Inhalable and respirable dust in metalliferous mining is generated from mining activities such as drilling, extraction, crushing, hauling, stockpiling and processing of minerals. The workers in mines and processing plants bear the brunt of exposure to crystalline silica and other metalliferous dust components which are potentially harmful when overexposed to mine workers. Some metalliferous ores uranium, silver and nickel contain toxic dusts cause chemical reactions within the respiratory systems or allow compounds to be absorbed into the blood stream through the alveolar walls. They are poisonous to body tissue or to specific organs. The toxicity and bioaccessibility of particulate matter depends on size mode, mass concentration, acidity, particle surface chemistry and area, particle chemistry such as metals and carbon and solubility. Many different factors affect the quantity and nature of dust generated at metalliferous mine sites, these include the mineralogy of the rock which influences rock-breakage characteristics, climatic conditions such as precipitation and wind gustiness which affect the extent of metalliferous dust dispersal. Descriptively, dust sources can either be point sources such as rock breakage circuits what are more easily controlled relative to fugitive sources such as unsealed haul roads. This second article in the series, seeks to pinpoint the sources of metalliferous dust in mining, understanding the different types of metalliferous dust and the challenges they pose to human health as well as establishing the correlation between metalliferous dust generation and efficacy of Global Road Technology innovative solutions that deal with different types of metalliferous dust at the source.
Sources of Dust
We will now take a closer look at the actual metalliferous mining-related activities which cause dust generation. Land clearing in surface mining operations requires topsoil and overburden to be removed and relocated. The dust generated from land clearing is nuisance dust as it results from diffuse sources. The larger size fraction of mineral dust generated from land clearing also limits the spatial distribution of dust generated. The process of drilling for blasting operations at metalliferous mines is a source of respirable dust with known high exposure levels for the metalliferous mine workers. The rate of dust generation during blast hole drilling increases with the diameter of the drill as well as rock hardness. Studies have shown that the rate of dust generation from sources in gold and platinum metalliferous mines is very high and that higher levels of mechanization also led to high dust generation rates. The first step in the comminution sequence to liberate valuable minerals from waste by initial rock fragmentation is blasting. Usually at open pit and strip mines blasting is a controlled operation, which occurs intermittently to fragment the rock being mined. Blasting results in the initial formation of a concentrated dust cloud that can affect the local region. The particle size distribution generated during blasting is highly variable but can produce a high proportion of fines.
Transport, Conveying and Crushing
The transportation of ore, waste rock, tailings and ore concentrate can generate significant quantities of fugitive dust. The transport of waste rock and ore at a mine site by haul trucks on unsealed roads can be one of the largest sources of particulate emission and requiring the most expensive dust control solutions. Dust emission at loading sites, unloading and stockpiling of processed materials such as iron ore can have significant impacts on the environment within the vicinity of such activities The grain size fraction of the final products is often fine and can be easily dispersed. Examples of areas of concern where metalliferous dust is generated is at ports during handling of iron ore. Crushing, milling and screening operations can be major sources of dust due to major reduction in grain size during comminution and segregation processes. It has been shown that dust emissions from common metalliferous mineral processes show a marked increase in emissions as the size of the material processed decreases although Australia has seen a number of fine grinding stirred mills being used for beneficiation of fine grained base metals ores such as zinc. Crushing occurs in a series of size reduction steps, which vary from hard ores such as copper, gold and iron which may require three stages of crushing to soft ores such as uranium that require litter to no crushing. Crushing can be done through different equipment either via slow compression of particles against rigid surfaces or by impact. Next is size reduction for mineral processing which normally involves wet grinding in tumbling mills where breakage occurs by compression, chipping and abrasion using an assortment of grinding media.
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Stockpiling and Storage
Stockpiling of ores for the short term, waste rock and tailings in the long term and other aggregate materials are an integral part to metalliferous mining operations. Stockpiles are usually left uncovered due to the access requirements for frequent material transfer. Dust emissions can occur throughout the storage cycle, during loading and unloading of metalliferous material and erosion due to strong winds. Dust emission from stockpiles will depend on the grain size of the aggregate material, age of the stockpile and moisture content. Fine grained tailings in tailings dams or other storage facilities are susceptible to wind erosion and therefore dispersion of dust. The particle size distribution of tailings is heterogeneous with sizes ranging from fine silt to sand and research shows that subsequent contamination happens to the surrounding environment by metal and metalloids bearing dusts sourced from abandoned tailings. The geochemistry and mineralogy of radioactive dust dispersed from tailings produced during uranium mining is known to cause lung cancer and fractions of dust sourced from oxidized ferruginous tailings have been found to be rick in metalloids.
Engineered solutions for managing dust
Global Road Technology offers dust control solutions metalliferous mining dust generated at different sources. Its innovative solutions target dust control at the source through use of environmentally friendly products that can effectively suppress dust. The generation of dust takes places at different areas and processes at the metalliferous mine and the chemistries related to the mechanism of action of dust control solutions provided by Global Road Technology take into account the environmental effects of their technologies from formulation to post life in the environment. It is important to tackle dust generation at the source and most importantly give the client active support in management of workplace health and safety in addition to adhering to the acceptable and allowable dust emission limits from the different dust generating activities at metalliferous mines in Australia.
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REFERENCES
McPherson, M.J. The hazardous nature of dusts. Chapter 19, Part 5.
