Specialized metallurgical processes – compositions for use therei – Processes – Electrothermic processes
Reexamination Certificate
2001-10-30
2004-10-19
Andrews, Melvyn (Department: 1742)
Specialized metallurgical processes, compositions for use therei
Processes
Electrothermic processes
C075S010460, C075S569000
Reexamination Certificate
active
06805724
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally relates to a method and apparatus used in metal melting, refining and processing, for example, steel making in an electric arc furnace (EAF), and more particularly, to a method and apparatus for the introduction of chemical energy and particulates, for example, carbon particles entrained in a carrier gas, in an EAF.
2. Description of Background Art
Electric arc furnaces (EAFs) make steel by using an electric arc to melt one or more charges of scrap metal which is placed within the furnace. Modern EAFs may also make steel by melting DRI (direct reduced iron) combined with the hot metal from a blast furnace. In addition to the electrical energy of the arc, chemical energy is provided by auxiliary burners using fuel and an oxidizing gas to produce combustion products with a high heat content to assist the arc.
If the EAF is used a scrap melter, the scrap burden is charged by dumping it into the furnace through the roof opening from buckets which also may include charged carbon and slag forming materials. A similar charging method using a ladle for the hot metal from a blast furnace may be used along with injection of the DRI by a lance to produce the burden.
In the melting phase, the electric arc and burners melt the burden into a molten pool of metal, termed an iron carbon melt, which accumulates at the bottom or hearth of the furnace. Typically, after a flat bath has been formed by melting of all the burden introduced, the electric arc furnace enters a refining and/or decarburization phase. In this phase, the metal continues to be heated by the arc until the slag forming materials combine with impurities in the iron carbon melt and rise to the surface as slag. During the heating of iron carbon melt it reaches the temperature and conditions when carbon in the melt combines with oxygen present in the bath to form carbon monoxide bubbles which is commonly termed as “carbon boil”. Generally, flows of oxygen are blown into the bath with either lances or burner/lances to produce a decarburization of the bath by the oxidation of the carbon contained in the bath.
The resulting decarburization reduces the carbon content of the bath to a selected level. If an iron carbon melt is under 2% carbon it becomes steel. Except for operations using the hot metal from the Blast furnaces, the EAF steel making processes typically begin with burdens having less than 1% carbon. The carbon in the steel bath is continually reduced until it reaches the content desired for producing a specific grade of steel, down to less than 0.1% for low carbon steels.
With the imperative to decrease steel production times in electric arc furnaces, it becomes necessary to deliver effective decarburizing oxygen to the iron carbon melt as early in the steel making process as possible. Conventional burners mounted on the water cooled side walls of the furnace generally must wait until the melting phase of the process is substantially complete before starting high velocity injection of oxygen for the decarburization process. These burners can not deliver effective high velocity oxygen to the bath early in to the melting cycle because unmelted scrap is in the way of the injection path and would deflect the oxygen flow. The bottom of the electric arc furnace is spherical shaped and the melted scrap forms the melt in the middle of the furnace first and then it rises filling up the sides.
Therefore, it would be highly advantageous to reduce the melting phase of an electric arc furnace so that high velocity oxygen and carbon could be injected sooner and decarburize the melt faster.
One way to shorten the melting phase is to add substantially more energy with the burners at early times in the melting phase to melt the scrap faster. There are, however, practical considerations with conventional side wall mounted burners that limit the amount of energy which can be introduced into the furnace and the rate at which it can be used efficiently. The location of a conventional burners is subject to flashback. When scrap is initially loaded into the furnace, because it is located very near the flame face and oxygen jet of the burner, the danger of a flash back of the flame against the side wall where the burner is mounted is significant. The panels where the burners are mounted are typically water cooled and a burn through of a water carrying element in an electric arc furnace is a safety concern, as well as a production loss. To alleviate this concern, many fixed burners are run at less than rated capacity until the scrap is melted some distance away from the face of the burner. Only after the burner face has been cleared does the burner operate to deliver its maximum energy.
Another problem to increasing the energy added during the early part of the melting phase is that the flame of the burner is initially directed to a small localized area of the scrap on the outside of the scrap burden. It is difficult to transfer large amounts of energy from the burner by this localized impingement to the rest of the scrap efficiently. Until the burner has melted the scrap away from its face and has opened a larger heat transfer area, increasing a burner to maximum output would result in overheating and melting scrap pieces together producing the problems for the next stage of the EAF operation.
Therefore, it would be advantageous to be able to increase the amount of energy applied by a burner during the early part of the melting phase which did not produce a risk of flash back for the water cooled panels of the upper shell of the furnace. It would also be advantageous to use this increased amount of energy more efficiently and to transfer increased portions of the energy to the scrap burden without scrap agglomeration.
Conventionally, oxygen is blown or injected into the iron carbon melt where it reacts with the carbon in the molten bath to lower the carbon content to the level desired for the end product. In general, the rate of decarburization in an electric arc furnace is determined by the carbon concentration of the iron carbon melt, the oxygen injection rate and the surface area of the reactions sites. At higher bath carbon concentrations, the reaction rate is not significantly limited by the availability of carbon to enter the reaction. However, as the bath carbon decreases to concentrations under approximately 0.15%-0.20% of carbon, it becomes increasingly difficult to achieve an acceptable rate. This is because the carbon concentration of the bath becomes the decarburization rate determining factor. The decarburization rate, after the critical carbon content has been reached, is dominated by mass transfer of the carbon and the carbon concentration.
The prior art practice to decarburize an iron carbon melt is characterized by the localized application of a large volume of oxygen by means of devices such as lances and burner/lances. Due to the localized nature of this process, the decarburization rate depends on the rate of oxygen injection to the bath, the carbon concentration and the mass transfer of carbon to the reaction area. At lower carbon level contents, the iron oxide concentration in the slag near to the oxygen introduction area reaches levels greater than equilibrium would allow, due to depleted local carbon concentration and poor mass transport. This causes greater refractory erosion, loss of iron yield, increased requirements for alloys, and a low efficiency of oxygen utilization.
Therefore, it would be advantageous to provide a method and apparatus to supply oxygen for efficient decarburization of the iron carbon melt at all carbon concentrations. A method that increased the number of reaction zones and supplied significantly more effective oxygen early in the process would be advantageous because it would shorten the duration of decarburization. Particularly important is the efficiency of the oxygen supply after the iron carbon melt reaches a low carbon content in order to maximize the decarburization rate, without over oxidizing the slag an
Andrews Melvyn
Process Technology International Inc.
Weatherly Mitchell G.
Weatherly & Associates, LLC
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