Bulk Bag Dusting - What Causes It?

A common rap against bulk bags is that unloading them can be a dusty mess.  In the first of a series of posts related to bulk bag discharging and dusting, let's examine what causes dust emissions.

1. Bulk bag design.  Unless a bulk bag has been specifically designed to contain the fines present in a given product, particles can sift through the bag fabric.  This looks like smoke escaping from the bag as it is being handled and emptied.  Depending on the amount of fines present in the product, the particle size and its shape, coated fabric may be all that is required to contain the dust.  However, in some cases polyethylene liners are required to prevent product sifting. 
2. Onset of product flow.  The point in time when the operator releases the outlet spout tie and product begins to flow can be a significant contributor to dust emissions.  If the product is flowable, when the outlet spout is released a down rush of product can occur - if the down rush occurs quickly this happens while the access door to your bulk bag unloader is still open.  The rapid drop of product causes an updraft of dust laden air that must be extracted properly or it will result in a cloud of dust being emitted into the atmosphere.  Look closely at how your bulk bag discharging equipment is designed to prevent this.
3. Product flow.  Bulk bag discharging equipment must provide suitable dust containment while the product is moving out of the bulk bag.  While this seems obvious, make sure your equipment can't loose its ability to extract dust during the discharge cycle due to the bag moving as it empties thereby altering the integrity of the 'seal' that the equipment relies on to maintain proper dust containment.
4. End of the discharge cycle.  As the final pounds of product empty from a bulk bag it may cause the discharging equipment's dust containment/extraction features to loose their ability to control dust.  This can result from the bag breaking its seal with the equipment as it looses its shape near the end of the discharging cycle.
5. Empty bulk bag removal.  Dusting can still occur even when the bag is 'empty'.  When a dusty product has been discharged from the bulk bag, the bulk bag is still full of dust laden air.  If the bag is removed from the discharger without re-tying its outlet spout the dust laden air will exit the bag in transit from the discharger and spread thorughout the immediate plant area.  Even if the outlet spout is retied prior to removing the empty bag, care must be taken in how it is handled when it reaches the disposal area - a bulk bag can't be folded or compressed without doing something with the dust laden air it contains.  Determine the best Standard Operating Procedure for your situation and ensure that your bulk bag unloading equipment supports it.  It may be necessary to use purpose-built bulk bag folders to make this step of the process dust free.

As can be seen dust containment and extraction must occur at multiple points in the bulk bag discharging cycle to maintain a clean and safe plant environment.  Careful equipment design and well thought out operating procedures are both required to optimize your bulk bag unloading operation.