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25REDUCTION OF OXIDE COMPOUNDS OF TUNGSTEN AND MOLYBDENUM BY CALCIUM VAPOURS
V. N. Kolosov*", V. M. Orlov", and M. N. Miroshnichenko"
aTananaev Institute of Chemistry - Subdivision of the Federal Research Centre Kola Science Centre of the Russian Academy of Sciences, Apatity, 184209 Russia *e-mail: tantal@chemy.kolasc.net.ru
DOI: 10.24411/9999-0014A-2019-10067
Tungsten and molybdenum are produced on an industrial scale only by powder metallurgy [1]. At the same time, the existing technologies are complex and have a limitation on the specific surface area of the obtained powders. The disadvantages of traditional technologies contribute to the search for alternative methods of production [2-5]. Earlier, it was shown that powders of tungsten and molybdenum with a specific surface area of up to 20 m2-g-1 can be obtained by reduction their oxygen compounds with magnesium vapor and then leaching MgO with solutions of mineral acids [6, 7].
The purpose of this work was to study the characteristics of tungsten and molybdenum powders obtained by reduction its oxide compounds with calcium vapors. As a reductant, calcium has a stronger affinity for oxygen than magnesium.
MgWO4, CaWO4, Ca3WO6, MgMoO4, and CaMoO4 were used as precursors, which were synthesized by the method [8]. The apparatus and techniques used to prepare and investigate tungsten and molybdenum powders were similar to those described previously [6]. The process was run under an atmosphere of calcium vapor and argon. A vessel containing calcium was mounted on the bottom of a reaction beaker. Weighed amounts (5 g) of precursors were loaded into metallic crucibles, which were placed over the calcium-containing vessel. Protective shields were placed over the crucibles. The space between the surface of the precursor and the shield was 27-30 mm. The reaction beaker was covered with a lid, which had an orifice in its center for the thermocouple sheath. The assembly was mounted in a stainless steel retort, which was hermetically sealed, pumped down, filled with an inert gas, and heated to a required temperature. The reaction beaker was kept closed to avoid calcium losses. The reduction process was run in the temperature range 800-860°C at a residual argon pressure in the range 5-10 kPa. The specific feature of the interaction of complex oxides of tungsten and molybdenum with calcium is the high thermal effect of the reactions (Table 1). The calcium vapor reduction of oxides makes it possible to control the rate of metal reductant delivery to the reaction zone, thereby maintaining the required process temperature.
Table 1. Thermodynamic characteristics of calcium reduction reactions of complex tungsten
and molybdenum oxides.
№ Reaction -AH°r kJmol-1 -AS°r, J-(mol-K)-1 AQr, kJkg1
1 MgWÜ4 +3 Ca = W+ MgO +3CaO 1519 37 3875
2 CaWO4 + 3 Ca = W + 4CaO 1351 12 3313
3 Ca3WÜ6 + 3 Ca = W+ 6CaO 1279 15 2458
4 MgMoO4 + 3Ca = Mo +MgO+3CaO 1563 38 5138
5 CaMoO4 +3Ca = Mo + 4CaO 1455 14 4545
Crucibles with reaction products and corresponding shields after the reduction of MgMoÜ4 and MgWÜ4 by calcium vapors are shown in Fig. 1. Also, as in the recovery of these compounds
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by magnesium vapor [5, 6], white matter deposits are clearly observed on the surface of the reaction mass, on the walls of the crucible above the load level and on the shield surface. According to X-ray diffraction data, the white substance on the inner walls of the crucible and on the shield surface is pure calcium oxide (Figs. 2b, 3b). At the same time, white crusts on the surface of the reaction mass after the reduction of MgWO4 and MgMoO4, were a mixture of CaO and MgO approximately in an equal mass ratio (Figs. 2c, 3c). Under the crust there was a homogeneous black powder, Mo or W with a small admixture of CaO and MgO (Figs. 2d, 3d). The presence of a significant amount magnesium oxide in the crust on the surface of the reaction mass is apparently due to the fact that the reaction of MgMoO4 and MgWO4 with calcium vapors involves the following exchange reactions:
MgWO4 + Ca = CaWO4 + Mg, MgMoO4 + Ca = CaMoO4 + Mg,
AH0r = -225,0 kJmol-AH0r = -165,6 kJmol-
(6) (7)
This is confirmed by the X-ray diffraction data of intermediate products of reactions (1) and (4), since the CaMoO4 and CaWO4 compounds are absent in the initial reagents (Figs. 2e, 3e). The magnesium metal forming in the course of reactions (6) and (7) is vaporized from the reaction mass, is deposited on its surface, and is involved in the electronically mediated reaction [9]. Therefore, the oxide layer on the surface of the reaction mass contains magnesium oxide, along with calcium oxide.
1
2
3
4
Fig. 1. (1, 3) Crucibles with reaction products and (2, 4) corresponding shields after reduction by calcium vapors; precursors: (1) MgWO4, (3) MgMoO4. Reduction temperature 860°C, residual pressure in the reactor 10 kPa.
Fig. 2. X-ray powder diffraction patterns of (a) MgWO4 and (b-e) its reduction products. The residual pressure in the reactor was 10 kPa, and the reduction time was (b-d) 5 h and (e) 1 h.
(d) (e)
Fig. 3. X-ray powder diffraction patterns of (a) MgMoO4 and (b-e) its reduction products. The residual pressure in the reactor was 10 kPa, and the reduction time was (b-d) 5 h and
(e) 1 h.
Crucibles with reaction products and corresponding shields after the reduction of CaMoO4, CaWO4, and Ca3WO6 by calcium vapors are shown in Figure 4. After the reduction of CaMoO4 on the part of the shield surface, on the walls of the crucible and on the surface of the reaction mass, white matter deposits are visible, which are pure calcium oxide (Fig. 5b). Under the CaO layer there is a homogeneous black molybdenum powder with a small amount of CaO impurity (Fig. 5c). At the same time, after the reduction of CaWO4 and Ca3WO6, the exfoliation of the components of the reaction products did not occur. The reaction mass is a homogeneous dark powder containing tungsten and calcium oxide in a stoichiometric ratio of the corresponding reaction (Fig. 5e).
In the reduction of double oxides of tungsten and molybdenum by calcium vapors, the specific surface of powders was 14-20 m2-g-1. The study of the porous structure of the powder showed that the specific surface is almost completely determined by the surface of the pores. Powder adsorption curves correspond to IUPAC type IV (Fig. 6). They are distinguished by the presence of a hysteresis loop and are characteristic of materials with a mesoporous structure.
12 3 4
30 mm I
Fig. 4. (1, 3) Crucibles with reaction products and (2, 4) corresponding shields after reduction by calcium vapors; precursors: (1) (left crucible) CaMoO4, (1) (right crucible) CaWO4, (3) Ca3WO6. Reduction temperature 860°C, residual pressure in the reactor 10 kPa.
(d) (e)
Fig. 5. X-ray powder diffraction patterns of (a) CaMoO4, (d) CaWO4, and (b, c, e) its reduction products. The residual pressure in the reactor was 5 kPa.
z >. r\eiauvB noaauio iu/u i /1 \
(a) (b)
Fig. 6. Nitrogen adsorption-desorption isotherms for tungsten powders obtained by reduction (a) MgWO4 and (b) CaWO4 by calcium vapor; powder surface: (a) 14 and (b) 16 m2-g-1.
Thus, the results of the studies performed showed the possibility of obtaining mesoporous
powders of tungsten and molybdenum with a high specific surface by reduction their complex
oxide compounds. At the same time magnesium or calcium can be used as a reductant.
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