Granulation of molten slag for heat recovery

A large amount of various molten oxides, such as steelmaking slag and lava, is cooled without heat recovery. For example, the amount of slag as a byproduct of steelmaking, over 1723 K in temperature, reaches 30 million tones annually. Although the systems for efficient heat recovery from molten slag have been proposed, the technology has not established yet. Conventionally, the treatments of a blast furnace (B.F.) slag were typically wet granulated or air/water cooled in open pits. This treatment is mainly used for producing a raw material of the B.F. cement, aggregate and, the roadbed. However, this method has some serious problems to be overcome, as follows: (1) Too much water is necessary to granulate the molten slag of high temperature. (2) An alkaline element in the slag is polluting the water. (3) Sulfide is emitted from the slag into an air by the water quenching. (4) A post process for drying slag is a must after the water quenching. (5) The thermal energy of high temperature slag is wasted without recovery. To solve these problems, we studied the dry granulation of the molten slag by rotary cup atomizer (RCA). The molten slag was supplied to the center of RCA. The rotating cup forces the slag outward to the cup lip where it is granulated with blasting. If reactant gas was blasted instead of air, sensible heat of molten slag can be recovered in the form of chemical energy (ex. C₄+H₂O□3H₂+CO ÄH= +206 kJ). The purpose of this paper was to study slag granulation for promoting heat exchange between slag and gas. In the experiment, the influence of the rotating speed and the shape of the cup on the slag drop size was mainly examined. The slag drops outward to the cup lip were collected by the receiver with separations and examined from viewpoints of shape, dimension and the flown distance of the drop. Most significantly, the molten slag was successfully granulated under the dry conditions without water impingement. The rotating speed of the cup influenced the diameter and shape of the slag drops very strongly. The higher rotating speed made the slag drops smaller, more spherical and uniform. Drops with 5 to 6 mm of average dimension were obtained at the rotating speed of 15 rps (900 rpm), and the drops with about 1 mm,- at that of 50 rps (3000 rpm). In the former case, the shape of drop obtained was distributed, changing from sphere to stick at the further place from the center of RCA. These results will be useful for establishment of the new heat recovery processes from the molten slag with many benefits.