While iron ore raw materials are used as the raw material for producing reduced iron as might be expected, recent price surge of iron ore raw materials due to strong demand has brought attention to recycling of iron raw material-containing dust produced in iron mills. In pig iron- and steel-making processes, powdery iron and iron oxide-containing dust blown up in blast furnaces, converters, melting furnaces, electric furnaces, and the like are recovered with dust collectors. The recovered powders (referred to as “steel-making dust” in Claims and the specification) are used as iron raw materials since they contain iron and iron oxides.
In a rotary hearth furnace, which is one of facilities for producing reduced iron, raw materials containing metal oxides and carbonaceous substances must be fed in the form of agglomerates to achieve homogeneous reduction reactions, and the agglomerates need to have a particular strength. However, when carbonaceous substances are incorporated, since carbon burns at a relatively low temperature, a technique of increasing the strength of the agglomerates by sintering, such as in the cases of sintered pellets and sintered ore, cannot be employed. Thus, the strength of the agglomerates has been enhanced by use of expensive binding agents (hereinafter also referred to as “binders”), such as starch and molasses.
Typically, either one of a pellet-making facility or a briquette-making facility is used as the facility to produce agglomerates, and, in either cases, agglomerates having high strength and as uniform size as possible must be produced. However, it has been extremely difficult to produce agglomerates from fine raw materials mainly for two reasons described below. The first reason is that the fine powder dust raw materials have a small bulk density and many gaps and thus do not readily form high-strength agglomerates. If the strength of the agglomerates is low, the agglomerates may collapse under pressure and thus cannot be stored in large quantities, and cracking and the like may occur during transportation. The second reason is the difficulty of homogeneously mixing a binder, which is mixed into the fine raw materials to increase the strength of the agglomerates. This is because a binder having an adhesive function has viscosity, and this viscosity obstructs homogeneous mixing. In addition, the binder materials are not only expensive but also can cause clogging of materials during manufacture or feeding (e.g., in an intermediate hopper or the like) of the agglomerates since the binder materials have the adhesive function. Once clogging of the raw materials occurs, the supply of raw materials to the rotary hearth furnace or the like stops or a large quantity of raw materials are suddenly fed, which renders the operation of the reducing furnace unstable. Thus, the amount of the binder used is preferably as small as possible.
In the description below, methods for producing pellets and briquettes of fine dust raw materials that have been known in the art are explained by citing examples.
Patent Document 1 teaches a method for producing pellets including kneading raw materials, which has been discharged from a raw material storage, by using a kneader and pelletizing the raw material, which includes grains containing a metal oxide and carbon, by using a pan pelletizer, as shown in FIG. 1 of Patent Document 1. In particular, a plurality of raw materials are fed onto a raw material conveyer from a plurality of material reservoir bins at a determined mixture ratio. The grain diameter of the raw material, the chemical composition, and the ratio of water to be mixed are determined. In particular, in order to adequately carry out reduction reactions, the ratio of the metal oxide to carbon is adjusted. The claims of Patent Document 1 disclose a method for producing pellets suitable for reducing furnaces, the method being characterized in that, in producing spherical pellets with a pan pelletizer from a powder containing a metal oxide containing steel-making dust and a carbon-containing powder, the powder contains 20 to 80% of grains having a diameter of 10 μm or less.
Patent Document 2 teaches a method for producing a solidified product from steel-making dust as shown in FIG. 1 of this patent document. As shown in this drawing, this system includes a granulating step of mixing the dust mainly composed of iron and its oxides produced during steel making with a powder mainly composed of carbon and pelletizing the resulting mixture, a water impregnation step of impregnating the mixture granules with water, a solidifying step of placing the water-impregnated pellets into forming dies and pressure-compacting the pellets to form briquettes, a transfer step of transferring the briquettes as the raw material for a melting furnace, and a dezincing step of concentrating zinc in the dust and dezincing the dust in the concentrated state after obtaining the dust from the melting furnace and before the solidifying step. According to this method, since impregnation with water is conducted immediately before molding, the surfaces of the mixture granules are softened to promote deformation of granules during briquetting, and the adhesion strength between the granules can be increased as a result.