Note: Descriptions are shown in the official language in which they were submitted.
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The present invention relates to a process for the
production of ferro-lime in a rotary lime kiln, more speci-
fically, the present invention relates to a process for the
production of ferro-lime in a rotary lime kiln by injecting
fine particles of iron containing material directly onto the
surface of the lime shortly after formation thereof, and
while the lime is at a temperature sufficiently high to
react substantially instantly with the iron containing
particles.
It is customary in smelting iron or steel to use
limestone or lime as a fluxing agent for the removal of such
impurities as silicon, phosphorus and sulphur. Both lime
and limestone have disadvantages. Limestone must be calci-
ned in the furnace which not only requires time but also
robs the furnace of heat. Lime, on the other hand, slakes
rapidly in a normal atmosphere, producing a dust problem,
also it is a relatively bulky material which increases
handling expense and charging time to the furnace.
It has been proposed to use ferro-lime as a
fluxing agent in place of the lime and limestone previously
used. Ferro-lime does not require calcining in the furnace
and does not slake as easily as lime and thus overcomes the
major disadvantages of these two prior fluxing agents.
Ferro-lime has a further advantage in that the ferric
content of the ferro-lime contributes to the production of
the furnace and it incorporates into the slag much more
rapidly.
` Ferro-lime has not been used to any appreciable
extent by the steel industry, presumably because no
efficient method of making the ferro-lime has been
available. It has been proposed to form the ferro-lime in a
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rotary kiln by various methods, for e~ample~ U.S. patent
2,159,977 issued May 30, 1939 to Nicholas teaches the
concepts of spraying iron oxide into the kiln in a mixture
with the fuel which is normally injected at the discharge
end of the kiln; of injecting iron oxide at the feed or
discharge end of the kiln by a separate air blast; or the
disclosed preferred concept of feeding the treating agent
(iron oxide) to the lime base as it is fed to the kiln.
None of these methods as broadly taught in this patent are
satisfactory.
The fines from a lime kiln operation are so~etimes
recovered by briquetting the fines to form a lime particle
or agglomerate of a size acceptable to the customer. It is
believed that a similar technique may be used for recovery
of ferro-lime fines, however this technique requires extra
capital expenditure for briquetting and related equipment.
It is an object of the present invention to
provide an improved system for the production of ferro-lime.
It is a further object of the present
invention to provide an improved system for recovery of
fines from a ferro-lime producing p~rocess.
Broadly the present invention relates to producing
a ferro-lime product in a rotary lime kiln by feeding
limestone to the inlet end of the kiln, calcining the
limestone to form a lime, heating the lime to a temperature
of at least 1250C whereby it will instantly react with
iron, injecting iron containing particles into the discharge
end of the kiln and directing said particles onto the lime
at said temperature whereby said iron reacts with said lime
substantially instantaneously to form a ferro-lime.
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The size of the injected particles ma~ be up to
1/4 inch maximum dimension, but generally will be less 1/~
inch the smaller the better to insure substantially instant-
aneous reaction between the iron and lime, but not so small
as to create a problem of entrainment by the kiln gases, and
care must be taken to ensure that substantial quantities of
the injected material are not carried by the gases through
the kiln and thus lost. To minimize problems in the kiln it
is very important that the injected material react substan-
tially instantaneously with the lime and that it be project-
ed down onto the surface of the lime between the discharged
end of the kiln and the flame. Clearly the particle size of
the injected material should not be so large as to signifi-
cantly reduce, by impact, the particle size of the lime
being treated.
If the method of the present invention is to be
used for the production of ferro-lime then the injected
material may be in the form of iron containing materials
such as magnetite, hematite, but preferably mill scale will
be used. The reaction temperature is slightly different
depending on the material injected. The stone fed to the
kiln may be any suitable calcinable stone such as limestone,
magnesite, dolomitic limestone or dolomite which will
produce ferro-lime and ferro-dolime.
In any process for the manufacture of ferro-lime
the resultant product will contain fines. These fines
contain both lime and iron, and may contain a substantial
quantity of the iron reacted with the lime because of the
preferential reaction of the iron with the lime fines. If
these fines are briquetted they will produce a high quality
product containing probably on the average more iron than
the regularly produced ferro li~e but probably will not be
able to command a premium price.
The present invention provides a method of
recovering ferro-lime fines and incorporating them into the
regular lime production in a rotary lime kiln, and at the
same time improving the distribution of the iron in the
ferro-lime product.
The ferro-lime fines may be recovered by injecting
these fines back into the kiln either with the iron contain-
ing material used to form the ferro-lime, or alternatively,
through a separate injector. Since ferro-lime ines do not
have to contact the lime at the reaction temperature between
the lime and the iron, they may be injected somewhat closer
to the discharge end of the kiln than the lime free iron
particles, however, they must be injected onto the ferro-
lime while the ferro-lime still is at a sufficiently high
temperature to sinter with the fine ferro-lime particles
being injected. Obviously care must be taken to insure the
ferro-lime fines are not gas entrained.
The recovery of fines as described hereinabove may
be used either with ferro-lime or ferro-dolime and is not
restricted to the above described process of producing
ferro-lime.
The injected iron containing particles such as
mill scale or lime containing particles must contact a
significant quantity of the lime in order to obtain proper
penetration and distribution. As the iron oxide reacts with
the lime to form ferro-lime, the iron tends to migrate
through and between the lime particles to reach more uniform
composition in the various lime particles and thus it is not
essential that the mill scale or iron containing particles
directly contact each piece of lime. It has ~een found
that if the jet containing iron particles is applied to the
lime for a distance along the length of the kiln at least
twice t:he pi~ch of the helix on which the lime travels
through the kiln, substantially all the lime will be treated
(assumi.ng sufficient iron containing particles are
injected).
Care should be taken to ensure that the iron con-
taining particles are projected down onto the bed of lime so
that the particles do not contact the kiln lining and to
better ensure that the particles are not easily gas
entrained.
If the iron containing particles are fines from
the ferro-lime operation it is not important that these
fines contact all of the lime in the kiln. These fines fuse
to the surface of the lime and thus the area of the jet on
the surface of the lime is not as critical and is basically
determined by the number of lime fines that have to be
reinjected into the kiln.
Further features, objects and advantages will be
evident from the following detailed description of speci~ic
examples of the preferred embodiments of the present inven~
tion taken in conjunction with the accompanying drawing in
which,
Figure 1 is a schematic illustration of a rotary
lime kiln with parts broken away to illustrate the present
invention.
Figure 1 illustrates a rotary lime kiln 10 having
an inlet end 12 and a discharge end 14. Rock to be calcined
is introduced at the inlet end 12 via the in-feed 16 and
feeds along the interior of the kiln 10, as the kiln rotates
in the direction of the arrow 18, along a helical path.
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The kiln 10 is heated by means of a suitable
burner such as the illustrated oil type burner 20 which
forms a flame 22 e~tending along the kiln from adjacent the
discharged end towards the inlet end. The last line of con- ~
tact between ~he flame 22 and the bed 24 indicated by the
line 26.
A lance 28 extends through the discharge end of
the kiln 14 and terminates adjacent the top 30 of the bed 24
of lime travelling through the kiln. Suitable means are
provided for entraining iron containing particles into an
air stream formed by the blower 32 to inject the iron
containing particles onto the surface 30 of the bed 24 in
the area designated by the dot dash lines at 34. It will be
noted that the area 34 is positioned between the front of
the flame at 26 and the exit end 14 of the kiln 10. It will
also be noted that the lance 28 is directed to project the
iron containing particles down onto the surface 30 of the
bed 24 so that there will be substantially no contact of the
particles and the kiln wall and a minimum amount of entrain-
ment of the small particles in the gases traversing thekiln. The length of the area 34 in the axial direction of
the kiln 10 will preferably be at least twice the pitch of
the helical path of the lime to insure proper contact
between the particles and the lime.
It will be apparent that the addition of the iron
containing particles may be stopped substantially instant-
aneously should some disruption occur in the kiln.
EXAMPLE 1
A 15 inch inside diameter kiln 30 feet long capa-
ble of producing 200 pounds of lime an hour with residencetime of up to four hours was used to prove the concept of
lnjecti~g iron containing fines onto the sur~ace of the hot
lime~ A stainless steel lance having a 1/2 inch inside
diametler was directed into the discharge end of the kiln so
that the end thereof was adjacent to the bed of lime between
the flame and the discharge end of the kiln. Iron oxide
fines (mill scale) were injected pneumatically tnrough the
lance onto the lime. To compensate for the air used to
inject the lime fines, the air to the oil burner was reduced
somewhat so that the air transporting the mill scale acted
as a secondary combustion air for the kiln.
Limestone having a particle size in the range of
about 3/8 to 1~ was fed to the kiln and a substantially
constant temperature of 1300C was maintained in the kiln at
the point of injection of the mill scale. The amount of
mill scale injected was varied during the test and the
ferro-lime produced varied in color from a light buff to a
jet black product with an iron content of less than 7
1/2~Fe.
The mill scale was added in two different size
ranges, namely minus 1~5 mesh and minus 20 mesh. The size
of the mill scale addition made no significant changes in
the resultant product.
An examination of the product indicated that
ferro-lime lumps had white cores unless the ferro-lime lumps
were less than about 1/2" in diameter.
In similar tests using dolomite the temperature of
the kiln at the point of addition of the mill scale was
maintained at about 1480C to obtain the instantaneous reac-
tion. Under such conditions similar results were obtained
as indicated hereinabove with respect to treatment of lime-
stone.
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EXAMPLE 2
,
5imilar tests were conducted using ~a) hematite
fines and (b) magnetite fines.
With the hematite fines there was an immediate
reaction but the temperature had to be increased to 1350C
to optimize the reaction~
The magnetite reacted in a similar manner at
essentially the same temperature as the hematite fines
namely 1350C.
In both the above tests the stone used was lime-
stone at a size in the range of 3/8 to 1"~
In order to test the concept of aglomeration of
ferro-lime fines on the ferro-lime product in the kiln the
following experiments were conducted.
EXAMPLE 3
One kilo of Jolette limestone approximately 1/2 to
1 inch mesh size was placed in a ceramic tray and dusted
with 50 grams of pulverized ferro-lime which contained 5%
iron. Two small piles of 5 grams each of loose pulverized
ferro-lime were also placed on the end of the tray. The
tray was then heated in the kiln to a temperature of 1400C.
The ferro-lime had reacted with the lime produced
and had penetrated 1/16 to 1/8 inch into the lumps. The
loose material had thoroughly sintered into a uniform black
lump.
It will be apparent from the above that the ferro-
lime fines can be recovered by injecting them into the kiln
and having them agglomerate with the ferro-lime product as
it is formed.
Having described the invention, modification will
be evident to those skilled in the art without departing
from the spirit of the invention as defined in the appended
Claims.
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