Environmental Engineering Reference
In-Depth Information
Figure 8.3.5
Middle: Photographs of electrolysis products from 20% Fe
2
O
3
or Fe
3
O
4
by mass in 800
◦
C
Li
2
CO
3
: following extended 0.5A electrolysis at a coiled wire (Pt or Fe) cathode with a Ni
anode. Left: cathode restricted CV in Li
2
CO
3
, containing 1:5 by weight of either Fe
2
O
3
or
Fe
3
O
4
. Right: The measured iron electrolysis potentials in molten Li
2
CO
3
, as a function
of the temperature, current density, and the concentration of dissolved Fe(III). Modified
with permission from Licht andWang 2010.
The two principal natural ores of iron are hematite (Fe
2
O
3
) and the mixed valence
Fe
2
+
/
3
+
magnetite (Fe
3
O
4
). We observe that, Fe
3
O
4
is also highly soluble in molten
Li
2
CO
3
, and may also be reduced to iron with the net electrolysis reaction:
E
◦
=
→
+
=
Fe
3
O
4
3Fe
2O
2
1
.
32 V, E
thermoneutral
1
.
45 V
(8.3.6)
Fe
3
O
4
electrolysis potentials run parallel, but
0.06 V higher, than those of Fe
2
O
3
in
Figure 8.2.1. The processes are each endothermic; the required electrolysis potential
decreases with increasing temperature. For Fe
3
O
4
in Figure 8.3.5, unlike the single
peak evident for Fe
2
O
3
, two reduction peaks appear in the CV at 800
◦
C. Following
the initial cathodic sweep (indicated by the left arrow), the CV exhibits two reduc-
tion peaks, again more pronounced at an iron electrode (grey curve), which appear
to be consistent with the respective reductions of Fe
2
+
and Fe
3
+
. In either Fe
2
O
3
,or
Fe
3
O
4
, the reduction occurs at a potential before we observe any reduction of the
molten Li
2
CO
3
electrolyte, and at constant current, iron is deposited. Following 1
hour of electrolysis at either 200 or 20 mA/cm
2
of iron deposition, as seen in the Fig-
ure 8.3.5 photographs, and as with the Fe
2
O
3
case, the extracted cooled electrode,
following extended electrolysis and iron formation, contains trapped electrolyte. Fol-
lowing washing, the product weight is consistent with the eight electron per Fe
3
O
4
coulombic reduction to iron.
The solid products of the solid reaction of Fe
2
O
3
and Li
2
CO
3
had been char-
acterized (Collongues and Chaudron, 1950; Wijayasinghe et al., 2003). We prepare
and probe the solubility of lithiated iron oxide salts in molten carbonates, and report
high Fe(III) solubilities, in the order of 50% in molten carbonates, are achieved via the
reaction of Li
2
O with Fe
2
O
3
, yielding an effective method for CO
2
free iron production.
Lithium oxide, as well as Fe
2
O
3
or Fe
3
O
4
, each have melting points above 1460
◦
C.
Li
2
O dissolves in 400-1000
◦
C molten carbonates. We find the solubility of Li
2
Oin
molten Li
2
CO
3
increases from 9 to 14 m from 750
◦
to 950
◦
C. Following preparation
of specific iron oxide salts, we add them to molten alkali carbonate. The resultant
∼
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