Specialized metallurgical processes – compositions for use therei – Processes – Electrothermic processes
Reexamination Certificate
1998-02-12
2001-06-05
Andrews, Melvyn (Department: 1742)
Specialized metallurgical processes, compositions for use therei
Processes
Electrothermic processes
C266S144000, C266S177000, C266S160000, C266S212000, C373S080000
Reexamination Certificate
active
06241798
ABSTRACT:
The invention relates to a plant for the production of iron melts, in particular steel melts, such as crude steel melts, as well as a process for producing such melts.
The present standard aggregate used for the production of electric steel is an a.c. or d.c. electric arc furnace. The iron carriers charged, which are comprised of
70 to 100% steel scrap, directly reduced iron-sponge iron in various quantitative ratios and sometimes also iron carbide (at present, up to about 10 to 15% of the total charge), and
0 to 30% liquid and/or solid pig iron
are melted by aid of one or several electric arcs using oxygen lance(s)—if desired, burner(s), nozzles and/or inert gas flushing—and under the addition of carbon carriers and slag formers. After this, the steel bath during a flat bath period (5 to 10 min) in an electric arc furnace is brought to the temperature and composition desired for tapping and is killed in the ladle during tapping. Energy and material consumption as well as plant productivity vary greatly as a function of the respective charging ratios and melting practice.
Due to the work-wide introduction of secondary metallurgical processes as well as a series of developments on the constructive, electric and technological sectors of electric arc furnaces, such as, e.g.,
cooled panels and lids
improved electrode qualities and the use of electrode cooling
the introduction of d.c. electric arc furnaces in addition to a.c. electric arc furnaces
elevated transfo outputs
the use of burners, lances, nozzles and/or flushing bricks for melting, heating, refining, blowing in solids and/or inert gas flushing
the use of current-carrying electrode brackets as well as electrode control
the optimization of the shape and size of a furnace (including tap opening)
a foamed slag operation
scrap preheating of various types
the use of sponge iron, if desired as a hot charge electric arc furnace melting has changed within the past two decades into a process both flexible and efficient in terms of charging substances an steel quality produced, more and more exhibiting substantial advantages over converter metallurgy and competing the same successfully.
With new process developments, important reductions of the melting time and the specific electric energy consumption and hence further reduction of the specific operating and investment costs of electric steel production in electric arc furnaces have been attained primarily by applying
integrated scrap preheating and/or hot charging of sponge iron/hot-briquetted directly reduced iron
continuous addition of a major portion of the charging substances (iron carriers, carbon carriers, fluxes, etc.) while minimizing the power-off time for carrying out charging operations
optimum foamed slag operation
cheaper primary energies (coal, natural gas, etc.) as a substitute for electric energy, including partial CO/H
2
afterburning within and/or above the foamed slag.
However, with the known processes for the production of electric steel by means of electric arc furnaces used as melting aggregates, the potential advantages of the above-mentioned process developments have been utilized to a limited extend only. Moreover, it has not been feasiable so far—despite an increasing demand—to process to liquid steel high portions of liquid pig iron and/or other carbon-rich iron carriers (sponge iron, iron carbide etc.) as well as problem scrap (used cars) of about 30 to 70% charged into electric arc furnaces, at a high productivity and energy utilization and, with car scrap, also without inadmissible loads on the environment. A technology and plant based on electric arc furnace technology and highly efficient under such conditions from an economic point of view are still missing.
The above-mentioned limitations with conventional electric arc furnaces are due exclusively to the configuration of the furnaces, which does not enable a quasi-stationary continuous process course. The operations of charging, melting, refining, heating and tapping take place on one side, by necessity more or less offset in time and with interruption(s) of the charge and current supply—at least before and after tapping—in order to obtain the desired composition and temperature (homogeneity and overheating in respect of the liquidus temperature) of the crude steel. The present process source in an electric arc furnace is discontinuous and hence limited in its performance. In this respect, the following is noted:
1) With already reached tap-to-tap times of 55 to 60 min for tap weights of 70 to 150 tons, the possibility of further reducing the power-off phases is strongly limited. The same holds for the power-on phases—since under such conditions the limits for an economic energy input per ton of charge and time unit—and hence for the overall melting time have almost been reached.
2) From a certain piece size, the continuous charging of scrap is hardly feasibly. Heavy and bulky scrap is charged by scrap baskets at power off.
3) In continuous charging as well as in refining and heating in the flat bath operation, which will take a substantially longer time with high charging portions of sponge iron and, in particular, of liquid pig iron and iron carbide (about 6.1% C), the actual transfo output, as a rule, is not completely utilized by electric arc furnaces.
From AT-B-295.566 a process for the continuous production of steel by melting prereduced ore and subsequently refining the melt of semi-steel to steel in an electric arc melting furnace comprising a melting hearth to which a refining zone and at least one slag depositing chamber are connected is known, in which prereduced iron ore is introduced into the electric arc zone of the melting heath in a lumpy or granular form, the metal is continuously agitated and set in a circulatory movement within the hearth and the metal is refined to steel while flowing through a refining zone by blowing in an oxygen-containing gas, whereas slag is caused to stream opposite to the metal at least along part of the length of the refining zone. The slag calms down in a slag depositing chamber without intensive mixing of the bath and then is tapped from the slag depositing chamber.
In that known process plant scrap and liquid pig iron may be charged, yet each in very limited amounts only. Discharging of the offgases takes place directly in the refining zone, i.e., not via the electric arc melting furnace. To prevent the melt from freezing in the refining zone, a high coke/coal addition is required in that known process. Consequently, that known process is applicable to a limited extent only, in the first place serving to produce crude steel from prereduced ore.
From DE-C 3 609 923 a process and an arrangement for continuously melting scrap to crude steel is known. In that process, which primarily is limited to scrap melting (no mention being made of charging liquid pig iron or directly reduced sponge iron), the heat of the furnace gases is utilized for heating the scrap. The scrap is preheated in a shaft centrally placed on the hearth-type furnace and is introduced centrally into the hearth-type furnace, thereby forming a scrap column supported on the bottom of the electric arc furnace under formation of a conical pile and capable of reaching up as far as to the scrap charging opening provided in the upper part of the scrap preheating shaft. Pivotable electrodes (preferably four electrodes) are symmetrically arranged about the scrap column in the electric arc furnace and assist in melting the scrap. The angle of inclination between the central axis of an electrode and a vertical line during scrap melting amounts to more than 20° for each of the electrodes. Thereby, the hearth-type furnace is exposed to a great thermal load, since the electric arcs are burning between the centrally introduced scarp column and the wall and lid of the hearth-type furnace. On the one hand, this causes an increased wear of the refractory lining and hence elevated material and time costs for doing repairs. In addition, a large portion of the input energy is imparted by radiation to the fu
Dimitrov Stefan
Fritz Ernst
Muller Heinz
Pirklbauer Wilfried
Ramaseder Norbert
Andrews Melvyn
Ostrolenk Faber Gerb & Soffen, LLP
Voest-Alpine Industriean-lagenbau GmbH
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