There are so many variables involved in dust generation that it should be considered on a site specific basis. Nevertheless, there are many underlying principles that apply to all surface mines and quarries.
The process by which dust on the ground becomes airborne is referred to as 'dust emission'. For this to happen, energy is required to overcome the gravitational and cohesive forces binding dust particles to the surface. Table 1 summarises the dust potential of different activities within sites.
Table 1. Dust emission from typical site activities (after Arup Environmental 1995).
|Activity||Relevance for Mineral Types||Duration of Activity||Potential for Dust Emission|
|Soil handling||Most minerals||Relatively short||Significant potential. Depends on silt/ clay content & dryness of material and transport to mounds on the edge of site.|
|Overburden handling||Most minerals, but quantities vary considerably||Varies, can be intermittent over life of site||Significant potential. Depends on nature of overburden during the unloading and haulage stages.|
|Drilling and blasting||Usually for hard rocks||Short, but can take place frequently||Drill rigs have significant potential unless suppression is used which localises dust generation. Properly designed/controlled blasts have limited potential.|
|Other extraction and handling activities||Most minerals||Varies considerably but usually long||Varies considerably depending on the equipment used.|
|Initial loading Activities||All mineral types||Ongoing during extraction||Can be significant but varies considerably depending on the nature of the material, whether it is wet or dry, volumes handled and equipment used.|
|Crushing and grading||Most minerals, not always at extraction||Varies, generally ongoing||Varies depending on type of equipment and exposure to wind.|
|Storage of minerals on site||Most mineral types||Usually ongoing during extraction||Varies depending on the volume, wet or dry and exposure to wind.|
|Transport and load-out within site||All mineral types||Usually ongoing||Mainly by road but varies. Not usually significant (except near site exits).|
|Transport off-site||All mineral types||Usually ongoing||Mainly by road but varies. Not usually significant (except near site exits).|
|Soil/overburden reinstatement/ restoration works||Most minerals||Can be relatively short - but may proceed intermittently in phases||Significant potential but depends on the state of the material and transportation from edges of the site.|
Dust particles are dispersed by their suspension and entrainment in an airflow. Dispersal is affected by the particle size shape and density, as well as wind speed and other climatic effects. Smaller dust particles remain airborne for longer, dispersing widely and depositing more slowly over a wider area. Large dust particles (greater than 30μm), that make up the greatest proportion of dust emitted from mineral workings will largely deposit within 100m of sources. Intermediate sized particles (10 - 30μm) can travel up to 200 - 500m. Concentrations decrease rapidly on moving away from the source, due to dispersion and dilution.
Haul road emissions
The emissions from unpaved haul roads is still not understood very well and can be a major problem. This is particularly relevant to sites that have to move considerable amounts of overburden to above original ground level, as is the case with most opencast coal sites, but it can also apply to other minerals. The need to develop effective and economic control of nuisance dust from haulage road dust emissions also addresses some of the following issues faced by quarry operators:
A major challenge is the growing need to reduce the amount of water that sites abstract from ground and surface water sources. The use of automated water sprays and water bowsers on haulage roads is a common industry practice. However, many mines and quarries can now no longer rely on a constant, 'free' supply of water. With increased competition from local communities, industries and agriculture coupled with more demanding regulatory standards, quarry operators are under increased pressure to reduce primary water abstraction and usage 458.
Dust Health Effects
Exposure to any dust in excessive amounts may create adverse health problems including respiratory problems. Various factors influence the health risks posed to the exposed party, including, particle size, dust concentration, dust composition, deposition location within the respiratory tract and the exposure duration 459. Therefore, the control of haulage road dust emissions is of major concern to protect the health of on-site workers, but should also be considered for nearby off-site residents.
For obvious safety reasons it is important that the drivers maintain excellent driver visibility to avoid truck collisions and avoid the trucks steering off the haul road. During periods of dry weather, these safety concerns are often compounded by increasing haulage road dust emissions, further reducing driver visibility and making conditions extremely problematic. In particular, hot and dry weather can result in very high evaporation rates, resulting in ineffective dust suppression programmes. A study 457 found that in a South African surface mine, water suppression (provided by a mobile water bowser) was required every 21 minutes to prevent the onset of hazardous driving conditions caused by reduced driver visibility.
This highlights the need for the development of an environmental monitoring and management system capable of constantly monitoring the road conditions ensuring site safety is never compromised.
Extent of the Problem
One report 452 estimates that 20% of the total PM10 emission from anthropogenic sources in the UK in 2005 was from quarrying activities, but the report recognises that there was a high degree of uncertainty (±50%). The amount of dust generated and emitted from a mineral working and the impact on the surrounding area varies with respect to the factors in Table 2, while the variation in potential dust impacts can be explained by the factors in Table 3.
Table 2. Factors controlling the generation of dust.
|Types and quantity of mineral and the method of working.|
|Types of processing activities undertaken on a site.|
|Topography, character and land use of the area surrounding the site.|
|Hydrogeology of the site and the vegetation cover.|
|Dust control measures employed on the site.|
Table 3. Factors controlling the magnitude of any potential dust impact.
|Scale of operations||Generally, the more extensive the scale of operations, the more likely that dust will be a concern.|
|Nature of the mineral||Although softer minerals crumble more easily during handling and may produce a greater number of dust particles, intensive handling of hard minerals may produce large amounts of dust due to higher energy inputs.|
|Colour and opacity of the mineral||High contrast dust from minerals, such as coal or chalk, is generally more likely to be noticed on deposition.|
|Length of operation||A potential dust problem may be more acceptable if it is known that operations will soon cease or move to another part of the site.|
|Chemical nature of the dust||This will affect the severity of the impacts upon soils and vegetation.|
|Type of activities undertaken within a site and the location and duration of those activities||All sorts of examples could be given here!|
In areas where mineral workings are close to residential properties or sensitive premises, their environmental performance must be such that they are viewed as acceptable neighbours for the period of the operation of the site. There are a wide variety of possible land uses around a site and Table 4 77 gives broad categories of uses in terms of their sensitivity to dust.
Table 4. Examples of dust sensitive facilities.
|High Sensitivity||Medium Sensitivity||Low Sensitivity|
|Hospitals and clinics
Painting and furnishing
Greenhouses and nurseries
Light and heavy industry
Influence of Weather
The potential for any site to emit dust is greatly influenced by weather. Rainfall decreases dust emissions, due to both surface wetting and increasing the rate at which airborne dust is removed from air. Rainfall of around 0.2mm per day is considered sufficient to suppress wind-blown dust emissions.
In contrast, strong, warm, drying winds increase the rate at which dust is lifted from an untreated surface and emitted into the air. They also have the effect of spreading dust over a large area.
The topography of a site and surrounding areas can have strong effects on localised wind patterns. The effect is most pronounced in or near to valleys or hills which can channel and direct winds. In addition, the presence of surface features, such as woodland or buildings can influence dust deposition patterns. Open, exposed sites lacking shelter and surface features are likely to be more susceptible to dust blow. At the same time, wind speeds increase with height, and large mounds which project well above ground level can thus be the subject of significant wind erosion. In contrast, activities within a quarry or void will be sheltered to some extent from external winds, restricting the potential for dust to disperse beyond the site.
It follows from the previous section that the placing of dust generating activities within worked out areas below ground level can significantly reduce potential dust impacts off site. Any entrained dust is more likely to experience increased circulation and greater number of impacts with surfaces, causing the entrained dust to be deposited.
It is obvious that potential sources of dust (such as haul roads and stock piles) should, where at all possible, be kept away from the site boundaries, or at least from sensitive receptors.
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