简介概要

Reduction characteristics of molybdenum trioxide with aluminum and silicon

来源期刊：Rare Metals2018年第7期

论文作者：Hang-Yu Zhu Rong Gao Wu-Tao Jin Long-Wu Qiu Zheng-Liang Xue

文章页码：621 - 624

摘要：Thermodynamics for reduction of molybdenum oxides by aluminum and silicon were calculated, and the results show that reduction reaction is feasible at a certain temperature region. Compared to the presence of CaO or CaCO₃, reduction products of molybdenum trioxide with aluminum and silicon at various temperatures were detected by X-ray diffraction（XRD）. Results show that molybdenum trioxide is reduced by aluminum or silicon step by step, and the intermediate product is MoO₂. At 1000 ℃, molybdenum trioxide could be reduced to metal Mo by aluminum, and in the presence of CaO, metal Mo as the reduction product appears even at 800 ℃. In contrast,silicon could barely reduce molybdenum trioxide to metal Mo even at 1200 ℃. In the presence of CaO or CaCO₃, reducibility of silicon increases significantly, and the reduction products are metal Mo and MoSi₂. Altogether, CaO or CaCO₃ performs two major roles in reduction process: restraining sublimation of MoO₃ and decreasing the temperature of reducing MoO₃ to metal Mo.

稀有金属(英文版) 2018,37(07),621-624

Reduction characteristics of molybdenum trioxide with aluminum and silicon

Hang-Yu Zhu Rong Gao Wu-Tao Jin Long-Wu Qiu Zheng-Liang Xue

State Key Laboratory of Refractories and Metallurgy, Wuhan University of Science and Technology

School of Materials and Metallurgy,Wuhan University of Science and Technology

作者简介：*Hang-Yu Zhu e-mail:zhuhy@wust.edu.cn;

收稿日期：8 April 2014

基金：financially supported by Hubei Provincial Natural Science Foundation of China (2015CFC838);the Open Research Fund of Key Laboratory for Ferrous Metallurgy and Resources Utilization of Ministry of Education, Wuhan University of Science and Technology (No. FMRU201309);

Reduction characteristics of molybdenum trioxide with aluminum and silicon

Hang-Yu Zhu Rong Gao Wu-Tao Jin Long-Wu Qiu Zheng-Liang Xue

State Key Laboratory of Refractories and Metallurgy, Wuhan University of Science and Technology

School of Materials and Metallurgy,Wuhan University of Science and Technology

Abstract：

Thermodynamics for reduction of molybdenum oxides by aluminum and silicon were calculated, and the results show that reduction reaction is feasible at a certain temperature region. Compared to the presence of CaO or CaCO₃, reduction products of molybdenum trioxide with aluminum and silicon at various temperatures were detected by X-ray diffraction(XRD). Results show that molybdenum trioxide is reduced by aluminum or silicon step by step, and the intermediate product is MoO₂. At 1000 ℃, molybdenum trioxide could be reduced to metal Mo by aluminum, and in the presence of CaO, metal Mo as the reduction product appears even at 800 ℃. In contrast,silicon could barely reduce molybdenum trioxide to metal Mo even at 1200 ℃. In the presence of CaO or CaCO₃, reducibility of silicon increases significantly, and the reduction products are metal Mo and MoSi₂. Altogether, CaO or CaCO₃ performs two major roles in reduction process: restraining sublimation of MoO₃ and decreasing the temperature of reducing MoO₃ to metal Mo.

Keyword：

Molybdenum trioxide; Reduction; Aluminum; Silicon; Metallothermic;

Received： 8 April 2014

1 Introduction

Molybdenum has many advantages such as high melting point,high elastic modulus,small expansion coefficient,good electrical and thermal conductivities,and excellent corrosion resistance.It is widely applied in aerospace,national defense industrial and agricultural production.Physical and chemical properties of molybdenum powder will largely determine the processability of molybdenum metal [ 1, 2, 3, 4] .Therefore,research on reduction characteristics of molybdenum trioxide is very important to improve processability and service performance.

Hydrogen reduction of molybdenum trioxide was studied extensively [ 5, 6, 7, 8] ;however,there are security risks for hydrogen in the process of production,transportation,storage,and usage.In the past few decades,much effort has been done on carbothermic reduction of metal oxides [ 9, 10] ,but these findings could not account for carbothermic reduction of molybdenum trioxide because of low melting point,high activity,and sublimation characteristic of molybdenum trioxide [ 11, 12] .Kirshenbaum and other researchers [ 13, 14, 15] studied reduction of molybdenum trioxide with various metals;however,aluminum and silicon were not involved.Reductions of molybdenum trioxide with aluminum and silicon are normally applied in catalysis field [ 16, 17] .

The purpose of this paper was to gain more understanding for the metallothermic reduction of molybdenum trioxide.Thermodynamics for reduction of molybdenum oxides by aluminum and silicon were calculated,and the results show that reduction reaction is feasible at a certain temperature region.

2 Experimental

2.1 Materials and characterization

Molybdenum trioxide,calcium oxide,calcium carbonate,aluminum powder,and silicon powder were used as raw materials.Calcium oxide powders (150μm,99%) were preheated at 900℃for 12 h to remove residual H₂O and CaCO₃.Molybdenum trioxides (particle size-150-+100μm,99%) were dried at 200℃for about 12 h.The particle size of A1 and Si powder was 150μm.These powders were analytically pure and supplied by Sinopharm Chemical Reagent Beijing Co.,Ltd.Sample mixtures were prepared by thoroughly mixing in a mortar.Molybdenum trioxide and aluminum,and molybdenum trioxide and silicon were mixed at molar ratios of 1:2 and 1.0:1.5,respectively,and the molar ratios of MoO₃/CaO/Al and MoO₃/CaO/Si,were 1:1:2 and 1.0:1.0:1.5,respectively.

Isothermal experiments were carried out in a conventional MoSi₂ furnace.The sample mixtures were placed in a corundum crucible and then lowered to the constant temperature region of the furnace when reaching the aimed temperature,and reaction samples were taken out 30 min later.Purified nitrogen was used in the furnace tube to ensure an inert atmosphere,and the flow rate was5 L.min^-1.At the end of each experiment,X-ray diffraction (XRD,PANalytical,X'Pert Pro) measurement was taken to identify the reduction products.

2.2 Thermodynamic data

Reduction of metal oxides by Al or Si is generally named as“metallothermic reduction,"which is the most common method to produce ferroalloy.In the present study,reaction equations of molybdenum oxides reduced by Al and Si are represented as follows:

Thermodynamic data for reduction of molybdenum oxides by Al and Si are calculated by means of HSC Chemistry 6.0,and the results are shown in Fig.1.It is observed that reduction reaction is feasible at a certain temperature region for Gibbs energy (ΔG⁰) in a very low level.The melting point of aluminum is 660℃while that of MoO₃ is 795℃,and aluminum melts with reaction temperature up to 660℃,and then,kinetics of reduction are in a good condition.Aluminum and M0O₃ are both liquid phases at above 795℃,and in this case,the reduction reaction becomes stronger.In addition,reduction of MoO₃ by Al or Si is an exothermic reaction.For the case of silicon,reduction reaction is possible in theory with a very low Gibbs energy even at lower temperature.However,silicon has a high melting point of 1410℃,and the reaction between silicon and MoO₃ is in poor kinetic condition.

Fig.1 Thermodynamic diagram (Gibbs energy vs.temperature) for reduction of molybdenum oxides by Al and Si

3 Results and discussion

3.1 Al-MoO3 system

XRD patterns for mixtures of Al and MoO₃ reacted at different temperatures are shown in Fig.2.It clearly shows that MoO₃ is reduced step by step,and MoO₂ is the intermediate product.At 800℃,there is no detectable trace of MoO₃ as well as Mo,and the major species are MoO₂ and Al₂O₃.Thus,it could be inferred that Al could not reduce MoO₃ or MoO₂ to Mo at this temperature.With the temperature rising,strong peaks for Mo appear while MoO₂ peaks become weaker.At 1200℃,reaction between Al and MoO₃ is complete for good kinetic condition,and from XRD patterns it is obviously found that the peaks are reduction products of Mo and Al₂O₃.

In the case of mixing CaO,there are some differences for reduction of MoO₃ by Al,and the results are shown in Fig.3.At 800℃,there are no MoO₂ peaks for CaO+-MoO₃+Al mixtures,and the reduction product is Mo compared to Al+MoO₃ mixtures at the same temperature.It could be inferred that the presence of CaO decreases the temperature of reducing MoO₃ to Mo.The reason could be that reduction product Al₂O₃ combines with CaO to produce CaO·Al₂O₃,and consumption of Al₂O₃ makes the reduction between MoO₃ and Al proceed further.

For the mixtures of CaCO₃+MoO₃+Al at 800℃,the result seems more different as shown in Fig.3.It clearly shows the presence of CaMoO₄ and strong peaks for CaMoO₄.The reason is that CaCO₃ and MoO₃ are more reactive at the starting reaction temperature of 450℃,which restrains the reduction between MoO₃ and Al.Once it reaches the CaCO₃ decomposition temperature of898℃,peaks for Mo could be identified.It is observed that XRD patterns of CaCO₃+MoO₃+Al at 1000℃are very similar to those of CaO+MoO₃+Al at 800℃,the reason could be that the combination of Al₂O₃ with CaO makes decomposition of CaCO₃ and reduction between Al and MoO₃ more available.

Fig.2 XRD patterns of Al-MoO₃ mixtures reacted at various temperatures

Fig.3 XRD patterns for reduction of MoO₃ by Al in presence of CaO or CaCO₃

3.2 Si-MoO3 system

XRD patterns for mixtures of Si and MoO₃ reacted at different temperatures for 30 min are depicted in Fig.4.At800℃,strong peaks for MoO₃ and Si are detected,and the major species are MoO₃ and Si.These findings indicate that reduction reaction between MoO₃ and Si could not occur at 800℃.With the temperature rising to 1000℃,MoO₃ disappears,and MoO₂ and unreacted Si are identified.At 1200℃,the XRD pattern is very similar to that at1000℃,and Mo could not be detected for both of these trials.In contrast to Al-MoO₃ system at 1200℃,Mo could not be produced through reducing MoO₃ by Si,the difference is attributed to high melting point of Si and its low activity.

In the presence of CaO or CaCO₃,peaks for Mo are found as shown in Fig.5.That means,MoO₃ is reduced to Mo by Si.The reasonable interpretation is that kinetic conditions are improved because CO₂ discharges with reaction between CaCO₃ and MoO₃.In addition,reduction product SiO₂ could combine with CaO to produce3CaO·3SiO₂,and consumption of SiO₂ makes the reduction between MoO₃ and Si proceed further.Like previous analysis for Al-MoO₃-CaO system,the presence of CaO or CaCO₃ could promote reduction reaction for MoO₃.Moreover,MoSi₂ and CaMo₆O₉ are detected except Mo in reduction products.The formation of MoSi₂ is due to the reduction product Mo and unreacted Si;however,the presence of CaMo₆O₉ seems more different from AlMoO₃-CaO system.CaMo₆O₉ is black powder,and it would be produced in the presence of Mo,MoO₂,CaMoO₄,and SiO₂ at certain temperature [ 18] .

Fig.4 XRD patterns for Si-MoO₃ mixtures reacted at various temperatures

Fig.5 XRD patterns of Si-MoO₃ system in presence of CaO or CaCO₃ at 1200℃

4 Conclusion

Molybdenum trioxide is reduced by aluminum or silicon step by step,and the intermediate product is MoO₂.In the case of CaO or CaCO₃ existing,CaMoO₄ appears as the intermediate product.At 1000℃,molybdenum trioxide could be reduced to metal Mo by aluminum,and metal Mo appears at 800℃in the presence of CaO.In contrast,silicon could barely reduce molybdenum trioxide to metal Mo even at 1200℃.In the presence of CaO or CaCO₃,reducibility of silicon increases significantly,and the reduction products are metal Mo and MoSi_2.

The presence of CaO or CaCO₃ has two advantages:firstly,formation of CaMoO₄ stabilizes MoO₃ and restrains sublimation of MoO₃,and secondly,CaO or CaCO₃decreases the temperature of reducing MoO₃ to metal Mo.