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.

(1) Screening

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. picture

Vibrating screen

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.

(2) Gravity sorting

Airflow sorting machine

There are many methods for gravity separation, which can be classified according to their principles of action as wind separation, jigs separation, heavy medium separation, inertial separation, etc. Gravity sorting is a process of separating solid waste materials based on the density differences between particles of different substances, which are subjected to the effects of gravity, medium dynamics, and mechanical forces in a moving medium, resulting in loose stratification and migration separation of particle groups, thus obtaining products of different densities. The medium for gravity sorting includes air, water, heavy liquids (liquids with higher density than water), heavy suspensions, etc.

The conditions for gravity sorting of garbage: there must be a density difference between particles in solid waste; The sorting process is carried out in a moving medium; Under the combined action of gravity, medium dynamics, and mechanical forces, the particle group becomes loose and layered according to density; The layered materials migrate and separate from each other under the driving force of the moving medium flow, and obtain final products with different densities. The most commonly used methods for gravity sorting are wind sorting and inertial sorting.

Wind sorting: It is a type of gravity sorting. It is a method of sorting solid waste particles by density and particle size using air as the sorting medium under the action of airflow. The basic principle is that airflow can carry lighter materials upwards or horizontally to farther places, while heavy materials settle due to the inability of the rising airflow to support them, or are thrown out horizontally for a closer distance due to inertia. Also known as "vertical airflow sorting" and "horizontal airflow sorting". The wind sorting method has a simple process. As a traditional sorting method, it separates the light components mainly composed of combustibles and the heavy components mainly composed of inorganic substances in urban garbage for separate recycling or disposal. According to the different directions of airflow blowing into the sorting equipment, the air sorting equipment can be divided into two types: upward airflow air sorting machine and horizontal airflow air sorting machine. The horizontal airflow sorting machine has a simple structure and easy maintenance, but its sorting accuracy is not high. It is rarely used alone and is often combined with crushing, screening, and updraft air selection machines for joint treatment processes.

Inertial sorting: Inertial sorting, also known as ballistic sorting, is a method of separating particles by horizontally ejecting them using high-speed conveyor belts, cyclones, or airflows, utilizing the different inertia differences formed by density and particle size, and the different properties of particles moving along parabolic trajectories. Ordinary inertial separators include ballistic separators, cyclone separators, vibrating plates, inclined conveyor belts, and rebound separators.

(3) Magnet sorting

drum magnetic separator

Magnet sorting is a method of separating materials in an uneven magnetic field by utilizing the magnetic differences of various substances in solid waste. The commonly used equipment is a magnetic separator, as shown in Figure 4-3. When the magnetic separator is working, the material falls from the feed port onto the magnetic cylinder. As the magnetic cylinder rotates, the material enters the magnetic field action zone of the cylinder. Non magnetic minerals are subject to inertia and gravity, and are thrown out by centrifugal force in the tangential direction at the front end of the magnetic separation zone. Then, through appropriate isolation by a partition plate, the iron removed product can be obtained. Under the action of magnetic field force, iron and iron oxides gradually leave the magnetic field zone with the magnetic cylinder, and iron and iron oxides fall into the collection hopper, thus achieving the entire process of iron removal.

There are two types of magnets commonly used in magnetic separators: electromagnetic, which magnetizes or polarizes ferromagnetic materials through electrification; Permanent magnet, using permanent magnet materials to form magnetic regions. Among them, permanent magnets are more commonly used, and the most commonly used magnetic separation equipment is introduced as follows:

CTN type permanent magnet cylindrical magnetic separator: can recycle iron and strong magnetic particles with a particle size of ≤ 0.6mm from construction waste.

Magnetic drum: This equipment is mainly used before crushing construction waste to remove iron from the waste and prevent damage to the crushing equipment.

Suspension magnet: It is also used to remove iron from construction waste and protect crushing equipment.

In addition to the several sorting methods introduced above, there are also friction sorting, jump bullet sorting, photoelectric sorting and other methods that need to be flexibly arranged according to needs.