459
J. Mater. Sci. Technol., Vol.25 No.4, 2009
Al Preparation from Solid Al2 O3 by Direct Electrochemical
Deoxidation in Molten CaCl2 -NaCl at 550◦ C
Hongwei Xie1) , He Zhang1) , Yuchun Zhai1)† , Jinxia Wang2) and Chengde Li1)
1) School of Materials and Metallurgy, Northeastern University, Shenyang 110004, China
2) School of Science, Northeastern University, Shenyang 110004, China
[Manuscript received October 21, 2008, in revised form December 13, 2008]
Al was prepared by a new method in molten salt at low temperature. Sintered alumina pellets were used as
cathode; graphite rod was employed as anode; and the molten CaCl2 -NaCl was the electrolyte. A constant
3.2 V voltage was applied in this experiment, and oxygen in solid alumina cathode was reduced by direct
electrochemical deoxidation at 550◦ C. In this process, the current gradually decreased with increasing time
and the alumina pellets became grey and porous. The metallic particles were obtained and characterized by
XRD (X-ray diffraction) and SEM (scanning electron microscopy).
KEY WORDS: Aluminium; Alumina; Electrochemical deoxidation; Low temperature
1. Introduction
It has been over a century since the establishment
of the fundamentals of the Hall-Heroult process in
electrolytic aluminium. The aluminium electrolysis
industry consumes a lot of energy. In addition, the
progress in materials science is producing new ‘inert’ materials for anodes in the electrolysis cell[1–3] .
Most of the inert anode materials come from cermet
or metal. For inert anodes, a low electrolyte temperature should be advantageous because the solubility
of the oxide material or the metal substrate material
that makes up the active anode surface decreases with
decreasing temperature. So it is necessary to search
a new method of aluminium electrolysis at low temperature. Researchers have spent many years for this
effort[4–8] . However, there has been little progress.
Recently, a new process in direct electrochemical deoxidation in molten salts has been proposed. Using
solid oxide as the cathode, researchers have successfully prepared reactive metals such as Ti, Si, Cr and
so on[9–20] . But study about aluminum electrolysis
has never been reported.
The temperature of the new method could be
much lower than that of Hall-Heroult0 s process because the solid oxide is not dissolved in the molten salt
as ionic state, instead being used as the cathode to be
electrochemically deoxidized directly. In this case, it
is not essential for the solid oxide to have high solubility in the molten salts. There are therefore more possibilities to choose low temperature electrolyte, which
can make electrolysis at low temperatures feasible.
In this study, solid alumina was used as raw material to gain solid aluminium by direct electrochemical
deoxidation in molten salt at 550◦ C.
2. Experimental
2.1 Material and equipment
Analytically pure anhydrous CaCl2 and NaCl were
used as electrolyte, which was mixed and dried at
† Corresponding author.
Prof., Ph.D.; Tel.:
+86 24
83687731; Fax:
+86 24 83687731; E-mail address:
[email protected] (Y.C. Zhai).
300◦ C. Alumina powder used as cathodic materials,
was pressed and sintered. Compact graphite rod was
used as anode. The graphite crucible was used as
container. A stainless steel insulator was used as reactor. Other equipments included furnace, programmable temperature control device, a DC stabilized
voltage/power source and a computer recording the
change of the current against time during the electrolysis process.
2.2 Experimental principle
The cathodic reaction is:
Al2 O3 + 6e → 2Al + 3O2
(1)
and the anodic reaction is:
and
C + O−
2 → CO + 2e
(2)
C + O−
2 → CO2 + 4e
(3)
The overall reaction is:
Al2 O3 + 3C → 2Al + 3CO ↑
(4)
2Al2 O3 + 3C → 4Al + 3CO2 ↑
(5)
and
2.3 Experimental process
Dried CaCl2 and NaCl with mol ratio of 1:1 were
put into graphite crucible. As shown in Fig. 1, cathode and anode were installed and were not inserted
into molten salt. The reactor was sealed and then
the pre-treatment to the mixed salts below its melting point was carried out, which further deprive H2 O
and O2 , followed by heating the mixed salt to melt
at 550◦ C. During this process, argon was injected to
avoid oxidation. Afterwards, both cathode and anode were inserted into molten salt to conduct preelectrolysis under the decomposing voltage of Al2 O3
to minimize the impact of the impurities.
After pre-electrolysis, the voltage was increased to
constant 3.2 V, until the current was kept around
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J. Mater. Sci. Technol., Vol.25 No.4, 2009
Fig. 1 Schematic diagram of the experimental device configuration
25
I / A
102
20
15
10
5
0
0
2
4
6
8
10
12
14
16
Time / h
Fig. 2 Relationship between current and time during
the electrochemical deoxidation of aluminium in
molten CaCl2 -NaCl at 550◦ C, 3.2 V
Fig. 4 Enlarged local photograph of cathodic product
sample after removing the salt
was taken out and then the metal and salt were
separated by immersing with kerosene. The cathodic product was melted and characterized using
SEM (scanning electron microscopy, SSX-550 Shimadzu, Japan) and XRD (X-ray diffraction, D/maxrb Rigaku, Japan).
3. Results and Discussion
Fig. 3 Photograph of the cathodic product samples after
removing the salt
0.1×10−2 A. The estimated time of electrolysis was
10–12 h. After the electrolysis process, the cathode
The relationship between current and time in the
electrolysis process is shown in Fig. 2. Figure 3 is
a photograph of the cathodic product after removing the salt and Fig. 4 is the enlarged photograph of
the local area of the cathodic product. SEM image
and XRD pattern of the melted cathodic product are
shown in Figs. 5 and 6, respectively.
As shown in Fig. 2, the current is higher at the
beginning of the electrolysis process, but it gradually
decreased until it stabilized at a small current. At
the beginning of the electrolysis process, the surface
area of the cathode was relatively large, so was the
contact surface with molten salt. In this case, there
were more surface areas for electro-reaction, resulting
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J. Mater. Sci. Technol., Vol.25 No.4, 2009
pared by direct electrochemical deoxidation of solid
Al2 O3 in molten CaCl2 -NaCl at 550◦ C.
(2) It offered a novel method for low temperature
aluminium electrolysis and was significant for saving
energy and application of inert anode to reduce green
gas due to the low electrolysis temperature.
Fig. 5 SEM image of the melted cathodic product samples
Acknowledgements
This work was financially supported by the National
Natural Science Foundation of China under grant No.
50674026, the Postgraduate Foundation of Northeastern
University in China under grant No. 18702016 and the
Innovational Experiment Plan of Undergraduate Student
of China under grant No. 071014506.
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25000
Intensity / a.u.
20000
15000
10000
5000
0
0
10
20
30
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2
50
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80
90
/ deg.
Fig. 6 XRD pattern of the melted cathodic product samples
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4. Conclusions
(1) The metal aluminium was successfully pre-
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