Plant for the production of metal melts

Details Plant for the production of metal melts Plant for the production of metal melts

1999-02-26

2001-07-24

King, Roy (Department: 1742)

Metallurgical apparatus

Means for melting or vaporizing metal or treating liquefied...

By separating metal in a molten mass from undesired...

C266S230000, C266S236000, C266S901000

Reexamination Certificate

active

06264883

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a plant for the production of metal melts, in particular iron melts, such as steel melts, crude steel melts or pig iron melts, and a process for the production of these melts.
2. Description of the Related Art
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
60 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 20% of the total charge), and
0 to 40% 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 world-wide introduction of secondary metallurgical processes as well as a series of developments on the constructive, electric and technological sectors, electric arc furnace melting has changed within the past few years into a process both flexible and efficient in terms of charging substances and 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 and
cheaper primary energies (coal, natural gas, etc.) as a substitute for electric energy, including an improved offgas-afterburning operation and more efficient utilization of heat.
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 extent only. Moreover, it has not been feasible 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. The commercial application of a technology and plant based on electric arc furnace technology and highly efficient under such conditions from an economic point of view is still wanting.
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 site, 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 course 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 min with conventional electric arc furnaces and ≦35 min for electric arc furnaces with shaft, respectively, 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. 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), thus also increasing the heat loss, the actual transformer 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 hearth 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. The refining zone is constructed as a channel-type reactor, resulting in a high specific surface area with high heat losses. Refining is carried out with a C-concentration gradient along the refining zone of the channel-type reactor without a concentration balancing tank, and therefore the C content is difficult to adjust or control. 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 and/or 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 scrap column and the walls 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 furnace walls and the furnace lid and thereby gets lost. Moreover, possible bridging within the scrap column—above the melt caverns melted into it by the electrodes—may cause precipitation of the scrap column (or parts thereof), which might lead to a break of t

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