Electrolytic steelmaking as a paragon for new electrolytic processes
development
Electrolytic Steelmaking
: a 21st Century Challenge
To understand
why the development of electrolytic
steel is occuring so lately in technology history, one must first
consider the history of steelmaking and electricity.
Iron - wrought iron initially - has been smelted for
the
first time around 5000 BC in Mesopotamia (Samarra, Irak) [1].
It happened most probably by chance in a furnace commonly used to
smelt copper
and silver or lead. These furnaces were based on charcoal combustion, a
crude, old and efficient method to obtain high temperature and
reductive atmosphere. It could have happened thanks to an iron
oxide-rich gangue that came with copper. Carbon
reacts with the oxide ions of the iron oxide to form carbon monoxide or
dioxide,
leaving behind a solid iron product entrapped in gangue (silica,
alumina).
Such crude description reveals one scientific truth: iron is relatively
easy to extract from it oxides. Indeed, carbothermic reduction is an
easy method to implement, notably thanks to the abundance of carbon
(charcoal, coal, natural gas) as a powerful reducing agent. The
exothermic properties of its combustion provides heat to reach high
temperatures, and its alloying lead to a material with exceptional
mechanical properties (its alloying also lowers the melting point of
the product, leading to a liquid product). Unfortunately, one of the
difficulty faced by steelmakers after this smelting operation is the
nature and amount of elements (carbon, sulfur, phosphorus, silicon...)
alloyed with iron, which lead to the development of steelmaking per se [2,3]. The
technology has moved from breakthrough to breakthrough in 2000 years,
but the fundamental carbothermic principle remained unchanged.
The abundance of carbon-containing raw materials and
the development of
pyrometallurgical techniques has therefore lead to a cheap and
efficient carbothermic method to produce steel.
In parallel
to this inexpensive and easy technique revolutionary
development, the discovery of electricity as an energy vector is late,
too late probably. This naturally hindered the electrolysis technique
development.
Considering scientific developments since electrolysis discovery in
early 19th century [4],
one must also realize that there is a scientific
challenge for its application to iron. Indeed, as chromium, manganese,
vanadium or titanium [5],
iron is stable in two valence state in
atmospheric conditions (20% oxygen), +II or +III. The electrolysis of
mixed valency ions is difficult because electron transfer can occur in
any direction depending on the electrode considered: Fe3+
can be
reduced to Fe2+ on the cathode, which can then reach the
anode to be
oxydized back to Fe3+. Such cycling induces an unacceptable
diminution
of the process selectivity. The method commonly adopted to avoid
this is
the use of a separator which represent an extra barrier for current
flow (ohmic
drop), significantly increasing the energy needs of the process. This
physical reality explains why, until the most recent developments,
electrolysis production of iron has not been successfully developed.
With the acknowledgment of environmental impacts of
human activities, and more particularly industries, new paradigms are
expected to emerge. One of the environmental concerns, GHG emissions,
particularly affects the steel industries which rely on carbon as
reducing agent and energy vector. The situation of this industry in
developed countries is indeed paradoxical: it has reduced by 20% its
energy consumptions and 40% its GHG emissions in the last 40 years, and
still emits more than 1.75 metric tonnes of CO2 per tonne of
steel. Experts
in this field estimate that the technique has reached a physical and
thermodynamical limit in terms of energy consumption. This industry is
however expected to cope with more drastic regulations in terms of GHG
emissions. This situation explains the steel industry need and interest
in breakthrough technologies [6].
Any electrochemist must stress that the
reduction in energy consumption and CO2
emissions in the last 40 years
is partly related to the widespread use of elecricity in Electric Arc
Furnace operations based on recycled steel. Most economical scenarios
for 2020 and beyond underlines that the cost of raw materials and
fossil fuels is expected to increase drastically, while electricity
will become less carbon-intensive and relatively more affordable [7,8].
The development of electrolysis for steelmaking is
therefore considered
as a possible alternative, specially since other metals are already
industrially produced thanks to this technique (24 Mt for aluminum, 30
Mt for copper refined and extracted, 10Mt for nickel).
[1]
J.C. Waldbaum, in The Coming of the Iron Age, T. A. Wertim and J. D.
Muhly Editors, Yale University Press, NewHaven & London,1980
[2] e.g. see Professor Joseph S. Spoerl from St Anselm (NH, USA), Brief Story of Steelmaking here.
[3] D.A. Fisher. The Epic of Steel, Harper and Row, New York, 1963
[4] A common date for the beginning of the electrochemical science is
1800, when Count Alessandro Volta discovered the battery. He indeed
provided the first readily available source of electricity for
physicists and chemists. e.g. see the work from Pavia University
(Italy) on Volta's influence on physics.
[5] C.L.Mantell, Electrochemical Engineering, McGraw and Hill, New York, 1960
[6] J-P. Birat, F. Hanrot and G. Danloy,
CO2 Mitigation Technologies in
the Steel Industry: A Benchmarking Study Based on Process Calculations,
Stahl und Eisen, vol. 123, n°3, 2003 (available here)
[7] see International Energy Agency/OECD, Energy Technology Perspectives, (2010)
[8] C. Rynikiewicz, The Climate Change Challenge and Transitions for Radical Changes in the European Steel Industry, Journal of Cleaner Production, vol. 16, p781, 2008
Back