Method of producing molten iron in duplex furnaces

Details Method of producing molten iron in duplex furnaces Method of producing molten iron in duplex furnaces

1999-10-29

2001-06-26

King, Roy (Department: 1742)

Specialized metallurgical processes, compositions for use therei

Processes

Electrothermic processes

C075S010630, C075S010660, C075S484000

Reexamination Certificate

active

06251156

ABSTRACT:

FIELD OF INVENTION
This invention relates to an improved method for the production of molten iron in a continuous duplex furnace operation. More particularly, this invention relates to a method of continuously processing hot direct reduced iron in an electric melting furnace.
BACKGROUND OF THE INVENTION
In 1983, in U.S. Pat. No. 4,395,285, Merkert taught a low density, porous compact of prepared mix containing silica fume, finely-divided carbonaceous reducing agents such as petroleum coke or coal, and optimally with iron and a binder.
In 1987, U.S. Pat. No. 4,701,214, Midrex taught reduction by utilizing off gas generated by a smelting furnace in a rotary hearth furnace. A method of operation was promoted which required less energy and a smaller smelting furnace by introducing gaseous reductants and fuel into the rotary hearth furnace.
In 1987, in U.S. Pat. No. 4,731,112, Hoffman taught a method of making a molten ferroalloy product in a melting furnace from a feed briquette of metallized iron, granulated alloy metal oxide, and a carbonaceous material.
In 1998, in U.S. Pat. No. 5,730,775, Midrex taught an improved method known by the trade name or trademark of FASTMET, and apparatus for producing direct reduced iron from iron oxide and iron bearing and carbon compacts that are layered no more than two layers deep onto a rotary hearth, and are metallized by heating the compacts to temperatures of approximately 1316° to 1427° C., for a short time period. For a general understanding of the recent art, U.S. Pat. No. 5,730,775 is incorporated herein by reference.
All major steelmaking processes require the input of iron bearing materials as process feedstocks. In a steelmaking process utilizing a basic oxygen furnace, the iron bearing feed materials are usually blast furnace hot metal and steel scrap. A broadly used iron source is a product known as Direct Reduced Iron (“DRI”) which is produced by the solid state reduction of iron ore or iron oxide to metallized iron without the formation of liquid iron. Metallized in this sense, and throughout this specification, does not mean coated with metal, but means substantially reduced to the metallic state.
Improvements are sought within the industry for furnace modifications and improved methods of operation that provide for efficient, continuous production of high purity iron with a range of carbon content in which iron oxides are efficiently reduced to purified iron in the process while slag components are separated from the purified iron.
Specifically, a high purity iron product with a specified range of carbon content, a specified range of silicon and manganese content, and low sulfur and low phosphorous content is sought by the steelmaking industry. Molten iron product of this quality is typically produced by a blast furnace or conditioned after blast furnace production. Other melters such as conventional electric arc furnaces or submerged arc furnaces produce molten iron having different chemistry, in which the preferred reduced silicon content is not achieved efficiently. The reason that alternative melters cannot meet the industry's chemistry requirements for hot metal is that these furnaces fail to provide the necessary simultaneous conditions of optimum thermodynamic process equilibrium, and rapid melting. The invented method provides the environment as well as the process flexibility such that the desired silicon content in the hot metal can be easily achieved (increased or decreased) by adjusting power input to the electric melter (temperature).
SUMMARY OF THE INVENTION
The invented method continuously feeds material containing iron oxide and carbon compounds into a sequence of hot process steps. The first hot process step employs a rotary hearth furnace, operating below the melting point of the material, which effects pre-reduction of the material. The exit material from the rotary hearth furnace is continuously and preferably hermetically introduced into an electric melter wherein the material is further reduced at temperatures above the melting point of the material. The material exiting the pre-reduction rotary hearth furnace is never exposed to air or cooled between the exit port of the pre-reduction furnace and entry into the electric melter. The invented method produces a high purity iron melt containing a specified percentage of carbon. Starting materials are introduced into the rotary hearth pre-reduction process in layers in the form of compacts (e.g., compressed material). Pre-reduced material from the rotary hearth step is fed continuously and directly into the central interior area of the electric melter. The electric melter is maintained at temperature exceeding the melting point of the material and the ingress of oxygen is minimized to guarantee efficient reduction. High purity iron product is periodically removed from the electric melter.
Utilizing a pre-reduction step of heating iron-bearing compacts in a rotary hearth furnace, then directly and continuously feeding the carbon-containing metallized iron into an electric melter effectuates a very high iron content product having high percentages of carbon. Moreover, melting process conditions are such that the sulfur content is minimized, some SiO
2
is reduced to silicon, and some MnO is reduced to manganese in the final product. Therefore, an extremely desirable high iron content product is provided for use by the steelmaking industry.
OBJECTS OF THE INVENTION
The principal object of the present invention is to provide a method of achieving efficient reduction of iron oxide bearing materials at elevated temperatures in a series of furnaces.
Another object of the invention is to provide a method of achieving efficient continuous production of high purity liquid iron having concentrations of about 1% to about 5% carbon at elevated temperatures in a series of furnaces with separation of slag components from the purified liquid iron-carbon end product.
An additional object of the invention is to provide a method of desulfurizing high purity iron and reducing contaminants in direct reduced iron by continuously feeding an electric melter.
The objects of the invention are met by a method for producing highly purified iron and high percentage carbon product from iron oxide bearing materials, comprising the steps of providing a furnace for direct reduction of iron oxide bearing materials containing carbon in the form of compacts, layering the iron oxide and carbon bearing compacts in the furnace, pre-reducing iron oxide and carbon compacts, accomplishing the pre-reducing step in a furnace having a rotary hearth surface, the pre-reducing step producing hot carbon-containing metallized iron, then using an electric melter furnace for receiving hot carbon-containing metallized iron from the pre-reducing step, the second hot process step includes placing said electric melter furnace in close proximity to the rotary hearth furnace. After the rotary hearth furnace step, the hot, solid carbon-containing metallized iron material is used to directly and continuously charge an electric melter. The charge is inserted into the central interior area of the electric melter nearest the molten iron bath/electrode interface, or in other electric melters, inserted into the region of minimum slag, effecting rapid heating of the carbon-containing metallized iron to liquefying temperatures while minimizing the ingress of oxygen to assure optimum reduction conditions. Lastly, high purity iron product from the electric melter is periodically withdrawn without interrupting the continuous operation of the furnaces. The method of utilizing a pre-reduction step of heating carbon-containing iron oxide compacts in a rotary hearth furnace, and directly, continuously and hermetically feeding the hot, solid carbon-containing metallized iron from this furnace into an electric melter provides a high iron content product having high percentages of carbon, with significant desulfurization of the product, significant reduction of silicon oxides to silicon, and reduction of manganese oxide to mangan

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