Specialized metallurgical processes – compositions for use therei – Processes – Producing or treating free metal
Reexamination Certificate
1999-05-26
2001-08-07
Kastler, Scott (Department: 1742)
Specialized metallurgical processes, compositions for use therei
Processes
Producing or treating free metal
C266S178000
Reexamination Certificate
active
06270552
ABSTRACT:
INDUSTRIAL FIELD OF THE INVENTION
The present invention relates to a method and an apparatus for operating a traveling hearth furnace suitable for the manufacture of reduced iron from iron ore.
DESCRIPTION OF THE RELATED ART
Production processes of crude steel are broadly classified into the blast furnace-converter process and the electric furnace process. An electric furnace manufactures steel from scrap or reduced iron as an iron raw material, by heating and melting the raw material by electric energy, and refining the same as required. Scrap is a main raw material at present. Because of the recent supply shortage of scrap and the tendency toward manufacturing high-grade products by the electric furnace process, however, there is an increasing consumption of reduced iron.
As one of the processes for manufacturing reduced iron, for example, Japanese Unexamined Patent Publication No. S63-108,188 discloses stacking a layer of an iron ore and a solid reducing material onto a hearth rotating in the horizontal direction, and heating the layer from above by radiant heat transfer to reduce the iron ore, thereby manufacturing reduced iron. This method provides advantages of a relatively low construction cost of facilities and a relatively low risk of operational troubles. In many cases, a form of rotary hearth as shown in FIG.
1
and
FIG. 2
of the drawings is adopted as the hearth. A layer (t) of a mixed powder comprising an iron ore (oxide powder mainly comprising a fine iron ore) and a solid reducing material transported via a charger (a) is stacked onto the rotary hearth
1
. The hearth
1
is covered with a furnace body
2
lined with refractory and a burner
3
(
FIG. 2
) serving as a heat source is provided on the upper portion of the inside thereof so as to reduce the iron ore on the traveling hearth
1
. A furnace temperature of about 1,300° C. is usually used. To prevent oxidation after discharge of the product outside the furnace upon completion of reduction, and to achieve a high handling convenience, it is the usual practice to cool the reduced iron by use of a cooler on the rotary hearth, and then discharge the same for recovery.
In this conventional manufacturing method, gangue and ash contained in the raw material ore and the carbonaceous materials remain in the reduced iron product, and are melted and reduced in an electric furnace (melting furnace) forming the next step. Gangue and ash contained in the feed stock ore or coal entrapped into the electric furnace, have caused the following problems.
In order to reduce sulfur contained in the hot metal, it is necessary to raise the CaO/SiO
2
ratio in the produced slag. It is thus necessary to supply a CaO source in a large quantity in the form of limestone and dolomite according as the SiO
2
content resulting from gangue and ash increases.
Addition of much limestone and dolomite requires compensation for decomposition heat and sensible heat. For this purpose, an increase in the electric power consumption of the electric furnace is inevitable, hence requiring use of an extra-fine powdery ore having a low gangue content and concentrated and a low-ash coal so as to minimize the quantity of slag entering the electric furnace.
As described above, iron and steel-making via an electric furnace, require high-grade ore and coal suitable for a traveling hearth furnace, causing an increase in lost. In addition, the amount of deposits of resources and the production capacity thereof have already reached limits. Under these circumstances, therefore, the demand is increasing for supplying iron sources suitable for an electric furnace on the basis of ordinary ore and ordinary coal existing in large quantities.
It is easily conceivable that it is possible to maintain a reduced iron containing gangue and ash at high temperatures, bring the same in a molten state, and remove slag therefrom, but these cannot be achieved by the conventional method because of the following restrictions:
Separation of slag and metal is achievable by keeping metal and slag at high temperatures until complete melting in a both-type melting furnace such as a melting reducing furnace, but this is difficult in a traveling hearth furnace because of restrictions in facilities. The lowest temperature, the holding time and other operating conditions for separating slag and metal in a radiant heating furnace such as a traveling hearth furnace and melting behavior for determining these conditions have not as yet been clarified.
In order to increase the temperature of a radiant furnace, it is necessary to generate a high-temperature combustion flame. Increasing the temperature of the combustion flame requires preheating combustion air and to accomplish combustion near the theoretical air ratio. For the prevention of reoxidation of the reduced iron, on the other hand, it is necessary to maintain a very strong reducing gas atmosphere. Under this gas condition, however, it is difficult to obtain a high-temperature combustion flame.
When melting the raw material in a traveling hearth furnace, a thin molten iron layer and a slag layer separated from each other, are present on the traveling hearth. Problems of erosion of the traveling hearth caused by molten slag containing FeO, and adhesion of slag to the hearth, are encountered.
In this respect, U.S. Pat. No. 3,443,931 discloses a technique of achieving a semi-molten state by adjusting the atmosphere to a maximum temperature within a range of from 1,260 to 1,426° C., and to contain at least 10% combustible gas on a traveling hearth. While the disclosed technique permits maintenance of satisfactory contraction and oxidation resistance, separation of the gangue is difficult.
Removal of gangue and ash by melting outside the furnace is naturally conceivable, but it is needless to mention that this requires facilities such as a new cupola.
SUMMARY OF THE INVENTION
The present invention solves the problems in the conventional technique.
The invention provides a method of operating a traveling hearth furnace, comprising stacking a layer comprising a mixture of fine iron ore and a fine solid reducing material onto a hearth traveling horizontally on a course, and subjecting the layer to reduction of oxides via radiant heat transfer from the upper portion of the hearth, melting the reduction product on the course to a discharge port at a temperature over 1,450° C., and separating slag from the reduction product through aggregation. In the present invention, the oxygen partial pressure of a gaseous phase in contact with the reduction product is adjusted to a reducing atmosphere, and a layer different from the mixture layer may be stacked on the mixture layer.
In the invention, when stacking a mixture layer comprising a fine iron ore and a fine solid reducing material onto a hearth traveling horizontally, and reducing the fine ore by radiant heat transfer from above the hearth, first a fine coal char is stacked prior to stacking the mixture layer onto the hearth, then melting the product resulting from reduction of the mixture on the hearth at least once, and a carbonized gas is produced upon manufacturing a fine coal char by a preliminary carbonization of the coal as a heating fuel of the traveling hearth furnace.
In the operating method of a traveling hearth furnace of the invention for stacking feed stock mainly consisting of a fine iron ore and a fine solid reducing material on a hearth traveling horizontally, the method comprises the steps of dividing a mixed powder of a fine iron ore and a fine solid reducing material or a mixed powder of fine iron ore, a fine additive and a fine solid reducing material into small blocks, dotting the hearth surface with these blocks via the fine solid reduction material so as to avoid direct contact of the blocks with the hearth, and melting the reduced iron on the hearth at least once.
REFERENCES:
patent: 3443931 (1969-05-01), Beggs et al.
patent: 5186741 (1993-02-01), Kotraba et al.
patent: 5637133 (1997-06-01), Munnix et al.
patent: 5730775 (1998-03-01), Meissner et al.
Kikui Tetsuya
Sawa Yoshitaka
Takeda Kanji
Yamamoto Tetsuya
Kastler Scott
Kawasaki Steel Corporation
Schnader Harrison Segal & Lewis LLP
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