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C combined reducers at non-isothermal conditions
Int. Journal of Refractory Metals and Hard Materials 51 (2015) 315–323
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DTA/TGA study of molybdenum oxide reduction by Mg/Zn & Mg/C combined reducers at non-isothermal conditions A.M. Baghdasaryan a,b, O.M. Niazyan a, H.L. Khachatryan b,⁎, S.L. Kharatyan a,b a b
A.B. Nalbandyan Institute of Chemical Physics NAS RA, P. Sevak str., 5/2, 0014 Yerevan, Armenia Yerevan State University, Department of Inorganic and analytical Chemistry, A. Manukyan str., 1, 0025, Yerevan, Armenia
a r t i c l e
i n f o
Article history: Received 3 February 2015 Received in revised form 18 April 2015 Accepted 30 April 2015 Available online 1 May 2015 Keywords: MoO3 Reduction Mg/C Mg/Zn combined reducer TG DTA
a b s t r a c t In this work the reduction mechanisms of molybdenum oxide by single (Mg, Zn, C) and complex (Mg/C, Mg/Zn) reducers are reported. It is revealed that in the case of Mg and Zn the reduction undergoes before Mg/Zn melting, i.e. by solid-state mechanism. Unlike these systems, in the case of carbon, a stepwise reduction takes place. First stage (b 600 °C) is exothermic and corresponds to MoO3 → MoO2 reaction; next one is endothermic reduction (N 900 °C) of MoO2 to the mixture of Mo and Mo2C. When combined reducer is used, such as Zn/Mg, then before reduction process the Zn melts and reduction takes place by liquid–solid mechanism. In this case apart MoO2 (and MgO ∗ MoO2) MgZn2 also is formed. When the MoO3 is reduced by Mg/C mixture, the reduction process starts by carbon at 520–650 °C forming MoO2. Next the Mg melts and molten Mg reduces MoO2 yielding Mo. Based on the results obtained by DTA/TGA investigations the effective activation energy values are calculated for reduction stages for all studied mixtures. © 2015 Elsevier Ltd. All rights reserved.
1. Introduction Metallic molybdenum and its alloy/compound possessing superior feature such as excellent mechanical, thermal and electrical properties, good corrosion resistance, etc. make them attractive for electronics, metallurgical, aerospace and electrical industries [1–3]. Especially, the unique combination of physical, chemical and mechanical properties of molybdenum makes it as an ideal material for a wide spectrum of engineering applications where high temperature capability, weight considerations and ductility are key issues [4,5]. Pure molybdenum and its alloys have also received increased interest for defense-related applications where mechanical properties under high-strain rate deformation are main requirements [2]. The most widely used process for producing metallic molybdenum is based on the processing of molybdenite ore (MoS2) [6]. Generally, roasting of MoS2 concentrates is realized in air at 500 °C and produced 85% MoO3 containing concentrate. MoO3 concentrate precipitates using ammonia to form ammonium molybdate which is subsequently calcined to form pure MoO3. Then, the latter is reduced with hydrogen [7–15] or carbon [16–18] to form metallic Mo or Mo2C. In some cases metallic calcium and silicon are used to reduce trioxide [19,20]. In [10] mechanism of hydrogen reduction of MoO3 to Mo at 400– 1100 °C under well defined reaction conditions is reported. It is
⁎ Corresponding author. E-mail address: [email protected] (H.L. Khachatryan).
http://dx.doi.org/10.1016/j.ijrmhm.2015.04.037 0263-4368/© 2015 Elsevier Ltd. All rights reserved.
manifested that the formation of MoO2 always undergoes through formation intermediates: MoO3 → Mo4O11 → MoO2. The next stage is reduction of formed MoO2 yielding Mo. M.J. Kenney et al. are reported that the activation energies and frequency factors for these two stages are: E = 113 kJ mol−1, A = 4.2 × 10−l s−1 and E = 98.6 kJ mol−1, A = 2.2 × 10−2 s−1 correspondingly [13]. In [21] the kinetics of the MoO3 reduction process by hydrogen, propene, butene-1 and CO are investigated. It is reported that the rate of reduction increases in the following order: CO b H2 b C3H6 b C4H8. To reduce the tungsten and molybdenum oxides, as well as ammonium and sodium tungstates and molybdates by yielding metallic endproducts, the combustion reactions are applied during last two decades [22–31]. In [25–31] work reduction of Mo and W is successfully performed in one macroscopic stage using (Mg + C), (Mg + Zn) combined reducing mixtures. It should be underlined that it is highly challenging to monitor and reveal mechanism of the combustion reaction due to its high velocity. To solve this issue one of the approaches is the modeling the process at “soft” conditions (e.g. low heating rates and tuning the process in time) using the DTA/TGA method. It is worth to emphasize that more accurate data may be obtained if the study is performed at controlled heating conditions. This approach provides an enhanced opportunity to reveal the stepwise nature of complex reactions. Moreover, performing the process with different heating rates allows to use various kinetic schemes developed for this purpose and to calculate kinetic parameters for separate stages [32–34]. Using this approach in [35] it is an explored reduction process of WO3 by Mg/C combined reducer. In [20] the reaction kinetics of Si–MoO2 system has been
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DTG DTA
DTG
TG mg
40 140 30 TG
110
DTA
80
20
10
50
0 100
300
500
700
900
Temperature/ o C Fig. 1. DTA/TG/DTG curves for the MoO3 + 3C mixture (Vh/20° min−1, mo = 100 mg).
(DTG) and DTA point were registered by a computer in every 1 s, sample 30–100 mg in A12O3 crucibles, with volume 1 ml, A12O3 powder as reference material. All measurements were conducted in argon flow (150 ml min−1). Heating rate was programmed to be 2.5, 5, 10, 20 °C min−1. In order to reveal the reaction mechanism, the process is terminated at preset temperatures, and the samples were cooled down in inert gas flow. The cooling rate in the temperature range, presenting interest was around 300 °C min−1. The products formed at intermediate stages and final products were examined by XRD method with monochromatic CuKα radiation (diffractometer DRON-3.0, Burevestnik, Russia). The microstructures are studied by scanning electron microscope (BS-300, Tesla, CZ). The following powders are used as raw materials: molybdenum (VI) oxide, MoO3 (High grade, Pobedit Company, Russia, particle size b15 μm); magnesium powder (MPF-2 mark, Russia, particle size 100–250 μm); zinc powder (PZR-1 mark, purity 99.5%, average particle size 50 μm, VMP, Yekaterinburg, Russia); and carbon black (P-803™, Russia, particle size b 1 μm). 3. Results and discussion
analyzed by a model-free Kissinger method. X-ray diffraction analysis has confirmed the formation of Mo metal and SiO2 as the slag phase after silicothermic reduction of MoO2. The activation energy for silicothermic reduction of MoO2 to Mo is calculated to be 309 kJ mol−1. The basic goal of this research is to reveal the reaction mechanism and kinetic features of the molybdenum oxide reduction by Mg, Zn, C and (Mg + C), (Mg + Zn) mixtures at non-isothermal conditions. As it was mentioned above, these reducers have been successfully used in combustion reactions in order to produce powdered molybdenum; hence understanding the reaction mechanism is critical for controlling the reduction processes in such conditions as combustion. For this purpose the DTA/ TGA method combined with XRD analysis of intermediate and final products is applied. By the best of our knowledge, there are no other studies in the literature related to the mechanism and kinetics of reduction of MoO3 by Mg or combined reducers; therefore obtained data will fill this gap and support controlling of combustion processes in these systems. 2. Experimental The experiments are carried out by differential thermal (DTA) and thermogravimetric (TGA) analysis methods using a Q-1500 instrument (Derivatograph Q1500 MOM, Hungary). Differential thermogravimetric
1 - MoO3 _ 47-1320 2 - MoO2 _ 65-5787
In order to obtain better understanding about the reaction mechanism in the MoO3–Mg/Zn and MoO3–Mg/C ternary systems, firstly binary systems ((MoO3 + Mg), (MoO3 + Zn), (MoO3 + C)) are explored in the same conditions. 3.1. MoO3 + 3C system Experiments in the MoO3–C binary system reveal that at linear heating (Vh = 20° min− 1, mo = 100 mg) the stepwise carbothermal reduction is registered. Fig. 1 shows that the first step is exothermic, while the second and third stages are endothermic. Besides, up to 550 °C there are no significant changes in the green mixture and according to the XRD analysis the sample consists mainly of molybdenum (VI) oxide (Fig. 2, curve a). When the temperature exceeds 550 °C and reaches to 690 °C an exothermic reaction undergoes having the maximal temperature shift at Tmax = 596 °C. Simultaneously around 14% (≈ 14.1 mg) mass loss is detected. The latter corresponds to reduction of the molybdenum trioxide to molybdenum dioxide by the following equation: 2MoO3 þ C ¼ 2MoO2 þ CO2
Δm ¼ 14:67% :
3 - Mo _ 42-1120 4 - Mo2 C _ 65-8766
3 4 4
(d) 3
3
2
IntensIty
2
3
2
(c) 3 2
2
2
2
3
2 2
2
(b)
1
1
1 1
10
20
(a)
1
1 30
1 40
1
1 50
60
1 70
80
Fig. 2. XRD patterns of the MoO3 + 3C reaction products quenched from different stages: (a) — T = 550, (b) — 710, (c) - 960, (d) — 1000 °C.
ð1Þ
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317
TG mg
DTA 30
100 20 Mg melting DTA 80 10 TG
0
60 100
300
500 Temperature/ C o
700
Fig. 4. DTA/TG/DTG curves for the MoO3 + 3Mg mixture (Vh/10° min−1, mo = 70 mg).
Fig. 3 illustrates the microstructure evolution during transformations starting from green MoO3 + 3C mixture to final product. As it shown, grain size is significantly decreased while reduction process is taking place (till 960 °C) following by grain growth just after the formation of carbide phase. It is well known that heating rate may directly affect on reaction pathway and composition of the end-product. In order to explore the influence of heating rate on the carbothermal reduction process the heating rate is altered within 2.5–20 °C min−1 range. It is shown that increasing the heating rates leads to the considerable shift of Tmax values towards a higher temperature range by about 80 °С and 40 °C accordingly for the first and second stages. 3.2. MoO3 + 3Mg system Fig. 4 depicts DTA/TG curves of the MoO3 + 3Mg stoichiometric mixture. It is clear that strong exothermic reduction starts when temperature exceeds 560 °C (heating rate: Vh/10° min−1, mo = 70 mg). The maximum shift of the DTA curve appeared at Tmax = 591 °C. The main process is practically completed just the before melting point of the Mg. It must be stressed that during the process there is no mass change. The latter is evidencing fact that the reaction undergoes by solid + solid scheme (Fig. 4). According to the XRD analysis of the intermediates obtained at T = 610 and 810 °C, the MoO3 reduction proceeds partially to form Mo metal by the following reaction: MoO3 þ 3Mg ¼ Mo þ 3MgO: Fig. 3. SEM micrographs of MoO3 + 3C mixture at various stages: a — initial mixture, b — T = 960, c — 1000 °C.
The XRD analysis of intermediates quenched at 710 °C confirms this assumption (Fig. 2, curve b). The second step starts when temperature exceeds 850 °C. The endothermic reaction undergoes until 960 °C and the maximal temperature shift appears at Tmax = 890 °C. In this step noticeable mass loss (Δm ≈ 19 mg) takes place. According to XRD analysis the products obtained at 960 °C consist of molybdenum and MoO2 (Fig. 2, curve c). At 970 °C the third stage starts with formation of Mo2C (result of XRD analysis for products obtained at 1000 °C: Fig. 2, curve d). Hence in the second and third stages the MoO2 formed by reaction (1) is partially reduced to metallic Mo (2-nd stage) and Mo2C (3-rd stage) by the following reactions: MoO2 þ C→Mo þ CO=CO2
ð2Þ
MoO2 þ C→Mo2 C þ CO=CO2 :
ð3Þ
ð4Þ
In this case apart from metallic Mo, MoO2 and MoO2 ∗ MgO compounds are also detected (Fig. 5, curves b, c). The partial reduction of the MoO3 is caused by violent magnesiothermal process which may result in explosion of reacting mixture and/or evaporation of reacting components. The influence of the heating rates on the reduction process is presented in Fig. 6. One can observe that increasing the heating rate from 2.5 to 20 °C min− 1 range leads to the considerable shift of Tmax towards a high temperature range by about 90 °C. As SEM analysis reveals (not shown) there is no significant changes in microstructure during transformations of green MoO3 + 3Mg mixture to the final product (mainly Mo). 3.3. MoO3 + 3Zn system It is assumed that due to the low melting point of Zn (Tm = 419 °C), it is possible to organize the reduction process at relatively lower temperatures near the Zn melting point. Fig. 7 represents DTA/TG curves of the MoO3 + 3Zn mixture (heating rate: Vh/5° min− 1, mo = 100 mg). Likewise to the MoO3 + 3Mg system in this case also a strong exothermal effect is observed before the melting point of the metallic
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3 - MgO_ 65-0476 4 - Mg _ 35-0821
1 - MoO3 _ 47-1320 2 - Mo _ 42-1120 2
IntensIty
5
5
5
5
2 5
(c)
5 - MoO2 *MgO _ 18-0845
3
5
2
3
2
3
3
5
2
3
2
(b)
1 1
1
4
1 4
10
20
30
1
40
1
4 1
50
14
4
60
(a)
14
70
2
80
Fig. 5. XRD pattern of the MoO3 + 3Mg (a) initial mixture and reaction products quenched from 610 (b) and 810 °C (c).
reducer, i.e. zinc. Once again the main reduction process occurs in the solid state but unlike the magnesiothermal reduction it undergoes at much lower temperatures adjoining to the melting point of Zn (360– 400 °C). The XRD analysis shows that the end products obtained at 650 °C mainly consist of Mo, ZnO and non-reduced MoO3/MoO2 oxides (Fig. 8, curve c).
3.4. MoO3 + Zn + 2Mg system Fig. 9 depicts thermal curves for the MoO3 reduction by Mg + Zn combined reducers (heating rate: Vh/10° min− 1, mo = 100 mg). As can be seen from DTA curve at 410 °C endothermic process is registered which corresponds to melting of zinc. The next observed weak exothermic effect (till 470 °C) is associated with interaction of melted Zn with MoO3. At further increasing the temperature a strong exothermal interaction takes place with Tmax = 582 °C. The reduction process in the MoO3 + Zn + 2Mg system is practically finished at T = 660 °C.
The XRD analysis reveals (Fig. 10) that intermediates obtained at 500 °C mainly consist of MoO3, Zn, MoO2 and ZnO while in the second stage (at 660 °C) apart from Mo, ZnO and MgO products, MoO2, MgO ∗ MoO2 and MgZn2 are detected as well. Formation of the MgZn2 could be expected from the Mg–Zn phase diagram [36]. Thus, based on the obtained results, it can be assumed that the reduction of MoO3 by (Mg + Zn) combined reducing mixture occurs by the following sequence: the reduction process starts by zinc melting and after that interaction between melted Zn and MoO3 occurs, which leads to the partially reduction of molybdenum oxide. Next, reduction takes place by Mg. It must be underlined that in this case also the MoO3 isn't fully reduced. It is worthy to note, that at low heating rates the first reduction stage doesn't appear clearly and observes only at higher heating rates. This phenomenon may be associated with formation of Zn–Mg eutectics which have lower melting point compare to Zn. Thus, at low heating rate formation of these eutectics takes place before starting the reduction by Zn; hence reduction step could not be detected. When the
DTA 100
80 V=20 o C/min 60
V=10 o C/min
40
V=5 o C/min V=2.5 o C/min
20
0 100
300
500 Temperature/ o C
700
Fig. 6. Transformation of DTA curves for MoO3 + 3Mg mixture depending on heating rates.
A.M. Baghdasaryan et al. / Int. Journal of Refractory Metals and Hard Materials 51 (2015) 315–323
DTA
DTA
TG mg 100
TG
60
319
TG mg
80
135
50 60 125
80
40 30
40
60
115
Zn melting DTA
20 DTA
20
40
10
105
TG
Zn melting
0
0 100
20 100
200
300 Temperature/ oC
400
95 200
300
400
500
o
600
Temperature/ C
Fig. 7. DTA/TG/DTG curves for the MoO3 + 3Zn mixture (Vh/5° min−1, mo = 100 mg).
Fig. 9. DTA/TG/DTG curves for the MoO3 + Zn + 2Mg mixture (Vh/10° min−1, mo = 100 mg).
system is heating up faster then formation of Zn–Mg eutectics don't occur and reduction step by Zn appears in DTA curve.
magnesium and molybdenum dioxide. The last reduction stage can be represented as follows: MoO2 þ 2Mg ¼ Mo þ 2MgO:
3.5. MoO3 + 0.5C + 2Mg system
The XRD analysis shows that intermediates obtained at the first stage (610 °C) contain MoO2 while end-products from second stage (760 °C) consist of only Mo and MgO (Fig. 12). It is clear that using (Mg + C) rather than Mg changes the reduction pathway. Thus, in this case reduction by Mg takes place after melting of the Mg and the molten Mg reduces MoO2 rather than MoO3. In this case the endproducts are solely Mo and MgO. Fig. 13 illustrates that the grain sizes of end-products are strongly reduced and much smaller compared to that obtained from other reduction processes while intermediates' size is equivalently closer to each other (please refer to Fig. 3).
Fig. 11 illustrates DTA/TG curves vs. temperature for the MoO3 reduction by (Mg + C) binary mixture (heating rate: Vh/20° min− 1, mo = 30 mg). As can be seen MoO3 reduction by (Mg + C) mixture significantly differs from Mg and carbon reduction processes separately. In the MoO3–Mg–C ternary system the reduction process starts by carbon at 520 °C and completed at 650 °C. According to the TG curve, the mass loss for this stage is Δm = 13.6%. One can conclude that in this step a carbothermal reduction took place with accordance of reaction (1). At further increase of temperature magnesium melts, but the intensive exothermal reaction with MoO2 is delayed and takes place only around Tmax = 680 °C. The reduction process practically finished at 760 °C. Summarizing the reduction pathway, three stages can be distinguished: the first stage corresponds to the formation of MoO2 by the relatively weak reducer (carbon) (reaction 1). The second stage is Mg melting and the last one is high exothermal interaction between molten
3
4
3.5.1. Calculation of kinetic parameters The influence of the heating rate on the reduction process is undertaken for all studied systems. The quantitative results are summarized in Table 1 and are used for calculating kinetic parameters.
1 - MoO3 _ 47-1320 2 - Mo _ 42-1120 3 - MoO2 _ 65-5787
2
4
4
IntensIty
4
4 4 3
4 - ZnO _ 36-1451 5 - Zn _ 04-0831 (c)
3
5
4 2
3
4
3
ð5Þ
4
4
2
4
2 45
4
(b)
5 1 1
1,5
1 1
1
10
20
30
40
(a)
1
5 50
1 60
70
80
Fig. 8. XRD patterns of MoO3 + 3Zn initial mixture (a) and reaction products quenched from 450 (b) and 650 °C (c).
320
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3 - ZnO _ 36-1451 4 - Zn _ 04-0831
1 -MoO3 _ 47-1320 2 - Mo _ 42-1120
5 - Mg _ 35-0821 6 - MgO _ 65-0476
2 8 IntensIty
1 7,1
1
1 5
1 1
10
(c)
9
2
3
2
6
5,7 4 4
(b)
5
1
5
20
9 - MgZn2 _ 65-3578 6
3
1
7 - MoO2_ 65-5787 8 - MoO2*MgO _ 18-0845
5
4 1,4
30
4
1
40
1
50
(a)
1
60
70
80
Fig. 10. XRD patterns of the MoO3 + Zn + 2Mg reaction products quenched from different stages: (a) — T = 380, (b) — 500, (c) — 660 °C.
DTG DTA
Based on the results obtained by DTA investigations the effective activation energy values are calculated for reduction stages for all studied mixtures. There are several approaches for the calculation of the effective activation energy for non-isothermal conditions. One of the best known methods is isoconversion method formulated by Kissinger [32] and well developed recently by Starink [33,34]. Note that both methods are based on the shift of temperature corresponding to the maximum advance in the DTA curve (TDTA max) [32] or Tf [33,34] depending on the heating rate, Vh, which is kept constant. The derived expression for determination of activation energy by Starink method takes the form:
TG mg DTG
80
40
60
Mg melting
20 DTA
40
TG
0 100
20 300
500
ln
700
Temperature/oC
¼ −A
Ea þC RT f
where A = 1.0070 − 1.2 ∙ 10 − 5 E A (E A in kJ/mol), V h is the heating rate (K min − 1 ), T f is the reduction temperature corresponding
Fig. 11. DTA/TG/DTG curves for the MoO3 + 0.5C + 2Mg mixture (Vh/20° min−1, mo = 30 mg).
1 - MoO3 _ 47-1320 2 - Mo _ 42-1120 3 - MoO2 _ 65-5787
Vh T 1:8 f
4 - MgO _ 65-0476 5 - Mg _ 35-0821 6 - MoO2 *MgO _ 18-0845
2
(d)
4
6
IntensIty
2
3
3
4
2
(c)
2 4
4
3 3 5
3
(b) 3
3
1
10
5
1 1
1
20
5
30
1
40
1
5
50
1
15
5 60
(a) 1 5 70
80
Fig. 12. XRD patterns of MoO3 + 0.5C + 2Mg initial mixture (a) and reaction products quenched from 610 (b), 650 (c) and 760 °C (d).
A.M. Baghdasaryan et al. / Int. Journal of Refractory Metals and Hard Materials 51 (2015) 315–323
Fig. 13. SEM micrographs of MoO3 + 0.5C + 2Mg mixture at various stages: a — initial mixture, b — T = 650, c — 760 °C.
321
to the maximum advance in the DTA curve (°K), E is the effective activation energy of the process, and R is the universal gas constant. Vh 1 plots for all studied systems, − DTA Fig. 14 presents ln DTA 1:8 T max ðT max Þ from which the effective values of activation energy are calculated. The estimated values of effective activation energy by the Starink method for all studied systems are collected in table 2. Summarizing the obtained data on the mechanisms of MoO3 reduction by various reducers the following facts should be stressed out: although reduction of metal oxides by combined reducing agents is important both practical and scientific viewpoints, kinetics and mechanism of these processes have not been studied. There are few works related to reduction of molybdenum and tungsten trioxides by combined Mg/Zn and Mg/C reducers; however these works have been done mainly under the combustion regime [25,26,28,29]. Only one work [35] is devoted to the study of the mechanism of WO3 reduction by Mg/C mixture using the thermal analysis method. Unlike the reduction process when hydrogen is used, in the case of solid reducers formation of intermediate phases such as Mo4O11 is not observed. Besides, comparing the effective values of activation energy the minimum value is calculated to be low temperature reduction of MoO3 by zinc. In this case depending on the heating rate the reduction undergoes around the melting point of zinc (360–430 °C) by solid + solid and solid + liquid schemes. A slightly higher effective value of activation energy is calculated to be for low temperature carbothermic reduction of MoO3 to MoO2 and then MoO3 reduction by Mg. In all cases the effective values of activation energy for the magnesiothermic reductions, i.e. MoO3→Mo and MoO2→Mo, are significantly higher than that for reduction by hydrogen (113 and 98.6 kJ mol−1) [10]. Meanwhile for full reduction of MoO2 to metallic Mo the activation energy is much lower than that for reduction by silicon (309 kJ mol− 1) [20]. This may be explained by the fact that the MoO2 and Si have higher a melting point; consequently activation of the MoO2 + Si solid mixture requires higher temperature. The best results for reduction of MoO3 are obtained when Mg/C mixture is used. In some sense this process is similar to the two-step hydrogen reduction of MoO3. In both cases, firstly reduction undergoes up to MoO2 then at higher temperature metallic Mo is formed. However in the case of hydrogen reduction the reducer is only hydrogen while in the case of combined reducers Mg/C, initial reduction (from MoO3 to MoO2) takes place by carbon and the second stage (to metallic Mo) — by Mg. It is important to outline that the hydrogen or carbon reduction is energy—and time consuming processes while in case of combined reducers the process can be undertaken under the combustion mode [25, 26,28,29].
Table 1 Influence of heating rate on Tmax for all systems under the study. System
Vh/C min−1
Tmax/°C
System
Vh/°C min−1
Tmax/°C
MoO3 + 3Mg
20 10 5 2.5 20 10 5 2.5
627 591 553 540 417 400 380 357
MoO3 + 3C (I stage)
20 10 5 2.5 20 10 5 2.5
596 579 556 517 619 583 558 542
MoO3 + 3Zn
MoO3 + 0.5C + 2Mg
MoO3 + Zn + 2Mg (II stage)
Vh/°C min−1
Tmax/°C I stage
II stage
20 10 5 2.5
612 582 567 521
698 681 645 633
322
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-ln(Vh /T1.8 ) 1,4,5,6
16
-ln(Vh /T1.8 ) (2,3)
17
4 6
5
15
16
2
14
15 1
13
14
3 1000/T, K
-1
12
13 1
1.1
1.2
1.3
1.4
1.5
1.6
Fig. 14. Determination of effective values of activation energy for the following reactive mixtures: MoO3 + 3Zn (1), MoO3 + 0.5C + 2Mg, I stage (2), MoO3 + 3Mg (3), MoO3 + 3C, I stage (4), MoO3 + Zn + 2Mg (5), MoO3 + 0.5C + 2Mg II stage (6).
Table 2 The effective activation energies calculated by Starink method.
[4]
N
System
Stage
E/kJ mol−1
1 2 3
MoO3 + 3C MoO3 + 3Mg MoO3 + 0.5C + 2Mg
I stage
4 5
MoO3 + 3Zn MoO3 + Zn + 2Mg
132 123 122 197 116 147
I stage II stage
[5]
[6] [7]
[8]
4. Conclusions [9]
Molybdenum oxide, MoO3 reduction processes by Mg, Zn, carbon and (Mg + C) and (Mg + Zn) mixtures are explored at nonisothermal conditions (heating rates 2.5–20° min−1) and following remarks are revealed: • The reduction processes in MoO3–Mg and MoO3–Zn binary systems started and practically finished in a solid state before Mg (Zn) melts; • When (Mg + C) and (Mg + Zn) complex reducers are used firstly interaction of MoO3 with weak reducer (C or Zn) is taking place forming mainly MoO2. Next, Mg melts and molten Mg fully reduces formed MoO2. The best reducer is estimated to be Mg–C complex reducer and using this reducer allows to fabricate pure Mo. • The activation energy values for magnesiothermal reduction are higher for (Mg + Zn) and (Mg + C) combined mixtures as compare to pure Mg. This phenomenon is associated with the fact, that in the case of complex reducer the Mg reacts with formed MoO2 while in case of single reducer it reacts with MoO3.
[10] [11]
[12]
[13] [14]
[15] [16] [17]
[18]
[19]
Acknowledgments [20]
The authors acknowledge the financial support of the State Committee of Science of the Republic of Armenia (Projects # 11_1d167 and # 13_1D192).
[21] [22]
References [23] [1] E.R. Braithwaite, J. Haber, Molybdenum: an outline of its chemistry and uses, Elsevier, New York, 1994. [2] E.L. Baker, A. Daniels, G.P. Voorhis, Verong T, in: A. Crowson, et al., (Eds.), Molybdenum and Molybdenum Alloys, TMS, Warrendale, PA, 1998. [3] T.S. Srivatsan, B.G. Ravi, A.S. Naruka, L. Riester, M. Petraroli, T.S. Sudarshan, The microhardness and microstructural characteristics of bulk molybdenum samples
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obtained by consolidating nanopowders by plasma pressure compaction, Int. J. Refract. Met. Hard Mater. 20 (2002) 181–186. J.A. Shields Jr., E.L. Baker, Molybdenum alloys and emerging applications, Adv. Mater. Process. 1 (1999) 61. S.R. Chen, P.J. Maudlin, G.T. (Rusty) Gray III, Mechanical Properties and Constitutive Relations for Molybdenum under High Strain-Rate Deformation, in: A. Crowson, E.S. Chen, J.A. Shields, P.R. Subramanian (Eds.),Molybdenum and molybdenum alloys, TMS 1998, p. 155. A.N. Zelikman, Molybdenum, Metallurgiya, Moscow, (in Russian), 1970. S. Majumdar, T.G. Sharma, T. Samajdar, P. Bhargava, Kinetic studies on hydrogen reduction of MoO3 and morphological analysis of reduced Mo powder, Metall. Mater. Trans. B 39 (2008) 431–438. S. Majumdar, G.B. Kale, I.G. Sharma, A study on preparation of Mo–30 W alloy by aluminothermic co-reduction of mixed oxides, J. Alloys Compd. 394 (2005) 168–175. K. Brookes, New ways to make molybdenum as it enters nano-phase production, Met. Powder Rep. 59 (2004) 18. V.W. Schulmeyer, H. Ortner, Mechanisms of the hydrogen reduction of molybdenum oxides, Int. J. Refract. Met. Hard Mater. 20 (2002) 261–269. T. Ressler, J. Wienold, R.E. Jentoft, Formation of bronzes during temperatureprogrammed reduction of MoO3 with hydrogen — an in situ XRD and XAFS study, Solid State Ionics 141–142 (2001) 243–251. G.S. Kim, Y.J. Lee, D.G. Kim, Y.D. Kim, Consolidation behavior of Mo powder fabricated from milled Mo oxide by hydrogen-reduction, J. Alloys Compd. 454 (2008) 327–330. M.J. Kenney, S.C. Bevan, Kinetic study of the reduction of molybdenum trioxide by hydrogen, J. Less Common Met. 36 (1974) 23–30. J. Dang, G.H. Zhang, K.Ch. Chou, R.G. Reddy, Y. He, Y. Sun, Kinetics and mechanism of hydrogen reduction of MoO3 to MoO2, Int. J. Refract. Met. Hard Mater. 41 (2013) 216–223. J. Dang, G.H. Zhang, K.Ch. Chou, Study on kinetics of hydrogen reduction of MoO2, Int. J. Refract. Met. Hard Mater. 41 (2013) 356–362. T. Leisegang, A.A. Levin, J. Walter, D.C. Meyer, In situ X-ray analysis of MoO3 reduction, Cryst. Res. Technol. 40 (2005) 95–105. G.P. Martins, T. Kangsadan, G. Scott, C. Wagner, J.V. Hoose, A 21st. Century perspective on molybdenum powder production by hydrogen reduction, Mater. Sci. Forum 561–565 (2007) 447–452. M. Bahgat, M.K. Paek, J.J. Pak, Utilization of mill scale and recycled MoO3 from spent acid for economic synthesis of nanocrystalline intermetallic Fe–Mo alloys, J. Alloys Compd. 477 (2009) 445–449. H.L. Gilbert, F.E. Block, Bomb reduction of molybdenum trioxide by calcium metal, J. Electrochem. Soc. 102 (1955). P. Bhaskar, K. Jugal, S.P. Chakraborty, A.K. Suri, Thermal analysis on conversion of MoO3 to MoO2 and its silicothermic reduction, J. Therm. Anal. Calorim. 112 (2013) 31–36. J. Sloczynski, Kinetics and mechanism of molybdenum (VI) oxide reduction, J. Solid Chem. 118 (1995) 84–92. S.G. Ko, Ch.W. Won, B.S. Chun, H.Y. Sohn, Magnesium reduction of WO3 in a selfpropagating high-temperature synthesis (SHS) process, J. Mater. Res. 10 (04) (1995) 795–797. I.P. Borovinskaya, T.I. Ignat'eva, V.I. Vershinnikov, N.V. Sachkova, Preparation of tungsten carbide nanopowders by self-propagating high-temperature synthesis, Inorg. Mater. 40 (10) (2004) 1043–1048. V.V. Gostishchev, V.F. Boiko, N.D. Pinegina, Magnesiothermal synthesis of W-WB powders in ionic melts, Theor. Found. Chem. Eng. 43 (4) (2009) 468–472. S.V. Aydinyan, Zh. Gumruyan, Kh.V. Manukyan, S.L. Kharatyan, Self-sustaining reduction of MoO3 by Mg + C mixture, Mater. Sci. Eng. B 172 (2010) 267–271.
A.M. Baghdasaryan et al. / Int. Journal of Refractory Metals and Hard Materials 51 (2015) 315–323 [26] Kh. Manukyan, D. Davtyan, J. Bossert, S. Kharatyan, Direct reduction of ammonium molybdate to elemental molybdenum by combustion reaction, Chem. Eng. J. 68 (2011) 925–930. [27] Kh.V. Manukyan, Kh.G. Kirakosyan, Y.G. Grigoryan, O.M. Niazyan, A.V. Yeghishyan, A.G. Kirakosyan, S.L. Kharatyan, Mechanism of molten-salt-controlled thermite reactions, Ind. Eng. Chem. Res. 50 (19) (2011) 10982–10988. [28] Kh. Manukyan, S. Aydinyan, A. Aghajanyan, Y. Grigoryan, O. Niazyan, S. Kharatyan, Reaction pathway in the MoO3 + Mg + C reactive mixtures, Int. J. Refract. Met. Hard Mater. 31 (2010) 28–32. [29] S.V. Aydinyan, A.M. Baghdasaryan, O.M. Niazyan, Kh.V. Manukyan, S.L. Kharatyan, Direct Reduction of Ammonium Paramolybdate to Mo and Mo2C Powders by SHS, Proceedings of Intern. Conference, Modern Technologies and Methods of Inorganic Materials Science June 4–6 2011, pp. 125–135 (Tbilisi, Georgia). [30] Kh.V. Manukyan, A.A. Voskanyan, S. Rouvimov, A.S. Mukasyan, S.L. Kharatyan, W and two-dimensional WO3/W nanocrystals produced by controlled self-sustaining reduction of sodium tungstate, J. Mater. Res. 28 (18) (2013) 2611–2621. [31] A.A. Aghajanyan, S.V. Aydinyan, O.M. Niazyan, Kh.V. Manukyan, S.L. Kharatyan, Direct Magnesiothermic Reduction of Sodium Molybdate in the Combustion Mode,
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Contents lists available at ScienceDirect
Int. Journal of Refractory Metals and Hard Materials journal homepage: www.elsevier.com/locate/IJRMHM
DTA/TGA study of molybdenum oxide reduction by Mg/Zn & Mg/C combined reducers at non-isothermal conditions A.M. Baghdasaryan a,b, O.M. Niazyan a, H.L. Khachatryan b,⁎, S.L. Kharatyan a,b a b
A.B. Nalbandyan Institute of Chemical Physics NAS RA, P. Sevak str., 5/2, 0014 Yerevan, Armenia Yerevan State University, Department of Inorganic and analytical Chemistry, A. Manukyan str., 1, 0025, Yerevan, Armenia
a r t i c l e
i n f o
Article history: Received 3 February 2015 Received in revised form 18 April 2015 Accepted 30 April 2015 Available online 1 May 2015 Keywords: MoO3 Reduction Mg/C Mg/Zn combined reducer TG DTA
a b s t r a c t In this work the reduction mechanisms of molybdenum oxide by single (Mg, Zn, C) and complex (Mg/C, Mg/Zn) reducers are reported. It is revealed that in the case of Mg and Zn the reduction undergoes before Mg/Zn melting, i.e. by solid-state mechanism. Unlike these systems, in the case of carbon, a stepwise reduction takes place. First stage (b 600 °C) is exothermic and corresponds to MoO3 → MoO2 reaction; next one is endothermic reduction (N 900 °C) of MoO2 to the mixture of Mo and Mo2C. When combined reducer is used, such as Zn/Mg, then before reduction process the Zn melts and reduction takes place by liquid–solid mechanism. In this case apart MoO2 (and MgO ∗ MoO2) MgZn2 also is formed. When the MoO3 is reduced by Mg/C mixture, the reduction process starts by carbon at 520–650 °C forming MoO2. Next the Mg melts and molten Mg reduces MoO2 yielding Mo. Based on the results obtained by DTA/TGA investigations the effective activation energy values are calculated for reduction stages for all studied mixtures. © 2015 Elsevier Ltd. All rights reserved.
1. Introduction Metallic molybdenum and its alloy/compound possessing superior feature such as excellent mechanical, thermal and electrical properties, good corrosion resistance, etc. make them attractive for electronics, metallurgical, aerospace and electrical industries [1–3]. Especially, the unique combination of physical, chemical and mechanical properties of molybdenum makes it as an ideal material for a wide spectrum of engineering applications where high temperature capability, weight considerations and ductility are key issues [4,5]. Pure molybdenum and its alloys have also received increased interest for defense-related applications where mechanical properties under high-strain rate deformation are main requirements [2]. The most widely used process for producing metallic molybdenum is based on the processing of molybdenite ore (MoS2) [6]. Generally, roasting of MoS2 concentrates is realized in air at 500 °C and produced 85% MoO3 containing concentrate. MoO3 concentrate precipitates using ammonia to form ammonium molybdate which is subsequently calcined to form pure MoO3. Then, the latter is reduced with hydrogen [7–15] or carbon [16–18] to form metallic Mo or Mo2C. In some cases metallic calcium and silicon are used to reduce trioxide [19,20]. In [10] mechanism of hydrogen reduction of MoO3 to Mo at 400– 1100 °C under well defined reaction conditions is reported. It is
⁎ Corresponding author. E-mail address: [email protected] (H.L. Khachatryan).
http://dx.doi.org/10.1016/j.ijrmhm.2015.04.037 0263-4368/© 2015 Elsevier Ltd. All rights reserved.
manifested that the formation of MoO2 always undergoes through formation intermediates: MoO3 → Mo4O11 → MoO2. The next stage is reduction of formed MoO2 yielding Mo. M.J. Kenney et al. are reported that the activation energies and frequency factors for these two stages are: E = 113 kJ mol−1, A = 4.2 × 10−l s−1 and E = 98.6 kJ mol−1, A = 2.2 × 10−2 s−1 correspondingly [13]. In [21] the kinetics of the MoO3 reduction process by hydrogen, propene, butene-1 and CO are investigated. It is reported that the rate of reduction increases in the following order: CO b H2 b C3H6 b C4H8. To reduce the tungsten and molybdenum oxides, as well as ammonium and sodium tungstates and molybdates by yielding metallic endproducts, the combustion reactions are applied during last two decades [22–31]. In [25–31] work reduction of Mo and W is successfully performed in one macroscopic stage using (Mg + C), (Mg + Zn) combined reducing mixtures. It should be underlined that it is highly challenging to monitor and reveal mechanism of the combustion reaction due to its high velocity. To solve this issue one of the approaches is the modeling the process at “soft” conditions (e.g. low heating rates and tuning the process in time) using the DTA/TGA method. It is worth to emphasize that more accurate data may be obtained if the study is performed at controlled heating conditions. This approach provides an enhanced opportunity to reveal the stepwise nature of complex reactions. Moreover, performing the process with different heating rates allows to use various kinetic schemes developed for this purpose and to calculate kinetic parameters for separate stages [32–34]. Using this approach in [35] it is an explored reduction process of WO3 by Mg/C combined reducer. In [20] the reaction kinetics of Si–MoO2 system has been
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DTG DTA
DTG
TG mg
40 140 30 TG
110
DTA
80
20
10
50
0 100
300
500
700
900
Temperature/ o C Fig. 1. DTA/TG/DTG curves for the MoO3 + 3C mixture (Vh/20° min−1, mo = 100 mg).
(DTG) and DTA point were registered by a computer in every 1 s, sample 30–100 mg in A12O3 crucibles, with volume 1 ml, A12O3 powder as reference material. All measurements were conducted in argon flow (150 ml min−1). Heating rate was programmed to be 2.5, 5, 10, 20 °C min−1. In order to reveal the reaction mechanism, the process is terminated at preset temperatures, and the samples were cooled down in inert gas flow. The cooling rate in the temperature range, presenting interest was around 300 °C min−1. The products formed at intermediate stages and final products were examined by XRD method with monochromatic CuKα radiation (diffractometer DRON-3.0, Burevestnik, Russia). The microstructures are studied by scanning electron microscope (BS-300, Tesla, CZ). The following powders are used as raw materials: molybdenum (VI) oxide, MoO3 (High grade, Pobedit Company, Russia, particle size b15 μm); magnesium powder (MPF-2 mark, Russia, particle size 100–250 μm); zinc powder (PZR-1 mark, purity 99.5%, average particle size 50 μm, VMP, Yekaterinburg, Russia); and carbon black (P-803™, Russia, particle size b 1 μm). 3. Results and discussion
analyzed by a model-free Kissinger method. X-ray diffraction analysis has confirmed the formation of Mo metal and SiO2 as the slag phase after silicothermic reduction of MoO2. The activation energy for silicothermic reduction of MoO2 to Mo is calculated to be 309 kJ mol−1. The basic goal of this research is to reveal the reaction mechanism and kinetic features of the molybdenum oxide reduction by Mg, Zn, C and (Mg + C), (Mg + Zn) mixtures at non-isothermal conditions. As it was mentioned above, these reducers have been successfully used in combustion reactions in order to produce powdered molybdenum; hence understanding the reaction mechanism is critical for controlling the reduction processes in such conditions as combustion. For this purpose the DTA/ TGA method combined with XRD analysis of intermediate and final products is applied. By the best of our knowledge, there are no other studies in the literature related to the mechanism and kinetics of reduction of MoO3 by Mg or combined reducers; therefore obtained data will fill this gap and support controlling of combustion processes in these systems. 2. Experimental The experiments are carried out by differential thermal (DTA) and thermogravimetric (TGA) analysis methods using a Q-1500 instrument (Derivatograph Q1500 MOM, Hungary). Differential thermogravimetric
1 - MoO3 _ 47-1320 2 - MoO2 _ 65-5787
In order to obtain better understanding about the reaction mechanism in the MoO3–Mg/Zn and MoO3–Mg/C ternary systems, firstly binary systems ((MoO3 + Mg), (MoO3 + Zn), (MoO3 + C)) are explored in the same conditions. 3.1. MoO3 + 3C system Experiments in the MoO3–C binary system reveal that at linear heating (Vh = 20° min− 1, mo = 100 mg) the stepwise carbothermal reduction is registered. Fig. 1 shows that the first step is exothermic, while the second and third stages are endothermic. Besides, up to 550 °C there are no significant changes in the green mixture and according to the XRD analysis the sample consists mainly of molybdenum (VI) oxide (Fig. 2, curve a). When the temperature exceeds 550 °C and reaches to 690 °C an exothermic reaction undergoes having the maximal temperature shift at Tmax = 596 °C. Simultaneously around 14% (≈ 14.1 mg) mass loss is detected. The latter corresponds to reduction of the molybdenum trioxide to molybdenum dioxide by the following equation: 2MoO3 þ C ¼ 2MoO2 þ CO2
Δm ¼ 14:67% :
3 - Mo _ 42-1120 4 - Mo2 C _ 65-8766
3 4 4
(d) 3
3
2
IntensIty
2
3
2
(c) 3 2
2
2
2
3
2 2
2
(b)
1
1
1 1
10
20
(a)
1
1 30
1 40
1
1 50
60
1 70
80
Fig. 2. XRD patterns of the MoO3 + 3C reaction products quenched from different stages: (a) — T = 550, (b) — 710, (c) - 960, (d) — 1000 °C.
ð1Þ
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317
TG mg
DTA 30
100 20 Mg melting DTA 80 10 TG
0
60 100
300
500 Temperature/ C o
700
Fig. 4. DTA/TG/DTG curves for the MoO3 + 3Mg mixture (Vh/10° min−1, mo = 70 mg).
Fig. 3 illustrates the microstructure evolution during transformations starting from green MoO3 + 3C mixture to final product. As it shown, grain size is significantly decreased while reduction process is taking place (till 960 °C) following by grain growth just after the formation of carbide phase. It is well known that heating rate may directly affect on reaction pathway and composition of the end-product. In order to explore the influence of heating rate on the carbothermal reduction process the heating rate is altered within 2.5–20 °C min−1 range. It is shown that increasing the heating rates leads to the considerable shift of Tmax values towards a higher temperature range by about 80 °С and 40 °C accordingly for the first and second stages. 3.2. MoO3 + 3Mg system Fig. 4 depicts DTA/TG curves of the MoO3 + 3Mg stoichiometric mixture. It is clear that strong exothermic reduction starts when temperature exceeds 560 °C (heating rate: Vh/10° min−1, mo = 70 mg). The maximum shift of the DTA curve appeared at Tmax = 591 °C. The main process is practically completed just the before melting point of the Mg. It must be stressed that during the process there is no mass change. The latter is evidencing fact that the reaction undergoes by solid + solid scheme (Fig. 4). According to the XRD analysis of the intermediates obtained at T = 610 and 810 °C, the MoO3 reduction proceeds partially to form Mo metal by the following reaction: MoO3 þ 3Mg ¼ Mo þ 3MgO: Fig. 3. SEM micrographs of MoO3 + 3C mixture at various stages: a — initial mixture, b — T = 960, c — 1000 °C.
The XRD analysis of intermediates quenched at 710 °C confirms this assumption (Fig. 2, curve b). The second step starts when temperature exceeds 850 °C. The endothermic reaction undergoes until 960 °C and the maximal temperature shift appears at Tmax = 890 °C. In this step noticeable mass loss (Δm ≈ 19 mg) takes place. According to XRD analysis the products obtained at 960 °C consist of molybdenum and MoO2 (Fig. 2, curve c). At 970 °C the third stage starts with formation of Mo2C (result of XRD analysis for products obtained at 1000 °C: Fig. 2, curve d). Hence in the second and third stages the MoO2 formed by reaction (1) is partially reduced to metallic Mo (2-nd stage) and Mo2C (3-rd stage) by the following reactions: MoO2 þ C→Mo þ CO=CO2
ð2Þ
MoO2 þ C→Mo2 C þ CO=CO2 :
ð3Þ
ð4Þ
In this case apart from metallic Mo, MoO2 and MoO2 ∗ MgO compounds are also detected (Fig. 5, curves b, c). The partial reduction of the MoO3 is caused by violent magnesiothermal process which may result in explosion of reacting mixture and/or evaporation of reacting components. The influence of the heating rates on the reduction process is presented in Fig. 6. One can observe that increasing the heating rate from 2.5 to 20 °C min− 1 range leads to the considerable shift of Tmax towards a high temperature range by about 90 °C. As SEM analysis reveals (not shown) there is no significant changes in microstructure during transformations of green MoO3 + 3Mg mixture to the final product (mainly Mo). 3.3. MoO3 + 3Zn system It is assumed that due to the low melting point of Zn (Tm = 419 °C), it is possible to organize the reduction process at relatively lower temperatures near the Zn melting point. Fig. 7 represents DTA/TG curves of the MoO3 + 3Zn mixture (heating rate: Vh/5° min− 1, mo = 100 mg). Likewise to the MoO3 + 3Mg system in this case also a strong exothermal effect is observed before the melting point of the metallic
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3 - MgO_ 65-0476 4 - Mg _ 35-0821
1 - MoO3 _ 47-1320 2 - Mo _ 42-1120 2
IntensIty
5
5
5
5
2 5
(c)
5 - MoO2 *MgO _ 18-0845
3
5
2
3
2
3
3
5
2
3
2
(b)
1 1
1
4
1 4
10
20
30
1
40
1
4 1
50
14
4
60
(a)
14
70
2
80
Fig. 5. XRD pattern of the MoO3 + 3Mg (a) initial mixture and reaction products quenched from 610 (b) and 810 °C (c).
reducer, i.e. zinc. Once again the main reduction process occurs in the solid state but unlike the magnesiothermal reduction it undergoes at much lower temperatures adjoining to the melting point of Zn (360– 400 °C). The XRD analysis shows that the end products obtained at 650 °C mainly consist of Mo, ZnO and non-reduced MoO3/MoO2 oxides (Fig. 8, curve c).
3.4. MoO3 + Zn + 2Mg system Fig. 9 depicts thermal curves for the MoO3 reduction by Mg + Zn combined reducers (heating rate: Vh/10° min− 1, mo = 100 mg). As can be seen from DTA curve at 410 °C endothermic process is registered which corresponds to melting of zinc. The next observed weak exothermic effect (till 470 °C) is associated with interaction of melted Zn with MoO3. At further increasing the temperature a strong exothermal interaction takes place with Tmax = 582 °C. The reduction process in the MoO3 + Zn + 2Mg system is practically finished at T = 660 °C.
The XRD analysis reveals (Fig. 10) that intermediates obtained at 500 °C mainly consist of MoO3, Zn, MoO2 and ZnO while in the second stage (at 660 °C) apart from Mo, ZnO and MgO products, MoO2, MgO ∗ MoO2 and MgZn2 are detected as well. Formation of the MgZn2 could be expected from the Mg–Zn phase diagram [36]. Thus, based on the obtained results, it can be assumed that the reduction of MoO3 by (Mg + Zn) combined reducing mixture occurs by the following sequence: the reduction process starts by zinc melting and after that interaction between melted Zn and MoO3 occurs, which leads to the partially reduction of molybdenum oxide. Next, reduction takes place by Mg. It must be underlined that in this case also the MoO3 isn't fully reduced. It is worthy to note, that at low heating rates the first reduction stage doesn't appear clearly and observes only at higher heating rates. This phenomenon may be associated with formation of Zn–Mg eutectics which have lower melting point compare to Zn. Thus, at low heating rate formation of these eutectics takes place before starting the reduction by Zn; hence reduction step could not be detected. When the
DTA 100
80 V=20 o C/min 60
V=10 o C/min
40
V=5 o C/min V=2.5 o C/min
20
0 100
300
500 Temperature/ o C
700
Fig. 6. Transformation of DTA curves for MoO3 + 3Mg mixture depending on heating rates.
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DTA
DTA
TG mg 100
TG
60
319
TG mg
80
135
50 60 125
80
40 30
40
60
115
Zn melting DTA
20 DTA
20
40
10
105
TG
Zn melting
0
0 100
20 100
200
300 Temperature/ oC
400
95 200
300
400
500
o
600
Temperature/ C
Fig. 7. DTA/TG/DTG curves for the MoO3 + 3Zn mixture (Vh/5° min−1, mo = 100 mg).
Fig. 9. DTA/TG/DTG curves for the MoO3 + Zn + 2Mg mixture (Vh/10° min−1, mo = 100 mg).
system is heating up faster then formation of Zn–Mg eutectics don't occur and reduction step by Zn appears in DTA curve.
magnesium and molybdenum dioxide. The last reduction stage can be represented as follows: MoO2 þ 2Mg ¼ Mo þ 2MgO:
3.5. MoO3 + 0.5C + 2Mg system
The XRD analysis shows that intermediates obtained at the first stage (610 °C) contain MoO2 while end-products from second stage (760 °C) consist of only Mo and MgO (Fig. 12). It is clear that using (Mg + C) rather than Mg changes the reduction pathway. Thus, in this case reduction by Mg takes place after melting of the Mg and the molten Mg reduces MoO2 rather than MoO3. In this case the endproducts are solely Mo and MgO. Fig. 13 illustrates that the grain sizes of end-products are strongly reduced and much smaller compared to that obtained from other reduction processes while intermediates' size is equivalently closer to each other (please refer to Fig. 3).
Fig. 11 illustrates DTA/TG curves vs. temperature for the MoO3 reduction by (Mg + C) binary mixture (heating rate: Vh/20° min− 1, mo = 30 mg). As can be seen MoO3 reduction by (Mg + C) mixture significantly differs from Mg and carbon reduction processes separately. In the MoO3–Mg–C ternary system the reduction process starts by carbon at 520 °C and completed at 650 °C. According to the TG curve, the mass loss for this stage is Δm = 13.6%. One can conclude that in this step a carbothermal reduction took place with accordance of reaction (1). At further increase of temperature magnesium melts, but the intensive exothermal reaction with MoO2 is delayed and takes place only around Tmax = 680 °C. The reduction process practically finished at 760 °C. Summarizing the reduction pathway, three stages can be distinguished: the first stage corresponds to the formation of MoO2 by the relatively weak reducer (carbon) (reaction 1). The second stage is Mg melting and the last one is high exothermal interaction between molten
3
4
3.5.1. Calculation of kinetic parameters The influence of the heating rate on the reduction process is undertaken for all studied systems. The quantitative results are summarized in Table 1 and are used for calculating kinetic parameters.
1 - MoO3 _ 47-1320 2 - Mo _ 42-1120 3 - MoO2 _ 65-5787
2
4
4
IntensIty
4
4 4 3
4 - ZnO _ 36-1451 5 - Zn _ 04-0831 (c)
3
5
4 2
3
4
3
ð5Þ
4
4
2
4
2 45
4
(b)
5 1 1
1,5
1 1
1
10
20
30
40
(a)
1
5 50
1 60
70
80
Fig. 8. XRD patterns of MoO3 + 3Zn initial mixture (a) and reaction products quenched from 450 (b) and 650 °C (c).
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3 - ZnO _ 36-1451 4 - Zn _ 04-0831
1 -MoO3 _ 47-1320 2 - Mo _ 42-1120
5 - Mg _ 35-0821 6 - MgO _ 65-0476
2 8 IntensIty
1 7,1
1
1 5
1 1
10
(c)
9
2
3
2
6
5,7 4 4
(b)
5
1
5
20
9 - MgZn2 _ 65-3578 6
3
1
7 - MoO2_ 65-5787 8 - MoO2*MgO _ 18-0845
5
4 1,4
30
4
1
40
1
50
(a)
1
60
70
80
Fig. 10. XRD patterns of the MoO3 + Zn + 2Mg reaction products quenched from different stages: (a) — T = 380, (b) — 500, (c) — 660 °C.
DTG DTA
Based on the results obtained by DTA investigations the effective activation energy values are calculated for reduction stages for all studied mixtures. There are several approaches for the calculation of the effective activation energy for non-isothermal conditions. One of the best known methods is isoconversion method formulated by Kissinger [32] and well developed recently by Starink [33,34]. Note that both methods are based on the shift of temperature corresponding to the maximum advance in the DTA curve (TDTA max) [32] or Tf [33,34] depending on the heating rate, Vh, which is kept constant. The derived expression for determination of activation energy by Starink method takes the form:
TG mg DTG
80
40
60
Mg melting
20 DTA
40
TG
0 100
20 300
500
ln
700
Temperature/oC
¼ −A
Ea þC RT f
where A = 1.0070 − 1.2 ∙ 10 − 5 E A (E A in kJ/mol), V h is the heating rate (K min − 1 ), T f is the reduction temperature corresponding
Fig. 11. DTA/TG/DTG curves for the MoO3 + 0.5C + 2Mg mixture (Vh/20° min−1, mo = 30 mg).
1 - MoO3 _ 47-1320 2 - Mo _ 42-1120 3 - MoO2 _ 65-5787
Vh T 1:8 f
4 - MgO _ 65-0476 5 - Mg _ 35-0821 6 - MoO2 *MgO _ 18-0845
2
(d)
4
6
IntensIty
2
3
3
4
2
(c)
2 4
4
3 3 5
3
(b) 3
3
1
10
5
1 1
1
20
5
30
1
40
1
5
50
1
15
5 60
(a) 1 5 70
80
Fig. 12. XRD patterns of MoO3 + 0.5C + 2Mg initial mixture (a) and reaction products quenched from 610 (b), 650 (c) and 760 °C (d).
A.M. Baghdasaryan et al. / Int. Journal of Refractory Metals and Hard Materials 51 (2015) 315–323
Fig. 13. SEM micrographs of MoO3 + 0.5C + 2Mg mixture at various stages: a — initial mixture, b — T = 650, c — 760 °C.
321
to the maximum advance in the DTA curve (°K), E is the effective activation energy of the process, and R is the universal gas constant. Vh 1 plots for all studied systems, − DTA Fig. 14 presents ln DTA 1:8 T max ðT max Þ from which the effective values of activation energy are calculated. The estimated values of effective activation energy by the Starink method for all studied systems are collected in table 2. Summarizing the obtained data on the mechanisms of MoO3 reduction by various reducers the following facts should be stressed out: although reduction of metal oxides by combined reducing agents is important both practical and scientific viewpoints, kinetics and mechanism of these processes have not been studied. There are few works related to reduction of molybdenum and tungsten trioxides by combined Mg/Zn and Mg/C reducers; however these works have been done mainly under the combustion regime [25,26,28,29]. Only one work [35] is devoted to the study of the mechanism of WO3 reduction by Mg/C mixture using the thermal analysis method. Unlike the reduction process when hydrogen is used, in the case of solid reducers formation of intermediate phases such as Mo4O11 is not observed. Besides, comparing the effective values of activation energy the minimum value is calculated to be low temperature reduction of MoO3 by zinc. In this case depending on the heating rate the reduction undergoes around the melting point of zinc (360–430 °C) by solid + solid and solid + liquid schemes. A slightly higher effective value of activation energy is calculated to be for low temperature carbothermic reduction of MoO3 to MoO2 and then MoO3 reduction by Mg. In all cases the effective values of activation energy for the magnesiothermic reductions, i.e. MoO3→Mo and MoO2→Mo, are significantly higher than that for reduction by hydrogen (113 and 98.6 kJ mol−1) [10]. Meanwhile for full reduction of MoO2 to metallic Mo the activation energy is much lower than that for reduction by silicon (309 kJ mol− 1) [20]. This may be explained by the fact that the MoO2 and Si have higher a melting point; consequently activation of the MoO2 + Si solid mixture requires higher temperature. The best results for reduction of MoO3 are obtained when Mg/C mixture is used. In some sense this process is similar to the two-step hydrogen reduction of MoO3. In both cases, firstly reduction undergoes up to MoO2 then at higher temperature metallic Mo is formed. However in the case of hydrogen reduction the reducer is only hydrogen while in the case of combined reducers Mg/C, initial reduction (from MoO3 to MoO2) takes place by carbon and the second stage (to metallic Mo) — by Mg. It is important to outline that the hydrogen or carbon reduction is energy—and time consuming processes while in case of combined reducers the process can be undertaken under the combustion mode [25, 26,28,29].
Table 1 Influence of heating rate on Tmax for all systems under the study. System
Vh/C min−1
Tmax/°C
System
Vh/°C min−1
Tmax/°C
MoO3 + 3Mg
20 10 5 2.5 20 10 5 2.5
627 591 553 540 417 400 380 357
MoO3 + 3C (I stage)
20 10 5 2.5 20 10 5 2.5
596 579 556 517 619 583 558 542
MoO3 + 3Zn
MoO3 + 0.5C + 2Mg
MoO3 + Zn + 2Mg (II stage)
Vh/°C min−1
Tmax/°C I stage
II stage
20 10 5 2.5
612 582 567 521
698 681 645 633
322
A.M. Baghdasaryan et al. / Int. Journal of Refractory Metals and Hard Materials 51 (2015) 315–323
-ln(Vh /T1.8 ) 1,4,5,6
16
-ln(Vh /T1.8 ) (2,3)
17
4 6
5
15
16
2
14
15 1
13
14
3 1000/T, K
-1
12
13 1
1.1
1.2
1.3
1.4
1.5
1.6
Fig. 14. Determination of effective values of activation energy for the following reactive mixtures: MoO3 + 3Zn (1), MoO3 + 0.5C + 2Mg, I stage (2), MoO3 + 3Mg (3), MoO3 + 3C, I stage (4), MoO3 + Zn + 2Mg (5), MoO3 + 0.5C + 2Mg II stage (6).
Table 2 The effective activation energies calculated by Starink method.
[4]
N
System
Stage
E/kJ mol−1
1 2 3
MoO3 + 3C MoO3 + 3Mg MoO3 + 0.5C + 2Mg
I stage
4 5
MoO3 + 3Zn MoO3 + Zn + 2Mg
132 123 122 197 116 147
I stage II stage
[5]
[6] [7]
[8]
4. Conclusions [9]
Molybdenum oxide, MoO3 reduction processes by Mg, Zn, carbon and (Mg + C) and (Mg + Zn) mixtures are explored at nonisothermal conditions (heating rates 2.5–20° min−1) and following remarks are revealed: • The reduction processes in MoO3–Mg and MoO3–Zn binary systems started and practically finished in a solid state before Mg (Zn) melts; • When (Mg + C) and (Mg + Zn) complex reducers are used firstly interaction of MoO3 with weak reducer (C or Zn) is taking place forming mainly MoO2. Next, Mg melts and molten Mg fully reduces formed MoO2. The best reducer is estimated to be Mg–C complex reducer and using this reducer allows to fabricate pure Mo. • The activation energy values for magnesiothermal reduction are higher for (Mg + Zn) and (Mg + C) combined mixtures as compare to pure Mg. This phenomenon is associated with the fact, that in the case of complex reducer the Mg reacts with formed MoO2 while in case of single reducer it reacts with MoO3.
[10] [11]
[12]
[13] [14]
[15] [16] [17]
[18]
[19]
Acknowledgments [20]
The authors acknowledge the financial support of the State Committee of Science of the Republic of Armenia (Projects # 11_1d167 and # 13_1D192).
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