Particle size is of the essence when it comes to dust suppression and its importance can never be underestimated. Wet dust control in particular involves fine atomized mist of water used at elevated pressures to coat buoyant dust nanoparticles that are already airborne whilst in the process enlarging the particle size resulting in settlement owing to gravity. We do mention water very sparingly and just as a common example as it is not the only wet dust suppressant available but can be combined with other dust palliatives such as surfactants and amphiphilic liquid polymers that can enhance its dust control properties. The importance of dust suppression can be critically highlighted in how respirable and inhalable dust nanoparticles can by-pass the respiratory system and enter the lower respiratory tract. Depending length and frequency of exposure and source of dust nanoparticle it can scar the lung tissues resulting in pneumoconiosis (coal) and silicosis (silica). The article will focus on dust nanoparticles and their effect on respiratory system, their sources, targeted methods of dust suppression of coal and silica. 

Dust in the nano range

The size range of particulate matter varying between 1 and 100 nanometers are known as nanoparticles. Their very small size renders them susceptible to dislodgement in the air and remain unnoticed and dispersed for a prolonged period of time as potential highly inhalable and respirable particles prone to attach and interact on the alveolar regions of the lungs. The role played by the pulmonary surfactant is critical in understanding how exposure to dust nanoparticles can lead to deterioration of pulmonary function as they are capable of reaching the deepest regions of the lungs. Nanoparticle size, surface charge and hydrophobicity play a key role in the extent of damage that can proliferate in the pulmonary system. The area of interest in our discussion is alveolar region consisting of alveolar ducts and sacs, which are covered with alveoli that supply oxygen to the blood. The alveoli that coat the surface of the alveoli region are blanketed in a thin layer of pulmonary surfactant which reduces the surrounding surface tension and provides a physical barrier to protect alveoli from collapse and particle interaction. Intricately, highlighting the biology of the lung is to emphasize the importance of why dust suppression is not a matter of choice but a must because exposure to dust nanoparticles can lead to acute, accelerated and prolonged lung disease which can even lead to loss of life. 

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Reviewing dust generation

Origin of dust can be related to larger masses of the same material subjected to mechanical deterioration and breakdown processes. Activities such as stockpiling, loading, transportation and mineral processing operations can result in dust generation. In drilling and blasting operations air flush from drilling and force of the blast can generate dust. The dropping of material from height produces dust in loading, dumping, draglines and conveyors. In mine haulage roads the dust is generated from transit of mine trucks, exhaust and cooling fans whereas in storage piles wind blowing and high-speed winds generates dust particles. Some processes are more prevalent than the others depending on the type of mine and mineral that is being produced. In general, for a particle to be airborne it is supposed to have an aerodynamic drag force larger than the sum of the particle weight and the interparticle forces. The smaller particles such as nanoparticles will behave as a gas and are influenced by molecular forces however the larger particles are usually affected by gravitational and inertial forces. From a health and safety perspective the pivotal factors chemical composition of the dust particle, the shape and mass of particulate matter in a unit volume of air (mg/m3).  

Coal mining as an example

They are 76 known elements in coal some of which are potentially hazardous to human health. Examples of some elements include arsenic, mercury and lead. Coal also consists of mineral matter and there are about 120 minerals in coal some of which have great impact on human health. The chemical functional groups found in coal include aromatic carbon backbone structures and aromatic carbon-hydrogen structures. Also present are aliphatic chains, hydroxyl, carboxyl and carbonyl groups. The most important chemical component in coal are free radicals which by nature have great reactivity which is beneficial to energy utilization but at the detriment of human health owing to diseases such coal workers pneumoconiosis. Effectively, surfactants are used to make water work when it comes to coal dust suppression. The hydrophobic nature of coal requires improved wetting ability of water hence surfactants lower the surface tension of water thus enabling the water particles to attract coal dust particles through the process of agglomeration. Most importantly the activated water droplets should be equivalent to the size of the coal dust particles so that the fast and buoyant coal dust particles do not flow around them. Making water work also comes with the incentive of using less water. 

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Silica and Silicosis

They are various forms of silica and these silica nanoparticles pose significant health risks to the human respiratory system leading to silicosis and lung cancer. Silica toxicity is not fully understood but owing to the nanoscale size of some silica particles studies postulate that interactions occur between silica nanoparticles and the lung surfactant. In order to achieve effective silica dust suppression it is key to determine the size, surface charge and hydrophobicity given the wide spectra of the types of silica. The dust suppression of silica dust can be effective on assessment of the determinant factors which for example a hydrophobic surface would be best controlled using an amphiphilic dust suppressant such as versatile liquid polymer or surfactant. Given particle size considerations the choice of dust suppressant should also factor in the mechanism of action. If buoyancy is what you intend to tackle then the particle size of the dust suppressant should match so that it can become effective. In cases were the silica dust surface is hydrophilic then a hydrophilic dust suppressant will increase with wetting capabilities of the dust suppressant therefore increasing efficacy of dust suppression. 

Size, shape and charge

Respirable particles of dust are a critical size that matters because if ignored they have the potential to cause pulmonary damage which can lead to silicosis, coal workers pneumoconiosis and eventually loss of life due to chronic ailments such as lung cancer. Mining activities ranging from shearing, excavation, stock piling, haul mine transit just to mention but a few contribute to the generation of dust nanoparticles. The shape, size and surface charge of the dust particles govern the mode of dust suppression to be utilized. The rationale behind the use of similar size molecules to bind the suspended dust nanoparticles is to enable the combined size to force the particles to settle under its own weight owing to gravity. Coal and silica dust nanoparticles matter because failure to suppress them using targeted and effective dust suppression methods can put mine workers lives at serious risk owing to build up of inhalable and respirable coal dust in the lungs. 

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