Screening is one of the most important parts in aggregate production. It takes place before and/or after the crushing stage (or stages) and produces the specifications required of the final products.
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The purpose of screening is to separate a flow of aggregate particles into uniform sizes. This is accomplished by using Vibrating Screens.
Vibrating Screens are machines used to separate aggregates by particle size by means of an exciting force applied to the machine. The vibration lifts the material, causing it to stratify, allowing the smaller material to move from the top of the depth of material to the bottom of the material, thus exposing them to the screening surface.
There are two main types of aggregate vibrating screens for dry screening applications:
The primary difference between Horizontal Screens and Inclined Screens is the Inclined Screen uses gravity to help with the flow and conveyance of the feed material.
However, both screens offer advantages for various applications.
Inclined Screens are the most popular for aggregate applications. They are installed on an incline (anywhere from 5-45 degrees depending on the type of screen and the required cut) and employ the use of gravity to propel material down the screen. Since gravity helps to move the material, this type of screen requires low energy and low stroke.
Horizontal Screens are run flat at zero degrees but can be inclined or declined as much as 10 degrees in either direction. Because they don't rely on gravity to move the material forward, Horizontal Screens require higher energy and a larger stroke.
These types of screens are ideal for low-profile applications where height requirements are limited, which makes them a great choice for portable plants. In stationary plants, they are often used as tertiary or finishing screens. Finer screening applications can greatly benefit from this type of screen.
Known for their efficiency and reliability, Horizontal Screens offer more accurate particle sizing. The high G-forces required to move the material ensure screen plugging is virtually eliminated, allowing for more continuous open area. The flat orientation of the Horizontal Screen allows material to be retained on the screen longer for higher efficiency. It also makes them a great choice for wet screening applications.
Because screening plays such an important role in producing the required specifications of a final product, it is imperative that the right screen is selected for the job.
Vibrating Screen selection is based on the following factors:
Vibratory Screens come in a variety of sizes. They can range from 4' to 12' wide and 6' to 32' long. The width of the screen determines the carrying capacity of the screen deck, while the length of the screen determines the overall efficiency of the screen. Typically, the length of the screen is 2.5 to 3 times the size of the width.
Every manufacturer has its own screen sizing formula. Screens are tailored to an operation based on the following factors:
Vibratory Screens can be configured with one, two, three or four decks. Some fine screening operations can even have as many as eight screening decks. Three decks are pretty common in the aggregate industry, but the number of screen decks depends on the number of products the site needs to make.
In a typical application where multiple products are being made, the top deck(s) of the screen makes the coarse cut, the middle deck(s) makes a middle cut and the bottom deck(s) make the fine cut.
Each screen deck is covered with screen media containing the openings for the particles to pass through. Screen media comes in many different forms, and the type of screen media plays an important role in screening efficiency.
Screen media is all about open area, or the number of openings in the screen. The more openings in the screen, the more opportunities the particles will have to pass through and the more efficient the screen will be.
However, more openings in the screen can decrease the wear life of the media. The correct screen media for an application will balance both wear life and efficiency.
Common types of screen media for aggregate applications include:
Wove wire cloth is the most consistent, versatile performing media product. It averages 50-70% open surface area in most configurations and provides the most flexibility for operations that need to make frequent media changeouts due to varying product specifications. Overall screening efficiency is good with this type of media.
Wire media
Polyurethane is better suited for wet/wash screens and dry applications with highly abrasive materials. It provides 30-40% less open area than wire cloth; however, polyurethane offers extended wear life over wire media.
Polyurethane media
Rubber media is used for heavier, coarser materials that can damage standard wire screens. It is most often used for dry applications or abrasive materials. Rubber media has less openings than wire cloth, and therefore less throughput; however, they last much longer than wire screens.
Rubber media
Hybrid media combines wire screen with reinforced rubber or urethane strips to fit screen supports. It is highly popular in dry screening applications with high moisture materials and a high proportion of fines. This type of media works to eliminate blinding and pegging, which ends the need to stop production to clean the screen. It provides more open area than polyurethane and lasts longer than woven wire. Depending on the application, production can increase 40% using hybrid screen media.
When sizing and selecting a Vibrating Screen for an aggregate operation, be sure to provide the manufacturer with as much accurate detail about the feed material and the application as possible. From there, they can recommend what type of screen (Inclined or Horizontal) is best for the site, as well as the number of decks required and the type of screen media that will provide the best wear life and efficiency.
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Vibrating screen is a mechanical equipment that uses vibrating screen panels to classify the mixture which contain different particles according to particle size.
According to different structure and use, vibrating screens usually be devided into many types by the vibrating screen manufacturers. Below wil introduce the top 10 vibrating screens, and how to choose the right vibratory screen?
1. linear Vibrating Screen
linear vibrating screen is driven by double vibrating motors. When the two vibrating motors are rotating synchronously and reversely, the excitation force generated by the eccentric block offsets each other in the direction parallel to the axis of the motor, and overlaps into a resultant force in the direction perpendicular to the axis of the motor, so the motion track of the linear vibrating screen machine is a straight line.
Working Principle: The two motor axes of the linear vibrating screen have an angle of inclination in the vertical direction relative to the screen panel. Under the combined force of the exciting force and the self gravity of the material, the material is thrown on the screen surface to jump or move forward in a straight line. Through the multi-layer screen panels, a variety of specifications of materials are generated, and discharged from their respective outlets, so as to achieve screening and classification.
linear vibrating screen is suitable for screening various dry powder or granular materials with particle size of 0.074-5mm, moisture content <7%, and no viscosity. The feed particle size is not more than 10 mm.
2. Circular Vibrating Screen
Circular vibrating screen is a new type of vibrating screen with multi-layer screen and high efficiency. According to the type of materials and the requirements of users, the high manganese steel woven screen, punched screen plate and rubber screen plate can be used. The circular vibrating screen is installed in the seat type. The adjustment of the screen surface angle can be realized by changing the position and height of the spring support.
Working Principle: The motion track of the screen box of the circular vibrating screen is circular. The circular vibrating screen uses the inertia exciter to produce vibration. The main shaft fixed on the screen box is driven by the motor to rotate at high speed, and the eccentric body installed on the main shaft rotates with it, generating centrifugal inertia force, so that the screen box that can freely vibrate will produce vibration similar to the circular track.
Circular vibrating screen is widely used in the materials classification of mining, building materials, transportation, energy, chemical industry and other industries because of its long flowing line and many screening specifications.
3. Elliptical Vibrating Screen
Elliptical vibrating screen is a vibrating screen with elliptical motion track (Elliptical Shale Shaker), which has the advantages of high efficiency, high screening accuracy and wide range of application. Compared with the ordinary sieve machine of the same specification, it has larger processing capacity and higher screening efficiency.
Triaxial elliptical vibrating screen is widely used for the screening operation of sand and stone materials in sand plant. It is the ideal screening equipment for all kinds of mines, quarries and mobile screening stations.
Working Principle: The power is transmitted from the motor to the main shaft of the exciter and the gear vibrator (speed ratio is 1) through the V-belt, so that the three shafts can rotate at the same speed and generate the exciting force. The exciter is connected with the high-strength bolts of the screen box, resulting in elliptical movement.
Materials on the screens do high-speed elliptical movement along with the screen machine, so as to achieve uickly separate, sift and move forward, and ultimately complete the classification of materials.
4. Roller Screen
The working surface of the roller screen is composed of a series of rolling shafts that arranged horizontally, on which there are many screen plates. When working, the fine material passes through the gap between the roller or screen plate, large blocks of materials are driven by rollers, moving to the ends and discharging from the outlets. Roller screens are mostly used in the traditional coal industry.
Working Principle: For the rolling shafts are arranged according to different working angles, the speed of the material is faster when it runs in the position with higher working angle; the speed of the material is slower when it runs in the position with lower working angle.
When two kinds of materials running at different speeds converge at a certain position on the screen surface, they start to move axially, so that the materials are evenly distributed on the screen surface, and the screening efficiency is improved.
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5. Rotary Vibrating Screen
Rotary vibrating screen is mainly used for the classification of materials with high screening efficiency and fine screening accuracy. Fully closed structure, no flying powder, no leakage of liquid, no blocking of mesh, automatic discharge, no material storage in the machine, no dead angle of grid structure, increased screen area.
Any particle, powder and mucus can be screened within a certain range. Sieve to 500 mesh or 0.028mm, filter to 5 microns minimum. It can be used for classification, classification and filtration in food, chemical, metal, mining and other industries.
With the help of the heavy hammer installed at the upper and lower ends of the motor shaft, the rotary motion of the motor is transformed into a horizontal, vertical and inclined three-dimensional motion, which is then transmitted to the screen surface to make the material do an outward involute motion on the screen surface.
Working Principle: After the rotary screen is started, the eccentric blocks of different phases at the upper and lower ends of the vibrating motor generate a composite inertia force, which forces the vibrating body of the rotary screen machine to rotate again and again, and the screen frame continuously moves to and fro under the action of the vibration force, and then drives the screen surface to vibrate periodically, so that the materials on the screen surface move in a positive and directional manner together with the screen box. Materials smaller than the screen meshes fall to the lower layer, and the materials larger than the screen meshes discharged from the discharge port.
6. High Frequency Vibrating Screen
High frequency vibrating screen is also called high frequency screen for short. High frequency vibrating screen (high frequency screen) is composed of exciter, pulp distributor, screen frame, supporting, suspension spring and screen, etc. High frequency vibrating screen is the most important screening machine in mineral processing industry, which is suitable for completely wet or dry raw materials.
Working Principle: Different from ordinary screening equipments, high frequency screen adopts high frequency, which destroys the tension on the pulp surface and makes the fine materials vibrate at high speed on the screen, accelerates the separation of useful minerals with large density (specific gravity), and increases the probability of contact between the materials with smaller than the separated particle size and the screen holes.
As a result, high frequency screen results in a better separation conditions, which makes the materials that smaller than the separation size (especially with larger specific gravity), and pulp pass through the screen holes together to complete the screening.
High-frequency vibrating screen is usually operated at an inclined angle 0°-25°, up to 45°, and the operating frequency range is -r/m.
7. Grizzly Screen
Grizzly screen is often used for pre-screening before coarse and medium crushing of materials. The mesh size is generally >50mm, but sometimes <25mm.
Grizzly screen has simple and solid structure, no power consumption, no moving parts, low equipment cost and convenient maintenance, but the productivity is low, the screening efficiency is not high, generally 50% ' 60%, and the mesh is easy to be blocked.
Working Principle: Generally, the grizzly screen is placed in an inclined position, and the materials are dumped at the high end. Under the action of its own weight, it slides down the screen surface, and the particles smaller than the gap or hole on the screen surface pass through the screen to achieve classification.
The inclination angle of the screen is related to the static friction angle of the material. Generally, the inclination angle is 35°-45°, and the viscosity material can reach about 50°.
8. Banana Screen
The banana screen adopts a screen plate with multiple sections and different dip angles, the longitudinal section is broken line and the whole screen is like banana shaped.
Banana screen is mainly suitable for the classification of large and medium-sized materials with high content of fine particles, and it can also be used for dehydration, demineralization and desliming.
Working Principle: During operation, the motor is connected with the vibration exciter through the V-belt or soft connection. The motor drives the eccentric shaft to rotate to generate centrifugal inertia force, which forces the screen box to vibrate. After the materials fall into the screen from the feeding inlet, they move forward rapidly under the action of the vibration force, loosely and pass through the screen.
The thickness of the material layer of banana screen from the feeding inlet to the discharging outlet is constant. The ratio of the material quantity to the flow speed on the screen of each section is stable, the material layer is high and uniform. The screening efficiency of banana screen is higher, which is 1-2 times higher than that of other screening machines with the same effective area.
9. Inclined Screen
Heavy inclined screen can be applied to the treatment of debris from quarry, mine and building demolition, the treatment of topsoil, the recycling of construction materials, the screening of gravel, and the screening of gravel and aggregates.
A variety of combination of screen box forms can be woven screen, welded screen, self-cleaning screen, piano wire screen, needle screen, perforated plate screen.
Working Principle: The screen box shaft is driven by the motor installed on the auxiliary frame through three V-belts, the auxiliary frame is rigidly connected with the machine underframe, and the screen box spring is used to support the screen box.
Inclined screen usually adopts 2-4-layer screen panels, and is fixed on the inclined frame at an angle between 15° and 30°. The material can be screened into 3-5 grades at the same time.
10. Horizontal Screen
Horizontal screen has the advantages of both inclined screen and linear vibrating screen. horizontal screen has the features of good screen permeability, large processing capacity and small installation height.
The installation angle of common vibrating screen is 15°-30°, while the installation of horizontal screen is parallel to the ground, or slightly inclined 0-5°. Horizontal screen is an ideal equipment for all kinds of mines, quarries and mobile screening stations.
Working Principle: Horizontal screen is designed with oval stroke. The advantage of this design is that it can change the oval big diameter length and angle of throwing material stroke to meet the best needs of vibrating screen. The oval stroke is easy to adjust by center gear, counterweight wheel and motor speed.
How To Choose Right Vibratory Screen
Different types of vibrating screens can be used for the same material to get different screening effects. The reasonable selection of vibrating screen is an effective way to improve vibration efficiency and maximize economic benefits. Generally, you need to consider the following 5 tips:
1. Characteristics of MaterialsWhen choosing vibratory screen, the material characteristics should be taken into account, including the content of material particles under the screen, the content of difficult screen particles, material moisture, the shape and specific gravity of the material, and the content of clay.
2. Structure of Vibratory ScreenTips: Heavy vibrating screen is used for materials above 200mm; circular vibrating screen is used for materials above 10mm; linear vibrating screen and high frequency vibrating screen are used for desliming, dewatering and grading.
When selecting the vibratory screen, the screen areas, layer numbers, shape, size and area ratio of the screen holes, as well as the motion mode, vibration frequency and amplitude of the vibrating screen should also be considered.
The screening efficiency of the vibratory screen is related to the motion form of the screens, so the fixed screen is lower than that of the moving screen.
3. Screening PurposeTips: In order to keep the screens under good working conditions, the ratio of screen length to width should be in the range of 2-3; when there is much viscous mud and high moisture in the material, double deck screens should be avoided as far as possible.
Vibratory screens need to be selected according to different screening purposes. If it is necessary to pre screen materials, circular vibrating screens are generally used; for the classification and screening of crushed materials, large vibrating screens and probability screens are selected; for the deionization and dehydration of materials, linear vibrating screens are better; if it is necessary to desliminate and clean up materials, probability screens are used.
4. Process RequirementsWhen selecting the shale shakers, it also needs to be considered according to the processing capacity of the production line, screening method, screening efficiency and the tilt angle of the shale shakers.
5. Professional Vibrating Screen ManufacturerProfessional vibrating screen manufacturers could provide competitive vibrating screen price, diversity customized vibrating screen models, timely after-sales service, spare parts and can continue to provide services for customers' whole production circle.
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