Note: Descriptions are shown in the official language in which they were submitted.
~i~5542
DIR~CT REDUCTION ROTARY KILN
WITH IMPROVED AIR INJ~CTION
Background of the Invention
The present invention relates to a direct reduction
process for the reduction of iron ores in a rotary kiln
provided with air injection pipes along its length and
using countercurrent f low of gas and charge, and more
particularly, to an improved method and means f or in-
troducing the air flow into the kiln
Various methods have been suggested and used for carry-
ing out the direct reduction process using high vola-
tile coal as heating agent and reductant in a rotary
kiln. For example, in some of these processes the coal
is fed into the kiln through the discharge end by mechan-
ical or pneumatic means, such as disclosed in U.S. Pat.
No. 3,505,060 to Heitmann, and in some it is fed at the
center of or along the kiln, such as disclosed in U.S.
Pat. No. 3,206,299 to Senior et al. However, consider-
able disadvantages have arisen in blowing all of the
high volatile coal into the kiln from the discharge
end, and in feeding such coal at the center of the kiln.
Because air is supplied to the kiln at a constant rate,
unless altered by the intervention of the operator, and
the composition of the chamber gas is subject to fluc-
tuation, the reducing and combustion processes are not
uniform, and the control of the process is adversely
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affected. When the reducing agent used has a high con-
tent of volatile matter or moisture, such as is the
case with many low grade coals, the pressure in the
rotary kiln is also subject to general and local fluc-
tuations which further affect control of the process
and which lead to a nonuniform discharge of solids from
the kiln. As the distribution of the coal throughout
the kiln is so highly critical, addition of all the
coal from the discharge end makes the process difficult
to control for simple mechanical and metallurgical rea-
sons.
Further, although benefits are derived in regard to
fixed carbon consumption when feeding all the fuel and
reductant requirements in the form of a high volatile
coal from the discharge end of the kiln, the control of
the operation can be very difficult due to the large
amount of fuel and reductant that has to be fed and the
need to have highly precise distribution of the fuel if
a high degree of reduction is to be achieved. It has
been found in practice that it is not possible to main- -
tain this fuel distribution, and conse~uently variable
reduction results. It has also been found that the in-
corporation of high volatile coal into the kiln bed at
the discharge end of the kiln results in impaired reduc-
tion capability in the kiln due to variations in the
CO/C02 ratio in the bed and in the chamber gas, and
that this situation tends to limit the degree of metal-
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lization to a level below that required for commercial
practicality.
On the other hand, feeding of a high volatile coal from
the feed end of a countercurrent flow system leads to a
loss of volatile material in the first section or pre-
heat zone of the kiln. These volatiles are removed by
the combustion gas flow and are thus lost to the process
and increase the heat value of the kiln off gas, and
only a portion of the gases from the low temperature
distillation of the coal can be used for the process.
The increased heat value of the off gas can further
cause operating difficulties in the off-gas exhaust or
processing system.
The disadvantages of these various approaches have been
overcome by feeding a portion of the coal from the dis-
charge end of the kiln sufficient to control the tem-
perature profile throughout the kiln and feeding the
remaining portions of the coal at the feed end while
ensuring that the coal from the discharge end is dis-
tributed in the kiln in such a manner that substan-
tially no coal lands in the reducing zone within the
last 15~ of the kiln length and is distributed to with-
in the feed end zone of the kiln. The rotary kiln is
fitted along its length with air injection devices
which blow air countercurrent to the general flow of
reducing gases within the kiln to produce mixture there-
between. A system of this type is disclosed in ~.S.
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Pat. No. 3,890,138 to Hockin, particularly for use in
reducing ilmenite. However, while this latter tech-
nique improves upon the other coal feeding methods in
the direct reduction process, when it is used for re-
ducing iron ore to sponge iron, certain problems still
remain in the content of the exhaust or off gases re-
quiring special attention in the off-gas processing or
cleaning system.
It has generally been the practice in the direct reduc-
tion art to direct the air supply within the kiln along
its length toward the feed end in keeping with the ear-
ly teachings of Moklebust, for example, in U.S. Pat. No~
2,829,042 and subsequently in U.S. Pat. No. 3,170,786,
as well as the teaching in the previously cited U.S.
Pat. No. 3,206,299 to Senior et al. However, Meyer et
al in U.S. Pat. No. 3,235,375 teaches the directing of
the air flow preferably toward the discharge end or in
either or both directions to achieve improved heat dis-
tribution and more effective combustion of carbon mon-
oxide gas while obviating localized overheating of the
charge mixture and formation of wall accretions.
The practice in the process of Hockin, U.S. Pat. No.
3,890,138, when reducing iron ore to sponge iron has
been to direct the air toward the discharge end of the
kiln countercurrent to the reducing gas flow to enhance
mixing. Such is the arrangement shcwn in the prior art
diagram of Fig. 1 which illustrates the components of a
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direct reduction plant for producing sponge iron in ac-
cordance with the HOCKIN process. Although the HOCKIN
process overcomes many of the problems af the prior
art, still as noted above, difficulties have been en-
countered in handling the off-gases. The present in-
vention involves certain i~provements which have been
discovered in the operation of the illustrated plant
for the HOCKIN process.
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Summary of the Invention
In accordance with the present invention, it has been
found that in a direct reduction process of the HOCXIN
type when the introduction of air into the kiln is di-
rected oppositely in the preheat zone from the reducing
zone, that is, toward the feed end rather than the dis-
charge end, substantially complete reaction of the air
with the combustible components in the preheat zone is
achieved, and thus the composition of the off-~as is
considerably improved, and improved preheating of the
kiln charge can be effected without the formation of
undesirable kiln accretions. More particularly, rather
than blowing all of the air countercurrent to the com-
bustion gases as in the past, with the present inven-
tion, while the air injection tubes in the reducing or
working zone are directed to blow the air toward the
discharge end of the kiln, one or more of the air injec-
tion tubes in the preheat zone are oriented to blow the
air toward the feed end. The improved condition of the
waste or off-gas resulting ~rom this improved air blow-
ing arrangement obviates the need for using an after-
burner in the off-gas system, as has been used in some
of the other direct reduction kilns wherein all of the
air has been directed toward the feed end of the kiln.
The approach of the present invention differs from that
of Meyer et al in previously-noted U.S. Pat. No.
3,235,3Z5 which, while suggesting the opposite directing
11~55~2
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of the introduced air flow, does not limit the opposite
directing of the air flow to and within either of the
different zones of the kiln. Unlike Meyer et al, no
supplementary fuel is used in the process of the present
invention, and by virtue of reversing the tubes only in
the feed end or preheating zone, relatively large quan-
tities of air are introduced to combust volatiles with-
out affecting the desired temperature profile of the
charge material in the entire length of the kiln and
without causing sticking of gangue material on the pel-
lets and/or sticking of particles of the charge on the
wall of the kiln, such as may occur in the Meyer et al
process.
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Brief Description of the Drawings
Fig. 1 is a diagrammatic view of a prior art direct re-
duction plant for the production of sponge iron, which
plant is of the type on which the present invention
improves.
Fig. 2 is a diagrammatic view of a reduction kiln of
the type used in the plant of Fig. 1, but which incor-
porates the present invention.
5~
g
Detailed Description of the Preferred Embodiment
A direct reduction plant for the production of sponge
iron of the type utilizing a rotary kiln operated in
accordance with the HOCKIN process is shown in Fig. 1.
The plant comprises an array of feed bins respectively
including: a bin 1 for supplying ore in the form of
iron oxide pellets; a bin 2 for providing limestone or
dolomite; a bin 3 for supplying recycled char; and a
bin 4 for providing a carbonaceous reducing agent in
the form of coal of less than 1 inch nominal diameter
particles. The iron ore pellets, coal r return char and
dolomite or limestone are accurately proportioned and
fed continuously as a charge to the feed end 6a of the
reduction kiln 6. A remaining bin 5 supplies coal of
less than 3/8 inch nominal diameter particles to the
feed end 6b of the rotary kiln 6, where carefully con-
trolled quantities are blown in together with carrier
air from a suitable source 7 through a coal injection
pipe 8 which can be adjusted to achieve the optimum
trajectory for this coal.
The reduction kiln 6 may be typically 11.5 feet (3.5
meters) in outside shell diameter and 148 feet (45
meters) long, sloped at 3%. It may be supported on two
tires driven by a 200 horsepower variable speed D.C.
motor and lined with 8 inches of castable refractory.
In addition to the introduction of carrier air through
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pipe 8, the kiln shell is equipped with a series of air
injection tubes 9 which are spaced along its length and
extend into the interior of the kiln for drawing air
from the outside and injecting it along the kiln axis.
Each of the tubes 9 is equipped with its own fan and
motor combination 10 so that the rate of injection may
be properly regulated along the kiln. ~n existing
plants of this type the air injecting ends 9a of the
tubes 9 are all directed toward the discharge end 6b
of the reduction kiln to produce air flow in that di-
rection for better mixing of the air with the counter-
current combustion and exhaust gases.
The hot waste or off-gases exhaust from Lhe feed end 6b
of the kiln and pass into an off-gas processing or
cleaning system. In a suitable cleaning system the
gases are passed first to twin refractory-lined scalp-
ing cyclones 11 and then to a 57 feet high by 11.5 feet
inside diameter spray cooling tower 12 where they are
cooled to 500 F before passing to an 8-cell bag house
13 equipped with glass fiber bags. The cleaned gases
exit via an induced draft fan and a 100 feet high
stack 14.
The material discharged from the discharge end 6b of
reduction kiln 6 consists of a mixture of sponge iron,
coal char, coal ash and desulfurizing agent. This ma-
terial is cooled in a rotary cooler 15 fitted with
lifters and cooled externally with water. The cooled
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mixture is then passed from the cooler 15 to a screening
system 16 and screened. The two oversized fractions are
subject to magnetic separation in respective magnetic
separators 17 and 18. Separator 17 removes the sponge
iron from waste material and delivers the sponge iron
to the product loadout area. The nonmagnetic fraction
is conveyed to the return char hin 3.
While a plant such as shown and described in connection
with ~ig. 1 has, with proper control of combustion con-
ditions over the kiln bed and reduction conditions in
the bed, yielded high rates of heat transfer and opti-
mum utilization of kiln volume with metallization of
iron consistently in a 9~%-95% range, still when high
volatile coal is used at the feed and discharge ends,
problems may arise with regard to the condition of the
off-gases.
As the high-volatile coal introduced at the feed end is
heated in the preheating zone of the kiln from ambient
to the temperature of operation in the reduction zone,
volatile hydrocarbons are distilled as gaseous mixtures
from the coal. These hydrocarbon gases are carried by
the process combustion gases out of the feed end of the
kiln and into the gas handling and cleaning equipment.
In this equipment their concentration by volume may be
sufficient to form combustible mixtures if air be acci-
dentally admitted therein creating an explosion risk.
In the absence of air they may condense inside the dust
~1~55~2
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cleaning equipment, e.g., on the baghouse bags, causing
impaired performance thereof, and/or they may condense
to a relatively stable aerosol suspended in the ambient
air adjacent to and mixed with the total gas flow from
the stack exit causing the opacity of the stack gas
plume to exceed the opacity permitted by environmental
regulations. In some prior art direct reduction sys-
tems it has become necessary to install afterburning
equipment in the off-gas system to deal with problems
of this type. However, it has been discovered that
these problems can be solved without the need for
afterburning equipment by modifying the direction of
air injection in accordance with the present invention.
More particularly, the improved modification of the
kiln 6 to conform to the present invention is shown in
greater detail in Fig. 2. The rotary kiln 6 fitted
with air injection devices 9, ten in number, is oper-
ated in accordance with the HOCKIN process, that is,
using a high volatile, non-caking coal as the reductant
and fuel and characterized in that part of the coal is
added from the discharge end 6b of the kiln in such
manner that substantially no coal is incorporated in
the kiln bed within at least the last 15%, and prefer-
ably the last 20%, of the kiln length and so that some
of the coal added from the discharge end of the kiln is
distributed to within the feed-end region or zone of
the kiln, and in that the remainder of the coal is
11~5~2
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added at the feed end 6a of the kiln. The amount of
coal fed into the kiln from the discharge end 6b to
maintain a satisfactory and controllable temperature
profile is preferably within the range of 15%-30% by
weight of the total coal feed. The rate of feed and
the particle size of the coal are suitably controlled
to make sure that the desired operating conditions are
obtained. As indicated in Fig. 2, operationally the
kiln is divided into two zones, that is, a preheat zone
toward the feed end of the kiln, which extends for ap-
proximately the first one-third, but perhaps as far as
the first one-half, of the kiln length, and a working
or reduction zone which extends through the remainder
of the kiln to the discharge end. In the preheat zone
the ore, limestone or dolomite, coal, and recycled char
are preheated gradually to the reduction temperature
of aLproximately 1800 to 1950 F. The volatiles from
the coal and the carbon monoxide formed by reduction in
the kiln bed are combusted progressively by air admitted
to the kiln through the spaced air tubes 9 mounted in
the wall of the kiln. The temperature profile within
the kiln is dependent on a number of factors and will
differ with the type of coal used, its fixed carbon
content, the volatile matter and its charring tempera-
ture and its ash softening temperature. The kiln tem-
peratures are measured with twelve thermocouples 19
along the kiln 6 which are designed to separately mea-
l~S5~2
-14-
sure the temperature of the charge in the kiln and the
gas temperature.
With the exit nozzles 9a of the air injection tubes 9
all directed to introduce air flow countercurrent to
the reducing gas flow in the kiln, that is, directed
toward the discharge end 6b of the kiln as in Fig. 1,
the composition of the off-gas from the feed end of the
kiln may be such as to cause problems in the off-gas
cleaning system as indicated above. It has been dis-
covered, however, that by reversing the orientation of
the nozzles 9a of one or more of the air injection
tubes in the preheat zone of the kiln, preferably the
three tubes 9', as shown in Fig. 2, the off-gas compo-
sition can be sufficiently improved to obviate any mod-
ifications to the off-gas cleaning system, such as the
use of an afterburner, while the optimum temperature
profile within the kiln can be maintained.
More particularly, it should be appreciated that with
the exit nozzles 9a of the three air injection tubes 9'
all directed in accordance with the prior practice to
introduce air flow countercurrent to the combustion gas
flow in the kiln, i.e., directed toward the discharge
end 6b of the kiln as in Fig. 1, the air volume flow in
the three tubes is required to be limited to the range
from ten to thirty percent of the total air volume flow
through all ten tubes of the kiln. This is necessary
to prevent excessive heating of the kiln internal sur-
~5S~2
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faces, by the otherwise complete oxidation and combus-
tion of the volatile hydrocarbon gases to carbon dioxide
and water vapor, which would cause sticking of the
charge particles to each other and to the kiln internal
surfaces, and to prevent overheating and consequent
damage to the metal equipment feeding the charge ma-
terials into the kiln. It also prevents loss of con-
trol of the optimum temperature profile of the charge
axially along the kiln in the preheating zone and t.~e
first one-third of the reduction zone.
However, with the exit nozzles 9a of the three air in-
jection tubes 9' all directed to introduce air flow
concurrent to the combustion gas flow in the kiln, i.e.,
directed toward the feed end 6a of the kiln in accord-
ance with the present invention as shown in Fig. 2, the
air volume flow in the three tubes 9' may advantageously
be increased to the range from 65~ to 85% of the total
air volume flow through all ten air tubes of the kiln,
thereby partially or substantially oxidizing and com-
busting the volatile hydrocarbon gases to carbon diox-
ide and ~ater vapor in such a manner that the tempera-
ture of the total gas and air stream at their exit (6a)
from the kiln does not cause excessive heating of the
kiln internal surfaces in the preheating zone. In ad-
dition, it does permit maintaining the optimum tempera-
ture profile of the charge axially along the kiln in
the preheating zone and the first one-third of the
~1~5542
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reduction zone. Substantially complete oxidation and
combustion of the volatile hydrocarbon gases by the air
from the three tubes 9' is effected witllin the pre-
heating zone of the kiln and/or outside the feed end 6a
of the ~iln so that the aforementioned hydrocarban
gases do not cause the problems and difficulties in the
gas cleaning system described above.
It will therefore be seen that by reversing the direc-
tion of injection of the air from one or more of the
air supply tubes in the preheat zone of a kiln operat-
ing in accordance with the described process, the con-
dition of the off-gases can be considerably improved
without affecting the process in the reduction zone and
ultimately the quality of the resulting product.
