Factors Influencing the Direct Electrochemical Reduction of Nb2O5 Pellets to Nb Metal in Molten Chloride Salts

摘要/Abstract

Abstract:

Powder compacted and sintered Nb₂O₅ pellets were cathodically polarised against graphite anode in calcium chloride melt at 1173 K to study the influence of various factors on the electrochemical reduction of the oxide. The parameters were; duration and temperature of electrolysis, open porosity of pellets, nature of anode, mode of electrolysis and configuration of the oxide cathode. The experiments were also conducted in KCl, KCl-25 mol% CaCl₂ and NaCl melts to understand the effect of melt composition on the electroreduction. Different Ca-Nb-O and Nb-O intermediates were found in the pellets electrolysed for different durations of time in CaCl₂ melt which eventually reduced to Nb. The current efficiency of the process decreased with increasing duration of electrolysis. Decrease in electrolysis temperature from 1173 to 1073 K led to the decrease in the rate of reduction of the oxide pellets. Pellets with high open porosity reduced faster. Carbon contamination of the melt was relatively less when pyrolytic graphite was used as anode. Of all the melts studied, the reduction was found to be better in calcium chloride melt, that too when alumina crucible was used as container of the melt.

Key words: Fray-Farthing-Chen (FFC) Cambridge process, Electrochemical reduction, Open porosity, Niobium pentoxide, Calcium chloride

. [J]. Acta Metallurgica Sinica (English Letters), 2017, 30(3): 218-227.

Maha Vishnu D. Sri, N. Sanil, Mohandas K. S., K. Nagarajan. Factors Influencing the Direct Electrochemical Reduction of Nb₂O₅ Pellets to Nb Metal in Molten Chloride Salts[J]. Acta Metallurgica Sinica (English Letters), 2017, 30(3): 218-227.

图/表 11

Table 1 Particulars of the different electroreduction experiments

S. No.		DOER experiment ID	Experimental details
1	Time of electrolysis	S-1 to S-7	Experiments for 2, 9, 22, 26, 35, 44 and 56 h (designated as S-1 to S-7, respectively)
2	Temperature of electrolysis	S-6 and S-8	Experiments at 1173 K (S-6) and 1073 K (S-8) for 44 h
3	Open porosity of pellets	S-6 and S-3 S-9 and S-10	Pellets sintered for 3 h at 1673 K (open porosity ~10%) (S-6 and S-3) and 1373 K (open porosity ~42%) (S-9 and S-10)—electrolysed for 44 h and 22 h
4	Nature of anode	S-11 and S-12 S-13 and S-14	High-density graphite (HDG) (S-11 and S-12) and pyrolytic graphite (PyG, density 2.22 g/cc) (S-13 and S-14) as anodes during electroreduction of Nb₂O₅ pellets for 20 h and 44 h
5	Cathode configuration	S-6 and S-12	Two configurations, one in which pellets were placed in a Ta cup (S-6) and the other in which pellets were wound with a Ta wire (S-12), were electrolysed for 44 h
6	Container for electrolyte	S-6 and S-15	Crucibles of alumina (S-6) and stainless steel (S-15) were employed as containers for molten salt
7	Melt compositions	S-6, S-16, S-17 and S-18	Salts such as CaCl₂ (S-6), KCl (S-16), KCl-25 mol% CaCl₂ (S-17) and NaCl (S-18) have been employed as electrolytes
8	Mode of electrolysis	S-12, S-19 and S-20	Two modes—constant voltage mode (3.1 V) (S-12) and constant current mode at lower applied currents (0.5 and 0.2 A) (S-19 and S-20)

Fig. 1 a Current versus time profiles for the electrochemical reduction of Nb2O5 pellets at 3.1 V for different durations in CaCl2 melt at 1173 K. Inset shows the photographs of pellets before electrolysis (B) and after electrolysis for 44 h (P1—polished) and 56 h (P2). b A plot of charge passed with duration of electrolysis for 44 h. The solid line shows the actual charge passed with time and the intersection point of the dotted line with Y-axis shows the net charge, excluding the background charge, passed during electrolysis for 44 h

Fig. 2 SEM images of cross sections of a Nb2O5 pellet sintered at 1673 K, b Nb metal obtained after 56 h of electrolysis, c the bulk of the pellet after 22 h of electrolysis, d EDX spectrum of (c)

Fig. 3 aI-t curves obtained during electroreduction of Nb2O5 pellets at 1073 K (S-8) and 1173 K (S-6). Inset shows the longitudinal sections of the two pellets from S-8 experiment. b, c are the SEM images of cross sections of the outer reduced metal and inner unreduced oxide regions, respectively

Fig. 4 aI-t curves of electrochemical reduction of dense (S-6) and porous (S-10 and S-9) Nb2O5 pellets at 3.1 V. Inset shows the photograph of the cross section (C) of the product (P) from S-10. SEM images of the pellet sintered at 1373 K b before, c after electrolysis. OP given in (a) indicates the open porosity of pellets

Fig. 5 Current variation during electrolysis of Nb2O5 pellets at 3.1 V in CaCl2 melt at 1173 K. Photographs of the two different cathode assemblies, a, b used in the two experiments are shown in the insets

Fig. 6 Current variation during electrolysis of Nb2O5 pellets at 3.1 V in CaCl2 melt taken in alumina (S-6) and stainless steel (S-15) crucibles using graphite anode at 1173 K. Inset: photograph of aluminium particles found at the bottom of the alumina crucible

Fig. 7 Current-time profiles of electrochemical reduction of Nb2O5 pellets in KCl, KCl-25 mol% CaCl2, NaCl and CaCl2 melts at 1173 K at an applied voltage of 3.1 V

Fig. 8 SEM images of the cross section of pellets after electrolysis for 44 h at 1173 K in a NaCl melt and b CaCl2 melt. c, d higher-resolution images from a, b respectively

Fig. 9 Cell voltage and half-cell potentials vs time curves obtained during constant current electroreduction of dense Nb2O5 pellets at 0.5 A (S-19) and 0.2 A (S-20) in CaCl2 melt at 1173 K

Fig. 10 Photographs of Nb2O5 pellet before electrolysis (a) and after electrolysis for 20 h (b) and 44 h (c) at 3.1 V. In the constant current mode of electrolysis, the pellet broke and was dislodged from the current collector. d, e products of electrolysis at 0.5 A for 40 h and 0.2 A for 44 h, respectively

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Factors Influencing the Direct Electrochemical Reduction of Nb₂O₅ Pellets to Nb Metal in Molten Chloride Salts

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