Molten salt applications in materials processing

Article abstract of DOI:10.1016/j.jpcs.2004.06.049

The science of molten salt electrochemistry for electrowinning of reactive metals, such as calcium, and its in situ application in pyro-reduction has been described. Calcium electrowinning has been performed in a 5-10 wt% calcium oxide-calcium chloride molten salt by the electrolytic dissociation of calcium oxide. This electrolysis requires the use of a porous ceramic sheath around the anode to keep the cathodically deposited calcium and the anodic gases separate. Stainless steel cathode and graphite anode have been used in the temperature range of 850-950 °C. This salt mixture is produced as a result of the direct oxide reduction (DOR) of reactive metal oxides by calcium in a calcium chloride bath. The primary purpose of this process is to recover the expensive calcium reductant and to recycle calcium chloride. Experimental data have been included to justify the suitability as well as limitations of the electrowinning process. Transport of oxygen ions through the sheath is found to be the rate controlling step. Under the constraints of the reactor design, a calcium recovery rate of approx. 150 g/h was achieved. Feasibility of a process to produce metals by pyrometallurgical reduction, using the calcium reductant produced electrolytically within the same reactor, has been shown in a hybrid process. Several processes are currently under investigation to use this electrowon calcium for in situ reduction of metal oxides.

Full text of DOI:10.1016/j.jpcs.2004.06.049

Journal of Physics and Chemistry of Solids 66 (2005) 396–401
www.elsevier.com/locate/jpcs
Molten salt applications in materials processing
*
Brajendra Mishra , David L. Olson
Department of Metallurgical and Materials Engineering, Colorado School of Mines,
Kroll Institute for Extractive Metallurgy, 1500 Illinois Street, Golden, CO 80401, USA
Accepted 4 June 2004
Abstract
The science of molten salt electrochemistry for electrowinning of reactive metals, such as calcium, and its in situ application in pyro-
reduction has been described. Calcium electrowinning has been performed in a 5–10 wt% calcium oxide–calcium chloride molten salt by the
electrolytic dissociation of calcium oxide. This electrolysis requires the use of a porous ceramic sheath around the anode to keep the
cathodically deposited calcium and the anodic gases separate. Stainless steel cathode and graphite anode have been used in the temperature
range of 850–950 8C. This salt mixture is produced as a result of the direct oxide reduction (DOR) of reactive metal oxides by calcium in a
calcium chloride bath. The primary purpose of this process is to recover the expensive calcium reductant and to recycle calcium chloride.
Experimental data have been included to justify the suitability as well as limitations of the electrowinning process. Transport of oxygen ions
through the sheath is found to be the rate controlling step. Under the constraints of the reactor design, a calcium recovery rate of approx.
1
50 g/h was achieved. Feasibility of a process to produce metals by pyrometallurgical reduction, using the calcium reductant produced
electrolytically within the same reactor, has been shown in a hybrid process. Several processes are currently under investigation to use this
electrowon calcium for in situ reduction of metal oxides.
q 2004 Elsevier Ltd. All rights reserved.
Keywords: A. Inorgainc compounds; A. Oxides; B. Chemical synthesis; D. Thermodynamic properties
1
. Introduction
such as lanthanides and actinides make use of molten salt
processing for extraction and refining. Additionally, the high
temperature carbothermic or metallothermic smelting
reduction methods for metal production are associated with
the generation of significant quantity of waste as slags. There
is a need to develop alternative processes that have low waste
or ideally a ‘zero-waste’ generation. Low temperature multi-
component molten salts [1], as well as room temperature
ionic liquids [2] have been developed for materials proces-
sing. Molten salts are also finding applications in fuel cell
technology. The process described in this paper for
producing metals is primarily aimed at complete recycling
of process waste. It is also anticipated that the suggested
scheme will lower the production cost, since the reductant is
electrolytically generated. Significant research effort has
been invested in recent times for producing titanium and
other refractory metals by molten salt processing. Implemen-
tation of this scheme with respect to titanium production is
demonstrated [3–5].
Molten salt processing provides a unique opportunity to
process and produce metals where gas-based pyro-reduction,
metallothermic reduction, hydrometallurgical methods or
aqueous electrochemical techniques are not feasible due to
thermodynamic or kinetic constraints. Industrial applications
of molten salts have been well recognized for more than a
century. In spite of the use of high temperature corrosive
liquids, molten salts offer unique opportunities. The steel and
other non-ferrous metal industries make use of molten salts
and slags for refining and precision heat treatment.
Commercial production of aluminum, magnesium, sodium,
potassium, lithium, beryllium, etc. make use of molten salt
reduction or electrolysis, since any other method is techno-
economically not feasible. Several other reactive metals,
*
Corresponding author. Tel.: C1 303 273 3893; fax: C1 303 384 2189.
E-mail address: bmishra@mines.edu (B. Mishra).
0022-3697/$ - see front matter q 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.jpcs.2004.06.049

B. Mishra, D.L. Olson / Journal of Physics and Chemistry of Solids 66 (2005) 396–401
397
Direct oxide reduction (DOR), molten salt extraction
MSE), and salt scrub (SS) reduction are some of
the liquid deposited calcium floats up along the cathode on
3
the salt (approx. density; 2.0 g/cm ). The dissolved, as well
(
the pyrochemical processes that use molten salt medium
in the production and purification of certain actinide metals.
Various salt mixtures of chlorides with additions of fluorides
are used as the media that generates a considerable amount
of contaminated waste that has to be processed before
recycling or disposal. Direct oxide reduction produces
calcium oxide saturated calcium chloride salts as a
contaminated by-product of the calciothermic reduction of
actinide oxide. Since calcium chloride is the primary
constituent of the salt, these processes typically operate in
the temperature range of 850–950 8C.
as the deposited calcium metal can be used in situ to reduce
another metal oxide:
M O CyCa Z xM CyCaO
(5)
x
y
However, if the metal exists as a chloride or has been
converted to a chloride form, calcium oxide will not be
required in the cell. Calcium chloride could be dissociated
(NaCl and KCl are more stable than CaCl ) by a similar
2
scheme as Eqs. (1)–(3), producing chlorine gas on the anode
and using calcium according to the following equation:
M Cl CyCa Z xM CyCaCl
(6)
x
2y
2
A number of research studies have been initiated to
prepare a waste-minimization strategy [6–9]. This work
describes two aspects of the overall metal production
program: (a) electrolytic recovery of calcium metal from the
DOR process effluent salt comprising calcium oxide and
calcium chloride and (b) use of electrolytically recovered
calcium metal in situ as a reductant in a hybrid reactor.
There are several advantages of the suggested hybrid
process. The process ideally produces ‘zero’ waste and the
metals can be recovered from inexpensive oxides/chlorides
without the need for an expensive reductant. Oxygen,
carbon dioxide and chlorine gases are the only process
effluent which can be easily contained. Operational costs
include graphite anode, electric power and recyclable salt
only. The process is also amenable to alloy production
directly by incorporating co-reduction of respective oxides.
Thus, besides the collected metal, M, the only by-product
of the entire process is the anodically generated gas, viz.
oxygen, carbon dioxide or chlorine. The other product of
calcium oxide (Eq. (5)) or calcium chloride (Eq. (6)) will be
further dissociated electrolytically (Eq. (1)), thus forming a
closed cycle. The metal, M, could theoretically be any
metal, since calcium oxide is one of the most stable oxides
and, therefore, calcium can reduce most other metals,
including actinide, lanthanides, refractory metals, rare
earths and various other reactive and precious metals such
as, titanium, lead, silver, tin, bismuth, etc. However, the
following restrictions would apply:
1
2
3
. Metal M should be insoluble in the salt.
. Metal M should not alloy with calcium.
. A solid deposit of M will form if its melting point is
higher than the cell temperature, e.g. silver, neodymium,
silicon, titanium, etc.
2
. Electrochemical principles
4
5
. Metal will settle at the bottom if its density is greater than
2
3
g/cm .
The principles are similar to the Hall cell for aluminum
. Liquid metal pool can be deposited for low melting
metals, thus the process could be continuous.
production but the aluminum process is essentially
restricted by the solubility of alumina in cryolite. A molten
flux electrolyte consisting of commercial grade calcium
chloride and 10 wt% [20 at.%] calcium oxide, with
additions of potassium and sodium chlorides for lowering
the melting point, density and viscosity of the salt, has been
used in the range of 825–950 8C. The oxide is electro-
lytically dissociated on a graphite anode and steel cathode:
The behavior of calcium electrowinning cell and the
parameters relating to metal halide and oxide reduction with
calcium have been discussed in this work. A schematic
hybrid cell design has been developed which will allow
both, electrowinning and thermal reduction. The features of
this cell have been described. Use of this scheme to produce
titanium has been originally developed as OS process [11].
2C
2K
CaO Z Ca CO
(1)
(2)
(3)
(4)
2C
K
Ca C2e Z Ca ðon cathodeÞ
3. Experimental
2K
K
O
Z 1=2O C2e
ðon inert anodeÞ
K
2
Fig. 1 shows the schematic diagram of the electrowin-
2
K
ning cell used for calcium recovery. The distinctive feature
of this cell is the ceramic diaphragm shown around the
anode. This sheath is required to prevent the anodic carbon
and the gases from mixing with the catholyte and lowering
the cell efficiency. It is also required to keep the deposited
calcium away from the anodic gases. Table 1 lists the
various process parameters used in the experiments.
C C2O Z CO C4e
ðon graphite anodeÞ
2
Calcium has 4–6 at.% solubility in the salt depending on
the composition and temperature [10]. Therefore, the
deposited calcium metal is initially dissolved in the salt
and is subsequently deposited on the cathode. Calcium has a
3
very low density (1.55 g/cm ; melting point 805 8C) and

398
B. Mishra, D.L. Olson / Journal of Physics and Chemistry of Solids 66 (2005) 396–401
Fig. 1. Schematic diagram of the cell showing the arrangement for 13 mm diameter anode rod and 50 mm diameter sheath.
Five weight percent of calcium oxide was dissolved in
to diffuse in and replenish the anolyte. The replenishment
was also monitored as a function of time at a given
temperature. Once the concentration of oxygen leveled off,
current was reapplied and calcium was won. This sequence
was continued for three to four times at different
temperatures for a given sheath porosity. Data was analyzed
to obtain a diffusion coefficient as a function of temperature
for a given porosity. Equivalent amount of calcium was
dissolved in the catholyte.
calcium chloride at 900 8C in a magnesia crucible.
Magnesia is generally stable if it is not exposed to calcium
directly at high temperatures over a prolonged period of
time. Initial deposition of calcium is soluble in the salt and
the subsequent deposition adheres to the stainless steel
cathode near the top of the salt bath due to its low density. A
ceramic sheath with known porosity was inserted in the
molten salt and the salt was allowed to infiltrate into the
sheath. The porous sheath worked as the anolyte compart-
ment. The dc current was applied to the cell and the anodic
gas composition was monitored for the evolution of
chlorine. The current was interrupted once an appreciable
level of chlorine was detected in the anodic gas. The cell
was held at a set temperature and oxygen ions were allowed
4
. Results and discussion
It was determined that the primary difficulty in obtaining
a cathodic calcium deposit is due to the use of graphite
anode and the evolution of carbonaceous anodic gases.
These gases encourage various back-reactions in the cell
through which the deposited calcium is lost [4]. However,
very few metals lend themselves as electrode materials in a
molten chloride salt without corroding severely and,
therefore, in this work a novel technique of incorporating
a porous ceramic anode-sheath was adopted. The surface
area, volume and porosity of the sheath were found to be the
critical parameters in controlling the rate of electrowinning.
The rates of calcium deposition and oxygen evolution are
given by the following Eq. [5]
Table 1
Cell operating data calcium electrowinning trials
Electrolyte
Atmosphere
Crucible
2
CaCl , 0–5 wt% CaO
Argon
MgO
Operating temperature
Current
825–900 8C
5–15 A
Voltage
5–25 V
Time
Variable
Anode
Material
Graphite
Size (cm)
Active area
Current density
12.7 mm dia.!305 mm long
2
11.4–26.6 cm
K6
K2
rCa Z 5:2!10 !i C
(7)
(8)
2.3–8.8 kA m
c
area
Cathode
Material
Stainless steel
K6
rO2 Z 2:6!10 !i A
a
area
Size (cm)
273!152 mm
0.0173–0.0312 m
2
Active area
Current density
K2
where rCa and rO2 are the rates in moles/s for calcium
deposition and oxygen evolution, respectively, i and i are
0.40–1.44 kA m
c
a

B. Mishra, D.L. Olson / Journal of Physics and Chemistry of Solids 66 (2005) 396–401
399
the cathodic and anodic current densities and Carea and Aarea
are the cathode and anode areas, respectively.
Table 2
Diffusion coefficient of ceramic sheath [7]
The applied voltage of the cell, Vappl. can be represented
Parameter
Anolyte volume (cm )
825 8C
875 8C
as
3
98.54
104.19
30
113.32
117.49
30
2
Sheath surface area (cm )
Porosity (%)
salt
drop
sheath
CIRdrop
Vappl: Z EO2 KECa ChCa ChO2 CIR
(9)
Sheath thickness (cm)
Bulk conc. (wt%) CaO
Min. conc. (wt%) CaO
0.3302
5.0
0.3302
5.0
O2
Ca
where EO2KE ZEo KE C0:029 log Q at 950 8C cell
Ca
o
temperature and hCa and hO2 are the cathodic and anodic
over-potentials, respectively. The IR drops due to electro-
lyte and ceramic sheath directly influence the cell voltage.
For a dissociation reaction with graphite anode (activity of
carbonZ1)
0.12
0.20
2
K6
K6
Diff. coeff. (cm /s)
8.12!10
11.7!10
evolution at the anode. Once the current was interrupted, the
oxide concentration, C , was monitored as a function of time
t
2
CaO CC Z 2Ca CCO2
(10)
till the rate of ‘back-fill’ was too slow and the concentration
driving force diminished. An average compositional driving
O2
o
Ca
o
and at a cell temperature of 950 8C, E KE Z1:62 V and
1=2
CO2
2C
2K
QZða !p =½Ca ꢀ$½O ꢀÞ.
Ca
force, C , between a small time interval, t, was used to
f
The rate of flow of ions through the sheath has a major
influence on the cell potential, since it determines the values
calculate an approximate diffusion coefficient, Deng.
According to Fick’s first law:
sheath
drop
of IR
and Q. The activity of calcium ions is assumed as
Deng Z ½C KC ꢀ=½C K1=2ðC CCminÞꢀ½Vd=Atꢀ
(11)
t
min
B
t
unity, since both the oxide and chloride electrolyte provide
the source for calcium ions. The activity of calcium metal is
determined by the solubility of the metal in the salt which is
found to be 2–4 wt%, depending on the oxide content and
the cell temperature. Beyond the solubility limit of calcium
in the salt, the metal will have an activity of unity. The
constitutive constant, Q, depends on the flow of oxygen ions
through the sheath and the efficiency of removal of oxygen
or carbon dioxide gas from the anode and is a function of
time. It was found that the rate of oxygen ion replenishment
in the sheath is the rate-controlling step.
where C Z½ðC CC Þ=2KC ꢀ, V is the volume of molten
f
t
min
B
anolyte, A is the surface area of the sheath, and d is the
thickness of the sheath. Fig. 2 shows a plot of ½C KC ꢀ=Cf
t
min
versus time and the slope of the line is used to calculate
diffusion coefficient D at different temperatures. Table 2
shows the values for V, A, d, temperature, C , C and the
B
min
diffusion coefficient. The optimizationof porosity is essential
since large porosity adversely affects the mechanical
integrity of the sheath. However, small porosity levels of
the sheath make the transport extremely difficult. Table 2
shows that the diffusion coefficients are typical for ions in
molten salts. Since one has to wait for oxygen transport, the
transport through the sheath is the rate controlling process.
The sheath size, geometry and porosity must be optimized to
achieve a calcium winning electrolytic cell.
A simple model, based on Fick’s first law, was applied to
calculate the diffusion coefficient of the porous ceramic
sheath. Experiments were conducted as a function of
temperature. Five weight percent of calcium oxide was
used as the starting concentration, C , for electrowinning,
B
which has been assumed constant in the bulk catholyte. The
electrolysis was terminated when the concentration of oxide
in the anolyte reached a minimum value, Cmin. The
minimum level was ascertained by the initiation of chlorine
5. Hybrid process
The technique applied for calcium deposition has been
optimized as a combination of two steps; (STEP A)
electrowinning to a desired low level of oxygen ion
concentration within the anolyte and (STEP B) holding
for a known length of time for oxygen ion diffusion through
the sheath. The technology for calcium oxide dissociation
and recovery of calcium has thus been developed. The
reduction of lead, tin, bismuth and silver compounds by
calcium in a calcium chloride salt medium has also been
investigated as a preliminary evaluation of the concept. It
has been stated that the CaO–CaCl2 salt mixture is
generated as a result of the direct oxide reduction of
reactive metals. In the present investigation, this reactive
metal oxide is also a radioactive metal. Therefore, lead
oxide, tin oxide, bismuth chloride and silver chloride were
used to surrogate for the radionuclide due to their
Fig. 2. Plot of ½C KC ꢀ=C versus time. Slope of the line is used
t
min
f
to calculate diffusion coefficient D at different temperatures.

400
B. Mishra, D.L. Olson / Journal of Physics and Chemistry of Solids 66 (2005) 396–401
Fig. 3. Schematic diagram of a hybrid reactor for electrowinning and metal oxide reduction.
similarities of certain chemical and physical behavior.
These metals were not chosen on the basis of their reactivity
or as potential metals for calciothermic reduction. However,
investigations have been carried out to produce several other
metals, such as titanium, tantalum, niobium, neodymium,
etc. by other researchers [12]. The hybrid process is a
combination that essentially addresses the issue of utiliz-
ation of cathodically deposited calcium as an in situ
reductant for another metal compound. Fig. 3 shows a
schematic reactor design that was used to increase the
calcium deposition rate by increasing the sheath diffusivity
and the anolyte volume. For the given reactor design, a
calcium winning rate of 140 g/h was estimated at 100%
current efficiency for electrowinning. The winning rate is
estimated using the following equation based on oxygen ion
diffusion into the anolyte:
through the charge hopper, during the period (STEP B)
where the current supply is turned off and oxygen ions are
diffusing through the sheath. Calcium reduction will occur
during this period and metal, M will settle in the reactor
bottom.
(B) M O insoluble in CaCl –CaO salt system. In this
x
y
2
case, the metal oxide can be introduced at any stage (STEP
A and/or STEP B) through the hopper in the catholyte
compartment depending on the level of calcium dissolved in
the salt. The metal, M will also settle in the reactor bottom.
It is important to introduce the metal oxides after a delay,
since sufficient calcium must exist in the system for the
reaction with oxides.
The process could be conducted under a cover of
nitrogen gas in the electrowinning and reduction chamber,
since the metals are not exposed to high temperature
atmosphere and are always contained within the salt phase.
Fig. 4 shows two such possible designs that have been
adopted to produce titanium metal (OS Process) by
electrolytically winning calcium and using it simul-
taneously to reduce titanium oxide [11]. In Fig. 4a, the
calcium chloride salt with electrodeposited calcium is
transferred into another reactor where the reduction of
titania takes place. In the design shown in Fig. 4b, titania is
K6
4
Rmax Z ½1:2!10 expðK6:0!10 =RTÞꢀðff=qÞ½ðC
B
2
KCminÞ=dꢀð2prl Cpr Þ
(12)
This rate of calcium electrowinning is based on the
availability of maximum oxygen into the anolyte via sheath
diffusion. The modified reactor outer dimensions were
maintained for use in the same furnace chamber. The
parameters used for the modified hybrid reactor (Fig. 3) are
listed in Table 3 in comparison to the original reactor
Table 3
Cell parameters for the hybrid cell
(
Fig. 1). These parameters are used in determining the rate
Cell parameters
Calcium win- Hybrid cell
using Eq. (12). The reactor, shown in Fig. 3, allows the two
processes of electrowinning and calciothermic reduction to
be combined into one. The process was identical to the
scheme used for calcium electrowinning except that metal
chloride/oxide powders were added through a ceramic tube
into the catholyte chamber and the reduced metallic button
was collected after the salt was cooled and separated from
the metal. The reduction of metal compound can be
accomplished by following two different schemes:
ning cell
Fig. 1)
(Fig. 3)
(
K2
K1
K3
Sheath radius, r (m)
Sheath height, l (m)
Sheath thickness, d (m)
Sheath porosity, f
2.25!10
0.10
1.5!10
0.20
K3
2.9!10
0.32
1.0!10
0.40
Open porosity fraction, f
Tortuosity factor, q
0.85
0.90
1.5
1.5
Cell temperature, T (K)
2
Diffusion coefficient, DO2K (m s
1123
2.0!10
1.85
1273
4.2!10
3.70
K1
-9
K9
)
K3
Bulk concentration, C
B
(kg mole m
)
(A) M O soluble in CaCl –CaO salt system. In this case,
x y 2
the reactive metal oxide will be introduced into the reactor
K3
Min. anolyte conc., Cmin (kg mole m
)
0.07
0.07

B. Mishra, D.L. Olson / Journal of Physics and Chemistry of Solids 66 (2005) 396–401
401
Fig. 4. Two reactor designs describing the OS Process for titanium production; (a) separate chamber for titanium reduction and (b) single
chamber for electrowinning and reduction [11].
introduced in the same chamber and the reduction id
affected by the calcium deposited on the iron cathode.
[3] R.O. Suzuki, T.N. Hanada, T. Matsunaga, T.N. Deura, K. Ono, Mater.
Met. Trans. B 30B (1999) 403–410.
[
4] K. Ono, R.O. Suzuki, A. New, Concept for producing Ti sponge:
calciothermic reduction, J. Mater., TMS Publ. 54 (2002) 59–61.
5] R.O. Suzuki, K. Ono, A new concept of sponge titanium production
6
. Conclusion
[
2
by calciothermic reduction of titanium oxide in the molten CaCl ,
Calcium can be electrolytically produced by dissociating
Proc. Electrochem. Soc. 2002-19 (Molten Salts XIII) (2003)
810–821.
calcium oxide in a molten calcium chloride electrolyte. A
porous ceramic diaphragm around the anode is essential for
separating the anolyte and catholyte to be able to
cathodically deposit calcium. The cell temperature, fluidity
of salt and porosity of the sheath are critical in recovering
calcium. Ionic diffusion through the sheath is the rate-
[
6] P.D. Ferro, B. Mishra, D.L. Olson, J.J. Moore, W.A. Averill,
Recovery of calcium from the effluent of direct oxide reduction
process, in: R.G. Reddy, W.P. Imrie, P.B. Queneau (Eds.), Residues
and Effluent: Processing and Environmental Considerations, TMS
Publication, Warrendale, PA, 1992, pp. 539–550.
[7] B. Mishra, P.D. Ferro, D.L. Olson, J.J. Moore, W.A. Averill,
Electrowinning of calcium for in-situ direct reduction of metal
oxides, in: Proceedings of the 180th Meeting of the Electrochemical
Society, Phoenix, AZ, 1991, pp. 284–285.
K5
controlling step. A diffusion coefficient in the range of 10
K6
2
to 10 cm /s is obtained for a 30% porous alumina sheath
for cell temperatures between 800 and 9008C.
[
8] B. Mishra, D.L. Olson, W.A. Averill, Application of molten salts in
pyrochemical processing of reactive metals, in: R.J. Gale,
G. Blomgren, H. Kojima (Eds.), Molten Salts, Electrochem. Soc.
vol. 92-16 (1992), pp. 184–203.
A hybrid process is investigated consisting of electro-
winning calcium from calcium oxide and in situ utilization
of calcium as a reductant within the same reactor. Silver, tin,
lead and bismuth can be produced by pyrochemical
reduction of their respective chlorides and/or oxides with
calcium in a calcium chloride medium. These metals were
produced as a surrogate for a certain radioactive metal.
[9] B. Mishra, D.L. Olson, W.A. Averill, Processing of effluent salt from
the direct oxide reduction process, in: S.R. Rao, L.M. Amartunga,
D.A.D. Boateng, M.E. Chalkley (Eds.), Proceedings of the
International Waste Processing and Recycling in Mining and
Metallurgical Industries, CANMET Publication, Montreal, Que.,
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992, pp. 279–291.
References
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(1991), pp. 19–24.
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(