A widely used method to suppress dust in cement plant operations, spray technology can be highly effective and cost-efficient when properly specified. Controlling dust through application of water is a simple concept, yet its implementation entails significant complexity. Dozens of factors must be considered before optimum spray system solutions can be identified for a quarry and plant. As even slight variations in performance can compromise effectiveness, create new problems, and increase operating costs, the margin for error in spray system specification is severely circumscribed. The following information details available options and explains how to ensure optimal spray system performance.

DUST SUPPRESSION BASICS

Dust is generated by wind erosion as well as by moving, crushing, grinding or milling materials to a finer size. It consists of solid particles larger than 1 micron (µm) in mean diameter. A particle 1,000 µm or larger, however, is no longer considered fugitive dust since it fails to remain airborne. Because they are respirable, dust particles smaller than 10 µm are the most dangerous.

Among basic methods to achieve dust control, wet dust-suppression spray systems increase humidity/moisture content in the product stream, helping to minimize dust overall and prevent it from becoming airborne. Moisture can be added to the material when it is stationary or moving. Such systems use nozzles that produce drops in the 200 to 300 µm range to create a light rain effect.

By contrast, airborne dust-capture spray systems produce very fine water drops to collide with dust particles in the air, forming agglomerates of dust and water too heavy to remain suspended. Drops that are too large will not collide with the finest, most hazardous dust particles. Drops that are too small evaporate quickly and release the captured dust. Nozzles that produce a wet fog consisting of drops in the 10 to 50 µm range or a dry fog consisting of drops in the 1 to 10 µm range are most effective. Specific drop size requirements are determined by dust particle dimensions (see Figures 1 & 2). Collision is most likely to occur when the drops and dust particles are similar in size.

In addition to drop size, effective spray system performance and dust suppression depend upon such spray characteristics as drop distribution, drop velocity, spray pattern and spray pressure. These factors will be addressed in the following sections.

DEVELOPING A STRATEGY

Spraying dust with liquid adds weight to the particulate matter and prevents it from becoming airborne or settles it quickly, if already airborne. Adding moisture to dust and materials during processing also helps minimize the amount of dust produced downstream. What, then, is involved in making this happen?

First to be determined is where spraying is needed. Crushers, unloading/transfer points, dump hoppers, belt conveyors, grinders, transfer chutes, stockpiles and dirt/gravel transport roads are locations that typically produce dust and/or require soil stabilization. The quantity of dust generated will depend upon the volume of bulk material, its age and moisture content, as well as the proportion of aggregate fines. Accordingly, each location is likely to require a different type of spray solution (see Figure 3).

Determining the type of dust suppression system for optimum results is the next step. Wet dust suppression systems can spray precise amounts of water, fog, water with surfactants, foam or encrusting agents that coat the surface of the material with a film durable, yet flexible enough to allow the substance to breathe. Since limestone readily accepts moisture, spraying water in a very controlled manner may be the most economical option for the majority of locations within an operation.

The following, however, must be considered:

• Water has little residual effect and may need to be applied continuously.

• Overwetting is a potential problem, as it can impact material and/or equipment, causing mud, sludge and other build-up that may be detrimental to process efficiency.

• Freezing is a potential problem for water spray systems in certain climates.

• If water is in short supply, alternate solutions should be explored.

Spraying a mixture of water and surfactants comprises a solution in some locations, although surfactants can alter the properties of certain raw materials and damage equipment, such as conveyors. The lower surface tension of surfactant water solutions increases shear and produces more and finer drops using less water.

Spraying encrusting agents or a combination of foaming surfactants, water and air serves to reduce the amount of water required and provides a residual effect. However, spray systems designed for encrusting agents and foam surfactants are more costly to install, operate and maintain; they may also cause material or equipment damage.

Airborne dust-capture spray systems use finely atomized water to produce very fine drops or fog. Most commonly used at transfer points with tight enclosures and low turbulence, the system design is dependent on specific conditions at each location. These systems have low water requirements, add little moisture to the material and are contaminant-free since only water is being sprayed.

CONFIGURATION CONSIDERATIONS

Once locations and methods for dust suppression have been identified, the next step entails designing the spray system. Most important among components in a system are the spray nozzles, which determine drop size, drop distribution, drop velocity, spray pattern, spray angle, flow rate and pressure. These variables must all be considered in evaluating nozzle type and placement.

• The nozzle manufacturer should be consulted for information about drop size and drop distribution at various operating pressures. Drop size decreases as operating pressure increases, so what to expect from spray nozzles under various operating conditions must be delineated.

• Generally, higher drop velocities are desirable, especially in areas with high-velocity air streams and/or where the nozzle is not located close to the target. Drop velocity is dependent upon drop size: Small drops may have a higher initial velocity, but the speed diminishes quickly. Larger drops retain velocity longer and can travel further. Again, the nozzle manufacturer must supply drop size information.

• Nozzles produce different types of spray patterns with varying characteristics.

  • Full-cone nozzles produce a round spray pattern and provide high velocity over a distance. They are commonly used when nozzles must be located a good distance away from the area where dust suppression is needed.

  • Hollow-cone nozzles produce a circular ring of water. Drops are generally smaller than other nozzle types and are used in locations where dust is widely dispersed. Reduced clogging is an added benefit of hollow-cone nozzles because its orifice is larger than that of other nozzle types.

  • Flat-spray nozzles produce a tapered-edge, rectangular or even spray pattern. These nozzles are typically used in narrow or rectangular enclosed spaces.

  • Hydraulic fine-spray nozzles and air atomizing nozzles produce a fine mist or fog. Hydraulic fine spray nozzles are preferred in most areas because operating costs are lower since compressed air is not required.

    Air atomizing nozzles, which produce finer drops, are effective in locations where dust particles are extremely small and the nozzles can be located in close proximity to the dust source.

  • Extensive research conducted on spray nozzles to determine the most effective types for dust suppression indicates that atomizing nozzles offer the best performance, followed closely by hollow-cone nozzles. Full-cone and flat-spray nozzles are approximately two-thirds as effective as hollow-cone nozzles. Nevertheless, the specifics of any operation will determine which nozzles are best suited for its dust suppression system.

• The nozzle's spray angle also needs to be factored into the equation. Spray angle and spray pattern will yield a specific coverage, determining how many nozzles are needed and their relative placement. In most applications, a spray angle between 55° and 70° offers the best coverage without compromising the creation of agglomerates.

• The flow rate of water through the nozzle depends on operating pressure. In order to determine the increase in moisture on materials being sprayed, the flow rate of the spray nozzles must be ascertained.

• To avoid incurring damage, nozzles should be positioned for easy maintenance access without falling directly in the path of moving material. In wet dust suppression systems, nozzles should be upstream of the site where dust emissions are created. In airborne dust-capture systems, the nozzles should be placed to maximize the amount of time the water and dust can interact.

• Using more nozzles at lower flow rates can help ensure that nozzles are aimed at the broken material rather than spraying into the air and adding moisture to adjacent metal or rock.

• Automation of a dust suppression system can result in better performance with less waste. Simple on/off operation to ensure the addition of adequate moisture without overuse of water can be achieved using solenoid valves. Total system automation — control of all system components including pumps, spray nozzles/headers and electrical/pneumatic devices — is possible via turnkey spray systems designed to optimize the performance of the spray nozzles. Automation can result in more precise spraying, a decrease in water and electricity consumption, and reduced maintenance time.

ENSURING OPTIMAL RESULTS

The best means to evaluate an operation and assess dust suppression system requirements is to obtain expert help. Contacting spray nozzle/spray system manufacturers is a good place to start. Of utmost importance, however, is finding a company that has significant experience providing dust suppression solutions, plus the ability to simulate a particular environment and conduct drop size/spray characterization studies, in case testing is required to predict performance. When the science of spray technology and dust control is left to the specialists, best results are obtained, and producers can focus on their area of expertise — producing cement.

ABOUT THE AUTHORS

Wes Bartell and Bob Jett are applications engineers for Wheaton, Ill.-based Spraying Systems Co. Each has more than 25 years' experience in spray technology, emphasizing dust suppression in aboveground and underground mining applications. — Spraying Systems Co., 800/95-SPRAY (U.S.) or 630/665-5000 (outside the U.S.), www.spray.com.

FIGURE 2: PRACTICAL INTERPRETATION OF DROP SIZE

PARTICLE SIZE MEDIAN VOLUME (µM)	REFERENCE	TIME FOR PARTICLE TO FALL 10 FT. IN SECONDS
2,000 to 5,000	Heavy rain	.9 to .85
1,000 to 2,000	Intense rain	1.1 to .9
500 to 1,000	Moderate rain	1.6 to 1.1
100 to 500	Light rain	11 to 1.6
50 to 100	Misty rain	40 to 11
10 to 50	Wet fog	1,020 to 40
2 to 10	Dry fog	25,400 to 1,020

APPLICATIONS BY SPRAY NOZZLE TYPE

Jaw crushers	Air atomizing, hydraulic fine spray and hollow-cone nozzles
Loading terminals	Air atomizing and hydraulic fine spray nozzles
Primary dump hoppers	Air atomizing and hydraulic fine-spray nozzles
Stackers, reclaimers	Full-cone and hydraulic fine spray nozzles
Stockpiles	Flat-spray and hydraulic fine spray nozzles
Transfer points	Hollow-cone, full-cone and hydraulic fine-spray nozzles
Transport areas/roads	Hollow-cone nozzles
Vibrating screens	Air atomizing nozzles