High carbon DRI – getting the most value from iron ore - Tenova

DRI module at
Vikram Ispat
India produces
900kt/y of DRI
and HBI
Times have changed. Just a few
decades ago it seemed unlikely
that the electric furnace
steelmaking route would account
for a third of the world's steel
production. Most EAFs used all scrap
charges and today’s modern high-powered
furnaces and continuous casting
technologies were just being developed.
Only a very few were convinced that one
day the industry would be producing even
the most demanding grades of steel using
this new route.
By the same token, the use of DRI, HBI
and pig iron in EAFs was something of a
novelty. Only those who needed to dilute
scrap residuals were charging alternative
iron units to their furnaces, and generally in
just enough proportion (5-15%) to reduce
residual elements down to acceptable
levels. Generally speaking there was a
premium associated with these iron units -
usually from $20 to 40/t over the top grades
of scrap, and with scrap prices around $70/t
it was a hard sell to increase the use of DRI
or HBI beyond required levels.
Fast forward to the present, with scrap at
$250-300/t, iron ore around $100/t –
excluding freight – and modern EAF shops
needing the highest quality iron units
available, the demand for all quality iron
units continues to grow. Growth in demand
for steel, fed in large part by demand in the
Middle East and more significantly by
China, has driven raw materials prices to
new highs. A major difference now is that
with higher demand and higher prices for
raw materials, it becomes increasingly more
difficult to find enough of the best quality
iron ores at suitable costs. For quality steel
products, the need for the highest quality
DRI has never been more critical.
DRI
Energiron is the innovative direct reduction
technology jointly developed and marketed
by Tenova HYL and Danieli & Co.
Through more than 50 years of research
and development and industrial
application, the technology has evolved
RAW MATERIALS
High carbon DRI – getting
the most value from iron ore
The Energiron DRI process uses autocatalytical reformation
within the shaft furnace to convert the gas supplied to
hydrogen for reduction of the ore. This enables high
sulphur ores to be treated without contaminating the DRI
produced and results in a high carbon content DRI which
provides a more efficient energy source to the EAF than by
carbon injection, and also prevents reoxidation of the DRI
during storage and transport.
BY THOMAS SCARNATI*
High carbon DRI produced by the Energiron
process
into what is today the most flexible and
economical direct reduction technology in
the industry. This flexibility, which has
always avoided the ‘cookie cutter’ approach
to DR plant design, has been able to satisfy
clients in a diversity of situations,
depending on their availabilities of raw
materials, fuel and power supply and
product requirements.
IRON ORE QUALITY
In Direct Reduction processes, the
characteristics and cost of available iron ores
play a very important role. The selection of
suitable raw materials will optimise
productivity, energy consumption and the
overall economy of industrial plants.
Since the use of economic iron ores is key
for the feasibility of direct reduction
projects, it is of prime importance for the
DR technology to have good flexibility in
processing different iron ores with
satisfactory results in productivity, product
quality, operating reliability and energy
consumption.
The Energiron technology has a long
history of success in this regard, due to the
characteristics of the process itself and the
design of the shaft furnace reducing unit.
So much so in fact that it is feasible to use
very high percentages of lump ore, which is
generally more economical than pelletized
ore. Some plants such as Usiba in Brazil
and Vikram Ispat in India have used up to
100% lump ore successfully, although
adjusting for operating economics the
practical level of lump use tends to fall in
the range of 80 to 85% – still a very
significant cost saving.
*Tenova HYL Mexico Tel +52 81 88652863, thomas.scarnati@mx.tenovagroup.com
30 Steel Times International January/February 2008
SULPHUR NO PROBLEM
Iron ores for direct reduction are selected
according to their chemical and physical
characteristics, reduction properties and the
overall economics for both reduction and
subsequent steelmaking. For the Energiron
process, there are no practical limitations
regarding the chemical composition of the
iron ore. Common impurities such as
sulphur and phosphorous, which can be
present in some particular ores in relatively
high concentration, can be used without any
technical limitation in Energiron plants.
Particularly regarding the sulphur content
in iron ores, the Energiron process is very
flexible for using high-sulphur feedstocks,
since the reducing gas is not recycled
through a reformer, and thus the possibility
of poisoning the reformer catalyst by
sulphur does not exist. Moreover, most of
the sulphur from the iron ore, converted to
H 2 S in the reduction reactor, is eliminated
from the process in subsequent separation
steps, ie the quench towers and CO 2
removal unit. Therefore, the DRI produced
has typically low sulphur levels.
HIGH PRESSURE
As regards the physical characteristics of
the ores, the high pressure operation of the
Energiron process provides additional
advantages. The operation at high pressure
in the reduction reactor has the main
advantage of feeding larger mass flow rates
for a given volume flow rate. Under these
conditions, it is possible to increase the
reactor productivity and to keep the gas
velocity through the reactor below the point
at which the burden could be fluidised. At
lower gas velocities, the pressure drop
through the reactor is also smaller.
Therefore, the processing of friable ores
can be carried out in better conditions. As
some of these ores generate an important
amount of fines, the operation at high
pressure assures that the amount of solid
material carried by the gas stream is kept to
a minimum. On the other hand, by
operating with an adequate fluidisation
factor when processing these materials, the
solids flow pattern and the gas distribution
are properly controlled.
Due to the high operating pressure, the
amount of metallic units lost in Energiron
plants is between 5 to 10 times less than the
losses for DR plants operating at
atmospheric pressure.

REDUCING GAS FLEXIBILITY
In an Energiron plant the reducing gas is
supplied in any of several ways: using
natural gas directly in the shaft reactor by
means of ‘in situ reforming’ reactions (the
ZR process), or from an external supply
such as a steam reformer, coal gasification
unit or by using COG or other gas sources.
The ratio between reforming and in situ
reforming can be varied to balance
production and investment costs
exigencies. The Energiron scheme can be
based on 100% external reforming to
100% in-situ reforming (ZR) or any
combination (small reformer + oxygen
injection). This is a unique characteristic
of the Energiron process flexibility. The
most adequate scheme will depend on the
local cost structure of energy and raw
materials. As an example, the scheme with
external reformer consumes a bit more
gas (0.03Gcal/t), but the power demand is
minimised (10-15kWh/t less), while the
ZR scheme minimises the natural gas
consumption but requires somewhat more
power. Also, the product quality has to be
considered: the scheme with 100%
external reforming produces DRI with up
to 2.4% carbon or up to 3.5% carbon if
there is some oxygen injection, while the
ZR scheme naturally produces DRI with
about 4% carbon. The most adequate
scheme must be selected based upon the
production cost analysis up to liquid steel,
to consider all factors.
This overall flexibility, unique to the
Energiron technology, allows DR plants
to be designed even for regions where
natural gas is not readily available or
where the cost is too high to be
economical. The ZR process
configuration further allows the
production of the highest value-added
product in the form of highly metalized,
high carbon DRI.
HIGH CARBON FOR STEELMAKING
The ZR process scheme has the
characteristic that it can produce a wide
range of DRI carbon content. This carbon,
in the form of iron carbide (cementite), is
combined in the reduced iron and is
therefore equivalent to adding chemical
energy to the EAF when oxygen practice is
used. The typical DRI from the ZR process
ranges from 2.8% to 5%, with optimum
results in the range of 3.8-4% and
metallization of >94%. This is significantly
more than traditional DRI products, which
have carbon levels of 1.3% or less. When
using high percentages of DRI in the
metallic charge, DRI carbon content has to
be set according to the meltshop needs to
maximise meltshop productivity. This
combined carbon is more efficiently and
completely used in the EAF, thus
minimising external carbon (graphite)
additions. While coal and graphite
additions to the EAF give yields of 40% or
less, due to particle blow-off and the
content of ash and other materials in coal,
the combined carbon in DRI has a yield of
100%. Furthermore, the conversion of
Fe 3 C into iron and carbon is an exothermic
reaction which improves the thermal
efficiency in the EAF.
1 Energy performance of EAF charged with two
grades of C content DRI and at different feeding
temperatures
(Top 2.2%C 25Nm 3 /tls O 2 , lower 4.0%C 42m 3 /tls
O 2
The benefits of charging hot, high carbon
DRI in meltshop operations have been
demonstrated in TerniumHylsa’s meltshop
while feeding up to 100% hot DRI with
about 94% metallization and 4% carbon.
In general, DRI carbon in the EAF
provides:
Chemical energy contribution; the
dissociation of cementite is an
exothermic reaction:
(Fe 3 C 3Fe + C + E -0.7 kWh/%C),
which improves the thermal efficiency in
the EAF thus decreasing electric power
requirements. Elemental carbon for
charging to the EAF normally costs more
and provides a much lower yield than the
carbon obtained from the Fe 3 C derived
from natural gas during DRI production.
Efficient use of carbon; as compared to
other sources of carbon injection, while
minimising external carbon (graphite)
additions, cementite (Fe 3 C) in DRI is
characterised by a higher recovery yield
in the EAF.
Easy foamy slag generation; as high
carbon DRI enters in contact with free or
combined oxygen.
The same system controls the feeding
rate of metallic charge and carbon
additions.
The impact of DRI carbon in the EAF is
presented in Fig 1. Graphite injection is
about 12kg/tLS when DRI with 2.2%
CONTACT
Tenova HYL, Av Eugenio Clariond Garza 155 Col Cuauhtémoc San Nicolás de los Garza NL 66452, Mexico
Tel +52 81 8865 2801 Fax +52 81 8865 2810 e-mail hyl@mx.tenovagroup.com
32 Steel Times International January/February 2008
The HYTEMP transport line between the DRI
module and the EAF at Ternium Monterrey,
Mexico
carbon is charged and 0.5 kg/tLS for DRI
with 4.0% carbon. For these operating
conditions, the change from 2.2% to 4%
carbon in cold DRI represents a decrease
of 11kg graphite and 58kWh/tLS energy
consumption. This power saving is a
result of the replacement of graphite by
cementite due to the improved yield and
its exothermic decomposition.
If the DRI is fed hot from the
reduction shaft furnace, additional
sensible heat is supplied to the EAF,
reducing power consumption and tap-totap
time, which is additionally increases
productivity. The overall effect of using
the highly-efficiency ZR module with its
minimum thermal and electricity
consumption, plus the use of hot and/or
cold high carbon DRI in the EAF, has an
important impact on the overall energy
demand for steel production, in turn,
decreasing overall plant emissions and
particularly CO 2 released to the
atmosphere.
PASSIVE SURFACE
There is an additional and no less
important advantage of this type of DRI –
the carbon which is chemically combined
with iron is concentrated in the outer part
of the reduced iron pellets, more so than
with conventional DRI. This forms a
‘shell’ which helps resist reoxidation,
making the product much more stable
than conventional DRI products.
As such, it becomes an attractive
product for merchant iron production
since it can more safely be transported
over distances by following reasonable
safety practices. Such a product can
eliminate the need for expensive
briquetting operations, especially for
products which will primarily be shipped
by land or for shorter ocean voyages.
ADDING VALUE
The value of iron ore is increased
significantly by transforming it to DRI and
then to liquid steel. It is the direct reduction
processes themselves that determine the
acceptable ranges of iron ore materials, with
some technologies more limited in range
than others. The Energiron technology has
longstanding experience with lower grade
iron ores, including lump ores and high
sulphur content ores, all of which can be
perfectly well reduced by the process.
The independence of the reducing circuit
makes the Energiron technology capable of
using a variety of reducing gas sources. In
areas where low-grade coal is abundant,
syngas from coal gasification plants can be
used as the reductant in an Energiron
module. Coke oven gas from an integrated
facility is also an option, providing even
more flexibility for reducing iron ores in
regions lacking in natural gas.
The quality of the DRI produced from
the Energiron plants is highly valued by
electric furnace operators. The combined
carbon provides chemical energy to the
EAF, increasing the overall efficiency and
reducing the need for carbon additions. It
has the further advantage of added stability
due to its iron carbide content, making it an
attractive merchant product as well. STI