Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
CA 02497734 2001-03-19
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PROCESS AND APPARATUS FOR AUTOMATICALLY
CONTROLLING SLAG FOAMING
This is a divisional application of Canadian Patent Application Serial No.
2,402,860 filed on March 19, 2001.
Field of the Invention
The present invention relates to a method and to an apparatus for controlling
slag
in an arc furnace. The present invention has particular applicability to
controlling the
nature and quality of the slag during the process for the preparation of metal
alloys to
1o improve reliability, consistency as well as the ultimate properties of the
manufactured
alloy. It should be understood that the expression "the invention" and the
like
encompasses the subject matter of both the parent and the divisional
applications.
Background Art
~ 5 Methods for processing steel in an electric arc furnace involve melting
mostly scrap
metal as the charge to form a liquid bath of the metal. Conventionally, the
scrap is added to
the furnace at the beginning of the process cycle and melted down by the
passage of electric
current through the furnace's graphite electrodes which causes an electrical
arc between the
electrodes and the metal. A layer of molten, non-metallic material known as
slag is present
20 over the upper surface of the molten steel formed from a mixture of the
impurities
separated from the metal during the refining operation. The slag is formed on
the
CA 02497734 2001-03-19
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upper surface of the charge during the initial melt-down and remains on the
upper
surface during the entire process cycle until tapping of the heat.
In conventional steel making, the slag is foamed by the adjustment of
furnace parameters and the addition of materials, e.g. liquid or gaseous
chemicals,
which are added during the refining process. The resulting layer of foamed
slag
over the upper surface of the molten steel serves the useful purposes of
regulating
the percentage content of alloy additives in the steel, isolating the molten
steel
from ambient oxygen and certain devices in the vessel containing the molten
steel, and thermally insulating the steel to facilitate maintaining the steel
in a
molten state until poured into a desired mold.
Conventionally, the slag is foamed by a furnace operator upon visual
observations of the slag itself or upon some other process parameters. For
example, U.S. Patent 5,827,474 to Usher et al. discloses an apparatus and
method
for measuring the depth of slag and molten metal in an electric arc furnace.
The
method includes moving a voltmeter probe through the molten metal, the
interface
between the molten metal and the slag foam. The change in the voltage signal
between the strata, marks the position of the boundary between the molten
metal
and the slag foam.
Initially, visual inspection of the slag is sufficient to determine the
quality
of the foam. Over the process time, however, the dynamic process of steel
making requires the constant adjustment of variables such as the addition of
carbon or oxygen to affect the foam. When furnace operators are adjusting
parameters, such as the injecting of carbon, their reaction time is relatively
slow
in relation to the dynamics of the foaming and alloying. To overcome, in part,
some of the deficiencies of human error in foaming the slag, several models
have
been developed for the steelmaking process. For example, WIPO Patent
Application No. WO 99/23264 to Welker discloses a method and a device for
controlling the formation of foamed slag in an arc furnace employing a model
to
predict foaming. The reference discloses the use of a neural net and a foamed
slag model to predict the correct amount of carbon to add to the furnace in
order
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to ensure that at least partial envelopment of the arc in the slag foam is
achieved.
German Laid Open Patent Application (Offenlegungsschrift) DE 198 O1 295 to
Sesselmann discloses an arc furnace in which a neural net is used to control
the
position of the arc electrode, the supply of electrical energy and the amount
of
carbon added to the furnace. The height of the slag foam is measured using
sonar.
Additional controls have been made to the furnace in the steel making
process. For example, U.S. Patent 5,714,113 to Gitman et al. discloses an
apparatus for electric steel making, which comprises a burner/injector for
introducing oxygen into the steel melt.
Sensors have also been added to the furnace; as disclosed in U.S. Patent
5,050,185 to Bourge et al. which discloses a process and apparatus for
measuring
the instability of an arc in an electrical arc furnace for processing liquid
metal.
This patent discloses that arc stability may be measured by taking the
derivative
of the signal corresponding to the arc cun-ent, splitting the derivative
signal into
high and low frequency components, amplifying these separate components, and
producing therefrom a signal relating to the quotient of the high and low
frequency components of the derivative signal. This value represents the
stability
of the arc and can be used by operators to estimate the amount of carbon and
oxygen to be injected into the furnace.
A method for on-line monitoring and control of the performance of an
electric arc furnace can be found in U.S. Patent 5,099,438 to Gulden, Jr. et
al.
The method of Gulden, Jr. et al. includes collecting signals relating to
various
furnace parameters including electrical, mechanical and physical parameters of
the furnace. Such parameters include pulse rates from watt/var, current and
potential transducers, transformer tap positions, arc length settings,
hydraulic
variables, positions of mechanical furnace components and scrap charge
weights.
These data are processed using programmable logic controllers (PLCs) and
microcomputers.
U.S. Patent 4,742,528 to Stenzel discloses a compensating device for use
in a vacuum arc furnace. The compensating device compensates for the weight of
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a power line in a vacuum arc furnace, where accurate weights of the arc
electrode
are to be determined. The method uses a computer to control the intensity of
current supplied to sensor electrodes.
Russian Patent RU 2086657 to Dolgonosova, et al. teaches a device for
reduction of metal oxides comprising a loading device for feeding metals into
a
blast furnace. The device measures gas pressure fluctuations, and uses these
signals via a computer to control the feed of oxygen gas into the furnace.
The aforementioned references, however, do not adequately correlate or
control the various dynamic aspects of the arc furnace or the dynamic
properties
of the molten steel or slag to improve the nature or quality of the foamed
slag in
the manufacture of steel. Accordingly, a continuing need exists for
dynamically
improving the quality and nature of the foamed slag during the steel making
process thereby improving consistency, reliability and throughput for the
finished
product.
~isclosur~ of the Invention
An advantage of the present invention is a method of dynamically and more
precisely controlling the foaming of slag in an arc furnace.
Additional advantages and other features of the invention will be set forth in
the description which follows and in part will be apparent to those having
ordinary
skill in the art upon examination of the following or may be teamed from the
practice of the present invention. Advantages of the present invention may be
realized and particularly pointed out in the appended claims.
According to the present invention, the foregoing and other advantages are
achieved in part by a method of dynamically controlling the foaming of slag in
an
arc furnace in the formation of an alloy, such as steel. The method comprises
obtaining one or more signals) related to variables representing the nature or
quality of the slag; feeding the one or more signals) into a controller
comprising
a logic program to generate an output signal representative of the quality or
nature
of the slag; and adding one or more materials to the electric arc furnace
through a
CA 02497734 2001-03-19
valve that has a flow rate that can be variably adjusted to dynamically
control the
quality or nature of the slag by continuously adjusting the addition of the
one or
more material in response to the output signal generated by the controller.
The
method of the present invention thereby dynamically and more precisely
controls
S the quality and nature of the slag, its foaming and the ultimate finished
product.
Embodiments of the present invention include employing signals relating
to arc stability, slag foam viscosity and temperature as indicators of slag
foam
quality and continuously controlling the amount of materials to be injected
into
the furnace to affect the nature and quality of the slag, as by a variable
flow rate
valve. Further embodiments of the invention employ oxygen, carbon, magnesium
oxides, calcium oxides and lime as inputs for controlling the nature or
quality of
slag foam. The invention also contemplates automatic and manual controls as
means for controlling the introduction of inputs to control of the slag foam.
A further aspect of the present invention is a method of manufacturing a
1 S metal alloy by fomung a molten metal liquid having a slag thereover in an
electric
arc furnace and dynamically adjusting the foaming of the slag during the
process by
injecting material, e.g. carbon, through a valve capable of continuously and
variable
adjusting the flow rate, e.g. from about 25 pound/minute (lbs/min) to about
200
lbs/min, of the injected material.
Another aspect of the present invention is an apparatus comprising an
electric arc furnace having a variable valve in fluid communication with the
furnace and a material source that can be controlled to variably adjust the
amount
of material added to the furnace during a metal processing. Embodiments
include
an apparatus comprising: an electric arc furnace capable of forming a molten
metal covered, in part, by a slag; a sensor in electrical communication with
the
furnace and capable of sensing the nature or quality of the slag; a material
handling and controlling equipment in fluid communication with the furnace for
housing and adding material to the furnace to foam the slag and including a
variable valve for varying the amount of material added to the furnace; and a
programmable logic controller electrically coupled to the sensor and the
material
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handling and controlling equipment for receiving and processing signals from
the
sensor as to the nature or quality of the slag and for regulating the amount
of
material added to the furnace by the material handling and controlling
equipment
through the variable valve.
Additional advantages and other features of the present invention will
become readily apparent to those skilled in this art from the following the
detailed
description, wherein only the preferred embodiment of the present invention is
shown and described, simply by way of illustration of the best mode
contemplated
for carrying out the present invention. As will be realized, the present
invention is
capable of other and different embodiments, and its several details are
capable of
modifications in various obvious respects, all without departing from the
present
invention. Accordingly, the drawings and description should be regarded as
illustrative in nature, not as restrictive.
grief Description of Drawines
Fig. 1 schematically illustrates a dynamic slag control system in
accordance with an embodiment of the present invention.
pescrjption of the Invention
The present invention advantageously enables the dynamic and precise
control of the quality or nature of the foamed slag during the manufacture of
metal alloys such as steel. The present invention automatically maintains the
optimum foaming levels of the slag over the molten metal by continually
monitoring one or more foaming parameters and continually and variably
adjusting process input parameters. The present invention contemplates
monitoring furnace parameters, such as, but not limited to, the stability of
the
furnace arc, furnace temperature, etc. as well as monitoring the slag
parameters,
such as slag viscosity. Based on the ever-changing status of the molten metal
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bath, a logic controller determines the type, time and amount of materials
that are
injected or otherwise made available to the molten metal or slag thereby
eliminating human error. In an embodiment of the present invention, the amount
of introduced material is regulated on a continuos and variable basis, rather
than
by the conventional on/off blast injectors, to provide a foamy, non-corrosive
slag.
The present invention enables the use of longer electrode arcs and, by burying
those arcs in the foamed slag, reduces or eliminates splatter of the charge
while
minimizing electrode oxidation and consumption and maintaining high
temperatures and reduces the waste associated the addition of too much
material.
Embodiments of the electrical furnace system of the present invention is
illustrated in FIG. 1 and comprises furnace 10 and a control system. The
furnace
contains at least one electrode therein and houses the molten metal and,
during
processing, a layer of slag over the upper surface of the molten metal (not
shown
for illustrative convenience). In an embodiment of the present invention, a
plurality of electrodes are used in the furnace.
In practicing the invention, an electrical arc is generated between the
electrodes and the metal causing the metal to liquefy. The slag which forms
substantially over of the liquid metal bath undergoes, during the refining of
the
metal, a foaming caused by the chemical reactions in the liquid metal and in
the
slag, which are facilitated or the result of injecting materials, such as
oxygen and
carbon, through at least one blast pipe 12 into the liquid metal or into the
slag or
both through port 14. The blast pipe provides a fluid communication between
furnace 10 and material handling and containing equipment 16 which holds one
or
more materials to be injected into the furnace.
The injection of oxygen, for example, causes the formation of iron oxide
which passes into the slag, where as the injection of carbon, such as from
charcoal, causes the formation of a carbon oxide, such as carbon monoxide or
dioxide, which causes the slag to foam. The arc between the electrodes and
metal
charge, thus, can be to a greater or lesser extent enveloped in the layer of
slag
during the refining process.
CA 02497734 2001-03-19
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Additional blast pipes can also be added in fluid communication with the
furnace for injecting multiple materials simultaneously or individually. The
present invention contemplates several blast pipes in fluid communication with
the furnace and corresponding material handling and containing equipment. As
S inputs for controlling the quality of slag foam, the materials that can be
injected
during the process include, for example, oxygen, carbon, iron oxides,
magnesium
oxides, calcium oxides, lime, etc.
Additional materials that can be added to affect the quality and nature of
the slag of the present invention comprise compositions useful in the
steelmaking
process, such as from about 15 to about 80 wt% of a carbon source with the
remainder comprising a source of an oxide of calcium. The source of an oxide
of
calcium that can be used includes, for example, standard lime (95 weight
percent
(wt%) Ca0), dolomitic lime (approximately 60 wt% CaO, 40 wt% Mg0) and
mixtures thereof, as well as other calcium oxide sources known in the
steelmaking
art. Either lime can be utilized in calcined or hydrated form. The carbon that
can
be used in the present invention includes any type of carbon and particularly
from
those carbon sources known in the steelmaking art, as for example, charcoal,
wood, petroleum coke, graphite, anthracite coal and mixtures thereof.
In an embodiment of the present invention, material handling and
containing equipment 16 includes variable valve 18 to regulate, in a variable
manner, the amount of material added to the furnace. The variable valve can be
placed anywhere between the furnace and the material to be added to the
furnace.
The present invention contemplates that the volume of material that is
continuously controlled through the valve will depend on the nature of the
material and the size of the charge.
In an embodiment of the present invention, the variable valve is a rotary
valve that is capable of adding the desired material to the furnace during the
process at a continuous and variable flow rate, rather than a conventional
discrete
on/off rate. In an embodiment of the present invention the valve can be
controlled
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to continuously adjust the flow rate of the inject material, e.g. carbon, over
the range
of up to 200 lbs/min or more, e.g. from about 2S lbs/min to about 200 lbs/min.
The present invention automatically maintains optimum slag foam levels
by continually monitoring furnace parameters such as, but not limited to, the
S stability of the furnace arc, the temperature and the slag viscosity. To
monitor the
development of the foaming of the slag and thus to permit interventions in the
course of the process, the invention provides an electrical system including
one or
more sensors 20 connected, for example, to one or more probes 21 or to one or
more electrodes through electrical furnace buss tube 19. The system can
include a
detector 22, e.g. a foaming indicator, in electrical communication 21 with
sensor
20.
Sensor 20 is capable of generating a detectable signal representative of the
nature or quality of the slag as, for example, generating a detectable signal
representative of the intensity of the current passing through the arc. In an
I S embodiment of the present invention, sensor 20 is of the inductive type
as, for
example, a Rogowski torus, that is to say an electromagnetic ammeter which is
capable of measuring the derivative of the current, and its harmonics. The use
of
a sensor of this type exhibits, in particular, the advantage of increasing the
accuracy of the measurement of the fluctuations of the current. The input from
the Rogowski coil to the detector can be from about 0 to about 10 volts.
The processing of the signal, for carrying out the process, is achieved by
one or more conventional programmable logic controllers (PLC's) and,
optionally,
with one or more conventional data processing microprocessors or logic
circuits,
shown as single block 24. A PLC enables continuous on-line monitoring of
2S various furnace conditions and also serves as an input processor for higher
level
computers which can, if desired, perform heavy-duty data processing for higher-
level on-line control. PLC's are generally configured to withstand the rigors
of
the furnace area environment. The PLC's function is to perform high speed data
collection of electrical, mechanical, and physical parameters of the furnace,
such
as, but not limited to, pulse rates from watt/var, current and potential
transducers,
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transformer tap positions, arc length settings, hydraulic variables, positions
of
mechanical furnace components, and scrap charge weights and present the
collected data to a data processing microprocessor or logic circuit in a
usable
format.
5 In an embodiment of the present invention, PLC 24 processes all data
received through an I/O Interface (not shown for illustrative convenience)
during
a time cycle. The data received and processed by PLC (C) includes, but is not
limited to, pulse strings representing the furnace's electrical energy
consumption,
analog and digital signals representing furnace status and/or condition, and
timing
10 values representing the duration or absence of an event or delay. The
processing
of all received data is accomplished by monitoring the incoming data over a
fixed
time cycle and converting the data into data sets or values representing
appropriate engineering units.
In addition to data collection and processing, the system can further
I S process the data, as by conventional computers having logic circuits or
microprocessors to receive the data transmitted by the PLC. It is contemplated
that the PLC can contain the logic circuits necessary to process the data in a
cost
effective manner and provide the operator with control of the information
corresponding to the above identified variables by way of reports, displays
and/or
commands. In an embodiment of the present invention, PLC 24 is electrically
coupled to a user interface 26. The input from the user interface to the PLC
can
be any range of electrical signal, such an 110 volt AC input.
In accordance with the present invention, PLC 24 is electrically coupled to
material handling and containing equipment 16. In an embodiment of the present
invention, the output from the PLC to equipment 16 is approximately 4-20mV for
flow control and about 0-10 V for on/off control. In practicing the invention,
PLC
24 obtains one or more signals related to variables representing the nature or
quality of the slag from sensor 20 and processes or feeds the obtained one or
more
signals) into a controller comprising a logic program to dynamically control
the
quality or nature of the slag by continuously sending an output signal to
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equipment 16 to regulate, in a variable manner, the amount of material to be
added or reduced to the furnace.
The present invention contemplates integrating at least one sensor, at least
one PLC and at least one material handling equipment to provide on-line data
collection and data communication between the programmable logic controller
with or without an extra data processing computer for controlling the furnace
together with controlling the time and amount of a particular additive
material to
the furnace to affect the foaming of the slag.
The present invention advantageously automatically maintains optimum
slag foam levels by continually monitoring furnace parameters such as, but not
limited to, the stability of the furnace arc, the temperature and the slag
viscosity.
Based on the ever-changing status of the molten steel bath, a logic controller
determines what types of materials should be injected into the furnace, when
they
are to be injected and in what amount. The controller activates a valve
capable of
I S adding the materials over a specified range rather than conventional
abrupt on/off
blast pipes thereby reducing or eliminating the waste and improving the
accuracy
and control over the process. The specified range wherein the valve is
activatable
depends upon the nature of the added material and the size of the charge.
The present invention advantageously eliminates the human error
associated with determining when the slag is foamed well. Also, in almost all
instances, adjustment of the injection of carbon and oxygen by the operator
are
the only means for controlling the slag foam. This invention will also be able
to
adjust the slag chemistry by the injection of materials such as, but not
limited to
MgO, lime and FeO.
A prototype of an arc furnace system was installed in a steel mill where
the furnace included a standard 6,000 lbs (refractory material) dry gun, a
programmable logic controller (P.L.C.), user interface switches, a foaming
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indicator and Rogowski coil. There were two settings for the user interface;
manual and automatic. In the manual setting, the injection of carbon is
controlled
by the operator. In the automatic setting, the carbon injection is controlled
by the
PLC. The system successfully controlled the slag foaming while in the
automatic
S setting.
After experimentation, it was discovered that the standard dry gun did not
have a large enough capacity as with nearly every heat, the gun had to be
refilled.
Accordingly, in an embodiment of the present invention a larger, e.g. greater
than
6,000 lbs, dry gun is used. It was further observed that if the system is
placed into
the automatic setting while there is still scrap in the furnace, the system
will
identify the scrap as poor arc stability due to a lack of foamed slag. The
system
will then inject excessive amounts of carbon in an attempt to foam the slag
which
results in a waste of carbon and excess carbon contamination within the steel
itself. To circumvent the misreading of the slag, an alarm system has been
written
into the P.L.C. program that will alert the operator when the system is in the
automatic settings and the level of foaming is not approaching optimum levels.
It has also been observed that the flow rate of carbon through a butterfly or
a knife-gate valve is either zero pounds per minute or the maximum volume that
the valve will allow (i.e., approximately 200 lbs/min). In an embodiment of
the
present invention, the flow rate of carbon is precisely controlled over a
range of
to 200 lbs/min by a rotary valve with differential speed control. In one
experiment, the flow rate was controlled to a maximum of 140 lbs/min instead
of
200 lbs/min to achieve control over the foaming of the slag.
Only the preferred embodiment of the present invention and but a few
25 examples of its versatility are shown and described in the present
disclosure. It is
to be understood that the present invention is capable of use in various other
combinations and environments and is capable of changes and modifications
within the scope of the inventive concept as expressed herein.
