Kinematic Characteristics Of Materials And Screening Distribution Law In Variable Trajectory Combined Equal-thickness Screening Process

Coal is an important fossil energy source and an important foundation for national economic development.Coal washing and processing is the first process of clean utilization of coal and the source technology of clean coal technology.Screening is widely used in coal classification,dehydration,demedium and desliming,which is an important part of coal preparation industry.With the gradual reform of national energy policy,the equipments of coal preparation plant are required to further promote large-scale,standardized and efficient.Traditional equal-amplitude vibrating screen used in coal preparation is characterized by small processing capacity per unit area,accumulation of materials at the feed end,low utilization rate of overall screen surface and poor accuracy of screening.In order to solve the above problems,this paper proposed to design a new type of variable trajectory combined equal-thickness screen,through theoretical analysis and screening test,optimize the vibration parameters of each section,realize high-efficiency fine equal-thickness screening.Based on the model of variable trajectory combined equal-thickness screen,the dynamic model of unit screen is established.Through theoretical analysis,the criterion of unit screen from start-up unstable state to self synchronization stable operation is obtained.When the synchronization index P?1,the unit screen realizes self synchronization.The operation of the screen is divided into three stages:start-up transition stage,stable operation stage and shutdown stage.The kinematical characteristics of the screen body under different excitation conditions in stable operation stage are analyzed.When D=0,k=1.0 and D=0.8,k=1.0,the two unit screens move in a straight line,and the x-axis motion has no significant effect on the overall motion.When D=0,k=1.0,the two unit screens move with equal amplitude.When D=0.8,k=1.0,the two unit screens move with variable amplitude.A_y of the first stage unit screen is larger than that of the second stage unit screen.When D=0.8,k=1.2,the effect of x-axis motion on the overall motion increases significantly,and A_y decreases along the screen.Motion trajectory changing with time of variable trajectory combined equal-thickness screen is studied,and the spatial evolution Lissajous diagram of each operation stage and the spatial trajectories of each measuring point in stable operation stage are obtained under different excitation conditions.It is found that the screen body tends to be stable gradually in the start-up transition stage,and presents regular motion in the stable operation stage.In the stable operation stage,when D=0,k=1.0,the screen body moves in a straight line with equal amplitude along the screen length direction.When D=0.8,k=1.0,the screen body moves in a straight line with variable amplitude,the amplitude of the first unit screen is greater than that of the second unit screen.When D=0.8,k=1.2,the screen body moves in a variable ellipse with decreasing amplitude along the screen length direction.The 6 mm screening experiment is carried out with the method of subsection sampling and classification analysis.The optimum parameters of excitation force and excitation frequency are studied under the condition of equal amplitude and equal vibration frequency.The influence of feed rate on screening performance is analyzed.The results show that when the excitation force F_t1/F_t2=3.8/3.8 kN,excitation frequency f₁/f₂=13.5/13.5 Hz and the feeding rate Q=4 kg/s,the screening efficiency?=92.45%and the total mismatch content M_o=3.70%.Through the study on the screening distribution law of materials in the process of variable trajectory combined equal-thickness screening,the excitation force gradient,the frequency distribution of the two unit screens and the unbalanced excitation force coefficient are optimized to realize the optimization of the excitation parameters.The results show that when the excitation force gradient D=0.8and excitation frequency f₁/f₂=15.00/11.37 Hz and unbalanced excitation force coefficient k=1.2,the material is mainly screened through the first unit screen,and checking screened through the second unit screen.The screening efficiency?=96.12%,and the total mismatch content M_o=1.82%.Compared with the two unit screens with equal amplitude and equal vibration frequency,the screening efficiency is increased by3.67%,and the total mismatch content is reduced by 1.88%,and the screening performance is the best.Kinematical characteristics of particles in the screening process are studied by means of high-speed dynamic photographing test.The results show that with the increase of particle size,the average moving time decreases and average velocity increases.When D=0,k=1.0,the average moving time of particles of the same particle size decreases and the average velocity increases along the screen length direction.When D=0.8,k=1.0,the average moving time and the average velocity of particles of the same particle size are close along the screen length direction.When D=0.8,k=1.2,the average moving time of the same particle size in the first unit screen is short and the average velocity is large;the average moving time is long and the average velocity is small in the second unit screen.The retention time of the material in the second unit screen increases,and the overall screening time increases.When D=0,k=1.0,the materials in the stable screening stage accumulate in the first unit screen,with the maximum material layer thickness of 78.4mm.Material layer in the second unit screen is thinner,with the maximum material layer thickness of 67.6 mm,the material is concentrated in the first unit screen.When D=0.8,k=1.0,the materials in the stable screening stage tends to be equal thickness in two unit screens,and the material layer thickness is relatively large.The maximum material layer thickness of the first unit screen is 79.3 mm,and the maximum material layer thickness of the second unit screen is 80.1 mm.When D=0.8,k=1.2,the thickness of the material layer in the stable screening stage of the two unit screens tends to be the same.The maximum material layer thickness of the first unit screen is 65.4 mm,and the maximum material layer thickness of the second unit screen is 65.9 mm,the thickness of the material layer is small,the material layers fully.The screening penetration in second unit screen is significantly increased,and the overall utilization rate of the screen surface is high.The screening performance is the best,and the high-efficiency equal thickness screening is realized.