Study On Efficiency Improving Mechanism Of Pulsating Air Flow On Steel Slag Ultra-Fine Grinding

The iron and steel industry is the core of global economic development and the sustainable development in modern society.As one of the industries with"high energy consumption and high emission",a large amount of waste slag is produced in the iron and steel production process,which accounts for about 15%-20%of the crude steel output.However,the comprehensive utilization ratio of steel slag in China is relatively low,and there is a large gap between the utilization level of China and the developed countries in the world.A large amount of steel slag discharge and accumulation will not only occupy valuable land resources,but also cause serious environmental pollution and ecological damage due to the release of alkali metals,heavy metal ions and dust.Ultra-fine grinding is an important means to realize the resourceful and high-quality utilization of steel slag.However,due to the hardness of steel slag,the complexity and high cost of ultra-fine grinding process,its comprehensive recovery and utilization are greatly limited.The steam jet milling process using industrial waste heat steam as the comminution power can significantly reduce the grinding cost of steel slag.In view of the problem that particles and iron-bearing phase minerals are easy to accumulate in the grinding chamber during the ultra-fine grinding of steel slag by steam jet mill,which resulting in high circulating load and low grinding efficiency,this paper introduces pulsating air flow from the bottom of the grinding chamber to improve the fluidization state and distribution of particles in the chamber,increase the gas-solid concentration in the comminution area,increase the probability of particle collision,and improve the energy efficiency.The pulsating energy drives the particle bed to move back and forth,and the particles are arranged in different beds according to the density,to increase the concentration of iron at the bottom of the grinding chamber.At the same time,it can improve the properties of the particles involved in the grinding in the comminution area,to improve the grinding efficiency of the system and improve the quality of the fine powder products.The enriched iron phase minerals can also be recycled through further processing.This study provides technical guidance for improving the output of steam jet milling,saving energy consumption,and practicing sustainable development in the steel industry.A single particle impact comminution test system was built to carry out the single particle impact comminution test for narrow size fractions,and three impact breakage models of steel slag particles based on Weibull distribution were fitted.Then,based on the kinetic energy carried by the high-speed airflow generated by nozzle as the energy input benchmark of the jet mill,three energy efficiency models for the jet milling process are established.Use the air jet milling system in the laboratory to conduct the conditional grinding test on the coarse and fine narrow size fractions,compare and verify the applicability of the three energy efficiency models.Finally,the betterη_EM model is used as the evaluation standard of energy efficiency.Installing a pulsating air flow generator at the bottom of the grinding chamber in the laboratory jet milling system.The influence of grinding conditions and pulsating air flow conditions on the comminution characteristics of steel slag particles was studied by single factor experiment,and the corresponding optimal factor level was determined.Response surface method was used to analyze the significance and interaction of various factors on the grinding efficiency and energy efficiency,and the mathematical prediction models between the grinding efficiency,energy efficiency and various factors were established respectively.The best operating conditions were obtained by using the model,and the accuracy of the prediction model was verified according to the grinding experiment.Finally,when there is no pulsating air flow,the grinding efficiency obtained at the optimal factor level is 2.86 kg/h,and the energy efficiencyη_EM is 0.43%.After applying pulsating air flow,the grinding efficiency obtained at the optimum factor level is 3.62 kg/h,and the energy efficiencyη_EM is 0.52%,increased by 26.57%and 20.93%respectively,with a significant improvement effect.Simulation analysis based on CFD shows that the impact of pulsating air flow on the high-speed jet in the comminution flow field is small.And the destruction of pulsating energy on the eddy current in the chamber improves the uniformity of the conveying flow field and the classification flow field.While,the introduction of periodic changes in the air flow leads to the fluctuation of the radial velocity of the classification wheel with time,which making the classification effect unstable.In addition,CPFD was used to study the particle motion and spatio-temporal distribution.The results showed that pulsating energy could promote the fluidization of the particle bed and increase the particle collision probability.At the same time,the periodic movement of the particle bed under the action of pulsating energy can separate the fine powder particles produced by comminution and bring them into the classification area in time,so as to avoid the high circulating load and energy consumption caused by their circulation in the comminution area and the conveying area.This reveals the main mechanism of pulsating airflow to improve grinding efficiency.Based on TFe as the evaluation index,the influence of pulsating air flow on the enrichment and migration of iron-bearing minerals was studied,and the distribution of iron content in the particle bed under two conditions was compared and analyzed.Combined with particle motion analysis and spatio-temporal distribution in grinding chamber,the particles move back and forth under the action of pulsating energy,so that they can arranged according to the density and the difference of the movement stroke.The application of pulsating airflow strengthens the enrichment of iron containing minerals（heavy particles）at the bottom of the grinding chamber,improves the particle properties involved in the comminution area,and enhances the grinding efficiency and energy efficiency of the comminution system.At the same time,it can reduce the migration of iron containing minerals into the grinding product and improve the quality of the fine product.