Industrial electric heating furnaces – Induction furnace device – Core-type
Reexamination Certificate
2002-12-20
2004-11-16
Hoang, Tu (Department: 3742)
Industrial electric heating furnaces
Induction furnace device
Core-type
C373S162000, C373S163000
Reexamination Certificate
active
06819705
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to induction furnaces used in the melting or smelting of metals and particularly to induction furnaces used in steelmaking.
BACKGROUND TO THE INVENTION
In recent years there have been moves in the steelmaking industry to develop new steelmaking processes that are radically different compared to the traditional iron blast furnace and steelmaking-furnace routes.
In the traditional route steel is basically produced in two stages. In the first stage, which occurs in the blast furnace, iron oxide is reduced to pig iron. In the second stage, which occurs in the steelmaking furnace, elements such as carbon and manganese are controlled to specific levels and elements such as silicon, sulphur and phosphorous are mostly eliminated. Steelmaking furnaces include furnaces such as basic oxygen and electric arc furnaces.
One of the problems with the traditional method of making steel is the need to transfer liquid iron between the two stages of the process. The transfer involves a costly capital investment in infrastructure and also carries with it the risk associated with transporting liquid iron. The traditional methods are also associated with gas emissions that are not environmentally friendly.
A significant development in this area has been the development of a channel type induction furnace that is charged with an iron-containing burden and produces crude steel. This is the type of process described in U.S. Pat. No. 5,411,570 and patent applications PCT/EP97/01999 and PCT/IB99/01334.
The furnace is a channel type induction furnace and consists of a shell lined with refractory material. Feed material, iron containing ore and carbon reductant, is charged through holes in the sides of the furnace and is then heated by combustion of the different gases that are formed when a carbon reductant and ore mixture is heated, and under certain conditions, combustion of additional fuel.
Induction heaters situated at the bottom of the metal bath heat the liquid metal in the furnace which in turn heats the burden further and melts it to form liquid slag and metal. These heaters are attached to the furnace in the conventional manner. This means that the furnace has appropriate openings in its shell and flanges around the opening for bolting the complementary flange of the induction heater to the flange of the shell. Both the furnace and the induction heaters are lined with refractory material.
The thickness of the refractory material of the furnace around the induction heater opening in the furnace determines the depth of the entrance or ‘throat’ to the induction heater.
Molten metal flows into the induction heater through the throat and also exits the induction heater through it. The metal closest to the inner surface of the induction heater is heated. This means colder metal flows into the induction heater channels on the outside and is heated as it passes against the inside of the channel. Flow of the molten metal is generated by the difference in densities between hot and cold metal. Electromagnetic forces can assist this effect, to modify the flow pattern of the molten metal.
The known channel induction heaters are of the type that consists of an electrical coil that is built into a refractory body with a channel formed in the refractory material around the coil. The coil is isolated from the channel by refractory material, water-cooling panel(s) and an air gap. The combined depths of the refractory material on the floor of the furnace; the thickness of the furnace shell; the thickness of the furnace flange; and the distance between the furnace shell and the furnace flange is commonly accepted as the depth of the throat to the induction heater. The throat is shaped to be substantially vertical and it leads directly into the channels of the induction heater.
In the channel type furnace several of the induction heaters are arranged in a row along the length of the furnace.
The charge in the furnace consists of the molten metal bath, a layer of slag on top of the metal and the solid burden at the top. The burden is basically divided into two continuous heaps extending for the greater part of the length of the furnace, as described in U.S. Pat. No. 5,411,570; or the furnace can be charged so that the two continuous heaps of burden meet in the centre of the of the furnace to close the gap between the two heaps of burden, as described in patent application PCT/EP97/01999.
The molten metal flows into an induction heater through its throat and also exits the induction heater through its throat. The exit stream from the induction heater is substantially vertical, thereby mixing with the metal directly above the opening. The colder metal drawn into the induction heater also substantially originates from the pool of metal directly above the induction heater. The rising hot metal exchanges heat with the descending cold metal in the throat.
This means that the pool of metal above each induction heater opening and in the throat is to a large degree circulated through the induction heater and repeatedly heated. This causes local hotspots above the induction heater openings, especially when the depth of the metal bath above the induction heater is shallow. This causes the metal in the induction heater to be heated to unnecessarily, and some times dangerously, high temperatures.
The existence of local hotspots is not ideal in this type of furnace for a number of reasons. The first is that hotspots cause some of the burden in the vicinity of the hotspot to be preferentially melted, resulting in underexposure of that material to the heat from the burning gasses relative to the part of the burden not preferentially melted. Areas of overexposure and areas of underexposure to the heat from the burning gasses therefore exist. This difference in exposure leads to excessive electrical energy consumption and under utilisation of the available energy for reduction in the burning gasses and the heated roof. It also results in heating of unreduced burden that is too fast, leading to gas evolution in the liquid steel and subsequent undesirable boiling action. The effect of this is that the power input through the induction heaters must be reduced and as a result the production rate decreases.
In this specification the term “throat” shall mean the communication channel between the furnace and an induction heater in the floor of the furnace.
In this specification the term “throat depth” shall mean the operatively and substantially vertical distance from the uppermost extremity of the throat to a centre line drawn through the length of a coil of an induction heater in the floor of the furnace.
In this specification the term “service length” shall mean the length of the furnace that each induction heater is required to heat during use, which is the operatively and substantially horizontal distance from the mid-point between an induction heater and an adjacent induction heater to the mid-point between the induction heater and an oppositely adjacent induction heater or to the end of the furnace.
In this specification the term “throat length” shall mean the horizontal distance from one side of the throat of an induction heater, across the channels and the coil of the induction heater to its other side; this distance is measured substantially parallel to the “service length” of the induction heater.
In this specification the term “throat width” shall mean the distance between sidewalls of the throat and this distance is measured transverse to the “throat length”.
In this specification the term “induction heater channel width” shall mean the approximate distance from one side wall of the induction heater channel to the opposite side wall, measured at the centreline of the induction heater and measured at right angles to the long axis of the induction heater.
In this specification the term “conventional throat depth” shall mean, for a conventional induction furnace used for a similar process than that of the invention, the combined thickness of the floor refractory, the furnace shell suppo
Darby & Darby
Hoang Tu
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