Control droplet size to mitigate dust

We all know how difficult it is to work in dusty environments, especially when the dust is airborne. Dust mitigation will help to reduce maintenance, machinery and cleaning costs, but also to improve the well-being of workers and neighbors and to increase visibility and minimize accidents.

European directive 2017/2398, which must be transposed into Spanish regulations before January 17, 2020, considers various agents, in their respirable fraction, as mutagens and carcinogens. One of them is crystalline silica dust. Therefore, the suspended dust produced in quarries, aggregates, mines or recycling tasks (which contains this crystalline silica) can cause cancer and silicosis to workers and the general public.
From the European Union, they have already begun to demand preventive actions from companies that are dedicated to carrying out work that entails these risks.

The strategy to follow is prevention: preventing dust from being generated and becoming airborne. The second is collective protection, that is, preventing the dust from reaching people. And as a final step, individual protection. In no case is individual protection alone sufficient; both collective protection and prevention are necessary, if possible.

On the other hand, the main measure that must be adopted is to act on the source of dust emission to prevent it from being suspended or mitigate it as soon as possible. One of the most effective systems for this is based on the nebulization of water by cannons, which can reduce up to 11 times the amount of dust.

The behavior of dust depending on its size

The respirable fraction of crystalline silica is 10 micrometers (µm) or less. The fraction visible to the human eye is from 30 µm. This means that the carcinogenic dust is invisible. The other larger dust particles, between 60 and 200 μm, can become suspended by air movements, but they would only rise to 1 meter or so under normal conditions and would fall to the ground on their own in no time.

However, those smaller than 60 μm would remain suspended for long periods of time and travel long distances in the air. Precisely, the respirable fraction (less than 10 µm) travels even longer distances. Remember that these particles are carcinogenic and mutagenic.

In mining, before the dust is put into suspension, water nozzles should be used very close to the source of emission (25 to 30cm) with drop sizes greater than 100 μm, preferably 200 to 500 μm to wet the materials.

Instead, if the dust is already airborne, the goal is to mitigate it. To achieve this, the water droplets must be similar in size to the dust particles. The intent is to cause the droplets to collide and stick (clump) to the dust particles, causing them to fall out of the air. In this case, droplets in the 2-20 μm range have been shown to be the most effective.

If the droplet diameter is much larger than that of the dust particle, the latter will simply follow the airflow around the droplet. If the water droplet is of a comparable size to the dust particle, contact will occur; this happens when the dust particle, following the airflow, collides with the droplet (Illustration 1). Therefore, for optimal agglomeration, the sizes of the water particles and droplets should be approximately equivalent.

Better more drops, less volume

In addition, the probability of impaction also increases as the size of the droplets decreases. This is simply because the smaller the size of the water droplets, the more water droplets there will be and the more surface area covered.

Next, Table 1 relates the size range of dust particles that can be produced in a mining job with the size range of common precipitation drops. The table also includes the time it takes for each kind of droplet to fall a distance of 10 feet (approx. 3 meters), to give the reader an estimate of how long these drops of various sizes will stay in the air.

The key: the mouthpieces

Many factors can affect droplet size: fluid properties, nozzle capacity, spray pressure or angle.
In Spain, the National Institute of Silicosis (INS) emphasizes the importance of choosing the ideal size of the water droplets, since, if the adequate size of the drop is not produced, the dust will not be mitigated. As can be seen in Illustration 2, with an average droplet size of 34,6 µm, the vast majority of the droplets are smaller than 20 µm.
Misting systems such as SprayStream, distributed by Anzeve in Spain, produce micro-droplets of water capable of trapping suspended dust particles without wetting the ground. These barrels are manufactured in different sizes and ranges and allow you to choose the right nozzles to achieve the desired size in each situation.
Now that you understand the importance of droplet size, all that remains is to contact an expert to be able to determine which style of nozzle and spray pattern suits your needs, what is the appropriate angle, what pressure variable to choose, with what impact to the surface, where to install them and what type of water to use.

A case study in New Delhi

This general air quality study was done during a construction process. The result is truly amazing: the drop in ambient air pollution is over 90% thanks to SprayStream.