Size control is done (i) to prevent undersize in the feed from blocking the next size reduction stage (scalping), (ii) to prevent oversize from moving into the next size reduction or operation stage (circuit sizing), and (iii) to prepare a sized product (product sizing). There are two methods dominating size control processes. They are (i) screening using a geometrical pattern for size control, and (ii) classification using particle motion for size control.
Screening using geometrical patterns for size control makes use of screening media made of bars, wires, and panels with holes usually rounds, squares, rectangle aligned to length, and rectangle aligned to width. Screening is the process of separating solids into two or more products on basis of their size. The objective of screening is size control.
Linear Vibrating Screen
The purpose of screening is to separate from granular substance particles that are smaller than the screen opening from those that are larger. This is not as simple as it sounds, and the difficulties compound as the opening becomes smaller. This can be done dry or wet.
The action of screening is aided when the screen is subjected to some kind of motion, reciprocating or gyratory in the horizontal plane, or shaken with a reciprocating motion having both vertical and horizontal components. The minus particles pass through the screen at a diminishing rate until all but the particles closest to the opening size have been separated out. The time duration of the shaking to reach this stage is roughly proportional to the amount of material on the screen.
Circular Vibrating Screen
The performance of screens falls back on three main parameters namely (i) motion, (ii) inclination, and (iii) screening media. It is measured by the screening efficiency. The most commonly used measure of screen efficiency is the cumulative weight of material that has passed the screen in any time interval, compared to the total weight of undersize in the feed, expressed in a percentage. This can be reversed, when the oversize is the product to be recovered; then efficiency is the weight percent of the material in the screened oversize fraction compared to the total weight of oversize in the feed.
The process of screening falls under two categories namely (i) screening by stratification, and (ii) screening by free fall. In the case of screening by stratification, screening is carried out by building up a material bed on a screen deck. The material gets stratified when the motion of the screen reduces the internal friction in the material. This means that the finer particles can pass between the larger ones giving a sharp separation. In the case of screening by free fall, the double inclination is used for stratification (from 10-15 degrees up to 20-30 degrees). During free fall, no particle layer can build up on the screen deck and the particles are sized directly via the screening media, giving a higher capacity, (or a more compact installation), but also less sharpness in separation. Freefall screening has optimal use when a large amount of fines is to be removed quickly.