This paper studies the possibility of developing a new heat recovery system from various hot wastes generated by the steelmaking industry, by utilizing the endothermic heat of reaction instead of sensible heat. In the proposed system, the waste heat of the gas was first stored using a Phase Change Material (PCM), and then supplied to an endothermic, methane-steam reforming reaction (MSR) as a heat source. The molten slag was granulated using a rotary cup atomizer (RCA) and the sensible heat of the slag was recovered using MSR. A heat and material balance model was developed to evaluate this system and to predict all its operating data. An exergy analysis and an economic evaluation were conducted on the basis of the predicted data. The results showed that the exergy loss in the proposed system was only 15 % from the total exergy losses in the conventional system, and that the annual cost benefit of the proposed system totaled US$ 409 million from heat recovery, and US$ 1 945 million from slag granulation.
At the present molten slag from a blast furnace (B.F.) is granulated by impinging much water without any recovery of its much sensible heat (1 823 K), polluting water and atmosphere. To solve these problems, we studied the dry granulation of molten slag by Rotary Cup Atomizer (RCA), in which the influence of the rotating speed of the RCA on slag drop size was mainly examined. In the experiment, the molten B.F. slag was supplied to the center of RCA with air blast. Slag drops flown from the cup lip due to centrifugal force were collected and examined from viewpoints of shape, dimension and the flown distance of the drop.Most significantly, molten slag was successfully granulated under the dry conditions without water impingement. The rotating speed of the RCA influenced the diameter and shape of slag drop very strongly. The higher rotating speed made the slag drops smaller, more spherical and more uniform. Drops with 5 to 6 mm of average dimension were obtained at the rotating speed of 15 rps (900 rpm), and drops with less than 1 mm, at that of 50 rps (3 000 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. The results revealed a possibility of alternative, new slag granulation process with many benefits.
Steelmaking is well known to be one of the highest energy-consuming industries, where high temperature molten slag is discharged without any heat recovery. This paper describes the hot experiments where a Rotary Cup Atomizer (RCA) is used to produce dry glassy slag without water impingement. In this, the properties for granulated slag were chiefly investigated. Molten slag was first poured onto the center of the rotating cup at various rotating speeds. Slag granulation was then observed using a video camera, and finally, the particles were collected for physical and chemical analyses. The results of XRD and DSC analyses demonstrate that all slag drops obtained by the RCA method are undoubtedly glassy. The particle size of the granulated slag is strongly controlled by both the diameter of the cup and the speed of rotation. The relationship between the particle size and the two parameters is expressed as D p ¼ 16:86=r!. Smaller particles that produced at a higher rotating speed seem to be more transparent or glassy and have compression strength twice higher in comparison with water granulated slag. The data obtained will provide valuable information not only for producing glassy slag, but also for exchanging energy between gas and molten slag efficiently.
A mathematical model of a spinning disk atomizer (SDA) was developed to produce glass beads from high-temperature molten slag. The model comprises three parts: 1) fluid flow model of molten slag on the spinning disk, 2) physical model of ligament formation of slag, and 3) heat transfer model of slag drops dispersed from the ligament. First, a 2-D fluid flow model was developed to evaluate the film thickness of slag at the edge of the disk and was calculated using the scalar equation method (SEM). Next, the number and diameter of the ligaments formed were evaluated using the physical model. Finally, the heat transfer model was employed to evaluate the quenching rate and temperature distribution within the drop. The model developed was experimentally validated by comparing the calculated and observed values that were in good agreement. Most significantly, the model also estimated the quenching rate required for slag vitrification.
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