Allanore
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being condensable and transportable for remote energy generation or safe storage. Developing such a technology is a challenge. Indeed, to substitute existing technologies, the new electrolysis processes need to exhibit the cost, scale and productivity highlighted in Table I and discussed above. To compete on capital costs, the production per unit of time and per unit of area needs to be comparable to existing technologies. Aluminum electrolysis technology, with its high productivity gains (see Fig. 3), shows that producing a liquid metal enables the separation, handling and separation of the metal from a cell without interrupting the current. For iron, an electrolysis cell technology of the same dimensions as today’s HH cell would have to produce 10 times more liquid metal per hour, every hour, every day, every year, and for several years. The energy budget available is dictated by the cost of electricity, with special restrictions to the various contributions of the cell voltage introduced in Ref. 13 and detailed in Ref. 28. Hereafter, a projection of the electrochemical engineering metrics necessary for Fe and Cu direct electrolysis to compete with existing technology is proposed, using today’s aluminum electrolysis performances as a baseline.
Engineering of an Electrolysis Cell Producing a Liquid Metal Commodity
Assuming the production of a liquid metal, the existing HH cell dimensions and efficiency can be used as a baseline to evaluate the features of a similar process to make Fe (Tm = 1538 °C) and Cu (Tm = 1085 °C). For a mitigation of environmental impact, the gaseous anodic product evolves on a non-consumable anode (O2 for Fe, S2 for Cu), meaning a significant departure and improvement of the anode design for gas evolution and removal (see Ref. 21). The aluminum cells today are self-heating reactors, where the engineering of irreversibility (Joule effect due to the ohmic drop) is used to maintain the temperature of the electrolysis cell. The cell operates at 960 °C, with energy losses of around 53%. Some of those heat losses are purposely engineered to allow the formation of a frozen side-wall to contain molten cryolite. Additional losses include heat transfer through the bottom, walls, top electrodes and gas leaving the cell.15 The current efficiency for aluminum is very high, greater than 95%, an important feature for affordable tonnage metals production. Knowledge of the energy and current efficiency allows one to evaluate the actual energy consumption anticipated to conduct Eq. 5 for Fe and Cu. Maintaining the existing productivity and footprint for those metals (see Table II for existing data) calls for a re-evaluation of the total current flowing in electrolysis cells, meaning operating at a different current density. The current and current density recalculated for Cu and Fe are shown in Table II (column 5), where a single HH cell is considered with a cathode area of 54 m2.14 For iron, the production rate has been multiplied by 10 to match today’s smelter capacity differences with Al. For copper, the productivity by square meter has been scaled by 1.13, to match the difference in smelter footprint.
The energy consumption estimated for Cu direct decomposition, at 4204 kWh/t using the existing performance of HH cells (53% losses) is not yet compatible with the electricity budget available at $0.05/kWh derived above (3840 kWh/t). For Cu, energy management improvements for electrolysis are required to be at par with today’s conversion costs and losses can only represent 45% of the electricity consumed. If losses can be reduce to 33%, direct copper electrolysis would enable reduced energy consumption for copper extraction (2949 kWh/t). Matching specific surface productivity of today’s operations call for a current density at around 0.23 A/cm2, around 3 times what is found today in copper aqueous electrowinning. For iron, the energy losses experienced today in HH cells are not compatible with the cost of the final product. Indeed, affordable iron made by Eq. 5 driven by electrolysis requires a maximum of 3900 kWh/t, which could not be achieved with 53% losses. A direct electrolytic decomposition cell for iron production would need to achieve less than 33% energy loss, a challenge that requires careful mastering of heat transfer at 1538 °C. Existing electrical processes at such high temperatures suggest that such performance is possible, though yet to be proven for electrolysis.30 In terms of productivity, matching existing performances for iron requires a current density of around 4 A/cm2, a figure pointing to a need for a breakthrough in electrochemical reactivity of metal ions. For Cu, Fe, and Al, the requirement for high productivity highlights the relevance of producing a liquid metal product, avoiding the limitations inherent to electro-crystallization. The current example of Hall-Héroult cells or the past amalgam Cl2/Na production with a liquid cathode at a current density up to 1.5 A/cm2 are strong indicators of the benefits of such an approach. 28
Consequences on Electrolyte and Electrochemical Engineering
Some of the figures calculated above are challenging for electrical and mechanical engineering. For example, as suggested in Table II, line 4, how to safely operate a 2000 kA cell at around 1600 °C for liquid steel production, with the corresponding recovery of 120 Nm3/ min of O2, all at a terminal voltage of less than 2.6 V? Electrochemists are also challenged to find conditions that could actually enable Eq. 5 to be driven at such a rate, with a selectivity greater than 95%. Considering the desire to produce liquid metal, high temperature molten electrolytes are needed. Table II shows the conditions of the HH process, which again can be used as an inspiration. Considering the cathode current density used in today’s HH cells corresponds to 70% of the limiting current density—it is common in electrodeposition not to run at the limiting current to avoid uneven growth of the metal—it is possible to evaluate the equivalent mass transfer conditions existing at the cathode via:
kD =
jlim 1 3F [ M 3+ ]
(6)
Using a bulk concentration of Al3+ equivalent to 10 mol/L,1 the equivalent mass transfer coefficient amounts to around 4.10−6 m/s, familiar to aqueous and molten salts electrochemists. Assuming
Table III. Important physical chemical data for some molten electrolytes (μ: viscosity in mPa.s; ρ: density in g.cm−3; κ: conductivity in S.cm−1). Pros
Cons
Halides (Ref. 36, 37, and 38)
high κ [0.02 – 6] low μ [1 – 6] low ρ [1.3 to 1.9]
- temperature limited to <1200 °C - low solubility for O2−, S2− - possible liquid metal solubility
Fluorides (Ref. 36)
high κ [0.1 – 10] low μ [1 – 20] low ρ [1.8 to 4]
- temperature limited to <1200 °C - corrosive - vapor pressure
Oxides (Ref. 31 and refs therein)
high solubility for oxide low vapor pressure
- high viscosity in presence of network former - electronic conduction
Sulfides (Ref. 39)
high solubility for sulfides high κ low µ
- vapor pressure - electronic conduction
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The Electrochemical Society Interface • Summer 2017 • www.electrochem.org