Specialized metallurgical processes – compositions for use therei – Processes – Electrothermic processes
Reexamination Certificate
1999-12-10
2002-04-16
King, Roy (Department: 1742)
Specialized metallurgical processes, compositions for use therei
Processes
Electrothermic processes
C075S010460
Reexamination Certificate
active
06372010
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally relates to an improved method and apparatus for metal melting, refining and processing, for example, steel making in an electric arc furnace (EAF), and more particularly, to an auxiliary burner or lance for the addition of thermal energy by the combustion of fuel, the injection of oxidizing gas for melt refining, foamy slag production or post combustion of carbon monoxide, and the injection of particulates for slag and foamy slag production.
2. Description of Background Art
Oxygen and carbon lances are known in the art of metal melting, refining and processing, particularly steel making, to be useful for the injection of carbon and oxygen for many steps in the process. In addition, auxiliary burners have been used to provide additional thermal energy and supersonic oxygen to these processes.
An electric arc furnace makes steel by first using an electric arc to melt one or more charges of scrap metal which is placed within the furnace. The scrap is charged by dumping it into the furnace through the roof from buckets which also could include charged carbon and slag forming materials. The arc melts the scrap into a molten pool of metal, an iron carbon melt, which accumulates at the bottom of the furnace. After a flat bath has been formed by melting of all the scrap introduced, the electric arc furnace enters a refining or decarburizing phase. In this phase, the metal continues to be heated by the arc until the slag forming materials combine with impurities in the melt and rise to the surface as slag. The charged carbon when the iron carbon melt reaches a boiling temperature combines with any oxygen present in the bath to form carbon monoxide bubbles which rise to the surface of the bath. Generally, at this time supersonic flows of oxygen are blown at the bath with from either lances or burners to produce a decarburization of the bath by the oxidation of the carbon contained in the bath. The carbon content of the bath is reduced to under 2% carbon where the iron carbon melt becomes steel and then further reduced until the grade of steel desired is produced, down to less than 0.2% for low carbon steels.
In higher power electric arc furnaces it is becoming a standard practice to use a long arc. When a long electric arc is struck, the radiative component of the arc above the bath can cause degradation to the furnace walls and the surrounding furnace equipment. The danger of this damage occurring is greatest after a significant portion of the scrap has been melted and the walls of the furnace are exposed to heat radiation from the arc and similarly thereafter during refining and flat bath conditions. Steelmakers have developed a foamy slag practice to protect the furnace components from the arc where the slag layer of impurities covering the iron carbon melt is foamed to increase its volume and rise above the arc. This foaming of the slag creates an insulative barrier between the arc and the furnace walls thereby protecting them from over heating. The overall process is also improved as the excess heat from the radiation of the arc is now used to heat the bath. Slag foaming practice differs for each EAF and operator preference. Slag foaming can begin as early as the first charge of scrap and can continue until the molten steel is tapped and the slag discarded.
The slag is conventionally foamed by bubbles of carbon monoxide caused from the oxidation of carbon with oxygen. To effectively foam slag these carbon monoxide bubbles should occur in the iron carbon melt, at the melt-slag interface or in the slag itself. If the slag has the right temperature, chemistry and viscosity, the carbon monoxide bubbles become entrained in the slag and cause its volume to increase rapidly to produce a lather or foam. Initially, the carbon which is charged with the scrap may form carbon monoxide bubbles by combining with oxygen which is in the bath or furnace during boiling or from additional oxygen blown into the bath with lances or burners. In addition, when the slag is hot, injected carbon particles either from lances or burners are used to form carbon monoxide bubbles by combining with FeO in the slag. The slag should be hot because the combination of carbon and FeO is an endothermic reaction which requires heat. Still further, oxygen may be present, either by injection from lances or burners or the furnace atmosphere, to oxidize carbon in the slag itself to carbon monoxide bubbles. In the latter case, the carbon in the slag may have been injected at the same time and from the same equipment as the carbon combining with the FeO to form carbon monoxide bubbles.
It is normal practice to inject the carbon and oxygen through burners or lances, or combinations thereof, at the same time, or nearly the same time, and directed to the same location, or nearly the same location. However, until the present invention, there have been some problems with directing the carbon and oxygen to the same, or nearly the same, location. The burners and lances previously used to provide this function were not very successful and efficient as they could be, because they direct the flows of carbon and oxygen in different directions.
Recently, there have been some attempts to combine oxygen and carbon injection lances with the oxy-fuel burner function. An important question for the integration of these functions into one apparatus has been whether to retain particulate injection capability or supersonic oxygen capability because both functions are the most advantageous if located along the central axis of the lance or burner.
Particulate injection is best done through a straight conduit which is located along the central axis of the apparatus used. A straight conduit is conventional because the particulates injected into a steel making furnace are highly abrasive and will wear out bends or other restrictions to their flow quickly. This is one of the reasons why particulates have not be injected through the same conduit as the one used for supersonic oxygen of a burner, the particulates would quickly wear out the converging restriction of the nozzle. A central conduit is preferred because it is highly disadvantageous to break the stream into more than one flow because one would like to concentrate particulates in a specific area. Also, the size of the particles and amount of particulates used for an injection is large in mass compared to other injected materials, such as gases, and a relatively large conduit is needed for reasonable flow rates.
Laval or supersonic nozzles are usually used in the production of high speed streams of oxidizing gas for injection into a steel making furnace. These supersonic gas flows are produced by the converging/diverging shape of the nozzle which at above a critical pressure causes the gas flow though the nozzle to become supersonic. Usually, an conduit is machined centrally in a lance or burner and then the passage is fitted with a converging/diverging section or nozzle. A large centrally located nozzle is desired because of the flow rates of supersonic oxygen desired.
It is also highly desirable to provide a subsonic flow of oxidizing gas for the burning of fuel, including regular fuel and carbon monoxide for post combustion, for the addition of auxiliary thermal energy, and the supersonic oxygen flow for providing oxygen in iron melt decarburization, assisting in foamy slag production or post combustion of carbon monoxide. A burner which provides subsonic and supersonic flows of oxygen through the same centrally located conduit is manufactured and commercially sold by Process Technology International, Inc. of Tucker, Georgia, the assignee of the present invention. The subsonic flow is produced by providing a pressure in the supply conduit lower than the critical pressure of the Laval nozzle being used in the conduit. When supersonic oxygen is needed, the pressure in the supply conduit is increased to above the critical pressure.
One attempt to combine the functions of carbon injection and supersonic oxygen
Pulliam Terry E.
Shver Valery G.
King Roy
Marvin William A.
McGuthry-Banks Tina
Process Technology International Inc.
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