High-frequency vibrating screens
High-frequency vibrating screens are the most important screening machines primarily utilised in the mineral processing industry. They are used to separate feeds containing solid and crushed ores down to less than 200 μm in size, and are applicable to both perfectly wetted and dried feed. The frequency of the screen is mainly controlled by an electromagnetic vibrator which is mounted above and directly connected to the screening surface. Its high-frequency characteristics differentiate it from a normal vibrating screen. High-frequency vibrating screens usually operate at an inclined angle, traditionally varying between 0° and 25° and can go up to a maximum of 45°. They should operate with a low stroke and have a frequency ranging from 1500 to 9000 RPM. Frequency in high frequency screen can be fixed or variable. Variable high frequency screen is more versatile to tackle varied material condition like particle size distribution, moisture and has higher efficiency due to incremental increase in frequency. G-force plays important role in determining specific screening capacity of screen in terms of TPH per sqm. G-force increases exponentially with frequency.
Pre-treatment of the feed is often required before the use of the high-frequency screen, as the apertures in the screen may become blocked easily.
Range of applications
High frequency screens have become more standardized and widely adopted in materials classification processes. They allow efficient cuts and fines separations, which can provide high purity and precise sizing control of the product. Common industrial applications include dewatering of materials, processing of powder in coal, ores and minerals, wood pelleting, fractionated reclaimed asphalt pavement, food, pharmaceutical and chemical industry. Fineness of the products and system capacities varies over a huge range between different models, to satisfy individual application requirements. It is also used effectively to process Manufactured sand for size segregation and removal of silt which is 75 microns below. For fine particle removal high G force is desirable and is achieved by higher frequency around 5000 to 6000 rpm.Fractionated reclaimed asphalt pavement
Most commonly, high frequency screens are used to separate "reclaimed" asphalt pavement into multiple sizes and fractions, which allow producers to take full advantage of the recycled materials. RAP is a recycle material that is reused in new pavement construction; any recycled products are worth as much as what they replace. As compared to conventional screening methods which are limited to producing unacceptable sizes in the products, high frequency screens can produce more efficient sizing to obtain a finer product. Another advantage of using high frequency screens to recycle the reclaimed materials is the available aggregate and oil that can be reused, and reducing the amount of new material required. Therefore, the capital cost for the process is lowered while maintaining a high quality of the asphalt mixture. Moreover, high frequency screen applies intensive vibration directly onto the screen media, such high RPM allows asphalt pavement material to achieve a higher stratification and separate at a faster rate.Mineral processing
In mineral processing, such as metals ore and nonferrous metals ores, high frequency screens have a crucial role. After the ores get comminuted, high frequency screens are used as a classifier which selects materials size that is small enough to enter the next stage for recovery. For example, the closed grinding circuit. Firstly, it screens out the coarse particles and recirculates them back to the grinding mill machine. Secondly, the fine-grained material will be unloaded in a timely manner, avoiding over-crushing caused by re-grinding. The benefits of using high frequency screens in mineral processing can meet the requirement of fineness easily for recovery and is able to achieve a smaller size separation, reducing capacity needed for comminution stage and overall energy consumption. Hence, improving the grade of the final product and providing a better recovery and screening efficiency.Advantages and limitations
The high frequency vibrating screens achieves a high efficiency of separation and differs from its counterparts since it breaks down the surface tension between particles. Also the high level of RPMs contributes to increasing the stratification of material so they separate at a much higher rate. Separation cannot take place without stratification. Furthermore, since the screen vibrates vertically, there is a 'popcorn effect' whereby the coarser particles are lifted higher and finer particles stay closer to the screen, thus increases the probability of separation. In some high frequency vibrating screens the flow rate of the feed can be controlled, this is proportional to the 'popcorn effect'; if the flow rate lowers, the effect is also decreased. Limitations of the high frequency vibrating screen are that the fine screens are very fragile and are susceptible to becoming blocked very easily. Over time the separation efficiency will drop and the screen will need to be replaced.An alternative to the high frequency vibrating screens is the rotary sifter. A rotary sifter uses a screen which rotates in a circular motion and the finer particles are sifted through the apertures. It is also generally used for finer separations; between 12mm to 45μm particle size. The rotary sifter will usually be chosen based on the nature of the substance being separated; whey, yeast bread mix, cheese powder, fertilizers. The rotary sifter is often preferred in the non-metallurgical industry and operates in a way to achieve a dust and noise free environment. The limitation for the rotary sifter is that it cannot handle a high capacity compared to the high frequency vibrating screen. Both equipment, however, achieve a high screening efficiency.
Available designs
Conventional and general design for a high frequency vibrating screen consists of mainframe, screen web, eccentric bock, electric motor, rub spring and coupler. The two most common types of vibrators which induce the high frequency vibrations are hydraulic or electric vibrators, these electric vibrators are either electric motors or solenoids. Common designs for screening decks are either single or double deck. In addition, another feature of high frequency vibrating screens are the static side plates which provide benefits such as smaller support structure, less noise, longer life, and hence less maintenance. In industry, the screens are operated at a tilted angle up till 40 º. The high frequency and low amplitude characteristics leads to the vertical-elliptical movement that rapidly transports oversized particles down the screen. Creating a thin bed of particles, this improves the efficiency and capacity of the screen.Stationary screens are typically used in plants and not moved around. In the mineral processing industry, equipment often has to be moved to different sites depending on the jobs taken up by a company. Mobile screens thus are another viable design for companies who have to move their equipment often. These include wheel-mounted and track-mounted plants which allow for easy transportation and movement of the screens. Typical mobile screen designs are shown in the diagrams on the right.
Main process characteristics
The screening performance is affected significantly by various factors such as equipment capacity and angle of inclination, in which the performance can be measured by screening efficiency and flux of the product.Flux is defined as the amount of a desired component that has carried over the screening media from the feed per time per unit area. Screening efficiency is expressed as the ratio of the amount of material that actually passes through the aperture, divided by the amount in the feed that theoretically should pass. Commercially perfect screening is considered to be 95% efficient if the process is operated with appropriate feed concentration and size particles. Generally, a suitable particle size difference between sieving and feed should be no more than 30%. High screening efficiency can reduce the qualified gain content in cyclic loading and screening and thus increasing the processing capacity of the mill.
The equipment capacity is almost directly proportional to screen width. This means that by increasing the length, there will be additional chances for passage, and will usually lead to increase in transmission and efficiency. In general, the standard size of screen length should be two to three times the width. However, certain special situations such as restricted space may require a different design.
Angle of inclination can be designed based on the desired mineral grain. For example, wet sieving angle is generally around 25 ± 2 ° for concentrator. Increasing the slope of a screen will effectively reduce the aperture by the cosine of the angle of inclination. At the same time, the materials also move across the screen faster which leads to more rapid stratification. However, the performance tends to decrease after a certain point since the slope of the deck is too high and most particles will remain on the oversized stream instead of passing through the aperture, thus, lower flux is yielded.
Table below presents relationship between inclined angle with desired product flux and efficiency.
| Inclined Angle | Flow Rate/Flux | Efficiency |
| 18 | 18.29 | 86.4 |
| 20 | 24.39 | 54.6 |
| 22 | 30.48 | 62.8 |
| 25 | 36.58 | 64.2 |
| 30 | 32.37 | 67.5 |
Characteristics assessment
Screen movement
The purpose of the vibrating screen is that particles are introduced to the gaps in the screens repeatedly. The frequency of the screen must be high enough so that it prevents the particles from blocking the apertures and the maximum height of the particle trajectory should occur when the screen surface is at its lowest point. Based on the principle, there is an optimum frequency and amplitude of vibrationTransmission refers to the fraction of desired particle that passes through the apertures in the screen. At low frequency, screening efficiency is high but blinding is severe. Blinding will decrease as frequency increases but the particles will have difficulty going through the apertures. When designing a high frequency vibrating screen, an optimum point of frequency and amplitude must be chosen, depending on the specific applications.