Coal-fired boilers in thermal power plants burn a certain proportion of coal gangue. The coal arriving from the mine has an uneven particle size distribution, with a large portion of the material being of acceptable size and requiring no further crushing. This portion typically accounts for about 35-75% of the total material. Currently, there are many screening equipment options available in China. Commonly used screening equipment can be divided into two main categories: vibrating conveyor type and forced conveyor type, each with its own advantages and disadvantages. It is best to select the appropriate type based on the specific parameters of the material.
Screening machines are divided into fixed screens and moving screens. Moving screens are further divided into rotary screens and vibrating screens. Fixed screens mostly use grates installed above the crusher inlet to prevent oversized materials from falling in. Rotary screens are mostly made by folding plate screens into polygonal or cylindrical bodies. They have a simple structure and long service life, but low productivity, are bulky and heavy, and are rarely used.
Screening machinery is commonly known as a sieve. There are many types of sieves, which are generally classified into the following types according to the structure and movement of the sieve surface: Fixed Screens Fixed screens are the simplest and oldest screening machines. The sieve surface consists of many parallel sieve bars, arranged in the same or perpendicular direction as the material flow on the screen. The sieve surface is installed horizontally (during dewatering) or at an angle, and remains stationary during operation. The material is screened by its own weight sliding down the sieve surface. Fixed screens are divided into two types: fixed grid screens and bar screens. Grid screens generally have square or round screen holes, while bar screens generally have slit screen holes; the sieve surface angle is generally between 25 degrees and 85 degrees. The former has coarser sieve bars and larger slits (generally ≥25 mm), used for coarse particle screening; the latter has smaller sieve bars (generally 0.25–1 mm), used for preliminary dewatering of slurries. Fixed grid screens are widely used in mineral processing plants, generally for pre-screening before coarse or medium crushing. They have a simple structure and are easy to manufacture. They require no power and can directly unload ore onto the sieve surface. The main disadvantages are low productivity and low screening efficiency, generally only 50-60%.
Arc screens and cyclone screens are new types of fixed screens. The screen surface of an arc screen is arc-shaped along the direction of material flow, and the screen bars are perpendicular to the direction of material flow. The screen surface of a cyclone screen is conical, and the screen bars are approximately parallel to the generatrix. They can be used for preliminary dewatering, desliming, or demediuming of slurries, and both utilize the centrifugal force generated when the slurry moves along the screen surface to enhance screening.
Fixed screens have a simple structure, long service life, and especially do not consume power, have no moving parts, and have low equipment and operating costs. Therefore, although their production capacity and screening efficiency are relatively low, they are still widely used.
Rolling screens: The working part of a rotary screen is a cylindrical or conical screen cylinder. A rotating shaft is installed along the axis of symmetry of the screen cylinder, and the entire screen rotates around the axis of the cylinder, which is generally installed at a small angle. A conical screen cylinder is installed horizontally. Material is fed in from the smaller end of the cylinder and is lifted up as the cylinder rotates. When it reaches a certain height, it falls due to gravity. Fine-grained materials pass through the screen holes on the cylindrical working surface, while coarse-grained materials are discharged from the other end of the cylinder. This continuous up-and-down motion achieves material screening. The cylindrical screen has a very low rotational speed, stable operation, and good dynamic balance. However, its screen holes are prone to clogging and deformation, resulting in low screening efficiency, a small working area, and low productivity. It can be used for screening and dewatering coarse and medium-grained materials. Mineral processing plants rarely use it as screening equipment.
Vibrating Screen: The vibrating screen structure consists of four parts: a high-frequency vibrating motor, a primary fixed grid screen and a secondary grading fixed grid screen, vibrating springs, and a casing. The screen box is supported or suspended using spring assemblies, and the vibration of the screen box relies on a vibrator. The vibrating screen is an elastic vibration system, and its amplitude can be changed by the feed rate and other dynamic factors. The characteristics of vibrating screens are high frequency and small amplitude. The inclination of the screen surface is inversely proportional to the screening rate; the greater the inclination, the faster the flow velocity. The material jumps on the screen surface, resulting in higher productivity and screening efficiency. Vibrating screens are suitable for various screening operations in coal preparation plants, and there are many types.
Types of vibrating screens based on the principle of vibration include: linear vibrating screens, horizontal vibrating screens, shaking screens, eccentric screens, rotary vibrating screens, circular vibrating screens, banana vibrating screens, probability screens, etc. According to their transmission mechanisms, they can be further divided into: eccentric vibrating screens, inertial vibrating screens, self-centering vibrating screens, and resonant screens.
Resonant screens were once widely used, but due to their complex structure, poor operational stability, difficult adjustment, and high maintenance requirements, they are no longer manufactured. Elliptical vibrating screens have good screening effects, but their complex structure limits their application. The center-determined circular vibrating screen, commonly known as an eccentric vibrating screen, semi-vibrating screen, or gyratory screen, operates with a circular motion in the center and an elliptical motion at the feed and discharge ends. Its main disadvantages are a large dynamic load transmitted to the foundation, the need for a crankshaft and numerous bearings, and its complex structure, leading to its obsolescence. The crankshaft of the eccentric shaft self-centering circular vibrating screen is also expensive to manufacture, hence its limited production.
