Dust Control Handbook for Industrial Minerals Mining and Processing

DESIGN CONSIDERATIONS FOR TRANSFER POINTSEnclosures at both the head and tail ends of a conveyor are a very common practice because theyare effective at controlling dust at these locations and provide guarding safety. While theelimination of dust generation at conveyor transfer points is not feasible, effective dust control is.Designing the proper size enclosure is a critical factor because as the ore is dumped onto theconveyor, it entrains a measurable amount of air (venturi effect), and this can pressurize theenclosure if it is undersized. Enclosures for both conveyor and transfer points can be either fullor partial, depending on the various components of the system.Although this seems like a very simple process, significant dust generation and liberation canresult from transfer points if they are not properly designed and installed. The following aresome important design considerations for an effective transfer point or chute used with anexhaust ventilation system (also see Figure 5.15):It has been shown that the exhaust port of the transfer point enclosure should be a least6 feet from the dump point to minimize the entrainment and pickup of oversized particles[MAC 1980]. The air velocity at the base of the exhaust port cone should also be keptbelow 500 fpm to avoid the pickup of larger particles [Yourt 1990].Transfer point enclosures should be designed to have a 200 fpm intake velocity at anyunavoidable opening to eliminate dust leakage from the openings [USBM 1974]. TheMining Association of Canada (MAC) recommends adding 25 percent to the 200 fpmguideline [MAC 1980].It is important to minimize openings and thus preserve intake velocity by using plasticstripping and other types of sealing systems. It is recommended that the transfer pointenclosure be made large enough so that the air velocity within the enclosure is below200 fpm (1 m/sec) [Goldbeck and Marti 2010]. The larger enclosure can serve as aplenum where air velocity can dissipate. If an enclosure is undersized, air will be forcedfrom the interior high pressure of the enclosure to the lower pressure area outside,carrying dust particles outside the system.Transfer chutes should be sized to allow ore to flow without clogging or jamming. Ageneral rule of thumb is that the chute width should be at least three times the maximumlump size to avoid clogging [USBM 1987].The skirtboard for the transfer point should be high and long enough to serve as a plenumso that the dust has a chance to settle. On belts with minor air movement, a good rule-of-thumb is two feet per 100 feet/min of belt speed [Goldbeck and Marti 2010]. When beltshave a greater calculated air movement, the transfer point should approach three feet per100 feet/min of belt speed [Goldbeck and Marti 2010].The dump point of the ore should be designed to impact on a sloping bottom or an impactbed. Impact beds are designed to allow ore product to build up so that ore contacts ore attransfer locations or chutes to reduce wear and abrasion (Figure 5.16).Fall height of ore should be minimized whenever possible. Some methods to performthis are through the use of rock ladders, telescopic chutes, spiral chutes, and bin-loweringchutes.Any abrupt changes in product direction or flow should be avoided.144 Conveying and Transport