At the discharge of screen and crushers some stones spill over from the belt conveyor and get accumulated on the floor. This spilled over material needs to be periodically removed or else the piled up material beyond the height of the system drive component may hamper the operation of the belt conveyors etc. Such spillages are so frequent and large in quantity that almost once in a day it needs to be cleaned. This spilled material also contains large quantity of stone dust. It is due to this reason, in order to provide easy access for removal of material, one side of the enclosure is generally left open or even if a door is installed it always remains in open condition.
Such spillages happen due to systems faults or design lacunae such as misalignment and improper angles of discharge and belt conveyors, use of locally available material like used truck tyre tread pieces joined together to form the belt conveyor, lack of provision of rubber skirting etc. The possible solution to overcome this problem could be to provide well designed layout, proper/standard belt conveyor material, rubber skirting on the side of the belts, proper alignment and angle of the crusher discharge chute and belt conveyor etc
The product spillages also occur due to holes formed in the discharge chute of crushers and screens at the point of free fall of stones as the steel plate gets punctured due to wear and tear and eventually gives way for spillages. This could be minimized by adopting a step wise design of the chutes to reduce free fall height or by strengthening the steel at the point of impact. In the step design, a layer of stones always remains above the steps and the free fall impact is taken by these accumulated stones which saves the steel from direct wear and tear and increases its life.
c) Enclosures for Belt Conveyor & Dust Hopper and Constraints Reported
Some crushers use Jute Cloth in place of GI steel sheets as material of construction for enclosures to reduce the cost. But wind current can pass through jute cloth enclosure thereby reducing the usefulness of the enclosure in terms of containment of dust. At times plastic/PVC sheets are used for enclosure especially for the belt conveyors and vibratory screens but it gets torned frequently due to high velocity wind currents and therefore not effective enclosures.
Many units provide an enclosed hopper for collection and storage of less than 6mm size dust product, but a large opening is kept at the point of discharge from
belt conveyor in to the hopper and fine dust escapes through this opening. To achieve effective containment the belt discharge opening should be kept closed as much as possible by providing rubber flab type flexible materials in contact with the moving belt conveyor.
Partial enclosures with GI sheets provided for Primary and Secondary Screens. However, large gaps/window opening etc is provided through which dust escapes outside the enclosure. Some smaller units have enclosures made of Gunny Bag Cloths or Plastic Sheet etc.
G.I. Sheet/Polythene Sheet provided over the belt conveyors in some large stone crushers. Small and medium size SCUs generally do not have such enclosures.
Hardly any Crushers have enclosures for crusher discharge area, from where substantial dust emission occur.
In the State of Andhra Pradesh, all the products of various sizes are prominently stored in fully enclosed elevated hoppers made of steel and the material is directly loaded from the hoppers into the trucks. This arrangement eliminates secondary dust emissions from open stock piles which generally is quite predominant in most states. However, the dust suppression arrangements for controlling the emissions from the crusher area is generally not satisfactory.