Dust Control & Environment

BULKtalk: What can you do to reduce dust?

In his regular BULKtalk column, Steve Davis discusses the hazards dust emissions add to a site, and how to combat them.

In his regular BULKtalk column, Steve Davis discusses the hazards dust emissions add to a site, and how to combat them.

Safety is critical in the industry, and we know that dust emissions bring with them several hazards. Inhalation of fine airborne dust, particularly silica and carbonaceous materials is a well-known cause of lung damage, and we all prefer not to have to wear respirators when working near conveyors. Other dusts to be careful around include base metals, wood materials, and asbestos.

Many dusts are also potentially combustible and can create large explosions when at specific concentrations in an enclosed space such as a chute or bin. Small explosions may lead to larger ones by causing a pressure wave that releases accumulated dust, and facilities are destroyed regularly from these events. Airborne dust accumulates on structures and equipment causing failure, bringing down the roof from excess loading and with a little moisture can create accelerated corrosion. 

Equal with safety is our focus on environmental impacts of our activities. Airborne dust that is harmful to people is usually harmful in the wider environment, killing fauna and flora and contaminating land. Even if not harmful we are concerned about perception and a large dust cloud or shadow is not acceptable in most situations. 

An example is shown in the Google view of a phosphate conveyor feeding from the Boucraa mine to El Marsa port in Morocco, where the white dust shadow is clearly seen over much of the 110 km route. 

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In the Pilbara and other locations, the dust shadow is the same colour as the landscape and is not as obvious. Dusty conveyors that are in proximity to other operations and to populated areas are liable to raise complaints.

A further problem with high dust environments is the potential for abrasive and corrosive damage to equipment. Sulphur dust for example mixes badly with electrical and electronic components. Any dust that builds up on seals will accelerate failure. Some dusts, like clinker, will solidify after a short period of time if any moisture is present. Over time this can build into a thick concrete like coating.

Dust occurs on conveyors when we transport many materials. Dust is the finer size fraction of the material conveyed that separates from the main body of the material when exposed to a suitable airstream. Higher velocity airstreams will carry away larger particles, so controlling airflow relative to stream flow is one method of reducing dust emissions. Dust storms and sandstorms are the ultimate example of the interface between moving air bodies and finer particles.  

Dust is increased through attrition during handling. We measured particle distribution through a multiple conveyor and storage system and noted an increase in the finer fractions after each transfer, leading to a significantly higher fines content at the delivery point. 

Common locations and causes for potential dust release, and which will need some attention to meet exposure and release limits include:

• Chute designs that do not control flow. If falling material is allowed to ricochet off chute walls with no control, or fall height is such that velocity opens the flow pattern, finer particles will form into dust clouds. Chutes of various types are available to keep material flow contained to a large extent and reduce dust separation. Some chute designs available that are designed to manage internal airflow such that dust is contained. Material drop height should be minimised as far as practical as this reduces impact forces, material spread and likelihood of dust generation. There are examples where change from a basic drop chute to controlled flow reduces visible dust by significant amounts. We have many practitioners who can predict flow, why not use them?

• Free discharges, such as conveyors feeding stockpiles, where the fall height can be many metres and separation of fines into dust is probable need some attention. The drop height can be managed by using a luffing feed to maintain the lowest practical fall. Rill towers might be effective for fixed points. Some materials are amenable to push stacking using a scraper stacker, which takes away the fall; be wary of potential attrition during push and reclaim as this may increase dust in downstream conveying.

• Chutes with open skirt system risk dust being generated after the load point. Loading chutes where material drop occurs, such as pulsed filters, often have incorrectly selected skirt systems that blow out and generate dust clouds. Incorrectly adjusted and worn skirts often allow fine particles to spray out of the contact zone, which creates both spillage and dust.

• Dump hoppers are a common dust source, as the action of dumping separates the material in free fall. As material flow fills the hopper, it displaces air and carries dust out of the hopper. Closed bins and silos have similar issues, but the dust is generally easier to contain.

• Belt speed can give rise to a steady emission of dust along a conveyor. In this situation the material is moving next to a static air body. We can easily measure dust lift off speed for any material and decide whether measures should be taken to mitigate dust generation.

• Carry back and spillage are common sources of fugitive dust even with wet material. As the spilled material dries out, wind or other air movements will carry surface fines away as dust. Cleaning up dried spills often releases more dust as the source is removed. The cost of cleaning and the potential value loss from product that cannot be recycled and often must be treated or discarded at an additional cost should be enough to focus on carry back and spillage prevention, not to mention the benefit to personnel from having a clean facility.

• Belt cleaners by design clean off the finest material from the belt and this cleaned stream will lead to dust if not managed well. Worn, incorrect type and poorly adjusted or installed cleaners are liable to be ineffective and allow carry back. Belt ploughs are effective at removing material from belts, but design does often not consider where the cleaned material is deposited leading to a dust source.

There are many methods for managing dust, and this would start with identifying the criticality associated with the dust and the volume expected. What is to be done with the dust; return it to the system for onward carry, return it to the system elsewhere, collect and dispose? Are there any issues to consider, such as potentially flammable or explosive dust, toxic dust? Is there a limit to how much water can be added, and are additives viable?  

The best way to limit dust emissions is to remove the fine particles. For most bulk materials this is not possible, and even if it were, some dust is almost certain to be generated through attrition in any multiple handling system.

Is it reasonable to assume that a conveying system, that is built to best design practice and maintained well, will not emit any dust without suppression or collection? Probably not, however the spend on installing and operating dust management on a well-designed conveying system will be much lower .

Suppression or collection? 

Suppression assumes that some form of suppressant will be added to the bulk material to keep the finer particles in the main material stream, so they are not dislodged into an airstream. Collection assumes the dust laden airstream will be extracted and collected in a system appropriate to the dust and the chosen method of disposal.

If sprays are the format to be used, then they should be designed to add a defined amount of wetting over the width of the material using nozzles that generate droplets small enough to bind with dust particles. Should the spray be in the chute, where penetrating the body of material in freefall allows more wetting? Should the spray include a wetting agent or surfactant to extend the life impact of the sprays, if so is there a process contamination problem? Many materials have limitations on how much water can be added.

The theory of suppression assumes very small droplets will collide and adhere to dust and coalesce into sufficient size that the airstream cannot float them away. Considering this, is a fogging system more appropriate. Fog nozzles produce much smaller droplets than sprays and are very effective at controlling dust. Water quality and capital and operating costs are an issue, but water consumption is low. Effectiveness may offset significant other costs in clean up and maintenance.

Foaming agents create a sticky blanket over the body of material to prevent dust lift off by binding small particles. Consumption of foam is roughly one tenth that of spray water to get similar effects. These systems are like fog systems in size and complexity and generally use SLS or similar soap to produce foam. Residual effects may control dust for some time. Foam is not always viable due to potential for material contamination.

Dust collection generally falls into two categories for conveyors, either dry filters or wet entrapment. For any dust collection system to work, the collector and all ducting and collection points must be designed and installed to match dust loading and pressure drop needs. Dust systems that cannot be easily accessed cannot be maintained. There are many dust collection systems for conveyor systems that are not well designed and are either turned off or assumed to be working well because no dust reaches the collector so there is no dust load in the discharge.

Filters are generally bags or cartridges and will have reverse pulse air or shakers to loosen dust and allow it to collect in a hopper. Insertable filters are the simplest and most economical and are generally installed in a covered skirt system or in the top of a transfer chute. When the bags or cartridges are cleaned dust falls back onto the conveyor or into the chute. Although the dust is captured and released at each transfer, it finds its way to the end of the system and does not become a separate return or disposal stream, and no value is lost. 

The alternate, particularly if dust is explosive, is to capture dust in a wet system such as a scrubber. In a scrubber the dust laden airstream is mixed with a liquid (often water), and the dust removed as a slurry. It may be possible to return the slurry to the conveyor, however it is usually necessary to dispose of the waste somewhere else in the process or off site.

One observation is that dust suppression and dust collection can work together for better overall control, however the two do not mix too well inside the same space. Dust collectors will remove spray or foam before it has a chance to work well, and it will be sucked into the dust ducting and cause blockage.

Dust impacts are being recognised as a conveyor problem, but good technologies to manage it well are readily available. Like many things, these technologies must be designed and installed to correct parameters and such that they can be easily and safely accessed for maintenance. 

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