Screens allow particles smaller than the screen’s aperture to pass through while retaining larger ones. Such mineral separations can be an efficient and cost-effective way to concentrate the desired mineral while discarding the unwanted material in certain mineral ores.
Various minerals are found in nature either in a dispersed state, where they exist as separate entities, such as native gold particles in silica rock, or in a combined form, where they are bound within a host rock.
Often, due to variations in hardness, friability, and crushability between the mineral and host rocks, minerals need to be “liberated” through repeated crushing and other separation processes. This liberation process results in the creation of particles with different sizes and shapes, which can be separated using screens.
Dry material separations using screens and sieves are typically attempted down to particle sizes of about 75 microns. Finer materials have a tendency to block the sieve openings. In such cases, screening in the presence of water can be helpful. Separating even finer-sized materials becomes challenging using traditional sieves. For such fine materials, alternative processes like classification need to be employed.
In the industry, a distinction is made between screening and sieving. The fundamental mechanism of size separation is the same for both, but screening generally refers to large-scale industrial size separations, while sieving is more commonly associated with smaller-scale laboratory operations.
2. Screen Types
2.1. Grizzly Screens
Grizzly screens are typically constructed by welding steel rails, rods, or bars into grids with a specific pattern. The selection of rails can vary in size, ranging from about 7.4 kg/m to approximately 225 kg/m. These rails generally run parallel to each other along the entire length of the screening surface. The spacing between the rails typically range from 5 to 200 mm. To facilitate the smooth flow of materials, the openings are often tapered, with the top being wider than the bottom.
Heavy-duty grizzly bars are commonly cast from manganese steel and feature double tapers. These grizzly screens are designed to receive large lumps of ore from sources such as railroad wagons, tipper cars, and other bulk material handling systems that discharge materials from considerable heights. As a result, they are constructed to be exceptionally robust to withstand the impact and handling of heavy materials.
Rail grizzlies, also known as grizzly screens, can be installed to operate in a horizontal flat plane, but they are often inclined to facilitate the transportation of ore across the screen. The inclination typically ranges from about 30 to 40 degrees. For sticky ores, the inclination may be increased to as much as 45 degrees. In cases where the ore is exceptionally sticky, vibrators may be used to aid continuous operation.
When rods are used to construct grizzlies, they are usually either free-rotating or mechanically driven. These rods are supported by bearings and rotate in the direction of material flow, essentially functioning like a conveyor. The space between the rotating rods forms the apertures of the grid.
When designing a grizzly for a specific purpose, the openings between the grizzly bars should be appropriate for the size of the receiving hopper where the product needs to be discharged. Typically, the maximum distance between the grizzly rails is about 0.9 times the maximum hopper opening, which feeds into a crusher, for example.
Grizzly screens can be designed with more than one deck, but usually not more than two. The top deck serves a scalping function, while the lower deck is aimed at producing the final desired size. The two decks generate coarse, middle, and finer fractions of the material. The coarse and middlings fractions may need to be re-crushed and re-screened to achieve the acceptable final size.
2.2 Vibrating screens
Vibrating Screens are the single most important and most versatile screening machine for mineral processing applications. The success of vibrating screens has rendered obsolete many of the older screen types in the industry, such as shaking screens, reciprocating screens and other screen types.
A vibrating screen is a rectangular screen with a feed and an oversize discharge on either side of the screen. It performs size separations down to 300 mm (45 μm) and is used in a wide range of sizing, graduations, scalps, dewaterings, wet screens, and washing operations. Most types of vibrating screens can be made with multiple screening decks. On multiple-screen systems, a feed is fed into the upper coarse screen and the undersize falls through to the bottom screen decks. This produces a range of sizes from one screen.
A vertical, circular or elliptical vibration is induced mechanically by the rotation of unbalanced weights or flywheels attached usually to a single drive shaft. Addition or removal of weight elements bolted to flywheels can change the amplitude of throw. The direction of rotation can be perpendicular or in-direction. Contra-flow slows down the material more and allows for better separation, while in-flow allows for better throughput.
Horizontal vibrating screens have a horizontal or nearly horizontal protective surface, therefore they require less space than inclined screens. Horizontal screens must be vibrated with linear or elliptical vibration generated by biaxial or triaxial vibration. Horizontal screens have better particle size accuracy than inclined screens; however, since gravity does not help transport the material down the screen, their load capacity is lower than that of inclined screens. Horizontal screens are used in sizing where screening efficiency is critical, as well as discharge and wash screens in heavy medium circuits.
Photo 1 : Horizontal Screen
Dewatering Screens can feed thick slurry and produces a dewatered sand product. Dewatering screens are often installed with a slight up-hill incline to ensure that water does not flow over with the product. A thick bed of particles forms, trapping particles finer than the screen aperture.
Novelty Structures supplies various type of Screens for Mineral Processing Plants.
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