Acta Metallurgica Sinica (English Letters) ›› 2017, Vol. 30 ›› Issue (3): 218-227.
• Orginal Article • Previous Articles Next Articles
Maha Vishnu D. Sri1,2(),N. Sanil1,Mohandas K. S.1,K. Nagarajan1
Received:
2016-06-06
Online:
2017-03-16
Published:
2017-05-16
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.
S. No. | Parameter under investigation | 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) |
Table 1 Particulars of the different electroreduction experiments
S. No. | Parameter under investigation | 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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