Article
Preparation of nano-sized tungsten carbide via fluidized bed

https://doi.org/10.1016/j.cjche.2019.10.001Get rights and content

Abstract

Ultrafine or nano-sized of tungsten carbide (WC) is the key material to prepare ultrafine grained cemented carbides. In this paper, nano-sized WC powders were directly prepared by using industrial nano-needle violet tungsten oxide (WO2.72) as the raw material, a fluidized bed as the reactor, and CO as the carbonization gas. The relationship between particle sizes and reaction temperatures, residence times, atmospheres has been investigated systematically. In addition, the physical–chemical indexes (such as residual oxygen, total carbon and free carbon) of the products were measured. The results indicated that the particle size of WC increased with the increase of temperature from 800 to 950 °C. As the residence time increased, the particle size decreased gradually, and then increased due to slight sintering. The introduction of hydrogen reduced the carbonization rate, and is not beneficial to obtaining nano-sized WC. Products that satisfy the standard were obtained when WO2.72 reacted with CO at 850 °C, 900 °C and 950 °C for 3.0 h, 2.5 h and 2.0 h, respectively. The particle sizes of the three samples calculated from the specific surface area were 46.4 nm, 53.2 nm and 52.1 nm, respectively.

Introduction

Tungsten carbide (WC) is an important part of hard alloy and metal ceramics. Its size has a significant impact on the performance of hard alloy and metal ceramics. As the grain size of WC decreases to nano scale, the strength, modulus and wear resistance of hard alloy can be significantly improved [1,2].

Industrially, the preparation process of tungsten carbide includes H2 reduction, carbon blending and carbonization at high temperature (≥ 1400 °C) [3]. Due to the generation of large amount of water in the H2 reduction stage, volatile WO2(OH)2 can be formed by the reaction of WO3 or W with H2O, which leads to the growth of W particles [4]. Moreover, the W or WC particles are easy to be sintered as W reacts with C at high temperatures (≥ 1400 °C), which is very unfavorable for obtaining nano-sized WC [5]. Consequently, it is difficult to prepare nano-sized WC. Analyzing the above processes, the purpose of hydrogen reduction and high temperature carbonization is to strengthen the rate of mass transfer and diffusion. Therefore, the difficulty of mass transfer and diffusion in the reaction process becomes the bottleneck that restricts the preparation of nano-sized WC. To solve this problem, two types of strategies including increasing the contact area between the reactants and strengthening the reaction process were mainly adopted. The concrete methods include wire explosion [6], ball milling [7], chemical vapor synthesis [8,9], plasma synthesis [10], carbon coated on nano tungsten surface [11], etc. However, there are still some shortcomings. For example, chemical vapor synthesis can cause the generation of hydrogen chloride gas when volatile WCl6 is used as the raw material. Consequently, to prevent the corrosion of acid gas under the high temperature condition, higher requirements for the material of the equipment are put forward. Moreover, volatile tungsten sources are usually more expensive, which leads to higher preparation cost. Although ball milling technology can increase the contact area by reducing the particle size, some impurities are easy to be introduced. In addition, to obtain fully carbonized WC, it usually needs to react for a few days due to the slow reaction rate. Plasma synthesis can strengthen the reaction process, but the process is complex and expensive. Therefore, it is necessary to develop simpler and easier methods for nano-sized WC production.

It is worth noting that the gas–solid fluidization reaction can significantly increase the contact surface between the reaction media, which is conducive to strengthening the heat and mass transfer in the reaction process. Therefore, Luidold [12] has used fluidized bed as a reactor, blue tungsten oxide as a raw material, and H2 and CO as the reducing gas to prepare WC. Compared with the traditional industrial technology, the blue tungsten oxide could be completely converted to WC at 950 °C for 3 h. Unfortunately, the size of the WC crystal was about 1 μm due to the existence of H2. Obviously, except for the reactor type, reaction gas also has an important influence on the crystal size of the product. According to the reports [13,14], the introduction of CO to tube furnace reactor could modify the reduction-carburization sequence and decrease the temperature to 900 °C that needed for complete carburization. That is to say, it is possible to obtain nanoscale WC at lower reaction temperature by using pure CO as carburetant.

Therefore, to strengthen the mass transfer rate and diffusion rate between the agent and the raw material, and to avoid the generation of volatile WO2(HO)2 caused by the introduction of H2, this paper used a fluidized bed as the reactor, pure CO as the reductant and carburetant, violet tungsten oxide as the raw materials to prepare nano-sized WC. The relationships between the reaction temperature, residence time and the crystal size were investigated, and the physical–chemical indexes of the products were also analyzed.

Section snippets

Materials

Violet tungsten oxide was supplied by Ganzhou Huamao tungsten materials Co. Ltd.. According to the XRD analysis (Fig. 1a), the material phase was mainly WO2.72 (PDF No. 00-005-0393). Surface morphology analysis (Fig. 1b) showed that the raw materials with irregular shape of micron particles was about 30–100 μm. It could be found that the micron sized particles were agglomerated by nano needle crystals. These WO2.72 powders had high aspect ratio (the length/the diameter) ranged from 10 to 50.

Relationship between temperatures and crystal size

During the course of experiments, the effect of the reaction temperatures from 800 to 950 °C on the structure of WC has been systematically investigated. Fig. 3 showed the XRD spectra of the products obtained at different temperatures. It could be seen that there were six main peaks with 2θ at 31.35°, 35.46°, 48.20°, 63.87°, 73.13° and 75.59°, respectively. These peaks attributed to the (001), (100), (101), (002), (111) and (102) planes of WC, respectively [4], the diffraction peaks of W, W2C

Conclusions

In this paper, nano-sized WC was successfully prepared by using industrial nano-needle violet tungsten oxide as raw material in a fluidized bed. It was found that particle size increased with the increase of temperature from 800 to 950 °C. As residence time increased, the particle size decreased gradually, and then slightly increased due to sintering. The introduction of hydrogen reduced the carbonization rate of the reaction process, and is unable to obtain the nano WC. When the blue tungsten

Acknowledgements

The authors wish to gratefully acknowledge the financial support from the National Natural Science Foundation of China (Grant No. 21878305). The authors would like to thank the raw materials support provided Huamao Tungsten Materials Co. Ltd.

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