oxide and tungsten metal powder;

[0015] c) Measuring the content of tungsten(IV) oxide (WO2) in reaction stream II;

[0016] d) Measuring the crystallite size of the tungsten metal powder in reaction stream II;

[0017] e) Comparing the values measured in steps c) and d) with predetermined target values;

[0018] f) Optionally, adjusting process parameters;

[0019] The characteristics of the above-mentioned technical solution are that the reaction stream is guided through at least one analysis unit to enable the measurement of the content of tungsten(IV) oxide (WO2) and the crystallite size of the tungsten metal powder during the production process.

[0020] Particularly, the present invention is characterized in that the crystallite size of the obtained tungsten metal powder can be continuously determined during the production process and can thus be used as a quality characteristic. Tungsten metal powder consists of primary particles that can form aggregates. Depending on the particle size, individual primary particles can be single-crystalline or polycrystalline. A particle region in which regularly arranged lattice units can be observed is called a crystallite (also referred to as "grain" in the literature). These regions can extend throughout the volume of the particle. However, there may also be two or more regions in the particle where the lattice units are regularly arranged and the direction of the main axis can be different between these regions. In this case, grain boundaries can be observed between the regions. The smaller the crystallite, the stronger the scattering of the incident X-ray beam, which results in broadening of the X-ray peaks generated by diffraction at the detector.

[0021] By means of the method of the present invention, the reaction progress and the crystallite size of the product can be continuously and instantaneously monitored during the ongoing production process, which cannot be achieved by random sampling and time-consuming laboratory tests of the prior art. In addition, the method of the present invention also provides the possibility of feedback, which is necessary for targeted regulation of the production process. Therefore, it is possible to effectively produce powders of the required quality without material loss.

[0022] The method provided by the present invention is particularly suitable for quality control in the production of nanoscale and fine tungsten metal powders. Therefore, in a preferred embodiment of the present invention, the average particle size of the obtained tungsten metal powder is 20 nm to 5 μm, preferably 50 nm to 3.5 μm, and the average particle size is measured using a Fisher Sub Sieve Sizer FSSS I.m. according to ASTM standard B330. In addition, in another preferred embodiment of the present invention, the specific surface area of the obtained tungsten metal powder is 0.05 m 2 / g to 10 m 2 / g, preferably 0.15 m 2 / g to 6 m 2 / g, and the specific surface area is measured according to the method of BET (DIN ISO 9277).

[0023] The present invention is based on the fact that, surprisingly, it has been found that the known correlation between the specific surface area (measured by the BET method according to DIN ISO 9277) and the average particle size of the powder can also be applied to crystallites.

[0024] Contrary to the common prejudice in the prior art, within the scope of the present invention, it has been found that the known relationship between the particle size or grain size of the powder and its BET-specific surface area can be represented by the following equation:

[0025] d = 6 / (ρ × BET) [-Equation 1-]

[0026] where d is the particle size, ρ is the physical density of the material, and the BET value is the specific surface area measured according to DIN ISO 9277.

[0027] The oxygen content of fully reacted tungsten metal powder is proportional to the specific surface area of the powder. Therefore, it can be used to characterize tungsten metal powder and can also be used as an indicator to measure the completeness of the metal conversion reaction. Two values expressed as a percentage of WO2, namely the crystallite size and the oxygen content, can both be measured using X-ray diffraction. Therefore, in a preferred embodiment of the present invention, the analysis unit used in the method of the present invention is an X-ray diffractometer. Preferably, the crystallite size of tungsten metal powder and the content of tungsten oxide are measured by X-ray diffraction. By selecting the crystallite size and the content of WO2 in the reaction stream as quality control parameters, it is demonstrated that the method provided by the present invention also has the following advantages: two material properties can be determined by the same measurement method, that is, quality control can be carried out in one step, thus eliminating the need to separately determine material characteristics in separate measurements.

[0028] The production of tungsten metal powder based on the present invention is achieved by reducing tungsten oxide. Surprisingly, it is found that the progress of the reaction can be tracked by observing the content of tungsten oxide, especially WO2. The lower the content of tungsten oxide in the reaction stream, the smoother the reaction proceeds. Therefore, in a preferred embodiment of the present invention, the content of tungsten oxide, especially WO2, in reaction stream II is used as a measure of the reaction progress.

[0029] The values measured in steps c) and d) of the method of the present invention are compared with predetermined target values to check the progress of the reaction and the quality of the obtained tungsten metal powder. The target values to be used can be selected according to requirements and specific specifications. To obtain a reliable comparison, the comparison in step e) of the method of the present invention is repeated at short time intervals, preferably by using an evaluation module, especially by computer-aided methods.

[0030] The method of the present invention is compatible with common reducing agents used for the production of tungsten metal powder. When hydrogen is used as a reducing agent, the best effects related to the conversion of tungsten oxide are observed. Therefore, in a preferred embodiment of the present invention, hydrogen is used as a reducing agent.

[0031] In addition to the possibility of comprehensive quality control, the method of the present invention is also characterized by its simplicity of operation and the possibility of immediate and continuous analysis of the reaction progress and product control, in particular the determination of the content of tungsten oxide and the microcrystalline size of tungsten metal powder can be carried out in the same analysis unit. Therefore, in a preferred embodiment, the measurement of the content of tungsten oxide, in particular WO2, and the microcrystalline size of tungsten metal powder in steps c) and d) of the method of the present invention are carried out simultaneously or immediately successively. Within the scope of the present invention, "immediately" means a time delay of not more than 3 minutes, preferably not more than 1 minute, and more preferably not more than 30 seconds. In an even more preferred embodiment, these two parameters are determined in the same measurement, preferably using one record, in particular using an X-ray diffraction pattern or a part of an X-ray diffraction pattern. The advantage of this embodiment is that only one measurement needs to be performed and only one sensor is required in the analysis unit. Then, the data obtained in the measurement can be evaluated separately by known methods. In another preferred embodiment, the determination of the content of tungsten oxide, in particular WO2, and the microcrystalline size of tungsten metal powder in steps c) and d) of the method of the present invention are carried out in separate measurements, but these measurements are performed by the same analysis unit.

[0032] The method of the present invention enables immediate feedback from the measurement results to the system parameters, whereby the process parameters can be continuously optimized during the production process. Therefore, in a preferred embodiment, the data determined in steps c) and d) are used as the basis for possible adjustment of the process and system parameters in step f); preferably, the adjustment of the system and process parameters is achieved in such a way that the content of tungsten oxide and the microcrystalline size of tungsten metal powder correspond to the predetermined target values. The process and system parameters can be adjusted according to the values determined in steps c) and d), and the said parameters are preferably pressure, temperature, temperature distribution, volume and mass flow rate, rotational speed, concentration, filling amount, cycle time and flow rate.

[0033] The method of the present invention allows for a large number of measurements to be carried out per unit time, enabling an immediate response to changes in product quality. Preferably, the number of measurement values generated per hour within the scope of the method of the present invention is from 1 to 120, more preferably from 5 to 12. The measurements are preferably carried out in the reaction stream of a continuously operating production facility and / or at the reaction stream of a continuously operating production facility, whereby a temporal course of the measurement signal and a time-precise image of the reaction can be obtained. Thus, sampling from the product stream can preferably be omitted. In this regard, it has further proven advantageous that the energy consumed per measurement (expressed as the product of the measurement time and the measurement power, such as the radiation power of a diffractometer) cannot be set too high so as not to affect the ongoing reaction. Thus, in a preferred embodiment of the present invention, in which the measurement is carried out at a radiation energy of 50 to 500 kJ, preferably the measurement is carried out at a radiation energy of 80 to 250 kJ. The stated radiation energy is calculated according to the following formula from the product of the accelerating voltage U [volts], the tube current I [amperes] and the radiation time t [seconds]:

[0034] E = U [volts] × I [amperes] × t [seconds]

[0035] The method of the present invention allows for the immediate monitoring and comprehensive assessment of the quality of tungsten metal powder throughout the production process. For example, the reaction stream to be analyzed can be guided through different analysis units located at different positions in the production process in order to be able to monitor the reactions and processes at different stages.

[0036] Preferably, at least one analysis unit is located at the position where the product is discharged from the production process, such as at the product outlet of a continuously operating industrial furnace.

[0037] Thus, in a preferred embodiment of the present invention, the reaction stream II is guided through more than one analysis unit. In another preferred embodiment, several analysis units are distributed along the reaction stream in order to enable continuous monitoring throughout the production process. Also preferably, in one embodiment, several analysis units are arranged adjacent to each other in the production process. Another preferred embodiment is to combine the above two embodiments, in which case the different analysis units are designed to be able to exchange information with each other and the data obtained is centrally controlled and read by a control unit.

[0038] The present invention also relates to a device for carrying out the method of the present invention, said device comprising at least one analysis unit for measuring the content of tungsten oxide, in particular WO2, in the reaction stream and the crystallite size of the tungsten metal powder, wherein the analysis unit is preferably an X-ray diffractometer.

[0039] The present invention is particularly applicable to quality assurance in the production of tungsten metal powder. Accordingly, the present invention also relates to a method for quality assurance in the production of tungsten metal powder, wherein the quality assurance of the tungsten metal powder is achieved by monitoring parameters of the microcrystalline sizes of the tungsten metal powder and tungsten(IV) oxide (WO2) in the reaction stream; preferably, the characteristics of these materials are determined by X-ray diffraction method. Description of the Drawings

[0040] Figure 1 Fig. shows a scanning electron micrograph (SEM) of tungsten metal powder embedded in resin and partially polished, which was produced using the method of the present invention; particles as well as single-crystalline and polycrystalline regions can be seen in the figure.

Claims

1. A method for producing tungsten metal powder by reducing tungsten oxide, said method comprising the following steps: a) Providing a reaction stream I containing tungsten oxide particles; b) Treating reaction stream I with a reducing agent to obtain a reaction stream II containing tungsten oxide and tungsten metal powder; c) Measuring the content of WO2 in reaction stream II; d) Measuring the crystallite size of the tungsten metal powder in reaction stream II; e) Comparing the values measured in steps c) and d) with a predetermined target value; f) Adjusting process parameters; It is characterized in that the reaction stream is guided through at least one analysis unit to measure the content of WO2 and the crystallite size of the tungsten metal powder during the production process.

2. The method according to claim 1, characterized in that The specific surface area of the tungsten metal powder is 0.05 m 2 / g to 10 m 2 / g, and the specific surface area is measured according to the BET (DIN ISO 9277) method.

3. The method according to claim 2, wherein The specific surface area of the tungsten metal powder is 0.15 m 2 / g to 6 m 2 / g, and the specific surface area is measured according to the BET (DIN ISO 9277) method.

4. The method according to claim 1, wherein The analysis unit used in the said method is an X-ray diffractometer.

5. The method according to claim 1, characterized in that, The content of WO2 in reaction stream II is used as a measure of the reaction progress.

6. The method according to claim 1, characterized in that, The measurement of the content of WO2 and the crystallite size of the tungsten metal powder in steps c) and d) is carried out simultaneously.

7. The method according to claim 6, wherein The measurement of the content of WO2 and the crystallite size of the tungsten metal powder in steps c) and d) is carried out in one measurement.

8. The method according to claim 6, wherein The measurement of the content of WO2 and the crystallite size of the tungsten metal powder in steps c) and d) is carried out by using an X-ray diffraction pattern or a part of the X-ray diffraction pattern for one recording.

9. The method according to claim 1, characterized in that, The number of measurement values generated per hour within the scope of the said method is 1 to 120.

10. The method according to claim 9, characterized in that, The number of measurement values generated per hour within the scope of the said method is 5 to 12.

11. The method according to claim 1, characterized in that, The measurements described in steps c) and d) are carried out at a radiation energy of 50 to 500 kJ.

12. The method according to claim 11, wherein The measurements described in steps c) and d) are carried out at a radiation energy of 80 to 250 kJ.

13. An apparatus for implementing the method according to any one of claims 1-12, characterized in that, The said device comprises at least one analysis unit for measuring the content of WO2 and the crystallite size of the tungsten metal powder in the reaction stream.

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