Study On Particle Dynamics Of Impact Separation For Coal And Gangue Underground

Gangue, the accompanying mineral of coal mining, is always transported to the ground and accumulated as hillock. The gangue hillock causes many adverse effects on the surrounding environment. Achieving separation for coal and gangue underground and backfilling gangue in situ is the method that can radically solve the environmental problems produced by discharge and accumulation of gangue. With regard to this, impact separation technique for coal and gangue underground is presented depending on the differences of physical and mechanical properties for coal and gangue. With the advantages of high security, great treating capacity, small volume and low cost, this technique can reduce the environmental pollutions and bring down the transportation cost of coal mine as well as solve the subsidence caused by over-extraction partially. Therefore, the impact separation for coal and gangue underground has the significance for realizing the green mining. To provide theoretical basis for popularization and application of the technique, the theoretical analysis, numerical simulation and experiments are combined to investigate the particle dynamics behavior during the impact separation process in this dissertation.According to the Drucker-Prager yield criterion and based on the Hertz theory and the bilinear hardening elastio-plastic model, the initial yield model and impact contact model for impact between quasi-brittle spherical particle and rebound plate are built. The main influence factors and the influence law for initial yield response, impact contact response and the rebound distance are developed. Simultaneously, the impact rebound behaviors of coal and gangue particles are simulated by using finite element method and the variance rules of the differences for rebound velocities and rebound distances are developed.Taking coal and gangue particles from working face in Xiezhuang mining of Xinwen Mining Group as study objects, rebound distance experiments are carried out. The results indicate that the theoretical distance quite agree with the experimental result when the initial velocity is low, but is larger than the experimental result when the initial velocity is high because of the particle crushing. For coal and gangue in Xiezhuang mining, partial gangue can be separated by the difference of rebound distance when the initial velocity is higher than 7.5 m/s.Based on fractal theory, weakest link model and impact dynamics theory, the fracture crushing probability model and the crushing granularity distribution model are built as well as the inherence relation between the fracture crushing probability and the impact velocity is developed. The granularity boundary of separation by crushing granularity is determined, while the calculation formula for the mass ratio of separated gangue is put forward. The impact crushing expriments are completed for coal and gangue from three working faces. The results show that the fracture crushing probability and the crushing granularity of coal and gangue particles in impact crushing process both have fractal feature. Furthermore, the analysis indicates that the fractal dimensions of the fracture crushing probability and the crushing granularity for gangue particles are both smaller than ones of coal particles from the same working face, which demonstrates that the primary separation for coal and gangue can be achieved using separation by the crushing granularity.The simulation analysis on continuously projecting and rebound behaviors of particle group with granularity between 50 mm and 100 mm are carried out using three-dimension discrete element method. The study shows that main approaches to increase the rebound distances of particle group are to raise the contact probability between feeding middle particles and rebound plate and to decrease the impact interference among feeding middle particles. Four indexes are proposed to describe status of contacts between particle and rebound plate and secondary impact behaviors of particle group, including mean contact plate probability, non-trouble contact plate probability, secondary impact probability and secondary impact frequency. The indexes variance rules with particle feeding amount, fracture crushing probability and projecting velocity are thoroughly discussed.For certain particle feeding amount and projecting velocity, the continuously projecting and rebound behaviors of particle group are simulated to determine the dimensions of the rebound plate and the boundary of separation by the rebound distance. The technical scheme of combining the rebound distance and the crushing granularity to realize separation for coal and gangue underground is put forward, which lays a foundation for the industry application of this technique.Three indexes including the mass ratio of separated gangue, the mass ratio of waste coal and the mass ratio of slack coal are proposed to evaluate the effect of impact separation for coal and gangue. Based on economic benefits, the calculation formula of comprehensive evaluation index for separation effect is obtained. Combining with three- dimension discrete element method and BP neural network model, the effect of impact separation for coal and gangue is predicted. All above provide foundation for qualitative and quantitative evaluation of impact separation for coal and gangue underground.