Flow control chute or passive chute designs can make great improvements in reducing dust levels. However, there are some drawbacks, including cost and available real-estate to locate these types of larger chutes. Reducing air movement and sliding the material onto the belt can drastically reduce dust generation but in many cases finer float dust (PM-10) will still escape out of the conveyor load point area. DSI has provided Dry Fog Systems to remove this float dust from the air on every major manufactures Flow Control Chutes, including Benetechs InteliFlo®, SAS Global and others. In many cases clients might be able to use their existing chute work and install a Dry Fog System on it with only minor skirting modifications, saving a lot of money with the same desired results.
In recent years some companies have sold what are essentially snow making machines for dust control. The machines are often referred to as Fog Cannons when in reality they do not produce water droplets small enough to be called fog. Dry fog is defined as droplets between 1-10 microns. The next droplet size category is Fine Mist, droplets between 11 and 100 microns. These machines typically put out a minimum droplet size of 90 microns up to 300 microns which is defined as Fine DRIZZLE, just short of RAIN in size! This distinction is important because if the droplet size is larger than the airborne dust particle then the particle can slip stream around the droplet resulting in no contact and therefore no agglomeration of the dust particles to make them fall out.
Water consumption for Cannons is measured in Gallons per MINUTE ranging from 12 to 30 GPM! With Dry Fog TM we measure our nozzles in gallons per HOUR, ranging from 3 to 13 GPH. What this means is you have high water consumption, possible wetting of material issues, BTU loss for fuels, unstable loads for ship holds as example copper concentrate being too wet, slip and fall hazards ...
We can use so little water because we create a true dry fog that is the same size as the dust particle. We estimate that we can cover the surface area of ½ a football field with fog using only 1 gallon of water!
Water Sprays or Garden Hose Technology is the oldest form of "dust control". These systems are designed to wet the material before dust is generated. However, there are several issues that makes these systems ineffective.
Misting Systems are not Fog Systems. Misters use very high water pressure (600 to 1200 psi) to squeeze the water through orifices as small as .006 of an inch. Our Dry Fog system essentially produces droplets in the 1-10 micron size range (dry fog is defined as 1-10 micron droplets). See Droplet Size Chart The dust that becomes airborne and is of concern to health and environmental officials is called PM-10 or particulate matter 10 microns or smaller. A like size droplet to dust particle size (PM-10) is essential as it is this like size that dramatically increases the opportunity for the dry fog/water droplet to impact the airborne dust. Larger droplets (mist 20 to 100 microns) have proven to be ineffective in treating airborne particulate due to the slip stream effect created by the droplet being larger than the dust. This concept was first explored by the Colorado School of Mines in in the 1970s with the results being published in a 1976 Issue of Coal Age Magazine
Chemical Dust Suppression systems use chemicals in an attempt to bind dust particles together or to break up the surface tension of the water, increasing its ability to penetrate the conveyed material. In 2009, the US Environmental Protection Agency received testimony from a variety of Coal Preparation Plants that moved the EPA to Exclude Chemical Dust Suppression Systems from its designation of Best Demonstrated Technology (BDT) for sub-bituminous coal. At the same time the EPA designated Fogging Systems as BDT for sub-bituminous coal.
The Dry Fog Advantages
1. No Chemicals to buy year after year as none are required therefore reducing your operating cost. Some customers will use their operating budget for buying chemicals to purchase a Dry Fog System and eliminate yearly chemical cost!
2. No Chemicals to transport or store and no MSDS requirements to comply with or records to maintain.
3. No Damage to conveyor belts by applying proprietary chemicals. Conveyor Belt Damage Article
3. No Product Wetting which for fuel sources lowers the BTU value (see BTU penalty chart by Martin Engineering). Wetting material and can create material handling issues.
4. No Pumps, Totes or MSDS. Dry Fog operates at low water pressure using plain water, so it typically Does Not Require Pumps eliminating moving parts that need maintenance.
Wet extraction systems have been in use for some time. Rotoclones, wet scrubbers and more recently dust extractors all require expensive duct-work and create a secondary waste-stream of waste water to dispose of. They all have moving parts that can fail as seen in the photo. Wet dust can collect on rotating fans, creating an imbalance that can destroy bearings in short order.
Disadvantages of wet extraction compared to Dry Fog include:
1. Higher Capital and Operating Cost
2. Moving Parts to Repair and Replace
3. Requires Expensive Duct Work
4. Creates a Secondary Waste Stream to dispose of, possibly requiring new permits for waste water discharge.
5. Will not be very effective on large open dump pockets unless combined with Dry Fog