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20JOINT REDUCTION OF NiO + WO3 OXIDES BY COMBINED Mg/C REDUCER. SYNERGETIC EFFECT
M. K. Zakaryan*"'A, Kh. T. Nazaretyan", S. V. AydinyanflC, and S. L. KharatyanflA
aA.B. Nalbandyan Institute of Chemical Physics NAS RA, Yerevan, 0014 Armenia bYerevan State University, Yerevan, 0025 Armenia cTallinn University of Technology, Tallinn, 19086 Estonia *e-mail: zakaryan526219@gmail.com
DOI: 10.24411/9999-0014A-2019-10198
It is known that nickel-tungsten alloys possess high hardness and good wear resistance. This makes such alloys suitable for the production of turbine blades, hot forging dies, friction materials for brake assemblies, substitutes for chromium coatings, repair materials (without dismantling) for steam generators of nuclear power plants, decorative jackets, and bearings. It is already reported that tungsten-nickel alloys might be obtained by joint reduction of their oxides under heating in a hydrogen atmosphere or by carbon monoxide [1, 2].
However, it is well known that refractory metal containing alloys are difficult to prepare by conventional methods due to large differences in melting points and limited mutual solubility. Hence, the development of new preparation methods of Ni-W composite materials with tailored physicomechanical properties and full bulk density are in the focus of modern research.
In our previous work [3], we performed joint reduction of nickel/tungsten oxides by energy saving combustion synthesis method using Mg + C mixture as combined reducer. The using of such reducing mixture allows to control the reaction temperature in a wide range and to synthesize Ni-W composite powders in a controllable combustion mode.
In this work the mechanism and kinetics of tungsten and nickel oxides joint reduction by Mg + C combined reducer was studied utilizing high-speed temperature scanner (HSTS). The latter provides an advanced opportunity to disclose the stepwise nature of complex reactions in the multicomponent systems at high heating rates, Vh (up to 104-min-1) and Tmax = 1300oC.
Firstly, binary (NiO-Mg) and ternary (NiO-Mg-C, NiO-WO3-Mg) systems were studied at the same conditions, considering only magnesiothermic or magnesio-carbothermic reduction reactions. The rest systems (WO3-Mg-C and WO3-Mg) were studied in [4].
It is worthy to note, that kinetic parameters and reaction pathway for the reduction processes of nickel oxide with simultaneous utilization of magnesium and carbon were established in the work [5] via DTA/DTG technique ("Derivatograph Q1500" MOM, Hungary; Vh = 5-20°C-min-1). NiO-Mg binary system
The exothermic interaction in the NiO + Mg system starts immediately after the magnesium melting (To = 680oC, Tmax = 1300oC) under Vh =300°C-min-1 heating rate conditions. According to XRD analyses results the full reduction of nickel oxide takes place at 1200oC (Fig. 1a). NiO-Mg-C ternary system
The reduction process in the 2NiO + Mg + C reaction at Vh = 300°C-min-1 heating rate conditions also starts later than magnesium melting (To = 720oC, Tmax = 1270oC). It should be noted, that in this case, the full reduction of nickel oxide proceeds at incomparably low 800oC temperature (Fig. 1b). Note that, according to XRD analyses results, the complete reduction of nickel oxide by carbon takes place at 1035oC.
iSHS 2019
Moscow, Russia
Fig. 1. Heating thermograms with the results of XRD analyses for NiO + Mg (a), 2NiO + Mg + C (b), NiO + WO3 + 4Mg (c), NiO + WO3 + 2.5Mg + 1.5C (d) mixtures, Vh = 300°0min-1.
NiO-WO3-Mg-C quaternary system
Experiments performed with the quaternary NiO + WO3 + 2.5Mg + 1.5C mixture showed that the combined reduction process starts also after magnesium melting. On the other hand, unlike low heating rates [4] at high heating rates the reduction stages are not separated (Fig. 1d). The intense interaction starts at 750oC with 1100oC maximum temperature. It's worthy to note that as it was shown in [3], NiO + WO3 + 1.8Mg + 2.2C mixture was chosen as optimum for joint and complete reduction of NiO and WO3 in combustion mode. At that the main products formed are NinW3 and W metal.
Experiments performed at various heating rates (Vh = 100-1200°0min-1) allow to calculate the kinetic parameters (effective values of activation energy) for the magnesiothermal reduction stage for the NiO + Mg, 2NiO + Mg + C, WO3 + NiO + 4Mg and NiO + WO3 + 2.5Mg + 1.5C reactions (Table) using the Kissinger method [5].
Table. The values of activation energies for magnesiothermic reduction reactions of oxides.
Reaction Activation energy, kJmol 1 Reference
WO3 + Mg WO3 + Mg + C (for WO3 + Mg stage) NiO + Mg NiO + Mg + C (for NiO + Mg stage) WO3 + NiO + 4Mg NiO + WO3 + 2.5Mg + 1.5C (for NiO + WO3 + Mg stage) 106 92 175 185 139 198 [3] [3] [this work] [this work] [this work] [this work]
It was revealed that the reduction of oxides by combined reducer (Mg + C) proceeds at lower temperature compared to the separate binary mixtures, which evidences about the particular synergetic effect in the ternary (2NiO + Mg + C, WO3 + Mg + C) and quaternary (NiO + WO3 + Mg + C) mixtures. Thus, full reduction of nickel by the reaction NiO + C is observed at 1035°C, in a mixture of NiO + Mg, at 1200°C, and in the case of 2NiO + Mg + C, at 800°C.
This work was supported by the Committee of Science MES of RA (Research grants 18A-1d12
and 18T-1D051), and the Estonian Research Council grant PSG220 (S. Aydinyan).
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