Dry Fog – Effective Dust Control Technology for Respirable Dust

Dry Fog – Effective Dust Control Technology for Respirable Dust

Dust control in recent times has been a focus of attention brought on by the horrific consequences of elevated dust levels, highlighted in particular in the Inquiry into the re-identification of Coal Workers’ Pneumoconiosis (CWP) in Queensland in May 2017.
Fugitive dust, usually liberated by mechanical means, will always be an issue in any form of mining or materials handling and dust mitigation techniques need to provide an adequate means of tackling this dust at the source of the dust generation to reduce the risk. Dust generated in bulk handling situations such as ROM bins, breakers, screens, conveyor transfers, train loading or loadout, stackers, reclaimers and shiploading or unloading will produce a dust cloud with widely varying particle sizes. In particular, respirable dusts, defined as dust with sub-10 micron particle size in diameter and smaller, are very slow to settle or dissipate and can pass through the body’s natural respiratory filters to be taken deep into the lungs. The inquiry report submissions pointed to ‘primarily exposure to invisible respirable dust that is responsible for various adverse health effects ranging from mild symptoms – such as eye, nose, throat irritation and shortness of breath – to more severe effects such as CWP, chronic obstructive pulmonary disease (COPD), emphysema and silicosis.’1

Physical elimination of this dust, i.e. removing the hazard or preventing it occurring in the first place, is the ‘best and most effective protection from harm’2. The issue with fugitive dust is that it has become airborne and will need to be removed from the atmosphere before it can impact on workers in the vicinity. A cost method of scrubbing out this dust effectively at source

Dry Fog Dust Control – A Unique Solution
In the 1970’s, the technique of dry fog dust suppression was created by Sonic Environmental Systems, Inc., a U.S. based company, for use in industrial dust suppression. This was accomplished through the special design of an ultrasonic nozzle that generated a sonic wave with compressed air and vapourised introduced water to produce droplets below 10um in size. Original research and testing undertaken by the University of Sweden and Colorado School of Mines3 on the nozzles’ effectiveness to suppress dust revealed that impaction and agglomeration between a dust particle and binding agent such as water will occur if the water droplet is the same size or smaller than the dust particle. On the contrary, if the water droplet size is much larger than the dust particles (for example 20-300um in size) – then the dust particle (1-15um) will follow the air stream around the water droplet and stay suspended in the air. This is what is typically seen when traditional water sprays are used to tackle fugitive dust – the airborne respirable dust will not be effectively suppressed.

Remarkably, lab and field testing of the Sonic nozzle also found Dry Fog has an ability to carry a positive charge, and studies have also demonstrated that most industrial pollutants acquire an electrostatic charge as they are dispersed into the air. If charged particulate material is exposed to an oppositely charged water fog, there is an increased probability of collision between the particulates and fog droplets. After contact is made, the particulates agglomerate rapidly and fall out of the atmosphere due to their increased weight.

This finding was tested with charged and uncharged fog droplets across a wide variety of industrial pollutants ranging from materials such as coking coal and iron ore crushing dust to other materials that are very lightweight and highly susceptible to moisture such as cement clinker and fly ash (Hoenig, 1977). For example, when testing dust density of fly ash in a controlled environment, it was found that the fog reduced the density of respirable material by fog by over 91%. There are two factors that attributed to this result. The first is that fly ash dust particles, lightweight and approximately 3um particle size are similar in size to the fog droplet, which facilitates a stronger collision and agglomeration between particles. The second is that the fly ash holds a negative charge which was effectively suppressed by positively charged fog.

The Ultrasonic Nozzle
The ultrasonic nozzle is the core component of the dry fog dust suppression system. Unlike water spray or misting systems, ultrasonic nozzles create fog droplets below 10um that most closely match and most effectively agglomerate with PM2.5 and PM10. This is accomplished using compressed air to forcefully push air and water into a convergent divergent venturi. This process creates a standing shock wave of 47k Hz, a high frequency sound wave. Air is accelerated beyond the speed of sound through the venturi creating shock waves, which pass into a resonator cavity and are reflected back to amplify the subsequent waves. The result is an intense field of sonic energy focused between the nozzle body and the resonator cavity. The diagram below shows a cross section of the orifice and resonator cavity.

The water particles are sheared by the incoming air down to smaller sized droplets and then enter the air stream. Upon entering the shock wave zone, the water droplets are shattered into very fine droplets below 10um in size. Due to the fact that the nozzle does not use high hydraulic pressure, the orifice opening for the water and air can be larger than normal atomization nozzles, meaning they are far less prone to blockages. Additionally, the sonic shock wave helps foster small vibrations that create a “self-cleaning” nozzle, and being stainless steel they don’t wear out or corrode.
Dry Fog Particulate Control Mechanisms
Dust particles and atomized water droplets (fog) fill the same area and come into contact. When this occurs, particle removal from the air stream takes place due to three primary mechanisms:
• Impaction: When the fog droplet flows in the path of the dust-laden air stream, the droplets and dust particles may collide depending on their initial trajectory and velocity. This collision is called impaction (in our case agglomeration). Due to inertia, the impact with the droplet will cause the dust particle to become encapsulated, increasing its weight causing it to fall out of the atmosphere.
• Interception: The finer particles moving within the air stream do not hit the fog droplets directly but rather graze the droplets and adhere to them. This mechanism also causes an increase in particle weight.
• Diffusion: When fog droplets are scattered among dust particles, the particles are deposited on the droplets by diffusion.
The impaction / interception efficiency increases as the amount of water droplets compared with dust particles increases, the particle size increases (or conversely the water droplet size decreases), and the velocity of the droplets is fast enough to create an airstream by which the particles will flow.

Dry Fog – Minimal Water Useage
It is also important to note in the above study, that as the particle sizes decreased, the surface area of the fog increased. This is easy to understand from a simple physics and calculation perspective. If we have a 1000 micron diameter drop of water in a simple sphere and split it into 8 smaller spheres the surface area increases, while the volume of water stays the same. Figure 4 demonstrates this concept:
By splitting the droplet into 8 smaller droplets, we have just doubled the size of the surface area. The ultrasonic nozzle creates droplets in the range of 1-10um, with an average droplet size of just under 5um. We would need to split the spheres 15 times to achieve these sized droplets. After 15 divisions of the spheres, we will have droplets of average size 5.9um, and a surface area of approximately 3848 meters squared versus the original sphere at 0.12 meters squared. Typically, Dry Fog will use less than 1/5th of the amount of water used in a water spray system.

Dry Fog Dust Suppression Applications

Transfer Points

Due to the air movement from the displacement by the material, the dust that is generated at a transfer point moves with the material flow. More Dry Fog nozzles are typically used at the impact point on the receiving conveyor due to the larger amount of dust generated at the impact point to prevent the dust from exiting the conveyor into the open air (see Figure 7). Nozzles are usually mounted on the head chute cover and fog is drawn down with the material with the generated airflow.

ROM Bins, Receiving Hoppers and Shiploading

Larger open areas for material handling such as ROM bins, receiving hoppers, and shiploaders, generate dust clouds during the dumping process and when the material strikes the recieval bin or hold below, the air is displaced and large plumes of dust rise up the sidewalls. Dry fog can be used to counteract the dust by creating a “blanket” of fog in the recieval area, which captures the dust as it pushes up the walls of the bin, settling the dust back into the bin and preventing it from escaping to atmosphere. As fog is light, it will also follow the uprush of air up the bin walls, and continues to act on any dust that may push through this blanket.

Where the ROM bin or hopper is exposed, dry fog can be used in conjunction with wind fencing to contain the dust and dry fog in the same area to allow the agglomeration to occur between the dust particles and the fog droplets.

Train Loading Unloading
Both rollover or belly dump systems as well as train loaders can be treated with Dry Fog. Both types of operation create a large displacement of air as the material hits the hoppers or cars below causing the dust to rise with the air flow. The hoppers or fill area is filled with fog prior to discharging the material to create a “blanket” across the open area, and when the material is discharged, the fog agglomerates with the dust prior to leaving the hopper or car, increasing the particle weight and returning it to the process. The air that flows out of the hopper or car will carry the additional fog, and this will evaporate.

Other Applications
Dry Fog can be successfully used plant wide in both underground and open cut operations, with rotary breakers and scalping screens, or a crushing and screening circuits (including mobile circuits), as well as stackers, reclaimers and drill rigs.

Is Dry Fog Effective
Dry Fog has been used in over 400 installations worldwide, for materials such as coal, copper, gold, bauxite, cement, clinker, phosphate and manhy others. The US EPA lists it as a best demonstrated technology for suppression of sub-bituminous and lignite coal dust and is listed as a control technology for new CCR Rules in the USA in relation to fly ash and coal residuals handling. It is a well understood and implemented technology especially in the US coal industry, and is slowly being recognised as an effective control in many other countries throughout the world. Various studies have been conducted to determine effectiveness, and results with effective PM10 reductions above 90% have been recorded in some areas as per the table below:

Application Before Fogging System After Fogging System
Crusher Feed Point 72.63 1.67
Crusher Discharge 192.98 5.22
Screen 10.80 0.42
Transfer Point 15.10 1.06

MARC Technologies has been successfully delivering dry fog and other dust control solutions to the mining and industrial sectors since 1995. MARC Technologies is a wholly owned subsidiary of the Brisbane based ALS, providing a significant presence already in most mines in Queensland, NSW and beyond. www.marctech.com.au, contact us on 08 9232 0430 or through your local ALS office.

1Professor David Cliff, submission 1, p 6, Inquiry into the re-identification of Coal Workers’ Pneumoconiosis in Queensland, Coal Workers’ Pneumoconiosis Select Committee, May 2017
2Caledon Coal, submission 19, p 14 (from WorkSafeBC, 2016), Inquiry into the re-identification of Coal Workers’ Pneumoconiosis in Queensland, Coal Workers’ Pneumoconiosis Select Committee, May 2017
3 Schowengerdt, F.D., Brown, J.T., 1976. Colorado School of Mines Tackles Control of Spirable Coal Dust, Coal Age. Journal Volume: 81:4. Golden Colorado.
4 Warrington, Glen. 1979. Using Agglomerative Dust Suppression for Dust Abatement in Crushing and Screen Plants. Aggregate Producers Association of Ontario. Ottowa, Ontario. CA.
Coal Workers’ Pneumoconiosis, chronic obstructive pulmonary disease (COPD), emphysema and silicosis