| Molten blast furnace(BF)slag is discharged during the iron-making process,and carried total energy of 1600 MJ/t with critically high temperature of 1450-1650 °C.In the traditional water quenching process,the molten BF slag is cooled fast to form vitreous product,which is used as substitute for portland cement clinker.However,this technique not only fails to recover the large thermal energy carried by the molten slag,but also consume a lot of water and contributes to the release of waste cooling water and waste sulfide gases.In order to overcome the disadvantages of water quenching method,some dry slag treatment technologies,coupling of dry slag granulation and waste heat recovery has been proposed by researchers.Since the dry slag treatment technologes have a much lower particle cooling rate than the water quenching technology.Reducing the particle size is the best way to increase particle vitreous content and improve resource utilization value,meanwhile enhance the heat recovery rate of the system.Among all these dry slag treatment technologes,centrifugal granulation technique is quite promising due to its advantages of simple and compact structure,fine slag particles,low energy consumption.Unfortunately,no commercial industrial application has been reported until now,because this technique still faces some critical challenges,such as formation of slag wool,serious slagging and producing of large particles,limited the industrial application of the technology.In this paper,experimental and numerical simulation studies were carried out on the centrifugal granulation mechanism and characteristics for molten BF slag.Which provides guidance for the designing of high-efficiency atomizer and anti-slagging granulation chamber in industrial application.In the present work,low temperature analog working medium was first adopted to study the effect of atomizer outer edge structure on the critical flow rate of fragmentation modes,as well as average droplet size.The granulator outer edge structure with the best granulation performance was determined.Then,authentic molten BF slag was used as working medium,and observed the granulation phenomena in different fragmentation modes.Moreover,the effect of rotating speed and slag mass flow rate on the particle size,slag wool mass fraction and slagging mass fraction was investigated.The resource utilization performance tests of granulated particles revealed that centrifugal granulation method for molten BFslag was practicable.Based on the experimental results,a threedimensional model of centrifugal granulation for molten BF slag was established to clarify the formation and fracture mechanism of ligament.Furthermore,a heat transfer enhencement method was proposed for the granulation chamber wall to suppress the slagging phenomenon.Meanwhile,the slag mass flow rate was increased to study the effect of operating conditions on average particle size and slagging mass fraction in film formation mode.Based on the experimental data,an empirical correlation was proposed to predict average particle size for molten BF slag in film formation mode.In addition,new centrifugal-blast air atomizer and new double layer rotary cups were developed,and the granulation performances were tested by using molten BF slag as working medium.The main results are as follows:For the low viscous working fluids(?? 0.043 Pa·s),the liquid film easily attached on the side wall of flat edge disk during the centrifugal granulation process.The side wall adhesion of liquid film greatly hindered the formation of film formation mode,and enhanced the critical volume flow rate significantly.The larger critical volume flow rate for the disks with the short-flat edge and long-flat edge allowed the produce of smaller droplets at operating condition with large fluid volume flow rates.The largest decreasing rate of average droplet size for the short-flat edge disk was ascertained as 60% for glycerol/water mixture(80%).The long-flat edge disk is suggested to granulate the low viscous liquid(?? 0.043 Pa·s).Otherwise,the short-flat edge disk is recommended to be employed to granulate the high viscous liquid(?? 0.085 Pa·s),such as the real molten BF slag.With the increasing of roating speed or slag mass flow rate,the fragmentation mode was gradually translated from ligament formation mode to film formation mode,during centrifugal granulation of molten BF slag.The majority of the centrifugal granulated slag particles were populated in the size range of 0.5-3 mm.The average particle size decreased with increasing rotating speed and decreasing slag mass flow rate.It was found that the rapid cooling and solidification of the ligament during the centrifugal granulation process induced the formation of slag wool.Increasing the slag mass flow rate was the most effectively way to reduce the slag wool.The insufficient cooling of the slag droplets during the flight resulted in the slagging of slag droplets on the granulation chamber wall,and the larger the slag mass flow rate,the higher the slagging mass fraction.The resource utilization performance tests of granulated particles show that the centrifugal granulated BF slag particles had a similar grindability but a weaker hydration activity in comparison with the water quenching BF slag.This indicates that the centrifugal granulated BF slag particles had a high resource utilization value.The established three-dimensional centrifugal granulation model can accurately predict the centrifugal granulation process of molten BF slag.The simulation results show that the increase of the rotating speed gave rise to the increasing of ligaments number,decreasing of ligament fracture length,reducing of the ligament fracture wavelength and ligament fracture diameter.Finally,the average droplet size was reduced.The slag mass flow rate had negligible affect on the ligaments number but the ligament fracture length almost increased linearly with the increasing of slag flow rate.Besises,the temperature of molten BF slag has little effect on the ligaments number and the ligament fracture length.Reducing molten BF slag temperature lead to the increase of ligament fracture wavelength,and the slightly increase of average droplet size.The fine granulation performance of film formation mode under large slag mass flow rate condition was ascribed to the special histioid disintegration of slag film,which contributed to the formation of smaller slag droplet.At large slag mass flow rate condition,the formation of slag wool was greatly hindered.The water-cooled plate on the inner wall of the granulation chamber greatly inhibited the slagging of slag droplets.However,a small fraction of slag droplets still bonded with each to form large particles.A larger granulation chamber radius results in lower slagging mass fraction and reduced the average particle size.Meanwhile,larger rotating speed and smaller slag mass flow rate results in samller average particle size and lower slagging mass fraction.The average particle size decreased with the increasing of atomizer diameter.Owing to the higher probability of increasing slagging mass fraction for a larger atomizer,a much larger granulation chamber should be adopted.In addition,it is found that the initial temperature of molten BF slag had little effect on the centrifugal granulation performance.But a lower initial temperature of molten BF slag results in a much lower slagging mass fraction and reduced the size of the granulation chamber.The most reasonable slag initial temperature was ascertained as 1400 °C.An empirical correlation to predict average particle size in film formation mode was proposed through experimental data fitting.The employing of blast air strengthened the ligament/film fracture process.With the presence of blast air,the length of produced slag ligaments was shortened in the ligament formation mode,the slag film was reticulated and the granulation processes was accelerated in the film formation mode.Under the granulation chamber wall heat transfer and large slag mass flow rate conditions.On one aspect,the slag droplets were dispersed ditributed in the space by the impinging effect of blast air,which diminished the slagging probability and reduced the average particle size.On the other aspect,the crushing effect of the blast air for the slag film/ligament directly reduced the size of slag droplets,thereby obtained smaller particles.The granulation tests of the double layer atomizer show that the double layer atomizer realized the stratified flow of slag film successfully,and increased the dispersion rate of granulated slag droplets in the granulation chamber,which inhibited the slagging of slag droplets.Moreover,the double layer atomizer directly reduced the size of the granulated slag droplets and increase the granulation capacity of a single atomizer. |
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