The road to better dust control

Many dust control operations currently used in most opencast mining operations are based on outdated hardware and procedures. Over time, in an effort to improve operations, hardware was updated based on the procedural requirements, and procedures were updated based on the capability of the hardware. The result is dust control operations that do not satisfy safety expectations and do not help the financial position of the mining operation in these challenging economic times.

How would mine haul road dust control differ if the hardware and procedures were engineered in 2016?

Problem

To recap, an opencast mine haul road network is always changing: unsealed haul roads are regularly constructed, modified and reclaimed as the pit evolves. A mining truck driving on these unsealed haul roads will generate dust. If too much dust is generated, other haul road users can lose sight of their surroundings – a scary concept when operating a 400 t+ vehicle. Dust can also have serious health implications on mine workers and the surrounding community.

Solution

To suppress dust, an effective and readily available product is water. When sprayed onto the road, the water will temporarily bond with the dust particles, preventing them from becoming airborne. Mine sites, however, must be cautious when applying water to the haul road as an oversupply of water will make for a potentially hazardous environment that can result in truck slides and vehicle roll overs. Overwatering will also increase the cost of maintaining the haul road, as the roads will degrade at a much higher rate, causing corrugations, pot holes and road deformities, which will effect the rolling resistance of a haul truck.

By trying to manage one risk (dust), another is created (overwatering). Any hardware solution or operating procedure implemented must now consider both of these risks.

Hardware specifications

Mining operations require a water application method that can adapt to the changing road configurations. A mobile water truck fitted with spray valves and a large tank capacity will be able to provide water to the entire haul road network. The road network length will determine the required number of water trucks and tank capacity to maintain effective dust suppression.

Critically, the water trucks will require a spray control system with the ability to select a specific water application rate. This application rate will be a user defined value expressed as mm/m2 and will not change regardless of other forces, including ground speed, truck gear and engine RPM, that would normally effect the operation of the truck-mounted water pump.

Water truck spray systems, such as ADE’s ECO Spray Premium, use a measurable water coverage rate for definable and consistent water application, but also include data logging and remote monitoring features with the ability to generate usage reports. Mine site operators can get a detailed insight into their water truck operations.

Operating procedures

Water truck operating procedures will determine how the water is sprayed onto the road, at what application rate and how often. The following matters must be considered when determining operating procedures:

• Haul road friction.
• Spray coverage.
• Road degradation.
• Deployment strategies.

There is a balance between too much water and not enough. Too much water and safety is compromised and road degradation occurs. Too little water and evaporation will quickly hinder dust suppression efforts.

Haul road friction

The friction of a vehicle’s tyre on an unsealed road surface can be dangerously reduced after water is applied. To ensure a safe level of friction is maintained, mine operations can perform a haul road profiling audit.

The haul road profiling audit is a protocol developed by RoadSafety Training Services. The audit will determine the maximum allowable water application rate on a section of road. Factors that can influence the rate are road material and geometry. After an audit is completed, a road will be assigned a specific road category and water trucks should not spray any more than the allowable defined water application rate, expressed as mm/m2.

Spray coverage

For improved safety, mine sites can choose to only water a limited area of the haul road. Two variants commonly used are spot spray and strip spray. Reducing the watered area of road provides an extra level of safety that ensures the vehicles onsite always have a dry area of road for emergency circumstances.

Some mine sites can achieve acceptable dust suppression by only watering a limited section of road. Implementing limited spray coverage will increase the capacity of the water trucks, allowing them to water more of the mine’s haul road network.

Spot spray operations are regularly used on water trucks that are unable to reduce water output at low speeds – typically up ramps – and, in an effort to reduce total water onto the ground, the watered spots are separated by dry spot. With an effective water truck spray system, this procedure is not required and is not recommended. Spot spray can increase the rate of evaporation and causes tremendous strain on the water truck spray components.

Strip spray operations provide more even and controlled water coverage and are suitable for use with a measurable water truck spray system. Evaporation from the haul road will be reduced and water truck component life will be increased, as it is not subjected to the harsh spot spray operating process.

Using a measurable water truck spray system maintains friction on the haul road. When strip spraying with a measurable spray system, both sets of rear tyres will still maintain adequate friction and will not effect drivelines components.

Road degradation

On roads with high friction levels even after watering, it can be tempting to spray at a higher water application rate to increase the time between required spray cycles. If the water application rate is set too high, it will still be considered overwatering. While friction will be maintained, the excess water can run off and erode the haul road surface and cause road deformities, including corrugations and pot holes. An excess of road deformities can increase the rolling resistance of the haul trucks, which increases fuel consumption, cycle times and equipment maintenance costs. Road degradation will require increased haul road maintenance and, in turn, incur additional costs to the mine.

This is an excerpt from an article that was first published in World Coal September 2016. To register and receive your free trial of the magazine, click here.

Read the article online at: https://www.worldcoal.com/special-reports/28032017/the-road-to-better-dust-control/