ARTICLE pubs.acs.org/IECR
Mechanism of Molten-Salt-Controlled Thermite Reactions Khachatur V. Manukyan,*,†,‡ Khachatur G. Kirakosyan,† Yeva G. Grigoryan,† Ofik M. Niazyan,† Armenuhi V. Yeghishyan,† Artavazd G. Kirakosyan,† and Suren L. Kharatyan†,‡ †
Laboratory of Kinetics of SHS Processes, A. B. Nalbandyan Institute of Chemical Physics, National Academy of Sciences of the Republic of Armenia (NAS RA), 5/2, P. Sevak Street, Yerevan 0014, Armenia ‡ Department of Inorganic Chemistry, Yerevan State University, 1, A. Manoogian Street, Yerevan 0025, Armenia ABSTRACT: The present work was undertaken to study the chemistry and phase formation mechanism in the salt-controlled MoO3 + Mg + NaCl thermite reaction. It was found that the structure and phase formation mechanism in the studied system primarily depend on the salt content in the initial mixtures. In salt-poor mixtures, nucleation of product particles takes place in the molten magnesium, whereas under salt-rich conditions, products are mainly formed in molten sodium chloride. Analyses of combustion temperature profiles and product microstructures and thermal analysis of reacting mixtures suggested that the molybdenum oxide reacts with the salt at early stages of the process. The formed intermediate molybdenum oxychloride and sodium molybdate then react with magnesium, yielding Mo, MgO, and NaCl phases. The low value of the activation energy (50 kJ/mol) of the combustion process also suggests that gaseous (liquid) intermediates play an important role in the phase formation mechanism.
1. INTRODUCTION The term “thermite reaction” is used to describe a class of reactions that involves a metal reacting with a metal or nonmetal oxide. This form of oxidationreduction reaction can be written in general form as M þ AO ¼ MO þ A þ ΔH where M (typically Mg, Al, Ti, Zr, Zn, etc.) is a metal, A (MoO3, WO3, Fe2O3, Cr2O3, TiO2, SiO2, CuO, etc.) is either a metal or a nonmetal, MO and AO are their corresponding oxides, and ΔH is the heat generated by the reaction.14 Because of the large exothermic effect, thermite reactions can generally be initiated locally and become self-sustaining, a feature that makes their use extremely energy-efficient. Many thermite reactions yield a molten product consisting of a heavier metallic phase and a lighter oxide phase that can be separated by gravity and surface tension forces.3 The latter makes these reactions potentially useful in a variety of metallurgical applications.47 More recently, thermite reactions have become important in the synthesis of refractory ceramics,8,9 composite materials,913 and metal powders.14,15 However, intense gas evolution due to the decomposition/vaporization of initial oxides and/or reducing elements coupled with high reaction temperatures make it difficult to control the microstructure of the obtained materials. Therefore, some approaches have been adopted to soften violent reaction conditions and tune the morphology of the products. One of the most recognized methods is the application of so-called inert diluents. Addition of diluents to thermite mixtures effectively reduces the combustion temperature and reaction rate because of the production of less heat and the longer transport distances between reactants. A modified process of conventional thermite reactions with halide salt additives is known as molten salt-controlled combustion synthesis.1315 The basic precursors for the process are known higher oxides of transition metals such as WO3, Ta2O5, MoO3, and TiO2. Metallic r 2011 American Chemical Society
magnesium and zinc are frequently used as reduction agents.14,15 For certain oxides (WO3, MoO3), sodium azide (NaN3), and sodium boron hydride (NaBH4) can also be used as reducing agents.15 Recently, it was shown that one can use this method to synthesize not only nanopowders of pure metals but also different carbides (e.g., TiC,16 WC17), silicides (e.g, MoSi213), and complex compositions such as WCCo.16 Two main factors are important in controlling the microstructure of the products in salt-controlled thermite reactions. The first factor is mild reaction conditions, such as low temperatures, which prevent intense grain growth. Second is the presence of a molten “inert phase” in the reaction zone. Because of the heat generated by self-sustaining reaction, the salt melts at about 800 °C, and further nucleation of product particles occurs in the molten salt environment, which protects them from agglomeration and grain growth. In all published works, however, the effect of sodium chloride on the chemistry of combustion process was not studied, and salt was always considered as only an inert diluent. This does not rule out the possibility that, in the initial stages of the reaction, metal oxides might react with salt yielding various intermediates. For instance, it is well-documented18,19 that MoO3 reacts intensely with NaCl at 400800 °C, forming MoO2Cl2 and Na2MoO4. Early research20,21 on the interaction of transition metal oxides, namely, Ta2O5, WO3, MoO3, and TiO2, with sodium chloride showed intense weight loss at 650950 °C, which is conditioned by evaporation of sodium chloride, volatile initial metal oxide, metal chlorides, and oxyclorides. The aqueous solutions obtained after water treatment of the metal oxide NaCl reaction products contains oxyanions and chloride species. The concentrations of soluble metal species varied from several Received: February 21, 2011 Accepted: August 23, 2011 Revised: August 2, 2011 Published: August 29, 2011 10982
dx.doi.org/10.1021/ie2003544 | Ind. Eng. Chem. Res. 2011, 50, 10982–10988
Industrial & Engineering Chemistry Research hundred parts per million to several thousand parts per million. The soluble metal species in the solutions were in the form of either metal chloride or MeO2 and MeO22. Products formed from the MoO3 + NaCl interaction showed relatively high concentrations of soluble species (e.g., Na2MoO4). Significant losses of tungsten conditioned by volatile tungsten oxychloride vaporization during the WO3 electroreduction in molten salt media at 900 °C were also reported.22 Therefore, studying the influence of sodium chloride on the initial stages of the process in the salt-controlled thermite reactions is of special interest. The present work focused on studying the effects of sodium chloride on the chemistry and phase formation mechanism of salt-controlled combustion reaction in the MoO3 + Mg + NaCl system. This system was selected because earlier research1921 showed that molybdenum trioxide reacts with sodium chloride more intensely than other transition metal oxides.
ARTICLE
Table 1. Adiabatic (Tad) and Measured Maximum (Tc) Combustion Temperatures, Amounts of Molybdenum Oxychlorides Formed (C), Heating Rates of Reagents in the Combustion Wave (Vh), and Flame Propagation Velocities (Uc) for MoO3 + 3Mg + nNaCl Mixtures n
Tad
C(MoOxCly)
Tc
V
Uc
(mol)
(°C)
(mol)
(°C)
(°C/s)
(cm/s)
0 2
3500 2260
0 0.025
2050
1500
0.33
3
1970
0.04
1760
350
0.15
4
1970
0.05
1330
50
0.07
2. EXPERIMENTAL PROCEDURE 2.1. Combustion Experiments. The precursors used in this study included MoO3 (technical condition of manufacturing no. 6-09-4471-77, Pobedit Co., Vladikavkaz, Russia, purity 99.5%, particle size
