Importance of Dust Suppression in Coal Mines

Australian regulations on dust suppression in coal mines have been at the centre of scrutiny following the resurgence of coal mine workers pneumoconiosis is popularly known as ‘black lung’ disease. Re-emergence of black lung disease from 2015 and beyond has been experienced in Queensland. The assumption from the coal mining industry was that the disease had been completed eradicated in the last 50 years, but evidence depicts a different story. The historical timeline from 2015 to date shows that the launch of the Black Lung campaign by the Construction, Forestry, Maritime and Energy Union (CFMEU) led protests against the complacency in the health system citing the need for rectification or perhaps even a complete overhaul. Available records of diagnosed cases were small relative to the true picture which meant the reported cases could have just been a drop in the ocean compared to the actual reality. A year later, CFMEU ramped up their efforts to get a voice in the implementing systematic changes by lobbying for experts to assess x-rays as black lung resurgence was a matter of urgency. Bowing to the pressure of CFMEU and other advocacy groups the government established a Senate enquiry the same year which led to health and safety reforms being enacted into law the following year through the Coal Mining and Health Regulation 2017. The article seeks to highlight properties of coal, where dust is generated in coal mines and what is used for coal dust suppression, what works and why it works. 

Why is coal dust is so risky?

Coal dust is both a health and safety issue. The high concentration of coal dust produced in either open-pit or underground coal mines can lead not only to the death of miners suffering from pneumoconiosis but also to coal dust explosions and even gas explosions, which can result in major personal injury and economic loss.  Legislation in Queensland states that a coal mine’s safety and health management system must provide ways of ensuring each coal mine worker’s exposed to respirable dust at the mine is kept to an acceptable level. It is a strict regulation that the worker does not breathe an atmosphere at the mine containing respirable dust exceeding an average concentration 3 milligrams per cubic meter air for coal dust which is calculated under AS 2985:2009 over an 8 hour period. Shifts that go beyond the stipulated 8 hours must make sure that exposure to respirable dust does not exceed that of a person working an 8-hour shift. It is stipulated that if ever the average concentration of coal dust in the atmosphere is above the levels stated then controls for minimizing dust must be reviewed. 

Coal mines utilize measures such as ventilation dilution, water spray suppression and stabilizing to prevent dusts although the main one is water spraying owing to its easy maintenance. However, due to the high surface tension of water, fine dust particles are difficult to wet using water this in addition to the high hydrophobicity of coal dust hence coal dust is not well suppressed. Efficiency of water spray is generally less than 50%. The poor hydrophilicity of coal dust with predominant hydrophobic properties renders water ineffective for coal dust but addition of surfactants in water improves wettability of coal dust. Surfactants effectively improve the dust wetting ability of water and there are four types of surfactants namely anionic, cationic, non-ionic and zwitterionic surfactants. Research has shown that coal dust wetting properties of anionic and non-ionic surfactants are better than cationic surfactants. Some other school of thoughts approach efficacy of surfactants not just from the ability to reduce the surface tension of water citing that surfactant solutions with low surface tension do not necessarily indicate that it works for the various coal dust particles. Therefore surface tension is a physical parameter of the surfactant solution and not related to coal particles under 38 microns which presents a limitation when just approaching it from a surface tension perspective. Hence we further investigate what about coal should be understood to achieve effective coal dust suppressions.

Challenges in managing coal dust

Coal dust particles are generated from the numerous production processes including drill and blast, shearing or loading, mining, coal stockpiling, transportation, processing and utilization. There is also coal dust generated from conveyor transport which is driven by collision and falling of coal at subsurface transfer points and through ventilation airflow in the coal mine and the natural wind from the outdoor environment. The degree of coal metamorphism varies in different mining areas which affects its wettability as we now tackle what needs to be understood about coal itself in order to achieve effective dust suppression. The physical chemistry characteristics of coal vary with different metamorphic stages of coal which has an effect on wettability. Chemically, the benzene rings, aromatic hydrocarbons with benzene rings, aliphatic hydrocarbons with methyl and methylene are hydrophobic. Oxygen containing functional groups represented by hydroxyl and carboxyl groups and silicates and carbonate minerals are hydrophilic. The chemistry of coal face is then governed by the primary hydrophobic sites coupled with secondary hydrophilic sites. 

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      Lin Yang(Ms.)

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