WHAT IS THE BEST DUST CONTROL PRODUCT FOR ROADS?
Editorial focus by Harold Hough
Spend an hour at an open pit mine and you will quickly discover the most popular dust control agent for dirt roads – water. Not only is it environmentally friendly, it is relatively cheap if the mine has its own well. Best of all, it’s effective as long as it stays on the dirt. However, most miners don’t realize that using water for dust control is penny wise and gallon foolish.
For all of its benefits, water isn’t the ideal dust control agent. On haul roads, the regular use of water will cost a mine thousands of dollars a year in additional gravel. When the roads are wet, the gravel is forced to the side of the road as the tires move through the mud. When the road is dry, the gravel is thrown off the road and often into the windshields of other trucks and cars. As a result, vehicle repair costs are higher and the gravel must be replaced frequently.
Another problem with water is that it evaporates quickly and the remaining crust of dirt is easily broken up by traffic, necessitating a new treatment. As a result, many mines require one or more tank trucks to wet the roads and several people to run and maintain the fleet. Although water is cheap, the cost of applying it is enormous.
According to studies by Chemstar Lime, labor and material costs of applying water are six times higher than applying dust control agents, if water must be applied once a day. If the mine has to apply water more often, then the costs skyrocket.
The final problem with water is that it can cause public relations problems. Even though water is environmentally safe, water shortages in some Western states have made water consumption a sensitive issue. Consequently, a mine that uses water for dust control may find itself labeled a water hog by local activists.
The haul road is considered one of the biggest dust control problems because of the constant use. But, no one agent offers the perfect answer. For instance, if the haul road is a packed dirt road with little dust, a thick product that binds dirt will work very well. If the road is dusty, the mine needs a lighter product that penetrates the dust and binds it. If the road handles heavy traffic, the mine needs to make several applications for the dust control agent to work effectively.
So, if water isn’t the answer, what other choices are there?
There are hygroscopic salts like calcium chloride (table salt) and magnesium chloride that draw water from the atmosphere and hold in the surface soil. Unfortunately, salts depend on moisture in the air, so aren’t as effective in dry climates. They can also corrode equipment over a long time.
A more environmentally friendly solution is organic sulfonates like lignosulfonate. They are byproducts of the lumber industry and control dust in the same manner as hygroscopic salts. The major difference is that the material, which looks like molasses is organic, non-toxic, and won’t control equipment.
The third group are emulsion products, which includes a wide range of natural and synthetic materials that bond the soil together like glue. They include latex, polymers, and petroleum products. They come as a concentrate, are mixed with water and sprayed on the road surface. Usually, the stronger the compound, the harder it is to penetrate the soil. Therefore, the dusty roads are treated with a weaker solution to bind all the soil within a few inches of the surface and then retreated later with a stronger solution that provides greater protection to the surface.
In order to find the best dust control agent, the Department of Defense set up a test at Fort Sill and Fort Hood in the late 1990s. They looked at four agents from the three groups and how they worked on dirt roads: polyvinyl acrylic polymer, soybean feedstock byproduct, calcium lignosulfonate, and a 38% calcium-chloride solution. They discovered that all four agents were equally effective during the first month by reducing dust by 50%. Generally the dust control agents continued to provide protect past 60 days. However, polyvinyl acrylic polymers tended to break down between the first and second month as heavy traffic tended to break down its surface sealing characteristics.
Another dirt road dust control study was done by California and focused on eliminating the fine dust that remains suspended in the air and is inhaled. They discovered that polyvinyl acrylic polymers reduced these fine dust particles by 90%. Lignosulfonates reduced dust by 20% and calcium chloride 10%.
When it comes down to it, there is no perfect dust control agent for haul roads. Mine managers have to consider several factors, including soil composition, local environmental regulations, and traffic.
Here’s an example. Lignosulfonates don’t last as long as chloride products, so they can cost more. However, they are more environmentally friendly and don’t corrode heavy equipment. But if lignosulfonates get on heavy equipment, it’s like trying to get pine tar off. Usually warm, soapy water does the trick, but that increases water usage and pollution problems. There are also some minor downsides to the smell and color of lignosulfonate. When you first put it down, it smells like pine tar, but that dissipates quickly. The lignin also turns the road black because of the color of the product.
Whatever the dust control agent you choose, they all offer advantages to water. They all reduce dust for months and keep haul roads in better condition than water. In the long run, a good dust control agent will save your mine more money than water.