Specialized metallurgical processes – compositions for use therei – Processes – Electrothermic processes
Patent
1997-01-23
1998-12-22
Andrews, Melvyn
Specialized metallurgical processes, compositions for use therei
Processes
Electrothermic processes
75 1042, 373 73, C21C 552, F27B 322
Patent
active
058512632
DESCRIPTION:
BRIEF SUMMARY
The invention relates to the smelting of steel in electric arc furnaces. More specifically, it relates to the method of injecting gaseous oxygen into the chamber of the furnace, the function of this oxygen being to accelerate the melting of the scrap and to achieve complete combustion of the carbonaceous off-gases from the bath of liquid metal.
Steelmaking electric arc furnaces serve to melt the iron-containing raw materials (scrap, cast iron or prereduced iron ore) which are intended to form the bath of liquid steel, as well as to carry out at least the first steps in adjusting the composition of this liquid steel prior to casting it. In the case of furnaces supplied with alternating current, the supply of energy necessary to melt the metal charge and to adjust its temperature is mainly provided by an electric arc established between the bath and three graphite electrodes which pass through the roof resting on top of the vessel of the furnace and which are arranged above the bath. In the case of furnaces supplied with direct current, the current flows between at least one graphite roof electrode and at least one electrode which is fitted in the hearth of the furnace and brought into contact with the bath.
Currently, there is an increasing tendency to substitute some of this supply of electrical energy, or to add to it, a supply of fossil energy based on the combustion by gaseous oxygen of the carbon initially contained in the solid charge, or even also of carbonaceous materials deliberately added to the solid charge and/or to the liquid bath. It is also known to heat the scrap by means of oxygen, fuel-oil or natural gas burners. Overall, this supply of fossil energy may form 25 to 50% of the total potential energy introduced during the melting and smelting of the charge.
The successive reactions for complete combustion of carbon, by virtue of which reactions the supply of energy by means of oxygen is produced, may be written as: 25.degree. C. 1600.degree. C. C. 25.degree. C. 1600.degree. C.
It is clearly the reaction of oxidation of CO to CO.sub.2, usually designated by the expression "postcombustion of CO", which represents the most useful energy supply for the metal charge. However, in practice there is quite often only a small part of the available carbon which is completely burnt into CO.sub.2 in the immediate vicinity of the solid or liquid metal. Decarburization of the liquid steel, whether this occurs naturally or as a result of blasting oxygen into the metal bath, forms CO which on escaping from the bath is not under conditions sufficiently favourable for its complete oxidation to CO.sub.2. In fact, this oxidation occurs significantly only in the upper part of the furnace, when the CO comes into contact with the air which penetrates the furnace in an uncontrolled manner via the openings in the furnace and via the interstices separating the roof from the upper edge of the vessel on which it sits. The preferred region for postcombustion of CO is therefore too far from the metal charge and the combustion temperature is too low for this postcombustion to contribute very significantly to the heating of the scrap being melted or of the bath of liquid metal.
It has been conceived (see the document EP 0,257,450) to fit oxygen lances in the side wall of the furnace (above the level of the liquid bath) so as to accentuate the phenomenon of postcombustion of CO to CO.sub.2. These lances are inclined at an angle of 30.degree. to 60.degree. with respect to the vertical and direct their jets tangentially to an imaginary cylinder defined either by the electrode itself, in the case in which the furnace is a single-electrode furnace, or by the cylinder in which the electrodes lie, in the case of a three-electrode furnace. These lances are aimed at those regions of the furnace which are usually the relatively coolest and where the melting of the scrap is slowest. However, experience has shown that the effectiveness of such equipment is not yet optimal.
The aim of the invention is to provide an effective method for
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patent: 4986847 (1991-01-01), Knapp et al.
patent: 5112387 (1992-05-01), Lazcano-Navarro
patent: 5375139 (1994-12-01), Bender
patent: 5531805 (1996-07-01), Worner
Bernet Thierry
Destannes Philippe
Roth Jean-Luc
Andrews Melvyn
Cole Thomas W.
Usinor Sacilor
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