Tongqing Li

Thermo-stress coupling field of friction lining during high-speed slide of wire rope in a mine friction-drive hoist

In order to seek the intrinsic reason for the serious high-speed slide accident in a mine hoist, the thermo-stress coupling field of friction lining was studied during the high-speed slide. First, the helical contact characteristics were analysed. Subsequently, the thermomechanical properties and the dynamic coefficient of linear expansion were studied, and the thermomechanical constitutive relation was obtained. Then, the theoretical model of thermo-stress was established with the consideration of the helical contact characteristics and the thermomechanical constitutive relation. Also, the numerical simulation was performed by the finite element analysis. Finally, the experiment was carried out on a friction tester. It is found that the temperature is the highest at the contact zone II and the friction heat focuses on the contact surface layer. The variation frequency of the stress is 6.98 Hz at 0.5 m/s. Besides, the catastrophe for the strain and coefficient of friction occurs at 3 m/s. The thermo-stress concentration occurs at contact zone II. The experiment results agree with the simulation ones, which validates the theoretical model of thermo-stress.

Discrete Element Method Simulations of the Inter-Particle Contact Parameters for the Mono-Sized Iron Ore Particles

Aiming at predicting what happens in reality inside mills, the contact parameters of iron ore particles for discrete element method (DEM) simulations should be determined accurately. To allow the irregular shape to be accurately determined, the sphere clump method was employed in modelling the particle shape. The inter-particle contact parameters were systematically altered whilst the contact parameters between the particle and wall were arbitrarily assumed, in order to purely assess its impact on the angle of repose for the mono-sized iron ore particles. Results show that varying the restitution coefficient over the range considered does not lead to any obvious difference in the angle of repose, but the angle of repose has strong sensitivity to the rolling/static friction coefficient. The impacts of the rolling/static friction coefficient on the angle of repose are interrelated, and increasing the inter-particle rolling/static friction coefficient can evidently increase the angle of repose. However, the impact of the static friction coefficient is more profound than that of the rolling friction coefficient. Finally, a predictive equation is established and a very close agreement between the predicted and simulated angle of repose is attained. This predictive equation can enormously shorten the inter-particle contact parameters calibration time that can help in the implementation of DEM simulations.

Effect of the Lifting Velocity and Container Shape on Angle of Repose of Iron Ore Particles

To investigate the impact of lifting velocity and container shape on angle of repose, the fixed-base cylinder method was performed using three types of container shape. The container shape was lifted a series of lifting velocities. Six size fractions of iron ore particles ranging from coarser to fine particles were used as the test materials. And the sand-pile calibration method was then used to calibrate the contact parameters of iron ore particles. Results show angle of repose decreased exponentially with the lifting velocity, while it appeared approximately to be invariant to particle shape, for all size fractions. The sand pile highly depends on the container shape at a low lifting velocity but appears to be invariant to particle size for a high lifting velocity. And then a predictive equation is established and a very close agreement between the predicted and measured angle of repose is attained. Finally, a series of DEM simulations considering the irregular particle shape are conducted by means of sphere clump method to calibrate the contact parameters and are in good visual agreement with the experimental results, indicating the “tuned” contact parameters as well as the applicability of the predicted equation.

Multi-layer kinematics and collision energy in a large-scale grinding mill-the largest semi-autogenous grinding mill in China

Using the largest semi-autogenous grinding mill in China as a model, collision energy was analyzed on the basis of the multi-layer kinematics of the steel balls. First, the kinematic equation of the steel balls was obtained by considering the multi-layer characteristics of the steel balls. Second, the collision energy of the inner-layer steel balls was addressed according to its kinematic characteristics. Finally, the total collision energy per unit time was obtained. Results show that the leaving angle decreases as the mill speed ratio increases and as the radius ratio increases, but the leaving velocity increases linearly. Moreover, a discontinuity point of the tangential collision velocity occurs at an angle factor β = 0, and the angle factor β is divided into two intervals: [−3, 0] and [0, 1.5]. The leaving angles corresponding to a tangential velocity equal to zero are calculated to be 1.185 and 0.7854 rad in the two intervals. In addition, the sum collision velocity increases when β is less than zero, but it decreases sharply above zero. The maximum total collision energy per unit time occurs at 84.2% of the mill speed corresponding to the optimal mill speed ratio.

Experimental study of charge dynamics in a laboratory-scale ball mill

To understand and describe the behavior of charge dynamics in mills, a series of dry and wet grinding tests were performed on a laboratory-scale ball mill. The comparisons between experimental results and grinding media trajectory simulations were addressed. Results show that the grinding media trajectory simulations exhibit a good agreement with the experimental results. The shoulder angle was proportional to mill speed and ball filling. The toe angle was inversely proportional to ball filling, but the impact point angle was appeared to invariant to ball filling and inversely proportional to mill speed. By means of motion analysis of the charge, a good grinding efficiency can be obtained when the ball filling ranging from 20% to 40% and the mill speed ranging from 70% to 80%. For dry tests, the orthogonal analysis indicates that the influence order of four factors on power-mass ratio is ball filling, mill speed, powder-grinding media ratio and lifter profile and the influence order of four factors on −0.074 mm yield is mill speed, ball filling, powder-grinding media ratio and lifter profile. The best dry tests are a combination of 70% of critical speed, 20% of ball filling, 0.8 of powder-grinding media ratio and waveform lifter. Correspondingly, the power-mass ratio can increase by 28.27% and the production of −0.074 mm can increase by 50.38%. For wet tests, the variations of −0.074 mm yield on mill speed and moisture content increase up to a maximum and then decrease rapidly. The −0.074 mm yield can reach a maximum at the 80% of mill speed and 50% of moisture content.

Impact Load Behavior between Different Charge and Lifter in a Laboratory-Scale Mill

The impact behavior between the charge and lifter has significant effect to address the mill processing, and is affected by various factors including mill speed, mill filling, lifter height and media shape. To investigate the multi-body impact load behavior, a series of experiments and Discrete Element Method (DEM) simulations were performed on a laboratory-scale mill, in order to improve the grinding efficiency and prolong the life of the lifter. DEM simulation hitherto has been extensively applied as a leading tool to describe diverse issues in granular processes. The research results shown as follows: The semi-empirical power draw of Bond model in this paper does not apply very satisfactorily for the ball mills, while the power draw determined by DEM simulation show a good approximation for the measured power draw. Besides, the impact force on the lifter was affected by mill speed, grinding media filling, lifter height and iron ore particle. The maximum percent of the impact force between 600 and 1400 N is at 70–80% of critical speed. The impact force can be only above 1400 N at the grinding media filling of 20%, and the maximum percent of impact force between 200 and 1400 N is obtained at the grinding media filling of 20%. The percent of impact force ranging from 0 to 200 N decreases with the increase of lifter height. However, this perfect will increase above 200 N. The impact force will decrease when the iron ore particles are added. Additionally, for the 80% of critical speed, the measured power draw has a maximum value. Increasing the grinding media filling increases the power draw and increasing the lifter height does not lead to any variation in power draw.

Discrete element method simulations of load behavior with mono-sized iron ore particles in a ball mill

Aiming at addressing the load behavior of iron ore particles in a ball mill, a design of experimental method was used to define a series of discrete element method simulation conditions with two factors being the mill speed and lifter height. The key feature locations of impact toe, bulk toe, shoulder, and head positions were identified visually to determine the load behavior of the charge. To allow the irregular particle shape to be accurately determined, a quick and accurate sphere-clump method was employed in modeling the geometrical model of irregular shape. The results show that the dependence of the impact toe and head on mill speed is higher than its dependence on the lifter height with a correlation of −0.923 and 0.97, respectively. The shoulder changes approximately invariant with the mill speed and varies little with the lifter height. The bulk toe of particles appears to be invariant to the mill speed as well as the lifter height, resulting in the approximately same inclination of the chord joining the shoulder and bulk toe.

Energy dissipation during the impact of steel ball with liner in a tumbling ball mill

Abstract The coefficient of restitution and the energy dissipation during the impact of the steel ball with liner in a tumbling ball mill was addressed. Critical speed ranged from 76 to 88% was concerned, and numerical solution using the finite element method for steel ball impacting with liner was investigated. Results indicated that the coefficient of restitution and impulse ratio increase in proportion to the critical speed increasing. Moreover, the contact force–displacement response is obtained and divided into two stages: elastic–plastic compression stage and elastic restitution stage. By representing the energy dissipation during collision, the encompassed area by the elastic–plastic compression stage and the elastic restitution stage decreases with the critical speed increasing. Finally, the kinetic energy loss ratio is given to quantitative represent the energy dissipation. It is shown that the kinetic energy loss ratio is inversely proportional to the critical speed.