Specialized metallurgical processes – compositions for use therei – Compositions – Solid treating composition for liquid metal or charge
Reexamination Certificate
2000-07-10
2001-10-16
Andrews, Melvyn (Department: 1742)
Specialized metallurgical processes, compositions for use therei
Compositions
Solid treating composition for liquid metal or charge
C075S010310, C075S484000, C075S504000, C075S656000, C075S961000
Reexamination Certificate
active
06302938
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention concerns a method of producing reduced iron pellets formed by reducing iron oxide pellets containing Zn ingredients. More in particular, the invention relates to a method of producing reduced iron pellets formed by reducing iron oxide pellets containing dusts (including sludges) containing Zn ingredients formed in iron works.
2. Description of the Related Art
In iron and steel making processes including blast furnaces, converters and electric furnaces, various kinds of iron oxide-containing dusts or sludges are formed in a great amount which are recovered and reused as iron sources (used in the form of pellets or sintered ores). However, iron oxide-containing dusts or sludges resulting from iron and steel making processes may contain noxious Zn ingredients such as Zn oxide, to result in a problem that iron sources of high Zn amount can not be used as the starting material (hereinafter, used as the meaning of feed material) for blast furnaces. In the blast furnace, if the starting material contains a great amount of Zn, the operability of the blast furnace is worsened, as well as it leads to a problem that Zn attacks refractories of furnace walls.
As described above, use of dusts or sludges containing Zn-containing iron oxide has been restricted and, particularly, iron oxide-containing dusts or sludges containing Zn ingredients formed in a great amount from blast furnaces have not been effectively recovered and reused as iron sources at present.
In view of the above, methods of producing reduced iron by removing Zn from pellets which are molded from iron oxide-containing dusts or sludges resulting from iron and steel making processes have been proposed in recent years. There can be mentioned, for example, a method of using a rotary kiln furnace (refer to Japanese Patent Examined Publications Sho 51-13083 and 55-21810) and a method of using a rotary hearth furnace (refer to Japanese Patent Unexamined Application Hei 5-125454 filed in Japan corresponding to U.S. Pat. No. 5,186,741).
The method of using a rotary kiln has a merit in that Zn can be decreased to as low as 0.01 mass % and metallization ratio can be improved to about 80% (about 87% as a reducing ratio). However, since pellets are tumbled and baked in a rotary kiln in this method, results in a problem that pellets are powderized during operation, which are melted and deposited to form kiln rings, thereby making the operation impossible. Furthermore, since the total amount of Zn in the starting material fed to the blast furnace is controlled (for example, 0.2 kg Zn/t-pig iron), and the amount of using reduced pellets containing Zn is restricted, the amount of Zn has to be further decreased for use as the starting material for the blast furnace also in this method.
Than, in a method of using a rotary hearth furnace described in Japanese Patent Unexamined Application Hei 5-125454, reduced pellets are produced by forming green pellets comprising a mixture of dusts from steel works, carbonaceous material such as coal or coke and an organic binder, feeding the green pellets on a layer of pellets baked in a rotary hearth furnace, drying them at a temperature lower than 900° C. for 10 to 15 min, thereby forming coked dried pellets and then reducing the pellets at a temperature lower than 1150° C. for 20 to 30 min. While Japanese Patent Unexamined Application Hei 5-125454 describes for the reduced pellets that iron oxide is reduced to a metallic state and Zn ingredient is removed from the reduced pellets, it does not teach about the actual extent of reduction ratio and the amount of Zn. Furthermore, the method does not define the contents of iron and Zn in the pellets and the amount of carbonaceous material to be added. As can be seen from the description that the reduced pellets, when discharged from a rotary hearth furnace at about 1000° C., may possibly include a considerable amount of carbon (as high as 12 mass %) when they are discharged from the rotary hearth furnace), it is suggested that a great amount of carbon (as much as 12% by weight) may remain after the reduction in this method. Residue of a great amount of carbon brings about a problem that not only the content of iron in the reduced pellets is decreased to lower the utilizing efficiency as an iron source, but also the strength of the reduced pellets per se is deteriorated. Particularly, when coal is added as the carbonaceous material, it tends to greatly deteriorate the strength of the reduced pellets per se. If the strength of the pellets is lower, it results in a problem that the reduced pellets are pulverized or crushed in the blast furnace, to lower the air ventilation in the blast furnace and worsen the blast furnace operation, so that the pellets can not be used as the starting material for the blast furnace.
SUMMARY OF THE INVENTION
An object of the present invention is to provide reduction pellets with less Zn content as noxious ingredients, having appropriate grain size and strength and with high reduction ratio as the starting material for the blast furnace, by using iron oxide-containing dusts (including sludges) formed from iron works, particularly, those blast furnace dusts (which contain carbonaceous material but can not be used as the starting material for the blast furnace because of Zn ingredients contained and finely particulate form thereof) and other dust containing Zn ingredients, as well as a method of producing them.
In the method of producing reduced pellets according to a preferred embodiment of the present invention, reduced iron pellets are produced by controlling a carbonaceous material such that the amount of carbon is from 7 to 60 mass % based on the total amount of iron and Zn in a starting material mixture containing one or more of dusts containing iron oxide and Zn oxide and a sufficient amount of a binder to bond the dusts and then adding water to prepare them into green pellets incorporated with the carbonaceous material. Then, reduced iron pellets are produced by drying the thus prepared green pellets into a reduction furnace, heating the dry pellets by heat conduction, mainly, irradiation such that a temperature elevation rate is from 3 to 13° C./sec within a temperature range of the pellets from 150 to 900° C., reducing Zn oxide and evaporating Zn and reducing iron oxide.
In this case, the temperature elevation rate of the pellets is increased and the amount of carbon charged in the pellets is optimized. Accordingly, the shape of the pellets upon reduction can be retained and, as a result, the reduction ratio of the reduced pellets can be improved and pulverization of the pellets can be prevented during reduction.
Further, it is preferred to reduce iron oxides and Zn oxides at 1100 to 1350° C.
In this case, since the Zn oxide is reduced preferentially to the reduction of the iron oxide at a temperature higher than 1100° C., the amount of Zn in the pellets can be decreased remarkably. Further, since the reduction ratio is improved and the sintering of metallic iron is proceeded, the strength of the reduced pellets can be increased.
Further, as the dusts, carbonaceous material-containing blast furnace dusts, converter dusts, sintering dusts, electric furnace dusts or a mixture thereof can be used. Use of the dusts can decrease the amount of industrial wastes.
It is preferred to use a carbonaceous material comprising a coke ingredient.
In this case, since the coke has no substantial volatile ingredients, volatile ingredients are not evaporated in the above-mentioned temperature range causing less reduction of pellets and sintering (150-900° C.). As a result, since the elevation of the gas pressure along with the evaporation of the volatile ingredients is not caused and the pellets are not pulverized, temperature elevation rate of the pellets can be increased. Particularly, when blast furnace dusts containing the coke ingredients as the carbonaceous material are used, use of additional carbonaceous material such as coal or coke is no more required, so th
Iwakiri Haruhisa
Kamijo Tsunao
Miyagawa Kazuya
Shimizu Masakata
Shirouchi Shoji
Andrews Melvyn
Kabushiki Kaisha Kobe Seiko Sho
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
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