Numerical Research Of Solidification And Heat Transfer Characteristics For A Molten Blast Furnace Slag Droplet

Due to energy shortage and environmental issues, it becomes inevitable for the development and utilization of new energy and the reduction of energy consumption. The steel industry is one of the industries own large energy consumption, nearly 10%-15% of the country whole energy consumption. Therefore to reduce steel industry energy consumption as a circular economy is significant. Blast furnace slag is a by-product of the steel industry, owning a high temperature between 1450?-1650?, which has a large amount of waste heat with high recovery value. Currently, quick cooling of the high temperature slag is realized by water quenching. However, distinct drawbacks are presented during the course of water quenching process, including a full waste of sensible and latent heat; discharge of harmful gases(such as hydrogen sulfide, sulfur dioxide and others) and huge waste of cooling water. Thus making the integrated dry centrifugal granulation and air cooling heat recovery technology has been a new process and has a good application prospect. For the centrifugal granulation method, its technical process can be described as follows. High-temperature liquid slag is directly poured into a high-speed container(cup/disk/cylinder) and then the slag is radially projected outwards due to the role of centrifugal force and subsequently broken into droplets due to the role of surface tension. At the same time air is blown to cool the droplets. This process can perfectly realize the purpose of the recovery from the slag, including the effective heat recovery and the reuse of the slag.As for the technology, the most important parts are the granulation of the slag and the recovery of the high grade heat. But the high temperature of the slag increases the difficulty for the granulation and the heat recovery process and becomes the main restriction factor for the laboratory study. Therefore, the heat recovery process still has not been experimently studied.As we know, the air cooling process of the slag is a very obvious phase change heat transfer problem. Consequently, a moving interface separating two different phases develops and absorption or release of latent heat happens in the vicinity of this interface. Mathematical modelling of such problems are always challenging task because of the complex boundary conditions, as well as varied thermal physical properties.Theoretic analysis on transient air cooling and solidification behaviors of a spherical molten slag particle is performed. With temperature model, the solidification behaviors are analyzed when the phase change occurs at a constant temperature; With enthalpy model, the solidification behavior are analyzed when the phase change occurs in a temperature range; Meanwhile, the solidification and heat transfer of air-cooling molten blast furnace slag droplet with two-dimension is numerically simulated by the volume of fluid(VOF) method coupling with the solidification/melting model. The characteristics of slag droplet heat transfer, solid-liquid interface movement of slag droplet, air flow and temperature field solidify are explored. The influence of the droplet size, the droplet initial temperature, air velocity and air temperature are discussed. The main conclusions of this paper are summarized as follows:(1)The variable thermal conductivity of the slag extends the solidification process, while the radiation significantly improves the cooling speed. Which makes the results are in better agreement with the practical physical process; Meanwhile, the solidification process is speeded up with increasing air flow rate and decreasing particle size. The cooling and solidification process is prolonged for the slag particle with higher initial temperature. However, as for the high initial temperature of the slag and the occurrence of the whole slag solidification process at a high temperature, the average ratio of radiation is 50%-60%, the increase of air velocities and the decrease of the initial air temperature has little influence on the solidification behaviors of the slag, but will play a important role in the following cooling process of the slag.(2)Compared with the constant phase change temperature, there exists a mushy zone, which is the coexistence of liquid phase and solid phase. The internal temperature cooling rate significantly decreases, resulting in a significantly longer time to complete the solidification process. The variation of the thermal conductivity with temperature shortens the solidification time of the slag particle, the consideration of radiation make the numerical results more reasonable for the high temperature BF slag. The increase of the spherical particle diameter reduces the moving speed of the phase change interfaces and significantly increases the solidification time. The higher velocity and the lower temperature of the cooling air, the higher the moving speed of the phase change interfaces and the shorter the solidification time of the slag particle. However, the influence is not significant as the radiation heat transfer owns a great proportion for its' high initial temperature.(3)For a single air-cooling slag droplet, the surface of slag is rapidly solidified with internal non-uniform solidification caused by the change of airflow field and thermal conductivity of the solid phase. The increase rate of the solid fraction will drops sharply after the 80% of slag completes the solidification, it will take a relatively longer time for the slag to complete the whole solidification process.(4)The diameters of the slag proves to a very important factors to the solidification process of the slag. For the smaller droplet, it takes short time for the solidification due to the strong heat transfer on the surface. The air velocity proves to be important for the solidification of the slag surface, but shows less influence on the whole solidification time of the slag. Therefore, the selection of the air velocity will take all the factors into consideration.