Understanding passive control of bulk material dust emissions in discrete feed sitution

Research Doctorate - Doctor of Philosophy (PhD)Fugitive emissions arising from the handling of particulate materials have occurred since these materials were first handled. As the volume of materials handled has increased so has the volume of fugitive materials; the fraction of the fugitive material focused on in this thesis, we generally call dust. In modern operations, dust represents a lost income stream, as the dust is often saleable product; increased cost of operations due to cleaning costs; a public nuisance for those working and living adjacent to material-handling operations; an explosion risk with some dust types; an environmental pollutant, and significantly a multitude of health risks due to slips, trips and inhalation of dust (Hamelmann & Schmidt). Government regulation through various environmental pollution agencies is continually reducing the so-termed acceptable limits for dust emissions associated with different operations and in different environments. As these limits reduce, a greater level of operational care and often cost is required by the operators to comply. Active dust control is often seen as an easy method to reduce emissions. In the simplest form, an air mover is used to pull dust laden air away from the process in question, before air separation and returning the fine product back to the process line. Active measures clearly carry an energy penalty to operate the air movers, but also have high costs associated with duct and pipe wear, air-mover filtration system maintenance and reliability issues. Further, there are many areas where active dust control is impractical. Passive dust control in contrast, through an understanding of the mechanics of dust generation, aims to reduce the total emissions through a range of mechanisms, without external energy input. This can be achieved for example by embedding the smaller particles fractions deeper within a flowing stream (Wheeler, Krull, Roberts, & Wiche, 2007), or careful positioning of baffles and stilling zones in equipment. Unfortunately with passive control systems, there is a much greater need to ‘understand the system’ before an appropriate passive solution can be implemented. This is unlike a high-cost active system where a simple up-scaling of equipment can often overcome a poor understanding of the system. This thesis presents a volume of work expanding the overall understanding of the underlying mechanics required for passive dust control, particularly related to transient events such as dump hoppers. Air entrained into a falling stream of bulk material under the influence of gravity is known as the main source of dust generation (R. Ansart, A. d. Ryck, & J. A. Dodds, 2009b). For calculating the amount of entrained air, several researches and formulas have been published. However, the accuracy of different formulas varies under different conditions. To understand the impact of different parameters on the behaviour of a material stream, this research was undertaken in a particular sequence, to enable detailed analysis in a controlled manner. In general, free falling largely contributes to the discrete bulkmaterial-handling case activities. Therefore, research was initiated on free-falling streams to better identify the impact of key influencing variables. This research provided an insight into the free-falling and influencing parameters such as particle diameter, particle solid density, stream diameter and drop height. Resulting from these investigations, a new approach to calculate the magnitude of air entrained into the free-falling stream of particles (AVCF - Air-Entrainment using Volume Conservation Theory in Free-Falling) has been developed (Esmaili, Donohue, Wheeler, McBride, & Roberts, 2013). Computational Fluid Dynamics (CFD) simulation was successfully applied to different free-falling conditions. These simulations were then verified with Particle Imaging Velocimetry (PIV), high-speed videos and particle-tracking techniques. A close correlation between the provided theory (AVCF), CFD and experimental data was observed. The knowledge obtained from the analysis of free falling was extended to transient bulkmaterial-handling cases. Dump hoppers are known to be one the major dust generation sources within the mining industry (Cecala, 2010), and was selected to extend the work completed on free falling air entrainment research. Scale model tests were conducted to provide air velocity readings, airflow patterns and high-quality footage for simulation verification purposes. The two-phase CFD simulation method is considered as a compromise solution, optimising accuracy, time consumption and ease-of-use considerations. The potential to further improve performance in the scale model tests were investigated using these CFD method. The influences of the drop height, particle size, particle density and the design of the dump hopper geometry, in relation to the air-flow pattern within the enclosure, and subsequently reducing dust emissions are discussed. Moreover, the effects of ‘air cells’, ‘curtains’, ‘baffles’ and introduced new technique (additional section) are investigated. The results reveal that the flow pattern of the air within the dump hopper geometry can be improved and the flow slowed down if air cells, baffles, curtains or any other technique is utilised appropriately. In addition to the above, the introduced theory work for the air-entrainment-magnitude calculation concept has been expanded to dump hoppers by utilising particle trajectory analysis. It has been concluded that this formula provides a close correlation to simulation