Motion And Separation Mechanism Of Particles In The Pulsating Air Flow Field

As a novel air separation technology, pulsating air separation utilises the motion of particles in unsteady air flows to overcome the difficulty of traditional air separation in the realisation of density based separation. Currently, it has already been applied to the recycling of electronic waste and industrial waste catalyst, and processing of non-metallic minerals. It also provides new idea and method to the mineral processing and recycling of secondary resource. However, since the unsteady flow renders the motion of air and particles complicated, there are still some unresolved issues with the mechanism of pulsating air separation, which hampers the further progress of this technology.The deficiency of existed pulsating air separation mechanism was analysed. One theory indicated that separation attributes to the acceleration difference of particles before reaching the terminal settling velocity. It is considered as lack of sufficient theoretical and experimental support and does not agree with practical separation process. Meanwhile, the theory of virtual mass force based on the dynamic equation of single particle cannot explain the density based separation of high density material. The correction of drag coefficient improve the theory though requires to be further studied. As a result, a new separation theory based on particle clusters was proposed in terms of the shortage of those existed theories. In this theory, dense phase zones in the separation column were considered to be responsible for the density based separation. The mechanism of pulsating air form was also discussed based on this theory.The large eddy simulation technology was applied to simulate the pulsating air flow over sphere. Drag coefficient in the pulsating air flow were obtained and compared to that in the steady air flow with same Reynolds number. The numeric results prove that the variation of drag coefficient depends on the Reynolds number and acceleration or deceleration of the air but not on the density of particles. The change of vortex structure in the pulsating air flow field was also investigated to analyse the reason for the change of the drag coefficient. The results show that the variation of drag coefficient is not the main reason for density based separation. To reduce the time in the low Reynolds number range and control the proportion of acceleration and deceleration are beneficial to the separation results.In order to further prove the theory based on particle clusters, the pulsating gas solid fluidised bed were used to investigate the motion of particles in a dense phase environment. Firstly, the effect of operational parameters on bed and bubble properties was investigated. The results show that pulsating air frequency plays a pivotal role in the steady of bed but does not affect the expansion rate of bed much. The increase of pulsating air frequency causes the rising generating frequency and size of bubbles. By observing the motion of foreign particles, bubbles were found to accelerate the falling of particles. The density based separation can be strengthened by virtual mass force which becomes increasingly significant with the rising pulsating air frequency and solids volume fraction.Finally,the separation experiment of-6+3mm hard-to-wash coal was conducted. 0.17 Ep value, 48.08% separation efficiency and 68.22% combustible recovery were obtained. Effect of pulsating amplitude, frequency and feed amount on the separation results were explained by the proposed separation mechanism. The concept of critical frequency and critical solids volume fraction were developed.Numeric simulation and experiment were combined to investigate the motion and separation of particles in the pulsating air flow field in this paper. A new pulsating air separation theory was developed and used to explain the density based separation mechanism of high density particles. The results can provide theoretical basis for the selection of operational parameters and device structure and process design. It also has some reference to other gas-solid flow which includes pulsating air flow.