Environmental Engineering Reference
In-Depth Information
As indicated in Figure 8.3.4, a molar excess, of greater than 1:1 of Li
2
OtoFe
2
O
3
in
molten Li
2
CO
3
, will further inhibit the Equation 8.2.1 disproportionation of lithium
carbonate. The right side of Figure 8.3.6 summarizes the thermochemical calculated
potentials constraining iron production in molten carbonate. Thermodynamically it is
seen that at higher potential, steel (iron containing carbon) may be directly formed via
the concurrent reduction of CO
2
, which we observe in the Li
2
CO
3
at higher electrol-
ysis potential, as Li
2
CO
3
O
2
, followed by carbonate regeneration via
Equation 8.2.3, to yield by electrolysis in molten carbonate:
→
C
+
Li
2
O
+
Steel Production
:Fe
2
O
3
+
2xCO
2
→
2FeC
x
+
(3
/
2
+
2x)O
2
(8.3.10)
From the kinetic perspective, a higher concentration of dissolved iron oxide improves
mass transport, decreases the cathode overpotential and permits higher steady-state
current densities of iron production, and will also substantially decrease the ther-
modynamic energy needed for the reduction to iron metal. In the electrolyte Fe(III)
originates from dissolved ferric oxides, such as LiFeO
2
or LiFe
5
O
8
. The potential for
the 3e
−
reduction to iron varies in accord with the general Nerstian expression, for a
concentration [Fe(III)], at activity coefficient,
α
:
E
Fe(III
/
0)
+
α
Fe(III)
[Fe(III)])
1
/
3
E
Fe(III
/
0)
=
(RT
/
nF)log(
(8.3.11)
This decrease in electrolysis potential is accentuated by high temperature and is a
∼
0.1 V per decade 10 increase in Fe(III) concentration at 950
◦
C. Higher activity coef-
ficient,
α
Fe(III)
>
1, would further decrease the thermodynamic potential to produce
iron. The measured electrolysis potential is presented on the right of Figure 8.3.5 for
dissolved Fe(III) in molten Li
2
CO
3
, and is low. For example 0.8 V sustains a current
density of 500 mA cm
−
2
in 14 m Fe(III) in Li
2
CO
3
at 950
◦
C. Higher temperature, and
higher concentration, lowers the electrolysis voltage, which can be considerably less
than the room potential required to convert Fe
2
O
3
to iron and oxygen. When an exter-
nal source of heat, such as solar thermal, is available then the energy savings over room
temperature iron electrolysis are considerable.
Electrolyte stability is regulated through control of the CO
2
pressure and/or by
dissolution of excess Li
2
O. Electrolyte mass change was measured in 7 m LiFeO
2
&
3.5 m Li
2
O in molten Li
2
CO
3
after 5 hours. Under argon there is a 1, 5 or 7 wt% loss
respectively at 750
◦
C, 850
◦
C or 950
◦
C), through CO
2
evolution. Little loss occurs
under air (0.03% CO
2
), while under pure CO
2
the electrolyte gains 2-3 wt% (external
CO
2
reacts with dissolved Li
2
O to form Li
2
CO
3
).
The endothermic nature of the new synthesis route, that is the decrease in iron
electrolysis potential with increasing temperature, provides a low free energy opportu-
nity for the STEP process. In this process, solar thermal provides heat to decrease the
iron electrolysis potential, Figure 8.3.5, and solar visible generates electronic charge
to drive the electrolysis. A low energy route for the carbon dioxide free formation of
iron metal from iron ores is accomplished by the synergistic use of both visible and
infrared sunlight. This provides high solar energy conversion efficiencies, Figure 8.2.2,
when applied to Equations 8.2.14 and (8.3.6) 20 in a molten carbonate electrolyte. We
again use a 37% solar energy conversion efficient concentrator photovoltaic (CPV) as
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