2.7 Controls

There are many controls that can be implemented in solving dust problems, including using water, chemicals or wind breaks, and redesigning materials handling equipment.

The most effective process for determining which controls to use for mitigating dust is to understand the controls and how they apply to a specific situation, and use the hierarchy of controls to make the final choice.

2.7.1 Hierarchy of controls

Efforts at controlling emissions of any type should follow the engineering hierarchy for control: source, then dispersion pathway, then receiver. The most beneficial controls are usually those applied at source, so that emissions are minimised or even eliminated. This is most effective for sources that have a small size and can be enclosed to enable filtration or other forms of capture and removal. Source control is not as straightforward for area sources or roads, where the application of water or chemicals is typically the most effective option.

Where emissions are unavoidable, there may be opportunities to reduce the downwind impact by installing windbreak systems, in the form of either vegetation or engineered structures. Such methods are most effective when they are close to the source, when the plume still has relatively small dimensions and thus can be more readily intercepted.

The least effective and least acceptable option is to mitigate effects at the receiver. This is rarely done, but can take the form of fully air-conditioning or paying for regular cleaning of the receiving premises.

2.7.2 Dust

Dust is by far the most prevalent problem air emission associated with mining and quarrying. The following mining or quarrying activities can lead to particulate emissions:

  • the movement of top soil, the raw material or product and waste or overburden
  • blasting, mining, hauling, conveying, stacking, loading (onto trains and/or ships) and reclaiming material
  • clearing of open areas
  • medium to heavy traffic on unsealed roads
  • use of tailings dams where the surface has dried but has not been treated for dust mitigation (particularly if equipment is driven across the surface).

Before the final choice of control or controls can be made to solve a specific situation, it is vital to gain an understanding of the essential characteristics of the dust that is to be controlled. These characteristics are the material characteristics of the dust, its moisture characteristics, size distribution and hydrophobicity.

Mineral characteristics

Some soil, waste and raw material types tend to be dustier than others. If clay minerals are present, not only will particulate dustiness be more prevalent but particulate emissions will also be higher. The size distribution also plays a role in the severity of the particulate dustiness of the raw material, waste and/or soil, as does the porosity, density and hydrophobic nature of the material. It is important therefore to examine what is known about the dust characteristics of the raw materials, wastes and soil types that will be disturbed.

The extent to which a material will produce fine particles is a function of the minerals present. For example, the grains of clay minerals tend to be finer than those of most other minerals, so clays typically produce finer particles and larger volumes than most other minerals. In general clays are softer, less dense and more porous, and can be broken down by water if exposed to it for some time. This means that clays will absorb more water per tonne than most other minerals. It is important therefore to understand the mineralogy of the raw materials, wastes and soils in the operational area.

Moisture

Moisture is the most significant mitigating agent available for controlling particulate emissions. In general, higher moisture means less dusty conditions. Unfortunately, there is also a point of moisture addition at which most materials become sticky, which can cause blockages in the plant and cause the material to ‘hang up’ in dump trucks and rail wagons.

Several moisture factors are tested for in order to understand the nature and behaviour of a material. Each raw material has two moisture limits that are critical to understanding the nature and behaviour of the material: the dust extinction moisture (DEM) and the materials handling moisture (MHM). The DEM, which is the lower moisture limit, is the moisture at which the material is no longer dusty. The MHM, which is the upper limit, is the moisture at which the raw material becomes sticky and begins to cause materials handling problems. These upper and lower limits are not exact figures and should be reported as ranges, not as a single point, in much the same way as tolerances are given with an engineering specification.

The two levels are determined during the same test program. The DEM and MHM can be determined by a suitably equipped materials handling testing facility such as Newcastle Innovation at the University of Newcastle. The figures below depict typical data sets associated with DEM and MHM determinations.

DEM (upper moisture limit).DEM (upper moisture limit). Source: John Visser, Rio Tinto.

Flowability index of a typical iron ore.Flowability index of a typical iron ore. Source: John Visser, Rio Tinto.

Because moisture is constantly being drawn from raw material by the sun and wind, it is prudent to ensure that the moisture level of any raw material that will be handled is kept at least 1 per cent or 2 per cent above the DEM, to ensure that particulate emissions are not generated. Moisture analysers are required to keep track of the moisture of the raw material as it is handled and water addition points should be placed at all available sites.

The DEM and MHM levels of each raw material body should be determined by test work, and a test program should be developed to perform this work. If the nature of the raw material changes (for example, if there is a change in the fine to coarse material ratio or the clay content) its DEM and materials handling moisture levels will also change, so the test work should be carried out again. When raw materials with known moisture limits are blended, the blended raw material behaves differently to the separate raw materials, so it is always best to perform the test work again using the planned raw material blends.

Particle size distribution

The next factor that affects the dustiness of products is the particle size distribution. In general, the finer the grain size the more dusty the behaviour of the material. Finer grains are lighter and can therefore be more easily blown about. Once suspended by the wind, finer grains tend to stay airborne more readily and are therefore also transported further from the source. Fineness

is of course a function of the mineralogy, but it also depends on the energy imparted into the material by blasting, crushing, grinding and so on.

Size of common particles (after SIMTARS & Wark K and Werner CF. 1981)
Fine particle sizing. These particle sizes are typical of most dusts, but mine dusts can easily be as small as 1 micron. Source: Air Pollution: Its origin and control; Wark, K and Warner, C 1981.

Hydrophobic characteristics

Very dry, very fine particulate emissions particles can often behave as if they are hydrophobic for three reasons:

  • Particulate emissions can be electrostatically charged, resulting in widely dispersed clouds of particles; if they have the same charge as the water droplets that are being used to wet them, the water droplets and the particulate emissions particles repel each other.
  • The particulate emissions particles can be hydrophobic for a chemical or surface tension reason.
  • The particles can behave in a hydrophobic manner because each particle has a high surface area, which means that coating the particles with water requires larger volumes of water per tonne of raw material than would normally be the case.

Appropriate test work is the only way to determine which of these three causes is affecting the materials that have been labelled hydrophobic.

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