| In the iron-making sector, vast amount of blast furnace(BF) slag is emitted at the temperature of 1450-1550 oC. The BF slag carrying substantial high-grade heat is regarded as the last untapped waste heat in iron and steel industry. Nowadays, the molten BF slag is conventionally treated by water quenching, in which the water spray is employed to fast cool the slag. The water quenching method can produce glassy slag suitable for cement manufacturing. However, it fails to recover the waste heat from molten BF slag, but also leads to freshwater consumption and pollution emissions. Therefore, dry slag granulation(DSG) has been proposed as an alternative to water quenching method. The DSG technology involves with two-step strategies:(i) turning the molten slag into small particles with high surface-to-volume ratio and(ii) recovering the heat from slag particles through air. Among the DSG technologies, centrifugal granulation integrated with heat recovery has been recognized as the most promising method for slag treatment. In principle, this method adopts rotary atomizer to crush the BF slag into particles and then harvest the waste heat from the as-produced particles. In this regard, the particle size is of considerable importance for subsequent heat recovery. Thus, a thorough knowledge of the influence of operational conditions, liquid properties and atomizer configurations on the granulation performance contributes to the development and deployment of DSG. Also, designing novel atomizers for efficient granulation paves the way to commercialization of the DSG technology.Unfortunately, the feasibility of DSG has never been testified and the understanding of centrifugal granulation mechanism still remains unclear. Thus, in this thesis, a feasibility analysis of DSG has been performed. Besides, both the experimental and computational investigations into the centrifugal granulation mechanism have been carried out. The main works include:(1) Feasibility analysis of DSG. Life cycle assessment has been implemented on the BF slag treated by DSG. The energy consumption, resource consumption, environment impact and economics involved in this technology have been obtained. A comparative study between the DSG and the water quenching methods including OPC, INBA, TYNA and RASA has been conducted. The results indicated that the DSG can help lower the energy and resource consumption, incur less impact on environment and enable better economic benefit.(2) Experimental study on the centrifugal granulation mechanism. The influences of operational conditions, liquid physical properties and atomizer configurations on granulation performance has been identified. For operational conditions, it can be found that the rotary speed almost dominates both ligament formation and droplet formation, while the liquid flowrate only matters to ligament formation. For liquid properties, as the liquid viscosity increases, the granulation modes would transit from direct droplet formation to ligament formation, and end in sheet formation. The droplet size is larger for liquids with higher viscosities and the corresponding critical flowrate from ligament formation to sheet formation is less. For atomizer configurations, it has been found that cup-like atomizer can inhibit film formation and decrease the droplets size. A follow-up hot experiment with rosin and wax mixture as the working media, has been carried out to analyze the granulation performance of BFS analogously. From the hot experiment, the slag wool formation is caused by ligament solidification. Increasing the liquid temperature, decreasing the liquid flowrate or increasing the rotary speed contributes to more slag wools. Finally, a rotary atomizer combined with crushing bars has been developed to inhibit the slag wool formation.(3) Simulation study on the centrifugal granulation mechanism of BF slag. A 3-D model has been established to reproduce the centrifugal granulation process of BF slag. One of the major findings is that the slag film thickness has a positive effect on the final particle size. Thus it's applicable to evaluate the granulation performance according to the slag film thickness. On the basis of 3-D results, another 2-D model has been promoted to investigate the film flow characteristics of BF slag more efficiently. The concept of atomizer-profile functions has been proposed to describe the atomizer configurations, and the slag film thickness variation on the atomizer with different configurations has been studied accordingly. For linear-function atomizers, rotary cups with an angle of 40-60o and depth of 10-12 mm can lead to thinner slag film. For nonlinear-function atomizers, the sinusoidal atomizer can surprisingly decrease the slag film thickness by more than 80% in comparison with that of rotary disc atomizer. The variation of liquid slag film thickness on atomizers with various configurations can be attributed to the different length of liquid motion trajectory on them. The theoretical analysis indicates that longer motion trajectory length leads to thinner film. This finding can serve as the guideline for designing an optimal atomizer targeting for good granulation. |
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