Dust solutions for underground mining conveyor transfer points

Dust can foul rolling components, machinery, and equipment air intakes, necessitating extra parts and labor for cleaning and maintenance.

These factors unnecessarily elevate operational costs, especially when there are methods and technologies designed to control and suppress dust emissions before they become airborne and create these risks.

While it is clear that one way to reduce mining dust emissions is to decrease the amount of dust generated during processing, this is not always practical or easy to achieve.

Numerous sources of dust must be managed, depending on the extraction, haulage, and storage methods used. Most of the dust present in bulk materials originates from the reduction in particle size due to crushing or grinding, as well as from transfers between steps in the production process, such as conveyor transfer points or during discharging onto a stockpile.

Surface vs underground mining operations

In underground extraction, water is frequently applied at the face and conveyor transfer points to control dust.

When water cannot be used, alternative methods such as mining salts, ventilation, and modular dust collection are effective options.

In surface operations, controlling dust is challenging because it occurs in open air, and the fracturing of the in-situ material generates dust. Typically, bulk material is loaded into haul trucks at the extraction point and transported to either a conveyor transfer point or a crusher.

Regardless of the source, as the material is dumped and crushed, the most effective method of dust control is applying water; however, if adding water causes issues, foam is used. Water is less effective than foam but is often chosen because of the cost of foaming chemicals. While water may have some residual effects, they are usually short-lived.

Causes of dust on conveyors

Conveyors are a significant source of dust emissions, yet they can also help reduce fugitive dust.

For instance, in pit crushing and overland conveying at a surface mine, total site dust generation is lower compared to truck haulage.

Some raw materials are easily wind-swept, and in some cases, an enclosed conveyor belt system may be necessary.

When the haulage involves a conveyor belt, dust generation depends on the loading and discharge processes, as well as the management of these processes.

Closed conveyors are highly effective at preventing contamination and shielding the cargo from environmental elements, but they must still be opened and closed for loading and discharge. Passive dust reduction strategies include:

• Shorter or directed drops: Transfer chutes over loading zones that minimise the impact of cargo on the belt below reduce the turbulence within the loading zone, thereby decreasing the amount of dust released.
• Managing the flow: While rock boxes can be effective, they are also susceptible to clogging. Therefore, experienced engineers recommend a sloping system that slows material to minimise impact and induced air, along with loads in the centre of the belt to reduce shifting and enhance belt training.
• Preventing belt sag between idlers: The belt can dip slightly between idlers, creating gaps between the belt and skirting that allow dust and fines to escape in the loading zone. Using an impact cradle with shock-absorbent polyurethane bars reduces impact strain on the belt and ensures an even belt plane without gaps between the skirting and belt. Cradles can extend along the entire length of the stilling zone.
• Fully enclosed transfers: By completely enclosing the loading and settling zone, dust is contained. Items like dust curtains and dust bags can then be added to control airflow and capture dust.

Balancing the decision to slow belts down

With lower belt speeds, the width of the belt must increase to convey the same tons per hour, creating a dilemma between capital costs and operating costs. Many sources suggest belt speeds of two metres per second (394 feet per minute) or less for reducing dust generation.

It is worthwhile to closely compare the capital savings from a higher-speed belt with the long-term costs of maintenance, cleanup, and safety.

There are clear relationships among increased cleanliness, fewer safety incidents, and more reliable production, so the trade-offs should be examined carefully.

‘Foundations for Conveyor Safety’ – a comprehensive textbook for safe conveyor operation written by Martin Engineering – provides a detailed methodology and data sources for including direct and indirect costs in the financial analysis in section six.

Paying attention to belt tension

At critical speed, the bulk material loses contact with the belt at the idler and is launched into the air, landing back on the belt at a slightly lower speed than the belt itself.

This splashing action opens the profile, creating induced air flows that can release dust, resulting in turbulence, impact, and degradation as the material lands and returns to belt speed.

Maintaining a belt sag of one per cent between idlers is a common specification. Typically, the concerns in conveyor design regarding these belt sag phenomena involve the increased belt tensions necessary to counteract the frictional losses.

Design choices often overlooked in a dust reduction strategy can minimise dust creation from the undulations of the bulk material on the belt during transport.

Managing belt tension to reduce sag between idlers mitigates the effects of material trampling and splash. Material trampling refers to the particle-to-particle movement that occurs when the bulk material profile changes as it passes over the idlers.

Both trampling and splashing can generate dust due to the numerous times cargo crosses over idlers each hour. The higher the belt tension, the lower the trampling loss.

Raw and processed material storage

Controlling dust at the storage location presents another challenge. Large stockpiles are impractical to enclose in buildings and are often stacked and reclaimed by machinery that generates additional fines.

Open stockpiles are subject to weather conditions, causing some bulk materials to degrade when exposed to the atmosphere, while other materials revert to a solid state upon exposure to humidity or rain. Those materials that can be wetted often utilise water sprays to reduce windblown dust. Other strategies include wind fences and compacting the pile.

Discharge onto the pile is a source of dust release as the material flows from the delivery equipment, often a conveyor, onto the pile.

Cascading or telescoping chutes can be used to reduce the release of dust in these cases. If the material is easily broken, the drop height from discharge to the pile or between cascade shelves can create additional dust from impact degradation.

One unexpected source of dust emissions can be the site layout. For example, if a slope conveyor going from the stockpile into a storage bin or building is oriented in line with the prevailing winds in a high wind locale, the wind flowing up the conveyor will overwhelm dust control strategies by creating positive pressure throughout the conveyor enclosures.

Control the air, control the dust

If the material stream can be constrained so that it does not open up when discharged, the amount of air induced into the transfer point is minimised. As the material particles disperse, they create a low-pressure zone in the spaces, which encourages airflow into the transfer point.

The amount of dust that can become airborne is directly proportional to the volume and speed of the airflow through the transfer point. If the openings in the chute are restricted to the practical minimum, the inward airflow is limited.

A useful dust control strategy is to capture the material shortly after discharge and keep the stream coalesced as tightly as possible to minimise induced air.

There are several discrete element modelling (DEM) software programs specifically designed for optimising material flow through chutes, and there are specialty chute manufacturers that focus on these techniques.

These chutes perform best with materials that have consistent size along with adhesive and cohesive properties, such as coal. Wear on the chute surfaces may accelerate; however, this can be mitigated by a maintenance-friendly design that allows for quick and easy replacement of wear surfaces.

Conclusion

Much emphasis is placed on planning the mine to maximise profitability, but little attention is paid during the initial feasibility studies to how the layout can affect dust creation and emissions.

Conveyor transfer points have a history of being drafted rather than designed. Design tools are now readily available to address these critical details. How the conveyor is operated and maintained also significantly affects dust generation and release.

This article was developed in partnership with Martin Engineering president emeritus Todd Swinderman. 

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