A LOW ENERGY CONSUMING IMPROVED PROCESS FOR THE
PRODUCTION OF DIRECT REDUCED SPONGE IRON IN A ROTARY KILN
AND AN APPARATUS FOR MAKING THE SAME
FIELD OF INVENTION
The present invention relates to a process for the production of direct reduced
iron (DRI) from iron ore and non-coking coal. This invention also relates to a
system (apparatus) for making direct reduced iron using non-coking coal. This
invention further relates to a process for the production of char and further
gasification of the same in making direct reduced iron.
The invention particularly relates to the production of direct reduced iron or
sponge iron in a rotary kiln using 100% non-coking coal as a fuel and reductant.
In this invention, sponge iron is produced using much less energy than the
conventional process.
PRIOR ART
In an earlier invention disclosed by the applicant in the Indian Patent No. 170432
developed by Krishna Kant Prasad et al where an energy efficient process was
claimed. The present invention is an improvement over the same where further
reduction of energy is effected in a cost effective manner so that the process can
be made viable at a much smaller scale than is hitherto before possible.
The system (apparatus) provided according to this invention incorporates all the
features of the said Indian Patent No. 170432 except the under-bed oil injection
system in the early part of the cooler which has been proved over the years to be
non-cost effective, i.e. the benefits due to carbon increase do not justify the
expense on account of oil usage. Thus, as per the present invention, the sponge
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iron produced is not of high carbon content compared with the sponge iron
produced as per the said prior Indian Patent No. 170432.
Prior art knowledge on the direct reduction of iron ore is available from the
published report entitled "Reduction Process outside the Blast Furnace" by
Battelle Institute, Frankfurt and in the book entitled "Direct Reduced Iron-
Technology and Economics of Production and Use" (published by Iron and Steel
Society of AIME). Out of the many known processes, the non-coking coal based
rotary kiln processes find greatest promise in countries like India. India faces
severe shortage of non-coking coal for operating the blast furnaces. However,
the process of making sponge iron in rotary kiln using non-coking coal suffers
from a major disadvantage of high energy consumption (requiring about 5.5 to
6.00 Goal/ton of sponge iron). The major reason for such high energy
consumption is that the waste gases take away 35 to 45 percent of the total heat
with it.
SUMMARY OF THE INVENTION
The object of the invention is to find a suitable process by which sponge iron can
be produced with much lesser energy as compared with the prior art. Another
object of the present invention is to produce char by using sensible heat of the
product and use of volatile matters so liberated within the reduction kiln. Still
another object of this invention is to devise an improved system (apparatus) by
which the said process for producing sponge iron can be accomplished with
substantial recovery of heat. A still further object of the invention is to avoid use
of any oil and also avoiding corresponding under-bed oil injection system in the
overall apparatus for the production of direct reduced sponge iron known in the
prior art.
The said objects are fulfilled in the present invention by the following way:
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1) by providing a waste heat recovery system in the overall apparatus which
heats iron ore, char and flux by off gases while simultaneously effecting
partial reduction of iron oxide or ore and comminution of coarse char
thereby making it more reactiveand its gasification.
2) by providing an air pre-heater, which heats the process air, and
3) by incorporating a system in the overall apparatus, of using sensible heat
in the product mix for carbonizing additional coal for making char in the
early part of cooler. The volatile mater so liberated is fed back into the
reduction kiln to supplement its energy requirement.
In the present invention, waste gases coming out of a sponge iron rotary kiln are
passed through another rotary kiln (preheating kiln) kept upstream with respect to
the flow of solids. Iron ore, re-circulated char and limestone get pre-heated to
600°C to 900°C in a preheating kiln with drying and partial reduction of iron ore
and drying and partial calcination of flux (limestone or dolomite). The waste
gases escape out of the preheating kiln at a temperature of 300°C to 500°C.
These gases are used to heat the process air. It has also been found that heating
the air up to 250°C by the waste gas coming out of the preheating kiln and using
the hot air instead of cold air in the kiln results in an energy recovery of 0.10 to
0.15 Goal per tonne of sponge iron. Most of the hot air (to the extent of 60 to
90%) thus obtained is supplied to the preheating kiln; the rest is supplied to the
reduction kiln. Heating of solids in the preheating kiln by the waste gases effects
recovery of energy by 0.4 to 0.5 Goal/tonne of sponge iron.
In the preheating kiln, iron ore lump acts as a medium of grinding or comminuting
the re-circulated char, which exposes fresh surfaces of the char. The carbon in
char gets gasified within the solid bed to carbon monoxide, which helps in
reducing partially the iron oxide in Fe2O3 form to FeO or wustite stage. This
partial reduction is endothermic and thereby further amount of heat is taken up
from the waste gases, which amounts to 0.15 to 0.25 Gcal/tonne sponge iron.
Thus, the combined heat recovery according to the present invention is 0.55 to
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0.75 Gcal/tonne of sponge iron, which is much higher than the corresponding
heat recovery of only 0.4 Gcal/tonne of sponge iron available according to the
applicant's prior Patent No. 170432.
Further, since the iron ore, char and fluxes are already pre-heated and partially
reduced the need for burning of additional coal for heating of charge and partial
reduction is obviated. Consequently, much less coal is needed in the reduction
kiln and waste gas volume is also consequently much lower.
After the reduction is over, the product mix is allowed to fall into the cooler at 850
to 1000°C. Then additional non-coking coal is sprayed on the product mix in the
transfer chute so that the sensible heat in the product mix is recovered to
carbonize the fresh coal and the volatile matter so liberated is led to the reduction
kiln to meet some of the energy requirement of the process. The char thus
prepared in the early part of the cooler is recovered downstream and ploughed
back into the process in charging in preheating and reduction kilns.
Thus, this invention provides for an improved process for the production of direct
reduced sponge iron in a rotary kiln using non-coking coal, of which process
comprises charging predetermined amounts (proportions) of iron ore, return char
and limestone or dolomite in a preheating kiln, preheating the mixture to a
temperature of 600 to 900°C using sensible heat of waste gases, further heating
the charge in a reduction kiln to a temperature of 850 to 1050°C using hot waste
gases of said reduction kiln, allowing the gases coming out of said preheating
kiln to pass through a heat exchanger for heating the process air up to a
temperature as herein described and using sensible heat of the product mix,
flowing out of the said reduction kiln, to carbonize fresh coal in an early part of a
cooler connected to the said reduction kiln so that the evolved volatile matter
flows back into the said reduction kiln to supplement its energy requirement and
finally separating the direct reduced sponge iron and char from the product
mixture by magnetic separation technique after screening.
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According to this invention the said preheating kiln is in the shape of a segment
or a frustum of a cone and is fitted on the inside at the highest end with guide
plates to drive the material forward with rotation of the preheating kiln so that a
high degree of fill of 25% to 50% is maintained in the said preheating kiln.
This invention further provides for an improved process wherein the iron ore (or
any other iron oxide feed) in the said preheating kiln is allowed to act as a
grinding medium or comminuting agent for coarse char to help it in gasifying into
carbon monoxide and thereby effecting partial reduction of ferric oxide into FeO
or wustite stage in the said preheating kiln itself. The temperature of the process
air is allowed to be increased to 200 to 300°C in the said heat exchanger and
thereby lowering the temperature of the waste gases between 150°C to 200°C.
In this process 60 to 90% of hot process air is allowed to be passed through the
preheating kiln and the remaining air is supplied to the reduction kiln.
Further, a portion of the coal is sprayed onto the hot product mix coming out of
the said reduction kiln, and using the sensible heat of product mix to produce
char and the evolved volatile matters are led back into the reduction kiln to
supplement its heat energy requirement.
This invention further provides for an apparatus for making direct reduced
sponge iron using non-coking coal comprising essentially a preheating kiln, a
reduction kiln, an air pre-heater, a heat exchanger and a cooler, said preheating
kiln being located upstream of said reduction kiln and is interconnected so that
hot waste gases leaving the said reduction kiln travel through the preheating kiln
characterized by conical or tapering shape in order to maintain the gas velocity
throughout the kiln constant with optimum utilization of kiln volume and the said
cooler, being connected to said reduction kiln, is provided with refractory lining in
the early part thereof which acts as carbonizing zone for fresh coal, the
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remaining part of the said cooler is not refractory lined and is provided with lifter
blades as a means for faster cooling.
In the apparatus, the said air pre-heater is shell-and-tube type. In the apparatus
nozzles are fitted for water spraying outside the cooler.
The present invention is now described hereinafter in detail with reference to the
accompanying drawing wherein:
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWING
Figure 1 diagrammatically represents the main units of the system and the
process.
DETAILED DESCRIPTION
Iron ore of predetermined size, char and flux are led through a feed pipe (10) into
a preheating kiln (12) after proper proportioning through weigh feeders (not
shown). Since gases in preheating kiln will undergo steep temperature drop, the
free board area can be continuously reduced to maintain same linear velocity.
Thus, the preheating kiln may have the shape of a part (frustum) of a cone and
would thus be economizing on the size and weight of the said preheating kiln.
The preheating kiln would further have guide plates fitted at the charging end of
suitable design to drive the charge material forward with rotation of the
preheating kiln so that a high degree of fill of 25 to 50% is maintained in the
preheating kiln, which is congenial for rapid heat transfer. The solid charge
comprising iron ore (iron oxide), flux (limestone or dolomite) and re-circulated
char encounters hot waste gases in the preheating kiln and gets preheated
before being discharged through a transfer chute (16) into a reduction kiln (21).
Gases generated from coal reduce the iron ore of the charge. A part of the coal is
charged through a feed pipe (20) in the first transfer chute (16). Coal is also
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charged in the reduction kiln through an inlet (23) using hot air as carrier. A third
part of coal is dropped by a chute (24) on the hot product mix in the second
transfer chute (25). In the early part (28) of the cooler (30) hot product mix
carbonizes the fresh coal and converts it into solid char and volatile gaseous
matter. The evolved volatile matter is allowed to flow back into the reduction kiln
(21) to supplement its energy requirement. The hot material is then cooled in the
remaining portion of the cooler (30) which is not refractory lined and is cooled
from outside by water spray (22). This portion of the cooler is provided with "lifter"
blades of predetermined design (not shown), which increases the efficiency of
cooling. The cooled product is treated in a conventional manner by screening
and magnetic separation to obtain sponge iron.
The entire non-magnetic portion, which contains mostly char with fairly high
carbon content, is sent for recycling. The coarser portion is fed into the
preheating kiln (12) while the finer fraction is fed to the reduction kiln (21). The
exhaust gases from the inlet hood (33) of the preheating kiln (12) are taken to
shell-and-tube type pre-heater (35) where it exchanges heat with counter current
flow of air from air compressor or blower (38). Waste gases pass through tubes
placed vertically while air flows through the shell. The cooled waste gases are let
off through a dry electrostatic precipitator (40), an induced draught (ID), fan (42)
and a chimney (50). The pre-heated air from air pre-heater (35) is fed into the
preheating kiln (12) through a pipe (14). A smaller quantity of preheated air is
also sent to reduction kiln through another pipe (55).
In the process as described herein above, four major improvements have been
noticed over the existing conventional rotary kiln based direct reduction
processes:
(a) In a conventional coal based rotary kiln DR process for making sponge
iron, the energy content in waste gases is 2 to 2.5 Gcal/tonne of sponge
iron. If this gas is used for preheating of iron ore, char and flux, only 0.4 to
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0.5 Gcal/tonne or 20% energy can be recovered. If partial reduction of
Fe2O3 to FeO stage is also effected in the preheating kiln a further amount
of 0.15 to 0.25 Gcal or 8 to 10% energy can be recovered. And if
preheated and partially reduced charge (iron ore etc.) reaches the
reduction kiln, the generation of gas is reduced since additional coal need
not be used for preheating and partially reducing iron ore. In the present
invention, which consists of preheating and reduction kilns, the energy
content of the waste is reduced to 0.3 to 0.4 Gcal indicating a recovery of
80 to 85%. This is largely due to reduction in gas volume. Waste gas
volume would be reduced to 5000 to 7000 Nm3/tonne of sponge iron to
2000 to 2500 Nm3/tonne of sponge iron.
If coal containing 25% ash and 10% moisture is used, the consumption
expected in a conventional commercial unit is 1100 to 1200 kg of
coal/tonne of sponge iron. If waste heat is recovered in a preheating kiln,
and char is re-circulated, the coal consumption should go down to 550 to
600kg/tonne of sponge iron. The energy consumption reduces from 5.5 to
6 Gcal/tonne of sponge iron in the conventional case to about 2.7 to 3
Gcal/tonne when waste heat recovery system of the present invention is
incorporated.
Since coal is not added in the preheating kiln, the volume of the
preheating part of the reactor can be much less. Consequently, per
volume unit of the reactor, the productivity would be higher. In
conventional cases, the productivity is about 0.5 tonne/( m3)/(day) i.e. an
increase of over 20%.
(b) In the system of the present invention, there are two stationary points
(represented by transfer chutes) where air can be conveniently injected
after preheating it. It may be possible to preheat almost the entire
quantity of air required except the air supplied through the shell
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air fans. In this process, the waste gases which have a temperature in
the order of 300 to 500°C, are used to preheat air in an air pre-heater of
the type usually used in the boilers of thermal power plants. Even if the
air is preheated on an average to 200°C, the waste gas temperature
would then come down to 150 to 200°C, i.e. an energy recovery of 7
to 10% is envisaged. Coal requirement would be further reduced by
about 50 kg since waste gas volume would be further reduced. This
would lead to a consumption of 500 to 550kg/tonne of sponge iron. In the
conventional processes, no system exists for heating the process air.
(c) For a conventional coal based rotary kiln sponge iron plant, it becomes
necessary in the long run to install a power generating plant or any
alternative unit to realize the full benefit of the energy invested in
operation. In the present invention, the energies in the waste heat and
solid wastes are used within the process and the need for large
investments in additional units is obviated.
(d) In a coal based process emission of carbon dioxide to the atmosphere
can't be eliminated. But the present invention restricts the carbon dioxide
emission to 0.95 to 1.05 tonnes per sponge iron as compared to1.5 to 1.8
tonnes per tonne sponge iron in the conventional process.
In the revised scheme the following extra units are required:
(i) a preheating kiln with all accessories;
(ii) an air pre-heater; and
(iii) an additional transfer chute
Parts of the cost to be borne for the above are compensated by:
a) lesser capacity of electrostatic precipitator;
b) lesser capacity of stack and I D fan
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c) lesser number of shell air fans (0 to 4) instead of conventional (6 to 12);
d) lesser capacity of coal handling system;
e) lesser capacity of drive system; and
f) no need for "after burning and cooling" chamber
Techno-economic evaluation of the new process indicates that while the cost of
production by the conventional process is about Rs. 9,000.00 / tonne of sponge
iron, the cost of production by this process comes down to about Rs. 8,000.00 /
tonne of sponge iron thus giving a net benefit of Rs. 1000/tonne of sponge iron
considering the prices prevailing in the year 2007 AD.
From the above disclosure, the object of the invention stated herein has been
completely fulfilled. Having now fully described this invention, it will be
understood by those of ordinary skill in the art that he same can be performed
within a wide and equivalent range of conditions and other parameters without
affecting the scope of the invention or any embodiment thereof which is governed
by the claims followed hereinafter.
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WE CLAIM:
1) An improved process for the production of direct reduced sponge iron in a
rotary kiln using non-coking coal, which process comprises charging
predetermined amounts (proportions) of iron ore, return char and limestone or
dolomite in a preheating kiln, preheating the mixture to a temperature of 600
to 900°C using sensible heat of waste gases, further heating the charge in a
reduction kiln to a temperature of 850 to 1050°C using hot waste gases of
said reduction kiln to pass through a heat exchanger for heating the process
air up to a temperature as herein described and using sensible heat of the
product mix, flowing out of the said reduction kiln, to carbonize fresh coal in
an early part of a cooler connected to the said reduction kiln so that the
evolved volatile matter flows back into the said reduction kiln to supplement
its energy requirement and finally separating the direct reduced sponge iron
and char from the product mixture by magnetic separation technique after
screening.
2) An improved process as claimed in claim 1, wherein the said preheating
kiln is in the shape of a segment or a frustum of a cone and is fitted on the
inside at the highest end with guide plates to drive the material forward with
rotation of the preheating kiln so that a high degree of fill of 25% to 50% is
maintained in the said preheating kiln.

(3) An improved process as claimed in claim 1, wherein the iron ore (or any
other iron oxide feed) in the said preheating kiln is allowed to act as a
grinding medium or comminuting agent for coarse char to help it in gasifying
into carbon monoxide and thereby effecting partial reduction of ferric oxide
into FeO or wustite stage in the said preheating kiln itself.
(4) An improved process as claimed in claim 1, wherein the temperature of
the process air is allowed to be increased to 200 to 300°C in the said heat
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exchanger and thereby lowering the temperature of the waste gases between
150°C to 200°C.
(5) An improved process as claimed in claim 1, wherein 60 to 90% of hot
process air is allowed to be passed through the preheating kiln and the
remaining air is supplied to the reduction kiln.
(6) An improved process as claimed in claim 1, wherein a portion of the coal
is sprayed onto the hot product mix coming out of the said reduction kiln, and
using the sensible heat of product mix to produce char and the evolved
volatile matters are led back into the reduction kiln to supplement its heat
energy requirement.
(7) An apparatus for making direct reduced sponge iron using non-coking
coal comprising essentially a preheating kiln, a reduction kiln, an air pre-
heater, a heat exchanger and a cooler, said preheating kiln being located
upstream of said reduction kiln and is interconnected so that hot waste
gasses leaving the said reduction kiln travel through the preheating kiln
characterized by conical or tapering shape in order to maintain the gas
velocity throughout the kiln constant with optimum utilization of kiln volume
and the said cooler, being connected to said reduction kiln, is provided with
refractory lining in the early part thereof which acts as carbonizing zone for
fresh coal, the remaining part of the said cooler is not refractory lined and is
provided with lifter blades as a means for faster cooling.
(8) An apparatus as claimed in claim 7, wherein the said air pre-heater is
shell-and-tube type.
(9) An apparatus as claimed in claim 7, wherein nozzles are fitted for water
spraying outside the cooler.
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(10) An improved process and apparatus for making sponge iron or direct
reduced iron substantially as herein before described.
Dated this 30th day of January, 2008


To,
The Controller of Patents,
The Patent Office, Kolkata.

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The present invention relates to An improved process for the production of direct reduced sponge iron in a rotary kiln using non-coking coal, which process comprises charging predetermined amounts (proportions) of iron ore,
return char and limestone or dolomite in a preheating kiln, preheating the mixture to a temperature of 600 to 900°C using sensible heat of waste gases, further heating the charge in a reduction kiln to a temperature of 850 to
1050°C using hot waste gases of said reduction kiln to pass through a heat exchanger for heating the process air up to a temperature as herein described and using sensible heat of the product mix, flowing out of the said
reduction kiln, to carbonize fresh coal in an early part of a cooler connected to the said reduction kiln so that the evolved volatile matter flows back into the said reduction kiln to supplement its energy requirement and finally
separating the direct reduced sponge iron and char from the product mixture by magnetic separation technique after screening An apparatus for making direct reduced sponge iron using non-coking coal.