Process And Mechanism For Centrifugal Granulation And Solidification Heat Transfer Of High Temperature Slag

Liquid blast furnace(BF)slag,as a by-product in the ironmaking production process,has large output and high discharge temperature,so it contains a large amount of waste heat that needs to be recovered urgently.The composition of the slag is similar to that of cement,and it can form a vitreous state under the condition of rapid cooling,which is used as an auxiliary raw material for cement production.The dry centrifugal granulation and waste heat recovery process with the advantages of low energy consumption,easy adjustment of parameters,and good granulation performance,is an optimal solution to realize the recovery of slag waste heat and resource utilization.The process is still in the laboratory stage,and there are still bottlenecks such as unclear granulation mechanism,slag particle compaction on the silo wall caused by deposition,and contradictory regulation between quality of slag particle and cooling air.In the present study,the centrifugal granulation of high-temperature liquid slag,the deposition of molten slag particles on the wall,and the mineral phase evolution of slag particles are respectively studied and correlated,in order to achieve the goal of "controlling particle size,preventing coalescence,and ensuring quality" and practical application for the process to provide basic data and theoretical support.Firstly,for the centrifugal granulation of liquid high-temperature slag,the volume-of-fluid(VOF)model is used to track the gas-liquid interface,and the shear stress transport k-ω model is adopted to deal with turbulent flow.Under the filament formation mode of liquid slag,the dynamic interface evolution of centrifugal granulation,the fracture mechanism of liquid filaments are clarified as well as the forming characteristics of granulated slag particles.Furthermore,blasting air is introduced to realize the centrifugal-air blast hybrid granulation,or the outer structure of the atomizer is adjusted to optimize the granulation performance.As a result.unstable capillary wave is the most fundamental cause of continuous breakage of liquid filaments.The perturbation and shearing effects of quenching air can optimize particle size,and flat-edge and arc-edge can be used to optimize particle size and suppress slag cotton,respectively.Secondly,for the deposition of molten slag particles on the wall,an experimental platform for droplet impact was first built.Relying on the viscous effect of the natural oxide film on the surface of gallium indium tin alloy,the cold experiment of substitute working medium was carried out.It was found that the droplet impingement experienced collision,spreading,retraction,and stabilization.The pinning effect would cause the front of the liquid film to be entangled in air.In addition,based on the VOF model and the solidification/melting model,the dynamic deformation characteristics of slag particles with high temperature,high viscosity and complex variable physical properties were obtained by simulation,which were in good agreement with the experimental results.Moreover,the selection of wall materials is clarified,and the heat transfer mechanisms such as temperature contours,solid-liquid phase distribution and heat transfer rate of the wall in the impingement process are clarified.Finally,the prediction correlations of spreading factor is established to characterize the maximum spreading of slag particles deposition on the wall.Then,for the mineral phase evolution of slag particles,the basic relationship between crystal phase content and cooling rate is introduced to the enthalpy method by self-programming,further realizing the coupling of cooling and crystallization under non-uniform cooling conditions.Considering the real characteristic temperature range and complex physical properties of BF slag,the crystallization behavior details such as slag particle cooling rate and crystal content distribution were obtained.Finally,the correlation formula for regulating and controlling the quality of slag particles is established.The results show that the surface cooling rate of the slag particles is approximately linear with the final crystal phase content within the working conditions of this study,and the critical value of the former is about 20 K/s.For the adjustment of various operating parameters and components,reducing particle size is very effective in improving the quality of slag particles.In addition,it is also very beneficial to modify the slag composition to reduce the optical basicity.Finally,for the parameters transfer and process association in the granulation chamber,discrete phase and realizable k-ε models are used.Considering the contact heat transfer of the slag particles impacting on the wall,the numerical simulation of the whole process of granulation-particle tracking-waste heat recovery in the granulation chamber is realized,which clarifies the importance of waste heat recovery in the initial cooling stage.Taking particle size as an intermediate parameter,two stages of centrifugal granulation and particle cooling are effectively connected in series.Finally,the critical indicators(particle size and vitreous phase content)of the pilot scale for the dry centrifugal granulation and waste heat recovery process were successfully predicted.