Synthesis and Characterization of Nano Tungsten Carbide from Ores

Abstract

Development of new materials with improved properties is the basic theme of materials science research. In order to do so new techniques are explored which include modification in processing route. Currently more emphasis is being given on development of new processes for the synthesis of nano materials. In order to make the processes economical, more emphasis is given on direct synthesis of high purity nano materials from the raw materials itself. This will enable to make use of these nano materials on large scale. The same is the case with nano WC where its production from pure tungsten precursors makes it costly product. It is well established fact that the use of nano WC gives rise to increased hardness and toughness as compared with the micro-crystalline counterpart having the same composition. The science behind increased physical properties is that nanocrystalline materials posses high density of core defects like grain boundaries, inter phase boundaries, dislocations etc. Moreover, nano WC finds new area of application like catalysis in many energy devices. Literature review indicates that all efforts on direct synthesis of pure single phase nanocrystalline WC from the ore has led to impure/mix phase formation. Keeping in view this technological challenge, direct synthesis of pure nano carbide (WC) from its ores (Scheelite and Wolframite) is taken up in the present work. Nano WC is synthesized from the ores of W where the impure phases are removed after the synthesis process. This is a reverse process when compared to currently used hydrometallurgical practice where prior removal of impurities is done. These unconventional routes have distinct advantages like, decrease in number of steps involved in synthesis, lesser use of acids/bases for removal of impurities (no digestion of the ore is required) and subsequently reducing the amount of effluent generation. Above all the overall process is economical and eco-friendly too. The as synthesized nano WC powders were consolidated in the matrix of Co. The composite powders showed grain growth with increase in sintering temperature. However, controlled sintering process leads to retention in nano phase WC with slight coarsening effect. The hardness of these composites are well above the acceptable limit as has been reported in the literature. The present work deals with the synthesis and characterization of nano WC directly from the Scheelite and Wolframite ores. The entire work of the thesis is divided into seven chapters. In Chapter 1, the brief history and development of tungsten carbide (WC), its crystal structure, properties and phase diagram is discussed. The hydro-metallurgical processes for the extraction of pure tungsten precursor from the ores, scheelite and wolframite are given in brief. Further, the conventional methods of production of WC and nano WC from pure tungsten precursors and ores are described. Study reveals that there is no route available for synthesis of nano WC directly from the ores. Also the diverse applications of nano tungsten carbide are discussed. Chapter 2 presents the detailed literature review of synthesis of monolithic WC and WC/Co nano powders. The literature review indicated that only solid state reaction was attempted for the synthesis of nano WC directly from ores that resulted in micron sized and/or mixed/impure phase formation. After reviewing all the segments of literature, the need for further development of solid state reaction method for direct synthesis of single phase nano WC from the ores was felt. The reported solid state reaction method requires the reduction temperature in the range of 1000 to 1100C. However, for the production of still smaller size WC particles (< 50 nm), the synthesis temperature has to be lowered to retain the nano size of WC. Based on these gaps, the aims and objectives of the thesis work was decided. Chapter 3 describes about the experimental procedures followed in the present work. On the basis of literature survey solid state reaction method was used for the synthesis of nano WC from the Scheelite and Wolframite ore. For low temperature method several experiments were planned, which include “Dry-autocalving” and was successfully used for the synthesis of single phase nano WC. For the reactions a specially designed autoclave was used. The details of synthesis and characterization techniques are described in this chapter. Chapter 4, deals with results and discussion part of the present work of experiments conducted from scheelite ore. All the experiments were conducted without any purification of scheelite ore. Removal of impurities was done after the synthesis of nanocrystalline WC. This chapter is divided into three sections; first one describes the results of solid state reaction method. The second one describes the results of thermo-chemical route. The third section gives the details of scaling experiments conducted by thermo-chemical route. In the first section, the details of systematic experiments conducted from the milled mixture scheelite ore with activated charcoal for different time periods are described. Reduction of 10h milled mixture of scheelite and activated charcoal was studied. The scheelite ore got converted into nano W and WC. The unreacted scheelite got converted into nano rods of 5 to 7 nm upon HCl leaching. Further experiments were carried out with the aim to synthesize single phase nano WC. The scheelite and activated charcoal was milled for 50h and 100h. The 100h milled sample showed complete reduction/carburization of scheelite at 1025C. After removal of impurities single phase nano WC was obtained. Further experiments were carried out on 150h milled sample, which resulted into smaller nano WC (10 to 25 nm) particles with some unreacted scheelite. The second part presents the results of synthesis by dry autoclaving. The nano WC was synthesized by heating the mixture of Scheelite, Mg and activated charcoal at 800C for 20h in an autoclave. After removal of reaction by products and impurities, single phase nano WC particles were obtained. Further, experiments were conducted for optimising temperature and time for synthesis of single phase nano WC. Single phase nano WC (14.10.1 nm average crystallite size) was obtained at low temperature (650C). The third section gives the details of scaling experiments conducted to get higher amount of nano WC. The lattice parameters of synthesized WC were calculated from refined crystal structure by Rietveld Method. In chapter 5, the results obtained from the Wolframite ore are discussed. This chapter is divided into two parts; first one describes the results by solid state reaction method. The second part describes the experiments conducted by thermo-chemical route. By solid state reaction method, nano W2C and Fe2W2C were obtained from the 50h milled mixture of wolframite and activated charcoal calcined at 1100C in argon atmosphere. In the second part, the results of synthesis of single phase nanocrystalline WC directly from the wolframite ore (containing nearly 50% impurity in as received ore) by thermo-chemical route are described. The mixture of wolframite, Mg and activated charcoal was heated at 800C for 20h in an autoclave. The resultant powders were washed by dilute HCl (1:1). The acid leached powders contained nano WC and SiO2 impurity which was leached out by HF. Furthermore, experiments were conducted at 800, 700 and 650C with holding time of 4 h. The sample heated at 800C for 4h, showed complete transition to product WC phase. After The powders were characterized by X-ray diffraction and high resolution transmission electron microscopy (HRTEM). The lattice parameters were calculated from Rietveld analysis using Fullprof program. The chapter 6 deals with discussion on the results obtained from the composite (10 and 15 wt.% Co) synthesized using WC nano particles obtained by solid state reaction method from scheelite ore. The sintering behaviour of these composites at different temperatures (1200, 1300 and 1400C) in argon atmosphere is discussed in detail. In chapter 7, the entire work of present study is summarised and concluded. The present work shows that single phase nano WC particles of different sizes can be synthesized directly from the ore concentrates; scheelite and wolframite. We have exploited acid resistant property of WC for our work over the conventional routes. Since WC is resistant to HCl attack, the major impurities can be leached out from the mixture of nanocrystalline WC and ore impurities. Impurities present in the ore do not interfere in the synthesis of nanocrystalline WC. Impurities also act as grain growth inhibitor thus facilitating in synthesis of WC nanoparticles, which are leached out after synthesis.