2.11 Operations phase

At the operational stage, the major emissions and air quality issues occur, requiring an ongoing management plan that is both rigorous and flexible.

2.11.1 Management plan and system

A dust or air quality management plan may be required as a condition of approval. Even if it is not mandatory, there are good reasons to have a plan in place to deal with any issues that could adversely affect the operation.

Even where there are no sensitive neighbours to consider, there may be value in limiting dust impacts on machinery wear or vehicle operator safety. Generally, however, the main focus will be on limiting impacts on neighbours (which, apart from residents, might include sensitive ecosystems or farmland) and staying in compliance with regulatory requirements.

A management plan’s success is gauged by measurable performance: it must be more than a document that sits permanently on a shelf; it should be a document that is of daily relevance, not only to the environmental manager but also to senior management and the site operators.

An essential part of the management plan is a well-structured system for monitoring, recording, quality checking and reporting information transparently and consistently. All data have a value, which in some cases may not be completely obvious until a later time when readily available, high-quality data is important for a purpose such as an air quality impact assessment.

2.11.2 Monitoring

Compliance monitoring may be required by licensing conditions, in which case the regulatory agency will specify the requirements. However, the specific location(s) for monitoring may require negotiation with landowners, and allowances may need to be made for finding suitably exposed sites, power supply, ready access and so on.

Once a monitoring site has been established, it is important that correct maintenance and calibration is carried out to maximise the return of reliable data. Capturing all information in a single data repository is also an important feature: many problems can occur when data is captured on disparate platforms and cannot readily be consolidated or tracked over time.

Compliance monitoring requires a system that complies with the relevant Australian Standards and regulatory requirements. For particle monitoring, this will typically entail high-volume, partisol or TEOM instrumentation.

In specific cases, specialised monitoring or analysis may be required: for example, for respirable crystalline silica, asbestos, radio nuclides or radon. In these cases, it is important to receive expert advice.

For many sites, there may be an additional requirement for real time monitoring, similar to the needs at the construction phase but perhaps on a larger scale.

The requirement may be to address approval conditions or may be a voluntary undertaking. In either case, it should be set out in the air quality management plan.

A real time system may entail an array of monitors set out to provide indications of dust levels over periods as short as 1 minute. The regulator or a consultant will be able to establish a suitable trigger level for the short-term concentration that signals a need for intervention, in order to maintain compliance with the 24-hour standard. Most real time monitors are capable of sending out electronic, audible or visual alarm signals. The siting of a real time system should aim to provide information about the current or imminent impacts on the most sensitive locations around the mine.

At the opposite extreme to real time monitoring, dust deposition measurement using standard dust gauges, which collect samples over 30 days, is a useful adjunct to the monitoring program and also may be required by consent conditions. Deposition measurement is relatively inexpensive, but measurements can be prone to problems of sample contamination and vandalism, and care needs to be taken over siting, security and liaison with neighbours.

Most dust nuisance occurs as a result of specific short-term events rather than a gradual background accumulation of dust. Because dust gauges can yield only a monthly deposition rate with no information on short-term events, their results may not correlate well with the level of annoyance or complaints being received.

Nevertheless, dust deposition monitoring remains a common way of obtaining an indication of nuisance impacts, and can also be important in determining whether the deposition rates of hazardous components, such as lead, are at acceptable levels. An array of deposition gauges around the boundary and near sensitive locations is recommended.

2.11.3 Control and mitigation

A variety of control and mitigation efforts come into play during the operational phase. The matrix in Table 2.3 can be useful as a starting point in designing an appropriate mix. The matrix provides a useful checklist for the selection of an appropriate mitigating control in various particulate emissions situations.

Table 2.3 Control and mitigation matrix

  Barriers Capture Chemicals Design Greening Moisture Replace Surface treatment
In the pit YES YES YES YES YES
Underground YES YES YES YES
Stockpiles YES YES YES YES YES YES YES
Transfer points YES YES YES YES
Roads YES YES YES YES YES YES
Materials handling plants YES YES YES YES YES YES
Smelting operations YES YES YES YES YES YES
Tailings dams, open areas YES YES YES YES YES

Note: Barriers = these could be trees, hills, or man-made barriers

Capture = collecting the dust using a bag house, precipitator or scrubber Chemicals = modifying the raw material characteristics

Design = this refers to the design stage during which engineering solutions can be applied to mitigate particulate emissions

Greening = planting trees, shrubs, grasses Moisture = using water to control dust

Replace = using either roads, rail, or conveyors whichever produces the least dust Surface treatments = using bitumen, concrete, chemicals etc.

2.11.4 Mine

If the moisture content of the product is modified in the mine there can be a beneficial knock-on effect all along the value chain. This is true because the sooner that newly exposed raw material is conditioned by water, the more easily it absorbs water later on. The benefits are different for different products but the following benefits can accrue:

  • Additional moisture will reduce the dustiness of the product, and can lead to some binding of the very fine particles which will cause them to behave as larger particles which are easier to manage.
  • When raw materials are particularly dry and prone to being dusty, spraying water onto the blast prior to mining not only conditions the raw material but improves safety, as it reduces the particulate dustiness caused by the operations of the shovel or front-end loader. By wetting the raw material prior to mining, the number of poor visibility events will be reduced, increasing mine availability.

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A typical mobile irrigator, used on an iron ore mine in the Pilbara region of Western Australia. Source: Southern Cross SX2500 3500 Operators Manual V2.

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The mobile irrigator being used to wet the iron ore in the pit. The direction of travel is towards the anchor. A rotameter driven by the water winds the cable up onto the cable reel. Source: John Visser, Rio Tinto.

2.11.5 Plant

The principles in this section are applicable to both process plant and any type of materials-handling plant. Transfer points, operating equipment and discharge points within the materials-handling systems provide the potential for significant dustiness at the operation. The topics in this section describe particular dust-related issues that arise at certain points in the mining operation.

Designing seals and enclosures

Following the hierarchy of control, the most successful solution to dust arising from operations lies in eliminating ultra-fine particles from the raw material stream. This can be accomplished by bringing the raw material stream up to the DEM point. However, this is not a perfect solution, because water addition systems can fail and maintaining the DEM does not produce a dustless raw material. Substitution, the next level in the hierarchy of control, is not an option in this case.

The third level of control, preventing dust from leaving the raw material stream, can be applied by ensuring that all the transfer points and operating equipment within the plant are enclosed, and that the seals of the enclosures operate in a way that takes into account the specific application. So, for example, the design and sealing method for the enclosure for a crusher are different to those for the enclosure of a transfer point.

It is important to ensure that, when the raw material stream enters a transfer hood/chute, there is as little open space in that entrance as possible. This limits the volume of air drawn into the chute, which in turn prevents the fine particles from being blown around when they become airborne while falling.

The design of the seals for enclosures in the plant should be practical and maintainable so that the operators use them and the maintainers keep them working and replace the covers after working on the chute. There are several companies that specialise in designing this kind of equipment.

When the material stream has a high moisture content prior to being separated into coarse and fine material, the dustiness of the stream is less than the dustiness of the separated coarse material and fine materials. There are two reasons for this:

  • The ultra-fines tend to adhere to the coarse particles, which provide stability to the fine particles.
  • Coarse particles provide localised wind breaks for the fine particles lying next to them, which reduces local particulate lift off.

These effects occur on conveyors, stockpiles and trucks.

Changing the flow of raw material

Impact plates create particulate dustiness by imparting energy into the moving raw material stream by changing the direction of flow in the same way that the flow direction is changed when the raw material hits the top of the stockpile. This results in particles bouncing away, opening up the raw material stream and releasing the fine particles which are caught up in the moving air stream that always follows a moving raw material stream. In addition, the larger particles with sharp points often lose those points when they impact on the impact plate, which causes more particulate emissions.

There are three ways to control the resulting particulate emissions, described in the section below on transfer chutes and stockpiles. The same solutions can be applied to impact plates in other plant, such as crusher and screen feed boxes.

Tipping the raw materials

Dust generation occurs at truck tipping points, such as the crusher dump pocket/grizzly feeder, because the material is allowed to free fall, it is disturbed from its resting place, there are always high impact zones associated with tipping points and the moisture of the material is usually lower than the DEM. This is an excellent point at which to modify the moisture of the raw material stream and/or add an appropriate chemical.

Using conveyors

There are three potential sources of dust resulting from the transfer of raw material along a conveyor:

  • the movement of the raw material over idlers
  • the resonations of the conveyor structure
  • the failure of belt-cleaning mechanisms.

The raw material may develop small localised dust clouds at particular points along the conveyor, as a result of the movement imparted into the raw material as the belt passes over the idlers and down into the trough between the idlers. This is usually insignificant, because the fine particles usually fall back onto the conveyor and are removed. However, if even a gentle breeze is blowing, these fine particles are taken away from the source and a dust cloud forms. There are three solutions to this problem.

The first and best way to stop this from occurring is to add water to the raw material stream at some point/s upstream of the source of dust. The ‘rules of thumb’ for adding water into a moving raw material stream are:

  • Add a maximum of 0.5 per cent by mass of water to the free- falling raw material stream at any one transfer point.
  • If adding water to the surface of the raw material stream lying on a conveyor, do not add more than 0.2 per cent per string or, on an overland conveyor, per 350 metres length.
  • Always remember that the sprays should be aimed to direct the water at the raw material stream with no direct impact on the conveyor parts or structure.

If there is insufficient water available, because of a lack of the resource itself, an inadequate delivery system or a lack of transfer points in the raw material handling system at which water can be added, other solutions are needed.

The second solution that can be employed to stop wind erosion from the surface of the raw material on the conveyor is to treat the raw material stream with a chemical that will bind the ultra-fine particles together without detrimentally affecting the raw material itself. The chemical should contain a wetting agent to aid in penetration of the raw material stream.

The last way to solve the problem is to install a wind barrier over the conveyor structure, with an open end on the leeward side of the conveyor belt to allow access for maintenance. This solution has two benefits: it is effective, and it provides clear proof to staff and the community that the company is committed to dust mitigation. The disadvantages of this solution are the capital costs and downtime (in the case of an active operation) that are required for installation.

A similar source of dust, if the conveyor structure is not well designed, is the resonation of the entire conveyor structure. The solution to this is to modify the structure and stop it from resonating.

The third cause of dustiness is the dust that clings to the conveyor belt and is systematically removed, portion by portion, at each return idler. This is best resolved by the installation of belt-cleaning stations that are robust and work. There are a few suppliers that provide solutions that consistently work without significant maintenance costs. They supply both dry and wet cleaning systems; the wet cleaning systems are the most effective in removing this clinging dust.

Transfer points

Transfer points involve a falling stream of raw material which generates dust as described under ‘Changing the flow of raw material’. Transfer points are to be found at junctions between conveyors and unit processes such as screens, crushers and so on or between one conveyor and another conveyor. They are usually the dustiest points in the entire plant.

Dust is created at transfer points in three ways:

  • Fine particles are liberated from the raw material stream as the raw material cascades down the transfer point
  • If impact plates are used to change the direction of the raw material as it cascades through the transfer point, dust will continuously be generated at this point. Small pieces of raw material are broken off larger rocks by the impact of the large rocks on the wear plates (and other steel members) of the transfer point. Impact plates create particulate dustiness by imparting energy into the moving raw material stream by changing the direction of flow in the same way that the flow direction is changed when the raw material hits the top of the stockpile. This results in particles bouncing away, opening up the raw material stream and releasing the fine particles which are caught up in the moving air stream that always follows a moving material stream
  • The raw material stream drags air along with it into the transfer chute and this air is expelled at the exit point, taking with it ultra-fine dust particles.

The solutions to this problem accomplish two objectives. The first is to reduce the energy imparted into the raw material stream while it is being directed through the transfer point, through chute design. The other objective is to prevent the moving stream of material from spreading out, either to the side or from becoming elongated. The two main chute systems are:

  • Hood and spoon chutes—There is much negativity surrounding hood and spoon type transfer chute systems, because of their perceived high capital and operating costs. However, if designed properly, these systems can provide a dustless operation. The benefits of a hood and spoon transfer point are that there is no impact point where dust can be generated, they are virtually noiseless, conveyor belt wear is reduced significantly and there are no housekeeping issues caused by dust build up.
  • Cascade rock box chutes—Cascade rock box chutes can be retrofitted into most existing chutes. They introduce a series of ‘rock boxes’ located in a close cascade from the top of the transfer station to the bottom, increasing the number of impact points but significantly reducing the energy imparted at each drop by reducing the drop height considerably. This type of chute is not designed to transfer fine material.

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Rock box style chute. Source: John Visser, Rio Tinto.

Any chute will become dusty if it is not properly maintained. Holes in the sides and wear plates allow the egress of fine particles from the chute.

Moisture splitting

During the screening process coarse and fine particles are separated from each other. After this separation any moisture remains in the fines, because of the significantly higher surface area inherent in fines, and the coarse material is dry. As a result, when coarse particles are broken in processes downstream of the separation they are prone to dustiness, which may mean that water needs to be added to the coarse material at that point.

This factor must be considered during the design phase of the plant.

Moisture mixing

In general when wet raw materials are mixed with dry raw materials, the moisture from the wet raw materials mitigates dustiness from the dry raw materials simply by being mixed into the dry fines. There is a secondary dust mitigating effect that aids in reducing the dustiness of a dusty raw material when it is mixed with a less dusty raw material: the ultra-fine particles in the dusty raw material will find additional particles to which they can adhere.

2.11.6 Roads

The most well-known dust sources on a mine site are the roads. On an annual basis, the roads at an operation will produce as much dust lift-off as the dust generated by the movement of the raw material.

Wearing course material

The selection of a mine road surfacing or ‘wearing course’ material is central to the functional design of a road. Functional design of a haul road is the process of selecting the most appropriate wearing course material or mix of materials, typically natural gravel or crushed stone and gravel mixtures, commensurate with safety, operational, environmental (dust generation) and economic considerations. The most common wearing course materials for haul roads are compacted gravel or gravel and crushed stone mixtures.

A well-selected wearing course material will not generate excessive dust. The specifications for such a material are based on an assessment of the wearing course material shrinkage product (Sp) and grading coefficient (Gc), defined in the following equations:

Sp = LS x P425

Gc = (P265 - P2) x P475 / 100

LS = Bar linear shrinkage
P425 = Percent wearing course sample passing 0.425mm sieve
P265 = Percent wearing course sample passing 26.5mm sieve
P2 = Percent wearing course sample passing 2mm sieve
P475 = Percent wearing course sample passing 4.75mm sieve

Other tools for selecting a suitable wearing course material include a selection chart.

The choice of wearing course should also be evaluated in the light of other material property limits identified as important in the generation of dust, as shown in Table 2.4.

Table 2.4 Recommended parameter ranges for selecting haul road wearing course material for reduced dust generation

Impact on functionality below recommended range Material parameter Recommended range Impact on functionality above recommended range
Reduce slipperiness but prone to raveling and corrugation Shrinkage product 85–200 Increased dustiness and poor wet skid resistance
Increased loose stones, corrugations and potential tire damage Grading coefficient 20–35 Increased raveling and poor dry skid resistance
Reduced dustiness but loose material will ravel Dust ratio 0:4– 0:6 Increased dust generation
Increased loose stoniness Liquid limit 17%– 24% Prone to dustiness, reduced raveling
Increased loose stoniness Plastic limit 12%–17% Prone to dustiness, reduced raveling
Increased tendency to ravel, loose stoniness Plasticity index 4–8 Prone to dustiness and poor wet skid resistance
Poor wet weather trafficability, churning, excessive deformation and cross-erosion, maintenance intensive Soaked California Bearing Ratio at 98% Mod American Association of State Highway and Transport Officials 80

Increased resistance to erosion, rutting and improved trafficability

Ease of maintenance, vehicle-friendly ride and no tyre damage Maximum particle size 40 mm Poor surface finish following maintenance, potholing and potential tyre damage

Water-only treatment vs. chemical treatment

Roads whose surfaces are treated with water alone increase in their dustiness with time. This occurs simply because the water keeps the dust particles in place without protecting them, allowing the traffic on the road to continually grind the particles finer until the dust size becomes ultra-fine. There is some mitigation of this when the roads are graded and resurfaced as part of the normal road maintenance.

In addition, roads that are too wet can become a safety hazard, as the road surface becomes muddy and slippery. Poorly surfaced roads cause increased operational and maintenance costs due to high tyre wear and high rolling resistance. Poorly maintained road surfaces also lead to high vehicle operating costs.

Indeed, even roads treated with most chemicals will become a source of dust, because they cannot be swept clean on a daily basis since the sweeping activity slowly destroys the treated surface. A spray-on reapplication is often required to control this dust source. However, roads that are treated with chemical palliatives and regularly swept clean will lead to less dusty and safer operating facilities.

Dust palliatives

In broad terms, the effectiveness of any dust suppression system depends on changing the wearing course material’s susceptibility to erosion. The wearing course silt and fine sand fractions (for example, 2 microns to 75 microns) are a good indication of its erodibility.

The motivation for using some additional agent to reduce a material’s inherent erodibility is based on increasing particle binding. The finer fraction, although contributing to cohesiveness, also generates much of the dust, particularly when the material is dry. The presence of larger fractions in the material will help to reduce the erodibility of the finer fractions, as will the presence of moisture, but only at the interface between the surface and the mechanical eroding action. This forms the basis of the water-based dust suppression techniques used most commonly on mine haul roads.

The consequences of dust generation include:

  • loss and degradation of the road pavement material, the finer particles being lost as dust and the coarser aggregates being swept from the surface or generating a dry skid resistance defect
  • decreased safety and increased accident potential for road users, due to reduced or obscured vision and reduced local air quality
  • higher vehicle operating costs, with dust penetrating the engine and other components and resulting in increased rates of wear and more frequent maintenance.

From a mining perspective, the following parameters would define an acceptable dust palliative:

  • spray-on application with deep penetration (the ability to penetrate compacted materials), or (less preferable) mix-in applications requiring minimal site preparation (rip, mix-in and recompact)
  • straightforward applications requiring minimal supervision, not sensitive nor requiring excessive maintenance or closely controlled reapplications
  • short product curing period, so that the road is trafficable within a maximum of 24 hours
  • availability in sufficient quantity at reasonable prices
  • adequate proven or guaranteed durability, efficiency and resistance to deterioration by leaching, evaporation, ultra-violet light and chemical reaction with the wearing course or spillage on the road
  • effectiveness over both wet and dry seasons
  • independently evaluated against local and international safety standards and environmental requirements.

The broad classes of products available are listed in Table 2.5, which also provides a tool to identify classes of palliative which would suit a certain application.

Table 2.5 Dust palliative products and application parameters

  Hygroscopic Salts Lignosulphonates Petroleum-based products Others a
Climatic limitations Salts loose effectiveness in continual dry periods with low relative humidity. Selection dependant on relative humidity and potential to water road surface. Retains effectiveness during long dry periods with low humidity. Generally effective, regardless of climate, but will pothole (small diameter) in wet weather where fines content of wearing course is high. Generally effective, regardless of climate.
Wearing course material limitations Recommended for use with moderate surface fines (max 10% to 20%<75 micron suitable for low-fines materials or high-shrinkage product/plasticity index, low CBR
b
or slippery materials.
Recommended for use where high (<30%<75 micron) fines exist in a dense graded gravel with no loose material. Performs best with low fines content (<10%<75 micron). Use low-viscosity products on dense fine grained material, more viscous products on looser, open-textured material. Plasticity index range 8–35 Fines limit 15%–55% < 75 micron. Minimum density ratio 98% MDD. Performance may depend on clay mineralogy (enzymes).
Treatment maintenance and self-repair capability Reblade under moist conditions. Calcium chloride is more amenable to spray-on application. Low shrinkage product materials may shear and corrugate with high speed trucks. Tendency to shear or form ‘biscuit’ layer in dry weather— not self-repairing. Best applied as an initial mix-in and quality of construction important. Low shrinkage product materials may shear and corrugate with high-speed trucks. Shear can self-repair. Requires sound base and attention to compaction moisture content. Slow speed, tight radius turning will cause shearing—not self repairing, but amenable to spot repairs. Mix-in application—sensitive to construction quality. Difficult to maintain/rework. Generally no problem once cured.
Tendency to leach out or accumulate Leaches down or out of pavement. Repeated applications accumulate. Leaches in rain if not sufficiently cured. Gradually oxidises. Repeated applications accumulate. Does not leach. Repeated applications accumulate. Efficacy depends on the cation exchange capacity of the host material. Repeated applications accumulate.
Comments A high fines content may become slippery when wet. Corrosion problems may result. Generally ineffective if wearing course contains little fine material or there is excessive loose gravel on the road. Long lasting—more effective in dry climates. Generally ineffective if material is low in fines content or where loose gravel exists on surface. Curing period required.

Notes
a Includes sulphonated petroleum, ionic products, polymers and enzymes.
b California Bearing Ratio (%).

Importantly, a poor wearing course material cannot be improved to deliver an adequate performance solely through the addition of a dust palliative. The haul road wearing course material should ideally meet the minimum specifications presented earlier. If not, the inherent functional deficiencies of the material will negate any benefit gained from using dust palliatives.

In road surfaces with too much gravel, dust palliatives do not appear to work effectively, more especially where a spray-on technique is used as opposed to a mix-in. The palliatives do not aid compaction of the surface because of the poor size gradation, nor form a new stable surface. New surface area is created from exposed untreated material while, with a mix-in application, poor compaction leads to damage and raveling of the wearing course, traffic-induced breakdown of the material and eventual dust generation. With regard to water- soluble palliatives, rapid leaching may be problematic in some climates.

In compact sandy soils, tar and bituminous emulsion products appear effective where leaching of water-soluble products may be problematic. However, in loose medium and fine sands, bearing capacity will not be adequate for the tar/bitumen products to maintain a new surface and degeneration can rapidly occur.

In road surfaces with too much silt, it is unlikely that a dust suppression program will be effective. Excessive silt or sand fractions may lead to slipperiness while poor bearing capacity leads to rutting and the need for road rehabilitation or maintenance, which destroys most products. Small-scale potholing has been observed on a number of pavements following spray-on application or reapplication, as a result of traffic lifting fine cohesive material from the road. Again, where no depth of treatment has built up, this will lead to the creation of new untreated surfaces.

In general, spray-on applications do not appear appropriate for establishment of dust treatments, especially with regard to depth of treatment required. A spray-on re-application or rejuvenation may be more appropriate, but only if penetration of the product into the road can be assured; otherwise, it will only serve to treat loose material or spillage build up, which will rapidly breakdown and create new untreated surfaces. A spray-on treatment is however useful to suppress dust emissions from the traffic-free roadsides, since it would be easier to apply and, because the material is typically uncompacted, would provide some depth of penetration and a reduction in dust emissions due to traffic-induced air turbulence.

Finally, vehicle speed can be controlled not only for safety reasons but also to improve the effectiveness of dust control measures. In general, vehicle speeds of 40 kilometres per hour and less do not result in dusty conditions being experienced.

Idle open areas

Active areas must be kept to the absolute minimum, with roads, parking areas and no-go zones specifically designated. Drivers, particularly those with 4X4 vehicles, should be educated about the need to drive only on designated roads. Keep the roads treated with a suitable chemical.

All open areas that are not required for vehicle access or construction should be isolated and sprayed with a hydromulch chemical. Hydromulch will provide seeds, the required nutrients for germination and immediate cover against dust lift-off.

If chemical veneers can be used on the open area in question, dust can be controlled very effectively by chemically treating the area. The application can be achieved with mobile irrigation systems or aeroplanes for large areas. Mobile irrigators can also be used to spray open areas. As shown in the figure in Section 2.11.4, the irrigator pulls itself along, which means that its use is not labour intensive.

    2.11.7 Stockpiles and stockyards

    Stockpiles are often dusty during the stacking and reclaim processes, and they are also a source of dust when the wind blows. There are many ways to stop dusty conditions in the stockpile area.

    Primary stockpile discharge points

    Stackers must have sprays installed onto the boom as a matter of course. Sprays that can modify the moisture of the material should be installed but used only when necessary. They should be directed into the falling material stream.

    Misting sprays should also be installed to provide a mist curtain that will behave in the same way that fine dust will behave when the wind is blowing. The dust– water interaction results in dust particles that become too heavy to continue to be blown away and fall to the ground. These sprays should be directed to form a curtain around the falling stream so as to trap the fine particles.

    image

    An example of a well-designed stacker spray system. A stacker boom requires spray at the discharge end only. Source: John Visser, Rio Tinto.

    If the raw material is already at DEM, the nozzles should be low-volume misting nozzles directed along the raw material stream to stop the immediate dust. This will prevent run-off (which creates housekeeping work) caused by too much water. If it is necessary to add water to the raw material at this point, both misting and water addition nozzles will be needed to stop the immediate dust as well as to increase the raw material moisture. Both sets of nozzles should form part of the water curtain around the raw material stream.

    The nozzles on the stackers should have automatic and local control valves. The objective of installing local control valves is to allow an operator/maintainer to perform work on the nozzle/spray bar while the stacker is operating. The valves should however remain in automatic setting under normal conditions, which means that the manual valves should always be in the open position. A spray system that includes misting nozzles and low-angle fan nozzles for trimming purposes is required.

    Mobile reclaimers

    Bucket wheel reclaimers generally require two sets of nozzles to manage dust. One set is needed to spray into the face of the stockpile immediately ahead of and behind the cutting wheel, to stop dust that is caused by the raw material as it rills down the face. The second set is needed to spray into the raw material stream as it cascades out of the buckets into the transfer chute and onto the boom conveyor. The water must not be allowed to impinge onto the structure of the boom because that will cause fine raw material to stick to and build up on the structure, which will in turn upset the balance of the machine.

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    A bucket wheel reclaimer spray system that requires an upgrade. Good curtain sprays deliver to the face above the bucket wheel, but additional sprays are needed to stop dust arising from the material drilling down the face. The nozzles are not directed at the source of the dust and there are no misting nozzles. This photo demonstrates how dry raw material will produce dust even if some sprays are installed and turned on. Source: John Visser, Rio Tinto.

    Drum reclaimers

    Drum reclaimers also require sprays that are directed at the material falling down the face of the stockpile as it is reclaimed. There should be no water directed at any structure or moving parts, since this will cause housekeeping issues and increase the wear on the equipment without improving the effectiveness of well-aimed water or mist sprays.

    Static reclaim systems

    Stockpiles that are designed with a reclaim tunnel are inherently less dusty than are surface stockpiles. A stockpile with a large, well-designed tunnel provides a safe and dust-free reclaim operation.

    The source of dust is the reclaim feeder and its delivery onto the reclaim conveyor. Dust can be mitigated by misting sprays directed into the falling raw material stream.

    Water trucks and water cannons

    Water trucks and water cannons provide different ways to deliver water to the surface of the stockpile. While the cannon is the mainstay of the delivery system, trucks can reach areas not covered by the cannon and can be deployed when the canon system is down.

    Stockyard water cannons play an important role in managing dust from the fine material stockpiles, because moisture continuously evaporates while the stockpile lies dormant. The key aspects of ensuring the effectiveness of the cannons include:

    • Spray pattern height—The water delivered by the cannon nozzles should be able to reach just over the top of the stockpile.
    • Spray pattern coverage—There is usually a portion of the side of the stockpile that is not covered with water by the sprays. This occurs at the midpoint between the cannons and is a function of the circular arc produced by the spray cannon motion. Such areas can be covered by a water truck with correctly installed nozzles.
    • Distance from stockpile—The cannon stands should be located as close to the stockpile as possible, ensuring that they do not foul the movement of equipment. Generally, the further the water jet has to travel the more it breaks up and loses its effectiveness.
    • Wind conditions—In a high wind, the spray is blown about and entirely controlled by the wind; therefore, water must be sprayed onto the stockpiles prior to any strong wind event. The Bureau of Meteorology website can be used to plan for these events, and specialised services can provide more detailed local information. Trend information on wind speeds produced by the local weather station can also be used to trigger the commencement of the cannon spray sequence.
    • Air—Droplet fineness can be increased by introducing air into the water nozzle.

    Chemicals

    If fines are a problem, adding a chemical into the water deluge nozzles in the crusher dump pocket and the grizzly underpan will modify the raw material’s characteristics to prevent fugitive dust from being generated.

    The use of a chemical veneer on stockpiles prior to any significant wind event will stop dustiness from the stockpiles. Chemical veneers bind the fine particles together but do not stop water ingress or evaporation from taking place. The veneer looks somewhat like a spider’s web when viewed close up.

    The chemical veneer can be sprayed onto the raw material using the cannon system in a reconfigured state, or using water trucks. The concentration of the chemical should be determined taking into account the estimated wind speeds and duration of the anticipated wind event, which means that the cost of the chemical spraying depends on the event.

    Foam can be employed to drop dust at the discharge points of transfer chutes and along conveyors. However, to date no tests have proven successful with this type of chemical.

      2.11.8 Water management

      Dust mitigation is most effectively achieved with water, which means that a representative of the operation’s water management team should be involved in discussions, plans and changes to any dust mitigation system. Someone from the water management team should also be present during a risk assessment that is focused on dust mitigation.

      2.11.9 An automated particulate emissions control system

      Managing particulate emissions effectively requires a coordinated, site-wide system, whether the system is automated or not. An automated system can make use of information such as:

      • weather prediction data (anticipated wind speed, precipitation, relative humidity and temperature)
      • local weather station data (measured wind speed, precipitation, relative humidity and temperature)
      • scheduling of raw material, waste and soil movement (blasts, mining, area clearing, delivery to and reclaim from stockpiles, plant operation, road use)
      • raw material, waste and soil characteristics (DEM, flowability)
      • moisture information from analysers and laboratory samples
      • weightometer readings
      • site real time particle monitors.

      This information is used to control the movement of material and the set-up of:

      • water addition and mist spray systems
      • chemical treatment systems
      • belt wash systems
      • particulate emissions removal systems.

      Particularly for larger operations, these systems should be linked together to create a whole-of-site particulate emissions management system which is monitored by the operator, internal and external particulate emissions monitors, moisture analysers and weather monitoring systems, and principally controlled by a programmable logic controller (PLC). The operator should turn the system off only when there is a system failure.

      Specific menus for each raw material type can be written for the PLC. They become the principal determinant of the water management scheme for that raw material type, using local weather conditions and anticipated storage time on the stockpile as variables that modify water addition and/or chemical treatment.

      There are many scenarios that can be set up to run automatically if a complete system is implemented. For example, an automatic scenario could mean that when rain is anticipated the use of water through the cannon system is reduced or turned off. In the case of predicted high winds it may be necessary to spray the stockpiles with either water or chemically treated water, depending on the strength of the winds, prior to the arrival of the windy period. If a period of dry, hot weather is expected the water cannon sequence start frequency can be increased to deliver more water.

      The particulate emissions control system at Dalrymple Bay Coal Terminal in Queensland is an example of a system that controls water dams, borehole and delivery pumps and water addition, using raw material movement, moisture analyser data, Bureau of Meteorology information and local weather station information as the inputs. Other systems now available incorporate the various data inputs into a real time model that not only shows current impacts but also uses forecast data to predict impacts on air quality (including dust, sulphur dioxide and other contaminants), enabling various control scenarios to be tested by modelling.

        2.11.10 Taking the wind into account

        Windbreaks can comprise fences, trees, hills and simply distance between the dust source and sensitive receptor.

        Trees, bush and grass

        The most effective way to create a barrier to wind erosion and, therefore, dustiness is to plant trees, bushes and grass both upwind and downwind of stockpiles, open areas and operating plants. These natural barriers provide both wind and noise protection. This is obviously a long-term solution.

        Wind fences

        Wind fences are used to stop dust from interfering with the local community in many places around the world, particularly, and very successfully, in Japan.

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        Hessian fences stopping dust being blown away on a local scale. Source: John Visser, Rio Tinto.

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        Hessian fences stopping dust migration. Source: John Visser, Rio Tinto.

        Wind fences positioned upwind can be used to prevent dust problems by lifting the wind up over areas that have dusty materials on them. The wind fence will change the wind speed on the ground at a distance away from the fence as far as 30 times the height of the fence. For example, in a wind speed of 14 metres per second, a 12 metre-high fence will stop dust lift-off for up to 200 metres. In general the dust lift-off wind speed is 10 metres per second.

        A fence is a more effective windbreak if the top of the fence is significantly higher than the highest point in the area which it is designed to protect. The fence porosity should be between 60 per cent and 90 per cent.

        Wind fences positioned downwind of the area with the dusty material will act as a dust barrier, halting dust that has been picked up by the wind and dropping it at the foot of the fence. The problem with this option is that, to be completely effective, the fence must reach the height that the dust cloud reaches, and this is usually impractical. However, this kind of wind fence provides partial protection, by:

        • dropping the lower layers of dust—these layers generally contain the nuisance dust that causes community issues
        • disrupting the wind—forcing the wind to go up and over the fence slows the speed of the wind on the leeward side of the fence, which causes more dust to be dropped at that point.

        2.11.11 Modelling

        With each terrain there are particular disruptions to the wind velocity profile that will either increase or decrease the effectiveness of the wind fence. CFD modelling can provide the means of understanding these effects and therefore optimising the effectiveness of a wind fence.

        Modelling the prevailing wind velocity and terrain interaction is critical to understanding the effect that a windbreak will have on reducing dustiness. The CFD modelling provides the wind velocity information and the dust plume modelling takes that information and demonstrates where and how the dust will be blown and deposited.

        CFD modelling should go hand in hand with wind tunnel test work, because there is always a need to calibrate the CFD model. Two different calibration types are necessary: the dust lift off factor and the wind velocity profile.

        Once a CFD model has been calibrated for a material, it can be used to model many scenarios on that site in future without significant cost per run. The total cost of performing CFD and wind tunnel modelling can be paid back in months, based on the likely costs if a wrong location is selected if no modelling is done.

        CASE STUDY: Blast overpressure prediction system

        Blast overpressure levels experienced from operations on open-cut mines depend on many factors, including the design of the blast, the distance from the blast to the receiver and the prevailing atmospheric conditions. The way in which temperature and wind vary along the path through which the overpressure wave travels from the source to the receiver is particularly important in determining the overpressure experienced at the receiver.

        Australian Coal Association Research Program Project 12036 provides mine operators with information on meteorological conditions that allows the effects of atmospheric conditions to be taken into account before making the decision to blast (Holmes & Lakmaker 2009).

        The approach taken uses the MM5 mesoscale meteorological model, with 24-hour hour forecasts from the Bureau of Meteorology supplemented by local observations of wind speed, wind direction and temperature in the lower atmosphere, to predict three-dimensional meteorological fields in the Hunter Valley. The local observations are provided by a sonic detection and ranging (SODAR) and radar acoustic sounding system (RASS) stations centrally located in the modelling domain. In addition, wind speed and direction data from ground-based (10 metre) sensors are supplied to the model.

        The model is run at least once a day and at user-selected intervals. The output is processed to extract information on the predicted vertical profiles of wind speed, wind direction and temperature, extending from ground level up to 1.6 kilometres. Each time the model is rerun it assimilates local meteorological observations provided by the SODAR, RASS and other surface stations.

        The model results are reprocessed to create tabular and graphical summaries for selected sites coinciding with mine sites and other sites of interest. Data are uploaded to a website where registered users can obtain data on wind and temperature profiles for any area of interest on the grid, in a format suitable for input to a blast overpressure prediction model, or for any other use such as predicting dust events.

        The model predictions are ultimately used to predict the enhancement of blast overpressure levels. This is done using a model provided by Terrock, which uses MM5 data to predict the level of enhancement (positive or negative) caused by meteorological conditions, and the absolute level of overpressure resulting from a blast. The results are presented as contour plots overlaid on a map of the area surrounding the blast. An example showing blast overpressure enhancement is provided in the figure below.

        The results of the project are promising and the system appears to be a useful tool, in its current state of development, to improve the management of impacts from blasting.

        The model predictions are ultimately used to predict the enhancement of blast overpressure levels. This is done using a model provided by Terrock, which uses MM5 data to predict the level of enhancement (positive or negative) caused by meteorological conditions, and the absolute level of overpressure resulting from a blast. The results are presented as contour plots overlaid on a map of the area surrounding the blast. An example showing blast overpressure enhancement is provided in the figure below.

        The results of the project are promising and the system appears to be a useful tool, in its current state of development, to improve the management of impacts from blasting.

        Predicted blast overpressure levels showing zone of reinforcement due to atmospheric conditions.

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        2.11.12 Real time and forecast weather information

        The Bureau of Meteorology website (www.bom.gov.au) helps operators to keep abreast of the wind and weather predictions for the next day and week. This information can be used to plan activities that involve land clearing, raw material movement and the like, and enables planning to avoid periods when there will be strong winds. In addition, the site’s real time weather station data is useful to verify conditions.

        Detailed wind and rain predictions tailored to specific sites can be helpful if Bureau of Meteorology data are not sufficiently targeted. Commercial services are available from some consulting firms, providing web-based data and predictions that utilise sophisticated models. An example of a frequently updated predictive model is the Australian Coal Association Research Program forecast system for the Hunter Valley.

        2.11.13 Complaints management and community liaison

        One of the aims of dust management is to avoid or minimise the level of complaints. Complaints are an indicator of problems; they should be taken seriously and investigated, and their causes should be addressed.

        However, because complaints represent a subjective response to specific events or series of events, you cannot simply use the number of complaints as a definitive measure of performance. Complaints may be withheld for various reasons, or may be exaggerated to make some (sometimes broader) point. The absence of complaints does not mean that people are not being annoyed by dust. Therefore, it is important to maintain regular contact with potentially affected neighbours to ensure that there are no hidden issues that should be addressed on site.

        Complaints should be documented in detail and recorded in a database that forms part of the air quality management system. Recorded information should include:

        • the time and location of the complaint
        • the details of the complainant
        • the nature of the complaint
        • the details of the person who took responsibility for interviewing the complainant, and what they reported
        • the investigations conducted on site to establish a cause
        • the actions identified and taken to rectify the problem
        • the communication of the actions back to the complainant.

        In rare cases, there may be a vexatious complainant who will demand a significant amount of attention without providing evidence of a real problem. It is important to assess such situations carefully. A vexatious complainant cannot be judged on the basis of a single complaint: a vexatious complainant can only be identified after a series of unwarranted complaints, forming a clear pattern of behaviour. Once such a pattern becomes clear, however, it is important to discuss the problem with the regulatory agency. A well- documented approach to dealing with this problem will be vital.

        With dust or other air quality complaints there is often a question of properly identifying the specific emission source. A weather station on site, recording wind and other data at intervals of up to 10 minutes, will provide a very useful basis for identifying the source. The short-term data will identify wind variations, which may be crucial in getting a good result, as often the travel time between source and complainant will be only minutes or tens of minutes.

        It is now possible to use real time systems to quickly display a calculated air trajectory from the site of the complaint back to a potential source. Such an approach is particularly useful where there are multiple possible sources of emission that could contribute to the problem—either different mines or industries in the area, or specific sources on an individual mine site.

        As part of keeping the community informed about environmental performance, some companies provide public access to websites that display current and recent air quality monitoring data. When making real time data available via the internet, it is important to make the distinction between validated and un-validated data, and inform the user that sometimes raw data will need to be corrected or discarded after quality checks.

        2.11.14 Reporting and performance reviews

        An important part of the air quality management plan is reporting and review. The data collected by monitoring systems should be reported in ways that clearly show compliance with imposed or voluntary targets and objectives, and provide explanations of causes and remedial actions associated with any non-compliance. While a suitably detailed report should be provided, it is also important to make a simplified summary that conveys the essential performance measures in an easily appreciated graphical format.

        Periodic, at least annual, internal review of performance is necessary to ensure that:

        • compliance efforts are effective, both technically and administratively
        • objectives remain appropriate
        • no new issues are emerging that require systematic attention
        • information is being communicated to stakeholders effectively.

        In addition, regular third-party audits of the program and performance are strongly recommended and may be part of site’s quality system requirements. A successful outcome from the audit process will require that the management plan and associated reporting are complete and transparent.

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