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Self-sustaining reduction of MoO3 by the Mg–C mixture
Materials Science and Engineering B 172 (2010) 267–271
Contents lists available at ScienceDirect
Materials Science and Engineering B journal homepage: www.elsevier.com/locate/mseb
Self-sustaining reduction of MoO3 by the Mg–C mixture S.V. Aydinyan a , Zh. Gumruyan a , Kh.V. Manukyan a,b,∗ , S.L. Kharatyan a,b a b
Department of Inorganic Chemistry, Yerevan State University, 1, A. Manoogian Str., Yerevan 0025, Armenia Laboratory of Kinetics of SHS Processes, A.B. Nalbandyan Institute of Chemical Physics NAS RA, 5/2, P. Sevak Str., Yerevan 0014, Armenia
a r t i c l e
i n f o
Article history: Received 22 February 2010 Received in revised form 15 April 2010 Accepted 22 May 2010 Keywords: Molybdenum oxide Magnesium Carbon Molybdenum Combustion synthesis
a b s t r a c t The possibility of the molybdenum oxide (MoO3 ) reduction by the Mg–C mixture was investigated under the self-sustaining combustion regime. Thermodynamic analyses for the MoO3 –xMg–yC system were performed. Optimal ranges of x and y coefficients to ensure complete reduction of MoO3 to metallic molybdenum were found. The influence of x and y values on the process parameters (e.g. temperature, front propagation velocity), phase composition and the structure of products were investigated experimentally. Optimum reduction conditions of MoO3 to fine (1–3 m) molybdenum powder were defined. Possible mechanism of the molybdenum oxide reduction to metallic molybdenum was proposed. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Molybdenum and Mo-based materials are widely used in many industries (metallurgy, aerospace, electrical industry, electronics, etc.) for the excellent mechanical, thermal and electrical properties and good corrosion resistance [1–9]. The widespread method for the preparation of molybdenum is the reduction of MoO3 by hydrogen [3,6,10–15]. It was shown [10,13] that two-stage reduction starts at 550 ◦ C. At the first stage MoO3 → MoO2 transformation takes place via Mo4 O11 intermediate oxide. At the second stage, MoO2 converts to Mo at 1000–1100 ◦ C. The reduction temperature may be lowered down to 800 ◦ C using nanosized MoO3 [11]. Other distinguished method is the metallothermic reduction of molybdenum oxide [2,16]. Aluminum seems to be suitable reducer rather than calcium because of lower cost and higher heat of reduction per mole of MoO3 . Although aluminum has more tendencies to reduce MoO3 than silicon, Al–Si mixture provides more molybdenum yields than pure aluminum [17]. Average conversion at 1200 ◦ C was 92%. Carbon is also used to reduce MoO3 by the MoO3 + 3C = Mo + 3CO reaction. The Gibbs energy change (Go ) of that reaction makes about 270 kJ/mol and usually it is performed at high temperatures 600–1200 ◦ C [18,19]. Therefore, an alternative inexpensive and simple method for producing molybdenum powder is of considerable interest. One possible approach
∗ Corresponding author at: Department of Inorganic Chemistry, Yerevan State University, 1, A. Manoogian Str., Yerevan 0025, Armenia. Tel.: +374 10 28 16 10; fax: +374 10 28 16 34. E-mail address: [email protected] (Kh.V. Manukyan). 0921-5107/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2010.05.028
is to prepare molybdenum powder by the combustion synthesis (CS) or self-propagation high-temperature synthesis (SHS) method [20–22]. SHS is an effective, low-cost method for production of various industrially useful materials including metallic powders [23–25]. Advantages of the SHS in comparison with conventional methods for metal powder production are low energy consumption for achieving high temperatures, high rates of conversion, the simplicity of equipment used and high quality of final products. It is worth to note that the self-sustaining reduction of MoO3 by magnesium is characterized by high values of Go (−1040 kJ/mol), enthalpy Ho (−1060 kJ/mol) and reaction temperature (about 3000 ◦ C). At these temperatures significant amount of initial reactants evaporates. The process proceeds in a violent uncontrollable regime. In order to soften the process conditions the reduction temperature should be decreased significantly. The partial substitution of magnesium for carbon will enable to perform MoO3 reduction under the controlled self-sustaining regime. It is believed that the key factor to govern the reaction conditions (e.g. thermal regime) is amounts of reducers in the mixture. An additional benefit of using Mg–C reducing mixture is high yield of molybdenum powder due to the low content of the formed MgO. As a result subsequent leaching process of Mo powder will be significantly simplified. The objective of the present research is studying the feasibility of self-sustaining reaction in the MoO3 –Mg–C mixtures yielding the molybdenum powder.
2. Materials and methods The raw mixtures were prepared using powders of MoO3 (Pobedit Co, Russia, particle size < 15 m), Mg (MPF-3, Russia, particle
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size 150–300 m) and carbon black (P-803TM , Russia, particle size < 1 m). A green mixture of the reactants was homogenized in a ceramic mortar for 0.5 h. Cylindrical samples with 1.0–1.2 g/cm3 density, 20 mm in diameter and height of 60–70 mm were prepared from the green mixtures. Experiments were carried out in a laboratory constant-pressure reactor (CPR-31) using nitrogen (purity 99.95%, oxygen content not more than 0.02%) atmosphere at pressures from 0.5 to 5 MPa. The prepared samples were placed into the reactor, then it was sealed, evacuated, and purged with nitrogen. The reactor was then filled with nitrogen to the desired pressure. Combustion was initiated by means of a tungsten wire located on the upper surface of the sample and was programmed to produce a voltage pulse of 18 V for 2.0 s. Two BN-coated W-5Re/W-20Re thermocouples with 100 m in diameter were placed into each specimen (∼10 mm in depth) perpendicular to the cylinder axis to measure the maximum combustion temperature (Tc ) and the temperature distribution within the combustion wave. The output signals of thermocouples were transformed by a data acquisition board at the rate of 2 kHz, and were recorded by a computer. The average values of combustion velocity (Uc ) were calculated by the Uc = l/t formula, where l is the distance between thermocouples, t is the time distance between the signals of thermocouples. The standard errors of measurements for Tc and Uc were ±20 ◦ C and 5%, respectively. The reacted samples were washed with 5 wt.% hydrochloric acid. The combustion products were studied by XRD analysis with monochromatic CuK␣ radiation (diffractometer DRON-3.0). SEM (BS-300 microscope) analysis was conducted to study the microstructure of combusted samples and washed powders. The gaseous products were analyzed by gas-chromatography (LXM-72) method. Total carbon amount in the leached Mo powder was determined by measuring evolved CO2 volume at annealing in oxygen. Free carbon content was determined by the same method after dissolving Mo powder in the HF–HNO3 mixture.
3. Results 3.1. Thermodynamic consideration One of the primary tasks to design the combustion synthesis system successfully is to carefully consider the thermodynamics. For estimating the value of adiabatic combustion temperature (Tad ) and equilibrium composition of the final products, preliminary thermodynamic analyses of the MoO3 –xMg–yC system were performed using the “ISMAN–THERMO” software developed for multicomponent heterophase systems [26]. Calculation of equilibrium characteristics is based on the minimization of the Gibbs free energy of the system, which accounts for the contributions of Gibbs free energies for all components and their concentrations. Firstly, calculations for MoO3 –xMg–yC mixtures (where x and y molar coefficients were varied from 1 to 2 and 0.8 to 2.1, respectively) were performed at 2 MPa. The results are shown in Fig. 1, where Tad is presented as a function of x and y values. According to the results, three areas in the diagram may be marked out. At small amounts of magnesium and carbon Tad is calculated to be in the range of 1300–2100 ◦ C (I area). Here main products of the reaction are Mo, MgO, MoO2 , CO and CO2 . In the II area the products contain Mo, MgO and carbon oxides. Tad is in the range of 1700–2500 ◦ C. For the III area the temperature changes from 1500 to 2700 ◦ C. In this case the products contain Mo, MgO, carbon oxides and molybdenum carbides (Mo2 C and MoC). The calculation shows that CO is the major compound in the gaseous products. To determine pressure influence on Tad and products compositions similar calculations were performed with optimized x and y values (II area in Fig. 1). The results suggest (not shown) that the
Fig. 1. Thermodynamic analysis of the MoO3 –xMg–yC system: I area—Mo, MgO, MoO2 , II area—Mo, MgO and III area—Mo2 C, MoC, Mo, MgO.
optimal pressure to obtain solid products containing Mo and MgO is 1.5 MPa. At lower pressures significant amounts of MoO2 may be formed too. 3.2. Combustion process in the MoO3 –Mg–C system Firstly, the influence of x and y values on the process parameters was investigated under 2 MPa gas pressure. In order to reveal optimal reducing conditions experimentally, the larger ranges of x and y values (1.08 ≤ x ≤ 1.67 and 1.25 ≤ y≤ 1.92) were used compared with thermodynamically optimized ones. The results obtained for four series of experiments are shown in Fig. 2. y–Tc curves in all series exhibit minimum values. Combustion front propagation velocity in all experiments is about 0.65–0.75 cm/s. Phase compositions of reacted samples are listed in Table 1. Inspection of the data suggests that optimal amounts of magnesium (x) and carbon (y) to produce Mo powder are 1.5–1.7 and 1.25–1.35, respectively. Typical XRD pattern of the product obtained at optimal x and y values is shown in Fig. 3.
Fig. 2. Combustion parameters for the MoO3 –xMg–yC system vs. carbon amount (y) at x = 1.08 (1), x = 1.25 (2), x = 1.5 (3) and x = 1.67 (4).
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Table 1 Phase composition of combustion products for the MoO3 –xMg–yC mixtures depending on x and y valuesa . y(C), mole
x(Mg), mole 1.08
1.25 1.33 1.42 1.5 1.58 1.67 1.92 a
Mo, MgO, MoO2 Mo, MgO, MoO2 – Mo, MgO, MoO2 Mo, MgO, MoO2 – Mo, MgO, MoO2
1.25 and Mo2 C and Mo2 C and Mo2 C and Mo2 C and Mo2 C
Mo, MgO, MoO2 Mo, MgO, MoO2 – Mo, MgO, MoO2 – Mo, MgO, MoO2 Mo, MgO, MoO2
and Mo2 C and Mo2 C and Mo2 C and Mo2 C and Mo2 C
1.5
1.67
Mo and MgO Mo and MgO Mo, MgO and traces of Mo2 C Mo, MgO and Mo2 C – – –
Mo and MgO Mo, MgO and traces of Mo2 C – Mo, MgO and Mo2 C – Mo, MgO and Mo2 C –
For all x and y values Mo is the major phase in the product.
Fig. 3. XRD pattern of the combustion product for the MoO3 –1.5Mg–1.25C mixture.
Chromatographic analysis shows that gaseous product at experimentally optimized area is CO. In other cases CO2 was also detected with concentrations about 30–50 times smaller than that of CO. SEM analyses show (Fig. 4) that at high x values (e.g., x = 1.67, when y = 1.33) the products contain spherical particles with 1–3 m in size. Faceted crystals with 5–10 m sizes are observed too (e.g., at x = 1.25 and y = 1.17). Well-defined areas obtained from melt solidification are detected in the combustion products, especially at higher Mg contents (x = 1.67 and y = 1.17). It may be seen in Fig. 4 that in each series of experiments increment of carbon amount leads to formation of fine particles (e.g., x = 1.08 and y = 1.92). In some cases (x = 1.67 and y = 1.5) samples contain whiskers with 1–2 m in diameter and 20–30 m in length. In order to determine pressure influence, combustion of samples made from MoO3 –1.5Mg–1.25C mixtures was investigated. The results suggest (Fig. 5) that the pressure increase leads to small growth of the combustion temperature. Uc increases from 0.25 to 0.75 cm/s. At pressures from 1.5 to 5 MPa the products contain molybdenum and magnesia. At P < 1.5 MPa some amount of MoO2 is detected too. Thus, the optimal pressure is more than 1.5 MPa.
Fig. 4. Microstructure of combustion products for MoO3 –xMg–yC mixtures.
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Fig. 5. Combustion parameters for the MoO3 –1.5Mg–1.25C system vs. gas pressure, P(N2 ).
nature of the process. The present study may essentially improve the production of molybdenum powder. The proposed approach provides controllable self-sustaining reduction of MoO3 to the target metal. Thermodynamically predicted and experimentally confirmed results suggest that to produce Mo powder optimal amounts of Mg and C in the MoO3 –xMg–yC green mixtures should be 1.5–1.7 and 1.25–1.35, respectively. The reaction temperature at optimized mixtures changes from 2000 to 2200 ◦ C. At higher or lower temperatures Mo2 C or MoO2 by-products were also formed. The increment of carbon amount in the mixtures firstly leads to a decrease in the temperature (Fig. 2) due to increase in the portion of low-caloric carbon reduction. Further increase in temperature is most likely caused by heightened amounts of adsorbed oxygen associated with carbon’s high surface area (35 m2 /g). Microstructure analyses (Fig. 4) suggest that at elevated temperatures (at higher magnesium contents) the product is formed mainly from melt solidification. The increment of y value results in fine particles, probably due to small sizes of the initial carbon. Whiskers observed at higher x and y values indicate the participation of gaseous intermediates at the structure evolution. Presence of only spherical crystals at the washed molybdenum powder (Fig. 6) confirms that the faceted crystals represent MgO. The grains of washed molybdenum powder are mainly in the range of 1–3 m. Evidently structure evolution within the combustion wave in the MoO3 –Mg–C system is realized by a complex mechanism. It is well-documented in the literature [19] that at 400–650 ◦ C the low-exothermic MoO3 reduction to MoO2 by carbon occurs by the following scheme: MoO3(s) + C(s) → MoO2(s) + CO2 (CO)(g)
(1)
Table 1 shows that magnesium-poor initial mixtures yield the products containing MoO2 . On the other hand, MoO3 melting (at 800 ◦ C), sublimation and boiling (at 1155 ◦ C) [27] enable to conclude that within the combustion wave the following reactions may occur:
Fig. 6. XRD pattern (a) and the microstructure (b) of washed products obtained after combustion of the MoO3 –1.5Mg–1.25C mixture.
MoO3(s,l,g) + 3Mg(l,g) = Mo(s) + 3MgO(s)
(2)
MoO2(s) + 2Mg(l,g) = Mo(s) + 2MgO(s)
(3)
DTA data [28] show that in the MoO3 –Mg mixtures intensive reduction of MoO3 starts with magnesium melting at 650 ◦ C. Consequently during the combustion of MoO3 –Mg–C mixtures the reactions (2) and (3) proceed at temperatures higher than 650 ◦ C. Obviously, these two reactions are responsible for heat generation. Heat allocated from (2) and (3) reactions provides MoO2 reduction by carbon according to the following endothermic conversion: MoO2(s) + 2C(s) = Mo + 2CO(g)
Microstructure and phase composition of the combustion products for the MoO3 –1.5Mg–1.25C mixture at an optimum pressure (P = 2 MPa) after the acid treatment are presented in Fig. 6. In spite of the presence of some coarse particles (up to 10 m), the average size of Mo grains varies in the 1–3 m range. Total and free carbon amounts in this powder are 0.42 and 0.15 wt.%, respectively. 4. Discussion Various conventional reduction methods of MoO3 , e.g. by hydrogen [3,6,10–15], carbon [18,19] and metallothermic [2,16,17] reduction have been proposed to produce molybdenum powder. Hydrogen or carbon reduction becomes possible only at external heating (500–1200 ◦ C) conditions. Metallothermic reduction, however, is rarely used because of non-controllable high-temperature
(4)
However, MoO3 reduction or carbidization by the following reactions may not be ruled out too: MoO3(g) + 3CO(g) = Mo(s) + 3CO2(g)
(5)
2MoO3(g) + 8CO(g) = Mo2 C(s) + 7CO2(g)
(6)
Whiskers observed at higher x and y values (Fig. 4) may be proof of this assumption. Most likely, they were formed from the gaseous phase. Almost constant values of Uc (Fig. 2) allow to assume that Mg melting temperature determines the combustion velocity. Threefold increase of Uc caused by gas pressure growth (Fig. 5) confirms the important role of gaseous intermediates. Apparently, pressure increase suppresses intensive magnesium evaporation.
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5. Conclusions The results of the present study may be of essential importance to improve the production process of Mo powder. The proposed approach provides controllable self-sustaining reduction of MoO3 to the Mo using Mg–C reducing mixtures. The results show that process parameters (e.g., reaction temperature and velocity) for the MoO3 –xMg–yC system, phase composition and the structure of products are depending on the x and y values and the ambient pressure. Thermodynamically predicted and experimentally optimized values of x and y coefficients are found to be 1.5–1.7 and 1.25–1.35, respectively, yielding fine molybdenum powder (mainly from 1 to 3 m) with particulate structure. The optimal pressure is shown to be more than 1.5 MPa to provide complete conversion of the initial oxide to the metal by MoO3 → MoO2 → Mo stepwise transformations. The initial stage of the process is found to be carbon reduction of MoO3 into MoO2 . Main reactions, however, responsible for heat generation and combustion wave propagation are magnesium reduction of MoO3 and MoO2 . Acknowledgements Authors would like to acknowledge the financial support of the State Committee of Science (SCS) of the Republic of Armenia (project No. 354) and joint project of National Foundation for Science and Advanced Technology (NFSAT), State Committee of Science (SCS) of the Republic of Armenia and USA Civilian Research and Development foundation (CRDF) No. ECSP-09-17/A-02. References [1] E.R. Braithwaite, J. Haber, Molybdenum: An Outline of its Chemistry and Uses, Elsevier, New York, 1994.
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[2] S. Majumdar, T.G. Sharma, T. Samajdar, P. Bhargava, Metall. Mater. Trans. B 39 (2008) 431–438. [3] S. Majumdar, G.B. Kale, I.G. Sharma, J. Alloys Compd. 394 (2005) 168–175. [4] P. Garg, S.-J. Park, R.M. German, Int. J. Refract. Met. Hard Mater. 25 (2007) 16–24. ´ [5] J. Słoczynski, J. Solid State Chem. 118 (1995) 84–92. [6] K. Brookes, Met. Powder Rep. 59 (2004) 18–21. [7] G.-S. Kim, H.G. Kim, D.-G. Kim, S.-T. Oh, M.-J. Suk, Y.D. Kim, J. Alloys Compd. 469 (2009) 401–405. [8] K.S. Kumar, H. Van Swygenhoven, S. Suresh, Acta Mater. 51 (2003) 5743–5774. [9] T.S. Srivatsan, B.G. Ravi, A.S. Naruka, L. Riester, M. Petraroli, T.S. Sudarshan, Int. J. Refract. Met. Hard Mater. 20 (2002) 181–186. [10] V.W. Schulmeyer, M.H. Ortner, Int. J. Refract. Met. Hard Mater. 20 (2002) 261–269. [11] G.-S. Kim, Y.J. Lee, D.-G. Kim, Y.D. Kim, J. Alloys Compd. 454 (2008) 327–330. [12] T. Leisegang, A.A. Levin, J. Walter, D.C. Meyer, Cryst. Res. Technol. 40 (2005) 95–105. [13] T. Ressler, J. Wienold, R.E. Jentoft, Solid State Ionics 141–142 (2001) 243–252. [14] G.P. Martins, T. Kangsadan, G. Scott, C. Wagner, J. Van Hoose, Mater. Sci. Forum 561–565 (2007) 447–452. [15] M. Bahgat, M.-K. Paek, J.-J. Pak, J. Alloys Compd. 477 (2009) 445–449. [16] T.K. Mukherjee, D.K. Bose, C. Gupta, J. Macromol. Sci. Phys. (1980) 19–31. [17] K. Jamshidi, H. Abdizadeh, S.A. Seyyed Ebrahimi, K. Hanai, Int. J. Refract. Met. Hard Mater. 22 (2004) 243–245. [18] W. Gruner, S. Stolle, K. Wetzig, Int. J. Refract. Met. Hard Mater. 18 (2000) 137–145. [19] S. Chaudhury, S.K. Mukerjee, V.N. Vaidya, V. Venugupal, J. Alloys Compd. 261 (1997) 105–113. [20] A.G. Merzhanov, A.S. Mukasyan, Solid Flame, Torus Press, Nauka/Moscow, 2007. [21] A.S. Mukasyan, K. Martirosyan (Eds.), Combustion of Heterogeneous Systems: Fundamentals and Applications for Material Synthesis, Research Signpost Publisher, India, 2007. [22] S.T. Aruna, A.S. Mukasyan, Curr. Opin. Solid State. Mater. 12 (2008) 44–50. [23] M.H. Yamukyan, Kh.V. Manukyan, S.L. Kharatyan, Chem. Eng. J. 137 (2008) 636–642. [24] M.H. Yamukyan, Kh.V. Manukyan, S.L. Kharatyan, Armenian Chem. J. 61 (2008) 159–166. [25] J.H. Lee, H.H. Nersisyan, C.W. Won, J. Solid State Chem. 177 (2004) 251–256. [26] A.A. Shiryaev, Int. J. SHS 4 (1995) 351–362. [27] I.L. Knunyanc (Ed.), Chemical Encyclopedia, vol. 3, Bolshaya Rossiiskaya Enciklopedia, Moscow, 1992, pp. 247–248. [28] Y. Grigoryan, O. Niazyan, S. Kharatyan, 10th International Symposium on SHS, Book of Abstract, vols. 243–244, Tsakhkadzor, Armenia, 6–11 July 2009, 2009.
Contents lists available at ScienceDirect
Materials Science and Engineering B journal homepage: www.elsevier.com/locate/mseb
Self-sustaining reduction of MoO3 by the Mg–C mixture S.V. Aydinyan a , Zh. Gumruyan a , Kh.V. Manukyan a,b,∗ , S.L. Kharatyan a,b a b
Department of Inorganic Chemistry, Yerevan State University, 1, A. Manoogian Str., Yerevan 0025, Armenia Laboratory of Kinetics of SHS Processes, A.B. Nalbandyan Institute of Chemical Physics NAS RA, 5/2, P. Sevak Str., Yerevan 0014, Armenia
a r t i c l e
i n f o
Article history: Received 22 February 2010 Received in revised form 15 April 2010 Accepted 22 May 2010 Keywords: Molybdenum oxide Magnesium Carbon Molybdenum Combustion synthesis
a b s t r a c t The possibility of the molybdenum oxide (MoO3 ) reduction by the Mg–C mixture was investigated under the self-sustaining combustion regime. Thermodynamic analyses for the MoO3 –xMg–yC system were performed. Optimal ranges of x and y coefficients to ensure complete reduction of MoO3 to metallic molybdenum were found. The influence of x and y values on the process parameters (e.g. temperature, front propagation velocity), phase composition and the structure of products were investigated experimentally. Optimum reduction conditions of MoO3 to fine (1–3 m) molybdenum powder were defined. Possible mechanism of the molybdenum oxide reduction to metallic molybdenum was proposed. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Molybdenum and Mo-based materials are widely used in many industries (metallurgy, aerospace, electrical industry, electronics, etc.) for the excellent mechanical, thermal and electrical properties and good corrosion resistance [1–9]. The widespread method for the preparation of molybdenum is the reduction of MoO3 by hydrogen [3,6,10–15]. It was shown [10,13] that two-stage reduction starts at 550 ◦ C. At the first stage MoO3 → MoO2 transformation takes place via Mo4 O11 intermediate oxide. At the second stage, MoO2 converts to Mo at 1000–1100 ◦ C. The reduction temperature may be lowered down to 800 ◦ C using nanosized MoO3 [11]. Other distinguished method is the metallothermic reduction of molybdenum oxide [2,16]. Aluminum seems to be suitable reducer rather than calcium because of lower cost and higher heat of reduction per mole of MoO3 . Although aluminum has more tendencies to reduce MoO3 than silicon, Al–Si mixture provides more molybdenum yields than pure aluminum [17]. Average conversion at 1200 ◦ C was 92%. Carbon is also used to reduce MoO3 by the MoO3 + 3C = Mo + 3CO reaction. The Gibbs energy change (Go ) of that reaction makes about 270 kJ/mol and usually it is performed at high temperatures 600–1200 ◦ C [18,19]. Therefore, an alternative inexpensive and simple method for producing molybdenum powder is of considerable interest. One possible approach
∗ Corresponding author at: Department of Inorganic Chemistry, Yerevan State University, 1, A. Manoogian Str., Yerevan 0025, Armenia. Tel.: +374 10 28 16 10; fax: +374 10 28 16 34. E-mail address: [email protected] (Kh.V. Manukyan). 0921-5107/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2010.05.028
is to prepare molybdenum powder by the combustion synthesis (CS) or self-propagation high-temperature synthesis (SHS) method [20–22]. SHS is an effective, low-cost method for production of various industrially useful materials including metallic powders [23–25]. Advantages of the SHS in comparison with conventional methods for metal powder production are low energy consumption for achieving high temperatures, high rates of conversion, the simplicity of equipment used and high quality of final products. It is worth to note that the self-sustaining reduction of MoO3 by magnesium is characterized by high values of Go (−1040 kJ/mol), enthalpy Ho (−1060 kJ/mol) and reaction temperature (about 3000 ◦ C). At these temperatures significant amount of initial reactants evaporates. The process proceeds in a violent uncontrollable regime. In order to soften the process conditions the reduction temperature should be decreased significantly. The partial substitution of magnesium for carbon will enable to perform MoO3 reduction under the controlled self-sustaining regime. It is believed that the key factor to govern the reaction conditions (e.g. thermal regime) is amounts of reducers in the mixture. An additional benefit of using Mg–C reducing mixture is high yield of molybdenum powder due to the low content of the formed MgO. As a result subsequent leaching process of Mo powder will be significantly simplified. The objective of the present research is studying the feasibility of self-sustaining reaction in the MoO3 –Mg–C mixtures yielding the molybdenum powder.
2. Materials and methods The raw mixtures were prepared using powders of MoO3 (Pobedit Co, Russia, particle size < 15 m), Mg (MPF-3, Russia, particle
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size 150–300 m) and carbon black (P-803TM , Russia, particle size < 1 m). A green mixture of the reactants was homogenized in a ceramic mortar for 0.5 h. Cylindrical samples with 1.0–1.2 g/cm3 density, 20 mm in diameter and height of 60–70 mm were prepared from the green mixtures. Experiments were carried out in a laboratory constant-pressure reactor (CPR-31) using nitrogen (purity 99.95%, oxygen content not more than 0.02%) atmosphere at pressures from 0.5 to 5 MPa. The prepared samples were placed into the reactor, then it was sealed, evacuated, and purged with nitrogen. The reactor was then filled with nitrogen to the desired pressure. Combustion was initiated by means of a tungsten wire located on the upper surface of the sample and was programmed to produce a voltage pulse of 18 V for 2.0 s. Two BN-coated W-5Re/W-20Re thermocouples with 100 m in diameter were placed into each specimen (∼10 mm in depth) perpendicular to the cylinder axis to measure the maximum combustion temperature (Tc ) and the temperature distribution within the combustion wave. The output signals of thermocouples were transformed by a data acquisition board at the rate of 2 kHz, and were recorded by a computer. The average values of combustion velocity (Uc ) were calculated by the Uc = l/t formula, where l is the distance between thermocouples, t is the time distance between the signals of thermocouples. The standard errors of measurements for Tc and Uc were ±20 ◦ C and 5%, respectively. The reacted samples were washed with 5 wt.% hydrochloric acid. The combustion products were studied by XRD analysis with monochromatic CuK␣ radiation (diffractometer DRON-3.0). SEM (BS-300 microscope) analysis was conducted to study the microstructure of combusted samples and washed powders. The gaseous products were analyzed by gas-chromatography (LXM-72) method. Total carbon amount in the leached Mo powder was determined by measuring evolved CO2 volume at annealing in oxygen. Free carbon content was determined by the same method after dissolving Mo powder in the HF–HNO3 mixture.
3. Results 3.1. Thermodynamic consideration One of the primary tasks to design the combustion synthesis system successfully is to carefully consider the thermodynamics. For estimating the value of adiabatic combustion temperature (Tad ) and equilibrium composition of the final products, preliminary thermodynamic analyses of the MoO3 –xMg–yC system were performed using the “ISMAN–THERMO” software developed for multicomponent heterophase systems [26]. Calculation of equilibrium characteristics is based on the minimization of the Gibbs free energy of the system, which accounts for the contributions of Gibbs free energies for all components and their concentrations. Firstly, calculations for MoO3 –xMg–yC mixtures (where x and y molar coefficients were varied from 1 to 2 and 0.8 to 2.1, respectively) were performed at 2 MPa. The results are shown in Fig. 1, where Tad is presented as a function of x and y values. According to the results, three areas in the diagram may be marked out. At small amounts of magnesium and carbon Tad is calculated to be in the range of 1300–2100 ◦ C (I area). Here main products of the reaction are Mo, MgO, MoO2 , CO and CO2 . In the II area the products contain Mo, MgO and carbon oxides. Tad is in the range of 1700–2500 ◦ C. For the III area the temperature changes from 1500 to 2700 ◦ C. In this case the products contain Mo, MgO, carbon oxides and molybdenum carbides (Mo2 C and MoC). The calculation shows that CO is the major compound in the gaseous products. To determine pressure influence on Tad and products compositions similar calculations were performed with optimized x and y values (II area in Fig. 1). The results suggest (not shown) that the
Fig. 1. Thermodynamic analysis of the MoO3 –xMg–yC system: I area—Mo, MgO, MoO2 , II area—Mo, MgO and III area—Mo2 C, MoC, Mo, MgO.
optimal pressure to obtain solid products containing Mo and MgO is 1.5 MPa. At lower pressures significant amounts of MoO2 may be formed too. 3.2. Combustion process in the MoO3 –Mg–C system Firstly, the influence of x and y values on the process parameters was investigated under 2 MPa gas pressure. In order to reveal optimal reducing conditions experimentally, the larger ranges of x and y values (1.08 ≤ x ≤ 1.67 and 1.25 ≤ y≤ 1.92) were used compared with thermodynamically optimized ones. The results obtained for four series of experiments are shown in Fig. 2. y–Tc curves in all series exhibit minimum values. Combustion front propagation velocity in all experiments is about 0.65–0.75 cm/s. Phase compositions of reacted samples are listed in Table 1. Inspection of the data suggests that optimal amounts of magnesium (x) and carbon (y) to produce Mo powder are 1.5–1.7 and 1.25–1.35, respectively. Typical XRD pattern of the product obtained at optimal x and y values is shown in Fig. 3.
Fig. 2. Combustion parameters for the MoO3 –xMg–yC system vs. carbon amount (y) at x = 1.08 (1), x = 1.25 (2), x = 1.5 (3) and x = 1.67 (4).
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Table 1 Phase composition of combustion products for the MoO3 –xMg–yC mixtures depending on x and y valuesa . y(C), mole
x(Mg), mole 1.08
1.25 1.33 1.42 1.5 1.58 1.67 1.92 a
Mo, MgO, MoO2 Mo, MgO, MoO2 – Mo, MgO, MoO2 Mo, MgO, MoO2 – Mo, MgO, MoO2
1.25 and Mo2 C and Mo2 C and Mo2 C and Mo2 C and Mo2 C
Mo, MgO, MoO2 Mo, MgO, MoO2 – Mo, MgO, MoO2 – Mo, MgO, MoO2 Mo, MgO, MoO2
and Mo2 C and Mo2 C and Mo2 C and Mo2 C and Mo2 C
1.5
1.67
Mo and MgO Mo and MgO Mo, MgO and traces of Mo2 C Mo, MgO and Mo2 C – – –
Mo and MgO Mo, MgO and traces of Mo2 C – Mo, MgO and Mo2 C – Mo, MgO and Mo2 C –
For all x and y values Mo is the major phase in the product.
Fig. 3. XRD pattern of the combustion product for the MoO3 –1.5Mg–1.25C mixture.
Chromatographic analysis shows that gaseous product at experimentally optimized area is CO. In other cases CO2 was also detected with concentrations about 30–50 times smaller than that of CO. SEM analyses show (Fig. 4) that at high x values (e.g., x = 1.67, when y = 1.33) the products contain spherical particles with 1–3 m in size. Faceted crystals with 5–10 m sizes are observed too (e.g., at x = 1.25 and y = 1.17). Well-defined areas obtained from melt solidification are detected in the combustion products, especially at higher Mg contents (x = 1.67 and y = 1.17). It may be seen in Fig. 4 that in each series of experiments increment of carbon amount leads to formation of fine particles (e.g., x = 1.08 and y = 1.92). In some cases (x = 1.67 and y = 1.5) samples contain whiskers with 1–2 m in diameter and 20–30 m in length. In order to determine pressure influence, combustion of samples made from MoO3 –1.5Mg–1.25C mixtures was investigated. The results suggest (Fig. 5) that the pressure increase leads to small growth of the combustion temperature. Uc increases from 0.25 to 0.75 cm/s. At pressures from 1.5 to 5 MPa the products contain molybdenum and magnesia. At P < 1.5 MPa some amount of MoO2 is detected too. Thus, the optimal pressure is more than 1.5 MPa.
Fig. 4. Microstructure of combustion products for MoO3 –xMg–yC mixtures.
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Fig. 5. Combustion parameters for the MoO3 –1.5Mg–1.25C system vs. gas pressure, P(N2 ).
nature of the process. The present study may essentially improve the production of molybdenum powder. The proposed approach provides controllable self-sustaining reduction of MoO3 to the target metal. Thermodynamically predicted and experimentally confirmed results suggest that to produce Mo powder optimal amounts of Mg and C in the MoO3 –xMg–yC green mixtures should be 1.5–1.7 and 1.25–1.35, respectively. The reaction temperature at optimized mixtures changes from 2000 to 2200 ◦ C. At higher or lower temperatures Mo2 C or MoO2 by-products were also formed. The increment of carbon amount in the mixtures firstly leads to a decrease in the temperature (Fig. 2) due to increase in the portion of low-caloric carbon reduction. Further increase in temperature is most likely caused by heightened amounts of adsorbed oxygen associated with carbon’s high surface area (35 m2 /g). Microstructure analyses (Fig. 4) suggest that at elevated temperatures (at higher magnesium contents) the product is formed mainly from melt solidification. The increment of y value results in fine particles, probably due to small sizes of the initial carbon. Whiskers observed at higher x and y values indicate the participation of gaseous intermediates at the structure evolution. Presence of only spherical crystals at the washed molybdenum powder (Fig. 6) confirms that the faceted crystals represent MgO. The grains of washed molybdenum powder are mainly in the range of 1–3 m. Evidently structure evolution within the combustion wave in the MoO3 –Mg–C system is realized by a complex mechanism. It is well-documented in the literature [19] that at 400–650 ◦ C the low-exothermic MoO3 reduction to MoO2 by carbon occurs by the following scheme: MoO3(s) + C(s) → MoO2(s) + CO2 (CO)(g)
(1)
Table 1 shows that magnesium-poor initial mixtures yield the products containing MoO2 . On the other hand, MoO3 melting (at 800 ◦ C), sublimation and boiling (at 1155 ◦ C) [27] enable to conclude that within the combustion wave the following reactions may occur:
Fig. 6. XRD pattern (a) and the microstructure (b) of washed products obtained after combustion of the MoO3 –1.5Mg–1.25C mixture.
MoO3(s,l,g) + 3Mg(l,g) = Mo(s) + 3MgO(s)
(2)
MoO2(s) + 2Mg(l,g) = Mo(s) + 2MgO(s)
(3)
DTA data [28] show that in the MoO3 –Mg mixtures intensive reduction of MoO3 starts with magnesium melting at 650 ◦ C. Consequently during the combustion of MoO3 –Mg–C mixtures the reactions (2) and (3) proceed at temperatures higher than 650 ◦ C. Obviously, these two reactions are responsible for heat generation. Heat allocated from (2) and (3) reactions provides MoO2 reduction by carbon according to the following endothermic conversion: MoO2(s) + 2C(s) = Mo + 2CO(g)
Microstructure and phase composition of the combustion products for the MoO3 –1.5Mg–1.25C mixture at an optimum pressure (P = 2 MPa) after the acid treatment are presented in Fig. 6. In spite of the presence of some coarse particles (up to 10 m), the average size of Mo grains varies in the 1–3 m range. Total and free carbon amounts in this powder are 0.42 and 0.15 wt.%, respectively. 4. Discussion Various conventional reduction methods of MoO3 , e.g. by hydrogen [3,6,10–15], carbon [18,19] and metallothermic [2,16,17] reduction have been proposed to produce molybdenum powder. Hydrogen or carbon reduction becomes possible only at external heating (500–1200 ◦ C) conditions. Metallothermic reduction, however, is rarely used because of non-controllable high-temperature
(4)
However, MoO3 reduction or carbidization by the following reactions may not be ruled out too: MoO3(g) + 3CO(g) = Mo(s) + 3CO2(g)
(5)
2MoO3(g) + 8CO(g) = Mo2 C(s) + 7CO2(g)
(6)
Whiskers observed at higher x and y values (Fig. 4) may be proof of this assumption. Most likely, they were formed from the gaseous phase. Almost constant values of Uc (Fig. 2) allow to assume that Mg melting temperature determines the combustion velocity. Threefold increase of Uc caused by gas pressure growth (Fig. 5) confirms the important role of gaseous intermediates. Apparently, pressure increase suppresses intensive magnesium evaporation.
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5. Conclusions The results of the present study may be of essential importance to improve the production process of Mo powder. The proposed approach provides controllable self-sustaining reduction of MoO3 to the Mo using Mg–C reducing mixtures. The results show that process parameters (e.g., reaction temperature and velocity) for the MoO3 –xMg–yC system, phase composition and the structure of products are depending on the x and y values and the ambient pressure. Thermodynamically predicted and experimentally optimized values of x and y coefficients are found to be 1.5–1.7 and 1.25–1.35, respectively, yielding fine molybdenum powder (mainly from 1 to 3 m) with particulate structure. The optimal pressure is shown to be more than 1.5 MPa to provide complete conversion of the initial oxide to the metal by MoO3 → MoO2 → Mo stepwise transformations. The initial stage of the process is found to be carbon reduction of MoO3 into MoO2 . Main reactions, however, responsible for heat generation and combustion wave propagation are magnesium reduction of MoO3 and MoO2 . Acknowledgements Authors would like to acknowledge the financial support of the State Committee of Science (SCS) of the Republic of Armenia (project No. 354) and joint project of National Foundation for Science and Advanced Technology (NFSAT), State Committee of Science (SCS) of the Republic of Armenia and USA Civilian Research and Development foundation (CRDF) No. ECSP-09-17/A-02. References [1] E.R. Braithwaite, J. Haber, Molybdenum: An Outline of its Chemistry and Uses, Elsevier, New York, 1994.
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