Int. Journal of Refractory Metals and Hard Materials 28 (2010) 601–604

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Int. Journal of Refractory Metals and Hard Materials

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Reduction of MoO3 by Zn: Reducer migration phenomena

David Davtyan a, Khachatur Manukyan a,b,⁎, Raman Mnatsakanyan c, Suren Kharatyan a,b

a Department of Inorganic Chemistry, Yerevan State University, 1 Manoogian Str, Yerevan, 0025, Armeniab Laboratory of Kinetics of SHS processes, A.B. Nalbandyan Institute of Chemical Physics NAS of Armenia, 5/2 Sevak Str, Yerevan, 0014, Armeniac Laboratory of Kinetics and Catalysis, A.B. Nalbandyan Institute of Chemical Physics NAS of Armenia, 5/2 Sevak Str, Yerevan, 0014, Armenia

⁎ Corresponding author. 1 Manoogian Str, Yerevan, 0016 10; fax: +374 10 28 16 34.

E-mail address: khachat@ichph.sci.am (K. Manukyan

0263-4368/$ – see front matter © 2010 Elsevier Ltd. Aldoi:10.1016/j.ijrmhm.2010.05.005

a b s t r a c t

a r t i c l e i n f o

Article history:Received 18 January 2010Accepted 12 May 2010

Keywords:MoO3

Zn reductionReducer migrationMo powder

Self-sustaining reduction of MoO3 by zinc was investigated. To optimize process parameters thermodynamicanalysis was performed. It was shown that zinc migration from the reacting zone takes place at definiteconditions. Experimental parameters (e.g. sample density and diameter and gas pressure) influencing onprocess conditions and product characteristics were revealed. Molybdenum powder with 0.1–1 μm particlesize was produced at the optimal conditions. Oxidation onset of obtained powder was found to be 250 °C.

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1. Introduction

Molybdenum is a widely used metal in the chemical industry,the metallurgy and the aerospace industry, etc. for the properties ofhigh yielding strength and hardness at high temperature, excellentthermal and electrical conductivity and good erosion-resistance[1–5].

Hydrogen reduction of molybdates or molybdenum oxides toproduce molybdenum powder is conventional [2,3,6–9]. Otherrecognized technology of molybdenum powder production is thermaldecomposition of Mo(CO)6 [10]. A novel route of powderedmolybdenum preparation from MoO3 at solid-state reduction byKBH4 as a precursor has been proposed [11].

Combustion-based technology known as combustion synthesis(or self-propagation high temperature synthesis) was proposed toproduce Mo powder [12,13]. Well-known molten salt (NaCl)assisted technique was applied to moderate reaction conditionsfor the MoO3+NaBH4 system [12]. As a result, nano-size Mo powderwas obtained at high concentration of salt. To obtain Mo or W powdersself-sustaining reactions in theMoO3+ZnandWO3+Znmixtureswereinvestigated [12–17]. The combustionprocess in theMeO3+Znpowdermixtures gives mainly metal and zinc oxide. The ZnO in the reactedsamples can be easily leached by acid treatment. Combustion process atthe MoO3+Zn mixtures was investigated only at 0.1 MPa inert gaspressure [13]. To avoid reducer shortage surplus amount (3.5 mol) of Znwas used.

At the combustion of WO3+Zn mixture the evaporation of thezinc (T=906 °C) was suppressed by increasing the gas pressure from0.1 to 2.5 MPa [14]. Further increase in pressure, however, resulted inthe combustion limit. The authors assumed that combustion limitcaused by either the heat removal from the sample surface due to gasdensity increase or heat transportation from the reaction zone byvaporized Zn removal.

In order to avoid intensive evaporation in [15,17] reactiveWO3+3Znmixturewas diluted by sodium chloride. Precursor componentswere drymixed and heated up to 450 °C under the constant argon gas flow topromote the combustion reaction. This approach enables to keep reactiontemperature at 750–800 °C and to produce W nano-powder. Thus, therole of reducer evaporation and influence of formedvapouron theprocessconditions is not understood yet.

The goal of the present study is to investigate the effect of zincevaporation on the combustion synthesis process for the MoO3+3Znsystem.

2. Experimental section

The raw mixtures were prepared using MoO3 (Tech. condition ofmanufacturing No. 6-09-4471-77, Pobedit Co., Russia) and Zn (PZR-1mark, purity 99.5%, average particle size 50 μm, VMP, Yekaterinburg,Russia) powders. A green mixture of the reactants was homogenizedin a ceramic mortar for a 0.5 h. Cylindrical samples of 20–55 mm indiameter and height of 70 mmwere prepared from the greenmixture.Experiments were conducted in a laboratory constant-pressurereactor (CPR-3l) using nitrogen (purity 99.95% and oxygen contentno more than 0.02%) atmosphere at pressure of 0.1 to 5 MPa. At thestart of the experiment, the combustion chamber was sealed,evacuated, and purged with nitrogen. The chamber was then filled

Fig. 1. Thermodynamic calculation for the MoO3+3Zn system: (1) Mo, MoO2, Zn, andZnO and (2) Mo and ZnO.

Fig. 2. Combustion temperature and k value vs gas pressure.

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with gas to the desired pressure. During the experiments sampleswere placed into the reactor and combustion was initiated bytungsten wire located on the upper surface of the sample and wasprogrammed to produce a voltage pulse of 18 V for 2 s. Power to thefilament was immediately discontinued after ignition of the sample.

The maximum combustion temperature (Tc) and the temperaturedistribution within the combustion wave were determined bytungsten rhenium thermocouples (W/Re-5, W/Re-20, 100 μm indiameter) previously covered with a thin layer of boron nitride.Two thermocouple holes (2 mm in diameter, 10–25 mm deep) weredrilled into each specimen perpendicular to the cylinder axis at aspacing of ∼15 mm. The output signals of thermocouples weretransformed by a data acquisition board at the rate of 1 kHz, andwere recorded by PC. The average values of combustion velocity werecalculated as follows: Uc= l/t, where l is the distance betweenthermocouples and t is the temporary distance between the signalsof thermocouples.

Reacted samples were kept in an inert atmosphere to avoidpossible contaminations from air. In order to remove zinc oxidefrom the samples they washed with 5 wt.% hydrochloric acid anddeionized water. The volume of evolved hydrogen during the acidtreatment was measured to determine non-reacted zinc content.Washed powders were dried under vacuum at 110 °C for 3 h.Produced powders were analyzed by XRD method with CuKαradiation (diffractometer DRON-3.0, Burevestnik, Russia). FESEM(Hitachi S-4800) analysis was conducted to study the microstruc-ture of the powders. Specific surface area of obtained molybdenumpowder was determined by BET method using nitrogen adsorption(Gasometer, GKh-1). Oxygen content in the molybdenum powderwas determined by LECO TC400 analyzer.

Oxidation of prepared Mo powder was examined by DTAtechnique (MOM 1500, Hungary) at heating rate 20 °C/min.

3. Results and discussion

3.1. Thermodynamic consideration

The MoO3+Zn system was analyzed by “ISMAN-THERMO”software that is specially designed to calculate adiabatic tempera-ture (Tad) and products' equilibrium composition in the heteroge-neous combustion processes [18]. Calculation of equilibriumcharacteristics is based on the minimization of the Gibbs freeenergy of the system, which accounts for the contributions of Gibbsfree energies for all components and their concentrations. Calculat-ed amounts of molybdenum in products depending on zincconcentration and ambient gas pressure are presented in Fig. 1.Two areas may be marked out:

1 – reaction temperature changes from 950 to 1300 °C and productscontain Mo, MoO2, Zn and ZnO,

2 – temperature is in the range of 1300–1600 °C, products are Mo andZnO.

Fig. 1 suggests that gas pressure is an important parameter toobtain the target metal. For the stochiometric MoO3+3Zn mixture1.5–2 MPa pressure is optimal.

3.2. Reducer migration phenomena

Combustion process of samples (diameter — 30 mm and relativedensity — 0.45) made from MoO3+3Zn mixture was studied atdifferent pressures. The results are shown in Fig. 2. At0.1≤P≤2 MPa interval combustion temperature continuouslygrows from 1000 to 1400 °C. Further increment of the pressure to5 MPa practically does not influence on the combustion tempera-ture. The combustion wave propagates throughout the sample in theoscillating regime.

During the combustion wave propagation some amount of zincevaporates, then condenses on the reaction vessel walls. The weightloss of the sample (k, %) associated with zinc evaporation calculatedby the formula: k=[(min−mpr) /min]×100% (where,min and mpr areweights of the sample before and after combustion, respectively) as afunction of gas pressure is shown in Fig. 2. At 0.1≤P≤1MPak valuefalls from 14 to 4%. Two-fold decrease in k value is observed atpressure increasing from 1 to 5 MPa. One may assume that increase inpressure will suppress zinc evaporation. However, it is highly possiblethat evaporated zinc may freely migrate from the reaction zone to theporous non-reacted part of the sample. To confirm this statement thephase composition and free (non-reacted) zinc concentration inproducts were analyzed.

XRD pattern shows (Fig. 3b) that sample reacted at 2 MPa containsmolybdenum and zinc oxide. At higher (5 MPa) or lower (0.5 MPa)pressures Mo, ZnO, Zn and/or MoO2 phases are identified (Fig. 3c, a,respectively).

Free zinc amount in products as a function of pressure is presentedin Fig. 4. Zn concentration firstly decreases from 11.2 to 1.5% and thenincreased to 11.0%. It is obvious that at 0.1≤P≤1 MPa due to low

Fig. 3. XRD patterns for combustion products obtained at combustion of MoO3+3Znmixtures: a) P=0.5 MPa, b) 2 MPa and c) 5 MPa.

Fig. 5. Zinc distribution along the direction of combustion wave propagation:diameter −30 mm and height −70 mm.

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process temperature (see Fig. 2) a part of reducer do not react withMoO3. Pressure increment suppresses the vapour removal from thesample (see Fig. 2). However, the average concentration of non-reacted zinc at the elevated pressures is still higher.

It is seen from Fig. 5 that for samples reacted at 1 and 5 MPa zincconcentration gradually increases along the wave propagationdirection. This result confirms that zinc migrate from the reactionzone to the cold porous initial mixture. This trend confirmed alsoby layer-by-layer XRD analyses of reacted samples. Most likely at4–5 MPa zinc migration is well-defined because of high reactingtemperature.

Fig. 6. XRD pattern of leached product obtained within the optimal conditions.

3.3. Optimal reduction condition of MoO3

In order to optimize reduction conditions, the influence of samplesdensity and diameter on combustion laws were investigated for theMoO3+3Zn mixture (P=1.5 MPa). Increasing the density from 0.45to 0.65 the combustion temperature tends to decrease by 100 °C. Atlow temperatures same amount of MoO2 was detected in theproducts. Thus, reduction should be organized for samples with lowdensity.

In the next series, the diameter (d) of samples was changed from20 to 55 mm. Results suggest that samples with low diameters(db25 mm) cannot combust because of high level of heat losses.

Fig. 4. Zinc concentration in reacted samples.

Reaction temperature increases from 1250 to 1400 °C when d growsfrom 25 to 55 mm. The combustion front propagation velocitydecreases from 0.18 to the 0.12 cm/s with d increasing. The incrementof diameter also affects on the wave propagation regime. At

Fig. 7. SEM micrograph of Mo powder.

Fig. 8. Thermogravimetric curves of molybdenum powdes: a) SHS-derived andb) commercial powder.

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d=55 mm oscillating regime transforms to the steady-state combus-tion one. The concentration of non-reacted zinc at d=55 is only0.8 wt.%.

The XRD pattern for washed sample obtained within the optimalconditions is presented in Fig. 6. The micrograph (Fig. 7) of obtainedMo powder shows particles with 0.1–1 μm in size. Specific surfacearea of powder is 2.5 m2/g. Determined oxygen content is 0.18%.

To determine oxidation behavior of obtained powder, DTAmethod was applied. Thermogravimetric curve show (Fig. 8) thatintensive oxidation in air starts at about 250 °C. Similar curve forcommercially available molybdenum powder (Pobedit, Russia,particle size 1–10 μm) is also presented in Fig. 8. Oxidation onsetof that powder is somewhere between 320–350 °C. Most likely lowoxidation onset of SHS-derived molybdenum powder is due to thesmall particle size.

4. Conclusions

Combustion conditions in the MoO3+3Zn system is oprimizedby thermodynamic analysis and experimental investigations. Itis shown that reducer migration from the reaction zone to thenon-reacted porous part of samples takes place. Results suggestthat 1–1.5 MPa pressure is optimal to reduce MoO3 by zinc. Otherimportant experimental parameters influencing the process con-ditions and product features are found to be density and diameter

of reacting sample. Powdered Mo with particle sizes 0.1–1 μm issuccessfully obtained. Oxidation onset of Mo powder was found tobe 250 °C.

Acknowledgments

The authors acknowledge the financial support of the StateCommittee of Science of Republic of Armenia (project no. 354).Authors also are grateful to Dr. Ofik Niazyan and Dr. Yeva Grigoryanfrom the Institute of Chemical Physics, Armenia and Albert Voskanyanfrom the Yerevan State University for DTA investigations and specificsurface area measurement.

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