Selection of screening equipment

Principles for selecting screening equipment The main factors to consider when choosing a type of screening machine include: the properties of the material to be screened (the proportion of particles smaller than the screen opening size, the proportion of particles that are difficult to screen, the amount of moisture and clay in the material, the shape of the particles, the specific gravity of the material, etc.); the structure of the screening machine (the area of the screen mesh, the number of layers of mesh, the size and shape of the screen openings, the percentage of the screen surface covered by openings, the way in which the screening machine moves, the amplitude and frequency of vibration, etc.); and the requirements of the mineral processing process (processing capacity, screening efficiency, screening method, the inclination angle of the screening machine, etc.). In addition to considering the aforementioned factors, the following basic principles should also be followed: (1) After determining the screening area, the width of the screening surface should be at least 2.5 to 3 times the size of the largest pieces of material, in order to prevent the sieve from being clogged by large pieces. (2) To ensure that the sieve operates under optimal conditions, the ratio of its length to width should be selected within the range of 2 to 3. (3) A suitable screen surface material and structure that are appropriate for the operating conditions should be selected. (4) For determining the screen opening size, when fine screens, arc screens, or linear vibration screens are used for fine-grained screening, the screen slit size should be 2 to 2.2 times the size of the particles to be separated. It should not exceed 3 times at most; for vibrating screens used for screening medium-sized materials, the mesh size is 1.2 times the size of the particles to be separated; when used for screening coarse materials, a mesh size of 1.05 times the size of the particles to be separated is sufficient; for probability screens, the mesh size is generally 2 to 2.5 times the actual size of the particles to be separated. (5) Determine whether to use a double-layer or multi-layer sieve. When the particle size range of the material to be screened is wide, using a double-layer sieve instead of a single-layer sieve can increase the processing capacity of the screening machine, while also protecting the lower sieve mesh and extending its service life. The selection of the sieve hole size for the upper layer of a double-layer sieve should generally be determined based on the particle size characteristics of the feed material. It is advisable to use a particle size corresponding to 55–65% of the original feed amount, as the amount that passes through the upper sieve mesh, as the sieve hole size. Note that when the proportion of particles below the sieve size in the raw material exceeds 50%, there are many difficult-to-sieve particles, and the material contains a high amount of sticky sludge and moisture, it is advisable to avoid using a double-layer sieve as if it were a single-layer sieve. (6) Determine the effective working area of the sieve. The screening area calculated according to the requirements of the production process is the effective area of the sieve, while the specification of the sieve refers to its nominal area. For vibrating screens used for screening materials of medium particle size, the effective screening area should be 0.8 to 0.85 times the nominal area of the sieve. Of course, this is closely related to the porosity of the sieve holes on the sieve surface. For materials larger than 200 mm, heavy-duty vibrating screens are commonly used; for materials larger than 10 mm, circular motion screens are typically employed. For mud removal, dehydration, and grading, linear vibrating screens and high-frequency vibrating screens are usually used. (8) Where conditions permit, seat screens should be preferred to facilitate inspection and maintenance. When a suspension screen must be used, the lifting height should be reduced as much as possible to minimize the swinging amplitude of the vibrating screen, thereby facilitating production operations.