Niobium Powder Production by Molten Salt Electrolysis

Niobium Powder Production by Molten Salt Electrolysis

ELECTROCHEMICAL PULVERIZATION OF BULK METAL
FOR PRODUCING FINE NIOBIUM POWDER
Boyan Yuan* and Toru H. Okabe
Associate Professor Institute of Industrial Science The University of Tokyo, *Graduate Student Department of Materials Engineering The University of Tokyo
Nb powder for capacitors
Niobium (inexpensive and abundant), a new material for capacitors.
Introduction
The Hunter process
Comparison with other studies
Electronically Mediated Reaction-Molten Salt Electrolysis (EMR-MSE)
T.H. Okabe, et al., J. Alloys Comp. 1999.
Reduction: niobium oxide reduction
Current
→ 2 Nb (s) + 5 O2- (l)
Cathode: Nb2O5 (s)+ 10 emonitor
Anode: 5 Ca (l)
→ 5 Ca2+ (l) + 10 eA
Nb2O5 Electrolysis: reducant production
Graphite
e
preform
2+ (l) + 10 e→ 5 Ca (l)
(anode)
Cathode:
5
Ca
(cathode)
e- e→ COx (g) + 2x eAnode: C (s) + x O2-
K2TaF7 (l) + 5 Na (l) → Ta (s) +
5 NaF (l) + 2 KF (l)
Niobium Powder Production by Molten Salt Electrolysis
Ta powder
(anode)
Ta2O5
(dielectric)
Ta lead
(anode)
MnO2
(cathode)
K2TaF7 powder
feeding port
Potential
Nb capacitors
stirrer
Liquid Na
feeding port
Boyan Yuan and Toru H. Okabe*
Ceramic
capacitors
Al capacitors
CaCl2
molten salt
Ca-Al-Ni liquid alloy
Ta capacitors (MnO2)
1
10
100
K2TaF7(l) NaF(l)
Ta(s) Na(l)
1000
Capacitance, C / µF
Graphite Ag Solder
Fig. Structure of the tantalum capacitor.
Fig. Target area of the niobium capacitors.
a: Reference: “Mineral Commodity Summaries”, U.S. Geological Survey, 2005.
b: Year 2000, converted into pure metal.
c: Converted quantity of pure niobium used as ferroniobium (Fe-65%Nb).
d: Powder for capacitors.
e: Aluminothermic reduction (ATR) niobium, followed by electron beam (EB) melting carried out thrice;
purity: 99.8%.
f: Varies significantly with powder purity and morphology.
Nb 10 Vf
Nb 20 Vf
Nb 30 Vf
Ta 10 Vf
Ta 20 Vf
Ta 30 Vf
300
200
0
→ n Dy2+ (l)
(Reduction of Nbn+)
e-
Dy2+ addition into molten salt
Nb rod Producing
ions by anodic dissolution Galvanostatic
method
Reduction of Nbn+ by Dy2+ in molten salt (i = 2 A)
Mg-Ag
liquid alloy
(cathode)
Nb powder with salt
Molten salt
containing Dy2+ ion
e-
CV measurement
Nbn+
External power source
Nb rod
(anode)
CV measurement
Fig.
Mild steel reactor
Features
◎ Fine and homogeneous powder can be produced.
〇 High-purity powder with low iron and oxygen
contamination can be produced.
〇 Particle size of the powder can be controlled.
〇 Yield greater than 90% can be achieved.
〇 Current efficiency is not low.
〇 Semicontinuous process.
〇 Minimum amount of chloride wastes.
△ Difficult to separate powder from solidified salt.
After
Exp.
1000
0
30.50
50.14
1296
1385
1049
10
Exp. A
6
No
Dy2+
4
2
1
10
100
10
In molten salt
Dy2+ ion
In molten salt
CONTAINING
Dy2+ ion
2 µm
Frequency, F (%)
Exp. B
Exp. A (left image); Exp. B & C (right image)
NOT containing
8
6
1 mol% Dy2+
4
0
0.1
Nbn+ ions
were reduced
at the cathode
Nbn+ ions
were reduced
in the molten salt
0
D'
-1
25.15 g Dy add.
50.14 g Dy add.
1
2
3
Potential, E / V vs. Mg-Ag liquid alloy
10 mm
Ref.
E˚Dy2+/Dy = -0.37 V vs. Mg2+/Mg at 1000 K
E˚Dy3+/Dy = -0.08 V vs. Mg2+/Mg at 1000 K
E˚Dy3+/Dy2+ = +0.39 V vs. Mg2+/Mg at 1000 K
(a) Schematic illustration of experimental setup for
cyclic voltammetry before Dy2+ ion addition.
(b) Schematic illustration of experimental setup for
electrochemical pulverization of niobium rod.
(c) Appearance of the niobium powder with salt obtained
in the powder collecting dish after experiment.
Dy2+ and
Mg were
produced
in situ.
Electrochemical Pulverization (EP) technique
●
: Nb JCPDS #34-0370
●
●
●
20
●
40
60
80
Angle, 2θ (deg.)
Cath.
●
100
K
53.88 33.07 4.56 0.52 0.09 7.88 <0.01 <0.01
Current
efficiency (%)
Nb2+ Nb3+
Dish
77.40 18.03 2.21 0.25 0.05 1.21 0.81 0.04
60
90
B
30.50
Dish
99.67 0.12 0.01 0.14 <0.01 0.06 <0.01 <0.01
58
87
C
50.14
Dish
95.50 2.93 0.90 0.23 0.44 <0.01 <0.01 <0.01
51
77
A
0
1
10
100
・Electrochemically dissolved Nbn+ ions were reduced by
Dy2+ ion in molten salt to produce niobium powder.
・By employing EP,
Niobium powder with an average particle size below
1 µm, a narrow particle size distribution, and a purity of
99.7 mass% was successfully obtained.
・Particle size of the niobium powder can be controlled by
the concentration of Dy2+ ion in the molten salt.
・EP is suitable for producing highly pure, fine, and
homogeneous powders for capacitors and other
electronic devices.
In molten salt
Nb(s)
Nbn+
Dy2+
e-
Nb(s)
Dy3+
(a)
Dy2+
NbCl5 (l, in salt)
NbCl4 (l, in salt)
NbCl3 (l, in salt)
NbCl2 (l, in salt)
8
6
2 mol%
4
2
0
0.1
Highly pure niobium powder with a low
iron concentration was produced in the
molten salt containing Dy2+ ion.
Discussion
Exp. C
log pCl2
2 µm
Frequency, F (%)
Mg-Ag
cathode
10 mm
C
Cl-/Cl2
Fig. Cyclic voltammograms of glassy carbon electrode
in molten salt:
(a) NaCl-36 mol%KCl-10 mol%MgCl2
(b) NaCl-36 mol%KCl-9/8 mol%MgCl2-1/2 mol%DyCl2.
Table Analytical results of the niobium powders.
Mass of Site of
Composition of element i, Ci* (mass%)
Exp.Dy add. deposit
wDy / g
Nb
Fe
Cr
Ni
Mg Ag
Na
2
10
10 mm
D
Dy2+/Dy3+
A'
*: Determined by X-ray fluorescence (XRF) analysis.
Exp. C
Nb
deposit
1
-2 A Mg2+/Mg
0
Nb + salt
XRF analysis
Exp. B
(b) Holder for Mg-Ag liquid alloy (cathode)
XRD analysis
8
0
0.1
1 mm
Molten salt:
NaCl-36 mol%KCl-9/8 mol%MgCl2-1/2 mol% DyCl2
Nb powder with salt
Fig.
3
Conclusions
Particle size distribution analysis
Exp. A
10 mm
2
16
14
20
2
(b)
Stainless steel powder collecting dish
Nb powders,
2 µm
10 mm
2
NaCl-36KCl-10MgCl2
NaCl-36KCl-9MgCl2-1DyCl2
NaCl-36KCl-8MgCl2-2DyCl2
Intensity, I (a.u.)
Before
Exp.
Molten salt:
NaCl-36 mol%KCl-9/8 mol%MgCl21/2mol%DyCl2
Galvanostatic (c)
electrolysis
Current, Time,
i/A
t / ks
Composition,
(mol%)
1
2
Mg-Ag liquid alloy (RE/cathode)
Molten salt
0
Potential, E / V vs. Mg2+ / Mg
Supporting rod for powder collecting dish
A new technique for producing fine and homogeneous powders.
SEM analysis
(a) Nb rod (anode)
1000
Frequency, F (%)
Anode and cathode, observations
A
B
C
-2
Nb rod (anode)
Exp. Temp., Mass of
T / K Dy add.,
#
Mass,
wM.S / g
wDy / g
Scan rate: 20 mV/s
WE: Glassy carbon
CE: Glassy carbon
Mg2+/Mg
-1
A
Glassy carbon electrode (CE)
Table Experimental conditions for a comparative study of the effect
of Dy2+ addition in the electrochemical pulverization technique.
C
Cl-/Cl2
B
0
Glassy carbon electrode (WE)
Flowchart for niobium powder production by
the electrochemical pulverization technique.
A'
1
Electrochemical interface
Experimental conditions
Nb powder
2
(b)
Nb powder
Molten salts, CV measurements
Molten salt: NaCl-36 mol%KCl-10 mol%MgCl2
(a)
Electrochemical pulverization (EP)
Leaching
Nbn+(l) Dy3+(l)
Nb(s) Dy2+(l)
CaCl2-NaCl molten salt
Results
Dy + Ag + MgCl2
→ DyCl2 + Ag-Mg
NaCl-36KCl-9/8MgCl2-1/2DyCl2 molten salt CV measurement
Dy, Ag
→ Nb (s, powder)
Features
◎ Simple process.
◎ (Semi)continuous process.
× Low current efficiency.
× Sensitive to iron and carbon contamination.
△ Difficult to control powder morphology.
△ Chloride wastes are generated.
Ni potential lead
Ar gas inlet/outlet
Stainless steel tube current lead
Rubber plug for sealing and insulating
Stainless steel chamber
Heating element
Ag shot
Dy lump
Mild steel reactor (electrode)
Glassy carbon electrode (WE)
Glassy carbon electrode (CE)
Ni (Mg2+/Mg) electrode (RE)
Molten salt:
NaCl-36 mol%KCl-10 mol%MgCl2
Ceramic insulator
MgCl2
NaCl-36KCl-10MgCl2 molten salt
Anode: Nb2O5 (s) + 10 e- → 2 Nb (s) + 5 O2Cathode: C (s) + x O2→ COx (g) + 2x e-
Graphite
(anode)
Dy2+ addition
Molten salt preparation
Dy2+)
Nb (s, bulk)
KCl
Ar gas
Nb2O5 pellet e
(cathode)
(a)
In molten salt: Nbn+ (l) + n Dy2+ (l) → Nb (s, powder) + n Dy3+ (l)
Overall:
Mg shot
Direct Electrochemical Reduction
X.Y. Yan and D.J. Fray, Light metals, 2002.
The Hunter process is not suitable for
producing niobium powder for capacitors.
Experimental Procedure
NaCl
(Dissolution of bulk Nb)
(Regeneration of reductant,
Mg vapor
◎ Highly pure tantalum powder with
morphology suitable for capacitors
can be produced.
× Batch-type process.
× Large quantities of fluorides and
chlorides wastes are generated.
△ Separation of metal powder from
a large amount of solidified salt by
mechanical and hydrometallurgical
methods is time and labor consuming.
Fig. Dependence of diameter of the metal
powder on the theoretical capacitance
of the fabricated capacitor.
Experimental
Schematic illustration of the sodiothermic reduction
of tantalum fluoride salt (the Hunter process)
for tantalum powder production.
Features
0.0
1.0
2.0
Particle size of metal powder, d / µm
→ Nbn+ (l) + n e-
n Dy3+ (l) + n e-
Fig.
100
Electrochemical Pulverization (EP) Technique
Nb (s, bulk)
Nb2O5 (s,l) + Mg (g) → Nb (s) + MgO (s,l)
Features
◎ Simple process.
〇 (Semi)continuous process.
〇 Highly pure powder can be produced.
〇 Highly efficient process.
〇 Environmentally friendly process.
× Difficult to control powder morphology.
△ Difficult to separate metal powder from by-products.
Ta powder
400
Current density,
j / A·cm-2
Capacitance, CV / µFV・g-1・103
Comparison between niobium and tantalum.
Ta
Nb
Atomic number
(VB) 73
(VB) 41
Melting point
˚C
˚C
2468
2980
Density
g / cm3
g / cm3 16.65
8.56
Dielectric constant 41
27
of pentoxide
4,400,000 ton Nb 43,000 ton Ta
Reservesa
Annual global
ton Nb 2,300
ton Ta
23,000
production volumeb (21,000)c ton Nb (1,400)d ton Ta
Price
$ / kg
～700f $ / kg
～50e
Major
Microalloy
Solid electrolytic
applications
element for steel
capacitors
Commercial
Aluminothermic
Sodiothermic
production
reduction (ATR)
reduction (Hunter)
process
(Nb/FeNb bar)
(Ta powder)
Nb2O5 powder
Table
Cathode:
× Difficult to control powder morphology.
△ Difficult to separate powder from by-products.
Reduction of Oxide with Gaseous Magnesium
L.N. Shekhter, T.B. Tripp and L.L. Lanin, U.S. Patent 6,171,393, 2001.
Ta capacitors (polymer)
Ta
Ta2O5
MnO2
Cathode
(anode) (dielectric) (cathode) layer
Current density,
j / A·cm-2
Ta capacitor
Anode:
Overall: Nb2O5 (s) + 5 Ca (l) → 2 Nb (s) + 5 CaO (l)
Features
Graduate student, Department of Materials
Engineering, The University of Tokyo
Molten salt
◎ High-purity powder production.
〇 Semicontinuous process.
wastes are generated.
*Associate Professor, Institute of Industrial Science, The University of Tokyo×× Chloride
Complicated process.
1
10
100
Particle size, d / µm
Fine and homogeneous niobium powder was
produced by using the molten salt containing Dy2+ ion.
(b)
Cl2 (g)
Bulk
Fine powder
DyCl3
(l, in salt)
-40
α
Nb (s)
-60
Cl 2
Dy
β
in
(l,
-20
0
In molten salt
α
Nb
Dy3+
e-
Dy2+
β
-60
-40
log -40
a -20
Cl(NbCln) Nbn+
)
e-
Dy (s)
-60
Dy
lt
sa
l og
Fig.
a Nb
e-
(a) Three-dimensional chemical potential diagram
of the Nb-Dy-Cl system at 1000 K.
(b) A mechanism for niobium powder production
using Dy3+/Dy2+ equilibrium in the molten salt.
・Three-dimensional
chemical potential
diagram demonstrates
the feasibility of reducing
Nbn+ by using Dy3+/Dy2+
equilibrium.
・Reduction of Nbn+ ions by
Dy2+ ion in the molten salt
involves local
electrochemical reactions.