Intention of this work is to investigate the falling behaviour of bulk solids regarding to fugitive dust emissions. Especially for coarse bulk solids fine particles will be produced during preliminary transport movements due to abrasion. These fine particles will be the source of fugitive dust emissions if the bulk solids undergo a falling movement.
To investigate this behaviour a model bulk material is used and tested in a falling apparatus. The model bulk material consists of a mixture of coarse and fine particles (steel balls, particle size 300-600µm, and Al2O3 powder, particle size 0.1-25µm). The model bulk solid falls through a pipe into a dust chamber which is equipped with a ventilation system to suck off the dust loaded air. The particle size density distribution is measured by a scattered light particle counter sizer (PALAS, PCS-2010).
Relating the particle size distribution from the model bulk material q0,m(x) to the pure Al2O3 powder q0,f(x) results in a separation function T(x) (fig. 1):
(1)
The dust concentration increases with decreasing the w% of Al2O3. If the w% of Al2O3 falls below 20% the T(x) curve forms a minimum. From these results and from optical and visual observations a model consideration to explain this behaviour can be as follows:
With increasing bulk material mass flow an additional downwards air stream is generated inside the tube. At the bottom of the dust chamber the additional air stream turns back up to a certain height and is sucked into the core of the falling bulk solid. This creates a cycle air stream - a mixing zone evolves - reaching from the bottom up to a certain height where fine particles are transported with turbulent air flow. The fine particles (Al2O3), which are initially coming into the tube, are falling down more or less as agglomerates. The large agglomerates of the falling bulk solid stream will be deposited at the bottom. The smaller ones - in the range of app. 10 µm - will be immediately sucked off. The next smaller agglomerates (app. 2 µm) are following the upwards cycle air stream, sucked into the core bulk material stream, hit by steel balls and will be disaggregated into finer particles. Falling tests with steel balls alone on a deposited Al2O3 bed at the bottom of the dust chamber show no relevant re-dispersing of the deposited material.
The conclusion is that destroying agglomerates happen during the falling process of the bulk solid in the region of a “mixing zone” and not as assumed earlier at the bottom of the dust chamber.
To know the location and width of the T(x) curve minimum is important to describe how minimization measures affect dust generation during a bulk solids falling process. Pre-preparation of bulk solids may affect this minimum. But also knowledge of this minimum may influence the choice of the following dust separation technique.
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