Haul Road Dust Control Chemicals
There are a large range of different chemicals used on mine haul roads for dust suppression. In this article we will discuss the more common ones and specifically how they work, where they work well, their limitations and safety and environmental issues.
For all dust control chemicals, the better the road structure and condition the better the performance – this is particularly the case for longer term solutions.
Water attracting salts such as magnesium chloride and calcium chloride work by absorbing moisture out of the atmosphere making the surface behave like it has been lightly watered. Their dust suppression performance is dependent on their ability to absorb moisture which is affected by the temperature and humidity (eg magnesium chloride starts to absorb moisture at 32% relative humidity).
Magnesium and Calcium Chloride are very effective in suppressing dust and can provide long terms dust suppression without the need for watering for months. They are also more tolerant than other dust suppression chemicals of the quality of the road surface due to their action.
In climates that are dry for long periods (ie low overnight humidity) the surface may require occasional light watering. Chloride salts are highly soluble and are easily leached out of the surface with over watering or significant rain events, particularly shortly after application. This makes their application unsuitable in areas with high or regular rainfall. Chloride salts also require a degree of clay type material in the surface to hold the salts.
Concentrated chloride salt solutions are very corrosive to steel, although some suppliers include a corrosive-inhibiting additive.
Care should be taken using the salts on ramps or declines because they can cause the road surface to become slippery when wet – particularly if there is a high fines content in the surface. Roads treated with chloride salts can also lose their structural strength if they become over wet due to the dissolving and leaching of the salt leaving voids which allow the penetration of water into the road surface.
Chloride salts can have environmental impacts if the salts can make their way into water ways or ground water, but the risks are generally low if there is adequate buffer zone between the treated area and water ways.
Lignin is a biproduct of the wood pulping process and is used in road dust suppression to bind the surface of the road together.
In dry conditions it improves the strength of the road surface and can be very effective for months. Due to the binding effect the better the road condition prior to application the better the product performs.
In very dry conditions lignin derivates can become brittle so light watering may be required. Lignin derivates are highly soluble in water so they are easily leached from the road surface with heavy watering or rain. This solubility also makes the road slippery when wet which makes them unsuitable for use on ramps and declines. Like Chloride Salts, the dissolving and leaching of the Lignin when the roads become wet can cause the roads to lose structural strength.
There are generally no environmental impacts if adequate buffer zones are between treated areas and water ways.
Asphalt emulsions bind the surface particles together creating a strong hardwearing and waterproof surface.
Asphalt emulsions can provide long term, quality road surfaces that do not require watering and can require minimal maintenance. The quality of road structure combined with the type and frequency of traffic impacts their practicality and maintenance requirements.
The performance of asphalt emulsions is very dependent on the quality of underlying road structure. If there are too many fines in the surface the asphalt can form an unstable crust and fragment under traffic. If the road structure is not stable, then movement can cause cracks in the surface. If water is able penetrate through cracks or small holes, then it can quickly damage the unprotected road base causing holes which can have sharp edges that damage tyres. Due to the lack of structure in many haul roads this means that asphalt emulsion surfaces can require ongoing maintenance.
Due to the environmental and OH&S requirements with these products they are typically applied and maintained by specialist personal. In a remote mining environment this and the maintenance requirement may require the asphalt emulsion supplier to maintain staff permanently on site.
Asphalt emulsions are petroleum-based products and can come from a range of sources some of which may be toxic. There are environmental risks with spills and leaching prior to the product ‘curing’. The handling of the raw product typically requires specialist PPE to manage fumes and contact risks.
Polymers work by binding particles in the road surface together when they dry. The strength of the polymer bonds and their ability to withstand environmental conditions varies between polymer types, polymer quality and application rate.
Polymers are cost effective and easy to and safe to use. They are frequently added to mine water carts as part of mines ongoing watering program.
The limitations with most polymers are their short duration performance on haul roads due to the heavy vehicles continual breaking the polymer bonds so new bonding material needs to be regularly added.
The early polymers used for dust suppression were based on PVA and vinyl acrylic. These are readily available, cheap and easy to use. These are less frequently used for mine haul roads, but are still used in high concentrations for longer term dust suppression of light vehicle roads.
In the past 10 years gelling type polymers have been used on mine haul roads. They are used at low application rates (as low as 1:50,000) as water extenders to reduce watering. They are not used in high concentrations due to their gelling behaviour which makes applying at high concentrations impossible. The low volume of product required made them a cost-effective option to reducing water truck use.
New polymer products (eg DustWorx GX) have been released recently which are suitable for longer term mine haul road dust suppression. These new polymers have the advantages over other long term solutions in that they are environmentally safe, easy to apply and do not becoming slippery and dangerous in the wet.
Polymers work best on well-constructed and compacted roads and their performance is enhanced by good road compaction which is facilitated by regular heavy vehicle movements shortly after application. The performance of polymers is reduced on loose surfaces particularly those with loose gravel and where vehicles are turning as this increases the breaking of the polymer bonds. In high concentrations some polymers are slimy when in solution, but in general they do not cause road slipperiness as when applied as they quickly bond to the road particles.
Polymers do not leach and are generally safe to use and safe for the environment with many considered biodegradable.
Surfactants / Wetting Agents
Surfactants and wetting agents act by reducing the surface tension of the water.
Mine haul roads can become very compacted which reduces the ability of water to penetrate the surface. Surfactants and wetting agents improve the waters ability to penetrate into the road surface. This has the effect of extending the time between watering.
Most surfactants are safe to use and safe for the environment with many biodegradable. Care does need with using surfactants with some ores as surfactants can impact processing that utilise flotation.
Surfactants are commonly used with polymers and other dust suppression chemicals to improve their penetration into the road surface.