Reduced iron obtained by reducing iron-making dust or sludge to remove impurities, such as zinc, is reused as a raw material or the like of a blast furnace. FIG. 9 is a view showing a rotary hearth-type reducing furnace 100 used for reducing iron-making dust or sludge.
The rotary hearth furnace 100 comprises a hollow annular furnace 200, an annular hearth 300 swiveling in the furnace, and a predetermined number of burners 400 provided on the side wall of the furnace 200. The furnace 200 is lined on the inner side of the iron-made outer wall 210 with an alumina-silica refractory 220 to with stand high temperatures of 1,000 to 1,300° C. in the furnace.
The composition of the refractory is disclosed in Unexamined Japanese Patent Publication (Kokai) No. 06-206764.
When iron-making dust or sludge caked into a pellet form is supplied to the hearth, hearth 300 with pellets 500 thereon swivels in the furnace. During swiveling in the furnace, impurities such as zinc contained in the pellet 500 are removed by the heating and warming from the burner 400. Then, the pellet 500 deprived of impurities is recovered.
The thus-recovered pellet 500 is recycled.
The iron-making dust or sludge contains an alkali component such as sodium (Na) or potassium (K) in the form of oxide, carbonate, chloride or the like, and therefore, when the pellet 500 is heated and warmed in the furnace, the alkali component in the form of oxide, carbonate, chloride or the like vaporizes as it is, or sodium (Na), potassium (K) or the like after decomposition vaporizes, and as a result, the gas in the furnace contains an alkali component. The alkali component vaporized intrudes into the refractory and reacts with alumina and silica in the refractory. When the alkali component reacts with alumina and silica in the refractory, kaliophilite (K2O.Al2O3.2SiO2), leucite (K2O.Al2O3.4SiO2) and the like are produced as reaction products causing great volumetric expansion. This volumetric expansion generated partially in the refractory 220 has a problem that the refractory 220 falls off and the life of the refractory 220 decreases.
For example, Japanese Unexamined Patent Publication (Kokai) No. 06-206764 discloses a refractory used in a high-temperature alkali atmosphere, where the component percentages are from 92 to 98 mass % of Al2O3 and from 2 to 8 mass % of CaO.
According to such a construction, the grain boundary bonding part which is 6Al2O3·CaO is supposed to coat Al2O3 forming the aggregate, so that the reaction and erosion of Al2O3 by the alkali component-containing gas atmosphere can be prevented from proceeding.
However, when the pellet 500 is heated and the temperature is elevated, water vaporizes from the pellet 500 and a problem arises that according to the component percentages disclosed in Japanese Unexamined Patent Publication (Kokai) No. 06-206764, CaO contained reacts with the water to cause volumetric expansion associated with slaking of the refractory and the structure of the refractory cannot be maintained. Accordingly, a refractory having high durability is demanded as the refractory of a furnace for reducing the iron-making sludge.
An object of the present invention is to provide a long-life refractory capable of maintaining durability under severe conditions, such as an alkali component-containing gas atmosphere at high temperatures.