Page 42 - HUB-4 Magazine Issue 85

Quarrying News
Conveyor Dust Emissions
The volume of conveyor dust emissions is dependent on the conditions and the application. Operators and designers should first know the material characteristics and how they change as they pass through the processing and conveying system. Examples of this can be found in most industries from cement to mining as raw material is reduced from large to small, processed and stockpiled.
Year-round weather conditions can cause dust, especially during dry seasons, which pull humidity out of the air, increasing emissions. Prevailing winds or changes in wind patterns can suddenly shift a dust-free operation into one with several violations.
Conveyor dust emissions are also derived from many sources including transition points, material impact and cargo disruption:
• Dustatthetransitionhappenswhenmaterialfallstothe belt, hitting the sides of the chute or rock boxes on the way down. As it falls, material not contained by a drop chute (such as onto a stockpile) separates, exposing the entire stream to ambient air currents allowing emissions.
• Dustonimpacthappenswhenmateriallandsonthebelt with no controls such as a rock box or curved drop chute. Depending on the height, weight, and density of the material, the impact on the belt causes air turbulence and shifting leading to airborne emissions.
• Dustfromdisruptionhappensalongtheconveyorpath, often between idlers where the belt slumps. This causes the cargo to shift and bounce slightly along the belt path, disrupting material, and causing dust emissions.
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Dust suppression can be engineered controls like enclosed transfer points. Dust collection include dust bags or mechanical air cleaners.
Loading the Belt
Loading zone design has shifted over the past decade in response to the need for dust reduction and greater efficiency and is an excellent example of an engineering control. Rather than transitions from conveyor to conveyor or storage container to conveyor that are straight drops from heights,
chutes direct and control material flow using spoon designs. These designs ensure that material is loaded in the center of the belt with little impact. This reduces dust, spillage, mistracking and belt damage commonly associated with conveyor transitions.
Copyright © Martin Engineering 2024
A controlled material flow with dust curtains help dramatically minimize dust emissions in the loading zone.
The impact of material on the belt can cause a splashing effect and produce air turbulence that seeks exit points from the chute through gaps between the skirt and the belt created by the slump between impact idlers. These rollers also tend to break under long-term pressure, causing them to seize. So, another innovation replaces impact idlers with bed of steel angles lined by energy-absorbing impact bars with a top layer of low friction, ultra-high molecular weight (UHMW) polymer or polyurethane. The bar design helps the skirtboard sealing systems consisting of a wear liner and skirting to retain a consistent seal at the loading point to reduce the amount of spillage and dust emissions.
Eliminating moving parts and the requisite lubrication of rolling components drastically reduces the amount of maintenance and improves safety by promoting a tight seal between the skirting and the bar. Some manufacturers have even developed innovative designs that mount the cradles on rails, allowing slide-out removal to reduce maintenance time and improve safety.
Dust During Transport
After cargo has been loaded, the stilling and settling zones of the transfer enclosure should be properly designed to have a sealed environment that controls airflow with negligible dust emissions. This design should include closely set idlers or idlers that transition between cradles. Another important element is continuous external skirting in single strips that run the length of the enclosure on either side. Inside the enclosure, strategically placed dust curtains slow airflow enough for particulates to settle back into the cargo stream. Dust bags and compact mechanical air cleaners, when installed on the enclosure, will also capture dust and ensure a dust-free exit from the enclosure.
Once the material has left the enclosure, wind can be an issue so many operators cover exposed conveyors. However, this does not control dust from material shifting, disruption over idlers, or mistracking. These actions can cause spillage and dust to fall along the length of the conveyor. Installing tracking devices along the belt path helps reduce spillage from mistracking even if cargo shifts.
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www.hub-4.com March/April - Issue 85