Note: Descriptions are shown in the official language in which they were submitted.
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BACKGROUND OF THE INVENTION
This invention relates to the production of tricalcium
silicate useful in the manufacture of Portland and other
cements and in particular to the manufacture of such materials
in electrie arc furnaces.
Portland cement is commonly prepared from certain
indefinite compounds of lime, silica, alumina, iron oxide,
magnesium oxide and small quantities of other ozides. The
raw materials useful in the preparation of Portland cement
inelude cement rock, limestone, marl, clay and shale, blast-
furnace slag, and gypsum sand. One eommon method for
preparing Portland eement eomprises erushing or grinding the
raw materials into a powder whieh is then proeessed in dry
form or as a slurry. The mixture of raw materials is placed
in a rotary kiln for caleinization and burning into elinker.
Temperatures of about 28006F are typieally used at the hottest
heating zone near the diseharge end of the kiln. Following
heating, the clinker is air-cooled, pulverized and blended
with various chemicals added to modify performance or setting
time and then bagged or otherwise stored for use. This
method is not wholly satisfactory beeause it requires a
large amount of energy and eontrolling the eomposition of
the final product is relatively difficult.
OBJECTS OF THE INVENTION
It is a primary objeet of the present invention to provide
a new and improved method of making Portland eement elinker.
Another object of the present invention is to provide a
method of manufacturing Portland cement clinker in an electric
arc furnace.
Yet another object of the invention is to provide a method
of manufaeturing Portland eement elinker in an eleetrie arc
,.
furnace wherein electrical parameters are controlled to
prevent contamination of the melt with carbon from the
electrodes.
A still further object of the present invention is to
provide a method of manufacturing Portland cement clinker
in an electric arc furnace wherein raw materials added to
the melt are in lump form.
Another object of the present invention is to provide
a method of manufacturing Portland cement clinker in an
electric arc furnace wherein limestone lumps are calcined
to lime before addition to the melt.
How these and other objects and advantages of the
present invention are accomplished wil] be described in the
following specification taken in conjunction with the drawing.
In general, the invéntion comprises a process which
includes the steps of charging a pre-reduced slag into an
electric arc furnace having at least two carbon electrode
and energizing the electrodes to maintain a temperature in
the furnace of about 3200F to maintain the conductivity of
the melt at about 175 mho/meter. Further, raw materials
are added during the process to insure the desired composi-
tion for the final metl. Preferably, the process is con-
tinuous with the molten product being tapped for cooling and
the feed rate of material to the furnace being at a rate
which does not lower the temperature below about 3200F or
extinguish the arc.
BRIEF DESCRIPTION OF THE DRAWING
The single figure of the drawing schematically illus-
trates an electric arc furnace in which the method of the
invention may be practiced.
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DESCRIPTION OF THE PREFERRED EMBODIMENTS
The single figure of the drawing schematically illus-
trates an electric arc furnace 10 in which cement clinker
may be produced according to the method of the present
invention. In general terms, electric furnace 10 includes
a metallic shell 11 and a refractory lining 12. A plurality
of electrodes 14, which are essentially carbon may be of
the prebaked or self-baking type, extend through suitable
openings 15 in the arched roof 16 of furnace 10. While two
electrodes 14 are shown, any suitable number may be employed.
The electrodes 14 may be energlzed from any conventional
source, symbolized by the single phase~ alternating current
transformer 17 which is coupled by a pair of conductors 18
to the electrode clamps 19 of each electrode. Those
skilled in the art will appreciate that each clamp 19 will
include conductive members for engaging the electrode
surface whereby electric current may be transferred
readily therebetween.
Each electrode 14 is supported for vertical movement
relative to the furnace 10 in any suitable manner such as by
means of a schematically illustrated positioning mechanism 20
which includes a control 21, a positioning motor 22 and a
cable assembly 23. As will be discussed more fully below,
the control mechanism 21 is operative to sense electrode
current and voltage and to provide control signals to
motor 22 which, in turn, adjusts cable assembly 23 so that
its associated electrode 14 is adjusted vertically. In
this manner, the electrodes 14 are positioned at the
desired distance from molten bath 24 within the furnace 10.
while only a single control 21, motor 22 and cable mechanism
23 is shown to be connected to one of the electrodes 14, it
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will be appreciated that there will be an identical control
system for the other electrode 14 as well.
The cable mechanism 23 includes a cable 26 coupled at
one end to its associated electrode 14 and extending upwardly
therefrom and over sheaves 28. The opposite end of cable 26
is connected to a drum 29 which may be reversibly driven by
motor 22.
The control 21 is generally conventional and is coupled
to conductor 18 for receiving a first signal functionally
related to electrode voltage and a second signal functionally
related to electrode current. Assuming the conductivity of
the bath 24 remains the same, those skilled in the art will
appreciate that the electrode current will be inversely
related to the distance between the electrode 14 and bath 24
while electrode voltage will be directly related. More
specifically, control 21 includes an isolation transformer 30
whose primary is connected to conductor 18 through resistor
32 so that a signal appears in the secondary of transformer
30 which is functionally related to the potential between
electrode 14 and ground. This signal is rectified and
provided to control 21 through conductors 33. Also coupled
to conductor 18 is a current transformer 34 which generates
a current signal frunctionally related to electrode current.
This signal is rectified and applied across resistor 36
so that a voltage signal functionally related to electrode
current is applied to control 21 through conductors 38.
Control 21 is operative to compare the voltage signals
delivered through conductors 33 and 38 and to provide an
output signal to motor 22 when the relationship between
the input signals deviates from a preselected value. The
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output signal from control 21 will vary in magnitude and
sense depending upon the manner and degree that the input
signal relation deviates from the preselected value.
Should the gap between electrode 14 and melt 24 increase
from a desired amount, electrode voltage will increase and
electrode current will decrease and there will be corres-
ponding changes in the signals at conductors 33 and 38.
When these changes are sensed by control 21, the latter will
then provide an error signal to motor 22 which becomes opera-
tive to lower the electrode 14 toward the bath 24 until thedesired electrical conditions are again achieved. Conversely,
should the electrode move closer to the bath than the pre-
selected desired position, the electrode current will increase
and electrode voltage will decrease. These parameter changes
will be sensed by control 21 which will signal motor 22 to
raise the electrode 14 until it is elevated to the desired
position.
Another parameter affecting electrode voltage and
current is the conductivity of the melt. Normally, the
conductivity of tricalcium silicate, the principle con-
stituent of the melt, will be about 175 mho/meter at
3200F. However, it will be appreciated that in a continuous
process, as molten material is withdrawn through tap 39,
additional material 40 to be melted will be provided to
the furnace from hoppers 41. However, the conductivity of
the material varies from a relatively low value at its
melting point to the value indicated above at the operating
temperature of 3200F. It will also be appreciated that the
temperature of the melt is sensitive to the feed rate and
particularly in the vicinity of the electrode tips. Should
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the material be fed too fast, therefore, so that there is
a decrease in conductivity, the electrode current will
decrease even though the gap between the electrode and the
bath may be relatively short. This condition would be
sensed by the control 21 which then attempts to lower the
electrode 14 further so that the graphite electrode might
tend to become immersed into the bath 30. The tricalcium
silicate bath, however, is very reactive with carbon elec-
trodes at temperatures of 2900F-3200F which is the normal
operating range of the furnace. This reaction of lime and
carbon forms calcium carbide in the melt. The formation of
even very small amounts of this carbide renders the sub-
sequent clinker useless as a cement making material because
of deleterious effects it has upon the crystalline proper-
ties of solidified material. For this reason, the chargematerial 40 which is fed into furnace 10 from hopper 41
must be fed at a controlled rate through valve 42 so as not
to lower the temperature of the melt below about 3200F. For
this purpose a control 43 is connected to receive electrode
voltage and current signals. These signals are employed to
determine the power delivered to the furnace 10 and to
provide an output control signal functionally related thereto.
The output signal is used to control the valve 42 which
may take the form of a vibratory or screw feeder. This
control is based upon the finding that the power consumed by
the furnace is related to the material feed rate. If the
material is fed too fast, the power drops off sharply and
conversely increases from a preselected value if material
is fed too slowly. Thus by measuring power delivered to
the furnace, the material feed rate can be controlled to
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insure that the power is maintained within preselected limits
and further to insure that the electrode tips do not make
contact with the melt.
As the charge material 40 falls into the furnace, it
will be heated within the melt by the electric currents in
the furnace and will be influenced by the thermal convection
current generally flowing away from the electrodes toward
the furnace walls. The location of the feed chutes may be
used to provide layering of the charge on the top of the
bath near the electrodes so that it is placed where it can
advantageously use the superheat in the top layer of the
melt for melting as it moves from the arc contact zone
toward the furnace walls. The ideally sized furnace bath
crucible is one in which the superheat is completely trans-
ferred to the feed on the surface by the time the convectioncurrent has transported the melt to the refractory lining.
When this condition is maintained, a portion of the charge
45 solidifies on the inner surface of the refractory lining
12 thereby providing a protective layer or self-lining.
Because the melt 24 is highly corrosive when in contact
with most known refractory materials, the self-lining 45
helps to prolong substantially the life of the furnace
refractory 12.
In accordance with the method of one embodiment of the
invention, a mixture of clay, which is principally silicon
dioxide, and the waste product produced when phosphate rock
is treated with sulfuric acid for the extraction of phos-
phorous are stored in hopper 41. This waste product consists
principally of gypsum (caS0-2~ O). The mixture is fed into
the furnace 10 at a rate controlled by valve 42. The
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electrodes are energized to provide a bath temperature of
about 2900F-3200F. At this temperature, the following
chemical reactions occur:
3CaSO 2H O+Si0 CaO-SiO + 3SO + H20
4 2 2 2 3
This reaction is rapid and virtually complete by the time the
mixture of sulfate and silicous materials is melted. After
the materials charged from hopper 41 into furnace 10 melt,
the molten material 24 may be tapped through tap hole 39.
Accordingly, this process can be continuous and in this par-
ticular embodiment permits the recovery of SO by means of
a smoke hood 46 which is connected at one end to an opening
47 in the roof 16 of the vessel 10 and whose other end is
connected to a storage facility 48. Means may also be
provided to produce a slight suction at the lower end of
hood 46 to facilitate the collection of SO gas. After
recovery in facility 48, the SO gas may subsequently be
used for the production of sulfuric acid which may then be
used in the phosphate rock treating process.
In this example of the method according to the invention,
a rectangular furnace 1' by 2' in plan v;ew and 1' deep
was employed. Two 3 inch diameter electrodes were connected
to a 150kVA power source with the furnace power requirements
being 5OkW. The furnace bath temperature was maintained at
about 3200F and the voltage between each electrode and
ground was maintained at about 45 volts. By-product gypsum
and clay were fed into the furnace in the following propor-
tions at the rate of 50 pounds per hour:
CaO 73%
S O 24%
1 2
Fe O 3%
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A satisfactory portland cement composition shown in Table 1
was produced with the electrodes being maintained a slight
distance above bath level.
Table 1
Melt 2 Melt 3
SiO 26.9 26.6
A123 7.1 10.3
Fe O 3.2 3.4
2 3
CaO 59.1 57.2
MgO 4.3 2.4
SO 0.23 0.21
L.O.T. 0.11 0.01
The importance of maintaining the electrodes in a spaced
relationship with respect to the melt may be illustrated by
reference to a series of tests conducted by the inventors.
Initially, limestone was utilized as the calcium containing
material and furnace slag was employed as the silica con-
taining material.
A first series of tests was conducted. In the first test
of this series~ a limestone to slag ratio of 56:44 was
employed and in the remaining two tests of this series a
limestone to slag ratio of l.S:l was used. In each of the
tests, the raw materials were employed in lump or nonpul- t
verulent form. In prior cement manufacturing processes, it
was considered impermissible to use particles larger than
finely ground particles.
The tests w~re initiated by adding a portion of the slag
material to the furnace and stricking an arc to form a molten
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pool. The electrodes were then immersed in the pool and
limestone and additional slag added by gravity at intervals~
The added materials were melted into the existing melt by
resistance heating. The limestone disintegrated and rapidly
went into solution.
Electrode consumption and energy requirements were
measured. The results indicated that a tricalcium silicate
clinker could be produced using the process, but energy
requirements were extremely high and electrode consump-
tion was unacceptable. Tests conducted of the clinker todetermine if it had usefully hydraulic cement properties
indicated an inordinate amount of magnesia (mgo).
A second series of tests was then conducted, the primary
differences being that burnt lime was employed instead of
limestone and a carbon lined crucible was employed. The
burnt lime was also employed in a lump or nonpulverulent
form. The burnt lime was prepared by the calcinization of
limestone to convert carbonate to calcium oxide or lime.
These tests led to several important discoveries. First, all
clinker produced in this second series of tests disintegrated
into the gamma form upon cooling, even when various cooling
techniques were attempted to control decrepitation of the
clinker. Second, carbinde formation was noted by the evolu-
tion of a gas having an odor like acetylene. Finally, the
melt had reacted with the carbon lining at the temperatures
employed to produce the melt.
A third series of tests was then conducted. The slag
material used in this series contained approximately 44.61%
calcium oxide and 45.71% silicon dioxide with the remainder
being relatively smaller amounts of the oxides of iron,
aluminum, magnesium, potassium, sodium, manganese, phosphorus~
and approximately 2.1% iron. The burnt lime used in the
final series of tests contained 94.38% clacium oxide with
minor amounts of silicon dioxide, iron oxide, and magnesium
oxide. The slag was received in large chunks and was crushed
into lumps of less than one inch in size.
The furnace itself was the same one employed in the
second series of tests except the lining was removed and a
new rammed Magnesite lining was installed. Sufficient lining
was installed to assure that the furnace feed material would
form its own refractory crucible under the power load
anticipated. The third series of tests was also carried out
in a manner which would prevent any significant contact of
the electrodes with the melt.
Various batch mixtures of the crushed slag and lump
burnt lime feed materials were prepared to produce various
tricalcium silicate and dicalcium silicate ratios for the
final clinker. An initial feed was added to the crucible
to form a melt and portions of the batch mixtures were added
to the melt at approximately 45 minute intervals. The
furnace operation was stable at 100 volts on the trans-
former secondary which produced 100 volts at the electrodes
and a power input of 75 kw. The electrode position under
these conditions was just slightly above the melt and, therefore,
produced a short arc from the electrode to the melt. The
bath was heated both by radiation from the arc and as a
result of current f~ow through the bath. A constant power
input of 1 kw/lb. was maintained and the tap temperatures
were maintained in the vicinity of 3040 to 3200F., a
temperaure sufficient to insure chemical combination of the
slag and lime into the desired hydraulic cement.
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On conclusion of the third series of tests, it was noted
that the furnace refxactory did not experience erosion as
has been seen in previous tests. Electrode consumption was
determined to be substantially reduced and carbide forma-
tion was effectively controlled by maintaining sufficientvoltage between the electrode tip and the bath to prevent
contact of the carbon with the melt. Testing of the
various clinker materials produced in the third series of
tests confirmed that they had the desired hydraulic proper-
ties for use as a hydraulic cement.
The charge constituents may be selected from a varietyof calcium and silica bearing materials such as lime, lime-
yielding materials, aragonite, blast and phosphorus furnace
slags, etc. and the teachings of the present invention may
be adapted for these and other materials by one skilled in
the art after reading the present specification.
While several embodiments of the invention have been
illustrated and described, the invention is not to be limited
thereby it is to be limited only by the scope of the
appendant claims.
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