| Coal is the main primary energy source in China.Coal preparation is the basis and prerequisite for highly efficient utilization and sustainable development of coal,as well as the most cost-effective method for clean coal technology.Screening operation is the key unit of coal preparation process.There are many problems such as material accumulation at the feed end,severe screen blinding,low screen surface utilization rate and poor classification performance when the traditional equal amplitude rigid screening method was used for dry coal deep classification.In order to solve the above problems,variable-amplitude equal-thickness elastic screening method is proposed by combining the variable-amplitude equal-thickness screening and the multi-degree-of-freedom elastic screening.This method can optimize the material distribution on the screen surface,improve the screen surface utilization rate,strengthen the loosening and stratification of moist coal,eliminate screen blinding,with which the efficient dry deep screening of moist coal is achieved.The dynamic model of variable-amplitude equal-thickness screen with large span was established through theoretical analysis.The three degree-of-freedom oscillatory differential equation of the motion of screen centroid was established based on the theory of multi-degree-of-freedom vibration,from which the displacement equation of the centroid is obtained,as well as the arbitrary position on the screen.The kinematic characteristics of the variable-amplitude screen with large span at various operation stages and the trajectories of different screen box regions along the material flow direction during steady-state operation were analyzed by vibration testing.Violent oscillatory and swing of the vibration screen appeared at the starting and stopping stage.The beat phenomenon showed and the amplitude decreased gradually to the working amplitude at the transitional operation stage.At steady-state operation,the trajectories of different screen box regions were elliptical and the amplitude showed no significant difference along the screen surface,while decreased gradually along the direction perpendicular to the screen surface,as well as the angle between the major axis and the screen surface.The amplitude along the material flow direction decreased by using large span and unbalanced excitation.Moreover,on the condition of high excitation force coefficient and large relative span of exciting beams,the amplitude paralleling to the screen increased gradually while amplitude perpendicular to the screen increased at feeding end but decreased at discharge end with a growing difference between them.With the method of high speed photography and image analysis,the kinematics characteristics of particles and the material distribution in different surface regions of the variable-amplitude equal-thickness screen were studied.The implementing mechanism of variable-amplitude equal-thickness screening was revealed.High vibration intensity at the feed end promoted the loosening and migration of the particles,thus the material spread rapidly on the screen surface.Low vibration intensity at the discharge end reduced the particle migration velocities and prolonged the material residence time.As a result,the thickness tended to be consistent.The method of multi-section sampling analysis was proposed to investigate the sieve passing of materials with various size in the different regions of screen surface,based on which the strengthening mechanism of material classification by variable-amplitude equal-thickness screening was revealed.With the increase of excitation force coefficient and relative span of exciting beams,the average migration velocity of particles,the screening time and the material thickness changed,which promoted the spatial-temporal evolution of the material distribution on the screen.The material thickness varied successively as follows: thick at feeding end but thin at discharge,consistent and thin at feeding end but thick at discharge.Consequently,the screening performance was significantly affected.Therefore,the variable-amplitude equal-thickness efficient and deep screening of low moist coal can be realized with the fact that the average migration velocity of particles and material thickness on the screen can be controlled by adjusting the excitation force coefficient and relative span of exciting beams based on the above research.The rigid-flexible coupled elastic rod screen surface was designed and the kinematic characteristics of the rigid-flexible coupled rod under harmonic displacement excitation were studied.The displacement equation of the elastic tube and the size range of the screen hole along the material flow direction were given.With high speed photography technology,the spatial motion of the rigid-flexible coupled rod and the spatial distribution and behavior of the moist coal during elastic screening were analyzed.Furthermore,the mechanism of plugging elimination was revealed.Collisions between the rigid rod and the side plate,as well as the elastic tube,improved the average acceleration of the flexible elastic tube significantly,which promoted the loosening of material and the removal of adhered fine particles from the coarse particles' surface.Because of the rotation of elastic tube,the agglomerated particles and the covering film were sheared.The displacements and elastic deformations of adjacent elastic tubes contributed to the quasi-periodic fluctuation of sieve size.The random direction of force acting on material by the elastic tube made the agglomerates and the covering films break and the blocking particles remove from the screen aperture.Then,the effects of process parameters and material properties on the 6 mm classification performance of equal amplitude rigid-flexible coupled elastic rod screen surface were studied and optimum operating parameters were determined by single factor itemized test.The variable-amplitude equal-thickness elastic screening method was proposed,the effects of exciting force coefficient and the relative span of excitation beams on the classification performance were investigated by single factor experiment,based on which the factor levels for optimal performance were determined.The effects of selected factors which have significant influences on the variable-amplitude equal-thickness elastic screening on the classification performance were analyzed with multivariate response surface methodology,as well as the interactions.And the correlation model between the elastic screening efficiency and the factors was established.The 6 mm screening test of moist coal sampled from Ordos,Inner Mongolia was carried out on variable-amplitude equal-thickness elastic screen with the optimal operating conditions determined by the correlation model.The results showed that the screening efficiency was 87.79% which is accorded with the value predicted by the model and the total misplaced materials content was 6.77%,verifying the accuracy the correlation model.The research demonstrated that variable-amplitude equal-thickness elastic screening technology can realize efficient dry deep screening of moist coal. |
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