Dust control needs to be treated as a site-wide issue in open-pit mines. This is because there so many dust generation points to contend with all at once, from haul roads to tipping points to blasting activity. Consequently, innovative solutions are required for each and every stage of operation.
In this blog post, we take a look at the sources of dust in open-pit mines as well as some of the most effective control measures mine operators should be implementing.
Sources of dust in open-pit mines
Dust from haul roads is a particularly prominent concern for environmental and safety managers as well as pit managers. If there’s one thing that every pit manager knows it’s this: “good roads produce more loads.”
Given that wheel generated dust from haul roads within mines is the biggest source of fine dust particles on most mine sites, contributing about 40% of total emissions, control measures need to be innovative and highly effective. Airborne dust creates significant visibility issues for haul truck drivers who need good sight of distance to be able to see hazards up ahead in order to react on time to avoid incidents such as collisions and prevent cuts to truck tyres caused by stray ore.
Additional sources of dust include when material is being dumped into crushers. When the material is then crushed in the crusher, further dust is generated. Once this crushed material is put onto a conveyor belt, even more, finer dust particles become airborne.
Without effective control measures in place, the dust generated during the various processes at open-pit mines can quickly become widespread and unmanageable.
Traditional dust control measures
Dust control measures in open-pit mines typically centre around the use of water carts to wet haul roads as well as low pressure watering systems to pre-wet materials. Even though water itself is sometimes considered to be ‘free’, wetting roads and materials in this way is actually expensive and also not entirely effective.
To begin with, roads which have been overwatered create a number of safety concerns such as reduction in tyre traction, degradation of road quality and increased risks of collisions. Not to mention that roads never stay wet for long and need to be continuously watered, often leading to washouts and collapsing of fill sections and slopes.
Additionally, low pressure watering systems for pre-wetting materials are only a ‘quick fix’ solution at best and consume enormous volumes of water. These control measures will prevent coarser dust (10 microns and larger) from becoming airborne and will reduce overall dust throughout an open-pit mine. However, low pressure watering does not actively subdue dust particles that are already airborne or respirable sized dust particles which are less than 10 microns (PM10 dust).
Long term exposure to PM10 dust is proven to cause many adverse health effects such as asbestosis, silicosis and coal pneumoconiosis (black lung disease). This is because these respirable dust particles are so small they can penetrate the lower respiratory tract.
More innovative solutions for controlling dust include:
1. Stabilising haul road surfaces
2. Utilising fog cannons for visible dust
3. Installing high pressure dust suppression systems
1 - Stabilising haul road surfaces
It is becoming increasingly accepted across the mining industry that the correct use of dust suppressants on haul roads will lead to a significant reduction in wheel generated dust emissions compared to using water alone.
This approach to dust control can be taken one step further by stabilising the design of haul roads to create a more permanent haul road solution.
A stabilised haul road consisting of a 50-75mm bonded wearing course with a bitumen seal layer will ensure less wheel generated dust as well as reduced loss of fines and less water ingress.
DASProduct is a proprietary bitumen emulsion technology, specifically designed and formulated to provide stabilisation of the wearing course layer on permanent haul roads as well as subsequent control of fugitive dust. It can be incorporated into the wearing course layer of a haul road and will actively work to reduce the need to employ water bowsers by 90+% and reduce operational and respirable dust by 90+% (compared to an untreated road).
2 - Utilising fog cannons for visible dust
Fog cannons are now a popular dust control measure in open-pit mines due to their versatility and innovative technology. They are ideal for use in scenarios where visible dust, rather than respirable sized dust, is a major problem.
These cannons use highly pressurised water which is pumped through a series of jet nozzles, turning the water into mist through atomisation. This fine mist is dispersed through the air by a powerful fan.
Each fog cannon has its own generator and is set up on skids, allowing it to be moved around as dictated by weather conditions and the type of dust being generated. Once connected to a water source, fog cannons can be strategically placed near sources of dust in open-pit mines, such as at dumping areas and near haul roads.
When dust particles collide with the mist, the particles are captured and the water dissipates before ever having had the chance to spread. The cannons can be manually or remotely activated when high levels of dust are generated combined with certain weather conditions.
The key to the success of fog cannons is not just about how much water can be blasted from the cannon or how far the water can reach. The spray nozzles need to be strategically selected so the correct sized water droplets are released into the air, depending on the individual operating conditions.
Fog cannons are very effective at suppressing airborne dust and contribute to significant reductions in water usage compared to low pressure watering systems.
3 - Installation of dust suppression systems
Unlike low pressure watering systems, high pressure mist systems use low volumes of water at high pressure (above 50 bar) to add humidity/moisture to the air, usually at transfer points such as conveyor belts and tipping areas to capture and subdue dust particles that are already airborne.
These types of systems are ideal for ore that is moisture sensitive, i.e. coal & kimberlites, as in open pit where it is not practical to pre-wet materials. These systems are also useful for water scares areas and where pooling of water is a concern during material handling.
The performance of these systems can be further improved by ratio feeding a concentrated surface-active agent like HydroPlus to the feed water line. This product lowers the surface tension of water particles and makes it easier for the water and dust particles to agglomerate, particularly when managing hydrophobic materials such as coal and other fine ores.
The secret to successful dust suppression is ensuring that the atomised water droplets and the target dust particles are of equivalent size. Spray nozzles need to be exactly calibrated to the right pressure and angle, depending on the type of material being treated. This allows the water droplets and the dust particles to collide and agglomerate increasing in size thus falling to the ground due to gravity. If the water droplets are too large, the fast-moving airborne dust will simply flow around them
The benefits of high-pressure mist systems in open-pit mines include the suppression of airborne PM10 dust by 80+%, minimisation of water usage and the prevention of excessive water pooling
Want to learn more about innovative dust control measures for open-pit mines? Talk to Dust-A-Side.
For more than 40 years, we’ve been world leaders in total dust control management.
To learn more about our cutting edge dust control services, call us on +27 (0) 12 648 8900. Or you can get in touch by clicking here.