ARTICLE pubs.acs.org/JPCC
STEP Iron, a Chemistry of Iron Formation without CO2 Emission: Molten Carbonate Solubility and Electrochemistry of Iron Ore Impurities Stuart Licht* and Hongjun Wu† Department of Chemistry, George Washington University, Washington, D.C. 20052, United States ABSTRACT: The unexpected solubility of iron oxides in lithium carbonate electrolytes, coupled with facile charge transfer and a sharp decrease in iron electrolysis potentials with increasing temperature, provides a new route for iron production. Iron is formed without an extensive release of CO2 in a process compatible with the predominant naturally occurring iron oxide ore, hematite, Fe2O3. In this study, we demonstrate that iron is an effective cathode media and that nickel and iridium are effective anodes in the electrolytic formation of iron in molten carbonates, and each can sustain current densities of 1 A cm2. Silicate and aluminate impurities found in common iron ores are soluble in molten lithium carbonate and do not adversely affect the iron formation by electrolysis. The solubility of lithium oxide in molten carbonate is found to be unexpectedly high (∼50 mol %), and the presence of high concentrations of lithium oxide decreases the potential required for the electrolytic formation of iron or carbon. Iron, a basic commodity, currently accounts for the release of one-quarter of worldwide CO2 emissions by industry. The endothermic nature of the new synthesis route provides an effective vehicle for the solar efficient, CO2-free, production of iron as a STEP process.
’ INTRODUCTION Recently, we introduced an alternative chemical mechanism for iron production based on the unexpected, high solubility of iron oxide salts in lithiated molten carbonate salts.13 In these molten salts, containing as much as 50% by mass of dissolved iron oxide, iron metal is produced by electrolysis at high rate, low energy, and without carbon dioxide release. In industry, iron metal is produced by the greenhouse gas intensive reduction of iron oxide ores by carbon coke. This CO2-emitting process generates a significant fraction (25%) of all carbon dioxide released by industry.46 Instead, solar thermal energy activates the new carbonate electrolysis process, which, for iron synthesis, has been termed STEP iron (Solar Thermal Electrochemical Production of iron). The electrolysis is endothermic.3,7 Higher temperature offers the advantage of lower electrolysis potential and facilitated charge transfer, and the high sustainable iron oxide concentrations in carbonate melts further decrease the electrolysis potential.7 Along with control of fire, the carbothermal reduction of iron is one of the founding technological pillars of civilization. Yet, it is also one of the major global sources of greenhouse gas release, and a CO2-free process to form this staple is needed. Hematite, Fe2O3, and magnetite, Fe3O4, are the principal ores currently used for the widespread carbothermal industrial production of iron. The processes use ores that generally contain less than 25% silicate or aluminate salts (silicates are the dominant impurity, and aluminate comprises the second most common impurity in these ores). This study explores the anodic and cathodic electrochemistry, as well as the physical chemistry of silicate and aluminate impurities effects on the new STEP iron process. The carbonate dissolution chemistry of silicates and aluminates is related to the solubility of lithium oxide in molten carbonates. r 2011 American Chemical Society
The solubility of alkali oxides in molten carbonates was previously reported as less than 0.5 mol % at temperatures up to 700 °C.8 Unexpectedly, in this study, we report that the solubility of lithium oxide in lithium carbonate is 2 orders of magnitude higher and increases with temperature, from 42 mol % at 750 °C to 51 mol % at 950 °C. From a historical perspective, the earliest attempt at electrowinning iron (the formation of iron by electrolysis) from carbonate appears to have been in 1944 in the unsuccessful attempt to electrodeposit iron from a sodium carbonate, peroxide, metaborate mix at 450500 °C, which deposited sodium and magnetite (iron oxide), rather than iron.9 Later attempts have focused on the electrodepostion of iron from molten mixed chloride/fluoride electrolytes, which has not provided a successful route to form iron.10,11 Aqueous electrowinning of iron has been attempted for at least a century, including electrodeposition from aqueous choride, sulfate, and from aqueous hydroxide solutions.1215 Because of the high thermodynamic potential (E°25 °C = 1.28 V) needed at room temperature, aqueous electrowinning requires high practical electrolysis potential, which impedes viability or widespread commercial use. At higher temperatures, after an unsuccessful attempt at iron deposition in alternative molten electrolytes, the focus turned to molten halide electrolytes, which have been problematic and not led to a commercial process. An overview of the prior attempts of the art of iron electrolysis at higher temperatures was provided in 2007 by Haarber, Kvalheim, Rolseth, Murakami, Pietrzyk, and Wang.16 Received: August 16, 2011 Revised: November 8, 2011 Published: November 09, 2011 25138
dx.doi.org/10.1021/jp2078715 | J. Phys. Chem. C 2011, 115, 25138–25147
The Journal of Physical Chemistry C
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Both hematite, Fe2O3, and magnetite, Fe3O4, are highly soluble in molten lithiated carbonates.1 We have probed the mechanism of Fe2O3 dissolution2 and observed high solubility both for lower-temperature eutectic carbonate melts (Li0.87Na0.63K0.50CO3) and for pure Li2CO3 (pure Li2CO3 melts at 723 °C). In Li2CO3, ferric, [Fe(III)], solubility increases from 7 to 12 m (molal = m moles per/kg Li2CO3) with a temperature increase from 750 to 900 °C, whereas in the eutectic, the solubility increases from 1 to 4 m Fe(III) as the temperature increases from 550 to 900 °C. The iron electrolysis is endothermic.3,7 Higher temperature offers the advantage of lower electrolysis potential and facilitated charge transfer; alternatively, lower temperature tends to decrease corrosion and facilitate cell stability. LiFeO2 can be formed from dry Fe2O3 and Li2CO3 at 1, would further decrease the thermodynamic potential to produce iron. Iron is deposited on iron or platinum cathodes, and as previously noted, iron or platinum cathodes exhibit similar overpotentials in lithiated ferric-containing carbonate electrolytes. The cathode constrained electrolysis potential (measured with an oversized anode as detailed in the Experimental Section) is presented in Figure 3 for dissolved Fe(III) in molten lithium carbonate and is low. For example, in 10 m Fe(III), dissolved as LiFe5O8, at 950 °C, 0.7 V sustains a current density of 500 mA cm2. Higher temperatures decrease the energy needed for electrolysis, and the higher concentrations accessed in this study result in another energy savings. For example, as we previously observed,2 at 500 mA cm2, 1.6 V is needed to drive the electrolysis of 3 m Fe(III) at 800 °C. As expected, the measured potentials are considerably less than the room-temperature 1.3 V thermodynamic potential required to convert Fe2O3 to iron and oxygen. When an external source of heat, such as solar thermal, is available, then the energy savings over room-temperature iron electrolysis are considerable. It is interesting to note that the observed electrolysis potential at low current density is considerably smaller than the expected thermodynamic potential of 0.8 V at 950 °C. This is at least partially explained by the high, nonunit activity of the dissolved iron (10 m) providing significant voltage and energy savings. Molten Electrolyte Advantages. Molten carbonates present several fundamental advantages compared with solid oxides for CO2 electrolysis.22 (i) In the absence of dissolved iron, the molten carbonate electrolyzer provides a 103 to 106 times higher concentration of carbonate as a reactant at the cathode surface than a solid oxide electrolyzer. Solid oxides utilize gas-phase reactants, whereas carbonates utilize molten-phase reactants. Molten carbonate contains 2 102 mol of reducible tetravalent carbon/cm3. The density of reducible tetravalent carbon sites in the gas phase is considerably lower. Air contains 0.03% CO2, equivalent to only 1 108 mol of tetravalent carbon/cm3, and flue gas (typically) contains 1015% CO2, equivalent to 2 105 mol reducible C(IV)/cm3. Carbonate’s higher concentration of active, reducible tetravalent carbon sites logarithmically decreases the electrolysis potential and can facilitate charge transfer at low electrolysis potentials. (ii) Molten carbonates can directly absorb atmospheric CO2, whereas solid oxides require an energy-consuming preconcentration step. (iii) Molten carbonate electrolyses are compatible with both solid and gas-phase products. (iv) Molten processes have an intrinsic thermal buffer not found in gas-phase systems. Sunlight intensity varies over a 24 h cycle, and more frequently with variations in cloud cover. This disruption to other solar energy conversion processes is not necessary in molten salt processes. For example, the thermal buffer capacity of molten salts has been effective for solar to electric power towers to operate 24/7. These towers concentrate solar thermal energy to heat molten salts, which circulate and, via heat exchange, boil water to drive conventional mechanical turbines. Anode Stability. The stability and electrochemical activity of several anodes are probed in molten lithium carbonate with and 25141
dx.doi.org/10.1021/jp2078715 |J. Phys. Chem. C 2011, 115, 25138–25147
The Journal of Physical Chemistry C
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Table 1. Mass Loss of Pure Nickel (250 μm McMaster 200 Shim Stock) Anodes in Molten Lithium Carbonate during Electrolysisa anode current density,
molten carbonate temperature, T, °C
[Li2O], m
[LiFeO2], m
mA/cm2
time, s
blank, 25 °C
mass loss,
Ni loss,
mg/cm2
μm
1.2
3
750
0
0
100
600
1.7
4
750
5.31
0
100
600
5.3
12
750
3.54
3.54
100
90
5.3
12
750
3.54
3.54
100
600
5.6
13
750
3.54
3.54
100
1200
5.8
13
750 750
3.54 3.54
3.54 3.54
100 1000
5400 360
6.2 8.2
14 20
950
3.54
3.54
100
90
13.7
31
950
3.54
3.54
100
600
14.8
33
950
3.54
3.54
100
5400
15.4
35
950
3.54
3.54
1000
360
16.0
36
The indicated mass loss is normalized by the 6 cm2 anode surface area. The mass loss is normalized by the nickel density (8.9 g cm3) to determine the nickel thickness loss (final column). a
without added lithium oxide, and with and without added lithiated ferric oxide (LiFeO2). While pure nickel is oxidized, albeit slowly, when utilized as an anode in molten lithium carbonate, iridium metal exhibits no evidence of any corrosion after extended use (∼100 h) as an anode in molten lithium carbonate electrolytes, with or without Li2O or LiFeO2 additives, and when used over a range of current densities (from 1 to 1000 mA cm2), and over the temperature range from 750 to 850 °C. As one example, during electrolytic cathodic production, this same iridium anode exhibited no evidence of any oxidation or change in metal thickness after 8 h at 1000 mA cm2 in molten lithium carbonate containing 3.54 m LiFeO2 and 1.72 m Li2O at 850 °C. Nickel is more cost-effective than iridium, and in the future, it will be of interest to probe whether anodes containing a low iridium content (for example, through surface addition of Ir islands to nickel or carbon surfaces, or Ir thin films, or Ir-containing alloys) will also provide favorable anode characteristics. Table 1 summarizes the stability or oxidation of a 250 μm planar nickel anode in molten carbonates, determined by mass loss (subsequent to electrode removal, cooling to room temperature, and washing in sonicated HCl until the electrode surface returns to a uniform metallic color), before and after use as an anode in a molten lithium carbonate. The nickel anode stability is studied as a function of electrolyte composition, temperature, anodic current density, and electrolysis duration. As seen in the table, the nickel loss primarily occurs within the first 90 s of the electrode oxidation. In an initial loss, nickel is oxidized (a nickel thickness of either 12 or 31 μm is initially oxidized, respectively, at 750 or 950 °C). Following this nickel loss, the nickel electrode is particularly stable at 750 °C, at both low and high currents (100 mA cm2) and high current density (1000 mA cm2). For example, at 100 mA cm2 using the 250 μm Ni anode (and after the initial oxidative 750 °C loss of 12 μm, or 31 μm at 950 °C), the Ni anode loss rate is, respectively, ∼1 or 3 μm h1 at 750 or 950 °C. Anode Overpotential. Figure 4 presents the nickel or iridium anode constrained electrolysis potential in a molten lithium carbonate electrolyte in the absence of iron (without added ferric species), whereas Figure 5 presents the anode potential in the presence of iron (with ferric oxide added as LiFeO2). As seen in Figure 4, in the absence of ferric oxide in the electrolyte, iridium or nickel electrodes exhibit a similar anodic overpotential
Figure 4. Anode constrained electrolysis potential measured in a molten lithium carbonate without added iron oxide as a function of current density. Electrolytes with and without added lithium oxide are compared.
increase with increasing anodic current density. It is also evident in the figure that the measured electrolysis potential decreases significantly when lithium oxide, Li2O, is added to the electrolyte. This unexpected electrolysis potential decrease will be discussed in the next section. The anodic overpotential at Ni or Ir electrodes is consistently less than those measured at platinum electrodes (not shown in the figure). In addition, although not shown in the figure, hafnium and zirconium electrodes were examined under similar conditions and exhibited considerably higher anodic overpotentials than the platinum electrode. A titanium electrode was also studied and was not useful, as it underwent rapid oxidative decomposition as an anode. As seen in Figure 5, the addition of reducible ferric iron (as well as the higher temperature of 850 °C) lowers the electrolysis potential by several hundred millivolts compared with the ironfree electrolyte in Figure 4. It is seen that the addition of lithium oxide to the electrolyte again decreases the electrolysis potential. With added lithium oxide, the electrolysis potential is 0.38 V versus a steel cathode at low current density and, even at a current density of 1 A cm2, is still considerably less than the 25142
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The Journal of Physical Chemistry C
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Figure 5. Anode constrained electrolysis potential measured in a molten lithium carbonate with added iron oxide as a function of current density. Electrolytes with and without added lithium oxide are compared.
Figure 7. Anode constrained electrolysis potential measured in a silicate-containing lithiated iron oxide molten carbonate electrolyte. Planar nickel, iridium, and platinum anodes are compared as a function of current density and temperature (750 or 850 °C).
Figure 6. Anode constrained electrolysis potential measured in an aluminate-containing lithiated iron oxide molten carbonate electrolyte. Planar nickel, iridium, and platinum anodes are compared as a function of current density and temperature (750 or 850 °C).
potential is negligible at low current density and relatively small (0.1 V of the measured electrolysis potential of ∼1 V) at a constant current of 0.5 A cm2. The measurement of electrolysis potentials moves from simple molten lithium carbonate electrolytes in Figure 4, to molten lithium carbonate containing dissolved lithiated ferric oxide in Figure 5, and finally to these ferric oxide electrolytes also containing salts derived from common impurities in iron ores. As a first approximation, the presence of either aluminate or silicate is not expected to significantly influence the observed electrolysis potential as the reduction of oxidized iron is thermodynamically favored compared with that of oxidized aluminum or silicon. Aluminate or silicate are, respectively, added in high concentration to the carbonate electrolyte in the anode constrained iron electrolysis measurements in Figures 6 and 7. Figure 6 compares, for an aluminate-containing molten carbonate electrolyte, the measured iron electrolysis potential, as constrained by either iridium, nickel, or platinum planar anodes in lithated ferric oxide. As previously observed in Figures 4 and 5 for carbonate electrolytes without added aluminate, iridium and nickel again exhibit similar anodic overpotentials, while it is seen at both temperatures (750 or 850 °C) that both are more effective (lower overpotential) than platinum over a wide range of anodic current densities. At 850 °C in this figure, the measured electrolysis potentials using a Ni or Ir anode are similar, but marginally higher (∼50 mV), compared to those observed in the absence of added aluminate in Figure 5, and this difference is consistent with the somewhat higher ferric concentration (3.5 m) compared in Figure 5 compared with the (2.8 m) ferric concentration in Figure 6. The high current densities of 1000 mA cm2 probed in Figures 6 and 7 are substantial, as they are already a bit higher than the 800 mA cm2 often used in the commercial aluminum industry. Futhermore, higher current densities and lower electrolysis potentials can be expected with increased microscopic surface area, such as through roughed or porous electrodes. Interestingly, in Figure 7, which compares (flat) anode effects in a silicate-containing molten carbonate electrolyte, the measured iron electrolysis is higher at low current density, and lower at high current density, than observed for electrolytes without
room-temperature standard potential for ferric reduction of E°(FeIII)/Fe(0) = 1.3 V. These low electrolysis voltages at high temperature are useful for the low-energy STEP iron production process. Because of the high conductance of the molten lithium salt electrolyte, any voltage increase due to the resistance drop through the electrolyte is relatively small, even in the high current density domain. The measured electrolysis potentials can be corrected for this relatively small electrolyte resistance effect. The anode is situated in the middle of the crucible, 1.5 cm from the crucible walls used as the large surface cathode. The conductivity of the pure Li2CO3 conductivity is high, 6 S cm1, at the melting point, and increases with increasing temperature,23,24 and the centrally placed anodes have a resistance drop
