Bulk Bag Discharge in Building Materials Plants

In a cement or plaster plant the bulk bag station is exposed to the two things that destroy equipment: abrasive dust and weather.

The powders are fine, hard and often hygroscopic, so they wear the contact parts, they escape through any gap, and a bag that has stood in a damp yard discharges as lumps rather than as powder.

The station is also often outdoors or in an open bay, which means the extraction has to work against draughts that a clean indoor plant never sees.

A station that empties abrasive, cohesive powder completely without releasing dust into a yard where it will be visible for a hundred metres.

Practically: a sealed connection, extraction that works in a draught, wear parts that can be changed quickly, and a frame that tolerates bags that have been stored outside.

Extraction that works in the conditions the station actually stands in. A hood sized correctly in still air does nothing in an open bay with a crosswind, and most stations in this sector are in exactly that position.

In chemical batching the same station is specified against the whole material list rather than against one powder, because a frame built for the first product meets the twelfth.

The bag is lifted into the frame, usually by forklift in this sector, and the spout is sealed into the hopper before it is untied.

Massaging is normally essential rather than optional: cement, lime and plaster consolidate in the liner, and a bag that has stood for weeks will not empty on its own. Paddles that work the lower third of the bag do most of the useful work.

Below the spout, a sealed hopper with a generous outlet takes the powder, and a screw or rotary valve meters it onward. The outlet is sized for the consolidated condition, not for the way the powder behaved when it was packed.

Extraction is placed at the connection and shielded from draughts. In an open bay, a hood that would work indoors loses the dust to the wind.

Moisture is the failure that is specific to this sector. Cement, lime and plaster react with water rather than simply caking, so a bag that has stood in rain contains hard lumps that massaging will not recover and that block the outlet. Where the yard has no cover, that is the design case and the station needs a lump breaker or a screen rather than more vibration.

• Seal first, open second, massage always.
• Outlet sized for the stood condition.
• Extraction shielded against draughts.
• Lump breaker or screen where bags are stored uncovered and hydraulic binders have reacted.
• Hood shielded against draughts, since most stations here stand in open bays.
• Respirable silica exposure measured, because the fraction that matters is invisible.

• Consolidation after storage, which decides the massaging requirement and the outlet size.
• Abrasiveness, which decides the wear parts and how quickly they can be changed.
• Moisture exposure of the bags before they arrive; hydraulic binders react rather than simply caking.
• Draughts at the station, which decide the hood design and the extraction volume.
• Explosion characteristics where the product is combustible, which not all building material powders are.
• Silica content, which turns the respirable fraction into a regulated exposure.
• Lump handling where hydraulic binders have reacted with moisture in storage.