Specialized metallurgical processes – compositions for use therei – Processes – Producing or treating free metal
Reexamination Certificate
1999-08-27
2001-06-26
Andrews, Melvyn (Department: 1742)
Specialized metallurgical processes, compositions for use therei
Processes
Producing or treating free metal
C266S177000
Reexamination Certificate
active
06251161
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method for operating a moving hearth reducing furnace, in which iron oxide agglomerates incorporated with a carbonaceous material are reduced to iron.
2. Description of the Related Art
A typical method for preparing reduced iron is a MIDREX process. In this method, a reducing gas, such as natural gas, is blown into a shaft furnace through a tuyere. The reducing gas flows in and comes contact with iron ore or iron oxide pellets filled in the furnace. Thus, iron oxide is reduced in a reducing atmosphere in the furnace to form reduced iron. This method using a large amount of expensive natural gas, however, inevitably results in high production costs.
Recently, processes for making reduced iron using inexpensive coal in place of the natural gas have attracted attention. For example, U.S. Pat. No. 3,443,931, which is hereby fully incorporated by reference, discloses a process for making reduced iron including pelletizing a mixture of powdered iron ore and coal and reducing iron oxide in a hot atmosphere. This process has some advantages: use of coal as a reducing agent, direct use of powdered ore, a high reducing rate, and ready control of the carbon content in a product.
In this process, a given amount or depth of pellets or briquettes of iron oxide agglomerates incorporated with a carbonaceous material (hereinafter referred to as simply “agglomerates”) is fed into a moving hearth reducing furnace, such as a rotary hearth furnace. The contents are moved and heated by radiant heat in the furnace. Thus, iron oxide is reduced with the incorporated carbonaceous material to form reduced iron. The reduced iron is discharged from the moving hearth of the furnace by a screw of a discharging apparatus. The process of U.S. Pat. No. 3,443,931 is shown in FIG. 12. In
FIG. 12
, the screw
1
of the discharging apparatus is supported by an elevator
3
and a bearing
4
, comes into contact with a moving hearth
2
by its own weight, and rotates to discharge the reduced iron from a discharging port
25
.
When the agglomerates are fed into the moving hearth furnace, parts of the agglomerates are pulverized by rolling, friction or dropping shock and the iron oxide powder is deposited on the moving hearth. As shown in
FIG. 13
, the iron oxide powder moves towards the screw
1
and is reduced to metallic iron powder
26
. The metallic iron powder on the rotating hearth is squeezed into the furnace face by the screw and is deformed to elongated metal powder
28
(see “Initial Forming Stage of Iron Sheet” in FIG.
13
). The elongated metal powder
28
squeezed into the furnace is barely oxidized in a reducing atmosphere. Thus, the elongated metal powder gradually grows by the pressure of the screw
1
and becomes an iron sheet (see “Iron Sheet Forming Stage” in FIG.
13
).
In a hearth surface of the rotary hearth furnace, there is a temperature difference of at least 300° C. between the heating and reducing region and the feeding region in the furnace. This temperature difference is transferred to the iron sheet
29
by the rotation of the rotary hearth, and thus the iron sheet
29
repeatedly expands and shrinks. As a result, cracks form in the iron sheet
29
. When pressure by the screw
1
is applied to the cracks of the iron sheet
29
, a warp forms in the iron sheet
29
. The iron sheet
29
having a large warp catches on the screw
1
and is detached from the hearth (see “Detachment of Iron Sheet” in FIG.
13
). A grown detached iron sheet
29
inhibits discharge of reduced iron
10
by the screw
1
and causes problems such as shutdown (see “Shutdown due to Detachment of Iron” in FIG.
13
).
Furthermore, pits are formed on the moving heath during the formation and detachment of the iron sheet. Since the agglomerates are deposited on the pits, the depth of the fed agglomerates is not stable, and the agglomerates are not uniformly heated. Accordingly, the quality of the reduced iron is deteriorated.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to provide a method for operating a moving hearth reducing furnace for reducing iron oxide agglomerates incorporated with a carbonaceous material, which method does not substantially form an iron sheet on a moving hearth, remove iron oxide powder from the agglomerates, and enables continuous stable operation.
A method for operating a moving hearth reducing furnace in accordance with the present invention includes feeding iron oxide agglomerates incorporated with a carbonaceous material onto a moving hearth of a moving hearth reducing furnace, reducing the iron oxide agglomerates to form reduced iron agglomerates, and providing a gap between a discharging apparatus for discharging the reduced iron agglomerates from the moving hearth reducing furnace and the surface of the moving hearth.
Since metallic iron powder formed by reduction of the iron oxide agglomerate is not squeezed into the surface of the moving heath, no iron sheet is formed. An iron oxide layer formed on the moving hearth can be easily scraped off to renew the surface of the moving hearth so that the furnace can be continuously operated.
Preferably, the discharging apparatus is raised continuously or intermittently from the surface of the moving hearth in response to the thickness of an iron oxide layer formed on the moving hearth by oxidation of iron oxide powder included in the iron oxide agglomerates.
Since the iron oxide powder deposited on the iron oxide layer is not squeezed into the iron oxide layer, no iron sheet is formed.
Preferably, the discharging apparatus is brought into contact with the iron oxide powder deposited on the iron oxide layer on the moving hearth or metallic iron powder formed by reduction of the iron oxide powder, during the operation.
Preferably, the amount of the iron oxide powder fed with the iron oxide agglomerates into the moving hearth reducing furnace per unit time is determined, the amount of metallic iron powder formed by reduction of the iron oxide powder is determined, the amount of the metallic iron powder is converted to a volume A, and the discharging apparatus is raised so that the ratio A/B is 50 or less, wherein B is the spatial volume defined by the product of the increment of the height of the discharging apparatus and the area of the moving hearth. Preferably, a gap is provided between the discharging apparatus and the surface of the moving hearth or the iron oxide layer and the gap is ¾ or less the average diameter of the iron oxide agglomerates.
Preferably, the iron oxide layer on the moving hearth is periodically scraped off. Preferably, the surface of the iron oxide layer is preliminarily oxidized using an oxidizing burner and is scraped off by a vertically movable cutter provided behind the oxidizing burner.
Preferably, iron oxide powder included in the iron oxide agglomerates, metallic iron powder formed by reduction of the iron oxide powder, and metallic iron powder formed when the reduced iron is discharged from the furnace are evacuated together with exhaust gas through a duct provided in the vicinity of the discharging apparatus and a feeder for feeding the iron oxide agglomerates.
Preferably, the reduced iron agglomerates, metallic iron powder formed by reduction of iron oxide powder included in the iron oxide agglomerates, and metallic iron powder formed when the reduced iron is discharged from the furnace are simultaneously discharged from the furnace through the discharging apparatus.
Preferably, the discharging apparatus is a header blowing an inert gas or a reducing gas, and the reduced iron agglomerates and the metallic iron powder are simultaneously discharged from the moving hearth reducing furnace by blowing the inert or reducing gas in the radial direction of the moving hearth reducing furnace through the header.
Preferably, the discharging apparatus is an electromagnet unit which reciprocally moves in the radial direction of the moving hearth reducing furnace, and which attracts and discharges
Harada Takao
Matsushita Koichi
Tanaka Hidetoshi
Tateishi Masataka
Andrews Melvyn
Kabushiki Kaisha Kobe Sieko Sho (Kobe Steel, Ltd.)
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
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