J. Temuujin & L. Telmenbayar

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Telmenbayar Journal of Thermal Analysis and Calorimetry An International Forum

1 Carbothermic reactions of mechanically milled coal and wolframite mixtures under Ar and N 2 atmospheres J. Temuujin & L. Telmenbayar Journal of Thermal Analysis and Calorimetry An International Forum for Thermal Studies ISSN Volume 121 Number 2 J Therm Anal Calorim (215) 121: DOI 1.17/s

2 Your article is protected by copyright and all rights are held exclusively by Akadémiai Kiadó, Budapest, Hungary. This e-offprint is for personal use only and shall not be selfarchived in electronic repositories. If you wish to self-archive your article, please use the accepted manuscript version for posting on your own website. You may further deposit the accepted manuscript version in any repository, provided it is only made publicly available 12 months after official publication or later and provided acknowledgement is given to the original source of publication and a link is inserted to the published article on Springer's website. The link must be accompanied by the following text: "The final publication is available at link.springer.com. 1 23

J Therm Anal Calorim (215) 121:597 61 DOI 1.17/s1973-15-461-5 Carbothermic reactions of mechanically milled coal and wolframite mixtures under Ar and N 2 atmospheres J. Temuujin L.

3 J Therm Anal Calorim (215) 121: DOI 1.17/s Carbothermic reactions of mechanically milled coal and wolframite mixtures under Ar and N 2 atmospheres J. Temuujin L. Telmenbayar Received: 1 December 214 / Accepted: 19 February 215 / Published online: 28 March 215 Akadémiai Kiadó, Budapest, Hungary 215 Abstract The thermal reactions of mechanically activated coal and natural wolframite mixtures have been studied using DTA TG. The reaction mixture, prepared on the basis of a thermodynamic simulation, was milled in a planetary ball mill for either 2 or. Unmilled and milled mixtures were subjected to differential thermal analysis under nitrogen and argon atmospheres with the crystalline phase composition determined with XRD. The main differences between nitrogen and argon atmospheres were observed in the raw samples. Milled samples produced primarily an iron tungsten phase, Fe 7 W 6. Under the nitrogen atmosphere, both samples showed a greater extent of reduction than under argon. Keywords Wolframite Coal Co-milling Thermal reaction Carbothermic reduction Introduction Cemented tungsten carbide (WC) is one of the super hard materials used in heavy industry, since the 192s [1]. Classic synthesis of tungsten carbide includes preparation J. Temuujin (&) Institute of Chemistry and Chemical Technology, Mongolian Academy of Sciences, Ulaanbaatar 51, Mongolia jtemuujin@yahoo.com L. Telmenbayar School of Mechanical Engineering, Mongolian University of Science and Technology, Ulaanbaatar, Mongolia of metallic tungsten powder, mixing the powder with carbon and calcining at C temperatures under a reducing atmosphere [2]. The metallic tungsten is usually prepared from natural wolframite (FeWO 4 ) or scheelite (CaWO 4 ) by dissolution, precipitation, conversion to ammonium tungstate, calcination to WO 3 and reduction to W using hydrogen [3]. Obviously, such a complex synthesis is tedious and expensive, and shortcut of the reaction step is always preferred. Recently, a considerable body of work has indicated that nanometric particles have enhanced physical properties over micron-sized and larger particles. Several authors have reported producing nanometric tungsten carbide particles by mechanical activation of the wolframite or scheelite with a carbon source [4 6]. When wolframite was mechanically activated with activated carbon or graphite was formed a dilute Fe C region, which facilitated formation of tungsten carbide [7]. Previous all mechanically assisted carbothermic reductions of wolframite were studied using either activated carbon or graphite, which are chemically processed pure products. Terry and Azubike [8] used coal in their carbothermic reduction and observed little reduction at 12 C. Therefore, mechanical activation seems to be an essential step in order to improve reaction between wolframite and coal at lower temperatures. In the present research, we examine effect of calcining atmosphere on thermal reactions of mechanically milled mixtures of wolframite with coal. Previous studies were focused on DTA TG study in Ar atmosphere. However, Ar is more expensive than N 2 that is very often used for DTA TG as inert atmosphere. If N 2 behaves same as Ar, then it can be used for carbothermic reduction at industrial scale. The goal of the present research is to compare the thermal reactions of mechanically activated coal and wolframite mixtures under Ar and N 2 atmospheres.

4 598 J. Temuujin, L. Telmenbayar Experimental Natural wolframite was obtained from the Biyant deposit (Bayan-Ulgii province, Mongolia) and washed coal from Ukhaa Khudag Mining (Tavan tolgoi coal deposit, Mongolia). The chemical composition of the wolframite was by mass%: WO 3, Fe 2 O 3,.28 Al 2 O 3, SiO 2,.92 CaO,.545 MnO. Mineralogically wolframite is a general term for a mixed crystal series of Fe(II) and Mn(II) tungstates. The minerals between pure iron tungstate and 2 % MnWO 4 are known as ferberite [9]. The content of the MnO in the present sample was negligible, and the main crystalline phase corresponds to ferberite. We have used term of wolframite instead of ferberite. Chemical analysis of the coal indicated that the content of the carbon (C), hydrogen (H) and nitrogen (N) of the coal on dry basis was 63.14, 3.39 and 1.78 %, respectively. The ash content of the coal was %. The chemical composition of the coal ash by mass % as follows: 47.9 SiO 2,15.6Al 2 O 3,12.95Fe 2 O 3,7.4CaO,3.84P 2 O 5, 3.81 SO 3, 1.7 MgO,.76 K 2 O,.61 TiO 2,.25 Na 2 O,.25 SrO,.25 MnO. A preliminary thermodynamic model of the system was constructed using HSC6 software [1]. Excess carbon was added to wolframite and the equilibrium composition calculated at temperatures up to 12 C; the results are summarized in Fig. 1. The thermodynamic simulation indicated that the major reaction predicted to occur was as follows: FeWO 4 þ 5C! Fe þ WC þ 4CO " ð1þ Consequently, mixtures of FeWO 4 and coal were prepared to give the above stoichiometry. Calculation was based on the real content of WO 3 in the wolframite and carbon in the coal. Powders were milled with a planetary ball mill (Fritsch Pulverisette 5, Idar-Oberstein, Germany) using a ball to powder mass ratio of 5:1 at a rotation speed of 25 rpm. Powders were milled for 2 and, respectively. Wolframite and carbon stoichiometric mixture was also ground in an agate mortar for 1 min and then passed via a sieve with 9 micron size and denoted as raw. Crystalline constituents of the raw and milled powders were determined by XRD (Shimadzu MAXima-X XRD- 7) using Cu-Ka radiation. Diffraction patterns were collected from 1 to 7 2H. The step size was.2 2H with a scan rate of 1 per minute. DTA TG measurements were carried out with DTA TG instrument (Hitachi, DTA/ TG 73). Measurements were conducted up to 12 C at a heating rate of 1 C min -1 under either high purity ( %) Ar or N 2 gas, and the heating chamber was purged for 5 min before commencing heating. The flow rate of the Ar or N 2 gas was 2 ml min -1. After cooling under flowing gas, the residues were characterized using XRD in order to elucidate the phases crystallized during heating. Results and discussion Figure 2 shows XRD patterns of the raw and milled mixtures. wolframite contains quartz and scheelite as the main impurities. With milling diffraction peaks of the phases, all broadened due to amorphization. There were no new peaks evident indicating that no reaction was occurring during milling. The major effects of milling were to reduce the crystallite size, accumulate microstrain in the phases present and mix the phases at an increasingly fine scale. In the raw and milled mixtures appeared a broad peak at &15, though, neither in the raw wolframite nor in the coal samples showed up. kmol 15 CO(g) 14 CO 2 (g) FeWO 4 9 Fe C WC W 1 Fe W 2 C 3 W Temperature/ C Intensity/a.u Θ/ quartz scheelite unidentified peak unmarked - wolframite Fig. 1 Thermodynamic simulation of wolframite and carbon reaction mixture, 5:1 C:FeWO 4 molar ratio Fig. 2 XRD patterns of the raw, 2 and milled wolframite and coal mixtures

5 Carbothermic reactions of mechanically milled coal and wolframite mixtures 599 Figures 3 and 4 show DTA TG graphs of the raw, 2 and milled powders measured under nitrogen and Ar atmospheres. The DTA TG traces measured under nitrogen and argon atmospheres are quite similar to each other implying that the carbothermic reduction reactions may have same reaction path under both atmospheres. This was not entirely surprising since neither tungsten nor iron form thermally stable nitrides. The difference between the traces of the raw and milled mixtures was much more significant. Mass loss from the raw mixtures started at *3 C and continued to above 12 C. Earlier work showed that pyrolysis of the coal and initial carbonization starts at 3 C with a secondary pyrolysis starting at 65 C [11]. The major mass loss event in both mixtures started at around 1 C and was accompanied by a weak endothermic peak at around 117 C. Interestingly, mass loss at around 1 C accompanied by the endothermic peak was observed in the reaction mixtures used activated carbon [4]. Apparently, this behaviour does not depend on initial carbon source type, suggesting that it is predominantly due to reactions involving wolframite. Up to 12 C, the unmilled mixtures showed mass losses of %, suggesting that the same sequence of reactions was occurring under both atmospheres. The extent of Endothermic Exothermic DTA TG Temperature/ C Mass loss/% Fig. 4 DTA TG traces of powders measured under argon atmosphere Exothermic Endothermic 2h DTA TG Temperature/ C Mass loss/% Fig. 3 DTA TG traces of powders measured under nitrogen atmosphere reaction was somewhat less than the 32 % expected from reaction (1). The onset of the main mass loss from the milled samples started at around 75 C and was independent of milling time. Compared to unmilled powders, milling reduced the onset temperature of the main mass loss of the milled samples. Milling also increased the mass loss kinetics as noted by the main mass loss from the 2 and milled powders being essentially complete above 1 and 94 C, respectively. For the milled samples, the mass loss of the milled samples became almost similar to theoretical mass loss. Decrease in mass loss onset temperature of the milled mixture suggests that a lower temperature was required to induce carbothermic reduction. A similar reduction in onset temperature was observed for milled mixtures of wolframite with activated carbon and graphite and was considered to be due to shortened solid-state diffusion paths due to increasingly intimate mixing of the wolframite and carbon phases [4, 6]. XRD patterns of the products after DTA TG are shown in Figs. 5 and 6. The XRD traces of the samples heated under nitrogen were very similar with all three showing the presence of Fe 7 W 6. A small peak for tungsten was also evident in the samples at *39, but further confirmatory peaks were absent. The broad peak at *15 was present in the original

6 6 J. Temuujin, L. Telmenbayar Intensity/a.u. Fe 7 W 6 W unidentified peak Θ/ 5 6 mixture, and the intensity of this peak is being almost same for all the heated mixtures. None of the traces shows any sign of wolframite, indicating that it had been reduced during the heating. The XRD traces for the samples heated under argon were somewhat different between the unmilled and milled products. The unmilled mixture showed formation of tungsten (W) occurred, whilst some wolframite also remained unreacted. The results for the unmilled sample suggest that the nitrogen atmosphere results in a greater extent of reduction than argon. This may be related to the slower rates of gaseous diffusion in argon than in nitrogen reducing the removal of CO from the reaction, thereby hindering the reduction reaction. Fig. 5 XRD patterns of the remaining samples after thermal measurement in N 2 atmosphere Intensity/a.u. FeWO 4 Fe 7 W 6 W X-unidentified peak F 2 C X X Θ/ Fig. 6 XRD patterns of the remaining samples after thermal measurement in Ar atmosphere In the milled samples, the traces were essentially identical to those samples heated under nitrogen with Fe 7 W 6, predominating that although the broad peak at *15 appeared in the raw and milled mixture was absent, other unidentified peaks were observed. This suggests that the 15 peak is stable in the nitrogen atmosphere and implies the phase that may also contain nitrogen. For all samples, the presence of the intermetallic iron tungsten phase (Fe 7 W 6 ) in both nitrogen and argon atmospheres suggests that this compound is the initial reaction product. The thermodynamically favourable reaction according to Eq. (1) did not occur. The starting mixture was taken according to stoichiometry. Possibly during milling occurred some decarbonization, and mixture had not enough carbon to react. In that case, F 7 W 6 could be a reasonable product. Possibly, there occurred reaction: 7FeWO 4 þ 28C! Fe 7 W 6 þ W þ 28CO " ð2þ Reaction path according to Eq. (2) suggests carbon deficiency in the reaction mixture. Clearly, it is necessary to increase carbon content of the reaction mixture for the reaction path (1). Certainly, static measurement could change the reaction path further. There is also possibility of influencing of reaction path with the inorganic matter of coal as the ash content was *26 %. However, the absence of any major phases which include any of the impurities present in the ash suggests that the influence of such impurities is small at best. Conclusions Mechanical milling of a coal and wolframite mixture greatly reduces the intensity of the wolframite reflections whilst broadening the peaks. Thermal analysis of mechanically activated coal and wolframite mixtures showed that they behaved similarly under both nitrogen and argon atmospheres. An unmilled mixture in argon atmosphere resulted in metallic tungsten, whilst in nitrogen atmosphere Fe 7 W 6 was formed. The milled mixtures all showed reduction of wolframite to Fe 7 W 6. For the unmilled sample, nitrogen atmosphere results in a greater extent of reduction than argon atmosphere. Acknowledgements Authors wish to thank Dr. Nicholas Welham for his critical suggestions and thermodynamic simulation. This work based on project Research on preparation of nano sized tungsten carbide from natural minerals with assistance of mechanical activation which obtained partial support from the Asia Research Center, Mongolia, and Korea Foundation for Advanced Studies, Korea.

7 Carbothermic reactions of mechanically milled coal and wolframite mixtures 61 References 1. Fernandes CM, Senos AMR. Cemented carbide phase diagrams: a review. Int J Ref Metals Hard Mater. 211;29: Toth IE. Transition metal carbides and nitrides. New York: Academic press; p Robinson C. Chromium, molybdenum and tungsten in Comprehensive inorganic chemistry. Oxford: Pergamon; p Temuujin J, Senna M, Jadambaa TS, Byambasuren D. Direct synthesis of tungsten carbide nanoparticles by mechanically assisted carbothermic reduction of natural wolframite. J Am Ceram Soc. 25;88: Welham NJ. Novel route to submicrometer tungsten carbide. A Inst Chem Eng J. 2;46: Temuujin J, Senna M, Jadambaa TS, Byambasuren D. Effect of mechanical activation on the synthesis of WC from wolframite (FeWO 4 ) and graphite. J Metastable Nanocryst Mater. 25; 24 25: MacKenzie KJD, Temuujin J, McCammon C, Senna M. Mechanochemical activation of mixtures of wolframite (FeWO 4 ) with carbon studied 57 Fe Mössbauer spectroscopy. J Eur Ceram Soc. 26;26: Terry BS, Azubike DC. Reduction carburization of wolframite (FeWO 4 ): part 1 reduction of natural wolframite concentrate. Trans Inst Miner Metall (Sect C Miner Process Extr Metall). 199;99: Lassner E. Tungsten, tungsten alloys, and tungsten compounds in Ullmann s encyclopedia of industrial chemistry, vol. 37. Weinheim: Wiley; 23. p Purevsuren B, Davaajav Y, Erdenechimeg P. Coal study of some Mongolian larger coal deposits. Ulaanbaatar: Toonotprint; 21. p

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