Sorting construction waste is an important part of realizing its resource utilization and reduction. Through sorting, useful waste is fully selected and utilized, while harmful waste is fully separated. Another important function is to divide construction waste into different particle size levels for use in different recycling processes.
The basic principle of sorting is to use the differences in physical or chemical properties of materials to separate them. For example, using the difference between magnetic and non-magnetic properties in garbage for separation, using the difference in particle size for separation, and using the difference in specific gravity for separation. According to different properties, various machinery can be designed and manufactured to sort solid waste. Sorting includes manual picking, screening, gravity sorting, magnetic sorting, buoyancy sorting, optical sorting, etc., which will be explained separately below.
Screening is the process of separating coarse and fine materials by using a sieve to pass fine particles smaller than the sieve hole through the sieve surface, while leaving coarse particles larger than the sieve hole on the sun surface. This separation process can be seen as consisting of two stages: material stratification and fine particle sieving. There are three types of screening equipment commonly used in construction waste treatment: fixed screens, vibrating screens, and drum screens.
Fixed sieve: The sieve surface is composed of many parallel arranged sieve strips, which can be installed horizontally or obliquely. Its characteristics are simple structure, difficult to use power, low equipment cost, and easy maintenance. Therefore, it is widely used in the sorting process. Fixed sieve can be divided into two types: grid sieve and bar sieve. The sieve is usually installed before the coarse crusher to ensure that the feeding particle size is appropriate. Rod screens are used to screen coarse waste particles larger than 50mm. They are generally used before coarse and medium crushing. When installing, the inclination angle should be greater than the friction angle of the waste on the screen surface, usually 30 ° to 35 °, to ensure that the material slides down along the screen surface. The mesh size of the bar sieve should be 1.1-1.2 times the required particle size for the material to be exposed to sunlight, and the width of the sieve should be greater than 2.5 times the maximum particle size in solid waste.
Vibration screen: Vibration screen is a widely used equipment in many industrial sectors. Its characteristic is that the vibration direction is perpendicular or approximately perpendicular to the screen surface, the vibration frequency is 600-3600r/min, the amplitude is 0.5-1.5mm, and the material undergoes segregation on the screen surface. Particles with high density but small force particle size enter the lower layer and reach the screen surface. The suitable inclination angle for a vibrating screen is generally 8 ° to 40 °. A small inclination angle will cause the material to move slowly, while a large inclination angle will cause the material to move too quickly and result in sieving. Due to the strong vibration of the screen surface, vibrating screens eliminate the phenomenon of clogging the screen holes, which is beneficial for screening wet materials. They can be used for screening coarse, medium, and fine particles of construction waste. There are mainly two types of vibrating screens: resonant screens and inertial screens.
Drum screen: also known as rotary screen, is a cylindrical screening surface that rotates slowly (usually at a speed of 10-15 r/min). The inclination angle of the screen cylinder axis is generally installed at 3 ° -5 °. The most commonly used screening surface is the impact screen plate, which can also be a screen woven from various materials, but it is not suitable for screening linear materials. During screening, the material is fed into the sieve from the slightly higher end and then continuously flipped inside the sieve along with the rotating drum. Fine particles eventually pass through the sieve holes. The inclination angle of the drum screen determines the axial running speed of the material, while the behavior of the material perpendicular to the drum axis is determined by the rotational speed.
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