Dust Suppression in Mines: Haul Roads, Fog Cannons and Drills

On an opencast mine, the haul trucks make most of the dust. Field studies at surface mines put the trucks’ share at 78 to 97 per cent of the fine dust (particles under 10 microns) given off on site, so dust suppression in mines starts on the haul road and then moves to the drills, the loading face and the crusher.

The tools are simple. Water from a tanker or a fixed spray, a fine mist from a fog cannon, water or a dust collector on the drill, covers on the trucks and a sealed cab for the operator. What makes them work is using each one at the right place, with the right size of droplet, often enough.

This guide explains where mine dust comes from, how water actually brings it down, how haul roads are treated, what fog cannons and drill collectors do, what the machines themselves carry against dust, and why dust sometimes keeps rising even after spraying. It carries no rupee figures.

What is dust suppression in mines?

Dust suppression in mines is the work of stopping rock and soil dust from getting into the air, or bringing it back down once it is up. Mining engineers split it into two jobs.

Prevention wets the material before it is disturbed, so the fine particles stay stuck to the bigger pieces. Spraying a muck pile before the excavator loads it is prevention. So is wetting a haul road before the trucks run on it.

Suppression, in the narrow sense, knocks down dust that is already floating. A fog cannon aimed at a tipping point does this. The droplets hit the dust, the two stick together, and the heavier lump falls.

Where both are possible, prevention usually does more. The NIOSH dust control handbook, a US government research manual on mine dust, says so plainly: prevention is generally more effective than suppression.

Why it matters goes beyond comfort. Fine rock dust carries crystalline silica, and breathing it for years causes silicosis, a lung disease that has no cure. The chronic form, the most common one, usually shows up 10 to 30 years after a worker first breathes the dust. Dust also blinds drivers on the haul road, clogs engine air filters and radiators, and settles on fields and homes around the mine.

Where does the dust in an opencast mine come from?

Dust comes from every step that breaks, moves or drops rock. Some steps make a lot of it for a short time. Others make less but never stop.

The haul road is the big one because it never rests. Loaded tyres grind the road surface into fine powder, and each truck’s wake lifts it. Close to the trucks, US field studies have measured respirable dust as high as 21.5 mg per cubic metre of air. Most of what a truck throws up is coarse and falls quickly, about 85 per cent of it larger than 10 microns, but the remaining fine part is the part that reaches the lungs.

Blasting looks dramatic, yet it happens only a few times a day at most, so it adds little to the fine dust over a shift. Bare ground and stockpiles start blowing once the wind is strong enough. For fine dry material of about 0.1 mm, that is a wind of about 21 km/h (13 mph).

How the mine is laid out decides where these sources sit. The guide to bench height and haul roads covers road width and gradient, and the guide to overburden removal shows how much material those roads carry.

How does water stop dust?

Water stops dust by making it heavy. A wet particle sticks to its neighbours and to the rock it sits on, and a clump is too heavy for moving air to carry.

The catch is droplet size. A big raindrop pushes air ahead of it as it falls, and a tiny dust particle simply rides that air around the drop and never touches it. A droplet close to the size of the dust particle cannot push it aside, so the two collide and stick. That one fact explains most of what works and what does not.

So there are two rules. To wet rock or a road before it is disturbed, use bigger droplets, above 100 microns and ideally 200 to 500, because they land where they are aimed. To catch dust that is already in the air, use droplets of 10 to 150 microns, close to the size of the dust.

Ordinary spray nozzles make droplets by forcing water through a small hole at high pressure. Air-atomising nozzles add compressed air and make finer droplets still, but they need a compressor, and their tiny openings block more easily. A well-designed fine spray adds very little water to the rock, usually no more than 0.1 per cent of its weight, which matters at a crusher where wet material can choke screens and chutes.

How are haul roads kept from making dust?

Mines treat haul roads in three ways: build the road so it makes less dust, keep its surface damp or bound together, and change how the trucks use it.

Build it well. A road made of hard, well-graded rock, compacted in layers and given a crown so water runs off, breaks down into powder far more slowly than a soft one. Soft rock such as shale or coal on the running surface grinds into dust quickly. Motor graders keep the crown and blade off ruts and potholes, and standing water left in a badly shaped road turns into potholes under loaded tyres.

Water it. Water is the most common treatment because it needs no preparation. A water tanker carries a tank, a pump and a row of fan-shaped spray nozzles at the back, and many also carry a front water cannon that the driver aims at a muck pile before loading. On the largest off-road chassis the tank can hold about 1.1 lakh litres (30,000 US gallons).

The weakness of water is that it dries. US EPA tests summarised in the NIOSH handbook show the pattern:

• about 0.6 litres per square metre cut road dust by 95 per cent, but only for half an hour;
• about 2.1 litres per square metre cut it by 74 per cent for the next 3 to 4 hours;
• at one road, watering once an hour cut dust by 40 per cent, and watering every half hour raised that to 55 per cent.

No published rule fixes how much water or how often, because it depends on the road material, the traffic, the sun and the wind. One study found that keeping the road at about 2 per cent moisture gave the lowest fine dust. Whatever the dose, water has to go down again and again through the shift, and more often on hot, dry, windy days.

Add something to the water. When plain water dries too fast or the mine is short of water, additives stretch it.

Every one of these works best on a road that is already in good shape. If the road surface breaks up under the trucks, the crust breaks with it.

Change how the trucks run. Slowing trucks from 40 km/h (25 mph) to 16 km/h (10 mph) cut the fine dust they raised by about 58 per cent in one study, though slower trucks also mean less output. On unpaved roads, keeping a gap of about 20 seconds between trucks cut the dust breathed by the driver behind by up to 52 per cent and lets the cloud clear before the next truck drives into it. Trucks leaving the mine on public roads go faster, so their loads are covered with tarpaulin and not heaped above the sides.

What is a fog cannon and where does it work best?

A fog cannon, also called a mist cannon, is a large fan with a ring of fine nozzles around its mouth. A high-pressure pump feeds the nozzles, which break water into droplets in the tens of microns, and the fan blows that fog out over the work area as a cloud. Dust in the air meets droplets of its own size, sticks and drops.

It fits places where you cannot put a spray right at the source: the face where trucks tip into the crusher hopper, a loading point, a stockpile being worked, the edge of a haul road. Makers rate the reach in still air, and the biggest models in one maker’s range are rated at about 100 m.

Three limits are worth knowing before buying one or blaming one.

• Wind decides a lot. Fine fog drifts with the air, so place the cannon where the wind carries the fog into the dust, and as close to the source as the work allows.
• It is a suppression tool. It catches dust that is already up. Wetting the material first, at the source, still does more with less water.
• It needs clean water. Fine nozzles block with silt, so water from a settling pond needs a strainer before the pump.

How is dust controlled while drilling and blasting?

A blast-hole drill blows compressed air down the drill rod to lift the rock cuttings out of the hole, and that air comes out of the hole full of dust. There are two ways to deal with it.

Wet drilling adds a small flow of water to that air. The water droplets catch the dust on its way up the hole, and tests have shown dust control of up to 96 per cent. The water needed is small: typically 0.4 to 7.6 litres a minute (0.1 to 2 US gallons). At one mine, control improved sharply between about 0.8 and 2.3 litres a minute, then levelled off, and near 3.8 litres a minute the wet cuttings got too heavy to blow out and the rod began to bind. The rule is to raise the water slowly until visible dust at the hole stops, and no further.

Water does have a cost on rotary drills. It wears tri-cone bits faster and can cut their life by half or more. A water separator fitted in the drill string strips most of the water out before it reaches the bit.

Dry drilling uses a shroud (a skirt) around the hole under the drill deck and a fan that pulls the dusty air through filters. A well-kept collector can catch up to 99 per cent of the dust, and it does not need water. The usual reasons it fails are torn or missing filters and a shroud that no longer seals against the ground.

Blasting dust is short-lived. Wetting the blast area beforehand helps, though in summer the water may dry before the blast. In underground mines, water-filled cartridges placed in the hole with the explosive have been reported to cut blast dust by 40 to 60 per cent. Explosives handling and licensing are outside this guide.

How are crushers, screens and conveyors kept clean?

Most of the dust made by crushing does not fly off on its own. It stays stuck to the freshly broken rock until the rock is dropped, screened or tipped onto a belt. That is why sprays work best just before the drop, at the feed hopper, the crusher discharge and each conveyor transfer point, and why the water should be mixed into the rock as it tumbles.

There is a limit. Each crushing stage opens up new dry faces that need more water, and too much water clogs screens and makes the product sticky. Clay-rich rock and shale clog at quite low moisture. So plants often pair light sprays with dry control: covers and skirts around transfer points, and a fan pulling dusty air into a bag filter, a dust collector that works like a big vacuum cleaner bag.

The guide to jaw, cone and VSI crushers explains what each crushing stage does and where the fines are made.

What does the machine itself do against dust?

Spraying protects everyone near the work. The machine’s own design protects its operator and its engine, and mining-class machines carry features built for dust.

A pressurised cab blows filtered air in so that the pressure inside stays higher than outside, and dust cannot leak in through gaps. The NIOSH handbook suggests at least 12.5 pascals (0.05 inches of water) of positive pressure, which holds against a wind of about 16 km/h. Well-kept cabs cut the dust reaching drill operators by 90 per cent or more, and retrofitted haul truck cabs by 59 to 84 per cent. The same tests show how easily it is lost: on one drill, opening the cab door between rods raised the dust inside nine times, from 0.09 to 0.81 mg per cubic metre, even though no dust cloud could be seen outside.

An air pre-cleaner spins or traps the heaviest dirt before the air reaches the engine filter, and a dust ejector dumps what the air cleaner collects without the operator opening it. A reversible cooling fan runs backwards for a short time to blow dust out of the radiator.

None of this replaces spraying. A sealed cab protects one person, and the drill helper, the blaster and the fitter standing outside still breathe what the spray did not catch. Compare more mining excavators and their cab and filter features before you choose one for a dusty pit.

Why is the dust still rising after spraying?

When a mine sprays and the dust keeps coming, the cause is usually one of a handful of faults, and most can be read from what you see.

Two habits prevent most of these. Inspect nozzles on a fixed schedule, since a blocked nozzle looks fine until you watch it spray. And judge the result by the air near people, not by how wet the road looks: fine dust you cannot see is the dust that harms the lungs.

What do environmental clearances ask a mine to do about dust?

In India, dust control on a mine is written into its environmental clearance and its consent to operate, and the conditions have read much the same for years. Two clearances ten years apart, one issued in 2005 for a limestone mine in Tamil Nadu and one in 2015 for a bauxite mine in Jharkhand, show the common set:

• control fugitive dust (dust that escapes into open air) from all sources, with water spraying on haul roads and at loading, unloading and transfer points, kept in working order;
• drills fitted with a dust extractor or a water injection system;
• crushers run with high-efficiency bag filters and water sprinkling;
• mineral carried in covered trucks that are not overloaded;
• respiratory protection and training for people working in dusty areas;
• air quality around the mine monitored for fine particles (PM10 and PM2.5) against the Central Pollution Control Board norms.

Both mines report reusing rainwater and pit water for this sprinkling. The guide to mine dewatering explains how that water is collected and settled first. How the consent to operate itself is applied for is covered in the post on consent to establish and consent to operate. The exact conditions differ from mine to mine, so always read your own clearance letter.

What changes in an underground mine?

Underground, the dust has nowhere to blow away to, so control leans on water at the source and on ventilation. Drilling is done wet wherever possible, water mists and wetting agents are added at the face, and sprays wet broken rock before it is loaded. Air moved by the mine’s ventilation system carries off what is left. In the past, underground headings ran an air-and-water fogger about 30 m (100 ft) from the face, switched on before the blast and kept running for 20 to 30 minutes after it.

The surface methods in this guide are the ones a quarry or opencast mine uses every day.

Which machines are part of a mine’s dust plan?

Dust control on a mine is partly a fleet job. Graders keep haul roads crowned and smooth so they make less dust and shed standing water. Dozers and loaders build and repair the road surface. Loaders and excavators with sealed cabs protect their operators where the dust is heaviest. Water tankers and fog cannons do the spraying.

Read the guide to mining equipment for the whole fleet, and find every mining topic under mining. When you are ready to choose, compare mining excavators and mining dozers, and connect with a dealer.

Prices, specifications and features are indicative, vary by variant, location and date, and should always be confirmed with the official OEM or authorised dealer before any purchase decision. DesiMachines is not liable for decisions taken on the basis of information that may have changed after publication.

References: NIOSH, Dust Control Handbook for Industrial Minerals Mining and Processing (RI 9689, 2012), chapters 2 (wet sprays and droplet size), 3 (drilling and blasting), 4 (crushing), 9 (enclosed cabs) and 10 (haul roads). Environmental clearance compliance reports for the Kallakudi limestone mine, Tamil Nadu (clearance of 24 October 2005) and the Kujam-II bauxite mine, Jharkhand (clearance of 21 July 2015, report for October 2023 to March 2024). Wikipedia, Fugitive dust. Fog cannon working principle and rated reach from a fog cannon maker’s published product description. Machine dust features are taken from the feature lists published against each machine.