Influence of Temperature on the APT Carborreduction Process for Obtaining Nanocrystalline WC Powder from Sheelite Concentrate

Abstract

Tungsten carbide (WC) is a highly relevant material due to its exceptional catalytic properties and applications in composite materials and cutting tools. The production of nanocrystalline and ultrafine WC powders contributes to enhanced properties in these applications. Therefore, this study aims to obtain nanocrystalline WC powders with a high surface area from ammonium paratungstate (APT) derived from scheelite concentrate. To achieve this, a gas-solid reaction (carbothermal reduction) of APT was carried out at different temperatures under a mixed H₂/CH₄ atmosphere. The results showed that the WC powders obtained at 850°C present a higher purity with a crystallite size of 15.8 nm composed of agglomerates of ultrafine particles of 159 μm.

Keywords:
WC; Ammonium Paratungstate; Gas-Solid Reaction; Nanocrystalline; Ultrafine

1. Introduction

Hardmetal is a material of great industrial importance due to its various applications in cutting tools¹,², composites³, and materials with catalytic properties⁴. Tungsten carbide (WC) exhibits a high melting point, exceptional hardness, low friction coefficient, chemical stability, and oxidation resistance⁵. Due to its remarkable properties and wide range of applications, extensive research has been conducted on the use of WC powders⁶-¹⁰. Another critical factor in achieving superior WC properties is the use of nanostructured powders¹¹. The incorporation of nanomaterials enhances the properties of hardmetals and tungsten alloys, facilitating the sintering process¹¹. Additionally, WC powders with a high surface area exhibit excellent catalytic properties in various reactions that improve sustainability and enable efficient energy conversion¹². Therefore, developing research focused on obtaining nanometric WC powders is of great importance.

Several techniques can be used to produce nanostructured WC powders from its precursor, including chemical vapor reaction¹³, spray conversion, high-energy milling¹⁴, sol-gel processing¹⁵, co-precipitation¹⁶, and carbothermal reduction or gas-solid reaction¹⁷. The attainment of improved WC properties is related to its purity, as well as particle and crystallite sizes¹⁸,¹⁹. The gas-solid reaction route offers a significant advantage, as diffusion mechanisms occur more rapidly with a higher mass transfer rate compared to traditional routes²⁰. The use of the gas-solid synthesis process results in the production of WC-Ni catalysts with a high surface area and crystallite sizes ranging from 16.2 to 47.7 nm²¹.

Among these methods, the gas-solid reaction process stands out as it enables the synthesis of WC powders with a high surface area. In this WC synthesis route, reactions occur at elevated temperatures (700 °C – 900 °C) in resistive furnaces under a mixed CH₄/H₂ atmosphere¹⁰,²²,²³.

In this context, this study aims to investigate the synthesis of nanostructured tungsten carbide powders from ammonium paratungstate (APT) obtained from scheelite concentrate from the Brejuí mine. The process is conducted via gas-solid reaction in a tubular furnace under a mixed CH₄/H₂ atmosphere.

2. Materials and Methods

The WC powders were synthesized from ammonium paratungstate (APT) provided by the CNPq project 383493/2022-6, obtained from scheelite concentrate from the Brejuí mine. For the carbothermal reduction process, 2 g of APT were placed in a tubular resistive furnace at temperatures of 750, 800, and 850 °C, with a heating rate of 10 °C/min and an isothermal holding time of 1 hour. During both heating and cooling, a mixed atmosphere of 5% CH₄ / 95% H₂ was used, conforme usado na literature¹⁸,²¹. Cooling was performed at room temperature. The obtained powders were characterized by X-ray diffraction (XRD) using Cu radiation, with a scan speed of 3°/min and a step size of 0.02°. The crystallite size was determined using the most intense peak of the phase and calculated based on the Scherrer equation (Equation 1), where D is the crystallite size, β is the full width at half maximum (FWHM), θ is the peak position, and λ is the wavelength of the radiation used.

$D=\frac{\mathrm{0,94}\lambda }{\beta \cos \theta }$ (1)

For morphology visualization and chemical analysis, the powders were characterized using high-resolution scanning electron microscopy (SEM-FEG) and energy-dispersive spectroscopy (EDS). Figure 1A shows that the APT exhibits a characteristic morphology with prismatic particles, while Figure 1B confirms that the XRD pattern is typical of ammonium paratungstate (APT).

Figure 1
Scanning Electron Microscopy (SEM-FEG) of the starting material APT (a) and (b) and X-ray powder diffraction patterns the APT.

3. Results and Discussion

3.1. Effect of the carbothermal reduction process on WC formation

Figure 2A presents the XRD peaks during the carbothermal reduction of APT at temperatures of 750 °C, 800 °C, and 850 °C. It can be observed that at 750 °C, WC formation does not occur, and metallic W is obtained at the end of the process. At this temperature, the elimination of water molecules takes place due to thermal decomposition. Additionally, the interaction between the powders and the hydrogen atmosphere promotes oxygen removal, leading to the formation of metallic tungsten. At 800 °C, methane in the atmosphere interacts with APT, initiating a further stage of carburization, which results in the formation of WC at this temperature. However, small W peaks are still detected at this stage (Figure 2B). With an increase in temperature to 850 °C, WC powders are formed as a consequence of the carbothermal reduction process. At this temperature, the previously observed W peaks are significantly reduced (Figure 2B), leading to the formation of powders with a higher concentration of tungsten carbide.

Figure 2
X-ray diffraction powder patterns (A) of powders synthesized at 750 °C, 800 and 850 °C (B) tungsten and carbide tungsten region in XRD.

Figure 3 shows the crystallite size of W at 700 °C and WC at 800 °C and 850 °C. It can be observed that tungsten carbide exhibits a significantly smaller crystallite size compared to metallic W, with a reduction of approximately 50% for WC obtained at 800 °C and 850 °C. Additionally, increasing the temperature from 800 °C to 850 °C results in only a slight increase in crystallite size. The slight increase in crystallite size observed may be a consequence of the increased atomic vibration, leading to diffusion and grain growth²⁴,²⁵.

Figure 3
Crystallite size of the powders sintered at 750 °C, 800 °C, and 850 °C.

3.2. Morphological changes during the carbothermal reduction process

The powders obtained through the carbothermal reduction process at 850 °C exhibited only the main WC peaks, as observed in the XRD results. Therefore, it is crucial to investigate the morphological characteristics of these powders. The Figure 4A-G presents the morphological characteristics of the powders obtained from APT at 750 °C (A-B), 800 °C (C-D), and 850 °C (E-G). It can be observed that the powders retain a prismatic morphology, characteristic of the initial APT powders. However, a distinct feature is noted—this prismatic morphology exhibits porosity (Figure 4B, D, F). This suggests that the removal of water and ammonia during the carbothermal reduction process leads to the formation of these porous structures. As the reduction temperature increases, the porosity of the structures also increases, as observed at 800 °C and 850 °C. In Figure 4G, it is observed that the prismatic morphology of WC obtained at 850 °C consists of agglomerates of ultrafine particles, approximately 159 µm in size. Additionally, the literature reports that these particles are actually small crystals (Figure 4H), as described by the Wulff model, and exhibit a morphology very similar to the γ-WC phase²⁶. According to the literature, the carbothermal reduction process results in powders with a high surface area¹⁷. The attainment of the characteristics of WC obtained at 850 °C is of great importance, as it leads to improved catalytic properties and enhances the sinterability of systems utilizing tungsten carbide²⁷,²⁸.

Figure 4
High-resolution scanning electron microscopy of powders synthesized at (A-B) 700 °C, (C-D) 800 °C, (E-F) 850 °C, (G) particle agglomerates of powders synthesized at 850 °C, and (H) crystallite particle at 850 °C following the γ-WC Wulff model²⁶.

3.4. Phase distribution in nanocrystalline WC

Figure 5 shows the EDS mapping, where red represents carbon and yellow represents tungsten, elements present in the WC phase, as observed in the XRD. It is evident from the EDS mapping that the WC obtained presents a high homogeneity between W and C, which in fact proves that carbon is associated with W.

Figure 5
EDS mapping of the WC obtained from the sintered powder at 850 °C, mapping region and elemental mapping.

Figure 6 in the EDS spectrum shows that the main peaks are associated with W and C, which shows that the powders obtained at 850 °C. The main peaks of the spectrum are associated only with the WC phase, with a quantification of 90% by mass of W, highlighting its high purity. The high purity of WC powders obtained through the gas-solid reaction process indicates that no free carbon formation occurred, leading to improved applications for WC powders.

Figure 6
X-ray energy dispersive spectrum of the powders synthesized at 850°C.

4. CONCLUSIONS

The carborreduction process of ammonium paratungstate powders obtained from scheelite concentrate from the Brejuí mine led to obtaining high quality nanostructured WC powders. The WC powders obtained showed a high Surface area formed by an agglomeration of ultrafine particles composed of γ-WC crystals according to the Wulff model. Obtaining WC formed by these particles is extremely important as it increases catalytic properties and favors the sintering process for the development of new materials.

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Publication Dates

• Publication in this collection
  12 May 2025
• Date of issue
  2025

History

• Received
  01 Feb 2025
• Reviewed
  19 Mar 2025
• Accepted
  11 Apr 2025