Note: Descriptions are shown in the official language in which they were submitted.
WO U1/90424. CA 02407401 2002-10-23 PCT/HEOl~~UU88
ION ORE REDUCTION PROCESS
AND INSTALLATION FOR ITS IMPLEMENTATION
The present invention relates to a process for
reducing iron ore and to an installation for .
implementing it.
In the sense of the present invention, iron
sponge is a ferrous material obtained from iron oxide
by a reduction operation referred to as direct
reduction operation. Iron oxide is traditionally
obtained from ores, in which it is accompanied by
various undesirable substances forming the gangue.
Currently, another interesting source of iron oxide is
also formed by the surface oxides collected at various
stages of steel-making processes, such as mill slivers
and wash sludges. This category of oxides does not
comprise gangue but often contains impurities, such as
oil or grease residues.
The following description will refer to the
general term "iron ore"; here, this term includes both
the customary iron ores and oxides derived from steel
making processes, either separately or in the form of
mixtures in any proportions.
Currently, iron sponge arouses growing interest,
particularly as regards its use in converters, in
alternative processes for the production of cast iron
and in electric steelworks furnaces. Up until now, the
metallic load of electric steelworks furnaces has been
mainly made up of scrap iron. However, it has been
observed that the quality of this scrap iron tends to
decline, especially because of its content of alloying
elements, which may be undesirable for the considered
steels. Moreover, the price of scrap iron varies
sometimes considerably depending not only on its
quality but also on its availability, something that
yV~ ~1.~90424. CA 02407401 2002-10-23 PCT/BEO1~UUU88
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may compromise supplies to electric steel plants in
particular.
Hence the growing interest in the supply of "clean"
iron through the addition of high-quality DRI or of
cast iron produced by melting DRI of inferior quality.
Numerous direct reduction processes are known in
the prior art . These processes are based on the use of
a reducing agent, which is generally either gaseous or
solid. The process of the invention belongs to the
category of processes based on the use of a gaseous
reducing agent, which is essentially a hot reducing
gas, preferably produced from coal.
The known processes in this field are
implemented in furnaces of various types, such as shaft
furnaces, fluidised-bed furnaces or rotary furnaces,
the productivity of which is low and which may require
significant investment and entail high operating costs.
A general tendency to seek an increase in yield while
reducing energy costs per tonne of iron sponge produced
is also observable.
Moreover, owing to the imposition of ever-
stricter legal standards on industry, the treatment of
waste in terms of its impact on the environment
inevitably had to be taken into consideration and
included within the framework of operating costs, said
costs being ever higher to meet legal requirements.
The present invention relates to a process for
manufacturing iron sponge based on the economically
acceptable use of a gaseous carbonaceous reducing
agent. Moreover, this process more easily fits the
framework of respect for environmental norms thanks to
the judicious use of the generated gaseous waste.
The extremely high productivity of the process
of the present invention allows to reduce the impact of
W~ X7./90424. CA 02407401 2002-10-23 PCT~BL''01~0~~88
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the investment pertaining to it when calculating the
cost price of the iron sponge produced.
In accordance with the present invention, a
process for reducing iron ores with a view to
manufacturing . iron sponge, in which a gaseous
carbonaceous reducing agent is used, is characterised
in that a load comprising iron ore is deposited on a
grate, in that said grate is displaced in order to move
said load into at least 3 separate treatment zones, the
first zone comprising an operation for the formation of
the load on the grate, the last comprising an operation
for unloading the load comprising reduced iron ore from
the grate, the intermediate zone or zones comprising,
on the one hand, operations for supplying gaseous
fluids, referred to as inlet gases, including a hot
gaseous carbonaceous reducing agent, and forcing said
gaseous flux through the load resting on the grate,
and, on the other hand, operations for collecting the
gaseous fluids, referred to as exit gases, resulting
from said above-mentioned forced passage in such a way
that said ore is raised to a temperature between 850°C
and 1300°C, preferably between 1050°C and 1150°C, and in
that said load is kept at this temperature until the
metallisation rate of the iron ore present is between
60% and 100%, preferably between 85% and 95%.
The metallisation rate is the ratio of the
percentage of metallic iron (Fe) to the total
percentage of iron.
The notion of "grate" should be considered as an
element for supporting the load of iron ore. Moreover,
due to its construction, said grate can, on the one
hand, be crossed by a gaseous flux and, on the other
hand, serve to transfer the load through the different
treatment zones.
WO X1./90424. CA 02407401 2002-10-23 pCT/BEOl~UUU88
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According to a preferred embodiment of the
process of the invention, the load deposited on the
grate successively passes through at least one zone in
which the temperature of the inlet gas is 450°C ~ 150°C,
at least one zone in which the temperature of the inlet
gas is 500°C ~ 150°C, at least one zone in which the
temperature of the inlet gas is 1200°C ~ 150°C, and at
least one zone in which the temperature of the inlet
gas is 1000°C ~ 200°C.
The load deposited on the mobile grate comprises
iron ore, which is preferably hematite that is
partially hydrated and appropriately prepared for
forming said load.
According to one embodiment of the process of
the invention, the load deposited on the grate in the
loading zone comprises ore mainly made up, in terms of
volume, of iron ore sized between 5mm and 40mm,
preferably between 5mm and lOmm.
Apart from the above-mentioned sized form, the
iron ore can also be incorporated in the load deposited
on the mobile grate in the form of pellets or granules,
for example.
In general, both the pellets and the granules
are obtained by a pelletising operation. However, the
pellets have a relatively homogeneous structure in
terms of volume, while the granules have an element
that acts as a nucleus and serves as a tying base for
finer particles.
According to another embodiment of the process
of the invention, the load deposited on the grate in
the loading zone comprises iron ore pellets with a
diameter between 5mm and 20mm, preferably between 5mm
and lOmm.
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According to yet another embodiment of the
process of the invention, the load deposited on the
grate in the loading zone comprises iron ore granules
with a size between 2mm and lOmm, preferably between
4mm and 7mm.
According to one embodiment of the process of
the invention, a load formed by a layer with a
thickness between 150mm and 600mm, preferably between
300mm and 500mm, is deposited on the grate.
According to one particular embodiment of the
process of the invention, in the context of the
presence of pellets or granules in the load deposited
on the grate, a layer, referred to as a protection
layer, is deposited on the grate and the load is
deposited on said protection layer.
The protection layer has a dual role, on the one hand,
preventing the passage of the loaded materials through
the grate and, on the other hand, avoiding adhesion of
the materials making up the load in the case of partial
melting of the materials.
According to one embodiment of the process of the
invention, the protection layer has a thickness between
30mm and 100mm, preferably between 40 and 60mm.
According to a preferred embodiment of the
process of the invention, the protection layer
comprises at least one of the following elements: baked
pellets, sized ore that may or may not have been pre
reduced or sized scrap iron, either alone or in
combination with one or more of the above-mentioned
elements.
According to a preferred embodiment of the
process, which is the subject of the present invention,
the protection layer made of constituents with a
particle size between 5mm and 40mm, preferably between
lOmm and l5mm.
WD Ul/90424. CA 02407401 2002-10-23 PCT/BE01./ODU88
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An advantageous procedure for achieving the
protection layer consists in using elements issued from
what is referred to as "the upper particle size range"
resulting from an grinding operation of at least part
of the iron ore. to be reduced. These elements are
"larger" and serve as obstructions in the protection
layer to prevent the elements of the load from falling
through the mobile grate.
According to a preferred embodiment of the
process, which is the subject of the present invention,
a carbonaceous substance, preferably coal or coke dust,
is incorporated into the load deposited on the mobile
grate in a proportion between lkg and 40kg of carbon
present in said carbonaceous substance per ton of iron
ore loaded before reduction.
This variant of the composition of the load
allows to obtain satisfactory conditions for the
reduction of said load of iron ore, i.e. with a
metallisation rate at the unloading higher than 60%,
even when gas fluxes forced through said load having a
low reducing potential, owing for example to the
presence of C02 and/or H20, are used.
The above alternative embodiment of the load is
of particular interest when using oxygen/coal burners
that generate reducing gases containing C02 in
proportions of 2% to 20% by volume of the gas issued
from the burner.
After the loading of the grate, the latter
serves as a means of transport for successively moving
the load deposited within zones in which the conditions
are controlled, both with regard to temperature and
with regard to the composition of the inlet gases
passing through the load.
According to a preferred embodiment of the
process of the invention, the flux of carbonaceous
WO 01/90424- CA 02407401 2002-10-23 PCT/HE01/00088
inlet gases through the load deposited on the grate is
directed downwards, preferably by creating a vacuum
between 500 and 2000mm water column underneath the
grate.
As the grate supporting the load progresses, the
iron ore heats up on contact with the inlet gases, i.e.
those that are drawn through the layer forming the
load, and it is successively reduced into magnetite,
wustite and finally metallic iron.
Given that the aim is to achieve high reduction
kinetics in the layer forming the load, said load needs
to be raised to temperatures between 850°C and 1300°C,
preferably between 1050°C and 1150°C.
According to one embodiment of the process of
the invention, the flux of inlet gases forced through
the load in at least one treatment zone is formed at
least partially by exit gas collected underneath the
grate, said collected gas has preferably undergone at
least one treatment, such as a scrubbing, a
desulphurisation, a drying, a dust-removal, a reheating
or a decarbonation operation.
The above-mentioned treatments of scrubbing/
drying and/or of decarbonation carried out on the exit
gas are intended to restore the reducing potential of
said gas before it is recycled towards a zone for
treating the load on the grate.
In the case where the collected gas is not
reheated before it is recycled towards a treatment zone
in which it is forced through the load, it is
advantageously mixed with gas issued from a coal
gasification stage with a view to cooling the latter to
a useful temperature for passing through the load, i.e.
of the order of 1200°C ~ 150°C or 1000°C ~ 200°C.
The
above-mentioned operation of mixing the gas with gas
issued from a gasification stage allows to reduce or
X1./90424- CA 02407401 2002-10-23 PCT/BEOl~UUU88
_ g _
even omit the addition of the steam that is generally
used as a cooling agent.
According to another preferred embodiment of the
process of the invention, the flux of inlet gases
forced through the load in a zone directly located
after the loading zone is at least partially formed by
fumes generated by the combustion of exit gas collected
underneath the grate, said exit gas collected is
preferably chosen among the exit gases collected with
the lowest reducing potential, e.g. with a low CO
content, said collected gas has advantageously
undergone at least one scrubbing or dust-removal
operation.
This way of proceeding has the economic
advantage of recovering in situ the latent heat of said
exit gases collected as a fuel for the production of
the fumes used to preheat the load.
An additional advantage is the fact that, after being
used as a fuel, it can be unloaded via a flue, since
the toxic elements have been neutralised during the
combustion generating the above-mentioned fumes, the
toxic CO has been transformed into non-toxic C02, for
example.
It is thus possible to preheat the load arranged
on the grate either directly by means of gases emerging
from the layer forming the load and collected
downstream in the displacement direction of the grate
relative to the preheating zone, either by means of the
fumes produced by the combustion of said above-
mentioned exit gases or by means of the two above-
mentioned instances in combination.
According to one embodiment of the process of
the invention, hot reducing gas is produced from coal
in a gasifier and said reducing gas obtained is used to
form, at least in part, the gaseous carbonaceous
X1/90424. CA 02407401 2002-10-23 PCT/BE01/~~~88
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reducing agent, which is forced to pass through the
layer forming the load, which is deposited on the
mobile grate, said gasifier being preferably supplied
either with oxygen-enriched air or tonnage or pure
oxygen.
The main advantage of using a gasifier in order
to generate the reducing gas consists in that it is
possible to produce a reduced iron or DRI of very high
quality since it has a low content both of gangue and
of sulphur. This result is linked with the possibility,
both theoretically and physically, to eliminate
directly at the level of the gasifier the ashes and
sulphur from the coal used in said gasifier, these no
longer being present in the DRI obtained on the mobile
grate.
According to another embodiment of the process
of the invention, a hot reducing gas is produced by
means of one or more "oxygen/coal" burners in at least
one zone for treating the load on the mobile grate,
these preferably being zones in which the temperature
of the inlet gas is higher than 800°C, said burners
using either oxygenated air, pure oxygen or a mixture
of the two, preferably supplied with pulverised coal,
and said obtained reducing gas is used to form, at
least in part, the gaseous carbonaceous reducing agent,
which is forced through the layer forming the load,
which is deposited on the mobile grate.
According to a preferred embodiment of the
process of the invention, steam is used to control the
temperature of the hot reducing gas produced in a
gasifier or by "oxygen/coal" burners during the
formation of the flux of gas forced through the load,
the steam is preferably generated by a steam boiler,
the fuel for which is formed, at least in part, by
gases emerging from the layer forming the load and
CA 02407401 2002-10-23 PCT/BE01/00088
- 10 -
collected underneath the mobile grate, said collected
exit gases preferably emerge from one or more zones in
which the exit gases are too low in CO to be used as
reducing gases, by being directly recycled for example,
and too rich in C02 to be decarbonated at low cost,
this typically being equivalent to 20% S % CO < 40% and
35 <_ % C02 < 55% on dry gas.
Collecting at least part of the gases emerging
from the layer deposited on the mobile grate and re
using it either as a mixture with the gaseous flux
forced through said layer or as a fuel to generate
steam or to generate hot fumes that can be used to
preheat the load, on the one hand, allows to optimise
the use of the reducing power and calorific value of
said gases and, on the other hand, allows to minimise
atmospheric emissions of pollutants such as CO.
According to one embodiment of the process of
the invention, the vacuum created underneath the grate
and/or the displacement speed of the mobile grate
is/are modulated in such a way as to obtain a reduced
ore with a metallisation rate between 60% and 100%,
preferably between 85% and 95%, in the unloading zone.
The reduced ore is commonly called DRI.
According to a preferred embodiment of the
process of the invention, the DRI obtained on the
mobile grate is directly unloaded towards a smelting
furnace .
The previous embodiment allows to optimise the use of
the sensitive heat of the DRI in a significant way
since, when it is unloaded, said DRI is at a
temperature between 800°C and 1200°C. Due to this, the
operation of direct transfer towards a smelting furnace
exerts a favourable influence on the energy balance of
said smelting furnace.
WO 01/90424. CA 02407401 2002-10-23 PCT/BE01/00088
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The following description relates to a preferred
embodiment of the process of the invention, in which
the mobile grate successively passes through 6 zones,
namely a loading zone, four zones for the treatment of
the load with a view to reducing the iron ore, and one
unloading zone, referred to as zones 1, 2 , 3 , 4 , 5 and
6, respectively. In the text, reference will be made by
means of letters and numbers to the sole attached
figure, which is merely enclosed as a non-limitative
illustration in the sense that some elements can exist
within the context of the process that is the subject
of the present invention without, however, being shown
in the f figure or can be shown in the f figure without a
reference in the text. Moreover, the different physical
states of the iron ore to be reduced have been
schematised and identified by the customary chemical
formulae. As it passes along the grate, the ore changes
from Fe203 to Fe204, then to Fe0 and, finally, to Fe.
The functioning of the process is considered in
the case of operations being underway, i.e. after a
start-up phase during which the process is loaded and
made to function until an operating state has been
obtained in which the whole mobile grate is supporting
a layer forming the load, both in the loading zone and
in the unloading zone.
According to a particular embodiment of the
process of the invention, in which a mobile grate (G)
successively passes through a loading zone, four
treatment zones, and one unloading zone, a layer
comprising iron ore is deposited, preferably
continuously and in a constant thickness, on the mobile
grate (G) in the first zone (Z1), referred to as the
loading zone, in order to form the load (C) to be
reduced, the grate (G) is displaced, preferably
continuously, so as to move the load (C) by means of
W~ ~1~90424 CA 02407401 2002-10-23 pCT/BEOl~UUU88
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the movement of the mobile grate (G) from the loading
zone (Z1) to the unloading zone (Z6), successively
passing through the treatment zones (Z2), (Z3), (Z4)
and (Z5), the temperature of the gaseous flux forced to
pass downwards through the load (C) deposited on the
mobile grate (G) in zones (Z2) to (Z5) is regulated to
450°C ~ 150°C in the case of zone (Z2), 500°C ~
150°C in
the case of zone (Z3), 1200°C ~ 150°C in the case of
zone (Z4) and 1000°C ~ 200°C in the case of zone (Z5)
respectively, the gases emerging underneath the grate
(G) from the layer are collected underneath the grate
(G) in each of said zones (Z2) to (Z5), and at least
part of the gases collected from at least one of the
zones (Z2) to (Z5) is recycled towards means that
control the gaseous fluxes forced through the load.
In the above context, the load (C) is deposited on the
mobile grate (G) and, after the loading phase,
undergoes preheating without reduction in one zone, the
rest of the heating is then combined with a reduction
operation in the following 3 zones until an ore with a
metallisation rate of at least 60% is obtained, this
involving the recycling of the gases collected
underneath the grate (G).
According to a preferred embodiment of the
preceding mode, the following operations are carried
out:
- a gaseous flux comprising hot reducing gas formed
from the gasification of coal into CO and H2,
preferably in a pulverised form, in the presence of
oxygen and steam is forced through the load (C)
arranged on the mobile grate (G) in zones (Z4) and
(Z5);
- at least part of the gas emerging underneath the
grate (G) is collected in zone (Z3), it is subjected
to a scrubbing operation, possibly also to a drying
X1./90424 CA 02407401 2002-10-23 pCT/BEOl~U~U88
- 13 -
operation, and part is then directed towards a steam
generator, in which the collected gas is used as
fuel, and another part is directed towards a
combustion chamber supplied with a significant excess
of air and in which fumes are generated at a
temperature of 450°C ~ 150°C, on the one hand, the
fumes are introduced into zone (Z2), and, on the
other hand, are mixed with exit gases collected in
zone (Z2) to obtain a gas at a temperature that is
above the acid dew point, the gas obtained being
dedusted and then evacuated to the atmosphere via a
flue;
- at least part of the gas emerging underneath the
grate in zone (Z4) is collected, it is subjected to a
scrubbing operation and possibly a drying operation,
and it is recycled, after decarbonation, towards
zones ( Z4 ) and ( Z5 ) in order to form at least partly
the gaseous flux forced through the layer forming the
load (C) in these zones (Z4) and (z5);
- at least part of the gas emerging underneath the
grate in zone (Z5) is collected, it is cooled,
possibly by introducing water into it, it is dedusted
and it is used in zone (Z3) to form the gaseous fluid
forced through the layer forming the load in said
zone (Z3) .
The collection and recycling of exit gas in zone
(Z4) has two significant advantages, said gas serves,
on the one hand, to dilute the reducing gas produced by
gasification of the coal and to control the temperature
of the mixture obtained and, on the other hand, it
reduces the consumption of coal by increasing the
quantity of reducing gas available per unit weight of
coal.
InT~ U1~90424 ~ CA 02407401 2002-10-23 pCT/BEOl~UU~88
14
The sole attached figure shows an installation
for implementing the process of the present invention
according to a preferred embodiment.
The installation for implementing the process according
to the present invention, an illustration of which is'
given in the attached figure, comprises at least the
following elements:
- a grate (G), preferably formed by mobile carriages
provided at their bottoms with elements, such as
bars, that promote the passage of a gaseous flux, the
grate being provided with means for displacing it in
the direction of the arrow,
- means (Ch) for depositing a material on the grate
(G) ,
- means for defining an atmosphere (A2), preferably a
hood (H2),
- means for defining an atmosphere (A3), preferably a
hood (H3),
- means for defining an atmosphere (A4), preferably a
hood (H4),
- means for defining an atmosphere (A5), preferably a
hood (H5) ,
- means (DC) for unloading the grate (G) and for
transferring the load (C) towards an outlet (S),
- dedusters (D1, D2) for treating a gaseous fluid and
protecting the fans,
- scrubbers (L1, L2) of gaseous fluids for removing
dust and condensing part of the steam,
- possibly dryers (not shown) for lowering and
controlling the steam content,
- means (E1,...) for decarbonating a gaseous fluid,
- means (P1, P2, P3, P4) for propelling a gaseous
fluid,
U1/90424 CA 02407401 2002-10-23 pCT/BEOl~UUU88
- 15 -
means (R2, R3, R4, R5) located underneath the mobile
grate (G) to collect the gases emerging from the load
(C) underneath said grate (G),
- a combustion chamber (R),
- a steam generator (V),
- means for supplying (H2) gaseous fluid coming from
the combustion chamber (R),
- means for supplying (H3) gaseous fluid collected by
(R5), which is dedusted in (D2) and compressed in
(P4),
- oxy-coal burners (B) mounted in (H4) and (H5),
- means for introducing pulverised coal (CP), oxygen
(02) and steam (VP) from the steam generator (V) into
(H4) and (H5) in order to produce reducing gas from
pulverised coal, oxygen and steam,
- means for supplying (H4) and (H5) with gaseous fluid
that is recycled and collected in (R4), then passes
into the scrubber (L2), possibly also a dryer, then
into the compressor (P3) and finally into the
decarbonator (E1),
- means for guiding the exit gas collected in (R2),
mixing it with fumes emerging from (R) and sending
the resulting gas towards a deduster (D1), then
towards a flue (O) via an extraction means (P1),
- means for guiding the exit gas collected in (R3) so
as to send it towards a scrubber (L1), possibly also
a dryer, a compressor (P2) and then towards the steam
generator (V) or the combustion chamber (R), where it
is used as a fuel.
According to a preferred embodiment of the
installation for implementing the process of the
invention, oxy-coal burners (B) are vertically placed
in the roof of the hood (H4 ) and (H5 ) of zones ( Z4 ) and
(Z5) on a number of lines parallel to one another and
W0 Ul/90424- CA 02407401 2002-10-23 pCT/BEOl~UDU88
- 16 -
parallel to the displacement direction of the grate
(G) .
This arrangement allows the ashes produced by
the combustion of the coal to settle in grooves,
5' between which the ore forming the load remains "clean"
and has better permeability to gases. In this way, a
phenomenon of complete or partial clogging of the layer
forming the load on the grate by the ashes of the burnt
coal is avoided, this phenomenon being highly
damageable to the heating and reduction rate of the
load (C) .
The attached figure also allows to explain a
process for implementing the process of the invention.
To simplify the explanation, the case of an
installation in operation is examined, it focuses on
the path of the exit gases collected underneath the
mobile grate (G).
Zones (Z4) and (Z5) are equipped with hoods (H4) and
(H5), which are provided with oxy-coal burners supplied
with pulverised coal, oxygen and steam so as to
produce, as a mixture with recycled gas, a hot and
highly reducing gas, i.e. at a temperature of 1200°C in
(H4) and 1000°C in (H5), the flux of which is forced
through the load deposited on the grate from the top
downwards.
After passing through the load on the grate, the hot
reducing gas, which is rich in CO and H2, gives rise to
an exit gas containing C02 and H20 derived from the CO
and H2, following the reduction process of the iron
ore.
The exit gas collected in zone (Z4) underneath the
grate is recycled in zones (Z4) and (Z5), after having
undergone the scrubbing, condensation of the steam and
finally decarbonation treatments.
H10 01/90424 CA 02407401 2002-10-23 pCT/HEO1~U~U88
17
The decarbonation is achieved by absorption in a
solvent such as methyl diethanolamine and, to this end,
the exit gas is compressed at a pressure of around 5
bar absolute, then expanded after treatment, e.g. in a
turbine on the same axis as the compressor so as to
minimise the energy consumption for the compression by
recovering the mechanical power generated during
expansion in the turbine.
This gives a scrubbed and decarbonated gas which
is highly reducing and which is reintroduced into zones
(Z4) and (Z5), where it participates, as a mixture with
the gases derived from the gasification of the coal, in
the reduction of the iron ore.
As regards the exit gas collected underneath the
grate in zone (Z5), it is generally at a temperature
between 500°C and 1000°C and, if necessary, it is
worthwhile to cool it, by injecting water for example,
before proceeding to dust-removal in a multicyclone,
and then to recycle it towards zone (Z3), where it acts
as an initiator for the reduction of the iron ore layer
forming the load by continuing the heating of the
latter that has begun in zone (Z2).
The exit gas collected underneath the grate in zone
(Z3) is first of all scrubbed and dried, then used as a
fuel, partly in the boiler for producing steam and
partly in the combustion chamber.
The fumes produced in the combustion chamber (R) are at
a temperature of around 600°C. They are partly directed
towards the hood (H2) to preheat the load deposited on
the grate and partly towards the collecting means (R2),
where they are mixed with relatively cool fumes (~
100°C) emerging underneath the grate so as to ensure
that the resulting gaseous mixture is at a temperature
higher than the acid dew point, e.g. 150°C, and as said
mixture only contains 02, C02, H20 and N2, i.e. in
W~ 01/90424- CA 02407401 2002-10-23 pCT/BEO1~~DU88
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principle no toxic component, it can thus be unloaded
into the atmosphere via a flue without special
treatment, apart perhaps from a dust-removal.
It has been observed that implementation of the
'5 process of the invention is particularly advantageous
when reducing ores sized within a particle size range
from ~ 8mm to ~ 40mm, brought by moderate milling to a
particle size around the 5 - lOmm fraction with a
maximum size of l5mm and the minimum possible fines of
less than 5mm. To recap, the ore ground so as to be
100% < l5mm can advantageously be re-screened to lOmm,
for example, in such a way as to separate the 10 - l5mm
fraction and use it as a constituent of the protection
layer that can be deposited on the mobile grate. The
ore is preferentially iron ore based on partially
hydrated hematite (combined water content of the order
of 2% to 6%). The hematite is more suitable for
processing than magnetite since it is more reducible,
and the effect of the above-mentioned combined water
content, after its loss during heating of the load to
600°C on the grate, is to give rise to a large specific
surface area and hence to promote high reactivity.
However, an excessive combined water content has the
disadvantage of producing excess crackling of the ore
grains, an effect that is deleterious as regards the
permeability of the layer and hence deleterious to the
productivity of the reduction process.
The vacuum applied underneath the grate in order to
assist the passage of the gaseous fluid through the
layer is typically of the order of 500 to 2000mm water
column. The hoods for guiding the gas into the
different zones should operate under a constant
relative pressure that is very slightly negative, of
the order of -2mm water column, in order to avoid any
W~ ~1~90424. CA 02407401 2002-10-23 PCT/BEO1~UUU88
- 19 -
risk that CO or H2 will escape into the working
environment.
Moreover, it is clear that any entry of parasitic air
into the hoods should be avoided so as to avoid burning
the reducing gas present there.
The DRI produced on the mobile grate is unloaded
in zone (Z6) at a temperature of the order of 1000°C.
It will be noted that the implementation of the
process of the invention allows the achievement of very
high productivity, which is between 5t to 20t of DRI
per m2 of mobile grate and per day of operation.
In addition to the above remarks linked to the
economic sector of the process of the invention, the
following advantages will also be noted:
- the ease both of loading and unloading the transport
element, formed in this case by a mobile grate;
- the thermal self-sufficiency, i.e. that the coal used
in the process is sufficient both to generate the
reducing gas required for the functioning of said
reduction process and to supply the heat required
both for the production of steam and the preheating
of the ore;
- the almost total absence or at least the presence in
very small quantities of "spent gas"; the term "spent
gas" is used to refer to a gas that has virtually
exhausted its reducing potential and has a very low
calorific value. Owing to these two elements, this
gas is difficult to make use of and is even
deleterious to the energy yield of the process if it
has to be exported out of the latter, and this
deleterious effect increases with the volumes of
spent gas to be treated;
- the possibility to use, for implementing the process
of the invention, an existing agglomeration or
pelletisation installation, both of them can be used
WO X1/90424 ~ CA 02407401 2002-10-23 PCT/BB01~~~088
- 20 -
for the DRI production process according to the
invention while entailing only relatively low costs
for the adaptation of the installations.
