电解钒的新方法

Electro-deoxidation of V2O3 in molten CaCl2-NaCl-CaO

Shu-lan Wang, Shi-chao Li, Long-fei Wan, and Chuan-hua Wang

School of Science, Northeastern University, Shenyang 110004, China (Received: 9 April 2011; revised: 1 June 2011; accepted: 8 June 2011)

Abstract: The electro-deoxidation of V2O3 precursors was studied. Experiments were carried out with a two-terminal electrochemical cell, which was comprised of a molten electrolyte of CaCl2 and NaCl with additions of CaO, a cathode of compact V2O3, and a graphite anode under the potential of 3.0 V at 1173 K. The phase constitution and composition as well as the morphology of the samples were studied by X-ray diffraction (XRD) and scanning electron microscopy (SEM). 3 g of V2O3 could be converted to vanadium metal powder within the processing time of 8 h. The kinetic pathway was investigated by analyzing the product phase in samples prepared at different reduction stages. CaO added in the reduction path of V2O3 formed the intermediate product CaV2O4. Keywords: vanadous oxide; electro-deoxidation; vanadium; molten salt; electrochemical cells

[This work was financially supported by the National Natural Science Foundation of China (Nos.51154002 and 50834001) and Panzhihua New Steel and Vanadium Co. Ltd.]

friendliness. The FFC Cambridge process has been utilized to prepare a range of metals and alloys, such as titanium [5-7], chromium [8], niobium [9], silicon [10], and other relevant alloys [11-13]. The reduction mechanism and ex-perimental techniques of the FFC Cambridge process have also been extensively investigated [14-21].

In the present work, the FFC Cambridge process was used to produce vanadium metal from V2O3 in the mixed molten salt of CaCl2 and NaCl containing 0.7wt% to 3.2wt% of CaO. The molten salt of CaCl2 and NaCl has the merits of low melting point and high electrical conductivity. The addition of a small amount of CaO in the molten salt can prevent the evolution of chlorine at the beginning of the application of a cell potential of 3.0 V [21]. Fully and par-tially reduced samples were prepared and were character-ized by X-ray diffraction to reveal the kinetic pathway of the electro-deoxidation of V2O3 in the molten salt.

1. Introduction

Vanadium is an important element in ferro-vanadium al-loys, hydrogen storage alloys, and nuclear reactor materials. Pure vanadium is still expensive, partly because of the com-plex and energy-consuming refining process currently in use. Suzuki et al. [1-2] utilized the two step process to prepare vanadium metal from its oxides: the electrolysis of CaO in molten CaCl2 to generate calcium metal and the calciother-mic reduction of vanadium oxide by calcium. The process has also been used to study the preparation of V-Ti-Cr al-loys [3-4].

The Fray-Farthing-Chen (FFC) Cambridge process is a novel electrochemical process [5-7] and can directly convert metal oxides to metals. In the FFC Cambridge process, metal oxide cathodes are electro-deoxidized and converted to metals by applying a cell potential of 3.0 V between two electrodes in a CaCl2-based molten salt. The FFC Cam-bridge process has significant advantages over other tradi-tional metal extraction processes. These advantages include low cost, easy operation, versatility, and environmental

Corresponding author: Shu-lan Wang E-mail: slwang@mail.neu.edu.cn

2. Experimental

V2O3 (97wt%, a commercially available powder from

© University of Science and Technology Beijing and Springer-Verlag Berlin Heidelberg 2012

S.L. Wang et al., Electro-deoxidation of V2O3 in molten CaCl2-NaCl-CaO 213

Panzhihua New Steel and Vanadium Ltd. Co. was the pre-cursor. V2O3 discs were prepared by uniaxial and isostatic pressing and were sintered in Ar (99.99% in purity) atmos-phere at 1373 K for 4 h. The majority of the discs used in the present study were 3 g by mass, 25 mm in diameter, 3 mm in thickness, and 24.6% in the porosity.

CaCl2 and NaCl (both 99wt% from Shanghai Chemical Co. Ltd.) were heated at 473 K for 48 h. CaO was obtained by the thermal decomposition of calcium carbonate at 1273 K for 4 h in air. The mixed salt of CaCl2 and NaCl (110 g, 90:10 in mole ratio) with the addition of CaO, giving a CaO concentration of 0.7wt%-3.2wt%, was placed in a graphite crucible (58 mm in inner diameter, 4 mm in thickness, and 85 mm in height).

The graphite crucible was kept in a furnace with an Ar (99.99%) flow at 473 K for 12 h before further heating treatment. The pre-electrolysis of the molten salt was per-formed by applying a cell potential of 1.0 V between two graphite electrodes (15 mm in diameter). The reduction ex-periments of V2O3 were performed in a two-terminal elec-trochemical cell at a potential of 3.0 V at 1173 K. The V2O3 disc was fixed on a molybdenum rod (1.5 mm in diameter) with a nickel wire (0.5 mm in diameter) and served as the cathode. The graphite crucible served as the anode. To iden-tify the phase and construction of the cathode at different stages of the electro-deoxidation of V2O3, a number of par-tially reduced samples were prepared by termination of the reduction experiments in molten CaCl2-NaCl-0.7wt%CaO at different processing time. After the reduction experiments, the samples were lifted out of the molten salt and cooled to room temperature in the furnace in Ar atmosphere. The samples were cleaned with distilled water and dilute HCl and were dried naturally. The partially reduced samples consisted of two parts: the grey reacted surface layer and the black un-reacted layer in the central part. X-ray diffraction (XRD) patterns of the fully and partially reduced samples were collected from the powder of the samples using a Philips PW3040/60 diffractometer at a scanning rate of 0.03°⋅min-1 for 2θ in the range of 10° to 80°. Scanning elec-tron microscope (SEM) images were captured using a scan-ning electron microscope (SSX-550) operated at an accel-eration voltage of 30 kV.

3. Result and discussion

Fig.1 shows the typical current versus time curves which were recorded during the first 8 h of electro-deoxidation

experiments performed with compact V2O3 in molten CaCl2-NaCl-0.7%CaO and CaCl2-NaCl-3.2%CaO. The plots commence with high currents and end with constant currents. The current in the molten CaCl2-NaCl-3.2wt%CaO is always greater than that in the molten CaCl2-NaCl- 0.7%CaO.

Fig.1. Current versus time curves during the electro-deoxi-dation of V2O3 in molten CaCl2-NaCl-CaO.

Fig. 2 shows the XRD patterns of the powders of the fully and partially reduced samples. Samples after 0.5 h re-duction were found to predominantly consist of V2O3 and vanadium suboxide VO. Samples recovered after 1 h were composed of V2O3, VO, and CaV2O4. Samples recovered after 2 h were comprised of vanadium metal, V2O3, VO, CaV2O4, sesquioxides V2O, and V16O3. In samples of 8 h reduction, only the patterns of vanadium metal existed. The X-ray diffraction analysis suggests that the reduction of V2O3 to vanadium metal in molten CaCl2-NaCl-0.7wt%CaO proceeds through a number of well-defined reaction steps and CaO is involved in the reaction by forming CaV2O4. This compound was also detected in Suzuki’s work on the preparation of vanadium metal from vanadium oxide [3]. The electro-deoxidation of V2O3 in molten CaCl2-NaCl- 0.7%CaO at a cell potential of 3.0 V under 1173 K was ac-companied by the decomposition of CaO as follows: CaO → Ca + 1/2O2 (1) 2CaO + C → 2Ca + CO2 (2) CaO + C → Ca + CO (3) The decomposition potential of CaO at 1173 K is 2.66 V and is lowered to 1.63 V to generate CO2 and 1.54 V to produce CO in the case of a graphite anode [22]. Therefore, in the process of electro-deoxidation of V2O3 at the potential of 3.0 V, calcium is also deposited on the cathode. The generated CO2 and CO can dissolve in molten CaCl2-

214 Int. J. Miner. Metall. Mater., Vol.19, No.3, Mar 2012

Fig. 2. XRD patterns of the reduced samples in molten CaCl2-NaCl-0.7wt%CaO: 1—V2O3; 2—VO; 3—CaV2O4; 4—V2O; 5—V16O3; 6—V.

NaCl-CaO. The solubility of CO2 in CaCl2 containing 0.2mol.% CaO at 1173 K under pCO2 = 100 kPa is 6.5% [23]. The dissolved CO2 and CO can further oxidize the deposited calcium. The Gibbs free energy change for both the oxida-tion reactions of calcium by CO2 and CO is negative at 1173 K [22]:

2Ca + CO2 → 2CaO + C,

ΔG (1173 K) = −1508 kJ⋅mol−1 Ca + CO → CaO + C,

ΔG (1173 K) = −348.4 kJ⋅mol−1

(4) (5)

CaV2O4 + 4e → V2O + CaO + 2O2−

(9)

In Eq. (7), the vanadium ion in oxidation state +III in CaV2O4 is directly reduced to vanadium metal. If Eq. (7) represents the real reduction step of CaV2O4, vanadium metal should be found in the sample after 1 h reduction. However, vanadium metal was not detected in the sample after reduction by X-ray diffraction. This indicates that Eq. (7) is not the reduction step of CaV2O4. Both VO and V2O were found in the samples after reduction; therefore, Eqs. (8) and (9) represent the reduction steps of CaV2O4.

According to XRD patterns in Fig. 2, the following elec-tro-deoxidation reactions were also involved in the reduction process of V2O3:

V2O3 + 2e → 2VO + O2− 2VO + 2e → V2O + O2−

8V2O + 10e → V16O3 + 5O2− V16O3 + 6e → 16V + 3O2−

(10) (11) (12) (13)

The fresh CaO formed on the cathode further reacts with V2O3 via the following reaction:

V2O3 + CaO → CaV2O4

(6)

In the initial stage of the electro-deoxidation of V2O3, the amount of CaO formed on the cathode was small. This is why we did not find the peaks of CaV2O4 on the X-ray pat-terns of samples recovered after 0.5 h reduction. In the XRD patterns of samples after 1 and 2 h reduction, the diffraction peaks of CaV2O4 were found.

CaV2O4 can be reduced by following three pathways: CaV2O4 + 6e → 2V + CaO + 3O2− CaV2O4 + 2e → 2VO + CaO + O2−

(7) (8)

Fig. 3 shows the SEM images of the precursor V2O3 and the reduced products. The morphology of the samples changed greatly with the processing time. The compact V2O3 after sintering at 1373 K in Ar atmosphere for 4 h was porous and had an average particle size of 20 to 50 μm (Fig. 3(a)). After 1 h reduction, the particle size in the samples

S.L. Wang et al., Electro-deoxidation of V2O3 in molten CaCl2-NaCl-CaO 215

Fig. 3. SEM images of the V2O3 precursor and the reduced samples in molten CaCl2-NaCl-0.7wt%CaO: (a) V2O3 precusor sintered at 1373 K for 4 h in Ar; (b) sample reduced for 1 h at 1173 K; (c) sample reduced for 8 h at 1173 K; (d) magnification of (c).

declined to about 5 μm (Fig. 3(b)). This indicates that during the electro-deoxidation of V2O3 the electrolyte permeated into the V2O3 cathode, resulting in the very fast reduction speed of V2O3. Further extending the processing time to 8 h, the fine vanadium powder, with an average size of 1 to 2 μm, was obtained (Figs. 3(c) and 3(d). The vanadium particles agglomerated owing to sintering during the reduction proc-ess.

4. Conclusions

In the present study, it has been demonstrated that the formation of vanadium metal by electro-deoxidation of compact V2O3 precursors in CaCl2-NaCl-CaO melts at a cell potential of 3.0 V. 3 g of vanadium oxide, V2O3, was con-verted to vanadium metal powder with an average particle size of 1-2 μm within the processing time of 8 h at 1173 K. The X-ray diffraction investigation on partially reduced samples revealed that the kinetic pathway of the elec-tro-deoxidation of V2O3 in CaCl2-NaCl-CaO is a step reduc-tion from V2O3 to vanadium metal through oxidation states of +III, +II, +I, +0.375 and the temporary formation of cal-cium vanadate, CaV2O4.

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