Introduction
Screening operations can produce high levels of dust because smaller-sized material is handled. Airborne dust is generated from the vibrating screen decks that accomplish the size separation. Additionally, material must fall some distance as part of the separation process, and some dust will be suspended by this process.
Dust control for screening
Dust control for screening systems is similar to that for crushers, although wet systems are generally not used due to blanking of the screen openings by the wet material. Also, screens are not normally subject to large surges of material flow as are crushers. This is due to the fact that screens operate most efficiently within a designed flow range.
Overloading screens, whether through excessive feed rate or large surges of material, can cause accelerated wear of components, reducing the efficiency of the operation and potentially increasing dust emissions. Screens should be totally enclosed, and water suppression systems (when compatible with the process) or dust collection and exhaust systems should be incorporated. Necessary openings in the screen enclosure must be minimized, and inspection and maintenance openings must be provided with tight-fitting closures.
Flexible materials (e.g., rubber or synthetic sheeting) must be used to seal openings between the moving screen components and stationary equipment and structures. Because the seal between moving and stationary components is under dynamic stresses whenever the screen is operating, it should be frequently inspected for signs of failure on a schedule that incorporates data from its operating history.
Some screen original equipment manufacturers offer parts and materials, such as covers and flexible sealing components designed specifically to fit their equipment well, and thus enabling creation of an effective dust control enclosure. Equipment vendors should be consulted to explore available control options.
Work Practices to Minimize Dust Exposure from Screens
The following practices should be observed where compatible with installed equipment and existing operating procedures.
- Clean equipment and area before and during work, when needed, preferably by washing or vacuuming.
- Stop material flow before cleaning screens if operationally feasible. The absence of flowing material minimizes the potential for dust emissions.
- When cleaning, only open the top or the bottom part of screen decks at a time. Do not open both at the same time. The larger the size of the opening, the less effective the ventilation system can be.
- Only open one screen at a time during cleaning. Do not open multiple screens simultaneously. Again, numerous openings decrease the capture effectiveness of a ventilation system.
- When cleaning screens, only use long-handled brushes that will provide more distance between the worker and the dust source.
- Consider variable-speed vibrators, which significantly reduce the frequency of manual cleaning of screens. The less time needed to clean screens, the less the potential for dust exposure.
- Open screen decks slowly to allow the internal exhaust system to function at the deck periphery. Opening decks rapidly can cause a swirling effect, with dust being released into the work area and potentially exposing personnel.
- Do not slam screen decks closed. Any material, no matter how slight, has the potential to become airborne due to the shock of deck closure.
- Decks must be kept clear of dust. A clean deck lessens the potential of dust emissions resulting from opening and closing decks, as well as from environmental factors such as wind.
- Maintain seals on screen decks where an airtight closure is desired. Decks that are properly sealed should remain sealed when all fasteners and sealing materials are in place.
- Closure and locking devices such as clamps and other fasteners must be well maintained. Fasteners are ineffective unless they are used as intended.
- Where compatible with the process, wash down areas around screens on a periodic basis. Periodic washing removes material before it accumulates to a degree where dry cleanup methods become necessary. Dry cleanup activities present a much higher potential for worker exposure.
Conclusion
To sum up, controlling dust in screening operations is essential for both safety and efficiency. By using proper enclosures, dust collection systems, or water suppression methods, and following best practices, dust emissions can be greatly reduced. Regular inspections, maintaining seals, and careful cleaning are important steps to minimize exposure and keep the workplace safe. When the right equipment and procedures are used, dust can be effectively managed, leading to better performance and a longer lifespan for screening systems.
Reference
NIOSH Mining Program Report of Investigations, Dust Control Handbook for Industrial Minerals Mining and Processing.
