A well-designed residual suppression system makes it possible to control fugitive dust over a wide area by applying the solution at a few strategic points.
Residual Chemical Suppression Agents
Surfactants wet the dust fines, so the particles agglomerate, thereby preventing them from becoming airborne. Once the solution evaporates, the suppression effect of normal surfactants is gone. In many cases, however, dust suppression is required not only as the materials move through multiple transfer points, but also after the materials reach the storage bins, railcars, barges or stockpiles. In these cases, it is wise to consider using a water/surfactant spray or foam system with a longer-lasting residual effect. Residual dust suppression is valuable when considering dust suppression for:
- Large areas with multiple application points
- Long distances between application points
- Stackers or trippers
- Crushers or mills
- Elevated transfer points where it would be difficult to apply dust suppression
A well-designed residual suppression system makes it possible to control fugitive dust over a wide area by applying the solution at a few strategic points. In contrast, using water and/or fog systems for large areas will require multiple application points, including several pump stations; longer water, chemical and air lines; higher pumping capacity; and more application nozzles—all of which can make the system considerably more expensive and, in some cases, not as effective.
Coal conveyed from unloaders to open storage piles might remain there for extended periods of time. Material stored in open stockpiles is subject to variations in climate, including wind, sun, and precipitation.
The heat of the sun can evaporate moisture out of stored material, making it more likely to become wind-blown. Wind erosion creates large amounts of dust that can settle on nearby houses and yards. When stored coal is reclaimed, it may be dry and present greater dusting problems than it did during initial handling. Dusty materials, such as calcined coke or iron-ore pellets, may require dust control from the point of production to the point of end use. This could amount to several weeks and several thousand kilometers (miles) apart. In such cases, it may be more economical to apply a residual surfactant/binder to the materials than to apply surfactants and water at multiple sites throughout the materials-handling system. There are a variety of residual binders available.
The objective of a residual, or binder, suppressant is to agglomerate fines to each other, or to larger particles, and then hold the structure together, even after the moisture evaporates. In some cases, a hygroscopic material—such as calcium chloride—is used, which retards the ability of moisture to leave the treated material. The advantage to this approach can be a low treatment cost. More conventional binders include lignin, tannin, pitch, polymers, and resins. When combined with surfactants to aid wetting, these compounds coat larger particles and then act as a glue to attract and hold dust fines.
Application of residual binders tends to be more expensive than surfactant applications, because they must be applied with higher concentrations. Although binders are less expensive per kilogram (pound), they are typically applied at dilution rates ranging from 50-to-1 to 200-to-1 (2.0 percent to 0.5 percent).
It is important to mention that, with the use of a residual chemical, a plant can reduce the number of application points required, reducing, in turn, the amount of maintenance required.
When choosing a binder, it is especially important to know the effects the binder will have on transfer equipment and conveyor belts. If the binder adheres well to the material, it may do the same to the handling equipment. Proper application of the product becomes critical, because overspray of the binder onto process equipment or empty belts can result in considerable production and maintenance problems.
An important consideration in selecting a binder is the effect the chemical will have on both the material being treated and the environment. If the binder is applied to material going into a stockpile, and that stockpile is exposed to rain, portions of a water-soluble binder may end up in the runoff and provide an environmental concern. Most chemical manufacturers provide only binders that are compatible with the environment; however, this is an issue that should be raised with the chemical supplier.