Handling Hazardous Ingredients In Bulk Bags

Increasingly, bulk bags are used to ship 'hazardous' dry bulk solid materials.  This post investigates what you need to know to handle them.  Note that we will address the topic of how bulk bag manufacturers make bags to meet relevant regulations related to robustness and package integrity in a subsequent post.

First, let's discuss what hazardous means.  The United Nations defines hazardous goods as follows:  "Definition of 'dangerous goods' covers articles or materials capable of posing significant risk to people, health, property, or environment when transported in quantity."

That's a broad definition, but for the purpose of this discussion we'll focus on three potential hazards that can arise during bulk bag handling:

1. Static discharge.  Bulk solids that within a bulk bag may generate enough static charge to iginite flammable vapors, gases or dust.
2. Explosive dust.  Bulk solids whose dust, if allowed to accumulate in a high enough concentration, may explode if exposed to a suitable ignition source.
3. Health hazard.  Bulk solids that present a health hazard to humans via direct contact.

Dissipate Static

There are two ways to mitigate the risk of static discharge: bag and equipment design.

Of the four types of bulk bag (Type A, B, C and D), typically only types C and D are used to prevent static discharge leading to explosions. 

Type C bulk bags feature conductive fabric or conductive threads or filaments woven into non-conductive fabric.  The key factor is that Type C bags must have a grounding point to which all fabric, threads or filaments are connected.  The grounding point must then be connected to an external ground point duing filling and discharging of the bulk bag.

Type D bulk bags are constructed from fabric that allows static charge to be dissipated without being connected to ground.  The advantage of Type D bags is that operators are not required to making a ground connection with the bag prior to filling and unloading - a task that can be forgotten sometimes resulting in disaster.

Equipment used in situations where static discharge could ignite flamable substances must, as a minimum, have ground lugs.  Depending on the Area Classification electrical components may have to be explosion rated.

Contain Dust

If the product you are filling or discharging can cause an explosion if a high enough concentration of dust is exposed to an ignition source, dust containment is critical.

Bulk bag fillers must have 'twin-tube' fill heads and provide a tight seal with the bulk bag inlet spout.  The outer tube of the fill head must then be connected to dust collection system so that dust laden air that is displaced during the filling process is safely extracted.

Bulk bag dischargers must have dust containment features suitable for the hazard level.  There are different levels of dust containment available (discussed in another blog post).

All equipment must feature electric components rated for the Area Classification.

Isolate Human Contact

Bulk solids that are health hazards include various refined metals that are carcinogens, chemical compounds that cause respiratory problems and so on.  When dealing with these products maximum dust containment is required.

Specialized bulk bag discharging equipment is available for this application.  It is critical that such equipment contain dust at the following critical steps in the bulk bag unloading process:

• Initial onset of product discharge.  The point in time when product begins to flow from the bulk bag can create a large amount of dust.
• Throughout bulk bag emptying.  Of course, dust must be contained as the bulk bag is emptying.
  
• End of discharge. Near the end of the discharge cycle the bulk bag will loose its shape.  Equipment must be designed to maintain dust containment integrity at this crucial stage.
  
• Bag removal.  Removing the empty bulk bag - still full of dust laden air - must be done safely without exposing operators to dust.

Further, bulk bags must be designed to prevent the escape of product particles during handling - otherwise referred to as 'sifting'.  Coated or lined bags can be specifically designed to suit the product and the hazard.

This is only a brief overview of handling hazardous goods in bulk bags.  Users and bulk bag and equipment manufacturers must coordinate closely to ensure safety.

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.