Screening Mechanism of High-Amplitude Screen

　　Dry screening of wet sticky raw coal smaller than 13mm is a challenging problem in domestic and international screening technology because wet sticky raw coal clumps together, making it difficult to loosen or causing it to adhere to the screen surface, reducing the effective screening area and leading to decreased screening efficiency. The raw coal does not loosen or stratify and moves as a whole, making the screening process difficult to complete. Therefore, deep screening of fine wet sticky raw coal is a major issue urgently needing resolution in the industry.

　　The high-amplitude screen uses large amplitude, high vibration intensity, relatively low vibration frequency, and a self-cleaning screen surface to complete the screening process of wet sticky raw coal.

　　Traditional screening theory holds that the screening process is divided into three stages: loosening, stratification, and sieving, with loosening being the prerequisite for stratification. Stratification is the condition for completing the screening process, and sieving is the purpose of screening.

　　When the moisture content of raw coal is low and the ash content is small, because there is no or very little adhesion between raw coal particles and between raw coal and the screen mesh, the raw coal flows well. As long as the particle size of the raw coal is smaller than the screen aperture and there is relative motion between the particles and the screen mesh, raw coal particles smaller than the screen aperture can pass through. However, the screening process for wet sticky raw coal is completely different. The increased viscosity of raw coal makes loosening and stratification difficult, and the flowability of raw coal worsens. At this time, sufficient acceleration between the raw coal and the screen mesh is required to loosen and stratify the raw coal. But acceleration alone is not enough because the sieving process of wet sticky raw coal also requires space and time. If the screen surface vibrates too fast, the raw coal cannot be thrown up, and screening is ineffective. Additionally, particle sieving requires time to complete. The large amplitude, high vibration intensity, and low vibration frequency of the high-amplitude screen precisely meet these screening process characteristics.

　　Theoretically, there is a limit to the adhesion force between raw coal particles and between raw coal and the screen mesh. When the excitation force generated by the screen surface exceeds the adhesion force between raw coal and the screen mesh, the raw coal will detach from the screen mesh, causing the raw coal particles to separate and become active, preventing screen blockage and enabling effective screening. However, the smaller the particles and the higher the viscosity, the smaller the inertial force obtained, which means smaller particles need greater acceleration to separate. In reality, overly fine particles do not exist independently; they always adhere to larger particles, sticking together. Large amplitude and high vibration intensity help break the adhesion between raw coal particles, forcing the raw coal to sieve through.

　　1. Amplitude

　　To effectively eliminate the blockage of fine raw coal particles in the screen apertures, the screen uses a larger amplitude. Because of the selection of screening process parameters, the primary consideration should be the screening rules of raw coal. The amplitude used during the screening process should be determined by experiments rather than arbitrarily set. Excitation intensity, amplitude, and frequency are interrelated and should be selected based on the movement rules of raw coal during screening. The amplitude adjustment range should be gradually approached during experiments to achieve better screening results. The amplitude range we selected can be adjusted between 15 and 25mm depending on the material properties.

　　2. Frequency

　　To achieve high screening efficiency and large processing capacity, wet sticky materials must be forcibly loosened by using a larger amplitude and lower frequency. Because raw coal sieving requires sufficient time and space, too high a frequency prevents proper stratification and affects screening results. We selected a frequency of 12HZ.

　　3. Vibration Intensity

　　Vibration intensity is an important indicator, defined as the ratio of the acceleration of the vibrating screen box to gravitational acceleration. Typically, the vibration intensity of vibrating screens in China ranges from 3 to 4.2. However, for wet sticky raw coal, this vibration intensity is too low. We selected a vibration intensity of 5-9. Screening wet sticky raw coal requires time and space. The inertial force generated when raw coal is thrown up should overcome the adhesion between wet sticky raw coal and the screen wires. Higher vibration intensity increases the relative motion speed between raw coal and the screen surface when the raw coal falls back. We want the material bed to loosen so that raw coal smaller than the screen aperture truly contacts the screen mesh, rather than the entire material layer moving up and down as a whole. From a screening process perspective, increasing vibration intensity improves screening results, which has been proven in practice.

　　4. Material Movement Speed

　　Raw coal particles detach from the screen surface when the upward movement speed reaches its peak. At this time, the speed of the screen surface is

　　V=Aω=0.012×(π×740/30)=0.93m/s

　　The movement speed of a conventional vibrating screen generally ranges from 0.15 to 0.3m/s, so the movement speed of the high-amplitude screen is more than three times that of a conventional vibrating screen.

　　5. Relative Movement Speed Between Material Falling Back to the Screen Surface and the Screen Surface

　　During the screening process, the speed that truly affects screening is the relative movement speed between raw coal and the screen surface. When particles start moving with the screen surface, the speed and acceleration of the particles and the screen surface are the same. However, at the detachment point, when the inertial force on the particles exceeds their own weight, the particles detach from the screen surface and perform projectile motion. The screen surface moves in a sinusoidal curve under the excitation force. At the landing point, the particles meet the screen surface again and move together with it. At the moment of contact, the screen surface still moves in a circular motion, while the particles move downward along a parabolic trajectory. Throughout this process, the relative movement speed between the screen surface and particles is directly related to screening effectiveness.

　　Movement of a single particle on the screen surface:

　　Vx=Aωsinωt

　　Vy=Aωcosωt

　　ax=Aω2cosωt

　　ay=Aω2sinωt

　　Velocity along the y-axis direction:

　　Vy=Aω=0.012×(π×740/30)=0.93m/s

　　The velocity of a conventional vibrating screen is:

　　V普=Aω=0.004×(π×850/30)=0.356m/s

　　Calculations show that the velocity at which raw coal is thrown up by the high-amplitude screen is 2.72 times greater than that of a conventional vibrating screen.

　　Practice has proven that fine wet sticky raw coal particles always adhere to other raw coal particles or screen wires during screening because the adhesion force is much greater than their own weight. However, when the aggregate reaches a certain size, its strength and stiffness decrease as its mass increases. Due to differences in coal properties such as moisture and degree of muddiness, the aggregate size also varies. If the aggregate particle diameter is 6mm, the inertial force generated by the screen surface movement in the high-amplitude screen is:

　　F=Aω2W/g=0.0297kg

　　Particle gravity is:

　　W=πr3=0.0013kg

　　The inertial force generated is 22.85 times the gravity.

　　Ordinary vibrating screens only have:

　　F=0.0042kg

　　The inertial force generated is 3.23 times the gravity.

　　Therefore, to overcome adhesion force, the high-amplitude screen is 7 times that of the ordinary screen.

　　With technological progress, high-amplitude screens have added various self-cleaning screen technologies to adapt to the deep screening of fine-grained wet sticky materials such as lignite, coal slime, and middling coal.

　　High-amplitude screens have achieved satisfactory results through extensive industrial applications and have made remarkable achievements both domestically and internationally, contributing to the advancement of screening technology.