High-frequency vibrating screens

Most commonly, high frequency screens are used to separate "reclaimed" asphalt pavement (RAP) 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.[2] 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.[3]

In mineral processing, such as metals ore (e.g. iron, tin, tungsten, tantalum etc.) and nonferrous metals ores (e.g. lead, zinc, gold, silver and industrial sand etc.), 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 (e.g. recirculating network with ball mill). 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.[4] 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.

Dependence of screening efficiency on the amplitude and frequency of vibration

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 vibration [5]

Transmission 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,[5] depending on the specific applications.

The separation efficiency is simply a measure of the amount of material removed by the screen compared to the theoretical amount that should have been removed. Screen efficiency can be obtained using different equation, which depends on whether the desired product is the oversize or undersize fraction from the screen.

The screen efficiency based on the oversize (E_o) is given by:

${\displaystyle E_{o}={\frac {Q_{ms}(o)\;[1-M_{u}(o)]}{Q_{ms}(f)\;[1-M_{u}(f)]}}}$

The screen efficiency based on the undersize (E_u) is then given by:

${\displaystyle E_{u}={\frac {Q_{ms}(u)\;M_{u}(u)}{Q_{ms}(f)\;M_{u}(f)}}}$

where Q_ms(o) is the mass flow rate of solid in the screen overflow, Q_ms(f) is the mass flow rate of solid feed, Q_ms(u) is the mass flow rate of solid in the screen underflow, M_u(o) is the mass fraction of undersize in the overflow,
M_u(f) is the mass fraction of undersize in the feed, M_u(u) is the mass fraction of undersize in the underflow.[6]

The overall efficiency (E) is given by:

${\displaystyle E=E_{o}\;E_{u}}$

The selection on the screen type will be based on the materials that the equipment will be used to process. A significant problem occurs with screens because if the screen is not suitable for the material fed to the screen, the materials will blind the apertures and regular maintenance will be required. Different types of screens have been developed to counter this problem. An example is the "self-cleaning" wire; these wires are free to vibrate and so resistance to blinding will increase. The particles will be shaken off the wires and apertures. However, there will be a trade-off with screening efficiency.[6]

The high frequency vibrating screens will often be used as a secondary screener as its purpose is to separate the finer minerals. This not only ensures good separation efficiency, it will also help to maintain the life-time of the screen. Blinding can occur significantly if particle sizes are not within the screens' designed criteria.[5]

Another problem that is often encountered is that the particles clump together due to moisture. This clumping will result in undesired increase in effective particle size, with the result that the clumped particles are not allowed to pass through the apertures into the product stream. It is recommended that screening at less than around 5mm aperture size is normally performed on perfectly dry materials.[6] A heated screen deck may be used to evaporate the moisture in the feed. It will also break the surface tension between the screen wire and the particles. An alternative is to run the feed through a dryer before entering the high frequency vibrating screen.

The research on high frequency screens has led to new developments in the field which enhance the operation and performance of the equipment. These new developments include the stacking of up to 5 individual screen decks placed on top of the other and operating in parallel. A divider system splits the feed slurry to each Stack Sizer screen, then to each screen deck on the machine. Each screen deck has an undersize and oversize collection pan which will respectively go into their common outlet. The stacking of the machines thus allows more production while using less space.[15] Another new development is the fabrication of Polyweb urethane screen surfaces that have openings as fine as 45 μm and open areas from 35% - 45%. This leads to the screen being able to separate finer particles. The screens can be used for both wet and dry applications and urethane formulation is still an ongoing process. Thus, research and development is still being invested in high frequency screening equipment to improve the overall separation efficiency and also to lower costs.[16]

To further optimize the performance for high frequency vibrating equipment, a "variable speed" hydraulic vibrator is being developed and used to drive the screen decks. It utilizes fluid hydraulic force which then can be converted into rotary power in order to generate high frequency vibration.[17] This modification allows equipment to operate at higher frequency range, up to 8200 RPM, compared to the conventional electric vibrators. Special Electric vibrator motors are also used to have variable frequency ranging from 3000 to 9000 rpm and have proved to be more efficient and trouble free with less maintenance. Besides that, the induced vibration also creates an excellent condition for separating finer particles and improves the contacting probability for the materials. Another variation that could be applied to the equipment is the "rotary tensioning system", in which it helps to provide a quicker screen media change.[10] Therefore, multiple applications can be achieved by single equipment, as with different size of feed material can be deal by replacing screens in a very small downtime. Hence, it improves the economic benefits of plants. One important change is multi-slope deck in high frequency screen making it more efficient with incremental throughput and efficiency in same screening area.