The Characteristics Of Flow And Heat Transfer Of Liquid Slag On Granulator And The Formation Of Slag Crust

Blast furnace slag is the main by-product in the process of steel production.Due to the high temperature and huge annual output of BF slag,which contains a large number of high-grade waste heat resources.the main technology of slag treatment is water quenching for the time being.This method not only wastes water resources and causes environmental pollution,but also fails to recover slag waste heat effectively.on account of this,a variety of dry processing processes have been developed.centrifugal granulation technique has been widely studied because of its advantages of waste heat recovery and material utilization,simple and compact equipment and low energy consumption.Centrifugal granulation is the most critical segment in the process of centrifugal granulation of liquid slag,among which,the liquid film temperature decreases and the granulator temperature increases when the molten furnace slag flows on granulator.The decreasing temperature of liquid film will affect the granulation performance.The increasing temperature of granulator has great influence on the safety and stability of granulator.there is a lack of research on the flow heat transfer characteristics and forming crust characteristics of slag liquid film on granulator up to now.Aiming at this problem,this study has done some researches to provide promising theoretical guidance for the selection of operating conditions in the process of centrifugal granulation.In the present study,the system of flow and heat transfer of low temperature working medium on granulator was designed and built.Taking mixture of the rosin and paraffin as the simulated working medium,and the experimental study on the flow and heat transfer of which was carried out,and then obtained the flow and heat transfer characteristics on granulator of high viscosity fluid with molten paste zone.Subsequently,the similar experiment was also carried out by using the molten slag as working medium,and the flow and heat transfer characteristics of liquid slag on granulator were obtained,and the influence of slag initial temperature,granulator rotation rate and flow rate were discussed as well.On this basis,refractory cement coat on granulator was made,and the effect of which on the thermal protection of granulator was investigated.In addition,the slag crust was measured and characterized,and the changes of the morphology,section structure,thickness distribution and crystal content of the slag crust were analyzed.The main conclusions are as follows:(1)For the simulated working medium,The temperature field of liquid film was influenced by the external heat transfer conditions and its own flow state.The temperature distribution of granulator was influenced by the heat transfer of liquid film and its own structure.In a non-inertial system,the trajectory of the liquid film was a spiral rising from the bottom of the granulator.The surface temperature of the liquid film gradually decreased outwards from the center at the bottom of granulator,and reached the lowest at the corner.when the liquid film turned to the side wall,its temperature rose rapidly and then decreased gradually.The temperature of the inner wall of granulator was the lowest at the corner,and at the other positions,the farther away from the corner,the higher the temperature of the inner wall of granulator was.Influenced by the changing of the temperature difference between film and wall and the physical property parameters of liquid film,with time increasing,the surface temperature of the liquid film rose slightly first and then gradually became stable,the temperature of the inner wall of granulator rose gradually,but the rate gradually decreased.Finally,the convective heat transfer coefficient was obtained,the results showed that the average convective heat transfer coefficient was the largest at the center of the granulator,and was the smallest at the corner.(2)For the liquid slag,the flow and heat transfer characteristics of which were similar to that of the simulated working medium,but had differences due to the production of slag crust.At the initial stage,the flow of liquid film was disordered,and the distribution was not uniform with the slag crust forming and falling off.The surface temperature of the liquid film reached the lowest at the corner and when it turned to the side wall,its temperature rose,but the degree of temperature increase is also related to the release of latent heat caused by the formation of slag crust.The distribution and variation of the wall temperature of the granulator were also similar to that of the simulated working medium,but the temperature rise rate was also affected by the generation and remelting of the slag crust.The heat transfer was dominated by convection heat transfer,and the heat transfer coefficient first decreased and then slightly increased with time increasing.The change of operating conditions would cause changes in the initial state of molten slag liquid film or heat transfer conditions,resulting in changes in the surface temperature of the liquid film,the temperature of granulator and the heat transfer coefficient.The addition of 5 mm refractory cement coat could effectively reduce the temperature of the granulator from 900? to 600?,and had a good thermal protection effect on the granulator.(3)The liquid slag solidified during the flow and heat transfer process on granulator and the slag crust is formed,and there was a fingerlike pattern on slag crust surface.Influenced by the solidification rate of slag film,there were many tiny pores on the surface of slag crust on the side wall,and the higher the initial temperature of slag,the greater the rotating rate and the larger the flow rate,the more pores there were,but the pores in slag crust on the bottom were mostly sealed.Influenced by the thickness and the surface temperature of the liquid film,the thickness of slag crust gradually increased outwards from the center at the bottom of granulator,and reached the maximum at the corner,and turning to the side wall,the thickness of slag crust continued to increase and then gradually decreased.The higher the slag initial temperature and the higher the granulator rotation rate and the larger flow rate,the thinner the slag crust was.The main crystal precipitated from the slag crust was akermanite-gehlenite.Affected by the cooling rate,the crystal content of the bottom slag crust was lower than that of the side wall slag crust.The higher the slag initial temperature and the higher the granulator rate and the less the flow rate,the lower the crystal content of slag crust was.