Specialized metallurgical processes – compositions for use therei – Processes – Producing or treating free metal
Reexamination Certificate
2001-11-02
2004-06-29
Andrews, Melvyn (Department: 1742)
Specialized metallurgical processes, compositions for use therei
Processes
Producing or treating free metal
C075S313000, C075S484000, C075S770000
Reexamination Certificate
active
06755888
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a facility for reducing metal oxides, a rotary hearth reducing furnace which is the facility mentioned above, a method for reducing a metal oxide using the same, and to a method for reducing metal oxide-containing dusts and sludge generated by the metal refining or processing industry.
BACKGROUND TECHNOLOGY
There have been proposed a variety of processes for producing reduced iron or an iron alloy. Of these, there is one known as a process of high productivity that that includes the use of a rotary hearth reducing furnace, with which the reduction of a metal is effected. The essential part of this process is the use of a rotary hearth reducing furnace for reducing a metal oxide, the furnace being a firing furnace of the type in which, under a fixed fire resistant ceiling and side walls, a center-lacking-disk-shaped fire resistant hearth rotates along a rail at a constant speed (hereafter, referred to as a rotary furnace). Generally, the diameter of the disk-shaped hearth is from 10 meters to 50 meters, and its width is 2 meters to 6 meters.
After having been mixed with a carbon-based reducing agent, a metal oxide-containing powder as a raw material is pelletized to make raw material pellets and fed to the rotary hearth. The raw material pellets are spread all over the hearth. Since the raw material pellets are held stationary in relation to the hearth, this process has the advantage that the raw material pellets are hardly broken up in the furnace and therefore it is free from the problem of the raw material breaking up to form a powder which sticks to the fire resistant hearth. Moreover, this process has the advantage of high agglomerate production yield. Furthermore, since its productivity is high and a less expensive coal-based reducing agent and powdered raw materials can be employed, in recent years, the number of cases employing this method are increasing.
The rotary hearth method is also effective in reducing dusts generated through the process of steel manufacture in blast furnaces, basic oxygen furnaces, and electric arc furnaces, and in reducing thickener sludge generated in a rolling step, and in removing impurities therefrom. This method has been adopted for treating dusts and is effective in recycling materials.
How the rotary hearth method is carried out is described below in outline. Firstly, after metal oxides such as ores, dusts, or sludge as a raw material are mixed with a certain amount
of a carbon-based reducing agent required for the reduction of the oxides, the resulting mixture is granulated into pellets of from several millimeters to several tens of millimeters by a granulator such as a pan pelletizer while supplied with water so as to have an average moisture content of approximately 10%. In the case where the particle size of an ore as the raw material or that of the reducing agent is large, after having been crushed by a pulverizer such as a ball mill, the materials are kneaded and granulated.
The pellets thus obtained are fed onto the hearth of a rotary furnace in layers. The pellets spread all over the hearth are quickly heated and calcined at temperatures as high as 1100 to 1300° C. for 5 to 20 minutes, whereby the metal oxide is reduced by the reducing agent mixed in with the pellets to provide a metal. Depending on what kind of metal is to be reduced, the degree of metallization varies. In the case of iron, nickel, or manganese, the degree of metallization reaches 95% or higher. Even if the metal to be reduced is chromium, the reduction of which is difficult, the degree of metallization reaches or exceeds 50%. Moreover, in the case of treating dusts generated by the steel manufacturing industry, their recirculation back to a blast or electric arc furnace is easy, because such impurities as zinc, lead, alkaline metals, and chlorine are volatilized as the reduction reaction proceeds.
As described above, in the method for reducing metal or dusts generated in the process of steel manufacture through the use of a rotary hearth, it is necessary to pelletize the raw material and the reducing agent. It is also essential that, as a pretreatment of the raw materials, the mixture of a powdered metal oxide and a reducing agent as the raw material be brought into a condition suitable for granulation. As the pretreatment, milling of the raw materials, kneading with a ball mill, or any of a variety of techniques are carried out.
DISCLOSURE OF THE INVENTION
As described above, the reduction of a metal oxide with the use of a rotary hearth method using a conventional method is excellent with respect to productivity or production cost, so that a metal is produced economically. However, in the prior art, it was important to mix a raw material and a reducing agent and then pelletize the resulting mixture. Therefore, it was necessary to employ a material of high granularity or to install an expensive pulverizer in the production line for crushing the raw material to improve its granularity. This resulted in a rise in the production cost.
In other words, when an ore such as iron ore was employed as the raw material, due to its large particle size, it was in many cases granulated to form pellets after having been milled so as to have an average particle size of about several tens to a hundred microns. This involved the use of a costly device for crushing, electricity to drive the pulverizer, and the maintenance costs due to the wear of the pulverizer.
In some cases, a microfine powder raw material was employed for saving the cost of milling. However, due to the strict limitations on, e.g., particle size in the selection of the raw material, such a method was not in wide use. For solving such problem, the use of a wet-beneficiated powdery ore or thickener dusts generated in a blast furnace or basic oxygen furnace, scale pit sludge produced in the rolling step, or sedimentary sludge generated in the acid cleaning step is effective. However, even in this case, there was a problem that the moisture content of such raw materials was so high that it was hard to granulate them. In other words, since these raw materials are finely divided powders each having a particle size of from about 1 micron or less up to a hundred microns and thus liable to be sludgy when moisturized, even if dehydrated by a vacuum dehydrator or filter presser, their moisture content cannot be lower than 20 to 50%. The moisture content of the raw material upon pelletization is suitably about 8 to 13% by mass and therefore these raw materials collected according to the wet process were too moist to be granulated as they were.
One way to solve such problems as described above is to completely dry these raw materials using a heat source such as hot air. However, since these powdery raw materials are quasi-coagulated in the drying step and thus unable to be granulated as they are, after having been brought back into a micro-powdery condition again through milling, they were hydrated together with, for example, coke breeze, granulated, and reduced on the rotary hearth.
Accordingly, in the case of utilizing any of these raw materials collected by the wet process in such manner as described above, after drying with a large amount of heating energy, the material is moistened again and evaporation of moisture upon granulation entails the use of a heat source again. This is not at all economical as a method for reducing metal.
Especially, dusts or sludge generated by the metal refining or processing industry exemplified by the steel industry and collected from wet dust collectors or sedimentation tanks have, at a maximum, a moisture content of 80%. Attempts at reducing them in accordance with the rotary hearth method were faced with problems related to the treatment in the drying step and the milling or crushing step after drying.
For solving these problems, for example, as recited in Japanese Patent Laid-Open Publication No. Hei. 11-12619, there has been proposed a method including using a raw material in a rotary hearth-type reducing furnace without granulation,
Abe Yoichi
Hiromatsu Takashi
Ibaraki Tetsuharu
Imura Shoji
Kondo Satoshi
Andrews Melvyn
Nippon Steel Corporation
Wenderoth , Lind & Ponack, L.L.P.
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