Phase Change Cooling And Impact Surface Characteristics Of High-temperature Molten Blast Furnace Slag Droplet

The iron and steel industry is not only the pillar industry of China's economic development,but also an industry with high energy consumption and high pollution.The energy consumption accounts for more than 16%of the total industrial energy consumption in China.Blast furnace(BF)slag is one of the chief high-temperature by-product in the iron and steel making industry.The annual output of the slag is about340 million tons,and the sensible heat is as high as 1260 MJ per ton.At present,water quenching method is widely used for rapid cooling of molten BF slag,and the obtained glass slag is applied broadly in producing of cement to realize its resource utilization.However,it wastes a large amount of high-temperature waste heat and consumes a lot of water resources.To achieve the goal of both the the waste heat recovery and resource utilization,after decades of research,dry centrifugal granulation method has attracted extensive attention due to its advantages such as small slag particle size,high glass content of cooled slag,low energy consumption of treatment system and small pollutant emission.However,the technology is still at the stage of laboratory research,and there is no industrial application at present.Some critical issues limit the industrialization of the centrifugal granulation technique,such as the conflict of high cooling efficiency with low-grade recovered heat under large air flow rate,the adhesion of the granulated slag droplets and the insufficient capacity of centrifugal granulating device.Therefore,the phase change cooling and impact wall surface characteristics of molten BF slag droplet is studied in this work,to provide theoretical guidance for the development of the liquid slag dry centrifugal granulation and waste heat recovery technology.Firstly,the phase evolution process of BF slag under different cooling/heating modes is investigated by hot wire method and DSC technology.Then,air-cooled solidification process of BF slag droplet is experimental studied,the relevant rule of heat transfer equation is put forward,and combining the crystallization kinetics equation,the coupling phase evolution model of molten BF slag droplets air cooling solidification is established,the effect of the technological parameter on the internal temperature of the slag and the distribution of glass phase is disclosed.The work provides design consideration for granulating slag cooling process.Secondly,visual experiments of molten BF slag droplet impact wall surface is employed to obtain the temperature and morphological evolution laws of the droplet.The influence mechanism of the materials and properties of the wall on the impact behavior of slag droplet is explored to clarify the design basis of anti-adhesion on wall surface of centrifugal granulation device.Finally,the dynamic process of droplet swarm impact wall surface in centrifugal granulation of molten BF slag is captured,the motion law of slag droplet impact vertical water wall is revealed,the adhesion probability model of BF slag droplet swarm impact wall is built,the adhesion characteristics of BF slag droplet swarm and heat transfer regulation is revealed,empirical correlation of heat transfer between the wall and the droplet swarm is discovered.The results can supply important references on the wall design and operation parameters control of centrifugal granulation device.The main conclusions are as follows:1.The TTT(Time Temperature Transformation)curve of the BF slag is in the shape of"C".As the increase of supercooling degree,the crystallization driving force increases,however the solute diffusion resistance increases,the nucleation and growth rate increases first and then decreases with the temperature.The crystallization process of slag can be predicted by using JMA crystallization kinetic model.The higher the cooling rate,the lower the initial crystallization temperature.Combined with the equivalent crystallization time treatment method in non-isothermal crystallization process,the regularity of crystallinity variation with time in the constant speed cooling process of BF slag can be predicted by the cooling crystallization kinetic model.The critical cooling rate of complete glass formation is 48.9°C·s^-1,and the maximum cooling rate of complete crystal formation is 13°C·s^-1.In order to prevent crystallization reaction in heat process,for glassy BF slag,the heating temperature should be avoided to rise above T_g.For semi-molten BF slag which has not crystallized but the temperature is still higher than T_g,crystallization is more likely to occur and heating should be avoided as far as possible.2.To realize both the rapid cooling of slag droplet to form glass slag and the increase of air temperature as much as possible,the design principle of slag droplets air cooling process can be summarized as follows:in the temperature range(about1012-1322 ^oC)where slag is prone to crystallization,cool air with low temperature and high speed should be used.When the slag droplet or particle is above or below this temperature range,the air temperature can be appropriately increased and the wind speed can be reduced,especially when the temperature is lower than the glass transition temperature,the slag can be cooled slowly to increase the air temperature.The higher the temperature of slag droplet will led to the higher air-cooled heat transfer coefficient.When the cooling rate is low,the glass content decreases from the outside to the inside in the radial direction,and the droplet diameter has the most significant influence on the crystallization behavior.Reducing the droplet diameter is the most effective measure to improve the glass content.3.Five morphological evolution modes are defined based on the final stage of the slag film as deposition,receding,prompt-splashing,retraction-deposition and retraction-splashing.An impact regime map is established with the Reynolds number and dimensionless initial temperatures as the coordinates and six boundaries dividing the different modes are identified in the regime map.The apparent temperature nephograms of the slag film are extracted by a home-developed optical calibration method.The dynamic spreading processes and temperature distributions of the different regimes are not simply determined by the initial impact parameters,the coupling effect of the flow of the high-viscosity liquid slag and the heat transfer at the interface will obstruct the consecutive morphological evolution.The key to avoid slagging is to prevent the droplet from fully spreading in the impact process,and the key to fully spreading of the slag droplet is to overcome the surface energy barrier.The dimensionless surface energy barrier of the molten BF slag droplet impact quartz glass is about 2.72.When droplets impact inclined wall surface,they would slip,and the spreading and breaking of the liquid film would be inhibited,but the spreading time would be prolonged.4.Under the condition of normal temperature,the apparent static contact angles between the slag droplet and different surfaces were greater than 90°.The wall temperature is higher,the apparent static contact angle is smaller and adhesion force between the slag droplet and the surface is greater.The 310s stainless steel with surface roughness of 0.1-0.35?m is a suitable wall material wall for the granulation device.When the surface is too smooth,the slag droplet will fully spread.When the surface is too rough,it will cause slag physical adhesion onto the wall.For the 310s stainless steel,the influence of the slag wettability on the dynamic behaviors of liquid film is dominant,while the influence of solid-liquid heat transfer is secondary.To prevent droplet wetting the wall and weaken droplet spreading and adhesion,the wall surface temperature should be maintained at least below 550 ^oC.The inclined wall can prolong the spreading time of the impacting droplet,then the cooling effect of the wall will be strengthened,and then the resistance of the liquid film in the retraction process will be increased.The result is not conducive to the droplets to break away from the wall surface as soon as possible.Vertical wall should be installed in the granulation device.5.In the centrifugal granulation system,the slag droplets impingent will go through the process of spreading,retraction and rebound.The maximum spreading area of slag droplet A_m and the corresponding time?_m,the total residence time of slag droplet on the wall?_a and the recovery coefficient e when falling off the wall are all related to droplet diameter D₀ and linear velocity of the granulator u₀.The relation between the spreading area of slag droplet and time A(?)can be described by Nelder function,and the relevant undetermined coefficients are related to D₀ and u₀.In this paper,the function relation between the above parameters and D₀ and u₀ are obtained.Based on the coalescence probability model of slag droplet swarm impact wall surface,the specific evaluation parameters,such as the probability of droplet collision with different diameters P_ij,the overall probability of droplets coalescence P,the corrected average diameterD₃'₂,and the droplets coalescence index Rs,are systematically proposed.For the centrifugal granulation process,it is recommended that the droplets coalescence index should not be higher than 0.1.In order to increase the capacity of centrifugal granulation and slow down the slag adhesion,external measures should be taken to increase the height of the wall impact area,and the diameter of granulation device can be appropriately increased.At the same time,the water wall should be effectively cooled to avoid increasing the residence time of slag on the hot wall.With the increase of linear velocity of the granulator,the slag temperature in the process of impact wall can drop 50-120°C.When the coalescence index is not higher than 0.1,the heat transfer between the granulated slag and the wall can be calculated by the linear velocity of the granulator u₀ and the slag flux,according to the proposed empirical.