Screening is a process of separating into groups of different products of various sizes. Each group consists of products of approximately same size. The mass, which remains, on a screen is called the oversize of the screen and the material, which passes through the screen is, called the undersize.
The screens in between the primary and the secondary crusher help in separating the oversize. The oversize goes to the secondary crusher while the undersize is discharged through a chute on to the same conveyor belt, which receives the discharge from the secondary crusher. This saves considerable power in the second crushing stage and also increases the capacity of the secondary crusher, as it eliminates possible choking of material in the crusher, particularly during wet weather or wet material.
Screening is generally classified into two types.
(i) Coarse screening, which is achieved through Grizzlies, Vibratory Screens, Revolving Screens or Shaking Screens.
(ii) fine screening which is achieved through Vibrating screens, Shaking Screens. Brief description of various screening technologies is given below.
Grizzlie type Screens
The Grizzlies are normally used to remove “undersize” before primary or secondary crushing. The width of a Grizzly should be about 3 times that of the largest piece of material. The bars are usually iron or steel castings, tapered towards the lower end to avoid clogging. Grizzlies are of three types, Stationary, Cantilever and Mechanically vibrated types.
Stationary Grizzlies:
Coarse screening of dry type material is done with the help of stationary grizzlies. It is inclined (plus or minus 300) so that the feed slides down appreciably. The grizzly is made self-clearing by having bars placed in such a way that there is maximum width at the top and minimum width at the bottom. The same can be achieved by having suitable cross-section of the bars.
Cantilever Grizzlies:
The grizzly is fixed at one end and the discharge end overhangs and is free to vibrate. The vibration of the bars result from impact of the material. This type of grizzly is more effective than the stationary type due to its design. (there are no cross rods which are found in the stationary type). The cross rods lead to clogging in the stationary grizzly. This type of grizzly can handle larger tonnages, it is simple to use and easy to repair and operate.
Mechanically Vibrated Grizzlies:
These grizzlies are simply bars mounted on the eccentrics. The entire assembly is given to and fro motions at some 100 strokes per minute. This is the most common type of grizzly used.
Vibrating Screen
Vibrating Screens save space and weight, and operate on little power because the screening surface may be actuated by vibrating, gyrating or pulsating movement of small amplitude, but at frequencies that normally exceed 3000 cycles per minute.
Combination of frequency of vibration (speed) and amplitude (throw), affects performance of any vibrating screen. A correct combination of slope and direction of mechanism of rotation is vital for inclined screens. The larger the opening the greater the amplitude needed for a screen. Vibrating screens are mainly divided into two classes. Mechanically vibrated and electrically vibrated.
The mechanical shaking screen comprises of a rectangular frame with perforated steel or wire cloth. It is usually inclined and suspended on loose rods or cables. They are used mainly for special tasks of coarse screening. The screening media that are normally in use are of many classes. Following are some of the commonly used media in the stone crushing industry.
• Bars or rods of the grizzly vibrator or feeder .
• Perforated screen plates, sheets, or plates of metal with round or squares openings.
• Wire cloth (woven of metal wires or fabric) for very fine screening,
• Rubber-clad perforated screen plate
• Profile screens having small parallel tapered bars for de-watering and fine screening • Polyurethane modular panels
• Polyurethane tensioned mats
• High tensile steel wire screen decks.
Rotary Screen
Rotary Screens are cylindrical shape perforated steel cylinders, which keeps rotating during screening operation. The crushed stones from the crusher are fed at one end and travels to the other end and during process material of various sizes gets screened as per the diameter of the perforated screen. The production capacity of the rotary screens is limited and therefore predominantly used by small crusher and these are hardly used by medium and large capacity stone crusher .
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Screening Technologies: Grizzlie type Screens, Vibrating Screen, Rotary Screen
Screening is a process of separating into groups of different products of various sizes. Each group consists of products of approximately same size. The mass, which remains, on a screen is called the oversize of the screen and the material, which passes through the screen is, called the undersize.
The screens in between the primary and the secondary crusher help in separating the oversize. The oversize goes to the secondary crusher while the undersize is discharged through a chute on to the same conveyor belt, which receives the discharge from the secondary crusher. This saves considerable power in the second crushing stage and also increases the capacity of the secondary crusher, as it eliminates possible choking of material in the crusher, particularly during wet weather or wet material.
Screening is generally classified into two types.
(i) Coarse screening, which is achieved through Grizzlies, Vibratory Screens, Revolving Screens or Shaking Screens.
(ii) fine screening which is achieved through Vibrating screens, Shaking Screens. Brief description of various screening technologies is given below.
Grizzlie type Screens
The Grizzlies are normally used to remove “undersize” before primary or secondary crushing. The width of a Grizzly should be about 3 times that of the largest piece of material. The bars are usually iron or steel castings, tapered towards the lower end to avoid clogging. Grizzlies are of three types, Stationary, Cantilever and Mechanically vibrated types.
Stationary Grizzlies:
Coarse screening of dry type material is done with the help of stationary grizzlies. It is inclined (plus or minus 300) so that the feed slides down appreciably. The grizzly is made self-clearing by having bars placed in such a way that there is maximum width at the top and minimum width at the bottom. The same can be achieved by having suitable cross-section of the bars.
Cantilever Grizzlies:
The grizzly is fixed at one end and the discharge end overhangs and is free to vibrate. The vibration of the bars result from impact of the material. This type of grizzly is more effective than the stationary type due to its design. (there are no cross rods which are found in the stationary type). The cross rods lead to clogging in the stationary grizzly. This type of grizzly can handle larger tonnages, it is simple to use and easy to repair and operate.
Mechanically Vibrated Grizzlies:
These grizzlies are simply bars mounted on the eccentrics. The entire assembly is given to and fro motions at some 100 strokes per minute. This is the most common type of grizzly used.
Vibrating Screen
Vibrating Screens save space and weight, and operate on little power because the screening surface may be actuated by vibrating, gyrating or pulsating movement of small amplitude, but at frequencies that normally exceed 3000 cycles per minute.
Combination of frequency of vibration (speed) and amplitude (throw), affects performance of any vibrating screen. A correct combination of slope and direction of mechanism of rotation is vital for inclined screens. The larger the opening the greater the amplitude needed for a screen. Vibrating screens are mainly divided into two classes. Mechanically vibrated and electrically vibrated.
The mechanical shaking screen comprises of a rectangular frame with perforated steel or wire cloth. It is usually inclined and suspended on loose rods or cables. They are used mainly for special tasks of coarse screening. The screening media that are normally in use are of many classes. Following are some of the commonly used media in the stone crushing industry.
• Bars or rods of the grizzly vibrator or feeder .
• Perforated screen plates, sheets, or plates of metal with round or squares openings.
• Wire cloth (woven of metal wires or fabric) for very fine screening,
• Rubber-clad perforated screen plate
• Profile screens having small parallel tapered bars for de-watering and fine screening • Polyurethane modular panels
• Polyurethane tensioned mats
• High tensile steel wire screen decks.
Rotary Screen
Rotary Screens are cylindrical shape perforated steel cylinders, which keeps rotating during screening operation. The crushed stones from the crusher are fed at one end and travels to the other end and during process material of various sizes gets screened as per the diameter of the perforated screen. The production capacity of the rotary screens is limited and therefore predominantly used by small crusher and these are hardly used by medium and large capacity stone crusher .